Direction-sensitive direct dark mattersearch experiment with a gaseous TPC

Hironobu Nishimura
Department of Physics, Faculty of Science,
Kyoto university
Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan

This thesis was submitted to the Department of Physics, Graduate School of Science,
Kyoto University in partial fulfillment of the requirements for the degree of Doctor ofPhilosophy in physics.
January 5, 2009

Abstract
A lot of observational results indicate the existence of dark matters in the universe. Weakly
Interacting Massive Particle (WIMP) is one of the leading candidates of the dark matter.
Some theories predict that WIMPs can be directly detected via elastic scatterings with
nuclei in detectors. Many dark matter search experiments have been performed so far andone of them claimed the detection of the dark matter signal, but no experiments have neversuported its result. We need other methods which should provide more reliable signals of
the dark matter. The direction-sensitive method is one of them.
NEWAGE is one of the direction-sensitive direct dark matter search projects. A time
projection chamber with a micro-pattern gas device (µ-TPC) was developed as a proto
type direction-secsitive dark matter detector, (NEWAGE-0.3a detector). We measured theperformance of the NEWAGE-0.3a detector and operated it in an undergroound labolatory.
We performed a dark matter search run after background studies and an optimization ofoperation parameters. We obtained a 90% C.L. spin-dependent WIMP-proton cross-section
limit of 5.4 × 103 pb for a WIMP mass of 150 GeV/c2 with a directional analysis.

Contents
1 Introduction
1.1 Dark matter problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.1 Cosmological dark matter . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.2 Galaxy rotational curves and dark matter . . . . . . . . . . . . . . . .
1.1.3 Local halo density and isothermal halo . . . . . . . . . . . . . . . . . .
1.2 Dark matter candidate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.1 Axion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.2 WIMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 SUSY dark matter - LSP neutralino . . . . . . . . . . . . . . . . . . . . . . .
1.3.1 SUSY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.2 Neutralino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4 Review of neutralino search experiment . . . . . . . . . . . . . . . . . . . . .
1.4.1 Direct search . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4.2 Indirect search . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4.3 Search by accelerator experiment . . . . . . . . . . . . . . . . . . . . .

5
6
6
7
8
11
11
11
11
12
13
15
15
16
18

2 Direct detection of neutralino
2.1 Dynamics of the Earth, the Sun and the neutralino . . . . . . . . . . . . . . .
2.2 Event rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Cross section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Nuclear form factor correction . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5 Distinct signal of the dark matter . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.1 Nuclear target dependence . . . . . . . . . . . . . . . . . . . . . . . .
2.5.2 Annual modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.3 Directional asymmetry . . . . . . . . . . . . . . . . . . . . . . . . . . .

19
19
20
23
30
33
33
35
35

3 Review of direct dark matter search experiments
3.1 Solid scintillator experiments . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Bolometers (cryogenic detectors) . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Liquid noble element detectors . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4 Superheated droplet detectors . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5 Gas detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5.1 Gas selection for the direction-sensitive WIMP search . . . . . . . . .
3.5.2 Gaseous detector experiments . . . . . . . . . . . . . . . . . . . . . . .

39
39
42
44
45
45
45
48

4 NEWAGE-0.3a detector
4.1 NEWAGE-0.3a detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.1 Property of CF4 gas . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.2 NEWAGE-0.3a detector . . . . . . . . . . . . . . . . . . . . . . . . . .

51
51
51
53

2

CONTENTS
4.2

4.3
4.4

4.5

Energy calibration and stability . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1 Drift velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.2 Energy cariblation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.3 Position-dependence of the gas gains . . . . . . . . . . . . . . . . . . .
4.2.4 Energy resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.5 Time-dependence of the gas gains . . . . . . . . . . . . . . . . . . . .
Gamma-ray rejection power . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Response to nuclear recoils . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.1 Absolute detection efficiency . . . . . . . . . . . . . . . . . . . . . . .
4.4.2 Directional-dependent efficiency . . . . . . . . . . . . . . . . . . . . . .
4.4.3 Angular resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Summary of detector performance . . . . . . . . . . . . . . . . . . . . . . . .

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62
64
66
67
70
70
72
72
76

5 Pilot runs and background studies in Kamioka mine77
5.1 Dark matter search in a surface laboratory . . . . . . . . . . . . . . . . . . . 77
5.2 Pilot runs in an underground laboratory . . . . . . . . . . . . . . . . . . . . . 79
5.2.1 Kamioka RUN-I ∼ first underground run . . . . . . . . . . . . . . . . 82
5.2.2 Kamioka RUN-II ∼ radon-rich run . . . . . . . . . . . . . . . . . . . . 84
5.2.3 Summary of the pilot runs . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.3 Background from outside the vacuum chamber . . . . . . . . . . . . . . . . . 87
5.3.1 Cosmic-ray muons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.3.2 Gamma-rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.3.3 Neutrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
5.4 Background from inside the chamber material (Contributions of α-particles ) 90
5.4.1 Contributions of Rn progeny in gas chamber A . . . . . . . . . . . . 93
5.4.2 Contributions of U and Th in the drift plane B . . . . . . . . . . . . 98
5.4.3 Contributions of U and Th in the GEM and the µ-PIC C D . . . . . 98
5.5 β-ray from 40 K inside the vacuum chamber . . . . . . . . . . . . . . . . . . . 102
5.6 Summary of the background study . . . . . . . . . . . . . . . . . . . . . . . . 104
6 Dark matter search in Kamioka mine106
6.1 Dark matter search run in Kamioka mine . . . . . . . . . . . . . . . . . . . . 106
6.2 Dark matter limits by a conventional method . . . . . . . . . . . . . . . . . . 116
6.3 Dark matter search by a direction-sensitive method . . . . . . . . . . . . . . . 116
7 Discussion
124
7.1 Remaining background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
7.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
7.2.1 Background rejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
7.2.2 Improvement of the angular resolution . . . . . . . . . . . . . . . . . . 127
7.2.3 Lowering the threshold . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
7.2.4 Track-sense measurement . . . . . . . . . . . . . . . . . . . . . . . . . 128
7.2.5 Future of the NEWAGE . . . . . . . . . . . . . . . . . . . . . . . . . . 128
8 Conclusion

132

9 Acknowledgments

133

A Radioactive isotpes in materials
134
A.1 238 U-chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
A.2 232 Th-chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
A.3 40 K . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
3

CONTENTS
B µ-PICs and µ-TPC
139
B.1 µ-PIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
B.2 µ-TPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

4

Chapter 1

Introduction
Recent results from Cosmic Microwave Background (CMB)[1], Large Scale Structure (LSS)
and Type Ia supernovae observations have established a standard model of cosmology. The
model favors a flat universe consisting of more than 70 % dark energy, about 22 % dark matter, and the remainder of the ordinary (Baryon) matter, as shown in Figure 1.1. Answering

Figure 1.1: Composition of the universe. The universe consists of 73% dark energy, 22%
dark matter and 5% ordinary matter.
the questions ”what is the dark energy” and ”what is thw dark matter” became one of themost challenging tasks for physicists in recent year. This thesis and underlying experimentare intended as an exploration of the dark matter problem. A discovery of the dark matterparticle will become both a strong support of the cosmological observations and predictions,and a piece of evidence for new physics beyond the standard model of the particle physics.
In this chapter, we first introduce the problem of the missing mass in the universe
and evidence from cosmological observations that suggest a large amount of matter in the5Chapter 1. Introduction
universe is dark. We then describe some theoretical explanations and models that may be
used to explain the mysterious dark matter problem focusing on candidates for the dark
matter.

1.1

Dark matter problem

Existence of the dark matter is suggested by some astrophysical observations; cosmologicalobservations and its theory, galaxy claster observcations, galaxy rotation curves, and so on.
We review some of them and a distribution model of the dark matter in this section.

1.1.1

Cosmological dark matterIn recent years, cosmological parameters such as the Hubble constant H0 , the mass density
parameter Ωm , the baryon density parameter Ωb and the total density parameter Ω0 havebeen precisely measured. In the cosmological model, the density parameters are defined as
the ratio of the density to the critical density ρc = 1.05 × 10−5 h2 GeVcm−3 , as shown inEq.(1.1).
Ωi ≡

8πG
ρi
ρi
=
ρc
3H0 2

(1.1)

Here h ≡ H0 /(100[km/sec/Mpc]) and G = 6.71 × 10−39 h̄c(GeV/c2 )−2 is the gravitational
constant. The total density parameter of the Universe, Ω0 , is directly related to its present
curvature. A flat Universe has Ω0 = 1, an open Universe has Ω0 < 1, and a closed Universe
has Ω0 > 1. These parameters were restricted by cosmic microwave background (CMB)
radiation measurements, observations of the Type Ia supernova, and the cluster galaxies.
The CMB radiation is the radiant left over from the big bang when the universe wascool. The radiation is highly uniform across the sky, but there are tiny fluctuations (alsocalled anisotropy) associated with fluctuations of the matter density in the early universe.
The NASA’s COBE satellite firstly detected this anisotropy in 1992, and the Wilkinson
Microwave Anisotropy Probe (WMAP) reported a more accurate result [1]. The WMAP
results determined the cosmological model parameters as the matter density of the universeΩm h2 = 0.1326 ± 0.0063, and the Hubble constant h = 0.719 ± 0.026. In addition, the
WMAP data and the deuterium observation data agree on the baryon density isΩb h2 = 0.02273 ± 0.00062,(1.2)

based on the Big Bang nucleosynthesis(BBN) [2]. This means that non-baryonic dark matteris required beyond any doubt. The measurements and observations of the distance-redshift
relation of Type Ia supernovae found that the expansion of the Universe is accelerated. This
acceleration suggests an unknown type of energy, dark energy. The Type Ia supernovae
observation data [3] suggests a total energy density ΩΛ , which is the dark energy, around
0.7 and the mass density Ωm around 0.3 for a flat (ΩΛ + Ωm = 1) cosmology.
The gravitational instability and small fluctuations of the initial density field of the
Universe cause the formation of a large-scale structure and the galaxy distribution in today’s
Universe. Thus the measurement of the distribution of galaxy clusters can be used todetermined the mass density of the universe, and this is particularly investigated by the
Sloan Digital Sky Surveys (SDSS)[5]. Combined with the CMB anisotropy measurement by
WMAP, SDSS results favor a low-density Universe with Ωm ∼ 0.30 ± 0.04.
Combining the high-redshift supernova survey, the galaxy cluster observations and thecosmic microwave background measurements, the normalized mass and energy density Ωm6

Dark matter problem
and ΩΛ nicely converge in the cosmological parameter space as shown in Fig. 1.2, whichindicates an inflationary and flat cosmological models.

Figure 1.2: Cosmological parameter space from high-redshift supernova survey [3], galaxy
cluster observation [4] and cosmic microwave background measurement.

1.1.2

Galaxy rotational curves and dark matter

The clearest evidence for the existence of the dark matter is observed by the difference
between the matter distribution implied by the observed luminosity and that implied by therotation velocity, vc , of the spiral galaxies.
The rotation velocity distribution of a spiral galaxy expected from Kepler’s law is
M (r)[vc (r)]2
=G 2
r
r

(1.3)

where r is the radial distance from the center of the galaxy, and M (r) is the total mass
within the radius r. Since the orbital velocity can be measured by the Doppler shifts of thespectrum of the stars, the 21 cm line of the HI gas (neutral hydrogen gas) and 3.6 mm lineof the CO gas, the mass distributions of the spiral galaxies are estimated from Eq.(1.3). On
the other hand, the luminous matter is found concentrated near the center of the galaxy. If
the total mass of the galaxy concentrate near the luminous core of the galaxy, v must fall
like r−1/2 . The measured velocities, however, remain constant out to the distance as far as
7

Chapter 1. Introduction
can be measured in almost all cases. This implies that M (r) ∝ r at a large radius, and there
exists a large amount of matter of a non-luminous mass which exceeds the luminous one.
Fig. 1.3 shows the rotational curve of the spiral galaxy NGC6503 [6]. The luminous disk
extends no further than approximately 5 kpc from the center of the galaxy. If the luminous
matter concentrated within this radius, the rotational curve would drop at large radius.
From the discrepancy between the observed rotational curve and that expected from theluminous disk and gas, the existence of a dark halo is inferred. Based on this method, thetypical mass density of the dark halo is estimated to beΩhalo ≥ 0.1(1.4)

On the other hand, the mass density of the luminous part of the galaxy is estimated to be,Ωluminous ≤ 0.01

(1.5)

Thus the comparison between Eq. (1.4) and (1.5) indicates that the dark matter exists atthe scale of a single galaxy. Some searches for baryonic non-luminous objects appeared that
there are some non-luminous mass in the galaxies but not enough to explain the galacticrotations.

Figure 1.3: Rotation curve of the spiral galaxy NGC6503 [6]. The points indicate themeasured rotation velocities as a function of the distance from the galactic center. The
dashed and dotted curves are the contribution to the rotation velocity from the observed
disk and gas, respectively. The dash-dotted curve is the contribution from the dark halo.
The solid line is a sum of them.

1.1.3

Local halo density and isothermal halo

Although the determination of a halo model in our Galaxy is currently a very active topic
in astrophysics, both a local halo density, ρ0 , and the dark matter velocity distribution
8

Dark matter problem
represent a significant systematic uncertainty. Therefore, the dark matter direct detection
community uses the simplest reasonable model, the isothermal halo model.
In determining ρ0 and v0 , the galactic rotational curve is the most important observational quantity. The rotational curve of our Galaxy cannot be measured with a same
precision as that of an external spiral galaxy, for instance NGC6503 in Fig. 1.3. However,assuming that our Galaxy is an ordinary spiral galaxy like NGC6503, the rotation velocity
of our Galaxy increases linearly from zero at the center to roughly vc =220 km/s in thesolar neighborhood and remains roughly flat all the way out to ∼ 25 kpc [7]. The observed
rotational curve for our Galaxy is shown in Fig. 1.4.
In the isothermal halo, the shape of the rotational curve could be explained by assumingthat the density distribution of the dark halo isρ(r) =

ρ0
1 + r2 /r0 2(1.6)

where ρ0 and r0 are fitting parameters. Using the observed rotation velocity in the solar
neighborhood, the local density of the halo dark matter is estimated to beρ(r) ∼ 0.3GeVc−2 cm−3

(1.7)

with an uncertainty of a factor two or even more [8].
The following values are adopted in this thesis as in many dark matter search experiments.
ρ(r) ∼ 0.3GeVc−2 cm−3v0 ∼ vc ∼ 220km/s

9

(1.8)

Chapter 1. Introduction

Figure 1.4: Rotation curve of our Galaxy [7]. Square points with error bars indicate measured rotation velocities as a function of distance from the galactic center. The different
lines represent the contributions from the bulge (dotted), the disk(filled circles), the HI layer(crosses), the H2 layer (circles) and from the dark halo (dashed). The solid line represents
the sum of the contributions.

10Dark matter candidate

1.2

Dark matter candidate

The cosmological and galactic observations indicated that non-baryonic dark matters exist
with a density parameter of ΩDM ∼ 0.2−0.3 since Ωb (∼ 0.022) is much less than Ωm (0.132).
Two types of non-baryonic dark matter candiates are considered. One is the Hot Dark
Matter(HDM), which travels with relativistic velocity. Another one is the Cold Dark Matter(CDM), which travels with non-relativistic velocity. The best candidate for the identityof the HDM is the neutrino, however, galactic cluster observations and WMAP results
restricted the neutrino density to 0.003 ≤ Ων ≤ 0.10[9]. On the other hand, physicists havesuggested many candidates of the CDM based on various theories. In this section, we review
two plausible CDM candiates; axion, and WIMP.

1.2.1

Axion

The axion was originally proposed as a neutral new light pseudo-scalar boson to solve theCP conservation problem of strong interactions [10]. The CP invariance of strong interactions is violated in the context of the quantum chromodynamics (QCD) theory, which was
not observed by any experiments. Although the theories do not predict the axion mass,laboratory experiments and implications from astrophysical and cosmological effects haveexcluded the most of the mass range, leaving only two windows, 10−6 eV ≤ ma ≤ 10−3 eV
and 2 eV ≤ ma ≤ 5 eV, for further explorations. The axion with its mass in the lighter
window could be a candidate of the CDM and one with its mass in the heavier windows
could be produced in the Sun (the Solar axion).
The CDM axion, has been searched with the Sikivie radio frequency cavity technique.
The most recent experiment searched the axion mass range of 2.3 µeV ≤ ma ≤ 3.4µeV andobtained no positive signal of the CDM axion[11].
The Solar axion has been searched with superconducting magnet telescopes using theinverse Primakoff effect [12] (a→← 2γ). Recent results from the CERN Axion Solar Telescope[13] and Tokyo helioscope group[14] imply upper limits on the axion-photon coupling, gaγ <1.16 × 10−10 GeV−1 at 95% C.L. for the axion mass of ma ≤ 0.02 eV, and gaγ < 5.6 − 13.4 ×
10−10 GeV−1 for the axion mass of 0.84 < ma < 1.00 eV, respectively (Figure 1.5).

1.2.2

WIMP

The widely discussed the non-baryonic dark matter candidatse are Weakly Interacting Massive Particles (WIMPs). ”Weakly interacting” means that these particles interact only
via gravity and any weaker force than electromagnetic force, ”Massive” means sufficiently
massive to solve the structure-formation problem brought on by a baryons-only, or even aneutrinos and baryons, the Universe. This would require a WIMP mass of at least 1 keV.
Incidentally, the particle physics provides some good generic rationale for the existence of
WIMPs. Supersymmetry (SUSY), a well-motivated and presently most-favored extension
from the Standard Model of the particle physics, implies WIMPs. The details of the SUSY
dark matter are described in Section 1.3. On the other hand, theories of the universal extra
dimensions in which Kaluza-Klein parity is conserved provide a second possible class known
as the lightest Kaluza-Klein particle (LKP)[16, 17, 18].

1.3

SUSY dark matter - LSP neutralino

The lightest new particle in the SUSY, neutralino, is the most plausible candidate of thedark matter because there are much more agreements between the expected property of the11Chapter 1. Introduction

Figure 1.5: Solar axion search results. 95% C.L exclusion lines of each experiment to theaxion-photon coupling constant as a function of the axion mass are shown. Region above
the limit lines are excluded and yellow region is the theoretical allowed region for axion.
neutralino and the cosmological requirement than other candidates and the proper rationaleof the existence of the neutralino.

1.3.1

SUSY

The standard model (SM), which is a non-Abelian gauge theory based on the gauge group
SU (3)color × SU (2)left × U (1)hypercharge of strong, electromagnetic and weak interactions acting on the quark and lepton multiplet, is an extraordinarily successful theory, and describes
all experimental results on the interactions between elementary particles with a surprising
accuracy. However, the SM has its natural drawbacks and unresolved problems. Some ofthe problems are listed below.
• The SM has a large number of free parameters.
• There is the strong CP problem in the SM.
• The SM is not unified with gravity.
• The flavor mixing and the number of generations are arbitrary in the SM.
• The hierarchy problem occurs in the SM.
• The grand unification is not realized.
SUSY theory introduces hypothtical ”symmetrical” particles and solve the ”hierarchy problem”. SUSY presupposes that each elementary particle with a spin j in the SM has asupersymmetric partner with a spin |j − 1/2| as shown in Table 1.1
From precise measurements of the coupling constants and the solution of the renormalization group equations for the evolution of the coupling constants, the possibility of the12

SUSY dark matter - LSP neutralino

Normal particles
Symbolq = u, c, t
q = d, s, b
l = e, µ, τνe , νµ , ντNameup quarks
down quarks
leptons
neutrinos

Spin
1/2
1/2
1/2
1/2

g
W±
H±

gluons
W bosons
charged Higgs

1
1
0

γ
Z0
h0 (H20 )H 0 (H10 )A0 (H20 , P0 )photonZ bosonlight scaler Higgs
heavy scaler Higgs
pseudoscaler Higgs

1
1
0
0
0

SUSYparticles
Symbolq̃u1 , · · · , q̃u6
q̃d1 , · · · , q̃d6
˜l1 , · · · , l̃6ν̃1 , ν̃2 , ν̃3Nameup squarks
down squarks
sleprtons
sneutrinos

Spin
0
0
0
0

g̃
±
χ̃±
1 , χ̃2gluinos
charginos

1/2
1/2

χ̃01 , · · · , χ̃04

neutralinos

1/2

Table 1.1: SUSY particles

unification with the gravity within the SM was excluded. On the other hand, SUSY particles affect the slope of the couplings, and unification of the coupling constants is realized at1016 GeV/c2 in the case of SUSY breaking scale, Msusy ∼ 1 TeV/c2 as shown in Fig. 1.6. In
general, Msusy must be in the range from 100 GeV/c2 to about 10 TeV/c2 to provide thisunification.

1.3.2

Neutralino

In the minimum supersymmetric extension of the standard model (MSSM), where the standard model is extended with only an extra Higgs doublet and partners required to form
supersymmetric multiplets, the interactions of SUSY particles with ordinary particles aregoverned by ”R-parity”. The R-parity is given by
R = (−1)3B+L+2S ,(1.9)

where B is the baryon number, L is the lepton number and S is the spin. The supersymmetricparticles are assigned odd R-parities and the ordinary particles are assigned even R-parities.
Conservation of the R-parity has following two consequences.
• The supersymmetric particles are created in pairs.
• The lightest supersymmetric particle (LSP) is stable.
The lightest neutralino (χ ≡ χ̃01 ), the lowest-mass linear combination of the photino (γ̃),
the zino (Z̃) and the higgsinos (H̃1 , H̃2 ), is the leading candidate for the LSP and also for the
non-baryonic cold dark matter. The photino and zino are expressed with the superpartners
of gauge bosons (Gauginos), Bino (B̃) and Wino (W̃ ) asγ̃ = cos θW B̃ + sin θW W̃3 Z̃ = − sin θW B̃ + cos θW W̃3 .
13(1.10)

α-1
( µ)
i

Chapter 1. Introduction

60

α-1
( µ)
1

50

40

α-1
( µ)
2

30

20

α-1
( µ)
3

10

α-1
( µ)
i

0

3

10

5

104 10610Standard Model8107 10

9

10

10 10

13

1011 1012 10151014 1016

10µ[GeV]

60

50

α-1
( µ)
1

40

α-1
2 ( µ)
30

20

α-13 ( µ)

10

0

3

10

4

10

5

10

6

10

MSSM Model

7

10

8

10

9

10

10 10

11

10

12

10

13

10

14

10

15

10

16

10
µ[GeV]

Figure 1.6: Running of couplings in the Standard Model(SM) (top) and in the Minimal Supersymmetric Standard Model (MSSM) (bottom). α1 , α2 and α3 are the U (1) hypercharge,
SU (2) electroweak and SU (3) strong couplings, respectibely. Only in the bottom case theunification of the coupling constants is realized. [19]14Review of neutralino search experimentThus, χ can be written asχ = a1 B̃ + a2 W̃3 + a3 H̃1 + a4 H̃2 .
The neutralino mass matrix is written as

M1
0

0
M
2

 −MZ cos β sin θW
MZ sin β sin θW
MZ cos β cos θW −MZ sin β cos θW−MZ cos β sin θW
MZ sin β sin θW0−µ(1.11)
MZ cos β cos θW−MZ sin β cos θW 
 (1.12)−µ0where M1 and M2 are the respective mass parameters of U(1) and SU(2) gaugino, µ is thehiggsino mass parameter, sin2 θW is the weak mixing angle, MZ is the Z bozon mass andtan β ≡ v2 /v1 , where v1 and v2 are the vacuum expectation values of the two Higgses. If
we assume the grand unification, M1 = (5/3)M2 tan2 θW ∼ 0.5M2 is obtained, thus theneutralino mass matrix is characterized by three parameters: µ, M2 and tan β. In general,
three basic configurations exist:• Gaugino dominated: If M1 , M2 ≪ µ, then the lightest neutralino is primarily thegaugino.
• Higgsino dominated : If µ ≪ M1 , M2 , then the lightest neutralino is primarily thehiggsino.
• Mixed: If M1 ∼ µ, then the neutralino is a roughly equal mixture of the gaugino andthe higgsino.
It should be noted that the bino, wino and higgsino have different interactions, so thecomposition affects both the annihilation cross section and thus the relic density as well as
the cross section for the neutralino-quark scattering.

1.4

Review of neutralino search experiment

There have been three types of neutralino search experiments: direct search experiments,
indirect searches, and searches by accelerator experiments. We will review these experimentsin this section.

1.4.1

Direct search

If the neutralino, WIMPs, exist as the dark matter, the earth encounters the dark matter
halo with the relative speed of our solar system against that of the Galaxy. The WIMPs fromthe halo can elastically scatter off the target nucleus of a detector in the laboratory. Thus,
the WIMPs can directory be detector as nuclear recoil signals. These signals are expcted toprovide information on the dark matter mass and the neutralino-quark cross sections withsome astrophysical assumptions (See Chapter 2).
The WIMP recoil event is very rare and the expected energy spectrum has a noncharacteristic spectrum of a simple exponential spectrum, while the ambient γ-rays andneutrons make background events with energy similar to that from WIMPs. Thus the main
challenge of the direct detection of dark matter experiments is the reduction and the discrimination of background events. Recently, the direct detection of dark-matter experiments areamong the most exciting experimental physics around the world. These experiments require
15

Chapter 1. Introductionχχ →annihilation channels
f f¯
W + W−
Z 0Z 0+ −W H , W −H +Z 0 A00 0
Z H , Z 0 h0
0 0
A A , H 0 H 0 , h0 h0 , H 0 h0
A0 H 0 , A0 h0
H +, H −gg, q q̄g
Z 0 γ, γγTable 1.2: The annihilation channel of the neutralino [15]. f and f¯ are the standard model
neutrinos, leptons and quarks, and A0 are axions.

a very low background environment, thus they are usually located in deep undergroundlaboratories. This thesis represents one of this type of experiments.
We will discuss the expected the WIMP event rate in Chapter 2 and the variety ofdetectors that currently search for the WIMP dark matter in Chapter 3.

1.4.2

Indirect search

The dark matter, neutralinos, is expected to be trapped in the core of the Galactic, thecenter of the Earth, and the Sun or may locally concentrate by some other reasons. In
such a neutralino concentration area, the annihilation of the neutralino may often occur andproduce γ-rays and also other particles since the neutralino is a majonara fermion. The
annihilation process has many channels as listed in Table 1.2 [15]. The detection of theannihilation products, such as γ- rays, positrons, anti-protons and neutralinos, etc., from
specific directions and locations will provide indirect information on the dark matter.
Gamma ray γ-rays may result from annihilation χχ → γγ or χχ → Zγ [15]. The rates ofthese processes are difficult to be estimated because of the uncertainties in SUSY parametersand halo density profiles. However, the γ-ray intensity from the neutralino annihilation in
the galactic halo has a characteristic direction dependence.
The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma
Ray Observatory has observed an about 60% higher integrated intensity for γ-rays from thegalactic plane above 1 GeV than a model calculation of the emission based on the dynamicbalance and the realistic interstellar matter and the photon distributions [28]. On the other
hand, Elsaesser and Mannheim compared the extragalactic γ-ray background from EGRET
data with high-resolution simulations of the cosmic structure formation, and found a best
fit value of neutralino mass around 515+110−75 GeV [29]. But it appeared that the result of[29] had a systematic uncertainty in the high energy region. The next generation γ-ray
observatory GLAST (Gamma-Ray Large Area Space Telescope) was launched in June, 2008
and renamed as ”Fermi”. Fermi satelite is expected to reach a sensitivity of 2×10−9 photons
cm−2 s−1 , which is 30 times better than EGRET, for γ-rays in enegy range of 100 MeV to
300 GeV after two year’s observation [30] .
16

Review of neutralino search experiment
Antimatter Self annihilation of WIMPs will also produce particles such as positron (e+ ),antiprotons (p̄), antideuterons (D̄) and other heavy anti-nuclei. The measured antiproton
or antideuteron spectrum can provide information on the light neutralino and can be used
to put stringent constrains on the supersymmetric configuration [20, 21].
The searches for antiprotons or antideuterons have been performed by the Balloon-borne
Experiment with Superconducting Spectrometer (BESS). Fir the antiprotons searche, theyfound the effect of the solar modulation to the antiproton spectrum was large and havenever achieved the dark matter contribution[22]. For the cosmic antideuteron search, no
candidates of antideuteron was found from data during four balloon flights from 1997 to
2000. An upper limit of 1.9×10−4(m2 s sr GeV/nucleon)−1 at the 95% confidence level wasderived for the differential flux of cosmic-ray antideutrons at the top of the atmosphere withenergy range of 0.17 and 1.15 GeV/nucleus [23].
The satellite projects of PAMELA[24] has be performing in the outer space orbits from2006. The PAMELA detector has a superconducting spectrometer and calorimeters. The
PAMELA collaboration reported a result of the measurement of the positron abundance inthe cosmic ray up to 100 GeV[25], that the positron fraction increased in the energy range
above 10 GeV (Figure. 1.7). According to [26], the annihilation of neutralino may affects
the amount of the cosmic positron in the energy region above 10 GeV.
The next generation antimatter detector GAPS (the General Antiparticle Spectrometer) identifies antimatter through the characteristic X-rays emitted by antimatter when it
is captured in the target and forms an exotic atoms. Its sufficient sensitivity to detect antideuterons will probe the SUSY parameter space for a sensitive indirect dark matter search[27].

Figure 1.7: Positron fraction measured by PAMELA (red points and error bars)[25]. Black
marks show results of other experiment measurements.

17

Chapter 1. Introduction
Muon neutrino Neutralinos scatter elastically off nuclei in the Earth, the Sun and theGalactic Center. If neutralinos velocities become less than the escape velocity in the process,they are gravitationally trapped in the core of them. The accumulated neutralinos can selfannihilate and produce various particles. Among the products, energetic muon neutrinos
can escape from the core and should be detected by the neutrino detectors on the Earth. Theenergetic neutrinos will undergo charged current interactions in the rock below the detector
or within the detector. Therefore, neutralinos are indirectly detectable as upward-going
muons. Since the energies of these muons will be typically 1/3 to 1/2 of the neutrino mass,
they cannot be confused with ordinary neutrinos [31].
Fluxes predicted for such muon neutrino events in SUSY models seem to fall for themost part between 10−6 and 10−2 events/m2 /yr. Several experiments are ongoing and have
set limits on the flux of upward-going muons from the Earth. The tightest limits havebeen reported by the Super-Kamiokande Collaboration [31]. The new generation neutralino
telescopes such as IceCube [32] and ANTARES [33] are under development.

1.4.3

Search by accelerator experiment

The neutralino searches have been also performed by accelerator experiments (LEP2 performed at CERN [34], for instance). Neutralinos could be produced in an e+ e− collider
through an s-channel virtual Z, or by a t-channel scalar electron (selectron, ẽ) exchange as
bellow.
e+ e− → χ̃01 χ̃02 or χ̃02 χ̃02

(1.13). Produced theχ̃02 will decay intoχ̃02 → χ̃01 Z ∗ → χ̃01 f f˜ orχ̃02 → χ̃01 γ.

(1.14)The signature is a missing energy due to the undetected χ̃10 and one or two photons, two or
four leptons, or one to four hadronic jets. Constrained Minimal Supersymmetric Standard
Model (CMSSM) motivated by Grand Unified Theory (GUT) is often used for the analysis
to simplify physics interpretations. The lower limit to Mχ̃01 mass for a certain parameter
set is 40 GeV c−2 [34]. However, searches for neutralino with lighter masses are still needed
because it is probable the GUTs may not work and neutralinos with a mass of a few GeV c−2
can exist [35].
Neutralino could also be produced in a pp collider through various channels. Tevatroncollider searched for the neutralino at the maximum collision energy of 2 TeV. They reported
the limit of neutalino life time [36]. The Large Hadron Collider (LHC), which has started
in September 2008 and will be operated at the maximum collision energy of 14 TeV, is
expected to produce many SUSY particles. Detectable neutralino events are expected fromthe production of the heavier SUSY particles, which in turn decay via a multi-step cascade to
the stable LSP. The LSP could be identifid by a missing mass of those interaction. The LHC
experiment will find a neutralino or give a much more stringent constrain to the neutralinoparameters [37].

18Chapter 2

Direct detection of neutralino
Neutralinos can be detected via elastic scatterings with ordinary matters. SUSY theory predicts two types of the interactions, namely spin-independent(SI) and spin-dependent(SD)interactions. Feynman diagrams of the spin-independent and the spin-dependent neutralinoquark elastic scatterings are shown in Figure 2.1 and Figure 2.2, respectively. Many experiments have been performed to reveal the existence of the WIMPs assuming these interactions. In this chapter, the theoretical framework of the direct detection of neutralino is
described based on ref. [38].

2.1

Dynamics of the Earth, the Sun and the neutralino

The motions of galaxies indicate that the dark matters exist as dark halo in the galalxies.
Thus the dark matter direct detection rate strongly depends on a realtive motion between
the detector on the Earth and the dark matter. In this section we discribe relative motionsof the Earth, the Sun and the neutralino dark matter with respect to our Galaxy. The
detector velocity on the Earth in the Galactic rest frame can be expressed as;vE = vc + vLSR + vES ,(2.1)

where vc is the rotational velocity around the Galactic Center, vLSR is the solar system’s velocity with respect to the Local Standard of Rest, and vES is the earth’s velocity with respect to the Sun. The velocity of the Sun (vSUN = vc + vLSR ) can be foundfrom the equatorial coordibatres of the Galactic Center (Sagitarius A*, 17h45m37.199s

 

χ

χ

χ

χ

H, h

q̃

q
q
q
q
Figure 2.1: Feynman diagrams of spin-independent(SI) interactions between a neutralino
and a quark.

19

 

Chapter 2. Direct detection of neutralino
χ

χ

χ

Z

χ

q̃

q
q
q
q
Figure 2.2: Feynman diagrams of spin-dependent(SD) interactions between a neutralino anda quark.

R.A., −28◦ 56′ 10.23”decl.(J2000.0)) and of the Galactic North Pole (12h51m26.275s R.A.,+27◦ 07′ 41.7”decl. (J2000.0)). They identify a point in the celestial sphere with equatorial
coordinates of 21h12m01,053s R.A., +48◦ 19′ 46.71”decl(J2000.0), which is the direction of
the constellation Cygnus. Since the component of vLSR parallel to the galactic rotation is
12 km/s, the absolute value of vSUN is considered to be ∼230 km/s. On the other hand, thecomponent of vES paralleled to the galactic rotaion, vESk is generally expressed asvESk = vorb cos ζ cos(ωt).

(2.2)

Here vorb ∼ 30 km/s denotes the Earth orbital speed velocity with respect to the Sun. The
angle ζ ≃ 60◦ is the inclination of the Earth orbital plane with respect to the Galactic planeand ω = 2π/year. t is the elasped time from June 2nd in years when vESk has the maximum.

2.2

Event rate

The differential event rate per unit target mass (kg) isdR =NAσχ−N vdn,
A(2.3)

where NA is the Avogadro number per unit mass (6.02×1026kg−1 ), A is the mass numberof a target nucleus, v=|v − vE |, which is the dark matter velocity relative to the target n is
the dark matter particle number density, and σχ−N is the neutralino-nucleus (χ − N) crosssection for zero momentum transfer. The cross section for non-zero momentum transfer,σχ−N(non−zero) is expressed asσχ−N(nonzero) = σχ−N F 2 (ER ),which is discussed later in this Section. The total event rate, R, is then expected asZN0N0σχ−N vdn =σχ−N n0 hvi,
R=
A
A

(2.4)

(2.5)

ρD
where n0 = M
is the mean dark matter particle density given by the ratio of the darkχmatter density, ρD , to the neutralino mass, Mχ , and hvi is the mean dark matter velocity.

20Event rate
A Maxwell-Boltzmann distribution,f (v, vE ) =123(πv02 ) 22

e−|v+vE | /v0 ,(2.6)

is practically assumed as the dark matter velocity distribution. Here v0 is the velocity
dispersion of the dark matter. For vE = 0 and the local Galactic escape velocity vesc = ∞,hvi is expressed as2
hvi = √ v0 .
π

(2.7)

From Eq. (2.5) and Eq. (2.7), the total event rate for vE = 0 and vesc = ∞, R0 is expressed
as
R0 =

ρD 2
N0
√ v0 .
σχ−N
A
Mχ π

(2.8)

R0 is conventionally expressed in units of kg−1 day−1 . Normalized to ρD = 0.3GeVc−2 cm−3and v0 = 220 km s−1 , Eq. 2.8 becomes




ρD
v0
σχ−N 
361
R0 =
(2.9)
Mχ MN
1pb
0.3 GeVc−2 cm−3
220 km s−1
with Mχ , MN in units of GeV c−2 . MN is the mass of the target nucleus (MN = 0.932 A).
as

The total event rate for vE 6= 0 and vesc 6= ∞ is expressed using Eq. (2.5) and Eq. (2.8)
k0 1
π hvi
= R0
R = R02 v0k 2πv04Zvf (v, vE )d3 v,

where k is the normalization constantZ vescZ 1
Z 2π
f (v, vE )v 2 dv.
d(cos θ)dφk=0(2.10)(2.11)

0−1

3

k0 = (πv02 ) 2 is the value of k for vesc = ∞. Eq. (2.10) is then,




2
2
R(0, vesc )
vesc
k0
−vesc
/v02
,
1− 1+ 2 e
=
R0
k1
v0
 


2
vE
1
vE
1
1 v0
R(vE , v∞ )
−vE
/v02
2
π
,
Erf ( ) + e
=
+
R0
2
v0
2 vE
v0


 2

2
2
R(vE , vesc )k0 R(vE , ∞)
1 vE
vesc
−vesc
/v02
,
=
−
+
+1 e
R0
k1
R0
v02
3 v02
√ Rx
where Erf (x) = 2/ π 0 exp(−t2 )dt. k1 is the value of k for vesc 6= ∞ written as




2
2 vesc −vesc
vesc
/v02
.
−√
e
k1 = k0 Erfv0
π v0(2.12)(2.13)(2.14)(2.15)The recoil energy of the nucleus with a mass MN caused by an elastic scattering of the21Chapter 2. Direct detection of neutralino
dark matter particle of the mass Mχ with the kinematic energy E is
ER = Er(1 − cos θ)/2,(2.16)

where θ is the scattering angle (in the center of mass) andr=

4Mχ MN
2(Mχ + MN ).

(2.17)

With an assumption that the scattering is isotropic, and where ER is uniformly distributed
over the range of 0 ≤ ER ≤ Er, then the differential event rate is
Z EmaxZ vmax 2
1
v0
1
dR
=
dR(E) =
dR(v),(2.18)
dER
E0 r vmin v 2Emin ER 2
v
where E0 = 1/2Mχ v02 = v02 E, Emin = ER /r is the lowest energy of the incident particle
1

which can produce a nuclear recoil of the energy ER and vmin = (2Emin /Mχ ) 2 is the dark
matter particle velocity corresponding to Emin . From Eq. (2.10),
Z vmax1
R0 k0 1f (v, vE )d3 v,(2.19)dR =E0 r k 2πv02 vmin v

then the differential spectral function is expressed using Eq. (2.18)
Z vmaxdR
R0 k0 11=f (v, vE )d3 v.
dER
E0 r k 2πv02 vmin v(2.20)For the simplest case of vE = 0 and vesc = ∞, Eq. (2.20) gives
R0 −ER /E0 r
dR(0, ∞)
=
e
.
dRR
E0 r

(2.21)

Eq. (2.21) shows that the distribution of the recoil energy is determined by the mass of thedark matter particle and target nuclei. For non-zero vE and finite vesc , Eq. (2.20) givesdR(0, vesc )
dER

=
=

dR(vE , ∞)
dER

=
∼

dR(vE , vesc )
dER

=


2
2
k0 R0  −ER /E0 re− e−vesc /v0
k1 E0 r2
2
R0 vesc
k0 R0 dR(0, ∞)−e /v0 ,
k1 E0 r
dER
E0 r





R0 π v0
vmin + vE
vmin − vEErf− Erf
E0 r 4 vEv0v0
R0 c2 ER /E0 r
c1
e
,
E0 r


2
k0 dR(vE , ∞)R0 −vesc
/v02
,
−
e
k1
dER
E0 r

(2.22)(2.23)(2.24)

where c1 =0.751 and c2 =0.561 are parameters in Eq. (2.23) for vE =244.0 km·s−1 .
The Earth velocity vE varies during the year as the Earth moves around the Sun.
vE ∼ 244 + 15 sin(2πy)km · s−1 ,22(2.25)

Cross section
where y is the elapsed time from March 2nd in year. The Maxwellian velocity parameter,
v0 , and the escape velocity, vesc , are discussed by several authors [39, 40], and
v0
vesc

=
=

(220 ± 40)km · s−1 ,(450 − 650)km · s−1 ,(2.26)(2.27)are commonly used values.

2.3

Cross section

Neutralinos interact with quarks spin-independently(SI) and spin-dependently (SD). The
cross section of each interaction can be calculated by assuming certain sets of parameters in
the MSSM model [15, 41]
Theoretical Framework
The theoretical framework we use in the context of the MSSM is reviewed in this section.
The MSSM Lagrangian leads to the following low-energy effective Lagrangian, Leff , which
is suitable for describing elastic neutralino-quark interactions[41].
χ̄γ µ γ 5 χq¯i γµ (α1i + α2i γ 5 )qi + α3i χ̄χq¯i qi

Leff =+α4i χ̄γ 5 χq¯i γ 5 qi + α5i χ̄χq¯i γ 5 qi + α6i χ̄γ 5 χq¯i qi,(2.28)

where qi denotes the quark and the subscript i labels up-type quarks (i = 1) and down type
quarks (i = 2). This Lagrangian is to be summed over the quark generations. Terms in
Eq. (2.28) with coefficients α1i , α4i , α5i and α6i make contributions to the elastic scatteringcross section that have a velocity dependence, and may be neglected for dark matters with
non-relativistic velocities. Coefficients α2i and α3i contribute to the SD and SI interactions,
respectively. α2i and α3i are written as follows.
α2i =12 − M 2)
4(M1i
χ

 2

|Yi | + |Xi2 | +
−

α3i =

1
2 − M 2)
4(M2i
χ
2





|Vi2 | + |Wi2 |

 3
g
2
2 Ti
,
|a
|
−
|a
|
3
4
4MZ2 cos2 θW
2

1
1
∗
∗
2 − M 2 ) Re [(Xi )(Yi ) ] − 2(M 2 − M 2 ) Re [(Wi )(Vi ) ]
2(M1i
χ
χ
2i
"


gMqi
1
1
′
−
Re(δ1i [ga2 − g a1 ])Di Ci − 2 + 2
4MW Bi
MH1
MH2
 2
#
Di
Ci2
′
+Re(δ2i [ga2 − g a1 ]),2 + M2MH
H1
2(2.29)(2.30)

where Mqi , is the quark mass. Eq. (2.29) and (2.30) are to be summed over q and i. M1iand M2i are the squark mass eigenvalues, Mχ is the neutralino mass, MZ is the Z bozon
mass, MW is the W boson mass, Ti3 , is the third component of the weak isospin, a1,2,3,4 are
the neutralino composition parameters in Eq. (1.11), MH2 < MH1 denotes two scaler Higgs
mass, and θW is the weak mixing angle, which is given by the ratio of the two weak coupling
23

Chapter 2. Direct detection of neutralino
constants, tan θW = g ′ /g.
gMqi a∗5−i∗− η12e i g ′ a1 ,2MW Biy∗i ′∗∗ gMqi a5−i
Yi ≡ η11,
g ai + gTi3 a2 + η12
2
2MW Bi∗∗ gMqi a5−i∗
Wi ≡ η21− η22
e i g ′ a1 ,2MW Biy∗i ′∗ gMqi a5−i∗
Vi ≡ η22+ η21
g ai + gTi3 a2 .
2MW Bi2∗
Xi ≡ η11(2.31)

Here yi denotes the hyper charge of the sfermion defined by ei = Ti3 + yi /2, η is the sfermion
mass matrix and defined for each flavor qi , by an angle θqi as sin θqicos θqiη11 η12=(2.32)− sin θqi cos θqi .
η21 η22
Other parameters in Eq. (2.29) and (2.30) areδ11 = a3 , δ12 = a4 , δ21 = a4 , δ22 = −a3 ,A1 = cos β, A2 = − sin β, B1 = sin β, B2 = cos β,

C1 = sin β, C2 = cos α, D1 = cos α, D2 = − sin α.

(2.33)(2.34)(2.35)(2.36)

Here α denotes the Higgs mixing angle, and tan β = v2 /v1 , where v1 and v2 are the vacuum
expectation values of the two Higgses. α, β and Higgs mass, MH are related by the following
equations.


1−k
2

 21

1+k
2

 12

.

(2.37)

 2
M2
c + r2 − 2r
, c = cos 2β, r = H2 .
2cr − 2rc
MZ(2.38)

sin α = −, cos α = −



Here
k=c

The cross section of the elastic neutralino-nucleon scattering is given byX
σχ−N ∼| hN | Leff |N i ,(2.39)q

where |ni is a wave function of the nucleus.

Cross Section
The neutralino-nucleus cross section at zero momentum transfer is expressed asσχ−N = 4G2F µ2χ−N CN ,24

(2.40)

Cross section
where GF = 1.166 × 10−5 GeV−2 (h̄c)3 is the Fermi coupling constant, µχ−N is the reduced
mass of the neutralino (Mχ GeV c−2 ) and the target nucleus (MN GeV c−2 ),µχ−N =Mχ MN.
Mχ + MN(2.41)

CN is a dimensionless number referred to as the enhancement factor that carries all theparticle physics model information, CN is expressed by the sum of SI and SD terms,
SISD
CN = (CN+ CN).

(2.42)SISD
In the following section, CN
and CNvalues are discussed for various nuclei. Since σχ−N
can be expressed asσχ−N = σχ−pµ2χ−N CNµ2χ−N CN=σχ−nµ2χ−p Cpµ2χ−n Cn

(2.43)

with the enhancement factor of proton Cp and neutron Cn . Here σχ−p and σχ−n are
neutralino(χ)-proton and neutralino-neutron cross sections, respectively.

Spin-Independent (SI) interaction
The enhancement factor of the SI cross section is written asi2
1 h (p)
(n)
SI
,
Zf
+
(A
−
Z)f
CN
=
πG2F

(2.44)

where A is the mass number and Z is the atomic number [41]. f (p) and f (n) are protonneutralino and neutron-neutralino SI couplings, respectively, while they are written as the(n)(p)
neutralino-quarks SI couplings, fTq and fTq , summed over the quarks that comprise nucleon
as described below.
X (p) α3q
X α3q
2
f (p)
=
+ f (p)
,
fTp
Mp
Mq
27
Mq
q=u,d,s(2.45)

q=c,b,tX (n) α3q
X α3q
f (n)
2
=
+ f (n)
,
fTp
Mn
Mq
27
Mq
q=u,d,s(2.46)

q=c,b,t(2.47)

where Mq is the quark mass, Mp and Mn are the proton and neutron masses, respectively.
(n)(p)
Parameters fTq and fTq are determined by the information of quark mass ratio and chiral
(n)

(p)

symmetry applied to baryons. The value of fTq and fTq are as follows,
(p)

(p)

(p)

(n)

(n)

(n)

fTu = 0.020 ± 0.004, fTd = 0.026 ± 0.005, fTs = 0.118 ± 0.062
fTu = 0.014 ± 0.003, fTd = 0.036 ± 0.008, fTs = 0.118 ± 0.062,
while
(p)

fT G = 1 −X

q=u,d,s25

fq(p) ,

(2.48)

Chapter 2. Direct detection of neutralino(n)fT G = 1 −(p)

Xfq(n) .

(2.49)

q=u,d,s(2.50)(n)SI∝ A2 from Eq. (2.44),
In most case, fTq ≃ fTq , therefore CN
SI
C SI
CN
= N
= A2
SI
Cp
CnSI

(2.51)

is a practical estimation. Eq. (2.43) and Eq. (2.51) give
SI
SI
σχ−p
= σχ−Nµ2χ−p 1.
µ2χ−N A2

(2.52)SI
SI
Eq. (2.52) can be used for the conversion of the obtained σχ−Ninto σχ−p. Eq. (2.52) indicatesthat materials with large A values are effective for SI-interacting neutralinos. Conversion
SI
SI
values of σχ−N/σχ−pof materials are shown in Figure 2.3.

9

10

128

8

Xe

10

σχSI-N/ σχSI-p

107

73

Ge

6

10

19

F

5

10

10

102
Mχ GeV/c2

SI
SI
Figure 2.3: Function of conversion values from σχ−pto σχ−Nfor each nucleus as a functionof neutralino mass.

26

Cross section
Spin-Dependent(SD) interaction

The enhancement factor of the SD cross section is written asSDCN=2 J + 1
8
ap hSp(N) i + an hSn(N) i,πJ(2.53)

where hSp(N) i and hSn(N) i are the expectation values of the proton and neutron spin in the
nucleus N [41]. ap and an are the neutralino-nucleon SD couplings, and J is the total spinof the nucleus. ap and an are expressed as
α2q
√
∆(p)
q ,
2G
F
q=u,d,sX
α√ 2q ∆(n)
an =
q ,
2GF
q=u,d,sap =(p)where ∆q(n)and ∆qX(2.54)(2.55)(2.56)

are the quark spin contents of the nucleon, and are calculated as
(p)

(p)
∆(p)
u = 0.78 ± 0.02, ∆d = −0.48 ± 0.02, ∆s = −0.15 ± 0.02.

(2.57)In the case of the neutron,
(p)

(n)

(p)
(n)
(p)
∆(n)
u = ∆d , ∆d = ∆u , ∆s = ∆s ,(2.58)

Eq. (2.53) is written as
SD
=
CN

8 2
λ J(J + 1),π(2.59)

where
λ≡


1
ap hSp(N) i + an hSn(N) i .
J(2.60)λ is referred to as the Landé factor. λ2 J(J + 1) values calculated on the basis of the odd
group model for various nuclei are listed in Table 2.1. Eq. (2.43), Eq. (2.59) and Table 2.1
give
SD
SD
σχ−p
= σχ−N

µ2χ−p
0.75
.
µ2χ−N λ2 J(J + 1)

(2.61)σSDSD
SD
Eq. (2.61) can be used for the conversion of the obtained σχ−Ninto σχ−p. Values of σχ−N
SD
χ−p

σSD

are shown in Figure 2.4 and Figure 2.5, respectivily.
and σχ−NSD
χ−nAs a ”model independent” analysis, the case for the pure proton coupling (an = 0) andthe pure neutron coupling (ap = 0) are also checked to interpret the experimental resultsin terms of the SD interaction λ2 J(J + 1) values calculated in the case of the pure proton
coupling and the pure neutron coupling for various nuclei are listed in Table 2.2 and Table
2.3.
27

Chapter 2. Direct detection of neutralino

19

F

102
127

σχSD
/ σ SD-N χ-p

I

23

Na

7

10

Li

1
10

102 2
Mχ GeV/c

3

10

SD
SD
Figure 2.4: Function of conversion values from σχ−pto σχ−N
as a function of the neutralinomass for each nucleus. Natural abundunce for each nucleus is considered.

Isotope
1
H
7
Li
19
F
23
Na
127
I
133
Cs
Isotope
3
He
29
Si
73
Ge
129
Xe
131
Xe
183
W

unpaired proton
Abundance(%) λ2 J(J + 1))
100
0.750
92.5
0.244
100
0.647
100
0.041
100
0.007
100
0.052
unpaired neutron
J
Abundance(%) λ2 J(J + 1))
1/2
1.3×10−4
0.928
1/2
4.7
0.063
9/2
7.8
0.065
1/2
26.4
0.124
3/2
21.2
0.055
1/2
14.3
0.003
J
1/2
3/2
1/2
3/2
5/2
7/2

Table 2.1: Values of λ2 J(J + 1) calculated in the basis of the odd group model for various
nuclei.[38]28Cross section

3

10

129

Xe

σχSD
/ σ SD-N χ-n

102

10

73

131

Ge

Xe

183

W
29

Si

1

10-1
10

102
Mχ GeV/c2

3

10

SD
SD
Figure 2.5: Function of conversion values from σχ−nto σχ−N
as a function of the neutralinomass for each nucleus. Natural abundunce for each nucleus is considered.

Isotope
19
F
23
Na
127
I
Isotope
29
Si
73
Ge
129
Xe
131
Xe

unpaired proton
Abundance(%) λ2 J(J + 1))
100
0.583
100
0.103
100
0.133
unpaired neutron
J
Abundance(%) λ2 J(J + 1))
1/2
4.7
0.000012
9/2
7.8
0.0011
1/2
26.4
0.00243/2
21.2
0.000014
J
1/2
3/2
5/2

Table 2.2: Values of λ2 J(J + 1) in the case of pure proton coupling.

29

Chapter 2. Direct detection of neutralino

Isotope
19
F
23
Na
127
I
Isotope
29
Si
73
Ge
129
Xe
131
Xe

unpaired proton
Abundance(%) λ2 J(J + 1))
100
0.036
100
0.00067100
7.87
unpaired neutron
J
Abundance(%) λ2 J(J + 1))
1/2
4.7
0.051
9/2
7.8
0.175
1/2
26.4
0.385
3/2
21.2
0.086
J
1/2
3/2
5/2

Table 2.3: Values of λ2 J(J + 1) in the case of pure neutron coupling.

2.4

Nuclear form factor correction

When the wavelength, h/q, with the momentum transfer q = (2MN ER )1/2 is no longer large
compared to the nuclear radius, the effective cross section begins to fall with an increasing
q. Here h is the Plank Constant. This effect is represented by a form factor, F , which isa function of the dimensionless quantity, qrn , where rn is an effective nuclear radius. The
neutralino-nucleus cross-section at a zero momentum transfer, σχ−N , is corrected with thisform factor asσ(qrn ) = σχ−N F 2 (qrn ).

(2.62)In the first Born (plane wave) approximation, the nuclear form factor can be defined asa Fourier transformation of ρ(r), the density distribution of ‘scattering centers‘
Z
F (qrn ) = ρ(r) exp(iq · r)dr.
(2.63)For small momentum transfers (qrn < 2), a commonly used approximation is2
j1 (qrn )exp −(qs)2 ,F 2 (qrn ) = 3
qrn(2.64)

F 2 (qrn ) = j02 (qrn ),(2.65)

where j1 (x) = [sin(x) − x cos(x)]/x2 , rn ≃ 1.14A1/3 fm and s ≃ 0.9fm represents the nuclear
skin thickness. The form factors as a function of the recoil energy for the SI interactions
are shown in Fig. 2.6. Energy spectra expected with fluorine targets for the SI-interacting
neutralinos are shown in Fig. 2.7, where the nuclear form factor correction, F 2 , is taken into
account.
For SD interactions, with the odd group model assumption, the form factor is approximated by the Bessel function partially filled with a constant value,

where j0 (x) = sin(x)/x and rn ≃ 1.0A1/3 fm. The form factors as a function of the recoil
energy for SD interactions are shown in Figure 2.8. Energy spectra expected with fluorine
targets for the SD-interacting neutralinos are shown in Fig. 2.9, where the nuclear form
factor correction, F 2 , is taken into account.

30

Nuclear form factor correction

19F

132 Xe

-1

10

74

Ge

F2

10-2

-3

10

10-4

-5

10 0

50

100

150 200 250 300
recoil energy (keV)

350

400

Figure 2.6: Form factors as a function of the recoil energy for the SI interactions for eachnucleus.

-3

10

19

F

19

counts/keV/kg/days

F

19

F

Mχ =50GeV
Mχ =100GeV
Mχ =200GeV

10-4

-5

10

-6

10

20

40

60

80 100 120 140 160 180 200
keV

Figure 2.7: Energy spectra expected with fluorine targets for the SI-interacting neutralino
for each mass of the neutralino.
31

Chapter 2. Direct detection of neutralino

1
7

Li

19

F

10-1

127

F2

I

10-2

-3

10

10-40

50

100

150 200 250 300
recoil energy (keV)

350

400

Figure 2.8: Form factors as a function of the recoil energy for SD interactions for eachnucleus.

19

F

19

1
counts/keV/kg/days

F

19

F

Mχ =50GeV
Mχ =100GeV
Mχ =200GeV

10-1

10-2

-3

10

20

40

60

80 100 120 140 160 180 200
keV

Figure 2.9: Energy spectra expected with fluorine targets for the SD-interacting neutralino
for each mass of the neutralino.
32

Distinct signal of the dark matter

2.5

Distinct signal of the dark matterWe need to observe a distinct signature in order to claim a clear detection of the dark
matter because the expected spectrum itself has featureless exponentioal shape as describedin Figure 2.7 and Figure 2.9. Distinct signatures can be caused by the nuclear-dependenceof the neutralino-necleus reactions, relative motions of Solar system to the galactic halo, andthat of the Earth. They can be observed as the signatures listed below.
• Nuclear target dependence of the event rate
• Annual modulation of the event rate and the shape of spectrum• Directional dependence of the event rate

2.5.1

Nuclear target dependenceThis signature arises from the difference of the cross sections σ0 F 2 (ER ) and the target mass
MN . The expected recoil spectra dR/dER of various nuclei in existing detectors for the SISI
interacting and the SD-interacting neutralino of Mχ = 100GeV/c2 and σχ−p
= 1 × 10−6 pb
are shown in Figure 2.10 and Figure 2.11, respectively.
Although these figures indicate that the target-dependences seem to be easily observable,
intrinsic backgrounds of target nuclei are also different, and thus reliable signatures are noteasily obtained from the target-dependences.

counts/keV/kg/days

10-2

-3

10

74
39

132 Xe

Ar

127 I

22

Na

-4

10

0

20

Ge

40
60
80
recoil energy (keV)

100

120

Figure 2.10: Recoil specra of various nuclei for the SI-interacting nuetralino with a mass of
SI
100 GeV/c2 and σχ−p=1×10−6pb.

33

Chapter 2. Direct detection of neutralinocounts/keV/kg/days

1

19

10-1

10-2

F

22
12

Na

20

40
60
80
recoil energy (keV)

7

I

7 Li

-3

10

100

120

Figure 2.11: Recoil specra for of various nuclei for the SD-interacting nuetralino with a massSDof 100 GeV/c2 and σχ−p=1 pb.

34

Distinct signal of the dark matter

2.5.2

Annual modulation

The annual modulation of the event rate is caused by the Earth’s motion around the Sun as
describe in Eq. (2.2). The order of modulation is less than o(vorb /vSUN ) ∼ 10% considering
the inclined orbit of the Earth[42]. The component of vES parallel to the vSUN reaches its
maximum in June 2nd and its minimum in December 4th every year. Consequently, not only
the total event rate, but also the shape of the recoil energy spectrum should vary. Figure 2.12
illustrates the eventual modulation of the recoil energy spectra of the florine target for theSD= 1pb. The observation of thisSD interaction with neutralino of Mχ =100 GeV and σχ−p2counts/keV/kg/days

1.8
1.6

Dec

1.4

Jun

1.2
1
0.8
0.6
0.4
0.2
0

20

40

60 80 100 120 140 160 180 200
recoil energy [keV]

Figure 2.12: Difference of the fluorine spectra between summer and winter for the SDSD
interacting neutralino with a mass of 100 GeV/c2 and σχ−p
= 1pb. Green line is expected
recoil specra in June, where vES has its maximum in the year. Red line is expected recoil
specra in December, where vES has its minimum in the year.
small modulation of a very rare signal requires a large target mass, a long time measurement
time, superb stability, extreme control of systematics. The results and discussions on theactual observations of this annual modulation signature are described in Section 3.1.

2.5.3

Directional asymmetry

The most convincing signature of WIMPs would be seen in the directional asymmetry of therecoil nucleus. The motion of the solar system relative to our galaxy makes an asymmetry inthe directions of the incoming dark matters observed on the Earth, which can be expressedas ”WIMP-wind”. Figure 2.13 shows the directions in the sky of the Cygnus calculated fora day as they are observed Kamioka Observatory. The ”WIMP-wind” is browing us fromthose directions.
35

Chapter 2. Direct detection of neutralino

degreezenith80

9/10 21:00

60

9/10 0:00

9/10 18:00

40
20

9/10 3:00

9/10 6:00

W 0
-50
-20

9/10 15:00

N 9/10 12:00
0E
50

9/10 9:00

degree

-40
-60
-80

Figure 2.13: Daily motion of the direction of Cygnus, which is the same direction of the”WIMP-wind”, expected in Kamioka Observatory(36◦25’N, 137◦ 18’E).

36

Distinct signal of the dark matter
Spergel [43] has derived the differential angular spectrum with respect to laboratory
recoil angle θ as
#
"
2
d2 R
1 R0
(vE cos θ − vmin ),(2.66)∼exp −dER d cos θ
2 E0 rv02

recoil energy [keV]

where a distribution of the isotheramal model Eq.(2.6) was assumed. Expected recoil angleSD
energy distribution of 19 F for the SD interacting WIMP cross section of σχ−p
= 1pb with amass of Mχ = 100 GeV is shown in Figure 2.14.
The order of magnitude of the effect should be O(vE /v0 ) ∼ 1 and the effect of the”WIMP-wind” is clearer in the recoil angle distribution dR/d cos θ. The expected anglularSD
distribution of 19 F for the SD interacting WIMP cross section of σχ−p
= 1pb with a massof Mχ = 100 GeV and a given recoil energy range of 100-120 keV is shown in Figure 2.15.
It shows a large asymmetry which is not easily mimicked by any isotropic background.
If the device has a direction sensitivity and is able to measure cos θ distribution, we candiscriminate dark matter signals from isotropic background events by fitting measured datawith the model. A small signale-to-background ratio should be enough to discriminate signal
from background event [44].
In this thesis we develop and study the direction sensitive dark matter search detector
with this detection methode.

400
350

3

300

2.5

250

2

200
1.5
150
1
100
0.5

50
-1 -0.8 -0.6 -0.4 -0.2 0

0.2 0.4 0.6 0.8 1[cosθ]

0

Figure 2.14: Expected recoil angle-energy distribution for the SD interacting neutralino withSD
a mass of Mχ = 100 GeV, and a cross section of σχ−p
= 1pb, where θ is the recoil angle ofthe nuclear recoil with respect to the WIMP wind direction. X-axis shows cos θ, where θ isnuclear recoil angle. Y-axis shows nuclear recoil energy. Color contor shows event rate inunit of [counts/keV/kg/days/cos θ].

37

count/keV/kg/day/cosθ

Chapter 2. Direct detection of neutralino

0.045

0.04

0.035

0.03

0.025

0.02

0.015

0.01

0.005

0

-1 -0.8 -0.6 -0.4 -0.20

0.2 0.4 0.6 0.8 1
cosθ

Figure 2.15: Expected cos θ distribution for the SD interacting neutralino with a mass ofSDMχ = 100 GeV, a recoil energy range of 100-120 keV, and a cross section of σχ−p
= 1pb,
where θ is the recoil angle of the nuclear recoil with respect to the WIMP wind direction.

38

Chapter 3

Review of direct dark mattersearch experiments
A number of experiments have been trying to detect WIMPs by observing neutralino-quark
elastic scatterings. Their detectors are designed to detect recoil nuclei via photons, phonons
or charges in these experiments. Detector systems and results of some of the experiments
are described in this section. Those results are summarized in Figure 3.1 and Figure 3.2.
In general, direct dark matter detectors should have following three characteristics.
• Low energy threshold
As shown in Section 2.2, a featureless, quasi exponentially decreasing energy spectrum
is expected by elastic scatterings. Therefore, the lower the energy threshold, the moreof the signal is expected. The relevant energy region will be typically below 100 keV.
• Large target mass
Since WIMP direct detection is a rare event search, one would need large target masses
generally in order to gain sufficient statistics in a reasonable observation time of theexperiment. A typical required mass scale is from kilogram to ton.
• Low backgroundTypical precautions of rare event searches such as a material selection and shielding
are essential to WIMP detectors in an underground laboratory. For nuclear recoil
events, a generic background contribution originates from neutrons. On the other
hand, the majority of background consists of electron recoils from photons (X-ray or
γ-ray radiation) or electrons (β-ray radiation). Techniques to discriminate between
these two types of energy deposition reduce the background obviously.

3.1

Solid scintillator experiments

Scintillators have advantages to constructing large mass detectors and well-studied theirproperties. Many types of scintillators, such as NaI(Tl), CsI(Tl) and so on, have been used
for dark matter searches.
DAMA group operated a dark matter search with a highly radio-pure 100kg NaI(Tl)
setup for seven years and claimed to have detected an annual modulation signature due tothe dark matter[45]. Allowed parameter regions of WIMPs are shown as enclosed regions
by black lines in Figure 3.1 and Figure 3.2. They also reported new results with a highly
39Chapter 3. Review of direct dark matter search experiments

SI 90% C.L. upper limits and allowed region

aI)

D(N

A
NAI

-5

10

DAMA(NaI)e)σ χ-p[pb]

S(G

-6

IS
WE
L
E
ED
Ar)
RP(
WA
O)
aW
T(C

10

SS

CRE

e)

10(X

N
ENO

X

10-7

)

(Ge

S
CDM

-8

10

102 2
Mχ [GeV/c ]3

10

Figure 3.1: Results of SI(spin-independent) WIMP searches. Regions enclosed by blackline is a region allowed by DAMA experiment[45]. Other experiments (NAIAD[47],EDELWEISS[55],CDMS[56], WARP[65], CRESST[59], and, XENON10[64]) exclude upper
region of each line.

40

Solid scintillator experiments

SD 90% C.L. upper limits and allowed region
3

10

e

A_X

DAM

σ χ-p[pb]

102

)

,Si

e
S(G

M

10

CD

O

SS

A
PIC

DAMA(NaI)

aF2

C
yo
Tok (F)
E
PL
SIM

(F)

)

(Xe

N

XE

1

NAIAD

(Na,I)s,I)

KIMS(CCOUPP(F)

10-1

10
ON

102 2
Mχ [GeV/c ]3

10

Figure 3.2: Results of SD(spin-dependent) WIMP searches. Regions enclosed by blackline is a region allowed by DAMA experiment[45]. Other experiments (DAMA(Xe)[60],
CDMS[56], Tokyo[49], PICASSO[71], SIMPLE[70], NAIAD[47], KIMS[48], XENON10[64],and COUPP[69]) exclude upper region of each line.

41

Chapter 3. Review of direct dark matter search experiments
radio-pure 250kg NaI(Tl) setup (DAMA/LIBRA), which reproduced the peculiarities of
the DAMA annual modulation signature as shown in Figure 3.3[46]. The DAMA/LIBRA
experiment accumulated the largest exposure (0.82 ton·year) and claimed the only positive
signature of the dark matter, while other experiments have not shown any results that havesupported the DAMA/LIBRA results.
NAIAD detector was a 55 kg NaI(Tl) scintillator developed by UKDMC group in theyear of 2000. They analyzed all data using a pulse shaping cut to reject background gammaray events. The pulse shaping cut applies a property, which a scintillation decay time fornuclear recoils is shorter than that for electron recoil. They derived an exclusion limits froman exposure of 44.9 kg·year (Green lines in Figure 3.1 and Figure 3.2).
KIMS collaboration constructed a 34.8 kg CsI(Tl) scintillator detector. They analyzed
their 3409 kg·days data using the pulse shaping technique. They set the best limit of 1.7 pb
to the SD WIMP-proton interaction for WIMPs with a mass of 100 GeVc−2 (Cyan line in
Figure 3.2)[48].
CaF2 scintillators do not show a good rejection power by the pulse shaping cut, while
they have a potential advantage to the SD sensitivity because fluorine has a large λ2 J(J + 1)
as shown in Table 2.1. Shimizu et al.[49] reported a WIMP-search result with highly radiopure CaF2 crystals (Yellow line in Figure 3.2).

3.2

Bolometers (cryogenic detectors)

The heat capacity of a dielectric crystal is proportional to T3 at low temperatures. Thus at
mK temperatures the small energy deposition from a nuclear recoil can yield a measurable
increase of a target crystal [50]. Some of the earliest WIMP search experiments were basedon the technique, where energy released by particle interactions can be observed as phonons
or a quanta of lattice vibrations. Works started on this idea in the 1980’s [51], of which
the original motivation were to obtain very low recoil energy thresholds and high energy
resolutions. However, it was soon demonstrated, first in Si [52] and then in Ge [53], thatthe phonon detection could be combined with a simultaneous detection of the ionization toprovide also a powerful means of the discriminatio n against electron recoils, on an eventby event basis. This comes from the feature that the ratio of energy observed in the twochannels depends on the energy deposition per unit length. A high dE/dx event, such asa recoil nucleus, produces proportionally more heat than the ionization (i.e. ionization isquenched). For instance, the ratio of the ionization to recoil energy (the ionization yield)
for Ge recoils is 0.3 times of the value for electron recoils above 20 keV [54].
While bolometers without ionisation collections already showed a good potential for adark matter search, the hybrid technique has obtained one of the best sensitivities in thedark matter experiments. Most notable experiments are EDELWEISS (at Frejus)[55] andthe CDMS collabolation (at Soudan mine)[56]. The EDELWEISS experiments used 320g
Ge crystals operated at 17 mK with NTD-Ge thermometric sensors attached for the heatsignal and aluminium electrodes used to collect the charge. A variety of detectors were triedin EDELWEISS-I with several runs from 2000 to March 2004. These measurements yieldeda total exposure of 62 kg·days, and the main result coming from three crystal with a recoil
energy threshold of 13 keV or better over 4 months of a stable operation. Some nuclear
recoil candidates were recorded, but they seem to be due to the remaining background or
surface electrons.
The CDMS experiment operates towers of Ge and Si crystals with thickness of 1 cm.
The mass of the Ge and Si crystals were 250 g and 100 g, respectively. Technique forthe ionisation collection was same as the EDELWEISS but here thermal phonons weredetected using superconducting transition edge sensors, formed by photolithography on the42

Bolometers (cryogenic detectors)

Figure 3.3: Residual rates maeasured by DAMA/LIBRA experiment in the (2-4), (2-5) and(2-6) keV energy intervals as afunction of the time. X-axises show days (t) from January
1st of the first year of data taking of the former DAMA/NaI experiment. The experimental points present the errors as vertical bars and the associated time bin width ashorizontal bars. The superimposed curves represent the cosinusoidal functions behaviours
A cos ω(t−t0 ) with a period 2π
ω = 1 yr, with a phase t0 = 152.5 day (June 2nd) and with modulation amplitudes, A, equal to the central values obtained by best fit over the whole data,that is: (0.0215±0.0026) cpd/kg/keV, (0.0176±0.0020) cpd/kg/keV and (0.0129±0.0016)
cpd/kg/keV for the 2-4 keV, for the 25 keV and for the 26 keV energy intervals, respectively. The dashed vertical lines correspond to the maximum of the signal (June 2nd), while
the dotted vertical lines correspond to the minimum. The total exposure is 0.82 ton·yr[46].

43Chapter 3. Review of direct dark matter search experiments
crystal surface. This design has a potential advantage to providing a sensitivity of thedepth position, via a measurement of the phonon pulse rise time, and hence a possibility ofrejecting surface electron events. The 15 Ge crystals were operated from Octobar 2006 to
July 2007 yielding an exposure of 121.3kg·days in the recoil energy region of 10-100 keV.
No events were observed and thus a 90% C.L. SI WIMP-proton limit of 4.6 × 10−8 pb for
WIMPs with a mass of 60 GeVc−2 were obtained (purple line in Figure 3.1) [57].
ROSEBUD[58] and CRESST[59] groups have developed ”scintillating bolometers” which
detect scintillation photons together with heat as an alternative method for the electronevent rejection. In the CRESST experiment, a silicon wafer of 30×30×0.4mm3 with tungsten
thermometer was used to detect the photon, while a 8 mm×8 mm×200 nm superconducting
evaporated tungsten film was used as the heat sensor. Results so far have been obtainedwith two 300 g CaWO4 crystals at Gran Sasso with a total exposure of 20.5 kg·days. 16
events observed in the recoil energy range of 12-40keV could be explained by the ambient
neutron background becasue they measured without a neutron shield. Due to a relatively
high sensitivity to the SI interactions of W nuclei, they derived a limits comparable to otherexperiments [59](yellow line in Figure 3.1).
All these cryogenic experiments are being upgraded to increase the target masses to theorder of 100 kg, while feasibility studies on the next scaling-up to 1 ton scale are also being
performed.

3.3

Liquid noble element detectors

While great efforts in the world have been devoted to cryogenic bolometers over many
years, a rapid growth in liquid noble gas technology for WIMP searches was achieved. The
liquid noble elements are excellent scintillators, while simultaneous ionization signals can becollected with an existence of a certain electric field. Most notable has been liquid xenon(LXe), started by DAMA/Xe [60], but also recently liquid neon and, in particular, liquidargon.
LXe has particularly good intrinsic properties for the WIMP detection, that are a large
atomic number (Z=54, A=131.3), high density, high radio purity, and, high light output.
ZEPLIN I experiment, which used a 3.2 kg of active LXe viewed by three photo multipliers(PMTs), accumulated a 293 kg·days exposure during an operation at Boulby mine until
2002, and published a result based on the pulse shaping analysis for the nuclear recoil
discrimination [61]. XMASS project [62] designed a spherical LXe detector based on a selfshielding with a large scale LXe (800 kg). γ-rays coming from outside of the detector within
the dark matter search energies are expected to be sufficiently shielded by the outer region of
the detector. These experiments with only the liquid state of xenon are commonly referred
to as ”single phase” detectors in contrast to the ”two phase” detectors which have a gaseous
phase simultaneously.
Two phase detectors detect not only a scintillation signal but also an ionization signal.
The ioninzation signal is detected as a gas amplification scintillation signal in the gas phaseregion caused by the primary electron drifted out of the liquid phase. Because the ionization signals for nuclear recoils are quenched significantly much more than the primary
scintillation signals, while the ionization signals for electron recoils are not quenched somuch, a ratio of the ionization signal and the primary signal provides a powerful electron
discrimination power. This technology is used by ZEPLIN II/III[63] and XENON 10/100
experiments[64]. These experiments aim to achieve higher sensitivities with small fiducialmasses than those expected with single phase detectors. XENON10 with a 15 kg detector
accumulated 165 kg·days of data until 2007. They applied a background subtraction andderived a very stringent SI WIMP interacting limit of 5.5×10−8pb for WIMPs with a mass44

Superheated droplet detectorsof 100GeV/c2 (blue line in Figure 3.1) [64].
Despite the current interest in LXe, the intrinsic discrimination power may not be as
good as that of bolometers. In this respect liquid argon (LAr) detectors may provide better
prospects. Some properties of LAr are inferior to LXe for the WIMP search, notably smaller
atomic mass (A=40) and the need to use a wavelength shifter for the vacuum ultra violet
scintillation light (128 nm). However, both the pulse shaping discrimination and the twophase signals discrimination are now known to be more powerful, which is expected to becapable in principle combined the discrimination factor up to 108 . LAr is also a factor 400
times lower in cost than LXe. Based on these properties, WARP collabolation has buildand deployed a 3.2kg LAr detector at LNGS and recently reported a sensitivity near 10−6
pb for WIMPs with a mass of 100 GeVc−2 and an exposure of 96.5 kg·days (Cyan line in
Figure 3.1) [65]. Other LAr experiments such as ArDM[66], CLEAN[67] and DEAP[68] arealso under construction.

3.4

Superheated droplet detectors

Superheated liquids, in the form of bubble chambers, were used in accelerator experiments.
In a bubble chamber kept in a delicate metastable superheated state, an ionizing radiation
particle makes a local vapor phase with the energy deposited along its path. Rapidly growing
bubbles are photographed and the chamber is then reset by a fast recompression to themetastable liquid phase. COUPP[69], SIMPLE[70], PICASO[71] experiments applied asuperheated droplet detector to the direct dark matter search using piezoelectric sensors as
the trigger.
These devices have advantages to the nuclear recoil discrimination, a large mass, and atarget selection for the SD WIMP searches, but have disadvantage to detecting the nuclear
recoil energy. The superheated liquid can be tuned to respond exclusively to particles with alarge stopping power, while muon and electron events all fall well below the bubble nucleationthreshold.
The COUPP project reported the lowest limit on the SD WIMP-proton cross section
for WIMPs with a mass of 10 GeV/c2 , assuming the amount of the background event due
to 222 Rn gas. This result excluded the result of DAMA observation completely in the SD
WIMP search. (Dark green line in Figure 3.2)[69].

3.5

Gas detectors

Low-pressured gas detectors have an advantage to a direction-sensitive dark matter search,while several challenges to overcome exist. The requirement for use of low pressure gas
implies large volume detectors of more than 1 m3 will eventually be needed. Another issue
is to discriminate the head and tail of a track, or the track sense. Orientation of a recoil
track alone provides significant directional information, which nearly an order of magnitude
greater sensitivity can be achieved in principle if the track sense are known.

3.5.1

Gas selection for the direction-sensitive WIMP search

There are some candidates for target gases of a directional WIMP search. A target gas
should have a large cross-section of WIMP-nucleus scatterings and a small cross-section of
the neutron interaction because it is impossible to discriminate the WIMP-nucleon interaction from neutron-nucleon one event by event. Since hydrogen has a large cross-section toneutron, a gas which contains one or more hydrogens (e.g. CH4 , C2 H6 ) should be avoid.
45

Chapter 3. Review of direct dark matter search experiments

Drift Velocity (µ m/ns)

Noble gases (Xe, Ar, and so on) are well-used gas for common particle detectors, and
Xe is the most proper nucleus of noble gases for the SI interaction search because it has alarge atomic mass (A = 131.29) and few radio isotopes in natural Xe as described in section
3.3. However, the electron transfer diffusion in noble gas are so large that an appropriate
position resolution can not be obtained with a large detector. Drift velocities and drift
diffusions calculated by the MAGBOLTZ simulator [82] are shown in Figure 3.4 and Figure
3.5, respectively. For example, in case of the Xe gas with an electric field of 2 kV/cm/atm,√
the electron transfer diffusion calculated by MAGBOLTS is larger than σdiff =400µm/ cm.
On the other hand, a negative ion time projection chamber (NITPC), where the primaryelectron is rapidly captured by electron negative gases and transferred as negative ions alonga strong electric field, has potential to realize a very small diffusion because, unlike electrons,
drifting ions remain in or near thermal equilibrium with the gas up to extremely high drift
fields. Transverse and longitudial
diffusions are therefore suppressed to the thermal levels,√
which is less than 100(µm/ cm) in case of CS2 .
CF4 √gas, although it is not electron negative gas, has small electron diffusions less than
200µm/ cm becasue the electron drift velocity in CF4 gas is fast. CF4 gas is easy to handle
because it is inert and harmless, while CS2 gas is ignitable and harmful. We note that CS2
gas has advantages to the SI interaction search (Atomic mass of sulfur is 32) and CF4 gas
has advantages to the SD interaction search.

CS2
CF4
Xe

140

120

100

80

60

40

20

0
0

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Drift field E/P (V/cm/atm)
Figure 3.4: Drift velocity of electrons of negative ions calculated by MAGBOLTZ for
CF4 (red), Xe(blue), CS2 (black) at a temperature of 300K.
The position resolution is also an important property of a direction-sensitive detector.
46

Gas detectors

Diffusion (µm/ cm)

1000

CS2 Transverse
CS2 Longitudial
CF4 Transverse
CF4 Longitudial
Xe Transverse
Xe Longitudial

900
800
700
600
500
400
300
200
100
0
0

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Drift field E/P (V/cm/atm)
Figure 3.5: Longitudial (dotted line)and Transverse (solid line) diffusion of electrons or
negative ions calculated by MAGBOLZ for CF4 (red), Xe(blue), CS2 (black) at a temperatureof 300K.

47

Chapter 3. Review of direct dark matter search experimentsmm

Roughly saying a direction-sensitive detector should have a position resolution better thana typical nuclear track length of interest. The lengths of nuclear tracks in several typesof chamber gas calculated by SRIM[77] as a function of the initial energy are shown in
Figure 3.6. Although Longer tracks are expected in lower pressured gas, a low pressured gas
detector needs a large volume and also often causes the unstability of the operation. We
need to overcome these difficulties in order to detect the very distinct signals of the dark
matter.
An overall discussion leads that the use of Xe gas requires much more technologicaldevelopment issues than those of CF4 and CS2 gas because a finer position resoltuion andlower pressure operation are required.

10

1

12

C 0.002atm in CF4
F 0.002atm in CF4
12
C 0.05atm in CF4
19
F 0.05atm in CF4
12
C 0.02atm in CF4
19
F 0.2atm in CF4
12
C 1atm in CF4
19
F 1atm in CF4
19

-1

10

10-20

50

100

150

200

250

300

350

400
keV

Figure 3.6: Length of recoil Carbon (dotted line) and Fluorine (solid line) nuclei in CF4
gas calculated by SRIM with each pressure (blue:0.002atm, green:0.05atm, red:0.2atm,
black:1.0atm).

3.5.2

Gaseous detector experiments

Since there are some issues to technologically overcome, any gaseous detector project has
never reached the sensitivity for the DAMA signal regions. We describe some gasous detectors and development status in the following paragraph.
US-UK DRIFT experiment [72] pioneered the study of the directional detection of thedark matter with 1 m3 of CS2 at 40 torr (170g fiducial mass) detectors. They demonstrated
the ability to reconstruct the direction of the incoming particles by detecting the direction48

mm

Gas detectors

10

1

10-1
32

S 0.002atm in CS2
Xe
0.002atm in Xe
32
S 0.05atm in CS2
Xe 0.05atm in Xe
32
S 0.2atm in CS2
Xe
0.02atm in Xe
32
S 1atm in CS2
Xe 1atm in Xe

10-2

-3

10 0

50

100

150

200

250

300

350

400
keV

Figure 3.7: Length of recoil Xe and S nuclei in Xe gas and in CS2 gas. They were calculatedby SRIM with each pressure (blue:0.002atm, green:0.05atm, red:0.2atm, black:1.0atm)

49

Chapter 3. Review of direct dark matter search experimentsof recoil nuclei in a gaseous detector. However, there existed an intrinsic limit of ability due
to their readout pitch of 2 mm.
DMTPC collaboration[74] proposed a new method to detect the track sense using a
CCD-readout TPC. They demonstrated that the projected images of nuclear recoil tracks
could be precisely taken via scintillation photons produced during the avalanche process in
CF4 gas. Though their detector is now small and there are many issues to develop in oredera dark matter search detector, their technique is very notable for the detection of the track
sense.
NEWAGE detector with CF4 gas and a fine pitch gasous detectors, which will be described in Chapter. 4 has potentials to scaled up to 1m3 with a low-cost mass-production.
Here we compare the NEWAGE detectors with the gaseous detectors of other projects,
DRIFT and DMTPC. Properties of the current and and developing detector of each project
are shown in Table 3.1. NEWAGE has advantages to the readout pitch and the effective
volume against DRIFT and DMTPC, respectively.
NEWAGE-0.3agas
pressure(torr)
volume(m3 )readout
dimension
pitch
ref

NEWAGE-0.3b

CF4
152

40

0.23×0.28×0.31

0.23×0.28×0.51

GEM&uPIC(charge)
3D
400µm
[75]

DRIFT-I

DRIFT-IIa

CS2
40
1×1×1
multi-wire(charge)
2D
3D
2mm
[72]

DMTPC

DMTPC2CF4
50
0.16 × 0.16×0.16

φ0.25 × 0.25

CCD-readout(light)
2D
256µm
[74]

Table 3.1: Comparison of gaseous directional sensitive dark matter detectors. Details of thethe DRIFT and the DMTPC can be found in the references.

50

Chapter 4

NEWAGE-0.3a detector
4.1

NEWAGE-0.3a detector

NEWAGE (NEw generation WIMP search with advanced Gaseous TPC Experiment) is adirection-sensitive dark matter search project. As described in Section 2.5, a directionsensitive method has a potential to provide strong evidence for the dark matter. However,there was no directional-sensitive detector with a sensitivity covering the DAMA region, letalone the MSSM region. Purposes of NEWAGE are to develop direction-sensitive detectors,
to search for the WIMP-wind, to detect a strong evidence, and finally, to study the propertiesof the dark matter. NEWAGE goals of the sensitivity for the spin-dependent WIMP-protoncross sections are shown in Figure 4.1 [75].
As a first step, we developed a prototype detector (NEWAGE-0.3a) and investigatedits performance precisely focusing on the requirements for a direction-sensitive dark mattersearch experiment. We describe the properties of CF4 gas and the properties and performance of the NEWAGE-0.3a detector in this chapter.

4.1.1

Property of CF4 gas

The TPC (Time Projection Chamber) gas is the most important material in the detector
because it determines the performance of the detector and also the sensitivity to WIMPs,
as described in Section 3.5. We use CF4 gas for the NEWAGE-0.3a detector.
Basic properties of CF4 gas are listed in Table 4.1. CF4 gas has an advantage to the
SD WIMP search, because a CF4 molecular has four fluorine nuclei which have a large
λ2 J(J + 1) (See Section 2.3). Since CF4 gas has a faster drift velocity, smaller drift diffusion
than noble gases (e.g. Ar, Xe. See Figure 3.5) is expected. This is an important property
to develop a large-sized detectors.
On the other hand, CF4 gas has two disadvantages: a large W-value and small Townsent
coefficient. The W-value, which is an average energy required to make an electron-ion pair inthe detector gas, of CF4 is about twice larger than that of Ar and Xe. This property means
the number of electrons is only a half of that expected in Ar gas. The townsend coefficient,a factor that governs the gas multiplication process, of CF4 is smaller than that of Ar and
Xe. This property indicates that we need to supply a higher voltage to anode electrodes
than the voltages with Ar or Xe gas. Therefore, the use of CF4 gas as TPC gas generally
requires a more intensive study than those of the other standard gases. This requirement
was slightly relaxed in our case because the muons and electrons are not our primary target,
but the recoil nuclei, with larger energy deposition per unit length, are.
51

Chapter 4. NEWAGE-0.3a detector

SD 3σ detection sensitivities
102
101
σWIMP -p[pb]

UKDMC(NaI)
010

0.3 m3year ST/FT

10-1

3 m3year ST/FT

10-2

300 m3year ST/FT

10-3

MSSM

10-4
101

102
MWIMP [GeV]103

Figure 4.1: Sensitivity goals of the NEWAGE project. Thick and dotted lines show expected
3σ asymmetry detection sensitivities by the ST(semi-tracking) and FT(full-tracking) modes,
respectively. Details are described in [75]. An experimental result of NAIAD (thin dashddotted line labeled UKDMC) and MSSM predictions(thin line labeled MSSM) are also shown[47],[76].

molecular forum
nameappearance
molecular massdensitymelting point
boiling point
W-value

CF4
Tetrafluoromethane, Carbon tetrafluoride
colorless odorless gas
88.01 g/mol
3.76g/l (15◦ C 1atm)-183.6◦C-127.8◦C
54

Table 4.1: The properties of CF4 gas.

52

NEWAGE-0.3a detector

4.1.2

NEWAGE-0.3a detector

The NEWAGE-0.3a detector is a µ-TPC (micro Time Projection Chamber) read by a
30.7×30.7 cm2 -sized µ-PIC (micro PIxel Chamber, TOSHIBA/DNP, SN060222-3) which
is a two-dimensional fine-pitch imaging device [78]. The properties of the NEWAGE-0.3a
are shown in Table 4.2 and its photograph is shown in Figure 4.2. The schematic view of

Figure 4.2: Photograph of the NEWAGE-0.3a detector, which consists of a TPC vessel andread-out circuits.
the µ-TPC is also shown in Figure 4.3. A µ-PIC is manufactured by printed circuit board(PCB) technology. PCB technology realizes an economical mass production, which is oneof the most important requirements for the fabrication of a dark matter detector. The µPIC has 768×768 pixels with a pitch of 400 µm read by 768 anode strips and 768 cathodestrips. The anode and cathode strips are orthogonally formed and thus we can detect thetwo-dimensional position of each event by taking the coincidence of the anode and the cathode strips. The schematic structure of µ-PIC is shown in Figure 4.4. Details of µ-PICs areshown in Appendix B.
The µ-PIC is used as a main gas-amplifier and as readout electrodes. A GEM [79] with
an effective area of 23×28 cm2 (Scienergy Co. Ltd.), is set 5 mm above the µ-PIC and used
as a sub-amplifier. We make an electric field with a length of 31 cm above the GEM withfield-shaping patterns on circuit boards to form a detection volume of 23×28×31 cm3 . A
volume of 20×25×31 cm3 is set as the fiducial volume for the dark matter search in theanalysis. The µ-TPC is set in a 3 mm-thick stainless-steel vacuum vessel filled with CF4
gas at 152 torr. A SORB-AC Cartridge pump (saes getter MK5) is attached to the vessel
to absorb contaminants in the chamber gas.
The µ-TPC signals from 768 anode strips and 768 cathodes strips are read by a dedicatedelectronic system [80]. The schematic structure of the data acquisition system for NEWAGE0.3a is shown in Figure 4.5. The signals are digitized in the amplifier shaper discriminator(ASD) chips (CXA3183Q), synchronized in a position encoding system with 100MHz clocks,
and recorded by a VME-bus memory board. Each digital signal or hit is a set of (Xmin ,Xmax, Ymin, Ymax, T), where Xmin and Xmax are the minimum and maximum positions
53

Chapter 4. NEWAGE-0.3a detector

V DRIFT =−8.2kV
drift plane (z=31cm)

8.25M Ω
31

Field shaping pattern
30
Z

V GEMT =−915V
V GEMB =−650V

Y

30M Ω

X

GEM
(Z=0cm)V µ−PIC=515Vµ −PIC(z=−0.5cm)
cathode strip
anode strip

768

768

Figure 4.3: The schematic structure of the NEWAGE-0.3a. VDRIFT is a voltage suppliedat the drift plane, VGEMT is at the top of the GEM, VGEMB is at the bottom of the GEM,
Vµ−PIC is at the anode of the µ-PIC.

Figure 4.4: The schematic structure of a µ-PIC.

54

NEWAGE-0.3a detector

Outer vessel
size
42.6×42.6×34.65 cm3thickness
3mm(top&side),16.5 mm(bottom)
material
stainless steel
Gas volume
Gas
CF4
pressure
152 torr
Drift volume
size
35.0×35.0×31.0 cm3material
Fluoroplastic∗1 (wall), Cooper(top)electric field∼240 V/cm at 152torr
Detection volume
effective volume
23 × 28 × 31 cm3
fiducial volume
20 × 25 × 31 cm3
fiducial mass
11.48greadout module(µ-PIC)
S/N
SN060222-3
size
30×30 cm2
operation gas gain ∼ ×700
readout pitch
400 µmGas preamplifier (GEM)
material
polyimidesize
23×28 cm2
hole pitch
140µmhole size
70µm
thickness
50µm
operation gas gain ∼ ×3
Table 4.2: Properties of the NEWAGE-0.3a detector*1:GP-500 [81]

55

nhit

Anode hit
(768bit)

Memory Board

ASD

64bit

Anode hit

Scaler

1kHz
Clock

BUSY

Xmax XminYmaxYminT

BUSY

Fadc trig

Phillips 740

12

Data aq trig

Analog FAN IN OUT(Analog SUM )64ch anlogsum

ex trig
Phillips 757

anlog sum
12

Interrupt

Reset(768bit)

1/3 attenuator(192ch anlogsum+attenuated)

4

100MHz Flash ADC

Clock

Digital IO(768bit)

NIM 12ASD

Xmax XminYmaxYmint

Position
EncorderLVDS 768ch

Discriminator
Phillips 706

cathode 768ch
56

Figure 4.5: Data acquisition system of the NEWAGE-0.3a detector

uTPC

anode 768ch

LVDS 768ch

Chapter 4. NEWAGE-0.3a detector

VME BUS SYSTEMNEWAGE-0.3a detectorof the anode strips which detected the TPC signals, the Ymin and Ymax are those of thecathode strips and the T is the clock counter. We introduced a new tracking procedure
which took account of the spread of the electron cloud by using a data set of (Xmin ,Xmax, Ymin, Ymax) for one clock, instead of ((Xmin+Xmax)/2,(Ymin+Ymax)/2) usedin our previous work [75]. In the previous tracking procedure we were not able to decidedirections of tracks that run parallel to X-Y plane because X-Y spreads in one clock wereignored. In this new method, we analyzed a data set of (Xmin , Xmax, Ymin, Ymax, T) as
independent 4 points (Xmin, Ymin, T), (Xmax, Ymin, T), (Xmin, Ymax, T), and (Xmax,
Ymax, T). We thus obtained 4 points for a clock and determined the direction by fitting allof these measured points with a straight line. The track-length is defined as the distanceof end-points of the fitted segment. Analog TPC signals of 768 cathodes are amplified andgrouped down to 4 channels and their waveforms are recorded by a 100 MHz flash ADC.
The energy deposition of a charged particle is thus known from the detected waveforms.
Typical operation parameters are shown in Figure 4.3, which are optimized to realize astable operation with a combined (µ-PIC×GEM) gas gain of 2000 and the drift field of
0.24 kV/cm.
The gas gains of the µ-PIC and the GEM for CF4 are measured by a previous work andare shown in Figure 4.6 and Figure 4.7. This measurement was performed by irradiatinga small-sized TPC which consisted of a 10×10 cm2 µ-PIC and a 10×10 cm2 -GEM with
5.6 keV X-rays from a 55 Fe source. The operation gas gain for the dark matter search isabout 2000, which is out of the range of these measurements, so we used these results as arough guide to determine the operating voltages. Absolute gas gain is measured by using
the α-ray calibration (See Section 4.2.2).

Total gas gain(gem =180V)

104

103

102

450

500

550

600

650

700
750
800
anode voltage (V)Figure 4.6: Gas gain as a function of µ-PIC anode voltage. The voltage between the top
and bottom electrodes of GEM is fixed at 180 V.
We defined the detector coordinate as follows. We set the origin at the center of the57

Total gas gain(anode =600V)

Chapter 4. NEWAGE-0.3a detector

104

103

100

150

200

250

300
350
400
GEM voltage (V)Figure 4.7: Gas gain as a function of a voltage between the top and bottom electrodes of
GEM. The µ-PIC anode voltage was fixed at 600V.
GEM, X-axis along the cathode strips, Y-axis along the anode strips and Z-axis along thedrift field as shown in Figure 4.3. The detector is set so that the µ-PIC plane is placed
horizontally. We also describe a position in the laboratory with the detector coordinate as(X(cm),Y(cm),Z(cm)).
The NEWAGE-0.3a has two type of the operation modes which differ in the trigger
signal. One is ”external-trigger mode”. We use this mode only for a measurement with a252
Cf source and we use a signal of a prompt gamma ray or a neutron detected by a plastic
scintillator set nearby the 252 Cf source as a trigger. We can measure the absolute Z positionof the recoil nuclear tracks in this operation mode. We measured the drift velocity and theangular resolutions in this mode. The other is ”self-trigger mode”. In this trigger mode, the
TPC analog signals from 768 anode strips are grouped down to 16 channels and are usedas the triggers. The absolute Z position is not measured, but only the relative position ismeasured. In this thesis we used the self trigger mode unless we mention otherwise.
The data taken by µ-TPC is analyzed with some event selections as follows.
• Nhit selection: We select the events which have at least three hit points. We implicitly
used this selection to all data in this thesis.
• Fiducial selection: We select the events whose hit points were all in the fiducial volume.
We implicitly used this selection to all data in this thesis.
• Energy selection: We select the events which satisfy the energy condition.
• Length selection: We select the events which satisfy the length condition.
58

NEWAGE-0.3a detector• Nuclear selection: This selection consists of an energy selection of E>100 keV and alength selection of L<1 cm. The length limit is to reject gamma rays, and the energy
threshold is determined to keep a performance of nuclear tracking. These details aredescribed in Section 4.3. This selection was used at the nuclear recoil measurements
in Section 4.4.1 and the dark matter search run.

59

Chapter 4. NEWAGE-0.3a detector

4.2

Energy calibration and stability

4.2.1

Drift velocity

counts

The electron drift velocity in the µ-TPC needs to be measyred to reconstruct a threedimensional track from the µ-TPC data set (Xmin , Xmax, Ymin, Ymax, T). Although adrift velocity of an electron in CF4 is calculated in Figure 3.4, it is very sensitive to theimpurity of gas. We measured the drift velocity with the external-trigger mode. We used aplastic scintillator and a 252 Cf source with an intensity of 2.2 MBq. (We mean this source
when we refer to as ”the 252 Cf source” throughout this thesis.) The 252 Cf source was set
at (0,0,33) and the plastic scintillator for a trigger was set by the source. Nuclear recoils
due to neutrons from the source were detected and a drift-time (T) distribution of the hit
points were obtained as shown in Figure 4.8. Because the detection volume was limited by
the drift plane, we can see a edge due to the drift plane. The drift velocity was calculated
to be 6.5cm/µs since the edge is at 4750 ns and a maximum drift length is 31cm which wasa length from the drift plane to the GEM.

240
220
200
180
160
140
120
100
80
60
40
20
0
0

100 200 300 400 500 600 700 800
clock(10ns)

Figure 4.8: Drift-time distribution of nuclear recoils in a drift velocity measurement

4.2.2

Energy cariblation

We calibrated the µ-TPC with α-particles. First, we used the 10 B(n,α)7 Li reaction (Q =2.7 MeV). The maximum energy of the α particle in this reaction is 1.5 MeV , and it runs
approximately 1.6 cm in CF4 gas at 152 torr. A glass plate with a size of 7.5×2.3 cm2 andcoated with a 0.6 µm-thick layer of 10 B was installed at the position (12.3, 8.0, 15.0) in thevessel as a part of the detector. We set the 252 Cf source, and then placed polyethylene blocks
with a thickness of 5 cm surrounding the source, and thus the 10 B-plate was irradiated withthermal neutrons. In an analysis of this measurement, the events which were fully containedin an area of 5 cm≤X≤14 cm and 3 cm≤Y≤12 cm were selected. Typical tracks of α particles
detected in this measurement are shown in Figure 4.9. Each mark indicates one digital hit,and different marks (colors) display different events. The gray region indicates the 10 B-platein the figure. It was clearly seen that these tracks came from the 10 B-plate. We determinedthe energy scale using the spectrum edge around 1.5MeV. The measured length-to-energy
correlation was compared with the one calculated by SRIM [77]. We fitted each track with
60

z cm

Energy calibration and stability

25
24
7.0

cm

23
22
21
20

2.3cm

19
18
17
12
y c1110m

B coated board

9

8

7

6

5

4

3

2 11

11.5

12

12.5

13

13.5

14

x cm

Figure 4.9: Typical tracks of α particles from the 10 B-plate. The 10 B-plate is set at theshaded area. Each mark indicates one digital hit and different marks (color) display different
events.

61

length[cm]

Chapter 4. NEWAGE-0.3a detector

5

p

4.5

103

4

α

3.5
3

102

2.5
2
1.5

10

1
0.5
0
0

500

1000

1500

2000
2500
3000
detected energy [keV α .e.]1

Figure 4.10: Correlation of the energy-length of the 10 B-run (color contour) and the calculated lines by SRIM. The dotted line (black) shows an energy-length correlation of the
protons and the solid line (purple) shows one of the α particles.

a straight line and calculated the position on the nearest to each hit point, and then defined
the track length as the distance between the both ends of the points on the fitted line. In
Figure 4.10, the measured data are shown by the color contour, while the correlations of the
protons and the α particles calculated by SRIM and were shown by dotted (black) and solid(purple) lines, respectively. The measured plots are consistent with those of the α-particle
tracks down to 100 keV. The energy scale determined by these alpha-particles are used as
keV α-particle equivalent ( keVα.e.) because it is important to consider a nuclear quenching
factor for low energy nuclear recoil detector, Fq , which is determined asFq =energy used for ionization of target molecules.
total energy deposited in target(4.1)

Using this Fq , a number of ion-electron pairs produced by a recoil particle with an energyE, where W is W-value of the gas. Nuclear
of E in a gas detector is calculated as Fq × W
quenching factors for several gas detectors were summarized in [83]. The nuclear quenching
factors of helium, carbon and fluorine ions in CF4 gas were calculated by SRIM as shownin Figure 4.11. These values are used when the energy deposition of ions in a detector are
calibrated by α-particle energy.

4.2.3

Position-dependence of the gas gains

We measured the position-dependence of the gas gains in order to apply the calibrationby the 10 B-plate to the whole area. Then we used background α particles from 222 Rnprogeny (See Appendix A) for these two measurements, where the higher energy data,
above 1 MeV, of a dark matter run performed with the self-trigger mode were used. We
divided the fiducial area into 4×5 regions, and made an energy spectrum of each region by
applying a length-selection of L > 3 cm. The background spectrum of the reference region62

nuclear quenching factor Fq

Energy calibration and stability

1
0.9
0.8
0.7
0.6
0.5
0.4

He

0.3

C

0.2

F

0.1
0
0

100 200 300 400 500 600 700 800 900 1000
recoil energy (keV)

Figure 4.11: Nuclear quenching factor for helium (red line), carbon(green line) and fluorine(red line) in CF4 gas. The number of ion-electron pairs produced by a particle with energy
Eof E in gas is calculated as Fq × W, where W is the W-value of the gas.

63

count/keV/kg/days

Chapter 4. NEWAGE-0.3a detector

20
18
16
14
12
10
8
6
4
2
0
0

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
α particle energy [keV α .e.]Figure 4.12: Spectrum of α particles due to the 222 Rn progeny in the reference region. The
solid line is a best-fit function.

(7.5 cm≤X≤12.5 cm, 5 cm≤Y≤10 cm), which is the nearest region to the 10 B-plate is shownin Figure 4.12. We fitted the spectrum with a sum of three Gaussians that had peaks at
5.49 MeV, 6.00 MeV and 7.69 MeV, which are energies of α-particles emitted from 222 Rnprogeny (See Figure A.2). We fixed the relative energy scales, relative energy resolutions
and relative intensities of three peaks, while the absolute energy scale, the absolute energy
resolution and the absolute intensity were treated as free parameters. The best-fit energy
scale agreed with the one obtained with the 10 B-plate calibration with an accuracy of ±3%(RMS). We then fitted 19 spectra of other regions by the same method. We calculated thegas gains of these 19 regions relative to the 10 B-region using the best-fit energy scales. We
show the relative gas gains of the whole area, or the gain map, in Figure 4.13. The gas gains
were normalized by the averaged one. The maximum gas gain and the minimum one were
1.4 and 0.35, respectively. This in-uniformity of the gas gain was due to the plating processof the µ-PIC, and is expected to be improved in the next fabrication. We used this gain
map to correct the gas gains in all analysis of the following measurements.

4.2.4

Energy resolution

We made an energy spectrum of the whole fiducial volume. We then fitted the correctedenergy spectrum in the same way as we fitted the spectrum of each region. The measured
spectrum and corrected spectrum are shown in Figure 4.14. The energy resolution was 45%(FWHM) around 6 MeV. This energy resolution was not a good value, which is probably
due to the local position-dependence of the gas gains within the regions and the positiondependence along the Z-axis.
64Energy calibration and stability

Y (cm)

10

1.4

8
6

1.2

4
1

2
0

0.8

-2
-4

0.6

-6
-8
-10

0.4

-10

-5

0

5

10
X (cm)

25

counts/keV/kg/days

counts/keV/kg/days

Figure 4.13: Measured relative gain map of the µ-TPC. The area enclosed with dotted lines
is the reference region. The gas gains were normalized by averaged value.

not corrected

20

15

25

20

15

10

10

5

5

0
0

corrected

01000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0keVα.e.

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

keVα.e.

Figure 4.14: Measured spectrum of α particles due to the 222 Rn progeny from whole fiducial
volume before (left) and after (right) the position dependent gain correction.

65

Chapter 4. NEWAGE-0.3a detector

relative gain

1.2

1

0.8

0.6

0.4

0.2

0
0

10

20

30

40

50
day

Figure 4.15: Time-dependence of the gas gain. The origin of the time axis is when we filled
the vessel with a CF4 gas and gas gains are normalized by the initial one.

The energy resolution around 100 keVα.e. was estimated as follows. There were fluctuations due to an electrical noise σnoise equal to about 55 keVα.e.(FWHM) against a gas gainof 2000, while that of position dependence σuni was less than 45% (FWHM) as measured
and that of statistical dependence σsta was 5.5% (FWHM) at 100keVα.e.. Thus the totalenergy resolution was estimated to be about 70% at 100 keVα.e. with a gas gain of 2000.

4.2.5

Time-dependence of the gas gains

Also we have to monitor the time-dependence of the gas gains in order to calibrate theenergy properly during a long-term measurement. The time-dependence of the gas gain
was monitores by the energy scale obtained by the procedure described in Section 4.2.2.
The measured time-dependence of the gas gain is shown in Figure 4.15, where the originof the time axis was when we filled the vessel with CF4 gas. A gradual 15% decrease of
the gas gain was observed in one month. The decrease was so slow that we could correcta calibration parameter with this gain monitor. However we needed a certain gas gain tomaintain sufficient performances, so we refilled the vessel with new CF4 gas at least every
two months.

66

length[cm]Gamma-ray rejection power

10
9
8
7

10

6
5

electron events

4
3

nucleus events

2
1
0
0

50

100

150

200

250
300
350
400
detected energy [keV α .e.]1

Figure 4.16: Correlation of the length-energy of the 252 Cf run. The nuclear-selection region isindicated by the vertical thick line (E>100 keVα.e.) and the horizontal thick line (L<1 cm).

4.3

Gamma-ray rejection power

Gamma rays are known to be one of the most dominant background sources for direct
dark matter search experiments. Many technologies have been developed to subtract themstatistically [47, 48, 69, 70, 71], or reject them by event-by-event methods [56, 55, 59, 61,
64, 65]. Obviously the latter method is more reliable for experiments with a poor statistics
of real events. In the NEWAGE-0.3a detector the length-to-energy correlation is used todiscriminate the electron tracks from the nuclear tracks by an event-by-event method. Recoil
nuclei are known to deposit much more energy per unit length than electrons. We measured
the responses of the µ-TPC to the gamma rays to measure the rejection power.
We set the 252 Cf source at a position of (0,0,40.5) so as to irradiate µ-TPC with fastneutrons and gamma rays. We then replaced the source with a 137 Cs gamma-ray source withan intensity of 0.9 MBq. Both measurements were performed in the self-trigger mode withthe full-fiducial volume. We determined the nuclear recoil region of the “nuclear selection”,
where few gamma-ray events were expected. Figure 4.16 shows a length-to-energy plotof a 252 Cf run. In the 252 Cf run, most events are seen in the nuclear recoil region along
the horizontal axis. We determined the nuclear selection as requiring both L<1 cm andE>100 keVα.e. The length limit was to reject gamma rays, and the energy threshold was
to keep certain angular resolutions. (See Section 4.4.3.) The detection efficiencies for the
nuclear tracks are discussed in Section 4.4.1. Figure 4.17 shows a length-to-energy plot of
the 137 Cs run. Almost all events were outside of the nuclear region, which indicated a good
gamma-ray rejection power.
We calculated the rejection factor of the electron tracks by a comparison between the137
Cs run and a data simulated by Geant4. We defined the rejection factor of the electron tracks as the probability that we mis-identify an electron event as a nuclear event.
Figure 4.18 shows the energy spectra of the nuclear track region of the 137 Cs run (square
and red line histogram) and the background run (triangle and blue line histogram). We
67

length[cm]

Chapter 4. NEWAGE-0.3a detector

10
9
8
7
6
10

5
4
3
2
1
0
0

50

100

150

200

250
300
350
400
detected energy [keV α .e.]1

count/keV/kg/days

Figure 4.17: Correlation of the length-energy of the 137 Cs run. The thick lines shows theselection described in Figure 4.16. Almost all events are outside of the nuclear region.

700

137

Cs RunBG Run

600
500
400
300
200
100
0

50

100

150

200

250

300
350
400
detected energy [keV]

Figure 4.18: Spectra of the 137 Cs run (square red) and the background run (triangle blue)in the nuclear recoil region.

68efficiency

Gamma-ray rejection power

×10

-6

10

8

6

4

2

0

50

100

150

200

250
300
350
400
detected energy[keV α .e.]Figure 4.19: Efficiency of the electron track detection, or the gamma-ray rejection power, ofµ-TPC in the nuclear recoil region.

subtracted the background spectrum from the 137 Cs run spectrum. We then derived therejection factor of the electron track by dividing the measured subtracted spectrum by thesimulated spectrum of the initial energy of the recoil electrons. The result is shown in
Figure 4.19. The measured rejection factor of the electron tracks is (8.1±1.9)×10−6 at
100 keVα.e., and less than 1×10−6 at an energy region higher than 200 keVα.e.

69

Chapter 4. NEWAGE-0.3a detector

count/keV/kg/days

measurement

Geant4

106

105

50

100

150

200
250
300
350
400
energy of nuclear recoil [keV α .e.]efficiency

Figure 4.20: Measured (triangle red) and simulated (square blue) spectra of the nuclear
recoil scattered by fast neutrons from the 252 Cf source.

1

0.8

0.6

0.4

0.2

0

50

100

150

200
250
300
350
400
energy of nuclear recoil [keV α .e.]Figure 4.21: Absolute detection efficiency of the µ-TPC for nuclear recoil.

4.4

Response to nuclear recoils

4.4.1

Absolute detection efficiency

It is very important to precisely investigate the responses of the µ-TPC to nuclear recoils
for a dark matter search. We measured the absolute detection efficiency by comparing the70Response to nuclear recoils

degreezenith (+Z)
2.5

80
60
2

40
20
0

1.5

-X

+Y

+X
1

-20
-40

0.5

-60
-80
-80

-60

-40

-20

0

20

40

60 80
degree0

Figure 4.22: Relative directional-dependent efficiency (100-400 keVα.e.), which is the detection efficiency of nuclear recoils depending on the directions of nuclear recoils. The verticalaxis is the elevation angle and the horizontal axis is the azimuth angle.

measured spectrum of nuclear recoils scattered by fast neutrons from the 252 Cf source witha simulated spectrum.
A Geant4 simulation indicated that the irradiations from positions of (25,0,15), (-25,0,15),(0,25,15), (0,-25,15), (0,0,40.5) and (0,0,-9.5) provide an isotropic distribution of the nuclear
recoil directions. We irradiated the detector with fast neutrons from the 252 Cf source placed
at one of the six positions described above, and took data in the self-trigger mode. We
repeated this measurement for the six positions. We applied the fiducial- and nuclearselections and combined six spectra. The combined spectrum and the simulated one areshown in Figure 4.20. The simulated spectrum was normalized taking account of the live
time of the measurements, the intensity of the source and the quenching factor of carbon andfluorine in Figure 4.11. Then, the absolute detection efficiency was obtained by dividing themeasured spectrum by the simulated one. As shown in Figure 4.21, the detection efficiency
was 80% at 100 keVα.e. This decrease was reasonable because we apply the nhit selection(NHIT>2) and the two-dimensional sampling pitch of the µ-PIC (400 µm) and the samplingclock (100 MHz ) was not fine enough compared to the track length (ex. 1.1 mm for an
100 keVα.e. for a fluorine nucleus in 152 torr CF4 ).
71

Chapter 4. NEWAGE-0.3a detector

4.4.2

Directional-dependent efficiency

For the same reason as the absolute efficiency decrease, there also exists an in-uniformityof the detection efficiency of the nuclear recoil depending on the direction of the nuclear
recoil (directional-dependent efficiency). For a directional dark matter search, we need adirectional-dependent efficiency map of the nuclear recoil. We made a map by comparing
the measured distribution of directions of the nuclear recoil with the simulated one. We used
the same isotropic scattering data described in Section 4.4.1. The direction of each nuclear
recoil was determined by the best-fit vector, which was derived from a fitted line of measured
nuclear track points. In the fitting process, we fixed the sign of the X component of the vector
to be positive, because we did not measure the track sense (head-tail discrimination) in thismeasurement. We made a distribution of the direction of the nuclear track with the energy
range of 100-400 keVα.e. (measured direction distribution). We also made a distribution of
the direction of simulated nuclear recoils under the same condition, which was isotropic, as we
had expected (simulated direction distribution). The relative directional-dependent efficiency
was derived by dividing the measured direction distribution by the simulated direction one.
We show the relative directional-dependent efficiency in Figure 4.22. The efficiency is shownin the celestial chart, where the vertical axis is the elevation angle, the horizontal axis is
the azimuth angle, and the color contour shows the relative directional-dependent efficiency
normalized by the mean value. Our detector had better efficiencies for tracks with moderate
elevations ( around 45◦ from the horizon) than for those close to the vertical (elevation=90◦ ).
This is mainly caused by the readout system and we are trying to decrease the directiondependence by improving the position encoding algorithm.

4.4.3

Angular resolution

The angular resolution of the tracking is one of the most important parameters of a directionsensitive dark matter detector. We defined the tracking angular resolution as the root meansquare (RMS) of the angles between the real tracks and the measured ones. We measured theangular resolution by comparing the distribution of the measured recoil angles of the nuclear
tracks with that of the simulated ones. Figure 4.23 shows simulated cos θ distributions,where θ is the angle between the direction of an incident neutron and a nuclear track. Eachcos θ distribution was a combined one of the results of the simulations in which the 252 Cf
source was set at (25,0,15), (0,-25,15) and (0,0,40.5), The tracking angular resolutions of 0◦ ,22.5◦ , 45.0◦, 67.5◦ and 90.0◦ were taken into account using two-dimensional Gaussians. The
relative directional-dependent efficiency shown in Figure 4.22 was also taken into account.
The shape of the distribution of the cos θ apparently depends on the angular resolutionof the tracking. We thus can measure the angular resolution by comparing the measured
distribution with the simulated distributions.
We set the 252 Cf source at the positions of (25,0,15), (0,-25,15) and (0,0,40.5), andmeasured with the external-trigger mode. We used this mode to detect the absolute positionof a nuclear recoil and thus the incoming direction of an incident neutron was precisely
measured. We set the sign of the X component of the direction of the nuclear recoil to bepositive by the same reason as in the previous measurement. The incident neutron vector
was calculated from the source position and the hit point nearest to the source position,
while the nuclear recoil track vector was measured by the µ-TPC. We then calculated theabsolute value of the cosine of a recoil angle between these two vectors.
One of the measured distributions is shown by a histogram with errors (red) in Figure 4.24, where the energy range was 100-400 keVα.e. We compared the measured histogram with simulated histograms with various angular resolutions in the range from 0◦
to 90◦ . The simulated one with an angular resolution of 46◦ fitted with the least value of
72

a.u.

Response to nuclear recoils

2.5

2

σ=0°
σ=22.5°σ=45°
σ=67.5°
σ=90°

1.5

1

0.5

0
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9
1
|cos θ|Figure 4.23: Simulated distributions of the absolute value of the cosine of the recoil angleof the nuclear tracks (100-400 keVα.e.) assuming some angular resolutions of the tracking.

χ2 , where χ2 /d.o.f was 1.829/9. This best-fit simulated histogram is shown by the solid
histogram (black) in Figure 4.24. The probabilities derived for the χ2 tests are shown in
Figure 4.25 as a function of the angular resolution. Thus, the angular resolution of the
tracking was known to be 46±3◦ in the energy range of 100- 400 keVα.e., where the error
was the 68% confidence level. We also derived the angular resolutions in the energy rangesof 100-200 keVα.e., 200-300 keVα.e., 300-400 keVα.e. and 400- 800 keVα.e. by the same
method. The result is shown in Figure 4.26. The angular resolution was better at higher
energy due to a longer track This trend indicates that better resolution is expected in alower pressure gas. We will operate the µ-TPC at a lower pressure in the near future forbetter angular resolutions.

73

cts/day/|cosθ|

Chapter 4. NEWAGE-0.3a detector

×10

3

2200
2000
1800
1600
1400
1200
1000
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9
1
|cos θ|Probability

Figure 4.24: Measured (red histogram with error bars) and the best-fit simulated (blacksolid histogram) distributions of cos θ .

1

0.8

0.6

0.4

0.2

0

35

40

45
50
55
60
angular resolution [degree]

Figure 4.25: Probabilities derived by the χ2 as a function of angular resolutions. We fitted
measured cos θ distributions with simulated ones.

74

angular resolution (degree)

Response to nuclear recoils

80

70

60

50

40

30

200

100

200

300

400

500
600
700
800
900
energy of nuclear recoil [keV α .e.]Figure 4.26: Angular resolutions of the nuclear tracking as a function of the recoil energy.
The errors bars indicate the 68% errors. The blue points show those with each energy andthe red one shows that with the energy in range of 100-400 keVα.e..

75

Chapter 4. NEWAGE-0.3a detector

4.5

Summary of detector performance

We summarized the performances of the NEWAGE-0.3a detector in Table 4.3.
parameter
energy resolution
γ-ray detection efficiency
energy threshold for nuclear recoil
Nuclear detection efficiency
Nuclear angular resolution

values45% (FWHM) at 6 MeVα.e.
70% (FWHM) at 100 keVα.e.
8.1×10−6 at 100keVα.e.
100 keVα.e.
80% at 100keVα.e.
55◦ (RMS) at 100keVα.e.

Table 4.3: Performances of the NEWAGE-0.3a detector.

76

section(4.2.4)(4.2.4)(4.3)(4.3)(4.4.1)(4.4.3)

Chapter 5

Pilot runs and backgroundstudies in Kamioka mine
We performed pilot runs for the dark-matter search with the NEWAGE-0.3a detector in asurface laboratory and an underground laboratory in Kamioka mine. Purposes of these pilot
runs are to demonstrate a long term stability of the detector and to understand background
events due to neutrons, γ-rays and α-particles in an underground laboratory. In this chapter
we will briefly introduce a surface run result, which was reported in [85], then report thepilot runs in the underground laboratory. We then discuss remaining background events
measured in these pilot runs.

5.1

Dark matter search in a surface laboratory

A dark matter search measurement was performed in a surface laboratory at Kyoto University (Lat. 35◦ 2′ N, Long. 135◦ 47′ E, third floor in a five-story building). The measurement
was performed from November 1st, 2006 to November 27th, 2006. The total live time was
16.71 days and the exposure was 0.151 kg·days. Figure 5.1 shows the obtained spectrum inthe energy range of 100-400 keV. The spectrum can be explained by a typical neutron flux
in a surface laboratory of O(1 × 10−2 ncm−2 s−1 ). We derived the limits of the WIMP-proton
SD cross section only from the spectrum(conventional method) and by a direction-sensitive
method as shown in Figure 5.2. This limit was the first limit derived with a gasous directionsensitive detector.

77

Chapter 5. Pilot runs and background studies in Kamioka mine

300

count/keV/kg/days

250

200

150

100

50

0
0

50

100

150

200
keV

250

300

350

400

Figure 5.1: Measured energy spectrum in the surface run. The detection efficiency was taken
into account.

SD 90% C.L. upper limits and allowed region
NEWAGE surface(spectrum only)

5

10

104

NEWAGE Surface(CF4,direction-sensitive)

3

10

e

σ χ-p[pb]

A_X

DAM

102
,Si)
F2
S(Ge okyo Ca
CDM
T
SO(F)
)
PICAS
0(Xe
ON1
)
F
N
(
E
E
L
X
SIMP
I)
(Na,
AD
NAI
s,I)
KIMS(C

10
DAMA(NaI)

1
COUPP(F)
-1

10

102
Mχ [GeV/c2]

3

10

Figure 5.2: Limits of SD cross section as a function of Mχ . NEWAGE surface run
experiment results (blue conventional method red direction-sensitive method). Allowed
region(DAMA(NaI)[45]) and limits of other experiments (DAMA(Xe)[60], CDMS[56],Tokyo[49], PICASSO[71], SIMPLE[70], NAIAD[47], KIMS[48], XENON10[64],COUPP[69])are shown for comparison.

78

Pilot runs in an underground laboratory

5.2

Pilot runs in an underground laboratory
NIM module
Z
Y
X

TPC

HV controler
VME
SystemASD

ASD

EncorderBoard

Terminal

Figure 5.3: Photograph of the NEWAGE-0.3a detector placed in Kamioka Observatory.
Pilot runs with the NEWAGE-0.3a detector were performed in an underground laboratory in Kamioka mine. The NEWAGE-0.3a detector was placed in Laboratory B, Kamioka
Observatory located at 2700m water equivalent (w.e.) underground (36◦ 25’N, 137◦18’E).
Since the aims of the NEWAGE-0.3a detector were to demonstrate a long-term operationand to study the background, we did not set any radiation shield. The detector was set sothat the µ-PIC plane was placed horizontally and the X-axis is aligned to the direction of
S87◦ E as shown in Figure 5.3. (For detector coordinates, see Figure 4.3).
The run properties are summarized in Table 5.1. Run IDs were reassigned when werun ID
RUN-I-1
RUN-I-2
RUN-I-3
RUN-I-4

date
Dec.7th -Dec.18th,2007
Dec.18th, 2007 -Mar.4th, 2008
Mar.6th -Apr.6th, 2008
Apr.9th -Jun.10th, 2008

RUN-II-1
RUN-II-2

Jun.12th -Jul.24th, 2008
Jul.25th -Sep.9th, 2008

live time [day] effective mass [g]
10.05
11.48
66.04
11.48
16.15
11.48
57.10
11.48
Kamioka RUN-I total exposure
21.46
11.48
30.99
11.48
Kamioka RUN-II total exposure

Table 5.1: Properties of the pilot runs in Kamioka mine.
79

exposure[kg days]
0.115
0.758
0.185
0.686
1.744
0.246
0.356
0.602

Chapter 5. Pilot runs and background studies in Kamioka mine

Drift velocity [cm/ µs]

refilled the chamber gas. The time dependencies of the drift velocity and the detection
efficiency at 100 keVα.e. are measured as shown in Figure 5.4, and Figure 5.5, by themethods described in Section 4.2.1 and Section 4.2.5, respectively. The red points in figures
show measured values and black error bars show interpolated value used in the analysis.
There were decrease due to the gas impurity and recoveries due to the gas refills in the both
figures. We also monitored the gas gain during the pilot run using the peak position around
6 MeV due to radons with the method described in Section 4.2.4, as shown in Figure 5.6.
There were decreases of the gas gain due to the gas impurity and recoveries due to thegas refills. The peak count rate around 6 MeV, Rpeak , which is a useful parameter tostudy the radon behaviors in the gas volume for background study later (Section 5.4.1), wasalso measured as shown in Figure 5.7. This variation seemed to be due to 222 Rn decay as
described in Section 5.4.1.
All the data of these pilot runs were analyzed with the nuclear selection cut and thefiducial volume cut with the method as described in Section 4.1.2. Since the detection
efficiency and gas gain decreased, we corrected the calibration factor and detection efficiency
with monitored parameters. Details of each run are described in the following sections.

14
12
10
8
6
4

RUN-I

2
0

12/12

01/11

02/10

03/12

RUN-II
04/11

05/11

06/10

07/10

08/09
09/08
Month/Day

Figure 5.4: Drift velocities during the pilot runs in Kamioka mine (Kamioka RUN-I and
RUN-II). Red points show measured values. Black error bars show interpolated values forthe analysis.

80Detection eficiency at 100~keV

Pilot runs in an underground laboratory

1
0.9
0.8
0.7
0.6
0.5
0.4
0.3

RUN-I

0.2

RUN-II

0.1
0

12/12

01/11

02/10

03/12

04/11

05/11

06/10

07/10

08/09

09/08

gas gain

Figure 5.5: Nuclear detection efficiencies at 100 keVα.e. during the pilot runs in Kamioka
mine (Kamioka RUN-I and RUN-II). Red points show measured values. Black error bars
show interpolated values for the analysis.

4000
3500
3000
2500
2000
1500
1000

Run-I

500
0

12/12

01/11

02/10

03/12

Run-II
04/11

05/11

06/10

07/10

08/09

Figure 5.6: Monitored gas gain in the pilot runs (Kamioka RUN-I and RUN-II). Decrease of
the gas gain was due to gas impurity. Recoveries of the gas gain were due to the gas refill.
222

counts/keV/kg/days

Rn peak height 6MeV

2

10

10

Run I
1

12/12

01/11

02/10

03/12

Run II
04/11

05/11

06/10

07/10

08/09
Month/Day

Figure 5.7: Monitored a peak count rate around 6 MeV due to 222 Rn progeny in pilot runs
(Kamioka RUN-I and RUN-II). The variation of peak count rate due to 222 Rn is discussed
in Section 5.4.1
81Chapter 5. Pilot runs and background studies in Kamioka mine

5.2.1

Kamioka RUN-I ∼ first underground run

count/keV/kg/days

We started the first underground measurement (Kamioka RUN-I) on December 7th, 2007.
Until June, 2008, we eventually achieved a total exposure of 1.74 kg·days. Obtained spectrum is shown in Figure 5.10. Measured event rate between 100 keVα.e. and 400 keVα.e.
region was about 40 % of that measured in the surface run. The background sources are
discussed from Section 5.3. We then plotted the directions of nuclear recoil tracks withan energy range of between 100 keVα.e. and 400 keVα.e. with the black points in Figure 5.9(top). We also plotted the directions of the solar motion, or the Cygnus from which
WIMP-wind is expected at each event time with purple circle marks. We calculated θ, theangle between the recoil direction and the WIMP-wind direction for each event, and drew
the |cos θ| distribution shown in Figure 5.9(bottom).

200
180

Length<1.0cm
Nhit>2
Energy 100-400 keVlive time 149.3 days
exposure 1.7 kgdays

160
140
120
100
80
60
40
20
0

50

100

150 200

250

300

350 400
keV α. e.

Figure 5.8: Obtained spectrum in Kamioka RUN-I.

82

Pilot runs in an underground laboratory

Zenithdegree

(A)
80
60
40
20

W0

N

E

-20
-40
-60
-80

counts/kev/kg/days/cosθ

0

20

40

60

80

100 120 140 160 180
degree

30

25

20

15

10
5

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1
|cosθ|

Figure 5.9: Obtained nuclear direction plots of events (top) and |cos θ| distribution(bottom)
in Kamioka RUN-I. An inuniformity in the event direction plots was due to the directionaldependent efficiency.

83

Chapter 5. Pilot runs and background studies in Kamioka mine

5.2.2

Kamioka RUN-II ∼ radon-rich run

count/keV/kg/days

We then the performed a radon rich run (Kamioka RUN-II) in order to study contributions ofradon to the dark matter search energy region from June 12th, 2008. Since α-particles from222
Rn progeny are one of the most probable candidates of remaining background origins, it
was important to study their behaviors(See Section 5.4.1 and Appendix A). We intentionally
mixed radon gas in the chamber gas by using the vacuum tube exposed to the radon-richatmosphere in the underground laboratory. Measured peak around 6 MeV due to α-particles
from radon progeny at the beginning of Kamioka RUN-II was ten times larger than that of
the Kamoika RUN-I as expected as shown in Figure 5.7. Time-variations of this peak andcontribution to the dark matter regions is discussed in Section 5.4.
Obtained spectrum in the energy range of 100-400 keVα.e., the direction map and the|cos θ| distribution drawn by the same manner in the previous section are shown in Figure 5.10, and Figure 5.11(top and bottom), respectively. The count rate at 100 keVα.e. was
1.5 times larger than that of Kamioka RUN-I.

200
180

Length<1.0cm
Nhit>2
Energy 100-400 keVlive time 52.5 days
exposure 0.6 kgdays

160
140
120
100
80
60
40
20
0

50

100

150

200

250 300

350 400
keV α. e.

Figure 5.10: Obtained spectrum in Kamioka RUN-II

84

Pilot runs in an underground laboratory

Zenithdegree

(A)
80
60
40
20

W0

E

N

-20
-40
-60
-80
0

20

40

60

80

100 120 140 160 180
degreecounts/kev/kg/days/cosθ

(B)60
50
40
30
20
10
0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1
|cosθ|

Figure 5.11: Obtained nuclear direction plot of events (top) and |cos θ| distribution(bottom)
in Kamioka RUN-II.An inuniformity in the event direction plots was due to the directionaldependent efficiency.

85

Chapter 5. Pilot runs and background studies in Kamioka mine

5.2.3

Summary of the pilot runs

The first underground run (Kamioka RUN-I in Figure 5.8) was performed at the same
condition as the surface run in a low background environment where neutron flux is approximately three orders of magnitude less than that in the surface laboratory (see Section5.3).
Measured count rate in Kamioka RUN-I was 40% of that of the surface run. This result explained that about half of the background sources in the surface run were ambient neutrons
and the measured spectrum in Kamioka RUN-I was caused by other sources. A flat |cos θ|
distribution (Figure 5.9 (bottom)) showed that unknown remaining backgrounds, not the
WIMP signals, should have dominated the measured events. The result of the radon-richrun (Kamioka RUN-II) implied the possibility of the contributions of the radon progeny. In
the following sections we will precisely discuss the candidates of this unknown background.

86

Background from outside the vacuum chamber

5.3

Background from outside the vacuum chamber

We discuss the remaining background measured in Kamioka RUN-I and RUN-II. We discuss
background originating outside the vacuum chamber and that originating inside the vacuum
chamber in this section and the next section, respectively.
In this section, we start with the cosmic-ray muon which is the most common backgroundsources in rare event searches. Then we discuss on the major two background sources inmost of dark matter search experiments: γ-rays and neutrons. β-rays and α-particles fromthe outside can be neglected because they are stopped by the vacuum chamber. Therefore
we estimated the contributions of muons, γ-rays and neutrons.

5.3.1

Cosmic-ray muons

The flux of the cosmic-ray muon at Kamioka observatory is ∼ 6 × 10−8 cm−2 s−1 sr−1 [86].
Muon flux as a function of depth in some underground laboratories in the world including
our laboratory (labelled by ”Super-Kamiokande”) is shown in Figure 5.12 [86]. A very
rough discussion explains that the muon contribution to the remaining background shouldbe negligible because the muon flux in the underground laboratory was less than 10−3 timesof that in the surface laboratory, while the background count rate in Kamioka RUN-I was
about 40% of that in the surface laboratory. More quantitatively, the muon rate passing
through the effective volume of the µ-TPC was calculated to be less than 2 × 10−4 cts/s.
If we assume the muon rejection power was the same order of that of electron (∼ 10−5 at
100 keVα.e.), the rate of a misidentified muon track as a nuclear track should be less than
2×10−1 counts/kg/days above 100 keVα.e. That count rate is much smaller than the one
in Kamioka RUN-I shown in Figure 6.6. The secondary particles produced by cosmic-ray
muons are studied as ambient gamma rays and neutrons in the following sections.

5.3.2

Gamma-rays

Ambient γ-rays are major background sources for direct dark matter searches. γ-rays aremainly produced in the decay of 40 K and the decay chains of 238 U and 232 Th shown in
Figure A.1 and Figure A.3, respectively. Although the NEWAGE-0.3a detector had a strong
rejection power against electron tracks as described in Section 4.3, mis-rejected γ-ray eventsmight not be negligible.
We measured the γ-ray flux in our laboratory with a 5 × 5 × 1cm3 CsI(Tl) scintillator
and a 5-inch photomultiplier(PMT R877, Hamamatsu) on October 23rd, 2008. The energy
resolution was about 10% at 662keV. We set the scintillator on the µ-TPC, and the PMT
signals were recorded by a PCI-3100 FADC with 20 MHz clock and the live time was recorded
by a PCI-6200 50 MHz scaler. Measured spectrum with a live-time of 0.512 days is shownin Figure 5.13. Since the scintillator was on the NEWAGE-0.3a detector, the spectrum
contained γ-rays emitted from the detector materials.
We then simulated the generation and propagation of γ-rays from 40 K, U-chain, andTh-chain in the rock. The incident γ-ray spectra from the U-chain and Th-chain are shownin Figure A.2 and Figure A.4, respectively. We considered the detector response and fitted
these simulated spectra with the measured one treating the contaminations of the 40 K, 238 U,
and 232 Th as free parameters. Estimated ambient γ-ray spectrum is shown in Figure 5.14,
where fitting parameters have errors of +100% and -50%. The total flux of this result wasconsistent with that of a previous work [87] measured near the Super-Kaiokande with agermanium detector within the error. We then simulated the response of the NEWAGE0.3a detector to this γ-ray flux. The result is shown by a red line in Figure 5.15. The
87

Chapter 5. Pilot runs and background studies in Kamioka mine

Figure 5.12: Cosmic ray muon flux as a function of depth in 103 hg cm−2 equivalent to km
w.e.[86].

88

counts/keV/kg/days

Background from outside the vacuum chamber

5

10

104

3

10

102

10
0

500

1000

1500

2000

2500

3000
keV

counts/keV/cm2/s

Figure 5.13: Ambient γ-ray spectrum measured in our laboratory with a CsI(Tl) scintillator.

10-1

10-2

-3

10

10-4

-5

10 0

500

1000

1500

2000

2500

3000
keV

Figure 5.14: Estimated spectrum of the ambient γ-ray flux in our laboratory.

89

Chapter 5. Pilot runs and background studies in Kamioka minebackground rate by the γ-rays was about 7.1+7−4 counts/keV/kg/days at 100 keVα.e., which
was about 6+6%oftheremainingbackground.
−3

5.3.3

Neutrons

The neutron flux inside the radiation shield in Kamioka Observatory was measured with a
He proportional counter by other group [88]. Results are shown in Table 5.2. Since the3location
Kamioka Observatory

thermal neutron< 4.8 × 10−7

fast neutron∼ 1.9 × 10−6

Table 5.2: Neutron flux (neutrons cm−2 s−1 ) in Kamioka Observatory measured with a 3 He
proportional counter[88].
.
cross sections of the neutron capture reaction are very small for both of carbon and fluorine
nuclei, the contribution of thermal neutrons to the background spectrum was found to be
negligible.
We then calculated an expected spectrum by ambient fast neutrons as follows. If thefollowing conditions are satisfied, the energy spectrum of ambient fast neutrons is represented
by the ”1/E law”.
• Neutron sources are distributed uniformly inside a boundless moderator.
• The moderator rarely absorbs neutrons.
The Kamioka Observatory satisfies the above conditions because the laboratory is surrounded by a base rock which contains a small amount of water. Thus the ”1/E law” wasassumed to calculate the non-thermal neutron flux in the laboratory. An expected background spectrum from fast neutrons is calculated with Geant4 as shown with a blue line
in Figure 5.15. The expected spectrum was three orders of magnitude less than measured
remaining background spectrum.
As a result of this session, we note that the ambient γ-ray is the major background sourcefrom outside the chamber and its contribution is 7.1+7−4 counts/keV/kg/days, while that ofneutron and muon are less than 1 counts/keV/kg/days. at 100 keVα.e.

5.4

Background from inside the chamber material (Contributions of α-particles )Radioactive contaminations within the detector components often determine the backgroundlevel in rare event measurements. The radioactive contaminations emit γ-rays, β-rays andα-rays. γ-rays and β-rays can be rejected with a rejection power of 1 × 10−5, however α-rays
can not be discriminated from nuclear recoils around 100 keVα.e. region.
Natural radioactive isotopes contaminated in most of the materials are 238 U, 232 Th and40
K. As described in AppendixA, there are many α-decays and β-decays associating γ-ray
emissions in the 238 U-chain and the 232 Th-chain. 40 K decays with a β-ray and a γ-ray. These
isotopes could be contaminated in most of the material used in the detector components
such as metal, plastic, glass-fiber, and so on. In addition, radons(Rn) in U-chain and Thchain can emanate from the material as gas. Therefore, Rn and Rn progeny, which are
the isotopes below Rn as shown in Figure A.1 and Figure A.3, might exist not only in thedetector components but also in the gas volume.
90

counts/keV/kg/days

Background from inside the chamber material (Contributions of α-particles )102

10measued in Kamioka RUN-I
estimated ambient γ component

1estimated ambient neutron component

10-1
50

100 150 200

250 300 350 400
keV α. e.

Figure 5.15: Estimated spectrum due to the ambient γ-rays (red line) and the ambient fastneutrons (blue line). Black histogram shows the measured spectrum in Kamioka RUN-I.

91

Chapter 5. Pilot runs and background studies in Kamioka mine
We studied the contributions of these contaminations to the observed spectra in the pilot
runs. Although the number of β-rays emitted from one U-chain and Th-chain is as the sameorder as that of α-rays, the detection efficiency of an electron track of 100 keVα.e. is lessthan 1.0×10−5. In addition, most of the γ-rays from U-chain, Th-chain and 40 K went outof the vacuum chamber and measured as parts of ambient γ-rays in Section 5.3.2. Thus wemainly studied the behavior of α-particles emitted from U-chain and Th-chain. Then we
discuss β-rays from 40 K.
Although α-particles emitted from the U and Th chains have initial energy of more than
4 MeV, they sometimes deposit only their partial energy in the detection volume and makebackground events of about 100 keVα.e. The background due to α-particles from the U
and Th chains detected with the NEWAGE-0.3a detector can be classified into five types
according to the positions of their energy depositions. The categorization is listed below
and is also shown in Figure 5.16.

Drift plane(Copper)
3

5
2

Fiducial volume
1

5

Veto volume

5

1

3
5

2
4

4

4

GEM(Copper,Polyimide)
Gap volume

4µ−PIC(Copper,Polyimide)

4

Figure 5.16: Five types of energy deposition of background α-particles. There are α-particles
which run only in the fiducial volume ( 1 ), from the fiducial volume into the drift plane or
the GEM( 2 ), from the drift plane or the GEM into the fiducial volume ( 3 ), through thegap volume between the GEM and the µ-PIC ( 4 ), and through the veto volume ( 5 ).
Particles with green arrows( 1 ) deposit all of the initial energy in the fiducial volume, while
those with red arrows( 2 , 3 ) deposit parts of the initial energy in fiducial volume. Energy
depositions of purple arrows( 4 ), are suppressed by the ”suppression factor”(see text). αparticles categorized in 5 are rejected by a fiducial cut.
1α-particles which deposit all of their energy in the fiducial volume. In this case, αparticles are emitted from Rn progeny in the fiducial volume and stop in the fiducial
volume.
92

Background from inside the chamber material (Contributions of α-particles )2α-particles which deposit parts of their energy in the fiducial volume and then deposit
rests of the energy in the drift plane or the GEM. In this case, α-particles are emitted
from Rn progeny in the fiducial volume and run into the drift plane or the GEM.

3

α-particles which deposit parts of their energy in the drift plane or the GEM and thendeposit the rests of the energy in the fiducial volume. In this case, α-particles are emitted
from U or Th progeny in the drift plane or the GEM and come into the fiducial volume.

4

α-particles which run through the gap between the µ-PIC and the GEM. Their observed
energies are ”suppressed” compared to the events categorized in 1 by the following threereasons. (i) They deposit only some parts of their energy in the gap volume, because
their track length are generally longer than the gap length. (ii) These events are notmultiplied by the GEM before being detected. Thus their energies are observed as if”suppressed” by the GEM gain compared to the events in 1 . (iii) They run so close to
the µ-PIC that the electrode-structure of the µ-PIC is seen. Thus the electron collection
efficiency depend on their path. Measured energies of some events are suppressed by thecollection inefficiencies.
Since we did not understand exact values of (ii) and (iii), we defined the combined
effects as ”suppression factor”. The energy depositions in the gap region were detectedwith suppressed energies by the ”suppression factor”. In this case, some α-particles are
emitted from U, or Th in the GEM and the µ-PIC, and come into the gap region between
the GEM and the µ-PIC, while some α-particles are emitted from Rn progeny in the gap
region and run into the the GEM or the µ-PIC.

5

α-particles which run through and deposit parts of the energy in the veto volume. We
rejected them by the fiducial cut. In this case, α-particles are emitted from any place inthe detector and run through the veto volume.

Some of the α-particles classified in 2 , 3 , and 4 , may make background events withan energy around 100 keV. Thus we calculated the partial energy depositions of α-particles
with Geant4 assuming each radioactive contamination listed below.
A

222

Rn abd 220 Rn progeny uniformly distributed in the gas volume

B

238
U-chain and 232 Th-chain progeny uniformly distributed in the drift plane (the upside of the detection volume; 3mm-thickness cooper plate)

C

238

D

238

U-chain and 232 Th-chain progeny uniformly distributed in the GEM (the downsideof the detection volume; 5µm-thick cooper and 50µm-thick polyimide plate)U-chain and 232 Th-chain progeny uniformly distributed in the µPIC ( 100µm-thickcooper and 1mm-thick polyimide plate)

Calculated results are shown in the following sections and we also discuss the potentials of
the contributions to the measured Kamioka RUN-I and RUN-II spectra.

5.4.1

Contributions of Rn progeny in gas chamber A

Calculated spectra for case A are shown in Figure 5.17. α-particles from Rn progeny classified in 1 make a peak around 6 MeV, while those in 2 make a flat continuous components
from 100 keV to 3 MeV. The peak around 6 MeV was obviously observed and were used
for the gain monitoring in Kamioka RUN-I and RUN-II, as shown in Figure 5.6. The flat
continuous component would also make some contributions to the background spectrum of
Kamioka RUN-I and RUN-II. Following discussions strongly indicate that this component
dominated the measured spectrum above 200 keVα.e.
93counts/keV/kg/day

counts/keV/kg/day

Chapter 5. Pilot runs and background studies in Kamioka mine

particles running above GEM

particles running under GEM

10particles running above GEM

particles running under GEM

10

1

0

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

01000 2000 3000 4000 5000 6000 7000 8000 9000 10000

keV

keV

Figure 5.17: The results for A : Expected spectra of α-particles emitted from 222 Rn- (left)
and 220 Rn- (right) progeny in the gas volume. We assumed their density of 3.81×10−15g/g
and 3.27×10−20g/g, respectively. These contamination were determined by the count ratearound 6MeV of the measured spectrum in Kamioka RUN-I. Black and blue histograms
show deposit energy above (the deposit position 1 2 ) and under the GEM (the deposit
position 4 .) , respectively.

Time variation of the peak around 6MeV in Kamioka RUN-I and RUN-II
We observed time variations of the peak around 6MeV in Kamioka RUN-I and RUN-II as
shown in Figure 5.18 and Figure 5.19, respectively.
These variations were explained by the following scenarios. We assume that there is
no Rn gas in the gas volume just after the gas refill unless we intentionally contaminate.
Then the radons are produced in the U and Th chains within the detector components andemanate into in the gas volume. We assume the production rates of 222 Rn and 220 Rn in
the material are constant, because U and Th chains in the materials are in the radiation
equilibrium. Thus the Rn emanation rate from the material to the gas volume were assumed
constant as 220 Rout and 222 Rout [cts/s]. Therefore the rates of Rn decay in the gas volume
are described by Eq.(5.1).
t
))
T1
t
R222 Rn decay (t) =222 Rout × (1 − exp(− )),T2
R220 Rn decay (t) =220 Rout × (1 − exp(−(5.1)

where t is time from a gas refill, T1 = 79.3s and T2 = 5.516days, are decay times of 220 Rn,
Rn, respectively. The initial condition gives R220 Rn decay (0) = R222 Rn decay (0) = 0. In
the case of 222 Rn progeny, since the decay times of isotopes down to 210 Pb are much shorter
than 5.516 days, these progeny become in the radiation equilibrium in a few seconds. Thus
the decay rates of those 222 Rn progeny are same as that of 222 Rn, while the decay rate ofprogeny after 210 Pb can be negligible because a life-time of 210 Pb is much longer than thatof 222 Rn.
222

Based on those assumption were applied the time variation in Kamioka RUN-I. We fitted
94

counts/keV/kg/days

Background from inside the chamber material (Contributions of α-particles )

20
18

16
14

4.006+5.087*(1-exp(-t/5.51))

12
10
8
6
4
2
0

10

20

30

40

50

60
days

Figure 5.18: Time dependence of the count rate around 6 MeV in Kamioka RUN-I-2. X-axis
is the days from a gas refill. A blue line is the fitted function of C1 + Rout × (1 − exp(−t/5.5),
where C1 = 4.0 ± 0.40 and Rout = 5.1 ± 0.44. This time-dependence indicates existence of
222
Rn.

95

Chapter 5. Pilot runs and background studies in Kamioka minecounts/keV/kg/days

100
90

4.391+94.92*exp(-t/5.51)+5.087*(1-exp(-x/5.51))

80
70
60
50
40
30
20
10
0
0

5

10

15

20

25
days

Figure 5.19: Time dependence of the count rate around 6 MeV in Kamioka RUN-II-1. Xaxis is the days from a gas refill. A black line is the best fitted function of C2 + Rinit ×exp(−t/5.5) + Rout × (1 − exp(−t/5.5)) where C2 = 4.4 ± 0.29 and Rinit = 94 ± 3.8 were best
fit parameters. Rout = 5.1 was a fixed parameter. The blue line is the fitted function in
Figure 5.18 as an expected component of 222 Rn emanations. The red line shows componentof the initial 222 Rn decay and non 222 Rn components.

96

Background from inside the chamber material (Contributions of α-particles )the time-dependence of the count rate shown in Figure 5.18 with a function of
Rpeak (t) = R222 Rn decay (t) + C1 =222 Rout × (1 − exp(−t)) + C1 ,
T2

(5.2)

where C1 is a constant component which does not have a variations in a time-scale of a day.
C1 should contains 220 Rn contribution but we were not able to measure the amount of 220 Rn
contribution. Thus we defined this kind of constant component as a non-222 Rn component.
We took 222 Rout and C1 as free parameters and got the best-fit function as shown by a blueline in Figure 5.18 with parameters of C1 = 4.0 ± 0.40 and Rout = 5.1 ± 0.44. The time
variations of the peak around 6 MeV in Kamioka RUN-I were explained well with theseparameters.
On th other hand, there were some radon in the gas volume just after the gas refill in
Kamioka RUN-II (5.19). Measured peak around 6 MeV due to α-particles at the beginningof Kamioka RUN-II was ten times larger than that of the Kamoika RUN-I as described in
Section 5.2.2. We assumed that Rinit is the amount of 222 Rn at the time of gas refill(t = 0)
and Rout is the same as that in Kamioka RUN-I. We fitted the time-dependence of the countrate shown of Figure 5.19 with a function of
R222 Rn decay (t) = Rinit exp(−tt) + Rout × (1 − exp(− )) + C2 ,
T2
T2

(5.3)

where C2 is a non-222 Rn component in Kamioka RUN-II. We took Rinit and C2 as freeparameters and fixed Rout =5.1, which was the best fit parameter of the Rout in Figure 5.18.
The best-fit function is shown by a black line in the Figure 5.19. This line consists of
two component: a decreasing component (blue) due to the initial Rn and an increasing
component (red) due to the emanation from materials. The C1 in Figure 5.18 agrees with
C2 in Figure 5.19 within fitting errors. Thus the only difference between the 5.18 and
Figure 5.19 is the term of the initial 222 Rn, Rinit exp(− Tt2 ).
We note these characteristic time variations strongly support existence of 222 Rn although
there still exist unidentified components such as 220 Rn around 6 MeV.
Correlation between the low energy and high energy count rates
We then studied a contribution of α-particles to the low energy spectrum. The calculated
spectrum shown in Figure 5.17 has a flat component below 3 MeV and the measured one in
Kamioka RUN-II has also a flat component around 200 keVα.e. shown in Figure 5.8. Thissimilarity implies a contribution of the 222 Rn to the low energy spectrum. We therefore,studied the correlation between the low energy count rate with that of the peak around
6MeV in Kamioka RUN-II. The correlation was observed as shown in Figure 5.20. This
correlation indicates that radon and its progeny contributed to the low energy spectrum as
the calculated spectrum indicated.
We finally discuss how the ratio of the 222 Rn to 220 Rn affects the low energy spectrum.
A simple assumption, that the contributions of 222 Rn to measured peak around 6 MeV was
100%, gives the contamination of 222 Rn of 3.8×10−15g/g by fitting the measured in Kamioka
RUN-I with the calculated spectrum around 6MeV. Figure 5.21 shows spectra measured in
Kamioka RUN-I (red histogram) and calculated (black and blue) in the energy range of100-400keVα.e. The calculated one has about 5 counts/keV/kg/days contribution above
200 keVα.e. and about 6 counts/keV/kg/days contribution at 100 keV without the contribution of gap region events. In the opposite where the case contribution 220 Rn was 100%
results a 6 counts/keV/kg/days contribution above 200 keVα.e. and 6 counts/keV/kg/days
contributions at 100 keVα.e. without the contribution of gap region events. Thus we es97

sum 200-400keV counts/keV/kg/days

Chapter 5. Pilot runs and background studies in Kamioka mine

70
60
50
40
30
20
10
0
0

10

20
30
40
50
60
70
peak 6MeV counts/keV/kg/days

Figure 5.20: Correlation between the high energy (6 MeV) count rate and that of low
energy(200-400keVα.e.) in Kamioka RUN-II.
timated contributions of the radons are dominant above 200 keVα.e. with a count rate ofabout 5counts/keV/kg/days and about 5counts/keV/kg/days at 100 keVα.e. independentof the ratio of 222 Rn to 220 Rn in the gas volume. Contribution of the gap region events is
described in Section 5.6.

5.4.2

Contributions of U and Th in the drift plane B

Calculated spectra for case B are shown in Figure 5.22. Most of the α-particles deposit theirenergy in the drift plane classified as 3 . The spectra are nearly flat in low energy under
400 keVα.e. in contrast to the measured spectrum in Kamioka RUN-I (See Figure 5.23).
Since the expected contributions of radons ( A )at 400 keVα.e. (6±2 counts/keV/kg/days)
dominated the measured count rate at 400 keVα.e.(6±1 counts/keV/kg/days), the upper
limit of the contributions of case B at 400 keVα.e. was estimated to be 3 counts/keV/kg/days.
Thus we estimated the contributions of case B at 100 keVα.e. is less than 5 counts/keV/kg/days,by the shape of spectrum.

5.4.3

Contributions of U and Th in the GEM and the µ-PIC C

D

Simulated spectra for case C and D are shown in Figure 5.24 and Figure 5.25. Events
between the µ-PIC and the GEM ( 3 ) contribute to the low energy spectrum as shown inthe blue line in the figures. The count rate of these events is 100 times larger than that in
the fiducial volume ( 2 , black line in the figures). This characteristic distribution is causedby the suppression factor as described in the beginning of Section 5.4. We note that there
are no α-particles in the U-chain which have energy high enough (∼ 8.5 MeV) to penetrate
both the gap region and the GEM, and reach to the fiducial volume, while 212 Po in theTh-chain emits an α-particle with energy of 8.785 MeV, which is high enough to bring the98counts/keV/kg/days

Background from inside the chamber material (Contributions of α-particles )particles running above GEM
particles running under GEM

2

10

measured Kamioka RUN-I

10

50

100 150 200

250 300 350 400
keV α. e.

1

counts/keV/kg/day

counts/keV/kg/day

Figure 5.21: Measured Kamioka RUN-I spectrum (red) and a calculated contributions of222
Rn (black and blue histogram). Th black line and the blue line are estimated contributions
from 222 Rn with deposit positions of 1 and 4 , respectively. We assumed 3.81×10−15g/g of222
Rn in the gas volume to fit the peak around 6 MeV with that of the measured spectrumin Kamioka RUN-I.

α -particles from drift plane238

U 1.00 ppm

1

232

Th 1.00 ppm

10-1

10-1

10-2

10-2

0

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0keV

α -particles from drift plane

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

keV

Figure 5.22: The results for B : Expected spectra of α-particles emitted from 238 U- (left) andTh- (right) progeny in the drift plane. We assumed that both of their contaminations
are 1.0×10−6g/g as references. All particles deposit energy as 3 .

232

99

counts/keV/kg/day

Chapter 5. Pilot runs and background studies in Kamioka mine

102

10

1

0

50

100

150

200

250

300

350

400

keV
Figure 5.23: Comparison of shapes of calculated spectrum for case B and measured spectrumin Kamioka Run-I.

100counts/keV/kg/day

counts/keV/kg/day

Background from inside the chamber material (Contributions of α-particles )102
particles running above GEM

10particles running under GEM

1

102
particles running above GEM

101α-particles from GEM

10-1

α-particles from GEM

238

10-1

U 1.00 ppm

10-2

223

Th 1.00 ppm

10-2

-3

10 0

particles running under GEM

-3

10 0

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

keV

keV

counts/keV/kg/day

counts/keV/kg/day

Figure 5.24: The results for C : Expected spectra of α-particles emitted from 238 U- (left) and232
Th- (right) progeny in the GEM. We assumed that both of their density are 1.0×10−6g/g
as references. Black and blue histograms show deposit energy above (the deposit position
3 ) and and under the GEM (the deposit position 4 ), respectively.

102
particles running above GEM

10particles running under GEM

1

102
particles running above GEM

101α-particles from GEM

10-1

α-particles from uPIC

238

10-1

U 1.00 ppm

10-2

223

Th 1.00 ppm

10-2

-3

10 0

particles running under GEM

-3

10 0

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

keV

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

keV

Figure 5.25: The results for D . Expected spectra of α-particles emitted from 238 U- (left)
and 232 Th- (right) progeny in the µ-PIC. We assumed both of their contaminations are
1.0×10−6g/g as references. Black and blue histograms show the energy deposited above(the deposit position 3 ) and under the GEM, (the deposit position 4 ), respectively.

101

Chapter 5. Pilot runs and background studies in Kamioka mine
α-particle into the fiducial volume. Thus, there is no event of 2 with black line in the left
panel of Figure 5.25, but some events are in the right panel of Figure 5.25(right).
Since there are uncertain parameters of the suppression factor, and the amounts of238
U and 232 Th in this calculation, the calculated spectrum around 100 keV also has much
uncertainty. Actually, the shapes of the ”suppressed” (blue) spectra are affected by the
suppression factor. Figure 5.26 shows expected spectra with the suppression factor of 3, 5,
10, and 15 respectively. We simulated the spectra due to the α-particles from the 238 U chain
in the µ-PIC assuming a 238 U density of 5×10−6 g/g. The red histogram shows measured
background spectrum in Kamioka RUN-I. It was seen that larger suppression factor gave
steeper spectra. In addition, since there might be a position-dependence of the GEM gas
gain like the total gas gain, the spectrum might be a combined one of the spectra withvarious suppression factors. And these events possibly contribute to the significant amountof the low energy background spectrum in Kamioka RUN-I. As a result of studies in thissection the contributions of the 3 - C and 3 - D have potential to explain the measured
spectrum at 100 keVα.e.

5.5β-ray from 40K inside the vacuum chamber

We discuss β-rays from 40 K inside the vacuum chamber. Since the maximum energy ofβ-ray from 40 K is about 1.3 MeV, they can not penetrate the walls of the detection volume.
Thus we consider inner parts that faced on the detection volume. For producing a countrate of 100 counts/keV/kg/days events at 100 keVα.e. in the NEWAGE-0.3a detector, 40 K
needs the number of O(10−5 )mol atoms in inner parts considered the detection efficiencyof 10−5 at 100 keVα.e. This amount is equivalent to 300 g of the natural K assuming thenatural abundance of 40 K is 0.0117%. The total mass of the considered parts was about
1.4kg (440g(wall)+140g(µ-PIC+GEM)+860g(Drift plane)). Since the typical K contamination was no more than 1% in those materials, the contribution of 40 K was less than
5 counts/keV/kg/days.

102counts/keV/kg/days

β-ray from 40 K inside the vacuum chamber

200

Kamioka RUN-I

180
160
140

simulated(

238

simulated(

238

simulated(

238

U 5ppm,GEMgain=3)
U 5ppm,GEMgain=5)
U 5ppm,GEMgain=10)

simulated(238U 5ppm,GEMgain=15)

120
100
80
60
40
20
0

50

100 150 200

250 300 350 400
keVα .e.

Figure 5.26: Measured spectrum of Kamioka RUN-I and some of calculated contributionsof the µ-PIC contamination with 5.0×10−6g/g 238 U for given suppression factors. (purple
×3, blue ×5 , green ×10, black ×15), Contaminations in the GEM would make a similar
contribution. This figure suggests the events in gap between the µ-PIC and the GEM cancontribute to the low energy spectrum.

103

Chapter 5. Pilot runs and background studies in Kamioka mine

5.6

Summary of the background study
particle
γ-ray
cosmic muon
fast neutron
thermal neutron

counts/keV/kg/days at 100keV∼7<∼ 10−1∼ 10−1
negligible

Table 5.3: Estimated contributions of the background sources outside of the vacuum chamber
at 100 keVα.e..
α-particles (counts/kg/keV/days)gas volume A drift plane B GEM Cµ-PIC D
fiducial only 1
none
none
none
none
fiducial+materials 2 3∼5<5∼X/100 ∼X/1000
gap region+materials 4
<∼5
none
X
X
with veto region 5
none
none
none
none
Section(5.4.1)(5.4.2)(5.4.3)β-particles (counts/kg/keV/days)
U-chains
negligible
Th-chains
negligible
40K< 5 (Section 5.5)
Table 5.4: Estimated contributions to the measured spectrum at 100 keVα.e. of inside
the vacuum chamber. Issues of columns show materials with contaminations which emit
α-particles. Issues of rows show the positions of the energy deposition. X is unknown but
can potentially to be as high as 100.
We summarize the effects of the background sources outside and inside the vacuum
chamber in Table 5.3 and Table 5.4, respectively. Although a confirmation by a further
measurement is still required, ”X” in the table can potentially be as high as 100. We foundthat contributions of C - 4 and D - 4 could be the most dominant components of the measured spectrum at 100 keVα.e. in Kamioka RUN-I, and second dominant components wereA - 2 and A - 3 . The contribution of the outside sources was found to be less than that ofinternal sources. We have to reduce the internal background in order to achieve a lower background. Figure 5.27 shows one of example spectrum to explain the measured one in Kamioka
RUN-I, where we assumed 222 Rn density in the gas volume region is 3.8×10−15g/g from acount rate around 6 MeV and calculated ambient gamma-ray contribution in Section 5.3.2.
238
U density in the GEM is 4.0×10−6g/g and the suppression factor of 15 were assumed to
fit the measured spectrum in Kamioka RUN-I and other contribution were ignored.
Reducing the contaminations in the detector components is the most fundamental solution, however, a large amount of technical efforts are required to develop radio-pure GEMs
and µ-PICs. Another approach is to realize a complete fiducial cut to reject all backgroundin Table 5.4, by detecting the absolute Z position of each event. However, we need sometechnical breakthrough to pursue this approach.
A quick way to confirm the discussions above and reduce the background rate at 100 keVα.e.
is to focus on the gap region 4 . This region causes 100 times larger background than thefiducial regions as discussed in Section 5.4.3. The spectrum shapes of C - 4 and D- 4 could
be controlled by the GEM gains as seen in Figure 5.26. A higher GEM gain would make thespectrum steeper and decrease the count rate at 100 keVα.e. If we can operate the GEM
104

counts/keV/kg/days

Summary of the background study

102

Kakmioka RUN-I
GEM contribution(gap volume)
Rn contribution(fiducial volume)
Rn contribution(gap volume)
Gamma-ray contribution(outside)
Sum of estimated spectra

1050

100 150 200

250 300 350 400
keV α. e.

Figure 5.27: Example spectra explaining measured one in Kamioka RUN-I. We assumed
222
Rn contamination of 3.8×10−15g/g in the gas volume (black solid and dotted lines),
calculated ambient gamma-ray contribution (solid green line ), 238 U contamination in the
GEM of 4.0×10−6g/g (blue lines), and the suppression factor of 15. Other contributions wereignored. The dotted lines strongly depends on the suppression factor, which was actually
uncertain.
with a gain ten times higher than that of Kamioka RUN-I and RUN-II, the contributionof C - 4 should be comparable with C - 3 . We had not confirmed a stable operation of
GEM with such a high gain, and the risk of damaging the GEM might be too high. Thus we
decided to operate the NEWAGE-0.3a detector with about twice lager GEM gain to confirm
the contribution of C - 4 and D - 4 and to perform a lower background dark matter search.

105

Chapter 6

Dark matter search in Kamioka
mine
6.1

Dark matter search run in Kamioka mine

Based on the results of the pilot runs and the background studies described in Chapter 5,
we optimized the operation parameters and performed a dark matter search run (Kamioka
RUN-III). Kamioka RUN-III was performed from September 11th, 2008 to Decsenmber 4th
2008. The position and alignment of the NEWAGE-0.3a detector were same as those in
Kamioka RUN-I and RUN-II.(See Section 5.2.) We operated the GEM with a higher gain
than that in Kamioka RUN-I in order to suppress the background α-ray events interacted
in the gap between the GEM and the µ-PIC (events categorized as 4 - A , 4 - C and 4 - D
in Table 5.4). Operation parameters and run properties of Kamioka RUN-III are listed inTable 6.1 and Table 6.2, respectively.

µ-PIC anodesµ-PIC gain
GEMup
GEMdown
GEMup - GEMdown
GEM gain
Drift field
Gas
Pressurefiducial volume

Placement
X-axis direction

RUN-I&RUN-II
RUN-III
515V
455V∼ ×700∼ ×300
-915V
-915V
-650V
-605V
-265V
-310V
∼ ×3
∼ ×8
240 V/cm
CF4
152 torr
20 × 25 × 31cm3◦
36 25’N, 137◦18’E 2700m w.e.
S87E

Table 6.1: Operation parameter of µ-TPC.
The time dependencies of the drift velocity and the detection efficiency at 100keVα.e.
were measured as shown in Figure 6.1, and Figure 6.2, by the method described in Section 4.2.1 and Section 4.2.5, respectively. The red points in the figures show measured
values and black error bars show interpolated values used in the analysis. Decreases due tothe gas impurity and recoveries due to the gas refills were observed in the both parameters.
106

Dark matter search run in Kamioka mine
run ID
III-1
III-2
III-3

date
Sep.11th -Oct.1st, 2008
Oct.2nd -Nov.11th, 2008
Nov.13th -Dec.4th, 2008

live time [day] effective mass [g]
17.81
11.48
10.01
11.48
17.90
11.48
Kamioka RUN-III total exposure

exposure[kg days]
0.204
0.115
0.205
0.524

Table 6.2: Properties of the dark matter search run (Kamioka RUN-III) in Kamioka mine.

Drift velocity [cm/ µs]

We monitored the gas gain during Kamioka RUN-III using the peak around 6 MeV due toradons by the method described in Section 4.2.4, as shown in Figure 6.3. Furthermore, thecount rate around 6 MeV was monitored as shown in Figure 6.4, as like in Chapter 5. We
unintentionally contaminated the chamber gas with radon gas when we started Kamioka
RUN-III-2. The radon-contaminated period (from October 2nd to October 24th, 2008)
obviously seen in Figure 6.4 was eliminated from the dark matter search analysis.
All data obtained in Kamioka RUN-III except for the radon-contaminated period wereanalyzed by the same manner we did in the pilot runs described in Section 5.2. We applied
the nuclear selection cut and the fiducial cut, then corrected calibration factors and detection
efficiencies with the monitored parameters. Obtained energy spectrum is shown in Figure
12
10

8

6

4

2

0

Rn contaminated period
09/11 09/18 09/25 10/02 10/09 10/16 10/23 10/30 11/06 11/13 11/20 11/27 12/04
Month/Day

Detection eficiency at 100~keV

Figure 6.1: Drift velocities during the dark matter run (Kamioka RUN-III). Red points show
measured values. Black error bars show interpolated values for the analysis.

1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2

Rn contaminated period

0.1
0

09/18

10/02

10/16

10/30

11/13

11/27

12/11
Month/Day

Figure 6.2: Nuclear detection efficiency at 100 keVα.e. during the dark matter run (Kamioka
RUN-III). Red points show measured values. Black error bars show interpolated and extrapolated values for the analysis.

107gas gain

Chapter 6. Dark matter search in Kamioka mine

5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0

Rn contaminated period
09/18 09/25 10/02 10/09 10/16 10/23 10/30 11/06 11/13 11/20 11/27 12/04 12/11
Month/Day

R peak counts/keV/kg/days

Figure 6.3: Monitored gas gain during the dark matter run(Kamioka RUN-III).

50
45
40
35
30
25
20
15
10
5

Rn contaminated period
09/18 09/25 10/02 10/09 10/16 10/23 10/30 11/06 11/13 11/20 11/27 12/04 12/11
Month/dayFigure 6.4: Monitored count rate around 6 MeV due to Rn progeny during the dark matterrun (Kamioka RUN-III)108

Dark matter search run in Kamioka minecounts/keV/kg/days

6.5. We also compared the energy spectrum in Figure 6.6 with those of the pilot runs. The
count rate in Kamioka RUN-III was about 1/3 of that in Kamioka RUN-I at 100 keVα.e. We
then plotted the directions of nuclear recoil tracks with an energy range of 100-400 keVα.e.
with black points in Figure 6.7(top) and also directions of the Cygnus from which WIMPwind are expected at each event time with purple circle marks as the same manner we havedone in Chapter 5. We calculated θ, the angle between the recoil direction and the WIMPwind direction for each event, and drew the |cos θ| distribution shown in Figure 6.7(bottom).
This direction result was divided into 15 distributions by the measured recoil energy of every
20 keVα.e. as shown in from Figure 6.8 to Figure 6.11.

100
90
80
Length<1.0cm

70

Nhit>2

60

Energy 100-400 keVα.e.
live time 45.7 days

50

exposure 0.5 kgdays

40
30
20
10
0

50

100

150

200

250 300

350 400
keV α. e.

Figure 6.5: Obtained spectrum in Kamioka RUN-III.

109

counts/keV/kg/days

Chapter 6. Dark matter search in Kamioka mine

Kamioka RUN-I

102

Kamioka RUN-II
Kmaioka RUN-III

10

50

100

150

200

250

300

350 400
keVα.e.

Figure 6.6: Obtained energy spectrum in the pilot and DM search runs (RUN-I, RUN-II
and RUN-III )at Kamioka observatory. RUN-I is operated with the same parameters as thesurface run, RUN-II is a radon-rich run and RUN-III is operated with optimized parameterslisted in Table 6.1.

110

Dark matter search run in Kamioka mine

degree(A)

Zenith

80
60
40
20

W0

N

E

-20
-40
-60
-80

counts/kev/kg/days/|cosθ|

0

20

40

0.1

0.2

60

80

100 120 140 160 180
degree

20
18
16
14
12
10
8
6
4
2
0

0

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1
|cosθ|

Figure 6.7: Obtained direction of the nuclear tracks plot (up) and |cos θ| distribution(down)with energy range of 100-400 keVα.e. in Kamioka RUN-III.

111

Chapter 6. Dark matter search in Kamioka mine(A)

Zenithdegreedegree

(A)
80

80

60

60

40

40

20

20

W0

N

E

W0

-20

-20

-40

-40

-60

-60

-80
0

40

60

80

100 120 140 160 180
degree0count/kev/kg/days/cosθ

count/kev/kg/days/cosθ

160
140
120
100
80

60

80

100 120 140 160 180
degree

35
30
25
20
15
10

20

5

0.1

0.2

0.3

0.4

(A)

0.5

0.6

0.7

0.8

0.9

0

1
cosθ

0

0.1

0.2

0.3

0.4

(A)

Zenithdegreedegree

40

40

40

80

0.6

0.7

0.8

0.9

1
|cosθ|

80
60

40

40

20

0.5

Zenith

60

20

W0

N

E

W0

-20

-20

-40

-40

-60

-60

-80
0

20

45

60

0

E

(B)50

180

0

N

-80
20

(B)200

N

E

-80
20

40

60

80

100 120 140 160 180
degree400count/kev/kg/days/cosθ

count/kev/kg/days/cosθ

Zenith

35
30
25
20
15

20

40

60

80

100 120 140 160 180
degree

20
18
16
14
12
10
8
6

10
4
5
0

0

2

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1
|cosθ|

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1
|cosθ|

Figure 6.8: The obtained direction of the nuclear tracks plots and |cos θ| distributions with energy range of 100-120 keVα.e.(top left), 120-140 keVα.e.(top right), 140160 keVα.e.(bottom left) and 160-180 keVα.e.(bottom right) in Kamioka RUN-III.

112

Dark matter search run in Kamioka mine(A)

Zenithdegreedegree

(A)
80

80

60

60

40

40

20

20

W0

N

E

W0

-20

-20

-40

-40

-60

-60

-80
0

40

60

80

100 120 140 160 180
degree

0

25

20

15

10

0

0.2

0.3

0.4

0.6

0.7

0.8

0.9

60

80

100 120 140 160 180
degree

20

15

10

0

1
|cosθ|

0

0.1

0.2

0.3

0.4

(A)

80

0.6

0.7

0.8

0.9

1
|cosθ|

80
60

40

40

20

0.5

Zenith

60

20

W0

N

E

W0

-20

-20

-40

-40

-60

-60

-80

N

E

-80
20

40

60

80

100 120 140 160 180
degree0counts/kev/kg/days/cosθ

30

25

20

15

10
5

0

40

25

degreedegree0.5

Zenith

0

20

5

0.1

(A)

0

E

30

counts/kev/kg/days/cosθ

counts/kev/kg/days/cosθ

30

0

N

-80
20

5

counts/kev/kg/days/cosθ

Zenith

20

40

60

80

100 120 140 160 180
degree

30

25

20

15

10
5

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1
|cosθ|

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1
|cosθ|

Figure 6.9: The obtained direction of the nuclear tracks plots and |cos θ| distributions with energy range of 180-200 keVα.e.(top left), 200-220 keVα.e.(top right), 220240 keVα.e.(bottom left) and 240-260 keVα.e.(bottom right) in Kamioka RUN-III.

113

Chapter 6. Dark matter search in Kamioka mine(A)

Zenithdegreedegree

(A)
80

80

60

60

40

40

20

20

W0

N

E

W0

-20

-20

-40

-40

-60

-60

-80
0

40

60

80

100 120 140 160 180
degree

0

25

20

15

10

0

0.2

0.3

0.4

0.6

0.7

0.8

0.9

60

80

100 120 140 160 180
degree

20

15

10

0

1
|cosθ|

0

0.1

0.2

0.3

0.4

(A)

80

0.6

0.7

0.8

0.9

1
|cosθ|

80
60

40

40

20

0.5

Zenith

60

20

W0

N

E

W0

-20

-20

-40

-40

-60

-60

-80

N

E

-80
20

40

60

80

100 120 140 160 180
degree0counts/kev/kg/days/cosθ

30

25

20

15

10
5

0

40

25

degreedegree0.5

Zenith

0

20

5

0.1

(A)

0

E

30

counts/kev/kg/days/cosθ

counts/kev/kg/days/cosθ

30

0

N

-80
20

5

counts/kev/kg/days/cosθ

Zenith

20

40

60

80

100 120 140 160 180
degree

30

25

20

15

10
5

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1
|cosθ|

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1
|cosθ|

Figure 6.10: The obtained direction of the nuclear tracks plots and |cos θ| distribution with energy range of 260-280 keVα.e.(top left), 280-300 keVα.e.(top right), 300320 keVα.e.(bottom left) and 320-340 keVα.e. (bottom right) in Kamioka RUN-III.

114

Dark matter search run in Kamioka mine(A)

Zenithdegreedegree

(A)
80

80

60

60

40

40

20

20

W0

N

E

W0

-20

-20

-40

-40

-60

-60

-80
0

40

60

80

100 120 140 160 180
degree

0

30

25

20

15

10

0

E

20

40

60

80

100 120 140 160 180
degree

30

25

20

15

10

5

0

N

-80
20

counts/kev/kg/days/cosθ

counts/kev/kg/days/cosθ

Zenith

5

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0

1
|cosθ|

0degree

(A)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1
|cosθ|

Zenith

80
60
40
20

W0

N

E

-20
-40
-60
-80

counts/kev/kg/days/cosθ

0

20

40

60

80

100 120 140 160 180
degree

30

25

20

15

10
5

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1
|cosθ|

Figure 6.11: The obtained direction of the nuclear tracks plots and |cos θ| distributions with energy range of 340-360 keVα.e.(top left), 360-380 keVα.e.(top right) and 380400 keVα.e.(bottom)

115

Chapter 6. Dark matter search in Kamioka mine

6.2

Dark matter limits by a conventional method
WIMP velocity distribution
Solar velocity
Maxwellian velocity dispersion
Escape velocity
Local halo density
Spin factor of 19 F
energy resolution at 100 keVα.e.

Maxwellianvs = 244kms−1
v0 = 220kms−1
vesc = 650kms−1
0.3GeVcm−3
2
λ J(J + 1) = 0.647
70%(FWHM)

Table 6.3: Astrophysical and nuclear parameters used to derive the WIMP-proton limit
We derived the limits of the WIMP-proton SD cross section only from the spectrum
shown in Figure 6.5 (conventional method). We calculated the expected spectrum of the
WIMPs as following. We used astrophysical and nuclear parameters given in Table 6.3, andfollowed Chapter 2. A spectrum of 19 F recoil by WIMP via SD-interactions was calculated
for each WIMP mass as shown in Figure 2.9. We then took account of the mass fraction offluorine nuclei in CF4 gas. We also then converted energy scale from keV to keVα. e. using
the quenching factor of the fluorine as shown in Figure 4.11. The expected spectrum of SD
interaction of WIMPs was derived by considering the detection efficiency of nuclear recoil
and the energy resolution. Expected spectra for WIMPs masses of 50 GeV/c2 , 100 GeV/c2 ,and 500 GeV/c2 , with WIMP-proton SD cross section of 1.0 pb are shown in Figure 6.12.
We then conservatively assumed that all the remaining events were caused by the WIMPs,
and the limit of σχ−N was calculated by comparing these remaining events with the expecteddark matter signal. We compared the measured spectrum with the expected ones by thesame manner used in[89] and derived the limit for a given WIMP mass. WIMP-proton SD
cross section limits are shown in Figure 6.16 by a blue thick dotted line. The upper limitof σ = 8.7 × 103 pb for WIMP mass of 180 GeV/c2 was obtained from the analysis with thespectrum information only.

6.3

Dark matter search by a direction-sensitive method

We next analyzed the data of Kamioka RUN-III by a direction-sensitive method. We performed the direction-sensitive analysis to compare the measured |cos θ| distribution withthat expected in the WIMP SD interactions for given WIMPs masses. The expected |cos θ|
distribution is obtained from the expected recoil angle-energy distribution without the detector responses shown in Figure 2.14. We also used the astrophysical and nuclear parameters
given in Table 6.3. We simulated WIMP-nucleus scattering events using the recoil angleenergy distribution, then make the |cos θ| distributions for some detected energy bins, taking
account the detector response as following procedure.
STEP 1: We prepared the distribution of Figure 2.14 and 15 |cos θ| histograms for 15 energy
bins The energy of 100-400 keVα.e. was divided into 15 bins.
STEP 2: We simulated a WIMP-nucleus scattering event for the given WIMP mass and thegiven Cygnus direction. A recoil angle θR and a recoil energy ER keV were given fromthe distribution of Figure 2.14. The azimuth angle, φR , was given randomly from aflat distribution.
STEP 3: The unit of the recoil energy (ER keV) was converted to keVα.e. using the quenching
factor. The detection efficiency and the mass fraction of fluorine nuclei in CF4 were116

couts/keV/kg/days

Dark matter search by a direction-sensitive method

10-2

Mχ=50GeV/c 2
Mχ=100GeV/c2
Mχ=500GeV/c2

v0=220.0 km/s
vesc=650.0 km/s
vE=232.0 km/s
A=19
σ p =1 pb

-3

10

10-40

50

100 150 200

250 300 350 400
keVα.e.

Figure 6.12: Expected energy spectra for the SD interaction of WIMPs in CF4 with a σχ−p =1pb. Energy resolutions, quenching factors, and nuclear detection efficiencies are taken
into account. Black, red, and green lines show the cases of Mχ =50, 100 and 500 GeV/c2 ,respectively.

117

Chapter 6. Dark matter search in Kamioka minemerged and applied as a directional independent efficiency ǫ. The energy fluctuation
due to energy resolution was taken account of. Detected energy was expressed as′ER
keVα.e.
STEP 4: We calculated the recoil vector, eR , in the detector coordinate from the Cygnus direction, eC , and the recoil angle, θR and φR . The directional-dependent efficiency
η(eR ) was applied for eR from Figure 4.22. The angular resolution was taken accountfor eR with the two-dimensional gaussian. A vector, edet , was obtained as a detected
direction.
STEP 5: |cos θdet | = |edet · eC | was calculated. We filled a value of |cos θdet | into each |cos θ|′histogram for each obtained energy ER
with a weight of the η × ǫ .
STEP 6: We repeated from STEP 2 to STEP 5 for many Cygnus directions to simulate thedaily modulation.
STEP 7: We repeated from STEP 1 to STEP 6 for some WIMPs masses.

counts/keV/kg/days/cosθThe calculated distributions by this procedure for some energy ranges and some WIMP
masses are shown in Figure 6.13 and Figure 6.14, respectively.

0.01
0.009
0.008

ER =100-120keVα.e.
ER =120-140keVα.e.
ER =140-160keVα.e.
ER =160-180keVα.e.

0.007
0.006
0.005

0.004
0.003
0.002
0.001
0
0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1|cosθ|

Figure 6.13: Expected |cos θ| distributions for some recoil energies (black 100-120keVα.e.,
red 120-140keVα.e., green 140-160keVα.e., blue 160-180keVα.e.). We assumed the SD crosssection of 1pb, WIMP mass of 100GeV/c2 , the tracking angular resolution of 55◦ (RMS), theenergy resolution of 70%(FWHM) and ”directional dependent efficiency ” (Figure 4.21).

118

count/keV/kg/days/|cosθ|

Dark matter search by a direction-sensitive method

0.01
0.009

A=19

Mχ=50GeV/c2

v E=232.0km/s
v 0=220.0km/s

0.008

Mχ=100GeV/c 2

ρ =0.3GeVc cm-2-3

Mχ=500GeV/c 20σSDχ-p =1.0pb

0.007
0.006

°

θrms =55

100keV<ER<120keV

0.005
0.004
0.003
0.002
0.001
0
0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1|cosθ|

Figure 6.14: Calculated |cos θ| distributions for the SD cross section of 1pb and WIMP
masses of 50, 100, 500 GeVc−2 , where the tracking angular resolution of 55◦ , energy resolution of 70%(FWHM) and the direction dependent nuclear detection efficiencies(Figure 4.21)
were considered. The histograms for the masses of 50 GeV/c2 and of 500 GeV/c2 are similar
because count rates of them are nearly same in a range of 100-120 keVα.e. in Figure 6.12.

119

Chapter 6. Dark matter search in Kamioka mine
We then compared the measured |cos θ| distribution of Kamioka RUN-III with these
expected WIMP |cos θ| distributions and the flat background model to the measured anddrew the limit line for the SD WIMP-proton cross section. We rebined the |cos θ| histograms
into two bins because the measured distributions shown in from Figure 6.8 to Figure 6.11
has a poor statistics. We fitted the measured distribution with the calculated distributions
with the cross section as a free parameter. Figure 6.15 shows the rebined histogram andthe best-fit distribution calculated for Mχ = 100GeV and 100-120 keVα.e. bin, The crosssection that gave the minimum χ2 was 5.5×103pb, where χ2 /d.o.f.=3.71/1. This best fitted

cts/kev/kg/days/cosθ

50
45
40
35
30
25
20
15
10

σ=5500 pb
χ2 /dof=3.71/1

500

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
cosθ

Figure 6.15: Measured |cos θ| distributions of Kamioka RUN-III in the energy range of 100120 keVα.e. and expected |cos θ| distribution with a cross section of 5.5 × 103pb and WIMP
mass of 100 GeVc−2 .
WIMP signal was rejected at the 90% confidence level by the χ2 test. We also fitted themeasured distribution with the flat background model, where we assume all events were due
to an isotropic background. The fitting result gave χ2 /d.o.f. = 0.110/1 independent of the
WIMP mass and the flat background model was not rejected at the 90% confidence level.
120

Dark matter search by a direction-sensitive method
We then tested for other WIMP masses by the same manner. The cross section thatwould give the minimum χ2 was calculated and the smallest cross section was taken as thelimit for the given WIMP mass. The WIMP-wind model was rejected at a 90% confidence
level for all the masses from 30 GeV/c2 to 1000 GeV/c2 , while the flat background model
was independent of the WIMP mass. The limits are shown with a thick-solid line (labeled”direction-sensitive”) in Figure 6.16. Calculated values of minimum χ2 with the WIMP
model, and with the flat background model are shown as black, and blue points, respectively,
in Figure 6.17. All black points are above 90% C.L. line. We obtained a 90% C.L. SD WIMPproton limit of 5.4 × 103 pb for WIMP with a mass of 150 GeV/c2 . This result marked newbest sensitivity record for a SD WIMP search with the direction sensitive method, although
its sensitivity was still far worse than that of the frontier of SD WIMP searches with othermethods.

121

Chapter 6. Dark matter search in Kamioka mine

SD 90% C.L. upper limits and allowed region

Surface run results

104This work(CF4,direction-sensitive red)(spectrum only blue)
3

10

e

A_X

σ χ-p[pb]

DAM

102
10

0

ON1

XEN

1

D
NAIA
S(Cs,I)DAMA(NaI)

)

(Na,I

KIM

COUPP(F)

10-1

Si)
F2
o Ca
y
k
o
T

(Ge,

S
CDM

102 2
Mχ [GeV/c ]

3

10

SD
Figure 6.16: Limits of SD cross section, σχ−p
, as a function of Mχ . The regions above
the curves are excluded. A blue thick dotted line is a result of conventional method anda red thick solid line is a result of directional-sensitive method in this work. We obtained
a 90% C.L. limit of 5.5 × 103 pb for WIMP with a mass of 100 GeV/c2 . Two black lines
labeled ”Surface run result” are shown surface run results by the conventional methode(upper) and by the directional methode (lower). Allowed region(DAMA(NaI)[45]) and limits of other experiments (DAMA(Xe)[60], CDMS[56], Tokyo[49], NAIAD[47], KIMS[48],
XENON10[64],COUPP[69]) are shown for comparison.

122

χ2

Dark matter search by a direction-sensitive method

5

4.5
4

WIMP wind model

3.5
390% C.L.

2.5
2
1.5
1
0.5
0

Flat BG model

102

3

10
WIMP mass GeVc-2

Figure 6.17: Minimum χ2 values for some WIMP masses. Measured |cos θ| distribution
was fitted with expected distributions of the WIMP wind model (black circle) and the flat
background model (blue square).

123Chapter 7

Discussion
We performed a dark matter search in Kamioka mine with the NEWAGE-0.3a detector.
Finally we set stricter upper limit for the WIMP-proton SD interaction than that of thesurface run. However, there were some issues to resolve in order to search the WIMP-windin the MSSM region.
In this chapter we discuss the remaining backgroud in Kamioka RUN-III and list theissues to imorove for the WIMP-wind search.

7.1

Remaining background

Although the background count rate in Kamioka RUN-III was about 1/3 of that in Kamioka
RUN-I at 100 keVα.e. as shown in Figure 6.6, there still remained so many events which
restricted the detector sensitivity for WIMP. We applied the discussions in Section 5.6 tothe measured spectrum in Kamioka RUN-III. Figure 7.1 shows the measured spectrum andestimated spectra where we assumed 222 Rn density of 3.8×10−15g/g (black solid and dottedlines) , calculated ambient gamma-ray contribution (solid gree line ), 238 U density in the
GEM of 4.0×10−6g/g (blue lines), and other contribution are ignored. Those conditions are
as same as in Figure 5.27, but the suppression factor was double of that in the previous
discussion.
We succeed reducing backgound count rate at 100 keVα.e. by increasing the surpression
factor in the gap region. We found that contributions of tha gap region between the µ-PIC
and the GEM (the C - 4 and the D - 4 categolized in Table 5.4) were the most dominantcomponent of the measured spectrum at 100 keV in Kamioka RUN-I, and could be still in
Kamioka RUN-III.

7.2

Future work

The upper limit was restricted by the statistical error of background events in this work.
Although this statistical error will decrease with longer operation, a prospect of the sensitivity with a exposure of 3kg·year is far away from even the DAMA region, not to say the
MSSM region(< 10−3 pb) (Figure 7.2). We need an exposure of 3000 kg·year to reach the
DAMA region assuming zero systematic errors, however it is not a reasonable experiment
to realize. So we propose to improve the following points of the NEWAGE-0.3a detector.
• Background rejection• Improvement of the angular resolution
124

counts/keV/kg/days

Future work

102
Kakmioka RUN-I
GEM contribution(gap volume)
Rn contribution(fiducial volume)
Rn contribution(gap volume)
Gamma-ray contribution(outside)
Sum of estimated spectra

1050

100 150 200

250 300 350 400
keV α. e.

Figure 7.1: Measured spectrum in Kamioka RUN-III and estimated spectra.

125

Chapter 7. Discussion

SD Prospct Sensitivity
This work upper limit (spectrum only blue)

104
direction-sensitive Red (0.512kgdays)

tector)

me de

3

10

ar

3kg yect(saprospeσ χ-p[pb]102

tector)ame dect(sprospe
r
a
e
y
g
k

3000

10
1

DAMA(NaI)s,I)

KIMS(CCOUPP(F)

10-1
10-2

102 2
Mχ [GeV/c ]3

10

Figure 7.2: Prospect sensitivity of the NEWAGE-0.3a detector with the same performanceand condition of this work for larger exposure operations.

126

Future work
• Lowering the threshold
We discuss each improvement plan in this section.

7.2.1

Background rejection

Dominant source of background events seemed to be α -particles in the µ-PIC-GEM gap(∼30 counts/keV/kg/days), α -particles from 222 Rn and 220 Rn progeny (∼5 counts/keV/kg/days),and ambient γ-rays(∼5 counts/keV/kg/days) although there still remain uncertainty factors.
We have already found that the effect of the α-particles in the µ-PIC-GEM gap can be suppressed by high gain GEM operations. Operations with an even higher gains should be
realized with a slight increase of possibilities of damaging the GEM. The background level
due to the gap events should be reduced by a factor of 100 in these higher-gain operations.
In addition to the gain control, we need to develop low radioactive-impurity µ-PICs andGEMs.
We have a plan to decrease α-particles from 222 Rn and 220 Rn progeny by eliminatingradon gas with a charcoal filter. A circulating system for the purification with a charcoal
filter will be reduce radons and the background event due to radons.
Misidentifications of γ-rays are not also negligible as shown in Section 5.3. We have toimprove the γ-ray rejection power. Causes of misidentification are bad energy resolution
and a bad tracking performance for electron tracks. They cause a bad dE/dx resolutions
and thus as insufficient γ-ray discrimination power. New tracking FPGA codes which havepotentials to detect more detail a shape of electron clouds are ready to improve the tracking
performancs. This improvement also should be effective against muon background events.
As a result, we found that reducing the radio-impurites in the chamber and the improvement of the tracking performance of µ-TPC are equally important. With these efforts, weshould achieve the background level O(10−1 count/keV/kg/day), where an ambient neutron
flux will dominate the background. Then we can reduce the neutron background by covering
the detector with appropriate shieldings (water or polyethylene) another order of magnitude.
The sensitivities should be improved by a factor of at least 100 when the background levelis O(10−2 count/keV/kg/day).

7.2.2

Improvement of the angular resolution

Angular resolution affect the sensitivities largely in a direction-sensitive dark matter search.
Bad angular resolution decrease the characteristic excess around cos θ = 1 in the |cos θ|
distribution as shown in Figure 4.23 and thus the sensitivity for the dark matter interaction
is worsened. Table 7.1 describes required signal-to-noise ratios (S/N) to reject the flat
background model with a 90% C.L. for the calculated |cos θ| distributions. The NEWAGEangular resolution
σ = 10◦σ = 20◦σ = 30◦σ = 45◦σ = 55◦required S/N with |cos θ|
2.14
2.42
3.37
7.20
14.27

required S/N with cos θ
1.89
1.95
2.00
2.25
2.65

Table 7.1: Required S/N for detecting dark matter signals in energy rage of 100-120 keV with
90% C.L. for Mχ =100GeV/c2. We took account the energy resolution, detection efficiency
of NEWAGE-0.3a detector and assumed zero background.
0.3a detector has a potential to improve the sensitivity more than 10 times by improving of127

Chapter 7. Discussion
the angular resolution. As describe in Section 4.4.3, the angular resolution was restrictedby the nuclear track length because the two-dimensional sampling pitch of the µ-PIC (400µm) and the sampling clock (100 MHz ) is not fine enough compared to the track length(ex. 1.1 mm for an 100 keVα.e. for a fluorine nucleus in 152 torr CF4 ). We need to operatewith lower pressure gas to get a longer nuclear track and higher angular resolution. Since
tracking angular resolutions at 600 keVα.e., for which a fluorine nucleus runs 4.5mm, was35◦ , the angular resolution should be improve to about 35◦ at 100 keVα.e. with 40 torr
CF4 gas. In addition, one of the new FPGA code should improve tracking performance forthe vertical component against µ-PIC plane, which dominated the angular resolution. Thus
angular resolution should be improved better than 30◦ at 100 keVα.e.
Furthermore, a gas detector with finer pitch than the µ-PIC, should realized much better
angular resolution even for lower energy (ex. 50 keVα.e.). A µ-PIC has a potential to befine as or finer than 300µm although some ptototype tests are needed to make the large sizeone (∼30×30cm2).

7.2.3

Lowering the threshold

Since the shape of the dark matter spectrum rises exponentially at low energy as we describedin Figure 2.9. We can expect more events with a lower threshold. If the detecor had lower
the threshold down to 50 keVα.e., ten times more dark matter events are expected, while thenumber on neutron background events increases only a foctor of three. The threshold in thiswork, Eth =100keVα.e., was restricted by the efficiencies for nuclear tracks. The efficiencies
near the energy threshold are restricted by the track length as we described in Section 4.4.1.
Thus a lower pressure operation also improve the detector threshold. We will try to operate
the µ-TPC with 40 torr CF4 gas to achieve a threshold of 50 keVα.e.

7.2.4

Track-sense measurement

Since we did not measure the track sense; the head and tail of each track, in this work,
the sensitivity for the dark matter was worse than a measurement using information of
the track sense. If we can measure the track sense, a smaller signal-to-background ratio(S/N) is sufficient discriminate a flat cosθ distribution from dark matter signals than ameasurement without sense as shown in Table7.1. The track sense is able to be determinedby measuring dE/dx along track precisely. Since energy depositions of a fluorine in CF4
gas decrease together with necleus energy along its track below 10 MeV, much more energy
depositions detected at the tail of track than at the head of track. Figure 7.3 shows thenumber of produced ion-electron pair in unit length by nuclear recoils as a function of therecoil energy[83].
In fact, DMTPC group observed this effect of a recoil fluorine using gas multiplication
mesh and CCD camera in 50 torr CF4 as shown in Figure 7.4 although only in two-dimention
projection plane [74].
In other hand, NEWAGE-0.3a detector need a much higher energy resolution and a muchhigher position resolution to measure the dE/dx variation along a track. The position resolution improvement is strongly related the improvement of the tracking angular resolution.
Now it is studyed that the methode to achieve much more performancis in this point.

7.2.5

Future of the NEWAGE

If we could achieve to reduce the background (O(10−2 )), to improve the angular resolution(∼ 30◦ ) and to operate with lower threshold (∼ 50 keVα.e.), (they should be realized withour current technology), we would get 103 times better sensitivity. Expected sensitivity
128

Future work

Figure 7.3: The Bragg-like curve estimated for recoil C and F ions in CF4. The area below
each curve expresses the number of ions produced [83].
with this improved detector are shown in Figure 7.5. We should achieve a sensitivity ofσχ−p =1.5 pb for the dark matter mass of 100 GeV/c2 , which is the current frontier for the
SD interaction search, with a exposure of 30 kg·year. Construction of the CF4 detector withtotal mass of O(10kg), which needs O(100m3 ) fiducial volume with a pressure of 40 torr, is
not impossible.
Furthermore, if we realize much lower threshold (∼ 30 keVα.e.), much better angularresolution (∼ 10◦ ), and the measurement of the track sense with some technical improvements, we will get 102 much better sensitivity and achieve to search the MSSM regions as
green lines in Figure 7.5 are shown.

129

Chapter 7. Discussion

Figure 7.4: Images of recoil tracks reported by the DM-TPC group [74]. The figure show
the CCD coordinates and pixel intensity. In all images neutrons are coming from the right.
Images in the right column have the wire planes rotated by 180◦ operator. The noticeable
asymmetry of the light yield along the wire indicates observation of the head-tail effect [74].

130Future work

SD Prospct Sensitivity
510This work upper limit (spectrum only blue)

104

direction-sensitive Red (0.512kgdays)3

10

σ χ-p[pb]102
10
1
COUPP(F)-1

1030kg year

10-2

ent)

provem10

MSSM prospectore impect(ms

ear pro30kg y

-3

10-4

ment)
spect(improves,I)
KIMS(Crovement)
prospect(imp

3kg year pro

DAMA(NaI)

spe

ear pro300kg y102 2
Mχ [GeV/c ]t)ovemenimpr
ct(more3

10

Figure 7.5: Prospect sensitivities of NEWAGE with current technical improvement (reddotted lines) and with next technical improvement (green dotted lines). ”Current
technical improvement” means 50 keVα.e. energy threshold, 30◦ angular resolution,
10−2 counts/keV/kg/days background level, and analysis without the track sense. ”Nexttechnical improvement” means 30 keVα.e. energy threshold, 10◦ angular resolution andanalysis with the track sense. Black lines labeled ”MSSM prospect” is a region predicted by
the MSSM.

131

Chapter 8

Conclusion
In this thesis, we proposed the NEWAGE experiment for a direction sensitive dark mattersearch and developed a prototype detector (the NEWAGE-0.3a). We measured the performance of the NEWAGE-0.3a detector and operated it in Kamioka mine. In the first
operation (Kamioka RUN-I), we found there were some unknown background origins otherthan the ambient neutron. We studied and estimated these unknown background origins
from the data of Kamioka RUN-I and Kamioka RUN-II. Based on these background studies,
the operation parameter of the NEWAGE-0.3a detector was optimized.
A dark matter search was performed with the optimized NEWAGE-0.3a detector. In
this dark matter search run, we obtained a 90% C.L. spin-dependent WIMP-proton limit of
5.4 × 103 pb for WIMP with a mass of 150 GeV/c2 and 8.7 × 103 pb for WIMP with a massof 180 GeV/c2 in the direction-sensitive and the conventional analysis method, respectively.
This directional result marked the new best sensitivity record for a SD WIMP search withthe direction sensitive method, although its sensitivity was still far worse than those of thefrontier of SD WIMP searches with other methods. A directional sensitive dark mattersearch with some technical improvements and 30kg·year exposure will reach to the MSSM
predict region.

132

Chapter 9

Acknowledgments
I wish to first thank Professor Toru Tanimori for his guidance and encouragements in thiswork. This thesis would not be completed without his support. I specially thanks the leaderof the NEWAGE projects, Dr. Kentaro Miuchi for all support of this experiment and grateful
discussions. His support was also indispensable.
I would like to thanks µ-PIC collaborators: Dr. H. Kubo, Dr. A. Takeda(ICRR), Dr. H. Sekiya(ICRR),
Dr. T. Nagayoshi(Rigaku), Dr. K. Tsuchiya(Police), Dr. R. Orito(MPI), Dr. S. Kabuki,
Dr. A. Takada(ISAS/JAXA), Y. Okada, K. Hattori, K. Ueno, S. Kurawasa, S. Iwaki,
S. Ida and M. Takahashi for many useful discussions and good advices. I especially thankDr. A. Takeda and Dr. H. Sekiya for their help for running this experiment in Kamiokamine.
I sincerely thank all of other staff of Kamioka Observatory, ICRR, University Tokyo for
their help in performing this experiment. I especially thank Dr. A. Minamino for useful
discussions

133

Appendix A

Radioactive isotpes in materials
We introduce major natural radioactive isotopes in this chapter. Most of data shown in this
Chapter based on [90].

A.1
238
206

238

U-chainU has a long half life of 4, 468 × 109 year, and it continuously decays into a stable nucleus,
Pb, as shown in Figure A.1. γ-rays emitted in this chain are shown in Figure A.2.

A.2

232

Th-chain232

Th has a longer half life than one of 238 U, 1.405 × 1010 year, and it continuously decays
into a stable nucleus, 208 Pb, as shown in Figure A.3. γ-ray emitted in this chain are shownin Figure A.4

A.3

40

K

The natural abundance of 40 K, is 0.0117%. 40 K has a lifetime of 1.27×109years. 40 K decays
with a β-decay (89.3%) or with an electron capture β + +e− (10.7%). The γ-ray with anenergy of 1460.8 keV is emitted as a result of the electron capture.

134

40

238
92

U

K

234

U

U
9

5

β 2.269

4.468 10 y

2.457 10 y(99.2%)
234

89

Ac

88

Ra

)

Th

%
234

234
90

1.17 m

6
.1

Pa

Pa

(0

91

α
4.198

Th

24.10 dβ 0.196(γ )β 0.472
(γ )

α
4.775
(γ )

Pa
6.75 h230

47.538 10 y

α
4.687
(γ )

226

87

Fr

86

Rn
At

Ra
3

1.600 10 y

α 4.784
γ 0.186

222

85

Th

Rn

3.824 d(0.02%)
218
84

Po

218

α
5.490
(γ )

At

1.5 s
β
α
6.694

Po

214

210

Po

Po

(RaF)
138.4d164.3 µs

3.10 m
(99.98%)83

Bi(99.98%)
214

Bi

α
6.003

β 1.071.51
3.27(99+%)
210

α
7.687
(γ )

19.9 mBi

β 1.16

α
5.304
(γ )

(RaE)
5.013 d−4

(1.3 10 %)(0.02%)
214
82

Pb

Pb

β
0.67
0.73

α 5.450(γ )26.8 m(99+%)
210

Pb

(RaD)
22.3 yβ 0.0170γ
0.0465

206

Pbstable−6(1.9 10 %)
210
81

Tl

Tl

Hg

Hg

β, γ

8.15 mFigure A.1: Decay scheme of the uranium chain

135

Tl

4.199 m1.30 m

206
80

206

α

β 1.53

%

Appendix A. Radioactive isotpes in materials

10
1
10-1
10-2
-3

10

10-4
-5

10

-6

10

0

500

1000

1500

2000

2500

3000

Figure A.2: Gamma rays emitted in the U-chain.

136

3500
keV

40

232
90

89

Th

1.405 10 y

Ac

α
4.012
(γ )

87

Ra

Ra

1.912 y228

Ac

β 1.16
1.73
2.07

α
5.423
γ
0.08446.15 h
β 0.0392

224

85

α
5.685
(γ )

Fr
Rn

Rn

55.6 s

α
6.288
(γ )

At
216

84

Ra

3.66 d5.75 y

220
86

Th

10

228
88

228

Th

Po

212

Po
212

α
6.779

Bi

Po

0.299 µs0.145 s(64.0%)

83

Bi

β 2.25
γ
0.727

α
8.785

60.55 m(36.0%)
212

82

Pb

Pb

β
0.335
0.574

α
6.051

10.64 hTl

Tl

3.053 mFigure A.3: Decay scheme of the thorium chain
137

208

Pb

stable208
81

K

β 1.80
1.29
1.52

%

Appendix A. Radioactive isotpes in materials

10
1
10-1
10-2
-3

10

10-4
-5

10

-6

10

0

500

1000

1500

2000

2500

Figure A.4: Gamma rays emitted in the Th-chain.

138

3000
keV

Appendix Bµ-PICs and µ-TPC
A Micro Pixel Chamber(µ-PIC) was used in the NEWAGE-0.3a detector as a main gasamplifier and a fine pitch read-out detector. In this chapter we introduce fundamental
performances of the µ-PIC and the µ-TPC (micro-Time Projection Chamber).

B.1

µ-PIC

µ-PIC is a two-dimensional gaseous detector which has 256×256 pixels with a pitch of 400µm in an area of 10×10 cm2 [91], [92], or 768×768 pixels with a pitch of 400 µm in an areaof 30×30 cm2 [78]. The µ-PIC has anode strips formed on one side of a polyimide board
with a pitch of 400 µm and cathode strips orthogonally formed on the other side with thesame pitch. Cathode strips have holes of 260 µm diameter and anode electrodes of 70 µm
diameter are formed at the center of these holes. The schematic structure and photographof µ-PIC shown in Figure B.1 and Figure B.2, respectively.

Figure B.1: Schmatic structure of µ-PIC.
Signals of pixels are read out through both anode strips and cathode strips. The µ-PIC
is made by the PCB (Print Circuit Board) Technology so that it has a hardness against adischarge and a potential of mass production at a low cost. We achieved a maximum gas
gain of 15,000 and a gas gain for a stable operation of 6,000 with a µ-PIC and a gas mixtureof argon and ethane. The non-uniformity of the gas gain is about 5% by sigma in an area
139

Appendix B. µ-PICs and µ-TPC

Figure B.2: Photograph (top ) and sectional side view (bottom) of a pixel of µ-PIC.

140

µ-TPCof 10×10 cm2 . The energy resolutions for an X-ray are 23% and 30% (FWHM) at 5.9 keVwith an area of 1.25×1.25 cm2 and 10×10 cm2 , respectively. The best spatial resolution ofthe µ-PIC for taking an X-ray image was 120µm with a gas mixuture of Xe and CO2 .

B.2

µ-TPC

Miuchi et al. [93] reportsed performances of a µ-TPC with a detection volume of 232831 cm3with a gas mixuture of argon and ethane. The µ-TPC was operated at a gas gain of 50,000,
using the GEM as preamplifier, and an energy resolution of 60% FWHM at 60 keV wasmeasured. A three-dimensional position resolution of against MIPs was 0.51 mm (FWHM).
More deteails are described in [93]. For CF4 gas a three-dimensional position resolution of
0.80 mm (FWHM) against α-particle was obtained in the NEWAGE-0.3a detector.

141

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145

List of Figures
1.1
1.2
1.3

1.4

1.5

1.6

1.7
2.1
2.2
2.3
2.4
2.5

2.6

Composition of the universe. The universe consists of 73% dark energy, 22%
dark matter and 5% ordinary matter. . . . . . . . . . . . . . . . . . . . . . .
Cosmological parameter space from high-redshift supernova survey [3], galaxy
cluster observation [4] and cosmic microwave background measurement. . . .
Rotation curve of the spiral galaxy NGC6503 [6]. The points indicate themeasured rotation velocities as a function of the distance from the galactic
center. The dashed and dotted curves are the contribution to the rotation
velocity from the observed disk and gas, respectively. The dash-dotted curveis the contribution from the dark halo. The solid line is a sum of them. . . .
Rotation curve of our Galaxy [7]. Square points with error bars indicate
measured rotation velocities as a function of distance from the galactic center.
The different lines represent the contributions from the bulge (dotted), thedisk(filled circles), the HI layer (crosses), the H2 layer (circles) and from thedark halo (dashed). The solid line represents the sum of the contributions. .
Solar axion search results. 95% C.L exclusion lines of each experiment to theaxion-photon coupling constant as a function of the axion mass are shown.
Region above the limit lines are excluded and yellow region is the theoreticalallowed region for axion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Running of couplings in the Standard Model(SM) (top) and in the Minimal Supersymmetric Standard Model (MSSM) (bottom). α1 , α2 and α3 are
the U (1) hypercharge, SU (2) electroweak and SU (3) strong couplings, respectibely. Only in the bottom case the unification of the coupling constants
is realized. [19] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Positron fraction measured by PAMELA (red points and error bars)[25].
Black marks show results of other experiment measurements. . . . . . . . . .

5
7

8

10

12

14
17

Feynman diagrams of spin-independent(SI) interactions between a neutralino
and a quark. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Feynman diagrams of spin-dependent(SD) interactions between a neutralino
and a quark. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
SISI
to σχ−Nfor each nucleus as a function
Function of conversion values from σχ−pof neutralino mass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26SDSD
to σχ−N
as a function of the neuFunction of conversion values from σχ−ptralino mass for each nucleus. Natural abundunce for each nucleus is considered. 28
SD
SD
Function of conversion values from σχ−nto σχ−N
as a function of the neutralino mass for each nucleus. Natural abundunce for each nucleus is considered. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Form factors as a function of the recoil energy for the SI interactions for eachnucleus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31i

LIST OF FIGURES
2.7

Energy spectra expected with fluorine targets for the SI-interacting neutralino
for each mass of the neutralino. . . . . . . . . . . . . . . . . . . . . . . . . .
2.8 Form factors as a function of the recoil energy for SD interactions for eachnucleus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.9 Energy spectra expected with fluorine targets for the SD-interacting neutralino for each mass of the neutralino. . . . . . . . . . . . . . . . . . . . . .
2.10 Recoil specra of various nuclei for the SI-interacting nuetralino with a massSIof 100 GeV/c2 and σχ−p=1×10−6pb. . . . . . . . . . . . . . . . . . . . . . . .
2.11 Recoil specra for of various nuclei for the SD-interacting nuetralino with aSDmass of 100 GeV/c2 and σχ−p=1 pb. . . . . . . . . . . . . . . . . . . . . . .
2.12 Difference of the fluorine spectra between summer and winter for the SDSD
interacting neutralino with a mass of 100 GeV/c2 and σχ−p
= 1pb. Greenline is expected recoil specra in June, where vES has its maximum in the year.
Red line is expected recoil specra in December, where vES has its minimumin the year. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.13 Daily motion of the direction of Cygnus, which is the same direction of the”WIMP-wind”, expected in Kamioka Observatory(36◦25’N, 137◦18’E). . . . .
2.14 Expected recoil angle-energy distribution for the SD interacting neutralinoSDwith a mass of Mχ = 100 GeV, and a cross section of σχ−p
= 1pb, where θ isthe recoil angle of the nuclear recoil with respect to the WIMP wind direction.
X-axis shows cos θ, where θ is nuclear recoil angle. Y-axis shows nuclear recoil
energy. Color contor shows event rate in unit of [counts/keV/kg/days/cos θ].
2.15 Expected cos θ distribution for the SD interacting neutralino with a mass ofMχ = 100 GeV, a recoil energy range of 100-120 keV, and a cross section ofSD= 1pb, where θ is the recoil angle of the nuclear recoil with respect toσχ−p
the WIMP wind direction. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1

3.2

3.3

3.4

31
32
32
33
34

35
36

37

38

Results of SI(spin-independent) WIMP searches. Regions enclosed by blackline is a region allowed by DAMA experiment[45]. Other experiments (NAIAD[47],EDELWEISS[55],CDMS[56], WARP[65], CRESST[59], and, XENON10[64])exclude upper region of each line. . . . . . . . . . . . . . . . . . . . . . . . . . 40
Results of SD(spin-dependent) WIMP searches. Regions enclosed by blackline is a region allowed by DAMA experiment[45]. Other experiments (DAMA(Xe)[60],
CDMS[56], Tokyo[49], PICASSO[71], SIMPLE[70], NAIAD[47], KIMS[48],
XENON10[64], and COUPP[69]) exclude upper region of each line. . . . . . . 41
Residual rates maeasured by DAMA/LIBRA experiment in the (2-4), (2-5)
and (2-6) keV energy intervals as afunction of the time. X-axises show days(t) from January 1st of the first year of data taking of the former DAMA/NaI
experiment. The experimental points present the errors as vertical bars andthe associated time bin width as horizontal bars. The superimposed curves
represent the cosinusoidal functions behaviours A cos ω(t − t0 ) with a period
2πω = 1 yr, with a phase t0 = 152.5 day (June 2nd) and with modulation
amplitudes, A, equal to the central values obtained by best fit over the whole
data, that is: (0.0215±0.0026) cpd/kg/keV, (0.0176±0.0020) cpd/kg/keV and(0.0129±0.0016) cpd/kg/keV for the 2-4 keV, for the 25 keV and for the
26 keV energy intervals, respectively. The dashed vertical lines correspondto the maximum of the signal (June 2nd), while the dotted vertical lines
correspond to the minimum. The total exposure is 0.82 ton·yr[46]. . . . . . . 43
Drift velocity of electrons of negative ions calculated by MAGBOLTZ for
CF4 (red), Xe(blue), CS2 (black) at a temperature of 300K. . . . . . . . . . . . 46ii

LIST OF FIGURES
3.5

3.6

3.7

Longitudial (dotted line)and Transverse (solid line) diffusion of electrons or
negative ions calculated by MAGBOLZ for CF4 (red), Xe(blue), CS2 (black)at a temperature of 300K. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Length of recoil Carbon (dotted line) and Fluorine (solid line) nuclei in CF4
gas calculated by SRIM with each pressure (blue:0.002atm, green:0.05atm,
red:0.2atm, black:1.0atm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Length of recoil Xe and S nuclei in Xe gas and in CS2 gas. They were calculated by SRIM with each pressure (blue:0.002atm, green:0.05atm, red:0.2atm,
black:1.0atm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4.1

Sensitivity goals of the NEWAGE project. Thick and dotted lines show expected 3σ asymmetry detection sensitivities by the ST(semi-tracking) andFT(full-tracking) modes, respectively. Details are described in [75]. An experimental result of NAIAD (thin dashd-dotted line labeled UKDMC) andMSSM predictions(thin line labeled MSSM) are also shown [47],[76]. . . . . . 52
4.2 Photograph of the NEWAGE-0.3a detector, which consists of a TPC vessel
and read-out circuits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.3 The schematic structure of the NEWAGE-0.3a. VDRIFT is a voltage suppliedat the drift plane, VGEMT is at the top of the GEM, VGEMB is at the bottomof the GEM, Vµ−PIC is at the anode of the µ-PIC. . . . . . . . . . . . . . . . 54
4.4 The schematic structure of a µ-PIC. . . . . . . . . . . . . . . . . . . . . . . . 54
4.5 Data acquisition system of the NEWAGE-0.3a detector . . . . . . . . . . . . 56
4.6 Gas gain as a function of µ-PIC anode voltage. The voltage between the top
and bottom electrodes of GEM is fixed at 180 V. . . . . . . . . . . . . . . . . 57
4.7 Gas gain as a function of a voltage between the top and bottom electrodes of
GEM. The µ-PIC anode voltage was fixed at 600V. . . . . . . . . . . . . . . . 58
4.8 Drift-time distribution of nuclear recoils in a drift velocity measurement . . . 60
4.9 Typical tracks of α particles from the 10 B-plate. The 10 B-plate is set at theshaded area. Each mark indicates one digital hit and different marks (color)
display different events. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.10 Correlation of the energy-length of the 10 B-run (color contour) and the calculated lines by SRIM. The dotted line (black) shows an energy-length correlation of the protons and the solid line (purple) shows one of the α particles. 62
4.11 Nuclear quenching factor for helium (red line), carbon(green line) and fluorine(red line) in CF4 gas. The number of ion-electron pairs produced by a particle
Ewith energy of E in gas is calculated as Fq × W, where W is the W-value ofthe gas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.12 Spectrum of α particles due to the 222 Rn progeny in the reference region. The
solid line is a best-fit function. . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.13 Measured relative gain map of the µ-TPC. The area enclosed with dottedlines is the reference region. The gas gains were normalized by averaged value. 65
4.14 Measured spectrum of α particles due to the 222 Rn progeny from whole fiducial volume before (left) and after (right) the position dependent gain correction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.15 Time-dependence of the gas gain. The origin of the time axis is when we filled
the vessel with a CF4 gas and gas gains are normalized by the initial one. . . 66
4.16 Correlation of the length-energy of the 252 Cf run. The nuclear-selection region
is indicated by the vertical thick line (E>100 keVα.e.) and the horizontal
thick line (L<1 cm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
iii

LIST OF FIGURES
4.17 Correlation of the length-energy of the 137 Cs run. The thick lines shows theselection described in Figure 4.16. Almost all events are outside of the nuclear
region. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4.18 Spectra of the 137 Cs run (square red) and the background run (triangle blue)in the nuclear recoil region. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4.19 Efficiency of the electron track detection, or the gamma-ray rejection power,of µ-TPC in the nuclear recoil region. . . . . . . . . . . . . . . . . . . . . . . 69
4.20 Measured (triangle red) and simulated (square blue) spectra of the nuclear
recoil scattered by fast neutrons from the 252 Cf source. . . . . . . . . . . . . 70
4.21 Absolute detection efficiency of the µ-TPC for nuclear recoil. . . . . . . . . . 70
4.22 Relative directional-dependent efficiency (100-400 keVα.e.), which is the detection efficiency of nuclear recoils depending on the directions of nuclear
recoils. The vertical axis is the elevation angle and the horizontal axis is theazimuth angle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.23 Simulated distributions of the absolute value of the cosine of the recoil angleof the nuclear tracks (100-400 keVα.e.) assuming some angular resolutions of
the tracking. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4.24 Measured (red histogram with error bars) and the best-fit simulated (blacksolid histogram) distributions of cos θ . . . . . . . . . . . . . . . . . . . . . . 74
4.25 Probabilities derived by the χ2 as a function of angular resolutions. We fitted
measured cos θ distributions with simulated ones. . . . . . . . . . . . . . . 74
4.26 Angular resolutions of the nuclear tracking as a function of the recoil energy.
The errors bars indicate the 68% errors. The blue points show those with eachenergy and the red one shows that with the energy in range of 100-400 keVα.e.. 75
5.1

Measured energy spectrum in the surface run. The detection efficiency was
taken into account. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Limits of SD cross section as a function of Mχ . NEWAGE surface run experiment results (blue conventional method red direction-sensitive method). Allowed region(DAMA(NaI)[45]) and limits of other experiments (DAMA(Xe)[60],
CDMS[56], Tokyo[49], PICASSO[71], SIMPLE[70], NAIAD[47], KIMS[48],
XENON10[64],COUPP[69]) are shown for comparison. . . . . . . . . . . . .
5.3 Photograph of the NEWAGE-0.3a detector placed in Kamioka Observatory. .
5.4 Drift velocities during the pilot runs in Kamioka mine (Kamioka RUN-I and
RUN-II). Red points show measured values. Black error bars show interpolated values for the analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5 Nuclear detection efficiencies at 100 keVα.e. during the pilot runs in Kamioka
mine (Kamioka RUN-I and RUN-II). Red points show measured values. Blackerror bars show interpolated values for the analysis. . . . . . . . . . . . . . .
5.6 Monitored gas gain in the pilot runs (Kamioka RUN-I and RUN-II). Decreaseof the gas gain was due to gas impurity. Recoveries of the gas gain were dueto the gas refill. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.7 Monitored a peak count rate around 6 MeV due to 222 Rn progeny in pilot
runs (Kamioka RUN-I and RUN-II). The variation of peak count rate due to222
Rn is discussed in Section 5.4.1 . . . . . . . . . . . . . . . . . . . . . . . .
5.8 Obtained spectrum in Kamioka RUN-I. . . . . . . . . . . . . . . . . . . . . .
5.9 Obtained nuclear direction plots of events (top) and |cos θ| distribution(bottom)
in Kamioka RUN-I. An inuniformity in the event direction plots was due tothe directional-dependent efficiency. . . . . . . . . . . . . . . . . . . . . . . .
5.10 Obtained spectrum in Kamioka RUN-II . . . . . . . . . . . . . . . . . . . . .
iv

78

78
79

80

81

81

81
82

83
84

LIST OF FIGURES
5.11 Obtained nuclear direction plot of events (top) and |cos θ| distribution(bottom)
in Kamioka RUN-II.An inuniformity in the event direction plots was due tothe directional-dependent efficiency. . . . . . . . . . . . . . . . . . . . . . . . 85
5.12 Cosmic ray muon flux as a function of depth in 103 hg cm−2 equivalent to km
w.e.[86]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
5.13 Ambient γ-ray spectrum measured in our laboratory with a CsI(Tl) scintillator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
5.14 Estimated spectrum of the ambient γ-ray flux in our laboratory. . . . . . . . 89
5.15 Estimated spectrum due to the ambient γ-rays (red line) and the ambient
fast neutrons (blue line). Black histogram shows the measured spectrum in
Kamioka RUN-I. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
5.16 Five types of energy deposition of background α-particles. There are αparticles which run only in the fiducial volume ( 1 ), from the fiducial volume into the drift plane or the GEM( 2 ), from the drift plane or the GEM
into the fiducial volume ( 3 ), through the gap volume between the GEM
and the µ-PIC ( 4 ), and through the veto volume ( 5 ). Particles with green
arrows( 1 ) deposit all of the initial energy in the fiducial volume, while thosewith red arrows( 2 , 3 ) deposit parts of the initial energy in fiducial volume.
Energy depositions of purple arrows( 4 ), are suppressed by the ”suppression
factor”(see text). α-particles categorized in 5 are rejected by a fiducial cut.
92
5.17 The results for A : Expected spectra of α-particles emitted from 222 Rn- (left)
and 220 Rn- (right) progeny in the gas volume. We assumed their density of
3.81×10−15g/g and 3.27×10−20g/g, respectively. These contamination were
determined by the count rate around 6MeV of the measured spectrum in
Kamioka RUN-I. Black and blue histograms show deposit energy above (thedeposit position 1 2 ) and under the GEM (the deposit position 4 .) ,respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5.18 Time dependence of the count rate around 6 MeV in Kamioka RUN-I-2.
X-axis is the days from a gas refill. A blue line is the fitted function of
C1 + Rout × (1 − exp(−t/5.5), where C1 = 4.0 ± 0.40 and Rout = 5.1 ± 0.44.
This time-dependence indicates existence of 222 Rn. . . . . . . . . . . . . . . 95
5.19 Time dependence of the count rate around 6 MeV in Kamioka RUN-II-1. Xaxis is the days from a gas refill. A black line is the best fitted function of
C2 + Rinit × exp(−t/5.5) + Rout × (1 − exp(−t/5.5)) where C2 = 4.4 ± 0.29 andRinit = 94 ± 3.8 were best fit parameters. Rout = 5.1 was a fixed parameter.
The blue line is the fitted function in Figure 5.18 as an expected component of222
Rn emanations. The red line shows component of the initial 222 Rn decay
and non 222 Rn components. . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.20 Correlation between the high energy (6 MeV) count rate and that of low
energy(200-400keVα.e.) in Kamioka RUN-II. . . . . . . . . . . . . . . . . . . 98
5.21 Measured Kamioka RUN-I spectrum (red) and a calculated contributions of222
Rn (black and blue histogram). Th black line and the blue line are estimated contributions from 222 Rn with deposit positions of 1 and 4 , respectively. We assumed 3.81×10−15g/g of 222 Rn in the gas volume to fit the peakaround 6 MeV with that of the measured spectrum in Kamioka RUN-I. . . . 99
5.22 The results for B : Expected spectra of α-particles emitted from 238 U- (left)
and 232 Th- (right) progeny in the drift plane. We assumed that both of theircontaminations are 1.0×10−6g/g as references. All particles deposit energy
as 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
5.23 Comparison of shapes of calculated spectrum for case B and measured spectrum in Kamioka Run-I. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100v

LIST OF FIGURES
5.24 The results for C : Expected spectra of α-particles emitted from 238 U- (left)
and 232 Th- (right) progeny in the GEM. We assumed that both of their density are 1.0×10−6g/g as references. Black and blue histograms show deposit
energy above (the deposit position 3 ) and and under the GEM (the deposit
position 4 ), respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
5.25 The results for D . Expected spectra of α-particles emitted from 238 U- (left)
and 232 Th- (right) progeny in the µ-PIC. We assumed both of their contaminations are 1.0×10−6g/g as references. Black and blue histograms show theenergy deposited above (the deposit position 3 ) and under the GEM, (thedeposit position 4 ), respectively. . . . . . . . . . . . . . . . . . . . . . . . . 101
5.26 Measured spectrum of Kamioka RUN-I and some of calculated contributionsof the µ-PIC contamination with 5.0×10−6g/g 238 U for given suppression
factors. (purple ×3, blue ×5 , green ×10, black ×15), Contaminations in
the GEM would make a similar contribution. This figure suggests the eventsin gap between the µ-PIC and the GEM can contribute to the low energy
spectrum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
5.27 Example spectra explaining measured one in Kamioka RUN-I. We assumed
222
Rn contamination of 3.8×10−15g/g in the gas volume (black solid and dotted lines), calculated ambient gamma-ray contribution (solid green line ), 238 U
contamination in the GEM of 4.0×10−6g/g (blue lines), and the suppression
factor of 15. Other contributions were ignored. The dotted lines strongly
depends on the suppression factor, which was actually uncertain. . . . . . . 105
6.1

Drift velocities during the dark matter run (Kamioka RUN-III). Red points
show measured values. Black error bars show interpolated values for the
analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
6.2 Nuclear detection efficiency at 100 keVα.e. during the dark matter run(Kamioka RUN-III). Red points show measured values. Black error bars
show interpolated and extrapolated values for the analysis. . . . . . . . . . . 107
6.3 Monitored gas gain during the dark matter run(Kamioka RUN-III). . . . . . 108
6.4 Monitored count rate around 6 MeV due to Rn progeny during the dark
matter run (Kamioka RUN-III) . . . . . . . . . . . . . . . . . . . . . . . . . . 108
6.5 Obtained spectrum in Kamioka RUN-III. . . . . . . . . . . . . . . . . . . . . 109
6.6 Obtained energy spectrum in the pilot and DM search runs (RUN-I, RUN-II
and RUN-III )at Kamioka observatory. RUN-I is operated with the same
parameters as the surface run, RUN-II is a radon-rich run and RUN-III is
operated with optimized parameters listed in Table 6.1. . . . . . . . . . . . . 110
6.7 Obtained direction of the nuclear tracks plot (up) and |cos θ| distribution(down)with energy range of 100-400 keVα.e. in Kamioka RUN-III. . . . . . . . . . . 111
6.8 The obtained direction of the nuclear tracks plots and |cos θ| distributions
with energy range of 100-120 keVα.e.(top left), 120-140 keVα.e.(top right),
140-160 keVα.e.(bottom left) and 160-180 keVα.e.(bottom right) in Kamioka
RUN-III. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
6.9 The obtained direction of the nuclear tracks plots and |cos θ| distributions
with energy range of 180-200 keVα.e.(top left), 200-220 keVα.e.(top right),
220-240 keVα.e.(bottom left) and 240-260 keVα.e.(bottom right) in Kamioka
RUN-III. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
6.10 The obtained direction of the nuclear tracks plots and |cos θ| distribution withenergy range of 260-280 keVα.e.(top left), 280-300 keVα.e.(top right), 300320 keVα.e.(bottom left) and 320-340 keVα.e. (bottom right) in Kamioka
RUN-III. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
vi

LIST OF FIGURES
6.11 The obtained direction of the nuclear tracks plots and |cos θ| distributions
with energy range of 340-360 keVα.e.(top left), 360-380 keVα.e.(top right)
and 380-400 keVα.e.(bottom) . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
6.12 Expected energy spectra for the SD interaction of WIMPs in CF4 with aσχ−p = 1pb. Energy resolutions, quenching factors, and nuclear detection
efficiencies are taken into account. Black, red, and green lines show the casesof Mχ =50, 100 and 500 GeV/c2 , respectively. . . . . . . . . . . . . . . . . . 117
6.13 Expected |cos θ| distributions for some recoil energies (black 100-120keVα.e.,
red 120-140keVα.e., green 140-160keVα.e., blue 160-180keVα.e.). We assumed the SD cross section of 1pb, WIMP mass of 100GeV/c2, the tracking
angular resolution of 55◦ (RMS), the energy resolution of 70%(FWHM) and”directional dependent efficiency ” (Figure 4.21). . . . . . . . . . . . . . . . . 118
6.14 Calculated |cos θ| distributions for the SD cross section of 1pb and WIMP
masses of 50, 100, 500 GeVc−2 , where the tracking angular resolution of55◦ , energy resolution of 70%(FWHM) and the direction dependent nuclear
detection efficiencies(Figure 4.21) were considered. The histograms for themasses of 50 GeV/c2 and of 500 GeV/c2 are similar because count rates ofthem are nearly same in a range of 100-120 keVα.e. in Figure 6.12. . . . . . 119
6.15 Measured |cos θ| distributions of Kamioka RUN-III in the energy range of100-120 keVα.e. and expected |cos θ| distribution with a cross section of
5.5 × 103 pb and WIMP mass of 100 GeVc−2 . . . . . . . . . . . . . . . . . . . 120
SD
6.16 Limits of SD cross section, σχ−p
, as a function of Mχ . The regions above
the curves are excluded. A blue thick dotted line is a result of conventional
method and a red thick solid line is a result of directional-sensitive methodin this work. We obtained a 90% C.L. limit of 5.5 × 103 pb for WIMP with amass of 100 GeV/c2 . Two black lines labeled ”Surface run result” are shown
surface run results by the conventional methode (upper) and by the directional methode (lower). Allowed region(DAMA(NaI)[45]) and limits of otherexperiments (DAMA(Xe)[60], CDMS[56], Tokyo[49], NAIAD[47], KIMS[48],
XENON10[64],COUPP[69]) are shown for comparison. . . . . . . . . . . . . 122
6.17 Minimum χ2 values for some WIMP masses. Measured |cos θ| distribution was
fitted with expected distributions of the WIMP wind model (black circle) andthe flat background model (blue square). . . . . . . . . . . . . . . . . . . . . . 123
7.1
7.2
7.3
7.4

7.5

Measured spectrum in Kamioka RUN-III and estimated spectra. . . . . . . . 125
Prospect sensitivity of the NEWAGE-0.3a detector with the same performance and condition of this work for larger exposure operations. . . . . . . . 126
The Bragg-like curve estimated for recoil C and F ions in CF4. The area
below each curve expresses the number of ions produced [83]. . . . . . . . . . 129
Images of recoil tracks reported by the DM-TPC group [74]. The figure show
the CCD coordinates and pixel intensity. In all images neutrons are comingfrom the right. Images in the right column have the wire planes rotated by180◦ operator. The noticeable asymmetry of the light yield along the wire
indicates observation of the head-tail effect [74]. . . . . . . . . . . . . . . . . 130
Prospect sensitivities of NEWAGE with current technical improvement (reddotted lines) and with next technical improvement (green dotted lines). ”Current technical improvement” means 50 keVα.e. energy threshold, 30◦ angularresolution, 10−2 counts/keV/kg/days background level, and analysis without
the track sense. ”Next technical improvement” means 30 keVα.e. energy
threshold, 10◦ angular resolution and analysis with the track sense. Blacklines labeled ”MSSM prospect” is a region predicted by the MSSM. . . . . . 131
vii

LIST OF FIGURES
A.1
A.2
A.3
A.4

Decay scheme of the uranium chain . . . . . . . . . . . . . . . . . . . . . . . . 135
Gamma rays emitted in the U-chain. . . . . . . . . . . . . . . . . . . . . . . . 136
Decay scheme of the thorium chain . . . . . . . . . . . . . . . . . . . . . . . . 137
Gamma rays emitted in the Th-chain. . . . . . . . . . . . . . . . . . . . . . . 138

B.1 Schmatic structure of µ-PIC. . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
B.2 Photograph (top ) and sectional side view (bottom) of a pixel of µ-PIC. . . . 140

viii

List of Tables
1.1
1.2

SUSY particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
The annihilation channel of the neutralino [15]. f and f¯ are the standard
model neutrinos, leptons and quarks, and A0 are axions. . . . . . . . . . . . . 16

2.1

Values of λ2 J(J +1) calculated in the basis of the odd group model for various
nuclei.[38] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Values of λ2 J(J + 1) in the case of pure proton coupling. . . . . . . . . . . . 29
Values of λ2 J(J + 1) in the case of pure neutron coupling. . . . . . . . . . . . 30

2.2
2.3
3.1

Comparison of gaseous directional sensitive dark matter detectors. Details of
the the DRIFT and the DMTPC can be found in the references. . . . . . . . 50

4.1
4.2
4.3

The properties of CF4 gas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Properties of the NEWAGE-0.3a detector . . . . . . . . . . . . . . . . . . . . 55
Performances of the NEWAGE-0.3a detector. . . . . . . . . . . . . . . . . . 76

5.1
5.2

Properties of the pilot runs in Kamioka mine. . . . . . . . . . . . . . . . . . 79
Neutron flux (neutrons cm−2 s−1 ) in Kamioka Observatory measured with a
3
He proportional counter[88]. . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
Estimated contributions of the background sources outside of the vacuum
chamber at 100 keVα.e.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
Estimated contributions to the measured spectrum at 100 keVα.e. of inside
the vacuum chamber. Issues of columns show materials with contaminations
which emit α-particles. Issues of rows show the positions of the energy deposition. X is unknown but can potentially to be as high as 100. . . . . . . . . 104

5.3
5.4

6.1
6.2
6.3
7.1

Operation parameter of µ-TPC. . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Properties of the dark matter search run (Kamioka RUN-III) in Kamioka
mine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
Astrophysical and nuclear parameters used to derive the WIMP-proton limit 116
Required S/N for detecting dark matter signals in energy rage of 100-120 keVwith 90% C.L. for Mχ =100GeV/c2. We took account the energy resolution,
detection efficiency of NEWAGE-0.3a detector and assumed zero background. 127ix