Direction-sensitive dark matter searchwith a gaseous micro time projection chamber
Kiseki Nakamura
2014.01.06

Abstract
Non-baryonic dark matter is widely believed to account for a large fraction of the mass inthe universe by a lot of observation of the universe. Weakly Interacting Massive Particle(WIMP) is the leading candidate of the dark matter. Many direct search experiments of dark
matter have been performed so far. Although some experiments have reported positive results,
no experiment has reached a widely agreed discovery. A more reliable method, directionsensitivity, is expected to provide a strong evidence of the dark matter. We developed adirection-sensitive dark matter detector, NEWAGE-0.3b’ to improve the sensitivity by one
order of magnitude from our previous measurement performed by NEWAGE-0.3a. NEWAGE0.3b’ was designed to have a twice larger target volume with low background material, a lowered
threshold of 50 keV, an improved data acquisition system, and a gas circulation system withcooled charcoal. A direction-sensitive dark matter search in Kamioka underground laboratory
with NEWAGE-0.3b’ was performed from July 17th, 2013 to November 12th, 2013. With an
exposure of 0.327 kg · days, a new 90% C.L. direction-sensitive SD cross section limit 557 pb for
WIMP mass of 200 GeV/c2 was obtained. This result improved the limit by ∼ 10 times fromprevious measurement, and marks the best direction-sensitive limit. From the detail study on
the remaining background, radon gas and the substrate of µ-PIC was found to be the dominantbackground source, and prospects for future improvements were discussed.

Contents
1 Introduction
1.1

1.2

1.3

4

Astrophysical dark matter : evidences . . . . . . . . . . . . . . . . . . . . . . .

5

1.1.1

Rotation curve of the galaxy . . . . . . . . . . . . . . . . . . . . . . . .

5

1.1.2

X-ray observation of the cluster of galaxy . . . . . . . . . . . . . . . . .

6

1.1.3

Weak gravitational lensing of Bullet Cluster . . . . . . . . . . . . . . . .

7

1.1.4

Cosmological parameters. . . . . . . . . . . . . . . . . . . . . . . . . .

8

1.1.5

Big Bang nucleosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . .

13

Particle physical dark matter : candidates . . . . . . . . . . . . . . . . . . . . .

16

1.2.1

WIMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

16

1.2.2

Axion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

18

1.2.3

Q-ball . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

19

1.2.4

Sterile neutrino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

21

1.2.5

Other candidates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

22

WIMP search . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

22

1.3.1

Direct search . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

22

1.3.2

Indirect search . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

23

1.3.3

Collider experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25

2 Method of Direct Dark Matter Search

27

2.1

Energy spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

27

2.2

Cross section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

30

1

2.2.1

Lagrangian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

31

2.2.2

Spin Independent (SI) cross section . . . . . . . . . . . . . . . . . . . . .

31

2.2.3

Spin Dependent (SD) cross section . . . . . . . . . . . . . . . . . . . . .

33

2.3

Form factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

36

2.4

Dark matter signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

39

2.4.1

Nuclear target dependence. . . . . . . . . . . . . . . . . . . . . . . . .

39

2.4.2

Annual modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

39

2.4.3

Directionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

41

3 Review of Direct Dark Matter Search
3.1

3.2

43

Conventional experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

44

3.1.1

Solid scintillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

45

3.1.2

Liquid Noble-gas scintillator . . . . . . . . . . . . . . . . . . . . . . . . .

45

3.1.3

Semiconductor detector (bolometers, CCD) . . . . . . . . . . . . . . . .

47

3.1.4

Bubble chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

48

Direction-sensitive experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . .

49

3.2.1

Gaseous detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

49

3.2.2

Emulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

54

3.2.3

DNA Tracking Chamber . . . . . . . . . . . . . . . . . . . . . . . . . . .

55

3.2.4

High pressure Xe detector . . . . . . . . . . . . . . . . . . . . . . . . . .

55

4 Detector

56

4.1

NEWAGE-0.3b’ detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

57

4.2

Data acquisition system (DAQ) . . . . . . . . . . . . . . . . . . . . . . . . . . .

61

4.3

Simulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

63

4.4

Detector performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

65

4.4.1

Drift velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

65

4.4.2

Energy calibration and energy resolution . . . . . . . . . . . . . . . . . .

66

4.4.3

Event selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

68

2

4.4.4

Angular resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

74

4.4.5

Direction dependent eﬃciency . . . . . . . . . . . . . . . . . . . . . . . .

76

5 Experiment

78

5.1

Dark matter search at Kamioka mine . . . . . . . . . . . . . . . . . . . . . . . .

78

5.2

Dark matter limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

84

6 Discussion
6.1

6.2

6.3

88

High energy backgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

91

6.1.1

Peak component . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

94

6.1.2

Continuous component . . . . . . . . . . . . . . . . . . . . . . . . . . . .

97

6.1.3

Summary of high energy background study . . . . . . . . . . . . . . . . 103

Low energy backgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
6.2.1

Environmental backgrounds . . . . . . . . . . . . . . . . . . . . . . . . . 106

6.2.2

Internal background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

6.2.3

Summary of low energy background study . . . . . . . . . . . . . . . . . 115

Future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

7 Conclusion

117

Acknowledgements

1193

Chapter 1

Introduction
Dark matter is one of the biggest puzzle of the universe. In 1960s, the modern cosmology
was started by both the theory of relativity and observations of Cosmic Microwave Background(CMB). In particular, recent observations of CMB[1], Baryon Acoustic Oscillation (BAO)[2],and far distant Type Ia Supernovae[3], favor a ﬂat universe consisting of 68.3 % dark energy,
26.8 % dark matter, and 4.9 % baryon (ordinary matter), as shown in Figure 1.0.1. According
to these ”understandings”, the dark matter is not ordinary baryonic matter and it is very likely
to a particle in new physics beyond the Standard Model (SM) of the particle physics. In thissection, studies on the dark matter from the viewpoints of astrophysics (Section 1.1) and particle
physics (Section 1.2) are discussed, then the dark matter search experiments (Section 1.3) arereviewed.

Figure 1.0.1: Composition of the energy of the universe. The universe was known to consist of 68.3 %dark energy, 26.8 % dark matter, and 4.9 % baryon (ordinary matter) from recent observations[1]

.
4

1.1

Astrophysical dark matter : evidences

The dark matter was introduced by Zwicky in 1930s[4]. He studied the motions of galaxies
in Coma Cluster and Virgo Cluster, and noticed that the velocities of galaxies were too highto keep them in a cluster by the gravity potential of visible galaxies. Thus he introduced”dark matter” as an invisible matter to keep them in a cluster. The mass of the dark matter
was thought to be 10 to 100 times larger than the visible matter. Owing to the following
observations of galaxies and clusters, we now have a lot of evidences of dark matter as invisible
mass. Also, non-baryonic mass is required in the whole universe from the measurements ofchemical component of primordial nucleosynthesis.

1.1.1

Rotation curve of the galaxy

Galaxy rotation problem was reported in 1970s. This problem is that a galaxy rotation
velocity in the outer part is too high to keep the stars within a galaxy. To solve this problem,
additional invisible mass providing suﬃcient gravitational potential was needed to explain therotation of the galaxy. The star’s rotation velocity in a galaxy is calculated from Kepler’s law
as

M (r)
vc2 (r)=G 2 ,(1.1)
r
r
where r is a radial distance from the center of the galaxy, vc (r) is a rotation velocity as thefunction of r, and M (r) is a total mass within the radius r. The rotation velocity was measured
from observations of the Doppler shift of bright lines in the spectra of stars, using 21 cm and3.6 mm lines of the HI gas (neutral hydrogen gas) and the CO gas. If a galaxy consisted ofonly visible stars, the rotation velocity v must have fallen as r−1/2 because observed stars
concentrated at the center of the galaxy. However, the measured rotation velocity was almost
constant even the outer region of the galaxy where the population of visible stars were very
low. This observational result implies that the mass distribution M (r) was proportional to r
even at a large radius, and the galaxy must contains large amount of invisible mass, or thedark matter.
Figure 1.1.1 is the rotation curve of NGC6503 galaxy measured by Doppler shift of 21 cm
line[5]. This galaxy is a typical spiral galaxy with a 2 kpc radius core and a luminous disk
expanding to more than ∼ 5 kpc from the center of the galaxy. Since the measured rotation
curve was ﬂat even up to the outer region, dark matter is required in the galaxy halo.

5Figure 1.1.1: The rotation curve of NGC6503 spiral galaxy. The black points are measured velocity
by Doppler shift of 21 cm line[5]. The dashed line and dotted line are the contribution to the rotation
velocity from the observed disk and gas, respectively. The dashed-dotted line is the contribution fromthe invisible mass at the galaxy halo. The solid line is a sum of them.

1.1.2

X-ray observation of the cluster of galaxy

A cluster of galaxies is a largest structure in the universe where 100 − 1000 galaxies concentrate. The existence of the dark matter in the cluster of galaxies scale was noticed from the
X-ray observation of the cluster of galaxies since 1980s.
X-ray observation for the cluster of galaxies enabled us to see the distribution of hightemperature gas. As a result, it was found out that clusters of galaxies was ﬁlled with hightemperature plasma of about 107 ∼ 108 K and that almost all baryons exist as plasma insteadof stars. Since such high temperature plasma becomes high pressure, the cluster of galaxies
needs a gravitational potential to keep galaxies inside. Calculated necessary mass was 10 − 100
times as much as the sum of the galaxies and plasma in a cluster. Thus, the dark matter was
suggested to sustain the distribution of cluster of galaxies scale.
In the X-ray observation of clusters of galaxies Abell1060 and AWM7 by ASCA (Japanese Xray satellite), the baryon fractions at clusters of galaxies was derived as shown in Figure 1.1.2[6].
Obtained baryon fractions were less than ∼ 0.2, and then it revealed the existence of dark
matter around a cluster of galaxies.

6Figure 1.1.2: The baryon fraction of galaxy clusters Abell1060 and AWM7 observed by ASCA[6]. Horizontal axis is distance from the center of galaxy clusters normalized by rvir . rvir is the 2 ± 0.5 Mpc and2 ± 0.4 Mpc for Abel1060 and AWM7 were taken, respectively. Solid line and dotted line show the best
ﬁt model corresponding to Abel1060 and AWM7, respectively. Dashed line and dot-dash line show themodel in case most compact and most expanded, respectively.

1.1.3

Weak gravitational lensing of Bullet Cluster

1E 0657-558, so-called ”Bullet Cluster”, is the region where galaxy clusters collide. Anotherdark matter evidence was observed in this region by weak gravitational lensing.
Gravitational lensing is the phenomenon that the light from a distant source (a backgroundgalaxy) toward an observer is bended by a distribution of the gravitational potential between thesource and the observer. Then the background object is observed to be multiplied or ampliﬁed
or warped. When a gravitational potential is not so strong, a background galaxy is just warped,
and this phenomenon is called ”weak gravitational lensing.” Using weak gravitational lensing,
we can evaluate the shape and the amount of the gravitational potential with a statisticalanalysis of the warps of galaxies.
The region around 1E 0657-558 was observed by Hubble Space Telescope to evaluate themass distribution through weak gravitational lensing observation and statistical analysis. Obtained mass distribution was the green contour in Figure 1.1.3. In addition, the optical and7Figure 1.1.3: Space distribution of 1E 0657-558 where galaxy cluster collided. Left color image, right
one and green contour represent space distribution measured by optical, X-ray and gravitational lensing,respectively. [7]. The green contour in both ﬁgure is the distribution of gravitational potential measured
by weak gravitational lensing.

X-ray radiation distribution observed by Magellan telescope and Chandra satellite are shownin the left and the right, respectively. The high temperature plasma observed by the X-ray
expanded about 0.72 Mpc and the right side cluster is moving away from the left side one at∼ 4700 km/s. As a line-of-sight velocity is only ∼ 600 km/s, the collision occurred nearly in theplane of the sky. From these observations, gravitational potential does not trace the plasma
distribution and obtained the picture that weakly interacting dark matter goes ahead in contrast to the viscous plasma. This result is one of the clear evidence of the existence of the dark
matter at a cluster of galaxies scale.

1.1.4

Cosmological parameters

Cosmology is the theory which covers the whole universe. The ΛCDM model, one of the Big
Bang cosmological model including a cosmological constant denoted by Λ and cold dark matter(CDM), is now considered as a standard model of cosmology. In the ΛCDM model,
the )energy(densities of the universe components are normalized by a critical density ρc =3c2 H028πG, andcalled cosmological parameters. The normalized parameters, the dark energy, the matter andthe radiation are referred to as ΩΛ , Ωm and Ωr , respectively. Only Ωr is accurately measured
directly as following. The radiation density is dominated by the energy of the CMB, and several
satellite observations determined its temperature to be T = 2.72548 ± 0.00057K, corresponding
to Ωr h2 = 2.47 × 10−5 [8], where h is Hubble parameter normalized by 100. Ωr is negligible
compared with Ωm after the transition from the radiation dominant era to the matter dominant
era at the early universe (z ∼ 104 ). When we consider baryon and dark matter separately,
matter(Ωm ) is divided into baryon(Ωb ) and dark matte(ΩDM ).
8

Here, we brieﬂy explain the cosmological parameters. The evolution of the scale factor a,representing the size of universe, is written as
( )2
{
}
ȧ
Ωr
Ωk
2 Ωm
= H0
+ 4 + ΩΛ − 2 ,
a
a3
a
a

(1.2)

where, H0 is Hubble constant and Ωk is the normalized curvature of the universe. Equation (1.2) is the normalized Friedmann equation and derived from general relativity assuming
a homogeneous isotropic universe. By solving this diﬀerential equation, we can evaluate thetime evolution of the universe and investigate the universe constriction/expansion, the ﬂatnessof the universe, and the existence of the Big Bang. The evolution depends on cosmological parameters and the observational results on cosmological parameters are shown in Figure 1.1.4.
There are several observations to determine the cosmological parameters; the observations ofType Ia Super Nova (IaSN)[3], Cosmic Microwave Background (CMB)[1], and Baryon Acoustic
Oscillation (BAO)[2] representatively. Each observation set the limit or allowed region of thecosmological parameters as shown in Figure 1.1.4.

Figure 1.1.4: Allowed regions of cosmological parameters (ΩΛ , Ωm ) by observations of IaSN (”Supernovae” in ﬁgure)[3], CMB[1], and BAO (”clusters” in ﬁgure) [2]. A ﬂat universe with (ΩΛ ,Ωm ) ≃ (0.7, 0.3) is favored by each three observations.

9A Type Ia Super Nova (IaSN) explosion occurs when a white dwarf in a binary system
getting mass accretion from a companion star reaches to the Chandrasekhar mass. Thus, themaximum luminosity of IaSN is considered to be constant in spite of individual situations. In
addition, a super nova is so luminous phenomenon that we can investigates a distant universe
over z ∼ 1. The redshift of IaSN spectrum gives the expansion velocity of the universe at the
distance of IaSN, and the measured intensity of a IaSN gives a luminosity distance dL using
L
F = 4πd2 , where F is observed ﬂux at the Earth and L is an intrinsic luminosity of a IaSN.
L

On the other hand, the luminosity distance can be written with cosmological parameters as∫]− 1
1+z z[dL (z) =ΩΛ,0 + (1 + z ′ )3 Ωm,0 − (1 + z ′ )2 Ωk 2 dz ′ .
(1.3)
H0 0
From the equation above, cosmological parameters can be evaluated. Figure 1.1.5 shows observed IaSN luminosity (converted to magnitude m) as a function of the distance (shown inunits of redshift z)[3]. From this ﬁgure, the model with Ωm = 0.25, ΩΛ = 0.75 was found to beconsistent to measurements.

Figure 1.1.5: Observed IaSN luminosity (converted to magnitude m) as a function of the distance (shownin units of redshift z)[3] Red points are measured and blue lines are models. Bottom ﬁgure shows theresidual from the model assuming Ωm = 0, ΩΛ = 0. The model with Ωm = 0.25, ΩΛ = 0.75 was foundto be consistent to the measurements.

10Cosmic Microwave Background (CMB) is the thermal radiation left from the Big Bang.
In the early and high temperature universe, where electrons, protons and photons exist withthermal equilibrium, photons cannot move straight due to Compton scatterings with electrons.
As the temperature decreased adiabatically with the universe expansion, electrons and protons
formed atoms, and photons can go straight. This phenomenon is called recombination andthe temperature of this time corresponds to the ionizing energy of a hydrogen atom which is∼ 3000 K. After the recombination, photon’s wavelength increased according to the universe
expansion. And now, we observe this remained photon as a microwave and its spectrum isconsistent to a black body of 2.725 K. Observed CMB is perfectly isotropic, but it has a little
ﬂuctuation in temperature. In order to study the ﬂuctuations for various scales, the spatial
distribution of the temperature T (θ, ϕ) is obtained by ﬁtting with spherical harmonics Ylm
using coeﬃcients alm as
T (θ, ϕ) =

∞ ∑
l
∑

alm Ylm (θ, ϕ),(1.4)

l=0 m=−l∑1|alm |2 .
2l + 1
l

Cl =

m=−l

Here Cl is called angular power spectrum and represents the amplitude of the ﬂuctuation at◦

the scale of l (∼ 180
in degree). C0 is the monopole and corresponds to the average temperl
ature, and C1 is the dipole moment and corresponds to the motion of the earth. At the ﬁner
scale, ﬂuctuations were induced by integrated Sachs-Wolfe eﬀect (ISW) and have informationof the recombination era (z ∼ 1100). ISW is the wavelength shifting eﬀect that causes red
shift at higher density region and blue shift at lower one due to a density ﬂuctuation which
was drawn out a quantum ﬂuctuation by inﬂation. Assuming the cosmological parameters, we
can evaluate the angular power spectrum by developing the temperature ﬂuctuation induced
by ISW according to the Boltzmann equation. Planck satellite observed the cosmic microwave
ﬂuctuations all over the sky[1]. Observed ﬂuctuation of CMB was quite tiny as ∼ 100µK.
Figure 1.1.6 shows the angular power spectrum measured by Planck. Comparing the measurement to the model, cosmological parameters were evaluated as Ωm ≃ 0.3, ΩΛ ≃ 0.7 as shownin Figure 1.1.4.
Baryon acoustic oscillations (BAO) is the ﬂuctuation in the density of the visible baryonicmatter of the universe, caused by acoustic waves which existed in the early universe. Now
theoretical model of the structure formation of the cluster of galaxy, we can predict how todevelop the density ﬂuctuation from acoustic waves to visible galaxies. Cosmological parameters
are estimated by comparing observed galaxies distribution to the calculated one. The density
ﬂuctuation is considered to be developed by the gravitational instability because gravity is
11

Figure 1.1.6: Angular power spectrum of CMB observed by Planck[1]. Blue points are measured data,
red line is the best ﬁt model. Bottom ﬁgure shows the residual from the best ﬁt model.

unique long-range force in four fundamental forces in the universe. In order to turn out thedensity ﬂuctuation, two types of dark matter were considered. One is the low mass dark matter,
which velocity is relativistic (hot). This type of dark matter is called hot dark matter (HDM).
The other is the massive dark matter, which is non-relativistic (cold). This type of dark matteris called cold dark matter (CDM). In case of HDM, a ﬁne structure is disturbed by the motionof the dark matter. In this case, a scenario that large scale structure is formed in the ﬁrst
place then divide into clusters and galaxies, is favored (top-down scenario). But this top-down
scenario is not supported because galaxies exist even at the early universe around z ∼ 6.
On the other hand, a bottom up scenario is favored in case of CDM and now this scenario
is widely supported. Sloan Digital Sky Survey (SDSS) is the project surveying galaxies toinvestigate the distribution of galaxies[2]. In order to compare observations with the structureformation model, ”two point correlation function” was used. Two point correlation function
indicates the diﬀerence from the random distribution and deﬁned as follows. Consider the
probability P (x1 , x2 )(dx1 )3 (dx2 )3 for galaxies contained to both of two volumes (dx1 )3 (dx2 )3around two points x1 , x2 at a distance r. If the distribution of galaxies is perfectly at random,
this probability is given as n̄2 (dx1 )3 (dx2 )3 using the number density of galaxies n̄. But, the12Figure 1.1.7: Measured two point correlation function of galaxies by SDSS[2]. Top, second and thirdlines show models assuming Ωm h2 = 0.12, 0.13 and 0.14, respectively, all with Ωb h2 = 0.024 andn = 0.98. The bottom line shows a pure CDM model assuming Ωb h2 = 0 and Ωm h2 = 0.105. Here, his Hubble parameter normalized by 100 and n is number density of galaxy. Inset is the enlarged view.

actual probability diﬀers from random as
P (x1 , x2 )d3 x1 d3 x2 = n̄2 [1 + ξ(r)] (dx1 )3 (dx2 )3(1.5)

and ξ(r) is the two point correlation function. Figure 1.1.7 is measured two point correlation
function of galaxies by SDSS. The peak around ∼ 100h−1 Mpc is the indication of BAO andcalled an acoustic peak. The amplitude of an acoustic peak is sensitive to the gravitational
components of the universe, and Ωm is estimated by BAO as shown in Figure 1.1.4.

1.1.5

Big Bang nucleosynthesis

Big Bang Nucleosynthesis (BBN) describes the production of light nuclei besides hydrogen
during the early universe. In 1940s, BBN was advocated by George Gamow to explain the ratioof elements in the universe, and after some discussions, the existence of CMB was predicted.
Hence the discovery of CMB strongly supported BBN. Baryon density Ωb is evaluated by BBN,
and the dark matter density can be estimated as ΩDM = Ωm − Ωb .
13

Production of heavier nuclei fusing nucleons needs a high temperature and high density
environment, such an ultimate environment is realized only at Big Bang(if it happened), innerstar, and Supernova. The fraction of elements in the universe is 75% hydrogen, 25% helium
and the rest with other elements. Similar fractions are measured anywhere in the universe. The
nuclei produced in inner stars or Super Novae are not only helium but also heavier elements.
In order to explain the general abundance of light elements, BBN is inevitable.
At Big Bang, when the temperature and the density was high enough, protons and neutrons
were at the equilibrium of weak interaction and they can convert to each other, where the ratio
of protons to neutrons became 1 : 1. As the temperature and density became lower due tothe universe expansion, the reaction velocity became slower than the expansion. When thetemperature is below ∼ 1 MeV, protons and neutrons are no longer in equilibrium. After that,neutrons one-sidedly decayed to protons, and the ratio of protons to neutrons became 4 : 1 at
∼ 100 keV. At this temperature, the number of photons of which energy resolves deuterons 2 D(binding energy of 2 D = 2.22 MeV) by photodegradation reaction was less than the numberof 2 D. As a result, suppressed nuclear fusion came to be superior, and then nucleons fused tomore stable nuclei. Since the binding energy of 4 He (28.3 MeV) is much higher than that of
2 D, almost all the generated 2 D fused to 4 He, and almost all the neutrons were ﬁxed in 4 He.

Also, 7 Li and 7 Be were slightly generated. As the universe expanding, the universe became too
cold and too low in density to keep nuclear fusions and then the fraction of elements was ﬁxed
at this time. The ﬁxed fraction is calculated by solving the Boltzmann equation and depends
on the baryon-to-photon ratio.
The number of photons and baryons decrease equally during the universe expansion, andthe baryon-to-photon ratio has been kept constant. Since the number density of photons ismeasured by CMB observations, the number density of baryons can be calculated using thebaryon-to-photon ratio. Figure 1.1.8 shows the relation of elements fraction to the baryon-tophoton ratio. The baryon-to-photon ratios measured from observed number density of somelight elements are almost consistent among these elements, and baryon density Ωb h2 = 0.0214
is obtained[9]. Considering normalized Hubble parameter h = 0.71+0.04−0.03 , the baryon density is
calculated as Ωb = 0.044 ± 0.004. Because this value is too small to explain the matter densityΩm ≃ 0.25, the existence of non-baryon dark matter of the amount of ΩDM ≃ 0.20 is supported.

14

Figure 1.1.8: The relation of elements fraction to the baryon-to-photon ratio. Horizontal axis shows
baryon-to-photon ratio and also written as baryon density. Vertical axis is elements fractions comparedto hydrogen in the early universe. Four lines are the calculation of the standard BBN model (SBBN).
The region surrounded by squares are the limit of number density from observation and corresponding
baryon-to-photon ratio. Comparing the measurement to the calculation, Ωb h2 = 0.0214 was obtained[9].

15

1.2

Particle physical dark matter : candidates

From astrophysical observations, the amount of non-baryonic dark matter was estimated tobe ΩDM ≃ 0.20. Also, the structure formation model (Section 1.1.4) strongly favors the CDM
than HDM. In this section, the candidates of the dark matter are reviewed.

1.2.1

WIMP

In particle physics, the Standard Model (SM) theory is well accomplished and can explain
physical phenomena at low energy scale (≲ 1 TeV). However, SM theory has several problems,
and this theory is considered to be the low energy approximation of an ultimate uniﬁed theory.
In this ultimate uniﬁed theory, the fundamental four interactions are considered to be uniﬁed
at Planck energy scale (∼ 1019 GeV). The existence of new theories describing the phenomena
between SM energy scale and Planck scale is widely believed. Weakly Interacting Massive
Particles (WIMPs) are predicted in some beyond-SMs, where the interaction must be weak
and the mass is heavier than SM particles. WIMPs are good candidates of the dark matterfrom their nature, and strongly motivated by particle physics. There are several theories which
produce WIMPs such as supersymmetry theory (SUSY), universal extra dimensions (UED),
little Higgs theory (LHT), and so on. Particularly, SUSY is strongly preferred as a beyond-SM
theory.
Supersymmetry theory (SUSY) is one of the these theories, which solves the hierarchy
problem (the secondary divergence of Higgs mass). In addition, a uniﬁcation of the coupling
constants is realized as shown in Figure 1.2.1. This theory introduces new partner particles
whose spins diﬀer from those of SM particles by 1/2 as listed in Table 1.2.1. The interactionof SUSY particles with SM particles are governed by the R-parity given as
R = (−1)3B+L+2S ,(1.6)

where B, L and S are the baryon number, the lepton number and the spin, respectively. SM
particles have odd R-parities, while SUSY particles have even R-parities. From the conservationof the R-parity, the lightest SUSY particle (LSP) must be stable because it cannot decay to
SM particles without SUSY particles. LSP is the lightest gluino or chargino or neutralino,
whose spins are 1/2. If LSP is the lightest neutralino, the neutralino can be a candidate ofthe dark matter. Lightest neutralino (χ̃01 ≡ χ) is the lightest linear combination of the photino(γ̃), the zino (Z̃) and the higgsinos (H̃1 , H̃2 ). The photino and zino are expressed with the16Figure 1.2.1: The relation of the coupling constants and energy in the standard model (SM) (top) andin the minimum supersymmetric standard model (SMMS) (bottom).α1 ,α2 and α3 are the U (1), U (2)and U (3) couplings, respectively.

superpartners of gauge bosons, the bino (B̃) and the wino (W̃3 ) asγ̃ = cos θW B̃ + sin θW W̃3 Z̃
= − sin θW B̃ + cos θW W̃3 .
Thus, lightest neutralino χ is written asχ = a1 B̃ + a2 W̃3 + a3 H̃1 + a4 H̃2 .

(1.7)

The lightest SUSY particle (LSP) is stable and satisﬁes the nature of dark matter if LSP isuncharged [10]. The method of neutralino search and the neutralino search experiments are
explained in Chapter 2 and Chapter 3.
Universal Extra Dimension (UED) is one of the theories to provide WIMP other than
SUSY. UED is the beyond SM theory to unify the gravity interaction and the electromagnetic
interaction. UED argues that the universe consists of ﬁve or more dimensions. In the low
energy limit (energy scale of SM), the universe is observed as four dimensions. UED requires
17Symbolq = u, c, t
q = d, s, b
l = e, µ, τνe , ν µ , ν τ

SM particles
Name
up quarks
down quarks
leptons
neutrinos

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

Symbolq̃u1 , · · · q̃u6
q̃d1 , · · · q̃d6
˜l6 , · · · ˜l6ν̃1 , ν̃2 , ν̃3

SUSY particles
Name
up squarks
down squarks
sleptons
sneutrinos

Spin
0
0
0
0

g
W±
H±

gluons
W bosons
charged Higgs

1
1
0

g̃
±
χ̃±
1 , χ̃2gluinos
charginos

1/2
1/2

γ
Z0
h0
H0
A0

photonZ bosonlight scalar Higgs
heavy scalar Higgs
pseudoscalar Higgs

1
1
0
0
0

χ̃01 , · · · χ̃04

neutralinos

1/2

Table 1.2.1: SUSY particles

the Kaluza-Klein particles which conserve the K-K-parity. The lightest Kaluza-Klein particle(LKP) is thus stable and can be a candidate of the dark matter[11].
Little Higgs theory (LHT) is one of the beyond-SM theories which naturally explains theelectroweak symmetry breaking. LHT requires a global symmetry at ∼ 10TeV scale and isbroken at ∼ TeV scale to cancel the second divergence of Higgs mass correction in the ﬁrst
loop. In this theory, new particles named T-partner and T-parity are introduced. SM particles
have positive T-parity, and T-partners have negative T-parity in contrast. The lightest Tpartner (LTP) is stable due to the conservation of the T-parity, and one of the candidates ofthe dark matter[12].

1.2.2

Axion

Axion was introduced as a new neutral light pseudo-scalar boson in order to solve the CP
violation problem of the strong interaction[13]. The mass of the axion is limited to 1 µeV −1 meV or 2 eV − 5 eV by the evolution of a star, observations of the neutrinos from supernova
explosions, cosmology and so on. A lighter mass region remains as the window for the cold dark
matter[14]. In spite of the light mass, axion is considered as a candidate of the dark matterbecause axions were produced at the phase transition of a vacuum around inﬂation era andaxions were not in the thermal equilibrium with other particles at the early universe.
Axions are widely searched for by the detection of photons generated by the Primakoﬀ eﬀect,
18

which is the conversion from an axion to a photon in a strong magnetic ﬁeld[15]. Figure 1.2.2
shows the results of axion searches in the lighter mass region. No positive signal has beenobtained and the limit of the axion-photon coupling for each mass of axion is settled[16].

Figure 1.2.2: 90%C.L. limit of the axion-photon coupling for each mass of axion [16].

1.2.3

Q-ballBaryon asymmetry problem is based on the fact that the amount of the antimatter is muchless than that of the matter in the universe. Aﬄeck-Dine mechanism is one of the models
to generate this baryon asymmetry. Though Aﬄeck-Dine mechanism does not match wellwith SM, Aﬄeck-Dine mechanism can work well with SUSY[17]. Aﬄeck-Dine mechanism withSUSY naturally contains the non-topological soliton called ”Q-ball”, and Q-ball can be thedark matter.
Q-ball is a single particle including a lot of squarks and sleptons. Since the mass per baryon
number is less than the mass of proton, the Q-ball can stably exist. Baryon number of a Qball is ∼ 1024 and the mass is ∼ 1 mg in spite of a single particle. The Q-ball interacts with
nucleus through the strong interaction, and emits two or three pions of ∼ 100 GeV. Because of19the quite large mass, a kinetic energy of a Q-ball is so high compared to the energy loss thata Q-ball goes straight interacting with matters. Though the estimated cross section of thisinteraction, 100 mb, is much larger than those of other candidates of dark matter, a Q-ball canbe a candidate of dark matter because of the quite low number density in the universe.
In order to detect the Q-ball, we need to detect several pions with ∼ 100 GeV emitted ina straight line. Water Cerenkov detectors (BAKSAN, Kamiokande) or high energy cosmic ray
detectors such as telescope arrays (TA) are suitable to search for the Q-balls. Figure 1.2.3
shows the results of the Q-ball searches. Severe limits are obtained but the perfect scenariothat explains both the baryon density and the dark matter density has not been denied yet.

Figure 1.2.3: The limit of the baryon number Q of Q-ball for the breaking scale of SUSY MF [17]. The
light blue region surrounded by (a-d) lines is the area where the Q-ball allowed to exist as the dark
matter. Particularly, all of the dark matter is explained by Q-ball along the pink dotted lines. The blueline is the ideal region that the baryon density reaches to the observations, and the light blue circle isthe best scenario. The regions labeled B, G, K-1,2,3 are the obtained limit by BAKSAN, Gyrlyand andKamiokande, respectively. The regions labeled TA and OA is the estimated limit by Telescope Arrayproject and OWL-AIRWATCH, respectively.

20

1.2.4

Sterile neutrino

By the experimental studies on neutrino oscillation in these a few decades, it is establishedthat there are three kind of neutrinos νe , νµ and ντ , and they are mixed with themselves. Now,
the fourth neutrino named sterile neutrino is proposed as the ﬁnal state of neutrino mixing to
explain the result of LSND experiment[18]. In addition, by assuming sterile neutrino, the pulsar
kick phenomenon of neutron stars cal also be explained[19]. Unlike other ordinal neutrinos,
the sterile neutrino does not interact via the weak interaction. Mass of the sterile neutrino is
estimated as 1 ∼ 30 keV. Then, sterile neutrino can be a candidate of dark matter.
Although the life time of the sterile neutrino is longer than the age of the universe, sterile
neutrino is considered to decay to X-rays. The sterile neutrino search using X-ray space observatories are performed and the result are shown in Figure 1.2.4. No X-ray signal originatedfrom sterile neutrinos has been observed and the limit of the mixing angle for each mass isobtained.

Figure 1.2.4: Obtained limits of the mixing angle θ1 as a function of sterile neutrino mass M1 [18]. The
red region is the constraint region by X-ray observations. The gray region is the constraint the sterile
neutrino dark matter.

21

1.2.5

Other candidates

In addition to the candidates described above, there are a variety of dark matter candidates
such as mirror matter[20], asymmetric dark matter[21], crypton[22], and so on. Please see thereferences for details.

1.3

WIMP search

There are three types of WIMP searches; direct searches (underground experiments), indirect searches (astrophysical observations) and collider experiments (production). As shown in
Figure 1.3.1, Feynman diagram of these WIMP searches are same in principle with the diﬀerence in the time evolution. Direct search seeks for the WIMP-nucleus elastic scatterings, andis expected to provide direct evidences of the dark matter. Indirect searches are to ﬁnd theevidence of the WIMPs by detecting gamma-ray, neutrinos and anti-particles emitted from theannihilations of WIMPs. Collider experiment aims to produce WIMPs directly by the collisionof accelerated particles. These three methods are complementary to the others.

Figure 1.3.1: Feynman diagram contributing to, direct searches, indirect search, and collider productionof dark matter[23]. Here, χ, q and Q represent nenutralino, SM particle and a force-mediating boson,respectively.

1.3.1

Direct search

Direct search pursues for WIMP-nucleus elastic scatterings, where the WIMPs in the haloof our galaxy are expected to be detected by the detector on the Earth. Direct search can

22

provide the direct evidence of the existence of the halo dark matter. However, expected event
rate is very low because the cross section of a WIMP interacting with an ordinal matter is verysmall. Therefore these experiments need to be performed in underground facilities. At present,a lot of direct search experiments are performed around the world. More information on direct
detection is in the next chapter; the method of direct detection is mentioned in Chapter 2 anddirect detection experiments is reviewed in Chapter 3.

1.3.2

Indirect search

Neutralino is considered to be a majorana particle, whose antiparticle is its particle itself.
Then neutralinos can annihilate with themselves[11]. The annihilation rate Γann depends on
the number density nχ , annihilation cross section σann and a velocity v asΓann ∝ n2χ < σann (v)v > .

(1.8)

At the high-gravitational-ﬁeld region, such as the center of a galaxy, the Sun or the Earth,annihilations of trapped neutralinos would constantly occur. The detection of the annihilation products, gamma-rays, neutrinos and antiparticles from speciﬁc locations would provide
indirect evidences of the neutralino.
Gamma-rays are generated by neutralino annihilation through process such as χχ −→ γγand χχ −→ Z 0 γ. The observable gamma-ray spectrum would consist of a peak corresponding to
the energy of neutralino mass and a continuous component of decayed gamma-rays of produced
hadrons and leptons. Since the annihilation rate depends on the SUSY parameters (σann (v)and branching ratio) and the halo density proﬁle (nχ and v), the estimation of a ﬂux and an
energy spectrum is diﬃcult. But gamma-rays from neutralino annihilation are expected to
be a radiation located in high density regions such as the center of the galaxy, and the timedependence is constant. Energetic gamma ray experiment telescope (EGRET) was a gammaray telescope on the CGRO satellite to observe 30 MeV − 30 GeV gamma-rays. From theobservation of the galactic plane, about 60% higher gamma-ray ﬂux than the model calculation
was detected above 1 GeV region[24]. This excess was found to be within the systematic
uncertainty and thus a clear discovery was not achieved with EGRET. Fermi satellite is a
gamma-ray telescope to observe 30 MeV−300 GeV gamma-rays, with 30 times better sensitivityof EGRET. An excess around 130 GeV in the energy spectrum was observed at the galactic
plane and galactic center with a local statistical signiﬁcance 4.5σ as in Figure 1.3.2[25]. This
peak might be explained that the excess originates from the dark matter annihilations. Fromcalculated reduced χ2 value, 130 GeV dark matter with 2DM → 2γ can ﬁt the data. This result
is one indication of existence of a particle-physical dark matter.
23

Figure 1.3.2: Energy spectrum of gamma-rays from the center of our galaxy measured by Fermi (red solidline). Gray region is 95%C.L. error band. Black dashed line is the best ﬁt model assuming dark matterannihilation. Left and right ﬁgures are ﬁtted with the 2DM → 2γ channel model and 2DM → 2V → 4γchannel model, respectively, where V is light hypothetical ﬁnals states. Best-ﬁt mass are 130 GeV and
145 GeV for left and right ﬁgure. respectively.

Neutrinos can be produced as a ﬁnal state of the annihilation of neutralinos. The energyof neutrino from the neutralino annihilation is about 1/2 ∼ 1/3 of a neutralino mass. Unlike
gamma-rays, the interaction of neutrino is very rare. Therefore, annihilations of neutralinos
trapped in the Earth and the Sun are observed using a neutrino probe where we cannot searchby gamma-rays. Since the Sun mostly consists of hydrogen, neutralino would be trapped by
SD interactions (Section 2.2). Thus, the observation of neutrinos from the Sun is sensitive tospin-dependent cross section. On the other hand, since the Earth contains atoms of the large
mass number, observation of neutrinos from the Earth is sensitive to SI cross section. SuperKamiokande (SK), constructed in the Kamioka mine, is a water-Cherenkov neutrino detector
with ∼ 50000 ton of water surrounded by ∼ 10000 of photomultipliers. The observation ofneutrinos from the Sun did not show any signiﬁcant excess, and the limit of SD cross section
was settled[26]. Another Cherenkov neutrino detector, IceCube, has ∼ 5000 of photo multipliersin the ice at the South Pole. Observations of neutrinos from the Sun and the galactic center
have been performed[27].
Neutralino annihilation also products particle-antiparticle pairs. It is considered that antiparticles in nearby universe is created by the cosmic ray spallation. Thus an excess of antiparticle in the cosmic ray maybe an indirect evidence of the dark matter. Since antiparticles havecharges, incoming direction cannot be observed. Thus, indirect detection by searching antiparticle is diﬃcult to discriminate a dark matter model from other model clearly. PAMELA (aPayload for Antimatter Matter Exploration and Light-nuclei Astrophysics)[28] and AMS (Alpha Magnetic Spectrometer)[29] are cosmic ray observation experiments which can distinguish24

antiparticles from normal particles by using magnetic ﬁeld. PAMELA, operated from 2006 to
2009, measured positron fraction Ne¯+ /(Ne− + Ne+ ) and found the excess from ∼ 10 GeV to
100 GeV (detector’s limit) energy region. AMS-02, on the International Space Station from2011, obtained a consistent result with a higher precision. Both results are shown in Figure 1.3.3. The excess can be explained by the annihilation of the dark matter and also by
nearby pulsars. Further observations are required to understand the origin of the excess.

Figure 1.3.3: positron fraction Ne¯+ /(Ne− + Ne+ ) in the cosmic-ray measured by AMS-02, PAMELA
and Fermi.

1.3.3

Collider experiment

An accelerator with a suﬃciently energy has a possibility to product SUSY particles. Produced SUSY particles would immediately decay to the lightest SUSY particle (LSP). If LSP
is neutralino, it is detected as a missing momentum and a missing energy. In addition, themass and cross section of neutralino can be estimated by measuring the event rate, the missing
momentum and the missing energy. Though accelerator experiments cannot argue that the neutralino is the astrophysical dark matter or not, a discovery of the neutralino and investigationsof its nature is important for other dark matter searches.
Large electron-positron collider (LEP) and LEP2 is circular accelerators colliding electronsand positrons, and the operation have already been completed. The maximum center-of-massenergy of LEP2 is 200 GeV. LEP2 did not discover the candidate event of neutralino and25obtained a lower limit of the neutralino mass as MD > 40 GeV [30]. Large hadron collider(LHC) is a circular accelerator colliding protons, started in September 2008. The designed
maximum center-of-mass energy is 14 TeV. By 2013, LHC has been operated at the energy of
8 TeV, and obtained no signal of neutralinos[31]. By 2015, LHC will be upgraded to be operated
at the energy of 13 to 14 TeV. International linear collider (ILC) is a linear accelerator colliding
electrons and positrons, and is at a planning stage. Since the interaction of leptons is more
simple than that of hadrons, the background rate of a lepton collider is low. Thus, a lepton
collider is suitable to measure more precise parameters of a neutralino. The maximum energy
achieved with circular accelerators is limited by synchrotron radiation. ICL will overcome thisproblem by using a linear accelerator, and is expected to measure detail parameters of theneutralino[32].

26Chapter 2

Method of Direct Dark Matter
SearchDirect detection pursues for the detection via WIMP-nucleus elastic scatterings. Direct
detection can provide the direct evidence of the existence of the dark matter in the halo of
our galaxy. In this section, the theoretical framework of the direct detection focusing on the
expected signal is reviewed[33].

2.1Energy spectrum

Direct detection method is to detect the energy deposition on the nucleus via WIMP-nucleus
elastic scatterings, and thus the calculation of the expected energy spectrum is the ﬁrst stepto estimate the signals. Because we are on the Earth, the relative velocity between the Earthand the halo of our galaxy aﬀects the expected energy spectrum. The solar system rotates inour galaxy at a distance of 8 kpc from the center of the galaxy, and the Earth rotates aroundthe Sun. Then, the velocity of the Earth relative to the galactic halo is written asvE (= |vE |) = vsun + vorb cos θorb sin 2πy(2.1)≃ 244 + 15 sin(2πy) [km/sec],
where vsun is the velocity of the Sun relative to the galactic halo, vorb is the orbital rotation
velocity of the Earth around the Sun, θorb is the inclination of the Earth orbital plane withrespect to the galactic plane and y is the time in year from March 2nd. Figure 2.1.1 shows theschematic view of the motion of the Sun and the Earth in our galaxy. Due to the rotation of the
Earth around the Sun, the relative velocity of the Earth to our galaxy become the maximumin June and the minimum in December.
27

Figure 2.1.1: The schematic view of the motion of the Sun and the Earth in our galaxy.

The number density of WIMPs, dn, with the velocity of v ∼ v + dv is written asn0 ≡∫ vesc
dn ,
0

n0
dn =
f (v, vE )d3 v,
k
∫ 2π
∫ +1
∫ vesck=dϕd(cos θ)f (v, vE )v 2 dv ,
0

−1

(2.2)

0

where k is the normalize constant, n0 is the total number of WIMPs in our galaxy, vesc is
the escape velocity of the galaxy, θ is the angle between vE and WIMP velocity, and ϕ isazimuth angle taking WIMP velocity vector as a zenith. Assuming a Boltzmann distribution,
the velocity distribution of the dark matter f (v, vE ) is written asf (v, vE ) = e−(v+vE ) /v0 ,
2

2

(2.3)

where v0 is the velocity dispersion of our galaxy and if the galaxy rotation velocity is ﬂat, it
becomes rotation velocity. The normalize constant k is derived in case vesc is inﬁnite or not ask = k0 = (πv02 )3/2
(vesc = ∞))}{ (2 vesc −vesc
vesc
2 /v 20− 1/2e(vesc ̸= ∞),k = k1 = k0 erfv0v0π∫ x −t22
dt is the error function.
where erf(x) ≡ π1/2
0 e

(2.4)
(2.5)

The diﬀerential event rate R per target mass per days is
dR =

NA
σvdn,
A

(2.6)

where NA is the Avogadro number (6.02×1026 ), A is the mass number of the target nucleus, v =|v − vE | is the dark matter velocity relative to the target, σ is the WIMP-nucleus cross section.
28

Strictly, σ depends on the momentum transfer. In this section, we discuss the cross section
with zero momentum transfer σ0 . The cross section taking account of the momentum transfer
will be discussed in the latter section 2.3. The total event rate R is derived by integrating
Equation (2.6) as
R=

N0
σ0
A

∫
vdn

(2.7)

The total event rate R0 for vE = 0 and vesc = ∞ is described as
R0 ==2 NA ρD
σ0 v0
π 1/2 A MD
(
)(
)(
)
σ0
ρD
v0
361
,
MD MN 1 pb
0.3 GeV/c2 /cm3
220 km/s

(2.8)
(2.9)

where ρD is the mass density of the dark matter, MD is the mass of the dark matter, MN (=0.932A) is the mass of the target nucleus. The Equation (2.9) is normalized by σ0 = 1 pb,
v0 = 220 km/s, ρD = 0.3 GeV/c2 /cm3 .
The recoil energy ER of the nucleus with a mass MN caused by an elastic scattering of the()
dark matter with the kinematic energy E = 21 MD v 2 in the laboratory system is
ER = Er (1 − cos θ) /2,r=

4MD MN,(MD + MN )2(2.10)(2.11)

where θ is the scattering angle in the center of mass system, r is the reduced mass. Assuming
the scattering is isotropic in the center of mass system, cos θ distribution become ﬂat and ERdR
is written as
also becomes ﬂat in the 0 ≤ ER ≤ Er region. Then the energy spectrum dE
R

dR
dER

∫ Emax
=
Emin

=

1
E0 r

1
dR(E)
Er

∫ vmaxvmin

v02
dR(v),
v2

where Emin = ER /r is the minimum kinetic energy of WIMP capable to give the energy ER toa target nuclei, Emax is the maximum kinetic energy of WIMP in a laboratory system, E0 isv2deﬁned as E0 = 21 MD v02 = v02 E. The energy spectrum is obtained using Equation (2.6), (2.2),and (2.3) as
dR
R0 k0
=
dER
E0 r k∫ vmaxvmin

1
f (v, vE )d3 v.
v(2.12)

By performing the integration of Equation (2.12), the speciﬁc formula of energy spectra are29

obtained asdR(0, ∞)dER
dR(0, vesc )dER
dR(vE , ∞)dERdR(vE , vesc )
dER

=
=
=
=

R0 −ER /E0 r
e
,
E0 r
[
]
k0 dR(0, ∞) −ER /E0 r
R0 −vesc
2 /v 2
0
e
−
e
,
k1
dER
E0 r
[ (
)
(
)]
R0 π 1/2 v0
vmin + vE
vmin − vE
erf
− erf
,
E0 r 4 vEv0v0[]
k0 dR(vE , ∞)R0 −vesc
2 /v 2
0 .
−
e
k1
dER
E0 r

(2.13)(2.14)(2.15)(2.16)

E0r dR(normalized event rate)
R0 dE

Figure 2.1.2 shows the normalized energy spectrum.

1

0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0

1

2

3

4

5

6

7
8
9
10
E (normalized energy)
E0r

Figure 2.1.2: The normalized energy spectrum of nucleus caused by an elastic scattering of the WIMP.
19
F is assumed for the target.

2.2

Cross section

In the previous section, the total rate R0 was shown to be proportional to the WIMP-nucleus
cross section. The cross section can be calculated by assuming certain sets of parameters in
the MSSM model [10, 34, 35]. In this subsection, the elastic scattering cross section of SUSY
neutralino and nucleus is discussed.
30

2.2.1

Lagrangian

The MSSM Lagrangian framework leads to the following low-energy eﬀective Lagrangian,Leﬀ , which is suitable for describing elastic neutralino-quark interactions[34].
Leﬀ()= χ̄γ µ γ 5 χq¯i γµ α1i + α2i γ 5 qi

(2.17)+ α3i χ̄χq¯i qi+ α4i χ̄γ 5 χq¯i γ 5 qi
+ α5i χ̄χq¯i γ 5 qi
+ α6i χ̄γ 5 χq¯i qi ,where χ and q denote neutralino and quarks. The Lagrangian should be summed over quark
generations, and the subscript i refers to up-type i = 1 and down-type i = 2 quarks. The
terms with coeﬃcients α1i , α4i , α5i and α6i depend on the velocity and are suppressed because
the velocity is non-relativistic in an elastic neutralino-quark scattering. Coeﬃcients α2i and
α3i contribute to the Spin-Independent(SI) and Spin-Dependent(SD) interactions, respectively,
and Feynman diagrams of each interaction are shown in Figure 2.2.1.

2.2.2

Spin Independent (SI) cross section

The cross section of SI interaction is written as
SI
σχ−N
=

4µ2χ−Nπ[Zfp + (A − Z)fn ]2 ,(2.18)

where Z and A are the atomic number and mass number of a target, respectively. µχ−N is the
reduced mass of the neutralino mass MD and a target mass MN ,µχ−N =MD MN.
MD + MN(2.19)

fp and fn are proton-neutralino and neutron-neutralino SI couplings, respectively. They are
(p)

(n)

written as the sum of the neutralino-quarks SI couplings, fTq and fTq as∑ (p) α3q
fp
2 (p) ∑ α3q
=
fTq
+ fTG
Mp
Mq
27
Mq

(2.20)∑ (n) α3q
fn
2 (n) ∑ α3q
=
fTq
+ fTG
,
Mn
Mq
27
Mq

(2.21)

q=u,d,sq=c,t,bq=u,d,sq=c,t,bwhere Mp , Mn and Mq are the mass of the proton, the neutron and quarks, respectively. The
(p)

(n)

(p)

(n)

numerical value of the fTq , fTq , fTG and fTG are calculated from the mass of quarks and31

Figure 2.2.1: Feynman diagrams of neutralino-quark elastic scattering. Upper set is SI and lower set isSD, respectively.

chiral symmetry applied to baryons, as
(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,
∑
(p)
(p)
fTG = 1 − q=u,d,s fTq ,
∑
(n)
(n)
fTG = 1 − q=u,d,s fTq .
(p)

(n)

SI
In most of the cases, fTq ≃ fTq , therefore σχ−N
∝ µ2χ−N A2 is practically derived. Thus, theSI ,cross section of SI interaction is written by using neutralino-proton SI cross section; σχ−p

SI
SI
σχ−N
= σχ−p

µ2χ−N
µ2χ−p

A2 .

(2.22)

Equation 2.22 indicates that a target element with large A is eﬀective for SI-interacting neutralinos. Figure 2.2.2 shows the SI cross sections normalized by neutralino-proton SI crosssection for some target nucleus.

32

SI
SI
/ σDM-p
σDM-N

9

10

19

8

10

F

73

Ge

128

Xe

107

6

10

5

10

102

3

10
MD [GeV/c2]

SI
SI
Figure 2.2.2: SI cross sections normalized by neutralino-proton SI cross section, σχ−N
/σχ−p
as a functionof neutralino mass. Black, red and blue line shows target diﬀerence as 19 F, 73 Ge and 128 Xe.

2.2.3

Spin Dependent (SD) cross section

The cross section of SD interaction is written as()2 J + 132SDσχ−N= G2F µ2χ−N ap ⟨Sp ⟩ + an ⟨Sn ⟩,π
J(2.23)

where GF (= 1.166 × 10−5 GeV−2 (ℏc)3 ) is the Fermi coupling constant, ⟨Sp ⟩ and ⟨Sn ⟩ areexpectation values of spin of proton and neutron in the nucleus, J is the total spin of thenucleus. ap and an consist of the sum of quark components as
∑ α2q
√
ap =
∆(p)
q ,
2G
F
q=u,d,s
∑ α2q
√
an =
∆(n)
q ,
2G
F
q=u,d,s(p)where ∆q(n)and ∆q(2.24)(2.25)

are the quark spin contents of the nucleon, and are calculated [34] as
(n)

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

33

Using Lande factor Λ =ap ⟨Sp ⟩+an ⟨Sn ⟩, the cross section of SD interaction is written as
J
SD
σχ−N
=

32 2 2G µ
Λ2 J(J + 1).
π F χ−N(2.26)The spin-dependent interaction can only occur for nuclei with nonzero spin. Only nuclei withan odd number of either protons or neutrons can have nonzero spin. As for such nuclei, Λ isapproximated using Sodd determined from measured magnetic momentum µmag , asaodd Sodd,J
l Jµmag − godd
Sodd ≈ s,lgodd − goddΛ≈ls
where subscript ”odd” refers an unpaired nucleon; p(proton) or n(neutron). godd
and godd
arethe orbital and spin nucleon g-factor; gpl = 1, gnl = 0, gps = 5.586 and gns = −3.826. The list of
the spin J, measured µmag and the calculated Λ2 J(J + 1)/aodd (≡ λ2 J(J + 1)) are in Table 2.2.1
SD as
for some target nucleus. For experimental use, the SD cross section is written using σχ−p

SD
SD
σχ−N
= σχ−pµ2χ−N λ2 J(J + 1)
µ2χ−p

0.75

.

(2.27)

SD /σ SD and σ SD /σ SD for some target nucleus are shown in Figure 2.2.3 andValues of σχ−N
χ−p
χ−n
χ−N

Figure 2.2.4, respectively.
Isotope
1H
7 Li
11 B
15 N
19 F
23 Na
127 I
133 Cs
3 He
17 O
29 Si
73 Ge
129 Xe131 Xe
183 W

J
1/2
3/2
3/2
1/2
1/2
3/2
5/2
7/2
1/2
5/2
1/2
9/2
1/2
3/2
1/2

Abundance(%)
100
92.5
80.1
0.4
100
100
100
100
1.0 × 10−4
0.0
4.7
7.8
26.4
21.2
14.3

µmag
2.793
3.256
2.689
−0.283
2.629
2.218
2.813
2.582
−2.128−1.890−0.555−0.879−0.7780.692
0.118

λ2 J(J + 1)
0.750
0.244
0.112
0.087
0.647
0.041
0.007
0.052
0.928
0.342
0.063
0.065
0.124
0.055
0.003

unpaired nucleon
protonprotonprotonprotonprotonprotonprotonprotonneutronneutronneutronneutronneutronneutronneutronTable 2.2.1: The spin J, measured µmag and the calculated λ2 J(J + 1) for some target nucleus.

34

SD
SD
/ σDM-p
σDM-N

102

10
7

Li

19

F

23

Na

127

I

1

102

3

10
MD [GeV/c2]SD
SD/ σDM-n
σDM-N

SD
SD
Figure 2.2.3: σχ−N
/σχ−p
as a function of neutralino mass. Red, black, blue and green line shows target
7
19
isotope diﬀerence as Li, F, 23 Na and 127 I, respectively.

3

10

29

Si

73

Ge

129

102

Xe

131

Xe

183

W

10

1

10-1

102

3

10
MD [GeV/c2]

SD
SD
Figure 2.2.4: σχ−N
/σχ−nas a function of neutralino mass. Black, red, blue, pink, green line shows
target isotope diﬀerence as 29 Si, 73 Ge, 129 Xe, 131 Xe and 183 W, respectively.

35

2.3

Form factor

When the de Broglie wavelength h/q with the momentum transfer q is no longer large√
compared to the nuclear radius, where h is the Planck constant and q = 2MN ER is themomentum of the recoil nuclei for a recoil energy of ER . The eﬀective cross section begins tofall for large q cases. This eﬀect is referred to as a ”form factor”, F , F is a function of thedimensionless quantity, qrn , where rn is an eﬀective nuclear radius. The eﬀective neutralinonucleus cross section is corrected with this form factor asσ(qrn ) = σ0 F 2 (qrn ),(2.28)

where σ0 is the cross section at a zero momentum transfer. In the ﬁrst Born (plane wave)
approximation, the form factor is given by Fourier transformation of the density distributionof the ’scattering centres’, ρ(r), as∫
F (qrn ) ==ρ(r)exp(iq · r)dr∫
4π ∞r sin(qr)ρ(r)dr.
q 0

For the SI interaction, since neutralino would interact with any of the nucleon, the nuclei
can be approximated as a solid sphere. In this case, the form factor is written using Bessel
function ji (x) = (sin(x) − x cos(x))/x2 as
(
)
(qs)2
j1 (qrn )exp −,F (qrn ) = 3
qrn
2(2.29)

where rn ≃ 1.14A1/3 fm, and s ≃ 0.9 fm representing the nuclear skin thickness. Figure 2.3.1
shows the SI form factors of some isotopes as a function of the recoil energy ER . Expectedenergy spectra of SI interaction with 19 F target is shown in Figure 2.3.2, where the form factor
is taken into account.
For the SD interaction, since neutralino would interact with the surplus nucleon, the nuclei
can be approximated as a thin shell, and this assumption is called the odd-group model. In
this case, the form factor is written using Bessel function j0 = sin(x)/x as
F (qrn ) = j0 (qrn ),(2.30)

where rn ≃ 1.0A1/3 fm. Figure 2.3.3 shows the SD form factors of some isotopes as a functionof the recoil energy ER . Expected energy spectra of SD interaction with 19 F target is shownin Figure 2.3.4, where the form factor is taken into account.
36

F2

1

10-1

19

F

74

Ge

132

Xe

10-2

-3

10

10-4

-5

10 0

50

100

150

200

250 300 350 400
recoil energy [keV]

spectrum [counts/keV/kg/days]

Figure 2.3.1: The SI form factors of some isotopes as a function of the recoil energy ER . Pink, blue andgreen lines show target isotopes of 19 F, 74 Ge and 132 Xe, respectively.

19

F MD=50GeV

-3

10

19

F MD=100GeV

19

F MD=200GeV

10-4

-5

10

-6

10 0

20

40

60

80 100 120 140 160 180 200
recoil energy [keV]SI
Figure 2.3.2: The expected energy spectra of SI interaction with 19 F target, where σχ−p
= 10−6 pb
is assumed. Red, green and blue lines show the cases for the neutralino mass of MD = 50, 100 and
200 GeV/c2 , respectively.

37

F2

1

19

F

10-1

7

Li

127

I

10-2

-3

10

10-40

50

100

150

200

250 300 350 400
recoil energy [keV]

spectrum [counts/keV/kg/days]

Figure 2.3.3: The SD form factors of some isotopes as a function of the recoil energy ER . Green, pink
and blue lines show target isotopes of 7 Li, 19 F and 127 I, respectively.

1019

F MD=50GeV

19

F MD=100GeV

1
19

F MD=200GeV

10-1

10-2

-3

10 0

20

40

60

80 100 120 140 160 180 200
recoil energy [keV]SD
Figure 2.3.4: The expected energy spectra of SD interaction with 19 F target, where σχ−p
= 1 pb isassumed. Red, green and blue lines show the cases for the neutralino mass of MD = 50, 100 and
200 GeV/c2 , respectively.

38

2.4

Dark matter signal

We need to observe a distinct signature in order to claim a clear detection of the dark
matter because the expected spectrum itself has featureless exponential shape as shown in
Figure 2.3.2 and Figure 2.3.4. Distinct signatures can be caused by the nuclear-dependence ofthe neutralino-necleus reactions, the relative motion of Solar system to the galactic halo, andthat of the Earth to the galactic halo. 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 the spectrum• Directional dependence of the event rate
These signals are discussed in the following subsections.

2.4.1

Nuclear target dependence

The shape of the expected energy spectrum dR/dER depends on the isotopes for the following reasons. First, the mass number A aﬀects the reduced mass as Equation (2.11), thus
the expected event rate becomes high for the target mass close to the assumed neutralino mass.
Second, the mass number and the reduced mass aﬀect the SI cross section as Equation (2.22),
thus the expected event rate becomes higher for larger mass number isotopes. Third, theλ2 J(J + 1) term and the reduced mass aﬀect the SD cross section as Equation (2.27), thus
some isotopes with the large λ2 J(J + 1) term, such as 19 F, 3 He and so on, have large SD
cross sections. Fourth, the mass number A aﬀects the form factor as Equation (2.29) andEquation (2.30), thus the eﬀective cross section become small for large mass number isotopes.
Figure 2.4.1 and Figure 2.4.2 are the expected energy spectra dR/dER of various targets for
the SI-interacting and the SD-interacting neutralino, respectively. Here MD = 100 GeV/c2 ,SI
SD = 1 pb are assumed. From Figure 2.4.1 and Figure 2.4.2, it canσχ−p= 1 × 10−6 pb and σχ−pbe judged that the diﬀerence of the event rate between isotopes can be a distinct signal of dark
matter. It should be noted, however, the background also depends on the isotopes, thus it isdiﬃcult to claim a dark matter detection strongly by using only this signal.

2.4.2

Annual modulation

The annual modulation of the event rate is caused by the Earth’s motion around the
Sun as described in Equation (2.1). The velocity of the Earth relative to our galaxy, vE ,39

event rate [counts/keV/kg/days]

SI, MD=100 [GeV], σ=1e-6 [pb]
19

F

22

Na

10-2

39

Ar

74

Ge

127

I

132

Xe

-3

10

10-4
0

20

40

60

80
100
120
recoil energy [keV]

Figure 2.4.1: The expected energy spectrum dR/dER of various targets for the SI-interacting neutralino
SI
with a mass of MD = 100 GeV/c2 and σχ−p
= 1 × 10−6 pb.

event rate [counts/keV/kg/days]

SD, MD=100 [GeV], σ=1 [pb]
19

F

7

1

Li

23

Na

73

Ge

127

I

10-1

131

Xe

10-2

10-3
0

20

40

60

80
100
120
recoil energy [keV]

Figure 2.4.2: The expected energy spectrum dR/dER of various targets for the SD-interacting neutralinoSDwith a mass of MD = 100 GeV/c2 and σχ−p
= 1 pb.

40

become maximum in June 2nd and minimum in December 4th, and the order of modulation
size is less than O(vorb /vsun ) ∼ 5%. Figure 2.4.3 illustrates the expected modulation of theenergy spectrum between June and December, where the target is 19 F for the SD interaction,
SD = 1 pb. In order to detect such a tiny annual modulation, we
MD = 100 GeV, and σχ−pneed to take large statistics using a large mass detector such as solid or liquid detector. The

spectrum [counts/keV/kg/days]

experimental result of annual modulation will be discussed in section 3.1.1.

2.2

Jun

2
1.8

Dec

1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0

20

40

60

80

100 120 140 160 180 200
recoil energy [keV]

Figure 2.4.3: The expected modulation of energy spectrum between June and December, where theSD= 1 pb.
target is 19 F for the SD interaction, MD = 100 GeV, σχ−p2.4.3

Directionality

The most convincing signal of the dark matter would be seen in the directional distributionof the recoil nucleus. Since the Cygnus constellation is seen in the forward direction of the
Solar system’s motion, dark matters would seem to be coming from the Cygnus direction like
”WIMP-wind”. Since the Cygnus direction is varying per hour and per day, the systematic
error due to the daily and seasonal environmental changes will be canceled.
In the laboratory system, expected angular spectrum of recoil nucleus is written as
]
[
d2 R
1 R0
(vE cos θ − vmin )2,(2.31)≃exp −dER d cos θ
2 E0 rv02
where angle θ is formed by the WIMP-wind direction and the recoil nuclei direction[36]. Figure 2.4.4 shows the expected angle-energy distribution, where the target is 19 F for the SD
SD = 1 pb. Figure 2.4.5 shows the expected cos θ distriinteraction, MD = 100 GeV, and σχ−pbution obtained by selecting the events with recoil energy of 100 − 120 keV in Figure 2.4.4.
41

Since an isotropic background makes ﬂat cos θ distribution, asymmetric cos θ distribution inrecoil energy [keV]

Figure 2.4.5 can be a strong evidence of the dark matter detection.

400

1.8

350

1.6

300

1.4
1.2

250

1
200
0.8
150
0.6
100

0.4

50

0.2

0
-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1cosθ

0[count/keV/kg/days/cosθ]

Figure 2.4.4: The expected angle-energy distribution, where the target is 19 F for the SD interaction,SD= 1 pb, θ is the angle formed by the WIMP-wind direction and recoil nuclei
MD = 100 GeV, σχ−pdirection. The color bar represent the event rate with an unit of [count/keV/kg/days/ cos θ].

0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
-1 -0.8 -0.6 -0.4 -0.2 -0

0.2 0.4 0.6 0.8 1
cosθ

Figure 2.4.5: The expected cos θ distribution (100 − 120 keV), where the target is 19 F for the SD
SDinteraction, MD = 100 GeV, σχ−p
= 1 pb, θ is the angle formed by the WIMP-wind direction and therecoil nuclei direction.

42

Chapter 3

Review of Direct Dark Matter
SearchA number of experiments have been carried out to detect WIMPs by observing neutralinonucleus elastic scatterings. In direct search experiments, detectors are designed to detect recoil
energy of typically less than 100 keV via photons, phonons or charges. In general, direct dark
matter detectors should have following three characteristics.
• Low energy threshold
The shape of the expected energy spectrum is exponential-like (Section 2.1). Then,the threshold energy is lower, the more signal is expected. A relevant energy region isconsidered to be below 100 keV.
• Large mass
Since the direct detection of the dark matter is a rare event search, a large target massis required to accumulate suﬃcient statistics in a reasonable observation time. Typicalrequired mass is from kilogram to ton for annual modulation detection method.
• Low backgroundTypical techniques of low radioisotope background such as a material selection andshielding are important to WIMP detectors. Basically, the detector is settled in theunderground laboratories to reduce the background induced by cosmic-rays. In addition,
particle identiﬁcations using adittional signals are useful to discriminate backgrounds.
Direct dark matter search experiments are roughly categorized into two types in termsof the signals obtained; the conventional type and the direction-sensitive type. Whereas aconventional detector measures only the energy of the nuclear recoil, a direction-sensitive type43

also obtains the directional information of nuclear recoil track which can provide a strong
evidence of the dark matter detection (Section 2.4.3). A gaseous detector with high position
resolution is required the direction-sensitive search in order to detect a short track less than
1 mm in atmospheric pressure gas. Almost all groups in the direct dark matter search ﬁeld adopt”conventional method” which needs large mass detectors. On-going direct search experiments
are listed in Table 3.0.1. Direct detection experiments are reviewed in this chapter.
Experiments
DAMA/LIBRA
NAIAD
KIMS
DAMA/LXe
XMASS
XENON100
LUX
WARP
CDMS
CoGeNT
CRESSTDAMICCOUPPSIMPLEPICASSO
DRIFT
DM-TPCNEWAGEMIMAC
NIT-group
DNA-group

Detector (material)signalConventional type
Solid scintillator (NaI(Tl))
photonSolid scintillator (NaI(Tl))
photonSolid scintillator (CsI(Tl))
photonLiquid scintillator (Xe)photonLiquid scintillator (Xe)photonLiquid scintillator (Xe)photon, charge
Liquid scintillator (Xe)photon, charge
Liquid scintillator (Ar)photon, charge
Bolometer (Si,Ge)
phonon, charge
Bolometer (Ge)phonon, charge
Bolometer (CaWO4 )phonon, photon
CCD (Si)chargeBubble chamber (CF3 I)
chargeBubble chamber (C2 ClF5 )chargeBubble chamber (C4 F10 )chargeDirection-sensitive type
Gasous TPC (CS2 )chargeGasous TPC (CF4 )chargeGasous TPC (CF4 )chargeGasous TPC (CF4 )chargeEmulsion (AgBr, C,N,O)
chargeDNA Tracking Chamber (Au)chargeref[37, 38, 39, 40]
[41]
[42]
[43]
[44]
[45, 46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
[54]
[55]
[56]
[57]
[58]
[59]
[60]
[61]

Table 3.0.1: Direct dark matter search experiments.

3.1

Conventional experiment

Almost all direct dark matter search experiments use solid or liquid detectors to obtaina large target mass. The recoil energy is converted to photon, heat and ionization. From
the diﬀerence of the signals, the detectors are categorized as follows; solid scintillators andliquid noble gas scintillators for detecting only the photon, liquid noble gas scintillators for44

detecting the photon and ionization, semiconductor bolometers for detecting the ionizationand heat, crystal bolometers for detecting photon and heat, bubble chambers for detecting theionization. The results are summarized in Figure 3.1.1. Details on the conventional detector
experiments are described in the following subsections.

3.1.1

Solid scintillator

Scintillators have advantages in constructing large mass detectors and well-studied theirproperties. Many types of scintillators, such as NaI(Tl), CsI(Tl), have been used for dark
matter searches.
DAMA group conducted a dark matter search with a highly radio-pure 100 kg NaI(Tl) setup
for seven years and claimed to have detected an annual modulation signature due to the dark
matter[40]. Allowed parameter regions of WIMPs are shown as enclosed regions in Figure 3.1.1.
They also reported new results from another seven years observation with a highly radiopure 250 kg NaI(Tl) setup (DAMA/LIBRA), which reproduced the DAMA annual modulation
signature as shown in Figure 3.1.2[37]. The DAMA/LIBRA experiment accumulated the largest
exposure (1.17 ton · year) and claimed a positive signature of the dark matter. However, there
is also the opinion that this annual modulation signal is arised from systematic error due to aseasonal ﬂuctuating background becacuse several experiments have obtained lower cross section
limits. Experiments using NaI in other sites[62, 63] are one method to conﬁrm the DAMA’s
signal. In addition, a direction-sensitive experiment can provide a distinct signal of dark matterwithout seasonal systematic error.
Other experiments using solid scintillators have been carried out. NAIAD[41] using 55 kg
of NaI(Tl) accumulated a 44.9 kg · year of exposure showed a sensitivity comparable to DAMA
without positive results. 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 hasa large λ2 J(J + 1). Shimizu et al.[64] reported a WIMP-search result with highly radiopure
CaF2 crystals. KIMS[42] using 34.8 kg of CsI(Tl) set a better SD limits with a 3490 kg · daysof exposure.

3.1.2

Liquid Noble-gas scintillator

Liquid noble gases are excellent scintillators, and furthermore, simultaneous ionization signals can be collected with an existence of a certain electric ﬁeld. Particularly, liquid xenon(LXe) have a good intrinsic properties; large mass number, high density, high radio purity, andhigh light output. Though the mass number of liquid argon (LAr) is less than that of LXe,45

SI 90% C.L. upper limits and allowed region
10-1
10-2
DAMIC

(Si)201
2

-3

4F
O(C
ASS

AS
)
Xe
1
20

ed

w
llo

-5

10

13

10-6

G

eN

T(

a

20
e)

G

Co

10-7
10-8

CR

-9

E

Ca
T(
SS

W

5
I)200
D(Na
)2011
NAIA
5
ClF
E(C2
L
P
SIM
08
(Ar)20
WARP
2012
F3I)
P(C
P
U
CO
0
)201
S(Ge
CDM
12
e)20
00(X
1
N
O
XEN
2013
(Xe)
LUX

3

σSI[pb]

PIC

S(

10-4

012

10)2

XM

10

w
llo
)a
I
Na wedA(
M a llo
A
1
D
01
)2
4
O

ed

10

1

10

Mχ [GeV/c 2]

3

102

10

SD 90% C.L. upper limits and allowed region
102
DAMA allowed(NaI)

000

e)2

X
Xe(

/L

10

DA

MA

06
) 20
9
aF2
)200
C
(
(Ge
o
2
y
S
Tok
CDM

0
e)20

8

10-2

12
I)20 11
CF3 F5)20
(
P
l
C
UP
CO LE(C2
P
S IM

σSD
χ-p [pb]

10-1

005
aI)2
D(N
A
I
NA
007
aI)2
12
S(N
)20
KIM
F10
4
C
(
SO
AS
PIC

1

0(X
ON1

XEN

10

102
Mχ [GeV/c 2]

3

10

Figure 3.1.1: Results of direct dark matter search experiments. Upper and lower ﬁgures are searchesof SI and SD χ-p interacting WIMPs, respectively See Table 3.0.1 for references. Region enclosed byline is allowed region (DAMA). Other upper regions of lines are excluded regions. Color indicates thetype of detector; blue for solid scintillator, purple for liquid noble gas detector, green for semiconductor
detector, and sky blue for bubble chamber.

46

Figure 3.1.2: The observed annual modulation of the residual event rate of 2 − 6 keV energy range
acquired over 13 annual cycles by DAMA and DAMA/LIBRA with a total exposure of 1.17 ton·year[37].
The horizontal axis shows the days from DAMA/NaI started. Solid line is the best ﬁt sine curve.

LAr has the advantage of being low cost. There are two types of liquid noble-gas dark matter
detectors. One is a single phase detector which detects only a scintillation signal by photon
detectors. Single phase detectors are constructed as simple as possible to reduce the background from detector itself. Environmental gamma-ray background can be rejected by the selfshielding, and a pulse shaping analysis is used for the nuclear recoil discrimination. The otheris a two phase detector which has a gas phase in addition to the liquid phase and detects not
only the scintillation signal but also the ionization signal. The ionization signal is detected asa gas ampliﬁcation scintillation signal in the gas phase caused by the primary electron driftedout of the liquid phase. The ionization signals for nuclear recoils are quenched much morethan the primary scintillation signals, while the ionization signals for electron recoils are notquenched so much. Therefore, the ratio of the ionization signal and the primary scintillation
signal provides a powerful electron discrimination power.
DAMA/LXe[43] and XMASS are the single phase detectors using LXe. DAMA/LXe using
6.5 kg of liquid 129 Xe performed the dark matter search with a 2500 kg · days exposure. As
two phase detectors using LXe, there are LUX[47] and XENON[45]. LUX using 118 kg of LXe
performed a dark matter search with a 10065 kg · days exposure and obtained the best limit
for the SI interacting WIMP in the mass range of 10 − 1000 GeV. WARP collaboration has
built and deployed a 3.2 kg LAr detector at LNGS and performed a dark matter search withan exposure of 96.5 kg · days[48].

3.1.3

Semiconductor detector (bolometers, CCD)

In the direct search using bolometers, the nuclear recoil energy is detected via phonons(temperature rising). The heat capacity of a dielectric crystal is proportional to T 3 at the47

low temperature, thus at the mK temperature, the small energy deposition from a nuclear
recoil can yield a measurable increase of a target crystal. Since the electron-hole pairs is
also generated at the same time in the semiconductor bolometers, electron background can bediscriminated by using the diﬀerence of ionization/phonon ratio between electrons and nucleus.
In addition, some groups have developed ”scintillating bolometers” which detect scintillation
photons together with heat as an alternative method for the electron event rejection.
CDMS-II experiment operated towers of Ge and Si crystals with a thickness of 1 cm cooled
down to 40 mK[49]. The mass of the Ge and Si crystals were 250 g and 100 g, respectively.
Since the depth of event can be measured from the diﬀerence of detecting time of phonon andcharge, CDMS detector can reject surface electron events. They accumulated an exposure of612 (140.2) kg · days with Ge (Si) detectors, after the event selection, 2(3) events remained for
the Ge(Si)-run, but it can be explained by the background and their statement is ”Though thisresult favors a WIMP interpretation over the known background-only-hypothesis, we do notbelieve this result rises to the level of discovery.” and further investigation is needed. CoGeNT
project uses 443 g of p-type point contact Ge detector with a low-threshold of 0.4 keV, andperformed a dark matter search with an exposure of ∼ 200 kg · days[50] . In the low energy
region, the annual modulation of event rate was observed for over one year at ∼ 2.8σ, andobtained the allowed region at low mass region as shown in the upper panel of Figure 3.1.1.
CRESST-II experiment uses 10 kg of CaWO4 crystals and read two types of signals, a phonon
signal and coincidently produced scintillation light for electron discrimination[51]. After an
exposure of 730 kg · days, due to the excess of the event rate as expected for background,
CRESST-II set the allowed region as shown in the upper panel of Figure 3.1.1. In order toexplain the ”positive” results described above, a low mass dark matter (∼ 5 GeV) was supposed.
However, the low mass region is diﬃcult to discuss because the sensitivity changes rapidly due
to the energy threshold. DAMIC experiment uses low noise CCD with a very low energy
threshold of 40 eV, and is dedicated to the search for low-mass WIMP[52]. They performed adark matter search using a CCD of 1 g with an exposure of 1.5 kg · day, and obtained the best
limit in the low-mass region.

3.1.4

Bubble chamber

Bubble chambers are the detector made of superheated liquids. Bubble chambers are kept
in a delicate metastable superheated state, and ionizing radiation particle makes a local vapor
phase with the energy deposition along its path. Rapidly growing bubbles are photographed
and the chamber is then reset by a fast recompression to the metastable liquid phase. These
devices have advantages of the electron discrimination, a large mass, and a target selection for48

the SD WIMP searches. But they have disadvantage of being a threshold-type detector, or noway to to detect the nuclear recoil energy. The superheated liquid can be tuned to respondexclusively to particles with a large stopping power, so that muon and electron events all fall
well below the bubble nucleation threshold.
COUPP[53], SIMPLE[54], PICASSO[55] experiments applied a superheated droplet detector for the direct dark matter search using piezoelectric sensors as the trigger. COUPP
performed a dark matter search using 4.0 kg of CF3 I bubble chamber with an exposure of553 kg · days. SIMPLE performed a dark matter search using 0.215 kg of C2 ClF5 bubble chamber with an exposure of 13.67 kg · days. PICASSO performed a dark matter search using 0.72 kg
of 19 F in C4 F10 bubble chamber with an exposure of 114 kg · days. Since 19 F large λ2 J(J + 1)
term is used for a target of these detectors, they have strong limit for SD interaction.

3.2

Direction-sensitive experiment

Direction-sensitive experiments are to measure the incoming direction of WIMPs with detectors to measure the direction of nuclear recoil tracks. Typical length of the nuclear trackwith an energy of 100 keV is less than 1 mm in the gas at 1 atm pressure. Thus, low pressure gaseous detectors are used in many cases. Detectors using emulsion or DNA which haveboth directionality and large-mass target have also been studied. Furthermore, by measuring
the recombination of the drift electron, high pressure noble-gas detector may provide somedirectional information[65].

3.2.1

Gaseous detector

Low pressure gaseous time-projection-chambers, commonly coupled with micro-patterned
gaseous detectors (MPGDs), namely µ-TPCs, are widely studied for direction-sensitive dark
matter searches. The requirement for the use of low pressure gas implies large volume detectorsof more than 1 m3 will eventually be needed. Although the target mass density is ∼ 1/1000
times less than that of conventional detectors, the directional signal can be a strong evidenceof the detection of the dark matter.
Typical feature of gaseous detectorsµ-TPC obtains a track of a charged particle in the following sequences. The detector gas
is ionized along the path of a charged particle, ionized electrons drift along the electric ﬁeldthat is applied along the z-axis in the gas, undergo gas ampliﬁcation if necessary, then reach49

the 2-dimensional detector. Using x and y determined by the 2-dimensional detector and z
determined by the time when the electron reaches to the 2-dimensional detector. A three
dimensional track is obtained.
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
recoil from the neutron recoil in principle. Since hydrogen has a large cross-section to neutron,a gas which contains one or more hydrogens (e.g. CH4 , C2 H6 ) should be avoided. The noble
gases such as Xe and Ar are often used in a gas detector, in particular, the sensitivity of SI
interaction is high for Xe due to the large mass number. However, the diﬀusion of electron
during drift in the noble gas is very large and it requires ﬁne optimizations to construct alarge detector measuring ﬁne tracks. MAGBOLTZ[66] calculations of the diﬀusion constantfor several gas are shown in Figure 3.2.1. A negative ion time projection chamber (NITPC),
the primary electron is rapidly captured by electron negative gases such as CS2 and drift as
the negative ions drift along an electric ﬁeld, has potential to realize a very small diﬀusion.
It is because drift ions remain in or near thermal equilibrium with the gas, and diﬀusions aresuppressed to the thermal levels. CF4 gas, although it is not an electron negative gas, has small
electron diﬀusions because the electron drift velocity in CF4 gas is high as shown in Figure
3.2.2. 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 an advantage to the SI interaction search with large
mass number of 32 S and CF4 gas has an advantage to the SD interaction search with a large
λ2 J(J + 1) term. The properties of CF4 gas are summarized in Table 3.2.1.
a chemical formula
nameappearance
molecular weight
densitymelting point
boiling point
W valueCF4
carbon tetraﬂuoride
colorless, odorless, inert and nonpoisonous
88.01 g/mol
3.76 g/l (15◦ C, 1 atm)
89.55◦ K
145.35◦ K
54eV

Table 3.2.1: Properties of CF4 gas.

Direction-sensitive detector should have a position resolution better than a typical nuclear track length of interest. The lengths of nuclear tracks in several types of gas calculatedby SRIM[67] as a function of the initial energy are shown in Figure 3.2.3. In the lower pressuregas, longer tracks are expected and thus a lower energy threshold is realized. But low pressure gas detector needs a large volume and also often causes the unstability of the operation.
50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 3.2.1: Gas diﬀusions of electrons or negative ions as a function of the electric ﬁeld at a temperature of 300K calculated by MAGBOLTZ. Dotted and Solid line are longitudinal and transverse
diﬀusion, respectively. Color shows the molecular; red, blue and black correspond to CF4 , Xe and CS2 ,respectively.

We need to overcome these diﬃculties in order to detect the very distinct signals of the dark
matter.
It is important to consider a nuclear quenching factor for low energy nuclear recoils. Nuclear
quenching factor, Fq = Eionized /Etotal , is the ratio between total energy loss and energy used
for ionization. Nuclear quenching factors for several gas detectors were summarized in [68].
The nuclear quenching factors of 4 He, 12 C and 19 F ions in CF4 gas were calculated by SRIM asshown in Figure 3.2.4. These values have nucleus-, energy- and also gas pressure-, dependence
as shown in the ﬁgure. They are used when we convert the measured (ionized) energy to therecoil (total) energy in dark matter search experiments.
Experiments with gaseous detectors
We review the experiments with gaseous detectors and development status in the following
paragraph.
DRIFT experiment pioneered the study of the directional detection of the dark matter with
1 m3 of gas detectors.

In particularly, they have been leading the ﬁeld with the background51

Drift Velocity (µ m/ns)

140

CF4
Xe

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 3.2.2: Drift velocities of electrons as a function of electric ﬁeld calculated by MAGBOLTZ for
CF4 (red) and Xe(blue).

mm

102

10

1

12

C in 0.01atm CF4
F in 0.01atm CF4
12
C in 0.1atm CF4
19
F in 0.1atm CF4
12
C in 1atm CF4
19
F in 1atm CF4
19

10-1

10-2
0

50

100

150

200

250

300

350

400
keV

Figure 3.2.3: Track length of recoil 12 C(dotted line) and 19 F(solid line) in CF4 gas calculated by SRIM
with each pressure (blue:0.01atm, red:0.1atm, black:1.0atm).

52

quenching factor Fq

100
90
80
70
60
50

He 0.1atm

40

He 1.0atm

30

C 0.1atm
C 1.0atm

20

F 0.1atm

1000

F 1.0atm

100 200 300 400 500 600 700 800 900 1000
keV

Figure 3.2.4: Quenching factor of various nucleus in CF4 gas calculated by SRIM[67]. Each line shows
diﬀerent nuclei-pressure combination as shown in the legend.

reduction techniques[56]. They performed a dark matter search with DRIFT-IId detector for
47.4-days live time with 139 g of a mixture of 30 torr CS2 and 10 torr CF4 gas. They conducted
a ﬁne-tuned cut analysis without direction information and obtained comparable limits to solid
detectors, 1.8 pb for a WIMP mass of 100 GeV/c2 (Figure 3.2.5). They are also starting R&Ds
to use MPGD readouts in order to overcome their potential weakness of the readout pitch of
2 mm.
DMTPC collaboration proposed a new method to detect the recoil tracks using a CCDreadout TPC[57]. 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.
Although only 2 dimensional projected track is obtained instead of 3 dimensional track due tothe mechanism of the readout, their technique is very notable for the detection of the track
sense. In 2011, they reported the surface dark matter search result, and improved the SD
direction-sensitive limit to 2.0 × 103 pb for 115 GeV/c2 dark matter (Figure 3.2.5).
MIMAC project[59] and D3 project [69] are also developing the gaseous tracker. MicroMEGAS[70]is used for MIMAC, and GEMs[71] with an ATLAS FE-I3 pixel chip[72] are used for D3.
NEWAGE with CF4 gas and a ﬁne pitch gaseous detectors has obtained ﬁrst directionsensitive limits in 2007, and improved to 5400 pb for WIMP mass of 150 GeV/c2 in 2010

53

(Figure 3.2.5)[58]. Properties of the current and developing detector of each project are in
Table 3.2.2. NEWAGE has advantages in the readout pitch and the eﬀective volume against
DRIFT and DMTPC, respectively.

Figure 3.2.5: Results of direction-sensitive direct dark matter search experiments. In left ﬁgure, blacksolid line shows the result of DRIFT(2012)(without direction-sensitive analysis)[56]. In the central
ﬁgure, black solid line shows the result of DM-TPC(2011)[57]. In right ﬁgure, red solid line shows theresult of NEWAGE(2010))[58]. For more information, see references.

detector
NEWAGE-0.3a
NEWAGE-0.3b’
DRIFT-IId
DRIFT-III
DM-TPC
DM-TPC 2

MIMAC

gas
CF4
CF4
75%CS2
25%CF4CS2
CF4
CF4
70%CF428%CHF3
2%C4 H10

pressure
0.2 atm
0.1 atmvolume (cm3 )23 × 28 × 31
30 × 30 × 41

deviceµ-PICµ-PIC

readout
pitch
400 µm
400 µm

track
3D
3D

headtailnono

40 torr
40 torr
50 torr
50 torr

100 × 100 × 100
100 × 100 × 100
14.6 × 14.6 × 19.7
ϕ25 × 25

MWPC
MWPC
CCD
CCD

2 mm
2 mm
256 µm256 µm

3D
3D
2D
2D

1D
1D
2D
2D

50 mbar10 × 10 × 25

micro
MEGAS

424 µm

3D

no

ref
[58]
this thesis

Table 3.2.2: Properties of gaseous detectors for direction-sensitive dark matter searches.

3.2.2

Emulsion

Nuclear emulsions. though they are solid detector, can potentially be used for direction
sensitive dark matter experiments because of their good position resolutions. Emulsion has
AgBr in gelatin, and tiny cluster of Ag is generated along the pass of a charged particle. Then,by developing the cluster of Ag, a track of charged particle is observed by microscope. Althoughemulsions have no time resolution, they have advantage in scalability. Nano imaging tracker(NIT) have been developed to detect the ﬁne track such as <∼ µm at Nagoya university[60],
54

[56]
[57]

and it is suitable for a direction-sensitive dark matter search. In addition, an undergroundexperiment at Gran Sasso is scheduled.

3.2.3

DNA Tracking Chamber

Detectors made of DNA (deoxyribonucleic acid) could provide nanometer resolution for
tracking and energy threshold of 0.5 keV[61]. The structure of DNA tracking chamber is thatthousands of strings of single stranded DNA (all with known base sequences) are hung in aplate of Au side by side like a curtain. When a charged particle comes into this detector,
DNAs are cut along the path of charged particles. DNAs that were cut are ampliﬁed and readby using well-known techniques of molecular biology. Of course DNA detector needs vigorous
R&D, it can be an ultimate detector with the highest position resolution.

3.2.4

High pressure Xe detector

Nygren proposed a new concept based on columnar recombination in dense xenon gas,sensing nuclear recoil direction relative to a TPC drift ﬁeld[65]. The central advance is that kindof directionality information is obtained through a comparison, event-by-event, of the ionization
signal and recombination signal that are produced prior to drifting the track ionization. The
optimum xenon density may be about ten bars, much higher than that of low pressure gas
detectors, and large mass experiment can be performed.

55

Chapter 4

Detector
NEWAGE (NEw general WIMP search with an Advanced Gaseous tracker Experiment)
is one of the direction-sensitive direct dark matter search experiments. As mentioned in Section 2.4.3, directional information is certainly a strong evidence of the discovery of the dark
matter. NEWAGE adopts a three dimensional gaseous tracking detector µ-TPC whose readout
is a µ-PIC, one of the micro patterned gaseous detectors (MPGDs). The µ-PIC has two advantages; ﬁne tracking and scalability. The ﬁner pitch of µ-PIC (400 µm) than that of MWPC
(2 mm), realizes a higher direction sensitivity. The large size µ-PIC can be madeat a low costbecause it is made by print circuit board technology. CF4 is selected for the target gas because
the gas diﬀusion is small and ﬂuorine has a relatively large SD cross section. There has beenno direction-sensitive detector with a sensitivity reaching to the MSSM region yet. Final goalof NEWAGE is to detect a strong evidence and to study the properties of the dark matter byreaching the MSSM region, in particular as shown in Figure 4.0.1[73].
Previous measurements were performed in Kamioka with NEWAGE-0.3a detector which
improved the direction-sensitive limit of spin-dependent WIMP-proton cross section down to5400 pb for 150 GeV[58]. In that experiment, the gas pressure was set to 0.2 atm, the energy
threshold was 100 keV with an angular resolution of 46◦ and an old data acquisition protocol(DAQ-mode1, show Section 4.2) was used. We developed a new detector, NEWAGE-0.3b’, toimprove the cross section limit by more than one order of magnitude by reducing the backgroundand increasing sensitivity with a lower energy threshold.
In this chapter, details of NEWAGE-0.3b’ detector are described. The structure of the detector is described in Section 4.1, and the data aquitision system is explained in Section 4.2. The
simulator is written in Section 4.3, and the typical performances are described in Section 4.4.

56

Figure 4.0.1: 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 [73]. An experimental result of NAIAD (thin dashed dotted line labeled UKDMC) andMSSM predictions (thin line labeled MSSM) are also shown [41, 74].

4.1

NEWAGE-0.3b’ detector

A new detector, NEWAGE-0.3b’, was designed and developed to improve the sensitivity by
one order of magnitude. NEWAGE-0.3b’ consists of a micro time projection chamber (µ-TPC),
its electronics system, and the gas circulation system (Figure 4.1.1).

Figure 4.1.1: The photograph of whole system of NEWAGE-0.3b’ in Kamioka underground laboratory.

57

Figure 4.1.2: The schematic image of the µ-TPC named NEWAGE-0.3b’. The coordinate system, with
its origin at the center of the detection volume, is also shown. Red, blue and green lines shows the x, y
and z axis. A glass plate with a thin layer of 10 B is set at (−5, 0, −12) cm for the energy calibration.

A schematic view of the µ-TPC and the internal structure are shown in Figure 4.1.2 andFigure 4.1.3, respectively. µ-TPC consists of a micro pixel chamber (µ-PIC) which is a twodimensional ﬁne-pitch imaging device[75], a gas electron multiplier (GEM)[71], and 30 × 30 ×
41 cm3 of detection volume. x, y and z axis are deﬁned in Figure 4.1.2, and the unit is cm inthis thesis unless otherwise mentioned.
A µ-PIC is manufactured using printed circuit board (PCB) technology. PCB technology
realizes an economical large-size detector production, which is one of the most important requirements for the fabrication of a dark matter detector. The 30.72 × 30.72 cm2 -sized µ-PIC
has 768 × 768 pixels with a pitch of 400 µm, which are connected by 768 anode strips and 768
cathode strips. The anode and cathode strips are orthogonally formed and thus we obtain thetwo-dimensional position of a hit pixel. The photographs and schematic the structure of theµ-PIC are shown in Figure 4.1.4. The µ-PIC works as a main gas-ampliﬁer and as readout
electrodes. In order to obtain a suﬃcient gas gain, a GEM is settled 4 mm above the µ-PIC
as a sub-ampliﬁer. The eﬀective area of the GEM is 31 × 32cm2 covering the whole area ofthe µ-PIC. The GEM area is segmented into 8 sub-areas to reduce discharge damages. The
substrate of the GEM is LCP of 100 µm thick and the hole size and pitch are 70 µm and 140 µm,
respectively. The drift length is 41cm, which was determined by the optimization between thetarget-increase advantage and the angular-resolution determination with a longer drift length.

58

Figure 4.1.3: The µ-PIC and the GEM during their installation (upper) and inner structure of TPC
including the 10 B-evapolated glass plate (lower).

The electric ﬁeld is formed by the drift plane and wires on the side walls of the drift region
with a spacing of 1 cm. Applied voltages for the µ-PIC, the GEM and the drift plane are listed
in Table 4.1.1, where a stable operation with a combined (µ-PIC×GEM) gas gain of 2500 was
obtained. A glass plate with a thin layer of 10 B is installed at the position of (−5, −12, 0) cm
for the energy calibration. The size of the 10 B layer is 2 × 2 cm and the designed thickness is
0.6 µm. The µ-TPC is placed on a 2.5 cm thick stainless-steel vacuum vessel ﬁlled with CF4
gas at 0.2 atm. As mentioned in Section 3.2.1, CF4 gas is suitable for a direction-sensitive spin
dependent dark matter search.

59

Figure 4.1.4: The photograph of µ-PIC (left top) and the schematic view of µ-PIC (left bottom). Upperright ﬁgure shows the enlarged photograph by microscope. Bottom right ﬁgure shows the cross-section
view of the µ-PIC structure.

µ-PIC
GEM-top
GEM-bottom
DRIFT

voltage
510 V−530 V−280 V
−3.91 kVcurrent∼ 10 nA< 1 µA< 1 µA
20.2 µA

Table 4.1.1: Supplied voltages for µ-PIC, GEM-top, GEM-bottom, and drift plane.

A gas circulation system with cooled charcoal was developed for NEWAGE-0.3b’. The
schematic drawing is shown in Figure 4.1.5. The aims of this system are to reduce radon,
which is a serious background source for dark matter searches, and to keep the gas quality (gasgain and drift velocity) during long time use (more than one month). The gas in the vessel
passes through the charcoal, where 100 g of charcoal (the product name is TSURUMICOAL2GS) absorb radon and impurities as well. The pump (XDS5 Scroll Pump (EDWARDS)) isan oil-free pump, and works with a low pressure gas at 0.1 atm. A needle-type valve was used
to control the ﬂow rate at 500 ml/min. A stable cooling at 230 K was realized by controllinga heater, while the cooler(CT-910 Cool Man Trap (SIBATA)) was always operated at its
maximum cooling power.

60

Figure 4.1.5: The schematic image of the gas circulation system for lowering radon backgrounds andkeeping the gas quality.

4.2

Data acquisition system (DAQ)

A data acquisition system (DAQ) with a dedicated electronics for the µ-PIC readout was
adopted for the NEWAGE-0.3b’ detector[76]. The schematic structure of the DAQ is shownin Figure 4.2.1. Two types data, namely ”track” by the memory board and ”charge” by the
ﬂashADC, are mainly recorded by the DAQ system.
The DAQ has two types of trigger modes. One is the ”self-trigger mode”, where the TPC
analog signals from 768 anode strips are grouped down to 16 channels and any one of their hits
is used as the trigger. In this trigger mode, the absolute z position is not measured, but only the
relative position is measured. The self-trigger mode is used otherwise mentioned in this thesis.
The other is ”external-trigger mode”, for a measurement using a 252 Cf source. The signal ofa prompt gamma ray or a neutron from 252 Cf detected by a plastic scintillator set nearby thesource is used as a trigger. In this mode, the absolute z position of the tracks are measured.
The drift velocity and the angular resolutions are measured with the external-trigger mode.
Two types of DAQ-mode for ”track” data with a diﬀerent measured parameters as listed
in Table 4.2.1 were used for this work. DAQ-mode5 records the addresses and time-over-61

Figure 4.2.1: Data acquisition system (DAQ) of NEWAGE-0.3b’ detector.

62

DAQ modex-y coincidence
strip address to record at each clock
Time over threshold (TOT)

mode1
take
Xmin , Xmax , Ymin , Ymax
not takemode5
not takealltakeTable 4.2.1: The list of the DAQ mode.

Figure 4.2.2: The schematic image of time-over-threshold (TOT). Red line shows the waveform of a
strip.

threshold of all hit strips (TOTs). TOT is the time duration of the waveform corresponding to
the energy deposit as shown in Figure 4.2.2. Here we use DAQ-mode5 with self-trigger mode.
DAQ-mode1 records by reducing the size of data by taking x-y coincidence and limiting thenumber of output addressed for the ”track” information. DAQ-mode1 was thus used for theexternal-trigger mode because the dead time of DAQ-mode5 is too long to operate with theexternal-trigger mode.
For ”charge” information, analog signal of 768 cathodes are ampliﬁed and grouped down
to 4 channels, then these waveforms are recorded by a 100 MHz ﬂash ADC. From the summed
waveforms (FADC-sum), the energy deposition of a charged particle is evaluated.

4.3

Simulator

A Monte Carlo simulation was used to evaluate the detector performance. The full process,starting from the generation of incoming particles, followed by the interaction of the particles
in the detector, the electron drift in the gas volume, the gas ampliﬁcation nearby the detection
plane, the ampliﬁcation and the discrimination by the preampliﬁer, the data reduction in the
FPGAs, and the data acquisition, was simulated.
A simulation package named Geant4[77], originally developed for high-energy physics experiments and now being used widely in related ﬁelds, was used for the generation, transportationand interaction of the incoming particles. Gamma-rays, electrons, neutrons, and α particles
were generated inside and outside of the detector for the performance evaluation (Section 4.4)63

and the background estimation(Section 6). The version and physics lists used in the simulator
are listed in Table 4.3.1.
The electron drift and the gas ampliﬁcation parts were simulated mainly based on MAGBOLTZ package[66]. MAGBOLTZ is a package to simulate the transportation of electrons
in gas mixtures. Electron clouds generated by a reaction in the detector volume are diﬀused
during the drifting towards the detection plane. The electron diﬀusion in the CF4 at 0.1 atm
gas was simulated in our code. The diﬀusion size orthogonal(σtransverse ) and parallel(σlongitudial )√√to the drift direction were 0.27 mm/ cm and 0.19 mm/ cm, respectively. The electrons werediﬀused, ampliﬁed in the GEM and around the µ-PIC pixels, then detected by the µ-PIC
detector. Positions and timings of the detected electrons were recorded.
After the charge detection, the data acquisition system was simulated by our original code.
The signal were ampliﬁed and discriminated by the preampliﬁres with the speciﬁcations listed
in Table 4.3.2. Then the DAQ system was simulated and the ”track” and ”charge” information
were recorded by the same format as the real data.
versionelectromagnetic process

physics lists

hadronic process

G4.9.2.p03
G4LowEnergyRayleigh
G4LowEnergyPhotonElectric
G4LowEnergyCompton
G4LowEnergyGammaConversion
G4LowEnergyIonisation
G4LowEnergyBremsstrahlung
G4eIonisation
G4eBremsstrahlung
G4eplusAnnihilation
G4hLowEnergyIonisation
G4EnergyLossTables
G4hIonisation
G4LElastic
G4LEProtonInelastic
G4NeutronHPElastic
G4NeutronHPElasticData
G4NeutronHPInelastic
G4NeutronHPCapture

Table 4.3.1: The version and physics lists used for the simulation.

64

parameterrise time
decay time
preamp gainmainamp gain

value
6 ns
70 ns
0.898 V/pC
×7

Table 4.3.2: The parameters of ASD used in the detector response included simulator.

4.4

Detector performance

The performance of NEWAGE-0.3b’ detector was precisely studied. Some of these performances were monitored during the dark matter run and summarized about its in Section 5.1.

4.4.1

Drift velocity

The electron drift velocity in the µ-TPC is needed to convert the arrival timing (clock) to
the zposition. We measured the nuclear recoil events in the µ-TPC using fast neutrons from
252 Cf source placed at (0, 0, 47.5).

In order to determine the starting time of drift, we used

external-trigger mode with a plastic scintillator as the external trigger detecting gamma rays
generated by ﬁssion of 252 Cf which simultaneously emits several fast neutrons. Because thetrigger rate of external-trigger is very high, we used DAQ-mode1 which works with less deadtime. Figure 4.4.1 shows the measured drift time distribution of nuclear recoils. The edge
structure at 4.3 µsec corresponds to the event occurred just below the drift plane. The drift

countsvelocity was calculated to be 9.5 cm/µsec since electrons took 4.3 µsec to drift 41 cm.

60
50
40
30
20
10
0
0

1

2

3

4

5

6

7

8

9

10
µsec

Figure 4.4.1: Drift time distribution of nuclear recoils in a drift velocity measurement.

65

4.4.2

Energy calibration and energy resolution

Since our purpose is to detect the recoil nuclei by dark matter, we calibrated the energy
scale of µ-TPC using α particles instead of γ-rays and β-rays to measure the detector responseto the nuclear tracks, because that is the case we expect in the dark matter nucleus scatterings.
A glass plate on which 10 B is deposited in 2 × 2 cm is placed at the position of (−5, −12, 0) cm
for the calibration (see Figure 4.1.2). By irradiating the 10 B plate with thermalized neutrons
from 252 Cf surrounded by polyethylene, α particles are generated in the µ-TPC by the following
reactions,10

B + n −→4 He +7 Li + 2.79 MeV(6%)

(4.1)

10

B + n −→4 He +7 Li + 2.31 MeV + γ(0.48 MeV)(94%).

(4.2)

Since the cross section of reaction (4.2) is ∼ 15 times larger than that of (4.1), neutron capture
reaction of 10 B emits prompt gamma-rays with 478 keV in most cases, and the 7 Li nuclei andα particle share 2.7 − 0.478 MeV, thus the generated α particle has 1.5 MeV of kinetic energy.
Because of the thickness (designed value was 0.6 µm) of the deposited 10 B, the 1.5 MeV α
particles make the shape of an edge rather than a peak. Figure 4.4.2 shows the result of theenergy calibration. By comparing the simulation (blue line) and measurement (red points), we

counts/keV/kg/days

determined the energy scale.

140

× 10

3

measurement120

simulation

100
80
60
40
20
0
0

500

1000

1500

2000

2500

3000
keV α.e.

Figure 4.4.2: The result of an energy calibration using α particles from 10 B(n, α)7 Li. Blue line issimulated energy spectrum and red points are measured one.

66

counts/keV/kg/days

0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0

2000

4000

6000

8000

10000 12000 14000
keV α.e.

Figure 4.4.3: The measured energy spectrum of α particles due to the radon progeny (blue histogram).
Red line is the ﬁtted line in case of 220 Rn.

In order to check the linearity, we checked the energy scale of ∼ 6 MeV in addition to
the 10 B calibration. The radon decay events within the TPC gas make peaks around 6 MeV.
Figure 4.4.3 shows the measured energy spectrum of α particles due to the radon progeny.
There are two types of radon as 220 Rn and 222 Rn. The energy of α particles from 220 Rn
and its progeny are 6.288 MeV, 6.779 MeV, 6.051 MeV and 8.785 MeV, and that of 222 Rn are
5.490 MeV, 6.003 MeV and 7.687 MeV. Therefore, both radon makes peaks at ∼ 6 MeV. Since
the fraction of the 220 Rn and 222 Rn is not known, both cases were examined and the diﬀerenceswere treated as a systematic error. The linearity up to 6 MeV was thus conﬁrmed within thesystematic error as shown in Figure 4.4.4. Though the energy calibration in the lower energy
region is important, direct measurement with a gas gain of 2500 is diﬃcult. A linearlity checkwith a small-sized detector operated at higher gas gain with an 55 Fe source was performed and
we rely on this measurement for the low energy linearlity[78].
We obtained the energy resolution from the width of the radon peak as σuni = 20 ± 5%.
The position dependence of the gas gain and the disappearance of electrons during the drift
are considered as the factors that make this energy resolution. Detail on the radon study is
described in Section 6.1.1. The electric noise component in the energy resolution was evaluatedwith the ﬂashADC data. The oﬀ-timing-of-trigger waveforms were processed by the samemanner as the on-timing data. Since the time-window for charge calculation depends on theenergy, several energies were assumed to estimate the noise resolution. Figure 4.4.5 shows
the result of the energy resolution due to the electric noise. Obtained energy resolution was
σnoise = 2 keV for 50 keV.
67

TOT-sum

40000
35000
30000
25000
20000
15000
10000
5000
0
0

1000

2000

3000

4000

5000

6000

7000

8000
keV

Eres(keV)

Figure 4.4.4: The relation of energy and FADC-sum. Red point is determined from 10 B calibration, andred line is the extension of this point. Blue point is obtained from radon ﬁtting. The error bar is thesystematic error due to the diﬀerence of 220 Rn and 222 Rn.

4
3.5
3
2.5
2
1.5
1
0.5
0
0

50

100

150

200

250

300

350

400

450

500
keV

Figure 4.4.5: Estimated energy resolution (σ keV) due to the electric noise. The energy dependence isdue to the width of waveforms.

4.4.3

Event selection

Several event selections are applied to the raw data to reduce several kinds of known background. Fiducial-cut, selecting 28 × 24 × 41 cm3 ”ﬁducial volume” in the detection volume
30.72 × 30.72 × 41 cm3 , is applied in the dark matter analysis in order to reject the charged
particles from the walls and the 10 B plate. This ﬁducial-cut are applied for all measurements
except for the 10 B calibration.

68Another three types of cut is applied mainly to reduce gamma-ray background which isone of the serious background for the dark matter search experiments. For these cuts, threeparameters, ”track-length”, ”TOT-sum”, and ”roundness”, were introduced. We explain theseparameters using the event samples in Figure 4.4.6-4.4.9. The track-length is the measured
range of rise-points (blue points in sample) which are the arrival points of electrons and havetrack-shape information. TOT-sum is the sum of the TOTs of all of the strips. Here, the length
between blue and black points for each strip corresponds to the time-over-threshold (TOT).
And thus the total number of blue, red, and black points is regarded as the TOT-sum of aevent. Roundness is deﬁned by following equations.
Nx∑roundnessx =(zrisex − ax x − bx )2

Ny
∑

,

roundnessy =(zrisey − ay y − by )2

Nxroundness = min(roundnessx , roundnessy ),

Ny

,
(4.3)

where Nx and Ny are the number of hit of x and y strips, respectively, zrisex and zrisey are thelowest z corresponding the blue points shown in Figure 4.4.6-4.4.9, (ax ,bx ) and (ay ,by ) are thebest-ﬁt straight line parameters by ﬁtting (x, zrisex ) and (y, zrisey ) (blue points), respectively.
We checked the energy dependence of the track length, TOT-sum, and roundness for nuclear
events by 252 Cf and gamma-ray events by 137 Cs as shown in Figure 4.4.10-4.4.12. Since theenergy loss of an electron is much smaller than that of a nuclei, electron events should be longand scratched. Then, we deﬁned three cuts to discriminate nuclear events of 252 Cf-run fromelectron events of 137 Cs-run as followings.
• length-cut : track-length[cm] > 0.6 + 0.004 × E[keV]for the rejection of long events. (for example Figure 4.4.7)• TOT-sum-cut : TOT-sum< 50
for the rejection of scratched events. (for example Figure 4.4.8)• roundness-cut : roundness < 0.05
for the rejection of the events remained in 137 Cs-run shown in Figure 4.4.12. (for example
Figure 4.4.9)
The nuclear event shown in Figure 4.4.6 survives these three cuts. These cuts eﬀectively
reduce gamma-ray events, while keeping certain eﬃciency for the nuclear tracks as shown in
Figure 4.4.10-4.4.12. In addition, in order to study the eﬀect of roundness-cut, we measured
the roundness as a function of the drift length by using external-trigger mode as shown in

69

-18

z(cm)

z(cm)

-19

-18
-19

-20

-20

252

Cf RUN

ene_low=100.783500 [keV]
-21

-21

-22

-22

-23

-23

-24
-5

-4

-3

-2

-1

0

1
x(cm)

-24
-1

length=0.689406 [cm]TOT-sum=250
roundness=0.055549

0

1

2

3

4

5
y(cm)

z(cm)

-17

z(cm)

Figure 4.4.6: Track sample measured with 252 Cf. Left and right panel shows the x − z andy − z projection of the digitized track acquired by DAQ-mode5. Blue and black points shows
the rise-time points and fall-time points. This event survives the length-cut, TOT-sum-cut androundness-cut.
-17

-18

-18

-19

-19

137

Cs RUN

ene_low=91.831500 [keV]
-20

-20

-21

-21

-22

-22

-23
-15

-14

-13

-12

-11

-10
-9
x(cm)

-23
-10

length=5.014193 [cm]TOT-sum=88
roundness=0.073630

-9

-8

-7

-6

-5

-4
y(cm)

z(cm)

-13

z(cm)

Figure 4.4.7: Track sample measured with 137 Cs, cut by length-cut.
-13

-14

-14

-15

-15

137

Cs RUN

ene_low=100.054500 [keV]
-16

-16

-17

-17

-18

-18

-19
10

11

12

13

14

15
16
x(cm)

-19
-2

length=0.814168 [cm]TOT-sum=55
roundness=0.021144

-1

0

1

2

3

4
y(cm)

z(cm)

-18

z(cm)

Figure 4.4.8: Track sample measured with 137 Cs, cut by TOT-sum-cut.
-18

-19

-19

-20

-20

137

Cs RUN

ene_low=79.837500 [keV]
-21

-21

-22

-22

-23

-23

-24
-1

0

1

2

3

4

5
x(cm)

-24
9

length=0.889054 [cm]TOT-sum=246
roundness=0.022808

10

11

12

13

14
15
y(cm)

Figure 4.4.9: Track sample measured with 137 Cs, cut by roundness-cut.

70

cm

5
4.5

252

4

137

Cf
Cs

3.5
3
2.5
2
1.5
1
0.5
0
0

50

100

150

200

250

300

350

400
keV

TOT-sum

Figure 4.4.10: Energy dependence of the track-length. Red and blue points are measured with
252
Cf and 137 Cs, respectively.
500
450
400
350
300
250

252

Cf

200

137

Cs

150
100
50
0
0

50

100

150

200

250

300

350

400
keV

roundness

Figure 4.4.11: Energy dependence of the TOT-sum after the length-cut. Red and blue points
are same to above ﬁgure.
0.3
252

Cf

0.25

137

Cs

0.2
0.15
0.1
0.05
0
0

50

100

150

200

250

300

350

400
keV

Figure 4.4.12: Energy dependence of the roundness after the length-cut and TOT-sum-cut.
Red and blue points are same to above ﬁgure.

71

chi2

0.25

2
1.8

0.2

1.6
1.4

0.15

1.2
1

0.1

0.8
0.6

0.05

0.4
0.2

0

-20

-15

-10

-5

0

5

10

15

20
z(cm)

0

Figure 4.4.13: The measured roundness as a function of drift length.

Figure 4.4.13. From this ﬁgure, roundness-cut is found to work as a weak ”z-ﬁducial-cut”. The
event shown in Figure 4.4.9 might be the background α particle (see also Section 6).
By applying these three cuts, we measured the detection eﬃciency for the nuclear events andthe rejection power for gamma-ray events. The measured detection eﬃciency of the nuclear
recoil events is shown in Figure 4.4.14. We deﬁned the eﬃciency by dividing the measured
energy spectrum after all cuts by the simulated one. An ideal simulation results, not including
the detector responses was used as the denominator. In order to cancel the position dependence
and measure an overall response of the detector, an averaged spectrum of 6 measurements byplacing a 252 Cf to 6 places; (25.5 cm, 0 cm, 0 cm), (−25.5 cm, 0 cm, 0 cm), (0 cm, 25.5 cm, 0 cm),
(0 cm, −25.5 cm, 0 cm), (0 cm, 0 cm, 47.5 cm) and (0 cm, 0 cm, −47.5 cm) was used. Since theenergy deposition of nuclear events (19 F and 12 C) are suﬃciently large , intrinsic detection
eﬃciency of nuclear recoil events should basically be 100%. The discrimination of the threshold
and the cut eﬃciency decrease the eﬃciency for low energy range. The eﬃciency does notreaches the intrinsic one in the high energy due to the track-length cut and roundness-cut. We
obtained the detection eﬃciency of nuclear events as 40% at 50 keV.
The detection eﬃciency of electron events, or the gamma-ray rejection power, was evaluatedby irradiating the detector with gamma-rays from a 137 Cs source and comparing the datawith the simulation results. The detection eﬃciency for a given energy bin was calculatedby dividing the measured event rate with the simulated event rate of the electron with that72

energy. Evaluated detection eﬃciency in the energy bin 50 − 100 keV was 2.5 × 10−5 . The

efficiency

energy dependence of the electron detection eﬃciency is shown in Figure 4.4.15.

1
0.8
0.6
0.4
0.2
0
0

50

100

150

200

250

300

350

400
keV

efficiency

Figure 4.4.14: The detection eﬃciency of nuclear events.

10-5

10-6

10-7

0

50

100

150

200

250

300

350

400
keV

Figure 4.4.15: The detection eﬃciency of the electron event, or gamma-ray rejection power.

73

4.4.4

Angular resolution

Angular resolution is the most important parameter for a direction-sensitive detector. We
measured the angular resolution of the nuclear tracks using the fast neutrons from a 252 Cf
source[79]. We evaluated the angular resolution by the comparison of measured and simulated
distributions of the recoil angle. The recoil angle is deﬁned in Figure 4.4.16. In order to measure
the actual interaction’s position, we used the DAQ-mode1 with an external-trigger mode.
First, we measured the direction dependence of the detection eﬃciency, or the response
to isotropic radiation, for nuclear tracks. We irradiated the detector with fast neutrons fromthe 252 Cf source placed at (25.5 cm, 0 cm, 0 cm), (−25.5 cm, 0 cm, 0 cm), (0 cm, 25.5 cm, 0 cm),
(0 cm, −25.5 cm, 0 cm), (0 cm, 0 cm, 47.5 cm) and (0 cm, 0 cm, −47.5 cm). We conﬁrmed that we
can emulate isotropic recoil (standard deviation is 23%) with a weighted sum of these six
measurements by Geant4 simulations. Then we calculated the elevation angle and the azimuth
angle as shown in Figure 4.4.17, where these angle are schematically shown in Figure 4.1.2.

[deg]

60

80

9

ele

8

60

8

12

40

7

10

20

6

80
14
60

φ

9

80

degreecenter-north(degree)

Figure 4.4.16: The schematic view of nuclear recoil. θ is the recoil angle.

7
40

40
6

20

20
5
0

8

0

4
-20

6

-20

3
-40

-40

-80

-60

-40

-20

0
20 40 60 80
west-north-east(degree)

3
2

2

1

-80

-80

0

4

-40
-60

-60

1
-80

5

4

2
-60

0
-20

-80

-60

-40

-20

0

20

40

60 80
degree0-80-60 -40

-20

0

20

40

60

φ

azi

80

0

[deg]

Figure 4.4.17: Direction-dependent eﬃciency, or the response to isotropic irradiation, for angular resolution measurement range measured with DAQ-mode1 and external-trigger mode. The energy rangeof left, center, and right ﬁgures are 50 − 100 keV, 100 − 200 keV, and 200 − 400 keV, The vertical axis
is the elevation angle and the horizontal axis is the azimuth angle. Color scale is counts normalized by
the mean value.

74

400

count/kg/days/cosθ

count/kg/days/cosθ

count/kg/days/cosθ

500

800
700
600
500

800
700
600
500

300
400
200

900

400

300

300

200

200

100

100

100

0
0

0
0

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

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
cosθ

1.4
1.2

σ=0[deg]
σ=22.5[deg]σ=45[deg]
σ=90[deg]

A.U.

1.6

A.U.

A.U.

Figure 4.4.18: | cos θ| distribution of the scattering angle in nuclear recoil events that occur by irradiation
with neutrons from 252 Cf. The energy range of left, center, and right ﬁgures are 50 − 100 keV, 100 −200 keV, and 200 − 400 keV, respectively. Measured and simulated distributions are shown by blue andgreen histograms respectively.
σ=0[deg]
σ=22.5[deg]σ=45[deg]
σ=90[deg]

1.8
1.6
1.4

σ=0[deg]
σ=22.5[deg]σ=45[deg]
σ=90[deg]

2.5

2
1.2

1

1

1.5

0.8
0.8

0.6

1

0.6

0.4

0.4

0.2
0
0

0.5

0.2
0
0

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

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
cosθ

χ2

χ2

χ2

Figure 4.4.19: Simulated | cos θ| distributions assuming angular resolutions of 0◦ (black), 22.5◦ (red),45◦ (green), 90◦ (blue). The energy range of left, center, and right ﬁgures are 50 − 100 keV, 100 − 200 keV,and 200 − 400 keV, respectively.
7

12

6.5

11

6

10

20

9

5.5

8

5

15

7

4.5

6

4

10

5

3.5

4

3
20

25

5
30

40

50

60

70
80
sigma [degree]

3

30

40

50

60

70
80
sigma [degree]

30

40

50

60

70
80
sigma [degree]

Figure 4.4.20: χ2 values obtained between measured and simulated | cos θ| distributions as a function ofangular resolution. The energy range of left, center, and right ﬁgures are 50 − 100 keV, 100 − 200 keV,and 200 − 400 keV, respectively.

75

As a next step, we measured the recoiled nuclear tracks by neutrons from the 252 Cf source,
at six positions, given above. Measured events were weighted by the direction-dependenteﬃciency and thus corrected | cos θ| is obtained The observed | cos θ| distribution is shown by
the blue histogram in Figure 4.4.18. To determine the angular resolution, we simulated theneutron-nucleus scatterings assuming several angular resolutions as shown in Figure 4.4.19, andcompared them to the measured distributions. We calculated a χ2 of | cos θ| distribution foreach simulated angular resolution, and the χ2 values plotted against the angular resolutions areshown in Figure 4.4.20. We obtained the minimum χ2 (≡ χ2min ) point using a ﬁtted polynomial
function. We then set 1σ error for the angular resolution as the range determined from [χ2min :χ2min + 1] [80].
We measured the angular resolutions in each energy ranges. The result is shown in Figure 4.4.21. By using the low pressure gas of 0.1 atm, we were able to measure the angularangular resolution σ [deg]resolution in the 50 − 100 keV energy range in addition to other energy ranges.

60
50
40
30
20
10
0
0

50

100

150

200

250

300

350

400
keV

Figure 4.4.21: Obtained angular resolution for each energy.

4.4.5

Direction dependent eﬃciency

We measured the overall direction-dependent eﬃciency in the 50 − 100 keV range by using the DAQ-mode5 with a self-trigger. The detector was irradiated with the neutrons fromthe 252 Cf source placed at (25.5 cm, 0 cm, 0 cm), (−25.5 cm, 0 cm, 0 cm), (0 cm, 25.5 cm, 0 cm),
(0 cm, −25.5 cm, 0 cm), (0 cm, 0 cm, 47.5 cm) and (0 cm, 0 cm, −47.5 cm). We conﬁrmed that we76

can emulate isotropic recoil (standard deviation is 23%) with a weighted sum of these six
measurements by Geant4 simulations. Figure 4.4.22 shows the calculated distribution of theelevation angle θele and the azimuth angle ϕele . We can see that the eﬃciency is high around(θele , ϕazi =0◦ , 0◦ ) and (0◦ , ±90◦ ). Since ϕazi = 0◦ and ϕazi = ±90◦ represent the direction of
the anode/cathode strips as shown in Figure 4.1.2, it is seen that the recoil direction tends
to be detected as that along the anode/cathode strips. Also, there are higher points along
the diagonal lines from the direction of the anode and the cathode strips. This is because thecurrent tracking algorithm tends to measure the diﬀused tracks in such directions. Measured
direction-dependent eﬃciency is used for weighting the expected direction distribution of recoil

elevation angle (degree)

nuclear track scattered by the dark matter for the direction analysis.

4

80
3.5

60
40

3

20

2.5

0
-90
-20

-45

0

45

90

2
1.5

-40

1

-60

0.5

-80
°

0

azimuth angle (degree) for elevation=0

Figure 4.4.22: Direction-dependent eﬃciency in the 50 − 100 keV range measured with DAQ-mode5 andthe self trigger. The vertical axis is the elevation angle and the horizontal axis is the azimuth angle.
Color scale is normalized counts by the mean value.

77

Chapter 5

Experiment
A direction-sensitive dark matter detector,NEWAGE-0.3b’, was developed to improve thesensitivity by one order of magnitude as described in the previous chapter. The detector was
set in the Kamioka underground laboratory in March, 2013, and a direction sensitive dark
matter search experiment was performed from July, 2013. The measurement properties andthe results are described in this chapter.

5.1

Dark matter search at Kamioka mine

A direction-sensitive dark matter search experiment was performed with the NEWAGE0.3b’ detector in an underground laboratory in Kamioka mine. The NEWAGE-0.3a’ detector
was placed in Laboratory B, Kamioka Observatory located at 2700 m water equivalent (w.e.)
underground (36.25 N, 137.18 E). The ﬂuxes of environmental backgrounds; cosmic-ray muons,
neutrons, and gamma-rays, in Kamioka are listed in Table 5.1.1. Since the µ-TPC is set in a
vacuum vessel, environmental radon gas is blocked and can be ignored.
particlecosmic-ray muons
thermal neutrons (< 0.5 eV)
non-thermal neutrons (> 0.5 eV)
fast neutrons (> 500 keV)
gamma-rays

ﬂux in Kamioka∼ 6 × 10−8 /cm2 /sec/sr
8.26 × 10−6 /cm2 /sec
1.15 × 10−5 /cm2 /sec
2.06 × 10−6 /cm2 /sec
∼ 4 × 10−3 /cm2 /sec

ﬂux in surface∼ 1 × 10−3 /cm2 /sec/sr
1.4 × 10−3 /cm2 /sec
1.3 × 10−2 /cm2 /sec

ref
[81]
[82]
[82]
[82]
[83]

Table 5.1.1: The ﬂux of backgrounds in Kamioka underground laboratory.

Any radiation shield was not set mainly for an easy access to the detector. Also it was
expected from previous measurements that the environmental background is still lower than78

the internal background even without any shield. The µ-PIC plane of the µ-TPC was placed
vertically and the z-axis is aligned to the direction of S30◦ E. The run properties are summarizedin Table 5.1.2. The ﬁrst sub-run started by the gas ﬁlling on July 17th, followed by the dark
matter run from July 20th to August 11th with a 17.1 live-days. The second sub-run, started
on October 17th, accumulated data from October 19th to November 12th resulting a 14.5
live-days dark matter exposure. The target gas was CF4 at 0.1 atm in the ﬁducial volume of
275 cm3 . Thus exposure of the sub-runs are 0.177 kg · days and 0.150 kg · days, making a total
exposure of 0.327 kg · days. The time dependencies of the drift velocity and the gas gain were
measured as listed in Table 5.1.3, by the same method described in Section 4.4. The energy
and track length are obtained using these calibration factors from the measured raw data.
RUN ID
RUN14-1
RUN14-2
RUN14

gas ﬁlling
2013/07/17
2013/10/17

period2013/7/20 − 8/11
2013/10/19 − 11/12

live time
17.1 days
14.5 days
31.6 days

mass
10.36 g10.36 gexposure
0.177 kg · days
0.150 kg · days
0.327 kg · days

Table 5.1.2: Conditions of underground measurement RUN14.

cal-runDM-runcal-runcal-runDM-runcal-runDM-runcal-runmeasured date gain
RUN14-1
7/18-202500
7/20-8/11
8/20-21
2500
RUN14-2
10/17-19
2500
10/19-10/26
11/1-22500
11/2-12
11/12-14
2350

Vdrift [cm/µsec]
8.9
8.5
9.5
9.5
9.5

Table 5.1.3: Measured values of gas gain and drift velocity during RUN14.

The data was cut by the ﬁducial-cut ﬁrst, then the three cuts deﬁned in Section 4.4.3 wereapplied. Figure 5.1.1-5.1.4 show the energy dependence of the length, the TOT-sum and theroundness of the event after each cut. Figure 5.1.4 shows the ﬁnal sample for the dark matter
analysis and its spectrum is shown in Figure 5.1.5 by the red histogram. Energy spectra at eachcut step are also shown in Figure 5.1.5 and is used for analysis of dark matter limit. The eﬀectof the length-cut and the TOT-sum-cut is remarkable for low energy region, where gammarays are mainly rejected. The roundness-cut aﬀects the whole energy range, particularly in79

Figure 5.1.1: Energy dependence of the length (left), the TOT-sum (center), and the roundness(right). The events are cut by only ﬁducial-cut. These events correspond to the black line in
Figure 5.1.5.

Figure 5.1.2: Energy dependence of the length (left), the TOT-sum (center), and the roundness(right). The events are cut by ﬁducial-cut and length-cut. These events correspond to thepurple line in Figure 5.1.5.

Figure 5.1.3: Energy dependence of the length (left), the TOT-sum (center), and the roundness (right). The events are cut by ﬁducial-cut, length-cut and TOT-sum-cut. These events
correspond to the blue line in Figure 5.1.5.

Figure 5.1.4: Energy dependence of the length (left), the TOT-sum (center), and the roundness (right). The events are ﬁnal sample with the ﬁducial-cut, length-cut, TOT-sum-cut androundness-cut. These events correspond to the red line in Figure 5.1.5.

80

100 − 400 keV range. It suggests the existence of background from −z (µ-PIC or GEM) androundness-cut is eﬀective to cut such events. Details of the background study is described arecounts/keV/kg/days

Section 6.

h0 (fiducial-cut only)104

h1 (h0 + length-cut)
h2 (h1 + TOT-sum-cut)103

h3 (h2 + roundness-cut)

102

10

1

10-1
0

50

100

150

200

250

300

350

400
keV

Figure 5.1.5: Obtained energy spectra in Kamioka RUN-14. Black line is the spectrum with only theﬁducial-cut. As show in the legend, pink, blue, and red histograms are the energy spectra at eachcut-step described in Section 4.4.3, and correspond to the events shown in Figure 5.1.1-5.1.4.

By taking into account the detection eﬃciency (Figure 4.4.14), we obtained the ”ﬁnal”
energy spectrum of RUN14 shown in Figure 5.1.6 together with the result from previous measurement, RUN5. We plotted the energy spectrum from 50 keV for RUN14 because the angularresolution down to 50 keV was measured as described in Section 4.4.4. Compared to the RUN5,
the background rate of RUN14 is reduced by ∼ 1/10 in the energy range of 100 − 400 keV. Obtained nuclear direction plots and corresponding | cos θcygnus | distribution for the 50 − 400 keVenergy range are shown in Figure 5.1.7 and Figure 5.1.8, respectively. The | cos θcygnus | distributions of the lowest 6 bins, 50 − 60 keV to 100 − 110 keV are shown separately in Figure 5.1.9.
The angle θcygnus is the angle between the WIMP-wind direction and the measured directionof a recoil nucleus. If we are observing the dark matter, the strong signal from the Cygnus
direction is expected. Obtained direction distribution was roughly isotropic, and it can be said
that the background is still dominating the results.

81

counts/keV/kg/days

80
70
60

RUN14

50

RUN5

40
30
20
10
0
0

50

100

150

200

250

300

350

400
keV

elevation angle [degree]

Figure 5.1.6: Obtained energy spectrum in Kamioka RUN-14 (red points). Blue points are the resultsof previous measurements RUN-5[58, 83]. Detection eﬃciency was taken into account.

80
60
40
20
0
-90

-45

0

45

90

-20
-40
-60
-80
azimuth angle [degree]

Figure 5.1.7: Obtained direction of the nuclear tracks of RUN-14 in the energy range of 50 − 400 keV82

counts/keV/kg/days/|cosθcygnus|

50-400keV

10
8
6
4
2
0
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9
1
|cosθcygnus|

140
120

80
60
40

60-70 keV

70
60
50
40
30
20

70
70-80 keV
60
50
40
30
20

20

10

10

0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1|cosθcygnus |0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1|cosθcygnus |0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1|cosθcygnus |40
3580-90 keV

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|cosθcygnus |30
2590-100 keV

20
15
10

counts/keV/kg/days/|cosθcygnus |counts/keV/kg/days/|cosθcygnus |

100

80

counts/keV/kg/days/|cosθcygnus |50-60 keVcounts/keV/kg/days/|cosθcygnus |160

counts/keV/kg/days/|cosθcygnus |counts/keV/kg/days/|cosθcygnus |Figure 5.1.8: Obtained | cos θcygnus | distribution of RUN-14 in the energy range of 50 − 400 keV

12
10
8

100-110 keV

6
4

5

2

0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1|cosθcygnus |0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1|cosθcygnus |Figure 5.1.9: Obtained | cos θcygnus | distribution of RUN-14 of the lowest 6 bins.

83

5.2

Dark matter limits

SD by a direction-sensitive method by
We derived the limits of the SD cross-section σχ−pcomparing the measured | cos θcygnus | distribution with a calculated one. In the calculation,
we used the astrophysical parameters, nuclear parameters, and detector responses listed inTable 5.2.1. The calculation includes nuclear quenching factor shown in Figure 3.2.4 anddetector responses: energy resolution, angular resolution and direction-dependent eﬃciency
described in Section 4.4. Because of the small statistics, measured and calculated | cos θcygnus |shown in Figure 5.1.9 distributions were rebined into 2-bin as shown in Figure 5.2.1. We
obtained the direction-sensitive exclusion limits by the following procedure.
STEP 1 Prepare the measured | cos θcygnus | distributions of the energy bin of interest (thelowest bin is 50 − 60 keV, bin width is 10 keV, at least one event in the bin).
STEP 2 Renormalize the measured | cos θcygnus | distribution. The unit changes from [counts/keV/kg/days]
to [count] by using the live time, ﬁducial mass, and the detection eﬃciency.
STEP 3 Calculate the expected | cos θcygnus | distribution by WIMPs by taking the projectionof calculated angle-energy distribution (Figure 2.4.4). Here, the WIMP mass is assumed
and the quenching factor, the energy resolution and the astrophysical and nuclear parameters listed in Table 5.2.1 are taken account of. Then the distribution is smared withthe angular resolution of 40◦ and direction-dependent eﬃciency (Figure 4.4.22) by Monte
Carlo method assuming random ϕcygnus , the azimuth angle when the cygnus direction isa zenith direction.
STEP 4 Renormalize the expected | cos θcygnus | distribution. The unit changes from [counts/keV/kg/days]
to [count] by using the live time and the ﬁducial mass.
STEP 5 Compare the measured and expected | cos θcygnus | distribution by scanning the crossSD . To evaluate the χ2 value with the Poisson error, we used count-base dissection σχ−ptribution obtained in STEP 2 and STEP 4. If the minimum χ2 /d.o.f. is larger than
χ2 /d.o.f. of 90%C.L. exclusion level (i.e. 2.7 for d.o.f. = 1), the cross section is taken as
a direction-sensitive dark matter limit. If the minimum χ2 /d.o.f. is less than χ2 /d.o.f. of
90%C.L. exclusion level, we evaluated the 90%C.L. upper limit by increasing the crosssection when the χ2 value becomes more than χ2 /d.o.f. of 90%C.L. exclusion level.
STEP 6 Repeat STEP 3-5 by changing WIMP mass.
STEP 7 Repeat STEP 1-6 by changing the energy bin, and take the minimum cross-sections
for each WIMP mass.
84

WIMP velocity distribution
Maxwellian velocity dispersion
Escape velocity
Local halo density
Spin factor of 19 F
Energy resolution at 50 keV
Angular resolution at 50 − 100 keV
direction-dependent eﬃciency at 50 − 100 keVMaxwellian
v0 = 220 km/sec
v0 = 650 km/sec
ρDM = 0.3 GeV/c2 /cm3
λ2 J(J + 1) = 0.647
7.8 keV40◦
Figure 4.4.22

Table 5.2.1: Astrophysical parameters, nuclear parameters, and detector responses used to derive the
WIMP-proton limit.

Figure 5.2.1 show the example of measured and calculated | cos θcygnus | distributions, again
normalized into [counts/keV/kg/days/| cos θcygnus |] unit. The left ﬁgure is the case of excluding the best-ﬁt calculated | cos θcygnus | distribution for 50 GeV of WIMP mass with suﬃcient
statistics of 50 − 60 keV energy bin. The right ﬁgure is the case of low of statistics, thus theupper limit of cross section was evaluated by increasing the cross section when the χ2 /d.o.f.
value becomes more than that of 90%C.L. exclusion level. Since the steeper energy spectrum
is expected for the lower WIMP mass as shown in Figure 2.3.4, the lower energy range limit
the low-mass WIMP. In this measurement, the energy bin of 50 − 60 keV limits the low-masscounts/keV/kg/days/|cosθcygnus|

counts/keV/kg/days/|cosθcygnus|

WIMPs (≤ 70 GeV), and the that of 100 − 110 keV limits the high-mass WIMPs (≥ 100 GeV).

100
80
60
40

MDM=50GeV
50-60keV20

σ=1960pb, χ2/dof=3.86435

4
3.5
3
2.5

MDM=200GeV
100-110keV
σ=557pb, χ2/dof=2.70559

2
1.5
1
0.5

0
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0
0

0.9
1
|cosθcygnus|

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9
1
|cosθcygnus|

Figure 5.2.1: cos θcygnus distributions win the energy range of 50−60 keV (left) and 100−110 keV (right).
Red histograms are measured | cos θcygnus | distribution. Blue and green lines are calculated cos θcygnus
distribution assuming WIMP-wind model with mass of WIMP 50 GeV (left) and 200 GeV (right). Blueline is the best-ﬁt. Green line is the | cos θcygnus | distribution when the model is excluded by 90%C.L..
SD , as a function of mass of WIMP
Figure 5.2.2 shows the limits of SD cross section, σχ−pMχ . The regions above the curves are excluded. Red solid line is a result of directional methodin this work. Blue thick dotted line is a result of conventional method (spectrum-only) for85

SD 90% C.L. upper limits and allowed region
5

10

NEWAGE surface run
4

10

N

0 ( RU
E 201
G
A
W
E

DMTPC

N5)

)
2012(F

3

10

THIS WORK (RUN14)

20

10

)

(F

13

σχ-p[pb]

T
IF
DR

102

DAMA allowed(NaI)

0
)200
e(Xe
A/LX 2006
DAMo(CaF2)
09
Toky
e)20
S2(G
CDM

1

e)2008

N10(X

XENO

10-1

12
3I)20 11
P(CF0
COUP (C2ClF5)2
LE
SIMP

10-2
10

005
(NaI)2
NAIAD
2007
I)
a
(N
KIMS
)2012
4F10
SO(C
S
A
PIC

102 2
Mχ [GeV/c ]

3

10

SD
, as a function of mass of WIMP mass Mχ . Red thick solid
Figure 5.2.2: Limits of SD cross section, σχ−pline is the result of a directional method in this work. Blue thick dotted line is a result obtained withthe conventional method for reference. Red thin lines labeled ”NEWAGE surface run” and ”NEWAGE
2010 (RUN5)” are previous results measured at surface and Kamioka, respectively[83]. Allowed region(DAMA[39]) and upper limits of other experiments are shown for comparison. Green lines are limits set
by liquid or solid detectors, blue lines are the limits set by gas detectors with conventional analysis.

reference. The largest systematic error is arised from the uncertainty of the energy resolution.
Since the shape of the expected energy spectrum is exponential (Figure 2.1.2), the lower limit
is obtained for the larger energy resolution. As described in the Section 4.4.2, the energy
resolution is sum of σuni = 20 ± 5% and σnoise = 2 keV. In order to obtain the conservative
limit, σuni = 15% is used for the analysis (total energy resolution is 7.8 keV for 50 keV). It
should be noted that the direction-sensitive analysis and conventional method provide similar
limits with this low statistic results. Figure 5.2.3 shows the calculated χ2 /d.o.f. values with the
WIMP model (red), and calculated that with the ﬂat-background model (black). The minimum
χ2 values are shown in the 20 − 70 GeV mass region. As for the 100 − 1000 GeV mass region,86

we evaluated the 90% C.L. upper limit by increasing the cross-section χ2 values until exceeding
2.7. We obtained a 90% C.L. SD WIMP-proton cross section upper limit of 557 pb for WIMP
mass of 200 GeV. This result marked new best sensitivity record for a SD WIMP search withthe direction-sensitive method, although its sensitivity is still worse than that of the frontier ofχ^2/dof

SD WIMP searches with other methods.

5
4.5
4
3.5

WIMP wind model3
2.5 90%C.L. (dof=1)
2
1.5
1
0.5
0
10

isotropic BG model

102

3

10
Mχ [GeV/c 2]

Figure 5.2.3: χ2 values in comparison with measured | cos θcygnus | and that of models. Red points areχ2 values with the WIMP wind model. Black points are the minimum χ2 values with the isotropic
background model.

87Chapter 6

Discussion
A dark matter search in Kamioka mine has been carried out with the NEWAGE-0.3b’
detector, and then a new lower upper limit for the WIMP-proton SD interaction than that ofthe previous run has been obtained. However, further reduction of background is necessary toreach the MSSM region. In addition, the origin of remaining background is studied in detail.
Radioactive contaminations within the detector components are well-known backgroundsources in rare event measurements. The radioactive contaminations emit γ-rays, β-rays andα-rays. In our case, γ-rays and β-rays were rejected with a rejection power of ∼ 10−5 , but
α-rays cannot be discriminated from nuclear recoils around 50 keV region. Dominant naturalradioactive isotopes contaminated in most of the materials are 238 U, 232 Th and 40 K. There
are several α-decays and β-decays associated with γ-ray emissions in the U-chain and theTh-chain as shown in Figure 6.0.1.

40 K decays with a β-ray and a γ-ray.

These isotopes

wouold be contaminated in most of the material in the detector components such as metal,
plastic and glass-ﬁber. In addition, radon in U-chain and Th-chain emanates from the material
as gas. Therefore, radon and radon progeny, which are the isotopes below radon as shownin Figure 6.0.1, might exist not only in the detector components but also in the gas volume.
Although α-particles emitted from the U/Th-chains have an initial energy of more than 4 MeV,
they sometimes deposit only their partial energy in the detection volume and make background
events of ∼ 50 keV.
The ﬁnal goal of this chapter is to identify the background sources in the energy range
relevant to the dark matter search; i.e. 50 − 400 keV energy range. Measured energy spectra
are shown in Figure 6.0.2. The red spectrum is the ﬁnal one with all the cut applied and theblue spectrum is the one with all but the roundness-cut applied. Since the roundness-cut has
a large uncertainty in terms of the cut eﬃciencies for short-drift events, the understanding of
the blue spectrum is studied instead of that of the red one. At ﬁrst, I mention on the results
88

238
92

U

234

U

U

9

5

β 2.269

4.468 10 y

2.457 10 y(99.2%)
234

α
4.198

Th

1.17 m

234

234
90

Pa

)

Pa

6%
.1
(0

91

Th

24.10 dβ 0.196(γ )β 0.472
(γ )

α
4.775
(γ )

232

Pa

90
230

α
4.687
(γ )

226

87

228

85

At
Po
Bi

β 0.0392

5.75 y

87

Fr

86

Rn

Rn
218

α
5.490
(γ )

85

At

84

Po

1.5 s

α
6.694

210

Po

164.3 µs

3.10 m
(99.98%)
214

Bi

α
6.003

β 1.07
1.51
3.27

Po
(RaF)

Bi
(RaE)

β 1.16

Pb

β
0.67
0.73

α
6.288
(γ )

α 5.450
(γ )

α
5.304
(γ )

5.013 d26.8 mPb(RaD)
22.3 yβ 0.0170γ
0.0465

Pb

Tl

212

Bi

82

Pb

β
0.335
0.574

α
6.051

10.64 h1.30 m

Tl

81

Tl

208

Pb

stable208

αβ 1.53α8.785(36.0%)

Pb

stable206

β 2.25γ
0.72760.55 m

212

206

Po

0.299 µs

α
6.779

Bi

(1.3 10 %)(99+%)
210

212

Po

0.145 s(64.0%)

83

−6

Tl

β 1.801.29
1.52

3.053 m4.199 m

206

Hg

Rn

55.6 s138.4d(99+%)
210

α
7.687
(γ )

19.9 m

210

80

α
5.685
(γ )

216

214

(1.9 10 %)Tl

Ra

3.66 d

At

β

Po

214

81

224

3.824 d

−4

Pb

α
5.423
γ
0.0844220(0.02%)

82

β 1.16
1.73
2.07

α 4.784
γ 0.186(99.98%)

83

Ra

Ac

6.15 h3(0.02%)
218
84

Ra

228

Ra

Fr
Rn

1.912 y1.600 10 y

222
86

α
4.012
(γ )

4

Ac
Ra

Ac

Th

10

Th

7.538 10 y88

881.405 10 y6.75 h

89

89

228

Th

Th

Hg

β, γ

8.15 mFigure 6.0.1: Decay chain of 232 Th and 238 U.

on the high energy background shown in Figure 6.0.3 as an ”independent input”. The highenergy spectrum consist of a peak-like component and a continuous component. By comparing
the dependence of the measured track length on energy to that of calculated one by SRIM,
background events of the high energy are considered to be due to α particles as shown in
Figure 6.0.3. When the directional distribution is concentrated to the higher at ±z direction,with a small fraction of isotropic component. The background sources of the high energy
events will be identiﬁed quantatively in Section 6.1, then the background identiﬁcation in the
low energy range is going to be discussed in Section 6.2.

89

counts/keV/kg/days

103
raw spectrum (without χ2 -cut)

102

raw spectrum (with all cuts)

10

1

10-1
0

50

100

150

200

250

300

350

400
keV

elevation angle [deg]counts/keV/kg/days

Figure 6.0.2: Obtained energy spectrum of RUN14 in the range of 50 − 400 keV.

2.5

2

1.5

80

140

60

120

40
100

20
80

0
1

0.5

length [cm]

0
0

-20

60

-40

40

-60

20

-80
2000

4000

6000

8000

10000 12000 14000
keV

-150

-100

-50

0

50
100 150
azimuth angle [deg]

0

50
180

H

45

He

40

160
140

35
120

30

100

25
20

80

15

60
40

10

F

5
0
0

2000

4000

6000

8000

10000 12000 14000
keV

20
0

Figure 6.0.3: Obtained energy spectrum (top-left), energy-length distribution (bottom), and direction
distribution (top-right) of RUN14 in the range of 500 − 15000 keV. Only ﬁducial-cut is applied.

90

6.1

High energy backgrounds

Figure 6.1.1 schematically shows the candidates of the high-energy background. Events
1 is removed by the ﬁducialcrossing the outer region of the ﬁducial area, or the veto area, as ⃝2 the events fromcut. Remaining events are categorized into three types; the events from +z (⃝),
3 ⃝’,3 ⃝”),
3
4 ⃝’,
4 ⃝”,
4 ⃝,
5 ⃝’,
5 ⃝,
6 ⃝,
7 ⃝’).
7gas (⃝,and the events from −z (⃝,These three types of
events diﬀer in the drift length. By the gas diﬀusion, a large TOT-sum is obtained for an eventwith a long drift length. Figure 6.1.2 shows the simulated TOT-sums assuming three origins
4 ⃝’
4 ⃝”,
4 ⃝,
5
of the background sources. The events generated from the −z region (red points; ⃝,
5 ⃝,
6 ⃝,
7 ⃝’),
7⃝’,
have a short drift length and a small diﬀusion, and result in small TOT-sums.
2 and generated
On the other hand, the events generated from the +z region (green points; ⃝)
3 ⃝’,
3 ⃝”)
3
from the gas region (blue points; ⃝,have a long drift lengths and a large diﬀusions,and result in large TOT-sums.

Figure 6.1.1: Candidates of high-energy background events (500 − 15000 keV).

91

TOT-sum [ch]

20000
18000
16000
14000
12000
10000
8000
6000
222

Rn in gas (CF )44000

U in µ-PIC (substrate)

238

2000
0
0

238

U in drift-plane (copper)

2000

4000

6000

8000

10000
keV

TOT-sum [ch]Figure 6.1.2: Simulated TOT-sum as a function of energy. Color indicates the origins of the sources.

20000

60

18000
50

16000
14000

40

12000
10000

30

8000
20

6000
4000

10

2000
0
0

2000

4000

6000

8000 10000 12000 14000
keV

Figure 6.1.3: Measured TOT-sum as a function of energy.

920Figure 6.1.3 shows the measured TOT-sum as a function of energy. By comparing with thesimulated TOT-sum in Figure 6.1.2, it can be judged that the events from −z region seems to be
dominant, and some events from gas region also exist. The events were divided into two regions
by the red dotted line to study the origin of each component. The obtained energy spectrum
and direction distribution for each component are shown in Figure 6.1.4 and Figure 6.1.5, wherea clear diﬀerence is seen. In the ”above the line” region, the energy spectrum has a ”peak” at∼ 6 MeV and the direction distribution is isotropic, while in the ”below the line” region, theenergy spectrum becomes ”continuous” and the direction distribution concentrated to the ±z

elevation angle [deg]counts/keV/kg/days

direction. We discuss on the origin of each component in the following sections.

0.5

0.4

9

60

8

40

7

20

6

0

5

-20

4

0.3

0.2

10

80

3

-40
0.1

2

-60

1

0
0

-80
2000

4000

6000

8000

10000 12000 14000
keV

-150

-100

-50

0

50
100 150
azimuth angle [deg]0elevation angle [deg]counts/keV/kg/days

Figure 6.1.4: Obtained energy spectrum (left) and direction distribution (right) above the red dottedline in Figure 6.1.3.

2.5

2

1.5

80

140

60

120

40
100

20
80

0
1

0.5

0
0

-20

60

-40

40

-60

20

-80
2000

4000

6000

8000

10000 12000 14000
keV

-150

-100

-50

0

50
100 150
azimuth angle [deg]0Figure 6.1.5: Obtained energy spectrum (left) and direction distribution (right) below the red dottedline in Figure 6.1.3.

93

6.1.1

Peak component

The peak component can be said to be due to the radon gas because 1) the peak energy
is close to the expected energy of radon gas, 2) measured energy to TOT-sum correlations inFigure 6.1.3 agree with the simulated ones in Figure 6.1.2, and 3) the directions are isotropic.
There are two radioactive radon isotopes; 220 Rn and 222 Rn, which decay as shown in Figure 6.0.1
making a peak at ∼ 6 MeV in both of the cases. Radons are generated from the decay of Uchain or Th-chain contaminated in the detector components. Radons are gas and comes outof the components and enter the detection region of µ-TPC. The life time of 238 U and 232 Th
is so long that they reach the radioactive equilibrium. There is no radon when gas is re-ﬁlled
in the µ-TPC. Then the number of radon NRn increases with a time constant of the radon life
time λRn (= ln 2/TRn ) as)
R0 (1 − e−λRn t ,
(6.1)
λRn
where t is the time from the gas re-ﬁll, R0 is a generation rate of the radon, TRn is the halfNRn =life of radon. Because the half-life of 220 Rn is so short as T220 Rn = 55.6 sec, the number of
220 Rn reaches an equilibrium within several minutes.

On the contrary, the number of 222 Rn

increases for several days because of its long half-life as T222 Rn = 3.824 day. We need to circulate
the gas in the µ-TPC in several minute if we were to reduce 220 Rn by a gas circulation system(Section 4.1), which is not realistic in terms of the speciﬁcation of the pump and the convectionin the µ-TPC. Therefore a fundamental reduction by a material selection is required to remove
220 Rn. On the other hand, because the long half-life of 222 Rn, the radon reduction by the gas

circulation system is expected. In case of reducing the radon by a gas circulation, the timedependence of the number of radon becomes
NRn =

[
]
R0
−(λRn +F P/VTPC )t
1
−
e
,
P
λRn + VFTPC(6.2)

where VTPC ≃ 125 ℓ is the volume of µ-TPC, F is the ﬂow rate, P is the adsorption probabilityof the charcoal.
Figure 6.1.6 shows the measured time dependence the of peak component (> 3000 keV).
Black points are measured without the gas circulation system and consistent to the time constant of 222 Rn. By using the gas circulation system, we obtained pink points and reduced radon
rate by a factor of ∼ 10 in 20 days. The pink line is ﬁtted with the same R0 to the black line,
where P is assumed as a free parameter. Then the material of the drift cage was replaced fromglass-reinforced plastic to PEEK (polyetheretherketone) as a low background material. The
blue and red points are measured results in RUN14-1 and RUN14-2, respectively. The radon
rate was reduced further by a factor of ∼ 5. The blue and red lines are ﬁtted by using same P
94

as the pink line, while R0 is assumed as a free parameter. When cooled charcoal is added togas circulation system and low radiative matirial is selected, the radon rate was reduced by aRadon rate [counts/kg/days]

factor of ∼ 50 in total.

105
no circulation, glass-plastic

104
cooled charcoal [500ml/min], glass-plastic

103

102

cooled charcoal [500ml/min], PEEK, RUN14-1
cooled charcoal [500ml/min], PEEK, RUN14-2

0

5

10

15

20

25
time [days]

Figure 6.1.6: The measured time dependence of the peak component (> 3000 keV).

Next, the amount of radon in the ﬁducial volume was evaluated. Because the amount of
222 Rn reach the equilibrium more quickly with the gas circulation system than its half-life,

it is diﬃcult to distinguish 222 Rn and 220 Rn using the time dependence anymore. Since thehalf-lifes of 218 Po, 214 Pb, 214 Bi and 214 Po are shorter than the that of 222 Rn, 222 Rn decays
to stable 210 Pb in a short period compared to the decay rate of 222 Rn. Thus, the α particles
from the daughter nucleus, 218 Po and 214 Po, are also detected when a 222 Rn decays in theµ-TPC. Similarly, α particles from 216 Po, 212 Pb, 212 Bi and 212 Po are also detected when a
220 Rn decays. More than 90% of daughter nucleus are known to be positively charged[84], andconsidered to drift to the drift plane where negative high voltage is applied. Since the half-lifeof 218 Po, several minute, is longer than the typical drift time of several micro seconds, 218 Po
is considered to stick to the drift plane and decays in the place. Thus, the probability that α
particles from 218 Po are emitted into the detection region of µ-TPC is halved; i.e. the otherhalf goes into the drift plane. The daughter nucleus 218 Po decays on the drift plane and theprobability is halved as well. For the series of 220 Rn, it is necessary to consider the branchingratio shown in Figure 6.0.1. In addition, since the track length of α particles of ∼ 6 MeV is95relatively long, the fully-contained detection eﬃciency is precisely estimated by the simulation.
By ﬁtting the measured peak with the summarized contributions of radon daughters whose
ratios are discussed above (listed in Table 6.1.1), we evaluated the content of radon for eachcase of 222 Rn and 220 Rn as 9.3 mBq/m3 and 14 mBq/m3 , respectively.
isotope
Energy of α collapse (MeV)
Branching ratio (%)
Probability of entering the gas
Detection eﬃciency by track length

U-chain
218 Po214 Po5.490 6.003 7.687
100
99.98 99.98
1
0.5
0.5
0.52
0.45
0.31

222 Rn220 Rn

6.288
100
1
0.40

Th-chain
212 Bi
6.779 6.051
100
36.0
0.5
0.5
0.36
0.44

216 PoTable 6.1.1: The parameters of radons and their progenies.

96

212 Po

8.785
64.0
0.5
0.23

6.1.2

Continuous component

2 ⃝7
The origin of the continuous components are identiﬁed to be some combinations of ⃝3 is estimated from the discussion in the previous section.
in Figure 6.1.1, while component ⃝First of all, we discuss on the continuous component by studying the direction of the events.
In the high energy region, a suﬃcient number of hit-strips enable to determine the head-tail ofthe tracks because the time-over-threshold (TOT) distribution provides the information on theenergy deposition dE/dx along the tracks. Figure 6.1.7 shows the simulated energy depositionsof α particles as a function of energy and length in the 0.1 atm CF4 gas by using SRIM, where
α particles are seen to have their energy-loss peak around 1 MeV. This means that α particle
with an energy more than ∼ 1 MeV increase its energy loss rate by going ahead, while α
particles with a energy less than ∼ 1 MeV decrease its energy loss rate by going. Thus, the”normal Bragg-curve” can be seen for high energy tracks (>∼ 4 MeV). On the other hand,the ”reversed Bragg-curve”, energy loss is smaller at the end of the track, can be seen for low
energy tracks (<∼ 1 MeV). Figure 6.1.8 shows an example of the high-energy track and TOT
distribution, where the ”normal Bragg-curve” in TOT distribution is seen. The direction ofthis event can be easily determined to be (+x, +y, −z) → (−x, −y, +z) resulting an elevation
angle of 62◦ . In order to examine the head-tail recognition statistically, obtained elevation
angle distribution of the continuous high energy background events are shown in Figure 6.1.9
by assuming the ”normal Bragg curve” regardless of the event energy. Since the ”normal Braggcurve” can be seen in 4 − 15 MeV, obtained elevation angle distribution indicates the events
run in the +z direction. Besides, since the ”reversed Bragg-curve” is expected in 0.5 − 1 MeV,
obtained elevation angle distribution also indicates the events are actually running towards
the +z direction. Thus, the direction of almost all continuous high energy backgrounds is +z
2 and ⃝”,
3 which predict α particles going −z
direction. This result disfavors the assumption ⃝direction.
Obtained elevation angles is high and have a peak around 70◦ . This result gives us further
information on the sources. The α background source should be located in the GEM or µ-PIC
4 ⃝’,
5 ⃝,
6 ⃝,
7 ⃝’)
7 and have to pass through the holes of GEM to make the peak around 70◦ .
(⃝”,If the copper of GEM is a background source, we should detect background from the surface
4 as well as the µ-PIC-side copper (⃝’,
4 ⃝”),
4
copper (⃝)
and more events of the low elevation
angle should have been observed. However, obtained elevation angle is strongly concentrated
to the large elevation angle. Therefore, the position of the background source is limited to the
5 ⃝’),
5
6 and the substrate ofinside of the LCP of the GEM (⃝,surface copper of the µ-PIC (⃝),
7 ⃝’),
7
the µ-PIC (⃝,and each case is discussed in the following paragraphs.

97MeV/(mg/cm2)

MeV/(mg/cm2)

1.6
1.4
1.2

1.6
1.4
1.2

1

1

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0
0

1000

2000

3000

4000

5000

0
0

6000
keV

20

40

60

80 100 120 140 160 180
mm

-8

z(cm)

z(cm)

Figure 6.1.7: Energy deposition of α particles as a function of energy (left) and length (right) in the
0.1 atm CF4 gas.

-10

-10

-12

-12

DM RUN14-14-14

ene_high=5468.707605 [keV]

-16

-16

θelevation=62.306578 [deg]

-18

-18

length=13.007051 [cm]

-20

-20

TOT-sum=5681-22roundness=0.046847

-22
-6

-4

-2

0

2 4
x(cm)

0

TOT [clock(=10ns)]-12 -10 -8

TOT [clock(=10ns)]

-8

40
35
30
25
20

2

4

6

8

10 12 14
y(cm)

30
25
20
15

15
10
10
5

5
0

-5.5

-5

-4.5

-4

-3.5

-3

-2.5
x(cm)

0

5

6

7

8

9
y(cm)

Figure 6.1.8: Example of a high energy (5470 keV) track and TOT distribution. Left-top and right-top
ﬁgure shows x-z and y-z projected track, respectively. Left-bottom and right-bottom ﬁgure shows TOT
distribution for x-strips and y-strips, respectively. The gap around x = −4 cm is due to the segmentation
gap of GEM and correspond to the gap around y = 6.7 cm.

98

500-15000[keV]300

countscountscounts

350

160

60

140
50

500-1000[keV]
120

250
40

100

30

80

200
150

4000-15000[keV]

60
100

20

50

10

0

0

40

-80 -60 -40 -20

020 40 60 80
elevation angle [deg]20-80 -60 -40 -20

0020 40 60 80
elevation angle [deg]-80 -60 -40 -20

020 40 60 80
elevation angle [deg]Figure 6.1.9: Measured elevation angle distribution of continuous high energy background. Left, centerand right ﬁgure shows the distributions correspond to the energy region of 0.5 − 15 MeV, 0.5 − 1 MeV,
and 4 − 15 MeV, respectively.

Inner LCP of GEM
In order to examine the particle emission from the inside of the LCP of the GEM as a
background source, we simulated the α particle emission from Th-chain in the LCP of the
GEM. Figure 6.1.10 shows the simulated energy spectrum and elevation angle distribution in
the energy of 0.5 − 15 MeV. Simulated elevation angle distribution was broad to the low angle(∼ 20 − 50◦ ), and it was not consistent to the measurements (left of Figure 6.1.9). This is5
because that the events passing through the GEM hole (⃝’),
which was thought to make ahigh elevation angle population, are less dominant than the events passing through the surface
5
copper of the GEM with a thickness of 10 µm (⃝).
Similar tendency was seen in case ofassuming U-chain. Thus, the contamination of Th-chain or U-chain in the LCP of the GEM

220

countscounts/keV/kg/days

5 ⃝’)5 is not found to be the main component of the high energy continuous background.
(⃝,

LCP of GEM

14

LCP of GEM

180

Th-chain 1ppm

Th-chain 1ppm

160

500-15000[keV]

12

16

200

140
10

120

8

100

6

80
60

4

40
2
0
0

20
2000

4000

6000

8000

0

10000 12000 14000
keV-80 -60 -40 -20

020 40 60 80
elevation angle [deg]Figure 6.1.10: The simulation result of the energy spectrum (left) and elevation angle distribution in
5 ⃝’).
5energy of 0.5 − 15 MeV (right) by assuming 1 ppm contamination of Th-chain in LCP of GEM (⃝,99

Surface copper of µ-PIC
Next, we simulated the α particle emission from Th-chain in the surface 10 µm-thick copperof the µ-PIC. This simulation was carried out as followings. First we performed a simulation
without the GEM, and then we selected the event that would cross the region corresponding to
the holes of the GEM 4 mm above the surface of the µ-PIC. Energy deposition and hit pointsof track in the detection volume were analyzed. Figure 6.1.11 shows the simulated energy
spectrum and elevation angle distribution in the energy of 0.5 − 15 MeV. Obtained elevation
angle is similar to the measurements but the energy spectrum turned out to peak at ∼ 4 MeV,
which was not consistent to the measurement (left of Figure 6.1.5). This is because that thesurface copper of µ-PIC is too thin and the energy loss in the copper is too small to makelow energy continuous component below 4 MeV. This result suggests that the contamination
may exist in the deeper position of the µ-PIC. Similar tendency is seen for the simulation of
6
U-chain. Thus, the contamination of Th-chain or U-chain in the surface copper of µ-PIC (⃝)

counts/keV/kg/days

countsis not found to be the main component of the continuous high energy background.

0.08
0.07

surface copper of uPIC

0.06

Th-chain 1ppm

14

surface copper of uPIC

12

Th-chain 1ppm

10500-15000[keV]

0.05

8
0.04

6

0.03
0.02

4

0.01

2

0
0

2000

4000

6000

8000

0

10000 12000 14000
keV-80 -60 -40 -20

020 40 60 80
elevation angle [deg]Figure 6.1.11: The simulation result of the energy spectrum (left) and elevation angle distribution inenergy of 500 − 15000 keV (right) by assuming 1 ppm contamination of Th-chain in the surface copperof the µ-PIC.

Substrate of µ-PIC
The α particle emission from Th-chain and U-chain in the substrate of the µ-PIC are
simulated taking account of the eﬀect of GEM hole in the same way as described above. Also
we covered a half of the surface area by copper of 10 µm as a cathode pattern. Figure 6.1.12 andFigure 6.1.13 show the simulated energy spectra and elevation angle distributions in the energyof 0.5 − 15 MeV for the contamination of Th-chain and U-chain, respectively. The simulated100

elevation angle distributions and energy spectra reproduce the measurement results in bothcases of the U-chain and Th-chain. Therefore, the substrate of µ-PIC is found to be a primary
candidate of the continuous background source. Actually, glass ﬁbers are used to reignforce
the substrate, which is known to contain impurities including radioactive background sources.
Here, the range of alpha particles with the energy of < 10 MeV in polyimide is less than 100 µm
as shown in Figure 6.1.14. Thus, the alpha particles from the the back side of the insulator(100 µm thick of polyimide) do not come into to the detectable reagion. Although we plan thefurther study of radio assay using High-purity Germanium detector, in this thesis the following

countscounts/keV/kg/days

discussion is based on the quiantative study from the high energy continuous spectrum.

0.9
0.8

substrate of uPIC

0.7

Th-chain 1ppm

0.6
0.5
0.4

70
60

substrate of uPIC

50

Th-chain 1ppm

40500-15000[keV]

30

0.3

20
0.2

10

0.1
0
0

2000

4000

6000

8000

0

10000 12000 14000
keV-80 -60 -40 -20

020 40 60 80
elevation angle [deg]countscounts/keV/kg/days

Figure 6.1.12: The simulation result of the energy spectrum (left) and elevation angle distribution in anenergy range of 0.5 − 15 MeV (right) by assuming 1 ppm contamination of Th-chain in the substrate ofthe µ-PIC.

substrate of uPIC

2.5

50

40

U-chain 1ppm

2

substrate of uPIC
U-chain 1ppm

30

500-15000[keV]

1.5

20

1

10

0.5

0
0

2000

4000

6000

8000

0

10000 12000 14000
keV-80 -60 -40 -20

020 40 60 80
elevation angle [deg]Figure 6.1.13: The simulation result of the energy spectrum (left) and elevation angle distribution in anenergy range of 500 − 15000 keV (right) by assuming 1 ppm contamination of U-chain in the substrateof the µ-PIC.

101

um

100
90
80
70
60
50
40
30
20
10
0
0

1

2

3

4

5

6

7

8

9

10
MeV

Figure 6.1.14: The calculated range of alpha particles in polyimide by SRIM. The highest energy ofalpha particle of U-chain and Th-chain is 8.785 MeV, and the thickness of the insulator (polyimide) is
100 µm. Thus, the alpha particles from the back side of the insulator (polyimide) do not come into to
the detectable reagion.

1026.1.3

Summary of high energy background study

The high energy background sources of the dark matter search experiment are found to beradon and U/Th in the substrate of the µ-PIC. Figure 6.1.15 shows the high energy background
spectrum of simulation results together with the one of measured data. In the ﬁgures, theradon component estimated in Section 6.1.1 is shown by red and orange ﬁlled histograms. The
contribution of U/Th in substrate of µ-PIC are stacked with black and green histograms. The
amount of U/Th was determined to explain the high energy spectrum. The case in the left
top (220 Rn + 232 Th-chain model) in Figure 6.1.15 explains the measured spectrum best, while
the other can still be mixed as minor components. The estimated amount of contamination for

counts/keV/kg/days

14.0[mBq/m3] 220Rn in gas
2.2[ppm] Th in uPIC substrate

2.5
214.0[mBq/m3] 220Rn in gas

2

1

1

0.5

0.5

4000

6000

8000

10000

12000

0
0

14000
keV

RUN-14 (high-energy, fid-cut only)3

9.3[mBq/m3] 222Rn in gas
2.2[ppm] Th in uPIC substrate

2.5
2

0.5

0.5

8000

10000

12000

0
0

14000
keV

8000

10000

12000

14000
keV

RUN-14 (high-energy, fid-cut only)
0.9[ppm] U in uPIC substrate

2

1

6000

6000

2.5

1

4000

4000

9.3[mBq/m3] 222Rn in gas

1.5

2000

2000

3

1.5

0
0

0.9[ppm] U in uPIC substrate

2.5

1.5

2000

RUN-14 (high-energy, fid-cut only)

3

1.5

0
0

counts/keV/kg/days

RUN-14 (high-energy, fid-cut only)

3

counts/keV/kg/days

counts/keV/kg/dayseach case is written in Table 6.1.2.

2000

4000

6000

8000

10000

12000

14000
keV

Figure 6.1.15: Energy spectrum of the high energy background. Each ﬁgure diﬀers in the type of radonand U/Th in substrate.

103

component
radon
U/Th in substrate

type
222 Rn220 RnTh-chainU-chainamount
14 mBq/m3
9.3 mBq/m3
2.2 ppm
0.9 ppm

Table 6.1.2: The estimated amount of high energy background sources. The spectrum study prefers
220
Rn + 232 Th-chain model the most, while the other components can be mixed as minor components.

104

6.2

Low energy backgrounds

The internal background sources was investigated based on the high energy spectrum study
in the previous section. Figure 6.2.1 shows the expected candidates of the low-energy background sources. The environmental background is shown as (A), which will be discussed in
Section 6.2.1. The α particles form radons in the gas make low energy background when they
deposit the partial energy by stopping at GEM or drift plane (B), which will be discussed in
Section 6.2.2. From the study of the high energy background as described in Section 6.1.2,
substrate in µ-PIC contains U/Th-chain background sources (C), and the eﬀect for low energy range will be described in Section 6.2.2. Although other places, surface copper of µ-PIC,
GEM, and drift plane, could contain background sources to a certain extent, we consider only
the dominant background source, substrate, in this thesis.
The energy of the event occurred in the region between µ-PIC and GEM, ”gap region”

Figure 6.2.1: Candidates of low-energy background events (50 − 400 keV). From the discussion of highenergy background as in Section 6.1, we only discuss about U/Th contamination in substrate of µ-PIC
as a background from material of the detector.

105

(B’,C’), is suppressed compared to the event in the µ-TPC (A,B.C) by the following reasons.
First, α particles deposit the partial energy by stopping at GEM or µ-PIC because their track
lengths are generally longer than the gap length of 4 mm. Second, since these events are notampliﬁed by the GEM before being detected, their energies are observed as if ”suppressed”
by the GEM gain. Third, they run so close to the µ-PIC that the electrode-structure of theµ-PIC is seen. Thus the electron collection eﬃciency of µ-PIC depend on the path of the track.
The α particles from radon in the gap makes low energy background, and will be describedin Section 6.2.2. The α particles from substrate in µ-PIC is likely to be the most serious
background, and will be described in Section 6.2.2.

6.2.1

Environmental backgrounds

In this section, the environmental background, which comes from the outside of the µTPC as shown (A) in Figure 6.2.1, is described. Cosmic-ray muon is a popular backgroundsources in rare event searches. Then other major two background sources for the most of dark
matter search experiments: neutrons and gamma-rays are investigated. β-rays and α-rays fromthe outside can be ignored because they are stopped by the vacuum chamber. Therefore thecontributions of muons, neutrons, and gamma-rays are estimated in this section.
Cosmic-muon ﬂuxes as a function of the depth in several underground laboratories in theworld including our laboratory (labeled as ”Super-Kamiokande”) are shown in Figure 6.2.2[81].
The ﬂux of the cosmic-ray muon at Kamioka observatory is ∼ 6 × 10−8 /cm2 /s/sr. The muon
rate passing through the eﬀective volume of the µ-TPC was calculated to be ∼ 6 counts/day. By
assuming the muon rejection power as the same order of that of electron (∼ 10−5 at 50 keV), therate of a misidentiﬁed muon track as a nuclear track should be less than 6×10−3 counts/kg/days
above 50 keV. That count rate is much smaller than the measured one shown in Figure 6.0.2.
The secondary particles produced by cosmic-ray muons are studied as ambient gamma-rays
and neutrons in the followings.
Environmental neutrons are generated in the series decay of U/Th-chain, spontaneous ﬁssionof U/Th and the spallation by the cosmic-ray muon. Generated neutrons are moderated by
the material around the laboratory. Since the cross sections of the neutron capture reaction
are very small for both of 12 C and 19 F, the contribution of thermal neutrons to the background
spectrum was found to be negligible. The ﬂux of fast neutrons (above 500keV) in the Kamioka
mine is measured by a 3 He proportional counter as 2.06 × 10−6 neutron/cm2 /sec (Table 5.1.1)[82]. When neutron source is distributed uniformly to a boundless moderator which rarely
absorbs neutrons, the energy spectrum of fast neutrons is known to be proportional to 1/E[85].

106Figure 6.2.2: Cosmic-ray muon ﬂux as a function of depth in 103 hg/cm2 equivalent to km w.e.[81].

The Kamioka mine satisﬁes the above conditions because the laboratory is surrounded by a baserock which contains a small amount of water. By using the above assumption, we determined
the ﬂux of environmental fast neutrons in Kamioka mine as a function of energy as shown in
Figure 6.2.3, and we simulated the background by neutrons. Figure 6.2.4 shows the estimatedenergy spectrum of nuclei recoiled by environmental neutrons. The estimated event rate at
50 − 60 keV is 0.3 counts/keV/kg/days. Comparing to the measurements in Figure 6.0.2, thecomponent of environmental neutrons is found to be negligible.
Ambient gamma-rays are major background sources for direct dark matter searches. Environmental gamma-rays are mainly produced in the decay of 40 K and U/Th-chain. Although
the NEWAGE-0.3b’ detector has a strong rejection power against electron tracks as describedin Section 4.4.3, unrejected gamma-ray events might not be negligible. Figure 6.2.5 shows theestimated environmental gamma-ray ﬂux in our laboratory based on the measurement by a
5 × 5 × 1 cm3 CsI(Tl) scintillator[83]. Figure 6.2.6 is obtained energy spectrum by simulation
due to the environmental gamma-rays. The gamma-ray rejection power (Fig4.4.15) is taken
into account. The estimated event rate at 50 − 60 keV is 2.4 counts/keV/kg/days. Comparing
to the measurements in Figure 6.0.2, the component of environmental gamma-rays is one orderof magnitude less than the measurement.
107

counts/keV/cm2/sec

10-7

10-8

10-9

0

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
keV

counts/keV/kg/days

Figure 6.2.3: Estimated ﬂux of environmental fast neutrons in Kamioka mine.

10-1

10-2
0

50

100

150

200

250

300

350

400
keV

Figure 6.2.4: Estimated energy spectrum of nuclei recoiled by environmental neutrons. The estimatedevent rate at 50 − 60 keV is 0.3 counts/keV/kg/days.

108

counts/keV/cm2/sec

1

10-1

10-2

10-3

10-4

10-5

0

500

1000

1500

2000

2500

3000
keV

counts/keV/kg/days

Figure 6.2.5: Estimated ﬂux of environmental gamma-rays from measurement of a CsI(Tl)scintillator[83]. The gamma-rays from 40 K, U-chain, and Th-chain in the rock are taken into account.

1

10-1

10-2

10-3
0

50

100

150

200

250

300

350

400
keV

Figure 6.2.6: Estimated energy spectrum by environmental gamma-rays.

109

The contribution of the environmental neutrons and gamma-rays are summarized in Figure 6.2.7.

counts/keV/kg/days

103

RUN-14 (with all cuts)
RUN-14 (without roundness-cut)

102

Enviromental gamma-rays
Enviromental neutrons

10
1

10-1
10-2
10-3
0

50

100

150

200

250

300

350

400
keV

Figure 6.2.7: Energy spectrum of the measurement and estimated environmental backgrounds. Grayline shows the measured spectrum with all cuts. Black line shows the measured spectrum with all but
the roundness-cut. Red line shows the estimated spectrum by environmental gamma-rays. Blue line
shows the estimated spectrum by environmental neutrons.

1106.2.2

Internal background

From the high energy background studies described in Section 6.1, we found that someamount of the radon in the gas and U/Th-chain in the substrate of the µ-PIC are main background sources. In this section, we explain the possibility of the contribution of the radon andthe substrate to the background in the energy range of 50 − 400 keV.
α particles from the radon in the gas makes low energy background when they deposit thepartial energy by stopping at GEM or drift plane (B in Figure 6.2.1). We simulated this eﬀect
Amount of radon in the gas is evaluated in the high energy background study in Table 6.1.1.
The energy spectra of the low background from these two types of radons, 220 Rn and 222 Rn, areshown in Figure 6.2.8. Althhough half of radon events which stop in the GEM will be rejectedby the roundness-cut, another half which stop in the drift plane remains, and the estimatedspectra was in the same order to the measured one in the energy range of 100 − 400 keV.
103

counts/keV/kg/days

counts/keV/kg/days

103
RUN-14 (with all cuts)
RUN-14 (without roundness-cut)

102

RUN-14 (with all cuts)
RUN-14 (without roundness-cut)

102

220

Rn in gas 14.0[mBq/m3]

10

Rn in gas 9.3[mBq/m3]

222

10

1

1

10-1

10-1

10-2

10-2
0

50

100

150

200

250

300

350

400
keV

0

50

100

150

200

250

300

350

400
keV

Figure 6.2.8: Estimated energy spectra of background from 220 Rn (left) and 222 Rn (right) in the detection
volume. Gray line shows the measured spectrum in RUN14 with all cuts. Black line shows the measured
spectrum in RUN14 with all but the roundness-cut.

Next, the contribution of the radon in the gap region (B’) was studied. These gap events do
not undergo the gas ampliﬁcation by the GEM. By using the amount of radon in Table 6.1.1,
energy spectra of background from the two types of radons, 220 Rn and 222 Rn, are estimatedas shown in Figure 6.2.9. Because the gap volume is 1/100 times smaller than the detection
volume, background of radon in the gap region was negligible.
As described in Section 6.1.2, the substrate in the µ-PIC contains U/Th-chain backgroundsource (C and C’), and we simulated the low energy contribution of the α particles in the detection volume from substrate by using Geant4 simulation (C). This simulation was performed in111counts/keV/kg/days

10-1

counts/keV/kg/days

10-1
3

220

Rn in gap 14.0[mBq/m ]GEM gain=5

10-2

10-3

3

222

Rn in gap 9.3[mBq/m ]GEM gain=5

10-2

0

50

100

150

200

250

300

350

10-3

400
keV

0

50

100

150

200

250

300

350

400
keV

Figure 6.2.9: Estimated energy spectra of background from 220 Rn (left) and 222 Rn (right) in the gap
volume. We assumed a typical gain of GEM.

the same way as the high-energy continuous background in Section 6.1.2. The contamination of
U/Th-chain in substrate is evaluated by high energy background study in Table 6.1.2 We evaluated the low-energy background due to the substrate in Figure 6.2.10, which explains the ﬂatcomponents of measured spectrum around 100 − 400 keV energy range. Here, α particles fromsubstrate would be removed by the roundness-cut, and almost all measured events remained
before roundness-cut were rejected by roundness-cut in the energy range of 100 − 400 keV (Figure 5.1.3 and Figure 5.1.4). This fact supports that the measured background is due to thesubstrate of the µ-PIC.
Finally, the background from the substrate of the µ-PIC in the gap region (C’) was studied.
This simulation was performed in the similar way as the high-energy continuous backgroundstudy in Section 6.1.2, except the selected events are not through the GEM hole but stopped
in the GEM. These gap events do not undergo the gas ampliﬁcation by the GEM, and the
GEM gain was not known precisely. Therefore, several simulations were performed within the
GEM gain uncertainty of 4 to 7. From the evaluated contamination values in Table 6.1.2, thelow-energy background due to the gap events from the substrate was estimated and the results
are shown in Figure 6.2.11, which successfully explains the rising of the background near thethreshold.
In addition, we simulated the | cos θcygnus | distribution assuming the background from thesubstrate of the µ-PIC depositing the energy in the gap region (C’) as shown in Figure 6.2.12.
Compared to the measured | cos θcygnus | distribution, similar structure is seen in simulated one.
This result is a cross check of the existence of background source in the substrate of µ-PIC.

112

TPC events of 2.2[ppm] Th in uPIC substrate

TPC events of 0.9[ppm] U in uPIC substrate
counts/keV/kg/days

103

counts/keV/kg/days

103
RUN-14 (without roundness-cut)

Th in uPIC substrate

102

RUN-14 (without roundness-cut)

U in uPIC substrate

102

10

10

1

1

10-1
0

50

100

150

200

250

300

350

10-1
0

400
keV

50

100

150

200

250

300

350

400
keV

Figure 6.2.10: The estimated energy spectra of the events that α particles from radioactive impurities
included in the substrate of the µ-PIC in the detection region (C in Figure 6.2.1). Left and right ﬁgureshow the results by assuming 2.2 ppm of Th-chain and 0.9 ppm of U-chain, respectively. The evaluatedconcentration amounts of each isotope-chain discussed in Section 6.1.2 is used. We put the measured
spectrum of RUN14 without the roundness-cut because the events from substrate is basically cut by theeﬀect of z-ﬁducilization of roundness-cut.

gap events of 2.2[ppm] Th in uPIC substrate

gap events of 0.9[ppm] U in uPIC substrate
103

counts/keV/kg/days

counts/keV/kg/days

103

RUN-14 (without roundness-cut)
GEM-gain=7
GEM-gain=6

10

GEM-gain=5

2

GEM-gain=7
GEM-gain=6

10

GEM-gain=4

10

1

1

50

100

150

200

250

300

350

10-1
0

400
keV

GEM-gain=5

2

10

10-1
0

RUN-14 (without roundness-cut)

GEM-gain=4

50

100

150

200

250

300

350

400
keV

Figure 6.2.11: The estimated energy spectra of the events that α particles from radioactive impurities
included in the substrate of the µ-PIC stopping in the gap region (C’ in Figure 6.2.1). Left and rightﬁgure show the results by assuming 2.2 ppm of Th-chain and 0.9 ppm of U-chain, respectively. The
evaluated concentration amounts of each isotope-chain discussed in Section 6.1.2 is used. We assumed
several typical gain of GEM because we cannot measure the accurate gain of GEM itself. We put themeasured spectrum of RUN14 without the roundness-cut because the events from substrate is basically
cut by the eﬀect of z-ﬁducilization of roundness-cut.

113

600
500

substrate of uPIC
Th-chain 2.2ppm gap GEM-gain=5400

50-100[keV]

300
200

counts/keV/kg/days/|cosθcygnus|

counts/keV/kg/days/|cosθcygnus|

700

100
0
0

700
600
500

RUN14 (without roundness-cut)

400

50-100[keV]

300
200
100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0
0

0.9
1
|cosθcygnus|

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9
1
|cosθcygnus|

Figure 6.2.12: The simulated (left) and measured (right) | cos θcygnus | distrubition without roundnesscut. The simulation is assumed the background from the substrate of the µ-PIC depositing the energy
in the gap region (C’).

114

6.2.3

Summary of low energy background study

The contribution of the internal background is summarized in Figure 6.2.13. The measured spectrum without the roundness-cut is shown because the events from the substrate arebasically rejected by the z-ﬁdducial-cut eﬀect of roudness-cut, and the estimation of the eﬃciency of roundness-cut for such low-z background events has a large uncertainty. As shownin Figure 6.2.13, the low energy background is explained well by radon and U/Th-chain in
substrate of µ-PIC, which are determined from the high energy background study. Obtained
component of background are summarized in Table 6.2.1. Thus, environmental backgrounds

3

RUN-14 (with all cut)
RUN-14 (without roundness-cut)
220

3

Rn in gas 14.0[mBq/m ]10

TPC evts of Th in uPIC substrate 2.2[ppm]2

3

RUN-14 (with all cut)
RUN-14 (without roundness-cut)
310

gap evts of Th in uPIC substrate 2.2[ppm]gap evts of U in uPIC substrate 0.9[ppm]

10

1

1

10

50

100

150

200

250

300

350

400
keV

3

RUN-14 (without roundness-cut)
3

222

Rn in gas 9.3[mBq/m ]10

TPC evts of Th in uPIC substrate 2.2[ppm]

2

10-10

10

RUN-14 (with all cut)

200

250

300

350

400
keV

250

300

350

400
keV

3

1

150

200

222
Rn in gas 9.3[mBq/m ]TPC evts of U in uPIC substrate 0.9[ppm]

2

1

100

150

RUN-14 (with all cut)

10

50

100

3

10

10-10

50

RUN-14 (without roundness-cut)

10

gap evts of Th in uPIC substrate 2.2[ppm]

220
Rn in gas 14.0[mBq/m ]TPC evts of U in uPIC substrate 0.9[ppm]

2

10

10-10

counts/keV/kg/days

10

counts/keV/kg/days

10

counts/keV/kg/days

counts/keV/kg/days

are not dominant at present.

10-10

gap evts of U in uPIC substrate 0.9[ppm]

50

100

150

200

250

300

350

400
keV

Figure 6.2.13: The estimated energy spectrum due to the radons and the radioactive impurities includedin the substrate of the µ-PIC. Each ﬁgure diﬀers in the type of radon and U/Th-chain in substrate.

115

particle
position
muon
cosmicneutronenvironment
gamma-rayenvironment
222
220
α ( Rn/ Rn) TPC → drift plane or GEM
α (222 Rn/220 Rn) gap
α (U/Th)
substrate of µ-PIC → TPC
α (U/Th)
substrate of µ-PIC → gap
RUN14 with all cut
RUN14 with all but the roundness-cut

counts/keV/kg/days at 50 − 60 keV<∼ 0.1
0.26
2.4
<∼ 1
<∼ 0.1∼ 5 × ef froundness−cut∼ 500 × ef froundness−cut
24.1
732

section
6.2.1
6.2.1
6.2.1
6.2.2, 6.1.1
6.2.2, 6.1.1
6.2.2, 6.1.2
6.2.2, 6.1.2
5.1
5.1

Table 6.2.1: The list of background components at the threshold (50 keV), where ef froundness−cut is thecut eﬃciency of the low-z event by the roundness-cut.

6.3

Future prospects

From the study of the remaining background the dark matter search experiment, the radioactive contamination in the substrate of the µ-PIC is found to dominate the background at
the threshold energy. In the ﬁrst place, a gamma-ray radio assay of the µ-PIC material by aGe-detector is necessary to conﬁrm the contamination of U/Th-chain. As a next step, a new µPIC with low-background material needs to be developed in order to improve the sensitivities.
GEM signals can be used as the coincidence signal to reject the gap events.

116

Chapter 7

Conclusion
We developed a direction-sensitive dark matter detector, NEWAGE-0.3b’ to improve thesensitivity by one order of magnitude from our previous search with NEWAGE-0.3a. NEWAGE0.3b’ was designed to have a twice larger target volume with low background material, a lowered
threshold, an improved data acquisition system, and a gas circulation system with cooled charcoal. By a detector study prior to the dark matter search experiment, the energy thresholdwith a direction sensitivity was conﬁrmed to be lowered from 100 keV to 50 keV.
A direction-sensitive dark matter search in Kamioka underground laboratory with NEWAGE0.3b’ was performed from July 17th, 2013 to November 12th, 2013. With an exposure of
0.327 kg · days, a new 90% C.L. SD cross section limit 557 pb for WIMP mass of 200 GeV was
obtained (Figure 7.0.1). This result improved the limit by ∼ 10 times from previous measurement, and marks the best direction-sensitive limit.
The identiﬁcation of the background soruce is very important to improve the low backgrounddetector. Remaining background was studied in detail, and the substrate of µ-PIC was foundto be the dominant background source. If a new low-background µ-PIC is developed, thebackground level will be limited by environmental gamma-rays and it will be reduced by an
improvement of the tracking algorithm. The expected limit with the above improvements willreach to the DAMA region as shown in Figure 7.0.1. Further sensitivity improvements will
be achieved by recognizing the sense of the track, improving angular resolution, and usinglarge-size detectors with long-term operation, then a direction-sensitive dark matter search of
the whole DAMA region will be performed.

117

SD 90% C.L. upper limits and allowed region
105
NEWAGE surface run

104
NEWAGE 2010 (RUN5)σχ-p[pb]

103
THIS WORK (RUN14)

102
DAMA allowed(NaI)
low-BG µ -PIC + gamma reduction

10

1
10

102
Mχ [GeV/c 2]

3

10

SD
Figure 7.0.1: Limits of SD cross section, σχ−p
, as a function of mass of WIMP mass Mχ . Red thick solidline is the result of a directional method in this work. Black thin lines labeled ”NEWAGE surface run”
and ”NEWAGE 2010 (RUN5)” are previous results measured at surface and Kamioka, respectively[83].
Brown thick dotted line is a expected limit by reducing the U/Th contamination in µ-PIC and enviromental gamma-rays. Allowed region (DAMA[39]) is shown for comparison.

118

Acknowledgements
I would like to express my great gratitude to Professor Toru Tanimori whose suggestions
and sincere encouragement made enormous contribution to this thesis. I would also like toexpress my gratitude to Associate Professor Kentaro Miuchi who provided carefully consideredfeedback and insightful comments. I specially thank the µ-PIC collaborators: Dr. Hironobu
Nishimura, Associate Prof. Hidetoshi Kubo, Dr. Atsushi Takada, Dr. Parker D. Joseph,Dr. Yoshitaka Mizumura, Dr. Tetsuya Mizumoto, Tatsuya Sawano, Yoshihiro Matsuoka,
Shotaro Komura, Dr. Kazuki Ueno. I thank to those who spent and discussed together asa similar generation student in Kyoto University: Shinya Nakashima, Takao Onishi, Yusuke
Konno, Ryunosuke Gando, Kojiro Taniue, Naoki Higashi. I also thank to all the memberof the cosmic ray laboratory in Kyoto University: Prof. Takeshi Tsuru, Hiroyuki Uchida,
Takaaki Tanaka, Masayoshi Nobukawa, Takayuki Saito, Shinya Sonoda, Dai Tomono, Kumiko
Nobukawa, Ryusuke Sugawara, Shogo Nakamura, Kenichiro Hatanaka, Masato Yasumi.
I appreciate to the member of the particle physics laboratory in Kobe University which
had been accepted me as a sponsored student for last year of PhD course and gave me acomfortable place to concentrate on the study: Prof. Hisaya Kurashige, Prof. Yasuo Takeuchi,
Associate Prof. Toshio Hara, Associate Prof. Yuji Yamazaki, Assistant Prof. Atsuhiko Ochi,
Assistant Prof. Atsumu Suzuki, Tomoe Kishimoto, Keishi Hosokawa, Yuki Inamaru, Yosuke
Onishi, Rie Kurumida, Ye Chen, Yushiro Yamaguchi, Fumiya Yamane, Tsuyoshi Takemoto,Ryota Yakabe, Makoto Hasegawa, Shota Nakaura, Tsuyoshi Utsugiri, Yuya Ebi, Takuya Nakai,
Takashi Hashimoto, Rei Fujita, Satoru Yamauchi, Hiroki Osato.
I thank to the member of Kamioka institution for helping underground experiments: Hiroyuki Sekiya, Atsushi Takeda, Yasuhiro Kishimoto.
I would like to extend my gratitude to all the people who supported and encouraged meduring my time in graduate school. I wish to express my deep gratitude to Chihiro Shindo,
Inazuma, Daakumatan, Ms. Cygnus and her mother for great spiritual support. Finally, I
would also like to express my gratitude to my family for their warm encouragements.

119

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