Development of InP detector for solar pp/7Be neutrino measurement
Y.Fukudaa T.Izawad Y.Kaminagaa Y.Koshinob Y.Miyakea S.Moriyamab T.Nambac R.Onoa T.Satoa M.Shiozawaba Miyagi University of Educationb ICRR, University of Tokyo

c ICEPP, University of Tokyo d Solid state division, Hamamatsu Photonics K.K.

supported by the Grant-in-Aid Scientific Research (B)17340065 of Japanese Society for the Promotion of Science, Inamori Foundation, and Asahi Glass Foundation

4. Spectrum analysis

1. Introduction◆ Purpose : Precise measurement of ν oscillation mixing angle θ12◆ Status of determination for ν oscillation parameter

events/sec/0.1fC◆Response from 57Co, 133Ba and 241Am

・ 96%C.L. allowed region obtained by global fit
LMA solution (blue) : Cl + Ga + SK (D/N spectrum)
KamLAND (green) → confirm Δm122

27°< θ 12<37°
mixing angle θ12 is not constrained compared with θ23 (obtained by Atm. ν)
events/sec/0.1fC

Survival probability tends to increase in 5MeV or less in case of LMA solution,and shapes depend on θ12.

Amount of pp/7Be ν should be observed

events/sec/0.1fC

Y.Suzuki@Neutrino2004● on-going solar pp/7Be experiment・ KamLAND (Liquid scintillator, electron elastic scattering (ES))
・ Borexino (Liquid scintillator, ES)● future solar pp/7Be experiment
・ XMASS (LXe, ES, DIRK MATTER)
・ LENS (Liquid scintillator loaded In/Ye, charged current (CC))
・ CLEAN (LNe, ES)・ MOON (100Mo,CC)・ SIREN (160Gd, CC)◆ Technique of low energy solar neutrino detection
R.S.Raghavan Phs.Rev.Lett37(1976)259● Advantage
・ direct counting
・ sensitive to low energy νe (Eν ≧ 125keV)・ measuring the energy (Ee = Eν － 125keV)・ Triple fold coincidence to extract neutrino signal
from huge BG (e1 +γ2 + γ3 )● Disadvantage
・ natural β-decay of 115In (τ1/2 = 6.4 × 1014 yr)
115In + ν → 115Sn* + e・ Possible BG due to correlated accidentale
115Sn*(4.76μs) → 115Sn + γ (115keV) + γ (497keV)
coincidence by Bremsstrahlung
12γ spectrum obtained by InP detector●Possible detector with 4tons of 115In assuming 5 years ope.
・ Number of expected events assuming no ν oscillation → 1885
・ Number of expected event assuming LMA solution with Ee≧100keV → 720

28.5cm1.0cm●Hybrid structure of InP and solid scintillator
・InP detector module 10mmX10mmX0.2mm cell・Solid scintillator surrounding InP detector to detect γs●4tons of 115In detector for solar ν experiment
InP : 5.1tons (2.1X106 modules)
solid scintillator (ex. CsI) : 934tons
total size : 6.3m X 6.3m X 5.3m

◆ How to increase the carrier drift length
1. Increasing mobility (μ)● μτ product value

2. Semi-Insulating(SI) InP semi-conductor

Conduct

Semi-insulation

DopantInP(Fe)Resistivity(4.9～5.2)×107Ωcm

Mobility(2.8～2.7)×103Ωcm

Thickness456～459μm

Diameter

50.00mm

Orientation(100)±0.03°

Cryostat for LN2 in preparation

DryIce (-79 ℃)・ Increasing μτ product value ・ Increasing mobility by coolingat lower temperature

Liquid N2(- 186℃)

Direct measurement of mobility using Hall Effectwill be done by Semiconductor Research Institute.

・Process

2. Higher bias voltage (V0)

In inner chamber controlled by steam pressure of As or P, covering material meltby B2O3 like LEC method and pulling up single crystal growth from it. Because ofcrystal growth in low temperature gradient, transition density can reduce.

・ Avoid electric noise due to dark current
・ Schottky barrier height: 0.37eV(Au) / 0.41eV(Pd) / 0.54eV(Ag) Ref: J.Phys.III 1(1991) 749-758

Pd or Ag)

5. Prototype detector

I-V characteristics◆ proto-type module

I [A] ∝T2/3 exp(-Eg/2kβT)
Dark current could be reduced
by lowering temperature

Eg : band gap (1.29eV)kβ : Boltzmann constantT : element temperature

I-V characteristics● Response for γ-rays

3. Test Module with the SI InP◆ Detection of radiation with SI InP solid state detector
・ Produced electron loses its energy, andcreates electron-hole pair as carrier.
・ Carrier moves along electric field.
・ If drift length is enough to the thickness,charge collection efficiency achieves 100%.
・ If the drift length is not enough, the carrier
might be trapped by the impurity before
the collection.
・ Drift length is proportional to carrier mobility
and electric field.
Size:
7mm×7mm×0.2mm
Electrode :・ Ohmic contact
・ Vacuum evaporationof Au base

Schematic view of InP detector

events/sec/0.1fC● Characteristic of SI InP semi-conductor is both target and detector●InP detector with cooling dewar

Change electrode material (Au・ size:10mm X 10mm X 0.2mm
・ same structure as testmodule
・ current is proportional toelectrode area.
・ just delivered since 12thof July !
・ first data has been takenwith DryIce cooling.

・No. of Events have been
increased by 3.1 times ascompared with 2.8 times
larger electrode area(57Co).

events/sec/0.1fC～5,000cm-2

Liquid N2
Detector

Ref:J.Costa and A.Peczalski, J.applo Phys.66(2),(1989)p674-479

・Product : Sumitomo Electric Industries, Ltd.

● Characteristic tableEPDlower

Extrapolated Low-Field Electron Drift Mobility for InP

events/sec/0.1fC

http://www.sei.co.jp/sc/technical/inp.html

Temperature

Temperature(k)● VCZ (Vapor Pressure Controlled Czochralski)

Inner chambar

57Co

MCAssuming Ld ~1000mm, photoelectric peak will be single peak withgood energy resolution◆ InP crystal growth method
high pressurechambar

Carrier drift length Ld should be longer

Drift Mobility (cm2/Vs)

3.0cm・ Assuming charge collection due to both depletion
layer formed at the ohmic contact between electrodeand InP and drift of carrier in remaining region・ Input parameter : thickness of depletion layer : 30μm
Drift length:250μm, e/h production energy : 3.5eV・ MC reproduces spectral shape of data well

Low CCE signal improved by increasing carrier drift lengthθ12 = 29°- 34°

Statistical and theoretical error in total → ~3.9 %Spectral analysis with MC・ Photo electric peak for 81keV (133Ba), 122keV (57Co)
and 60keV(241Am) were observed,・ However, two peaks were found.
・ Higher peak corresponds to photo electric peak withcharge collection efficiency (CCE) 100%.
・ Average energy of electron/hole pair production is
obtained by 3.5eV.
・ Lower peak obtained by carrier drift is equivalent to
CCE 56%(57Co) and 72%(133Ba), respectively
・ Intrinsic energy resolution σ=5%@122keV

Effective area spread.
・Proto-type module has same
performance as test module.

◆ Prototype detector for solar neutrino experiment● proto-type detector consists of 4 modules● Measurement of 115In β-decay spectrum andbremsstrahlung in Kamioka mine
● estimate effect of accidental coincidence due to backgrounds.
● establish counting methods.
● Kyoudou-riyou program of ICRR, Univ. of Tokyo● experiment will start at the end of this year or early of next year

Schematic view of experimental setup
Experimental hall

Oxygen free copper

CsI

Lead Shield