InP solid state detector for measurement of low energy solar neutrinos(Indium Project on Neutrino Observation for Solar interior : IPNOS)Y.Fukudaa T.Izawad Y.Koshinob S.Moriyamab T.Nambac M.Shiozawaba Miyagi University of Educationb ICRR, University of Tokyo

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

4.Response for γ-rays

1. Introduction◆ Purpose (1) Determine ν oscillation mixing angle θ12・ 96%C.L. allowed region obtained by global fit● γ spectrum measurend by InP detector @-79 ℃
・ Measured clear photopeak, but two peak structure
・ Lower peak: inducedcharge (Ld~120mm)
Higher peak: full Q collection
・ Energy of electron-hole
pair production : 3.5eV
Energy resolution :25%@122keV

LMA solution (blue) : Cl + Ga + SK (D/N spectrum)
KamLAND (green) → confirm Δm12227°< θ 12<37°・ mixing angle θ12 is not fixed compared with θ23 (obtained by Atm. ν)・ Survival probability would increase at 5MeV or less in case of LMA
solution, and the shapes depends on the value of θ12.
pp/7Be solar neutrino spectrum will fix precise θ 12
● on-going solar pp/7Be experiment・ KamLAND (Liquid scintillator, electron elastic scattering [ES])・ Borexino (Liquid scintillator, ES)

Y.Suzuki@Neutrino2004● future solar pp/7Be experiment
・ XMASS (LXe, ES, DARK MATTER)
・ LENS (Liquid scintillator loaded In/Ye, charged current [CC])・ CLEAN (LNe, ES)・ MOON (100Mo,CC)・ SIREN (160Gd, CC)

InP detector works as
expected, but lessenergy resolution◆ Purpose (2) Solve new solar problem
small descrepancy between observation of Helioseismology and prediction of standard solarmodel will be solved by CNO cycle
only neutrinos from CNO cycle tells us definite answer◆ Technique of low energy solar neutrino detection● Advantage
・ large cross section (~640SNU)
・ direct counting for solar neutrinos
・ sensitive to low energy region (Eν ≧ 125keV)・ energy measurement (Ee = Eν － 125keV)・ triple fold coincidence to extract neutrino signal
from huge BG (e1 +γ2 + γ3 )● Disadvantage
・ natural β-decay of 115In
115In + ν → 115Sn* + e(τ1/2 = 4.4 × 1014 yr , Ee≧498keV)
e115Sn*(4.76μs) → 115Sn + γ (115keV) + γ (497keV)・ possible BG due to correlated accidental
12coincidence by radiative Bremsstrahlung

R.S.Raghavan Phs.Rev.Lett37(1976)259◆Possible detector with 4tons of 115In assuming 5 years operation
・ Number of expected events assuming no ν oscillation → 1885
・ Number of expected event assuming LMA solution with Ee≧100keV → 720

5. Background measurement◆ Measurement the natural β decay of 115In and demonstrationfor the triple fold coincidence●Configuration of InP detector and CsI scintillator
InP detector

CsI scintillator

leadoxygen free copper●Measure the β-decay spectrum in InP detector
・Observed b-decay specrum

θ12 = 30°- 34°

Statistical and theoretical error in total → ~3.5 %◆InP solid state detector (as both target and detector) with scintillator●Hybrid structure of InP and solid scintillator
3.0cm

28.5cm
1.0cm

・ CsI(Tl) scintillator : to detect Bremsstrahlung
and other coincidence events with InP detector
・ CsI crystal size : 50mm×50mm×20mm
・ radiation shield : lead in 5cm thickness andoxygen free copper in 1cm thickness
・ 4-π active veto plastic counter : to veto cosmic
ray muon

・InP multi-pixel detector (10mmX10mmX0.2mm cell)・solid scintillator surrounding InP detector to detect γs●4tons of 115In detector for solar ν experiment
InP : 5.1tons (2.0X106 modules with ΔE/E~10% )solid scintillator (ex. CsI) : 934tons
total size : 6.3m X 6.3m X 5.3m

InP・U/Th contamination in SI InP wafer

Observed spectrum has
different shape from theone expected by βdecay of 115In

According to Ge
measurement,
amount of U/Th
are estimated by
5X10-11 g/g and3X10-11g/g,
respectively .

Events with E<100keV
seem to be vibration noise・Observed events above 300keV●Measure the BG spectrum in CsI scintillator

2. Semi-Insulating(SI) InP semi-conductor● VCZ (Vapor Pressure Controlled Czochralski)
・Product : Sumitomo Electric Industries, Ltd.
・Process
In inner chamber controlled by steam pressure of As or P,
Inner chambar
covering material melt by B2O3 like LEC method and pulling up
single crystal growth from it. Because of crystal growth in low
http://www.sei.co.jp/sc/technical/inp.html
temperature gradient, transition density can reduce.

CsI

high pressurechambar● Characteristic table
EPD～5,000cm-2

Conduct

Semi-insulation

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

Mobility(2.8～2.7)×103cm2V-1s-1

Thickness456～459μm

Diameter

50.00mm

Orientation(100)±0.03°

amount of U/Th contamination
was evaluated by ~10-10g/g

Whole shape is consistent
with 115In + U/Th β−decay●Coincidence BG events between InP (Si) detector and CsI scintillator

I-V characteristics■ InP/CsI

I [A] ∝T2/3 exp(-Eg/2kβT)
Dark current could be reducedby lowering temperature■ Si/CsI as reference

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

・Observed spectrum by CsI scintillator are consistent with the U/Th γ (not inside of CsI)・Observed spectrum by InP(Si) are expected by U/Th β−decay (inside and outside of InP(Si))
・No clear evidence for radiative Bremsstrahlung

3. SI InP cell detector●InP detector in vacuum dewar

Several peaks of γ rays causedby U/Th contamination wereobserved in self trigger

Schematic view of InP detector
Size:
10mm×10mm×0.2mm
Electrode :・ Ohmic contact
・ Au vacuum evaporation
・Insulator (SiN) to avoid
leak current●Effect of triple fold coincidence for detecting solar neutrinos
Assuming accidental triple coincidence in InP hybrid detector (ev#1) and solid scintillator (ev#2with E~116keV and ev#3 with E~497keV) within 10μs gate width, 5×10-6 events/day/1 hybrid
detector = 10 events /day/whole detector (2.0X106 ) is evaluated (S/N~0.2)

Amount of U/Th contamination in material should be reduced by order of 1/10)

6. Next stepcooled by Dry-Ice (-79 ℃) or LN2 (-185 ℃)● Improve energy resolution●Charge collection scheme
Procedure to detect charge
1) Produced electron loses its energy, andcreates electron-hole pair as carrier.
2) Carrier moves along electric field.
3) If drift length is enough to detector
thickness, the charge collection efficiency
achieves 100% as usual SSD.
4) If the drift length is not enough, thecarrier might be trapped by the impurity
before achieving the electrodes.
5) In above case, most collection done byinduced charge.

・reduce dark current
Metal-Insulator-Semiconductor(MIS) structure
・Increase drift length
improve quality and reduce
impurity using wafer produced
by Vertical Bridge method●demonstrate proto-type
detectorThicker detector (0.5μm)
reproduces the spectral
shape with corresponding
electric field.
MIS structure detector
collect same amount ofcharge as usual detector.

Confirm induced charge ,but same energy resolution

We will test 1st proto-type detector (~100g InP)
in 2009 and start mini-exp. (~10kg) in 2011