InP solid state detector
for a measurement of low
energy solar neutrinos
TAUP2009 conference
Solar, Reactor and other Low-Energy Neutrino Physics
July 1st, 2009
Miyagi University of Education Y.Fukuda
ICRR, Univ. of Tokyo S.Moriyama, M.Shiozawa, Y.Koshio
ICEPP, Univ. of Tokyo T.Namba
Hamamatsu Photonics K.K. S.S.div. T.Izawa, M.Asakura

Motivation
Allowed region obtained by
combined results and KamLAND

Survival probability for solar
matter oscillation below 1MeV
Y.Suzuki@Neutrino2004

Neutrino energy
mixing angle θ12 is not well determined compared with θ23 obtained by Atm. ν.
Survival probability could increase at 5MeV or less in case of LMA solution,and the value of probability depends on θ12.
pp/7Be solar neutrino spectrum gives us precise θ 12
July 1st, 2009

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

2Capture of low energy solar neutrinos by 115In
R.S.Raghavan Phs.Rev.Lett37(1976)259

Nuclear Physics A 748 (2005) 333-347

νe → 115Sn* + e115Sn*(4.76µs) → 115Sn +γ1(115keV) + γ 2(497keV)
115In +July 1st, 2009● 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 neutrinosignal from huge BG (e1 +γ2 + γ3 )● Disadvantage
・ natural β-decay of 115In(τ1/2 = 4.4 × 1014 yr , Ee≧498keV)・ possible BG due to correlated
coincidence by radiative Bremsstrahlung
Requirement for the detector

1.
2.
3.

Good energy resolution :10%(FWHM)
Fine segmentation (104-105)
High efficiency γ detection

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

3Semi-Insulating InP semi-conductor
high pressurechambar

Product :Sumitomo Electric Industries, Ltd.
Crystal growth method:
Vapor Pressure controlled
Czochralski (VCZ)

Inner chambar

http://www.sei.co.jp/sc/technical/inp.html
EPD～5,000cm-2Conduct

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°

July 1st, 2009

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

4Possible InP detector for solar neutrinosscintilator

Multi-pixel structure for large area detector
 High Z scintillator surrounding InP detector detect γs
 4tons of 115In detector for low energy solar νIndium Project on Neutrino Observation for Solar interior(IPNOS) experiment
July 1st, 2009

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

5Semi-insulating InP cell detector

Mounted in vacuum chamber


SI InP cell detector using VCZ-InPwafer (product of SumitomoElectric K.K.)
Cooled by dry-ice (T = -79 degree)
Response for gammas from radioactive sources

July 1st, 2009

Surface size:
10mm×10mm×0.2mm(6mm×6mm×0.2/0.23/0.28/0.45mm)
Electrode :・ Ohmic contact
・ evaporated Au base metal
・ Insulator (SiN) to avoid leak current

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

6Principle of charge collectionInduced charge : dQ = qdx/dUsing Hecht formula,For full collection (Le+Lh～d) Q=Q0

July 1st, 2009

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

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γ spectrum measured by InP detector

July 1st, 2009

InP detector should be cooled(-79 degree using Dry-Ice)Clear photo-peak wasobserved, but two peakstructure
Lower peak: induced charge
generated by drift of carrier(electron and hole)
Higher peak: full chargecollection
Energy of electron-hole pair
production : 3.5eV
Energy resolution :25%@122keV for induced
charge peak (intrinsic : 3%)

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

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Spectral shape and simulation

Assuming,Le~200µmand Lh~30µm, two peak structure could be
reproduced by induced charge and full charge collection.

July 1st, 2009

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

9Optimization for detector thickness

July 1st, 2009

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

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Observation of internal 115In β decay andcorrelated backgrounds
CsI(Tl) scintillator : detect
radiative Bremsstrahlung
and other coincidence eventswith InP detector
 CsI crystal size :50mm×50mm×20mm
 radiation shield : lead in 5cmthickness and oxygen freecopper in 1cm thickness 4-π active veto plastic
counter : veto cosmic raymuon
July 1st, 2009

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

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115In β-decay signal in InP detector

 Observed spectrum

has different shapefrom the expected
one by β decay of115In.
 Events with E<100keV

seem to be noise dueto the vibration
 Events with E>300keV
seems to be anotherbackgrounds TAUP2009 Solar, Reactor and otherJuly 1st, 2009

Low-Energy Neutrino Physics

12U/Th contamination in SI InP wafer According to BG

measurement using
low-BG Ge detector,
amount of U/Th
contamination areevaluated by 5X10-11g/g and 3X10-11g/g,
respectively .

July 1st, 2009

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

13β−decay spectrum with U/Th backgrounds
 Assuming U/Th

backgrounds, thespectral shape with
E>300keV seems tobe consistent with theobserved spectrum.
214Pb(Emax=670keV)
214Pb(Emax=730keV)
212Pb(Emax=334keV)
234Th(Emax=106keV)

 Another source of β−decay with
E>300keV?
July 1st, 2009

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

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Backgrounds inside of Shield
Measurement of large area Si
detector

Measurement of CsI scintillator
detector

No peak of gammas （βspectrum of U/th?）Several peaks ofgammas from U/Thdecay

July 1st, 2009

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

15

Observed coincidence backgrounds
Coincidence background
was observed.
 CsI scint. detects external
gammas from U/Th decay
 InP (Si as a reference)
detects internal/external
βs from U/Th decay.
 No significant radiative
Bremsstrahlung from 115In
β decay was observed.
need more statistics
July 1st, 2009

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

16Uncorrelated BG for solar ν experiment
InP signal (ev#1) andscintillator signals (ev#2with E~116keV and ev#3
with E~497keV) within
10ms Gate Uncorrelated BG: 5×10-6
events/day/module= 10 events /day/wholedetector =2.0X106
modules
 U/Th in InP wafer should
be reduced ~1/10


July 1st, 2009

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

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Conclusion
InP detector observed clear peak of γs Induced charge due to drift of carrier (electron and hole)
generated by radiation.
 Average energy of carrier production : 3.5eV
Vertical Bridgeman method
 Energy resolution : 25%
 No significant backgrounds related to radiativeneed more statistics
Bremsstralung of 115In
 Amount of Internal U/Th contamination should be
reduced by 1/10 in order to keep S/N~1


Next step : IPNOS phase-I (10kg InP in LXe)
 Low background (&low temperature) environment inside of LXe A few events per year for pp solar neutrinos, but…
 Demonstrate actual performance for low energy solar ν
July 1st, 2009

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

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Prototype of multi-pixel InP detector

July 1st, 2009

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

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New concept for IPNOS phase-I experiment
InP multi-pixel detector inside ofLiquid Xenon.

July 1st, 2009

30cm cubic chamber (like XMASS
100kg prototype) includes ~10kg
InP detector

TAUP2009 Solar, Reactor and other
Low-Energy Neutrino Physics

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