Development of InP detector for solar pp/7Be neutrino measurement
Y.Fukudaa T.Izawad Y.Koshinob S.Moriyamab T.Nambac S.Sakaiharaa 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.Response for γ-rays

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

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 = 4.4 × 1014 yr , Ee≧498keV)
115In + ν → 115Sn* + ee
115Sn*(4.76μs) → 115Sn + γ (115keV) + γ (497keV)・ Possible BG due to correlated accidental
12coincidence by Bremsstrahlung◆Possible detector with 4tons of 115In assuming 5 years operation

γ spectrum obtained by InP detector
・ Photo electric peak for 81keV (133Ba), 122keV (57Co) and 60keV (241Am) were observed.
・ However, two peaks were found.
・ Lower peak position moved by increasing bias voltage.
・ Lower peak was obtained by the carrier drift at outside of depletion layer.
・ Higher peak corresponds to photo electric peak with charge collection.
・ Average energy of electron/hole pair production is assumed by 3.5eV-4.0eV.
・ Intrinsic energy resolution was obtained by MC σ=5%@122keV

Succeeded to develop InP detector
used for hybrid detector module.

・ Number of expected events assuming no ν oscillation → 1885
・ Number of expected event assuming LMA solution with Ee≧100keV → 720

θ12 = 29°- 34°

Statistical and theoretical error in total → ~3.9 %

3.0cm

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

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

5. Background measurement◆ Purpose : Measurement the natural β decay of 115In and theBremsstrahlung●Expected number of eventsβ decay : 68events/hour

Bremsstrahlung : 0.0006 - 0.714 events/hour●Configuration of InP detector and CsI scintillator
InP detector

CsI scintillator

・ CsI(Tl) scintillator : to detect Bremsstrahlung andother coincidence events with InP detector
・ CsI crystal size : 50mm×50mm×20mm
・ radiation shield : lead in 5cm thickness and oxygen
free copper in 1cm thickness
・ 4-π active veto plastic counter : to veto the cosmicray● VCZ (Vapor Pressure Controlled Czochralski)
high pressurechambar

・Product : Sumitomo Electric Industries, Ltd.

・Process
Inner chambar

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

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.

● Characteristic table
EPD～5,000cm-2

Conduct

Semi-insulation

DopantInP(Fe)Resistivity(4.9～5.2)×107∧cm

Mobility(2.8～2.7)×103Ωcm

Thickness456～459μm

Diameter

50.00mm

Orientation(100)±0.03°

I-V characteristics

leadoxygen free copper●Observed energy spectrum in
InP detector●Observed energy spectrum in
CsI scintillator

Observed spectrum
has different shapefrom expected by βdecay of 115In

Several peaks of γrays caused by U/Th
series were foundInP
I [A] ∝T2/3 exp(-Eg/2kβT)
Dark current could be reduced
by lowering temperature

Eg : band gap (1.29eV)kβ : Boltzmann constantT : element temperatureThis could be explainedby following noise・ 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.

●InP detector with cooling dewar

Size:
10mm×10mm×0.2mm
Electrode :・ Ohmic contact
・ Vacuum evaporationof Au base

Background amount
was calculated as afew 10-10g/g
・Observed events below 100keV

・Observed events above 300keV
Semi-Insulating InP
has Fe dopant● Characteristic of SI InP semi-conductor is both target and detector

3. Proto-type detector with SI InP

CsIInP

60Fe → 60Co ＋ e- ＋ ν(Ee≦3.978MeV ,τ1/2 =1.5×106 yr)

eAssuming 4×10-10 g/g 60Fe contaminationconsistent with 60Fe of β decay

CsI
Low energy events contain human walking noise
might be due to the vibration●Energy spectrum of coincidence events for CsI scintillator and InPdetector■ β , γ coincidence scheme

CsIInP
Some γs could escape from CsI scintillator
and then InP detector detected them.

・Observed CsI scintillator spectrum was higher energy than expected spectrum by Bremsstrahlung.
・Radiative Bremsstrahlung might not be able to explain the dip around 200keV.
・Simulation assuming above scheme can explain the shape of spectrum both CsI scintillator andInP detector.
Schematic view of InP detector

Most of coincidence events might be explained by U/Th series in CsI scintillator