InP solid state detector and the observation of low
energy solar neutrinos
Y. Fukuda
Faculty of Education, Miyagi University of Education,
149, Aobaku-aza-aoba, Sendai, Miyagi 980-0845, JapanT. Izawa
Solid State Devision, Hamamatsu Photonics K.K.
1126-1, Ichino-cho, Hamamatsu, Shizuoka 435-8558, JapanY. Koshio, S. Moriyama, M. Shiozawa
Kamioka Observatory, Institute for Cosmic Ray Research,
University of Tokyo
Higashi-Mozumi, Kamioka-cho, Hida, Gifu 506-1205, Japan

T. Namba
ICEPP, International Center for Elementary Particle Physics,
University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
Abstract. A large volume radiation detectors using a semi-insulating Indium Phosphide(InP) wafer have been developed for Indium Project on Neutrino Observation for Solar interior(IPNOS) experiment. The volume has achieved to 20mm3 , and this is world largest size among
the detector observed gammas at hundred keV region. Although the depletion layer, most of
charge are generated by an induction, and the charge collection efficiency achieves 50 to 60%, which is determined by the detector thickness and the carrier drift length (Ld = 120µm).
The energy resolution is obtained by 25 %. We measured actual backgrounds from 115 In beta
decay, and also the effect of radiative Bremsstrahlung from those betas. No significant event
was found in the measurement and the radiation such as Bremsstrahlung from InP detector
could be negligible.

keyword:low energy solar neutrinos (低エネルギー太陽ニュートリノ）
neutrino oscillation （ニュートリノ振動）
stellar evolution （恒星進化）
indium (インジウム）
solid state detector （半導体検出器）

1. Introduction
In 1998, Super-Kamiokande has reported an evidence of atmospheric µ neutrino oscillation[1] for the first of the world, and the K2K experiment has confirmed the oscillation using νµbeam produced by KEK 1 GeV Proton Synchrotron[2] in 2005. On the other hands, SuperKamiokande and Sudbery Neutrino Observatory experiment has established the νe oscillationfrom solar neutrinos in 2001[3, 4], and a long way problem so called Solar Neutrino Problem
in past 30 years was almost solved by LMA oscillation. Independently, KamLAND experimentconfirmed the oscillation using reactor νe in sense of ∆m2 [5]. However, the oscillation mixing
angel (θ12 ) was not pointed out as well as θ23 observed in the atmospheric neutrino data. Next
step of neutrino physics should be to measure the precise oscillation parameter and CP phase.
For instance, a valuable θ13 measurement should be done as soon as possible by T2K experimentand the reactor experiment such as Double Chooz. For the future solar neutrino experiment,a precise θ12 measurement would be done with 1% accuracy. In this point of view, newexperimental technique to measure not only flux but also energy of solar pp/7 Be neutrinos will
be necessary due to the direct observation of upturn of the neutrino observation. On the otherhands, Helioseismology shows us the information of the interior of the Sun using observationsof slight motion on the surface. Most of analytic results are consistent with predictions of thestandard solar model, however, still small difference of frequency less 0.5% in the intrinsic mode
between the observation and the model remains. Therefore, low energy neutrinos from not
only pp − chain but CNO cycle are also important for direct investigation of solar interior on
the stellar evolution theory. We are going to plan the experimet of low energy solar neutrino
observation, so called, Indium Project on Neutrino Observation for Solar interior (IPNOS), forthese objects.
2. Low energy solar neutrino experiment using 115 In
Traditional technique used in KamLAND, Borexino [6] and Super-K is an elastic scattering ofelectron with induced neutrinos. Radio-chemical technique used in 37 Cl and 71 Ga is a neutrino
capture using special nuclei which has a large cross section. Former case could measure both fluxand energy of incident neutrinos with relatively higher energy, but later could only averaged flux.
Most difficult problem is caused by natural radio activity backgrounds which emit low energy
gammas and betas. In 1976, R.Raghavan proposed new technique for the measurement of low
energy pp/7 Be solar neutrinos [7] via following reaction;
115

In + νe →115 Sn∗ + e− .

(1)

The prompt electron has an energy with Eν −125 keV, here Eν is an energy of incident neutrinos.
Therefore the neutrino spectroscopy can be realized. An excited state of 115 Sn shown in Eq.(1)decays into the ground state with a lifetime of 4.7 µs, and emits two gammas (116 keV and 487
keV). This signature is also able to use for a triple-coincidence to extract neutrino signal fromhuge backgrounds. However, 115 In itself has natural beta decay into the ground state of 115 Snwith a lifetime of 4.4 × 1014 years. The radiative Bremsstrahlung could produce fake coincidence
for neutrino signal. To avoid this, a fine segmented with well energy resolution detector isnecessary [8].
Many possible detectors using indium were designed in last few decade, however, no realistic
detector has been made. In 1988, Suzuki and Fukuda developed the InP solid state detector
with the pn-junction and the detector observed the gammas from the radioactive sources [9].
The detector size was very small(1mm2 × 10µm), however, the prototype detector could observe
the low energy 60 keV gammas with the energy resolution of 5.5%. For large volume detector, aliquid scintillator solved indium was developed by Suzuki [10] and LENS project [11]. Recently
LENS group presented the feasibility of realistic way to build a big scale detector [12], but there
might be exist the difficulties of the transparency with respect to the weight of solvent.

New detector using InP semi-conductor has been re-evaluated for last several years. The semiinsulating (SI) InP wafer is commercially produced for the optical devices. Typical dopant is Fe
and the crystal is usually grown by the liquid encapsulated Czochralski (LEC). Some radiation
detectors have been developed by the SI InP wafer. The ESTEC group have characterized a 0.18
mm thick of InP at -60 ◦ C achieving 8.5 keV FWHM at 60 keV[13], and Italy group obtained 11
keV FWHM at 122 keV with 0.25 mm thick SI InP at -60 ◦ C[14]. There is other crystal growthmethod, namely the Vertical Gradient Freeze (VGF), which is relatively higher resistance andnaturally smaller EPD than LEC. A radiation detector using the VGF SI InP wafer is developedby UK group[15], and our group. All developed detectors, however, have a small volume of theorder of 1mm3 or less except us, and it seems to hard to use for solar neutrino experiment.
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Figure 1. Possible detector design for IPNOS experiment. Left figure shows one module asmulti-pixel combination, and right figure shows the hybrid structure consist of multi-pixel InP
module surrounded by scintillator for detection of two gammas.
It is possible to be solar neutrino detector using InP solid state detector, if the detector have amassive. If the InP detector have an order of 1g, which corresponds to 10mm × 10mm × 0.2mm,
then 106 order of detector are necessary for the actual detection of solar neutrinos. We needto combine such small detector as multi-pixel as shown in left side of Fig.1. According to thisidea, 5 × 5 detectors are combined as one module, so same bias voltage should be applied andalso the signal line also should be common. Another feature is the detection of gammas fromνe capture process by 115 In nuclei. These 116 keV and 497 keV gammas could escape from theoriginal wafer of InP, so other calorimeter should detect them efficiently. The IPNOS detector
will consist of a hybrid structure that InP modules surrounded by the ultra-low background(solid) scintillator as shown in right side of Fig.1.
3. Performance of InP detector
We have chosen another wafer produced by Sumitomo Electrical Industry Co. LTD withthe method of Vapor Czochralski (VCZ). Hamamatsu Co. LTD developed InP detector with
10mm × 10mm in surface, and 0.2mm in thickness. The electrodes consist of Cr-Au with 1µm
thickness for top and Au-Ge/Ni/Au with 0.13/0.015/0.5 µm thickness for bottom as shown in
Fig.2. The junction between electrode and InP are ohmic contact in room temperature, however
actually a Schottky barrier could be formed at low temperature, because of the rectification in
the measurement of Hall effect.
The performance of InP detector was measured by using gammas emitted by usual radio
active sources. Carriers generated by the energy deposit of an electron via photoelectric process
or Compton scattering are drifted along to electric field. Assuming a drift chamber in case ofsolid state, the charge are induced by the polarized carrier at the surface of electrode. The

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shows that the observed charge distribution measured at -79 ◦ C for several radio-isotopes. There
found two peaks in each spectra. For instance, photo peak for 122 keV gamma in 57 Co appears
around 0.3 × 10−14 C and 0.55 × 10−14 C. Higher peak is produced by the charge collectionin the depletion layer (thickness = x0 ), and it is consistent with other sources assuming by anaverage energy for electron/hole pair production to be 3.5 eV. This assumption is also confirmedby the fact that the higher peak position was not changed as increasing bias voltage. On theother hands, lower peak moves to higher position as increasing bias voltage. This is naturally
explained by above formula. The collection efficiency for lower peak corresponds to 50% to 60%.
According to a simulation using Eq.(2) with the energy resolution of 25% at 122 keV for a lower
peak, the spectral shape could reproduce assuming both Ld ∼ 120µm and x0 ∼ 20µm as shownin Fig.3. An intrinsic energy resolution for a higher peak was also determined by 3% at 122
keV in this simulation. This induced charge were also found in the another sample, which had a
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Xtal Technology Inc. The observed charge spectra were well reproduced by same simulation but
no depletion layer as shown in Fig.4.
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Figure 4. Observed and simulated charge distributions for gammas obtained by VGF InP
detector (6mm × 6mm × 0.5mm) are shown.

4. Measurement of backgrounds
As described in section 2, 115 In decays naturally with beta emission, and it could be possible
background in the observation of solar neutrinos. In order to measure the β decay from 115 In
and the effect of Bremsstrahlung, we used InP solid state detector for measurement of β spectra
and CsI(Tl) scintillator which was produced by SCIONIX for the measurement of radiative
Bremsstrahlung. The CsI size was 50mm × 50mm × 20mm. These two detectors are locatedby face to face and set inside of radio-active shield which consist of the lead in 5cm thickness
and the oxygen free copper in 1cm thickness. The 4-π active veto plastic counter surrounded
the shield rejected backgrounds due to the cosmic ray and the external radiation.

Figure 5. Configuration of InP and CsI detector, radio-active shield and 4-π active veto plastic
counter surrounded the shieldIn the measurement of β decay of 115 In, the events without any coincidence between InP
and CsI detector nor veto counter. The spectra of observed events and expected β decay are
shown in Fig.6. The event rate of β decay was expected by 68 per hour. For low energy events
blow 100 keV, the backgrounds due to the vibration seems to dominated. The InP detector was
so cooled that a floated capacitance between the InP detector and the charge amplifier madesensitive to external vibration. Observed events above 400 keV was still inconsistent with the
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Figure 6. Observed energy distribution of InP detector and the expected β decay spectrum.
Residual events above 400 keV was consistent with the 60 Fe β decay.
spectrum of β decay of 115 In. According to the measurement of U/Th activity using ultralow background germanium detector located in Kamioka mine, the semi-insulated InP wafer
contains those activity as order of 10−11 g/g. This amount of backgrounds could not explain
both number of these events nor a spectral shape. Another possibility was iron (Fe) which isused for the dopant of semi-insulating InP crystal. The 60 Fe nuclei decays into 60 Mn with β −decay (Ee ≤ 3.978 MeV, T 1 = 1.5 × 106 years). Assuming 60 Fe contaminated an order of 10−102g/g, then the expected energy spectrum in InP detector are almost consistent with the observed
spectrum as shown in Fig.6.
For a measurement of radiative Bremsstrahlung from β decay of 115 In, it should take acoincidence between InP and CsI detector within 10 µsec. The radiative Bremsstrahlung isdifferent from usual one, because of the radiation emits due to β nuclei, not a free electron. The
Coulomb potential is so stronger that an energy of the radiation could be higher (∼ 100 keV) thanusual X-ray with less 10 keV. In case of 10 hours measurement, totally 105 events were found.
The observed number of events looks larger than the expected number of events of β decays.
Before concerning of coincidence events, it is necessary to take into account the U/Th natural
backgrounds in CsI scintillator, Figure 7 shows the energy spectrum of CsI detector obtained by
the self trigger. It is clearly seen that the photo-electric peak due to some nuclei in the U/Th
decay chain. The amount corresponds to several 10−10 g/g order of U/Th contaminated in the
CsI scintillator or the photo multiplier. This amount is quite adequate, however it becomes
non-negligible backgrounds. Those nucleus generally decay with both gamma and beta, so thatsome gammas could escape from CsI scintillator and enter into InP detector as described in
Fig.7. Applying the simulation using this scheme, the energy spectrum of InP and CsI detector
taken the coincidence was obtained as shown in Fig.8.
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Figure 8. Energy spectrum of coincident events for CsI and InP detector for 32 hours data,
and the expected spectrum from above scheme is overlapped.
According to these figures, most of coincident events between InP and CsI detector looks
consistent with β-γ coincidence of U/Th backgrounds in CsI scintillator, and no clear evidence
for the effect of radiative Bremsstrahlung could be observed. However, this was not completely
confirmed by the radiative Bremsstrahlung, because of difficulties to separate them. We have tomeasure same spectrum using other solid state detector such as CdTe, and confirm them causedby U/Th series in CsI scintillator. However, these radiative Bremsstrahlung backgrounds could
be eliminated by the timing difference, because of few µ seconds in the case of 115 In decay.
Another possible background should be concerned by neutron capture (n, γ) reaction by
115 In. The cross section is quite large (an order of 100 barn) and some resonances have a peakvalue (an order of 1000 ∼ 10000 barn) 116m In nuclei decays into 116 Sn with β − decay (T 1 = 54.2
2min) and emits gammas such as 1.29 MeV, 0.42 MeV and 2.11 MeV. We will measure this effectusing natural neutron background and nuclear fission neutron like 252 Cf, however it will not bealso severe background for IPNOS experiment due to an efficient neutron shield.
5. Conclusion
An InP solid state detector has been developed and obtained suitable performance. Observed
charge spectra could be explained by the carrier drift length as Ld = 120µm, and the collection
efficiency is achieved by 50 to 60 %, which corresponds to the induced charge. This is the firsttime to demonstrate InP detector with a bulk size crystal, and detector performances including
the background measurements show us that an InP detector is actually possible to use for IPNOS
experiment.
Acknowledgments
This work was supported by KAKENHI Grant-in-Aid Scientific Research (B) 17340065 ofJapanese Society for the Promotion of Science, Inamori Foundation, and The Asahi GlassFoundation.
References[1] Y.Fukuda et.al, Phys. Rev. Lett. 81 (1998) 1562.
[2] K2K collaboration, Phys. Rev. Lett. 94 (2005) 081802.
[3] S.Fukuda et.al, Phys.Rev.Lett. 86 (2001) 5651.
[4] SNO Collaboration Phys.Rev.Lett. 87 (2001) 071301.
[5] KamLAND collaboration, Phys.Rev.Lett. 90 (2003) 021802.
[6] Borexino Collaboration, Phys. Lett. B 658 (2008) 101.
[7] R.S.Raghavan Phys. Rev. Lett. 37 (1976) 259.
[8] R.S.Raghavan hep-ex/0106054.
[9] Y.Suzuki, Y.Fukuda, Y.Nagashima, and H.Kan Nucl. Instr. and Meth. A275 (1989) 142.
[10] Y.Suzuki et. al, Nucl. Instr. and Meth. A293 (1990) 615.
[11] R.S.Raghavan Phys. Rev. Lett. 78 (1997) 3618.
[12] D.Motta et. al, Nucl. Instr. and Meth. A547 (2005) 368.
[13] A.Owens et. al, Nucl. Instr. and Meth. A487 (2002) 435.
[14] P.G.Pelfer et. al, Nucl. Instr. and Meth. A458 (2001) 400.
[15] H. El-Abbassi, S. Rath and P. J. Sellin Nucl. Instr. and Meth. A466 (2001) 47.