InP solid state detector for a measurement of low
energy solar neutrinos
Yoshiyuki Fukuda
Faculty of Education, Miyagi University of Education,
149, Aobaku-aza-aoba, Sendai, Miyagi 980-0845, Japan
E-mail: fukuda@staff.miyakyo-u.ac.jp
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
InP detector observed γs at one hundred keV region. Most of charge are induced by moving
carriers, and the charge collection efficiency and the energy resolution are obtained by 60% and25%, respectively. We measured actual backgrounds related to 115 In β decay, and no significant
background was found.

1. Introduction
Super-Kamiokande and Sudbery Neutrino Observatory experiment has established νe oscillationin their solar neutrino data in 2001[1, 2], and a long way problem so called Solar Neutrino
Problem in past 30 years was almost solved by the LMA oscillation. Independently, KamLANDexperiment confirmed the oscillation using reactor νe in sense of ∆m2 [4]. Next step of neutrino
physics should measure both precise oscillation parameters and CP phase in the MNSP matrix
elements. For the future solar neutrino experiment, a precise θ12 measurement with 1% accuracy
will help the determination of θ13 [3] and the direct investigation of solar interior on the stellar
evolution theory, respectively. In this point of view, new experiment Indium Project on Neutrino
Observation for Solar interior (IPNOS) should measure full scale of energy spectrum of solar
neutrinos including pp neutrinos.
2. Low energy solar neutrino experiment using 115 In
In 1976, R.Raghavan proposed new technique for the measurement of low energy pp/7 Be solar
neutrinos [5] via following reaction;
115

In + νe →115 Sn∗ + e− .

(1)

The prompt electron has an energy with Eν −114 keV, here Eν is an energy of incident neutrinos.
Therefore the spectroscopy will be realized in the solar neutrinos. An excited state of 115 Sn shownin Eq.(1) decays into the ground state with a lifetime of 4.76µs, and emits two γs (116 keV and497 keV). This signature will be also able to use for a triple-coincidence to extract neutrinosignal from huge backgrounds. However, 115 In itself has natural β decay into the ground stateof 115 Sn with a lifetime of 4.4 × 1014 years and the end point energy is 499keV. The radiativeBremsstrahlung could produce fake coincidence for the neutrino signal. In order to avoid thesebackgrounds, a fine segmented with well energy resolution detector is necessary [6].
We have chosen semi-insulating wafer produced by Sumitomo Electrical Industry Co. LTD
with the method of Vapor pressure Controlled Czochralski (VCZ). Hamamatsu Co. LTD
developed InP detector with 10mm × 10mm in surface, and 0.2mm in thickness. The electrodesconsist of Cr-Au with 0.03 ∼ 1.0µm thickness for top and Au-Ge/Ni/Au with 0.13/0.015/0.5µm thickness for bottom. The junction between electrode and InP are ohmic contact in room
temperature, however actually a Schottky barrier has been formed at -79 ◦ C because of therectification in the measurement of Hall effect.
3. Performance of InP detector
The performance of InP detector was measured by using gammas emitted by usual radio active
sources. Carriers generated by the energy deposit of electrons via photoelectric process orCompton scattering are drifted along to electric field. The expected charge could be evaluated− d−xxby Hecht formula; Q[C] = Q0 { Lde (1 − e− Le ) + Ldh (1 − e Lh )} Here, Le,h is drift length ofelectron/hole, d is thickness of the detector, and x is distance from the electrode. Generally
speaking, the carrier drift length is expressed by Le,h ≡ µe,h τe,h Vd0 , here µe,h is the carrier
mobility and τe,h is life-time of the carrier trapping for electron/hole, and V0 is the bias voltage.
Figure.1 shows that the observed charge distribution. There found two peaks in each spectra.
For instance, the peak for 122keV γ-ray in 57 Co appears around 0.3×10−14 C and 0.55×10−14 C.
Higher peak is produced by full carrier collection due to reach the electrode, and it is consistent
with other γs assuming by 3.5eV of an average energy for electron/hole pair production. Thecollection efficiency for lower peak is obtained by the 60% in case of 122keV γs, and this peak
was formed by the induced charge as explained by Hecht formula. According to a simulation,
the spectral shape could be reproduced assuming both Le = 200µm and Lh = 30µm as shownin bottom of Figure.1.
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4. Measurement of backgrounds
As described in section 2, 115 In decays naturally with β emission, and the radiative
Bremsstrahlung due to 115 In nuclei might be possible background in the observation of solar
neutrinos. For the measurement of those effect, we used InP detector for measurement of β

event and CsI(Tl) scintillator for the measurement of radiative Bremsstrahlung. The CsI size
was 50mm × 50mm × 20mm. These two detectors are located by face to face and set inside ofradio-active shield which consist of the lead in 5cm thickness and the oxygen free copper in 1cmthickness. The 4-π active veto plastic counter surrounded the shield was used for rejection ofcosmic-ray backgrounds. The energy threshold for InP and CsI detector was 100keV and 50keV,respectively.
In order to detect the radiative Bremsstrahlung, we took coincidence between InP andCsI detector within 10 µsec. In case of 17.3 hours measurement, 46 events were observed.
Before concerning of the coincidence events, it is necessary to take into account U/Th
natural backgrounds. According to measurement of U/Th activity using ultra-low backgroundgermanium detector located in Kamioka mine, the InP wafer contains them as order of 10−11 g/g.
Also right figure of Fig.2 shows the energy spectrum of CsI detector obtained by self trigger. It
is clearly seen that the photo-electric peak due to some nuclei in the U/Th decay chain suchas 214 Bi. The amount corresponds to order of 10−10 g/g in the CsI scintillator or surroundedmaterial. According to left of Fig.2, most of coincidence events between InP and CsI detector
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observed energy in keVFigure 2. Energy spectrum of coincidence events for CsI and InP detector for 17 hour data,
and U/Th background observed in CsI detector by self trigger.
looks consistent with β-γ coincidence of U/Th backgrounds in those detectors as indicatedby 214 Bi γ observed in CsI, and no clear evidence for the effect of radiative Bremsstrahlung
was found. An accidental triple coincidence between InP detector and the scintillator shouldbe evaluated by 5 × 10−6 events/day/unit detector, which corresponds to 10 events/day/4tondetector in case of IPNOS detector. Therefore, we have to reduce the amount of U/Th
contamination order of 0.1, however it is not so hard to control the production of materials.
5. Conclusion
An InP detector has been developed, and obtained good performance. Observed charge spectra
could be explained by carrier drift with 250µm and collection efficiency is achieved by 60%. No
clear evidence of radiative Bremsstrahlung was found.
Acknowledgments
This work was supported by the Grant-in-Aid Scientific Research (B) 17340065 of JapaneseSociety for the Promotion of Science, Inamori Foundation, and The Asahi Glass Foundation.
References[1] Super-Kamiokande collaboration, Phys.Rev.Lett. 86 (2001) 5651.
[2] SNO Collaboration, Phys.Rev.Lett. 87 (2001) 071301.
[3] G.L.Fogli, E.Lisi, A.Marrone, A.Palazzo and A.M.Rotunno, Phys.Rev.Lett. 110 (2008) 141801[4] KamLAND collaboration, Phys.Rev.Lett. 90 (2003) 021802.
[5] R.S.Raghavan, Phys. Rev. Lett. 37 (1976) 259.
[6] R.S.Raghavan, hep-ex/0106054.