Development of InP detector for pp/7Be solar
neutrino measurement
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 radiation detectors used a semi-insulating Indium Phosphide (InP) wafer formeasurement of solar pp/7 Be neutrinos have been developed in last a few years. A volume ofthe detector has achieved to 20mm3 . This detector observed γ-rays, and measured a peak forthe photoelectric effect at -79 ◦ C. The charge seems to be mainlly collected due to a carrier
drift along to the electric field at outside of the depletion layer, and the efficiency is achievedby 60 to 70%. No clear Bremsstrahlung coincidence was seen in the background measurement.

1. Introduction
In 1998, Super-Kamiokande has reported an evidence that the atmospheric µ neutrinos oscillate
into τ neutrinos [1], and the K2K experiment has confirmed the oscillation using νµ beam
produced by KEK 1GeV Proton Synchrotron[2] in 2005. On the other hands, Super-Kamiokande
and Sudbery Neutrino Observatory experiment has established the νe oscillation for the solar
neutrino observation in 2001, and a long way problem called Solar Neutrino Problem inpast 30years was almost resolved by LMA oscillation. Independently, KamLAND experimentconfirmed the oscillation in the LMA using reactor νe in sense of ∆m2 [5]. However, theoscillation 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 J-PARC accelerator experiment nor the reactor experiment such as Double Chooz.

For the future solar neutrino experiment, a presise θ12 measurement should be done with 1%
accuracy for restriction of GUT model. For that purpose, not only a flux but also the spectral
shape of solar pp/7 Be neutrinos will be necessary.
2. Solar pp/7 Be neutrino experiment using 115 In
Recently Borexino has reported that the first detection of 7 Be neutrinos signal using low
background liquid scintillator [6]. Because of precise analysis for low energy background, they
will give us clear spectrum of recoiled electrons in near future. However it seems to be very
difficult to detect pp neutrinos even though they establish the finite value of 7 Be neutrinos flux.
In 1976, R.Raghavan proposed the measurement of low energy solar pp/7 Be neutrinos [7]via following reaction 115 In + νe →115 Sn∗ + e− . The prompt electron has an energy withEν − 125keV , here Eν is an energy of incident neutrinos, therefore the neutrino spectroscopy
would be realized. An excited state of 115 Sn decays into the ground state with a lifetime of
4.7 µs, and emits two gammas (116keV and 487keV). This signature is also able to use for a
triple-coincidence to extract neutrino signal from huge backgrounds. However, 115 In itself has
natural beta decay into the ground state of 115 Sn with a lifetime of 4.41 × 1014 years. The
Bremsstrahlung could produce fake coincidence for neutrino signal. Therefore a fine segmented
with well energy resolution detector is necessary [8].
3. InP solid state detector
Many possible detectors using indium were designed in last few decade, however, no realistic
detector has been made. A liquid scintillator solved indium was developed by Suzuki [9]et.
al and LENS project [10]. Recently the LENS group presented the feasibility of realistic way
to build a big scale detector [11], but there might be exist the difficulties of the transparency
with respect to the weight of solvent. In 1988, Suzuki and Fukuda developed the InP solidstate detector with the pn-junction and the detector observed the gammas from the radioactive
sources. [12]. The detector size was very small(1mm2 × 10µm), however, the prototype detector
could observe the low energy 60keV gammas with the energy resolution of 5.5%.
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Figure 1. Possible detector design for pp/7 Be solar neutrino measurement. Left figure shows
one module as multi-pixel combination, and right figure shows the hybrid structure consist ofmulti-pixel InP module surrounded by scintillation counter for detection of two gammas.
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 is hard to use for solar neutrino experiment.
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 497keV gamma-ray could escape fromthe original wafer of InP, so other detector should detect them efficiently. Therefore we design
a hybrid structure that the scintillation counter surrounded InP module as shown in right sideof Fig.1.
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Figure 2. Left figure shows the schematic view of the proto-type detector. The detector size is
10mm × 10mm in surface, and 0.2mm in thickness.
We have developed large volume detectors using another wafer produced by Sumitomo
Electrical Industry Co. LTD with the method of Vapor Czochralski (VCZ). The schematic
view and actual surface view of InP detector is shown in Fig.2. Hamamatsu Co. LTD actually
produced two types of InP detector with 7mm × 7mm and 10mm × 10mm in surface, and0.2mm in thickness. The detector volume corresponds to 10mm3 and 20mm3 , respectively. The
electrodes consist of Cr-Au with 1µm thickness for top and Au-Ge/Ni/Au with 0.13/0.015/0.5µm thickness for bottom side. The junction between an electrode and the InP wafer are ohmic
contact in room temperature, however actually a Schottky barrier has been formed at Dry-Ice
temperature (-79 ◦ C), because of the rectification in the Hall effect measurement. The InP
detector is assembled in the vacuum chamber due to cooling by Dry-Ice as shown in photographof Fig.2.
The leakage current as a function of the bias voltage which were measured at room
temperature is shown in Fig.3. Typical current value is 40µA at 100V bias. The dark current
2E
could be reduced by lowering the temperature as followed formula; I ∝ T 3 exp(− 2kBgT ); here
I[A], T [K], Eg , and kB show dark current, temperature, band gap (1.29eV in case of InP), andBoltzmann constant, respectively. Typical value of dark current at -79◦ C is 5nA at 400V bias.

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Figure 3. The leakage current as a function of bias voltage. Basically the current is same eventhough the positive or negative bias voltage.
4. 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 and reached the electrode. The expected
R

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charge could be evaluated by Q[C] = 0R dE/dx
dx × e. Here, Ld is carrier drift length, ²² eis an average energy for electron/hole pair production, R is electron range, and r(x) is distancefrom electrode. Generally speaking, the carrier drift length is expressed by Ld ≡ µτ Vd0 , hereµ is the carrier mobility, τ is life-time of the carrier trapping. V0 is the bias voltage, and d isthickness of the wafer. In the depletion layer, carrier life time is long enough to the drift time, so
the observed charge should be same as Neh × e. For the measurements of gammas, the detector
should be cooled by -79 ◦ C using Dry-Ice.
Figure.4 shows that the observed charge distribution of 122keV, 81keV and 60keV gammas
using 7mm × 7mm detector at several bias voltage. At lower bias voltage, there is no clear
peak from the photo-electric interaction. As higher bias voltage applied, there found two
peaks. For instance, the peak for 122keV gamma-ray appears around 0.3 × 10−14 C and
0.55×10−14 C. Higher charge peak is produced by the charge collection of depletion layer, and it
is consistent with the charge collection assuming by 3.5eV for an average energy of electron/holepair production. This assumption is also consistent with the fact that the peak position was notchanged as increasing bias voltage. On the other hands, lower charge peak appeared above 400V
and moves higher charge position rapidly. The collection efficiency for lower peak is obtainedby the 56% in case of 122keV γ, and this peak was formed by charge collection due to carrier
drift outside of the depletion layer.
Also Figure.5 shows that the observed and simulated charge distribution of each gamma using
20mm3 detector at 500V bias voltage. The spectral shape are almost same as ones obtainedby 10mm3 InP detector, and therefore the charge collection should be same. According to theMonte Carlo simulation, the spectral shape could reproduce assuming both Ld = 250µm and20µm for the thickness of depletion layer as shown in Fig.4 and Fig.5. The number of events
form each isotopes are increased by 3.1 times as compared with 2.8 times larger surface area(solid angle). Therefore, an effective area actually spread out as expected, and the performanceis stable.

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Figure 4. Observed and simulated charge distributions for gamma-rays are shown.
5. Background measurement
Using 20mm3 InP detector, we tried measured the natural beta decay of 115 In and the effectof Bremsstrahlung as fake coincidental backgrounds. In order to detect Bremsstrahlung andother coincident events from InP detector, we used CsI(Tl) scintillator produced by SCIONIX
for detection of such backgrounds. The CsI size is 50mm × 50mm × 20mm and assembled by thephotomultiplier. Figure6 shows the configuration of InP detector and CsI scintillation counter.
These two detectors are located by face to face for measurement of 115 In β decay and the effectof Bremsstrahlung. For the accidental events, these are located by back to back. These detectors
are located in the radio-active shield which consist of the lead in 5cm thickness and the Oxygen
free copper in 1cm thickness for the low radio-active background environment. Also, in order toavoid the background due to the cosmic ray and the external radiation, 4-π active veto plastic
counter surround the shield.
For the measurement of β decay of 115 In, the isolated events were observed in InP detector
as seen in Fig.7. The coincident events are rejected in this figure. The measurement time is 10
hours, and the expected number of β decay is 680. The observed events are clearly larger thanthe expected number of β decay. The expected energy spectrum for 115 In β decay is also shownin left of Fig.7. For low energy events blow 100keV, we assume those as backgrounds due to thevibration. The detector was so cooled that the floated capacitance between InP detector and thecharge amplifier affected to generate noise which is sensitive to the vibration. This is confirmed

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Figure 5. Observed and simulated charge distributions for gamma-rays are shown.

Figure 6. Configuration of InP and CsI detector. These are located face to face to detect 115 In β
decay and the Bremsstrahlung from InP detector. For low radio-active background environment,
the shield consist of lead in 5cm thickness and oxygen free copper in 1cm thickness. 4-π active
veto plastic counter surrounded the shield in order to veto the cosmic ray and external incident
radiation.
by main amount of fraction in those events due to the accidental vibration happened in otherperiod as shown in right of Fig.7. Assuming the events blow 50keV to be noise events dueto the vibration, the expected energy distribution is consistent with the observed distribution
as shown in Fig.8. Observed events above 400keV in InP data is still inconsistent with thespectrum from β decay of 115 In. According to the measurement of U/Th natural activity using
ultra-low 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 these events. Another possibility is iron (Fe) which is used for the dopant of InP crystal.
60 Fe nuclei decays into 60 Mn with β − (E ≤ 3.978MeV, T = 1.5 × 106 years). Assuming 60 Fe1e2

radio-active background contaminated an order of 10−10 g/g, then the expected energy spectrumin InP detector are very consistent with the observed spectrum as shown in right of Fig.8.

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spectrum from β-decay of 115 In. Lower energy events with E ≤ 100keV are consistent with thebackgrounds due to the vibration.
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Figure 8. Observed energy distribution on InP detector assuming the noise due to the vibratio
for the events blow 50 keV, and assuming 60 Fe nuclei decays.
For a measurement of Bremsstrahlung effect from βdecay of 115 In, it should take a coincidence
between InP detector and CsI counter. In case of location of those detectors as face to face,
there was 105 events found in the 10 hours measurements as shown in Fig.9. On the otherhands, the coincidence was taken however the location of those detectors as back to backshould be accidental noise as shown in Fig.10. Number of those events was 15 during 10
hours. This is consistent with number expected by the accidental coincidence of event rate
for each detector. The energy spectrum of InP detector also indicates that those should be noisedue to the vibration. Before concerning of events shown in Fig.9, it is necessary to take into
account the U/Th natural backgrounds in CsI counter, Figure 11 shows the energy spectrumof single triggered by CsI counter. It is clearly seen that characteristic photo peak due tospecified U/Th nuclei as listed in Fig.11. This means that the several 10−10 g/g order of U/Th
could be contaminated in CsI counter. The amount is usual however it becomes non negligible
backgrounds. Those nuclei generally decay with both γ and β, so that the some gamma could

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Figure 10. Energy spectrum of coincident events for CsI counter and InP detector located byback to back. These events should be accidental.
escape from CsI counter and enter into InP detector as shown in right figure of Fig.11. Using thisscheme, Monte Carlo simulation was done inside of CsI counter to the InP detector. Figure12
shows the energy spectrum of coincident events between InP detector and CsI counter andthe simulated energy spectrum are overlapped. According to these figures, most of coincident
events between InP detector and CsI counter are caused by the U/Th natural backgrounds in
CsI counter, and no clear evidence of Bremsstrahlung effect which should be observed blow
50keV was appeared. Another possible backgrounds to be concerned is radiative β decay from115 In. This is similar to Bremsstrahlung however the much higher energy could be carried to
the radiation. We concluded that this effect should be small because of spectral shape of InP
detector for coincidence events is different from the expected shape of β decay, and small amountevents can be seen in lower energy region. In the actual case for the measurement solar neutrinos,
these kinds of related β decay backgrounds should be negligible due to a few µ seconds of timedifference between InP detector and surrounded scintillator.

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data, and the expected spectrum from above scheme is overlapped.
6. Conclusion
An InP detector with large volume has been developed, and obtained good performance asconcerned with radiation detector. Observed charge spectra could be explained by the carrier
drift with 250µm and the collection efficiency is achieved by 60 to 70 %. This is the first time todemonstrate InP detector with a bulk size crystal, and the detector performance shows us that
it is possible to use for solar pp/7 Be neutrino measurement. Backgrounds in InP detector seems
to be very small even though amount of 60 Fe is order of 10−10 g/g. No effect of Bremsstrahlung
can be seen in the background measurement, and it is very important that the low level of radio
activity for surrounded scintillator should be selected.
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.

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