InP solid state detector for measurement of low energy solar neutrinosYoshiyuki Fukudaa , Toshiyuki Izawab , Yusuke Koshioc , Shigetaka Moriyamac , Toshio Nambad , Masato Shiozawaca Faculty of Education, Miyagi University of Education,

149, Aobaku-aza-aoba, Sendai, Miyagi 980-0845, Japanb Solid State Devision, Hamamatsu Photonics K.K.
1126-1, Ichino-cho, Hamamatsu, Shizuoka 435-8558, Japanc Kamioka Observatory, Institute for Cosmic Ray Research,University of Tokyo
Higashi-Mozumi, Kamioka-cho, Hida, Gifu 506-1205, Japand ICEPP, International Center for Elementary Particle Physics,
University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, JapanAbstract
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 hundred keV region. Although the depletion layer, mostof charge generated by electron hole pair production are collected by an induction, and the charge collection efficiency
and the energy resolution are obtained by 60% and 25%, respectively. We measured actual backgrounds related to 115 Inβ decay, and no significant background was found.
Key words: solar neutrinos, neutrino oscillation, stellar evolution
PACS: 13.85.Tp, 14.60.Lm
1. Introduction
Super-Kamiokande and Sudbery Neutrino Observatory
experiment has established νe oscillation in their solar neutrino data in 2001[1, 2], and a long way problem so called
Solar Neutrino Problem in past 30 years was almost solvedby the LMA oscillation. Independently, KamLAND experiment confirmed the oscillation using reactor νe in sense of∆m2 [3]. 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 measure both precise oscillation parameter and CP phase. For
the future solar neutrino experiment, a precise θ12 measurement would be done with 1% accuracy. In this pointof view, new experimental technique to measure not only
flux but also energy of solar pp/7 Be neutrinos will be necessary due to direct observation of upturn of the neutrinoenergy spectrum.
Helioseismology shows us the information of the interior of the Sun using observations of motion on the surface.
Most of analytic results are consistent with predictions of
the standard solar model, however, still small differenceof frequency less 0.5% in the intrinsic mode between theobservation 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 thestellar evolution theory. We are going to plan the experiment of low energy solar neutrino observation, so called,
Preprint submitted to TIPP09 Proceedings in NIMA

Indium Project on Neutrino Observation for Solar interior(IPNOS), for these objects.
2. Low energy solar neutrino experiment using 115 In
In 1976, R.Raghavan proposed new technique for themeasurement of low energy pp/7 Be solar neutrinos [4] via
following reaction;
115

In + νe →115 Sn∗ + e− .

(1)

The prompt electron has an energy with Eν − 118 keV,
here Eν is an energy of incident neutrinos. Therefore theneutrino spectroscopy can be realized. An excited state of115
Sn shown in Eq.(1) decays into the ground state with alifetime of 4.76µs, and emits two γs (116 keV and 497 keV).
This signature is also able to use for a triple-coincidence toextract neutrino signal from huge backgrounds. However,115
In itself has natural β decay into the ground state of115
Sn with a lifetime of 4.4 × 1014 years and maximum
β energy of 495keV. The radiative Bremsstrahlung could
produce fake coincidence for the neutrino signal. In order
to avoid this, a fine segmented with well energy resolution
detector is necessary [5].
Many possible detectors using indium were designed
in last few decade, however, no realistic detector has beenmade. In 1988, Suzuki and Fukuda developed the InP solidstate detector (1mm2 × 10µm) using the pn-junction andthe detector observed the γs from the radioactive sources
April 20, 2009

[6]. For large volume detector, a liquid scintillator solved
indium was developed by Suzuki [7] and LENS project [8].
Recently LENS group presented the feasibility of realistic
way to build a big scale detector [9], but there still exist
some difficulties of the transparency with respect to theweight of solvent.
New detector using InP semi-conductor has been reevaluated for last several years. The semi-insulating (SI)InP wafer is commercially produced for the optical devices. Typical dopant is Fe and the crystal is usually grownby the liquid encapsulated Czochralski (LEC). Some radiation detectors have been developed for X-ray detector.
The ESTEC group have characterized a 0.18 mm thick ofInP at -60 ◦ C achieving 8.5 keV FWHM at 60 keV[10],and Italy group obtained 11 keV FWHM at 122 keV with
0.25 mm thick SI InP at -60 ◦ C[11]. There is other crystal growth method, namely the Vertical Gradient Freeze(VGF), which is relatively higher resistance and naturally
smaller EPD than LEC. A radiation detector using theVGF SI InP wafer is developed by UK group[12], and ourgroup. Most of developed detectors by UK group, however, have a small volume of the order of 1mm3 or less,and it seems to hard to use for solar neutrino experiment.
On the other hands, our group tried to produce 7mm3 size
InP detector, however, no clear peak from γs was found.
;PXOWLSL[HO,Q3PRGXOHIRU,3126H[SHULPHQW,Q3PXOWLSL[HOPRGXOH,Q3GHWHFWRU

We have chosen another 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×0.2mm.
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 forbottom as shown in Fig.2. The junction between electrodeand InP are ohmic contact in room temperature, however
actually a Schottky barrier could be formed at low temperature, because of the rectification in the measurementof Hall effect.
PPVTXHUH

PP&U$X µP

6L1 µP,Q3 µP$X*H1L$X
µP

Figure 2: Left figure shows the schematic view of the InP detector.
The detector size is 10mm × 10mm × 0.2mm.

+\EULGGHWHFWRUIRU
,3126H[SHULPHQW

PPPP3. Performance of InP detector

The performance of InP detector was measured by using γs 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. As usual solid state detector, the charge
could be induced by the polarized carrier at the surface ofelectrode. The induced charge is evaluated by
Z R
dE/dx Ld
Q[C] =dx.
(2)²d0aFPXOWUDORZEDFNJURXQG
VFLQWLOODWRU

Figure 1: Possible detector design for IPNOS experiment. Left figure
shows the multi-pixel InP detector, and right figure shows the hybrid
structure which consist of multi-pixel module and scintillator.

Here, Ld is the carrier drift length, ² is an average energy
for electron/hole pair production, R is the carrier range,and d is thickness of InP detector. Generally speaking, thecarrier drift length is expressed by Ld ≡ µτ Vd0 , here µ is
the carrier mobility, τ is life-time of the carrier trappingand V0 is the bias voltage. In order to get longer life time,the detector should be cooled.
Figure.3 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 γ of 57 Co appears around 0.3 × 10−14 C and0.55 × 10−14 C. Higher peak is produced by the chargecollection which the carrier reaches at the electrode, and
it is consistent with an average energy for electron/holepair production to be 3.5 eV. This assumption is also confirmed by the fact that the higher peak position was notchanged as increasing bias voltage. On the other hands,
lower peak moves to higher position as increasing bias voltage. This is naturally explained by Eq.(2). The collection

It is possible to be real solar neutrino detector, if InP
solid state detector has a massive. Assuming a mass of InP
detector to be 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 need to
combine such even small detector as multi-pixel as shownin left side of Fig.1. According to this idea, 5 × 5 detectors
are combined as one module, so same bias voltage should
be applied in each detector and the signal line also should
be unified. Another feature is the detection of γs fromνe capture process by 115 In nuclei. These 116 keV and497 keV γ could escape from the original InP wafer, so
other detector should observe them efficiently. Therefore
the IPNOS detector will have a hybrid structure of InP
multi-pixel modules surrounded by low background (solid)
scintillator as shown in right side of Fig.1.

2

30050500.20.40.60.8

1.0x 10-140100

QXPEHURIHYHQWV

0.2

0.40.60.8

1.0-14x 10%D%D

1600
1400

1400

120012001000

1000

ELDV9800
600

600

400

400

400

0.2

0.40.60.8

1.0x 10 -14

REVHUYHGFKDUJH>&@
250

000.2

0.4

0.60.8REVHUYHGFKDUJH>&@

1.0x 10-14250

$P

225
2000$P

200

150

150

1250

ELDV975

0.2

0.4

0.60.8REVHUYHGFKDUJH>&@

1.0-14x 100%D

0.20.40.601.0-14x 100.80.20.40.6observed energy in keVCsIPHDVXUHPHQWWLPHKRXUV

50000



7K NH940000
30000
20000



5D NH9
 3E NH9
3E NH9
3E NH9
%L NH9observed energy in keV

observed energy in keV

Figure 4: 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.

1.0x 10-14
VLPXODWHGFKDUJH>&@

0.8

$P

ELDV975$P

50

250609keV(214Bi)100000&01005025

ELDV912510050

1.0-14x 10
VLPXODWHGFKDUJH>&@

186keV(226Ra)60000

0200150

ELDV90.8

REVHUYHGFKDUJH>&@

175

750.6

%D

225

175

1000.4

CsI

250



225

175

1250.2200

200001.0x 10-140.8



800

600

00.6

1600

1000

2000.41800

1200

ELDV90.2

1cmX1cmX0.2mm
bias 300V
17.3 hour2000

1800

1400

8000

REVHUYHGFKDUJH>&@2000

1600

0InP

&R

0&500

REVHUYHGFKDUJH>&@

2000
1800

ELDV9150QXPEHURIHYHQWV

1000

ELDV9150100&R

200

number of events

ELDV9150

250

number of events

200

REVHUYHGFKDUJH>&@QXPEHURIHYHQWV&R

DUELWDOVFDOH

2502000300

&R

DUELWDOVFDOH

QXPEHURIHYHQWV250

DUELWDOVFDOH

3000&

2500.2

0.4

0.6

0.8

1.0-14x 10
REVHUYHGFKDUJH>&@0material. According to Fig.4, most of coincidence events
between InP and CsI detector looks consistent with βγ coincidence of U/Th backgrounds in those detectors as
indicated by 214 Bi γ observed in CsI, and no clear evidence
for the effect of radiative Bremsstrahlung was found. Thisscheme was also confirmed by Si instead of InP detector.
Moreover, an accidental triple coincidence between InP
detector and the solid scintillator was evaluated by 5 ×
10−6 events/day/1 hybrid detector, which corresponds to
10 events/day/4ton detector using this data. This meansthat we need to reduce the amount of U/Th contaminationin the detectors or surrounded materials only 1/10, if S/Nratio will be assumed by an order of 1.

00.2

0.4

0.6

0.8

REVHUYHGFKDUJH>&@

1.00-14x 100.2

0.4

0.6

0.8

1.0-14x 10
VLPXODWHGFKDUJH>&@

Figure 3: Observed and simulated charge distributions for several γsobtained by the InP detector.

efficiency for lower peak corresponds to 60%. According toa simulation with the energy resolution of 20% at 122 keV
for a lower peak, the spectral shape could reproduce assuming both Ld ∼ 120µm as shown in most right of Fig.3.
An intrinsic energy resolution for a higher peak was also
assumed by 3% at 122 keV in this simulation.

5. Conclusion
An InP solid state detector has been developed andobtained suitable performance. Observed charge spectra
could be explained by induced charge, and the collection
efficiency is achieved by 60%. No significant background
was found in the actual setup for solar neutrino observation. This is the first time to demonstrate InP detector
with a bulk size crystal.

4. Measurement of backgrounds
As described in section 2, 115 In decays naturally with
β emission, and the radiative Bremsstrahlung might bepossible background in the observation of solar neutrinos. For the measurement, 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 of radio-active shieldwhich 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 dueto the cosmic ray. The energy threshold for InP and CsI
detector was 100keV and 50keV, respectively.
In order to detect the radiative Bremsstrahlung, wetook coincidence between InP and CsI detector within 10µsec. In case of 17.3 hours measurement, 46 events wereobserved. Before concerning of the coincidence events, it isnecessary to take into account U/Th natural backgrounds.
According to measurement of U/Th activity using ultralow background germanium detector located in Kamioka
mine, the InP wafer contains them as order of 10−11 g/g.
Also right figure of Fig.4 shows the energy spectrum of CsI
detector obtained by self trigger. It is clearly seen thatthe photo-electric peak due to some nuclei in the U/Th
decay chain such as 214 Bi. The amount corresponds toorder of 10−10 g/g in the CsI scintillator or surrounded

Acknowledgments
This work was supported by KAKENHI Grant-in-Aid
Scientific Research (B) 17340065 of Japanese Society for
the Promotion of Science, Inamori Foundation, and The
Asahi Glass Foundation.
References[1] S.Fukuda et.al, Phys.Rev.Lett. 86 (2001) 5651.
[2] SNO Collaboration Phys.Rev.Lett. 87 (2001) 071301.
[3] KamLAND collaboration, Phys.Rev.Lett. 90 (2003) 021802.
[4] R.S.Raghavan Phys. Rev. Lett. 37 (1976) 259.
[5] R.S.Raghavan hep-ex/0106054.
[6] Y.Suzuki, Y.Fukuda, Y.Nagashima, and H.Kan Nucl. Instr. andMeth. A275 (1989) 142.
[7] Y.Suzuki et. al, Nucl. Instr. and Meth. A293 (1990) 615.
[8] R.S.Raghavan Phys. Rev. Lett. 78 (1997) 3618.
[9] D.Motta et. al, Nucl. Instr. and Meth. A547 (2005) 368.
[10] A.Owens et. al, Nucl. Instr. and Meth. A487 (2002) 435.
[11] P.G.Pelfer et. al, Nucl. Instr. and Meth. A458 (2001) 400.
[12] H. El-Abbassi, S. Rath and P. J. Sellin Nucl. Instr. and Meth.
A466 (2001) 47.

3