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, Japan
E-mail: fukuda@staff.miyakyo-u.ac.jp
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 7mm3 . This detector observed γ-rays, and measured a peak forthe photoelectric eﬀect at -79 ◦ C. The charge collection is mainly done by a carrier drift along
to the electric ﬁeld at outside of depletion layer, and the eﬃciency is achieved by 60 to 70%.

1. Introduction
In 2001, Super-Kamiokande [1] and Sudbery Neutrino Observatory [2] using the charged current
process have established the νe oscillation in their solar neutrino measurement. KamLAND also
conﬁrmed the oscillation using reactor νe in sense of ∆m2 [3], however the mixing angel θ12 is
not restricted as well as θ23 observed in the atmospheric neutrinos. In order to obtain precise
θ12 , it is necessary to measure not only the ﬂux but also the spectral shape for solar pp/7 Beneutrinos.
2. Solar pp/7 Be neutrino experiment using 115 In
In 1976, R.Raghavan proposed the measurement of low energy solar pp/7 Be neutrinos [4] via
following reaction 115 In+νe →115 Sn∗ +e− . The prompt electron has an energy with Eν −125keV ,here Eν is an energy of incident neutrinos. Therefore the neutrino spectroscopy can 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 toextract neutrino signal from huge backgrounds. However, 115 In itself has natural beta decay into
the ground state of 115 Sn with a lifetime of 4.4 × 1012 years. The Bremsstrahlung could produce
fake coincidence for neutrino signal. Therefore a ﬁne segmented with well energy resolution
detector is necessary [5].
3. InP solid state detector
Semi-insulating InP wafer is usually grown by the liquid encapsulated Czochralski (LEC). We
have chosen another wafer produced by Sumitomo Electrical Industry Co. LTD with the methodof Vapor Czochralski (VCZ). Hamamatsu Co. LTD developed InP detector with 6mm × 6mm
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. The junction between

electrode and InP are ohmic contact in room temperature, however actually a Schottky barrier
could be formed at -79 ◦ C.
4. Performance of InP detectorThe 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(x)

%Q

1.2

MG8γ

1RJQVQGNGEVTKE

0.8

RGCM

0.6
0.4
0.2
0DKCU8
00.2

0.4

0.6

0.8

1.0-14x 101098765432
101.2MG8γ

RJQVQGNGEVTKE
RGCM

DKCU80QDUGTXGFEJCTIG=%?

CTDKVCNUECNG

%Q

0.2

0.4

0.6

0.2

0.4

0.6

0.8

1.0-14x 10

UKOWNCVGFEJCTIG=%?

1.0-14x 10#O

0.80.6

MG8 γRJQVQGNGEVTKE

0.4

RGCM

0.2
DKCU8
000.2

0.4

0.6

$C

0.20.40.6

0.8

1.0-14x 10

UKOWNCVGFEJCTIG=%?

1.0-14x 10



#O

/%

/%
00.8

QDUGTXGFEJCTIG=%?

/%
00.8

%%'1QDUGTXGFEJCTIG=%?

CTDKVCNUECNG



$C

%%'GXGPVUUGEH%

%%'CTDKVCNUECNG

1.4

GXGPVUUGEH%

GXGPVUUGEH%

Ldcharge could be evaluated by Q[C] = 0R dE/dx
dx × e. Here, Ld is carrier drift length, ²² e
is 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 dis thickness 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, thedetector should be cooled by -79 ◦ C. using Dry-Ice. Figure.1 shows that the observed charge
distribution. There found two peaks in each spectra. For instance, the peak for 122keV gammaray appears around 0.3 × 10−14 C and 0.55 × 10−14 C. Higher charge peak is produced by thecharge collection of depletion layer, and it is consistent with other gammas assuming by 3.5eVof an average energy for electron/hole pair production. This assumption is also conﬁrmed thepeak position was not changed as bias voltage increasing. On the other hands, lower charge
peak moves to higher position rapidly. The collection eﬃciency for lower peak is obtained by
the 56% in case of 122keV γ, and this peak was formed by charge collection due to carrier drift
outside of the depletion layer. According to a simulation, the spectral shape could reproduce
assuming both Ld = 250µm and 20µm for the thickness of depletion layer as shown in Figure.1.

00.2

0.4

0.6

0.8

1.0-14x 10

UKOWNCVGFEJCTIG=%?

Figure 1. Observed and simulated charge distributions for gamma-rays are shown.

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 to 70%. This is the ﬁrst time to demonstrate 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.
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] 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.