Development of InP solid state detector and liquid
scintillator containing metal complex formeasurement of pp/7Be solar neutrinos andneutrinoless double beta decay
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

Shigetaka Moriyama
Kamioka Observatory, Institute for Cosmic Ray Research,
University of Tokyo
456, Higashi-Mozumi, Kamioka-cho, Hida, Gifu 506-1205, Japan
E-mail: moriyama@icrr.u-tokyo.ac.jp
Abstract. A large volume solid state detector using a semi-insulating Indium Phosphide (InP)
wafer have been developed for measurement of pp/7 Be solar neutrinos. Basic performance suchas the charge collection eﬃciency and the energy resolution were measured by 60% and 20%,
respectively. In order to detect two gammas (115keV and 497keV) from neutrino capture,
we have designed hybrid detector which consist InP detector and liquid xenon scintillator forIPNOS experiment. New InP detector with thin electrode (Cr 50Å- Au 50Å). For anotherpossibility, an organic liquid scintillator containing indium complex and zirconium complex
were studied for a measurement of low energy solar neutrinos and neutrinosless double beta
decay, respectively. Benzonitrile was chosen as a solvent because of good solubility for thequinolinolato complexes ( 2 wt%) and of good light yield for the scintillation induced by gammaray irradiation. The photo-luminescence emission spectra of InQ3 and ZrQ4 in benzonitrile wasmeasured and liquid scintillator cocktail using InQ3 and ZrQ4 (50mg) in benzonitrile solutions(20 mL) with secondary scintillators with PPO (100mg) and POPOP (10mg) was made. Theenergy spectra of incident gammas were measured, and they are first results of the gamma-ray
energy spectra using luminescent of metal complexes.

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, KamLAND
experiment confirmed the oscillation using reactor νe in sense of ∆m2 [3]. 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 [4] 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) would measure full scale of energy spectrum of solar
neutrinos including pp neutrinos.
On the other hands, 96 Zr is one of the expected nuclei for neutrinoless double beta decay withhigher Q-value (3.72MeV). In order to avoid gammas from 208 Tl decay, it is better to have highQ-value than 2.6MeV. Several experiments are planned such as COBRA which used CaZnTesolid state detector and Super-NEMO with Mo and Se. Generally speaking, those experiments
use target nuclei with order of 100kg, however we need ton-scale target nuclei to reach 0.1eV
order for the neutrino mass. In this point of view, a large target (≥ 500kg) experiment suchas SNO+ (150 Nd) and KamLAND-Zen (136 Xe) should have a possibility to observe neutrinolessdouble beta decay events. In order to realize ton-scale target experiment, we have chosen 96 Zr
and developed the Zr complex which could be solved at order of 10% in the organic scintillator.
2. InP detector with thin electrodeWe have chosen semi-insulating wafer produced by Sumitomo Electrical Industry Co. LTD
with the method of Vapor pressure Controlled Czochralski (VCZ) and the vertical bridge (VB).
Hamamatsu Co. LTD developed InP detector with 6mm × 6mm in surface, and 0.2mm inthickness. The performance of InP detector was measured by using gammas emitted by usual
radio active sources and no background from the radiative Bremsstrahlung was found [6]. The
average energy for electron/hole pair production is 3.5eV, and the charge collection eﬃciency isobtained by 60%, and the energy resolution is almost 20% at 122keV.
Recently we had new idea to observed the scintillation light due to delayed gammas. The
IPNOS detector was designed by InP detector for observing the prompt electron emitted byneutrino capture, and also observing scintillation lights emitted by liquid xenon scintillator
surrounded by the InP detector. The conceptional detector design is illustrated by Fig.1 (a).
The InP detector was located at the wall which is filled with liquid xenon. In order to observe thescintillation light from liquid xenon, top electrode should be formed with thin layer as illustrated
in Fig.1(b). The thickness of electrode is formed with Cr 50 Åand Au 50Å.

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7L3W$Xύύύ,Q3$X*H1L$XµPύύύ72SDFNHJH(a) IPNOS(b) InPFigure 1. (a) Conceptional design for IPNOS experiment.(b) InP detector with thin electrode(Cr 50Å- Au 50Å) to observe scintillation light from liquid xenon.

3. InQ3 and ZrQ4 loaded liquid scintillator
A liquid scintillator solved indium was developed by Suzuki et al[7] and LENS project [8].
Recently LENS group presented the feasibility of realistic way for a proto-type detector (Mini-LENS) [9], which will use MVA indium complex in the Liner Arkyl Benzen (LAB) [10].
We had a new idea to observe such a low energy solar neutrinos and also neutrinolessdouble beta decay using liquid scintillator. Metal complex usually has large solubility for theorganic solvent, and some complexes also have a luminescence, so that it could be role assecondary scintillator. We have chosen A tris (8-quinolinolato) indium (InQ3 ) and a tetrakis(8-quinolinolato) zirconium (ZrQ4 ) as such complex. These are not commercially produced, so
we have to synthesize using metal salt compound and quinolinolato regands.
We have searched for an organic solvent which should have high solubility of InQ3 and ZrQ4complex, and have found benzonitrile (C6H5CN: PhCN) which has an order of a few % solution.
The maximum photo luminescent wavelength of PhCN is 291nm, a flash point is 75 ◦ C, and theattenuation length is 66cm for the 0.5wt% dissolution.
Maximum emission wavelength of InQ3 and ZrQ3 are found at 559.3nm (excitation wavelength: 397nm) and 548.0nm (excitation wavelength : 387nm), respectively. The absorption ofInQ3 and ZrQ4 complex also measured by the spectrophotometer. The maximum absorption
wavelength were located at 394.7nm for InQ3 and 383.3nm for ZrQ4 complex, respectively. The
obtained quantum yield were also measured by using standard material, and obtained by 0.050
and 0.011 for InQ3 and ZrQ4 , respectively.
4. Performance of liquid scintillator containing metal complex
For the measurement of performance, we used gammas from 137 Cs (662keV) and 60 Co (1.17MeV
and 1.33MeV). Usual gammas interact with electrons in liquid scintillator, and emit an electronvia photo-electric eﬀect or Compton scattering. In order to transfer the energy deposited byelectron to the photomultiplier, we resolved 2,5-Diphenyloxazole (PPO) which has absorptionmaximum at 309.7nm and fluorescence maximum at 368.0nm in PhCN. In addition, 1,4-Bis(5phenyloxazol-2-yl)benzene (POPOP) would help the energy transfer due to overlap between theemission of POPOP and the absorption of InQ3 and ZrQ4 .
The liquid scintillator was put into 20mL quartz vials, and two 2-inch photomultiplier(Hamamatsu H6410) are connected to the vial through acrylic light guide for the coincidence
method for avoiding backgrounds. Comparing of light yields, PhCN with PPO 100mg andPOPOP 10mg cocktail scintillator has 63% relative to the standard liquid scintillator (BC505).
Here, the wavelength of emitted lights for both scintillators are almost same (∼ 420nm). The
quantum eﬃciency of Hamamatsu H6410 photomultiplier is 25% at 420nm.
We measured the energy spectrum of induced gamma-ray from radio isotopes using 50mg
of InQ3 and ZrQ4 solved in PhCN with PPO 100mg and POPOP 10mg as shown in Fig.2 (a)and (b). The scintillation light yield of ZrQ4 looks smaller than that of InQ3 . This should
be explained by the diﬀerence of quantum yield of those complexes. The quantum yield ofquinolinolato complex could be evaluated by diﬀerence of scintillation photon yield between theonly PhCN cocktail scintillator and PhCN cocktail scintillator containing complex. Becauseof diﬀerent wavelength of those scintillator, we have to correct the light yield by the quantum
eﬃciency of H6410 photomultiplier. The emission wavelength of InQ3 and ZrQ4 in PhCN is559nm and 548nm, respectively. The quantum eﬃciency of PMT is obtained by 12.6% and9.3%, respectively. Therefore, the quantum yield of InQ3 and ZrQ4 dissolved in PhCN cocktail
scintillator are obtained by 15.6% and 11.5%, respectively. These values are larger than thevalue obtained by the photo luminescence.
This reason was explained by following. The photon emission from POPOP has slightly
longer tail than the absorption distribution of InQ3 and ZrQ4 complex. This means that theresidual photon could be contaminated in the emission of InQ3 and ZrQ4 complex. In order toevaluate the amount of POPOP residual light, we measured the emission of scintillation lightfrom InQ3 dissolved in PhCN cocktail scintillator via the sharp cut filter (FujiFuilm : SC-48).
This film has transparency of 92% above 530nm and quickly decrease the eﬃciency below 530nm

478keV(661keVγ comptonedge)

137

Cs

Number of events

Cs

Number of events

Number of events

137

478keV(662keV γcompton edge)
137

Cs

478keV(661keVγ comptonedge)

1.04MeV(1.17MeV /1.33MeVγ compton edge)

ADC channel [0db](a) InQ3 50mg

60Co

Number of eventsCo

Number of events

Number of events

60

ADC channel [0db]

ADC channel [0db]

ADC channel [0db]1.04MeV(1.17MeV /1.33MeV
γ compton edge)

60

Co

1.04MeV(1.17MeV /1.33MeV
γ compton edge)
ADC channel [0db](b) ZrQ4 50mg

ADC channel [0db](c) InQ3 50mg via
SC-48 filter

Figure 2. Observed energy spectrum of (a) InQ3 50mg (b) ZrQ4 50mg and (c) InQ3 50mg vi
SC-48 filter dissolved in PhCN cocktail scintillator for 136 Cs (top) and 60 Co (bottom).
(50% at 482nm and 10% at 470nm). The observed energy spectrum of InQ3 dissolved in PhCN
cocktail scintillator showed that the photon light yield was lost about 34% as shown in Fig.2(c).
On the other hands, 23% of the light yield of PhCN cocktail scintillator still remained even via
SC-48 filter. Therefore the diﬀerence of quantum yield between the photo luminescence and thegamma irradiation could be explained by the residual light from POPOP, and the amount of
those are expected by 10% at maximum.
5. Conclusion
An InP detector with thin electrode (Cr 50Å- Au 50Å) has been developed, and an organic
liquid scintillator containing InQ3 and ZrQ4 complex was demonstrated. We need quinolinolato
complex with new substitution group which will improve the quantum yield to be 0.533, andthe maximum emission wavelength to be shifted at 490nm [11].
Acknowledgments
This work was supported by KAKENHI Grant-in-Aid Scientific Research (C) 22540303 of Japan
Society for the Promotion of Science and Grant-in-Aid for Scientific Research on Priority Areas
22011001 of the Ministry of Education, Culture, Sports, Science and Technology (MEXT).
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