Development of liquid scintillator containing
8-quinolinolate indium complex for a measurement of
low energy solar neutrinos
Yoshiyuki Fukuda and Kunihiko Watanabe
Faculty of Education, Miyagi University of Education,
149, Aobaku-aza-aoba, Sendai, Miyagi 980-0845, Japan

Shigetaka Moriyama
Kamioka Observatory, Institute for Cosmic Ray Research,
University of Tokyo
456, Higashi-Mozumi, Kamioka-cho, Hida, Gifu 506-1205, Japan
Abstract. An organic liquid scintillator containing indium complex was studied for ameasurement of low energy solar neutrinos. The energy of the emitted electrons from those
processes could be transferred to the molecular of complex via the organic solvent molecules,
and the luminescence from the indium complex would be detected by the photo-multiplier. Here
we report some results obtained by photo-luminescence and the γ’s induced energy spectra oftris (8-quinolinolate) indium (InQ3 ) complex. Benzonitrile was chosen as a solvent becauseof good solubility for the quinolinolate complexes ( 2 wt%) and of good light yield for the
scintillation induced by γ’s irradiation. The photo-luminescence emission spectra of InQ3 of 30
M solution in benzonitrile was measured. The emission maxima for InQ3 is found at 559nm
as excitation wavelength: 397nm. We prepared InQ3 (50mg ∼ 200mg) of benzonitrile solutions(20 mL). Two secondary scintillators, 100 mg of 2,5-diphenyloxazole (PPO) and 10 mg of 1,4bis[2-(5-phenyloxazolyl)]benzene (POPOP), are also dissolved in above solutions and they playa role of wavelength shifter from the UV region emitted by excited benzonitrile molecules to thevisible region emitted by InQ3 complex. The energy spectra of electrons emitted by Compton
scattering of incident γ’s using 60 Co radio isotope was obtained. These results are the firstobservation of the γ’s energy spectra using luminescent metal complexes.

keyword:low energy solar neutrinos （低エネルギー太陽ニュートリノ）
liquid scintillator （液体シンチレータ）
8-quinolinolate complex （キノリノール錯体）
1. Introduction
Super-Kamiokande has discovered the evidence of atmospheric muon neutrino oscillation in
1998[1], and the K2K experiment has confirmed the oscillation at 99.995%C.L. using νµ beam
produced by KEK 1 GeV Proton Synchrotron in 2005[2]. Therefore the neutrino physics lead
to beyond the standard model of elementary particle physics.

Also Super-Kamiokande and Sudbery Neutrino Observatory experiment have establishedelectron neutrino oscillation in their solar neutrino data [3, 4]. 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 anti-neutrino in sense of ∆m2[5]. Even though Borexino reported the first results of 7 Be solar neutrino flux [6], the oscillation
mixing angel (θ12 ) obtained by global fit using all solar neutrino data was not restricted as wellas θ23 observed in atmospheric neutrino data. For the future solar neutrino experiment, a precise
θ12 should be measured with 1% accuracy due to the observation for solar pp/7 Be neutrinos inorder to obtain finite value of θ13 in case of only upper limits obtained by T2K nor the reactor
experiments.
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 [7] 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.
An excited state of 115 Sn shown in Eq.(1) decays into the ground state with a lifetime of 4.76µs,
and emits two γ’s (116 keV and 497 keV). This signature is also able to use for a triple-coincidence
to extract neutrino signal from huge backgrounds. However, 115 In itself has natural β decay into
the ground state of 115 Sn with a lifetime of 4.41 × 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 detector with both good vertex resolution (a few cm) and well energy resolution(10% @ 100keV) is necessary [8].
Many possible detectors using indium were designed in last few decade, however, no realistic
detector has been made. In 1988, Suzuki and Fukuda developed the InP solid state detector(1mm2 × 10µm) using the pn-junction and the detector observed the γ’s from the radioactive
sources [9]. New detector using InP semi-conductor has been re-evaluated for last several years,
and a realistic size of InP detector was established by using semi-insulating substrate[11].
For large volume detector, a liquid scintillator containing indium was developed by Suzuki
et al.[10] and LENS project [12]. Recently LENS group presented the feasibility of realistic way
for a proto-type detector (Mini-LENS) [13], which will use MVA indium complex in the Liner
Arkyl Benzen (LAB) [14].

N
In3+
O-

3

Figure 1. tris (8-quinolinolate) indium complex.
A tris (8-quinolinolate) alminum complex is very famous as an organic electron luminescent(OEL) device, and the chemical characteristics were studied in 1970s. The metal coupled withthe ligand could be exchanged by other metal such as Gallium and Indium. The optical property
was also studied [15] and every complexes have almost same maximum wavelength (∼ 550nm)
for the luminescence. We used tris (8-quinolinolate) indium complex as shown in Fig.1 for not
only solute of liquid scintillator but the fluorescent material in the scintillator system.

3. Synthesis of InQ3 complex
A tris (8-quinolinolate) alminum complex (molecular mass : C27H18N3O3Al F.W.=459.44)
is commercially produced as an organic electron luminescent device, however, a tris (8quinolinolate) indium complex (molecular mass : C27H18N3O3In F.W.=547.28) is not popular.
Therefore, we have to synthesize using metal salt compound and quinolinolate ligand. A 2.1773gof 8-quinolinolate ligand and 1.1562g of ammonium acetate were added in 25mL of H2 O, andthen a 35mL of ethanol was also added. This solution was heating with 50 ◦ C in order todissolve them. In this solution, a 1.7744g of indium(III) nitrate trihydrate with 8mL of H2 O
was added. Immediately, the yellow powder was precipitated with primary complex, and it
was agitated during 10 minutes. Then the precipitated material was filtrated and put into thevacuum desiccator. The primary yield was obtained by 2.8596g, and it corresponds to 100%
eﬃciency. The obtained primary complex could include an impurity, so that we had sublimatedit (0.3201g) under the vacuum (7.5 × 10−4 Torr). The temperature was almost 350◦ C. After
2 times of sublimation, we obtained 0.2449g of a tris (8-quinolinolate) indium complex. The
eﬃciency was achieved by 77%.
This obtained complex was analyzed by two methods. One is the elemental analysis (EA)
and the other is the high resolution mass spectrometry (HRMS). The elements of InQ3 complex
consists of C :59.26, H:3.32, and N.7.68. Found elements by EA are C:59.233, H:3.534, andN:7.652, respectively. They are completely consistent with the elemental structure. Also the
HRMS (ESI) with the m/z calcd for C27H18N303NaIn [M+Na+ ] expected total mass 570.0283.
The found mass is 570.0279. Therefore, the impurity of synthesized InQ3 complex should
achieved above 99%.

700

emission wavelength [nm]emission wavelength [nm]4. Photo luminescence of InQ3 complex and quantum yield
We have searched for an organic solvent which should have high solubility of InQ3 complex.
Some typical aromatic solvent such as toluene or xylene, which were generally used for the baseof liquid scintillator, could not dissolve InQ3 nor AlQ3 complexes so much. These are not a polar
molecular. We have found benzonitrile (C6H5CN: PhCN) and benzyl alcohol (C6H5CH2OH:
PhCH2 OH) have an order of a few % dissolution for AlQ3 and InQ3 complexes. The maximum
photo luminescent wavelength are 291nm and 270nm in UV region, respectively. For the current
development, we have chosen PhCN as solvent because of larger photo yield. The PhCN has aflash point of 75 ◦ C, and the attenuation length is 66cm for the InQ3 0.5wt% concentration.

640
580
520
460

700
648
596
544
492

400
340

372

404

436

468

500

excitation wavelength [nm](a) AlQ3

440
340

360

380
400
420
440
excitation wavelength [nm](b) InQ3

Figure 2. 2-dimensional plot of fluorescence of (a) AlQ3 and (b) InQ3 complex dissolved in
PhCN are shown. Horizontal and vertical axis shows the wavelength of excitation and emission
light.

intensity

intensity

We used HORIBA FluoroMax-4 spectrofluorometer for scanning the photo luminescence.
Figure 2 shows the 2-dimensional plot of the wavelength between an excitation and an emission
light for (a) AlQ3 and (b) InQ3 complex dissolved in PhCN, respectively. The concentration ofboth complexes was 3 × 10−5 mol/L. The emission light yield is also shown in the color on thecontour.
Figure 3 shows photo luminescence and absorption of of AlQ3 and InQ3 complex in
PhCN solution. Maximum emission wavelength of AlQ3 and InQ3 complex are found at
529.9nm (excitation wavelength : 383nm) and 559.3nm (excitation wavelength : 397nm),
respectively. The absorbance of AlQ3 and InQ3 complex also measured by HITACHI U-3000
spectrophotometer. The maximum absorption wavelength were located at 389.6nm for AlQ3
and 394.7nm for InQ3 complex, respectively.

AlQ 3 emission

InQ 3 emission

excitation : 397nm

excitation : 383nm

AlQ 3 absorption

wavelength [nm]intensity

intensity

wavelength [nm]InQ 3 absorption

wavelength [nm]wavelength [nm](a) AlQ3(b) InQ3

Figure 3. Photo luminescence and absorbance of (a) AlQ3 and (b) InQ3 complex dissolvedin PhCN are shown. The maximum emission wavelength of AlQ3 and InQ3 were found at
529.9nm and 559.3nm with the excitation wavelength at 383nm and 397nm, respectively. Also
the maximum absorption wavelength were located at 389.6nm and 394.7nm, respectively.
The quantum yield of tris (8-quinolinolate) complex was obtained by the standard
material. Quinine ((6-Methoxyquinolin-4-yl) [(2S,4S,5R)-5-vinyl-1-aza-bicyclo[2.2.2]oct-2-yl](R)-methanol) was used for the reference material which has known quantum yield 0.55 andrelative light yield to the maximum at 100% as a function of the wave number are shown in
Ref.[16]. The emission spectra in Fig.3 was not corrected by the quantum eﬃciency of the photo
multiplier which is used in FluoroMax-4. Using relative light yield for measured fluorescence ofQuinine, we have corrected the emission spectra, and have obtained the area of each spectrum.
The quantum yield was defined by following formula;Φc = Φ r ×

Ar
Sc
×
Sr
Ac

(2)

Here, Φr / Φc , Sr / Sc , and Ar / Ac is the quantum yield, area of spectrum, and the maximum
absorbance of Quinine and tris (8-quinolinolate) complex, respectively. Φr is referred as 0.55 in[16]. The obtained quantum yield were 0.17 and 0.05 for AlQ3 and InQ3 complex, respectively.
These values are quite consistent with the published value as shown in Ref.[17].

5. Performance of liquid scintillator containing InQ3 complex
The γ’s from sources of 137 Cs (662keV) and 60 Co (1.17MeV and 1.33MeV) could interact
with electrons in liquid scintillator, and emit an electron via photo-electric eﬀect or Compton
scattering. Light nuclei such as Hydrogen and Carbon has larger cross section of Compton
scattering than that of photo-electric eﬀect. The energy of emitted electron could be depositedby the electron-magnetic interaction with other electrons during going through the material.
Those electrons received the energy would be transited to the exited state, and after proper time,the electron would be back lower energy state with the photon emission such as scintillation.
PPO absorption
wavelength [nm]PPO emission

excitation wavelength 320nm

wavelength [nm]POPOP absorption
wavelength [nm]POPOP emission

excitation wavelength 346nm

wavelength [nm]InQ3 absorption
wavelength [nm]excitation wavelength 397nm

InQ3 emission
wavelength [nm]Figure 4. Absorption and emission of PPO, POPOP and InQ3 in PhCN.
PhCN has larger scintillation light yield than PhCH2 OH, however, the wavelength ofluminescence did not match the absorption of InQ3 as shown in Fig.4. In order to transfer theenergy to the photomultiplier, we dissolved 2,5-Diphenyloxazole(PPO), which has absorption
maximum at 309.7nm and fluorescence maximum at 368.0nm, as a secondary scintillator in
PhCN. In addition, 1,4-Bis(5-phenyloxazol-2-yl)benzene (POPOP) or 1,4-Bis(2-methylstyryl)benzene (Bis-MSB) would help the energy transfer due to more overlap with the absorptionof InQ3 complex. Figure 4 shows photon absorption and emission of PPO, POPOP and InQ3
complex, respectively.
The liquid scintillator was put into 20mL quartz vials. Two 2-inch photomultiplier(Hamamatsu H6410) are connected to the vial through acrylic light guide. The signal was
attenuated because of wide dynamic range for observed scintillation light yield. The coincidence
method using two signals from those photomultiplier was used for avoiding backgrounds. The
setup for data acquisition is shown in Fig.5.
Figure 6 shows the energy spectrum of electrons emitted by induced γ’s for both standard
scintillator BC505 and PhCN with PPO 100mg and POPOP 10mg (PhCN cocktail scintillator).
Clear Compton edges of γ’s was observed for each sources. While two γ’s are emitted
simultaneously from 60 Co source, those gamma-rays do not have same direction. Therefore,light guide
Hamamatsu H6410

Hamamatsu H6410

Photomultiplier

Photomultiplier

1800V

Repic RPH-032

2000V

X10

30mV

sensitive ADC

Gate
Generator

Coincidence
RegisterDisriminator

Ampliﬁer

Attenuator

Delay (10m)

LeCroy VME 1182 charge

20mL vial

200ns

100ns

478keV(662keV γ
compton edge)

137

Cs

Number of events

Number of events

Figure 5. Data acquisition for scintillation lights through the photomultiplier.

1.04MeV(1.17MeV /1.33MeV
γ compton edge)

60

Co

ADC channel [36db](a) BC505

137

Cs

ADC channel [18db]
Number of events

Number of events

ADC channel [24db]

478keV(662keV γ
compton edge)

1.04MeV(1.17MeV /1.33MeV
γ compton edge)
60

Co

ADC channel [24db](b) PhCN cocktail

Figure 6. Photon spectrum of electrons emitted by γ’s interaction using 136 Cs and 60 Co sources
for both standard liquid scintillator BC505 and PhCN cocktail scintillator.
one of two γ’s could be entered into the liquid scintillator. Since those energy should be too
close to distinguish each γ’s due to poor energy resolution, we observed as single γ’s withaveraged energies.
The PhCN cocktail scintillator had 63% of light yield relative to that of BC505. Here, thewavelength of emitted lights for both scintillators are almost same (∼ 420nm). The quantum
eﬃciency of Hamamatsu H6410 photomultiplier is 25% at this wavelength.
We measured the energy spectrum of induced γ’s from sources for 50mg of AlQ3 and InQ3
dissolved in PhCN cocktail scintillator as shown in Fig.7. The scintillation light yield of InQ3

137

Cs

Number of events

Number of events

looks smaller than that of AlQ3 . This should be caused by the diﬀerence of quantum yield of
those complexes.

Co

1.04MeV(1.17MeV /1.33MeV
γ compton edge)

ADC channel [6db]ADC channel [0db]Number of events

Number of events

ADC channel [0db](a) AlQ3 50mg

Cs

478keV(661keVγ comptonedge)

478keV(661keVγ compton edge)

60

137

60

Co

1.04MeV(1.17MeV /1.33MeV
γ compton edge)

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

Figure 7. Observed energy spectrum of (a) AlQ3 50mg (b) InQ3 50mg dissolved in PhCN
cocktail scintillator for γ’s from 136 Cs and 60 Co.
The quantum yield of γ’s irradiation could be evaluated by diﬀerence of scintillation photon
yield between the PhCN cocktail scintillator and PhCN cocktail scintillator containing thecomplex . Because of diﬀerent wavelength of the scintillation light, we have to correct thelight yield with the quantum eﬃciency of H6410 photomultiplier. The emission wavelength of
AlQ3 and InQ3 complex in PhCN is 530nm and 560nm, respectively. The quantum eﬃciency is
12.6% and 9.3%, respectively. The quantum yield of AlQ3 and InQ3 dissolved in PhCN cocktail
scintillator, therefore, are obtained by 24.7% and 15.6%, respectively. These values are larger
than that obtained by the photo luminescence as described in Section 3.
According to the Fig.4, the photon emission from POPOP has slightly longer tail than
the absorption distribution of InQ3 complex. This means that the residual photon could becontaminated in the emission spectrum of InQ3 complex. In order to evaluate the amountof POPOP residual light, we measured the emission of scintillation light from InQ3 dissolvedin 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 (50% at
482nm and 10% at 470nm). The observed energy spectrum of InQ3 complex dissolved in PhCN
cocktail scintillator was shown in Fig.8 (a). The photon light yield was lost about 34%. On theother hands, 23% of the light yield of PhCN cocktail scintillator still remained via SC-48 filter asshown in Fig.8 (b). Therefore the diﬀerence of quantum yield between the photo luminescenceand the γ’s irradiation could be explained by the residual light from POPOP, and the amount of
those lights was assumed to be ≤ 10% of original PhCN cocktail scintillation yield, at maximum.

137

Cs

Number of events

Number of events

478keV(662keV γ
compton edge)

137

Cs

478keV(661keVγ comptonedge)

1.04MeV(1.17MeV /1.33MeV
γ compton edge)

60

Co

ADC channel [0db]Number of events

Number of events

ADC channel [0db]60Co

1.04MeV(1.17MeV /1.33
MeV γ comptonedge)

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

ADC channel [6db](b) PhCN cocktail

Figure 8. Observed spectrum of γ’s via SC-48 filter. (a) The light yield was lost about 34%
in case of PhCN cocktail scintillator containing InQ3 complex. On the other hands, (b) 23% of
the light yield of PhCN cocktail scintillator still remained via SC-48 filter.
6. Conclusion
An organic liquid scintillator containing InQ3 complex was developed using the photo
luminescence of complex. The InQ3 complex dissolved in PhCN has maximum photo
luminescence at 560nm and the attenuation length was about 66cm at 0.5wt% dissolution. The
PhCN cocktail scintillator containing InQ3 has actually luminescence for γ’s irradiation. The
quantum yield were 5% and 15% for the photo luminescence and γ’s irradiation, respectively.
The diﬀerence could be explained by the residual light from the fluorescence of POPOP. The
developed liquid scintillator contaning InQ3 compex was successfully worked to detect γ’s.
However, for the actual solar neutrino detector, it is still necessary more light yield to getbetter energy resolution, and more solubility to order of 5wt%. In order to achieve former point,
we will produce new InQ3 complex with the substitution group, tris (5-aryl-8quinollinolate)metal complex. The AlQ3 complex with 1a substitution group improved the quantum yield tobe 0.533, and the maximum emission wavelength was shifted to 490nm [18]. Later means that
better quantum eﬃciency of the photomultiplier will help to get more scintillation light yield.
The another metal such as zirconium (Zr) instead of aluminum or indium could be formedby 8-quinolinolate complex. We have already succeed to synthesize ZrQ4 complex, recognizedsame solubility for PhCN as InQ3 , and measured the photo luminescence. The 96 Zr is one of
the target nuclei for the neutrino-less double beta decay (0νββ) with higher Q-value. Recently
SNO+ group has decided to use zirconium instead of neodymium (Nd). Therefore, new technique
to dissolved zirconium in liquid scintillator will be very important for realizing ton-scale 0νββdecay experiments.

Acknowledgments
This work was supported by KAKENHI Grant-in-Aid Scientific Research (C) 22540303 of
Japanese Society for the Promotion of Science. We would like to thank Prof. Muramatsu
for a measurement of fluorescence using HORIBA FluoroMax-4 spectrofluorometer, and Prof.
Ikeyama for a measurement of absorbance using HITACHI U-3000 spectrophotometer.
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