Performance of a liquid scintillator containing a zirconium β-keto ester complex developed for the ZICOS experiment

A liquid scintillator containing a zirconium β-keto ester complex has been developed for the ZIrconium Complex in Organic Scintillator (ZICOS) neutrinoless double beta decay experiment. We are aiming to develop a detector which has a good energy resolution (4% at 2.5 MeV), a large light yield (60% that of BC505) and a low background rate (0.1 counts/tonne・year) with several tonnes of 96Zr isotope, so we have investigated the zirconium β-keto ester complexes tetrakis (isopropyl acetoacetato) zirconium and tetrakis (ethyl acetoacetato) zirconium, which have high solubility (over 10 wt.%) in anisole. We measured the performance of liquid scintillators containing these zirconium β-keto ester complexes and obtained 40% of the light yield of BC505 and energy resolution of 4.1% at 2.5 MeV assuming 40% photo coverage of the photomultiplier in the ZICOS detector. Thus we almost achieved our initial goal. Preliminary investigations indicate that tetrakis (diethyl malonato) zirconium will give us no quenching of the light yield and an energy resolution of 2.9% at 2.5 MeV. This will be a suitable complex for the ZICOS experiment, if it has a large solubility

Direct measurement of spectral shape of Cherenkov light using cosmic muons

The spectral pulse shape of Cherenkov lights was directly measured by using cosmic muons. The observed decay times for early and late timing were 5.0 and 5.2ns, respectively. They were actually shorter than the time of scintillation lights which were also measured as 9.3ns and 9.2ns, respectively. However we could not see the difference of the rise time between scintillation and Cherenkov lights. This was due to the slow response of our DAQ equipment, photomultiplier and FADC digitize

Precise pulse shape measurement of Cherenkov light using sub-MeV electrons from Sr-90/Y-90 beta source

The precise spectral pulse shape from Cherenkov lights was directly measured by using sub-MeV electrons from 90Sr/90Y beta source. The observed shape was clearly different from the shape of scintillation light. The pulse rise and fall （decay） time for Cherenkov light were 0.8 ns and 2.5 ns, respectively. They were actually shorter than those times of scintillation light which were also measured by 1.6 ns and 6.5 ns, respectively. This clear Thisclearclear difference of rise time will be used for the pulse shape discrimination in order to select PMTs which receive Cherenkov lights, and the topological information due to Cherenkov light will be used for the reduction of backgrounds from 208Tl beta decay which should be major backgrounds observed around Q-value （3.35MeV）of 96Zr neutrinoless double beta decay

ZICOS : New project for neutrinoless double beta decay experiment using zirconium complex in liquid scintillator

　A liquid scintillator containing a zirconium β-keto ester complex has been developed for the Zirconium COmplex in liquid Scintillator (ZICOS) experiment which is new project of neutrinoless double beta decay search. We are aiming to develop a detector which has a good energy resolution (3.5% at 3.35 MeV), a large light yield (60% that of BC505) and a low background rate (0.1 counts/tonne・year) with several hundred kg of 96Zr isotope, so we have investigated the zirconium β-keto ester complexes such as tetrakis (isopropyl acetoacetato) zirconium and tetrakis (ethyl acetoacetato) zirconium, which have high solubility (over 30 wt.%)in anisole. We measured the performance of liquid scintillator containing these zirconium β-keto ester complexes, and obtained 48.7+-7.1% of the light yield of BC505 and the energy resolution of 4.1+-0.6% at 3.35 MeV assuming 40% photo coverage of the photomultiplier in the detector. This results reached our initial goal, so we estimated that ZICOS experiment should be sensitive to < 0.2 - 0.3 eV assuming gA = 1.25, gPP = 1.11 and QRPA model, if a radius of the balloon is 1.5 m and the balloon is filled with a liquidscintillator containing 10 wt.% concentration of a zirconium β-keto ester complex with an enriched 96Zr

HUN-ZICOS 検出器を用いたチェレンコフ光の位相幾何学情報の直接観測

The topological information of Cherenkov light from low energy electron was directly measured by HUNIZICOS detector. A 1.484 MeV electron with fixed direction to the center of hemispherical surface of the detector was generated by Compton back scattering with 100 degree from 88Y 1.836 MeV gamma. The observed averaged angle of Cherenkov light emitted from this electron was clustered around 40 degree assuming the vertex position to be at the center of truncated icosahedron photomultiplier jig. It was not Cherenkov angle around 47 degree as obtained by old simulation and the vertex to be the center of light yield for hitted photomultiplier. According to the HUNI-ZICOS simulation, the averaged angle of Cherenkov light was also clustered around 40 degree. On the other hands, the simulated averaged angle of scintillation was clustered around 49 degree, which is consistent with the averaged value of angle between the direction to center of hemisphere surface and each photomultiplier from the center of the jig. The obtained hitmap seemed to have same non flat structure as that of simulation due to Cherenkov ring. This is an evidence that Cherenkov lights emitted from 1.484 MeV electron should really have their topology. Therefore, we concluded that we will be able to reduce 208Tl background using the averaged angle for 96Zr neutrinoless double beta decay search.HUNI-ZICOS検出器を用いて低エネルギー電子が放出するチェレンコフ光の位相幾何学情報の直接観測を行った。単一方向・単色エネルギーを持つ電子を用いてチェレンコフ光の均角を観測したところ、光電子増倍管を設置する切頂20面体の治具の中心をバーテックスと仮定すると、40度に事象が集中していた。また、シミュレーションによると、チェレンコフ光の平均角は同様に40度に集中していた。これに対して、シンチレーション光の平均角は49度に集中していた。これは、切頂20面体の中心から半球の中心方向と、各光電子増倍管との間の角度の平均値と一致している。また、観測された光電子増倍管のヒットマップはチェレンコフ光リングにより一定の構造にはなっておらず、シミュレーションのヒットマップの構造とも合致している。これらの事実は、低エネルギー電子から放射されたチェレンコフ光は位相幾何学情報を維持していることの証拠である。つまり、平均角を用いればニュートリノを放出しない二重ベータ崩壊事象の探索で問題となる208Tlの背景事象を除去できると結論づけた

Development of pulse shape discrimination for Cherenkov lights in liquid scintillator

With a liquid scintillation used for ZICOS experiment, we measured pulse shapes in case of several radio isotopes, 60Co, 137Cs, 133Ba, and 57Co. Taking FADC timing at 60 nsec for the peak position, FADC spectra from 58.5 nsec to 80 nsec were almost same shape for each RI, however, before 58.5 nsec, we have found that those were different shape. Especially, in case of 57Co, the energy is lower than Cherenkov threshold, so that the spectra should not include Cherenkov light. Using those spectra between 57.0 nsec and 58.0 nsec（3 bins）, we calculated simply χ2 and it was clearly discriminated that χ2 ≥ 0.1 should be include Cherenkov lights. This was also confirmed by Compton electrons with fixed energy and fixed direction. Obtained detection inefficiency of Cherenkov lights was observed by 21.4 ± 9.6 %. According to Compton edge events which have almost same direction as the incident γ and backgrounds events which should have isotropic direction, the detection inefficiency were 10.4 ± 0.5 % and 49.1 ± 1.4 %, respectively. They were quite different values and the inefficiency of both fixed energy and Compton edge events were statistically same. This is a direct evidence that Cherenkov lights should keep their topology even if they are emitted by around 1 MeV electron

チタンホスホネートクラスターの合成と性質およびその有機―無機ハイブリッド材料への展開に関する研究

東京理科大学201

Pressure Effects with Incorporated Particle Size Dependency in Graphene Oxide Layers through Observing Spin Crossover Temperature

This research highlights the pressure effects with the particle size dependency incorporated in two-dimensional graphene oxide (GO)/reduced graphene oxide (rGO). GO and rGO composites employing nanorods (NRs) of type [Fe(Htrz)2(trz)](BF4) have been prepared, and their pressure effects in the interlayer spaces through observing the changes of the spin crossover (SCO) temperature (T1/2) have been discussed. The composites show the decrease of interlayer spaces from 8.7 &#197; to 3.5 &#197; that is associated with GO to rGO transformation. The shorter interlayer spaces were induced by pressure effects, resulting in the increment of T1/2 from 357 K to 364 K. The pressure effects in the interlayers spaces estimated from the T1/2 value correspond to 24 MPa in pristine [Fe(Htrz)2(trz)](BF4) NRs under hydrostatic pressure. The pressure observed in the composites incorporating NRs (30 &#215; 200 nm) is smaller than that observed in the composite incorporating nanoparticles (NPs) (30 nm). These results clearly demonstrated that the incorporated particle size and shape influenced the pressure effects between the GO/rGO layer