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Register a new userAn accurate measurement of PMT TTS based on the photoelectron spectrum
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The water Cherenkov detector array (WCDA) for the large high altitude air shower observatory(LHAASO) will employ more than 3600 hemisphere 8 inch photomultiplier tubes (PMT). The good time performance of PMT, especially the transit time spread (TTS), is required for WCDA. TTS is usually defined as the TTS of single photoelectron, and usually determined by using single photoelectron counting technique. A method using the photoelectron spectrum is researched for the measurement of TTS. The method is appropriate for multi-photoelectrons and makes it possible to measure the TTS of different photoelectrons at the same time. The TTS of different photoelectrons is measured for Hamamatsu R5912 with the divider circuit designed in specifically. The TTS of single photoelectron is determined to 3.3 ns and satisfies the requirement of WCDA.
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Results are presented from radioactivity screening of two models of photomultiplier tubes designed for use in current and future liquid xenon experiments. The Hamamatsu 5.6 cm diameter R8778 PMT, used in the LUX dark matter experiment, has yielded a positive detection of four common radioactive isotopes: 238U, 232Th, 40K, and 60Co. Screening of LUX materials has rendered backgrounds from other detector materials subdominant to the R8778 contribution. A prototype Hamamatsu 7.6 cm diameter R11410 MOD PMT has also been screened, with benchmark isotope counts measured at <0.4 238U / <0.3 232Th / <8.3 40K / 2.0+-0.2 60Co mBq/PMT. This represents a large reduction, equal to a change of times 1/24 238U / times 1/9 232Th / times 1/8 40K per PMT, between R8778 and R11410 MOD, concurrent with a doubling of the photocathode surface area (4.5 cm to 6.4 cm diameter). 60Co measurements are comparable between the PMTs, but can be significantly reduced in future R11410 MOD units through further material selection. Assuming PMT activity equal to the measured 90% upper limits, Monte Carlo estimates indicate that replacement of R8778 PMTs with R11410 MOD PMTs will change LUX PMT electron recoil background contributions by a factor of times1/25 after further material selection for 60Co reduction, and nuclear recoil backgrounds by a factor of times 1/36. The strong reduction in backgrounds below the measured R8778 levels makes the R11410 MOD a very competitive technology for use in large-scale liquid xenon detectors.
The $alpha$-particle light response of liquid scintillators based on linear alkylbenzene (LAB) has been measured with three different experimental approaches. In the first approach, $alpha$-particles were produced in the scintillator via $^{12}$C($n$,$alpha$)$^9$Be reactions. In the second approach, the scintillator was loaded with 2% of $^{mathrm{nat}}$Sm providing an $alpha$-emitter, $^{147}$Sm, as an internal source. In the third approach, a scintillator flask was deployed into the water-filled SNO+ detector and the radioactive contaminants $^{222}$Rn, $^{218}$Po and $^{214}$Po provided the $alpha$-particle signal. The behavior of the observed $alpha$-particle light outputs are in agreement with each case successfully described by Birks law. The resulting Birks parameter $kB$ ranges from $(0.0066pm0.0016)$ cm/MeV to $(0.0076pm0.0003)$ cm/MeV. In the first approach, the $alpha$-particle light response was measured simultaneously with the light response of recoil protons produced via neutron-proton elastic scattering. This enabled a first time a direct comparison of $kB$ describing the proton and the $alpha$-particle response of LAB based scintillator. The observed $kB$ values describing the two light response functions deviate by more than $5sigma$. The presented results are valuable for all current and future detectors, using LAB based scintillator as target, since they depend on an accurate knowledge of the scintillator response to different particles.
Many current and future dark matter and neutrino detectors are designed to measure scintillation light with a large array of photomultiplier tubes (PMTs). The energy resolution and particle identification capabilities of these detectors depend in part on the ability to accurately identify individual photoelectrons in PMT waveforms despite large variability in pulse amplitudes and pulse pileup. We describe a Bayesian technique that can identify the times of individual photoelectrons in a sampled PMT waveform without deconvolution, even when pileup is present. To demonstrate the technique, we apply it to the general problem of particle identification in single-phase liquid argon dark matter detectors. Using the output of the Bayesian photoelectron counting algorithm described in this paper, we construct several test statistics for rejection of backgrounds for dark matter searches in argon. Compared to simpler methods based on either observed charge or peak finding, the photoelectron counting technique improves both energy resolution and particle identification of low energy events in calibration data from the DEAP-1 detector and simulation of the larger MiniCLEAN dark matter detector.
Detectors using liquid xenon as target are widely deployed in rare event searches. Conclusions on the interacting particle rely on a precise reconstruction of the deposited energy which requires calibrations of the energy scale of the detector by means of radioactive sources. However, a microscopic calibration, i.e. the translation from the number of excitation quanta into deposited energy, also necessitates good knowledge of the energy required to produce single scintillation photons or ionisation electrons in liquid xenon. The sum of these excitation quanta is directly proportional to the deposited energy in the target. The proportionality constant is the mean excitation energy and is commonly known as $W$-value. Here we present a measurement of the $W$-value with electronic recoil interactions in a small dual-phase xenon time projection chamber with a hybrid (photomultiplier tube and silicon photomultipliers) photosensor configuration. Our result is based on calibrations at $mathcal{O}(1-10 , mathrm{keV})$ with internal $^{37}$Ar and $^{83text{m}}$Kr sources and single electron events. We obtain a value of $W=11.5 , ^{+0.2}_{-0.3} , mathrm{(syst.)} , mathrm{eV}$, with negligible statistical uncertainty, which is lower than previously measured at these energies. If further confirmed, our result will be relevant for modelling the absolute response of liquid xenon detectors to particle interactions.
Since the discovery of nuclear gamma-rays, its imaging has been limited to pseudo imaging, such as Compton Camera (CC) and coded mask. Pseudo imaging does not keep physical information (intensity, or brightness in Optics) along a ray, and thus is capable of no more than qualitative imaging of bright objects. To attain quantitative imaging, cameras that realize geometrical optics is essential, which would be, for nuclear MeV gammas, only possible via complete reconstruction of the Compton process. Recently we have revealed that Electron Tracking Compton Camera (ETCC) provides a well-defined Point Spread Function (PSF). The information of an incoming gamma is kept along a ray with the PSF and that is equivalent to geometrical optics. Here we present an imaging-spectroscopic measurement with the ETCC. Our results highlight the intrinsic difficulty with CCs in performing accurate imaging, and show that the ETCC surmounts this problem. The imaging capability also helps the ETCC suppress the noise level dramatically by ~3 orders of magnitude without a shielding structure. Furthermore, full reconstruction of Compton process with the ETCC provides spectra free of Compton edges. These results mark the first proper imaging of nuclear gammas based on the genuine geometrical optics.
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