arXiv subjects

B. T. Khai

Publications and source records attributed to B. T. Khai.

5 recordsLinked to original sources

arXiv2021

Low background measurement in CANDLES-III for studying the neutrino-less double beta decay of $^{48}$Ca

We developed a CANDLES-III system to study the neutrino-less double beta (0$νββ$) decay of $^{48}$Ca. The proposed system employs 96 CaF$_{2}$ scintillation crystals (305 kg) with natural Ca ($^{\rm nat.}$Ca) isotope which corresponds 350\,g of $^{48}$Ca. External backgrounds were rejected using a 4$π$ active shield of a liquid scintillator surrounding the CaF$_2$ crystals. The internal backgrounds caused by the radioactive impurities within the CaF$_2$ crystals can be reduced effectively through analysis of the signal pulse shape. We analyzed the data obtained in the Kamioka underground for a live-time of 130.4\,days to evaluate the feasibility of the low background measurement with the CANDLES-III detector. Using Monte Carlo simulations, we estimated the background rate from the radioactive impurities in the CaF$_{2}$ crystals and the rate of high energy $γ$-rays caused by the (n, $γ$) reactions induced by environmental neutrons. The expected background rate was in a good agreement with the measured rate, i.e., approximately 10$^{-3}$ events/keV/yr/(kg of $^{\rm nat.}$Ca), in the 0$νββ$ window. In conclusion, the background candidates were estimated properly by comparing the measured energy spectrum with the background simulations. With this measurement method, we performed the first search for 0$νββ$ decay in a low background condition using a detector with a Ca isotope, in which the Ca present was not enriched, in a scale of hundreds of kg. The $^{48}$Ca isotope has a high potential for use in 0$νββ$ decay search, and is expected to be useful for the development of a next-generation detector for highly sensitive measurements.

hep-ex↗

arXiv2021

A study on energy resolution of CANDLES detector

In a neutrinoless double-beta decay ($0νββ$) experiment, energy resolution is important to distinguish between $0νββ$ and background events. CAlcium fluoride for studies of Neutrino and Dark matters by Low Energy Spectrometer (CANDLES) discerns the $0νββ$ of $^{48}$Ca using a CaF$_2$ scintillator as the detector and source. Photomultiplier tubes (PMTs) collect scintillation photons. At the Q-value of $^{48}$Ca, the current energy resolution (2.6%) exceeds the ideal statistical fluctuation of the number of photoelectrons (1.6%). Because of CaF$_2$'s long decay constant of 1000 ns, a signal integration within 4000 ns is used to calculate the energy. The baseline fluctuation ($σ_{baseline}$) is accumulated in the signal integration, thus degrading the energy resolution. This paper studies $σ_{baseline}$ in the CANDLES detector, which severely degrades the resolution by 1% at the Q-value of $^{48}$Ca. To avoid $σ_{\rm baseline}$, photon counting can be used to obtain the number of photoelectrons in each PMT; however, a significant photoelectron signal overlapping probability in each PMT causes missing photoelectrons in counting and reduces the energy resolution. "Partial photon counting" reduces $σ_{baseline}$ and minimizes photoelectron loss. We obtain improved energy resolutions of 4.5-4.0% at 1460.8 keV ($γ$-ray of $^{40}$K), and 3.3-2.9% at 2614.5 keV ($γ$-ray of $^{208}$Tl). The energy resolution at the Q-value is estimated to be improved from 2.6% to 2.2%, and the detector sensitivity for the $0νββ$ half-life of $^{48}$Ca can be improved by 1.09 times.

physics.ins-det↗

arXiv2020

Pulse Shape Discrimination of CsI(Tl) with a Photomultiplier Tube and MPPCs

In this study, we evaluate and compare the pulse shape discrimination (PSD) performance of multipixel photon counters (MPPCs, also known as silicon photomultiphers - SiPMs) with that of a typical photomultiplier tube (PMT) when testing using CsI(Tl) scintillators. We use the charge comparison method, whereby we discriminate different types of particles by the ratio of charges integrated within two time-gates (the delayed part and the entire digitized waveform). For a satisfactory PSD performance, a setup should generate many photoelectrons (p.e.) and collect their charges efficiently. The PMT setup generates more p.e. than the MPPC setup does. With the same digitizer and the same long time-gate (the entire digitized waveform), the PMT setup is also better in charge collection. Therefore, the PMT setup demonstrates better PSD performance. We subsequently test the MPPC setup using a new data acquisition (DAQ) system. Using this new DAQ, the long time-gate is extended by nearly four times the length when using the previous digitizer. With this longer time-gate, we collect more p.e. at the tail part of the pulse and almost all the charges of the total collected p.e. Thus, the PSD performance of the MPPC setup is improved significantly. This study also provides an estimation of the short time-gate (the delayed part of the digitized waveform) that can give a satisfactory PSD performance without an extensive analysis to optimize this gate.

physics.ins-det↗

arXiv2020

The energy calibration system for CANDLES using (n, γ) reaction

CAlcium fluoride for the study of Neutrinos and Dark matters by Low-energy Spectrometer (CANDLES) searches for neutrino-less double beta decay of $^{48}$Ca using a CaF$_2$ scintillator array. A high Q-value of $^{48}$Ca at 4,272 keV enabled us to achieve very low background condition, however, at the same it causes difficulties in calibrating the detector's Q-value region because of the absence of a standard high-energy $γ$-ray source. Therefore, we have developed a novel calibration system based on $γ$-ray emission by neutron capture on $^{28}$Si, $^{56}$Fe and $^{58}$Ni nuclei. In the paper, we report the development of the new calibration system as well as the results of energy calibration in CANDLES up to 9 MeV.

physics.ins-det↗

arXiv2018

$μ$TCA DAQ system and parallel reading in CANDLES experiment

A new $μ$TCA DAQ system was introduced in CANDLES experiment with SpaceWire-to-GigabitEthernet (SpaceWire-GigabitEthernet) network for data readout and Flash Analog-to-Digital Converters (FADCs). With SpaceWire-GigabitEthernet, we can construct a flexible DAQ network with multi-path access to FADCs by using off-the-shelf computers. FADCs are equipped 8 event buffers, which act as de-randomizer to detect sequential decays from the background. SpaceWire-GigabitEthernet has high latency (about 100 $μ$sec) due to long turnaround time, while GigabitEthernet has high throughput. To reduce dead-time, we developed the DAQ system with 4 "crate-parallel" (modules in crates are read in parallel) reading threads. As a result, the readout time is reduced by 4 times: 40 msec down to 10 msec. With improved performance, it is expected to achieve higher background suppression for CANDLES experiment. Moreover, for energy calibration, "event-parallel" reading process (events are read in parallel) is also introduced to reduce measurement time. With 2 "event-parallel" reading processes, the data rate is increased 2 times.

physics.ins-det↗