Radioactivity control strategy for the JUNO detector

JUNO is a massive liquid scintillator detector with a primary scientific goal of determining the neutrino mass ordering by studying the oscillated anti-neutrino flux coming from two nuclear power plants at 53 km distance. The expected signal anti-neutrino interaction rate is only 60 counts per day (cpd), therefore a careful control of the background sources due to radioactivity is critical. In particular, natural radioactivity present in all materials and in the environment represents a serious issue that could impair the sensitivity of the experiment if appropriate countermeasures were not foreseen. In this paper we discuss the background reduction strategies undertaken by the JUNO collaboration to reduce at minimum the impact of natural radioactivity. We describe our efforts for an optimized experimental design, a careful material screening and accurate detector production handling, and a constant control of the expected results through a meticulous Monte Carlo simulation program. We show that all these actions should allow us to keep the background count rate safely below the target value of 10 Hz (i.e. ∼1 cpd accidental background) in the default fiducial volume, above an energy threshold of 0.7 MeV.

Natural Gamma-Ray Background Characterization in Pyhasalmi Mine

<p>We present the results of measuring the gamma-ray background performance of Pyhasalmi mine, the deepest one in Europe. Two underground facilities in Lab 2 (1440 m) and Lab 5 (1410 m) were investigated. Based on measurements made in Lab 2 with a low-background HPGe spectrometer, we determined the integral gamma-ray background count rate in the energy range of 40 keV to 2.7 MeV to be 0.095 s<sup>–1</sup> kg<sup>–1</sup>. The minimum detectable activities of some natural and artificial nuclides were less than 0.071 Bq/kg (<sup>226</sup>Ra), 0.77 Bq/kg (<sup>40</sup>K) and 0.012 Bq/kg (<sup>137</sup>Cs). The specific activities of natural nuclides in the shotcrete covering the walls of the Lab 2 were higher than those in the rock: 100.3 Bq/kg (<sup>232</sup>Th), 161.7 Bq/kg (<sup>226</sup>Ra) and 1171 Bq/kg (<sup>40</sup>K) in the shotcrete covering and 47.6 Bq/kg (<sup>232</sup>Th), 83.1 Bq/kg (<sup>226</sup>Ra) and 1513 Bq/kg (<sup>40</sup>K) in the rock. The measurements showed that the gamma-ray background level in Lab 5 is significantly lower than that in Lab 2. The integrated gamma-ray background count rate for the energy range of 40 keV to 2.7 MeV was 0.028 s<sup>–1</sup> kg<sup>–1</sup> for Lab 5. Purging the measuring chamber of the gamma spectrometer with nitrogen gas at a rate of 0.15 L/h allowed to further improve this parameter to 0.021 s<sup>–1</sup> kg<sup>–1</sup>. In general, the results of this study confirm that the level and energy spectrum of background gamma radiation in the underground facility within the studied energy range is defined mainly by the composition of the walls of the Labs.</p><p> </p>

Precision Measurements of the Minimum Detectable Activity of Radionuclides by Plastic Scintillators for Radiation Monitoring

This work is devoted to the precision evaluation of the minimum detectable activity (MDA) measurement of gamma radiation radionuc­lides 137Cs and 60Co using plastic scintillators (PS). Scintillators are part of the detectors, which are sensitive elements of radiation portal moni­tors (PM). PS were produced at the Institute for scintillation materials of the National Academy of Sciences of Ukraine (ISMA NAS of Ukraine). The evaluation was generate in accordance with State standard of Ukraine ISO 5725-2:2005 [1] through statistical processing of experi­mental material. This made it possible to take into account the factors that determine the scattering of results without a model equation. Influence of PS sizes, energy radiation detected and distance from PS to the source of ionizing radiation (SIR) on the value of MDA precision measurement was studied. Precision coefficients of radionuclides MDA measurement evalua­ted when changing PS sizes from 500×300×50 mm (7500 cm3) up to 500×500×50 mm (12500 cm3), changing the energy radiation detected from 662 keV (137Cs) until 1332 keV (60Co), changing the distance from the PS to SIR from 10 to 50 cm. The results are presen­ted in Fig. 6—9 and Table. Found, that the greatest contribution to the increase in MDA precision coefficient measurements makes a change in the irradia­tion geometry. It is shown that when distance from PS to SIR increases from 10 to 50 cm, precision values increase: when registering 137Cs, from 3,9 to 7,0 % (V = 7500 cm3) and from 3,6 to 6,3 % (V = 12500 cm3); when registering 60Co, from 3,1 to 4,3 % (V = 7500 cm3) and from 3,2 to 3,8 % (V = 12500 cm3). It is established, that the obtained results are due to the change in the contribution of background count rate to PS count rate under different measurements conditions. This contribution affects on the value of sensitivity coefficient and consequently on the value of MDA and precision coefficient.

A Low Background Gamma Ray Spectrometer with Anticosmic Shielding

The article describes a gamma ray spectrometer protected by a lead shield (Model 747E Canberra lead shield) and an active shield made of an 80~cm \(\times\) 80~cm \(\times\) 3~cm plastic scintillator plate in anticoincidence on top of the lead shield. The detector used as low background gamma-spectrometer is a high purity Germanium crystal of model GC2018 Canberra. The background count rate currently achieved (30-2400 keV) is 1.27 cps without anticoincidence. The level of background suppression obtained from the active protection is 0.80 overall and about 0.43 for the 511 keV gamma line. The gamma ray spectrometer is installed and operated in the Nuclear Laboratory, Department of Nuclear Physics, University of Science, HCMC-Vietnam National University.

The optimization of measurement device independent quantum key distribution

Measurement device independent quantum key distribution (MDI-QKD) is a promising method for realistic quantum communication which could remove all the side-channel attacks from the imperfections of the devices. Here in this study, we theoretically analyzed the performance of the MDI-QKD system. The asymptotic case rate with the increment of the transmission distance at different polarization misalignment, background count rate and intensity is calculated respectively. The result may provide important parameters for practical application of quantum communications.

A Simple, Extremely Stable Single-Tube Liquid Scintillation System for Radiocarbon Dating

This paper describes a simple and compact liquid scintillation radiocarbon dating system, ICELS, and demonstrates its long-term stability and reproducibility to a precision level rarely presented before, better than 0.04% (3 14C yr). Inexpensive systems of this kind may, in the future, help to meet increasing demand for high precision (±16 to ±20 14C yr) and strict quality control. ICELS comprises a compact detector unit, where a 3-mL dome-shaped vial, with an optimal light reflector, sits on the top of a vertical 30-mm photomultiplier tube. Sample changing is manual. The high voltage is set at the balance point for each sample, securing maximal counting stability. The quench correction method used (spectrum restoration) corrects with 0.04% precision for all parameters that can normally shift the 14C spectrum. For 3 mL of benzene at 71% 14C counting efficiency (recent carbon 23 cpm), the background is 1.72 cpm behind a 5-cm-thick shield of lead (27 kg) and 1.53 cpm behind 10 cm of lead. The background count rate corrected for atmospheric pressure variations was completely stable over 47- and 57-d testing periods for the 2 systems.

Performance of the Packard Tri-Carb® 2770TR/SL Liquid Scintillation Analyzer for 14C Dating

We present results that demonstrate the potential of the Packard Tri-Carb® Model 2770TR/SL for radiocarbon dating. For 2 g of sample benzene, a stable background count rate of 0.307 cpm and a stable counting efficiency of 64.78% were determined using a 13–75 keV counting window. Changes to the mathematical routines for t-SIE (quench indicating parameter) calculation and a reduction in the activity of the external standard have enabled stability of the t-SIE to be achieved, and combined with the use of a suitable balance point counting window; all of these factors give the stability of performance required for 14C dating. Calculations based on the above parameters indicate that the limit of detection for 2 g samples, counted for 5000 min, is >48,900 yr bp. The great advantage of this system is that these data were acquired using inexpensive standard 7-mL low potassium borosilicate glass vials. Vial holders manufactured from BGO reduced the background to 0.15 cpm with a minimum effect on efficiency (64.46% for 13–75 keV). A similar calculation of the limit of detection gave >51,700 bp. The use of the BGO vial holders in other instruments employing time-resolved liquid scintillation counting (TR-LSC) (Models 2250CA and 2260XL) also brought about significant improvements in detection limits.