Progress in HXR diagnostics at GOLEM and COMPASS tokamaks

Scintillation detectors are widely used for hard X-ray spectroscopy and allow us to investigate the dynamics of runaway electrons in tokamaks. This diagnostic tool proved to be able to provide information about the energy or the number of runaway electrons. Presently it has been used for runaway studies at the GOLEM and the COMPASS tokamaks. The set of scintillation detectors used at both tokamaks was significantly extended and improved. Besides NaI(Tl) (2 × 2 inch) scintillation detectors, YAP(Ce) and CeBr3 were employed. The data acquisition system was accordingly improved and the data from scintillation detectors is collected with appropriate sampling rate (≈300 MHz) and sufficient bandwidth (≈100 MHz) to allow a pulse analysis. Up to five detectors can currently simultaneously monitor hard X-ray radiation at the GOLEM. The same scintillation detectors were also installed during the runaway electron campaign at the COMPASS tokamak. The aim of this contribution is to report progress in diagnostics of HXR radiation induced by runaway electrons at the GOLEM and the COMPASS tokamaks. The data collected during the 12th runaway electron campaign (2020) at COMPASS shows that count rates during typical low-density runaway electron discharges are in a range of hundreds of kHz and detected photon energies go up to 10 MeV (measured outside the tokamak hall). Acquired data from experimental campaigns from both machines will be discussed.

Measurement of absolute light yield and quantum efficiency of LaBr3:Ce scintillator detector using SiPM

This brief communication presents our work to determine the absolute light yield and quantum efficiency of LaBr3:Ce scintillator by comparison and also by direct pulse measurement method using SiPM. The first part presents use of the simpler comparison method to determine the light yields of different scintillators using the known yield of NaI(Tl) as reference. In the second part we have determined the absolute light yield and quantum efficiency of LaBr3:Ce crystal by using a SiPM photo detector. Our measured value is in good agreement with the light yield reported by previous measurements using PMT and excitation fluorescence spectroscopy. Quantum efficiencies for scintillation detectors have been determined by using both PMTs and photo detectors, namely APDs by previous authors. This communication is possibly the first report on the determination of quantum efficiency of LaBr3:Ce using SiPM photo detector. The simple and effective method presented here would allow to determine the light yield of any scintillation detector.

INVESTIGATING THE CHARACTERISTICS OF LARGE-VOLUME PVT SCINTILLATION DETECTORS IN THE RADIATION PORTAL MONITORS USING MONTE CARLO SIMULATIONS

This paper presents the characteristics determination process of the large-size Polyvinyl Toluene (PVT) scintillation detectors using MCNP5 simulation code. The energy spectra using a 137Cs calibration source, absolute efficiency in the energy range of 50 ÷ 3000 keV, and the angular response of the EJ-200 50×50×5 cm3 and 25×25×5cm3 are investigated. The simulated energy spectra are in good agreement with the experimental spectra. The results of determining the absolute efficiency show that the EJ-200 50×50×5 cm3 and 25×25×5cm3 plastic detectors have detection efficiencies of 16,3% and 9,2%, respectively, at 10cm source-to-detector distance, and down to 0,6% and 0,17% at 100 cm source-to-detector distance. The angular responses of the detectors show that the detection efficiency value reached ≥ 90% of the maximum value with the incident angle less than 5π/6. The results can be applied in the process of design optimization of plastic-based radiation portal monitors.

Calculation of the Effective Energy Release Centerʼs Position of Inorganic Scintillation Detectors for Calibration at Small “Source–Detector” Distances

Inorganic scintillation detectors are widely used to measure of dose rate in the environment due to their high sensitivity to photon radiation. A distinctive feature when using such detectors is the need to take into account of the position of the effective energy release center. This peculiarity is actual when using measuring instruments with inorganic scintillation detectors as working standards during calibration at short “source–detector” distances in conditions of low-background shield or using a facility with protection from external gamma radiation background in the dose rate range from 0.03 to 0.3 μSv/h (μGy/h). The purpose of this work was to calculate the position of the effective energy release center of NaI(Tl) scintillation detectors and to take it into account when working at short “source–detector” distances.An original method of determining the position of the effective energy release center when irradiating the side and end surfaces of inorganic scintillation detector with parallel gamma radiation flux and point gamma radiation sources at small “source–detector” distances using Monte Carlo methods is proposed. The results of calculations of the position of the effective energy release center of NaI(Tl) based detectors of “popular” sizes for the cases of parallel gamma radiation flux and point sources of gamma radiation at small “source–detector” distances are presented. The functional dependences of the position of the effective energy release center of NaI(Tl) based detectors on the distance to the point gamma radiation sources and the energy of gamma radiation sources are presented.As a result of the study it was found that for scintillation NaI(Tl) detectors of medium size (for example, Ø25×40 mm or Ø40×40 mm) the point gamma radiation source located at a distance of 1 m or more, creates a radiation field which does not differ in characteristics from the radiation field created by a parallel flux of gamma radiation. It is shown that approaching the point gamma radiation source to the surface of scintillation detector leads to displacement of the position of the effective energy release center to the surface of the detector.