Comparison of LaBr3 and LaCl3 inorganic scintillators as HXR monitors for SPARC*

Tokamaks emit hard X-rays (HXR) when electrons are accelerated to relativistic energies, entering the so-called runaway regime, and undergo bremsstrahlung interaction with plasma ions and plasma facing components. This can happen at any time during a plasma discharge, but poses more concerns during plasma start-up and disruptions. SPARC¹ will be equipped with a HXR monitor system that, in case a start-up runway beam is detected, will send an alarm to the plasma control system to shut down the discharge and protect the structural integrity of the machine. The preliminary design of the HXR monitor² considers a LaBr3 inorganic scintillator operated in current mode. A new scintillator material, LaCl₃, is currently emerging as HXR detector. In this poster, we compare the performance of this material with a LaBr₃ crystal. Using semi-analytical cross sections and the MCNP code³, we present an estimate of the HXR and neutron signal levels at the diagnostic’s position. We then compare LaBr3 and LaCl3 in terms of energy resolution, total efficiency and dynamic range. Finally, we study the activation of the two materials due to DT neutrons using FISPACT⁴, from which we will estimate their neutron attenuation needs and discuss the possible application of the diagnostics for post-disruption REs monitoring.

¹A. J. Creely et al., Journal of Plasma Physics 86.5 (2020)

²E. Panontin et al. proceedings of HTPD2024 (submitted)

³C. J. Werner et al.  (2018) 10.2172/1419730

⁴J-Ch. Sublet et al., Nuclear Data Sheets 139 (2017) 77-137