Development and commissioning of a LaBr3:Ce, Sr-based collimated hard x-ray spectrometer for MAST-U

A new collimated hard x-ray (HXR) spectrometer has been developed and installed on the Mega Amp Spherical Tokamak Upgrade (MAST-U) to support future studies of runaway electron generation and provide a diagnostic capability to plasma-facing components monitoring. The diagnostic is based on an Sr-codoped LaBr3:Ce scintillator (2″ × 2″) coupled to a fast data acquisition system capable of recording raw scintillation waveforms in both pulse and continuous modes, allowing detailed reconstruction of individual events using algorithms for optimization of spectrometric parameters and pulse-shape discrimination. The detector was energy-calibrated up to 4.8 MeV using standard spectroscopy sources and a 66Ga radioisotope. Its performance was then evaluated at the MC40 cyclotron (University of Birmingham), demonstrating stable operation at 1.1 Mcps, with gain shifts of less than 4%. Following successful validation, the diagnostic was installed using an existing midplane line-of-sight configuration of the MAST-U, passing through both plasma-facing components and the central column region. A neutron flux map was simulated to estimate the radiation environment within the diagnostic field of view defined by a 15 cm diameter penetration through 1.5 m of concrete biological shielding acting as a collimator, together with an additional 10 cm polyethylene layer. Following installation, the collimated HXR diagnostic demonstrated reliable operation by recording gamma-ray events and hard x-ray bursts during both ohmic and neutral beam injection heated plasma discharges, confirming the successful commissioning of the diagnostic on MAST-U.