Identifying the origins of possible nonlinear scintillator response under ultra-high dose rate electron beams

Purpose/objective

Previous studies of the Exradin-W2 (Standard Imaging, Middleton, WI, USA) organic scintillator under ultra-high dose rate (UHDR) electron beams, have shown signs of saturation at high pulse repetition frequencies (PRF) and high doses per pulse (DPP). This study investigated the hypothesis that the saturation observed in the W2 is due to limitations of the electrometer rather than the scintillator itself.

Material/methods

We conducted a series of measurements to assess the performance of a 1 × 3 mm W2 detector and a 1 × 2 mm BCF-12 scintillator under 9 MeV electron beams. The experiment was carried out using a spectrometer capable of handling a wide range of signal intensities. The area under the curve was computed for each spectrum and used to compare across all measurements. To investigate the behavior of the detector to variations in PRF, we adjusted the PRF values from 10 Hz to 120 Hz. The linearity to integrated dose was validated by varying the number of pulses per acquisition. Additionally, we tested the linearity of the detector's response to DPP by changing the surface-to-source distance up to a DPP of 5.4 Gy. Finally, the W2 was connected to a long clear fiber extension and was used with the MAX SD electrometer under the same beam parameters as the previous measurements.

Results

The signal sensitivity variation for BCF-12 remained under 6 % for all pulse width (PW) values tested across all PRFs, whereas for the W2, the maximum decrease in signal sensitivity (3 %) was for a PW of 2 μs at 60 Hz. The linearity of the detector's response was maintained across all pulse numbers tested. Both detectors' responses remained linear with respect to both integrated dose and DPP. The W2 with the clear fiber extension showed a decrease in signal as the PRF increased.

Conclusion

These findings suggest that the previously observed nonlinear effects are likely attributable to limitations of the electrometer, which was not designed for use with UHDR beams, rather than intrinsic properties of the W2 scintillator.