Session Item

New devices for real time monitoring of beam delivery are needed to study the parameters triggering the FLASH effect in Ultra High Dose-Rate (UHDR) irradiations. The overall goal is to overcome the limits of ionization chambers with new devices featuring high temporal and spatial resolution, beam transparency, large response dynamic range, radiation hardness. Within the INFN FRIDA project, the University and INFN Torino are studying planar thin silicon sensors for electron beam monitoring in high dose-rate regimes.

Silicon sensors of 45 µm active thickness, segmented in strips of 4 x 0.5 mm² each and inversely polarized at 200V, have been readout by both an oscilloscope (10 GS/s sampling rate) and a TERA08 chip. Originally developed to readout gas monitors in charged particle therapy, TERA08 is a 64 channels current to frequency converter with a maximum output frequency of 20 MHz/channel and a charge quantum of 200 fC in the present configuration. First tests of silicon sensor’s response linearity vs dose-per-pulse have been performed at the ElectronFlash accelerator (EF) of the Centro Pisano Multidisciplinare sulla Ricerca e Implementazione Clinica della Flash Radiotherapy (CPFR) on 9 MeV electron beams. Measurements were performed using a 4 cm diameter PMMA applicator, 13 mm solid water slab in between the applicator and the silicon sensor and few cm air separating the solid water slab from the silicon sensor surface.

Preliminary tests were performed for different dose-per-pulse EF settings, verified with a PTW FLASH diamond detector. The integrated charge of one silicon sensor strip readout with the oscilloscope showed a linear response (R value > 0.99) up to the maximum reachable dose-per-pulse (4 Gy per 4 µs pulse duration), as shown in Figure 1. The present configuration of TERA08 allows a maximum sustainable instantaneous current of 256 µA, when the strip signal is split among all the 64 channels. Therefore, an RC circuit of  tau=2 ms (R=2kOhm and C=1µF) was connected in between the strip output and the TERA08 input to extend the signal duration. The linearity of the integrated charge over 10 pulses (at 1 Hz frequency) in respect to the dose-per-pulse was also verified. Studies of the electric field variation inside the silicon sensor as a function of the dose-per-pulse EF settings and sensor bias voltage are ongoing and will be reported. 

The first experimental tests demonstrated the potential of silicon sensors with reduced active thickness to measure the flux of electron beams in FLASH dose-per-pulse regimes up to few Gy/pulse. Tests of different sensor geometries, different active thicknesses, with increasing dose-per-pulse (i.e. using EF applicators smaller than 4 cm diameter) and improving the readout electronics are envisaged for studying beam shape and uniformity measurements capabilities of silicon sensors as quality assurance (QA) and/or real time monitors in FLASH electron beams.