Performance of LGAD strip detectors for particle counting of therapeutic proton beams

Abstract

Objective: The performance of silicon detectors with moderate internal gain, named Low-Gain Avalanche Diodes (LGADs), was studied to investigate their capability to discriminate and count single beam particles at high fluxes, in view of future applications for beam characterization and on-line beam monitoring in proton therapy.
Approach: Dedicated LGAD detectors with an active thickness of 55 μm and segmented in 2 mm2 strips were characterized at two Italian proton-therapy facilities, CNAO in Pavia and the Proton Therapy Center of Trento, with proton beams provided by a synchrotron and a cyclotron, respectively. Signals from single beam particles were discriminated against a threshold and counted. The number of proton pulses for fixed energies and different particle fluxes was compared with the charge collected by a compact ionization chamber, to infer the input particle rates.
Main results: The counting inefficiency due to the overlap of nearby signals was less than 1 % up to particle rates in one strip of 1 MHz, corresponding to a mean fluence rate on the strip of about 5⸱107 p/(cm2⸱s). Count-loss correction algorithms based on the logic combination of signals from two neighboring strips allow to extend the maximum counting rate by one order of magnitude. The same algorithms give additional information on the fine time structure of the beam. 
Significance. The direct counting of the number of beam protons with segmented silicon detectors allows to overcome some limitations of gas detectors typically employed for beam characterization and beam monitoring in particle therapy, providing faster response times, higher sensitivity, and independence of the counts from the particle energy. 

First experimental time-of-flight-based proton radiography using low gain avalanche diodes

“…Depending on the detector technology, the number of pile-up events in a single channel, the dead area between the individual channels and the number of simultaneously fired channels per particle hit can vary, which directly impacts the rate capability. In clinical environments, especially in a synchrotron-based facility with thin pencil beam scanning, particle fluxes in the order of 10 9 -10 10 p s −1 cm −2 can be expected (Grevillot et al 2019, Monaco et al 2023. In order to cope with such high particle fluxes, the detector geometry has to be carefully chosen to guarantee efficient single-particle tracking.…”

“…In order to cope with such high particle fluxes, the detector geometry has to be carefully chosen to guarantee efficient single-particle tracking. Monaco et al (2023) have studied the rate limitations of an LGAD-based particle counting system consisting of LGAD strip sensors with a strip area of 15 mm × 150 μm and a pitch of 216 μm. While their sensor was not significantly affected by crosstalk between the channels due to charge sharing or capacitive coupling, a particle flux limit of 4 × 10 8 p s −1 cm −2 was identified for the given geometry.…”