Pileup effects due to the overlap of signals within the system deadtime (tau) influence the counting capability of radiation detection devices, necessitating the use of correction algorithms to compensate for the countlosses at high radiation rates. Countrate linearity is especially critical for clinical applications like Xray imaging or beam monitoring in particle therapy. In particular, in proton therapy the number of delivered particles must be measured online during the treatment session with a maximum error of 1 % up to an average input beam flux of about 10^10 cm2s1. For a segmented detector used to identify and count the single beam particles, assuming a channel area of 1 mm2 and a deadtime tau = 2 ns, a maximum counting inefficiency of 1 % is required up to tau*f = 0.2 for each detector channel, where f represents the input rate. Moreover, the beam is often delivered in bunches with higher instantaneous particle rates, and the saturation model of the detector and electronic chain could not be easily determined. Similar considerations are applicable for pixelated detectors used for photon counting. Two methods are proposed to mitigate counting inefficiencies with radiation sources of variable timestructures. Both methods are based on the collection of logic signals provided by two independent detector channels exposed to the same radiation field after discriminating the detector analog outputs with a fixed threshold, assuming that the duration of the discriminator output signal corresponds to the system deadtime. The correction algorithms employ the measurements of the time durations, the number of signals from the two channels and of their AND/OR combinations. The methods provide countloss corrections without the need to know the deadtime model. The performances of the proposed algorithms are evaluated by using simulations of ideal boxcar signals of fixed duration.., distributed randomly in time to emulate the deadtime behavior of the system. Both methods provide an effective countloss correction with a maximum deviation of 1% for different input rates up to tau*f = 1, assuming a uniform random time distribution of input events for both paralyzable and nonparalyzable systems. The simulations of pulsed radiation fluxes provide the same results as a function of the instantaneous input rates. These results are similar to those obtainable by the standard livetime correction algorithm. However, the latter algorithm can only be applied to continuous particle fluxes, while the proposed algorithms work also for pulsed beams, without any hypothesis on the bunch duration or frequency. The robustness of the algorithms with respect to the resolution of the time measurement is studied and the potential limitations in more realistic systems are discussed. The algorithms can be easily implemented in standard logical circuits with multiple input signals provided by segmented detectors. Even if the methods are intended for realtime correction in beam particle counting, they could be applied in a wider range of applications of radiation measurements.
Twochannel combination methods for countloss correction in radiation measurements at high rates and with pulsed sources
Monaco, V^{ Cofirst};Abujami, M;Bersani, D;Data, EM;Galeone, C;MontalvanOlivares, DM;Richetta, E;Staiano, A;Vignati, A;Cirio, R^{Colast};Sacchi, R^{Colast}
20220101
Abstract
Pileup effects due to the overlap of signals within the system deadtime (tau) influence the counting capability of radiation detection devices, necessitating the use of correction algorithms to compensate for the countlosses at high radiation rates. Countrate linearity is especially critical for clinical applications like Xray imaging or beam monitoring in particle therapy. In particular, in proton therapy the number of delivered particles must be measured online during the treatment session with a maximum error of 1 % up to an average input beam flux of about 10^10 cm2s1. For a segmented detector used to identify and count the single beam particles, assuming a channel area of 1 mm2 and a deadtime tau = 2 ns, a maximum counting inefficiency of 1 % is required up to tau*f = 0.2 for each detector channel, where f represents the input rate. Moreover, the beam is often delivered in bunches with higher instantaneous particle rates, and the saturation model of the detector and electronic chain could not be easily determined. Similar considerations are applicable for pixelated detectors used for photon counting. Two methods are proposed to mitigate counting inefficiencies with radiation sources of variable timestructures. Both methods are based on the collection of logic signals provided by two independent detector channels exposed to the same radiation field after discriminating the detector analog outputs with a fixed threshold, assuming that the duration of the discriminator output signal corresponds to the system deadtime. The correction algorithms employ the measurements of the time durations, the number of signals from the two channels and of their AND/OR combinations. The methods provide countloss corrections without the need to know the deadtime model. The performances of the proposed algorithms are evaluated by using simulations of ideal boxcar signals of fixed duration.., distributed randomly in time to emulate the deadtime behavior of the system. Both methods provide an effective countloss correction with a maximum deviation of 1% for different input rates up to tau*f = 1, assuming a uniform random time distribution of input events for both paralyzable and nonparalyzable systems. The simulations of pulsed radiation fluxes provide the same results as a function of the instantaneous input rates. These results are similar to those obtainable by the standard livetime correction algorithm. However, the latter algorithm can only be applied to continuous particle fluxes, while the proposed algorithms work also for pulsed beams, without any hypothesis on the bunch duration or frequency. The robustness of the algorithms with respect to the resolution of the time measurement is studied and the potential limitations in more realistic systems are discussed. The algorithms can be easily implemented in standard logical circuits with multiple input signals provided by segmented detectors. Even if the methods are intended for realtime correction in beam particle counting, they could be applied in a wider range of applications of radiation measurements.File  Dimensione  Formato  

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