Pileup correction of microdosimetric spectra

Microdosimetric spectra were measured at the Fermilab neutron therapy facility using low pressure proportional counters operated in pulse mode. The neutron beam has a very low duty cycle (<0.1%) and consequently a high instantaneous dose rate which causes distortions of the microdosimetric spectra due to pulse pileup. The determination of undistorted spectra at this facility necessitated (i) the modified operation of the proton accelerator to reduce the instantaneous dose rate and (ii) the establishment of a computational procedure to correct the measured spectra for remaining pileup distortions. In support of the latter effort, two different pileup simulation algorithms using analytical and Monte-Carlo-based approaches were developed. While the analytical algorithm allows a detailed analysis of pileup processes it only treats two-pulse and three-pulse pileup and its validity is hence restricted. A Monte-Carlo-based pileup algorithm was developed that inherently treats all degrees of pileup. This algorithm can be used in an iterative manner to correct pileup distorted spectra. The pileup simulation and correction codes compared well with experimental data. A set of pileup distorted spectra that were collected under different setup configurations at the Fermilab facility were corrected for pileup. Analysis of the spectral variations between the spectra is consistent with findings reported in the literature.