Nuclear Instruments and Methods in Physics Research A 579 (2007) 256–259 Development of a pulse processing system for real-time analysis of microdosimetric spectra S.H. Byun à , K. Chin, W.V. Prestwich, Z. Liu, S. Saxena Department of Medical Physics and Applied Radiation Sciences, McMaster University, Hamilton, ON, Canada L8S 4K1 Available online 6 April 2007 Abstract A new pulse processing system was developed for real-time analysis of microdosimetric spectra. Microdosimetric spectra from a tissue- equivalent proportional counter are usually extracted through complicated analysis steps after measuring event spectra with conventional pulse-height analyzers. The new pulse processing system consisted of a logarithmic and a linear peak-sensing ADCs and accumulates a dose spectrum directly without any post analysis. The logarithmic ADC was made by modifying a Wilkinson ADC and showed a satisfactory operation in the logarithmic scale. The new system was tested at the mixed neutron-gamma field of the McMaster Accelerator Laboratory. A conventional system was operated in parallel to estimate the reliability of the new system. The incident proton energy was varied from 1.8 to 2.3 MeV, which produced different neutron-gamma ratios through 7 Li(p,n) and 7 Li(p,p 0 g) reactions. The counting rate during measurements changed from 560 to 1400 cps as the incident proton energy was increased by fixing the current at 45 mA. Microdosimetric spectra from the new system were consistent with those from the conventional system for various fields except for a little deviation induced by the nonlinearity of the logarithmic ADC. Both neutron and gamma-ray doses from two systems showed good agreement within 2%. Estimations regarding the dead time and tolerable counting rate will be continued. r 2007 Elsevier B.V. All rights reserved. PACS: 07.05.Hd; 29.40.Cs; 87.53.Rd Keywords: Microdosimetry; Tissue-equivalent proportional counter; Pulse processing 1. Introduction Experimental microdosimetry aims for measuring a dosimetric spectrum at a microscopic volume of tissue [1]. To date the tissue-equivalent proportional counter (TEPC) has usually been employed due to its excellent adaptabil- ities for two fundamental requirements in microdosimetry, tissue equivalence and capability of simulating a micro- scopic tissue size. Since a TEPC detector can separate different linear energy transfer (LET) components in a mixed radiation field, it has been widely adopted in medical applications [2,3] and radiation protection [4]. Recently, many trials have been carried out with different approaches in order to expand the operational limits of standard TEPC detectors toward either extremely high dose rate environments or much smaller simulation site sizes [5,6]. However, the pulse processing architecture of TEPC has remained relatively unchanged as compared to various efforts on detector improvements. In conven- tional pulse processing systems, the signal from a TEPC detector is branched and fed into two or three set of pulse processing electronics to cover 4–5 decades of lineal energy range [1,7]. Each set of electronics is composed of a spectroscopy amplifier and a peak-sensing ADC as in general radiation spectrometry. To distinguish different LET components, a microdosimetric spectrum must be plotted in a logarithmic scale, which requires a redistribu- tion step of the measured event spectrum into an equal logarithmic interval. Moreover, the event frequency must be multiplied with the corresponding lineal energy to convert to a dose spectrum. The standard procedure is quite inconvenient and complicates real-time analysis of a microdosimetric spectrum. Some trials to overcome the shortcomings in analysis were reported in Refs. [8,9] using a nonlinear amplifier or a ARTICLE IN PRESS www.elsevier.com/locate/nima 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.04.050 à Corresponding author. Tel.: +1 905 525 9140; fax: +1 905 522 5982. E-mail address: soohyun@mcmaster.ca (S.H. Byun).