MULTILEVEL MODELING OF THE RESPONSE OF THE ULTRAMINIATURE PROPORTIONAL COUNTER - GAS GAIN PHENOMENA AND PULSE-HEIGHT SPECTRA

The ultraminiature proportional counters, UMC, unique radiation detect ors for monitoring high intensity therapy fields, designed by Kliauga and operated at Columbia University (USA), have yielded a number of pu lse height distributions for photons, neutrons and ions at simulated d iameters of 5-50 nm. Monte Carlo calculations of the gas gain in such a counter questioned the possibility of achieving proportionality at s uch low simulated diameters. The response of the UMC has now been mode lled taking into account both fluctuations of energy deposited in the counter volume and its calculated gas gain. Energy deposition was calc ulated using the MOCA-14, MOCA-8 and TRION codes, whereby distribution s of ionisations d(j) after irradiations with Cs-137, 15 MeV neutrons and 7 MeV.amu(-1) deuterons were obtained. Monte Carlo calculations of electron avalanches in UMC show that the size of the single-electron avalanche P(n) reaching the anode wire depends strongly on the locatio n of the primary ionisation within the counter volume. Distributions o f the size of electron avalanches for higher numbers of primary ionisa tions, P-j(n), were obtained by successive convolutions of P(n). Fina lly, the counter response was obtained by weighting P-j(n) over d(j) distributions. On comparing the measured and calculated spectra it was concluded that the previously proposed single-electron peak calibrati on method might not be valid for the UMC due to the excessive width an d overlap of electron avalanche distributions. Better agreement betwee n the measured and calculated spectra is found if broader electron ava lanche distributions than those used in the present calculations, are assumed.