Microdosimetric Studies of Non-Ionising* Radiation Using Cylindrical Walled and Wall-less Proportional Counters

A wall-less proportional counter of unit elongation, based on Wilson (1970) design, has been constructed. The boundary of the sensitive volume has been delineated using a field plotting technique and a beam of collimated alpha particles from Am-241 source. The effect on the sensitive volume due to the potentials applied to the beam collimator and the container has been studied. The characteristics of the wall-less counter have then been studied and compared with those of the walled proportional counter of the same dimensions; these characteristics have also been compared with those expected from 4 theory of the conventional proportional counter. The transit time measurement of single electrons produced from the cathode by ultraviolet light has been carried out for the special counter design and gas mixture used in this project as a function of E/p.
Indirectly ionising particles have been used for the microdosimetric studies - Am - 241, Cs - 137 and Ra - 226 as gamma radiation sources and Cf-252 as a source of mixed field of radiation (neutron + gamma). The energy deposition spectra have been studied for each source in both the walled and the wall-less counter. The average site diameter of about 0.5 um to 2 um tissue equivalent have been simulated.
From the pulse height distribution of low LET radiation, the frequency probability density, yf(y), energy probability density, y-£(y) distributions and their averages yF and yD, respectively, have been calculated. The wall-effect, obtained from a comparison of YF for the walled and wall-less counters, has been estimated. This walleffect has been found to lie between about 84% to about 102% except in one case when it was about 108%. A method of extrapolation for the event frequency distribution, N(y) Vs.y, to 30 eV has been suggested and used. Some of the results have been compared with those obtained by using the method proposed by Booz (1976).
In the case of Cf-252 source, the gamma-ray contribution has been separated from that of neutrons by making use of the method proposed by Maier et al (1975). Comparison of YF for the walled and wall-less counters is inconclusive because of the large effect of the gamma-ray contribution on yF, but a comparison of yf(y) Vs y spectra shows no significant difference between the walled and wall-less counters in the lineal energy range of about 10 keV/μ to about 180 keV/μ.