Page 56 - 00. Complete Version - Progress Report IPEN 2014-2016

56 Application of Ionizing Radiations | Progress Report

the photocathode is proportional to the num- is also needed for correctly comparing with
ber of light photons from the crystal and the microdosimetric distributions obtained from
electric current generated by the photomul- Monte Carlo simulations of charged particles
tiplier tube. This way, the height of the elec- tracks. Since the interactions of electrons with
trical pulse from the photomultiplier tube matter are at the core of any charged particle
is proportional to the radiation energy ab- transport code, the knowledge of electron-col-
sorbed by the crystal. This allows that the en- lision cross sections with their energy depen-
ergy from different radionuclides can be dis- dence in real tissues or in TEG mixtures is im-
tinguished from each other by pulse height. portant. Likewise, experimental values of the
Measurements of pulse height using gam- first Townsend ionization coefficient, a key pa-
ma radiation sources of 54Mn (835 keV) and rameter for modelling the avalanche growth
137Cs (662 keV) are show in Figures 12 and 13. at high electric field strengths, are essential
in the design of proportional counters. Mea-
Gaseous Detectors

The reference detector in experimental mi-
crodosimetry has been the tissue-equiva-
lent proportional counter (TEPC) whose walls
and filling gases have elemental composition
and mass stopping powers similar to that of
human tissues. Tissue-equivalent gas (TEG)
mixtures, that also allow stable operation
of proportional counters with high gas gain,
have been employed in TEPCs operated at
low pressure scaled to simulate the real size
of microscopic tissue sites, as well as to en-
sure the validity of the cavity-chamber prin-
Figure 14. Gas chamber with the RPC
ciple. Meeting these dosimetric requirements like cell and the complete apparatus.

surements of the first Townsend ionization
coefficient have been carried out in a small
planar chamber, with a resistive glass an-
ode and a metallic cathode, using the Pulsed
Townsend technique. With this setup (Fig.14),
reliable sets of data were obtained in pure gas-
es, such as nitrogen, methane, carbon dioxide,
isobutane, butane, all of them under contin-
uous flow at low and atmospheric pressure.
In order to extend these researches towards
to gaseous microdosimeters, measurements
of the first Townsend ionization coefficient
Figure 15. Density-normalized first Townsend ionization co-
efficient, α/N, for the CH4-TEG mixture as a function of E/N in several TEG mixtures were performed for
together with Magboltz results (continuous and dashed the first time (Fig.15). The main focus of in-
lines). For comparison, the measurements in each compo-
nent of the referred TEG mixture are also included. terest was on those composed by hydrocar-

Instituto de Pesquisas Energéticas e Nucleares