1. Factors Limiting the Linearity of Response of Tissue Equivalent Proportional Counters Used in Micro- and Nano – Dosimetry
T.Z. Kowalski i
AGH, University of Science and Technology, Fac. of Physics and Applied Computer Science, Krakow, Poland
Gas gain as function of applied high voltage (left) for methane based TEG mixture was measured. The values of the gas amplification factors were determined using the current method as the ratio of the current value at given high voltage to the current value in the ionization chamber mode. Gas gain was determined for varying intensity of X-rays, the current was always below 2 nA, (left) and for constant high radiation soure intensity (right). In the region below the arrow no changes in gas gain with count rate are observed. Above the arrow there is already an influence of space charge of positive ions in the avalanche multiplication region, which reduces the gas gain.
Values of currents over which the 5% reduction in gas gain is observed as the function of mixture pressures are shown. The current, I, flowing through the counter can be expressed by: I = ( ΔE/W) ×A×R×e, ΔE- energy deposited by registered particle, W – energy required to produce ion-electron pair, R- count rate and e – elementary charge. So ΔE×R is the energy deposited by radiation per second so it is the radiation dose rate, P, [P = (I×W)/(A×e)] and
is the maximal measurable radiation dose rate. Radius of electron avalanches as function of mixture for fixed, indicated on right figure, values of gas gains determined from the equation of Diethorn. Higher radius of avalanche, higher critical current.
The ratio of the peak position of Cd-109 source to the peak position of cathode X-ray fluorescence (CuKα – line) as a function of gas gain. For high gas gain rapide decrease in the ratio due to space charge effect is observed. The slope of the gas gain curves from graphic differentation and from Dierthorn equation. The big difference in the slope for high gas gain for high mixture pressure is caused by self-induced space charge effect for smaller pressure both by self-induced space charge effect and by photon feedback.
REMARKS
Maximal values of the gas gains, not affected neither by self-induced space charge effect no by secondary effects as the function of mixture pressures.
Work supported partially by the Polish National Science Centre, grant no. DEC-2013/10/M/ST7/00568