A technique for the absolute measurement of the W-value for X-rays in counting gases

Abstract

A technique was developed for the absolute measurement of the W-value (the mean energy for the production of an electron-ion pair) for low-energy X-rays in a wide range of gases at atmospheric pressures, with a standard uncertainty better than 1%. This technique is based on the absolute measurement of the primary ionization charge produced by X-ray photons from a constant intensity monoenergetic X-ray source, e.g. a long lifetime radioactive source. The ionization charge is calibrated by the number of X-ray photons absorbed in the gas, counted with a photon detector. For this purpose, a hybrid detector system was tested and its use in W-value measurements was investigated. The technique was applied to pure xenon at 825 Torr with 5.9 keV X-rays and a W-value of 21.61> \ eV was obtained for a 68% con"dence level. The required corrections and the di!erent factors contributing to the accuracy of the results are discussed. The advantages and limitations of this technique are explored and future developments are discussed.

Experimental Study of the $w$-Values and Fano Factors of Gaseous Xenon and Ar-Xe Mixtures for X-Rays

“…The first one [10], [11] was used to determine their -values while the Fano factors were obtained with the second one.…”

“…For the experimental measurement of the -values we used a method previously developed by our group [11] based on a hybrid detector capable of working as an ionization chamber, a proportional counter or a gas proportional scintillation counter (see Fig. 1 of [11]).…”

“…The intensity of the beam was measured by counting the number of X-ray photons absorbed in the gas with a single channel analyser and an electronic counter. Since the counting rates were fairly high (about ) dead time corrections had to be made with a dead time paralysable model [11], [17]. Corrections for losses due to the escape of Ar L-and Xe M-fluorescence X-rays were also made since X-ray spectra were obtained during this second step using a multichannel analyser.…”

“…1 of [11]), eventually leading to charge collected in the first step, but not being counted as absorbed events in the second step.…”

“…In Section II, we present the results for the measured -value of Ar-Xe mixtures using an experimental technique developed before [10], [11]. A preliminary partial version of this work was presented to the 1999 IEEE NSS [12], following previous experiments [13]- [16].…”