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

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-ra ys in a wide range of gases at atmospheric pressures, with a standard uncer tainty 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 sour ce. The ionization charge is calibrated by the number of X-ray photons abso rbed 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 investi gated. The technique was applied to pure xenon at 825 Torr with 5.9 keV X-r ays and a W-value of 21.61(-0.10)(+0.14) eV was obtained for a 68% confiden ce level. The required corrections and the different factors contributing t o the accuracy of the results are discussed. The advantages and limitations of this technique are explored and future developments are discussed. (C) 2000 Elsevier Science B.V. All rights reserved.