63Ni β range and backscattering in confined geometries

To know the spatial distribution of ion pairs resulting from 63Ni β radiation in the gas phase is important for a variety of theoretical and practical reasons, in particular those concerning the electron capture detector. Literature estimates of this distribution vary by about one order of magnitude, yet this parameter is necessary for the modelling of this detector. The 63Ni-induced, initial ion pair distribution was therefore measured in a variety of gases with two techniques: a conventional one based on the electrical saturation current at variable interelectrode distances, and an unconventional one based on luminescence from a plastic scintillator. The data are analyzed in terms of two ranges, d50 and d95, that describe the distances from a planar radioactive foil within which 50% and 95% of the total gas-phase ionization occur. The data from the electrical measurement show unexpected evidence of strong β backscattering and secondary electron emission from the counter-electrode. Under these (non-exponential) conditions, d50 values in the common detector gases nitrogen and argon/methane vary from 0.5 to 1.0 mm, depending on the nature of the counter-electrode. Calculations based on the quasi-exponential range found at longer distances in electrical measurements yield values of about 2.5 mm (which are low because of geometric measurement bias). In contrast, the data from the luminescence measurement are almost completely exponential and d50 values for argon (+5% methane) and nitrogen are 2.8 and 3.8 mm, respectively. The d95 values vary from 12 to 16 mm for the luminescence, to 6 and 9 mm for the (less reliable) electrical measurement; all at ambient conditions. The luminescence data are considered closer to the "true" (unimpeded) charge distribution, while the ionization data may be closer to the initial charge topography inside an electron capture detector of confining geometry. All range data, however, are short enough to advise modelling the detector as a system with strongly heterogeneous charge distribution. No evidence was found for some of the very large range estimates found in the literature.