Studies and Speculations on an Alternative Mechanism for the Response of Direct-current Electron-capture Detectors

The classical theory of electron-capture detection does not allow for hypercoulometric response such as has been found recently under conditions of elevated pressure in some electron-capture detectors (ECDs), nor does it predict certain other experimental observations. In an effort to understand why and how hypercoulometric performance can occur, measurements have been made of the ionization region and characteristic current-voltage relationships in real and simulated ECDs. The centre-of-ionization was found much closer to the foil than is commonly assumed, namely at approximately 1 mm for 63Ni and approximately 0.2 mm for 3H in nitrogen at ambient conditions. A hypothetical response mechanism, alternative and, in certain ways, complementary to the classical theory was therefore developed. It sees detector response, distinct from the initial electron-capture reaction, as the effect of a space charge formed by migrating negative ions, which decrease the field gradient (and therefore increase electron-positive ion recombination) in the ionization region. Estimates of the counterfield established by the migration of anions show that it can indeed produce hypercoulometric response and that, in agreement with experimental observations, increased pressure leads to higher response. Some simple experimental tests of the proposed mechanism have been carried out and others, more crucial ones, are suggested for future investigation. © 1980