Delta electron production and the ultimate energy resolution of liquid argon ionization detectors

An intrinsic energy resolution of 26+or-2 keV has been measured for the 976-keV conversion electrons of a /sup 207/Bi source in a gridded ionization chamber filled with pure liquid argon, at a drift field of 11 kV-cm/sup -1/. The measurement of the /sup 207/Bi spectrum at different electric fields was repeated several times, with very-good liquid purity and optimized chamber geometry, reproducing the results. Data were also taken with a xenon-doped liquid argon filling in an attempt to improve the resolution. Enhanced ionization was observed as expected from the presence of excitons in liquid argon, but there was no improvement in the energy resolution. It is shown that the data are consistent with the assumption that recombination straggling from low-energy delta electrons, produced abundantly along the path of the primary ionizing particle, is mostly responsible for the degraded experimental resolution.<>