Use of a low ionization potential dopant in a kinestatic charge detector: experimental findings

The broadening of the line spread function (LSF) in the drift direction with increasing drift distance in the kinestatic charge detector is substantially reduced when small amounts (less than 1%) of trimethylamine [(CH3)3 N] are added to the x-ray detection medium (krypton or xenon). The LSF of a mixture of Kr and 0.01% trimethylamine (TMA) was measured as a function of distance at 15, 25, and 35 atm absolute pressure. The full width at half-maximum (FWHM) of the LSF was reduced from about 1.0 mm to less than 0.5 mm at a drift distance of 4.0 mm for the three pressures. The LSF's of mixtures of xenon and TMA at concentrations ranging from 0.0004% to 0.4% in one run and 0.06% to 4.0% in a second run were measured at a constant pressure of 20 atm. The FWHM of the LSF was reduced from 0.6 to 0.4 mm at 4.0 mm for the xenon measurements. The optimum concentration of TMA in Xe was found to be in the neighborhood of 0.1%. The use of TMA reduced the drift distance-dependent LSF broadening to the level expected from ionic diffusion, space charge repulsion, and electric field nonuniformity, and it may be possible to reduce the 0.4-mm FWHM plateau through the use of an improved Frisch grid design. Observation of negative charge carriers showed that electron attachment increases with increasing TMA concentration, although this could be caused by impurities in the TMA. The implications of these results are discussed in terms of extending the maximum drift distance attainable in a kinestatic charge detector.