Sensitive High-Temporal-Resolution Tof Electron Drift Tube; Asymmetrical Current Pulse Observation and Determination of V_d, D_Land D₃

Abstract

A unique and sensitive time-of-flight (TOF) drift tube has enabled the first observation of an asymmetric arrival-time-spectra (ATS) of the electron current pulse. From this skewness - a feature predicted by theory but never experimentally observed - the higher-order diffusion coefficient related to the skewness of the pulse is determined. The drift velocity (V_d), longitudinal diffusion (D_L) and skewness diffusion (D_S) are reported for the noble gases as well as for the gases hydrogen azide (HN₃) and hydrogen sulfide (H₂S). All gases tested exhibit a skewness of the ATS in contrast to the symmetric Gaussian ATS obtained by a more traditional TOF apparatus. The asymmetric ATS is well represented for small skewness by extending the analysis to the second-order diffusion approximation of the density gradient expansion (DGE) theory. In the case where a large amount of skewness is observed, such as that seen in the heavier noble gases (Ar, Kr, Xe), the second-order diffusion approximation analysis of the experimental pulse diverges, indicating that the next higher-order of the DGE may be necessary. The highly sensitive current detection and temporal response of this drift tube has been made possible by combining those components traditionally found in TOF drift tubes - a pulsed back-illuminated photocathode current source, guard rings and an anode - with a very sensitive, fast-current amplifier to resolve the electron current pulse. This virtual ground, current-to-voltage amplifier has a gain of 4.4 x 10⁷V/A (23pA/mV) with a 3 dB bandwidth of about 3 MHz. Also, preliminary results for the inference of HN₃cross-sections are to be presented.

Supported by the Air Force Office of Scientific Research, Boiling AFB, D.C.