Time Response of a Gridded X-ray Beam Ionization Chamber*

Recently, Quick-Scanning Extended X-ray Absorption Fine Structure Spectroscopy (QEXAFS) has become an important tool for in-situ characterization of materials and measurement of associated electronic structure for where samples have a short life due to X-ray damage. In this case, the time response of the ionization chamber affects the measurement resolution and therefore overall performance of the QEXAFS system. Common parallel-plate ionization chambers have a step-response rise time of about 0.1 sec, which does not meet the requirements of QEXAFS. To speed up the response, we constructed a gridded ionization chamber with variable bias voltage and optional background gas (N2 or He, respectively). To characterize the system we used a high-frequency beam chopper upstream of the ionization chamber and a highspeed, low-noise preamplifier to measure the step response of the chamber as a function of bias voltage and background gas conditions.