OPTICAL IMAGING OF CHARGED-PARTICLE TRACKS IN A GAS

The development of a new detector designed to optically image the trac k of a charged particle in a gas is outlined. In the detector, a pulse d high voltage (up to almost-equal-to 30-40 kV), high frequency (f = 2 7.126 MHz) RF field is temporarily applied (pulse duration almost-equa l-to 1-3 mus) across a pair of electrodes, immediately following, or a lternatively, just prior to the passage of a charged particle through the chamber. The pulsed RF field excites the subexcitation electrons l eft along the particle's path leading to excitation and ionisation of the surrounding gas and the emission of light. The track is then image d by a fast intensified digital camera (shutter speed almost-equal-to 0.1-5 mus). The image is recorded in a two-dimensional pixel array (51 2 x 512 pixels) within the camera, and. transferred to a computer for later analysis. The detector has been operated over the total gas pres sure range 2.5-100 kPa (20-750 Torr) using a gas mixture of 2-10% N2 i n Ar. Images of both alpha and beta tracks obtained with this detector are discussed to demonstrate the usefulness of the present technique in charged particle track analysis for dosimetry and microdosimetry ap plications.