Electronic Discrimination of the Effective Thickness of Proportional Counters

We developed and tested a new method of electronic discrimination of the effective thickness of proportional counters and applied this method in a proportional counter camera to adjust its effective aperture, i.e., the combination of counter efficiency and spatial resolution without changing the pinhole aperture, the collimator, or the distance between the subject and camera. The discriminator virtually divides the thickness of the electron drift volume of the camera into two regions and separates the photons detected in each region. Thus, two sets of data are acquired and displayed simultaneously during an exposure: one with good spatial resolution (< 2 mm fwhm) using photons detected in the drift volume close to the entrance window, and one with high detection efficiency (> 50% for 60-keV photons) but poorer resolution (~5 mm fwhm) using all detected photons. Simultaneous acquisition of two sets of data with different apertures reduces the exposure time in applications, such as nuclear medicine, where a sequence of images often is required to select the optimum aperture for different portions of radioisotope distribution images.