Window versus Windowless Detector Efficiency at Various Sample Depths
The best possible sensitivity is achieved using a windowless system with ultra-low system background. Using the efficiency figures for Sr90 above, a typical beta background of 0.45 cpm, and 1/8 inch sample depth, a PIC IPC-9025 system has a Figure of Merit of: 63*63/0.45 = 8820 (windowless) or 47*47/0.45 = 4500 (w/80 microgram window). In practical terms, this means that the windowless system will count the same sample to the same sensitivity (MDA), in half the time of the system with a window installed.
This efficiency data was collected on a Protean Instrument Corporation IPC-9025 windowed/windowless system. The sources are all calibrated, and in a form to eliminate as many geometry variables as possible. Each source was counted at four depths, “top,” 1/8, 1/4, and 5/16 inches. Top placement is done by using a 1/8 inch planchet inverted in a 1/8 inch insert. The source is then at the minimum distance possible (approximately 1/16 inch). The upper table is for a system with a standard 80 micrograms/cc² window installed. The lower table is for the same system with the window removed. A system with a window shows a great change in efficiency, depending on sample depth. This effect is the same regardless of system type. A windowless system shows a substantial increase in efficiency compared to a windowed system, and very little efficiency change due to sample depth. The efficiencies listed for the system with window installed are the same for all Protean alpha/beta counting systems using a gas flow proportional detector with a detector window.