A number of instruments have window and threshold controls that allow single isotope analyzing when used with a scintillation or proportional detector. The window control has no function with a GM detector, since the amplitude of the output pulse from a GM detector does not vary with energy.
The scaler-ratemeters have ten turn potentiometers for the threshold and window controls. This allows for the threshold dial setting, plus one-half of the window width, to be equivalent to the peak energy of the source.
The window control is calibrated such that one turn on the control is equal to one turn on the threshold control. That is, if the threshold control is calibrated to 100 keV per turn, the window control is equal to 100 keV/turn. Example: 137Cs energy is 662 keV. If the window setting is at 1.00, the threshold dial would be set at 6.12 (6.62 minus 0.50 window).
To find the peak energy of an isotope, the operating characteristics of the detector must be determined. A plateau of 50 volt increments should be taken with the window out and plotted for both background and a reference source, such as 137Cs to calibrate at 100 keV/turn, or 241Am to calibrate at 10 keV/turn. The peak energy of the source is located where the count from the source "just starts" to appear above background. Note the high voltage in this area. This will be the region of the operating point for the reference source. Place the window in-out switch to in, and set the window and threshold controls. Reduce the high voltage below the point where the source count is not seen above background. Expose the source to the detector. Slowly increase the high voltage. The source count should increase, then decrease with increasing high voltage. Reduce and refine the high voltage until the source count peaks. The source count with the window in should be 1⁄10 to 1⁄2 of the plateau count with the window out.
If in doubt, plot count vs. high voltage with the window in. The count should show a peak below the knee of the plateau, taken with the window out.
Other window widths may be selected. A window of 50 keV would result in a window dial setting of 0.50 and a threshold dial of 6.37 for 137Cs. Once the instrument is peaked, lock the high voltage control. Changing the threshold control for another energy may require tweaking the high voltage to re-peak the instrument.
To calibrate the threshold for 10 keV/turn, repeat the above procedure using an 241Am source (60 keV). Selecting a window width of 2.00, the threshold dial would be set at 5.00, which would be equivalent to 50 keV. The 60 keV peak then would centered in the window. It follows that the threshold dial for 125I (35 keV) would be set at 2.50 or 25 keV with the window at 2.00.
For instruments without dials, it will be necessary to use a pulser to measure input sensitivity of the threshold and window settings. Typically for a Model 16, set the threshold for 30 mV and the window for 40 mV. Place window to out position, and select appropriate range on meter. If there is no meter indication, increase amplitude on Model 500 so that meter counts/min reads the same as the Model 500 counts/min. Place the window switch to the in position. Increase pulser amplitude. Counts should appear at 30 mV and disappear at 40 mV. With the window out, run a plateau for background and reference source count. Remember that the high voltage, when peaked, will be below the plateau voltage. Now place the window in and peak the source by adjusting high voltage. (Note: the range selector may have to be positioned to the next lower range.) Energy relations can now be calibrated in MeV/mV using the pulser as a substitute for dials.
To calculate system resolution, use a narrow window (50 keV). Record source count with window centered at 662 keV (137Cs). Decrease the threshold setting until the count rate declines by 1⁄2. Record the threshold keV at this point. Now increase the threshold setting above the peak until the source count again declines by 1⁄2 from the peak value. Record the threshold keV at this point. Resolution may be calculated as:
E0.5H is the energy level where the count rate declines by one half from the peak count on the high side. E0.5L is the energy level below the peak when the count rate drops to one half of the peak level. EC is the energy level for the peak reading. This is calculated by threshold energy plus 1⁄2 of window width.