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This part of the HSS program allows the operator to examine the current operating parameters and to change to new operating parameters for the In Vivo Counter as may be needed.
The program is entered by selecting the “Parameters Modification” option from the Main Menu, shown in Figure 10-1, above. To do this:
Regardless of which of these methods is chosen, the result will be the screen shown in Figure 10-2, below.
From this menu you may choose to do the routine “Energy versus Channel” calibrations, or you may choose to do the occasional “Efficiency” calibrations. The “Rotational Speed Calibrations” are used only for the Helgeson Waste Analyzers. The “Linear Speed Calibrations” are used for the Classic Shadow Shield Whole Body Counter, the “Do-It-Yourself Whole Body Counter,” and for the vertical speed calibrations of the High Purity Germanium Barrel Counter.
The “Quicky,” Models I and III In Vivo Counter use four NaI(Tl) detectors. The “BRC” Waste Analyzer uses four NaI(Tl) detectors. The “Quicky,” Model VI In Vivo Counter uses six NaI(Tl) detectors. In each of the different models the output signal from each detector is sent to its own individual amplifier. Each photomultiplier tube ages at a different rate. This means that over the passage of time, the amplification characteristics of the photomultiplier tube change. If the gain of the detector is not adjusted frequently, the apparent resolution of the detector system deteriorates significantly, making isotopic identification impossible. Therefore, it is important to check the gain of each detector at least once daily. The “Do-It-Yourself Whole Body Counter” uses only one NaI(Tl) detector, but it is still a good idea to check the gain of the detector on a daily basis.
A thallium activated sodium iodide crystal, abbreviated “NaI(Tl),” is not precisely linear in its response to various gamma energies versus the channel in which the photopeak falls. Therefore, if a nominal 5 keV/channel gain is used, the 59.537 keV from 241-americium actually falls in channel 14.2693, not channel 59.537/5 = 11.9074. Similarly, the photopeak from the 1,173.2 keV gamma from 60-cobalt falls in channel 230.408, not channel 1173.2/5 = 234.64. The technician's task is to adjust the amplifier gain and the ADC zero so that these conditions are met. Fortunately, the ADC zero is very stable and seldom needs adjustment. The high energy photopeak, however, drifts due to environmental temperature changes. This drift could be as large as several channels in the region of channel 230. Therefore, the technician should check the system gain periodically. Table 10-1 shows the variation of pulse height as a function of the energy at a nominal 10 keV per channel.
Table 10-2 shows the calculations for determining the center channel of 241-americium, 60-cobalt, and 40-potassium. These calculations involve linear interpolation between the data points.
10.1.2. Choice of Sources
The energy calibration of the detector should be checked at both the low energy and high energy ends of the spectrum. The best results are obtained when these two photopeaks are outside the range of the radionuclides you would normally expect to find in people who work at nuclear power stations. Helgeson Scientific Services routinely provides a 241-americium source for the low energy end and a 60-cobalt source for the high energy end. The 60-cobalt source should have a strength of approximately 0.5 to 1.0 microcurie. The permanently installed 241-americium source has a typical strength of 15 nanocuries per source. One 241-americium source is used for each detector.
An excellent alternative to 60-cobalt is several hundred grams of C.P. Grade (Chemically Pure) potassium chloride. The advantage of the potassium is that it does not require a Radioactive Materials license to possess it and it can be handled by anyone. The source strength need not be known precisely although it should be in the range of 0.5 to 1.0 microcurie. We strongly recommend that the same source(s) be used all the time on the same counter. Keep the source(s) away from the counter until the operator is ready to begin the routine energy calibration check. (All references in this manual will be to 60-cobalt.)
10.1.3. Source Placement
If you are calibrating a “Quicky” Counter, note that four to six “Velcro” pads are mounted in front of each detector. These are called “counting positions 1 through 4 (or 6).” There also is a “Velcro” pad mounted on the back shield and centered vertically between the two middle detectors. This location is called “counting position 5 (or 7).” By using these various counting positions you will be able to obtain consistent results for counting rate and resolution measurements. If you are calibrating a “Do-It-Yourself Whole Body Counter,” the source should be placed on the counter bed. To ensure that the source is directly beneath the centerline of the detector, mark a spot from each end of the bed. The energy calibration work is done without moving the bed. If your source is weak, it may be mounted on a stand, placing it closer to the detector. Be sure that the stand and source are placed in the same geometric relationship every time. The Waste Analyzers have their own source calibration locations. It is important to keep the geometrical source-to-detector relationships the same regardless of the type of instrument you are using. 10.1.4. Determination of Counting Time Allow the system to count for about 1 minute; the stronger the source, the less time is needed. You should accumulate about 1,000 counts in the center channel. However, do not use a source which is so strong that the dead time is more than a few percent. For example, a 10 microcurie source is much too strong. Try 1 microcurie. This is the end of Chapter 10, “Calibration - Characteristics of NaI(Tl) Detectors.”
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