What is Gain?

Gain allows you to set the gamma spectrum of your PHA. By applying HV to different gain scales you can set the a specific keV or MeV scale to the a gamma spectrum. Today we well at True Gain. True Gain has numerical values starting at 1. If you set your True Gain 1 it will set the gamma spectrum between 0.0 MeV to 2.0 MeV. Each time you double your gain value the gamma spectrum divides in half. Another words, a gain of 2 will read a 0 to 1000 keV and a gain of 4 will read a spectrum between 0 keV to 500. Hence each time the value of gain doubles the gamma spectrum is reduced by ½.

How is Gain applied in Nuclear Medicine Instrumentation?

It depends on the gamma energy you are reading. As an example, if your energy gamma was greater than 1.0 MeV you would have to set the gain of 1. To use any other gain setting would not allow you to read the gamma that is greater than 1.0 MeV. You could say, it would be off the "scale."

However, let us take a more realistic example. It us say you want to count ^99m Tc, which gain setting would be most appropriate? Usually it is the scale, which is closest to, but is not less than the energy of the gamma being counted. Therefore a gain of 8 is suggested since it would have a scale of 0 to 250 keV.

Gain setting vs. Gamma Spectrum

How do channels relate to keV settings?

PHAs do not necessarily have keV settings, but channel setting. To determine the amount of keV per channel note the above table. If you were to calculate the amount of keV per channel, you would divide the total amount of channels into the total amount of keV. Therefore, a gain of 2 would be determined as follows:

How do you set the gain?

Adjust your well counter or probe to read in the appropriate gain setting pending the energy gamma you intend to record.

As an example, we will look at several spectrum readouts from a spectrum channel analyzer connected to a well counter. Prior to reading ¹¹¹In you must for first complete QC with the system with ¹³⁷Cs. Concurrent readings on a daily bases insures that the system is, as noted below:

1. The system is counting correctly. You could also say that you are testing to see that HV is consistent.
2. Once you save the data to your system the analyzer will use the 660 keV peak from ¹³⁷Cs to set all other entire gamma spectrum.

Today we will work with ¹¹¹In and determine the appropriate window settings. Let us first consider the energy gammas for ¹¹¹In and they are 173 keV and 247 keV. What happens if you collect this data at a gain of 1? By looking at the diagram above you will notice that the peak is far to the left and most of the 256 channels are not being utilized. In addition, the lower peak of 173 keV is highlighted and that is the peak being collected. The center window is set at channel 22. The system is also set at a 20% window, which means that only 2 channels to left and right of center are being occupied. This window is too small and with so many keV per channel some overlapping of the window could result in reading too much scatter radiation below LLD cutoff. Hence, it is obvious, from this image that you would want the gamma spectrum to fill in more of the total spectrum or scale (the 256 channels).

Now look what happens when you set the ¹¹¹In window to a gain of 4. In this case you have more control over the window and your peak is better defined. Why did we not use a gain setting of 8?

Finally, setting the regions off peak on ¹¹¹In identifies were the ¹¹¹In peaks should be. This can be noted by looking at dotted line of where the peak is suppose to be, however, as you can see the dark gamma peaks are not in line with where you would expect the energy peaks to be. Hence the PHA is not counting within the appropriate window.

Finally, taking a background count in the ¹¹¹In window reveals no significant peak activity.