Answer to Question #12333 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

The following question was answered by an expert in the appropriate field:

Q

I have a question about a multichannel analyzer (MCA) used with a High Purity Germanium (HPGe) detector. The MCA has options of 4 k, 8 k, 16 k, and 64 k. I want to change the channel from 4 k to 8 k. How can this be done? The MCA is digital Orion MCA. Secondly, at 4 k channels the maximum energy range is 1,300 keV, so to cover the range of 60Co and 40K I need at least 2,000 keV. So how can this energy be extended—by adjusting gain or anything else? Please guide me.

A

The selection of the number of channels over which the energy distribution of pulses is spread (designated 4 k, 8 k, 16 k, and 64 k, but actually 4,096; 8,192; 16,384; or 65,536) is determined through the conversion gain that is selected. I don't know exactly how the Orion MCA that you are using displays this, but it is commonly available through accessing the digital signal processing (DSP) gain selection screen. Pick the specific conversion gain that you want. Switching to a higher conversion gain, as you intend, can be helpful in spreading the pulse height distribution out over a wider range, thus making it easier to discern closely spaced peaks. At the same time, it generally leads to somewhat poorer statistical data associated with photo peaks, especially lower yield peaks, since each channel will have fewer counts at the higher conversion gain and the relative error in each channel count will be higher than when a lower conversion gain is used.

There are also amplifier gain controls available, often coarse and fine controls, again often accessible through the DSP gain settings. These controls allow peak positions to be shifted to higher channel locations by increasing gain and to lower channel locations by decreasing the gain. By adjusting the gain appropriately, you can position the peaks of interest where you want them. Various other techniques may be used to provide alternative displays of data. For example, by increasing the lower level discriminator one can eliminate the lower energy part of a pulse height distribution to display only higher energy peaks. One may then change the conversion gain to spread out or compress this higher energy distribution, depending on what your aims are. 

Different manufacturers use different software, and you should consult the manual for your instrument if you have difficulty accessing any of the parameters mentioned. Good luck in your pursuits.

George Chabot, CHP, PhD

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