ORTEC How to Choose the Right Photon Detector

How to Choose the Right Photon Detector

Low and Very Low Count Rates. Low Background Detectors

Reduced and low background detectors are available, constructed with different degrees of low background specification. The background in the detector comes from natural emitters found in nearly all materials. By carefully selecting the materials used in the detector, these natural radionuclides can be significantly reduced. No detectors (low or otherwise) have non-mode radionuclides in the material. The background has both peaks and continuum.

The Standard Options From ORTEC Are:

LB — Low Background
XLB — Low Background with lead backshield
LLB — Low Background with remote preamplifier
PLB — Reduced Background in PopTop

Low background (Ref. 7) options require specially selected materials, and therefore can add considerable cost to the detector. Before deciding that such a step is necessary, consider the following:

What problem are you trying to solve or mitigate? The basic principle should be removal of Interference Lines which degrade the results. The background continuum is not usually the problem.

Examples:

In a lung burden system, the major source of background is the 40K Compton background from the subject. If uranium is being measured, then some attention should be paid to removing sources of this line (including Compton-scattered events from the subject) from the spectrum.

Figure 18. Low background (LB), reduced background
PopTop (PLB), and standard detector.

In an In-situ measurement, the source of background is not the detector itself. You should first shield the detector from external radiation sources such as nearby containers.

Figure 18 shows the background in different detectors. If you are measuring uranium via the 186 keV line, then the reduced background PopTop or low background option rather than the standard detector is needed. This is due to uranium being present in the standard detector aluminum endcap. The LB and PLB detectors use selected magnesium endcaps.

While an LLB configuration may represent the ultimate in low background, you should discuss the measurement with a detector expert, who may be able to offer a cost-effective solution without extreme background reduction measures. In most cases, your budget is fixed and it is important to spend money where it will give you the best performance improvements.