Answer to Question #12276 Submitted to "Ask the Experts"

I am an inspector for a state radiation protection program. My question is about energy response and lower limit of detection for our portable survey meters.

Energy response is the range of energies the probe will detect with a certain assurance of accuracy (e.g., plus or minus 15% at 60 kiloelectronvolts [keV]). But my question is what is the lowest energy detectable by a specific meter/probe? In other words, what is the minimum energy at which the needle will be deflected? Is this even important? If you detect something below the energy-response curve, is the reading so inaccurate that it is useless information? I am thinking in terms of measurements of scattered, low-energy x rays, like those from a low-energy, open-beam analytical device.

I am having a hard time finding answers so I may not be thinking about this correctly and therefore not asking the appropriate question.

Your question appears to be concerned only with photon radiation—gamma rays or x rays—so I shall assume this in my response.

As you know, the energy responses of different instruments vary, and manufacturers typically stipulate accuracy boundaries, such as you cite, over specific energy ranges. Very often the manufacturer of the instrument/probe will have more information available as to the energy response below the lowest stipulated energy for which the accuracy is specified. Sometimes such information may be given in the form of an energy-response curve that may be available in the literature supplied with the device and/or on the internet, although the curves often extend only to about the lower specified range limit. For example, a popular pancake-type Geiger-Mueller probe has an available energy-compensating filter that flattens the energy-response curve to photons, and the response curves for this probe are shown on this web page. In cases where the response data are not provided on a web page or in the available literature, such information may still be available by contacting the manufacturer. If you cannot obtain the information, it may be possible, if you know the specific physical characteristics of the detector, to estimate, by calculations that account for attenuation of photons in the detector wall(s), the projected responses at energies below the lowest cited value. Such estimations may be subject to considerable uncertainty if you do not account properly for all materials that the radiation encounters, especially when the energies are low and small changes in material composition and/or thickness can have rather large effects on response.

When you cannot obtain energy-response information for energies below the lower end of the cited range for a given instrument, it is generally not appropriate to use the instrument in radiation fields that are known to include significant contributions from photons below the cited low end of the range. Knowing the energy distribution of photons in the field(s) to be measured is a critical piece of information in selecting an appropriate instrument. It is often not especially helpful to know what the lowest energy photons are that will produce a given deflection or some minimal reading on the instrument since, in a typical dose/dose-rate measuring instrument, you may have no way of knowing what energy photons are responsible for a reading. If you do know that all the possible photons at a given location are below the manufacturer's specified low-energy range limit for a given accuracy, but above some minimal energy where the instrument is capable of responding, any reading you obtain is still not very useful if you have no valid accuracy information.

I should make one other point regarding your observation on scattered radiation from low-energy devices, such as analytical instruments: namely, when low-energy photons scatter in a single event, unlike higher energy photons, they do not lose a large fraction of their energy, even at large scattering angles. For example, a 30 keV photon scattered at the most extreme energy-loss angle of 180^o yields a scattered photon of about 27 keV, an energy loss of only about 10%. This can be compared with a 1 megaelectronvolt (MeV) photon scattering at 180^o and producing a scattered photon of 0.2 MeV, an energy loss of 80%. Thus, if an instrument is acceptable, in terms of accuracy, for measuring the primary radiation from a low-energy source, it may very likely be suitable for measuring scattered radiation from the same source. Of course, you must know something about the nature of the scattering processes that apply in order to make a sound judgment about the likely energies. For example, if it is likely that photons reaching a point of interest may undergo multiple scattering events, especially at large angles, they may ultimately get reduced below the energy limit for accurate measurement.

There is more to say about this, but I don't think I can add much more helpful information without very specific cases at hand.

George Chabot, PhD, CHP