Answer to Question #12471 Submitted to "Ask the Experts"

My source emits only beta radiation along with the gamma background. I want to compensate for gamma background radiation. What are various methods to achieve this? Moreover, I am looking for methods to discriminate between beta and gamma radiation. What are various ways for beta and gamma radiation discrimination?

Thank you for the question. There are various ways to account for gamma radiation in the presence of beta radiation. The particular method you employ will depend on a number of factors, including the physical nature of the source, the energies of the beta and gamma radiations, the relative intensities of the beta and gamma radiation, the particular quantities you are attempting to measure (e.g., dose rate, fluence rate), the type(s) of instrumentation you have available, and the magnitude of the error you are willing to accept in your results. Since you do not describe the particular situation of interest, I will attempt to provide an overview of a number of possible approaches.

If all of the gamma radiation is coming from background radiation, with no gamma contribution from the beta-emitting source, and the source can be moved to a location away from the detector, clearly it is a simple matter to make a measurement for a fixed time with the source in place to obtain a gross count (or other measurement quantity) and then to remove the source and operate the detector again for the same length of time to establish the contribution from the background. The background value is then subtracted from the gross value to obtain the net beat count or contribution to whatever is being measured. I assume you would have done this if it were possible.

If there is a contribution of gamma radiation from the beta-emitting source or if the physical characteristics of the source are such that the source cannot be separated from the background (as might be the case when assessing widespread contamination in a particular area), and if the gamma radiation is fairly energetic (as, along with the high-energy muons, is typical for background radiation), perhaps the simplest way to make a reasonable discrimination is simply to cover the source (or cover the detector if the source cannot be conveniently covered) with sufficient material to attenuate all of the beta radiation. The radiation detected will then represent the gamma contribution to the quantity being measured. It is best to use a low-atomic-number material (such as polyethylene or polymethyl methacrylate plastic) to attenuate the beta radiation so as not to enhance the production of bremsstrahlung radiation. If the beta radiation is rather high in energy (perhaps >1 millielectronvolts [MeV]), the necessary thickness of attenuating material may be sufficient to attenuate significant gamma radiation, and this may be problematic, depending on the accuracy required. For example, if the beta radiation is less than about 1 MeV in energy, a thickness of about 4 to 5 millimeters (mm) of polyethylene or polymethyl methacrylate (Lucite™, Plexiglas™) is sufficient to stop the beta particles and would likely produce less than a 5% reduction in the gamma contribution.

If you have an appropriate detector and associated energy spectrometry system (e.g., an appropriate silicon surface barrier detector and analyzing system) and the source cannot be physically separated from the background counting environment, it may be possible to resolve the gamma contribution to the observed pulse-height distribution by subtracting the continuum of gamma-induced pulses from the gross distribution. For a relatively thin active volume detector, the gamma distribution of pulses from natural background will likely appear as a continuum of counts on which the beta pulse-height distribution is imposed. The continuum of counts under the beta distribution can be subtracted either by using software or by doing the process manually. Using this method relies on your ability to translate the accumulated counts in the net beta distribution to whatever quantity you require, e.g., activity, dose rate, etc. Similar measurements can be made with a plastic scintillation detector of sufficient thickness to stop the beta radiation and an associated multichannel analyzing system. Relatively larger detector thickness may be required for a plastic scintillator compared to a silicon surface-barrier detector, and gamma-induced backgrounds may be somewhat higher as well, depending on specific detector characteristics.

If you are familiar with coincidence/anticoindidence counting techniques, it may also be possible to implement such approaches to discriminate between the beta and gamma radiation. If the gamma radiation is unrelated in time to the beta radiation, which it presumably would be if the only gamma radiation was coming from external background radiation, it may be possible to use two detectors, a relatively small, thin, low atomic number, beta-sensitive detector and a much larger volume, thicker, higher atomic number, gamma detector operated in close physical proximity with the gamma detector covering of sufficient thickness to prevent entry of beta radiation. I will not go into detail here, but the usual approach is to establish a reasonable delay-time acceptance criteria such that if pulses are produced simultaneously in each detector (within the specified delay time between the beta detector response and the gamma detector response), we would assume that the radiation causing both events was gamma radiation, and such pulses would be rejected from the accounting so that the final recorded pulses from the beta detector would be those associated only with the beta radiation. This technique may fail if the gamma intensity is so high that a high rate of gamma-induced pulses are produced in the gamma detector such that random coincidence events occur between beta detector pulses and gamma detector pulses and different gamma rays are yielding pulses in both detectors within the specified delay time, resulting in the rejection of more gamma events than appropriate.

Another technique, not often available to many users, is to use a magnetic or electrostatic field to deflect the beta radiation from the source into an appropriate detector (charged-particle spectrometers are built on this principle). The negatively charged beta particles can be deflected in the magnetic/electric field to intercept an appropriate charged-particle detector while the gamma radiation is unaffected by the field. The field strength and the detector placement must be such that no beta radiation is detected when no magnetic/electric field is applied. If the source is a large extended source, such as a contaminated floor or wall, it would be necessary to collimate part of the source and/or the detector so that only beta radiation incident on the detector face is effective. Because beta-emitting radionuclides emit a range of beta particle energies from zero to the maximum beta energy, it may be necessary to make measurements with the beta detector in different locations, to vary the field strength to make multiple counts to cover the distribution of beta particle energies, or to use other algorithmic approaches that allow measurement of a particular limited range of energies that can then be related to total emission rate through energy spectral corrections. Measurements with the magnetic/electric field applied would represent response to beta plus gamma radiation, and measurement with no field applied would represent gamma radiation. The latter reading would be subtracted from the former to get net beta response, which would have to be converted to the quantity and units of interest through appropriate calibration and algorithms.

Other discrimination methods may be possible, but I believe the above should be adequate for you to develop a method that would be suitable to your specific situation. I wish you well in your pursuits.

George Chabot, PhD, CHP