Answer to Question #9568 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

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


I would like to know what instrumentation and survey techniques you would recommend for 7Be.


Beryllium-7 is produced in nature by cosmic radiation interactions in the atmosphere and can also be produced by various man-initiated nuclear interactions. It decays by electron capture, and the only radiation produced in sufficient yield to be useful in measurement is a gamma ray, about 478 keV in energy and with a yield of 0.103 gammas per disintegration of the 7Be.

The choice of instrument for detecting/measuring this gamma ray depends on the types of surveys you are conducting—e.g., assessment of fixed or removable contamination on surfaces, evaluations of the radionuclide in air or other media, etc.—as well as the expected quantities or concentrations being assessed and whether other radionuclides are present along with the 7Be. The type of survey used will depend on what your specific needs and objectives are.

If you are assessing removable surface contamination you would likely be taking paper wipes of affected surfaces, and if 7Be is the only radionuclide present that will contribute to counts, and it is present in relatively high activity, then you might get by using liquid scintillation counting. At 478 keV, the detection efficiency of a liquid scintillation system, however, will be rather low, and this will restrict your ability to measure relatively low levels of contamination. Additionally, you will have difficulty doing any useful energy discrimination if other radionuclides are present. Similar considerations would apply in trying to apply liquid scintillation to direct measurements of 7Be on air filters or in other media, such as water. In the case of air sampling you have the added possible concern that radon and thoron progeny collected on the filter may interfere with counting.

A better detector for assessing contamination on wipes or air filters or in other media would be a sodium iodide scintillation detector, NaI(Tl), or a germanium semiconductor detector. These solid detectors in reasonable sizes (e.g., 5.08 cm × 5.08 cm or greater for NaI(Tl) detector) have reasonable efficiencies for detecting the 478 keV photons and, if used in conjunction with a multichannel analyzer (or even a single-channel analyzer), have the great advantage of allowing gamma energy discrimination so that the specific gamma ray of interest may be measured  to the exclusion of other gamma emitters of different energies.

For field measurements of 7Be on surfaces or distributed in other media, a portable NaI(Tl) detector (often 2.54 cm × 2.54 cm or 5.08 cm × 5.08 cm) and associated ratemeter or digital scaler would be a good choice. Some such instruments are simple systems that record count (rate) in response to incident gamma radiation with no energy discrimination. These are appropriate if no other significant gamma emitters are present on or in the media being assessed. There are also portable gamma spectrometer instruments available for field measurements that allow energy discrimination to separate the radionuclide of interest from other possible interfering radionuclides. The most portable of these are commonly NaI(Tl) based but others, including portable germanium systems, are available. Germanium offers the advantage of higher energy resolution to separate closely lying gamma-ray energies.

Without more specific information about your particular situation, I am not able to make any more specific conclusions or recommendations. I hope the above is sufficient to get you started in the right direction.

George Chabot, PhD, CHP

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