Answer to Question #10957 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
I would like to conduct a study on environmental monitoring with CaSO4:Dy disks. Can you please advise me on the energy that the disks should be calibrated to? Should the disks be calibrated to 60Co for background radiation counts?
As you are likely aware, CaSO4:Dy is a dosimeter commonly used for environmental measurements of ionizing radiation dose; it is especially useful because of its high sensitivity compared to some other phosphors. It does have one particular weakness—because the effective atomic number of the material is notably greater than that of soft tissue, the phosphor exhibits an inherent overresponse to lower-energy photons (when being used to estimate doses to people).
The overresponse may exceed a factor of 10 at energies between about 20 and 30 keV and will decrease with increasing energy. This necessitates using some material to cover the active phosphor to attenuate many of the lower-energy photons so as to flatten the energy response. The material most commonly used has been metallic copper, which may be wrapped around the active material, depending on the configuration of the dosimeter. This process of using an attenuator to flatten out the energy response is a form of energy compensation. If it is used and demonstrated to be effective, then the process of calibrating the dosimeter to read dose is relatively simple, often requiring just a single radionuclide source.
One radionuclide that has sometimes been favored for calibration of dosimeters to be used in environmental monitoring is 226Ra. It is sometimes preferred because the major photons emitted by the source (226Ra with progeny in secular equilibrium) are the same as many of those encountered in the environment as a consequence of the gamma emissions from the radioactive progeny of 238U present in the earth. The use of 226Ra has been reduced in many facilities and calibrations using more common sealed sources such as 137Cs (662 keV) and 60Co (1.25 MeV) have been adopted.
If your dosimeters have been energy compensated, as noted above, and have already been demonstrated to provide an acceptably uniform energy response, then the use of single calibration sources is often adequate. If you are using commercially supplied dosimeters, the manufacturer has likely already done the energy-response evaluation, and using one or two sources should be sufficient. In addition to using a common, relatively high-energy source such as 137Cs or 60Co, I would recommend also using an 241Am source, which emits 60 keV photons, to check the response at a lower energy; 241Am is the most commonly used radionuclide source for such determinations.
If the dosimeters are energy compensated but have not been evaluated, then you will have to perform additional evaluations to show that the response is acceptable. This requires irradiations of test dosimeters to several photon energies. In particular, it is desirable to test the dosimeters from perhaps 20 or 30 keV up to at least 1.25 MeV. Most operational facilities do not have a variety of calibrated sources of different energies available and frequently prefer to have an accredited laboratory perform the necessary irradiations. In the United States, the National Voluntary Accreditation Program (NVLAP) offers various calibration services and supports a number of accredited laboratories that may be capable of performing some of the kinds of irradiations necessary to do the dosimeter evaluation. You can review information on the National Voluntary Laboratory Accreditation Program website to find a list of accredited labs that deal with ionizing radiation.
George G. Chabot, Jr., PhD, CHP