Importance of radionuclide standards in testing laboratories

This editorial has reference to an article in this issue, namely “Development and characterization of standard Am-241 sources.” I do not generally incline to write an editorial based on any particular article in the RPEJ issue and instead take up an topic related to radioactivity and discuss the problems and suggest mechanisms to address them. This time, I thought it is fit and necessary to discuss the proper use of sources by the professionals. The radionuclide standards are of beta/gamma emitters or alpha emitters commonly used for the calibration of radiation counting systems. The geometry (physical characteristics including size/shape, physical state, density, dimensions, etc.,) plays a crucial role in the determination of counting efficiency or the fraction of counts recorded with respect to the number of radiations emitted from the source. In this editorial, the discussion is focused on the alpha standard source of Am-241 or Pu-239 or Cm-244 or any other. Alpha standards are prepared using several techniques such as electrodeposition, microprecipitation, direct evaporation or use of solvents, and spreading agents. Among all, the electrodeposited source has the least or near-zero thickness and can be used for many years (few decades). This type of standard is used for evaluating the energy resolution of high-resolution Si alpha detectors in addition to the determination of efficiency. The efficiency determined using these sources can only be used for the samples prepared only by the electrodeposition technique. The microprecipitation technique involves the deposition of microliters of alpha-emitting material on a platinum or stainless-steel substrate, which also gives a thin standard suitable for alpha spectrometry with adequate activity concentration. Sources prepared by other methods may not be useful for spectroscopy purposes as the source thickness inhibits alpha particles reaching the detector and degrades the resolution. They are commonly used for the calibration of alpha-counting systems where energy resolution is not an important factor. The samples prepared for alpha counting using direct evaporation techniques or deposition of chemically separated precipitates usually have a thickness ranging from few mg/cm2 to 10 mg/cm2 or more. These thicknesses introduce very high self-absorption, in addition to inhomogeneity in sample deposition. Correction factors are required to be generated to evaluate the final activity concentration in testing samples like water. Most laboratories use efficiencies determined using electrodeposited alpha sources for evaluation of activity concentration in their samples, although their sample preparation is not electrodeposition type. The efficiency for electrodeposited sources generally is in the range of 25%–30%, whereas the evaporation technique samples will have thickness >5 mg/cm2 and the efficiency for this thickness gets reduced by more than 50%. Proper care is thus essential in using the type of source for a given specific purpose. The standard source (Am-241) discussed in the article published in this issue is prepared by electrodeposition technique, and the range of activity concentration used was 26 Bq to 412 Bq. The electrodeposition efficiency was consistent in the range of 83.7%–91.4% for six standards. The deposition uniformity was also determined to establish the homogeneous distribution of activity over the stainless-steel planchette. It is important to mention here, that the author’s institute, Bhabha Atomic Research Centre (Radiation Standards Section, Radiation Safety Systems Division), is the Designated Institute (DI) for the country in respect to radiological activities, radioactivity standards, and calibration. Any noncertified source/standard is required to be evaluated and certified by the DI to have global/national acceptability and traceability.