Gamma radionuclide standards for gamma spectrometry

Gamma spectrometry is relatively a simple technique to identify and quantitatively determine the gamma emitting radionuclides as compared to most other methods of radionuclide identification and quantification. Although simple, accurate determinations are only possible, if proper calibrations are carried out and appropriate corrections are implemented. Professionals use NaI(Tl) detectors or HPGe detectors for the purpose of routine measurements. Experienced and advanced laboratories apply standard procedures and often evaluate their capabilities by participating in international proficiency testing programs. Difficulties do arise in testing laboratories who start setting up of gamma spectrometry system, mostly employ NaI(Tl) detectors, owing to their lower cost, to begin their studies. They neither get proper guidance from their experienced professionals nor undergo rigorous training to perform gamma spectrometry leading to inaccurate results. They also send their results of the studies in the form of papers and get them published too. The two basic requirements for performing gamma spectrometry are energy calibration and efficiency calibration. The gamma standards used for energy calibration are the pure sources of radionuclides (without impurities of other radionuclides) whose characteristic gamma emission energies are known like for Cs-137, Co-60, and Ba-133. The activity concentration of these sources is immaterial (need not be accurately known) except that it should be adequate to give clear peaks with enough peak counts (500–1000 counts) in about 500 s. Generally, sealed disk sources are sufficient as these are frequently used to check the stability of the system. The other calibration for quantitative determination of identified radionuclide is efficiency calibration and is important to generate the accurate results. The essential requirement is sample geometry and the standard geometry must be identical in shape, size, and density. For instance, when a polyethylene (PET) cylindrical bottle of 5 cm diameter and 7.5 cm height with sample filled up to 7 cm height, the gamma standards (mixed or individual) also must be taken in the polyethylene (PET) bottle of same dimensions and be filled up to 7 cm height only. If the sample is powder and sieved through 2-mm sieve, the standard also should be of powder with similar particle sizes (<2 mm). The efficiencies can be determined for different energies in the range of 200 keV–2000 keV (for NaI detector), a curve can be drawn and fitted to obtain the efficiencies for other energies for whom radionuclides standards may not be available. I had come across during the review of papers submitted to this journal that people use sealed standards of disk type with ~5 mm thickness, ~25 mm diameter for efficiency calibration and use those factors for samples taken in a petri dish containing about 50 g soil, which is going to give erroneous results in addition to the poor detection capability. Another example of negligence on the part of authors can be found in a published article regarding their efficiencies for a 4” × 4” NaI(Tl) detector for a plastic bottle geometry. The efficiencies quoted are 3.51%, 2.09%, 0.67%, and 0.38% for 660 keV, 1175 keV, 1333 keV, and 2503 keV, respectively. On close observation, either 2.09% or 0.67% must be wrong. Overall, the efficiencies are also slightly on the lower side for a 4” × 4” detector for a 200 mL–250 mL capacity bottle geometry and it appears that 0.67% is wrong. Is it a case of 1.67% typed as 0.67%? Even if it is a typographic error, it should not go unnoticed as the paper goes through 3–4 reviews including proof before it gets published. This paper was published in a famous peer-reviewed journal having certain impact factor. It is due to this type of reasons, we are making it compulsory to mention their efficiencies for respective energies along with the details of standards used and geometry adopted. We are advising the authors for necessary corrections as and when needed ensuring that such mistakes/errors are avoided in the paper and also in their work. Some of the key/important points that should be adhered to in gamma spectrometry, although are fundamental, are given below. The standards should be reference materials/traceable standard sources spiked in selected materials Sample and standard should be of same geometry (shape and size), identical in every known aspect Correction factors, wherever necessary, for attenuation for density variations, summing processes should be applied Participation in interlaboratory exercises and proficiency testing programs to ensure the validity of calibration, calculation, and procedures Most importantly, the publication material needs to be reviewed by all the authors, although it is always implied that all the authors have contributed in some aspects of paper making.