Answer to Question #7401 Submitted to "Ask the Experts"

I am currently doing my PhD research on gamma radiation detection and I would like to know a simple method (no geometry) of calibrating the efficiency of my portable 7.62 cm × 7.62 cm NaI detector. I am interested in taking measurements on soil, water, and vegetables.
 

It is not clear what you mean by "no geometry," since geometry plays a critical part in the efficiency calibration of a NaI detector. If you are collecting samples of soil, water, and vegetation that are to be measured in the laboratory, then each sample must be in a container that provides a reproducible geometry.

If you are interested in only a few radionuclides you may be able to obtain standardized solutions of those radionuclides from a creditable standards laboratory that can be used in making up your standards. If you are concerned with multiple radionuclides that exhibit a number of different gamma-ray energies, you might want to consider using one or more radionuclides that emit multiple gamma rays of different energies. One of the most commonly used radionuclides in such instances is ¹⁵²Eu, which emits multiple gamma rays that range in energy from about 120 keV to about 1,460 keV. This radionuclide is probably more useful if you use a high-resolution detector, such as germanium, because a number of energies will overlap when evaluated using a NaI detector. Efficiencies are generally determined based on net counts in the full photopeak region, and the efficiency at a given energy would be expressed in units of counts per gamma ray emitted. The counting efficiency for a particular radionuclide then depends on the number of gamma rays of the energy of interest emitted per disintegration.

Water standards are the easiest to prepare since one can generally make up the standard in a water volume that simulates the actual samples in volume, geometry, and attenuation characteristics, especially mass density. Depending on the radionuclides used, the water samples may be kept somewhat acidic and/or small amounts of stable carrier may be added to minimize plateout of the radioactivity on the container walls.

Standards to be used to obtain efficiencies for soil and vegetation may be much more challenging to prepare. It is naturally desirable that the standards mimic the actual samples, as closely as possible and practical, especially in terms of volume and mass. Soil used in making the standards should have a mass density close to the soil to be measured. When making up soil standards, one approach is to add the radionuclides in liquid solution to a small portion of soil, which is then dried and subsequently mixed into the larger volume of soil to distribute the activity uniformly throughout the volume. For the standards it is generally desirable to use soil or sand that has a rather narrow distribution of particle sizes as this tends to provide for greater long-term stability of the prepared standards (less separation of particles based on gravitational settling). A 1996 paper by R. Colle and F.J. Schima ("A Quantifiable, Verifiable, and Efficacious Protocol for Spiking Solid, Granular Matrices with Radionuclidic Solutions" in Radioactivity and Radiochemistry, 7(3):32–48; 1996) describes a method for preparing suitable standards for soil analysis. Containers such as Marinelli beakers that surround the detector are often used to improve geometry and counting efficiency.

Relatively large volumes of vegetation may be necessary to obtain a sufficient amount of radioactivity for measurements. These volumes may be reduced by either dry ashing or, especially if dealing with relatively volatile radionuclides, by wet ashing procedures using rather large volumes of oxidizing acids to effect such wet ashing. The process of ashing results in a large reduction in mass and volume and can produce much more favorable geometries for counting. Again, when preparing the vegetation standards you must strive to have the standards simulate the actual samples to the extent possible.

Gamma detectors, including both NaI and germanium systems, have also been used for in-situ evaluations of gamma emitters in soil. Some such methods have used mathematical determinations of detection efficiencies. You can find various references to such techniques by typing "in-situ gamma spectroscopy" into some of the common internet search engines.

You should probably review the literature to get a better understanding of some of the techniques and considerations that apply to preparation of standards for measuring radioactivity in environmental samples. Good luck.

George Chabot, PhD, CHP