Answer to Question #6789 Submitted to "Ask the Experts"

How can I calibrate an RIA (radioimmunoassay) gamma counter for measuring ¹²⁵I? Which protocol shall I use? What are the standard-reference sources that I have to use? How often do I use them? Are there any documents that I can read in order to start such a procedure?

I am not familiar with any specific protocols recommended by government or other standards-setting or recognized recommending groups for calibrating gamma counters. There are other protocols and reports that relate to calibration of systems, such as scintillation detectors with analyzers used for gamma energy analysis, such as the 2007 Standard IEC 61453 of the International Electrotechnical Commission that might be of interest to you. I will attempt to address some of the considerations that do apply to calibration of the gamma counter.

You can calibrate the gamma counter for counting ¹²⁵I using an appropriate standard of ¹²⁵I. The standard should be prepared in a geometry and composition as similar as possible to the samples that you will be counting. The iodine emits rather low-energy photons in the interval from about 27 keV to 35 keV, and the tube or container used to hold the standard should also be the same as that used in counting so as not to produce wall attenuation effects that are different from that of the sample. You may be able to get the supplier to make up the standard according to your needs. If not, you may have to purchase the standard as a calibrated solution and use a portion of it to make up the standard in the necessary form. The activity of the standard should be traceable to an accepted standards laboratory, such as the National Institute of Standards and Technology or a secondary laboratory.

The instrument must be set up so that the major ¹²⁵I counts will fall in the counting window specified for ¹²⁵I. Once this has been done, count the standard long enough to obtain enough counts to obtain an acceptable error—e.g., neglecting background, 100,000 counts yields a relative standard deviation of 0.3 percent. The amount of activity commonly used for calibration in this type of system is usually between 0.05 microcuries and 0.1 microcuries. This is usually sufficient to produce a statistically valid number of counts in a one-minute counting interval without producing high-enough count rates to saturate the system. It is generally desirable to take a series of consecutive counts that can be used to generate the mean count and, perhaps, to perform one or more appropriate performance tests (see below).

n
          χ² = Σ(x_i - x_m)²/x_m,
i=1

where x_i is the i^th count, x_m is the mean of all the n counts, and the summation of the squared differences from the mean is for all of the counts. The ideal value of χ² is equal to the number of degrees of freedom for the data set (for this case the number of degrees of freedom is n-1 or 19 for a set of 20 counts). You can review a table of the values of χ² in order to specify a range of acceptable values consistent with your level of acceptance (for more discussion see Question 5982 and its answer on the Health Physics Society Ask the Experts Web site). For example, for a rejection level of 5 percent (i.e., a willingness to conclude 5 percent of the time that good data are not acceptable), the range of acceptable χ² values is from 8.91 to 32.7, and any value between these two would indicate acceptable performance of the counter.

When you do the initial calibration you might also want to count a source of ¹²⁹I, a radionuclide often used as a stand-in for ¹²⁵I for routine operational checks and secondary calibrations. The ¹²⁹I has a very long half-life, and using it for future instrument performance checks and calibrations is often desirable since the ¹²⁵I standard will decrease in activity appreciably with time and soon require replacement. The ¹²⁹I is often referred to as mock ¹²⁵I because it emits photons that are quite similar in energies to those from ¹²⁵I and provides a reasonable simulation of the ¹²⁵I.

The ¹²⁹I source should be used to test the operation and consistency of performance of the gamma counter every day that the instrument is used. This is part of the routine quality-assurance program that should be in operation. If time permits, I would recommend performing a series of 10 or 20 counts each day and performing a Chi-square test of the counter. The frequency of calibration varies with the kind of use that the instrument gets. If it is being used heavily by multiple users, then calibrations should be performed more frequently than for cases involving one or two users and/or light use. Calibration frequencies may vary from about weekly to annually. Here is a link to a service manual for the Isocomp I gamma counter from MGM Instruments that describes some statistical recommendations and shows typical data that might be obtained in the calibration process, which is recommended on a weekly basis.

Since the calibration process is quite straightforward and requires rather little time to accomplish, once the initial calibration with the ¹²⁵I has been done, I would feel comfortable using the ¹²⁹I source on at least a monthly basis to perform a secondary calibration. In any case, be sure that your daily checks with the ¹²⁹I source are conducted to confirm consistent performance. Naturally, if other radionuclides are also being assessed with the gamma counter, you would have to perform calibrations for them as well. I hope this is of some help to you.

George Chabot, PhD, CHP