Answer to Question #6829 Submitted to "Ask the Experts"

There are a number of radiation detecting and measuring instruments that are "self-calibrating," although one must be careful that the manufacturer's definition of calibration in such cases is consistent with the user's needs and regulatory requirements.

The detector/instrument types are probably those that operate as event detectors and yield a count or count rate as the output. In these instances it is possible to use a single source that may be built into the instrument to expose to the detector in a reproducible location so that the count rate, and sometimes other parameters, can be determined and compared to an expected value. The process of self-calibration is not necessarily completely automatic, but may require input from the user. Such approaches have been used in instruments such as gamma counters, liquid scintillation systems, and even gamma spectrometry systems; here is an example from MGM Instruments of a gamma counter that uses ¹²⁹I to mimic ¹²⁵I in a "self calibration" routine.

Joseph Shonka has written about instruments, designed for surface contamination monitoring, that are described as self-calibrating. Here is a link—Shonka Research Associates—to several of his papers.

A tissue-equivalent proportional counter (TEPC) is an example of an instrument that incorporates an alpha-emitter, often ²⁴⁴Cm, into its design so that the instrument can be self-calibrating. The TEPC is basically an LET (linear energy transfer) chamber, and the source can serve to provide radiation of known LET that is useful in setting up and calibrating the system. Here is an example of one such instrument from Far West Technology, Inc.

Other instruments have used an internal pulse generator in a self-calibrating mode to provide a known pulse rate, and operating parameters can be adjusted to yield the expected count rate. Systems of this type are not actually calibrating the detector, since the electronic pulser bypasses the detector, and one may still have to do independent radiation calibrations to verify proper operation of the entire instrument. This Web page from Excellent Technology shows a Model 3327B autoranging survey meter (no detector) that is self-calibrating for count rate.

Many of the common portable instruments used for dose—or dose rate—related measurements often cannot be adequately calibrated using a single internal source because calibration requirements may call for calibration on several ranges, often with more than one point on each range to demonstrate acceptable behavior. While instrument manufacturers have taken advantage of dramatic advances in the technology of electronic devices, especially microprocessors that can allow sophisticated signal processing and data manipulation, there are many aspects of calibration that require radiation sources that cannot be incorporated into the instruments, and external calibration will continue to be required.

I am sure there are more examples of instruments that invoke varying degrees of self-calibration that you can find if you wish to pursue your search on the internet or through individual manufacturers. Good luck.

George Chabot, PhD, CHP