Answer to Question #7972 Submitted to "Ask the Experts"

What kind of features in radiation detection instruments allow them to produce a reading in dose units such as mSv h^-1? I know that most detectors detect the radiation and generate pulses as voltage is applied and then give out counts. Is there some sort of software in the instruments that internally convert the counts into dose units?

Ultimately, all instruments rely on a proper calibration in a known field to establish the proper response of the instrument. Some instruments do use built-in firmware or software to convert the instrument signal to some desired quantity, such as ambient dose or equivalent dose. Certain instruments, such as some tissue-equivalent proportional detectors, are intended to interpret various dose-related quantities—such as linear energy transfer, effective energy, absorbed dose, and dose equivalent—and may use an internal source in association with firmware in establishing a proper calibration and allowing interpretation of the quantities of interest.

In many instruments, however, the correlation between the fundamental instrument output and the reading that is displayed is established through an appropriate calibration carried out in an appropriate radiation field associated with a source external to the detector. For example, if you have a Geiger-Müller (G-M) instrument that reads out in exposure rate units, mR h^-1, and you wish to use the instrument for measuring exposure rate, you must calibrate the detector in a gamma or x-ray radiation field of known exposure rate. The instrument is then adjusted, usually through turning one or more built-in potentiometers, so that the reading corresponds to the actual exposure rate. The G-M detector is an event-type detector, and for a given exposure rate for a particular source the detector feeds events (pulses) to the scaler/ratemeter at a particular rate; this is referred to as the count rate, and many such instruments have alternate or primary scales that are marked in count-rate units (e.g., counts per minute).

The G-M detector commonly exhibits an inherent photon energy-dependent response that is appreciably different from that of soft tissue or air, both of which have effective atomic numbers significantly smaller than that for common G-Ms, and calibration with photons of one energy may not guarantee accurate response to photons of a different energy.

Some other event-type detectors, such as the NaI(Tl) scintillation detector, may have energy dependencies that are more extreme and care must be taken in using such instruments for dose-related measurements; calibration at one energy may produce a response that may be very different from that obtained at a different energy.

Some instruments, such as many ionization chambers used for dose measurements, do not operate as event detectors, but rather operate as mean-level devices in which the average signal from multiple events is the output—e.g., a typical ionization chamber used in the rate mode outputs a current produced by continuous collection of charge, produced by ionization of the gas in the detector by the ionizing radiation, that varies directly with air kerma rate, exposure rate, or dose rate. These instruments also require calibration in a known field in order to establish the correct response. Properly designed air ionization chambers are well suited to exposure or dose-related measurements over a rather wide range of photon energies. 

In general, detectors that are fabricated of media that are similar to soft tissue or air in effective atomic number will tend to undergo interactions with the radiation that are similar in type and relative frequency to what would occur in tissue and are better suited for dose-related measurements.

The bottom line is that proper calibration ties the detector output to the desired dose quantity, and some detector types are much better suited to dose measurements than are others. I hope this clarifies some of your concerns.

George Chabot, PhD, CHP