Answer to Question #12254 Submitted to "Ask the Experts"

I'm looking for an inexpensive device that can give me an instant exposure/kerma reading for x-ray exposures. I'll be using the device to do experiments with radiography students. For example, I plan to have them make exposures at changing distances and have them predict the measured exposure using the inverse square law.

There are measuring devices that provide direct assessments of air kerma; e.g., kerma-area product (KAP) chambers/monitors that use thin transmission windows on an ionization chamber and associated readout instrumentation. These are quite expensive; possibly, but maybe not likely, your facility may have one available that is not in use.

Any appropriate ionization chamber having acceptable kerma/dose-rate capability and proper wall thickness for diagnostic beam energies could be used along with an electrometer. A couple of examples are:

• A small-volume (typically 5 to 10 centimeters³ [cm³]), cylindrical, air ionization chamber, such as the Radcal chamber.
• A thin-window, parallel-plate chamber, such as either the Exradin A600 or A650.

Please note that these are provided only as examples and should not be interpreted as specific recommendations on my part.

With either of these options, you would have to convert charge or current reading to air kerma or air kerma rate, respectively, but this is not difficult. Either of these approaches would cost several thousand to tens of thousands of dollars if you had to purchase the equipment new, but your facility may have some equipment available. If your facility has a radiation therapy department, they may well have at least some appropriate equipment; for example, they would certainly have electrometers, and you may be able to purchase an appropriate ion chamber for diagnostic energies if none is available.

I should also note that it is possible that you could make reasonable measurements, depending on your accuracy requirements, with somewhat less-expensive portable ionization chambers, designed as handheld instruments (often for health physics applications), that could be used in an integrate mode to record dose/kerma at specific locations from the x-ray target. Such ion chambers are commonly limited in dose-rate capability, so the x-ray machine output may have to be reduced to abide by such limits, often about 0.5 gray per hour (Gy h^-1). Such an instrument will likely cost in the vicinity of $3,000. Here is an example of one such instrument.

Another possibly lower-cost device is an electronic personal dosimeter, most often based on a silicon diode detector, that will record and display dose or dose rate with accuracy levels on the order of ±20% over a fairly wide energy range. If this accuracy is acceptable, you may want to investigate its possible use. The cost of such a device may be in the range of a few hundred dollars, but you should check with the manufacturer as to any restrictions or special requirements regarding calibration and single-unit use. Also make sure the device will cover the x-ray energy range of interest.

Another possible option is to use passive devices to measure dose/kerma. In particular, I am thinking about thermoluminescent dosimeters (TLDs) that may already be in use in your facility, especially if radiation therapy is being done. Properly calibrated, TLDs can measure dose or air kerma and, assuming a TLD reader is available, can be read out quickly. These also provide the option of exposing a group of TLDs, with selected TLDs placed at different distances from the x-ray target, to collect data that can be used easily in your inverse square demonstration. This is an inexpensive option if the reader is available. It does not yield an instant reading, but it can be quite fast.

George Chabot, PhD, CHP