Answer to Question #9085 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Equipment

The following question was answered by an expert in the appropriate field:

Q

Can you suggest a meter that has a flat energy dependence in diagnostic radiology energies range (20-140 kV) and a fast response (less than two seconds)? I am interested in confirming that we have adequate shielding in our x-ray departments. Most of the meters I came across have a slow response that the x-ray machine cannot cope with.

A

I usually use an ion chamber survey meter that can be operated in the integrate mode instead of the usual dose-rate mode. Pressurized ion chamber survey meters can measure lower dose and dose rates, but the robust chamber construction required to contain the pressures results in a nonlinear response at diagnostic x-ray energies (below ~140 kilovolt potential [kVp]). In diagnostic x-ray use, the typical x-ray exposure time is usually metered in fractions of a second. An ion chamber cannot stabilize the electrometer dose rate reading that quickly. In addition, even if the meter could respond that quickly, you humanly cannot read the peak meter reading in a fraction of a second. So, the way around this problem is to operate the ion chamber in the integrate mode. Exposure rates give results in milliroentgen/hr (mR/hr)[SI units of Coulomb/kg/hr (C/kg/hr)]; the integrate mode will give results, usually in microroentgen (µR)[SI units of Coulomb/kg (C/kg)] per exposure. To assess the shielding of an x-ray facility, you would place the ion chamber in the integrate mode at a point of interest outside the room and then operate the x ray at some known kVp and milli-Amp-seconds (mAs) setting. After the exposure, the ion chamber will display the µR (C/kg) leakage for that exposure and you can then estimate the shielding leakage in units of µR/mAs (C/kg/mAs) at that kVp.

Unfortunately, due to today's current National Council on Radiation Protection and Measurements and International Commission on Radiological Protection shielding recommendations of less than 0.02 millisieverts/week (mSv/week) in unrestricted areas, even with this technique, it is very difficult to determine if the shielding will meet the specifications. So, another method commonly employed to confirm the initial ion chamber measurements is to post a dosimeter at a point of interest outside the x-ray room and leave it there for a month or more. Dosimeters like film badges, thermoluminescent dosimeters (TLD), and optically stimulated luminescent (OSL) dosimeters can integrate doses over long periods of time, allowing you to measure very low dose rates. If the results come out less than 0.08 mSv/month, you can document that the shielding meets the 0.02 mSv/week standard.

Mike Bohan, RSO
Yale-New Haven Hospital

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
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