Meter to evaluate shielding in diagnostic radiology department

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