T.O. 33B-1-1
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an individuals dose. Occupationally exposed personnel shall NOT wear their dosimetry devices while undergoing
medical or dental X-ray procedures.
6.9.10
Measuring Exposures Rates: Ionization Chamber Type Survey Instruments.
6.9.10.1
Basic Operating Principle.
Radiation exposure is most accurately measured with ionization chamber type survey instruments. These detectors use
an air filled chamber across which an electric field is applied. When X-ray or gamma radiation interacts with the air in
the chamber, it creates positive and negative ions that drift apart under the influence of the electric field. As the ions
are collected on the electrodes within the chamber, a small current is generated which is measured by the instrument
and related directly to the radiation exposure rate in air.
6.9.10.2
Characteristics.
Radiation exposure measurement instrumentation must have a range suitable for the conditions of use. Accordingly, all
survey instruments used for industrial radiography "shall have a range such that two milliroentgens per hour through
one roentgen per hour can be measured" (10 CFR 34.24).
6.9.10.2.1
Portable survey instruments are affected by such factors as ambient temperature, configuration of radiation source (i.e.,
round, square, rectangular, etc.), isotope source, atmospheric pressure and relative humidity, direction of radiation
beam, radiation quality (effective energy or radiation spectra), and instrument susceptibility to radio frequency (RF)
radiation. Instrument response variations due to temperature and pressure usually do not exceed
5% for survey
instruments. Instrument directional dependence is negligible when the instrument's sensitive volume is pointed in the
direction of the radiation origin. Instrument susceptibility to radiofrequency radiation (RFR) may significantly affect
ionizing radiation measurements in the presence of RF radiation. If RF interference is suspected, it can often be
confirmed by placing a piece of leaded (Pb) rubber or similar shielding material over the ionization chamber of the
instrument to filter out the gamma or X-radiation, while observing the instrument reading. If no change is noticed in
the reading when the lead is placed over the chamber, the reading obtained previously was due primarily to RF
interference.
6.9.10.2.2
An X-ray machine operating at a given tube potential (kVp) produces a spectrum of X-ray energies. Since industrial
X-ray machines do not contain primary beam filtration (except the X-ray tube window), the X-ray spectrum contains a
relatively large portion of low energy X-rays (below 50 keV) regardless of the tube potential (kVp) setting employed.
Therefore, it is important that the survey instrument used in determining the exposure rate produced by such X-ray
machines be energy independent or, in other words, is capable of accurately measuring the exposure rate over a wide
range of X-ray energies.
6.9.10.3
Recommended Instruments for Exposure Measurements.
CAUTION
Under no circumstances shall Geiger-Mueller (GM) tube type instruments such as
the AN/PDR-27, AN/PDR-77, and ADM-300, and AN/VDR-2 be used during X-
ray operations or X-ray radiation protection surveys. The response of GM-type
instruments to the relatively low effective energies typical of X-ray operations is
extremely variable. This extreme variability together with lack of adequate
response to low X-ray energies could lead to serious personnel overexposures. For
example, at 32 keV the AN/PDR-27 measures only 1% of the true exposure rate.
