T.O. 33B-1-16-125an individuals dose. Occupationally exposed personnel shall NOT wear their dosimetry devices while undergoingmedical or dental X-ray procedures.6.9.10 MeasuringExposuresRates:IonizationChamberTypeSurveyInstruments.6.9.10.1 BasicOperatingPrinciple.Radiation exposure is most accurately measured with ionization chamber type survey instruments. These detectors usean air filled chamber across which an electric field is applied. When X-ray or gamma radiation interacts with the air inthe chamber, it creates positive and negative ions that drift apart under the influence of the electric field. As the ionsare collected on the electrodes within the chamber, a small current is generated which is measured by the instrumentand 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, allsurvey instruments used for industrial radiography "shall have a range such that two milliroentgens per hour throughone roentgen per hour can be measured" (10 CFR 34.24).6.9.10.2.1Portable 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 radiationbeam, 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 surveyinstruments. Instrument directional dependence is negligible when the instrument's sensitive volume is pointed in thedirection of the radiation origin. Instrument susceptibility to radiofrequency radiation (RFR) may significantly affectionizing radiation measurements in the presence of RF radiation. If RF interference is suspected, it can often beconfirmed by placing a piece of leaded (Pb) rubber or similar shielding material over the ionization chamber of theinstrument to filter out the gamma or X-radiation, while observing the instrument reading. If no change is noticed inthe reading when the lead is placed over the chamber, the reading obtained previously was due primarily to RFinterference.6.9.10.2.2An X-ray machine operating at a given tube potential (kVp) produces a spectrum of X-ray energies. Since industrialX-ray machines do not contain primary beam filtration (except the X-ray tube window), the X-ray spectrum contains arelatively 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-raymachines be energy independent or, in other words, is capable of accurately measuring the exposure rate over a widerange of X-ray energies.6.9.10.3 RecommendedInstrumentsforExposureMeasurements.CAUTIONUnder no circumstances shall Geiger-Mueller (GM) tube type instruments such asthe 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-typeinstruments to the relatively low effective energies typical of X-ray operations isextremely variable. This extreme variability together with lack of adequateresponse to low X-ray energies could lead to serious personnel overexposures. Forexample, at 32 keV the AN/PDR-27 measures only 1% of the true exposure rate.
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
