Using radiation surveys to demonstrate license compliance
December 3, 2008
Radiation and radioactive materials are often used in industry. Some of these uses include (but are by no means limited to) density gauges, sterilization, quality control and nondestructive testing. Depending on the amount and type of radioactivity and radiation used, the facility may operate under a license from the Nuclear Regulatory Agency or a local state agency.
Radiation surveys demonstrate license compliance, that both workers and the general public are not exposed to unacceptable levels of radiation, and that the devices themselves are not unwittingly releasing radioactive material.
The radioactive materials discussed here are used for the gamma radiation they emit. Beta particles happen to accompany the gammas in many of the sources used in these applications (e.g. Cesiums-137, Cobalt-60). X radiation can be treated as gamma radiation; the only difference between gamma and x radiation is that gamma emanates from the nucleus of an atom while x-rays are produced outside the nucleus. Of course, with x-ray machines, the radiation goes away when the beam is powered off â€” exposure to x-rays does NOT in turn make a material radioactive.
Radiological surveys come in three flavors: general area surveys, wipe surveys, and total surface contamination surveys.
General area surveys
The first of these is a “general area” survey and really only applies to gamma (and x) radiation. The basic purpose is to assess the potential for dose to humans who may be in proximity to these areas. The other types of radiation simply don’t (generally) have the range in air to pose a credible dose scenario.
A radiation detector capable of measuring gamma dose rate (or more often, exposure rate) must be used. The displayed units will usually be mrem/h or µrem/h (milli- or micro-rem per hour) for dose rate or mR/h or µR/h (milli- or microroentgens per hour) for exposure rate. There are real differences between the meanings of dose and exposure, but for demonstrating compliance with regulatory limits, it is accepted practice to consider them equivalent.
In performing the survey, the instrument is held at about waist level and the total reading is recorded. Background radiation, usually in the range of 10 to 20 µR/h, is not subtracted, but is measured and recorded on the survey form. Many instruments (those with “thin windows”) used for measuring gamma radiation are also sensitive to beta radiation. It is not appropriate to directly measure emissions from a beta source and report that value as “exposure rate.”
Measuring exposure rate can be performed using ion chambers, scintillation detectors, and Geiger-Mueller detectors. Ion chambers, while they have many desirable characteristics, are generally geared for relatively high levels of radiation and are therefore not appropriate for laboratory use.
Scintillation detectors are excellent instruments for measuring exposure/dose rate, especially at the low (i.e., approaching background levels) we are typically interested in. They tend to be a little expensive, however, and do not have additional application in the other types of surveys.
Geiger-Mueller (GM) detectors are the most often used because of their versatility for additional types of surveys and relative low cost.
The second flavor of radiological survey is the “wipe” survey. The basic purpose of a wipe survey is to assess removable surface contamination that could potentially become airborne, spread to uncontrolled areas, or become eventually ingested through contact. Wipe surveys are performed by wiping a paper or cotton disk on the surface to be assessed. An area of 100 cm2 should be wiped, as contamination limits are given in units of dpm/100 cm2 (for disintegrations per minute per 100 cm2). This area can be approximated by rubbing the wipe in an “S” shape about 7 to 10 inches long. Only moderate pressure should be applied and the wipe should be dry.
The wipes must then be analyzed for activity by an appropriate instrument. Most beta/gamma emitters can be assessed using a GM thin-entrance window “pancake” detector.
The wipes must be placed at a fixed distance from the detector window, usually 1 cm. A wipe test plate accomplishes this fixed counting geometry. Activity on the sample is calculated by taking the total counts per minute from the sample reading, subtracting the background counts per minute, then dividing the difference by the instrument’s efficiency for the beta particle energy you are measuring. This efficiency is obtained from the instrument’s calibration certificate, and the final results are now in the appropriate units of dpm/100 cm2.
Total surface contamination surveys
The third and final flavor of radiological survey is the “total surface contamination” survey, performed by direct measurement of a surface. The direct measurement represents the total of both fixed and removable contamination.
In performing the surveys, the active face of the detector is held at a fixed distance from the surface to be surveyed (usually 1 cm), and then scanned slowly across the surface at a rate of no more than 2 inches per second. The surveyor is looking for a significant increase in counting rate. Activity is calculated in the same manner as for wipe surveys, only the units will be dpm/detector area.
I’ve only touched on the basics of radiological surveys and the most common instruments, but regardless of the specific instrument, the same principles remain intact. Radiation surveys must be performed with an instrument capable of detecting the specific radioactive material(s) being used.
In some cases, this may mean that the same area will need to be surveyed more than once using additional instrumentation. In all cases, the instrumentation used must have been calibrated within the previous 12 months in order for the survey data to be considered valid by any regulatory authority.