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Use of Radiation Detectors in Laboratory Environments
By: Paul R. Steinmeyer

January 2006

Introduction

Radioactive materials find use in many laboratory situations. These materials are almost always licensed, whether by the federal Nuclear Regulatory Commission or a local State agency, and as such surveys must be performed. The purpose of these radiological assessments is to detect whether radioactive materials have spread to locations where they should not exist, i.e., become “contamination.” Either regulation or the specific radioactive materials license specifies the type and frequency of surveys that must be performed. Note that the surveys described in the license effectively are the law for your facility and failure to perform them, even if no radioactive materials are currently being used, is a violation.

The isotopes used most often in research are often referred to as “CHIPS.” This is an acronym for the radioactive elements used: Carbon-14, Hydrogen-3 (also called tritium), Iodine-125 (also Iodine-131), Phosphorus-32 (also Phosphorus-33), and Sulfur-35. There are hundreds of additional radionuclides that could potentially exist at a given laboratory. Devices that generate x rays are also common.

The radioactive materials discussed here emit beta, gamma, or a combination of beta and gamma radiation. 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—exposure to x rays does NOT in turn make a material radioactive!

Isotopes also exist that emit alpha only, gamma plus alpha, and gamma plus neutrons but their detection is beyond the scope of this article. However, the general principles discussed here will still apply.

We can broadly categorize radionuclides based on the type of radiation they emit. This will also determine the class of radiation detector required to detect them. These categories are gamma (I-131 from the list above), low-energy gamma and x ray (I-125), beta (C-14, S-35, P-32, P-33), and low-energy beta (tritium, Ni-63). There is crossover among these categories. For example, I-131 is listed here as a gamma emitter but it also simultaneously emits beta radiation. In another example of crossover, C-14 is categorized as a beta emitter but is low enough in energy that it could arguably be placed in the low-energy beta category. X rays would fall under the gamma class.

General Area Surveys
Radiological surveys come in three flavors. 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 such as the Ludlum Model 19 MicroR and the Bicron MicroRem 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. Prime examples of GM detectors are the SE International Inspector and the Ludlum Model 3 with a 44-9 probe.

Wipe Surveys
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 (occasionally called a “smear”) 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; it should never be moistened with water or alcohol except in special circumstances.

The wipes must then be analyzed for activity by an appropriate instrument. For low energy beta emitters (tritium and Ni-63), the only appropriate instrument is a liquid scintillation counter (LSC) These are rather large laboratory instruments in which each wipe (or other sample) is placed into a small vial, scintillation cocktail is added, and the vials are loaded into the LSC.

LSCs are extremely expensive; if your facility has an LSC it is likely that assaying wipe samples is a secondary task. If you use these isotopes and do not have an LSC then you will need to have the samples analyzed by and outside laboratory. LSCs have an additional advantage in that they are sensitive to pretty much all radioisotopes, not just the low energy beta emitters.

Wipe samples for low-energy gamma emitters, specifically I-125, can also be assessed using a thin-crystal scintillation detector. These detectors use a scintillation crystal such as thallium-activated sodium iodide (NaI[Tl]) with a thickness of only a millimeter or so. This optimizes them for detecting low-energy gammas while minimizing interference from higher energies.

Beta emitters are best assessed, assuming no LSC is available, using a GM thin-entrance window “pancake” detector. The SE International Inspector has a GM pancake detector integrated into the unit, and the Ludlum 44-9 is an external probe that contains a GM pancake detector.

The wipes must be placed at a fixed distance from the detector window, usually 1 cm. A Wipe Test Plate is available for the SE International Inspector that 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.

There is no common instrumentation available for routine direct measurement of low-energy beta emitters. For low-energy gamma emitters, only a thin-crystal scintillation detector is appropriate. GM pancake detectors work very well for just about all other isotopes.

In performing the surveys, the window 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.

Multi Channel Analyzers
In some instances where gamma-emitting nuclides are used, it is desirable to know which specific isotope is present in a given location (or sample). A Multi Channel Analyzer (MCA, also sometimes called a gamma spectrum analyzer) detects the individual gamma energies present into a “signature” used to identify specific isotopes, analogous to a chromatograph. A full discussion of the “ins and outs” of gamma spectroscopy is outside the scope of this brief article, but the SE International URSA-II is probably the least expensive and most versatile.

Summary and Conclusion
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.

About the Author
Paul R. Steinmeyer is a health physicist and Vice President, Operations for Radiation Safety Associates, Inc. of Hebron, Connecticut. He has performed consulting, radiation surveys, decontamination, and training throughout the United States for the past 18 years. He currently spends most of his time designing radiological instrumentation and software. Paul co-authored Mathematics Review for Health Physics Technicians and has been published in several journals including Radiation Protection Management and Radiation Safety Officer Magazine.