Radiation Safety Office

SECTION V

RADIATION MONITORING AND DETECTION PRINCIPALS

A. DETECTION OF FIXED AND REMOVABLE CONTAMINATION

In facilities where radioactive materials are handled, personnel should regularly monitor themselves and their work area for fixed and removable contamination in order to:

Ensure that contamination control is maintained;
Ensure that contamination is not transferred to non-radioactive areas;
Provide feedback as to the effectiveness of contamination control measures;
Prevent unnecessary personnel exposure resulting from intake of contamination.

Contamination can be removable, fixed, or a combination of the two. Removable contamination represents a greater hazard. Methods of detection should be used which can detect fixed or removable contamination, and differentiate between the two.

The appropriate frequency and thoroughness of monitoring will depend on a variety of factors, including levels of activity handled, degree of containment, and control exercised. Individuals should monitor themselves and their clothing during and after each use. Laboratories where radioactive materials are used should be surveyed weekly (see program requirements in VII F). Facilities that are shared with other users should be surveyed before and after each use.

Areas and items that should be monitored include hands, wrists, protective clothing, personal belongings, work surfaces, radioactivity processing and storage areas, heavy traffic areas, untidy areas, and equipment and materials leaving radioactivity handling areas.

B. SELECTION OF APPROPRIATE INSTRUMENTS

Identify those radionuclides which are handled, and select the appropriate instrument(s) based on the need for portability, and the sensitivity of the instrument for the characteristic radiations emitted by the radionuclides in question. Use the following table as a guide.

TABLE 10 INSTRUMENT SELECTION GUIDE
CHARACTERISTIC RADIATION	PORTABLE INSTRUMENTS	NON-PORTABLE INSTRUMENTS
Low energy beta³H, ⁶³Ni	None	Liquid Scintillation Counter
Medium energy beta¹⁴C, ³⁵S	GM Probe (thin window only)	Liquid Scintillation Counter
High energy beta³²P	GM Probe Gamma (NaI) Probe	Liquid Scintillation Counter
Low energy gamma¹²⁵I	GM Probe Gamma (NaI) Probe	Gamma Counter
Med-high energy gamma¹³¹I, ⁵¹Cr	GM Probe Gamma (NaI) Probe	Gamma Counter
Additional instrument selection parameters include background radiation levels, ruggedness needed, and minimum detectibility.
Portable instruments are often dedicated to a monitoring station where wipes and other items are brought for contamination monitoring.
Note: The Radiation Safety Office has available a list of recommended survey instruments for use in isotope laboratories.

C. INSTRUMENT OPERATION

Familiarize yourself with instrument operating instructions. Check for proper instrument function, including recent calibration, battery strength or operating voltage, and response to check sources.

D. MONITORING FOR CONTAMINATION

To directly monitor a surface for contamination, bring the probe to within about one-half inch of the surface, being careful not to damage or contaminate the probe. If available, use the audio output to quickly locate contamination. Move the probe slowly over the surface (3-4 in/sec). Adjust meter range selector and observe the reading from a point directly above the meter face to ensure proper alignment of needle and scale. Since the readout will fluctuate, look for the average value. High background levels may be overcome by partially shielding the probe.

To test for removable contamination, for example on surfaces previously found contaminated with a portable survey meter, firmly smear the surface in question with a paper filter circle or any clean paper towel. A 1 inch diameter filter is a convenient and commonly used size. Avoid touching potentially contaminated surfaces and wear gloves. Wipes can then be totally immersed in an appropriate scintillation cocktail, dirty side out, agitated, and counted by liquid scintillation counting.

Alternately, bring the wipe to the window of a GM or NaI probe and rate-meter and note any increase in the count rate. Do not move the probe if doing so causes a change in the background count rate. Wiping a standard surface area, such as 100 cm², yields results which can be compared from survey to survey. A wipe survey is a qualitative, not a quantitative indication of surface contamination. Maintaining a uniformity in the procedure will permit comparisons between wipes and between surveys.

The amount of radioactivity that has been detected by direct monitoring or by wipe survey can be readily estimated by subtracting the background count rate (bkg counts/min) from the observed count rate (gross counts/min) and then dividing the net count rate by the counting efficiency (counts/disintegration) for the radionuclide in question.

net disintegrations/min (dpm) = (gross cpm)-(bkg cpm) / efficiency

Always express contamination levels in standard units, such as dpm or microcuries/100 cm². If the identity of the radionuclide detected is not known, the most conservative (lowest) efficiency of the possible radionuclides should be used.

Section VI
Table of Contents

Radiation Safety Office
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Revised February 13, 2006