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Radiation Safety Training Module
Radiation Safety Training
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ISU Technical Safety Office, Campus Box 8106
Pocatello, ID 83209
(208) 282-2311/2310

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interNAL Exposure

The presence of radioactive substances inside the human body generates internal exposure. Surface contamination can generate airborne contamination (particle re-suspension) that in turn, is ingested or inhaled.

Radioactive contamination may be fixed or removable/transferable.

Fixed contamination is not easily transferred from one place to another. It usually becomes fixed by physical or chemical absorption or by entrapment in physical irregularities of the surface material. If only fixed surface contamination is present, it can be quantified. Typically this is done using a Geiger Muller (GM) detector. Often those are referred as GM pancake probes. The information displayed on the meter's scale is in counts per minute (cpm). However, because cpm is strongly dependent on the radionuclide and a source to detector distance, one should compute the surface contamination in disintegration per minute (dpm) which is independent of the type of contaminant. The contamination in dpm is equal to the value in cpm divided by the efficiency (e) of the detector for that type of radiation:

 

dpm=cpm/e

 

It is important to emphasize that the evaluation of contamination depends on the knowledge of two quantities cpm and e. We can read cpm on the meter's scale but we have to know efficiency (e)  from the calibration. Because e has values between 0 and 1 results that dpm > cpm. Only for an efficiency of 100% (hard to be obtained) dpm = cpm.

Portable meters are not of much help in case of low beta energy radionuclides (14C and 32S) and inapplicable in case of tritium (3H).

Removable/transferable contamination may readily be transferred among objects. Cleanup activities where radioactive dust or dirt is present may lead to airborne contamination due to the mechanical action of sweeping or bagging activities. 

Examples of transferable contamination include:

(1) surface contamination, which can be spread by contact; 

(2) airborne contamination, which can be spread by grinding or burning, by air currents, and by evaporation;     
(3) hot particles, which are small pieces of radioactive material with a very high radioactivity level. Hot                  particles may be especially hazardous to the skin or the extremities due to their short range and the                intensity of the radiation emitted.

In research facilities, no removable contamination shall be tolerated indefinitely, therefore a Removable Contamination Limit (RCL) is established for each lab. The RCL in dpm per 100cm2 (shipping regulations use dpm/300cm2 or Bq/cm2) is the maximum amount of removable contamination allowed for each individual radioisotope. The removable contamination is measured by rubbing a filter paper over a  100 cm2 area of the suspected contaminated surface. This survey is referred to swipe or smear test. If both fixed and removable contamination are present, the swipe test will give the removable component only and the GM pancake meter will give the total contamination (fixed plus removable). The RCL is also based on the Annual Limit on Intake (ALI).

If a worker becomes contaminated, a health physicist should be consulted for proper decontamination procedures.  The process is NOT the same as chemical decontamination.  The decontamination methods used depend upon the location and the form of the contamination.  Normal cleaning techniques for external decontamination usually involve washing with soap and lukewarm water, although the aid of other techniques and medical personnel may be needed.  Reduction of internal contamination depends on the radioactive half-life of the particular contaminant and the normal biological elimination processes such as urination, exhalation, defecation, and perspiration. These processes can be enhanced under proper medical supervision.

Prevention of intake of radioactive material
Ingestion of radioactivity must be prevented by avoiding mouth contact with any items handled in a radioactive material laboratory (pipettes, pencils, etc.), by prohibiting eating, drinking and smoking in radionuclide handling areas and by careful attention to personal hygiene.

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