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.
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:
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
(1) surface contamination, which can be spread by contact;
contamination, which can be spread by grinding or burning, by air currents, and
(3) hot particles, which are small pieces of radioactive
material with a very high radioactivity level. Hot
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
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.