University of Vermont

Cr-51 SOP



version: September 26, 1991


Chromium-51 is a commonly used radionuclide with a half-life of 27.7 days, emitting gamma rays with a maximum energy of 0.320 MeV (Million Electron Volts). See Appendix A for the decay rate information for Cr-51.


The major concern with using Cr-51 is radiation exposure in air over an unshielded vial. The dose rate at the opening of an unshielded vial containing 1 millicurie of Cr-51 can be 180 millirems per hour.

One millicurie = 2.22 x 109 dpm (disintegrations per minute)

This means that the quarterly NRC limit of 18,250 millirems for the hands would be reached in 101 hours.

The quarterly limit of 1,250 millirems for the whole body (assume 3 feet from the vial) would be reached in 62,500 hours.


Lead foil or lead bricks are the best shields for gamma rays from Cr-51. The half value layer for Cr-51 gamma rays in Lead is 1.7 mm., i.e. every 1.7 mm. of Lead reduces the Cr-51 gamma ray beam by 50%.


A tiny drop of contamination containing Cr-51 can be easily detected with a survey meter that has a Sodium Iodide (NaI) detector.


The following equipment and supplies must be available:
  • Geiger Counter with a Sodium Iodide crystal.
    • We recommend the Ludlum Model 3-98 or an equivalent.
  • Lead foil or bricks for shielding.
    • Lead bricks are available from many companies and usually cost $25 - $35 each. The Instrument Model facility (IMF) Office at UVM also has them.
    • Lead foil is fairly inexpensive.
  • Disposable latex or plastic gloves.
  • Film badge and ring badge.
  • Full-length lab coat.
  • radioactive waste receptacle
  • Pipettes dedicated to the use of Cr-51.
  • Commercial decontaminate, i.e. DuPont's "Count Off".


If the following safety precautions are used, personnel radiation exposure will be as low as reasonably achievable.
  • Designate a specific area of the lab for Cr-51 handling.
  • Place the Lead shielding near a wall (not toward another work area on the other side of the bench) away from the main flow of traffic in the lab.
  • All persons in the laboratory must wear a whole body film badge when in the lab, even those who are not handling Cr-51.
  • All persons handling Cr-51 must wear a ring badge on the hand which is most frequently used to handle vials, samples, pipettes, etc. containing Cr-51.
  • Full-length lab coats must be worn by all persons who handle Cr-51.
  • Protect your hands from becoming contaminated from spills by wearing two pairs of disposable gloves.
  • A Geiger Counter must be in operation during the experiment, and preferably at all other times.
    • To avoid contaminating the detector, place a thin sheet of plastic (i.e., Saran Wrap) around the detector
  • Place all vials and test tubes containing 51Cr behind a Lead brick shield.
    • Check the radiation level in front of the shield to determine if additional Lead foil should be added.
  • Do not work directly over an open container of Cr-51.
  • Never pipette Cr-51 by mouth.
  • Only use pipettes which have been dedicated to your specific use of Cr-51. (Pipettes will easily become contaminated and therefore, should not be shared with others.)
  • Check your gloves frequently for contamination with a Geiger Counter. (If contamination is found, immediately dispose of the gloves in the radioactive waste container.)


    After handling Cr-51

    • Use the Geiger Counter to check your hands, lab coat, shoes, clothing, work bench, floor, centrifuges, water baths, and other lab equipment.
    • If any contamination is found on your shoes and/or clothing, contact the Radiation Safety Office (RSO). You will likely have to remove the item temporarily until the radiation decays. The RSO has some disposable clothing that you can wear home. We do not have any disposable shoes.
    • If any contamination is found on your hands, wash thoroughly with soap and water. This will usually be sufficient to remove the surface contamination. If it does not, contact the RSO for assistance.
    • If any contamination is found on work spaces, use a commercial radiation contamination remover (i.e. Count Off) with paper towels to clean up the equipment.
    • Place the towels in the radioactive waste receptacle.
    • If contamination cannot be removed, place a "radiation" label on the equipment indicating that it is Cr-51, maximum cpm found, and the date you measured the level.
    • If contamination cannot be removed, contact the RSO to obtain shielding materials. Inform your fellow lab workers if any unremovable contamination is found.
    • Check the normal trash container to make sure no radioactive waste has been accidentally placed there.
    • Store the waste temporarily in containers which are sufficient to absorb Cr-51's gamma rays.
    • Send a Radiation Contamination Survey Report to the RSO.
    • Call the RSO if you have any questions about where to survey, or how to fill out the form.
    • Wash your hands thoroughly.
    • Bring the waste to the RSO on Fridays. Call 656-2570 for an appointment. The RSO will store the waste until it has decayed sufficiently to be disposed.

    Any questions about these procedures ? Call the Radiation Safety Office, 656-2570



              Days Elapsed      % of Activity Remaining   Decay Factor
                   0                    100.0               1.00
                   1                     97.5               0.975
                   5                     88.2               0.882
                  10                     77.9               0.779
                  15                     68.7               0.687
                  20                     60.6               0.606 
                  25                     53.5               0.535
                  27                     50.9               0.509
                  28                     49.6               0.496
                  30                     47.2               0.472
                  35                     41.7               0.417 
                  40                     36.8               0.368
                  45                     32.4               0.324
                  50                     28.6               0.286 
                  55                     25.3               0.253 
                  60                     22.3               0.223
                  65                     19.7               0.197
                  70                     17.4               0.174 
                  75                     15.3               0.153 
                  80                     13.5               0.135
                  85                     11.9               0.119
                  90                     10.5               0.105
                  95                      9.3               0.093
                 100                      8.2               0.082
                 105                      7.2               0.072
                 110                      6.4               0.064
                 115                      5.6               0.056
                 120                      5.0               0.050
                 125                      4.4               0.044
                 278 (10 half-lives)      0.1               0.001
    For example, if your vial contained 500 microcuries of Cr-51 on 7/1/90, the amount of activity remaining on 7/16/90 (15 elapsed days) would be:
                 Activity  x  Decay Factor 
         = 500 microcuries x 0.687 = 344 microcuries         

    Last modified January 21 2003 09:34 AM

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