Cr-51 Standard Operating Procedures (Version: May 1997)

Introduction

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.

Concerns

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 ALARA quarterly limit of 1,875 millirem for the hands would be reached in 10 hours.
  • The quarterly ALARA limit of 125 millirem for the whole body (assume 3 feet from the vial) would be reached in 86 minutes.

Shielding

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%.

Detection

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

Equipment / Supplies

The following equipment and supplies must be available:

  • Geiger Counter with a Sodium Iodide crystal or GM tube.
  • Lead foil or bricks for shielding.
  • 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".
  • Absorbent bench covering or tray.

Safety Rules

If the following safety precautions are used, personnel radiation exposure will be as low as reasonably achievable.

  1. Designate a specific area of the lab for Cr-51 handling.
  2. 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.
  3. All persons handling Cr-51 must wear a whole body film badge and a ring badge on the hand which is most frequently used to handle vials, samples, pipettes, etc. Containing Cr-51.
  4. Full-length lab coats must be worn by all persons who handle Cr-51.
  5. Protect your hands from becoming contaminated from spills by wearing two pairs of disposable gloves with ring badge inside.
  6. 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.
  7. 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.
  8. Do not work directly over an open container of Cr-51.
  9. Never pipette Cr-51 or any radionuclide by mouth.
  10. 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.)
  11. Check your gloves frequently for contamination with a Geiger Counter. (If contamination is found, immediately dispose of the gloves in the radioactive waste container.)

Post-Use Procedures

After handling Cr-51:

  • Use the sodium iodide detector or 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.
  • Call the RSO if you have any questions about where to survey.
  • Wash your hands thoroughly.
  • Bring the waste to the Radwaste Room on Thursdays or in an emergency at other times by arrangement. The RSO will store the waste until it has decayed sufficiently to be disposed

Any questions about these procedures?

Call the Radiation Safety Office: 464-6510


APPENDIX A
DECAY RATE OF Cr-51
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
96 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