STANDARD OPERATING PROCEDURES
Chromium-51
version: September 26, 1991
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 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.
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) detector.
EQUIPMENT / SUPPLIES
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".
SAFETY RULES
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.)
POST-USE PROCEDURES
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.
º Any questions about these procedures ?
Call the Radiation Safety Office, 656-2570 º
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
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