Carbon-14 is a commonly used radionuclide with a half-life of 5730 years, emitting only beta particles with a maximum energy of 0.156 MeV (Million Electron Volts) and an average energy of 0.049 MeV. The beta particles from C-14 travel a maximum of 22 cm.
The major concern with using C-14 is that it cannot be easily monitored during its use, therefore, special precautions are needed to keep the work environment clean. The regular use of wipe testing is the only way to insure that your work space is not contaminated. Contamination on the skin will not likely cause a significant dose to the dead layer of skin, however, it could lead to the internal absorption of C-14. The maximum permissible body burden to the whole body is 0.4 mCi.
Glass and plastic are the best shields for beta particles from C-14.
A tiny drop of contamination from C-14 can be easily detected with a wipe test from a Liquid Scintillation Counter. Most Geiger Counters will not efficiently detect the presence of C-14.
The following equipment and supplies must be available:
- A Liquid Scintillation Detector.
- Disposable latex or plastic gloves.
- A full-length lab coat.
- Radioactive waste receptacle
- Pipettes dedicated to the use of C-14.
- Commercial decontaminate
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 C-14 handling
- Full-length lab coats must be worn by all persons who handle C-14
- Protect your hands from becoming contaminated from spills by wearing two pairs of disposable gloves
- Never pipette C-14 by mouth
- Only use pipettes which have been dedicated to your specific use of C-14
- Pipettes will easily become contaminated and therefore, should not be shared with others
- If you have reason to believe that your gloves are contaminated, immediately dispose of them in the radioactive waste container
- Conduct a wipe test and count the wipes in a Liquid Scintillation counter
- Check all equipment, centrifuges, water baths for contamination
- If any contamination is found, use a commercial radiation contamination remover 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 C-14, maximum cpm found, and the date you measured the level
- Check the work bench and floor
- If contamination is found, it can usually be removed easily
- If it cannot be removed, contact the RSO at email@example.com 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 marked with labels "Radioactive Waste-Do Not Empty"
- Wash your hands thoroughly
- Beta Energy: 156 keV (maximum)
- 49 keV (average) (100% abundance)
- Physical Half-Life: 5730 years
- Biological Half-Life: 12 days
- Effective Half-Life: 12 days (Bound)
- Effective Half-Life: 40 days (Unbound)
- Specific Activity: 4460 mCi/gram
- Maximum Beta Range in Air: 24.00 cm
- Maximum Beta Range in Water/Tissue: 0.28 mm1
- Maximum Range in Plexiglas/Lucite/Plastic: 0.25 mm
1Fraction of C-14 beta particles transmitted through dead layer of skin: At 0.007 cm depth = 1%
- Critical Organ: Fat Tissue
- Routes of Intake: Ingestion, Inhalation, Skin Contact
- External exposure: deep dose from weak C-14 beta particles is not a radiological concern
- Internal exposure and contamination: primary radiological concerns
- Committed Dose Equivalent (CDE): 2.08 mrem/uCi (ingested), Fat Tissue: 2.07 mrem/uCi (puncture), 2.09 mrem/uCi (inhalation)
- Committed Effective Dose Equivalent (CEDE): 1.54 mrem/uCi (ingested)
- Annual Limit on Intake (ALI)1: 2 mCi (ingestion of labeled organic compound), 2000 mCi (inhalation of carbon monoxide), 200 mCi (inhalation of carbon dioxide)
- Skin Contamination Dose Rate: 1090-1180 mrem per 1.0 uCi/cm2 (7 mg/cm2 depth)
- Dose Rate to Basal Cells from Skin Contamination 1.0 uCi/cm2 = 1400 mrad/hour
- Immersion in C-14 Contaminated Air = 2.183E7 mrem/year per uCi/cm3 at 70 um depth of tissue and 4.07E6 mrem/year per uCi/cm3 value averaged over dermis
1[1.0 ALI = 2 mCi (ingested C-14 organic compound) = 5,000 mrem CEDE]
- Can detect C-14 using a thin-window G-M survey meter; survey meter probe must be at close range (1 cm)
- G-M survey meters have very low counting efficiency for C-14 (5%)
- Liquid scintillation counter (indirect counting) may be used to detect removable C-14 on wipes
- Not Needed (beta energy too low)
- C-14 Beta Dose Rate: 6.32 rad/hr at 1.0" air per 1.0 mCi C-14
- Skin Contamination Dose Rate: 13.33 mrad/hr per uCi on skin
General Radiological Safety Information
- Urinalysis: Not Required; however, prudent after a C-14 radioactive spill or suspected intake
- Inherent volatility (at STP): Not Significant
- Possibility of organic C-14 compounds being absorbed through gloves
- Care should be taken NOT to generate 14CO2 gas which could be inhaled
- Internal Dose is the concern: Skin contamination, ingestion, inhalation, and puncture
- Always wear a lab coat and disposable gloves when working with C-14
- The concentration of carbon in adipose tissue, including the yellow marrow, is about 3 times the average whole body concentration. No other organ or tissue of the body concentrates stable carbon to any significant extent
- The fractional absorption of dietary carbon (uptake to blood) is usually in excess of 0.90
- Three main classes of carbon compounds may be inhaled: organic compounds, gases (CO or CO2), and aerosols of carbon containing compounds such as carbonates and carbides
- Organic Compounds - Most organic compounds are NOT very volatile under normal circumstances; the probability of these being inhaled as vapors is therefore small. In circumstances where such substances are inhaled, it would be prudent to assume that once they enter the respiratory system they are instantaneously and completely translocated to the systemic circulation without changing their chemical form
- Gases - The inhalation of CO and its retention in body tissues has been studied extensively.
- Since gas has a relatively low solubility in tissue water, doses due to absorbed gas in tissues are insignificant in comparison with doses due to the retention of CO bound to hemoglobin. CO2 in the blood exists mainly as a bicarbonate
- Carbonates & Carbides - It is assumed that inhaled or ingested C-14 labeled compounds are instantaneously and uniformly distributed throughout all organs & tissues of the body where they are retained with a biological half-life of 12-40 days