Sealed Source Users Instruction Course Material
Sealed source use at CU
State and federal regulations control the use of radioactive materials at the University of Colorado. The University has been issued a license that allows the use of radioactive materials and also requires the University to control and monitor the use of these materials. The safe use of radioactive materials is best accomplished when the end user and radiation safety personnel act in cooperation
Sealed sources are radioactive materials sealed inside metal or plastic and can take many different forms. All forms share some type of encapsulation that prevents their radioactive contents from leaking or dispersing – barring tampering or a severe accident. In some forms, the radioactive material is an inherent part of the source and cannot be separated. Almost all “sealed sources” can be handled without concern that the radioactive material will rub-off or be dispersed onto hands or clothing. There is, however, reason to be concerned about exposure to the radiation emitted from the sealed source.
Sealed source forms include:
- Plated sources
- In this form, the radioactive material coats a disk or planchette. This coating may be covered, depending upon the type of radiation, by mylar, alumnium, steel, or plastic.
- In this form, a capsule usually made of metal surrounds the radioactive material. These sources are often placed onto the end of metal or plastic handling rods. Another example of a capsule is when a mixture of radioactive compounds is placed into a container and welded or sealed closed.
- Activated metal
- In this form, a metal wire or foil has been exposed to a neutron flux to irradiate the metal and create a radioactive isotope from the original material. This form of sealed source may have a plastic or epoxy coating to protect the activated metal. In some instances, however, the metal is not protected.
Many commonly used laboratory devices also contain sealed sources, such as gas chromatographs with electron capture detectors, liquid scintillation detectors, and static eliminators.
Authorized users of sealed sources
In order to possess or use radioactive sealed sources (or devices containing sealed sources) at the University of Colorado, your Principal Investigator (or P.I., or “Licensee”) must have a radioactive materials license approved by the CU Radiation Safety Committee.
To obtain a license, your P.I. must possess minimum experience and training requirements. This training packet is one of those requirements. The P.I. and everyone in a laboratory must satisfactorily complete the quiz associated with this packet before they begin to work with sealed sources. This training must be refreshed every three years.
By issuing a radioactive materials license to your P.I., the Radiation Safety Committee recognizes that your P.I. has assumed certain responsibilities, including assuring that everyone in the lab will have the training and equipment necessary to safely use the radioactive source(s).
The safe use of sealed sources
Sealed sources present an external radiation hazard as opposed to a contamination hazard. Sealed sources can emit any type of ionizing radiation, including alpha particles, beta particles, gamma rays, x-rays, or neutrons.
When working with any radioactive materials, the principal concern is controlling exposure to radiation. Since any radiation exposure presumably involves some risk to the individual involved, the level of exposure received should be worth the result that is achieved. In principle, the objective of radiation protection is to balance the risks versus the benefits from activities that involve radiation. Different uses of ionizing radiation warrant consideration of different exposure guidelines or means to reduce exposure.
An essential facet of radiation protection practices is the ALARA (As Low As Is Reasonably Achievable) philosophy. The ALARA concept gives primary importance to the principle that exposure should always be kept as low as practicable.
There are several simple ways that radiation exposure can be reduced when working in the lab. The main principles are Time, Distance, and Shielding. Each is described below.
- When working with radiation you should be aware that radiation exposure is directly proportional to the time spent in the field. If the time spent in a given radiation field is doubled, the workers exposure is doubled. What thought processes or actions will assist with this exposure reduction tool? These might include: not removing the source from its storage area until the last possible moment, thoroughly understanding the experiment by completing several “dry-runs” prior to introducing the source to the experiment, and understanding where the radiation beam is present, i.e.:
- Does the radiation stream from the source in 360 degrees or is the beam collimated in a particular direction?
- Does the experimental apparatus include adequate “beam stops”?
- Is the source removed from the experiment at the earliest opportunity?
- Has everyone who might come into contact with the experiment or work in the vicinity been informed of the presence of radiation?
- In many cases this control is more important than controlling time in the radiation field. For those sources that can be treated as a “point” source (i.e. the distance between you and the source is greater than the largest diameter of the source), the dose received is inversely proportional to the SQUARE of the distance of separation. Thus the distance of separation between a person and a source has a greater relative influence on dose than does the time factor.
- As an example, consider a person who is exposed to a radioactive source for 5 seconds at one meter or for 10 seconds at two meters. Which condition would provide the lowest exposure? If we consider that Exposure = (Time)/(Distance)2.
- For the one-meter example: Exposure = 5/(1)2 = 5 units.
- For the two-meter example: Exposure = 10/(2)2 = 2.5 units. Clearly, the two-meter distance leaves the worker with the lower radiation exposure.
- What thought processes or actions will assist with this exposure reduction tool? They might include the use of handling tongs to handle sources (this should only be done, of course, when it does not jeopardize the integrity of the source or create handling problems) and standing well away from the radiation beam or source.
- A simple, yet effective, way of reducing radiation exposure in conjunction with or when the previous methods cannot be used is by placing appropriate shielding between you and the source. To properly utilize this method, it is necessary to understand what shields work best for which types of radiation. Selecting the most appropriate shielding material depends upon a number of factors including the type of radiation, the energy of the radiation and the density of the materials used as shielding. For this reason, Health Physics should be involved in helping your laboratory design necessary shielding for your source(s).
- Shielding pure alpha sources is not necessary in the laboratory (keep in mind, however, that many alpha sources often have a secondary gamma or x-ray emission associated with them). While alpha particles can be quite hazardous if ingested, nearly all alpha radiation will be stopped by the dead layer of skin on your body and therefore is not a concern from an external standpoint.
- Shielding beta sources is best accomplished with low atomic number materials. Less dense materials such as Plexiglas or plastic provide nominally thick shields that attenuate the beta particles and minimize the production of Bremsstrahlung x-ray radiation (a type of x-ray produced by stopping beta particles with dense materials). All but the most energetic beta sources can be adequately shielded with a centimeter or two of Plexiglas. Dense materials such as lead and steel are discouraged for use with beta sources and can actually increase your radiation exposure due to the production of the Bremsstrahlung x-rays. Beta particles can penetrate up to a centimeter into your skin and so should be considered an external radiation exposure hazard.
- When shielding gamma or x-ray sources, the energy of the source and the density of the shielding material must be taken into consideration. For all but the most energetic sources, thin lead foil or moderately thick lead sheet can be used to shield the source. Metals such as steel or iron can also be used to shield gamma or x-ray sources. The exact design of gamma shielding can be quite complex and surveys may need to be performed to ensure the adequacy of the shield design. Gamma rays and x-rays emissions are very penetrating to the human body and are a significant external radiation exposure hazard.
All users of sealed sources or machines containing sealed sources are required to complete this packet and quiz before using the radiation. In addition, this packet and quiz must be re-taken every three years as a refresher. This is a requirement of the campus-wide Radiation Safety Committee and is a part of your P.I.’s radioactive materials license.
Anyone working with radiation should be familiar with the experimental procedures for which radiation will be used. The Licensee should provide these procedures, in written form, to each person involved in the experiment. If you have any questions about proper operating procedures for working with radiation or if you have questions about the services provided by Health Physics please refer to the Radiation Safety Handbook that is available in each laboratory or call Health Physics at (303) 492-6523.
Sources at the University of Colorado are divided into two sections: Tier I and Tier II. Tier I sources must be leak-tested periodically by Health Physics (either every three months or every six months, depending on the source), according to state regulations. Both Tier I and Tier II sources are inventoried every six months by Health Physics.
All sealed sources should be maintained within storage cabinets, safes, or at a designated location within the laboratory. Each storage location should have an inventory, provided by Health Physics, of the sources stored at that site. Whether or not the storage location should contain shielding material depends on the type of radiation emitted by the source(s). For many gamma or x-ray emitters, some form of shielding integral to the source holder or maintained within the storage cabinet itself will be necessary.
Each time a source is removed from its storage location, it must be documented on the Sealed Source Sign-out Log, kept near the storage location. Sources may only be used for a maximum of three months at a time and may only be used within the same building (preferably the same room). Sources should be returned to the locked cabinet when not in active use in order to lessen the risk of losing the source. The log should be updated when the source is returned to its storage location.
Only Health Physics employees may transport radioactive sources between non-contiguous buildings. As much time as possible (at least 4-6 weeks) should be given to Health Physics prior to the transport to allow for preparation of required paperwork and transport conditions.
Each laboratory using or storing radioactive materials at the University of Colorado is required to post warning signs such that members of the public (or students) will be aware of a radiation hazard in their vicinity. For some sources located in machines, the visual warning may be applied to the machine as opposed to the outer door of the laboratory.
Signs on laboratory doors or machines must contain the universal tri-foil radiation symbol and the words “Caution — Radioactive Materials” in the colors magenta (red) and yellow. Signs are available from Health Physics.
Outer doors to laboratories should also have a white and red “Emergency Notification” sticker listing emergency contacts in the case of a fire or other problem, including a NON life-threatening emergency.
Leak Tests & Inventory of Sources
The degree of regulatory control for sealed sources is dependent upon the type of radiation emitted and the activity of the source. Tier I sources are leak-tested by Health Physics at certain intervals determined by regulation. Alpha emitters are leak-tested every three months, while beta and gamma emitters are leak-tested every six months. Because of this, the recommended use period for Tier I and II sources is no more than 3 months.
Leak tests are accomplished by wiping a piece of filter paper (“wipe smear”) across a source or source housing. Radioactive contamination leaking from the source can then be measured by analyzing the filter paper in a liquid scintillation counter.
The following limits are used for leak-tests at the University of Colorado:
- No action is taken if a leak-test on a source reveals contamination below 0.001 uCi per smear
- If source wipes reveal contamination above 0.001 uCi per smear, but below 0.005 uCi per smear, Health Physics recommends that the source be removed from use.
- If the leak-test reveals contamination above 0.005 uCi per smear, the source is required to be removed from use and either repaired or disposed of appropriately. This level of contamination requires notification of state regulatory agencies.
At the same time leak tests are taken, Tier I and Tier II sources are inventoried by Health Physics. If a source is found to be missing, Health Physics must be notified as soon as possible so that a search for the source can begin. The loss of most sources requires notification of state regulatory agencies. An inventory with current source activities is provided each time that Health Physics inventories a storage location.
Dose and Exposure Control
Allowed levels of radiation exposure to laboratory personnel using radioactive sources at the University of Colorado is governed by federal regulations. The measurement of biological effect of radiation requires that a quality factor be used to take into account the different degrees of biological damage produced by equal doses of different types of radiation. This dose equivalent is typically measured in units of rem or Sieverts. The maximum whole body exposure for occupationally exposed personnel is 5 rem per year. For comparison, the average whole body exposure to all CU radiation safety workers in 2007 was 15.85 millirem (there are 1,000 millirem per rem). At the same time, the average amount of background radiation from cosmic and man-made sources in the United States is 360 mrem/year. The occupational limit is for radiation dose above the background level.
Regulations require that anyone likely to receive more than 10% of the allowable dose limit must be provided with an exposure-monitoring device (or dosimeter). At the University, this means that anyone working with radioactive materials energetic enough to penetrate through the epidermal layer of skin are provided with a dosimeter. Please contact Health Physics at (303) 492-6523 for assistance in determining your requirement to be monitored for radiation exposure.
Whole body dosimeters are correctly worn on the front of the body, between the neck and the waist. Extremity (ring) dosimeters should be worn on the hand most likely to come into contact with radiation. The white chip should face toward the inside of the palm. Dosimeters do not protect anyone from radiation effects and nor do they provide instant feedback on dose level, but rather are used as part of an ongoing program of monitoring occupational dose to radiation.
Contamination is not normally an issue when working with sealed sources and most can be handled without concern for the contamination of skin and clothing. However, if there is any reason to suspect that a source is leaking or that contamination is present in the laboratory, a wipe smear or a survey instrument (such as a geiger counter) may be used to detect radioactive contamination. All survey instruments at the University of Colorado are required calibrated every annually to ensure accurate measurements. Wipe smears are available from Health Physics, as are suggestions for types of survey instruments which would be appropriate for use in your laboratory and calibration services.
If a situation representing a life-threatening emergency occurs in the lab, lab personnel should call 911 from a safe telephone. If the situation is not a life-threatening emergency, Health Physics should be contacted at (303) 492-6523. Health Physics should be notified of any emergency involving radioactive materials, including but not limited to: radioactive contamination of a laboratory, loss of a radioactive source, or overexposure of a person to radiation. After normal business hours, Health Physics personnel can be reached by calling the CU-Boulder Police Department at (303) 492-6666 (or 911 from the Boulder campus).Take Quiz