Environmental Health and Safety

Radiation Safety Training Manual

Work Practices

1. ALARA

It is the responsibility of every radiation worker to keep their exposure to radiation as low as reasonably achievable or ALARA. Three ways to reduce your exposure are time, distance and shielding. Minimize your time in areas where exposure levels are likely to exceed background. Maximize your distance from sources of radiation. Exposure levels from x or gamma ray sources are reduced inversely, proportional with the square of the distance. Doubling your distance will reduce the exposure rate to 1 fourth (X ≈ 1/D2). Remember to use shielding which is appropriate for the type of radiation.

Note: It is important to keep in mind that time, distance and shielding all work together to reduce your exposure, but depending on the situation, one method may be more practical and effective than another.

2. Signs and Labels

All entries to laboratories where radioactive materials are approved to be used or stored are posted by EH&S with a sign stating “Caution Radioactive Materials” and displaying the radiation emblem. The purpose of the signs is to alert persons to the possible presence of radioactive materials. Persons entering these areas should not touch objects unless they know them to be free from contamination. These areas do not pose a radiation hazard to individuals by their presence in the area.

When radioactive material is removed from the stock container, an obvious indication must be made to alert other persons that radioactivity is present. Typically, adhesive tape which is imprinted with the words “radioactive material” is used to denote work spaces or items which contain radioactivity.

Areas which have levels of radiation warranting restricted access are labeled as “Radiation Areas.” Access to radiation areas is limited to authorized persons who have been issued dosimetry.

3. Protective Apparel

Whenever working with loose radioactive material, you are required to wear disposable gloves and a lab coat. The benefit of gloves as contamination control apparel is defeated as soon as you handle any clean item with contaminated gloves. Gloves should never be worn while handling items expected to be clean. Wearing sandals or open toed shoes is always discouraged. Your feet should be covered to prevent skin contamination from spills or splashes.

If you discover that your lab coat is contaminated, it should either be disposed as solid radioactive waste or placed in a bag for decay. Advise the RSO if your lab coat is contaminated to determine if decay is appropriate and to verify that your skin has not been contaminated.

4. Process Area

It is prudent practice to restrict the dispensation and manipulation of loose radioactive materials to as few locations within the lab as possible. The process areas should have laminated absorbent paper on the lab bench to insulate the surface from contamination and limit the spread of spills. The area should be identified as a process area by defining the perimeter with caution tape or other obvious indication. Clear definition of the process area is useful in determining where clean items such as notebooks and calculators may be kept. Items which are expected to be clean should never be within the process area. Shielding material which is appropriate to the type of radiation is to be present in the process area. Although the process area is where you would expect to find radioactivity, perpetual contamination should not be allowed.

5. Monitoring

Post procedure monitoring is required of radiation workers following the completion of a procedure with loose radioactive materials. With the exception of tritium, the laboratory should have a portable instrument to check for contamination. The purpose of post procedure monitoring is to identify and remove contamination. It is the responsibility of each radiation worker to perform decontamination when contamination is found. After cleaning the area, some residual contamination may still be present. Wipes of the surface may need to be taken to determine if the contamination is “fixed” (not removable). If fixed contamination is present, the area should be labeled as radioactive with the count rate or activity, the date and isotope, if known.

6. Detectors

When using a Geiger counter or other count rate instruments, it is important to remember it should not be used to quantify the amount of radioactive material, but simply to establish the presence of it. Its function is to count ionization events and it is calibrated for count rate measurements. The count rate observed, usually expressed in counts per minute (CPM), will always be less than the actual emission or disintegration rate of the material. The ratio of count rate observed to disintegration rate of the material is the efficiency of the instrument for that isotope. The efficiency of the instrument to detect the presence of radiation varies with the type of radiation and its energy. The efficiency of a portable count rate instrument will never approach 100‰. The table below lists commonly used radioisotopes at SDSU and their approximate detection efficiencies.

Detector Efficiency = CPM/DPM x 100 (Expressed as a percentage)

Conventional portable instruments are incapable of detecting the presence of tritium.

If a procedure involves the use of tritium, wipes will have to be taken of the area to determine the presence of loose contamination. The wipes must be evaluated using a liquid scintillation counter (LSC).

Before using a contamination monitor, make sure that there is a current calibration sticker and that the batteries are good. Take note of the background count rate before proceeding. When scanning surfaces, the instrument should be set to the lowest counting scale possible. Typically, for Geiger counters, the background rate varies between 20 and 80 counts per minute. The probe should be held close to the surface being checked but not in contact with it. Move the probe slowly to check other areas of interest, no faster than 2 inches per second. Areas which have a count rate twice the background rate should be considered contaminated and cleaned. When contamination is discovered it should be cleaned up immediately and rechecked.

If a laboratory has an exemption to permit food and beverages in an isolated section of the lab, further contamination checks are required. Consult the Radiation Safety Office for details regarding the clean area policy.

Radiation Safety Staff performs periodic monitoring of laboratories to supplement the efforts of laboratory personnel. Monitoring frequency is determined by the radiological safety index listed on the RUA and laboratory history.

7. Inventory of Radioactive Material

An inventory log must be maintained by authorized users of all radioactive material received under the Radiation Use Authorization. The receipt date, isotope, activity and chemical form must be stated on the inventory. The log entry should be done promptly upon receipt of the material. In addition, each removal of radioactive material from the stock container must be indicated by noting the date of removal, the activity removed and the initials of the person removing the material. If volumetric units are used to track usage, the concentration (activity per unit volume) must be indicated. The records must be legible and clearly comprehended by persons not affiliated with the lab. Records must be maintained by the laboratory for at least a year.

At the beginning of each calendar quarter the Radiation Safety Office distributes an inventory log of all radioactive materials received by each PI. Using the inventory record maintained by the lab, the inventory routed to the PI must be amended to reflect usage of radioactive material.