Module 7 - Working Safely with Sealed Sources

The sealed and plated sources referred to in this set of modules consist of radioactive material that is encased in metal or plastic (sealed sources) as in fig. 1 or radioactive material that has been plated as a thin film onto metal or plastic (plated sources) as in Fig. 2.  Fig. 3  shows various types of sealed and plated sources. Because the radioactivity is encapsulated or plated onto a surface, sealed sources do not present a significant contamination hazard under normal conditions.  However, sealed and plated sources may present an external exposure hazard, depending on the properties of the radioisotope or the amount of activity present.

While most sealed sources are stand-alone sources used to calibrate or check instrumentation, to irradiate materials, or for experiments in student science labs, sometimes analytical equipment such as gas chromatographs and liquid scintillation counters contain sealed sources.  The persons responsible for such equipment should be aware of the presence of sealed sources.

(Fig. 1)

(Fig. 2)

(Fig. 3)

Decay Schemes

The radioisotopes listed in the following tables are among the most common used in sealed sources at University of Washington.  Detailed fact sheets for each of these radioisotopes, as well as many other radioisotopes, are available upon request from Radiation Safety.

Radioisotope Half-Life Significant Radiations Comments
Sodium-22 2.6 yr  0.54 MeV positron; 0.51 and 1.27 MeV gammas  high beta and gamma dose rates
Iron-55 2.7 yr Various low energy x-rays and Auger electrons (<6 keV)  does not present a significant external hazard
Cobalt-57 271.8 days  Gammas (< than 0.13 MeV)  
Cobalt-60 5.3 yr 0.31 MeV beta; 1.17 and 1.33 MeV gammas high beta and gamma dose rates
Nickel-63 100 yr 0.066 MeV beta does not present a significant external hazard
Strontium-90 29.1 yr 0.54 MeV beta from Sr-90; 2.26 MeV beta from Y-90 decays to short-lived Y-90; very high beta dose rates
Cesium-137 30.1 yr 0.51 MeV beta; 0.661 gamma from Ba-133m decays to short-lived Ba-133m
Polonium-210 138.4 days 5.3 MeV alpha does not present a significant external hazard
Radium-226 1600 yr 4.8 MeV alpha; various alphas, betas and gammas from decay products
decays to Rn-222, with a long decay chain following 
Americium-241 432.7 yr 5.6 MeV alpha; various alphas, betas and gammas from decay products decays to long-lived Np-237, decaying in turn to Pa-233 and U-233.  Alpha emissions are the greatest concern.

Dose Rates

Radioisotope Beta dose rate
at 30 cm from source*
(millirem/hr per millicurie)
Gamma dose rate
at 30 cm from source
(millirem/hr per millicurie)
Sodium-22 370  13.3 
Cobalt-57 0 0.94 
Cobalt-60 48  14.4
Strontium-90 740 0
Cesium-137 777 4.1 
Americium-241 0 0.56
*Keep in mind that the annual skin dose limit is 50,000 mrem.  Working for extended periods in close proximity with certain radioisotopes can lead to large doses.

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The following guidelines incorporate the classic techniques of minimizing time, increasing distance and using shielding to minimize radiation exposure.  Increasing distance from a source is an effective way to minimize dose because radiation intensity follows the inverse square law:

The Inverse Square Law

  • The intensity of radiation emitted by a radioactive point source follows the inverse square law, 
    i.e., as distance from a point source increases, the intensity decreases proportionally to the square of the change in distance.
  • For example, if the dose rate is measured to be 10 mrem/hour at 10 cm from a point source, the dose rate at 20 cm from the source will be 2.5 millirem/hour:
  • Dose rate at 20 cm = 10 mrem/hr x (10 cm/20 cm)2  =  2.5 mrem/hour
  • Consequently small changes in distance near a source mean large changes in radiation exposure.

(diagram courtesy of  Center for Nondestructive Evaluation, Iowa State University, Ames, IA)

Handling the Source

  • Minimize the time spent handling a source or in the vicinity of a source.
  • Handle high activity sources with handling tools, such as tongs or remote-handling tools
  • Do not touch the active surface of a plated source with your fingers
  • Wash hands after handling a plated source

Eating & Drinking

  • Do not eat or drink in rooms where sealed and plated sources are used or stored.  Although the risk of contamination is low, Washington State Department of Health, Division of Radiation Protection regulations prohibit eating and drinking in areas where any radioactive materials are used or stored.

Labeling Sources

  • Sealed and plated sources must be labeled with a Caution: Radioactive Material label that lists the radioisotope, the amount of radioactivity, and the date the source was fabricated.

Shielding and Radiation Surveys

  • Use appropriate shielding.  Gamma and x-ray emitters should be shielded with lead.  Because bremsstrahlung production is a concern with beta emitters, beta emitters should be shielded with a primary shield of plexiglass or similar low Z material.  If sufficient bremsstrahlung is produced in the low-Z primary shield, a secondary shield of lead should be placed outside the primary shield.
  • High activity sources in storage must be shielded appropriately when in use or in storage.
  • Sources should be stored away from normally occupied areas
  • If you are uncertain about whether a source should be shielded or is sufficiently shielded, contact the Radiation Safety Office for information at 206.543.0463.

Radiation survey meters

Radiation survey meters are required to be calibrated once a year.  Teh Radiation Safety Office performs electronic calibrations of portable Geiger-Mueller and scintillation meters, and many ion chambers, exposure rate and dose rate meters. The RSO can arrange for off-site calibrations of instruments that must be calibrated by NIST or the device manufacturer.   The calibration date and the date that the next calibration is due is posted on a label on the side of the meter.  Do not use meters that are out of calibration.  If you find a meter that is out of calibration, contact the RSO. 

Before you use a survey meter, perform a preoperational check.  The preoperational check consists of:

  • a battery check
  • a background check (background for a G-M meter should be 25-75 cpm; background for a scintillation meter should be 200-300 cpm)
  • verifying that the meter has been calibrated within the last 12 months
  • a performance check using a check source.  The expected check source reading is posted on the calibration label on the side of the meter.  If the meter does not read within +20% of the expected check source reading, do not use the meter.  Contact the RSO to arrange for repairs.

Cloud Chamber Sources

Radioactive sources used in cloud chamber experiments typically are rods or needles plated with very small amounts of Sr-90 (for beta activity) or Po-210 or Pb-210 (for alpha activity).  The activity of these sources is low enough that shielding is not necessary, but it is important to handle the source by holding the cork, rather than by touching the needle part of the source on which the radioactivity is plated.

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Sealed sources are typically small in size and may be readily portable.  It is important to pay extra attention to ensuring that sealed and plated sources are secured.
University of Washington policy for securing radioactive materials requires that:

  • Sealed sources must be locked in a secured container or secured storage area when not in use.
  • Any room in which an sealed source is being used must be locked when unattended.

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Under the terms of the University's of Washington Radioactive Material Licenses, the RSO must perform leak tests of many of the sealed sources to ensure that the integrity of the source encapsulation is intact.  Such leak tests are required at three- or six-month intervals depending on the nature of the source.  Leak testing is not required for some low-activity sources, for example, if they contain 100 microcuries or less of a beta/gamma-emitter or 10 microcuries or less of an alpha-emitter, however, a semiannual inventory of these sources is required.

If you suspect that a sealed or plated source has been damaged, notify the RSO and do not use the source until Radiation Safety staff have leak tested the source.

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Missing source

If you discover that a sealed or plated source is missing, notify the RSO promptly at 206.543.0463.  Once you suspect that a source is missing, it may be reasonable to take a short time to attempt to find it, but do not take more than a few hours to notify Radiation Safety.  Under some circumstances, the University must notify the Washington State Department of Health, Division of Radiation Protection when a radioactive source cannot be located.  The RSO will make the determination whether notification is necessary and will assist in efforts to locate the source.

Overexposure to a Source
If you suspect that you have been received a significant exposure to a radioactive source, contact Radiation Safety immediately at 206.543.0463.

Emergency Response Guidelines
For assistance in dealing with a radiological or chemical incident or any other incident, consult the Emergency Response Guidelines for Laboratory Personnel, which are posted in each lab.

Contacting Radiation Safety Outside of Normal Business Hours
Call the University Police at 206.685.UWPD (8973), who can relay the incident details to the UW Environmental Health and Safety after hours Staff On Call.  The Staff On Call will contact Radiation Safety staff through home phones or pagers.

 

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This is the end of the Working Safely with Sealed Sources Module, which is the last of the seven Sealed Source Radiation Basics modules.  You may now go to the test or you may go to any of the previous modules:

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