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Sealed Source Radiation
Safety Training
Module 7: Working Safely with Sealed
and Plated
Sources
Characteristics of Sealed Sources and Their Radioisotopes
Guidelines and Requirements for the Safe Use
of Sealed and Plated Sources
Security
Leak Testing
Radiation Incidents
Characteristics of Sealed Sources and Their Radioisotopes
(top)
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
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Fig. 2
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Fig. 3 |
Decay Schemes
The radioisotopes listed in the following tables are among the most
common used in sealed sources at Princeton University. Detailed fact
sheets for each of these radioisotopes, as well as many other radioisotopes,
are available upon request from EHS.
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 (< 0.13 MeV) |
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| 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 dose rates |
| Cesium-137 |
30.1 yr |
0.51 MeV beta; 0.661 gamma from Ba-133m |
Decays to shortlived 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
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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 |
Guidelines and Requirements for the Safe Use of Sealed
and Plated Sources (top)
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 millrem/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.
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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, NJ Department
of Environmental Protection regulations prohibit eating and drinking
in areas where state-licensed radioisotopes are used or stored. Many
of these sources are state-licensed.
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 plexiglas 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 is 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 EHS to perform a radiation survey.
Radiation survey meters
Radiation
survey meters are required to be calibrated once a year. EHS performs electronic calibrations of portable Geiger-Muller
and scintillation meters and arranges for off-site calibrations
of exposure rate and dose rate meters. 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 EHS. 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 the check source attached to the
side of the meter. 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
EHS to arrange for repairs.
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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.
Security (top)
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.
Princeton University 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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Leak Testing (top)
Under the terms of the University's Nuclear
Regulatory Commission (NRC) and the NJ
Department of Environmental Protection (NJDEP) licenses, EHS 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 .
If you suspect that a sealed or plated source has been damaged, notify
EHS and do not use the source until EHS has leak tested the source.
Radiation Incidents (top)
Missing source
If you
discover that a sealed or plated source is missing, notify EHS
promptly at 8-5294. 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 EHS. Under some circumstances,
the University must notify the NRC or the NJDEP when a radioactive
source
cannot be located. EHS 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 EHS immediately at 8-5294.
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 EHS Outside of Normal Business Hours
Call Public Safety at 8-3134. Public Safety will contact EHS personnel
though home phones or pagers.
This is the end of the Working Safely with Sealed Sources
Module, which is the seventh of the eight Sealed Source Radiation Basics
modules. If you wrok with the americium-beryllium source, go to the next module. If you do not work with the americium-beryllium source, you may now go to the test or you may go to any of the previous modules::
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