Produced by the Health and Safety Department, the University of Edinburgh
RP GN001: Safe Working with Gamma Irradiators
Risk
A gamma irradiator is a device that uses a large activity source of gamma
radiation to expose a test object to a high dose of radiation. Most irradiators
use either caesium-137 or cobalt-60 as the gamma source, although the short
half-life of the latter makes it less convenient. They are self-contained devices
that enclose the radioactive source in sufficient lead shielding to ensure that
the user is never exposed to significant radiation. Subjects for irradiation
include small solid or liquid samples or small animals.
The design of irradiators falls into two categories. One category has
irradiators with the sources in a fixed position within shielding. The sample
chamber is located within a rotor system so that, after loading, an electric
motor rotates the chamber to put the sample in the exposed position. In the
other category irradiators have moving sources where the sample is loaded in
a fixed position within the irradiation chamber. When the chamber is closed,
the radiation source is moved into position, with an interlock system to make
sure that the chamber cannot be opened until the source returns to its fully
shielded position. The source may be moved by rods or chains or other
arrangement. The background dose rate close to the shield may rise slightly
while the source is moving or in the exposed position.
Gamma irradiators can contain very large activities, in the tens of
terabequerels. These sorts of activities, if completely unshielded, would
provide a LD50 (i.e. the dose likely to kill 50% of an exposed population) at a
distance of 1m in about 2 hours. The design of the irradiator is therefore
required to provide sufficient shielding to ensure that external radiation levels
are low. Interlocks, either electrical or mechanical or a combination, prevent
the opening of any access door while the sources are exposed and also
prevent movement into the "irradiate" position while an access door is open.
In some cases, it might be necessary to declare a controlled area around the
unit, and hence restrict access to persons entering under a written scheme of
work.
The government also impose certain security arrangements for these devices.
Special Considerations
•
•
•
The safety of gamma irradiators is almost entirely determined by
engineering means. Dose limitation should not rely on any procedural
controls.
To keep all personal exposures as low as reasonably practicable, it is
good practice to minimise the time spent close to the irradiator.
Users must be familiar with the specific irradiator. They must be trained
on the:
o design and safe operation of the unit; operating procedures;
o security arrangements; and
Created on 09/07/2009
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Produced by the Health and Safety Department, the University of Edinburgh
•
•
•
•
•
o emergency procedures
The unit must be adequately maintained.
The unit must not be modified in any way that could affect the safety
features.
Sources must be replaced at the end of their working lives unless the
relevant special form certificate has been renewed or the activity of the
source has fallen below the following activity values:
o Co-60: 60 GBq
o Cs-137: 10 GBq
Despite their size and weight, the presence of the source(s) has to be
checked and recorded every month.
The source capsule(s) have to be tested for leakage every two years
and after any incident that might have damaged it/them.
Monitoring
•
•
•
The unit must be surveyed for radiation levels annually and after any
incident that might have damaged it. Other monitoring should not be
necessary.
Although the risk of high radiation levels becoming accessible is
remote, the severity of the result could be very high. Therefore
irradiator rooms should be equipped with a fixed radiation alarm. This
device monitors the locality and sounds the alarm if radiation levels
exceed a certain fixed level. The actions to take in this event should be
included in the relevant local rules.
Exposure to stray radiation from the irradiator should be extremely low
and personal dosemeters are not required for routine use.
Physical Data and properties
Decay data
Caesium-137
Cobalt-60
Half life (t½ )
30.2 years
5.3 years
Decay constant (λ)
0.023 y-1
0.13 y-1
Daughter product
Radioactive (Ba-137m) Stable (Ni-60)
(1)
Created on 09/07/2009
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This document is intended for use by the University of Edinburgh staff and students only
The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336
Produced by the Health and Safety Department, the University of Edinburgh
Emissions
Caesium-137
Energy
Cobalt-60
Abundance Energy
Abundance
Beta emission
0.51 MeV 95%
1.17 MeV 5%
0.31 MeV
Gamma emission
0.66 MeV 86%
(2)
1.17 MeV 100%
1.33 MeV
100%
Dose rate from a point 0.10 mSv h-1
source of 1 GBq at 1m
distance (3), (4)
100%
0.35 mSv h-1
Transmission
Cs-137
through materials
(5)
Co-60
10
100
mm mm
thick thick
HVL TVL 10
100
mm mm
thick thick
HVL
TVL
Lead
0.37
2.6x10- 8
5
mm
24
mm
~0.6
4.6x10- 16
3
mm
46
mm
Steel
0.75
2.0x10- 29
2
mm
72
mm
~0.7
5.5x10- 36
2
mm
93
mm
Concrete (6)
-
~0.42
210
mm
-
~0.5
85
mm
Typical
Dose
Rate Caesium-137
Monitor Readings (7)
Background count rate
Cobalt-60
Mini E
Mini EP15
Mini E
Mini EP15
1
1
1
1
5.8
3.0
6.5
Count
rate
above 2.6
background equivalent
to a dose rate of 1 µSv
h-1.
Created on 09/07/2009
~110 280
mm mm
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This document is intended for use by the University of Edinburgh staff and students only
The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336
Produced by the Health and Safety Department, the University of Edinburgh
Notes:
1. The half-life of Ba-137m is only 6 minutes, and so it is normally in
equilibrium with its parent radionuclide.
2. This gamma radiation actually arises from the Ba-137m.
3. This excludes any possible bremsstrahlung.
4. Since these radionuclides are normally used in the sealed form, the
contribution due to the beta radiation is normally ignored.
5. HVL – Half-value Layer – is the thickness of a material that will reduce
the dose rate to half of its original value. TVL – Tenth-value Layer – is
the thickness that will reduce it by a factor of ten.
6. Values are for a concrete density of 2.35 t m-3.
7. Dose-rate readings for these two radionuclides should be undertaken
with an instrument scaled in dose-rate units and using a compensated
GM detector. The values in the table are for instruments that could be
used as a backup if a dose-rate meter is not available.
For advice on any of the above topics please contact Mr Colin Farmery, the
University Radiation Protection Adviser [email protected]
Created on 09/07/2009
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This document is intended for use by the University of Edinburgh staff and students only
The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336