6. Restriction of exposure - Hazard control
Contents
6. Restriction of Exposure - Hazard Control
6.2.1 Contamination Monitoring
6.3 Decontamination Procedures
6.4 Testing and Maintenance of Engineering Controls
6. Restriction of Exposure - Hazard Control
In order to minimise the exposure to ionising radiations, one needs to appreciate the properties of the ionising radiations and the two principal hazards which they can present.
We can be irradiated by a source of radiation which is outside the body - this presents us with an external hazard, or we can accidentally incorporate radioactive materials into the body - this presents us with an internal hazard. All hazards from ionising radiations can be minimised by using the lowest activity source or energy of X-rays consistent with experimental requirements.
6.1 External Hazard
External hazards can arise from any source of penetrating radiation, e.g. X-ray sets, gamma emitters, neutron sources and hard beta emitters. The more penetrating the radiation, generally the greater the hazard, although in some ways hard betas and soft X-rays, which are absorbed by the surface layers of tissue, can be more hazardous than more penetrating radiations which might go straight through the body (medical X-rays). Weakly penetrating particles do not pose an external hazard. There can only be a hazard from X-ray sets whilst the set is switched on. Sealed sources, used under normal conditions, should only present an external hazard, but this will always be present. Open sources can also present an external hazard. Doses can be minimised by:-
(a) use of effective shielding;
(b) keeping one's distance and
(c) exposing oneself for the minimum of time.
6.1.1 Inverse Square Law
The dose rate associated with any point source of gamma or X-radiation is inversely proportional to the square of the distance from the source.
D proportional to 1/r2
Therefore, doubling the distance from the source, reduces the dose rate by a factor of 4.
Always remember closeness endangers - distance protects.
A useful expression for calculating the approximate dose rate from a gamma source is:-
D = ME / 6r2
D is dose rate in µSvh-l
M is activity in MBq
E is energy/disintegration in MeV
r is distance from source in metres
This of course assumes no shielding, and a monitor should always be used to establish the true dose rate.
The table below gives some examples of dose rates at 1 m from 10 MBq sources, using information on the energy and intensity of their gamma emissions from ICRP publication 38.
| Isotope | Gamma dose rate at 1 metre in µSvh-l for 10 MBq source |
| Na-22 | 3.65 |
| Cr-51 | 0.05 |
| Fe-59 | 1.98 |
| Co-57 | 0.21 |
| Co-60 | 4.17 |
| I-125 | 0.07 |
| I-131 | 0.63 |
6.1.2 Shielding
Beta particles are best shielded by materials of low atomic number to prevent the production of Bremsstrahlung radiation. Perspex makes good shields, because it is robust and easily worked. Glass is also very effective, and thick walled glass vessels are particularly useful. The table below shows the thickness required for complete shielding. Beta particles are completely stopped in the shielding material, therefore, if an appropriate thickness of shield is used, all betas are attenuated irrespective of the activity present.
Note: The small amount of Bremsstrahlung radiation produced when beta particles interact with the relevant shielding should not be ignored.
E max (MeV) |
0.5 | 1.0 | 2.0 | 3.0 |
| Glass | 1mm | 2mm | 4mm | 7mm |
| Perspex |
2mm |
4mm |
7mm |
12mm |
Gamma rays and X-rays are far more penetrating than beta particles of the same energy and require dense shielding materials - lead is the material which is usually used. They are attenuated exponentially, and a knowledge of the half-value layer (HVL) or tenth-value layer (TVL) is useful in determining the amount of shielding required. 1 HVL is the thickness required to reduce the intensity to one half the incident value and 1 TVL is the thickness needed to reduce the intensity to one tenth the incident value. Some approximate values of HVL and TVL are given in the table below.
Gamma rays and X-rays cannot be completely stopped, thus shielding only reduces the dose received.
Gamma energy |
Millimeters of lead shielding | |
| MeV | HVL | TVL |
| 0.5 | 4 | 12.5 |
| 1.0 | 11 | 35 |
| 1.5 | 15 | 50 |
| 2.0 | 19 | 60 |
The good working practices required for work with sealed sources are given: in the Laboratory Rules (see Appendix 15); in the conditions of individual Project Application; and in some cases in additional detailed operating instructions. Detailed precautions for work with X-ray crystallographic equipment are given in Appendix 17.
6.2 Internal Hazard
When working with open or unsealed sources of radioactive material, as well as having a possible external hazard to contend with, one is faced with the possibility that radioactive material might find its way into the body. One would then be faced with an internal radiation hazard, and shielding, distance and time would no longer afford protection. Only by a combination of physical half-life and biological half-life can the material be eliminated from the body - some may remain there forever. It can easily be appreciated that small amounts of radioactive material inside the body can be more harmful than much larger amounts outside the body. Every effort must be made, therefore, to prevent radioactive material from entering the body. Routes of entry into the body are via the mouth by inhalation or ingestion and through the skin via cuts or absorption. Internal contamination can be avoided by adopting good working practices, and by following some basic precautions, such as:-
- Use of materials of minimum radiotoxicity;
- Presence in the laboratory of the minimum quantities;
- Containment, to prevent spread of contamination;
- Cleanliness and good housekeeping; and
- Use of appropriate protective equipment.
The good working practices required for work with unsealed sources are spelt out in the Laboratory Rules (see Appendix 16) with further information in the guidance notes.
A useful indication of the radiotoxicity of an isotope is its annual limit of intake (ALI). This is the amount which, if taken into the body in a year, will result in the individual receiving the full annual dose limit from this source of radiation alone. A list of the most restrictive ALI's for the common isotopes is given below:-
| Isotope | ALI (Bq) |
| H-3 (Water) | 1.1 x 109 |
| H-3 | 4.7 x 108 |
| Cr-51 | 5.3 x 108 |
| C-14 | 3.4 x 107 |
| S-35 | 1.5 x 107 |
| P-33 | 1.4 x 107 |
| P-32 | 6.2 x 106 |
| I-125 | 1.3 x 106 |
The importance of regular monitoring cannot be overstressed as it is the only way of ensuring that the other precautions you have taken have been effective in minimising contamination.
6.2.1 Contamination Monitoring
There should always be a suitable contamination monitor available in areas where unsealed sources are used. Every time you handle radiochemicals, you should use the appropriate contamination monitor to monitor yourself and your immediate work area - bench top, equipment, bench front and floor - at the end of each work session complete the appropriate form (see Appendix 29). Any contamination found should be removed immediately, or if this is not practicable, a suitable warning notice should be displayed. On no account should contamination, which would pose a hazard to others, be left unmarked.
At regular intervals, a full monitoring survey should be carried out to establish that:-
(a) the area is correctly designated (on grounds of contamination); and
(b) any contamination that has occurred has been dealt with efficiently and has not been spread to - fridge doors, cupboard doors, floors, door handles, etc.
These monitoring surveys have to be recorded.
If you are working with tritium, then conventional monitoring will not be any use, as the very weak beta emissions from tritium cannot be detected by the contamination monitors. You therefore have to perform wipe tests using moistened filter papers and sampling a known area.
6.3 Decontamination Procedures
All contamination should be removed as soon as possible after it has occurred, except in the following circumstances:-
(a) the contaminated item is disposable and can legitimately be put in the solid radioactive waste drum; and
(b) the isotope is of such short half-life (no more than a few hours) that, if the contamination was left, it would rapidly decay away.
In case (b) above, the concern is not to unnecessarily expose personnel to hazardous radiations during the cleaning process. Any area left with a high level of contamination should be clearly and prominently marked, to keep people away from it until the isotope has safely decayed.
For all personal decontamination, the possible need to seek medical advice should be borne in mind.
For contamination of the skin e.g. arms, hands etc., the first step is to wash the affected area with soap and water as normal. If the contamination persists, it should be washed and scrubbed gently, using a soft brush, with a deep cleansing soft soap or liquid soap e.g. 'Clearasil' or 'Dermactyl'. Care must be taken not to break the skin.
If the contamination still persists after several wash and scrub treatments and the contamination is restricted to parts of the hands, these may be cleaned with a saturated potassium permangenate solution. This will remove a superficial layer of skin and care must be taken to ensure that no undissolved crystals are present. The brown discolouration left by the permangenate can be removed with a 10% solution of sodium metabisulfate. If any other parts of the body are contaminated and the contamination is not easily removed by ordinary washing, then specialist help should be obtained.
If serious injury, cuts and wounds are associated with the contamination, these should be irrigated and first-aid measures taken before dealing with the contamination. Body openings such as eyes, ears, nose and mouth should always be decontaminated first. Decontamination of any 'hot spots' on other parts of the body should be dealt with next. Care should be taken to ensure that washings do not contaminate other areas. If the casualty has to go to hospital for treatment of wounds, only superficial contamination should be removed as a first-aid measure.
Emergency showers are rarely the best solution for dealing with a contaminated person as this can spread the contamination. For hand, arm and head contamination, it is better to use a hand wash basin and for legs a foot bath
If an individual is heavily contaminated the person should be decontaminated as far as reasonably practicable and the emergency procedures outlined on the back page should be followed.
6.4 Testing and Maintenance of Engineering Controls
Where any design features and/or devices are used as control measures in limiting exposure to radiation, then the School/Unit should draw up and implement a testing and maintenance program and keep written records.