The Society for Radiological Protection - Services

Rules of Thumb and Practical Hints

Introduction Disclaimer
Properties of Radiation
Instrument Response & Operational Monitoring
Other practical techniques
Shielding

Introduction
This document has been compiled by the Practical Radiation Protection Topic Group who thank all contributors. These Rules of Thumb and Practical Hints are intended as a resource and have been derived entirely from practical experience. They provide a rough practical method of calculation and may contain errors. We welcome comments and further examples by email or the form.

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Properties of Radiation

It requires an alpha particle of at least 7.5 MeV to penetrate the protective layer of skin, 0.07 mm thick.
It requires a beta particle of at least 70 keV to penetrate the protective layer of skin, 0.07 mm thick.
The range, R, of beta particles in g/cm2 is approximately equal to the maximum energy, E, in MeV divided by two (i.e. R = E/2).
The range of beta particles in air is about three and a half metres per MeV.
The activity of any radionuclide is reduced to less than 1% after 7 half lives.
For material with a half-life greater than six days, the change in activity in 24 hours will be less than 10%.
For gamma energies between 60 keV and 1.5 MeV the dose rate from a source A MBq and total energetic gamma emission per disintegration of E MeV
= 0.14 AE microGy/h at 1 m
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The bremsstrahlung from 37 GBq of Phosphorus-32 aqueous solution in a glass bottle is about 10 microSv/h at one metre.
For a point source of beta radiation (neglecting self- and air-absorption) of known activity in gigabecquerels, the doserate at one foot is approximately equal to 100 GBq Sv/h. The variation with energy is small over a wide range.
All alphas are not the same.
Short half-life naturals 8 MeV
Nuclear material 5 to 5.5 MeV
Long half-life naturals 4 MeV
This results in a big difference in range.
Alpha range in air = (1.24 E - 2.62) cm where E = energy in MeV.
Alpha range in anything (Rm) compared to air
Rm (g cm-2) = 0.56 A^1/3 R
Where A = atomic number of medium and R is the range in air (in cm)
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The higher the energy, the shorter the half life, but watch out for
Long half-life parent – Short half-life daughter
A measured HVL at 80kV of 3mm Aluminium indicates a total tube filtration of 3mm Aluminium.
One Bq of uranium is big enough to be visible, but 1 TBq of pure Tc-99m is not.
A 3mm diameter droplet of T20 contains ~ 1.5 TBq
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Instrument Response & Operational Monitoring
Dose rate (microGy/ h) on the outside of a large mass (drum or loader bucket) of evenly contaminated material = 0.3 AE, where A is the activity in Bq/g and E is the energetic gamma emission per disintegration in MeV. This ignores absorption in the container.
When carrying out contamination monitoring use the ear to detect and the eye to steer.
When taking smears, smear in a spiral pattern moving into the centre of the smear area so as not to spread any contamination.
The typical ion chamber correction from indication to contact dose rate (H'(0.07), averaged over 1 cm2) is around 50 for a point source or 5 for a large uniform area.
Reliable alpha monitoring by probe is only possible on a surface that looks clean enough to eat off.
When assessing alpha contamination in grimy conditions, for the transuranics, use the L x-rays (13 to 20 keV) and a thin sodium iodide scintillator or xenon filled proportional counter. The minimum detectable activity is a few becquerels/cm2. For naturally occurring radioactive material most alphas are followed by an energetic beta. Use that.
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When carrying out floor surveys with a probe, disconnect the probe holder on the meter and attach it to the bottom of a crutch. Using the crutch with the probe on the end allows one to carry out the survey without stooping.
When sending transit control film badges through the post, place a paper clip over the film. This can help to identify where the "out of holder" exposure took place.
Do not expect to get the same result if you make the same measurement twice.
On a horizontal gamma contaminated surface most gamma rays are incident on the detector between 45º and 85º to the vertical.
Monitoring of probably clean or low activity surfaces to probe distance of 3 mm. Use a trailing gloved finger supporting the probe on the surface. Check the gloved finger regularly. Do not use on non-smooth surfaces.
Contamination by positron emitters – treat as betas of the same energy. Only use the gamma rays for bulk activity.
Typical beta contamination monitors have a gamma detection efficiency of about 1% (counts/photon striking the window).
The typical beta contamination monitor background is 1 count per second per 20cm2.
The typical energetic gamma sensitivity of beta contamination monitors is about 0.25 counts per second per cm2 probe area per microsieverts/hour.
For energetic betas (greater than 0.7 MeV Emax), X-rays and gamma radiation greater than 20 keV, drawing back from the surface has little effect on the count rate from very large area surface contamination. It only widens the effective averaging area. Small areas of contamination will follow the inverse square law.
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Do not swing probes by their cables. Use a lanyard between probe and ratemeter.
When using thin sodium iodide detectors for X, gamma contamination, only remove the plastic end cap for energies below 30 keV.
When using instruments in high rf fields. All instruments are guilty until proven innocent. Two effects can take place. One is an enhanced background, the other is more sinister in that the operation of the instrument can be inhibited. Therefore test first with a check source.
Spend at least 10% of the cost of an instrument on the carrying case.
Have plastic protective caps lying around in boxes. One end window GM saved is equivalent to around 30 lost caps.
Do not trust 900V operating voltage GMs at high dose rates without good evidence that they work correctly.
Pulse counting instruments and pulsed sources. The indication is normally trustworthy up to the point where the count rate = 30% of the pulse repetition frequency. Hence you need to know both the pulse repetition frequency of the sources and the calibration value of the detector i.e., the value used to convert counts/s to microSv/h. If in doubt use an ion chamber for X- and gamma radiation.
Batteries vary in length and shape from manufacturer to manufacturer. For instruments that use flat spring connectors, stick to one type.
Remember battery performance falls off at low temperatures.
When you perform a battery check ensure that the needle is in the "healthy" zone and stable.
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Battery Check does not prove the condition of rechargeable batteries.
Instruments do not like being left in parked cars. In winter they get cold and condensation forms when taken into warm rooms. In summer the internal temperature can rise above component ratings.
Background is a very variable thing. The range in activity in building materials is about a factor of 10.
Often you know what the gamma energies of the source are but for shielded sources you do not know the energy that gets into the working area. Intact shielding, particularly lead, will preferentially remove the lower energies. Mazes and other scatter paths will reduce the energy.
Any soft beta detector, i.e. one that responds to Carbon-14, will detect alphas.
Always carry monitors switched on. Quite a few unexpected contaminated areas and unexpected sources have been found that way – you will have plenty of warning of entry into a radiation area.
Some organisations allow the judicious patching of light leaks in alpha and beta scintillation probes. Solvent based typist's correction fluid works best.
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When surveying for low doserate X and gamma radiation often it is quicker to detect first, using a very sensitive sodium iodide scintillator, and then measure the doserate later, using an energy compensated GM or an ion chamber.
It is difficult to make meaningful estimates at less than 3 counts per second. Either the indication is unsteady or the response time is very long.
Measurements of surface contamination for tritiated water are of limited, qualitative value only. Measurements of airborne concentration of Tritiated water are straightforward and relate directly to human exposure.
For normal operational health physics purposes, the skin dose per activity per unit area can be approximated as 1 uSv/h per Bq/cm2.
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Other practical techniques
Checking cut metal for temperature when working in plastic suits – use a candle stub. If it doesn't soften the metal can be handled.
Good decontamination techniques for the hands
- Lots of barrier cream and rubber gloves. Leave for some hours, remove the gloves and wash.
- Make pastry. The rubbing action, fat, slight abrasion and sweating will help remove activity.
For any activity employing gamma radiation, bleepers will tend to lower worker doses as they give a continuous reminder of the radiation level. However, once they start bleeping too often, they become irritating.
To stimulate mucus for nose blows, extra strong mints are effective.
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Shielding
An approximate HVL for 1 MeV neutrons is ~3 cm of polythene or water, ~7 cm for 5 MeV neutrons.
The air scattered radiation (sky-shine) from a 3.7 TBq Co-60 source placed one foot behind a one metre high shield is about 1 mSv/h at two metres from the outside of the shield.
For beta shielding low atomic number materials give you less bremsstrahlung production but also less bremsstrahlung attenuation. The optimum shielding solution requires a balance.
At shielding of high gamma energies it is mass/unit area that is important, not atomic number.
The lower the Z of the material, the worse it scatters X- and gamma radiation.
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For hard gamma radiation the denser the shielding material, the lower the mass of the shield for a source container. Density is more important than Z.
For americium/beryllium neutron sources 37 GBq is equivalent to 2.2E6 neutrons/second. The neutron doserate is typically 20 uSv/h at 1m from a similar size source.
For Cobalt-60 gamma rays the HVL is approximately:
1 cm of lead
2.5 cm of steel
7.5 cm of concrete
15 cm of water
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Disclaimer
These Rules of Thumb and Practical Hints may contain errors. They are not claimed to give accurate results or rigorous analyses. The Society refuses all claims relating to these Rules of Thumb and Practical Hints.

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