Re: Safe Working Distance for RT what enviroment are you working in? source strength,collimator material ie tungsten or lead. there are a multitude of things to take into consideration your question is too brief to give a definitive answer
01:44 Dec-04-2007 Nigel Armstrong Engineering, - Specialist services United Kingdom, Joined Oct 2000 1094
Re: Safe Working Distance for RT You must comply with the law and to do this there are no short cuts. The first paramount step towards safety is, without a doubt, to have all radiation work performed in a properly-designed radiation exposure bay. But I presume you are talking about site work where no bay is available. If its on a busy project a remote site should have been chosen by the Company Radiation Safety Officer together with any Compliance Officers where all transportable spool pieces are removed for radiography.
The next most critical item (perhaps the most critical) is that the work is performed by two safety-trained, qualified and diligent radiographers working together in constant communication (with radios, preferably).
Your first required piece of information is the dose rate limit for non-classified personnel (non-radiation workers not issued with dose measuring devices) in the country in which you are working -not all countries have the same limits. You calculate the distance at which to set your barriers from an uncollimated source (the collimator has an aperture from which the full-energy of the source is emitted). You will need the source strength (Becquerels) and the Radiation Intesity Factor (Becquerels per second) appropriate to the source being used. This calculation will give you a guaranteed legal safe-working distance no matter where on the barrier's perimeter measurements are taken. Look at the result and is it sensible - if it reads 4m or 400 m then a mistake may have crept in so re-check yoour result.
Now to mitigate the admittedly daunting prospect of erecting and patrolling such a large perimeter. Always shoot skywards if clear above and possible. Next always shoot downwards. If the joint is a tie-in then plan the shoot according to the number of exposures required to minimise incidence upon any busy working areas. Have the barriered area cleared of personnel and perform a test shot. During this exposure monitor the barrier perimeter in direct line with the collimator aperture (which could give a cone of uncollimated radiation subtending an angle of 30 - 40 degrees), just to confirm that your calculation has no major error in it such as wrong source strength. Depending on the geometry of your set-up and how much attenuating material stands between the emitter and the measuring device you may be able to move your barrier inwards. Certainly in those areas outside of the subtended cone of uncollimated radiation you should be able to move the barriers in a considerable distance. The trickiest shots with regards to radiation safety are small bore vertical pipes (horizontal welds) as then shooting skywards or earthwards is not possible. It may be necessary to fix a thick sheet of lead behind the radiographic film to prevent uncollimated radiation polluting the area behind the shot butt.
I always suggest to radiation contractors that they walk round the night's work to identify any safety-problematic welds for radiography and then work out the safe and legal method to radiograph those butts.
So the old daysof "job and knock" where the night's work of butts were bombed as quickly as possible so that the radiography crew could be down the pub before closing time should be long gone, but I doubt it!
----------- Start Original Message ----------- : what enviroment are you working in? source strength,collimator material ie tungsten or lead. there are a multitude of things to take into consideration your question is too brief to give a definitive answer ------------ End Original Message ------------
02:25 Dec-11-2007 jean Staton NDT Inspector, - - Instructor USA, Joined Aug 2006 4
Re: Safe Working Distance for RT ----------- Start Original Message ----------- : How would you calculate the safe working distance for a 25 curie source of Iridium using a collimator? ------------ End Original Message ------------
Re: Safe Working Distance for RT ----------- Start Original Message ----------- : : How would you calculate the safe working distance for a 25 curie source of Iridium using a collimator? : : By using the Inverse Square Law ------------ End Original Message ------------
Safe working distance depends on your definition of safe. Different countries have different regulatory requirements for dose rates for classified workers or general public. If you are going to do a lot of radiography you may want to reduce these values even more. You need to know the dose rate at a certain distance from the source, usually 1 foot or 1 metre, this is available from published sources or from the source manufacturer. Find out the material and thickness of the collimator and use the half value layers to calculate reduction in dose rate due to the collimator. Then using the regulatory figure for acceptable dose rate use the inverse square law to calculate distance. Or go on the net and look for www.radprocalculator.com which will do it for you.
One thing to keep in mind is that although you are using a collimator, there is a hole in it. It doesn't matter what the collimator is made of or how thick it is, it still has a hole in it. As exposures are made using a collimator you are changing the position of the collimator and the hole. Keep in mind the direction the hole in the collimator will point during you exposures and calculate your safe working distances based on the hole in the collimator (uncollimated exposure).
Is your question regarding being directly in the beam of a collimated source or working in proximity to a collimated source ?
Either way, your answer should be the same.
As Nigel and Gerald have stated you make your safety calculations based on an uncollimated source.
In addition to Nigels statement about taking in the direction your collimator is being pointed you must also consider additional means of exposure.
If you are working in close proximity to an active source the amount of radiation received will be based on the attenuation factor of the collimator material that the secondary radiation is passing through (eg. lead, tungsten, depleted uranium etc) and the thickness of the collimator.
Obviously this will be much, much less than if you stood in the direct path of the primary radiation.
However, two things to consider (even if the collimator is pointing directly at the ground) - when the source travels from the camera to the collimator it is fully exposed and there will also always be radiation coming out the back of the collimator (where the delivery tube is inserted).
I will qualify my statements above by noting I have not performed RT in many years and equipment may be totally different nowadays.