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Technical Discussions
RT Guy
RT Guy
07:09 Dec-03-2007
Safe Working Distance for RT

How would you calculate the safe working distance for a 25 curie source of Iridium using a collimator?


    
 
 
gary whitworth
gary whitworth
03:43 Dec-03-2007
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




    
 
 
Nigel Armstrong
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1094

Nigel Armstrong

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1094
01:44 Dec-04-2007
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 ------------




    
 
 
jean Staton
NDT Inspector, - - Instructor
USA, Joined Aug 2006, 4

jean Staton

NDT Inspector, - - Instructor
USA,
Joined Aug 2006
4
02:25 Dec-11-2007
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 ------------


By using the Inverse Square Law



    
 
 
Dave
Dave
02:06 Dec-12-2007
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.


    
 
 
ez does it
ez does it
08:36 Dec-12-2007
Re: Safe Working Distance for RT
square root of [(source activity x output)/ max. permissible dose)] and divide that by 8 if you are using a lead collimator

----------- Start Original Message -----------
: How would you calculate the safe working distance for a 25 curie source of Iridium using a collimator?
------------ End Original Message ------------




    
 
 
Abhijit Gaikwad
Abhijit Gaikwad
13:25 Dec-14-2013
Re: Safe Working Distance for RT
In Reply to RT Guy at 07:09 Dec-03-2007 (Opening).

what will be the safe distance for RT with 25 curie Ir. souurce for steel pipe in open area.

    
 
 
John
John
23:04 Dec-14-2013
Re: Safe Working Distance for RT
In Reply to Abhijit Gaikwad at 13:25 Dec-14-2013 .

The answer is still to use the inverse square law
You must know the length of exposures

    
 
 
Gerald R. Reams
Engineering,
Industry, USA, Joined Aug 2012, 178

Gerald R. Reams

Engineering,
Industry,
USA,
Joined Aug 2012
178
01:06 Dec-15-2013
Re: Safe Working Distance for RT
In Reply to John at 23:04 Dec-14-2013 .

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).

Regards,
Gerald

    
 
 
samir sahoo
samir sahoo
21:00 Jul-10-2015
Re: Safe Working Distance for RT
In Reply to RT Guy at 07:09 Dec-03-2007 (Opening).

Why calculate the curie of RT

    
 
 
meeran
meeran
13:38 Sep-12-2015
Re: Safe Working Distance for RT
In Reply to gary whitworth at 03:43 Dec-03-2007 .

i need safe working distance of RT and calculation.please do the needful

    
 
 
kenneth wagbara
kenneth wagbara
17:19 Aug-09-2016
Re: Safe Working Distance for RT
In Reply to RT Guy at 07:09 Dec-03-2007 (Opening).

how to calculate safe working distance for a 30 curie source of iridium using a tungsten collimator

    
 
 
Nigel Armstrong
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1094

Nigel Armstrong

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1094
17:36 Aug-09-2016
Re: Safe Working Distance for RT
In Reply to kenneth wagbara at 17:19 Aug-09-2016 .

See Gerald Reams response in this thread from 2013 (3 posts up)

    
 
 
Shane Feder
, Quality Co-ordinator (SubSea)
Thailand, Joined Dec 2014, 80

Shane Feder

, Quality Co-ordinator (SubSea)
Thailand,
Joined Dec 2014
80
06:35 Aug-10-2016
Re: Safe Working Distance for RT
In Reply to kenneth wagbara at 17:19 Aug-09-2016 .

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.

    
 
 

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