Re: update/summary real time extremity dosimetry

[JGinniver@AOL.COM]

In a message dated 25/11/04 5:38:49 pm, sandyfl@earthlink.net writes:





> I would be very interested in how the manufacturers of all these real-

> time extremity monitoring systems state how they calibrate their

> units in that the accurate dose is highly dependent on the energy

> calibration factor, and, what their energy response is across the

> various radiation types, i.e., photon and beta.

> 

I have had some limited data from Unfors (via their UK distributor) which 

shows the instruments response for Hp(10) and Hs(0.07) for photons, and how this 

varies for photon energy. This particular unit is quite 'peaky' around 60 keV 

but otherwise has a reasonably flat energy response.   However I am following 

up it's response to beta radiations and what tests were done to provide the 

data used in the existing graphs.





>  I would also be

> interested in how their units perform in mixed fields. I would also

> be very interested in how thin the manufacturers can make the

> attenuation material over the detector, since that would preclude

> certain types of radiation with specific energies from penetrating

> down to the detector.

> 

Again this is something I am following up but the Unfors unit appears to 

respond adequately down to around 30 keV.   

On another note here, (as I'm sure Sandy knows) there are significant 

differences in the UK in the way that Dosimetry Services are checked/approved by the 

regulator.   In the UK you can only supply dosimetry services if you have been 

"Approved" by the UK regulator.   The regulator runs bi-annual (I Think) 

performance tests for each lab to ensure that they continue to perform adequately. 

  However these tests are standard throughout the UK and don't necessarily 

reflect the different radiation fields that could be encountered at various sit

es and for different activities e.g. fuel fabrication, reprocessing, power 

generation at a Gas Reactor, power generation at a PWR, medical exposures.   I 

believe that US tests require dosimetry serves to demonstrate a much greater 

understanding of the radiation fields applicable to their facilities.   .   In 

addition in the UK it is the employer who is responsible for providing the 

dosimetry and that their staff could work on sites, under the control   of another 

operator, where they (the employer) don't fully understand the likely radiation 

fields.   Whereas in the US I understand that it is the site operator who 

provides the dosimetry to all staff (includiing contractors) who work at the site 

and consequently are able to provide dosimetry that is tailored to the 

radiation fields present at that site (again if I'm wrong on this - please feel free 

to let me know).



>  I think many companies state that they can

> detect and report extremity dose, but the devil is in the details. It

> would be interesting to see how these real-time detectors perform in

> an IEC or ANSI Performance Test Environment, where photons and beta

> are used, individually or in mixed fields.

> 

The AEGIS unit from John Caunt Scientific has only recently completed type 

testing in the UK.   I'm awaiting the results, but the testing was done by the 

UK National Radiological Protection Board (an independent UK Government agency) 

and so I'm   familiar with the types of tests they undertake and these are 

usually based around IEC documents.   I'm hoping that that they will have mostly 

used IEC 1283 (Radiation Protection Instrumentation - direct reading personal 

dose equivalent (rate) monitors - X, gamma and high energy beta radiation) as 

the basis for much of the testing.   I do know however that the NRPB 

contributed to the design of the unit through feedback to the manufacturer and that 

this included many of the team that jointly developed the Thermo (Siemens) EPD.



>  In the USA the ANSI N13.32-

> 1995 Performance Test is currently voluntary for NVLAP accredited

> labs. Extremity reporting can be quite accurate for today's passive

> dosimeters, but the accuracy is based solely on the various

> correction factors incorporated in the algorithm. I expect that the

> reason that you aren't provided much data is because that data shows,

> yes, they can measure extremity dose, but the performance isn't quite

> that good, unless the exposure of interest just happens to be that of

> what was used to calibrate their instrument.

> 

> When looking at manufactures data I always try to maintain a healthy 

scepticism, hence my preference for independent testing undertaken to establish 

'standards' e.g. IEC.   This reduces the opportunity for manufacturers to present 

the data in a way that draws attention to the good areas and hides (excludes) 

the bad.



I think it is important to remember that while we as a company would like to 

use real time extremity dosimeters, their purpose will be as control 

dosimeters and not 'legal' dosimeters which provide the data for the individuals dose 

record.   The benefit will come from the individual being able to adjust their 

working procedures to reduces doses and to provide day to day information as 

the 'legal' dosimeter is issued for a number of days rather than being read on 

a daily basis.   An ALARA/ALARP tool if you like.



Its noteworthy that even after more than 10 years of operation few 

organisations have switched to using the Thermo EPD as their 'legal' dosimeter.



Regards,

Julian