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Submitted by Pete Burgess, Radiation Metrology Ltd
   Commenting as an individual
Document Operational Quantities for External Radiation Exposure
 

I am concentrating on X, gamma and beta exposures in nuclear, industrial and research applications. It is possible (likely?) that I have miss-understood the document in places. If so, please enlighten me. However:


Predicting the consequence of radiation exposure has the major uncertainty of a lack of understanding, particularly at the organ level, of the consequences of low level radiation exposure. In that circumstance, other uncertainties become unimportant. The proposed changes will have no effect on our ability to predict consequences.


 


However, accepting the biological limitations, what we require is consistency, in the sense that the same radiation exposure will produce comparable results on well-designed instruments and dosemeters. We currently have consistency. Introducing these changes will lead to loss of consistency during the change-over period, which will take many years.


 


My most fundamental reservation is that there has been no credible attempt at cost/benefit analysis. If the authors are looking for acceptance of these changes they must list the benefits, identify their value in monetary and safety terms and set these against the costs. We are rightly expected to manage radiation exposures at minimal cost. How do the authors suggest we sell these changes to the people who pay or approve payment for the changes? This has been totally neglected as far as I can see.  


 


The authors appear not to have recognised that the use of radiation survey instruments is almost never for the control of effective dose. They are used to:




  • Demonstrate that an area is properly classified




  • Provide an upper estimate of the dose accumulation rate on someone’s dose meter at that position




  • Allow the calculation of an acceptable residence time for high dose tasks




  • Confirm understanding of the situation, given the sources and shielding present and the consequences of any changes made




  • Drive ALARP




  • Confirm it is basically the same as the last time, i.e. no one has stolen the shielding




  • Provide a value the regulator can check, thus giving them confidence you are on top of the situation




  • Determine things such as Transport Index


     




The calculations are based on standard phantoms. These are not a good representation of the western radiation worker population and, contrary to normal modelling practice, no sensitivity analysis has been made. Hence we have no idea how quickly the values change with weight, in particular. It seems bizarre that the calculations are based on tiny voxels given the sheer variability of humanity. Where does this move to tiny voxels actually produce a genuine improvement for anything other than that tiny fraction of the population that is close to standard man or woman?


 


As the authors identify, the proposed quantities are vulnerable to changes in the organs and weighting factors used to calculate E. This is a backward step. It could inhibit desirable changes to organs and weighting factors. The current quantities are not susceptible.


 


The RBE at lower photon and electron energies is known to be significantly greater than 1. If we are aiming to control effective dose, reducing the dose/kerma at low energies is wrong.


 


The proposed changes will be expensive:




  • Adjusting the calibration settings of every instrument and dosemeter by about 15 %




  • Rewriting procedures to do this




  • Modifying the energy response of survey instruments and dosemeters. I am, I think, qualified to do comment as I''m responsible for the designs of large numbers of detectors including most of the Centronic GM tubes, the Mirion G64 silicon diode based gamma alarm, the EPD, the invention of the energy compensated end window GM and several ion chamber types. I have probably done more than anyone else. Where has the modification been considered in the document? An initial estimate tells me we would have to add a 0.15 mm copper sheet with a hole in the centre with a diameter equal to 1/5th of the detector diameter on the slide on an ion chamber and on an end window energy compensated GM. The sides of ion chambers will also require the same change, only this time the additions will be two cylinders. For steel walled energy compensated GMs, a narrow ring of 0.15 mm copper will need to be added over the centre gap and discs fitted to the ends. The same process will have to be applied to the vast majority of dose rate meters, such as proportional counters and plastic scintillators. The only one where the change can be made using software are sodium iodide (generally) based “isotope identifiers” which have a dose rate function.




  • For single detector silicon diode based active dosemeters, generally the energy compensation filter will need to be thicker. These are difficult to work with as they are not designed to be taken apart and it is also not easy to stick extra material on the outside as these fit into readers, chargers etc. which expect very consistent dimensions




  • For dual detector silicon diode based instruments such as the EPD and the DMC2000 and 3000 series, it may be possible to modify the response by reducing the contribution of the low energy diode and by adjusting energy thresholds. But the word is “may”. As these often form parts of approved dosimetry services, this needs to be examined well in advance of any change. The authors may not be aware how many of these there are. For the EPD, there are in excess of 400,000. The DMC series has been made in larger numbers.




  • GM based dosemeters are preferred for their higher sensitivity. They make better ALARP tools. These can have remarkably good energy and polar responses. Users expect a reasonable level of agreement between these and any “legal” dosemeter. The more ambitious ones have a useful response down to less than 40 keV. These will also require modification




  • How do the authors propose dealing with passive dosemeters? Following the same reasoning, holders for LiF based dosemeters will require a copper disc with a centre hole fixed below the 10 mm of tissue equivalent part of the holder.




  • What proportion of badged workers and which proportion of surveys work in radiation fields where the new quantities will give “better” answers? I suspect both are small and that the dose rates and delivered doses are generally very small, with the possible exception of interventional radiology. But manufacturers will be obliged to change designs for all instruments




  • Operators will benefit from the increased exposure per unit dose in many situations, given that high doses are generally delivered by high gamma energies. They will be able to get more hours per worker before hitting dose limits or constraints. However, the workers will need a great deal of convincing that this increase is valid.




  • Similarly, radiation workers who have consistent dose histories will be very suspicious when their doses suddenly drop by 15 %. This is a far from trivial consideration and will demand much time from the RP staff.




  • The same consideration is relevant when dealing with survey results. Again acceptance may be a problem




 


Summary


I am vehemently against the introduction of these quantities, at least until a good cost/benefit analysis has been performed.