AW: [ RadSafe ] Allowable Dose Rates

Tue Mar 28 08:27:31 CST 2006

Robert:

In an occupational setting, the German Radiation Protection Ordinance generally does not specify separate limits for dose-rate.

For radiation workers - those that are 'at risk' to receive more than 1 mSv per year from their regular occupational activities - some dose rate constraints are derived by dividing annual limits (e.g. 50 mSv effective dose) by a nominal annual working time of 2000 hours.

An exception is the dose-rate limit of 3 mSv/h. A region where under normal operating conditions 3 mSv/h can be surpassed defines an "exclusion area" which must be sealed off and marked as such and which may be entered only under supervision and if compelling reasons do necessitate this.

Otherwise, separate limits for dose-rate are given to define 'license free' radiation sources. Sources for which "under normal operating conditions" at a distance of 10 cm from their "tangible surface" the dose rate stays below 1 micro-Sv/h are exempt from the necessity of authorisation. For ion accelerators this limit is 10 micro-Sv/h.(?)

Regards, Rainer 

Dr. Rainer Facius
German Aerospace Center
Institute of Aerospace Medicine
Linder Hoehe
51147 Koeln
GERMANY
Voice: +49 2203 601 3147 or 3150
FAX:   +49 2203 61970

-----Ursprüngliche Nachricht-----
Von: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] Im Auftrag von ROBBARISH at aol.com
Gesendet: Dienstag, 28. März 2006 12:41
An: yravello at ipen.gob.pe; radsafe at radlab.nl
Betreff: [ RadSafe ] Allowable Dose Rates

Mr. Ravello writes:

Dear Sirs,

I have to mention one thing about my consult in the  list: the dose rate that we measured was at control booth. If we take the  NCRP recommendations, the dose in a wk must be less than 0.1 mGy. We  calculate the dose in function of the workload: the result was that the  shielding is good. But usually, the dose rate we measured is less than 100  uSv/h [kerma]. So, when we measured the 500 uSv/h, we asked: is there a dose  rate limit for practice purpose?

Here is my assessment: 

My daily work now consists almost exclusively in the design of  radiation shielding for medical facilities worldwide. In this regard,  I point out that in the United States the acceptable dose is only  determined by the workload and occupancy at the point of interest, not by dose  rate. In the United Kingdom, however, the regulatory authorities have  decided to consider the "instantaneous dose rate" (IDR) from medical x-ray  equipment in deciding on the acceptability of shielding.

The current standards that are expected to be met are elaborated in a document with the title: Medical and Dental Guidance Notes: A Good Practice  Guide on All Aspects of Ionising Radiation Protection In the Clinical  Environment. 
This document is published by the Institute of Physics and  Engineering in Medicine. My copy is dated 2002.

Briefly put, that document does not specify a maximum permissible dose rate in "supervised areas" (equivalent to what we in the USA call "restricted areas"). So the brief answer to Mr. Ravello is that there is no dose-rate-limit at the control booth. He needn't worry.

The situation is quite different for "unsupervised public areas" (what we  in the States call "unrestricted areas"). The guidance document calls for an IDR with a maximum of 7.5 uSv per hour in those public spaces. For  those of us who think in old units, that's an instantaneous dose rate of only  0.75 mrem per hour!  In those same areas the recommended annual dose is 300  uSv (30
mrem) per year, about a third of the 1 mSv (100 mrem) annual dose limit  applied in US facilities. 

For a shielding design in the UK, the IDR plays a significant  role in the design criteria. Just as a brief example, if a 15 MV medical linear  accelerator is running at a dose rate of 6 Gy per minute at the  isocenter, and a horizontal beam impacts a person at a distance of 6.5 meters  away, the shielding required to meet the IDR standard would be just over  8.5 feet of concrete. Here in the USA, with identical geometry, for an annual  dose of 100 mrem and a typical workload of 500 Gy per week with a use  factor of 1/4 toward the wall, the required shielding would be only a bit more  than 7 feet of concrete. 

What's interesting about this, aside from the fact that the National Health Service apparently doesn't mind paying for this extra shielding, is that the document contains the following quote: "In estimating adequate protection at the  design stage, the following future developments should be considered:
(a) increases in dose rates."

In designing here in the USA, if a new breed of accelerator has double the dose rate of existing units, the patient treatment time would be halved, so the  shielding would still be adequate given the fact that actual beam-on time is a  small fraction of the total time devoted to each patient. In the UK, the IDR  from this hypothetical new machine would double, requiring an extra HVL of  shielding. The irony is in the request to estimate this technological leap in  the design stage. My psychic powers don't allow me to predict what the dose rate  of a future generation of linacs will be, so, I confess to this forum, I  really can't meet the requirements of the IPEM document when I design a British facility. I hope I don't lose any jobs there as a result of this confession.

Anyway Mr. Ravello, as long as your control booth is meeting the weekly integrated ALARA dose limit of 0.1 mGy (10 mrad), I think that it's more than adequate.

If there is a subscriber to the list from a country where there are IDR restrictions and the permitted IDR is less than 7.5 uSv/hr in unsupervised  public areas I would be interested in knowing where that is, and what those  limits are!

Robert Barish, Ph.D., CHP

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