AW: [ RadSafe ] Allowable Dose Rates
Rainer.Facius at dlr.de
Rainer.Facius at dlr.de
Tue Mar 28 08:27:31 CST 2006
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.(?)
Dr. Rainer Facius
German Aerospace Center
Institute of Aerospace Medicine
Voice: +49 2203 601 3147 or 3150
FAX: +49 2203 61970
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:
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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