ANNUAL DOSE, DOSERATE and the MAXIMUM INDIVIDUAL

[Quintino De Notariis <qdenotariis@novacon.it>]

If the ANNUAL dose limit for the public is an EFFECTIVE DOSE (ED) of 1

mSv/year, why some radsafers seem to take for granted that this IMPLIES a

DOSERATE limit of 0,114 microSv/h (1000 microSv/year x 8760 hours/year) ?



So if, for example, in an X-ray facility we MEASURE 0,200 microSv/h above

the background in correspondence of a door on an alley with a very limited

or no time occupancy by members of the public (say, ONLY 1 hour/year),

should we conclude that that facility is out of low, no matter that the

total ED in a year would be ONLY 0,200 x 1 = 0,200 microSv ?



Regulations give an ANNUAL dose limit NOT a DOSERATE limit.



Somewere I've read that requiring the limit of 0,114 microSv/h INSTEAD of 1

mSv/year is the consequence of the hypothesis of the "MAXIMUM INDIVIDUAL"

(that's to say, a person who stays behind that door 24 hours a day for 365

days = 8760 hours/year).

Isn't this hypothesis absurd? Shouln't we consider the EXPECTED TIME

OCCUPANCY? Or are we about to apply the LNT even to dead matter?



Another related question :

I don't think that one could MEASURE EFFECTIVE DOSE with an instrument,

because of the definition of ED through the tissue weighting factor WT :

                            ED = … WTxHT

where HT is the equivalent dose to tissue or organ T.

ONLY in the circumstance of UNIFORM irradiation of the body we can say that

ED and H values coincide :

                            ED = H

where H is the equivalent dose to the body uniformly irradiated (… WT = 1).

If PARTS of the body are striked by radiation we have

                            ED < HT

So, if we read 0,200 microSv/h Equivalent Dose above the background at the

level of the lungs (WT=0,12), even in the hypothesis of the maximum

individual we don't exceed 0,114 Effective Dose, because

                0,200 x 0,12 = 0,024 microSv/h.

In conclusion, we shouldn't take it for granted that a doserate > 0,114

microSv/h above the background could mean that we have exceeded the annual

limit of 1 mSv/year, because we should consider not only the EXPECTED TIME

OCCUPANCY by the members of the public IN A YEAR but also the GEOMETRY of

the irradiation (uniformity or non uniformity).



Please, radsafers, help me to clarify and correct me if there is something

that I miss.



Quintino De Notariis

Health Physicist   

 



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