[ RadSafe ] Dose rate limits

[Parry, Don (MDCH)]

Michigan did at one time have a criteria for new  linear accelerator
shielding designs that said:

"The instantaneous dose equivalent rate, as measured with a rate type
survey meter, in controlled and uncontrolled occupied areas does not
exceed 5 millirem per hour, normalized to a beam output of 200 rads per
minute at isocenter."

This was a criteria we used when evaluating new shielding designs.  It
allowed higher instantaneous rates for higher output machines.  If the
instaneous rate was exceeded it did not mean the plan would not be
approved, but we would look closer at the calculations for the weekly
integrated dose equivalent to ensure the facility design met the 2
millirem per week for uncontrolled areas and 10 millirem per week for
controlled areas.

It was a rule of thumb provided years ago by one of the therapy
physicist to aid us evaluating the adequacy of shielding.  Our current
criteria can be found at 

http://www.michigan.gov/mdch/0,1607,7-132-27417_35791_35793_35801-69837-
-,00.html

and instead of an instaneous dose rate it indicates that 

"The dose in any unrestricted area does not exceed 0.02 mGy in any one
hour of operation with the occupancy factor equal to one."

in addition to 

"The maximum integrated weekly radiation levels in occupiable areas
outside the room, taking the workload, use factor, and occupancy factor
into account, do not exceed the following: 

0.1 mGy per week in air kerma average (5 mGy per year) for controlled
areas

0.02 mGy per week in air kerma average (1 mGy per year) for persons in
uncontrolled areas."

Don

____________________________________

Donald E. Parry, MHP, CHP
Health Physicist
Radiation Safety Section
Michigan Department of Community Health
Phone : 517-241-1989  Fax: 517-241-1981
mailto:dparry at michigan.gov
Web Site:  www.michigan.gov/rss



-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On
Behalf Of Stabin, Michael
Sent: Wednesday, March 29, 2006 9:27 AM
To: radsafe at radlab.nl
Cc: ROBBARISH at aol.com
Subject: [ RadSafe ] Dose rate limits



>for restricted areas such as the x-ray  control booth you discuss,
there are no dose rate limits in any jurisdiction that I know

I have heard that in the USA, two states, MI and PA, have imposed
additional protection limits (above the standard US-wide annual dose
limits) based on instantaneous dose rates, that are used in shielding
calculations:

MI: 13.9 nSv/s controlled or uncontrolled
PA: 5.6 nSv/s for uncontrolled 

Am I correct about that? Thanks.


Mike

Michael G. Stabin, PhD, CHP
Assistant Professor of Radiology and Radiological Sciences 
Department of Radiology and Radiological Sciences 
Vanderbilt University 
1161 21st Avenue South
Nashville, TN 37232-2675 
Phone (615) 343-0068
Fax   (615) 322-3764
e-mail     michael.g.stabin at vanderbilt.edu 
internet   www.doseinfo-radar.com


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