[ RadSafe ] Re: Linac Shield calculation/NCRP 151 and others..

[Mark Ramsay]

Thanks Joseph

I did consider this and looked at attenuation factors and explicitly
build-up. 

However, I believe the NCRP 151 (and other references) use TVT and TVTe
(equilibrium), where TVTe accounts for spectral  changes in the inner
part of the shield (and it is used in a similar way to buildup). So I
think this has been considered but is not the cause of the issue. The
literature also quotes that primary barrier TVT's etc are
'conservatively safe'.

Since writing the initial query I did go back and look at density. A
density of 2.2 g/cm3 (instead of the required 2.35) would produce the
effect I have seen. Therefore I have asked to look at the density data
again to see what error margin they work to (the difference in density
might not look much to them but its effect on TVT / TVTe is enough to
double the dose rate). 

I am also aware that the 'dose equivalent' calculated in NCRP 151 is not
the same as effective dose equivalent (the quantity measured by the ion
chamber) - so I am not expecting something that is numerically the same,
but was expecting something closer than I got. One hunch I have is
simply that most people build bunkers with enough factor of safety such
that changes of a factor of 2 (in IDR) are just not noticeable (or are
ignored). i.e. if the IDR had been say 1 micro Sv/h then doubling this
to 2 micro Sv/h would still have been in the normal variability of the
ion chamber close to background levels.

Thanks

Mark


-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On
Behalf Of JPreisig at aol.com
Sent: 06 July 2009 14:54
To: radsafe at radlab.nl
Subject: [ RadSafe ] Re: Linac Shield calculation/NCRP 151 and others..

Hi Radsafers and Mark Ramsay,

     Perhaps, for the case of gammas/photons interacting with a concrete

shield
wall, one might need to include Buildup factors in the hand calculation.
For information on Buildup factors, please refer to Cember, the original

Rad Health
Handbook <USA>, Schleien's version of the Rad Health Handbook, etc.
A factor of 2 might sound right for such a Buildup factor.

     Please do this calculation yourself if possible.  

     Best wishes...        Regards,    Joseph R. <Joe> Preisig, Ph.D.


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