Gamma Doserate Modeling

[hodgdon@homer.yankee.com (hodgdon@yankee.com)]

An alternate method to test the accuracy of the model for targets 
above the array is the semi-space (half infinite) source equation.  The dose
rate in Rad/h is one half of 5.76E-5 times source power in MeV per 
gram second.  This is based on the principle that in an infinite
medium the energy released is the energy absorbed.

Assuming a source of 1 g/cc air emitting 1 MeV photons at a rate 
of 1.7E+4 ph/sec cc (conveniently the inverse of 5.76E-5) the equation 
predicts 0.5 rad/h.  

Microshield, for a target 10 cm from a rectangular volume source 100 cm 
deep, by 1000 by 1000 cm and integration parameters 20x40x40, predicts 0.48, 
which is close enough.

It is likely, given the density of the individual sources, that you can
estimate the dose rate from the array by a measurement made in contact 
with a single drum.

Given the size of the array any method you use should provide results
that are relatively stable with small changes in target location.  Be 
wary of point kernel methods especially adjacent to the side of the outer
most drums.  In this location dose rate could be dominated by scatter 
that may require more advanced techniques.

I can email the derivation of the equation and the microshield runs to 
anyone who wants them.  If I get more than a few requests I'll just post
it here (only a few pages of text.)

Andrew Hodgdon
hodgdon@yankee.com


RESPONDING TO

>Date: Tue,  2 Jul 96 14:09: 7 GMT
>From: phf@cap.urenco.co.uk
>Subject: Gamma Doserate Modeling

>To All

>I have to model an array of 220L drums 
>containing Uranium the array is 110 m long by 
>75 m wide by 3 m high. I tried to do this 
>using Microshield assuming the array formed 
>one large rectangular unit. This gave strange 
>answers that I do not believe. Can you please 
>reply to me if you have any experience using 
>other models ( I have read that Oak Ridge have 
>used a Monte Carlo code in the past ) that I 
>can obtain easily or if you have any thoughts, 
>ideas or previous experiences that you would 
>think useful. Please reply to 
>phf@cap.urenco.co.uk or to all if you think it 
>will be of interest.


>Thank You

>Paul Fazey