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

[Mark Ramsay]

Morning all.

 

A little mystery I am trying to solve - would be useful if anyone who
works with medical linacs could give me their views. 

 

This should be so simple - but the results have not been as expected.

 

Basic data:

 

10MV linac

6 Gy/min at Iso Centre

Distance between ISO and inner concrete primary wall = 5.8m

Concrete thickness is 210cm

Concrete density is 2.35

Use factor (Gantry) 0.25 (but not really needed here).

 

OK, using NCRP 151 and other references I make the calculated IDR
(Instantaneous dose rate) on the outer (safe) surface of the shield to
be 21 micro Sv/h.

 

A colleague using different references also calculated a IDR of around
21 micro Sv/h.

 

However, the maximum actual dose rate measured using two independent and
calibrated ion chambers   was 44 micro Sv/h.

 

Calculations normally errr on side of caution - so what is going?

 

So far:

 

Concrete density - checked

Thickness -  checked

Distances - checked

Linac output  - checked

Concrete pour quality (not checked  - but this occurs on both primary
shields so I would not expect the pour on both separate sides to be
equally poor).

 

 

For the record, I did not design this shielding and would have preferred
something a bit thicker. Furthermore, I know we are talking here about
IDR and in actual fact the 'dose-rate-in-any-one-hour' is acceptable.

 

But physics is physics and this is not rocket science - so what am I
missing here?

 

Any comments, advice would be helpful. Anyone willing to spend just s
couple of minutes repeating the above calculation using would also be
useful.

 

Thanks

 

Mark

 

 

Mark Ramsay

Radiation Protection Adviser

 

Ionactive Consulting Limited

Email: mark.ramsay at ionactive.co.uk <mailto:mark.ramsay at ionactive.co.uk> 

Web: www.ionactive.co.uk <http://www.ionactive.co.uk> 

 

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