broad beam conditions

[Chris Alston <alstonc@odrge.odr.georgetown.edu>]

>Content-Disposition: inline
>Date:         Thu, 21 May 1998 17:14:33 +1000
>Reply-To: Medical Physics Mailing List <MEDPHYS@LISTS.WAYNE.EDU>
>Sender: Medical Physics Listserver <medphys@lists.wayne.edu>
>From: David Thiele <thieled@HEALTH.QLD.GOV.AU>
>Subject:      broad beam conditions
>To: MEDPHYS@LISTS.WAYNE.EDU
>
>Hi all
>
>I have a question about broad beam conditions.  We are measuring the
>attenuating properties of a fibre cement building panel manufactured
>locally.  I am attempting to determine the field size to satisfy broad
>beam conditions (i.e. increase the field size until the attenuated
>exposure remains constant).  However, I find the unattenuated exposure
>also increases with field size! (10cmx10cm up to 80cmx80cm)  Indeed the
>ratio of attenuated to unattenuated exposure is relatively constant
>irrespective of field size.  Now I am confused!  I don't think the
>change in the unattenuated exposure is due to backscatter - the beam is
>pointing horizontally.  Am I seeing off-focal radiation?  If so, how do
>I know whether broad beam conditions are satisfied?  Any help
>appreciated!
>
>Best regards
> ===================================
> David Thiele PhD
> Manager, Statewide Medical Physics
> Biomedical Engineering & Health Technology Services (7D)
> Royal Brisbane Hospital
> Herston  Qld  4029
> Australia
>  ----------------------------------------------------
> Phone: +61 7 3253 7896
> Fax: +61 7 3253 3518
> Email: thieled@health.qld.gov.au
> ===================================
>
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