Re: broad beam conditions

["Tom Johnson" <tjohnson@radtrain.com>]

The quantity of scattered radiation will increase directly with the
scattering area (field size).  "Introduction to Health Physics" by Herman
Cember has a discussion of this on page 438 and 439 of the third edition (it
is in  Chapter 10 if you are using the second or first edition). If it is
available, you may also want to look at the NCRP 49 discussion on the
development of secondary barrier equations for this matter and broad beam
considerations for scattered radiation.

Tom Johnson
tj61@juno.com

-----Original Message-----
From: Chris Alston <alstonc@odrge.odr.georgetown.edu>
To: Multiple recipients of list <radsafe@romulus.ehs.uiuc.edu>
Date: Thursday, May 21, 1998 9:36 AM
Subject: broad beam conditions


>>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
>> ===================================
>>
>>                    *****
>>Frequently Asked Questions (FAQ) for Medphys at:
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