[ RadSafe ] "Two-scatter" rule in medical x-ray protection

[Falo, Gerald A Dr KADIX]

Hi all,
 
I don't do much medical health physics but many of my colleagues have
and continue to do so.  Between speaking with colleagues and reading
NCRP-147, I never realized that "the booth shall be positioned that no
unattenuated primary or unattenuated single-scattered radiation will
reach the operator's position in the booth." The same or similar text
appear in NCRP 49 and other documents.
 
I'm wondering why "two-scatter" rule exists. If the sum of the leakage
and scatter dose is less than the limits, then why does the
"two-scatter" rule exist? As far as I can reason, scatter radiation is
of a lower energy an lower intensity than either the primary or leakage,
so it's not a question of radiation quality. Is it simply an ALARA
consideration?
 
Thanks for your time and patience.
 
Jerry Falo
 
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Gerald A. Falo, Ph.D., CHP
Kadix Systems 
U.S. Army Center for Health Promotion and Preventive Medicine - Health
Physics Program
jerry.falo at us.army.mil
410-436-4852