High Radiation Area, basis, purpose

["Les Slaback, Health Physics" <SLABACK@micf.nist.gov>]

There has been an ongoing discussion among a few select persons regarding the
application of the High Radiation Area definition where only a small portion of
the body can be exposed, e.g., for radiation beams.  I offer this as food for
thought since extreme geometries/situations tend to help clarify understanding
of general principles.
First...
 -note that the 'area' definitions are strictly regulatory entities, and hence
basically are subject to the whims (excuse me, deeply and carefully considered
interpretations) of the regulatory agency.
 -being control/warning devices there presumably is not a deep philosophical
basis related to their origins.  The issue presumably is one of effectiveness
of achieving their purpose, whatever that is.
 -the NRC Q&As on the 'new' 10CFR20 have many questions relating to the
interpretation of the HRA rule, and provide very useful discussions.

So...
 -what is their purpose? Dose control? Information? Part of the ALARA program?
Overexposure prevention?  Is the underlying purpose different for the different
'area' definitions (RA, ARA, HRA, VHRA)?  Perhaps the only purpose is the fact
that one must comply with federal regulations, and they are part of the
regulations.  Does anyone recall seeing a stated purpose for these definitions?
Can you provide a reference?
 -does it matter how large a fraction of the body is exposed?  Note: NRC
uses a dose definition related to the highest exposed point of the body, not an
effective dose, for external exposures.  But if an HRA is simply a warning
device, not a dose control device, then the definition of equiv. dose may not
be relevant (i.e., perhaps air-kerma is the unit of choice).
 -NRC attempted to limit the application of the HRA definition by adding the
30 cm provision to the rule and using the word 'accessible'.  So clearly every
101 mrem/h field is not an HRA. But the Q&A responses muddy this somewhat by
responding that sometimes the 30 cm provision does not apply.  So what types
of 100+ mrem/h fields are not HRAs?  For example, 1: two parallel shield walls,
each shielding line sources and emitting radiation, with an accessible area
29 cm wide; 2: a beam 1 mm square thru open air over a path length of 3 meters;
3: a planar beta source of such an energy that the deep dose rate at 30 cm is
less than 100 mrem/hr (but the 10 cm dose rate might be many rem/h).
 -for those that qualify for exlusion does it matter how much in excess of
100 mrem/h the field is?

Other questions...
 -aside from proper regulatory compliance is this worth contemplating?  That
is, do these definitions significantly contribute to people safety? If each
person working in a restricted area wore an alarming electronic dosemeter
would these rules serve any purpose?
 -does anybody (e.g., state rules) control x-ray diffraction beams as HRAs?
 (Note- in-beam dose rates can be >>1000 rem/h, albeit over mm square areas.)
Note that this discussion is not a substitute for an official interpretation
for your pertinent regulatory authority.  It is more in the nature of 'in
a perfect world how should it be'.

Also please note: the dreaded g* word did not appear in this message.

SLABACK@MICF.NIST.GOV
   ...a little risk, like a bit of spice, adds flavor to life