environmental dose rate

[Slaback@micf.nist.gov]

I want an instrument with a sensitivity adequate
to see changes in dose rate of 0.01 microrem/h
(0.1 nSv/h).  The purpose is to directly measure
public dose from the facility Ar-41 emissions.

The 16 L Reuter-Stokes ion chamber has a sensitivity
of 0.1 urem/h.

NaI and similar spectroscopy techniques require fairly
complex spectrum unfolding methods, particularly for a
mixed spectrum from an elevated plume.  It's possible
that a proportional detector would do this, but I have
not seen one.

Having just toured the HPS exhibits with no luck, I thought
I would see if anyone knows of commercially available
instrumentation with this sensitivity.  Of course, the
system has to be able to operate outside in cold/hot weather
(a minor challenge for instrument designers of today's caliber).

Given the specialized nature of this query please reply to me.
I will summarize responses for the list.

A little risk adds spice to life.
slaback@MICF.NIST.gov