Re: Low energy NaI detection

[Bernard L Cohen <blc+@pitt.edu>]

On Thu, 9 Sep 1999, Neon John wrote:

> 
> I was asked one of those simple little questions today that just
> kinda stopped me cold and made me draw a blank.  The question was,
> can a NaI/PM detector be used for 0.5 keV X-ray detection?  My
> impulse answer was no.  My experience tells me that conventional
> detectors as used in effluent monitors typically have a 50-60 keV
> equiv noise floor.  But before I answered the question with a
> possibly wrong answer, I thought I'd ask this esteemed list :-) to
> see if there is some technique I'm not aware of.  There's no
> particular context to this (vacuum, exposed crystal in a dry box,
> etc.), just kind of a blue-sky question.

	In the most favorable scintillation system, there is one
photoelectron on average produced at the photocathode for each 110 eV of
energy deposited. Thus the pulses produced would average only 4
photoelctrons. This limits the energy resolution, delta E / E, to about
100%; that is, the full width at half maximum of the peak would extend
roughly from 0.25 to 0.75 KeV.
 The other problem is noise which, in ordinary systems, is about 10 Kev or
higher. This could be reduced by cooling. The importance of noise
depends on the strength of your signal, so if your source is strong
enough, noise becomes less important. But getting down to 0.5 KeV seems
very questionable to me.
	Ordinarily, NaI crystals are canned to keep out moisture, and this
would attenuate the gamma rays. This problem could be eliminated by
special enclosures or by operating in a very dry atmosphere.
	There certainly are better ways to detect 0.5 KeV X-rays.


> 
Bernard L. Cohen
Physics Dept.
University of Pittsburgh
Pittsburgh, PA 15260
Tel: (412)624-9245
Fax: (412)624-9163
e-mail: blc+@pitt.edu


************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html