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From: stiatl!john@gatech.edu (John DeArmond)
Newsgroups: sci.military
Subject: Re: Shielding Nukes
Keywords: finding backpack nukes?
Message-ID: <8692@cbnews.ATT.COM>
Date: 31 Jul 89 12:28:58 GMT
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From: stiatl!john@gatech.edu (John DeArmond)

In article <8676@cbnews.ATT.COM> sigma!bill@beaver.cs.washington.edu (William Swan) writes:
>A question:  Given the potential prospect of terrorist nukes, which would be

>far more likely to be smuggled in than delivered by missile, is it possible
>with this detection equipment to:
>	a) detect, and
>	b) locate
>said nuke, say, one of a size and mass (including shielding, if any) readily
>surreptitiously transported, within a typical city? What amount of shielding
>is sufficient to hide it, if any? Is this classified secret information? :-)

Yes (it can be detected and yes some parts are classified)

The amount of shielding would vary with the geometry and size of the
core.  Shielding attenuation is not hard to calculate.  Sensitivity of
equipment used for detection is as far as I know, still classified.
This is necessary in order to deny a potential terrorist the comfort of
being able to know exactly how much shielding would be necessary.

An important thing to remember is that shielding of gamma rays does not
stop the rays like it does particle beams.  It ATTENUATES the beam.  
The chore is to select the amount of shielding to attenuate the beam
to the desired level.  To protect against detection, one would have
to reduce the gammas to below ambient background.  Since the 
equipment used is capable of discriminateing gamma ray energy, one 
would have to take into account the energy spectrum of background.

One other note.  Lead emits a characteristic fluorscent X-ray when
excited by gamma radiation.  Therefore lead alone is not sufficient
to shield a nuke from detection.

Like I mentioned in another posting, the group charged with protecting
the public against abuse of SNMs (special nuclear materials) is the
NEST (Nuclear Emergency Strike Team).  Since I have done classified
work relating to this and since I really don't want to try to figure
out what portions of my knowledge is in the clear, I'll simply refer you 
to publicly available literature on NEST.  I will comment, though,
that they do their job exceedingly well.

John
-- 
John De Armond, WD4OQC                     | Manual? ... What manual ?!? 
Sales Technologies, Inc.    Atlanta, GA    | This is Unix, My son, You 
...!gatech!stiatl!john    **I am the NRA** | just GOTTA Know!!!