[ RadSafe ] Portal monitors to detect SNM

[A Karam]

Detectors looking strictly at radiation can be foiled by shielding.  When surveying the nuclear weapons on my submarine (unshielded), I could measure both gamma and neutron radiation at low levels, and in close proximity (perhaps 10 cm away).  Using a large NaI detector, I could read elevated count rates from a few meters away.  However, the gamma radiation emitted by nuclear weapons is fairly easy to shield and I would consider it an unreliable way to detect them.  Similarly, the neutrons emitted are fairly easily shielded and I would also consider that an unreliable detection method.  We have to assume that terrorists will be intelligent enough to install a few cm of lead and a few tens of cm of borated plastic around a nuclear weapon to reduce the chance of detection.  Similarly, we must assume that a terrorist importing RDD materials would use sufficient shielding to make detection very difficult.  As a case in point, the blood bank irradiator at my former employer's facility contained over 5000 Ci of Cs-17, yet was so well-shielded that a dosimeter placed directly on it consistently recorded no exposure each quarter.  We must remember, too, that a few tons of shielding in a cargo container is not going to alarm anyone - we expect cargo containers to be heavy.
 
The most promising technology I have seen is the Los Alamos system using cosmic ray muons.  Muons are very penetrating and their scattering angle depends on the atomic number of the material through which they are passing.  By carefully analyzing the scattering angles of muons passing through a container, LANL researchers were able to reliably detect high-atomic number materials, and even to obtain images (in one case, a C-clamp; in another case, lead bars cut to form the letters LANL).  The muon flux and their detection efficiency was such that they performed this with a 1-minute integration time.  They wrote their results up in Nature a few years ago, and a recent press release noted that this work is continuing and being expanded.  This promises to be the best way to "look" inside of containers for specific elements of interest.
 
Andy

________________________________

From: radsafe-bounces at radlab.nl on behalf of Rick Orthen
Sent: Wed 4/13/2005 9:07
To: 'Matt Wald'; radsafe at radlab.nl
Subject: RE: [ RadSafe ] Portal monitors to detect SNM



Matt--Your excellent question has fell on deaf ears.  It appears that this
list is now populated by less than a half-dozen people who are obsessed with
anything other than radiation protection or health physics.  If you have
questions about wind farms, teratogenic sperm, mythology and such, hang
around.

As far as the cargo portals, unfortunately you'll have to look elsewhere for
an informed response.  Perhaps the manufacturer, SAIC-Exploranium?  And I
would doubt (for obvious security reasons) you'll find anyone willing to
disclose the technical limitations of the portals.

Rick Orthen