[ RadSafe ] Detecting DU at a distance via beta particle emission.

[Robert J. Gunter]

Other factors would be the geometry of the source.  A projectile, or
fragment would be much different than a slab of DU.  You are probably not
looking for bricks, but maybe so.  In addition, if you are looking for them
with a detector (in other words they are not visible right there in front of
you), they may be covered with dirt or debris, or oxidized which would
attenuate the betas.  A simple solution to find what you can see is to
assume a point source and do a solid angle calculation assuming 4pi
emanations and what would "hit" your detector.  Figure in your efficiency
(by the time it travels through air, the E will be diminished), and compare
this to your background to see if you can distinguish it.

Another way is to put your probe on a string and get it closer to the ground
(maybe with some "feet"), thereby saving your knees....



Good luck.

Rob


Robert J. Gunter, CHP
CHP Consultants
rjgunter at chpconsultants.com
www.chpconsultants.com
www.chpdosimetry.com
Tel:  +(865) 387-0028
Fax:  +(865) 483-7189



-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On Behalf
Of Geo>K0FF
Sent: Monday, January 07, 2008 1:07 PM
To: radsafe at radlab.nl
Subject: [ RadSafe ] Detecting DU at a distance via beta particle emission.

 
Detecting DU at a distance via beta particle emission.


DU (Depleted Uranium) is uranium with the majority of the U-235 removed,
leaving behind U-238 and U-234. 
I prefer to think of DU as refined U-238. The metal has many industrial and
scientific uses, mainly because of its high density, being 1.7 times as
dense as lead. A few common uses would be counterweights and radiation
shielding. DU is used in projectiles because of its unique density,
pyrophoricity and "self-sharpening" characteristics.

The only progeny present that can be detected at a distance would be
Pa-234m, a beta emitter. 
With a maximum energy of  2.28 MeV, the betas would travel approximately
20-25 feet in air, using the rule of thumb of 10-12 feet per MeV.
A sensor would have to be appreciably closer than the maximum distance to
detect the beta particles.

Because of self-shielding, DU metal's surface beta rate is in ratio of the
surface size, not the thickness or 
volume of the sample. In other words a thin sheet would have the same beta
surface emission rate as a thick sheet.
Indeed, DU Slabs are used in the lab to calibrate probes. 

The best "calculations" are done in the lab with a DU slab and a detector!
Using a 2 pound cylinder of DU metal and 
a pancake probe, about 6 feet is as far as you can get and still obtain
usable (statistically significant) readings. Any closer and the readings 
ramp up quickly. With a 100 Cm^2 alpha-beta scintillator on a Thermo ELECTRA
alpha-beta meter, the range is extended to about 10 to 12 feet. 

George Dowell
GEOelectronics at netscape.com
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