H-3 - another theory

[ELMCGUIRE@life.uams.edu]

Let me throw another possibility out:

Electrons, contrary to what is commonly believed, or at least what is
convenient to assume, DO NOT have a finite range.  Other processes other than
the CSDA range are involved - straggling, etc. See Turner or any of Berger's
work. The upshot is that you can have some penetration of electrons at depths
>> CSDA range. Think of it like a tunnelling effect. For example, see Table 9 
of MIRD 5; the x90 for a 20 kev 
electron is 6E-4 cm in water. Now, a 1 mg/cm^2 wall is about 10E-4 cm, assuming
unit density.  So, if you have a large enough activity source, even with few
particles at or near Emax, and a container and detector with walls > range,  
I'll bet you're detecting some H-3 beta particles.  

I've seen this phenomena with P-32; don't try shielding 100 mCi with 1 cm
lucite - you'll need more.
   

Lynn McGuire
Univ. of Ark. Med. Sciences