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In fact the normal use of the Na22 here is as a source of
annihilation gamma-rays. Cheng-Ming, however, had deposited a small
amount of material on the outside of a metal pin. I *think* that we
should have had at least some positrons.
But the absence of tracks coming from the source is not the problem.
As I said, I cannot prove that we had anything other than gammas
available which would make a marvelous explanation for the absence of
tracks. ***What is remarkable is that the stray tracks we saw in the
detector every five seconds or so in the absence of any source ceased to
show up when the Na22 was placed in the chamber.*** Why?