Re: Cloud chamber mystery

[Maurice Barnhill <mvb@UDel.Edu>]

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?

[The source is inserted into the chamber just above the liquid alcohol
on a pin attached to a rubber cork which then closes the insertion hole,
so the source is in direct contact with the vapor inside the chamber.]

brian whatcott wrote:
> At 14:02 2/4/98 -0800, you wrote:
> >       If that was a sealed source I doubt that you will see many
> > positron tracks....they will mostly annihilate in the material around the
> > source.  To see them you need to deposit a very thin layer of a positron
> > emitter on a substrate.
> >
> > Mark
> >
> > Dr. Mark H. Shapiro
>
> My knowledge of this topic is not great.
> But enough to suggest that most positrons are thermalized before
>  annihilation - production of which is a prime use of 11Na22 -
> the prototypical artificial radio isotope.
>  I would expect about the same range as from an electron -
> enough to penetrate the capsule.
>
> Brian
> brian whatcott <inet@intellisys.net>
> Altus OK