Does anybody have any suggestions about how a beta+ source could
suppress tracks in a cloud chamber? Maybe some details are in order.
David Daughton, one of my advisees and an honors-level freshman, saw a
recent television program which pair-production in a cloud chamber, and
he wanted to see the experiment live for himself before he was
completely convinced. I'm a theorist, so I got help from one of our
experimentalists, Cheng-ming Fou. Our lab technician found an old
instructional cloud chamber and Cheng-ming supervised David in cleaning
and setting it up. It uses methyl alcohol sealed inside a very small
tank whose sides have absorbant paper to help with the evaporation of
the alcohol. The chamber, once working, shows tracks continuously, and
if you get too many and can't tell what is going on you can clear the
container with an electric field. It has two very weak radioactive
sources, a beta emitter and an alpha emitter. When David set it up, we
could clearly see tracks coming from the source and other tracks that we
assume were cosmic rays. Detecting and identifying electrons was no
problem. Cheng-ming found a magnet from another demonstration setup
which David could use to provide a magnetic field, and you could
(barely) see the curvature of the various tracks. However, there were
only two apparent pair-production events, one I saw and one David saw,
both near tracks that did not appear to come from the demo source.
David wanted to be sure he had seen positrons, so Cheng-ming got a small
beta+ source [Na22] from our radiation safety office. It was much
stronger than the demo sources. Na22 yields (1/2)-Mev positrons.
[Cheng-ming is used to handling sources and the source was nowhere near
strong enough to raise serious safety concerns.] When the source was
inserted into the chamber, a brief, continuous cloud a millimeter or so
thick was generated about helf a centimeter from the source. No tracks
were seen then or later. In fact, none of the source-independent tracks
were seen when the beta+ source was in the container, even though they
had been readily apparent immediately before inserting the source.
Anybody have any idea how the beta+ source suppressed the cosmic-ray
tracks?
To anticipate one question, we detected radiation from the source with a
counter, but we have no evidence that we were detecting beta+ radiation
as opposed to, say, annihilation gammas.
--
Maurice Barnhill, mvb@udel.edu http://www.physics.udel.edu/~barnhill/
Physics Dept., University of Delaware, Newark, DE 19716