We find that the
electron rest mass is equal to 511 keV of energy. Since the restmass of the
positron is identical, the collision will need to have a surplus of energy
of at least 1022 keV for pair creation to take place.
The other method for generating
positrons is the one we use at the ATHENA Positron Accumulator because it
is technically simpler. It only involves having a radioactive isotope of
the right kind. Most people who have had physics in school know that in radioactive
β decays what is really emitted is a high energy electron. This is more
correctly called a β- decay since there is another kind as well. There are
a few isotopes that decay by what is know as a β+ decay. This is similar
to β- decays except that the particle emitted is not an electron but a high
energy positron. The most commonly used isotope for making positron beams
is Na-22. This is convenient as it has a lifetime of 2.6 years, which
is long enough that you don't need to get a new radioactive source
all the time, yet short enough that the specific activity is rather high.
Unfortunately, regardless of
whether one gets one's positrons from one or the other mechanism, they
both yield positrons with a very wide variety of energies. In order to
make a positron beam, a narrow energy spread is necessary, the narrower
the better. This was a stumbling block that delayed the development of positron
beams for so long. The solution turned out to be to stop the positrons in
a solid. The trick is that once the positrons have stopped in the solid
they will start a random walk diffusion around the solid. If the solid was
made with the right thickness, there is a small chance that these slow positrons
reach the surface of the solid before they annihilate. Once they reach the
surface, a surprisingly wide variety of materials will kick them out. This
is surprising because materials do not normally kick out electrons. After
all there are electrons everywhere and they don't seem to leak out unless
the material has been put under high voltage or something similar. So solids
are generally very good at holding on to electrons but precisely because
of this they are not very good at holding on to positrons; after all they
have the opposite charge. Solids that do this trick are called positron
moderators.
The main technological development
of positrons beams since the 1970s has consisted of finding materials that
are ever more efficient at slowing down and emitting positrons. At present
the best such solid is Neon.
Unfortunately neon is a gas at room temperature
and only becomes a solid at very low temperatures. Therefore our radioactive
source and its surroundings are kept at about 5-6 K or -268 °C.
We then let in neon gas and a layer of solid Neon grows on the cold surfaces.
In this way about 3-4 ‰ of the positrons
made in the radioactive source emerge and are captured in what we now can
call a mono-energetic positron beam. This beam is guided to the main part
of the Positron Accumulator by a magnetic field. Using a radioactive source
with a source strength of 40 mCi we end up with a positron beam of about
5 million positrons per second.
Since a positron beam originating
from a radioactive source is by nature continious we can not use a method
with opening and closing a gate electrode like we do for the antiproton pulses.
Therefore we must use another method for capturing the positrons. Basically,
to trap a particle it has to loose energy so it is no longer free to fly
back out of the trap. This energy loss we achieve in the Positron Accumulator
by having the positrons collide with gas molecules following a method first
developed by the Surko research group at the University of California, San
Diego.
The ATHENA Positron Accumulator accumulates its positrons from a
slow positron beamline that is a technological evolution of those early concepts
of how to make a positron beam. Positrons are not a common occurrence in
nature, just like any other form of antimatter. However, because electrons
have a much lower restmass than e.g. protons, it is relatively easier to
create positrons. Now as then there are two ways of generating positrons:
By pair creation or from certain kinds of radioactive sources. Pair creation
involves collisions with high enough energy that an electron-positron pair
can be formed. If we use Einstein's famous formula:
