2011 Hertford College Magazine (Issue 91)

Farnsworth-Hirsch Fusor. You had a vacuum
chamber, which you pumped down
to around 10-6 torr. Then you injected the
gases you want to fuse, keeping the pressure
below 10-4 torr. Inside the vessel were
two electrodes, which were concentric
wire spheres. You put about 20,000 volts
across them, with the inner electrode negatively
charged, and the electric field would
accelerate the positively charged gas particles
towards the centre. Some would collide
with the inner electrode, but many
would pass into the centre, where they
would collide with each other, and hopefully
fuse. You would have your own personal
star in a bottle on a desk. You needed
little more than a small vacuum vessel,
a pump, a power supply, and some carefully
crafted wire basket weaving, unlike
the multinational collaborations that built
JET in Culham and ITER in Cadarache.
But then, they were all trying to achieve
something that I had no intention of attempting.
They were trying to get energy
out of their fusion reactors, a goal that has
been publicized as being twenty years in
the future ever since the 1970s. As a clean,
safe and relatively cheap energy source,
nuclear fusion has been the Holy Grail of
energy production since it was first proposed.
And like the Holy Grail, after years
of searching, it is still far beyond our grasp.
“ You would have your own
personal star in a bottle on
a desk ”
This design would break long before
you got close to getting energy out it, but
that didn’t mean it wasn’t useful. Whilst
getting energy out of it was nigh on impossible,
getting neutrons out of it looked relatively
straightforward, and could be produced
from dozens of possible reactions.
You can use neutrons for all kinds of
HERTFORD COLLEGE MAGAZINE
Hertford past and present: My own personal star
Phil’s reactor
useful things. For instance, you can inject
cancer patients with boron-11, which
is absorbed by tumours, and then blast
them with small doses of neutrons. The
low doses do little damage to the patient,
but are absorbed by the boron, which
breaks down and emits alpha particles,
killing the tumour. Or, you can fire them
at samples. You can look at how the neutrons
are scattered to see the structure of
the sample, a process known as diffraction
analysis, or look at the gamma rays emitted
to see what’s in it, known as neutron
activation analysis. You could even fire it
at airport luggage and look for 10.8 MeV
gamma rays. These are indicative of nitrogen,
which you tend to get only in explosives
and fertilizer, and you shouldn’t
have either of those on a plane. Neutrons
are surprising difficult to get hold of, given
that they are one of the most common
21.