The
ATHENA Detector 
Once antihydrogen is formed in the trap, it
will no longer be contained by the electric and magnetic fields, since it
is now a neutral atom. It will move in a straight line out of the system.
Once it collides with the electrode wall or with a background gas atom, both
the positron and the antiproton will annihilate almost simultaneously, within
a few nanoseconds of each other.
The antihydrogen detector surrounds the vacuum
system of the recombination trap and is 'wedged' between the cryogenic vacuum
system of the traps and the bore of our 3 Tesla superconducting magnet. The
detector detects the products of both the antiproton and the positron annihilations.
The annihilation of antihydrogen is different from that of unbound antiprotons
and positrons by the fact that with antihydrogen both the antiproton and
the positron annihilate at the same point in time and space.
The annihilation of an antiproton on a nucleon
produces on average 3 to 4 charged pions in the 50 to 900 MeV energy range.
Silicon strip detectors, arranged in two layers around the recombination trap,
measure two points of each of the trajectories. Each layer of strip detectors
consists of 16 detector modules arranged around the circumference with each
module having 128 strips on one side (r-φ) and 128 pads (z) on the other side. The vertex
of the antiproton annihilation is determined by the intersection of the lines
extrapolated from the measured points on the strip detectors. The error on
the vertex position is largely dominated by the curvature of the pion tracks
in the 3 Tesla magnetic field of our experiment.
The annihilation of a positron produces two
511 keV back-to-back gamma rays. They are detected by 192 CsI crystals, arranged
in 16 rows with 12 crystals in each. The crystals surround the Silicon strip
detectors. Each crystal is the size of a slightly overgrown sugarcube, being
about 13 mm on each side. The photo at the top of this page shows the installation
of the CsI crystals. If two crystals register energy deposits compatible
with 511 keV gamma rays within about 2 microsecond of an antiproton annihilation,
it is assumed that they originate from within a straight line between the
two crystals. For antihydrogen annihilation, it is then required that the
vertex position determined by charged pions lies within the errors in determining
this line.
We could thus say that our detector is a
miniature version of the big detectors that are common at big high energy
physics accelerator facilities such as CERN. However, there are a few things
that make this detector unique besides its compact size. First of all the
detector is operating inside a 3 Tesla magnetic field. This is 60,000
times stronger than the Earths magnetic field and means that special electronics
must be used. Also the detector is thermally anchored to the bore of the magnet
which is at liquid nitrogen temperature (77K or -196°C) and thus kept
very cold. Since the electronics of the detector generate some heat, the
operating temperature of the detector is about 120K (~ -150°C). The inside
of the detector has several layers of thermal super insulation to insulate
it from the 10K temperature of the trap cryogenic vacuum.
The figure of merit of the detector is the ability
to pinpoint the vertex of these annihilations in space and time. This detector
can locate the vertex of the pions to σZ ~ 4 mm, and σrφ ~ 3 mm. The annihilation site of the positron is
located somewhere along a line connecting the geometric center of the two
CsI crystals hit by the gamma rays. Obviously the gamma rays do not have
to hit the center of the crystals to light it up and this gives rise to an
uncertainty in determining the line made by the back-to-back gamma rays of
about σL ~ 7
mm.
LVJ - Last modified
September 16, 2002