In addition to the physics goals of MicroBooNE, there are a number of
technical challenges to be met in building a large scale liquid argon TPC
for neutrino physics. The experience gained in MicroBooNE will help inform
future LAr-based neutrino experiments such as
DUNE and the
short-baseline neutrino program at Fermilab.
The MicroBooNE detector. The high voltage feedthrough enters on the right and supplies voltage to the cathode plane. One side of the field cage can be seen on the face of the cut-away (supported by the "X" braces). The sense and induction wires are on the left side of the vessel. Behind the wire planes is the support structure for the PMT array (not shown).
The MicroBooNE detector is a 170 ton (total volume) liquid argon (LAr)
time projection chamber (TPC). It is currently the largest LAr TPC operating
in the U.S.
The liquid argon serves as the neutrino target. This is desirable since it
is dense, inert, and relatively cheap compared to other noble liquids. The
argon must be kept extremely cold (87K) to remain in liquid form and be
extremely pure for the ionization electrons to drift across the TPC volumes
so that their signals can be recorded.
The TPC consists of a cathode plane on one side, a field-shaping cage
around the drift perimeter, and three planes of wires on the opposite end to
record the signals from the drifting ionization electrons. The cathode plane
is powered to a very high voltage (roughly -100 kV) to create an electric field
aross the TPC volume. The ionization electrons will travel through this field
and their signals measured by the three wire planes. Each plane is offset by
60 degrees from the others providing spacial reconstruction ability in the
y-z plane. Knowing when the charge reaches the wires allows for determination
of the x component.
TPC wires connected to a wire carrier board.
Further timing information will be provided by a photomultiplier tube (PMT)
array (see photos on left) measuring the prompt scintillation light produced
during the excitation or ionization of argon atoms.
The support structure holding the PMTs that provide precise timing information. The structure is located behind the TPC wire planes so that the PMTs can view inside the TPC volume.
A single PMT. The full array will have 32 of these 8 inch diameter special cryogenic PMTs.
The MicroBooNE detector was filled with liquid argon
in the summer of 2015. The argon was successfully purified using the
MicroBooNE filtration, recirculation, and purification system. MicroBooNE saw
cosmic ray tracks in the TPC in August 2015 and started collecting
neutrino beam data in October 2015!
The D0 assembly building where the TPC was assembled.