Using Liquid Argon Time Projection Chambers in Fermilab's Neutrino Beams

May 21, 2007 David A. Finley / Fermilab

There are two main efforts in the US on liquid argon TPC (LArTPC)
development: 1) Demonstrating the purity required for a large detector
can be achieved without evacuation, and 2) T962, an approximately
one-half ton TPC to be placed in the MINOS near detector hall. The
purity demonstration is intended to end with a 150 ton LArTPC placed
outside the MINOS near detector service building. This 150 ton TPC is
intended to be a true prototype of a 5kton to 10kton detector envisioned
to be placed at Ash River to enhance the physics of NOvA. The prototype
would be constructed in the same manner as an Ash River detector and
thus clearly demonstrate the capability of this particular "no
evacuation needed" approach. It will also demonstrate what level of
cosmic ray shielding is needed to see and fully analyze neutrino events,
and perhaps most importantly, it will clearly show that liquid argon
TPCs can compete.

There have been discussions with people from Europe and the US about
putting an LArTPC in the SciBooNE hall in the Booster Neutrino Beam
(BNB). Recent calculations show a 4 to 8 ton detector would easily fit.
A possible purpose of such a detector would be similar to SciBooNE
itself. It sits in a neutrino beam appropriate for the T2K energy
spectrum but, unlike SciBooNE, it would help understand what would be
seen by a 2 km LArTPC near detector for T2K (which one assumes will be
built in due time). This could be a basis for an expanded collaboration
between Japan, the US, and Europe. The timescale would be after
SciBooNE, in 3 or so years.

There have been discussions with people from Europe and the US about
putting an LArTPC in the MiniBooNE hall. This hall receives neutrinos
from both the BNB and the NuMI beam. A liquid argon detector with a mass
slightly in excess of 1kton would easily fit in the hall. Of course
MiniBooNE would have to be removed first. It is likely that part of the
roof of the enclosure, which also serves as a 3 meter cosmic ray shield,
would be cut away to facilitate removal of the MiniBooNE detector
plates, and the construction of the LArTPC. Exactly how this
construction is done should have a view to how the construction at Ash
River would be done, which may include a sunken enclosure with a cosmic
ray overburden; this is similar to the MiniBooNE hall. This LArTPC
effort could begin seriously (after appropriate approvals, of course) as
soon as it becomes apparent the equipment in the MiniBooNE detector
would be more useful in another application. This application may or may
not be identified before SciBooNE is done.

Some think 1kton is a nice intermediate size between 150 tons and 10
ktons; others think the 1kton prototype is a better next step than the
150ton prototype step. One physics goal for the 1kton detector could be
to help understand what MiniBooNE has observed at low energy, in
particular to understand the various backgrounds better. If there is a
LArTPC in the SciBooNE hall, it would serve as a near detector in the
BNB (and based on the same technology), and this should help to better
understand the beam background from electron neutrinos. LArTPC
proponents claim it can distinguish single photons from electrons, and
if true, this will help eliminate another background (so called "dirt
events") in MiniBooNE's low energy range. (The elimination of another
MiniBooNE background, the pi-zero background, is assumed for LArTPCs,
although this too is undemonstrated as part of a real physics
experiment.) Of course one expects any PAC considering the use of
resources to mount such an experiment "just to understand what MiniBooNE
sees at low energy" would hope to see MiniBooNE thoroughly understand
things first.

The scales of these two efforts are similar to \"typical\" fixed target
experiments at Fermilab. They make use of two Fermilab resources that
are unique in the world: The Booster Neutrino Beam, and the NuMI beam.
These kinds of efforts appear to fit nicely into the mid-term planning
process, both in cost and timescale. And if LArTPCs start showing up in
several applications around the world AND SUCCEED, then people will be
much more willing to consider using them in applications such as
discussed in the Long Baseline report. Getting approval for the physics
basis of these efforts is another question, but the point of this
one-pager is to list some of the technical and political advantages of
such a program, and to challenge others to come up with better ideas.