9. FY06 PROGRESS FOR THE EXO DOUBLE-BETA-DECAY R&D
PROGRAM
by Peter Rowson
Appendix B Self-Evaluation FY2006
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A SLAC group (M. Breidenbach, C. Hall, D. MacKay, A.
Odian, C. Prescott, P.C. Rowson and K. Wamba) have been
collaborating with the Stanford Physics Department group
of G. Gratta, and with others, in an R&D program to test
the feasibility of a novel large-scale double-beta-decay
experiment. This experiment, known as EXO (for Enriched
Xenon Observatory) proposes to use a large quantity (>1
ton) of Xenon enriched in the Xe136
isotope as both a decay and detection medium. The double
beta decay process,
Xe136
→ Ba136++
+ e−
+ e−
(+ 2ν)
can proceed in the two neutrino(2νββ) mode expected from
the Standard Model (and which has already been observed
in several nuclei other than Xe136),
or possibly in the neutrinoless (0νββ) mode. The 0νββ
process is expected to occur only if neutrinos are
Majorana particles, and at a rate proportional to the
square of an “effective” neutrino mass, and hence its
observation would serve a mass measurement and as the
first demonstration that Majorana neutrinos occur in
nature. Xenon’s excellent calorimetric properties
(necessary to distinguish the broad beta spectrum of the
electron energy sum in the 2νββ process from the line
spectrum in the two-body 0νββ decay), readily achievable
high purity, and lack of worrisome radioactive isotopes
make this element an attractive candidate for a low
background experiment. In addition, we propose to
operate the rare decay search in a coincidence mode, by
identifying the Barium daughter nucleus of double beta
decay on an event-by-event basis. Barium identification
is accomplished by a laser florescence technique that is
sensitive enough to observe a single ion and, in
principle, to distinguish the various Barium isotopes.
The site for such an experiment must be deep underground
to minimize cosmic ray backgrounds
To date, the R&D efforts at SLAC and Stanford have
focused on a liquid xenon (LXe) TPC design, where the
Barium identification would be accomplished by removing
the ion from the LXe using a electrostatic probe, and
then delivering the ion to an as-yet-unspecified laser
system. The campus group has successfully constructed
and operated a laser-illuminated ion trap for Barium and
has observed single Barium ions. In addition, they have
demonstrated state-of-the-art energy resolution in LXe
(which occurs at electric fields >4 kV/cm) and have
preliminary results showing resolution enhancement when
the 175 nm scintillation light produced in xenon, in
addition to ionization, is collected.
The logistical problems connected with the procurement
of a large amount (~10 tons) of isotopically enriched Xe136
were dealt with under the Nuclear-Non-Proliferation
programs of DOE. To date, we have obtained 200 kg of
xenon isotopically enriched to 80% in Xe136
for use in a prototype experiment. The prototype, which
does
not
employ Barium identification, is presently being built.
The experiment, known as “EXO200”, will be placed in the
DOE operated underground facility WIPP (Waste Isolation
Pilot Plant) in Carlsbad NM. EXO200 will collect useful
data for TPC performance, should definitively observe
2νββ in Xe136
for the first time, and should accumulate the large
number of 2νββ decays needed to characterize this
important background. . In addition, a design goal is
that the prototype has sufficient sensitivity to test
with one or two years of data the recent and very
controversial claims from the Moscow-Heidelberg Ge76
experiment that they have observed 0νββ events.
SLAC
Activities : R&D
At SLAC, a xenon purification system was constructed
that is operated at ultra-high vacuum along with a xenon
purity monitor (XPM). The purifier employs a heated
Zirconium metal getter to remove non-noble gas
contaminants (nominally to the 0.1 ppb level), as well
as distillation capability (to remove Argon). The XPM
drifts electrons produced from a UV-laser-illuminated
cathode in LXe across a gap and measures the transport
efficiency. The XPM was upgraded this year to include a
longer drift region (60 mm was increased to 109 mm) for
improved sensitivity to impurities We have confirmed
electron lifetimes as high as 4 ms in purified LXe in
this way (more typically, results are ~1 ms), and have
reproduced electron drift velocities available in the
literature. In addition, we have recently replaced our
cold-finger/liquid-nitrogen (LN) cooling system for the
XPM with a refrigerator that cools HFE-7000, a
hydroflouroether, into which the XPM is submerged. The
HFE may serve as both a coolant and a radiation shield
for the prototype detector, and also alleviates safety
concerns regarding large volumes of LN at the WIPP
underground facility. The new HFE-based system is
working well.
Aseries of experiments was performed to test the
feasibility of electrostatic ion extraction from xenon.
The “probe-test cell” incorporated a movable
electrostatic probe, and an instrumented (PMTs, Si
barrier detectors) volume for LXe or gaseous Xe
containing a pair of HV electrodes. One of the
electrodes holds a weak U230
source which undergoes two α decays and emits Th228
and Ra222
ions into the Xe. We have seen that the probe tip, if
set to negative potential, collects radioactive ions
(thorium and radium α decays confirm presence of the
species). The apparatus was used to measure ion mobility
in LXe, an important issue as the barium ions will be
produced in an electric field, and this work has been
published in NIM :
http://xxx.lanl.gov/PS_cache/cond
mat/pdf/0503/0503560.pdf
. A“cryoprobe” equipped with internal plumbing that
functions as a Joule-Thompson expansion cooler using
high pressure argon gas, was first designed and tested
at SLAC. The probe tip cooled to below the freezing
point of xenon, and ions are trapped in xenon ice. By
this means, we demonstrated that captured ions may be
released by thawing the xenon ice, preventing
irreversible attachment to a bare metal or dielectric
probe tip. A second approach using a “hot probe” is
under study at SLAC. We have seen in the literature how
an appropriately chosen metal surface (eg, platinum) can
have a work function that favors the release of adsorbed
metal atoms (eg barium) in a ionized state when heated
to modest, ~500C, temperatures. This work now continues
at Stanford, along with a series of alternative
ion-capture experiments, in an apparatus designed
ultimately to test the laser identification and ion
capture simultaneously for the first time.
The SLAC group had coauthored a paper, submitted to
Phys.Rev.B, on observations of ionization and
scintillation correlation effects in LXe performed by
the Stanford campus group (available in the LANL E-print
server at
http://xxx.lanl.gov/PS_cache/hep-ex/pdf/0303/0303008.pdf
SLAC Activities : EXO200
A substantial effort is now focused on development of
the 200 kg prototype for installation at WIPP, “EXO200”.
The EXO200 TPC detector will incorporate a ~17 cm drift
region, a maximum electric field of ~3 kV/cm, and a
detection plane consisting of wire grids and/or pads and
LAAPDs
The design of the EXO200 apparatus is complete, and many
major components are already being fabricated. The
custom made modular cleanrooms that will house the
experiment at WIPP are already complete and installed in
the Stanford HEPL facility, with the large low-radioactivity copper
cryostat installed including its low-radioactivity
shielding lead. The refrigeration systems, and xenon
handling systems, which were designed by the SLAC team,
are nearly complete. In September, the first cooldown of
the cryostat to ~175K was successfully tested, and
checkout continues. Extensive testing of the entire
system at HEPL is planned prior to disassembly of the
six cleanroom modules, which will be loaded with the
apparatus by that time for shipment to WIPP.
A major effort is underway now to complete construction
of the xenon pressure vessel and the TPC electrode
structure that it will contain. This effort is co-lead
by the SLAC physicist and mechanical engineering team,
along with our colleagues at Stanford. Machining of the
copper pressure vessel has begun at Stanford. Assembly
of the vessel and TPC will be done in a separate
cleanroom at HEPL that is presently nearly complete.
A test setup for the numerous (~600) large area
amplification photodiodes (LAAPDs) used for 175 nm light
collection in the EXO 200 TPC is now operating at HEPL
The SLAC group has, thanks to assistance from local
electrical engineering manpower, designed and started
production of the ~200 channels of low noise charge
sensitive preamps, and associated digitization, control
modules as well as rack-mounting and cooling hardware
for EXO200.
A SLAC group lead the effort to produce a complete
detector monte carlo, and in addition, event
reconstruction software to be used for the prototype. A
first pass version is ready now and has been extensively
used.
The design of a full scale device incorporating Barium
identification will follow pending the results of our
R&D and prototyping effort.
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SLAC National
Accelerator Laboratory, Menlo Park, CA