Numerous astrophysical observations point to the existence of dark matter,
making up about a quarter of the mass-energy budget of the Universe.
Although its exact nature is unknown,
the quest for dark matter has been a long-standing journey
in physics.
The most popular dark matter model is the
weakly massive interacting particle (WIMP).
The majority of the direct detection dark matter experiments
searches for rare nuclear collisions between the WIMPs
and the target nuclei
in low background detectors located deep underground.
No conclusive detection of nuclear recoil (NR) dark matter interactions
has been reported up to date.
A compelling feature predicted by various dark matter models
is the annual and diurnal modulations in the interaction rates measured by the dark matter detectors
as a result of the Earth’s motion in the halo.
The DAMA experiment has observed an annual
event rate modulation in the low energy (2-6 keV) regime of the
electron recoil (ER) events
for two decades. Motivated by that,
a search for both annual and diurnal rate modulations in the low energy
ER events was performed by the
Large Underground Xenon (LUX) experiment.
LUX is a liquid xenon (LXe)-based dark matter detector located
at the 4850 ft level of the Sanford Underground Research Facility (SURF)
in Lead, South Dakota, USA. LUX operated in WIMP search mode
between 2013 and 2016, finding no evidence for WIMP NR interactions.
The analysis steps and results of the search for
annual and diurnal modulations in the LUX experiment are presented
in great detail.
LUX-ZEPLIN (LZ)
experiment is a second generation dark matter direct detection experiment, which is
currently being constructed at SURF and on track for commissioning in 2020.
LZ will contain
about 10 tonnes of LXe, which is more than that of LUX by nearly a factor of 30,
in order to reach an unprecedented WIMP detection sensitivity.
Another distinctive characteristic of LZ is
the large gadolinium-loaded liquid scintillator (GdLS) volume ($\sim$17 tonnes)
held by acrylic tanks surrounding the LXe volume.
The main goal of this outer detector (OD)
is to veto the neutrons that escape the LXe after single-scattering.
They are mostly ($\sim$88\% of the time) captured by the gadolinium isotopes ($^{155}$Gd and $^{157}$Gd)
in the medium that have large capture cross sections. The liquid scintillator converts the
energy released following the neutron capture into detectable light. Thus, the NR collision from a neutron
in the LXe
mimicking a WIMP interaction is rejected thanks to the coincident OD signal.
During the final design review for the OD acrylic vessels,
the reviewers noted that GEANT4 has a shortcoming
of depicting the Gd deexcitation cascade
and that GEANT4-based
evaluations of the LZ OD neutron efficiency
were most likely inaccurate, and they encouraged the OD group to
find and implement an improved simulation.
We adopted the DICEBOX (a nuclear physics software)
simulation of the deexcitation
cascade after neutron capture
on $^{155}$Gd and $^{157}$Gd.
The DICEBOX model addresses various
issues in the default GEANT4 final state
neutron capture model.
The implementation of the
improved
$\gamma$-cascade model after neutron capture on $^{155}$Gd and $^{157}$Gd is reported.
GEANT4 simulations indicate that neutrons tarry in the acrylic tanks
prior to the capture in the OD. In the LZ experiment
searching for rare WIMP interactions,
this can create the peril of confusing the
NRs from neutrons that are followed by a late capture signal (outside the veto window)
with those from WIMPs. In order to attack this problem,
LZ will carry out a tagged neutron source calibration.
In a common neutron source, such as AmBe,
the neutron emission
is usually followed by a high energy $\gamma$, which is prompt.
This 4.4 MeV $\gamma$ detected by a fast, dense and inorganic scintillator crystal
will serve as a tag for the neutrons within 5-10 ns of their birth.
The objective is to accurately measure
the neutron detection efficiency and the OD neutron capture time
in comparison with the GEANT4 predictions
in a way that is independent from the LXe detector.
The design and the simulation of the in-situ tagged AmBe neutron source for the OD
are presented. Furthermore,
the results of the neutron tagging experiments performed
at UCSB with the tagging detector built for this calibration are
discussed.