Dark Matter Experiments: XENON and DARWIN
Nikhef established a direct detection dark matter program in 2010. Direct detection
experiments aim to measure the elastic scattering of Weakly Interacting Massive Particles
(WIMPs) off a target material. WIMPs are compelling dark matter candidate particles
and 2011 was an interesting year for dark matter searches. Two direct detection
experiments, CoGeNT and CRESST-II, claimed (controversial) evidence for detection of
dark matter in a WIMP parameter region that had already been excluded by other
experiments. At the same time, LHC provided first data on supersymmetry related
searches, further constraining WIMP parameters. Data in the coming years are bound to
provide important insights, hopefully identifying the dark matter particle.
The XENON Experiments
The XENON-series of direct detection experiments use xenon as the WIMP target
material. Xenon is an excellent choice as a target material, since it is heavy, inert and has
good scintillation properties. The XENON collaboration operates the running
XENON100 experiment with 160 kg of very pure xenon of which the central 48 kg is the
target. This experiment has provided the worlds most stringent WIMP dark matter limits
in 2011, see figure XX-1, limits that exclude the new claims from CoGeNT and
CRESST-II and the older detection claim from the DAMA experiment. The Nikhef group
participates in analysis and shift taking and has prepared an upgrade to the high-voltage
system to reduce electronic noise that will be installed after the present run (three of six
selected WIMP candidates in the 2011 analysis were identified as noise events; of the
other, 1.80.6 are background events and the result is therefore consistent with no WIMP
observation). XENON100 will continue operation in 2012, but the main focus of the
collaboration is switching to the next-generation experiment, XENON1T.
XENON1T was approved for construction in Hall B of the Gran Sasso underground lab
(Italy) in April 2011. This experiment will have a tenfold increase in mass relative to
XENON100, roughly 2.5 t of xenon, and two orders of magnitude lower background
contamination, with the aim of reaching a background of less than one event per ton per
year. Apart from relying on the excellent self-shielding properties of xenon, the
collaboration is performing an aggressive program for material selection through
radioactive screening of materials. Placing the experiment underground reduces the
cosmic muon flux and the associated production of neutrons and radioactive isotopes by
five orders of magnitude. Nevertheless, the remaining neutrons can mimic WIMP-like
nuclear recoils. The XENON1T detector will therefore be placed inside a 9.6 m diameter
water tank. The water moderates and captures neutrons and is also an active muon
Cherenkov veto for muons traversing the water. A number of important design choices
were made in 2011. The collaboration decided to use conventional PMTs for light
readout instead of novel QUPIDs to reduce risk. We are finalizing the detector
dimensions (optimized through Monte Carlo simulations) and are in the process of
completing the design for the water tank. On the management side, the collaboration
finalized and signed a Memorandum of Understanding (MoU) that will facilitate the
governance of the experiment.
Nikhef is involved in the design of the double vessel cryostat housing the liquid xenon
and time projection chamber (TPC). Since the cryostat is in close proximity to the
detector, it has to fulfill stringent background requirements. Titanium is a promising
candidate for the construction of the two vessels. We are in the process of investigating
different titanium grades for radioactive contamination. We are also investigating
backgrounds through Monte Carlo simulations.
Another Nikhef responsibility is the design of the cryostat support structure holding the
cryostat inside the water tank. The main design challenge here is to keep material away
from the cryostat, while maintaining the detector stationary to better than a hundred
microns. We are investigating various options of suspending the cryostat from structures
either inside or outside the water tank, see Figure XX-2. This effort also involves Monte
Carlo to optimize the design.
Finally, Nikhef hosted a XENON mini-collaboration meeting in July, when many aspects
of the MoU and detector design were discussed.
DARWIN Design Study
The Nikhef dark matter group is also investigating experiments beyond XENON1T, to be
operational around 2018 – 2020. DARWIN, funded through ASPERA, is a design study
for R&D towards a dark matter facility that uses both argon and xenon as the target. The
goal is to probe the cross section region below 10-47cm2, or to provide a high statistics
measurement of WIMP interactions in case of a positive detection by one of the earlier
experiments, such as XENON100 or XENON1T. Nikhef is involved in various aspects of
this experiment, the main being an investigation of the use of the Nikhef-developed
GridPix detector in a cryogenic argon or xenon environment. We had lots of activity on
this front in the first half of the year, when we tested a GridPix detector in a dual-phase
argon detector in collaboration with ETH Zürich. While the operation was successful, the
patient died. During the test we were able to confirm that the electronic circuits of the
chip work at liquid argon temperatures (T=-185 degC) and that the gas amplification in
the grid continued to operate. We are now investigating several possibilities to improve
the design. Another highlight was Nikhef hosting the annual DARWIN collaboration
meeting in September (see Fig. XX-3).
WIMP-Nucleon Cross Section [cm2]
10-39
XENON100 (2011)
DAMA/Na
10-40
observed limit (90% CL)
Expected limit of this run:
± 1 s expected
± 2 s expected
CoGeNT
DAMA/I
CDMS (2011)
10-41
CDMS (2010)
XENON10 (S2 only, 2011)
10-42
EDELWEISS (2011)
XENON100 (2010)
10-43
10-44
Trotta et al.
Buchmueller et al.
10-45
6 7 8 910
20
30
40 50
100
200
300 400
1000
WIMP Mass [GeV/c2]
Figure XX-1: The blue line shows the 2011 WIMP-nucleon cross section versus WIMP
mass observed exclusion limit from the XENON100 experiment. The analysis was done
on 100.9 days of data, using only the central 48 kg of xenon target. The allowed regions
from the DAMA and CoGeNT experiments are shown in the upper left corner (the
CRESST-II result is not shown). Other exclusion limits from various experiments are
also shown.
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Figure XX-2: Nikhef is studying different cryostat support structures (both internal to the
water tank and external are being considered). An important requirement is to have good
vibrational damping while minimizing the amount of material in close proximity to the
detector.
Figure XX-3: DARWIN collaboration meeting in Amsterdam in September 2011.
DARWIN is a design study for a multi-target dark matter detector to be built after a few
years of XENON1T running.