Physics, P-DO
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Detection of Dark Matter and Low-Energy Solar Neutrinos with Liquid Neon
M.G. Boulay, A. Hime, J. Lidgard, D.-M. Mei (P-23) Introduction
Current searches for dark matter attempt to detect WIMPs from our galaxy’s dark-matter halo through their elastic scattering on target nuclei. Because the interaction between WIMPs and matter is extremely weak, event rates in terrestrial detectors are low. Current experimental searches are limited by the total mass of target material achievable, with limits on the WIMP interaction rate on the order of events per kilogram of detector material per day. Extreme care must be taken to reduce sources of background contamination in these experiments. The goal for next-generation experiments is to improve sensitivity by several orders of magnitude, with target masses on the order of tons.
Cryogenic Low-Energy Astrophysics with NeonThe CLEAN (Cryogenic Low-Energy Astrophysics with Neon) experiment (Figure 3) will be sensitive to the low-energy pp solar neutrinos (νx) and to WIMP particles (χ) through their elastic scattering from electrons and neon nuclei, respectively: νx + e- → νx' + e-' (1) χ + Ne → χ' + Ne' (2)
To evaluate the detector’s capability, we performed detailed Monte Carlo simulations. Nominal properties associated with the production and propagation of scintillation photons (scintillation yield, scattering lengths, etc.) and PMTs with currently achievable background levels were assumed in the simulation. PMT glass contains small traces of uranium, thorium, and potassium, which can decay and generate scintillation photons that could then be mistaken for signal events. Many of the properties of scintillation light in neon are not well known, and part of the current experimental program at LANL is to improve our knowledge of these. Scintillation-Event-Position Reconstruction
Background ContaminationPurification of the neon is expected to reduce background contamination from internal sources of radioactivity significantly because at the very low temperature of liquid neon most impurities will bind efficiently to carbon and can thus be removed by cold traps. The PMTs and associated hardware will contain the largest amount of radioactive contamination near the inner detector volume, and these are mitigated by applying position reconstruction algorithms described above. The dominant internal source of background for CLEAN is expected to be krypton-85 because it has a relatively short half-life (approximately 11 years), decays through e- emission with energies in the same range as the νx neutrinos (Q-value = 687 keV), and is present in the atmosphere. Several other naturally occurring radioactive contaminants will need to be removed from the neon to achieve acceptable background levels for neutrino detection. Projected Sensitivity
ConclusionsWe have demonstrated the possibility for a simultaneous dark-matter and low-energy neutrino experiment using liquid neon, assuming nominal scintillation characteristics and background contamination levels. Assuming the required background contamination levels can be achieved, the large target mass possible with neon may lead to the best sensitivity for detecting dark-matter particles. The current research and development program at LANL focuses on providing precise measurements of some of the fundamental scintillation properties in liquid neon and achievable background contamination levels, both of which are critical to the feasibility of the experiment. A test cell of approximately 5 kg of neon is being designed to measure the precise scintillation time distribution for both electron and nuclear recoils in liquid neon. A system currently under construction will clean neon gas of impurities using cold traps, with the goal of ultimately demonstrating the background requirements needed for the full-scale detector. Studies are under way to design a small-scale prototype that could be used to further assess scintillation and background properties and provide initial limits on WIMP interactions. References
AcknowledgmentThis work is supported with funds from the LANL Laboratory-Directed Research and Development program. For further information, contact Mark Boulay, 505-665-3821, mboulay@lanl.gov. |
The origin of dark matter in our universe and the study of low-energy solar neutrinos are two of the foremost topics in particle astrophysics. It has long been known that much of our universe is composed of an undetected form of matter, known as dark matter. Initial evidence for dark matter came from rotation curves of galaxy clusters and galaxies, an example of which is shown in Figure 1(b). Objects at very large distances from galactic centers are found to have velocities too large to be gravitationally bound by only the visible matter in the galaxy. Additional matter must be present to account for the fact that these objects are indeed bound by gravitational forces. A currently favored hypothesis is that dark matter consists of a new type of particle, a weakly interacting massive particle (WIMP), which has so far remained undetected.
Since the experimental discovery of the neutrino by a team of LANL researchers, the field of neutrino physics has become rich with new insights into the fundamental properties of the neutrino. A wide range of the solar neutrino spectrum (Figure 2) has been probed by solar-neutrino experiments during the past four decades. LANL has played a leading role in the Sudbury Neutrino Observatory (SNO), recent results of which have conclusively shown that electron neutrinos (νe) emitted in the sun undergo transformation into other neutrino flavors (νμ,ντ), solving a decades-old problem of missing solar neutrinos.3,4 Future experimental efforts on solar neutrinos will focus on very precise measurements of the lowest-energy solar neutrinos. The flux of pp neutrinos is very well predicted by standard models of solar evolution and is tightly constrained by the observed solar luminosity. Measurement of this component of the solar spectrum with high precision and in real time will shed light onto both the solar models and the fundamental properties of the neutrinos themselves.
The recoiling electrons (νx') or neon nuclei (Ne') lead to the production of scintillation photons in the liquid neon (approximately 15,000 photons per MeV of kinetic energy), which can then be detected by the photomultiplier tubes (PMTs). The concept of a liquid-neon scintillation detector on which this work is based was first put forward by McKinsey and Doyle.6 The proposed experiment consists of a large volume of liquid neon surrounded by 1842 PMTs that detect scintillation photons produced by the recoiling electrons (e-') or Ne' from reactions (1) and (2). A key to the CLEAN experiment is the difference in emission times of scintillation photons from reactions (1) and (2), allowing discrimination between these two reactions. At the projected sensitivity of the CLEAN experiment, low-energy solar neutrinos occur at rates much greater than the WIMP scattering rates, and separating these event types is critical to the success of the experiment. CLEAN is projected to measure the dominant pp component of the solar-neutrino flux with 1% precision.
Scintillation events from radioactive decays in the PMTs or other sources of radioactivity external to the neon are a potential background to the solar-neutrino or WIMP scintillation signals. Reducing this background to an acceptable level requires the reconstruction of scintillation-event positions based on the PMT data, which samples the scintillation photons. We have developed a new position reconstruction algorithm based on our detector simulation. The algorithm shows significant improvement over earlier geometrical reconstruction algorithms by including PMT timing information (Figure 4). The reconstruction of scintillation-event positions is critical to the success of CLEAN because it allows us to use a large target mass of neon necessary for WIMP sensitivity with a very low-energy threshold (approximately 12 keV) essentially free of PMT backgrounds.
By evaluating the detector response from Monte Carlo simulations and including the effects of background contamination from PMTs, internal radioactivity, and solar neutrinos, we have evaluated the ultimate sensitivity to WIMP dark matter (Figure 5). The cross sections (interaction strength) for WIMP-nucleon interactions and the WIMP’s mass are both unknown, and both affect the signal seen in CLEAN so that the sensitivity depends on these two parameters. For a 300 cm radius detector, we find an experimental sensitivity to dark matter that is several orders of magnitude better than current searches and competitive with proposed searches.