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DEAP
DEAP (Dark matter Experiment using Argon Pulse-shape discrimination) is a direct dark matter search experiment which uses liquid argon as a target material. DEAP utilizes background discrimination based on the characteristic scintillation pulse-shape of argon. A first-generation detector (DEAP-1) with a 7 kg target mass was operated at Queen's University to test the performance of pulse-shape discrimination at low recoil energies in liquid argon. DEAP-1 was then moved to SNOLAB, 2 km below Earth's surface, in October 2007 and collected data into 2011.
DEAP-3600 was designed with 3600 kg of active liquid argon mass to achieve sensitivity to WIMP-nucleon scattering cross-sections as low as 10−46 cm2 for a dark matter particle mass of 100 GeV/c2. The DEAP-3600 detector finished construction and began data collection in 2016. An incident with the detector forced a short pause in the data collection in 2016. As of 2019, the experiment is collecting data.
To reach even better sensitivity to dark matter, the Global Argon Dark Matter Collaboration was formed with scientists from DEAP, DarkSide, CLEAN and ArDM experiments. A detector with a liquid argon mass above 20 tonnes (DarkSide-20k) is planned for operation at Laboratori Nazionali del Gran Sasso. Research and development efforts are working towards a next generation detector (ARGO) with a multi-hundred tonne liquid argon target mass designed to reach the neutrino floor, planned to operate at SNOLAB due to its extremely low-background radiation environment.
Since liquid argon is a scintillating material a particle interacting with it produces light in proportion to the energy deposited from the incident particle, this is a linear effect for low energies before quenching becomes a major contributing factor. The interaction of a particle with the argon causes ionization and recoiling along the path of interaction. The recoiling argon nuclei undergo recombination or self-trapping, ultimately resulting in the emission of 128 nm vacuum ultra-violet (VUV) photons. Additionally liquid argon has the unique property of being transparent to its own scintillation light, this allows for light yields of tens of thousands of photons produced for every MeV of energy deposited.
The elastic scattering of a WIMP dark matter particle with an argon nucleus is expected to cause the nucleus to recoil. This is expected to be a very low energy interaction (keV) and requires a low detection threshold in order to be sensitive. Due to the necessarily low detection threshold, the number of background events detected is very high. The faint signature of a dark matter particle such as a WIMP will be masked by the many different types of possible background events. A technique for identifying these non-dark matter events is pulse shape discrimination (PSD), which characterizes an event based on the timing signature of the scintillation light from liquid argon.
PSD is possible in a liquid argon detector because interactions due to different incident particles such as electrons, high energy photons, alphas, and neutrons create different proportions of excited states of the recoiling argon nuclei, these are known as singlet and triplet states and they decay with characteristic lifetimes of 6 ns and 1300 ns respectively. Interactions from gammas and electrons produce primarily triplet excited states through electronic recoils, while neutron and alpha interactions produce primarily singlet excited states through nuclear recoils. It is expected that WIMP-nucleon interactions also produce a nuclear recoil type signal due to the elastic scattering of the dark matter particle with the argon nucleus.
By using the arrival time distribution of light for an event, it is possible to identify its likely source. This is done quantitatively by measuring the ratio of the light measured by the photo-detectors in a "prompt" window (<60 ns) over the light measured in a "late" window (<10,000 ns). In DEAP this parameter is called Fprompt. Nuclear recoil type events have high Fprompt (~0.7) values while electronic recoil events have a low Fprompt value (~0.3). Due to this separation in Fprompt for WIMP-like (Nuclear Recoil) and background-like (Electronic Recoil) events, it is possible to uniquely identify the most dominant sources of background in the detector.
The most abundant background in DEAP comes from the beta decay of Argon-39 which has an activity of approximately 1 Bq/kg in atmospheric argon. Discrimination of beta and gamma background events from nuclear recoils in the energy region of interest (near 20 keV of electron energy) is required to be better than 1 in 108 to sufficiently suppress these backgrounds for a dark matter search in liquid atmospheric argon.
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DEAP
DEAP (Dark matter Experiment using Argon Pulse-shape discrimination) is a direct dark matter search experiment which uses liquid argon as a target material. DEAP utilizes background discrimination based on the characteristic scintillation pulse-shape of argon. A first-generation detector (DEAP-1) with a 7 kg target mass was operated at Queen's University to test the performance of pulse-shape discrimination at low recoil energies in liquid argon. DEAP-1 was then moved to SNOLAB, 2 km below Earth's surface, in October 2007 and collected data into 2011.
DEAP-3600 was designed with 3600 kg of active liquid argon mass to achieve sensitivity to WIMP-nucleon scattering cross-sections as low as 10−46 cm2 for a dark matter particle mass of 100 GeV/c2. The DEAP-3600 detector finished construction and began data collection in 2016. An incident with the detector forced a short pause in the data collection in 2016. As of 2019, the experiment is collecting data.
To reach even better sensitivity to dark matter, the Global Argon Dark Matter Collaboration was formed with scientists from DEAP, DarkSide, CLEAN and ArDM experiments. A detector with a liquid argon mass above 20 tonnes (DarkSide-20k) is planned for operation at Laboratori Nazionali del Gran Sasso. Research and development efforts are working towards a next generation detector (ARGO) with a multi-hundred tonne liquid argon target mass designed to reach the neutrino floor, planned to operate at SNOLAB due to its extremely low-background radiation environment.
Since liquid argon is a scintillating material a particle interacting with it produces light in proportion to the energy deposited from the incident particle, this is a linear effect for low energies before quenching becomes a major contributing factor. The interaction of a particle with the argon causes ionization and recoiling along the path of interaction. The recoiling argon nuclei undergo recombination or self-trapping, ultimately resulting in the emission of 128 nm vacuum ultra-violet (VUV) photons. Additionally liquid argon has the unique property of being transparent to its own scintillation light, this allows for light yields of tens of thousands of photons produced for every MeV of energy deposited.
The elastic scattering of a WIMP dark matter particle with an argon nucleus is expected to cause the nucleus to recoil. This is expected to be a very low energy interaction (keV) and requires a low detection threshold in order to be sensitive. Due to the necessarily low detection threshold, the number of background events detected is very high. The faint signature of a dark matter particle such as a WIMP will be masked by the many different types of possible background events. A technique for identifying these non-dark matter events is pulse shape discrimination (PSD), which characterizes an event based on the timing signature of the scintillation light from liquid argon.
PSD is possible in a liquid argon detector because interactions due to different incident particles such as electrons, high energy photons, alphas, and neutrons create different proportions of excited states of the recoiling argon nuclei, these are known as singlet and triplet states and they decay with characteristic lifetimes of 6 ns and 1300 ns respectively. Interactions from gammas and electrons produce primarily triplet excited states through electronic recoils, while neutron and alpha interactions produce primarily singlet excited states through nuclear recoils. It is expected that WIMP-nucleon interactions also produce a nuclear recoil type signal due to the elastic scattering of the dark matter particle with the argon nucleus.
By using the arrival time distribution of light for an event, it is possible to identify its likely source. This is done quantitatively by measuring the ratio of the light measured by the photo-detectors in a "prompt" window (<60 ns) over the light measured in a "late" window (<10,000 ns). In DEAP this parameter is called Fprompt. Nuclear recoil type events have high Fprompt (~0.7) values while electronic recoil events have a low Fprompt value (~0.3). Due to this separation in Fprompt for WIMP-like (Nuclear Recoil) and background-like (Electronic Recoil) events, it is possible to uniquely identify the most dominant sources of background in the detector.
The most abundant background in DEAP comes from the beta decay of Argon-39 which has an activity of approximately 1 Bq/kg in atmospheric argon. Discrimination of beta and gamma background events from nuclear recoils in the energy region of interest (near 20 keV of electron energy) is required to be better than 1 in 108 to sufficiently suppress these backgrounds for a dark matter search in liquid atmospheric argon.
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