ArDM - Argon Dark Matter experiment

The ArDM (Argon Dark Matter) Experiment aims at direct Dark Matter detection based on a ton-scale liquid argon (LAr) double-phase time projection chamber (TPC). The experiment is located at the Laboratorio Subterráneo de Canfranc (LSC).

The ArDM-1t detector is the largest two-phase liquid argon detector for Dark Matter Searches in the world.

What is the Dark Matter composition ?

Astronomical observations give strong evidence for the existence of non-luminous and non-baryonic matter, presumably composed of a new type of elementary particle. A possible candidate is the Weakly Interacting Massive Particle (WIMP). If they exist, WIMPs should form a cold thermal relic gas, which could be detected via elastic collisions with nuclei of ordinary matter. The detection of these WIMPs is based on the capability of measuring the recoils of target nuclei with kinetic energy in the range of 10-100 keV. The signal is therefore quite elusive and is expected to be a rare event given the weak coupling between WIMPs and ordinary matter.

Noble liquids

Noble liquid detectors using Xenon or Argon can efficiently act as targets for Weakly Interacting Massive Particles (WIMP) detection. Xenon or Argon provide a high event rate because of their high density and high atomic number and large target masses are readily conceivable. They have high scintillation and ionization yields because of their low ionization potentials. Both scintillation and ionization are measurable and can be used to very effectively discriminate between nuclear recoils and gamma/electron backgrounds. The use of noble liquid gases to detect WIMP dark matter is currently the subject of intense R&D carried out by a number of groups worldwide. In these detectors, one relies on the simultaneous detection of the ionization charge and of the scintillation light produced during a nuclear recoil event. A main subject for any such detector is the method of the readout for the ionization and scintillation.

Nuclear recoils

Elastic scattering of Dark Matter particles (hypothetical WIMPs) off target argon nuclei is measurable by observing photons from scintillation and free electrons from ionisation, which are produced by the recoiling nucleus interacting with neighboring atoms. The ArDM detector is designed to measure both signals in its double-phase (liquid-vapour) TPC operation mode.

Scintillation light schematics

Detecting nuclear recoils

Recoiling nucleus interacting with neighboring atoms is measurable by observing photons from scintillation and free electrons from ionisation. The ArDM detector is designed to measure both signals in its double-phase (liquid-vapour) TPC operation mode. The ionisation electrons drift up to the liquid surface and are extracted into the saturated argon vapour above the liquid, thanks to strong electric fields (~kV/cm — this requires a very high voltage of ~100 kV in the system). The extracted electrons further interact with argon gas atoms producing secondary scintillation light (S2), its intensity proportional to the charge. The charge signal thus is recorded as S2 photons by “photomultipliers” (PMTs), while the primary scintillation light (S1) is recorded promptly by the same PMTs. The time interval between S1 and S2 (the electrons’ drift time) is proportional to the distance from the event vertex to the liquid surface, and is used to know precisely the vertical coordinate of the vertex position. With the horizontal coordinates that can be reconstructed from the distribution of the S2 photons falling onto the PMT array, one can obtain 3D position information.
Scintillation light schematics

Primary scintillation and secondary ionisation

Primary scintillation light is produced promptly by the passage of ionising particles through the liquid argon volume. The formation of excimers in either singlet or triplet states, which decay radiatively to the dissociative ground state with characteristic fast and slow lifetimes of approximately 6 nanoseconds, and 1.6 microseconds in liquid argon with the so-called second continua emission spectrum peaked at 128 nanometers.
Secondary scintillation light is produced in the gas phase of the detector when electrons, extracted form the liquid, are accelerated in the electric field. Because the gas is less dense and the mean free path of the electron is longer than in liquid, the extracted electrons can gain enough energy to excite argon atoms in collisions so that scintillation light appears. For appropriate electric fields, the amount of light is directly proportional to the amount of charge.
Schematics of the detector

Operating principle of ArDM detector

The detector is a ton-scale liquid argon (LAr) double-phase time projection chamber (TPC). Elastic scattering of Dark Matter particles (hypothetical WIMPs) off target argon nuclei is measurable by observing photons from scintillation and free electrons from ionisation, which are produced by the recoiling nucleus interacting with neighboring atoms. The ArDM detector is designed to measure both signals in its double-phase (liquid-vapour) TPC operation mode. The ionisation electrons drift up to the liquid surface and are extracted into the saturated argon vapour above the liquid, thanks to strong electric fields (kV/cm — this requires a very high voltage of 100 kV in the system). The extracted electrons further interact with argon gas atoms producing secondary scintillation light (S2), its intensity proportional to the charge. The charge signal thus is recorded as S2 photons by “photomultipliers” (PMTs), while the primary scintillation light (S1) is recorded promptly by the same PMTs. The time interval between S1 and S2 (the electrons’ drift time) is proportional to the distance from the event vertex to the liquid surface, and is used to know precisely the vertical coordinate of the vertex position. With the horizontal coordinates that can be reconstructed from the distribution of the S2 photons falling onto the PMT array, one can obtain 3D position information.
LSC building

The Laboratorio Subterráneo de Canfranc (LSC)

The experimental halls of the Laboratorio Subterráneo de Canfranc (LSC) have been excavated in the rock 850 m deep under the Mount Tobazo in the Spanish side of the Aragon Pyrenees. The rock filters the cosmic radiation providing the "cosmic silence", which is necessary to study rarely occurring natural phenomena such as the interactions with an atomic nucleus of neutrinos of cosmic origin or of particles of the invisible "dark matter".
The LSC’s total area is about 1.250 m² corresponding to a volume of about 10 000 m³ and it has two experimental halls (40x15x12 m³ and 15x10x7 m³) in which the experiments are distributed as well as offices, a clean room a mechanical workshop and gas storage room.

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