Within the DarkSide program, the active medium for detection of dark matter WIMPs is liquid argon (LAr), a cryogenic material with excellent scintillation and ionization properties. If WIMPs exist, they are expected to collide with atomic nuclei, causing the nuclei to recoil with kinetic energies up to about 100 keV. The recoiling nuclei produce short tracks of ionized and metastable excited argon atoms.
After this initial ionizing event, a sequence of reactions occur that involve the recombination of electron-ion pairs and ends with the formation of short-lived excited diatomic argon “molecules”. These then decay with a characteristic emission of 128 nm scintillation light. But not all ionization electrons recombine with the ionized nuclei – some free electrons remain, which are drifted up through the LAr by an applied uniform electric field.
There are key differences in the response of LAr to low ionization-density events (such as β or γ interactions from residual radioactivity in detector materials) compared to the sought-after heavily-ionizing nuclear recoil events. Low density ionization leads to less recombination of the ionized electrons, and therefore, more free electrons than a nuclear recoil track of the same total energy. The ratio of ionization to scintillation thus allows a method for separation of background events – those due to electron recoils versus those due to nuclear recoils.
The difference in ionization density also produces a significant difference in the time profile of the scintillation light for low and high-ionization-density events. Argon scintillation light is emitted from two nearly degenerate “molecular” states, a long-lived (τ ~ 1.6 μs) triplet state, and a short-lived (τ ~ 6 ns) singlet state. The long-lived state is found to be non-radiatively quenched in tracks with high ionization-density. Thus, electron recoils have a longer scintillation duration, compared to nuclear recoils. This turns out to be a significant effect, and is the working principle behind “pulse shape discrimination” (PSD), a method by which we can identify whether an event is an electron recoil or a nuclear recoil, with a mis-identification probability on the order of 10-8. The combination of discrimination by the scintillation to ionization ratio, combined with PSD, provides a powerful background rejection technique that is unique to liquid argon.
Exploiting these powerful background suppression techniques requires a two-phase LAr time projection chamber (LAr TPC) made of low radioactivity components and with the capability for efficient collection of both the scintillation light and ionization charge.
The DS-50 LAr TPC design is schematically shown in the accompanying picture. The inner detector contains the active LAr volume, which is viewed by arrays of photo-multiplier tubes (PMTs) from the top and bottom. The inner surfaces of the active volume are coated with a vacuum-evaporated thin film of tetra-phenylbutadiene (TPB) wavelength shifter (WLS), which shifts the 128 nm UV primary scintillation (S1 signal) into light visible by the PMTs.
To detect the ionization, DarkSide uses a two-phase TPC configuration, which contains a small region of gaseous argon above a larger region of liquid argon. A uniform, 200 V/cm electric field is produced by a “field cage” consisting of a cathode plane, field-shaping rings, and an extraction grid. This uniform field drifts the ionization electrons upward to the surface of the liquid. There, a collinear electric field of ~3 kV/cm extracts the electrons into the gas phase, where they produce secondary scintillation photons by a process called “electroluminescence” (EL). The resulting secondary photons (S2 signal) are detected by the PMT arrays as a delayed coincidence, relative to the primary scintillation signal (S1).
The LAr TPC allows events to be accurately localized in three dimensions. Since diffusion during the long drift is negligible in dense noble liquids, the delay (drift) time between the S1 and S2 signals accurately defines the vertical position of each event, with millimeter precision. The distribution of light over the top photo detector array gives the horizontal position, with cm-like precision.
Two-phase LAr TPCs filled with Underground Argon, utilizing low-background photosensors will allow construction of affordable multi-ton detectors operating with zero accepted background in multi-year exposures. The suppression is achieved by fully exploiting the rich information content of the combined ionization and scintillation signals available from the two-phase TPCs. This technology is at the heart of the DarkSide program.