Scintillating Bubble Chamber (SBC)

Scintillating Bubble Chamber (SBC): The SBC collaboration is working on developing liquid-noble bubble chambers for a large exposure, quasi-background-free detection of low energy nuclear recoils. SBC addresses a current lack of scalable, low-threshold nuclear recoil detection technology, enabling searches for low mass (1 GeV/c^2) dark matter to the neutrino fog and for high-statistic measurements of low-energy neutrino coherent scattering. SBC combines the electron-recoil background discrimination of a bubble chamber with the event-by-event energy resolution of a liquid scintillator detectors. This, along with the lack of bubble-nucleating  energy-loss mechanisms available to recoiling electrons in liquid nobles, allows for much stronger electron-recoil discrimination that existing bubble chamber dark matter detectors. In turn, this allows for background=free sensitivity to nuclear recoils at lower energies that can be achieves in any current working detection technique, reaching thresholds low as 100 eV while remaining blind to electron-recoiling backgrounds. The SBC collaboration's first physics-scale device, the 10kg liquid argon bubble chamber SBC-LAr10, is now commissioned and will undergo a series of source calibrations at Fermilab to measure both the discrimination power and low energy nuclear recoil sensitivity of the device, calibrating the detection threshold at 10 eV. Future SBC efforts will include a ton-year dark matter search targeting the solar neutrino fog at 1GeV/c^2 particle mass along with the deployment of a SBC device to measure neutrino cross sections at 1MeV neutrino energy.

[Scintillating Bubble Chambers: Liquid-noble Bubble Chambers for Dark Matter and CEvNS Detection https://arxiv.org/abs/2207.12400]

SBC Operation: The liquid-noble SBC is an extension of the Freon-based chamber technique used by COUPP and PICO for dark matter searches. The advantage of the SBC for DM searches is the intrinsic insensitivity to electron-recoil (ER) backgrounds, due to the thermodynamics of bubble nucleation. Nucleation requires the formation of a proto-bubble to overcome the free energy barrier arising from the surface tension of the bubble before growing to a visible size. Through the Seitz Hot Spike Model, a particle interaction can create this proto-bubble. In Freon detectors, the electron nuclear recoil insensitivity diminishes significantly below 1 KeV, resulting in γ -induced nucleation indistinguishable from nuclear recoils. Bubble chambers are threshold detectors, where a bubble is nucleated by any energy deposition above the Seitz threshold. If energy deposited is unknown, there will be an inability to differentiate between high and low energy nuclear recoils. Liquid-noble SBC chambers address both of these issues, using LAr (liquid argon) as a target fluid. LAr scintillates in the vacuum-UV under ionization radiation, allowing for the measurement of the total energy deposited and discrimination between low and high energy nuclear recoils. 

Unlike Pico , whose acoustic signal accompanying bubble nucleation gives MeV energy resolution, this energy reconstruction in LAr has a resolution in KeV. SBC is driven by a fundamental additional suppression of electron-recoil-induced bubble nucleation in superheated noble liquids, allowing for background free operation at nuclear recoil detection thresholds at magnitudes lower than those achievable in Freon-based detectors. The absence of bubble nucleation by electron recoils in superheated noble liquids is a consequence of the lack of molecular degrees of freedom for recoiling electrons to excite. Lacking energy loss due to molecular vibrational modes, particle interactions can locally heat the fluid only through center of mass motion of individual atoms. Although this is achieved through the Lindhard effect, the kinematics of elastic electron-atom collisions make this an extremely inefficient means of energy loss for electrons, so electron recoil energy is almost entirely carried away by scintillation light, IR radiation, and recombining ionization.

[Scintillating Bubble Chambers for Rare Event Searches https://www.mdpi.com/2218-1997/9/8/346]

Left: Schematic and annotated solid model of the SBC-LAr10 detector

Middle: Test of the SBC-LAr10 inner assembly consisting of the inner and outer fused silica vessels, bellows guides and support structure, stainless steel bellows, and spring-energiezed PTFE silicia-to-metal seals.

Right: Partially assembled HDPE castle isolating the warm target region from the cold, stable region, enclosing the copper SIPM support structure that surrounds the target volume. 3 of 8 piezoelectric acoustic sensors are visible at the bottom of the castle.