Speaker
Description
The He-40%CF₄ gas mixture has been proposed for directional dark matter searches. A collision between Weakly Interacting Massive Particles (WIMPs) and gas atoms induces a nuclear recoil, producing an ionization trail in the gas. From this trail, the 3D topological signature - including direction, dE/dx, and total energy - can be reconstructed. The Earth's motion relative to the expected WIMP halo in our galaxy, results in a relative WIMP direction from the Cygnus constellation. Thus, identifying the nuclear recoil direction would provide unambiguous evidence of a WIMP signal, effectively distinguishing it from background nuclear and electronic recoils induced by cosmic radiation and natural radioactivity from materials surrounding the sensitive volume. Additionally, recoils from solar neutrino scattering - an irreducible background in non-directional searches, known as the "neutrino fog" - can also be discriminated.
The electronic ionization signal from radiation interactions within the gaseous sensitive volume must be amplified for readout. This amplification is typically achieved by drifting ionization electrons toward a Gas Electron Multiplier (GEM) stack or another micropatterned electron multiplier, where electron avalanches occur. Optical readout of the photons generated in these avalanches offers significant advantages over charge readout, including improved signal-to-noise ratio and the capability to place photosensors remotely from the detector's sensitive volume.
The scintillation photons produced by the electron multiplier can be imaged using a sCMOS or CCD camera, or a 2D SiPM array. This 2D image, representing the projection of the ionization track onto the azimuthal plane, can be combined with the track's extension along the drift longitudinal direction. The longitudinal information is typically obtained from the scintillation signal arrival time profile measured, for instance, with a PMT. Consequently, the complete 3D topology of the ionization track can be reconstructed.
CF₄ is selected for its established scintillation properties in both the UV and visible spectra. The low mass of helium nuclei allows to sensitivity to WIMP-nuclei collisions down to a WIMP mass range of a few GeV. The inclusion of hydrogen-rich molecules, such as hydrocarbons, is considered to further extend detector sensitivity into the sub-GeV WIMP region. Previous studies evaluated the impact on scintillation output from adding a few percent of isobutane or methane to He-40%CF₄, determining the absolute scintillation yield under the assumption that additives would minimally affect the spectral emission characteristics of the base mixture [PLB and to be submitted].
In this work, we perform a detailed spectral analysis of scintillation emission from He-40%CF₄ and mixtures containing small percentages of CH₄. The actual impact of CH₄ addition on the He-40%CF₄ scintillation spectrum is assessed and discussed. For these studies, we employed a compact CCD spectrometer with a 200–1000 nm range and a wavelength resolution of 2 nm, including calibration and amplitude correction.
| Workshop topics | Front-end electronics and readout |
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