This year 2023 has been a busy time for the DAMIC group at UW. Here are the scientific highlights so far:

• The excess of ionization events in the bulk of our CCDs, first observed in our 2020 WIMP search, was confirmed with much more sensitive skipper CCDs that we installed in DAMIC at SNOLAB. The preprint is on the arXiv. The origin of the excess remains a mystery. There is no good "known-physics" explanation and the most straightforward dark matter interpretation is in tension with results from other experiments.
• Measurements of the ionization spectrum at lower energies provided world-leading constraints on the existence of dark matter particles with masses in the MeV scale. For this measurement,  we operated two CCDs very similar to those at SNOLAB in the Low Background Chamber (LBC) at the Modane Underground Laboratory. The first spectral analysis was published in PRL, while a second analysis that considers the time evolution of the possible signal to extend the sensitivity to smaller dark matter masses is on the arXiv.
• In an effort led by graduate student Kellie McGuire, we developed a technique to discriminate between nuclear and electronic recoils in the CCD target. The preprint is on the arXiv. This technique has huge potential since it may allow future CCD dark matter detectors to separately study the different interaction channels (i.e., with either nuclei or electrons) of dark matter particles.
• Prof. Chavarria's proceedings from Low Radioactivity Techniques 2022 conference were published. The paper succinctly summarizes the background suppression capabilities of CCDs and the hybrid CMOS imagers that we develop for Selena.

Prof. Chavarria's review article "Even Lighter Particle Dark Matter" was accepted for publication in the proceedings for the 14th International Conference on Identification of Dark Matter.

The article summarizes the current world-wide effort to search for sub-GeV dark matter particles from their interactions with electrons. Please find the preprint on the arXiv.

We seek a creative, conscientious, and highly motivated candidate to spearhead the activities in the construction and commissioning of DAMIC-M. In addition, the appointee will have the opportunity to participate in the data analysis of the prototype detectors, and the R&D program at the UW to develop next-generation skipper CCD detectors (Oscura) and CMOS/selenium detectors for neutrino physics (Selena).

Read the full advertisement and apply here.

We recently posted on the arXiv our measurement of the cosmogenic activation of tritium in silicon.

Tritium (³H) produced in the bulk silicon of CCDs by cosmic rays can be a problem for silicon dark matter experiments because the background from the low-energy beta particles can interfere with the dark matter signal. Tritium will accumulate in the silicon crystal starting when the ingot is pulled, throughout wafering and CCD fabrication, until the final devices are brought underground for the dark matter search. Thus, we measured the activation rate of cosmogenic tritium in silicon to determine the maximum time that the CCD silicon can spend on the surface before tritium decays become a limiting background for the DAMIC-M experiment.

For this measurement, we irradiated three 8-megapixel CCDs at Los Alamos Neutron Science Center (LANSCE). The neutron flux mimics the cosmic-ray energy spectrum but with much higher intensity. We acquired dark images with the irradiated CCDs to perform spectroscopy of the beta decays detected in the bulk silicon of the devices and measured the tritium decay rate. By scaling the quantity of tritium activated in the CCD by the relative neutron fluence, we obtained a cosmogenic neutron activation rate of (112±24) atoms/kg/day.