News

Results from a 11 kg-day exposure of DAMIC at SNOLAB!

DAMIC's results on the search for low-mass weakly interacting massive particles (WIMPs) was posted yesterday on the arXiv! Link.

We probed with the same nuclear target, and with an order-of-magnitude improvement in energy threshold, the parameter space that corresponds to the WIMP-signal interpretation of the excess of nuclear-recoil events recorded by the CDMS silicon experiment in 2013.

We observed an excess of ionization events above threshold that is inconsistent with the standard WIMP-signal interpretation of the CDMS result and demands further investigation. Although this excess leads to a weaker-than-expected exclusion limit at small mχ, our data still provide the most stringent exclusion limit from a silicon target experiment for WIMPs with mχ < 9 GeV c−2.

            

Upper limit (90% CL) on the spin-independent WIMP-nucleon elastic scattering cross section σχ−n as a function of WIMP mass mχ obtained from the 11 kg-day exposure of DAMIC (solid red line). The expectation ±1σ band if only known backgrounds are present in our data set is shown by the red band. For comparison, we also include 90% CL exclusion limits from our 2016 result with a 0.6 kg-day exposure, other experiments, and the 90% CL contours for the WIMP-signal interpretation of the CDMS silicon result.

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Cosmogenic activation of silicon measured!

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.

Fit to the energy spectrum of ionization signals in the irradiated CCD with Monte Carlo templates that include a realistic model of the CCD response. The hump of events below 18 keV from the radioactive decay of tritium is evident.

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Single electron response achieved!

We have recently demonstrated, for the first time, single-electron response with a 675 um-thick skipper CCD fabricated on high-resistivity silicon. This is the first techonolgical milestone of the DAMIC-M program.

The extremely low noise of the skipper CCD will allow us to reach unprecedently low energy thresholds with DAMIC-M for an increase in sensitivity by several orders of magnitude in the search for light dark matter particles.

The prototype 1kx6k CCD was designed by Berkeley Lab and packaged at the University of Washington (UW). The data acquisition system and interface electronics were also developed at UW with the support of CENPA technical staff.

Left: 1kx6k DAMIC-M development CCD packaged at the University of Washington. Right: Distribution of pixel values in a CCD image, demonstrating a noise level of 0.07 e- RMS, which allows the device to resolve single charges per pixel.

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Dark matter-electron scattering result from SNOLAB!

DAMIC's latest results on the search for light dark matter particles (MeV-scale masses) was posted today on the arXiv! Link.

These are the best exclusion limits on the existence of dark matter particles with masses in the 0.5–5.0 MeV range:

The 90 % C.L upper limits on the DM-electron free scattering cross section (σe) as a function of DM mass (mχ) for three different dark-matter form factors (FDM) obtained by DAMIC at SNOLAB (solid blue line).​ Other lines present results from competing experiments.

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Wire bonder works!

Yesterday we completed the first DAMIC 4k x 2k CCD package at the University of Washington. The device is half the area of the CCDs currently taking data at SNOLAB but otherwise identical. This CCD is a candidate to go to Los Alamos over the summer for activation by a neturon beam. The goal is to perform an experimental estimate of the tritium that is activated in silicon by cosmic neutrons.

Left: The first 4k x 2k DAMIC CCD to be wire bonded at the University of Washington. Thanks to Bret from Luna Tech Sales for his assistance. The package is now complete. Right: The resulting wire bonds as seen through a microscope: 78/78 bonded in automatic mode, 100% success rate!

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Clean lab is ready!

After a few months of hard work our clean CCD packaging and testing lab is ready!

Left: View from outside the clean room. Center: K&S 1470 automatic wire bonder from Luna Technical Sales and inspection microscopeRight: CCD test chamber under commissioning and CCD mock-ups.

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Website is up!

Welcome to the website of the DAMIC Group at the University of Washington. We have included a brief description of our experiment, the specific activities at UW, and our member profiles. Please use the tabs above for navigation.

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