Periodic Reporting for period 2 - DAMIC-M (Unveiling the Hidden: A Search for Light Dark Matter with CCDs)
Période du rapport: 2020-03-01 au 2021-08-31
Dark matter (DM) is a ubiquitous yet invisible presence in our universe. It dictated how galaxies formed in the first place, and now moves stars around them at puzzling speeds. The DM mass in the universe is known to be five times that of ordinary matter; yet its true nature remains elusive.
Weakly interactive massive particles (WIMPs), relics from the early universe, are a compelling explanation chased by sensitive experiments in deep underground laboratories. However, searches for heavy WIMPs (≈100 times the proton mass), the most theoretically natural candidates, have been so far unsuccessful. Alternative scenarios are now under scrutiny, as the existence of a hidden sector of lighter DM particles which interact, differently than WIMPs, also with electrons.
DAMIC-M (Dark Matter In CCDs at Modane) will search beyond the heavy WIMP paradigm by detecting nuclear recoils and electrons induced by light DM in charge-coupled devices (CCDs). The 0.5 kg detector will be installed at the Laboratoire Souterrain de Modane, France. In this novel and unconventional approach - CCDs are commonly employed for digital imaging in astronomical telescopes – the ionization charge will be detected in the most massive CCDs ever built with exquisite spatial resolution (15 μm x 15 μm pixel). The crucial innovation featured in these devices is the non-destructive, repetitive measurement of the pixel charge, which results in the high-resolution detection of a single electron and unprecedented sensitivity to light DM (≈ eV energies are enough to free an electron in silicon). By counting individual charges in a detector with extremely low leakage current – a combination unmatched by any other DM experiment – DAMIC-M will take a leap forward of several orders of magnitude in the exploration of the hidden sector, a jump that may be rewarded by serendipitous discovery.
The nature of Dark Matter constitutes one of the most fundamental questions in science still unanswered. A discovery by DAMIC-M would have extraordinary implications in our understanding of the universe and of the laws that govern it. In addition, we expect that the development of new technologies by DAMIC-M will have broader impacts, since low-noise digital imaging detectors have many scientific (e.g. in Astronomy, Medicine, Environmental and Quantum science) and commercial (e.g. cell phone cameras) applications which ultimately benefit the society at large.
Weakly interactive massive particles (WIMPs), relics from the early universe, are a compelling explanation chased by sensitive experiments in deep underground laboratories. However, searches for heavy WIMPs (≈100 times the proton mass), the most theoretically natural candidates, have been so far unsuccessful. Alternative scenarios are now under scrutiny, as the existence of a hidden sector of lighter DM particles which interact, differently than WIMPs, also with electrons.
DAMIC-M (Dark Matter In CCDs at Modane) will search beyond the heavy WIMP paradigm by detecting nuclear recoils and electrons induced by light DM in charge-coupled devices (CCDs). The 0.5 kg detector will be installed at the Laboratoire Souterrain de Modane, France. In this novel and unconventional approach - CCDs are commonly employed for digital imaging in astronomical telescopes – the ionization charge will be detected in the most massive CCDs ever built with exquisite spatial resolution (15 μm x 15 μm pixel). The crucial innovation featured in these devices is the non-destructive, repetitive measurement of the pixel charge, which results in the high-resolution detection of a single electron and unprecedented sensitivity to light DM (≈ eV energies are enough to free an electron in silicon). By counting individual charges in a detector with extremely low leakage current – a combination unmatched by any other DM experiment – DAMIC-M will take a leap forward of several orders of magnitude in the exploration of the hidden sector, a jump that may be rewarded by serendipitous discovery.
The nature of Dark Matter constitutes one of the most fundamental questions in science still unanswered. A discovery by DAMIC-M would have extraordinary implications in our understanding of the universe and of the laws that govern it. In addition, we expect that the development of new technologies by DAMIC-M will have broader impacts, since low-noise digital imaging detectors have many scientific (e.g. in Astronomy, Medicine, Environmental and Quantum science) and commercial (e.g. cell phone cameras) applications which ultimately benefit the society at large.
DAMIC-M is a complex detector which integrates an innovative technology into an apparatus with stringent constraints on radioactive contaminants. During this reporting period, we have advanced on many aspects of the project and achieved major milestones, highlighted in the following.
The raw material for the CCDs – silicon – is produced in the form of ingot and then sliced into wafers. After fabrication, cosmic rays interactions in the silicon produce tritium, a radioactive isotope whose decay can mimic a dark matter signal. It was thus of paramount importance to minimize exposure to cosmic rays at the very beginning of the project. The silicon ingot was produced in Denmark, wafers were sliced in the UK and then shipped to Canada, where CCDs are fabricated. Logistics included storage at the Boulby (UK) and SNOLAB (Canada) laboratories (1.1 km and 2 km underground, respectively) and shipping by sea in a custom 16 ton shielding container. The equivalent exposure to cosmic rays was only 14 days, a factor two better than originally planned.
The CCD fabrication has started with the first lot of 25 wafers in advanced stage of processing at the time of this report. A suite of vacuum chambers where the CCDs are cooled down to -140 C have been built and are ready to perform the validation test of this first lot before starting the full production.
Significant effort has gone in the design of the mechanical support for the CCDs and the DAMIC-M cryostat, the ultrapure copper vessel where CCDs will be kept at low temperatures under vacuum. A first test of the mechanical support, where CCDs are glued on a ultrapure silicon substrate with electrical lines to operate the CCD patterned on its surface, and of the cooling system yielded positive results.
Another important area of work has been the control of radioactive backgrounds from materials surrounding the detector. Materials for the mechanical package and shielding have been measured to be extremely pure, including electroformed copper and ancient lead from a Roman ship, where the radioactive lead isotopes have decayed away. A detailed Monte Carlo simulation of the apparatus – where radioactive contaminants in materials were simulated according to measured values - was developed to find the most important sources of background and guide the design of the detector. The dominant source was identified in the CCD cable and our efforts have focused on finding adequate radiopure technologies, with two possible solutions already fulfilling requirements.
A new electronics has been developed to operate the skipper CCDs. This includes a readout ASIC chip – to be placed close to the CCD to amplify its extremely small signal – and a CCD controller, which provides the voltage levels required to move the charge across the CCD and to perform the charge readout. A first version of the ASIC has been fabricated and tested with satisfactory results. The different boards which make the CCD controller have been produced and individually tested. A first integration of these boards has been successful, with a DAMIC-M CCD controller operating a CCD.
A large clean room has been installed at the Laboratoire Souterrain de Modane for DAMIC-M. It will first host the Low Background Chamber (LBC), a low background test setup to be used mainly for validation of the DAMIC-M components. The LBC design is finished and all of its parts are already fabricated or in advanced stage of production, including its support structure, the cryostat and CCD package, the ancient and standard lead shielding, poly shielding, and the CCD electronics.
The raw material for the CCDs – silicon – is produced in the form of ingot and then sliced into wafers. After fabrication, cosmic rays interactions in the silicon produce tritium, a radioactive isotope whose decay can mimic a dark matter signal. It was thus of paramount importance to minimize exposure to cosmic rays at the very beginning of the project. The silicon ingot was produced in Denmark, wafers were sliced in the UK and then shipped to Canada, where CCDs are fabricated. Logistics included storage at the Boulby (UK) and SNOLAB (Canada) laboratories (1.1 km and 2 km underground, respectively) and shipping by sea in a custom 16 ton shielding container. The equivalent exposure to cosmic rays was only 14 days, a factor two better than originally planned.
The CCD fabrication has started with the first lot of 25 wafers in advanced stage of processing at the time of this report. A suite of vacuum chambers where the CCDs are cooled down to -140 C have been built and are ready to perform the validation test of this first lot before starting the full production.
Significant effort has gone in the design of the mechanical support for the CCDs and the DAMIC-M cryostat, the ultrapure copper vessel where CCDs will be kept at low temperatures under vacuum. A first test of the mechanical support, where CCDs are glued on a ultrapure silicon substrate with electrical lines to operate the CCD patterned on its surface, and of the cooling system yielded positive results.
Another important area of work has been the control of radioactive backgrounds from materials surrounding the detector. Materials for the mechanical package and shielding have been measured to be extremely pure, including electroformed copper and ancient lead from a Roman ship, where the radioactive lead isotopes have decayed away. A detailed Monte Carlo simulation of the apparatus – where radioactive contaminants in materials were simulated according to measured values - was developed to find the most important sources of background and guide the design of the detector. The dominant source was identified in the CCD cable and our efforts have focused on finding adequate radiopure technologies, with two possible solutions already fulfilling requirements.
A new electronics has been developed to operate the skipper CCDs. This includes a readout ASIC chip – to be placed close to the CCD to amplify its extremely small signal – and a CCD controller, which provides the voltage levels required to move the charge across the CCD and to perform the charge readout. A first version of the ASIC has been fabricated and tested with satisfactory results. The different boards which make the CCD controller have been produced and individually tested. A first integration of these boards has been successful, with a DAMIC-M CCD controller operating a CCD.
A large clean room has been installed at the Laboratoire Souterrain de Modane for DAMIC-M. It will first host the Low Background Chamber (LBC), a low background test setup to be used mainly for validation of the DAMIC-M components. The LBC design is finished and all of its parts are already fabricated or in advanced stage of production, including its support structure, the cryostat and CCD package, the ancient and standard lead shielding, poly shielding, and the CCD electronics.
Notable progress beyond state of the art has been the development of large-size CCDs which feature an innovative “skipper” readout - the charge of a pixel is measured non-destructively multiple times, drastically improving the resolution in the charge measurement. Our skipper CCDs identify a single electron of charge with resolution better than 10%, allowing an unprecedented low threshold in the detection of DM interactions. We have fabricated and successfully operated the world-largest skipper CCDs (54 cm2), demonstrating that single electron charge resolution can be reliably and consistently achieved with this novel technology. Optimization of the skipper CCD operating parameters and selection of the best performing readout amplifier have been major achievements of this reporting period.
For rest of the project we will focus on the integration of the skipper CCDs in a low-background apparatus for dark matter detection. An intermediate step to the full DAMIC-M detector will be the assembly, commissioning and operation of the Low Background Chamber. The LBC will be instrumental to validate the design of the DAMIC-M components and provide, at the same time, first scientific results with a small detector. By the end of the project we expect to have completed the construction and commissioning of the full DAMIC-M detector, and started our search for light dark matter with unprecedented sensitivity.
For rest of the project we will focus on the integration of the skipper CCDs in a low-background apparatus for dark matter detection. An intermediate step to the full DAMIC-M detector will be the assembly, commissioning and operation of the Low Background Chamber. The LBC will be instrumental to validate the design of the DAMIC-M components and provide, at the same time, first scientific results with a small detector. By the end of the project we expect to have completed the construction and commissioning of the full DAMIC-M detector, and started our search for light dark matter with unprecedented sensitivity.