Periodic Reporting for period 4 - ULTIMATE (Towards the ultimate dark matter detector) Período documentado: 2021-11-01 hasta 2022-10-31 Resumen del contexto y de los objetivos generales del proyecto There is evidence that most of the matter in the Universe is made yet unknown form of dark matter that builds large structures in the Universe but does not interact with light. The existence of dark matter, that might interact very weakly with normal matter, is one of the strongest indications that there must be physics beyond the standard model of particle physics, as no known particle can be associated with dark matter. The “dark matter problem” is one of the most important open issues in modern physics as it suggests that mankind actually only properly understands a tiny fraction (in terms of mass) of the Universe. One strategy to detect dark matter is to search for their rare interactions with atomic nuclei. The huge background from cosmic rays and natural radioactivity requires that the experiments are well shielded and that the materials used to construct the detector, as well as the dark matter target itself, do not contain radioactive contaminants. The ultimate background for the dark matter search stems from neutrinos which produce a signal that is identical to that from a dark matter interaction. The most sensitive dark matter detectors so far are dual-phase time projection chambers (TPCs) filled with cryogenic xenon gas in liquid form (LXe). ULTIMATE’s goal was to explore how one can build the ultimate LXe-based dark matter detector that is capable to explore the entire accessible parameter space, down to the limit from neutrinos. The background of such detector needs to be dominated by neutrino-induced interactions, i.e. all other background sources need to be suppressed well below this level. To reach the design sensitivity and background level, a cylindrical TPC of about 2.6m height and diameter, containing 40t of LXe, is required. We focused on several crucial aspects related to the background and how such large detector can be constructed mechanically. In addition, we studied several science channels beyond dark matter which can be addressed by a 40t LXe TPC as well.The ULTIMATE objectives were1. To experimentally demonstrate whether background from radon-222, dominating the background of current detectors, can be mitigated by a novel hermetic TPC design which separates the active from the passive LXe.2. To investigate the intrinsic radiopurity of PTFE, an important structural and optical material in LXe TPCs. The level of purity will directly affect the detector’s background level.3. Since a low-background LXe TPC of 2.6m diameter has never been built before, a test-platform was developed to allow testing full-scale detector components in a cryogenic LXe environment.4. To investigate whether the charge signal from a particle interaction can be amplified in a novel way in liquid xenon (instead of in xenon gas) as this would simplify detector design and operation.5. To investigate science channels beyond dark matter which could be explored with the ultimate dark matter detector. Trabajo realizado desde el comienzo del proyecto hasta el final del período abarcado por el informe y los principales resultados hasta la fecha The objectives were addressed in five work packages (WPs). WP1: We built a small-scale hermetic TPC prototype and successfully demonstrated that radon background can be reduced in such detector. Even moderate levels of hermeticity can lead to significant background reduction. The publication on the study is currently under review. Since the hermetic TPC concept is so promising we already started a follow-up project with U Nagoya. We also submitted a publication on the LXe platform used for this test which includes measurements of some fundamental LXe properties. WP2: We have performed several studies to investigate and possibly reduce the background from PTFE. (i) We measured the light transmission of PTFE and showed that 40% thinner PTFE panels than previously used are sufficient to block the light. (ii) We successfully constructed and now operate detector to measure the emanation of the radioactive radon-222 from surfaces. (iii) Within WP5 we showed that the radiopurity level of PTFE used in the current generation of detectors is not sufficient for the next stage. (iv) We studied the radiopurity of commercially available PTFE and found that PTFE with a radiopurity level required for the ultimate detector is not available as wrought material. The result of (i) has been published, publications on (ii) and (iv) are in preparation and the result of (iii) was published in a PhD thesis.WP3: Very early it became clear that it is very important for the development of a 2.6m-scale TPCs to have a platform which allows testing full-scale detector components in cryogenic LXe. We could design and procure a test-platform of 2.7 m inner diameter using independent start-up funds. It will be used to develop and test TPC components and will be available for other DARWIN groups as well. A publication on the platform is being prepared.WP4: Proportional scintillation signals generated in liquid xenon are significantly faster than proportional signals produced in xenon gas. We conducted the first study exploring the benefits of the faster timing; the study has been published. We also designed and successfully operated the first full single-phase TPC prototype. We studied the charge signal and compared the single-phase performance to dual-phase data acquired in the same detector. The performance of the dual-phase detector has been published; the publication on the single-phase data is in preparation. Even though the achieved single-phase amplification is below the expectation, we plan to continue this promising research for a better understanding of the process and to improve the signal size.WP5: We studied the background level of DARWIN, the best-defined ultimate LXe detector, and concluded that the ultimate detector can reach it science goal for dark matter but also for other channels at the LNGS laboratory, provided that a sufficiently large shield is used. We led a study on the sensitivity of DARWIN to the neutrinoless double beta decay of 136Xe and provided the background model for a study on solar neutrinos (both studies published). We provided the detector model for an optical simulation study of variations of the conventional detector design (published) and explored DARWIN’s sensitivity to the neutrinoless double-electron capture of 124Xe. The study is currently being finalized. Avances que van más allá del estado de la técnica e impacto potencial esperado (incluida la repercusión socioeconómica y las implicaciones sociales más amplias del proyecto hasta la fecha) In particular the topics addressed in WP1 and WP4 explored new ideas which have never been investigated before. We could demonstrate that a (even a semi-)hermetic TPC is indeed able to reduce xenon-intrinsic backgrounds in the dark matter target volume and we could successfully operate a fully functional single-phase TPC. However, the observed charge signals are currently still smaller than required for a full dark matter detector.Within WP3 a unique large-scale liquid xenon detector test platform has been constructed and commissioned. It is now available to develop large components of future detectors. A facility of this size does not exist elsewhere.WP2 and WP5 addressed a number of questions with established methods, however, the detailed questions had not been addressed before. A member of the ULTIMATE team working in the cleanroom on a TPC prototype. Documentos relacionados working_on_hermetic_tpc.JPG
