Periodic Reporting for period 1 - LowRad (Low radon and low internal radioactivity for dark matter and rare event xenon detectors)
Reporting period: 2022-11-01 to 2025-04-30
Our understanding of the universe and the standard model of particle physics has reached an incredible level of depth and accuracy. Nevertheless, there are still very fundamental questions, for example, what is the nature of dark matter or why does the universe consist of matter and not in equal parts of antimatter. Experiments in underground laboratories with xenon-based detectors play a crucial role in answering these questions.
One major challenge is that the sensitivity of the detectors increases with their size, thus also increasing the rate of background signals due to radioactivity in the detector construction materials. Therefore, the purity of the detector materials must be reduced further and further, which is achieved first by careful material screening and selection. Secondly, the actual detector medium, liquid xenon, is shielded from the detector construction materials by sophisticated methods. This leaves the radioactive noble gases (radon and some isotopes of krypton and argon) in the liquid xenon as the main problem.
The goal of the LowRad project (Low radon and low internal radioactivity for dark matter and rare event xenon detectors) is to develop the technologies for reducing these radioactive noble gas impurities in liquid xenon (1 rad on atom per more than 100 moles xenon) for the next generation of a xenon-based observatory such as XLZD/DARWIN that they each cause a background rate an order of magnitude lower than that of the unshieldable solar and atmospheric neutrinos. In particular, cryogenic distillation methods combined with ultrasensitive diagnostic methods are being developed. The devices used must have a very low radioactive background rate and, in particular, an extremely low radon emanation rate. Therefore, practically all parts of the apparatus must be specially developed for this purpose. To provide the enormous cooling capacity required, a cryogenic heat pump technology is being developed. In a final demonstrator, these new methods will be combined with the removal of electronegative impurities from xenon and with calibration methods using short-lived radioactive noble gases.
The outcomes of this project will be crucial to significantly increase the sensitivity of the next generation of xenon-based detectors in the search for dark matter, neutrinoless double beta decay and other rare or exotic events. In addition, the development of high-purity instrumentation and equipment and the various methods can also be applied in other fields.
One major challenge is that the sensitivity of the detectors increases with their size, thus also increasing the rate of background signals due to radioactivity in the detector construction materials. Therefore, the purity of the detector materials must be reduced further and further, which is achieved first by careful material screening and selection. Secondly, the actual detector medium, liquid xenon, is shielded from the detector construction materials by sophisticated methods. This leaves the radioactive noble gases (radon and some isotopes of krypton and argon) in the liquid xenon as the main problem.
The goal of the LowRad project (Low radon and low internal radioactivity for dark matter and rare event xenon detectors) is to develop the technologies for reducing these radioactive noble gas impurities in liquid xenon (1 rad on atom per more than 100 moles xenon) for the next generation of a xenon-based observatory such as XLZD/DARWIN that they each cause a background rate an order of magnitude lower than that of the unshieldable solar and atmospheric neutrinos. In particular, cryogenic distillation methods combined with ultrasensitive diagnostic methods are being developed. The devices used must have a very low radioactive background rate and, in particular, an extremely low radon emanation rate. Therefore, practically all parts of the apparatus must be specially developed for this purpose. To provide the enormous cooling capacity required, a cryogenic heat pump technology is being developed. In a final demonstrator, these new methods will be combined with the removal of electronegative impurities from xenon and with calibration methods using short-lived radioactive noble gases.
The outcomes of this project will be crucial to significantly increase the sensitivity of the next generation of xenon-based detectors in the search for dark matter, neutrinoless double beta decay and other rare or exotic events. In addition, the development of high-purity instrumentation and equipment and the various methods can also be applied in other fields.
We developed a Python program with a simple graphical user interface for designing cryogenic distillation plants using the McCabe-Thiele method and cryogenic heat pumps using the Clausius-Rankine process. This allows all project participants to design the various systems.
In the cryogenic distillation process for the removal of krypton and argon, the so-called offgas is separated, which contains the more volatile components krypton and argon. This results in significant xenon losses. To minimize these losses in long-term operation over many years at the next generation xenon-based detector, the concentration of the more volatile components in the offgas must be increased further. Therefore, we have designed a special distillation unit (“Krypton Concentrator”) that enables a particularly high concentration increase by a factor 1000, which is installed as a second stage behind a normal distillation unit. This unit has been fully assembled and successfully put into operation. The high concentration of the more volatile components in the offgas of the Krypton Concentrator is ideal for determining the krypton (and argon) concentration in situ consisting of a xenon separation using a liquid nitrogen-cooled trap and a residual gas analyzer. With calibrated concentration factors for the entire system, it will be then possible for the first time to determine the krypton (and argon) concentration continuously in the actual liquid xenon detector.
To test a complete cryogenic heat pump with xenon as the working fluid, a demonstration and test facility was developed, built and put into operation. The focus here is on the effectiveness of the heat exchangers and the phase transitions from gaseous to liquid and from liquid to gaseous.
We have demonstrated, that we can measure the radon decays by the scintillation light produced in liquid xenon with the help of compact SI photomultipliers.
Furthermore, the development of ultra-pure xenon gas and liquid xenon pumps has been driven forward. The first prototypes are already working.
Electrical impurities impede the drift of charge carriers in liquid xenon, which destroys the functioning of a xenon detector based on the principle of the time projection chamber. In order to monitor the possible introduction of electronegative impurities by our new systems and to control the removal of these impurities, a novel detector for electronegative impurities in xenon is being developed.
In the cryogenic distillation process for the removal of krypton and argon, the so-called offgas is separated, which contains the more volatile components krypton and argon. This results in significant xenon losses. To minimize these losses in long-term operation over many years at the next generation xenon-based detector, the concentration of the more volatile components in the offgas must be increased further. Therefore, we have designed a special distillation unit (“Krypton Concentrator”) that enables a particularly high concentration increase by a factor 1000, which is installed as a second stage behind a normal distillation unit. This unit has been fully assembled and successfully put into operation. The high concentration of the more volatile components in the offgas of the Krypton Concentrator is ideal for determining the krypton (and argon) concentration in situ consisting of a xenon separation using a liquid nitrogen-cooled trap and a residual gas analyzer. With calibrated concentration factors for the entire system, it will be then possible for the first time to determine the krypton (and argon) concentration continuously in the actual liquid xenon detector.
To test a complete cryogenic heat pump with xenon as the working fluid, a demonstration and test facility was developed, built and put into operation. The focus here is on the effectiveness of the heat exchangers and the phase transitions from gaseous to liquid and from liquid to gaseous.
We have demonstrated, that we can measure the radon decays by the scintillation light produced in liquid xenon with the help of compact SI photomultipliers.
Furthermore, the development of ultra-pure xenon gas and liquid xenon pumps has been driven forward. The first prototypes are already working.
Electrical impurities impede the drift of charge carriers in liquid xenon, which destroys the functioning of a xenon detector based on the principle of the time projection chamber. In order to monitor the possible introduction of electronegative impurities by our new systems and to control the removal of these impurities, a novel detector for electronegative impurities in xenon is being developed.
The developments made in the LowRad project are primarily intended to continuously remove radioactive noble gas isotopes from the liquid xenon of ultra-sensitive detectors for the search for dark matter and other rare and exotic processes to a previously unattainable low level. The methods used and the instruments developed have the potential to be used in other areas as well. For example, we are developing hermetically sealed, magnetically coupled pumps made of high-purity materials. They are certainly also of interest for the transport of gases and liquids in the cleanest areas, such as food or medical technology, or for the transport of corrosive substances. In developing an instrument to determine the concentration of electronegative impurities in liquid xenon, we are also breaking new ground with a very compact design that could also be used in other gas or liquid detectors based on the ionization principle.
The development of the quasi-loss-free removal of krypton from xenon is novel, so that it promises uniquely low krypton concentrations. Likewise, the cryogenic distillation of xenon with cooling by a second cryogenic xenon-based heat pump circuit is an energy-reducing, promising approach that has not yet been chosen in astroparticle physics.
The results achieved so far in the LowRad project are summarized below:
- A novel design tool for cryogenic distillation plants based on the McCabe-Thiele principle and for cryogenic heat pumps based on the Clausius Rankine principle, operated via a graphical user interface.
- Design and construction of a distillation column “Krypton Concentrator” to increase the krypton-in-xenon concentration, so that a loss-free continuous distillation and the in-situ determination of the krypton concentration in the actual detector is possible.
- Design and construction of a test stand for cryogenic xenon-based heat pumps, which is to be used later to cool the “Krypton Concentrator”.
- Demonstration of the measurement of radon decays via the scintillation light in liquid xenon using a silicon photomultiplier
- Design of a novel, compact monitor for determining the concentration of electronegative impurities in liquid xenon
- Design, construction and initial tests of ultra-pure xenon pumps with a new magnetically coupled drive concept
The development of the quasi-loss-free removal of krypton from xenon is novel, so that it promises uniquely low krypton concentrations. Likewise, the cryogenic distillation of xenon with cooling by a second cryogenic xenon-based heat pump circuit is an energy-reducing, promising approach that has not yet been chosen in astroparticle physics.
The results achieved so far in the LowRad project are summarized below:
- A novel design tool for cryogenic distillation plants based on the McCabe-Thiele principle and for cryogenic heat pumps based on the Clausius Rankine principle, operated via a graphical user interface.
- Design and construction of a distillation column “Krypton Concentrator” to increase the krypton-in-xenon concentration, so that a loss-free continuous distillation and the in-situ determination of the krypton concentration in the actual detector is possible.
- Design and construction of a test stand for cryogenic xenon-based heat pumps, which is to be used later to cool the “Krypton Concentrator”.
- Demonstration of the measurement of radon decays via the scintillation light in liquid xenon using a silicon photomultiplier
- Design of a novel, compact monitor for determining the concentration of electronegative impurities in liquid xenon
- Design, construction and initial tests of ultra-pure xenon pumps with a new magnetically coupled drive concept