Tag Archives: 2017

Intrinsic backgrounds from Rn and Kr in the XENON100 experiment

XENON1T is currently the largest liquid xenon detector in the search for dark matter. To fully exploit the capabilities of the ton-scale target mass, a thorough understanding of radioactive background sources is required. In this paper we use the full data of the main science runs of the XENON100 experiment that were taken over a period of about 4 years to asses the target-intrinsic background sources radon (Rn-222), thoron (Rn-220) and krypton (Kr-85). We derive distributions of the individual radionuclides inside the detector (see Figure below) and quantify their abundances during the main three science runs. We find good agreement with external measurements of radon emanation and krypton concentrations, and report an observed reduction in concentrations of radon daughters that we attribute to the plating-out of charged ions on the negatively biased cathode.

The preprint of the full study is available on arXiv:1708.03617.

Figure: Spatial distributions of the various radon populations identified in XENON100.

Search for WIMP Inelastic Scattering Off Xenon Nuclei With XENON100

Most direct detection searches focus on elastic scattering of galactic dark matter particles off nuclei, where the keV-scale nuclear recoil energy is to be detected. In this work, the alternative process of inelastic scattering is explored, where a WIMP-nucleus scattering induces a transition to a low-lying excited nuclear state. The experimental signature is a nuclear recoil detected together with the prompt de-excitation photon. We consider the scattering of dark matter particle off 129Xe isotope, which has an abundance of 26.4\% in natural xenon, and when excited to it lowest-lying 3/2+ state above the ground state it emits a 36.9 keV photon. This electromagnetic nuclear decay has a half-life of 0.97 ns.

The WIMP inelastic scattering  is complementary to spin-dependent, elastic scattering, and dominates the integrated rates above 10 keV of deposited energy. In addition, in case of a positive signal, the observation of inelastic scattering would provide a clear indication of the spin-dependent nature of the fundamental interaction.

The search is performed using XENON100 Run-II science data, which corresponds to an exposure of 34×224.6 kg×days. No evidence of dark matter is found and a limit on dark matter inelastic interaction cross section is set. Our result, shown in the Figure, is the most stringent limit for the spin-dependent inelastic scattering to date, and set the stage for a sensitive search of inelastic WIMP-nucleus scattering in running or upcoming liquid xenon experiments such as XENON1T, XENONnT, LZ, and DARWIN.

Full details may be found in this article: Phys. Rev. D 96, 022008 and on the arxiv.

SAIP 2017: Latest results from XENON1T

At the 62nd annual conference of the South African Institute of Physics (SAIP), hosted by the University of Stellenbosch, Jacques Pienaar presented the results of our first science run with XENON1T. While a dark matter particle candidate still eludes us, we are able to demonstrate that for the first time a tonne-scale liquid Xenon dark matter detector is not only operating, but doing so very successfully.

The work done up to this point has given us a thorough understanding of the electronic and nuclear recoil response in our detector, which we can use to look for dark matter candidates. This of course is just the start. In this first result we had an exposure of only 0.1 ton.years, but our design goal is 2 ton.years. Therefore much work still lies ahead to probe for dark matter, and indeed we have more than 3 times as much data available already to push the bounds of our knowledge further. Stay tuned!

Modulation results from Xenon100 presented at PASCOS 2017 conference.

On Tuesday 20th of June, we presented our latest results on Electronic Recoil Modulations with 4 years of Xenon100 data at the PASCOS 2017 conference held in Madrid. After a short introduction, by M.L. Benabderrahmane, to the dark matter modulation as a signal, the main results have been presented, namely the test statistics of unbinned profile likelihood to search for the modulation period using three different sets of data. The first set contains the single scatter events in the energy range 2-5.8keV, the second set contains Multiple scatter events in the same energy range and the last one contains single scatters in the energy range 6-20keV. The last two samples are used as a sideband. The results of the likelihood gives a period of 431 days which is different from the one observed by the DAMA/LIBRA collaboration. Our single scatter modulation at 431 days has a global significance below 2sigma. The local test statistics for one year period gives a 1.8sigma. Similarity of the spectra between the two control samples and the signal sample disfavors the possibility for a modulation due to Dark Matter interaction.

Talk at WIN2017: First Results from the XENON1T experiment

This talk by Sara Diglio (slides) was presented at the WIN2017 conference at the University of California in  Irvine, from June 19 – 24, 2017.

ReStoX is an original cryogenic system designed for experiments that make use of high quantities of liquid xenon. It allows to store the total amount of xenon in gaseous, liquid or solid phase and to fill it into the detector vessel under high purity conditions. The system is crucial in case of emergencies that might require a fast recovering of the whole xenon present in the detector. ReStoX is currently being used by the XENON1T experiment and a future upgrade for XENONnT has already started.

The traditional approach for WIMP nucleus interaction studies in direct detection experiment is to consider just two types of interactions, the spin independent (SI) and the spin dependent (SD) ones. However, these are not the only types of interactions possible. In recent years, a non-relativistic effective field theory approach has been studied. In this framework, 14 new interaction operators arise. These operators include the SI and SD ones among others. Some of these new operators are momentum dependent and predict a non-exponential event rate as function of energy, in contrast to the traditional expected signals. Moreover, some of these operators predict energy recoils above the upper threshold of the standard analyses done in direct detection experiments. For XENON100, this threshold is 43keV (nuclear recoil).

In this analysis, we extend the upper energy threshold up to ~240 keV. This value is dictated by low statistics in calibration data above it. We divide our signal region into two regimes, low recoil energy, on which we perform the same “standard” analysis done for the SI and SD cases, and high recoil energy, which is the main focus of this work.

Summary of regions of interest, backgrounds, and observed data. ER calibration data, namely 60Co and 232Th data, is shown as light cyan dots. NR calibration data (241AmBe) is shown as light red dots. Dark matter search data is shown as black dots. The red line is the threshold between the low and high energy channels. The lines in blue are the bands. For the low energy channel these are operator and mass dependent, but are shown here for a 50 GeV/c^2 WIMP using the O1 operator. For the high-energy region, the nine analysis bins are presented also in blue lines.

We find that our data is compatible with background expectations. Using a binned profile likelihood, we thus produce 90% CL exclusion limits for both elastic scattering and inelastic WIMP scattering for each operator. Find the preprint of this study on the arxiv.

The XENON100 limits (90% CLS) on isoscalar dimensionless coupling for all elastic scattering EFT operators. The
limits are indicated in solid black. The expected sensitivity is shown in green and yellow (1σ and 2σ respectively). Limits from CDMS-II Si, CDMS-II Ge, and SuperCDMS [30] are presented as blue asterisks, green triangles, and orange rectangles, respectively.

On Tuesday, May 30, we presented the first XENON1T results in a seminar at LNGS, the laboratory where our experiment is hosted. The seminar was presented by Marco Selvi (INFN Bologna) in the Fermi room, the main auditorium at LNGS, and introduced by the LNGS director prof. Stefano Ragazzi in front of about 40 scientists.

After a short introduction on Dark Matter (you may guess that at LNGS they are well aware of the details of this physics puzzle! ), we described the construction and commissioning phase of the various systems crucial to run our detector.

We then focused mainly on the performances of XENON1T in the first science run,
where we reached the lowest ER background ever achieved in a dark matter experiment.

Also our sensitivity is very good, being it also the best out of the various direct search dark matter experiment, even with just 34 days of data acquisition.

With our result, XENON1T (and LNGS with) is back at the frontline of the race to finally detect dark matter particles … we look forward to analyse the already acquired >70 days of data !

 

XENON1T, the most sensitive detector on Earth searching for WIMP dark matter, releases its first result

[Press Release May 2017 – for immediate release. Preprint is on the arxiv]

The best result on dark matter so far! … and we just got started!”.

This is how scientists behind XENON1T, now the most sensitive dark matter experiment world-wide, hosted in the INFN Laboratori Nazionali del Gran Sasso, Italy, commented on their first result from a short 30-day run presented today to the scientific community.

XENON1T at LNGS

XENON1T installation in the underground hall of Laboratori Nazionali del Gran Sasso. The three story building on the right houses various auxiliary systems. The cryostat containing the LXeTPC is located inside the large water tank on th left, next to the building. (Photo by Roberto Corrieri and Patrick De Perio)

Dark matter is one of the basic constituents of the Universe, five times more abundant than ordinary matter. Several astronomical measurements have corroborated the existence of dark matter, leading to a world-wide effort to observe directly dark matter particle interactions with ordinary matter in extremely sensitive detectors, which would confirm its existence and shed light on its properties. However, these interactions are so feeble that they have escaped direct detection up to this point, forcing scientists to build detectors that are more and more sensitive. The XENON Collaboration, that with the XENON100 detector led the field for years in the past, is now back on the frontline with the XENON1T experiment. The result from a first short 30-day run shows that this detector has a new record low radioactivity level, many orders of magnitude below surrounding materials on Earth. With a total mass of about 3200kg, XENON1T is at the same time the largest detector of this type ever built. The combination of significantly increased size with much lower background implies an excellent dark matter discovery potential in the years to come.

The XENON1T TPC

Scientists assembling the XENON1T time projection chamber. (Photo by Enrico Sacchetti)

The XENON Collaboration consists of 135 researchers from the US, Germany, Italy, Switzerland, Portugal, France, the Netherlands, Israel, Sweden and the United Arab Emirates. The latest detector of the XENON family has been in science operation at the LNGS underground laboratory since autumn 2016. The only things you see when visiting the underground experimental site now are a gigantic cylindrical metal tank, filled with ultra-pure water to shield the detector at his center, and a three-story-tall, transparent building crowded with equipment to keep the detector running, with physicists from all over the world. The XENON1T central detector, a so-called Liquid Xenon Time Projection Chamber (LXeTPC), is not visible. It sits within a cryostat in the middle of the water tank, fully submersed, in order to shield it as much as possible from natural radioactivity in the cavern. The cryostat allows keeping the xenon at a temperature of -95°C without freezing the surrounding water. The mountain above the laboratory further shields the detector, preventing it to be perturbed by cosmic rays. But shielding from the outer world is not enough since all materials on Earth contain tiny traces of natural radioactivity. Thus extreme care was taken to find, select and process the materials making up the detector to achieve the lowest possible radioactive content. Laura Baudis, professor at the University of Zürich and professor Manfred Lindner from the Max-Planck-Institute for Nuclear Physics in Heidelberg emphasize that this allowed XENON1T to achieve record “silence”, which is necessary to listen with a larger detector much better for the very weak voice of dark matter.

XENON1T first results limit

The spin-independent WIMP-nucleon cross section
limits as a function of WIMP mass at 90% confidence
level (black) for this run of XENON1T. In green and yellow
are the 1- and 2σ sensitivity bands. Results from LUX
(red), PandaX-II (brown), and XENON100 (gray)
are shown for reference.

A particle interaction in liquid xenon leads to tiny flashes of light. This is what the XENON scientists are recording and studying to infer the position and the energy of the interacting particle and whether it might be dark matter or not. The spatial information allows to select interactions occurring in the central 1 ton core of the detector. The surrounding xenon further shields the core xenon target from all materials which already have tiny surviving radioactive contaminants. Despite the shortness of the 30-day science run the sensitivity of XENON1T has already overcome that of any other experiment in the field, probing un-explored dark matter territory.  “WIMPs did not show up in this first search with XENON1T, but we also did not expect them so soon!” says Elena Aprile, Professor at Columbia University and spokesperson of the project. “The best news is that the experiment continues to accumulate excellent data which will allow us to test quite soon the WIMP hypothesis in a region of mass and cross-section with normal atoms as never before. A new phase in the race to detect dark matter with ultra-low background massive detectors on Earth has just began with XENON1T. We are proud to be at the forefront of the race with this amazing detector, the first of its kind.”

As always, feel free to contact the XENON collaboration at contact@xenon1t.org.

Material radioassay and selection for XENON1T

To attain the high sensitivity needed to detect a dark matter particle with a xenon time-projection chamber, all other sources of particle interactions need to be eliminated or minimized. These interactions are classified as background events. Radiogenic backgrounds, in particular, come from radioactive isotopes within the detector materials that decay and lead to alpha, beta, or gamma emissions. Neutrons from spontaneous fission of heavy isotopes or from secondary reactions within the detector materials also contribute to the radiogenic background and can mimic a dark matter signal.

To minimize the radiogenic background, the goal of the XENON1T radioassay program is to measure the radioactivity of all materials that are needed to build the detector and to select only the most radiopure materials for the final construction. To do this, we use mass spectrometry techniques and high-purity germanium spectrometers that are capable of measuring radioactivity at the level of 10-6 decays per second in a kilogram of material (Bq/kg). As comparison, a typical banana has an activity of ~102 Bq/kg!

Because natural radioactivity is present in the soil, the water, and in the air, it is also present in the XENON detector materials. The Figure shows a measurement obtained with a germanium spectrometer of the gamma rays emitted from a sample of photomultiplier tubes. The background (purple spectrum) is subtracted from the sample (pink spectrum) in order to quantify the expected activity from a XENON1T component or material sample.

A high-purity germanium spectrometer measurement of gamma rays emitted from a sample of XENON1T photosensors. Some prominent isotopes from different sources are labeled: primordial uranium and thorium decay chains (green), potassium (red), man-made (orange) and cosmogenic (orange) isotopes.

The most common radioactive isotopes present in the Earth are primordial uranium and thorium, each of which decays into a series of other radioactive isotopes (marked in green in the Figure). Potassium (red) is also a common, primordial isotope that is found in soil, and subsequently in food and in your body. Other isotopes that are found in detector materials come from interactions with cosmic rays (yellow) or from man-made activities (blue), i.e. industrial or medical use, nuclear power plant emissions, nuclear accidents, and military testing.

The measured activities of each material selected for detector construction are used in simulations of XENON1T to determine the expected background. This allows for a prediction of the attainable sensitivity of the detector to dark matter interactions. The radioassay measurement results from over 100 material samples are presented in our new paper “Material radioassay and selection for the XENON1T dark matter experiment”.