Tag Archives: 2015

Lowering the radioactivity of the XENON1T photosensors

E. Aprile et al (XENON Collaboration), Lowering the radioactivity of the XENON1T photosensors, arXiv:1503.07698, Eur. Phys. J. C75 (2015) 11, 546.

The XENON1T experiment employs 242 photomultiplier tubes (PMTs) in the time projection chamber, arranged into two circular arrays. Because the overall background goal of the detector is incredibly low, with less than 1 expected event in a tonne of liquid xenon and one full year of data, the PMTs must be made out of ultra-pure materials. These materials were selected for their content in traces of 238-U, 232-Th, 40-K, 60-Co, 137-Cs and other long-lived radionuclides.

The XENON collaboration joined efforts with Hamamatsu to produce a photosensor that meets the strict requirements of our experiment. The sensor is a 3-inch diameter tube that operates stably at -100 C and at a pressure of 2 atmospheres. It has a high quantum efficiency, with a mean around 35%, for the xenon scintillation light at 178 nm and 90% photon collection efficiency.

PMT_schematicsThe sensor, shown schematically in the left picture, features a VUV-transparent quartz window, with a low-temperature bi-alkali photocathode deposited on it. A 12-dynode electron multiplication system ensures a signal amplification of ~3 millions, which is a crucial feature to detect the tiny signals induced by the rare collisions of dark matter particles with xenon nuclei.

Before the tubes were ready to be manufactured, the construction materials were inspected with gamma-ray spectroscopy and glow-discharge mass spectroscopy (GDMS). For the former, we employed the world’s most sensitive high-purity germanium detectors, GeMPI and Gator, operated deep underground at the Gran Sasso Laboratory. GDMS can detect trace impurities in solid samples and the results were compatible with those from germanium screening. We measured many samples to select the final materials for the PMT production. As an example, specific 226-Ra activities around or below 0.3 mBq/PMT were seen in most of the inspected materials. Such an activity corresponds to 3 x 10-4 226-Ra decays per second and tube, or about 26 decays per day.

BarChart_blogThe relative contribution of the selected materials to the trace contaminations in U, Th, K, Co and Cs of the final product, seen in the left picture, also tells us how to improve further sensor versions for the XENONnT upgrade. Most of the nuclides in the 238-U and 232-Th chains, especially dangerous for their emission of alpha particles, that can the produce fast neutrons in (alpha,n) reactions, are located in the ceramic stem of the tube. In consequence, finding a new material to replace the ceramic might drastically improve the background expectations.

pmts_gatorOnce the final production started, and the tubes were delivered in several batches to our collaboration, they were measured in the Gator detector. Its inner chamber can accommodate 15 PMTs at a time, as seen in the left picture. Each batch was screened for about 15 days, and theobserved activities were mostly consistent from batch to batch. For all measured PMTs, we obtain contaminations in uranium and thorium below 1 mBq/PMT. While 60-Co was at the level of 0.8 mBq/PMT, 40-K dominates the gamma activity with about 13 mBq/PMT. The information from screening was considered in the final arrangement of the PMTs in the XENON1T arrays. PMTs with somewhat higher activities are placed in the outer rings, where they are more distant from the central, fiducial xenon region of the detector.

The average activities per PMT of all trace isotopes served as input contaminations to a full Monte Carlo simulation of the expected backgrounds in XENON1T. The results show that the PMTs will provide about 1% and 6% of the total electronic and recoil background of the experiment, respectively. We can therefore safely conclude that the overall radioactivity of the sensors is sufficiently low, and they will certainly not limit the dark matter sensitivity of the XENON1T experiment.

XENON1T First Light

Today XENON1T has seen its first light:

firstlightThis is literally the first event recorded by the detector in that is is a single photon that was detected by one of the photomultipliers and recorded by the whole XENON1T data acquisition setup. What you can see from the picture is that our noise is indeed very low compared to the smallest possible signal – that of a single photon! And this is even without any fine-tuning of our electronics yet.

The detector is still empty and we are checking the photomultipliers one by one before making first background measurements. Filling with liquid xenon will happen as soon as those tests are concluded successfully.

Search for Event Rate Modulation in XENON100 Electronic Recoil Data

E. Aprile et al. (XENON Collaboration), Exclusion of Leptophilic Dark Matter Models using XENON100 Electronic Recoil Data, Science 2015 vol. 349 no. 6250 pp. 851, and Search for Event Rate Modulation in XENON100 Electronic Recoil Data, Physical Review Letters 115, 091302 (2015) and arxiv.1507.07748

The annual modulation signature

Although we believe that Dark Matter is Out There, we are completely oblivious to the impact of Dark Matter on our daily lives. On the human scale Dark Matter is nearly impossible to detect, the faintest whisper of the galaxy. The vast majority of the time Dark Matter particles pass right through us as if we don’t exist.

It is hypothesized, however, that we may be able to tune our ears to hear the unique song of Dark Matter here on Earth. Doing so successfully would constitute direct proof that Dark Matter exists.

Rather than the swelling symphony that you might expect from the most abundant matter in the Universe, this song will be a random melody, plucked out in individual notes. The tempo of these notes, that is the rate of events in a Dark Matter detector, should vary over the course of one year.

Evidence suggests that both the Sun and the Earth are enveloped by the Dark Matter halo of the Milky Way. As the Earth’s velocity relative to the Sun varies over its one-year orbit, so does it’s velocity relative to the Dark Matter. This should result in the so-called “WIMP wind” that blows harder in June, and softer in December.

This variation itself becomes the song of Dark Matter, repeating every year like clockwork – the annual modulation signature.

 

Illustration of the expected “WIMP wind” due to the motion of the Sun relative to the DM halo of the Milky Way. Figure from arXiv:1209.3339

Illustration of the expected “WIMP wind” due to the motion of the Sun relative to the DM halo of the Milky Way. Figure from arXiv:1209.3339

XENON100 was the first instrument using liquified xenon that was able to search for such a signature. The liquid xenon that fills the detector emits light when particles interact with it. We take pictures of the light with extremely sensitive devices, and use them to identify the energy and type of interaction. We took data with this detector from February 2011 to March 2012, long enough to observe more than one full cycle of the Dark Matter annual modulation.

What will Dark Matter events look like?

In XENON100, more than one type of event is identifiable. The type depends on whether Dark Matter interacts with the nuclei of the atoms in the detector, or with the electrons surrounding these nuclei. Typically, we assume the interactions of Dark Matter are with the nuclei.

For our newest study, we considered the possibility that Dark Matter instead interacts with the electrons in XENON100, and looked for an annual modulation signature.

One challenge of such a study is that many things can potentially make the rate of events in the detector vary in time, for example random noise in the instrument itself or the decay of radioactive particles. We examined all these possibilities carefully, and determined to what extent they might affect the rate of events in the detector.

The results of our study show some evidence for a rate of events varying periodically over the course of roughly one year, or perhaps longer. This slight change in rate – about half of the average rate in the detector, which is itself very small – can not yet be explained. There’s a one in a thousand chance that it is just a statistical fluke.

Before you go extolling the news from the rooftops, however, take note that our observation is not what we would naively expect from Dark Matter.

Our data shows that the rate of multiple-scatter events (interactions with more than one atom) varies almost as much as that of single-scatter events. Since Dark Matter interacts extremely rarely, we would never expect it to cause multiple-scatter events. In addition, the date of the peak rate in our detector does not match up with what we expect due to the motion of the Earth through the Dark Matter halo.

New perspective on an old claim of Dark Matter discovery

The DAMA/LIBRA collaboration has observed an annual modulation signal in their NaI detectors for more than a decade. They claim that it can be interpreted as a direct detection of Dark Matter. Meanwhile, many experiments that are more sensitive than DAMA/LIBRA (including XENON100) have found no comparable evidence of Dark Matter interacting with atomic nuclei.

However, given the fact that the NaI detectors are unable to differentiate between different types of events, one way to resolve this tension between the different experiments is if the interactions in DAMA/LIBRA are with the electrons.

Although our study shows that XENON100 sees some hint of a signal varying over long periods, the size of that signal is still much smaller than what we would expect to see if we were, in fact, detecting the same signal as DAMA/LIBRA. Thus, we find that it is extremely unlikely to be the case that DAMA/LIBRA observes an annual modulation due to interactions with electrons. The data from XENON100 exclude this possibility with a statistical significance of 4.8σ, corresponding to a probability of about one in a million.

Best-fit amplitude and phase of annual modulation signal in XENON100 from a profile likelihood study. Expected signal from DAMA/LIBRA and expected phase from the standard Dark Matter halo overlaid for comparison.

Best-fit amplitude and phase of annual modulation signal in XENON100 from a profile likelihood study. Expected signal from DAMA/LIBRA and expected phase from the standard Dark Matter halo overlaid for comparison.

Our study answers an important question about how to interpret the DAMA/LIBRA annual modulation signal, but raises many more. Why haven’t we discovered the annual modulation of Dark Matter? What causes the annual modulation in DAMA/LIBRA? What causes the slight variation of rate in XENON100?

More data has since been taken by XENON100 that will hopefully allow the last question to be answered. As to the nature of Dark Matter, well, we will have to keep listening.

Liquid Level Measurement in the XENON1T TPC

Knowing the exact level of the interface between the liquid and the gaseous phase in the XENON1T TPC is crucial for the operation of the detector, and very important to understand its response. Reason for this is the so-called S2 signal, which is the second signal one measures after an event happens in the detector. It originates from electrons, which are produced when a particle scatters off the xenon, and which rise up in the electric field of the TPC until they reach the liquid-gas interface. There, an even stronger electric field is extracting them from the liquid and accelerates them towards the top of the detector. The field is strong enough that, while drifting through the xenon gas, the electrons hit xenon atoms on their way, exciting each of them to emit an ultraviolet photon. A single electron will thus produce an amplified signal of up to 300 photons, of which about 20 will be ultimately detected.

The proportional scintillation light produced by this electron avalanche is detected by the top PMT array of the detector. The size of the resulting signal is proportional to the number of electrons produced. The meshes which apply the electric fields in the detector are at fixed positions. Hence, a lower or higher level of the liquid-gas interface has direct influence on the drift length of the extracted electrons in the gas and thus a direct influence on the size of the S2 signal. The size of the S2 signal in turn is a very important parameter which is used in many different ways in the data analysis. So a very good understanding is required of where the liquid level is.

To get that information, we have designed special instruments to measure the liquid level inside the TPC. Those levelmeters work capacitively, which means that they are basically hollow capacitors, which change their capacitance proportional to the level they are immersed in liquid xenon. In normal operation mode, the system is in a thermal equilibrium, so there are no changes in the liquid level. The TPC is designed in a way that one can manually adjust the liquid-gas interface to a higher or lower level. This dynamic range of the XENON1T TPC is about 5mm. Hence the levelmeters are of similar height.

The capacitance of a capacitor increases with the area of its electrodes. To achieve the highest possible capacitance change from the lower end of the capacitor to its upper end, a detailed simulation has been performed at the University of Mainz in Germany for different shapes and sizes of capacitors. It turned out that a triple-plate capacitor of 61mm length and 10mm height is the best compromise of having a large capacitance change per unit height, while still being small enough to enable a point-like measurement of the level in the TPC. The three plates of the capacitor are 0.5mm thick and are separated 1mm from each other. To prevent the capacitors from the large electric fields surrounding them, they are shielded by a copper cage. In addition, since the levelmeters are very close to the detector, they are made out of high purity copper to prevent introducing additional radioactive backgrounds. The levelmeters change their capacitance by ~1pF per mm that is filled with liquid xenon. This translates to a resolution of an amazing ~3µm to measure the liquid xenon level! Four of those devices are distributed around the TPC. This gives us the possibility to level the detector in µm precision. The capacitor signals are read out via a pair of 15m long coax cables and an electronic circuit that is connected to the slow control system of XENON1T.

DSCF1953_resized_cut DSCF1952_resized_cut
The short levelmeters for the XENON1T experiment. Three capacitor plates inside a copper cage provide a precise measurement of the liquid level inside.

Another use case for levelmeters is the monitoring of the filling process of the cryostat. In order to do this, two 1.4m long double-walled stainless steel cylindrical capacitors are located at the outside of the TPC, covering its full height. As for the short levelmeters, the long ones also work in a way that their capacitance is changing according to how high the liquid xenon rose inside them. Here, the compromise between having a large capacitance change per height value versus very small space requirements had to be made. The diameter of the outer conductor was designed to be 6mm, for the inner conductor to be 3mm. This leads to a capacitance change of 0.10 pF/mm and enables a resolution of ~30µm for measuring the absolut level of liquid xenon in the TPC.

The XENON1T levelmeters are well designed sensors by its own and have been developed over more than one year. After production in June 2015, they are shipped to LNGS, where they will do their job over the next years during the run-time of the XENON1T detector.