Tag Archives: S2

First Signals in the XENON1T Time Projection Chamber

While the functionality of each of the 248 PMTs had been tested during the different commissioning stages of the XENON1T dark matter detector, the signal detection with both PMT arrays and the full data acquisition system remained to be tested. For this, and for the LED_event1_cutsubsequent calibration of the time projection chamber (TPC), an LED illumination system has been set up with 3 individual channels, each branching out into six optical fibers distributed in a circumference around the TPC. Light shining through the fibers is collected by the PMTs, whose output signals are then magnified by a factor 10 with operational amplifiers and digitized with fast analog-to-digital converters.

The figure on the right shows the first detection of blue LED light by the XENON1T PMT arrays. A time delay between the LEDs has been set, resulting in the three peaks seen in the top panel, which correspond to the combined waveforms of all PMTs. The bottom panel shows the signals detected by each individual channel.

On March 17th, the TPC was filled with warm xenon gas for the first time, allowing to acquire the first scintillation signals with the detector. For these measurements, only the PMTs have been biased and no electric drift field was applied. The figure below shows the detection of an event occurring between the so-called screening mesh in front of the top PMT array and the photosensors (see the January 19 post for details on the TPC structure) and constitutes the first detection of an S2-like signal in XENON1T. The left panels show the hit pattern on the top and bottom arrays, while the right top and bottom panels display the summed waveform and the individual PMT hits, respectively.


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.