A Time-Projection Chamber (TPC) detector is a type of detector used for particle and nuclear physics. It generally consists of a detection volume of gas or liquid, to which a strong electric field is applied, a position-sensitive electron collector and a read-out system. If a high-energy particle hits the sensitive volume, it will leave a trail of ionized electrons behind its trajectory: the TPC is able to provide a complete, three-dimensional picture of the energy loss of the incident particle, thus enabling its identification.
The use of noble elements in in their liquid phase, especially argon and xenon (LAr and LXe), in the detection volume, is due to their high atomic number and density, which enables the construction of large, but compact and homogenous detectors. Also, noble gasses respond to the passage of radiation with a scintillation signal: among noble gasses, argon and xenon have the highest ionization and scintillation yield. Ionization and scintillation are two complementary and anti-correlated signals; their simultaneous detection can provide a complete and precise reconstruction of the incident particle’s properties.
LAr and LXe TPCs have been used for both accelerator and non-accelerator experiments, such as proton and double-beta decay; neutrinos and dark-matter searches and as imaging detectors.
Hamamatsu Photonics played an important role in the technological advance of this field, especially concerning the development of Vacuum Ultra Violet (VUV) Photomultiplier Tubes (PMTs).
Besides responding to an incident radiation with both the production of charge carriers and scintillation light, argon and xenon have vanishing electronegativity, and therefore the electrons created in the ionization trail of the incident particle will not be re-absorbed while heading to the detector read-out system.
The evolution of the LAr and LXe TPCs detectors field has grown under the aegis of the Imaging Cosmic and Rare Underground Signals (ICARUS) and XENON detectors for LAr and LXe TPCs respectively.
ICARUS is a 760-ton detector composed of two 3.5x3.9x19.9 cubic metres’ modules, filled with liquid argon (LAr). It was employed to study neutrino oscillations at Gran Sasso Laboratories (Italy), using a beam of neutrinos produced at CERN, from 2010 to 2014.
The ICARUS physical operating principle is fairly simple; at the centre of the detector, a uniform electric field is applied through a high-voltage cathode. When an energetic particle passes inside the volume, it will create an ionizing radiation along its track. The electrons thus created will drift towards the sides of the detectors, where there is a read-out system made of three layers of parallel wire planes, oriented at different angles (namely the TPC). Each plane will register the arrival time and the position of the drifted electrons, and together with the drift time they can reconstruct a three-dimensional image of the event.
The drift time can be determined knowing the ionization time of the particle: this is given by PMTs positioned behind the wire planes that almost instantaneously detect the scintillation light produced by the particle passing through the detector.
The simultaneous detection of ionization and scintillation signals allows the identification of primary particle interacting in the liquid.
The XENON dark matter project comprises a series of progressively bigger and more sensitive LXe dual-phase TPC named XENON10, XENON100 and XENON1T, where the numbers refer to the fiducial target Xe mass in kg.
In a dual-phase TPC chamber, a region of gaseous xenon resides above the usual liquid phase. The incident particle will produce an ionization and scintillation signal. The scintillation light will be detected mainly by the PMTs located in the liquid phase. Ionization electrons drift up through the detector volume and are accelerated into the gaseous region near the top of the cryostat by a strong electric field, which is able to extract electrons from the liquid phase. Once extracted, the electrons emit proportional scintillation light (through a process called “electroluminescence” or “proportional scintillation”) detected by the PMTs located in the gas part of the detector.
While the PMTs in the gas provide the x – y localization of the incident particle – by looking at the number of photons registered in every single PMT, the third coordinate is inferred by the time difference between the first and the second scintillation signals, as the drift velocity of the electrons is uniform in the liquid.
Several characteristics of this type of detector make it suitable especially for dark-matter particle search, the electrons can be drifted over a longer distance – therefore bigger detectors can be built; the efficiency of the three-dimensional reconstruction of the event is enhanced because the amplified charge signals can be shared between independent charge collection planes; the background signal can be efficiently suppressed since one can distinguish nuclear from electron recoils using the different ratio of ionization and scintillation light, it’s also possible to discriminate against the background signal created, for example, by gamma and beta rays in dark matter searches.
Most of the early LXe detectors in the past have exploited only the ionization process due to difficulties in efficiently detecting the scintillation. In the mid-1990s, the XENON Collaboration started a R&D program with Hamamatsu Photonics., aimed at the development of new sensitive PMTs operating immersed in LXe. Thanks also to this collaboration on PMTs, the sensitivity of the XENON detectors lowered by a factor of 1000 from XENON10 to XENON1T. Already at XENON100 the background was lowered by a factor of 100, thanks to, among other things, the Hamamatsu R8520-06-AL PMTs. XENON1T, whose construction started at the Gran Sasso Laboratories in 2014, will feature some Hamamatsu R11410-21.
The improvements in detectors for dark matter searches and neutrino studies is closely linked to the evolution of scintillation light detectors. Hamamatsu has developed a variety of silicon photomultipliers, or Multi-Pixel Photon Counters (MPPCs), that are sensitive to scintillation light down to 120 nm.
Moreover, for rare event search experiments (such as the Mu to E Gamma (MEG) experiment, which is dedicated to the measurement of the rare decay of a muon into an electron and a photon), the MPPCs require a low-noise background level.
The silicon photomultipliers developed by Hamamatsu have ultra-low radioisotope (RI) content in their constituent materials, and they are also cryogenically compatible with the working temperatures of LAr and LXe detectors, as they can perform at temperatures as low as 77K (-196,15 °C).
Please consult with your local Hamamatsu office for more information.
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