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A large-area detector for precision time-of-flight measurements

Final Report Summary - TORCH (A large-area detector for precision time-of-flight measurements)

Particle identification (PID) in experimental particle physics is an essential tool in determining the particle species produced in high-energy collisions. Identifying the species of the particle, be it a pion (an up-down quark combination), a kaon (an up-strange quark combination), a proton, or a point-like electron or muon, is vital for piecing together the underlying dynamics of the collision process. The measurement of the particle’s time of flight (ToF) is an effective and well-known technique for low-momentum PID. This involves determining the time difference due to the passage of a particle between two locations: namely the initial collision point at which the particle was produced, and a second point typically several metres from the collision. The velocity of the particle is then deduced from the ratio of this flight distance and the ToF and, with an independent measurement of momentum (mass times velocity) from the bend of the particle's path in a magnetic field, the mass of the particle can then be calculated. A ToF precision of order 15 picoseconds (15 million-millionth of a second) is required over a flight path of around 10m, in order to identify particle species up to 10 GeV/c momentum. Note that a particle travelling at the speed of light, the fastest speed possible, travels only 5 mm in 15 ps.

TORCH (Time Of internally Reflected CHerenkov light) is a novel detector concept to measure ToF in future particle physics experiments. A plane of quartz, 1 cm thickness, is located about 10m from the particle collision region, covering a total area of around 30 m^2. When a charged particle passes through the quartz medium it produces Cherenkov photons (quanta of light) which are emitted when the speed of the particle is greater than the speed of light in the medium. The photons are emitted in a cone around the particle track, an effect similar to the shock wave produced by a supersonic aeroplane. Photons then travel through the quartz medium to the periphery of the detector (up to 2.5 m away) by total internal reflection in the quartz, and their arrival times and position are measured with fast photon detectors. A 1 cm thickness of quartz can result in 30 detected photons per track, after corrections due to efficiency losses. In order to calculate the ToF of the particle correctly, it is important to also take into account the time of travel of each Cherenkov photon through the quartz, and this requires a correlation with its wavelength and its speed inside the quartz.

A 1 milli-radian measurement precision on the angle of each emitted photon is necessary to meet the TORCH requirements. To achieve this, and to ensure the desired timing performance, a novel type of Multi-Channel Plate Photomultiplier (MCP-PMT) detector has been developed with industrial partners, Photek Ltd, UK. This detector has a 53 x 53 mm^2 active area with a timing resolution of order of 30 ps, and is segmented into 64 x 8 pixels to match the angular requirement. The MCP-PMT detector converts a single photon at the entrance window into a fast electrical pulse of around 1 million electrons at the output. The TORCH MCP-PMT must also be tolerant against radiation, and the developed tubes operate up to an integrated charge of 5 Coulombs per square cm by using novel Atomic Layer Deposition (ALD) techniques. Another innovative feature of this MCP-PMT is the utilization of charge division to improve the spatial accuracy to around 100 microns. To meet the state-of-the-art timing and spatial resolution requirements we have also had to develop a highly performant customised electronics readout system. Ten MCP-PMTs with electronics readout have been produced for TORCH.

We have constructed a scaled-down prototype of the full-sized TORCH module. This has a single MCP-PMT which has been tested in a charged-particle beam at CERN. The detector has performed to design specifications, with close to a timing resolution of 70 ps measured per detected photon. Combining the anticipated 30 photons in a fully instrumented system will then lead to the required 15 ps resolution for the TORCH application. For the next phase of tests, a full-size quartz plate of 1250 mm length, 660 mm width and 10 mm thickness has been delivered, together with a quartz focusing element. We look forward to successfully demonstrating the full-scale TORCH module in the coming year, to pave the way for a TORCH-type detector to be constructed for a major particle-physics experiment of the future.