Detectors providing the particle time of flight (TOF), in conjunction with momentum measurement, have been used in high energy physics experiments since ever to identify charged particles. Colliding beam experiments have so far required large area TOF detectors at a few meters distance from the interaction region, with an overall time resolution ∼100 ps able to separate charged particles in the momentum region of few GeV/c. The challenges of future HEP facilities, like experiments at a future high-energy electron-positron collider, have aroused the necessity to extend the identification of charged particles to higher momenta; in this type of experiments, TOF counters would also be essential to cover the overlap region in dE/dx or dN/dx particle ID. Along with the collider luminosity increase, timing will also be compulsory for pile-up suppression and 4-D tracking.
Consequently, TOF systems based on faster sensors, by an order of magnitude better than the current ones, are being actively developed. These devices must be radiation hard, nearly edgeless for reducing the dead regions, and segmented to allow independent timing measurements of several particles.
This WP has a large parallelism with DRDT 4.3: continuous reciprocal coordination, consultations and information exchange will be needed to rationalize the work.

 

Task 4.1 - Study the coupling of a thin Cherenkov radiator to a single-photon detector array, for TOF of charged particles

Cherenkov light is prompt and therefore ideal for fast timing. Although the amount of light is small compared with that produced by scintillators, such limitation can be overcome by using Cherenkov devices capable of detecting single Cherenkov photons. Indeed, if the Cherenkov detector has an intrinsic time resolution SPTR (Single Photon Time Resolution), the time resolution due to the photoelectron statistics with Npe photoelectrons is then σt=SPTR/√ N pe. In this WP, we plan to develop high precision timing (σt∼10-15 ps) systems based on high refractive index solid Cherenkov radiators coupled to arrays of silicon photomultipliers (SiPMs) or multichannel plates (MCPs) as photon sensors. The entrance MCP window or a thin (a couple of mm) slab of a high refractive index transparent material added in front of the SiPM array acts as a Cherenkov radiator providing about a hundred photons per single charged particle traversing it. The independent measurement of the time for each of the photoelectrons resulting from the conversion of the Cherenkov photons in the MCP or the SiPMs will enable to achieve a precise timing with a sub-25 ps resolution. SiO2 (fused silica) with n=1.47 at 400 nm, and MgF2 with n=1.4 at 400 nm are potential radiator materials, although also Sapphire (Al2O3) (n = 1.7 at 400 nm) and Corning glass (n = 1.8 at 400 nm), both with a higher refractive index than fused silica, could also be used resulting in more Cherenkov photons. The transmission of these materials extends down to 350 nm.

Goals:
The timing performance of the proposed device depends not only on the type of Cherenkov radiator material, photon sensor geometry, pixel size, and read-out electronics but also by the optical coupling between the radiator window and the photon detector since the way the Cherenkov photons propagate introduces either a loss of signal or a spread in the timing.

Description of work:
Carry-out detailed GEANT4 simulations emulating the time spread introduced by the optical cross/talk (OCT) to neighbouring pixels and lab tests with SiPMs and MCPs equipped with different window materials.

Milestone:

• M4.1 Selection of suitable material for the Cherenkov radiator and its coupling to the sensor (M18)

Deliverable:

• D4.1 Fully characterized prototype TOF detector, with a report on design and performance (M36)

 

Task 4.2 - Develop a SiPM array for single-photon detection, with mm-scale pixelation, suitable for use in TOF prototypes

This task concerns R&D on SiPM detectors and electronics to provide mm-scale position sensitivity and fast timing of Cherenkov light at the very high rates expected with HL-LHC and future colliders. Whereas the remit of task 3.3 is broader, covering systems comprising PMT/MCP/SiPM detector arrays, task 4.2 is aimed at developing a high granularity SiPMbased system only. It will focus on the system aspects of combining SiPM arrays with radiation hardness, mm-scale pixelation, and cooling, integrated with multichannel readout electronics such as the FastIC ASIC family, with the overall aim of achieving the best possible time resolution. Development of the sensors themselves is undertaken within DRDT4.1.
Large arrays of 1 × 1 mm2 SiPMs, the smallest size generally available, would be complex and risky to manufacture, so larger SiPMs with segmentation providing several pixels per device are a preferred option. Additionally, event timing to ∼10-20 ps will require close integration of the sensor and electronics package, and the power dissipation in the sensor due to the microcell Geiger discharge at the very high count rates anticipated for HL-LHC will require active cooling of the SiPM to maintain stable device gain and prevent increase in dark noise.

Goals:
The preliminary tests obtained on single SiPMs should be generalised on larger array systems, equipped with more optimised front-end electronics.

Description of work:
A vigorous research program will be pursued for the deployment of arrays of SiPMs that will allow to make a step forward in TOF systems.

Milestones:

• M4.2 Demonstrate performance for single-photon detection of SiPM array with FastIC readout (M18).

Deliverables:

• D4.2 Prototype of array of cooled, mm-scale segmented SiPMs with integrated readout, with report on design and performance (M36)

 

Task 4.3 - Develop lightweight mechanical supports for DIRC-type TOF detectors

The aim of this work package is to develop lightweight mechanical supports for DIRC-type TOF detectors. The Cherenkov radiators typically comprise a set of highly polished quartz plates several tens of square metres in area, coupled with other optical components to the detectors, all manufactured to very high precision. All optical components require adjustable and highly accurate positioning by means of suitable, lightweight mechanical supports which should minimize both contact with optical surfaces and material in the detector acceptance. The supports must also maintain geometrical distortion to a minimum while adequately supporting the considerable overall weight of the quartz and services such as detectors, and potentially some of the electronics. Mechanical designs using various of lightweight materials, such as carbon-fibre based composites, including removable handling jigs for installation, will be investigated and reported, leading to a candidate design from which a prototype lightweight mechanical structure will be developed and its performance verified.

Description of work:

Develop lightweight mechanical supports for DIRC-type TOF detectors, supporting the polished quartz radiator and services.

Milestones:

• M4.3 Report on material choices and mechanical design for a lightweight TOF module (M18)

Deliverables:

• D4.3 Prototype of a lightweight mechanical support for a DIRC-type TOF detector, with report on design and performance (M36)

 

Task 4.4 Develop techniques for measuring the optical properties of optical components for TOF detectors

This work package focuses on the development of techniques for the measurement and characterization of the quartz Cherenkov radiators and techniques used for coupling of optical components. To achieve the necessary performance characteristics, the radiator requirements are demanding, including a very high level of polish to achieve very low surface roughness required together with a high degree planarity. These are both challenging to achieve during manufacture and difficult to accurately measure accurately afterwards with the required accuracy. Facilities already exist within the project partners for evaluating the surface quality of polished fused silica bars or plates. An existing setup housed in a temperature-stabilized optical lab comprises motion-controlled stepper motors and diodes, as well as polarized laser beams with six different wavelengths, to determine the coefficient of total internal reflection and the bulk attenuation of TOF/DIRC bars or plates, which will be available on request to collaborators within Task 4.4. We will develop these techniques further to measure the specifications of all optical components with a suitably high degree of precision. Additional measured parameters will include component planarity, optical transmission and scattering, defect size and density, radiation darkening, together with parameters specifically relating to the optical coupling materials used.

Description of work:

Develop techniques for measuring the optical properties of polished quartz radiators, and the coupling of optical elements in DIRC-style TOF detectors.

Milestones:

• M4.4 Report on progress in setting up optical laboratory for characterizing TOF radiator (M18)

Deliverables:

• D4.4 Completion of commissioning of an optical laboratory for characterizing the performance of a DIRC-style quartz radiator plate (M36)