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Modern detector systems in hadron physics require the complete identification and reconstruction of the full final state of a reaction. Extremely efficient particle identification capabilities covering the complete angular and momentum range are thus mandatory. The particle detection and identification devices in these experiments are imaging Cherenkov counters, ultra-fast time-of-flight measurements based on the detection of Cherenkov light or calorimeters based on Cherenkov and scintillation light. In the momentum region of the upcoming, new high precision experiments in hadron physics, e.g. the PANDA anti-proton annihilation experiment at FAIR (Darmstadt, Germany) or WASA at COSY (Jülich, Germany), imaging Cherenkov counters designed to measure the image of the Cherenkov cone offer the best performance. They can be built either as conventional Ring Imaging Cherenkov (RICH) counters or detectors in the innovative, compact Detection of Internally Reflected Cherenkov light (DIRC) technology. Both designs provide the necessary performance and discrimination power between particle species needed in these experiments. It is argued, that DIRC detectors provide a superior solution meriting a joint research effort to build up European expertise.

DIRC detectors rely on the reconstruction of the Cherenkov cone from photons trapped inside the radiator material. The radiator material of choice has a refractive index of about 1.5, which allows building thin radiators while maintaining a high photon yield for the image reconstruction. DIRC detectors can therefore be built very thin and are comparatively easy to accommodate in large detector systems with full solid angle coverage. It is imperative to solve two problems in the construction to exploit the performance of DIRC systems fully. The reconstruction of the Cherenkov angle relies on the conservation of this angle with every reflection on the surface. A photon will, depending on where in the detector it is produced, undergo up to 100 reflections. At each of these reflection points the angle has to be conserved and the reflection losses have to be kept under control. This, in term, requires a surface machined to high optical standards. The development of the polishing techniques and the accompanying quality assurance are one of the primary objectives of this work package.

The use of a solid radiator usually comes at the cost of using a material with a large dispersive variation of the refractive index. The optical dispersion of the radiator material will thus impose a stringent limit on the resolution in the Cherenkov angle if wavelength blind photon detection systems are used. Two methods could be employed to mitigate this problem. Putting a material with a different dispersion curve in the light path between the photon creation and detection allows the dispersive effects to be of the same order or significantly smaller than all other parameters affecting the resolution of the Cherenkov counter. Alternatively, a wavelength dependent photon read-out system, e.g. using dichroic mirrors and tailored photo-sensitive materials would allow for a correction of dispersive effects in the reconstruction algorithms. Both methods are suggested to be studied in this work package.

DIRC detectors yield intrinsically a high number of photons per charged particle traversing the radiator volume. Additionally, the upcoming experiments in hadron physics are planned to run at very high interaction rates for precision spectroscopy of known particles and searches for new hadronic states. Taking both together, this implies a large rate of Cherenkov photons which have to be detected, stored and reconstructed, and calls for a very fast, high rate capable photon detection system. Additional demands toward the time-resolution of the photon detection system and accompanying read-out stem from one of the possible methods of dispersion correction using a wave-length dependent read-out with Time-of-Propagation (ToP) measurements. A very good time resolution is anyway desirable as it allows a better event correlation and rejection of background signals. So far, these demands have only been met in bench tests with a few read-out channels using state-of-the-art photon detectors and very fast electronics. The same demands have now to be met for several thousand read-out channels in a full fledged detector system. Designing such a large scale, very fast photon detection system is one of most important objectives of this workpackage.

Last not least, all the components will have to be assembled into a prototype system to prove the validity of the design choices made. The prototype will be built on the experiences gathered in the tasks described below and be used and tested in suitable hadron beams.

We propose a Joint Research Activity (JRA) to concentrate European expertise on a joint Research and Development effort aimed at advancing current Cherenkov detector technology to design and build the next generation of particle identification detectors for hadron physics experiments. Compact detector design calls for solid radiators. Their use in high luminosity experiments with large solid angle coverage implies high photon count rates. Dispersive effects will need to be addressed more rigorously. Also, due to the more involved optical system, pattern recognition and thus the reconstruction of the Cherenkov cone poses a special challenge.

The proposed project aims at taking the existing technologies used to build Cherenkov-based particle identification detectors to a new level. The JRA focuses on the development issues surrounding Cherenkov detectors using solid radiators. The technological issues are:

• Identification and test of materials for optical dispersion correction (optical properties, proof-of-concept, radiation hardness).

 

• Development of the technological expertise to polish optical surfaces to the required quality.

 

• Investigation of wavelength dependent detection systems (dichroic mirrors, photon detection systems with optimised wavelength response).

 

• Improvement of multi-pixel photo-electron-multipliers using micro-channel-plates (quantum efficiency, time response, life-time, performance in magnetic fields). Industrial partner: Photonis-Burle, France.

 

• Test of a dedicated read-out system for both photon detector candidates.

 

• Construction of a prototype and test at a test-beam facility using hadron beams.

Task 1: Construction of Radiator Disc

One of the most important tasks when building a DIRC detector is the design and manufacturing of the radiator disc. This includes the following steps:

1. Evaluation of the required surface parameters (flatness, surface roughness, parallelism)
2. Mechanical construction of the disc and its mounting structure
3. Development of gluing techniques
4. Development of polishing techniques
5. Design of a prototype detector.

Task 2: Dispersion Correction

Central to this proposal is the enhancement of the particle identification performance by correcting the Cherenkov image for dispersive effects. Two approaches are proposed, one using time of light-propagation information, the other using optical methods. The work on dispersion correction is broken down as follows:

a.       Performance simulations of optical dispersion correction using different material and focusing optics

b.      Performance simulation of dispersion correction by Time-of-Propagation

c.       Design and prototyping of focusing light guides and optical dispersion correction schemes

d.      Prototyping of dichroic mirror system for wavelength dependent read-out

e.       Classification of read-out requirements for both options.

Task 3: Photon Detection System and Electronics

Existing products cannot yet meet all demands on the photon detection system and accompanying read-out electronics. Both candidate photon detection devices described above were proven to deliver the fast timing required for application in the next generation of Cherenkov counters. However, their rate capabilities, gain characteristics and life-time, require further study. Additionally, VLSI-Electronics that preserve good timing characteristics have still to be developed and tested. Task 3 is split up as follows:

a.       Definition of performance criteria for electronics and photon detection

b.      Measurement of photon detection performance in a high magnetic field environment

c.       Tests of the photon detection system life-time

d.      Test of the photon detection system time-response

e.       Design of the read-out electronic system

f.       Test of the rate capability and timing properties of the read-out electronics

g.      Test of the rate capability and timing properties of the full prototype system.

Task 4: Test-beams and Evaluation

The performance of the detector will be evaluated in a test-beam campaign using suitable a hadron beam. The test-beam period will synthesise the results of Tasks 1, 2 and 3, and feed back to them. Additionally, several sub-tasks have to be undertaken to complete the test beam successfully:

1. Design of the test-beam set-up and procurement of test-beam time
2. Procurement and testing of monitoring devices
3. Simulation of expected test beam performance
4. Set-up of data acquisition and reconstruction software
5. Prototype construction
6. Evaluation of test-beam results.

Role of participants:

The main deliverables of this project are the construction and the test of a DIRC prototype detector. These tests shall be carried out using electron and hadron beams to fully explore the detectors capabilities. The results shall be in time for an upgrade of the WASA detector at COSY and for the PANDA DIRC detectors. A successful prototype comprises (in order or priority):

1. A radiator disc of excellent and tested optical quality
2. A photon detection system of sufficient granularity, efficiency and lifetime
3. An electronic system guaranteeing high rate capability and excellent timing
4. Elements for dispersion correction enhancing detector performance
5. Tested performance of the prototype detector in a hadron beam.

The success of this development will be documented in peer-reviewed publications and Technical Design Reports for WASA and PANDA.

The research proposed in this JRA will put the participating European institutions at the forefront of current trends in the design and development of Cherenkov-based particle identification detectors. The only detector of the DIRC-type built so far is working at the BaBar Experiment at SLAC in barrel geometry. Disc DIRC detectors, or Time-of-Propagation DIRC detectors have not yet been built. The DIRC detectors to be developed in this JRA will give the European hadron physics community an edge in the development of novel particle identification detectors. In addition it is worth mentioning that the development of micro-channel plate photon detection systems together with the proposed industrial partner, Photonis-Burle, in France, will provide expertise in the field of very fast photon timing both to the participating research institutions and leading European industrial partners.

The results of the detector developments proposed here will be used for the construction of particle detectors at existing facilities in Europe (e.g. WASA at COSY), the European contributions to international laboratories (e.g. hadron physics experiments at Jefferson Laboratory, USA), and future European research infrastructures (PANDA at FAIR).