COMPASS RICH-1 Research Articles

Ion backflow in thick GEM-based detectors of single photons

Photon detectors based onmicropattern gas detectorsrepresent a new generation of gaseous photon detectors.In the context of a project to upgrade the gasphoton detectors of COMPASS RICH-1, weare performing an R&D programme aimed both to establishthe principles and to develop the engineering aspectsof photon detectors basedon multi-layer arrangements ofthick GEMs electron multipliers coupled to aCsI photoconverter.In this context, a reduced rate of the backflow ofthe positive ions generated in the multiplication processis required to overcome the critical issues relatedto the bombardment of the CsI photoconverter by ions.Our studies devoted to develop detector architecturesable to provide reduced ion backflow rates are reported.

Development of THGEM-based Photon Detectors for COMPASS RICH-1

Abstract An R&D project is presented, aimed to develop a high performance gaseous detector of single photons, for the upgrade of the Ring Imaging Cherenkov Counter RICH-1 of the COMPASS Experiment at CERN SPS. The detector has to stably operate at high gain and high rate, to provide good time resolution and insensitivity to magnetic field, and to offer the possibility to cover very large areas at affordable cost. The proposed solution is based on the use of a novel and robust electron multiplier, the Thick GEM (THGEM), arranged in a multilayer architecture, where the first layer is coated with a photosensitive CsI film. A systematic study of the response of THGEMs with various geometrical and production parameters and in different conditions was performed, leading to the choice of a set of optimal parameters. Prototypes of THGEM-based photon detectors able to efficiently detect Cherenkov photons have been built, tested in laboratory and operated in test beam exercises with typical gain of 10 5 and time resolution better than 10 ns. The engineering aspects of building large area (600×600 mm 2 ) THGEM-based photon detectors are presently being investigated.

Particle identification with COMPASS RICH-1

RICH-1 is a large size RICH detector in operation at the COMPASS experiment since 2001 and recently upgraded implementing a new photon detection system with increased performance. A dedicated software package has been developed to perform RICH-1 data reduction, pattern recognition and particle identification as well as a number of accessory tasks for detector studies. The software package, the algorithms implemented and the detector characterisation and performance are reported in detail.

Mirror alignment control for COMPASS RICH-1 detector

The focusing system of the COMPASS RICH-1 detector consists of two segmented spherical mirror surfaces, formed by 68 hexagonal and 48 pentagonal individual elements. All individual mirrors have two degrees of freedom to adjust the angular alignment in order to obtain a continuous spherical surface. Relative angular misalignments can be monitored on-line by the CLAM method, based on the optical reconstruction of line images. Complementing the CLAM measurements with photogrammetry, the absolute mirror orientation and position in space can be determined too. The method is described, as well as the algorithms used to access the relative and absolute mirror alignment.

The experience of building and operating COMPASS RICH-1

COMPASS RICH-1 is a large size gaseous Imaging Cherenkov Detector providing hadron identification in the range from 3 to 55 GeV/ c, in the wide acceptance spectrometer of the COMPASS Experiment at CERN SPS. It uses a 3 m long C 4F 10 radiator, a 21 m 2 large VUV mirror surface and two kinds of photon detectors: MAPMTs and MWPCs with CsI photocathodes, covering a total of 5.5 m 2. It is in operation since 2002 and its performance has increased in time thanks to progressive optimization and mostly to a major upgrade which was implemented in 2006. The main characteristics of COMPASS RICH-1 components are described and some specific aspects related to the radiator gas system, the mirror alignment, the MWPC electrical stability and the readout electronics are discussed. Some key features of the event reconstruction and the PID analysis are presented together with results from the COMPASS RICH-1 performance characterization study.

Particle identification with the fast COMPASS RICH-1 detector

A new photon detection system for the COMPASS RICH-1 detector has been designed and installed. In the central region, the project is based on multi-anode photo-multiplier technology accompanied by charge sensitive, high resolution and dead-time free time digitization. In the outer area, only the readout electronics for the existing photon detectors has been replaced. Details on the detector upgrade and its performance are presented.

Design and construction of the fast photon detection system for COMPASS RICH-1

New photon detectors, based on the use of multi-anode photo-multiplier tubes coupled to individual lens telescopes and read out with a dedicated read-out electronics system, equip the central region of the Cherenkov imaging counter RICH-1 of the COMPASS experiment at CERN SPS. They are characterised by high photon yield, fast response and high rate capability and are successfully in operation since the 2006 COMPASS data taking. The photon detection system fully matches the expected performance. The design and construction of the photon detectors are described in detail.

The COMPASS RICH-1 detector upgrade

The COMPASS experiment at CERN provides hadron identification in a wide momentum range employing a large size gaseous Ring Imaging CHerenkov detector (RICH). The presence of large uncorrelated background in the COMPASS environment was limiting the efficiency of COMPASS RICH-1 in the very forward regime. A major upgrade of RICH-1 required a new technique for Cherenkov photon detection at count rates of several 106/s per channel in the central detector part, and a read-out system allowing for trigger rates of up to 100 kHz. To cope with these requirements, the photon detectors of the central region have been replaced with a fast photon detection system described here, while, in the peripheral regions, the existing multi-wire proportional chambers with CsI photo-cathodes have been equipped with a new read-out system based on APV preamplifiers and flash ADC chips. The new system consists of multi-anode photomultiplier tubes (MAPMTs) coupled to individual fused silica lens telescopes, and fast read-out electronics based on the MAD4 amplifier-discriminator and the dead-time free F1 TDC chip. The project was completely designed and implemented in less than two years: The upgraded detector is in operation since the 2006 CERN SPS run. We present the photon detection design, constructive aspects and test studies to characterise the single photon response of the MAPMTs coupled to the read-out system as well as the detector performance based on the 2006 data.

The COMPASS RICH-1 fast photon detection system

A fast photon detection system has been built as a part of the upgrade of the COMPASS RICH-1 detector: it is based on 576 multi-anode photomultiplier tubes (MAPMTs) coupled to individual fused silica lens telescopes and fast readout electronics. This system has replaced the MWPCs with CsI photo-cathodes in the central region ( 1.3 m 2 , 25% of the total area) of the COMPASS RICH-1 photon detectors and has successfully been operated during the data taking in 2006 and 2007. We report about the fast photon detection system design, construction and commissioning, in particular about the design optimization and the validation tests of the lens telescopes. Preliminary values for the increased performances of COMPASS RICH-1 after the upgrade are also presented.

The characterisation of the multianode photomultiplier tubes for the RICH-1 upgrade project at COMPASS

A major upgrade of the Cherenkov photon detection system of COMPASS RICH-1 has been performed and it has been in operation since the 2006 physics run. The inner part of the photon detector has been replaced by a different technology in order to measure Cherenkov photons at high photoelectron rates, up to several times 10 6 per second and per channel. Cherenkov photons from 200 to 750 nm are detected by 576 multianode photomultiplier tubes (MAPMTs) with 16 channels each, coupled to individual fused silica lens telescopes and fast, high sensitivity and high time resolution electronics read-out. To guarantee an optimal performance of the complete system, parameters like dark current, gain, uniformity, relative quantum efficiency have been measured for a totality of more than 600 MAPMTs (about 10 000 channels) in a fully automated test-stand, developed for this purpose. The ideal working point for each individual pixel could be ascertained by these measurements. In 2006 the newly equipped detector exhibited an excellent performance: about 56 detected photons per ring at saturation and a time resolution of better than 1 ns. We report about the MAPMT characterisation and the quality control set-up, protocol and results.

Remote alignment of large mirror array for RICH detectors

Image focusing in large RICH detectors is obtained by composite systems of mirror elements. Monitoring and adjusting the alignment of the mirror elements during data taking are important handles to improve the detector resolution. Mirror adjustment via piezoelectric actuators can combine unprecedented accuracy and match some fundamental requirements: the detector material budget can be kept low and the high purity of the gas radiator can be preserved, a prerequisite when UV photons are detected. A system based on this principle, well suited for COMPASS RICH-1 mirrors, is proposed.

On-line mirror alignment monitoring method for COMPASS RICH-1

COMPASS RICH-1 is a large size gaseous Cherenkov detector. Two reflecting spherical surfaces, covering a total area of about 21 m 2 hosted in the radiator vessel provide image focusing. The segmented surfaces are formed by 116 mirrors, with individual degrees of freedom for angular adjustment. An original method has been developed and applied for the on-line monitoring of the mirror alignment. Relative tilts between neighbouring mirror elements create discontinuities in the image of continuous lines reflected by the mirror wall. The measurement of the discontinuities can also provide information about the absolute tilt with a resolution of 0.1 mrad. An independent check and calibration of this method has been performed in laboratory using, as reference, a laser interferometer. We present the technical solutions adopted to implement the dedicated setup, the results of the method calibration by laser interferometer, and the preliminary results obtained measuring the alignment of the RICH-1 mirror wall.

Pattern recognition and PID for COMPASS RICH-1

A package for pattern recognition and PID by COMPASS RICH-1 has been developed and used for the analysis of COMPASS data collected in the years 2002–2004, and 2006–2007 with the upgraded RICH-1 photon detectors. It has allowed the full characterization of the detector in the starting version and in the upgraded one as well as the PID for physics results. We report about the package structure and algorithms, and the detector characterization and PID results.

Read-out electronics for fast photon detection with COMPASS RICH-1

A new read-out electronics system has been developed for the fast photon detection of the central region of the COMPASS RICH-1. The project is based on multi-anode photomultipliers read out by the high-sensitivity MAD4 preamplifier-discriminator and the dead-time free F1 TDC chip characterised by high time resolution. The system has been designed taking into account the high photon flux in the central region of the detector and the high rate requirement of the COMPASS experiment. The system is described in detail together with the measured performances. The new electronics system has been installed and used for the 2006 data taking; it entirely fulfils the expected performances.

The fast photon detection system of COMPASS RICH-1

A fast photon detection system has been built for the upgrade of COMPASS RICH-1, the large size gaseous RICH detector in use at the COMPASS Experiment at the CERN SPS since 2001. The photon detectors of the central region have been replaced by a new system based on multi-anode photomultipliers coupled to individual fused silica lens telescopes and a fast readout electronics system, while in the outer region the existing MWPCs with CsI photocathodes have been equipped with a new readout system, based on the APV chip. RICH-1 has been successfully operated in its upgraded version during the 2006 run. We report on the upgrade design and construction, and on the preliminary characterization of the upgraded RICH-1 performances: at saturation about 60 photons per ring have routinely been obtained and the background level has drastically been reduced thanks to the good time resolution of the new system, leading to a 2 σ π –K separation above 55 GeV/ c.

Fast photon detection for particle identification with COMPASS RICH-1

Particle identification (PID) at high rates is an important challenge for many current and future high-energy physics experiments. The upgrade of the COMPASS RICH-1 detector requires a new technique for Cherenkov photon detection at count rates of several 10 6 per channel in the central detector region, and a read-out system allowing for trigger rates of up to 100 kHz. To cope with these requirements, the photon detectors in the central region have been replaced with the detection system described in this paper. In the peripheral regions, the existing multi-wire proportional chambers with CsI photocathode are now read out via a new system employing APV pre-amplifiers and flash ADC chips. The new detection system consists of multi-anode photomultiplier tubes (MAPMT) and fast read-out electronics based on the MAD4 discriminator and the F1-TDC chip. The RICH-1 is in operation in its upgraded version for the 2006 CERN SPS run. We present the photon detection design, constructive aspects and the first Cherenkov light in the detector.

Fast photon detection for COMPASS RICH-1

A fast photon detection system has been built and assembled for the upgrade of COMPASS RICH-1. The system is based on multianode photomultipliers coupled to fused silica lenses to collect the light from a larger surface and to guide it to the photocathode, preserving the position information. The emphasis is on the fast response and high rate capability of the detectors and the associated electronics. The photon detection system is now ready and it will be employed in the 2006 COMPASS data taking. We report about the system design and construction.

Design and status of COMPASS FAST-RICH

In the context of the upgrade of COMPASS RICH-1, we are developing a fast photodetection system for RICH counters, based on UV extended Multi-Anode PhotoMultiplier Tubes (MAPMT) and a custom, low dead-time electronic readout system. Photons are concentrated on the MAPMT photocathode by an optical system that preserves the position information. The ratio between the collection and the photosensitive surfaces is ∼ 7.5 in our design, larger than in previous applications. A new front-end electronics, based on a modified version of the MAD4 discriminator chip, is being realized to digitize the MAPMT signals. We report about the design of the photodetection system and of the associated electronic readout system, and on the preliminary test beam results.

CLAM, a continuous line alignment and monitoring method for RICH mirrors

A method is proposed for the angular alignment of RICH mirrors and for its monitoring, in particular for the COMPASS RICH-1 mirror system. Observing (by means of four cameras) apparent discontinuities in the images of continuous linear objects reflected by the mirrors surface, a relative misalignment of adjacent mirrors can be deduced and then corrected. The method can attain a sensitivity of at least 0.1 mrad, and can also be applied on-line to keep under control the stability of the mirrors during data taking.

Status and characterisation of COMPASS RICH-1

COMPASS RICH-1, a gas-radiator RICH detector with large transversal dimensions, which makes use of MWPC with CsI photocathodes as photon detectors, is in operation at COMPASS since 2001. The behaviour of the detector components is optimised and fully understood. The characterisation of the detector is now complete. We report about the detector status and its response in the COMPASS environment.