LHCb Ring Imaging Cherenkov Detectors Research Articles

The “green” use of fluorocarbons in Cherenkov detectors and silicon tracker cooling systems: challenges and opportunities in an unfolding era of alternatives

Saturated fluorocarbons (SFCs) of form CnF(2n+2) are chosen for their optical properties as Cherenkov radiators, with C4F10 and CF4 currently used at CERN in the COMPASS and LHCb ring imaging Cherenkov detectors. Their non-conductivity, non-flammability and radiation-resistance also make SFCs ideal coolants: C6F14 liquid cooling is used in all LHC experiments, while C3F8 is used for the evaporative cooling of TOTEM and the ATLAS silicon tracker. These fluids, however, have high global warming potentials (5000–10000*GWPCO2), and represented around 36% of CERN’s CO2-equivalent emissions in 2018. There is thus an impetus to reduce their use, losses in purification and wastage through leaks, through improved monitoring and closed circulation system design. Newer spur-oxygenated fluoro-ketones, for example from the 3 M NOVEC® range, with CnF2nO structures, can offer similar performance to SFCs with but with very low, or zero GWP. Although these fluids do not yet exist in large quantities over the full CnF2 “matrix” the radiation tolerance and thermal performance of NOVEC 649 (C6F12O) was sufficiently promising for it to be chosen as a C6F14 replacement for cooling silicon photomultipliers. Additionally, subject to optical testing, NOVEC 5110 (C5F10O) could (if blended with nitrogen) replace both C4F10 and CF4 in Cherenkov detectors. Lighter molecules (for example C2F4O, with similar thermodynamics to C2F6)—if and when available in industrial quantities—might allow lower temperature operation than evaporative CO2 in future silicon trackers operated at very high luminosity. Ultrasonic gas mixture analysis is very sensitive to concentration changes of a heavy vapour in a light carrier, and is used—in the only such fluorocarbon coolant leak monitoring system operating at LHC—for real-time monitoring of C3F8 coolant leaks from the ATLAS pixel and SCT silicon trackers into their nitrogen-flushed environmental volumes. A typical C3F8 sensitivity of better than 10−5 is achieved. Advanced new ultrasonic algorithms allow measurement of the concentrations of a pair of gases of particular interest on top of a varying known baseline of other gases. The technique is thus of considerable value in leak monitoring and could be used to blend fluoro-ketones with nitrogen or argon to reduce the GWP “load” of large volume atmospheric pressure gas Cherenkov radiators without the recourse to higher-pressure noble gas approaches. This paper outlines an approach to GWP reduction with fluoro-ketone fluids and the blending of heritage SFCs or fluoro-ketones with lighter gases using ultrasonic monitoring and control. Possible avenues for the use of fluoro-ketones in liquid phase and evaporative cooling of silicon trackers are discussed.

Characterisation of signal-induced noise in Hamamatsu R11265 Multianode Photomultiplier Tubes

Signal-induced noise is observed in Hamamatsu R11265 Multianode Photomultiplier Tubes, manifesting up to several microseconds after the single photoelectron response signal and localised in specific anodes. The mean number of noise pulses varies between devices, and shows significant dependence on the applied high-voltage. The characterisation of this noise and the mitigation strategies to perform optimal single-photon counting at 40 MHz, as required by the LHCb Ring-Imaging Cherenkov detectors, are reported.

Behavior of multi-anode photomultipliers in magnetic fields for the LHCb RICH upgrade

A key feature of the LHCb upgrade, scheduled for 2019, is to remove the first level trigger and its data reduction from 40MHz to 1MHz, which is implemented in the on-detector readout electronics. The consequence for the LHCb Ring Imaging Cherenkov (RICH) detectors is that the Hybrid Photon Detectors need to be replaced as the readout chip is inside the detector vacuum. The baseline for replacement are Multi-anode Photomultiplier tubes (MaPMT) and new readout electronics. The MaPMTs will be located in the fringe field of the LHCb dipole magnet with residual fields up to 25G. Therefore, their behavior in magnetic fields is critical. Here we report about studies of the Hamamatsu model R11265 in a magnetic field in an effort to qualify it for use in the LHCb RICH upgrade. Comparisons to the known model R7600 are also made. Measurements of the collection efficiency and gain were performed for all three space directions as a function of the magnetic field strength. In addition to measurements with bare tubes, measurements with different mu-metal shielding configurations were performed to optimize the configuration. This is important input for the layout of the upgraded LHCb RICH detector.

Characterisation and magnetic field properties of multianode photomultiplier tubes

We report on studies of the Hamamatsu model R11265 Multianode Photomultiplier as part of the effort to qualify their use in the upgrade of the LHCb Ring Imaging Cherenkov Detectors. Comparisons with the known model R7600 are also made. Of particular interest is the behaviour of the MaPMT in magnetic fields comparable to the residual fringe field of the LHCb bending magnet ranging up to 25Gauss.

Performance of the RICH Detectors of LHCb

Hadron identification, in particular the ability to distinguish charged kaons and pions, is crucial to many of LHCb core physics analyses. LHCb Ring Imaging CHerenkov (RICH) detector fulfils this role by providing charged particle identificationin the momentum range between1and 100 GeV/c. The calibration and monitoring of the RICH detectors is achieved using samples of D∗, KS0, Λ and φ events, which are plentiful in the data and can be cleanly isolated through their decay kinematics. The particle identification performance of the LHCb RICH detectors, measured in data taken during the 2010 and 2011 LHC runs, will be presented along with the strategy for aligning and calibrating the detector. Finally this performance will be placed in context by highlighting the impact of the RICH performance on some of LHCb benchmark analyses.

The Pixel Hybrid Photon Detector of the LHCb RICH

The LHCb experiment at the CERN Large Hadron Collider employs two Ring Imaging Cherenkov (RICH) detectors for particle identification. The first covers the charged particle momentum range 1 - 60 GeV/c using aerogel and C 4 F 10 radiators and the second covers up to 100 GeV/c using CF 4 . Cherenkov light will be detected using a novel type of position sensitive device: the pixel Hybrid Photon Detector (HPD). In the pixel HPD, photoelectrons are produced in a multialkali photocathode, accelerated and focused by an electrostatic field, and then detected by a pixelated silicon detector which is bump-bonded onto a 40 MHz read-out chip. This paper reports on the development of the LHCb RICH detectors and the performance of the HPDs.

Hybrid Photon Detectors and Ion Feedback

The long-term development of the Ion Feedback rate of Hybrid Photon Detectors in the LHCb Ring Imaging Cherenkov detectors was studied as a sensitive measure of the vacuum quality. From a phenomenological understanding of the development reliable predictions can be made for individual photon detectors. Eighty percent of them will operate well for the lifetime of LHCb. In a 20% subsample a faster-than-expected degradation of the vacuum quality was found. A HPD repair and replacement programme guarantees a good performance of the LHCb RICH detector at all time during data taking.

The RICH system of the LHCb experiment: Status and performance

The LHCb experiment is a single forward arm spectrometer at the LHC at CERN. Particle identification (pion, kaon and proton discrimination) is a crucial requirement to meet the physics goals of the experiment. This is provided by two Ring Imaging Cherenkov (RICH) detectors providing particle identification coverage between 1 and 100 GeV/ c. After a brief description of the LHCb experiment and RICH detectors, the results from the latest beam test using the final detector components and readout system will be given and the anticipated particle identification performance of the RICH detectors from Monte Carlo simulations will be presented. Finally, a technique to calibrate the particle identification performance of the RICH system with data using D * decays will be described.

Production of 500 pixel hybrid photon detectors for the RICH counters of LHCb

The pixel Hybrid Photon Detector (HPD) has been developed to detect the light produced by the two Ring Imaging Cherenkov (RICH) detectors of the LHCb experiment. The device is a vacuum photon detector equipped with cross-focussing electron optics and a custom anode for photoelectron detection. The production of the 500 such devices that are required to cover the photon detection surface of the LHCb RICH detectors is ongoing. A pre-series of HPDs has been manufactured and their performance assessed in the laboratory. Successful operation has also been confirmed in a test beam. Accelerated ageing tests performed on one pre-series tube show no degradation of performance.

System test of a three-column LHCb RICH-2 prototype detector

A three-column prototype of the downstream LHCb Ring Imaging Cherenkov detector (RICH-2) has been constructed. The prototype module contains six 8 cm diameter Hybrid Photon Detectors (HPDs), mounted in a close-packed arrangement, and read out at the full LHC speed of 40 MHz. Pairs of HPDs are read out with LHCb prototype on-detector RICH electronics utilizing radiation-tolerant FPGA technology, optical links and 40 MHz trigger-timing and control (TTC). The columns have a modularized mechanical arrangement with compact high- and low-voltage distribution systems. A full system test of the readout has been verified. The three-column structure has been mounted in a Cherenkov vessel and Cherenkov rings have been observed from N 2 and C 4 F 10 gas radiators in a CERN pion test beam.

Silicon detectors and electronics for pixel hybrid photon detectors

Silicon sensors and their corresponding readout electronics have been designed in the framework of the LHCb Ring-Imaging Cherenkov detector project. They are specified to detect low-energy photoelectrons created by Cherenkov light incident on a hybrid photon detector. The design of the sensor and electronics will be described, in particular the specific steps taken to meet the special requirements of the detector. Measurements on the electronics will be presented, together with results from assemblies constructed with sensor and electronics chips using a special bump-bonding technique tailored to match the detector fabrication process.

Performance of hybrid photon detector prototypes with encapsulated silicon pixel detector and readout for the RICH counters of LHCb

These proceedings report on the performance of the latest prototype pixel hybrid photon detector in preparation for the LHCb Ring Imaging Cherenkov detectors. The prototype encapsulates a silicon pixel detector bump-bonded to a binary read-out chip with short (25ns) peaking time and low (<2000e−) detection threshold. A brief description of the prototype is given, followed by the preliminary results of the characterisation of the prototype behaviour when tested using a low intensity pulsed light emitting diode. The results obtained are in good agreement with those obtained using previous prototypes. The proceedings conclude with a summary of the current status and future plans.

Performance of a cluster of multi-anode photomultipliers equipped with lenses for use in a prototype RICH detector

A cluster of multi-anode photomultiplier tubes (MaPMTs) equipped with focusing lenses in front of the tubes was tested in a prototype ring imaging Cherenkov (RICH) detector in a charged particle beam. The readout electronics were capable of capturing the data at 40 MHz. The effects due to charged particles and magnetic field on the MaPMT performance were also studied. The results are used to evaluate the MaPMT as a possible photodetector for the LHCb RICH detectors.

Pixel readout chips in deep submicron CMOS for ALICE and LHCb tolerant to 10 Mrad and beyond

The ALICE1LHCB chip is a mixed-mode integrated circuit designed to read out silicon pixel detectors for two different applications: particle tracking in the ALICE Silicon Pixel Detector and particle identification in the LHCb Ring Imaging Cherenkov detector. To satisfy the different needs for these two experiments, the chip can be operated in two different modes. In tracking mode all the 50 μm×425 μm pixel cells in the 256×32 array are read out individually, whilst in particle identification mode they are combined in groups of 8 to form a 32×32 array of 400 μm×425 μm cells. Radiation tolerance was enhanced through special circuit layout. Sensitivity to coupling of digital signals into the analog front end was minimized. System issues such as testability and uniformity further constrained the design. The circuit is currently being manufactured in a commercial 0.25 μm CMOS technology.

Pixel readout electronics development for the ALICE pixel vertex and LHCb RICH detector

The ALICE1LHCB pixel readout chip emerged from previous experience at CERN. The RD-19 collaboration provided the basis for the installation of a pixel system in the WA97 and NA57 experiments. Operation in these experiments was key in the understanding of the system issues. In parallel the RD-49 collaboration provided the basis to obtain radiation tolerance in commercial submicron CMOS through special circuit layout. The new ALICE1LHB chip was developed to serve two different applications: particle tracking in the ALICE Silicon Pixel Detector and particle identification in the LHCb Ring Imaging Cherenkov detector. To satisfy the different needs for these two experiments, the chip can be operated in two different modes. In tracking mode all the 50 μm×425 μm pixel cells in the 256×32 array are read out individually, whilst in particle identification mode they are combined in groups of 8 to form a 32×32 array of 400 μm×425 μm cells. The circuit is currently being manufactured in a commercial 0.25 μm CMOS technology.

Development of large area hybrid photodiodes for the LHCb ring imaging Cherenkov detectors

We report on the development of large area hybrid photodiodes (HPD) which are one of the proposed photodetectors for the RICH counters of the LHCb experiment. The HPD's consist of a cylindrical vacuum envelope of 127 mm diameter capped with a spherical borosilicate UV-glass entrance window. Focusing electrodes demagnify the image on a silicon detector of 50 mm diameter comprising 2048 pads with a surface of 1 mm/sup 2/ each. The analogue readout electronics is integrated in the vacuum tube. As an intermediate step a HPD with a UV sensitive CsI photocathode has been produced which allowed to verify the electron optics of the HPD. A large UHV evaporation plant for the production of HPD's with visible light transmissive bialkali photocathodes (K/sub 2/CsSb) has been built and successfully operated. The evaporation process is optimized for maximum quantum efficiency and life time of the photocathodes. A cold indium sealing technique developed for a minimum thermal load of the photocathode and the silicon sensor has proven to provide excellent vacuum tightness.

LHC-B ring imaging Cherenkov detector

The progress towards the realisation of the LHC-B Ring Imaging Cherenkov detector is reported.