Hybrid Photon Detectors Research Articles

Status of the LHCb RICH detector and the HPDs

The status of the Ring Imaging Cherenkov system of the LHCb experiment is presented, with particular emphasis on the recent achievements in the development of the photon detectors, the pixel Hybrid Photon Detectors.

Particle identification with the LHCb RICH system

The LHCb experiment uses a Ring Imaging Cherenkov (RICH) system to provide particle identification over the momentum range 2– 100 GeV / c . Two RICH detectors are employed. The upstream detector, RICH 1, utilizes both aerogel and C 4 F 10 gas radiators whilst the downstream RICH 2 uses a CF 4 gas radiator. The RICH 2 detector has been fabricated and is installed in the LHCb interaction region; RICH 1 has a programme of phased design and construction. Novel Hybrid Photon Detectors (HPDs) have been developed in collaboration with industry to detect the Cherenkov photons in the wavelength range 200–600 nm. The HPDs are enclosed in iron shielding and Mumetal cylinders to allow operation in magnetic fields up to 50 mT. The performance of pre-series HPDs and the results obtained from a particle test beam using the full LHCb readout chain is presented. The production of a total of 484 HPDs required for the two RICH detectors has recently commenced. The expected performance of the LHCb RICH system, obtained from realistic simulation, is described.

Performance Studies of Pixel Hybrid Photon Detectors for the LHCb RICH Counters

The Pixel Hybrid Photon Detector is a vacuum tube with a multi-alkali photo cathode, high voltage cross-focused electron optics and an anode consisting of a silicon pixel detector bump-bonded to a readout CMOS electronic chip fully encapsulated in the device. The Pixel HPD fulfils the requirements of the Ring Imaging Cherenkov counters of the LHCb experiment at LHC. The performances of the Pixel HPD will be discussed with reference to laboratory measurements, Cherenkov light imaging in recent beam tests, image distortions due to a magnetic field.

Simulation of LHCb RICH detectors using GEANT4

LHCb is one of the experiments at the Large Hadron Collider at CERN. It aims to make precision measurements of CP violation in the B-meson decays. The detector simulation of LHCb was developed using the GEANT4 software toolkit interfaced to the LHCb software framework named GAUDI. This is now being used for a large production run of 210 million events. An important feature of the LHCb experiment is the use of Ring Imaging Cherenkov Counters (RICH) for particle identification. The Cherenkov photons created in this detector are focused onto an array of photodetectors made of Hybrid Photon Detectors. A brief overview of the GEANT4 based LHCb simulation program named GAUSS will be given with the main emphasis on RICH simulation and its verification.

Overview of LHCb RICH detector development

For particle identification, LHCb will use two RICH detectors, one covering the charged particle momentum range 1–65 GeV/ c using aerogel and C 4 F 10 radiators and the other covering up to 100 GeV/ c using CF 4 radiator. A Hybrid Photon Detector (HPD) with a 80 mm photocathode diameter has been developed, to detect the Cherenkov photons in the wavelength range 200–600 nm. To shield the HPD arrays from magnetic fields up to 600 Gauss in one detector and 150 Gauss in the other, a shielding system has been designed with Mumetal cylinders enclosing each HPD. The engineering design of both RICH detectors has been completed and the construction and assembly of several components has started. The installation of these detectors in LHCb is expected to be completed in 2006. The design and current status of the LHCb RICH system will be reviewed and the expected performance from realistic simulations will be shown.

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.

Characterisation and compensation of magnetic distortions for the pixel Hybrid Photon Detectors of the LHCb RICH

The LHCb experiment at LHC, optimised for the study of CP violation, has two RICH detectors to provide particle identification in the momentum range ∼ 2 – 100 GeV / c . The stringent requirements on the photon detectors are met by the custom-made pixel Hybrid Photon Detector. The photon detectors need to operate in the fringe field of the LHCb dipole magnet which will produce distortions of the image detected on the pixel chip which is encapsulated inside the HPD. This paper reports on the experimental characterisation of the image distortions caused by an external magnetic flux density. These measurements allow for the development of a parameterisation of the effects and a compensation algorithm, which are also presented.

Magnetic shielding studies of the LHCb rich photon detectors

The LHCb experiment will use two RICH detectors to provide particle identification. The Hybrid Photon Detectors (HPDs) adopted for these detectors are required to operate in the fringe field of the 4 Tm LHCb dipole magnet. In fields in excess of 15 G, photoelectrons are lost from the active area of an HPD. Shielding the HPDs from the fringe field is therefore essential for the efficient operation of the RICH detectors. This paper describes the technique used to calculate the residual field inside the relevant magnetic shields. The size of the problem, together with the disparity in the geometric scales involved, makes the calculations intractable using a finite element model of the entire magnetic environment. As a result, a sub-modelling method has been used. The model indicates that the photon detectors will operate efficiently.

A large spherical HPD for a novel deep-sea neutrino experiment

An underwater neutrino experiment has been proposed which provides precise measurements of the neutrino mixing parameters θ 23 and Δ m 23 2 and permits an increase of sensitivity for the small angle θ 13 by more than one order of magnitude. A Cherenkov detector of about 1.5 Mt active mass, deployed in the Gulf of Taranto, utilizes the CNGS beam in off-axis configuration which represents an essentially mono-energetic source of muon neutrinos. A unique feature of the experiment is the possibility to move the detector and therefore exploit different baselines around 1200 km where the oscillation pattern is fully developed. The conceptual detector design consists of O(30,000) large area and acceptance photosensors arranged in a matrix of ∼300×300 m 2 size. Hybrid photon detectors are considered as promising candidates as they provide clean signal characteristics and uniform collection efficiency. We discuss the design and expected performance of a large spherical HPD with 380 mm diameter, which is housed in a high-pressure glass container. A scaled prototype HPD of 208 mm diameter is currently under development using the existing CERN HPD facility.

X-ray-sensitive hybrid photon detectors with Be-windows

A 0.3-mm thin YAlO/sub 3/(Ce) scintillating crystal plate of 22 mm diameter was mounted inside an electrostatic-focussed hybrid photomultiplier tube (HPMT). The photocathode was evaporated directly on this scintillating plate opposite to the HPMT-anode. The HPMT was vacuum-sealed with a 1-mm thick Be radiation entrance window. Photoelectron numbers at their peak energies of photoabsorption and energy resolutions were measured between 3.69 and 59.6 keV. The average number of photoelectrons produced per keV radiation energy was determined to be 4.457 phel/keV and the energy resolution at 59.6 keV was 17%. Introducing in the same way such a thin YAlO/sub 3/(Ce) plate inside an imaging silicon pixel array (ISPA) tube would improve its spatial resolution, since it depends on the thickness of the X-ray detector.

Readout of the LHCb pixel hybrid photon detectors

The LHCb experiment will use two Ring-Imaging Cherenkov counters equipped with pixel hybrid photon detectors (HPDs) to measure Cherenkov light generated by high-energy particles traversing a radiative medium. Data from these detectors must provide efficient pattern recognition whilst coping with the environmental demands of LHCb: 25 ns time precision, up to 4% channel occupancy and 1 MHz selective frame readout rate. Dedicated CMOS electronics have been developed to meet the requirements of the HPD, together with special packaging and interconnect techniques which withstand the harsh production processes of the detector. Successful prototypes have been fabricated and tested and results are presented. Large-scale production has started in collaboration with European hi-tech industry and the individual steps in the process are described.

Pixel hybrid photon detectors for the ring imaging Cherenkov detectors of LHCb

A Pixel Hybrid Photon Detector (pixel HPD) has been developed for the LHCb Ring Imaging Cherenkov (RICH) detectors. The pixel HPD is a vacuum tube with a multi-alkali photocathode, high-voltage cross-focused electron optics and an anode consisting of a silicon pixel detector bump-bonded to a CMOS readout chip; the readout chip is thus fully encapsulated in the device. The pixel HPD fulfils the stringent requirements for the RICH detectors of LHCb, combining single photon sensitivity, high signal-to-noise ratio and fast readout with an ∼8 cm diameter active area and an effective pixel size of 2.5 mm×2.5 mm at the photocathode. The performance and characteristics of two prototype pixel HPDs have been studied in laboratory measurements and in recent beam tests. The results of all measurements agree with expectations and fulfill the LHCb RICH requirements. In readiness for production of the ∼500 pixel HPDs for the RICH detectors, a test programme was designed and implemented to ensure component quality control at each stage of the manufacturing process.

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 study of new pixel hybrid photon detector prototypes for the LHCb RICH counters

A pixel hybrid photon detector (HPD) has been developed according to the specific requirements of the LHCb ring imaging Cherenkov counters. This detector comprises a silicon pixel detector bump-bonded to a binary readout chip to achieve a fast 25-ns readout and a high signal-to-noise ratio (SNR). The detector performance was characterized by varying the pixel threshold, the tube high voltage, the silicon bias voltage, and the determination of the photoelectron detection efficiency. Furthermore, accelerated ageing and high pixel occupancy tests were performed to verify the long-term stability. The results were obtained using Cherenkov light and a fast pulsed light emitting diode. The results of all measurements agree with the expectations and fulfill the design goals.

Feasibility of a novel design of high resolution parallax-free Compton enhanced PET scanner dedicated to brain research**All the authors are members of the CIMA Collaboration.

A novel concept for a positron emission tomography (PET) camera module is proposed, which provides full 3D reconstruction with high resolution over the total detector volume, free of parallax errors. The key components are a matrix of long scintillator crystals and hybrid photon detectors (HPDs) with matched segmentation and integrated readout electronics. The HPDs read out the two ends of the scintillator package. Both excellent spatial (x, y, z) and energy resolution are obtained. The concept allows enhancing the detection efficiency by reconstructing a significant fraction of events which underwent Compton scattering in the crystals. The proof of concept will first be demonstrated with yttrium orthoaluminate perovskite (YAP):Ce crystals, but the final design will rely on other scintillators more adequate for PET applications (e.g. LSO:Ce or LaBr3:Ce). A promising application of the proposed camera module, which is currently under development, is a high resolution 3D brain PET camera with an axial field-of-view of ∼15 cm dedicated to brain research. The design philosophy and performance predictions based on analytical calculations and Monte Carlo simulations are presented. Image correction and reconstruction tools required to operate this transmissionless device in a research environment are also discussed. Better or similar performance parameters were obtained compared to other known designs at lower fabrication cost. The axial geometrical concept also seems to be promising for applications such as positron emission mammography.

Novel design of a parallax free Compton enhanced PET scanner

Molecular imaging by PET is a powerful tool in modern clinical practice for cancer diagnosis. Nevertheless, improvements are needed with respect to the spatial resolution and sensitivity of the technique for its application to specific human organs (breast, prostate, brain, etc.), and to small animals. Presently, commercial PET scanners do not detect the depth of interaction of photons in scintillators, which results in a not negligible parallax error. We describe here a novel concept of PET scanner design that provides full three-dimensional (3D) gamma reconstruction with high spatial resolution over the total detector volume, free of parallax errors. It uses matrices of long scintillators read at both ends by hybrid photon detectors. This so-called 3D axial concept also enhances the gamma detection efficiency since it allows one to reconstruct a significant fraction of Compton scattered events. In this note, we describe the concept, a possible design and the expected performance of this new PET device. We also report about first characterization measurements of 10 cm long YAP:Ce scintillation crystals.

HPD imaging properties in Cherenkov telescopes

We discuss the imaging properties of Hybrid Photon Detectors (HPDs) in the framework of an Imaging Air Cherenkov Telescope experiment. We present also some preliminary results on Night Sky Background measurements, made with an HPD located in the focal plane of a single CLUE telescope.

Design, fabrication and performance of the 10-in. TOM HPD

The first sealed TOM Hybrid Photon Detector (HPD) with 10-in. diameter has been fabricated and successfully tested at CERN. This HPD has a spherical entrance window and a bialkali photocathode. The fountain focusing optics produces a demagnified image ( D=4) on the round segmented silicon sensor. The signals of the 2048 cells are read out through analog front-end electronics encapsulated in the vacuum envelope. We report on the design, fabrication technique and the experimental results obtained with laboratory test benches. The large TOM HPD is a prototype tube developed for the CLUE cosmic ray experiment. The final tubes, now under development, will be equipped with a solar-blind Rb 2Te photocathode and self triggering front-end electronics.

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.

Hybrid photon detectors

Abstract Hybrid photon detectors detect light via vacuum photocathodes and accelerate the emitted photoelectrons by an electric field towards inversely polarized silicon anodes, where they are absorbed, thus producing electron–hole pairs. These, in turn, are collected and generate electronic signals on their ohmic contacts. This review first describes the characteristic properties of the main components of hybrid photon detectors: light entrance windows, photocathodes, and silicon anodes. Then, essential relations describing the trajectories of photoelectrons in electric and magnetic fields and their backscattering from the silicon anodes are derived. Depending on their anode configurations, three families of hybrid photon detectors are presented: hybrid photomultiplier tubes with single anodes for photon counting with high sensitivity and for gamma spectroscopy; multi-anode photon detector tubes with anodes subdivided into square or hexagonal pads for position-sensitive photon detection; imaging silicon pixel array tubes with finely segmented anodes for photon-sensitive imaging devices. Some of the hybrid photon detectors’ applications and achievements in radiation detection are discussed and compared with competing devices such as photomultipliers, image intensifiers, photodiodes, silicon drift chambers, charge coupled devices, visible light photon counters, and photographic emulsions.