GaAsP Photocathode Research Articles

Response time characteristics of a transmissive uniformly doped GaAsP photocathode.

In this paper, the matrix difference method is used to calculate the photoelectron continuity equation and the outgoing electron flux density equation. The effects of the GaAsP/AlGaAsP recombination rate, electron diffusion coefficient, and activation layer thickness on the time-resolved characteristics and quantum efficiency of a GaAsP photocathode are systematically studied, and the accuracy of the theoretical calculation is verified by experiments. The response speed and quantum efficiency of the GaAsP photocathode can be greatly improved by adjusting the thickness of the GaAsP activation layer reasonably.

Evolution of the transverse and longitudinal energy distributions of electrons emitted from a GaAsP photocathode as a function of its degradation state

We present measurements of the transverse and longitudinal energy spread of photoelectrons emitted from a GaAsP photocathode as a function of its degradation state. The cathode was initially activated to a state of negative electron affinity in our photocathode preparation facility, achieving a quantum efficiency of 3% at a wavelength of 532 nm. It was then transferred under XHV conditions to our transverse energy spread spectrometer, where energy spread measurements were made while the photocathode was progressively degraded through a controlled exposure to oxygen. Data have been collected under photocathode illumination at 532 nm, and the changing photoelectron energy distribution associated with the changes in the level of electron affinity due to quantum efficiency degradation through an exposure to 0.25 L of oxygen has been demonstrated. Our experiments have shown that GaAsP boasts a significantly higher resilience to degradation under exposure to oxygen than a GaAs photocathode, though it does exhibit a higher level of mean transverse energy. Coupled with the favourable published data on GaAsP photoemission response times, we conclude that GaAsP is a viable candidate material as a particle accelerator electron source.

Characterization of fast microchannel plate photomultipliers for the ITER core LIDAR Thomson scattering system

In the new ITER core LIDAR Thomson scattering system under development, detectors with pulse response as low as 330 ps and 10 mm diameter active area, covering with good sensitivity the spectral range 380–1100 nm will be necessary. For the visible region 400–750 nm, fast microchannel plate photomultipliers with a GaAsP photocathode that fulfill the characteristics of speed, sensitivity and active area are already commercially available and have been recently characterized for operation in the JET edge LIDAR Thomson scattering system. In this paper we present a further characterization of these devices to evaluate their usefulness in the more demanding operating conditions of the ITER core LIDAR Thomson scattering system. The characteristics of these detectors with regard to linearity, gain recovery time, pulse repetition rate, quantum efficiency and response time for large F-number illumination, have been measured. Linearity and recovery time data have been interpreted according to a new, time dependent, microchannel plate operational model. The results show that these detectors are suitable also for use in ITER.

Degradation of GaAsP photocathodes under bombardment by ions of residual gases

Processes occurring in the near-surface region of semiconductor photocathodes based on an epitaxial multilayer GaAsP structure activated by a CsO monolayer have been investigated via secondary ion mass spectroscopy, X-ray luminescence spectroscopy, and electron microscopy. The main mechanisms of photoemitter degradation have been revealed. The main cause of degradation is low-energy heavy ion implantation into the near-surface region of the cathode, which changes the electron structure of the surface.

Very high QE HPDs with a GaAsP photocathode for the MAGIC telescope project

Hybrid photon detectors are one of the few low light level sensors that can provide an excellent single and multiple photoelectron amplitude resolution. The recently developed hybrid photon detectors for the Major Atmospheric Gamma-Imaging Cherenkov (MAGIC) telescope project with an 18-mm-diameter GaAsP photocathode (the Hamamatsu R9792U-40) have a peak quantum efficiency of more than 50% and a pulse width of ∼ 2.3 ns . In addition, the afterpulsing rate of these tubes is ∼ 300 times lower compared to that of conventional photomultiplier tubes. Photocathode aging measurements showed lifetimes of more than 10 years under standard operating conditions of Cherenkov Telescopes. The temperature dependence of the avalanche gain can be reduced to the same level of a photomultiplier by a simple compensation circuit. Here we report on the recent progress with the above-mentioned hybrid photon detectors.

Algol – CPNG: photon counting cameras for interferometry in visible wavelengths

Images in visible interferometry are characterised by their low coherence time, and except for brightest stars, the flux on the detector is much less than one photon per pixel per image. Algol and Comptage de Photons Nouvelle Generation (CPNG) are new photon counting cameras developed for high angular resolution in the visible. They are intensified CCDs built to benefit from improvements in photonic commercial components, and personal computer processing power. We present how we achieve optimal performances (sensitivity and spatiotemporal resolution) by the combination of proper optical and electronics design, and real-time elaborated data processing. The number of pixels is 532 × 516 and 768 × 640 read at a frame rate of 262 Hz and 50 Hz for CPNG and Algol respectively. The dark current is very low: 5 × 10 -4 electron.pixel -1 .s -1 . Quantum efficiencies reach up to 36% in the visible with the GaAsP photocathodes and and 26% in the red with the GaAs ones, thanks to the sensitivity of the photocathodes and to the photon centroiding algorithm; they are likely the highest values reported for ICCDs.

Development of a TOP counter for the Super B factory

We will in this paper discuss a novel concept for a ring imaging Cherenkov detector. It measures the time of propagation of the Cherenkov photons in a quartz radiator. The photon detectors are micro-channel-plate photo-multipliers with GaAsP photo-cathodes. The performance has been estimated to give a separation better than 3.5 σ between K and π at 4 GeV / c . This detector, together with a compact focusing system to correct the chromatic effect, will have an even better resolution. We will show that it can achieve more than 4.2 σ separation between K and π at 4 GeV / c . This detector is well suited for experiments at the Super B factory.

High Speed HPD for Photon Counting

We have succeeded in the development of a highspeed Hybrid-Photodetector (HPD), by using reduced electron lens and newly developed avalanche diode (AD) with very low capacitance. The HPD shows fast time response of less than 400 ps in both rise and fall times, and good timing resolution of 23 ps in sigma for single photons at full illumination on photocathode of Phi 8 mm, after deducting the waveform of the input light pulse. Limiting factor of the timing resolution was further investigated, and clarified to be the transit time difference of electrons in vacuum along the photocathode. The timing resolution was estimated to be less than 10 ps for the spot illumination of Phi 1 mm on the central part of the photocathode, where small transit time difference was expected by the simulation. This resolution is the world highest in the HPD, and matches for those of MCP-PMTs. Both GaAsP photocathode having close to 50% quantum efficiency in visible region and bialkali photocathode having 34% in UV region were fabricated in view of various applications. In this paper, we report the results of the evaluation including discussion about the limiting factors of the timing resolution for the new HPD.

New generation photon-counting cameras: algol and CPNG

Algol and Comptage de Photons Nouvelle Génération (CPNG) are new generation photon counting cameras developed for high angular resolution in the visible by means of optical aperture synthesis and speckle interferometry and for photon noise limited fast imaging of biological targets. They are intensified CCDs. They have been built to benefit from improvements in photonic commercial components, sensitivity, and personal computer workstations processing power. We present how we achieve optimal performances (sensitivity and spatiotemporal resolution) by the combination of proper optical and electronics design, and real-time elaborated data processing. The number of pixels is 532 x 516 and 1024(2) read at a frame rate of 262 and 100 Hz for CPNG and Algol, respectively. The dark current is very low: 5.5 x 10(-4) e(-) .pixel(-1). s(-1). The saturation flux is approximately 7 photon events /pixel/s. Quantum efficiencies reach up to 36% and 26% in the visible with the GaAsP photocathodes and in the red with the GaAs ones, respectively, thanks to the sensitivity of the photocathodes and to the photon centroiding algorithm; they are likely the highest values reported for intensified CCDs.

Hybrid photodetector for single-molecule spectroscopy and microscopy.

We report benchmark tests of a new single-photon counting detector based on a GaAsP photocathode and an electron-bombarded avalanche photodiode developed by Hamamatsu Photonics. We compare its performance with those of standard Geiger-mode avalanche photodiodes. We show its advantages for FCS due to the absence of after-pulsing and for fluorescence lifetime measurements due to its excellent time resolution. Its large sensitive area also greatly simplifies setup alignment. Its spectral sensitivity being similar to that of recently introduced CMOS SPADs, this new detector could become a valuable tool for single-molecule fluorescence measurements, as well as for many other applications.

Development status of High-QE HPDs with 18-mm GaAsP photocathode for IACTs

A new type of Hybrid PhotoDetectors (HPDs) with an 18-mm-diameter GaAsP photocathode was developed in order to lower the energy threshold of the MAGIC-II Project. The Quantum Efficiency (QE) reaches more than 50% at around 500 nm and is enhanced in the UV region by the wavelength shifting technique. The total light conversion efficiency could be practically improved by about a factor 2 compared to the conventional PhotoMultiplier Tubes (PMT). Photocathode ageing measurements indicate that GaAsP photocathode have a sufficiently long lifetime for being used in the MAGIC telescope imaging camera.

Evaluation of the light emission efficiency of LYSO:Ce scintillator under X-ray excitation for possible applications in medical imaging

Lutetium–yttrium-based scintillators, such as LYSO:Ce, have a high effective atomic number, are non-hygroscopic, fast emitting materials, and promising candidates for use in positron emission imagers. The present study investigates the light emission characteristics of (Lu, Y) 2SiO 5:Ce (LYSO:Ce) single-crystal scintillator under X-ray imaging conditions. Also, the parameters related to the luminescence emission spectrum and emission efficiency were studied using experimental methods. Various X-ray tube voltages currently employed in X-ray imaging techniques were used. Measurements were performed using an experimental set-up based on a photomultiplier coupled to an integration sphere. In addition, the emission spectrum under UV and X-ray excitation was measured using an optical grating monochromator to determine the spectral compatibility of optical photon detectors incorporated in medical imaging systems. The absolute efficiency of LYSO:Ce was found to increase with increasing X-ray tube voltage (from 2.2 EU at 22 kVp to 22.4 EU at 140 kVp), while its spectrum, peaking at about 430 nm, was found compatible with most optical detectors (photodiodes, photocathodes, charge coupled devices, etc.). The matching factor was estimated to range from 0.76 to 0.92 (for a silicon photodiode and for a GaAsP photocathode, respectively).

Development of HPDs with an 18-mm-diameter GaAsP photocathode for the MAGIC-II

A new type of Hybrid PhotoDetectors (HPDs) with an 18-mm-diameter GaAsP photocathode was developed in order to lower the energy threshold of the MAGIC-II project. The peak value of the Quantum Efficiency (QE) reaches 51% at around 500 nm. Application of the wavelength shifting technique can further enhance the sensitivity in the UV region. Compared to the currently used classical PhotoMultiplier Tubes (PMTs) with bialkali photocathodes, the new HPDs provide practically twice as high photon conversion efficiency. Simulation studies of the new HPDs indicate that they have a sufficiently long lifetime for being used in the MAGIC telescope imaging camera. In this report we present the evaluation results and the performance of these new photodetectors.

Aging studies on hybrid photon detectors with Cs activated monocrystalline photocathodes

Hybrid photon detectors are one of the preferred candidates for a light detector with high quantum efficiency in the visible range and single photoelectron resolution. To reach a high sensitivity Cs-activated monocrystalline GaAs or GaAsP photocathodes are used. Prototypes of such HPDs have shown a peak QE up to 45% at 500 nm, but no extensive studies on its aging properties have been performed up to now. Ion feedback effects and instabilities on the activation layer will be conceptually studied as the most important aging factors.

Very high quantum efficiency PMTs with bialkali photo-cathode

Since the mid-1960s and until today the classical PMTs with semitransparent bialkali photo-cathode provide peak Quantum Efficiency (QE) of ∼25%. About 2 years ago we started a program with the PMT manufacturers Hamamatsu, Photonis and Electron Tubes for boosting up the QE of bialkali PMTs. In the mean time we have obtained several batches of experimental PMTs from the above-mentioned manufacturers and measured few samples with QE values as high as 32–36% in the peak. Also, we want to report on the modest (5–7) % increase of the QE of the PMTs with flat input window after sandblasting. Earlier we have reported that by coating the hemi-spherical input window of bialkali PMTs with a milky layer we could enhance their QE by ∼10–20% for wavelengths ∼320 nm. Assuming that the industry can reliably produce PMTs with 32–35% QE in the peak, by applying the milky layer coating technique to the PMTs with hemi-spherical input window one shall be able to achieve peak QE values of ∼35–40%. Being by an order of magnitude cheaper and providing a matching level of QE such PMTs will become strong competitors for hybrid photo-diodes (HPD) with GaAsP photo-cathode.

Development of an ultrafast single photon counting imager for single molecule imaging.

We have begun developing an innovative ultra-fast single-photon counting imager which comprises a mega-pixel CMOS array and a newly-designed Image Intensifier. It is expected to have single photon sensitivity with 100 psec time resolution, operational at a total counting rate exceeding 1MHz. The readout is based on dead-time-free flash ADC, running at 1-2GS/s, followed by a FPGA for real-time parallel data processing. Such a device has not been realized before and is expected to revolutionize time-resolved fluorescence imaging and spectroscopy from a single-molecule to whole animal level. To evaluate the design principle, an Image Intensifier with a GaAsP photocathode (>40% quantum efficiency at 400-600 nm) followed by double MCP was evaluated together with an existing CMOS camera. In our future design, the image from CMOS Camera will be combined with the MCP output, followed by a set of FPGA and CPU for real time data processing. This stream line method will allow ultra fast single-photon counting with 100 psec time resolution and 20 µm position resolution (1M pixel imaging). In this paper, we present the design principle and preliminary results on its performance. Our future plan and the design goals are also described.

Evaluation of a new high QE photomultiplier for air Cherenkov telescopes

We report on the progress of a prototype hybrid photomultiplier (PMT), comprising a high QE, red extended GaAsP photocathode and an avalanche diode as electron bombarded anode with internal gain. These PMTs, with a larger cathode, are planned for the upgrade of the 17 m diameter MAGIC telescope for ground-based gamma (γ) astronomy.

イメージセンシング技術とその応用 ＧａＡｓＰ−Ｉ．Ｉ．を用いたＶＴＲ一体型ハイビジョン超高感度カメラ

This ultrahigh-sensitivity HDTV color camcorder uses a proximity-type image intensifier employing 3rd-generation GaAsP photocathodes. In order to accomodate a video cassette recorder (VCR) within the camera, thereby reduce the weight and increase the image quality, a compact optical unit consisting of an enlarger lens and tapered fiber optics plate (FOP) was developed. The camcorder can be equipped with a high-speed gate shutter with speeds up to one per microsecond by pulsing the voltage to the photocathode. The camcorder provides good color reproducibility under moonlight, and a signal level of 100% was obtained at 0.2 lx, f/2 (signal-to-noise ratio of 56dB). Its sensitivity is about 400 times greater than that of current HDTV CCD camcorders, mak-ing it particularly well suited for shooting faint objects such as auroras and nocturnal animals.

An evaluation of the new compact hybrid photodiodes R7110U-07/40 from Hamamatsu in high-speed light detection mode

The main parameters of hybrid photodiodes (HPD) are constantly improving. In the new 20mm ∅ HPDs from Hamamatsu an avalanche diode (AD) serves as anode. Due to high gain of the used ADs the HPDs show a total gain of 6−8×104 at nominal HV. This allows one to use HPDs with very fast low-noise voltage amplifiers and to detect fast pulses (down to the single photoelectron level) with very good amplitude resolution. Of special interest are HPDs of the type R7110U-40 with GaAsP photocathode which provide a quantum efficiency of ∼40% in maximum at ∼500nm. Results on the evaluation of the above-mentioned HPDs and their comparison with HPDs from INTEVAC will be presented.

Very high quantum efficiency fast light sensors for cosmic ray detectors

Low level light (LLL) sensors are widely used in cosmic ray detectors. Common LLL sensors are the classical photomultiplier tubes (PMT), which provide a peak quantum efficiency ( QE ) of 20–25 % in the wavelength range ~ 320 – 420 nm . Usually one tries to achieve a high detection efficiency and a low energy threshold for a cosmic ray detector. This is the case in experiments measuring fluorescence or Cherenkov light emission of relativistic particles in a given medium. A very cost-effective method to enhance the efficiency and to lower the threshold is the use of light sensors with higher QE . Here we discuss the recent progress in the field of hybrid PMTs applying GaAsP photocathodes, which provide a QE of ~ 45 % in the wavelength range 500–600 nm and a significantly better amplitude resolution compared to classical PMTs. By application of wavelength shifting technique (WLS) their QE can be enhanced close to the maximum level also in the blue and near ultraviolet parts of the spectrum.