Quantum Efficiency Of Photocathode Research Articles

Effects of NEA GaN photocathode performance parameters on quantum efficiency

Using research on the negative electron affinity GaN photocathode photoemission mechanism, we obtained the reflective-type and transmission-type GaN photocathode quantum efficiency formulas. The influence on quantum efficiency and sensitivity of integral of cathode performance parameters such as electron surface escape probability P, electron diffusion length LD, absorption coefficient α, back-interface recombination rate Sv and cathode thickness Te, were analyzed using these formulas. It was found that to obtain negative electron affinity GaN optoelectronic cathodes with high quantum efficiencies, we must constantly improve cathode activation technologies and the surface escaping probability of cathode. Also, we must increase the electronic diffusion length, reduce the rate of compounding, and find the optimal thickness of the cathode transmit layer based for the specific electronic diffusion length.

Quantum efficiency of GaN photocathode under different illumination

GaN samples are activated by Cs/O under illumination of deuterium lamp, 300 nm monochromatic light with power of 70 μW and 300 nm monochromatic light with power of 35 μW, respectively. Photocurrent is detected before activation under illumination of deuterium lamp. Quantum efficiency (QE) is tested after activation. The results indicate that GaN activated under 300 nm monochromatic light have higher QE than that under deuterium lamp, and no obvious difference is detected between different power 300 nm monochromatic light. The photocurrent before activation inhibits the adsorption of Cs on the GaN surface, which decrease the QE of GaN.

Influence of varied doping structure on photoemissive property of photocathode

The built-in electric fields within a varied doping GaAs photocathode may promote the transport of electrons from the bulk to the surface, thus the quantum efficiency of the cathode can be enhanced remarkably. But this enhancement, which might be due to the increase in either the number or the energy of electrons reaching the surface, is not clear at present. In this paper, the energy distributions of electrons in a varied doping photocathode and uniform doping photocathode before and after escaping from the cathode surface are analysed, and the number of electrons escaping from the surface in different cases is calculated for the two kinds of photocathodes. The results indicate that the varied doping structure can not only increase the number of electrons reaching the surface but also cause an offset of the electron energy distribution to high energy. That is the root reason for the enhancement of the quantum efficiency of a varied doping GaAs photocathode.

Electron-Bombarded CMOS Image Sensor in Single Photon Imaging Mode

Solid-state devices utilizing “photonic events amplification” (PEA) are used for low-level light imaging (LLLI) and are exploited in military, scientific, astronomy, surveillance, and other applications. The PEA imagers are more sensitive by a few orders of magnitude than regular CCD cameras and by an order of magnitude than most sensitive scientific LLLI CCD cameras. The Electron-Bombarded CMOS Image Sensor (EB-CMOS-IS) is a novel PEA technology and has just recently become commercially available. The EB-CMOS-IS technology is a best price/performance combination among concurrent technologies such as EBCCD, EMCCD, Intensified CCD, and Intensified CMOS image sensors. Although the EB-CMOS-IS-based applications demonstrate outstanding sensitivity, they are exploited today far from their maximal potential. In this study, we developed a comprehensive model of the EB-CMOS-IS used for simulation of the sensor performance as a function of the device parameters, e.g., photocathode quantum efficiency, acceleration bias, electrons-to-voltage conversion factor, and operation parameters, e.g., amplifier gain, offset, and exposure time. We selected parameters enabling a single photon imaging (SPI) mode and performed imaging simulations for an object under various low-level illumination conditions. We present a method of the EB-CMOS-IS operation in the SPI mode for low-light-level imaging of a stationary object, boosting the sensor sensitivity to a level better than 10-7 lux.

Comparison between gradient-doping and uniform-doping GaN photocathodes

In order to enhance the quantum efficiencies of negative electron affinity (NEA) GaN photocathodes, gradient-doping reflection-mode GaN photocathodes are grown by metal organic chemical vapor deposition (MOCVD)at doping concentrations of 1×1018cm-3, 4×1017cm-3, 2×1017cm-3 and 6×1016cm-3 from the body to the surface, with the thickness of each doping region being about 45nm and the total thickness of GaN 180 nm. The gradient-doping GaN photocathodes are activated in an ultra-high vacuum system and are compared with two kinds of uniform-doping GaN photocathodes whose thicknesses are both 150 nm and doping concentrations are 1.6×1017cm-3 and 3×1018cm-3 separately. The results show that both the photocurrent growth rate and the maximum photocurrent of the gradient-doping GaN photocathodes are greater than those of the uniform-doping GaN in the Cs/O activation process, and the multi-test system measured maximum quantum efficiency of the gradient-doping NEA GaN photocathode is about 56% which is as high as the double of the uniform-doping. Calculations show that the energy band bendings of the gradient-doping GaN photocathodes are 0.024eV, 0.018eV and 0.031eV from the body to the surface, a larger electron drift and diffusion length are gained due to the built-in electric field formed by the energy band bending, because of the 0.073eV total energy band bending the photoelectrons reaching the surface have higher energies and pass through the surface barrier more easily. Therefore the gradient-doping NEA GaN photocathodes have greater quantum efficiencies.

Quantum efficiency recovery of reflection-mode NEA GaN photocathode

In order to investigate the decay tendency and the recovery status of the quantum efficiency of reflection-mode NEA GaN photocathode, the quantum efficiency curves have been studied after the photocathode was fully activated, stored in system and supplemented with Cs. The quantum efficiency decay and recovery processes of reflection-mode NEA GaN photocathode were observed and the mechanism was discussed. The quantum efficiency value of reflection-mode NEA GaN photocathode can be recovered up to more than 94% of the best value in the shortwave region between 240nm and 300nm, and more than 88% in the long wave region between 300nm and 375nm after Cs supplement. Based on the changes of surface potential barrier profiles of the reflection-mode NEA GaN photocathode before and after the quantum efficiency degradation and the quantum yield formula, the decay characteristic and the recovery status of quantum efficiency curve after supplement with Cs have been related to the changes of surface barrier shapes.

Improvement methods of reflective photocathode QE of X-ray frame camera

Time-resolved diagnosis of the transient process using X-ray frame cameras (XRCs) is an important means in inertial confinement fusion (ICF) experiments. The sensitivity of the photocathode is a key parameter of the entire camera system. This letter aims to raise the quantum efficiency (QE) of the photocathode. With the changes in the deposition parameters, such as deposit angle, thickness of the coating in the channel, and vacuum of evaporation, the QE results are different. After testing and theoretical calculation, we find that there is a best matching value among these parameters. When the coating parameter meet this best value, the gain of the XRC can be improved significantly.

Middle-ultraviolet laser photoelectron emission from vertically aligned millimeter-long multiwalled carbon nanotubes

We demonstrate photoelectron emission from millimeter-long forests of vertically aligned multiwalled carbon nanotubes using 266 nm light, which illuminates the forests from the side. We have measured quantum efficiencies in the order of ∼10−5 at low fields (pure photoemission) and ∼10−3 at an applied field of 0.3 V μm−1, which are 2–4 orders of magnitude higher than those obtained from films of randomly oriented nanotubes, and approach the quantum efficiency of semimetal photocathodes. Through optical simulations we show that 266 nm light is absorbed within the first few layers of the nanotube forest.

A near ultraviolet photon counting imaging detector

Photon counting imaging detector is classified to direct collection mode and induction mode, based on the readout mode of microchannel plate (MCP) output charge. Previously, we discussed the detector with direct collection mode. A near ultraviolet (NUV) photon counting imaging detector with induction readout mode is presented here. The detector consists of a cesium telluride (Cs 2Te) photocathode, a v-stack MCP, a semiconductor germanium layer on ceramic substrate, and wedge and strip anode (WSA). Quantum efficiency of the detector photocathode is 12% at (230 nm), dark count rate is 10.7 counts/(s cm 2) and spatial resolution is 110 μm.

Tertiary Scintillation Gas Proportional Scintillation Counter (TS-GPSC): First Experimental Results$^{\ast}$

A new concept for a gaseous radiation detector is presented: the Tertiary Scintillation Gas Proportional Scintillation Counter (TS-GPSC). In this detector the electric field induced secondary scintillation is first detected by a CsI-coated GEM-like structure where it releases photoelectrons which are transferred through the GEM holes, with no charge multiplication, to another region where further field induced scintillation (tertiary) is produced and then again detected on a planar CsI-coated photocathode at the backplane of the detector. The electrons released therein are collected at a grid and constitute the detector's signal. Since there is no avalanche charge multiplication in the detector, the gain will be quite stable and, moreover, as the field induced scintillation yield is very high, and in spite of the low photocathode quantum efficiency, the gain will be high and with low fluctuations, so an improvement in the energy resolution as compared to other types of Gas Proportional Scintillation Counters is expected. The first prototype of the tertiary scintillation detector was tested in a xenon atmosphere for hard X-rays and experimental results are presented. Energy resolutions of 8.2% were achieved for 22.1 keV X-rays.

Focus on spectroscopy

Focus on spectroscopy

Study of spectral response characteristics of negative electron affinity GaN photocathode

Aiming at determining the quantum efficiency of fully activated negative electron affinity(NEA) GaN photocathodes, we have investigated the spectral response of NEA GaN photocathodes by using a dedicated ultraviolet spectral response measurement instrument. The quantum efficiency curve of reflection-mode NEA GaN photocathode has been obtained in the band region from 230nm to 400nm. The experimental results show that the quantum efficiency of reflection-mode NEA GaN photocathode reaches up to 37.4% at 230 nm, and at 365 nm, which is the threshold of GaN photocathode, the quantum efficiency remains 3.75%. A sharp cutoff characteristic with over two orders of magnitude degradation from 230 nm to 400 nm has been observed. Based on the former research results, the factors influencing quantum efficiency were also comprehensively analyzed in the paper.

Erratum: Quantum efficiency and thermal emittance of metal photocathodes [Phys. Rev. ST Accel. Beams12, 074201 (2009)

Erratum: Quantum efficiency and thermal emittance of metal photocathodes [Phys. Rev. ST Accel. Beams<b>12</b>, 074201 (2009)

Equivalent Method of Solving Quantum Efficiency of Reflection-Mode Exponential Doping GaAs Photocathode

The mathematical expression of the electron diffusion and drift length LDE of exponential doping photocathode is deduced. In the quantum efficiency equation of the reffection-mode uniform doping cathode, substituting LDE for LD, the equivalent quantum efficiency equation of the reffection-mode exponential doping cathode is obtained. By using the equivalent equation, theoretical simulation and experimental analysis shows that the equivalent index formula and formula-doped cathode quantum efficiency results in line. The equivalent equation avoids complicated calculation, thereby simplifies the process of solving the quantum efficiency of exponential doping photocathode.

Quantum efficiency and thermal emittance of metal photocathodes

Modern electron beams have demonstrated the brilliance needed to drive free electron lasers at x-ray wavelengths with major advances occurring since the invention of the photocathode gun and the realization of emittance compensation. These state-of-the-art electron beams are now becoming limited by the intrinsic thermal emittance of the cathode. In both dc and rf photocathode guns details of the cathode emission physics strongly influence the quantum efficiency and the thermal emittance. Therefore improving cathode performance is essential to increasing the brightness of beams. It is especially important to understand the fundamentals of cathode quantum efficiency and thermal emittance. This paper investigates the relationship between the quantum efficiency and the thermal emittance for metal cathodes using the Fermi-Dirac model for the electron distribution. We use a consistent theory to derive the quantum efficiency and thermal emittance, and compare our results to those of others.

Single-photon counting at 950–1300 nm: using InGaAsP photocathode–GaAs avalanche diode hybrid photomultiplier tubes

We describe the single-photon counting performance of a hybrid photomultiplier tube (HPMT) near-infrared (950–1300 nm) detector with a transfered electron InGaAsP photocathode and a GaAs Schottky avalanche diode anode. These devices have a lower photoelectron multiplication gain than conventional photomultiplier tubes, but offer a greater linear dynamic range and electrical bandwidth. With the use of a low-noise preamplifier, they can detect single photons with a greater than 20% quantum efficiency (QE) and a reasonably low dark-noise count rate. The avalanche diode at the anode operates in a low gain analog mode and has no afterpulsing. As a result, these HPMTs can detect single photons continuously at high count rates without gating. The relatively large photocathode active area (1 mm diameter) is also attractive to many applications including laser altimetry, ranging, and free-space communications through the atmosphere. We measured 25% photocathode QE and nearly the same single-photon detection efficiency at 1064 nm wavelength with a dark count rate of 60,000 per second at −22°C. The output pulse width in response to single-photon detection is about 0.8 ns. The maximum count rate exceeded 100 million counts per second and was limited only by the speed of the electronics. The rms timing jitter of the HPMT output was measured to be about 0.5 ns. The jitter is dominated by the electron diffusion time within the photocathode and can be improved by reducing the photocathode thickness at a small loss in photocathode QE. We evaluated several of these HPMTs and detailed measurement results are reported in this paper.

Study on the model of quantum efficiency of reflective varied doping GaAs photocathode

The theoretical model of quantum efficiency of the reflective varied doping GaAs photocathode is proposed, which adopts the weighted mean method based on the quantum efficiency formula of exponential doping and uniform doping photocathode. By this model the quantum efficiency curves of a reflective GaAs photocathode under varied doping structure have been fit and we obtained the precise best fit in different incident photon wavebands. The phenomena that the contributions of doping structures to the cathode quantum efficiency are different was discovered. This phenomena has been analysed and discussed detailedly in the paper.

Calculation of quantum efficiency of alkali halide photocathode materials in the extreme ultraviolet region

In order to meet the requirement of microchannel plate position-sensitive photon counting detector in the extreme ultraviolet region, the quantum efficiency of alkali halide photocathode was studied. Based on the fact that the photoemission from photocathode is formed by secondary electrons, the theoretical model of secondary electron emission for alkali halide photocathode was presented， and the formula of secondary electron yield was obtained. For photon energy ranging from 30 to 250 eV, the effect of the thickness of photocathode and the incidence angle on the secondary electron yield was calculated and analyzed，the result showed that when the thickness was higher than 100 nm and the grazing angle was larger than the critical angle of photocathode material, high secondary electron yield was obtained. Finally, according to the derived formula, the spectrum response of the secondary electron yield for twenty alkali halides was analyzed， and the result showed that the position of the peak yield was in accordance with the resonant absorption of photocathode materials.

KM3NeT: Optimization studies for a cubic kilometer neutrino detector

In the KM3NeT Collaboration an important task is the simulation of the detector performance which aims at the optimization of the detector design. In this work the effect of two important items are studied: the PMT photocathode quantum efficiency and the effect of the bar length of a three-dimensional detection units. Simulation studies including the improved PMT photocathode quantum efficiency show that, especially at energy below 100 TeV, the increasing of the neutrino effective area is important. Moreover, simulations that optimize the geometry of one of the detection units proposed in KM3NeT are presented. In particular an increase in the effective areas for longer bars is observed.

An overview of the status of the LHCb RICH detectors

The LHCb experiment will make precision measurements of CP violation and rare b-hadron decays. Efficient particle identification with high purity over a wide momentum range is vital to these aims. The experiment employs two ring-imaging Cherenkov (RICH) detectors with three radiators, silica aerogel, C 4F 10 and CF 4, to cover the momentum range from around 1–100 GeV/ c. The RICH system employs a number of innovative techniques, both in hardware and software. A total of 484 custom-built pixel Hybrid Photon Detectors (HPDs) will be used to measure the spatial positions of Cherenkov photons with wavelengths in the range 200–600 nm, covering an active area of around 3.3 m 2. The production of the HPDs has now been completed and the tube quality, including the photo-cathode quantum efficiency, far exceeds expectations. The installation of RICH 1 is almost complete; RICH 2 has been installed and aligned, and commissioning is almost complete. Reconstruction studies incorporating realistic backgrounds indicate excellent kaon efficiencies of around 97% and pion misidentification probabilities of around 6%, averaged over the full momentum range. This paper provides a general overview of the status of the LHCb RICH project, with emphasis on the readiness for LHC start-up and for physics.