Photocathode Thickness Research Articles

Theoretical calculation of response sensitivity of CsI photocathode of soft X-ray streak camera

The response sensitivity of CsI photocathode is an important parameter for the quantitative diagnosis of X-ray spectroscopy by soft X-ray streak camera, and its theoretical calculation has important guiding significance. The current theoretical analytical models are based on the one-dimensional random walking model of secondary electrons generated by thin film photocathodes, including the Henke model under the condition of normal incidence of X-rays and energy greater than 1 keV, and the Fraser model under variable angle incidence and photocathode thickness greater than 100 nm, which have certain limitations. In this paper, the basic expression of the probability of secondary electron transmission is introduced, and the general expression of the response sensitivity of CsI photocathode is deduced, which varies with X-ray energy <i>E</i>, photocathode thickness <i>t</i>, and angle <i>θ</i> between X-ray and cathode surface in a larger parameter range (X-ray energy 0.1–10 keV, photocathode thickness 10–200 nm). Finally, the theoretical calculation results of this paper are discussed by comparing it with those from the Henke model, Fraser model, literature data and experimental data on Beijing synchrotron radiation facility, which verifies the accuracy and universality of the computational model, and a theoretical reference is provided for optimally designing the CsI photocathode in high-time-resolution spectrometric quantitative measurement.

Fabrication and characterization of silver polycrystalline photocathode

Silver is considered the most noble metal after gold, which makes it a potential candidate for the metallic photocathode. In this article, we report on the fabrication of a polycrystalline silver photocathode by using a physical vapor deposition (PVD) technique. For the first time, silver photocathodes in both the reflection and transmission modes were fabricated on a super quartz (JGS1) substrate and characterized. The polycrystalline nature of the fabricated sample was confirmed by X-ray diffraction (XRD) analysis. The work function of the fabricated photocathode was measured at around 4.73 eV using photoemission yield spectroscopy in the air (PYSA). Photocurrent measurement and quantum efficiency (Q.E.) calculations were performed using a picoammeter and a 250 nm high power ultraviolet (UV) LED source. Silver photocathode yielded a Q.E. of (8.77±0.90)×10−6 in reflection mode, and (1.20±0.12)×10−6 in transmission mode. After being exposed to air for up to 72 h, no sign of degradation in its photocurrent/Q.E. was observed. The silver photocathode thickness was estimated to be (138 ± 5) nm for the reflection mode and (35 ± 1) nm for the transmission mode. The developed photocathode will be ultimately used for the study of the proposed novel photodetector concept called silicon photomultiplier tube (SiPMT).

Calculation of CsI photocathode spectral response in 10-100 keV X-ray energy region

CsI photocathode is widely applied to high energy X-ray detection. And the spectral response is an important character of CsI photocathode. In this paper, the interaction process of high energy X-ray with CsI is analyzed and the spectral response of CsI photocathode is calculated in a 10-100 keV range. The influences of Compton scattering, X-ray fluorescence radiation and Auger emission on the spectral response are analyzed in accordance with the physical process of high energy X-ray interaction with CsI photocathode. These influences prove to be negligible in comparison with photo-ionization influence. Thus only the photoelectric transition is taken into account in calculation. According to the analyses of the processes of the photoelectron creation, transition and escaping, the formula for CsI spectral response is deduced as a function of secondary electron mean escape depth and photocathode thickness. The formula of secondary electron mean escape depth is then deduced as a function of X-ray energy. These formulae indicate that the mean escape depth of the secondary electrons increases markedly with the rise of X-ray energy and has a remarkable influence on the CsI spectral response. The spectral responses for different CsI thickness values are then calculated in a range of 10-100 keV. The results show that 1000 nm CsI has the best response under 20 keV, while 10000 nm CsI has a higher response over 60 keV. Then the calculation data are compared with experimental data of Hara's and Khan's hard X-ray streak camera measurements. These data agree well with each other and prove that our calculation of CsI spectral response for high energy X-ray is reliable. The spectral responses to CsI thickness for 17.5 keV and 60 keV are also calculated and shown in figures. These calculation data match experimental data of Frumkin and Monte-Carlo simulation data of Gibrekhterman. The measurement error of Frumkin's experiment and the uncertainty of the secondary electron mean escape depth are considered to be the reasons for the deviations of calculation and experimental data. The figures of spectral responses to CsI thickness also reveal the optimal thickness values of CsI for different X-ray photon energies. It is shown that 1 m is the optimal thickness for 17.5 keV X-ray detection, and 10 m is optimal for 60 keV. Finally the spectral response of CsI photocathode in a 10-100 keV range is calculated and the formulae prove to be reliable. According to these formulae and calculations, the optimal thickness of CsI photocathode can thus be given for designing and optimizing the high energy X-ray imaging detectors.

Transport characteristic of photoelectrons in uniform-doping GaAs photocathode

The transport of photoelectrons in a uniform-doping transmission-mode GaAs photocathode is calculated by establishing the models of atomic configuration and ionized impurity scattering. And the influence of the doping concentration of photocathode, the photocathode thickness, the electron diffusion length on the diffused circle and the ratio of the number of photoelectrons reaching the emit-surface to the number of exited photoelectrons at the back-interface of GaAs photocathode are analyzed. The calculated results show that the limiting linear resolution is 769 mm-1 with the cathode thickness being 2 m, the electron diffusion length 3.6 m and the uniform-doping concentration 11019 cm-3. The research on the transport of photoelectrons is worthwhile for preparing the high-performance GaAs cathode and improving the resolution of intensifier image.

Characterization of x-ray photocathode in transmission mode for imaging application

The performance of an Al x-ray transmission photocathode was experimentally characterized by measuring the total electron yield from backsurface photocathodes of varying thickness. It was demonstrated that the backsurface electron yield is proportional to the x-ray photocurrent transmitted through the photocathode for thicknesses thicker than the optimum thickness. The optimum photocathode thickness with the highest conversion efficiency was found to be approximately 70nm at 2.963keV. An escape depth of the secondary electrons was determined to be approximately 13nm from the yield-versus-thickness data fitted with a semiempirical equation. Using this parameter, the dependence of the optimum photocathode thickness on the x-ray energy was calculated for the 30–10000eV range. As one example of imaging applications, different photoemission images in the transmission and reflection modes are also presented.

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.

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.

Low-temperature study of 35 photomultiplier tubes for the ZEPLIN III experiment

A set of 35 photomultiplier tubes (ETL D730/9829Q), intended for use in the ZEPLIN III Dark Matter detector, was tested from room temperature down to −100°C, with the aim of confirming their suitability for detecting xenon scintillation light at 175 nm while immersed in the cryogenic liquid. A general improvement of both gain and quantum efficiency at the xenon scintillation wavelength was observed with cooling, the best combined effect being 40%, while little change was noted in the timing properties and dark current. Saturation of response due to accumulation of charge in the resistive bialkali photocathodes was seen at an average photocurrent of 10 8 photoelectrons/s for the device with best quantum efficiency, whereas an order of magnitude higher current was required to saturate the least sensitive one. Variations in photocathode thickness from tube to tube could account for this behaviour, as well as the fact that the quantum efficiency improves the most for devices with poorest efficiency at room temperature.

Absorption of optical power in an S-20 photocathode

By considering a monochromatic plane wave obliquely incident upon a planar layer of S-20 photocathode material, deposited upon a non-absorbing glass substrate, the distribution of optical power absorbed within the layer can be resolved. This is important to the question of photocathode efficiency, as the absorbed light excites photoelectrons within the photocathode which then may pass from the photocathode into the vacuum of the photomultiplier tube and be collected and multiplied. The calculation uses the measured complex permittivity of an extended red S-20 photocathode in the wavelength range, 375–900 nm. The results show that thin film effects are important within the photocathode, as they give rise to interesting power absorption profiles. This information is invaluable in predicting optimum photocathode thickness for wavelength selective applications. Electromagnetic waves that are obliquely incident upon the photocathode are also considered in both transverse electric and transverse magnetic polarizations.

10. Optical method for investigating alkali antimonide photocathodes

Some physical aspects of S-type photocathodes have been studied for many years in different laboratories. Among these investigations, optical methods are of great importance. As a matter of fact, an optical measurement can be used for investigating alkali antimonide photocathode deposition inside an actual tube. For instance, optical reflectance and/or transmittance of the layer measure during processing can be performed. It is shown that the optical reflectance gives information about several physical parameters of the photocathode under growth: o (i) the photocathode thickness can be continuously measured and, therefore the growth speed of the layer controlled; (ii) simultaneously, some composition information may be obtained. The mixture of two phases such as SbNa 3 and SbNa 2 K can be analysed as well as the deposition of pure Sb or SbNa 3 layers on the top of an SbNa 2 K layer. Moreover, chemical displacement between two alkalis inside the compound itself can be observed in this way. Whatever the kind of alkali antimonide photocathodes considered, it will be shown that the optical reflectance and/or transmittance are very worthwhile tools to control the growth and its reproducibility, as complementary measurements to photoemissivity generally used to monitor the growth of S-type photocathodes.

LPE GaAs/(Ga, Al)As/GaAs transmission photocathodes and a simplified formula for transmission quantum yield

GaAs/(Ga,Al)As/GaAs transmission photocathodes were fabricated by a multiple layer liquid epitaxy technique using GaAs substrates. The GaAs substrate was subsequently removed by bubble etching. Transmission mode white light sensitivity of 340 μA/lm and an electron diffusion length of 5 μm or more can be routinely achieved. The reflection sensitivity was found to be lower than that of a similarly prepared layer grown directly on the GaAs substrate. The spectral response, when analyzed with the yield formula, indicated negligible recombination at the GaAs / (Ga,Al)As interface. Also, despite the almost perfect lattice match between GaAs and AlAs, some bowing of the GaAs / (Ga,Al)As samples was observed, suggesting the presence of internal stresses. A simplified formula is derived for the transmission quantum yield which enables graphic solution of surface recombination velocity at the photocathode input surface and the ratio of photocathode thickness to the electron diffusion length without tedious computer calculations.

Interference-Enhanced Photoemission

In order to study the enhancement of the ir response of photocathodes, a model describing the effects of absorption and interference within a semiconductor photocathode supported by a reflecting metallic substrate is developed. It is shown that the addition of a dielectric spacer between the metallic substrate and the cathode emitter to give a combined optical thickness of quarter wavelength enhances the quantum efficiency of photocathodes over a wider spectral range, and extends their maximum response toward the ir. With the choice of dielectric spacer thickness established by an optimum photocathode thickness, a broad spectral response is obtained with reasonably thin dielectric and thin photocathode layers. The absorption coefficient very strongly determines the degree of enhancement which is observed to increase rapidly at the longer wavelengths.