CsTe Photocathodes Research Articles

Mean transverse energy, surface chemical and physical characterization of CERN-made Cs-Te photocathodes

Cesium telluride photocathodes are known to offer high quantum efficiencies under UV illumination combined with good lifetimes compared to other semiconductor photocathodes, making them very popular electron sources for particle accelerator applications. The development of photocathode preparation, characterization, and related expertise at a single accelerator laboratory can be challenging, expensive, and time consuming. Recognizing this, we explored the use of a custom-designed ultrahigh vacuum suitcase for transportation of CERN-made (Switzerland) cesium telluride photocathodes to Daresbury Laboratory (UK) for characterization. We report the synthesis and characterization of a batch of four cesium telluride photocathodes corresponding to our second attempt of transport, following design and process improvements through lessons learned from our first attempt. The photocathode characterization involved, where possible, measurements of the surface elemental composition using x-ray photoelectron spectroscopy (XPS), surface roughness with an in-vacuum scanning tunneling microscope (STM), and quantum efficiency (QE) measurements. Transverse energy distribution curves were obtained over a wide range of illumination wavelengths using the transverse energy spread spectrometer (TESS) at room- and cryogenic temperatures, and the values for mean transverse energy (MTE) were extracted. The photocathodes exhibited distinct thicknesses ranging from ∼50 to ∼120 nm and significant MTE beyond the photoemission threshold which is attributed to the presence of CsxO and Cs phases, as confirmed by XPS analysis. The photocathode that exhibited no carbon or oxygen contamination was measured to have the highest QE of 2.9% at a wavelength of 265 nm at the end of the performance characterization process. The results presented herein offer an insight into the achievements possible through international collaborations by successfully utilizing long-distance transportation of photocathodes by land under ultrahigh vacuum conditions. Published by the American Physical Society 2024

Cs-Te photocathode preparation with Te intermittent and Cs continuous deposition based on improved preparation success rate and quantum efficiency

In order to prepare high-quantum-efficiency semiconductor Cs-Te photocathode which can produce a high-quality electron source, based on the INFN-LASA Cs-Te photocathode preparation method, the Cs-Te photocathode preparation method with Te intermittent, Cs continuous deposition is developed. The Cs-Te photocathode with quantum efficiency greater than 5% under 265 nm UV irradiation is successfully prepared in the photocathode preparation device of SINAP and SARI, and the fabrication success rate reaches 100%. As long as the preparation chamber vacuum degree is better than 10<sup>–8</sup> Pa, the Cs-Te photocathode with high quantum efficiency can be prepared by this preparation method, which will not be different due to the changes of preparation equipment and operators.

Study on Durability Improvement of Cs-Te Photocathode by Means of Alkali Halide Protective Films

Study on Durability Improvement of Cs-Te Photocathode by Means of Alkali Halide Protective Films

Beam brightness from Cs–Te near the photoemission threshold

We describe measurements of the mean transverse energy (MTE) of Cs–Te photocathodes near the photoemission threshold. The MTE displays an unexpected non-monotonic behavior as the drive laser's wavelength is tuned to threshold and changes significantly as the photocathode is cooled to cryogenic temperatures. We show that a simple analytical model of photoemission from multiple compounds with a work function below that of pure Cs2Te may describe this behavior. We identify the additional compounds as Cs5Te3 and metallic Cs, and by calculating the MTE numerically within the three step model, we reproduce both the wavelength and temperature dependence of the observed MTE. In our model, the MTE changes with temperature arise from realistically small changes in the workfunctions of both compounds and Cs5Te3's bandgap energy. These results suggest the existence of an illumination wavelength that is optimal for beam brightness and show that even trace impurities can dominate the MTE for near-threshold photoemission.

Durability improvement of Cesium Telluride photocathode for an rf-gun

At Waseda University, we have been studying for high quality electron beam generation using 1.6 cell Cesium Telluride (Cs-Te) photocathode rf-gun. We use photocathode as the electron source, which can generate high-quality electron beam such as low emittance, and short bunch. The performance of photocathode is evaluated mainly in terms of quantum efficiency (Q.E.) and lifetime. Cs-Te photocathode is known for high Q.E. about 10% with UV light and relatively longer lifetime among semiconductor photocathodes. Since it is a hard environment for photocathode inside the gun, it is necessary to replace the photocathode every several months. In other words, in order to achieve long-term operation of rf-gun, it is necessary to establish highly durable photocathode recipe. In this report, we report the evaluation results of Cs-Te photocathode by co-evaporation, CsBr coated photocathode performance in an rf-gun and future prospects.

Study on a long-life photocathode with an CsBr protective layer for an rf electron gun

A photocathode with a high quantum efficiency (Q.E.) and long lifetime can be fabricated by evaporating a protective layer onto a high Q.E. semiconductor photocathode. In this study we tried to improve the stability and lifetime of a Cs-Te photocathode by coating a CsBr protective layer of 20 nm in thickness over the surface. After the protective layer was applied, we measured the Q.E. of the photocathode and the lifetime in the evaporation chamber. The Q.E. of the coated Cs-Te was confirmed to be about 0.80% and to remain unchanged for a period of longer than 25 d. The effectiveness of the protective layer against exposure of the Cs-Te photocathode to oxygen gas was also confirmed for the first time. Our experiments were the first to show a lifetime improvement of a photocathode installed inside an rf gun compared to the same type of photocathode with no protective coating.

Novel single photon detectors for UV imaging

In this study we present an UV single-photon imaging detector based on a thin-gap Resistive Plate Chamber, featuring a very good position resolution of 30 μ m in a digital readout mode and a reasonable quantum efficiency of 0.3–8% in the spectral interval 200–140 nm. These detectors are inherently spark-protected and can operate without feedback problems at gas gains up to 10 5 . Attempting to extend the sensitivity of gaseous detectors to longer wavelengths, we successfully tested the first sealed parallel-plate gaseous detectors with CsTe photocathodes, reaching a gas gain of 10 3 .

Gaseous photomultipliers with solid photocathodes for the detection of sparks, flames and dangerous gases

In many applications, it is necessary to detect sparks or flames in daylight conditions or in illuminated areas. Most flames emit strongly in the ultraviolet spectrum (180–280nm), and this property can be used for reliable identification of flames. We have developed new spark and flame detectors based on gaseous photomultipliers with CsI, CuI or CsTe photocathodes. A modified version of the detector can also detect smoke and dangerous vapors. These detectors are able to perform complex monitoring and detection functions. Some of their advantages are: low cost, high sensitivity, large output signal and operation under battery power. Gaseous photomultipliers can be position sensitive and, if necessary, be used in combination with various optical systems, for example for monitoring flames from space.

Absolute number of scintillation photons emitted by alpha particles in rare gases

In order to determine absolute scintillation yields due to alpha particles in high pressure rare gases, the number of scintillation photons N/sub p/ was measured by using a VUV sensitive photodiode (PD) with a MgF/sub 2/ window and a Cs-Te photocathode with spectral quantum efficiency q/sub e/(/spl lambda/) measured as a function of wavelength /spl lambda/. The number of photoelectrons from the photocathode N/sub pe/ was measured absolutely by using charge-sensitive preamplifier calibrated in numbers of electrons. A collection efficiency F/sub ce/ at the photocathode for scintillation photons can be determined from solid angles subtended by the photocathode at a scintillation point under the condition that there is no photon reflected at surrounding wall. Then, N/sub p/ was determined from N/sub p/=N/sub pe//(Q/sub e/F/sub ce/), where Q/sub e/ is effective quantum efficiency calculated from q/sub e/(/spl lambda/) and a relative intensity I(/spl lambda/) of scintillation in rare gases at wavelength /spl lambda/. Although luminescence spectrums from rare gases emitted by radiation have been measured by many researchers, these spectrums were scarcely corrected by an efficiency of apparatus (e.g. efficiency of monochromator and scintillation detector) for /spl lambda/. In order to exactly determine the luminescence spectrums, these were also measured on our own terms. And, since it was reported that scintillation intensity from rare gases change with a pressure of rare gases, this experiments was carried out in a pressure range from 1.0/spl times/10/sup 5/ Pa to 1.0/spl times/10/sup 6/ Pa. The measurements were carried out in gaseous argon, krypton and xenon. In xenon of 1.0/spl times/10/sup 5/ Pa, N/sub p/ was measured to be 1.6/spl times/10/sup 5/.

N2 Lyman‐Birge‐Hopfield dayglow from ATLAS 1

Spectral measurements of the far ultraviolet dayglow were made from the ATLAS 1 shuttle mission using an intensified‐CCD imaging spectrometer array. The instrument imaged relatively large (177 Å) segments of the spectrum simultaneously and had the capability to image altitude in the second dimension of the two‐dimensional focal plane detector when the entrance slit of the instrument was oriented perpendicular to the limb of the Earth. The FUV channel of the instrument used a CsTe photocathode rather than the more conventionally used and more solar blind CsI photocathode material. As a result, the spectra were also measured with good sensitivity at FUV wavelengths longer than 1500 Å. In this paper, data are selected of specific interest to a study of the N2 Lyman‐Birge‐Hopfield system using two shuttle attitudes: In the first the field of view was held at a fixed tangent ray height of ∼140 km while scanning in wavelength, providing repeated FUV spectra over a range of latitudes and allowing comparison with our thermospheric airglow model. In the second observing sequence the line of sight of the instrument was scanned down through the atmosphere but covered about 75% of the full wavelength range. This sequence allows comparison of height dependence, including O2 absorption effects, with the thermospheric model. In all cases, relatively good agreement is obtained with the model (run strictly in a predictive mode) in comparisons in which the line of sight must be integrated along a path that spans a considerable range of conditions (height, local time, latitude, solar zenith angle). In the case of the data sets examined in this study the vibrational population distributions show mixed results. In one case the distribution agrees well with a theoretical model based on direct electron impact excitation without a significant cascade contribution. A second case indicates a lower population in ν′ = 3 and a higher population in ν′ = 6 than would be expected from direct impact excitation alone. This supports previous indications that conditions resulting in the cascade mechanism may not always be operative.

Test of BaF 2 scintillators coupled to a photomultiplier with a CsTe photocathode

Basic properties of BaF 2 scintillators coupled to a photomultiplier with a CsTe photocathode have been investigated. The signal from the photomultiplier shows a drastic reduction of the slow component of the BaF 2 scintillation light while preserving the good properties associated with the fast component.

Calibrating the light pulse shape of a hydrogen flashlamp using synchrotron radiation as a standard of excitation

Advantage has been taken of the measured pulse width of synchrotron radiation and its independence of wavelength to determine the delta-pulse response of a vacuum uv photomultiplier. This photomultiplier was then used to establish the true time profile of a nanosecond H(2) flashlamp. Two numerical techniques (the exponential series method and the fast Fourier transform method) were used to deconvolute the data arising from these experiments. The results indicate that the H(2) flashlamp probably has the same profile in the many-line region, lambda < 1800 A, and in the continuum region, lambda > 2100 A, and the delta-pulse response of the PMT appears consistent with known properties of the Cs-Te photocathode.

Performance Characteristics of Proximity Focused Ultraviolet Image Converters

Performance characteristics of Bendix type BX 8025-4522 proximity focused image tubes for uv to visible light conversion are presented. Quantum efficiency, resolution, background, geometric distortion, and environmental test results are discussed. The converters use magnesium fluoride input windows with Cs-Te photocathodes and P-11 phosphors on fiber optic output windows.

Solid Organic Filters for 2000 Å to 3000 Å

Earlier work on the development of organic, narrow bandpass solution filters with transmittance maxima in the 2000-3000-A range is extended to the fabrication of solid filters using a polymethyl siloxane resin as the host material. Three transmission filters are described having maxima at 2250 A, 2265 A, and 2840 A, respectively, with bandwidths <or=225 A. Transmittance and spectral response curves with CsTe photocathodes are given. Results of environmental tests show that these filters are well suited for balloon and space missions.