Cesium Telluride Photocathodes Research Articles

Photocathodes for the electron sources at FLASH and European XFEL

FLASH at DESY (Hamburg, Germany) and the European XFEL photoinjectors are operated by laser driven RF-guns. For both user-facilities cesium telluride (Cs2Te) photocathodes are successfully used since several years. We present recent data on the lifetime and quantum efficiency (QE) of the current photocathode at FLASH #105.2, operated before and after a long shutdown. In addition, data for the cathodes that recently have been exchanged at the European XFEL will be presented.

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.

Experimental demonstration of the correction of coupled-transverse-dynamics aberration in an rf photoinjector

The production of electron bunches with low transverse emittance approaches the thermal emittance of the photocathode as various aberrations are corrected. Recently, the coupled transverse dynamics aberration was theoretically identified as a significant source of emittance growth and a corrector magnet was proposed for its elimination [D.H. Dowell, F. Zhou, and J. Schmerge, PRAB 21, 010101 (2018)]. This aberration arises when the beam acquires an asymmetric distribution that is then rotated with respect to the transverse reference axis thus introducing a correlation in the vertical and horizontal planes. The asymmetry is introduced by a weak quadrupole field in the rf gun or emittance compensation solenoid and the rotation is caused by the solenoid. This Letter presents an experimental study of the coupled transverse dynamics aberration in an rf photoinjector and demonstrates its elimination by a quadrupole corrector consisting of a normal and a skew quadrupole. The experimental results agree well with theoretical predictions and numerical simulations. The study also demonstrates the emittance of a low charge beam can be preserved during transportation at its thermal value, which was 1.05 mm mrad/mm, for the cesium telluride photocathode and 248 nm UV laser used.

Observation of two-photon photoemission from cesium telluride photocathodes excited by a near-infrared laser

We explore the nonlinear photoemission in cesium telluride (Cs2Te) photocathodes where an ultrashort (∼100 fs full width at half max) 800-nm infrared laser is used as the drive-laser in lieu of the typical ∼266-nm ultraviolet laser. An important figure of merit for photocathodes, the quantum efficiency, we define here for nonlinear photoemission processes in order to compare with linear photoemission. The charge against drive-laser (infrared) energy is studied for different laser energy and intensity values and cross-compared with previously performed similar studies on copper [P. Musumeci et al., Phys. Rev. Lett. 104, 084801 (2010)], a metallic photocathode. We particularly observe two-photon photoemission in Cs2Te using the infrared laser in contrast to the anticipated three-photon process as observed for metallic photocathodes.

Stand-Off Radioluminescence Mapping of Alpha Emitters Under Bright Lighting

Remote detection of alpha emitters is achieved by measuring the secondary radioluminescence light (air fluorescence) that is induced by alpha particles when absorbed in air. A telescope was used to collect the radioluminescence photons to a photomultiplier tube, which is operated in the photon counting mode. Careful matching of photocathode response and filter pass-band allows the sensing of a faint radioluminescence emission in a brightly illuminated environment, which is essential for operative use. A minimum detectable alpha activity of 4 kBq was reached at 1 m distance in 10 s time, when ultraviolet-free lighting is present, and 800 kBq under bright fluorescent lighting. These sensitivities are realized using an ultra-bialkali and cesium-telluride photocathodes in the aforementioned environments respectively. The presented approach is a robust and affordable solution to remotely detect and localize moderate alpha activities in a field environment, providing a means for automated alpha contamination mapping. Moreover, it is shown that a signal increase of more than two orders of magnitude (150–420) can be achieved in deep ultraviolet (close to 260 nm), if nitrogen or argon purge are used to enhance the detection.

Measurements of copper and cesium telluride cathodes in a radio-frequency photoinjector

Radio-frequency (rf) photoinjectors are commonly used to generate intense bright electron beams for a wide range of applications, most notably as drivers for X-ray Free-Electron Lasers. The photocathode, mounted inside an rf gun and illuminated by a suitable laser, thereby plays a crucial role as the source of the electrons. The intrinsic emittance and the quantum efficiency of the electron source are determined by the properties of the photocathode's surface material. We present measurements of the intrinsic emittance and the quantum efficiency performed with copper and cesium telluride cathodes in the same rf photoinjector, thus comparing, for the first time, the performance of metal and semiconductor cathodes under the same conditions. Our results are consistent with theoretical expectations and show that the difference in intrinsic emittance for the two types of material is not significant in view of accelerator applications. We conclude that cesium telluride photocathodes provide a much higher quantum efficiency at essentially negligible degradation in beam emittance.

Kelvin probe studies of cesium telluride photocathode for AWA photoinjector

Cesium telluride is an important photocathode as an electron source for particle accelerators. It has a relatively high quantum efficiency (>1%), is sufficiently robust in a photoinjector, and has a long lifetime. This photocathode is grown in-house for a new Argonne Wakefield Accelerator (AWA) beamline to produce high charge per bunch (≈50nC) in a long bunch train. Here, we present a study of the work function of cesium telluride photocathode using the Kelvin probe technique. The study includes an investigation of the correlation between the quantum efficiency and the work function, the effect of photocathode aging, the effect of UV exposure on the work function, and the evolution of the work function during and after photocathode rejuvenation via heating.

Formation and acceleration of uniformly filled ellipsoidal electron bunches obtained via space-charge-driven expansion from a cesium-telluride photocathode

We report the experimental generation, acceleration, and characterization of a uniformly filled electron bunch obtained via space-charge-driven expansion (often referred to as ``blow-out regime'') in an L-band (1.3-GHz) radiofrequency photoinjector. The beam is photoemitted from a cesium-telluride semiconductor photocathode using a short ($<200\text{ }\text{ }\mathrm{fs}$) ultraviolet laser pulse. The produced electron bunches are characterized with conventional diagnostics and the signatures of their ellipsoidal character are observed. We especially demonstrate the production of ellipsoidal bunches with charges up to $\ensuremath{\sim}0.5\text{ }\text{ }\mathrm{nC}$ corresponding to a $\ensuremath{\sim}20$-fold increase compared to previous experiments with metallic photocathodes.

Cs2Te photocathode fabrication system at Peking University

A cesium telluride (Cs2Te) photocathode with a quantum efficiency of 13% at 253.7 nm (radiant incidence 200 μW/cm2) is fabricated by tellurium and cesium vapor deposition onto a stainless-steel substrate. The cesium telluride cathode will be used to provide a high-brightness electron beam source for the 3+1/2 photo-injector at Peking University. The design of the system, the fabrication procedures and the preliminary experimental results are presented in this paper.

Cs–Te photocathode RF electron gun for applied research at the Waseda University

A compact electron accelerator based on photo-cathode RF electron gun is under development for applied research at the Waseda University. Higher charge and higher energy are key issues for applications such as pulse radiolysis experiment and laser-Compton X-ray generation. The new RF-gun cavity with cesium telluride (Cs–Te) photo-cathode which has higher quantum efficiency (Q.E.) compared with metal cathodes has been installed. The evaluation of Q.E. of the photo-cathode has been performed and the value obtained is more than 5% at the preparation stage. For understanding the Cs–Te cathode properties and the higher Q.E. operation, fundamental studies by single bunch beam operation have been performed resulting in more than 6 nC bunch charge with a high Q.E. of 2.9%. Additionally, the new RF-gun cavity was improved for obtaining a higher Q value. Higher beam energy, i.e. shunt impedance compared with old type cavity, was observed. The shunt impedance increased by about 20%. These successful results will considerably contribute to the applied research.

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.

Progress toward a high-brightness photoemission source for multiple-electron beam lithography

A high-brightness photoemission source is being developed for use in a multiple-electron beam lithography system suitable for mask patterning. Cesium telluride (Cs2Te) photocathodes have achieved stable brightness of 2–3×106 A/cm2 Sr and current densities of 30–60 A/cm2. Magnesium photocathodes have achieved stable brightness of 1–3×106 A/cm2 Sr and current density of 40 A/cm2. Long term current stability of better than the required specification of 17%/5 min has been obtained at photoyields of ∼30 nA/mW.

First operation of cesium telluride photocathodes in the TTF injector RF gun

During the run 1998/1999 a new injector based on a laser-driven RF gun was brought in operation at the TESLA Test Facility (TTF) linac at DESY, in order to produce the beam structure and quality required either by TeV collider and SASE FEL experiments. High quantum efficiency cesium telluride photocathodes, prepared at Milano and transferred to DESY, have been successfully operated in the RF gun. A bunch charge of 50 nC, only limited by space charge effects, was achieved. The photocathodes have shown an operative lifetime of several months. A new cathode surface finishing has showed a promising decrease of the photocathode dark current. Measurements of dark current, quantum efficiency and lifetime are reported.

Modern lightweight UV spectrographs for in situ studies of the exosphere and photo-ion population about Mercury

Important resonance transitions of most neutral species, either already known to be present in the exosphere of Mercury or likely to be found there, fall in the UV portion of the spectrum as do the principal resonance transitions of the ions of these same elements. The design and fabrication is presented of a new lightweight UV spectrograph, under development at Southwest Research Institute, which can easily detect these known or expected species. The instrument is an athermal design constructed for the most part of aluminum. It weighs less than 3 kg, and draws less than 3 W of power. The spectral coverage is 1800–3300 Å, which is sufficient to reach resonance lines of all the important species identified in this spectral passband. The predicted spectral resolution for a point source at infinity is ∼9 Å at 1800 Å, ∼13 Å at 2500 Å, and ∼15 Å at 3000 Å ( λ Δλ ∼200 ). The spectrograph employs a 2-D microchannel plate wedge-and-strip detector with either a bi-alkali or cesium telluride (solar-blind) semi-transparent photocathode. For most of the important transitions expected, this instrument should be able to reach detection limits (signal-to-noise ratio ∼5) near 0.1 R in a 10 2 s exposure with a cesium telluride photocathode, or 0.08 R with a bi-alkali photocathode.

Cesium telluride photocathodes

Cesium telluride (Cs2Te) photocathodes, with quantum efficiencies (QEs) of 15%–18% at 251 nm, were fabricated by vapor deposition of Te and Cs onto a Mo substrate and used as an electron source for the Los Alamos Advanced Free-Electron Laser. In the fabrication chamber, the spectral response from 251 to 578 nm was measured before and after a controlled exposure of several photocathodes to air. The 251-nm QE dropped by about a factor of 20 when exposed to 2×10−4 Torr of air for 1 h. Heating degraded photocathodes to 150–200 °C partially rejuvenated their QEs to about 60% of the value before air exposure. The performance of Cs2Te as a source of electrons for accelerators was evaluated in the photoinjector stage of the Advanced Free-Electron Laser. The response time, saturation level, and dark current of cesium telluride photocathodes and the emittance and energy spread of the resulting electron beam were determined to be sufficient for free electron laser applications.

Performance of cesium telluride photocathodes as an electron source for the Los Alamos Advanced FEL

The Los Alamos Advanced FEL was successfully operated with a Cs2Te photocathode driven by a frequency quadrupled Nd:YLF laser as the electron source. Lasing was achieved at 5–6 μm. Cs2Te photocathodes with quantum efficiencies of 15–18% at 251 nm were fabricated in an ultrahigh-vacuum chamber and transferred under high vacuum to the FEL. 263 nm light from the drive laser was focused to an 8 mm spot on the center of the photocathode. We estimated the operational lifetime of Cs2Te photocathodes to be at least 20 times that for K2CsSb photocathodes. We have found that the dark current, emittance of the extracted beam, and amount of charge extracted is sufficient for FEL applications.

The use of synchrotron radiation for detector calibrations

The National Bureau of Standards (NBS) VUV radiometric detector program calibrates photodiode detectors as radiometric transfer standards over the wavelength range between 50 and 2500 Å (5–250 eV photon energy). Our calibration uncertainty is 10% or better over this full range. The source of radiation used for calibration in the spectral region below 600 Å is synchrotron radiation from the NBS Synchrotron Ultraviolet Radiation Facility (SURF) while a duoplasmatron source is used above 600 Å. A noble gas double ionization chamber is used as the absolute detector between 400–1000 Å while a single ionization chamber is used below 400 Å and a thermopile referred to the double ionization chamber is used above 1000 Å. At wavelengths above 1200 Å magnesium flouride windowed photodiodes with cesium or rubidium telluride photocathodes serve as transfer standards. For wavelengths below 1200 Å specially prepared Al 2O 3 photodiodes are used. Their photocathodes are made by evaporating about 1800 Å of aluminum onto a quartz disc. Then a 150 Å thick layer of Al 2O 3 is formed through anodization to serve as the photoemitting surface. The properties of these diodes prove to be very sensitive to contamination especially at wavelengths below 200 Å. It is particularly important that hydrocarbon oils not be cracked on their surface since this tends to form a carbonaneous photoemitting surface. We are also investigating tungsten photocathodes as a possible transfer standard for the shorter wavelengths. In collaboration with groups at LASL and NRL studies have been made of the photoefficiency of several “practical” photocathode materials at wavelengths between 50–584 Å. Results will be shown for natural aluminum, gold, and carbon. The studies include evaluation of the stability of photoefficiency with time and the sensitivity of the photoefficiency to contamination with use in plasma machine diagnostics. In collaboration with DoE and the University of Hawaii a study was made of some high photoefficiency materials for radiometric diagnostic work. Results for CuI and CsI will be reported as will a study of some “disposable” radiometric detectors.

Field-enhancement of photoemission from cesium telluride

The responsive quantum efficiency of semitransparent cesium telluride photocathodes has been found to be significantly increased by the application of high electric fields (a few MV m(-1)) such as are encountered in proximity-focused image converters. Measurements of the magnitude of the effect are presented as functions of applied field and of wavelength over the 120-800 nm range. An interesting anomaly was found in the pair-production region in the vacuum uv; otherwise, the results are fairly consistent with the effects of a simple Schottky lowering of the surface potential barrier of the photoemitter.

Photomultiplier window materials under electron irradiation: fluorescence and phosphorescence

The fluorescence and phosphorescence of photomultiplier window materials under electron irradiation have been investigated using a Sr(90)-Y(90) beta emitter as the electron source. Spectral emission curves of uv-grade, optical-grade, and electron-irradiated samples of MgF(2) and LiF, and of CaF(2), BaF(2), sapphire, fused silica, and uv-transmitting glasses were obtained over the 200-650-nm spectral range. Fluorescence yields, expressed as the number of counts in a solid angle of 2pi sr/MeV of incident electron energy deposited [MeV(-1) (2pi sr)(-1)], were determined on these materials utilizing photomultiplier tubes with cesium telluride, bialkali, and trialkali (S-20) photocathodes, respectively. Typical yields observed with a uv/visible sensitive bialkali cathode range from 10 MeV(-1) (2pi sr)(-1) for uv-grade MgF(2) to approximately 200 MeV(-1) (2pi sr)(-1) for CaF(2). For comparison, sodium-activated cesium iodide, one of the most efficient scintillator materials, yields about 700 MeV(-1) (2pi sr)(-1). High-purity fused silica has the lowest yield, approximately 6 MeVW(-1) (2pi sr)(-1). Optical-grade MgF(2)and LiF, as well as electron-irradiated uv-grade samples of these two materials, show enhanced fluorescence due to color-center formation and associated emission bands in the blue and red wavelength regions. Large variations in fluorescence intensities were found in uv-grade sapphire samples of different origins, particularly in the red end of the spectrum, presumably due to various amounts of chromium-ion content. Phosphorescence decay with time is best described by a sum of exponential terms, with time constants ranging from a few minutes to several days. Phosphorescence intensity expressed as a fraction of the steady-state fluorescence intensity is an extremely sensitive measure of crystalline perfection and purity. This fraction ranges from a high of approximately 10(-2) for some fluoride samples to a low of </=2 x 10(-6) for fused silica. Application of the parameters obtained in this work to the analysis of recent flight observations on low light-level experiments yields good quantitative agreement with flight data from OAO-3, OSO-7, and Apollo 17.