Optical and photoemissive properties of multi-alkali photocathodes

Beguchev, V. P.; Shefova, I. A.; Musatov, A. L.

doi:10.1088/0022-3727/26/9/025

Cited by 10 publications

(5 citation statements)

References 3 publications

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“…E e was expected to increase by 0.15 eV when the cathode has been cooled to the temperature of liquid nitrogen based on Ref. [20]. By using this analytical model, when irradiated with a 532 nm laser, a 50% drop in QE could be expected when the cathode is cooled from room temperature to that of liquid nitrogen.…”

Section: Analytical Modelmentioning

confidence: 92%

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Experimental measurements and theoretical model of the cryogenic performance of bialkali photocathode and characterization with Monte Carlo simulation

Xie

Ben‐Zvi

Rao

et al. 2016

Phys. Rev. Accel. Beams

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High-average-current, high-brightness electron sources have important applications, such as in highrepetition-rate free-electron lasers, or in the electron cooling of hadrons. Bialkali photocathodes are promising high-quantum-efficiency (QE) cathode materials, while superconducting rf (SRF) electron guns offer continuous-mode operation at high acceleration, as is needed for high-brightness electron sources. Thus, we must have a comprehensive understanding of the performance of bialkali photocathode at cryogenic temperatures when they are to be used in SRF guns. To remove the heat produced by the radio-frequency field in these guns, the cathode should be cooled to cryogenic temperatures. We recorded an 80% reduction of the QE upon cooling the K 2 CsSb cathode from room temperature down to the temperature of liquid nitrogen in Brookhaven National Laboratory (BNL)'s 704 MHz SRF gun. We conducted several experiments to identify the underlying mechanism in this reduction. The change in the spectral response of the bialkali photocathode, when cooled from room temperature (300 K) to 166 K, suggests that a change in the ionization energy (defined as the energy gap from the top of the valence band to vacuum level) is the main reason for this reduction. We developed an analytical model of the process, based on Spicer's three-step model. The change in ionization energy, with falling temperature, gives a simplified description of the QE's temperature dependence. We also developed a 2D Monte Carlo code to simulate photoemission that accounts for the wavelength-dependent photon absorption in the first step, the scattering and diffusion in the second step, and the momentum conservation in the emission step. From this simulation, we established a correlation between ionization energy and reduction in the QE. The simulation yielded results comparable to those from the analytical model. The simulation offers us additional capabilities such as calculation of the intrinsic emittance, the temporal response, and the thickness dependence of the QE for the K 2 CsSb photocathode.

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Section: Analytical Modelmentioning

confidence: 92%

“…Based on prior literature [20], the band gap of K 2 CsSb increases with the lowering of temperature. To determine the QE as a function of temperature, we used the band gap as a fitting parameter.…”

Section: Monte Carlo Simulationmentioning

confidence: 96%

Experimental measurements and theoretical model of the cryogenic performance of bialkali photocathode and characterization with Monte Carlo simulation

Xie

Ben‐Zvi

Rao

et al. 2016

Phys. Rev. Accel. Beams

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“…Photomultiplier (PM) tubes have existed for more than half a century and continue to play a key role in photon detection throughout modern science. Nevertheless, as currently produced and operated with alkali and semiconductor photocathodes [1][2][3][4][5][6], they perform well below their real potential [7][8][9]. Therefore, particularly with the advent of other high performance types of photon detectors, there is an incentive to reconsider their design so as to exploit more 4 Author to whom any correspondence should be addressed.…”

Section: Introductionmentioning

confidence: 99%

Optimization of photomultiplier spectral sensitivity with a graded thickness photocathode

Townsend

Valberg

Momchilov

et al. 2008

J. Phys. D: Appl. Phys.

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The efficiency of S20 multialkali photocathodes is a function of thickness, wavelength, polarization and angle of incidence of light into the cathode. Therefore no single set of conditions can maximize performance over the entire spectral range. Consequently, two prototype photocathodes have been made with a gradation in thickness so that during monochromatic analysis the optical beam can be addressed to different thickness regions at preferred angles of incidence. This potentially enables a spectrum to be recorded under optimal conditions at a single interaction event. Relative to normal commercial S20 photomultipliers the quantum efficiency (QE) has been significantly raised across the entire spectral range. The current data were primarily obtained between 450 and 800 nm and at 450 nm it resulted in values of at least 65% QE, which is the highest value ever cited at this wavelength. Signals at longer wavelengths, for example at 750 and 800 nm, were recorded with up to 20 and 10% QE, respectively. Once again these are new record values that match performance from multiple interactions in waveguide cathodes. The data from this new design of photocathode underline the potential for improvements in efficiency for non-normal incidence in graded thickness photocathodes and indicate that current S20 technology could be significantly enhanced. Alternative enhancement methods are mentioned, particularly for spectrally dispersed signals. The enhancements are compared with data for a standard high quality S20 photocathode.

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“…Over the last 60 years new cathode materials have enhanced the sensitivity and offered detection of lower energy photons. Much of the earlier work is mentioned in a recent review [1] or in items cited there [2][3][4][5][6][7][8][9]. One of the most common red sensitive photocathodes uses a multialkali composition, and is termed the S20.…”

Section: Introductionmentioning

confidence: 99%

Designs for waveguide and structured photocathodes with high quantum efficiency

Townsend¹,

Downey²,

Harmer³

et al. 2006

J. Phys. D: Appl. Phys.

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Conventional S20 multialkali photocathodes have a wide wavelength coverage from < 200 to > 850 nm, but their high transparency and the surface work function result in low quantum efficiencies at longer wavelengths. Theoretical modelling of the photon and excited electron interactions that define the cathode performance provides a realistic prediction of the measured response. The theory emphasizes that the basic light absorption is strongly sensitive to the cathode thickness, wavelength, polarization and incident angle. Parameters can be selected which predict that even at long wavelengths (e.g. 900 nm), absorption may be increased from ∼1% to ∼100%. Cathode topographies can be designed to exploit these responses and offer increased absorption at the longer wavelengths. Alternative designs, which include waveguiding of light within the cathode window, or in structured surfaces, can similarly lead to almost total absorption of the incident light by increasing the number of interactions. These concepts of optimal incidence and waveguiding have been both theoretically modelled and demonstrated in newly fabricated cathode designs. The methods have variously reached quantum efficiencies in excess of 50% at wavelengths in the range from 200 to > 750 nm under different operational conditions. The improvement factors relative to normal incidence on planar cathodes increase for longer wavelengths, and examples of 20–50 times by ∼900 nm were noted. Whilst the absolute S20 efficiency values at long wavelengths are still small, the improvements offer a usable sensitivity even beyond 1 µm, as demonstrated by spectroscopy data up to at least 1140 nm.

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Optical and photoemissive properties of multi-alkali photocathodes

Cited by 10 publications

References 3 publications

Experimental measurements and theoretical model of the cryogenic performance of bialkali photocathode and characterization with Monte Carlo simulation

Experimental measurements and theoretical model of the cryogenic performance of bialkali photocathode and characterization with Monte Carlo simulation

Optimization of photomultiplier spectral sensitivity with a graded thickness photocathode

Designs for waveguide and structured photocathodes with high quantum efficiency

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