Dark current and sensitivity measurements for structured S20 photocathodes

Abstract

Measurements of the temperature dependence of the dark current for standard commercial photomultipliers with multialkali photocathodes (type S20) have been contrasted with data from new prototype tubes with greatly enhanced long wavelength performance. The analyses suggest that for the larger structures part of the cathode is closer to a bialkali composition. This reduces the activation energy of the dark current from ∼1.1 eV for normal S20 cathodes down to ∼0.8 eV. Many tubes show a lower activation energy component near 0.6 eV which may represent a sensitization process from monolayers of molecular oxygen. Similarly, monolayers of water can play a significant role, but do so by reducing cathode sensitivity. Several tubes display a sharp transition in gain at the limiting temperature for water desorption (i.e. a sublimation transition). The commercial structured cathodes can show evidence for up to 70% improvements in quantum efficiency by operation at low temperature which can be immediately exploited. Prototype tubes with greatly improved performance were studied but for very small area illumination spots there can be problems of local variations in gain and wavelength response across the structural features of pyramidal modified cathode windows. Overall the data suggest that not only is improved performance already available with existing photomultiplier tubes but also with structured photocathode windows, and/or the introduction of surface ‘defects’ in the window, even further improvements are feasible.

First test of a cryogenic scintillation module with a CaWO4 scintillator and a low-temperature photomultiplier down to 6 K

“…It increases with cooling in the short wavelength range [2, 9,26], which is explained by the reduction of lattice scattering of photoelectrons driven towards the surface of the photocathode [2,9]. Conversely, the sensitivity of a bialkali photocathode decreases in the red [4,7,9,23] due to a change of the energy band structure of the photocathode material. Since the emission band of the CaWO 4 scintillator with a maximum at 420 nm overlaps with the central part of the quantum efficiency curve of the bialkali photocathode neither the long wavelength cut-off nor the short wavelength enhancement feature prominently as a temperature effect.…”

“…The dark count rate is predicted to reduce with cooling due to a decrease of thermally induced emission of electrons from the photocathode in the absence of optical stimulation [23]. However this effect is important merely for temperatures of >100 K since the process requires relatively high activation energies.…”

Tutorial on compressed ultrafast photography

“…The quantum yield of the photocathode is moderate for the visible light and decreases dramatically for near-infrared light. 161 , 162 The phosphor screen also has a relatively low conversion efficiency, especially for the fast-responding types. 163 The limited overall efficiency makes the image-converter streak tubes less suitable for imaging faint transient events.…”

“…The efficiency of image-converter streak tubes is also inherently limited by the photon–electron–photon conversion. The quantum yield of the photocathode is moderate for the visible light and decreases dramatically for near-infrared light 161 , 162 . The phosphor screen also has a relatively low conversion efficiency, especially for the fast-responding types 163 .…”

Optimization of photomultiplier spectral sensitivity with a graded thickness photocathode

“…In principle, internal fields need not be limited to the thick semiconductor design cathodes but can exist with the faster response thin S20 photocathodes. For example, we have suggested [14] that fields can be self generated by charge trapping of electrons in defect sites of the glass window, as seen with an example with a cooled S20 tube, which allowed stable charge trapping in the glass and doubled low light sensitivity. This benefit was disguised in conventional QE measurements with very high light intensities.…”