With these problems in mind it was believed that the Picoframe framing camera design could be employed. Initially it was realised that the higher secondary electron emission energy spread of a UV or x-ray-sensitive photocathodes (some 4eV as compared to 0.3eV for standard multi-alkali cathode) would cause problems due to the fact that the electron trajectory paths would stray much further from the electron-optical lens axis causing increased spatial and temporal distortion [4]. It was also pointed out that the electr on beam crossover would have a larger diameter which would
understandably cause problems at the anode aperture due to a reduction in electron transmission. Similarly, the deflection plate structure and framing aperture may all be regions of interest in the evaluation of an improved electron optical lens for the x-ray framing camera. A new design, slightly modified from the visible version was
proposed [4] to overcome some of these problems. This design involved a doubling of the cathode-mesh separation and the applied potential difference, a small reduction in length of the last electrostatic lens cylinder to alter the crossover position and a reduction of the anode aperture-to-framing aperture separation. Although these modifications were implemented the performance of the camera was not improved to any appreciable degree; the frame time was increased by -70%, the magnification was
increased from -1.4 for the visible camera to -1.9 and the spatial resolution was only marginally improved. The problem is that the secondary electron energy spread increase from -0.3eV from a typical S20 photocathode to -4eV for a typical x-ray-sensitive gold photocathode is so large that drastic redesign would have to be undertaken to
substantially improve the performance.
2 5 Construction of the Electron-Optical Lens
The camera geometry adopted was similar to that of the visible-sealed off version which had been previously assessed [1]. The only modification made was to increase the anode aperture to 4mm in order to maintain the usable cathode area while this ensured that the electron beam could hot impinge on the framing deflectors whose separation was maintained. In fact it has been found that the usable cathode area for the demountable camera is quite large (6 mm by 6 mm). The camera electrostatic lens electrodes were constructed out of aluminium and supported on four machinable glass ceramic rads to provide electrical insulation. The cathode plate was mounted using ceramic spacers and nylon bolts to the first cylindrical electrode. The mesh employed was identical to that of the sealed-off tube (60 cells per mm). With a cathode-mesh separation of 2.5mm and potential difference of 5kV the extraction field was very low (20 kV/cm) compared to that of streak tubes and so the camera could be operated at a vacuum of 4x10-5 Torr without any incidence of electrostatic breakdown. The entire lensing section was placed in an aluminium cylinder of 20cm diameter.
The electron-optical magnification of the constructed demountable UV /
x-ray-sensitive framing camera was measured to be -1.4 with a static limiting resolution of 201p/mm referred to the input cathode. The drift region was simply constructed in interchangeable aluminium segments while the anode aperture and framing slot (rather then aperture) were machined into 1.6mm thick steel plates which were screw mounted within the sections. The deflection plates were constructed out of 0.5mm thick, fiat copper sheet cut and bent into shape and then polished. The demountable Picoframe I camera posed no difficulties in mounting the plates which were fixed directly onto the 50fit 'BNC bulkhead sockets which were mounted through the drift tube wall. This
system offered good mechanical stabihty while introducing very little parasitic reactance. The plate separation was easily altered and tlie 11mm long by 11mm wide framing plates were separated by 4.4mm (measured using slip gauges). Similarly, the 14.8mm long 7.8mm wide compensation plates were separated by 4.2mm in order to maintain equal dc sensitivity and capacitance.
2.5.1 The Vacuum System
The entire system was evacuated using a turbo-pump (type Pfeiffer Duo 1.5A), a Vac-valve was used to isolate the camera at night so that the camera was maintained under vacuum as much as possible to extend cathode life and prevent contamination of the internal components. The Vac-valve also had the effect of aperturing the input radiation slightly (valve internal diameter was -2cm) which improved the
signal-to-noise ratio by reducing the amount of light internally reflected within the aluminium vacuum T-piece. A diagram of the complete demountable framing camera system employed as indicated in figure 2.2
Lensing Section Main Vac-Valve
\i
Vacuum Gauge Turbo Pump Framing Deflector Section Compensation and Shift Deflector t a g — a SectionHI
Intensifier BNC ConnectorsAir Admit Valve Magnetic Screen
Camera Back
2.5.2 Environmental Isolation of the Electron-Optical Lens
Initially spurious magnetic fields associated with power supphes and other equipment in close proximity to the demountable camera caused electron beam deflection to such an extent that the beam was being partially intersected by the anode and framing apertures. This problem was remedied by placing a ferrous (iron) shield around the lensing section of the system. One unfounded concern was that the metal sleeve surrounding the lensing section would provide electrostatic boundary conditions which were not compatible to those initially modelled. This did not seem to be the case as the system focussed at the correct voltages and provided a static resolution
comparable to that modelled for a gold UV-sensitive photocathode.