Picoframe I X-Ray-Sensitive Camera in Single And Double Frame Modes of Operation
4.2 The Picoframe H Camera
To overcome the shortcomings associated with the Picoframe I single-aperture design, the Picoframe II camera was devised. This camera is basically identical to the Picoframe I except that it has an additional framing aperture and set of compensation deflectors. A schematic is given as figure 4.1,
Framing
Apertures Comp.
Photographic Film Deflectors
Framing Comp. 2 Intensifier
Photocathode
Deflectors Deflectors
Figure 4.1 Schematic diagram of the Picoframe II electrostatic lens and deflection geometry.
Two aspects are of prime importance in the design of this image tube deflection region. Firstly, capacitive cross-coupling of the compensation deflectors and the capacitive coupling of the deflectors to earth must be kept to a minimum, and secondly the separation of the framing apertures must be kept to a minimum to reduce the inter-ffame time (which is defined by the scan speed and the inter-aperture spacing only)
4.3 The Proposed Triple-Aperture Design
The initial design proposal was for a three aperture / compensation deflector construction [1] which would provide three frames from the application of linear voltage waveforms to the deflectors. Unfortunately this design is virtually impossible to implement due to the space available and amplitude of voltage wavefoims easily
generated. In a recent report [2] a similar three aperture design has been discussed, but multiple dc biased shift plates were used to avoid the capacitive cross-coupling
problems mentioned. Only moderate dynamic spatial resolution was reported, probably due to the accumulated fringing field effects associated with the complex multi-deflector structure.
4.4 Initial Evaluation of the Picoframe H Design
The double aperture Picoframe H configuration designed initially by Eagles enabled the generation of two frames directly. This design was realised in a sealed-off visible-sensitivity camera image tube, but some misalignment of the electrodes either during construction or bake-out resulted in severe astigmatism. The ’undeflected' election beam was non-axial and in fact was able to go through one of the apertures positioned some 5mm off the axis. By using electromagnets positioned around the glass envelope the electron beam could be brought into line with the tube axis and preliminary studies were undertaken to evaluate the feasibility of the design. Frame doublets were achieved, but these had low dynamic spatial resolution (< 4 Ip/mm per frame ) and frame and inter-frame times o f-150 ps (FWHM) and ~800 ps were
indicated. More importantly it was confirmed that the compensation deflector cross-talk could be eliminated and independent compensation of the streaked images achieved. It
was therefore decided to construct suitable components to convert the Picoframe I demountable image tube into a Picoframe II camera.
4 5 The Vacuum Demountable Picoframe n
The aperture plate consisted of two framing slots, 1.8mm wide and separated by 9mm which was the minimum that could be achieved due to the compensation plate positioning. These were 14mm long and separated by 4mm. Compensation plate isolation was achieved by introducing an earth screen between the two sets of
deflectors. Machinable glass ceramic was used for mounting the plates and copper braid used for the electrical connections as shown in figure 4.2
C e r a m i c Mounting B lo c k s A p e r t u r e P l a t e - R.F. S h ie ld Mounting Holes 04 D e f l e c t i o n P l a t e s F ra m in g / ‘A p e r t u r e s ’ B raid To "BNC C o n n e c to r s
Figure 4.2. Construction of the compensation deflector arrangement.
4.5.1 The Static Spatial Resolution of the Picoframe H
To measure the limiting static spatial resolution of the Picoframe H camera design a dc bias deflection voltage must be applied to the framing deflectors. The two images resulting from the two framing apertures were inspected by illuminating the US AF test chart masked UV-sensitive photocathode with a mercury arc lamp. The framing and compensation deflectors were connected in inverse polarity and a suitable symmetrical
dc deflection bias voltage applied. The image intensifier was operated at a gain of -100 in order that the resultant images were sufficiently intense to be inspected by eye. The resultant spatial resolution is shown graphically as figure 4.3 (a), (b) corresponding to the two images. The deflection voltages indicated refer to the positive component of the deflection potential and so it may be seen that a deflection voltage of approximately ±750 V is required to sweep the photoelectron beam from the centre of one aperture to the other. With this deflector geometry it is clear that only moderate static spatial resolution was attained, although one image was of a higher quality then the other due to the asymmetry of the apertures about the undeflected photoelectron trajectories.
E E D. 15 -1 c .2 5 1 0- 1 nt •
I
E 3 0 0 4 0 0 5 0 0 6 0 0 Deflection Voltage (V) (a) E E ë c oI
I-
E E Li 10 9 8 7 6 5 200 3 0 0 4 0 0 Deflection Voltage (V) (b)Figure 4.3 (a), (b) showing the limiting static spatial resolution of the two photoelectron images resulting from the two framing apertures with respect to the
symmetrical deflection voltage applied to the framing and compensation deflection plates. Non symmetry of the two framing apertures about the undeflected photoelectron beam is evident by the different deflection voltages required.
Cai'eful analysis of the electron trajectories (achieved by electron 'ray tracing’) revealed that the photoelectrons approached the outer of the two compensation plates near their exit. It was felt that electrostatic fringing fields (which are non uniform electrostatic fields at the entrance and exit of the deflection plates) were the main reason of the degradation of the spatial resolution.