Polythene windows for vacuum systems

October, 1953

Vacuum, Vol. I I I No. 4

Fig. 3.

(Stainless Steel Trap)

P o l y t h e n e Windows for V a c u u m S y s t e m s Sommaire UNE MI~THODEEST d6crite brievement pour la fabrication des fen~tres ~ partir de ruban de polyth~ne en tubes qui p e u v e n t servir comme fen~tres de tubes ~ rayons-X et donnent une distribution tr~s large d'irradiation de rayons X qui est d~sirable dans ceaaines recherches sp~ciales.

A HOLE OF THE required size is made in the glass tube by grinding with a suitable grinding paste. The sharp edges are then removed with a fine carborundum stone. An ' O '-ring is selected of such a size that it will just rest on the circumference of the hole. The cross-sectional diameter of the ' O '-ring should be approximately 0.140 inch. A track ½ inch wide around the hole is then treated with adhesive S 21, (a specially prepared rubber adhesive). This is done by lightly painting with S 21, allowing to dry 3-4 minutes and then painting with S 21 again. The ' O '-ring is pressed into position and a slight pressure is maintained for approximately 10 minutes. The ' O '-ring will now take the shape of the contour of the glass tube. Windows of 0.010 inch polythene tape can be prepared by wrapping the polythene tape around the tube over the sealing ring and spot welding the two ends together on the opposite side of the tube. The

spot welding can be done quite well with a soldering iron. Test Procedure. Both ends of a tube similar to the one described in Fig. 2 were sealed and pumped through a smaller tube situated on the side of the main tube (see Fig. 2). A vacuum of 5 × 10-8 mm. Hg was readily attained. The polythene sheet can be easily removed and refitted or replaced by a new sheet and the tube can be rapidly evacuated after the operation. Fig. 1 shows a 35 mm. diameter lead glass tube with a hole of 1 inch diameter and with a rubber sealing ring fixed to the circumference of the hole with S 21 adhesive. Fig. 2 shows the same tube as Fig. 1 but in this case the 0.01 inch polythene tape has been wrapped over the ' O '-ring and around the tube; both ends have been spot welded together on the opposite side. The tube has been fitted with a smaller diameter side tubulation for evacuation purpose. Application. This method has proved very useful as a method of constructing X-ray tube windows in cases where the window is required to be as near as possible to the source from which the X-rays are being emitted. As soon as the tube has been evacuated the polythene is drawn inwards and a window of the size described above (using 0.01 inch tape) would move inwards by as much as 0.15 inch at the central portion of its effective area. In the particular case in which Polythene Window

~e 2

414

Vacuum, Vol. I I I

No. 4

October, 195£

GLASSNG /SHEATHI -"-

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Fig. I.

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R.E ENERATOR

(Screen Inspection Method)

this window is used on a demountable X-ray tube this movement inwards leads to the following advantages : - (1) The surface of the window comes slightly nearer to the target. (2) The angle of distribution of the X-ray radiation is wider. (3) The junction between the polythene and the ' O '-ring is perfectly vacuum tight. Operational Test. An X-ray tube was fitted with this type of window and was operated with 13 kV on Fig. z.

(Screen Inspection Method)

GLASSHING SHEAI J

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the anode and a target current of 1 mA for 10 seconds. This was repeated 60 times at ½minute intervals. The target voltage was left on all the time but the filament was turned off completely during each rest period. The 0.010 inch polythene window was quite satisfactory after this test which involves, of course, only restricted irradiation by the X-ray beam. I f the tube was operated continuously at 5 mA the window would fail after about 60 minutes operation. On the other hand, if the tube was allowed to remain under vacuum but without production of X-rays, the window would last almost indefinitely. The polythene window is therefore not at all durable as a window for a demountable X-ray tube, but it has a long enough life to enable certain experiments to be carried out, in which it is necessary to use a powerful beam of very low voltage X-rays and of very wide angle of distribution for certain special investigations.

~G ENE~ATOR RE

J. HODKINSON. Physical Laboratory, Ferranti Ltd., Moston, Manchester 10, England. 2nd March, 1955.

An InspectionMethodfor LuminescentScreens Sommaire

TIlE DECHARGEde gaz ~ basse pression sous haute tension dam un tube cathodique excite l'rcran luminescent. Ce phrnom~ne a 6t6 etudi6 pour son emploi dans une mrthode d'inspection d'rcran. THE DETAILEDNATUREof screen luminescence excited by one agent can be very different from that excited by another. The exact nature of luminescence depends on : - - T h e energy distribution, or energy spread of the exciting agent; the kind of exciting agent, whether photons, electrons, or other particles; and also on the direction from which the exciting agent is applied. In this sense, since the screen, in normal use, will be bombarded from the rear with electrons having only a narrow energy spread, the ideal would be to excite the screen in precisely this manner. Other tests, such as ultra-violet light directed onto the front of the screen, may or may not show imperfections, which, in turn, may or may not show under operating conditions. Ultra-violet light frontal illumination has in fact