Patent GB-672944-A (GB)
Title: Improvements in or relating to electroscopes
IPC: H01J47/04
Filed: 1949-07-20
Abstract: 672,944. Electroscopes. CARMICHAEL, H., SALMON, P. G., and SCIENTIFIC & INDUSTRIAL RESEARCH, HONORARY ADVISORY COUNCIL FOR. July 19, 1950 [July 20, 1949; March 15, 1950], Nos. 19048/49 and 6453/50. Class 37. [Also in Groups XL (a) and XL (b)] An electroscope comprises, in combination, a casing having an internal conducting surface, a member having a conducting surface mounted within the casing in insulated relationship thereto, a flexible conducting indicator fibre disposed adjacent to one of the said surfaces constituting a repulsion surface in electrical connection therewith, the fibre having an extremity capable of angular deflection with respect to the repulsion surface, a scale within the casing carried adjacent to the deflectable extremity of the fibre in measuring relationship to deflection thereof, an objective lens supported from an aperture in the casing in image-forming relationship to and spaced from the deflectable extremity of the fibre and scale by an optical distance less than its focal length, and means for illuminating the fibre and scale by light external to the casing whereby enlarged virtual images thereof may be viewed at the lens. The invention is directed particularly towards provision of miniature electroscopes suitable for use as detectors of gamma, beta or other radiation emanating from radioactive materials. Pocket or pen-holder electroscopes.-In Figs. 1 to 2 a tubular casing 70 having an inside conductive surface 72 and fitted to a pen-holder or detachable handle 74 defines an ion-chamber which contains a collector electrode 76 to which is conductively secured a flexible indicator fibre 79 and a rigid repulsion plate 77. When the instrument is charged for use the indicator fibre 79 is repelled from the fixed plate 77 and takes up a zero position in relation to a transparent scale 82, Fig. 2. With exposure to radiation the charge leaks away and the indicator fibre 79 is deflected along the scale 82 and through a lens 88, Fig. 1, the user observes a magnified virtual image of the scale reading against the background illumination of a window 89 in alignment with the optical axis of the lens 88. The transparent scale 82 is half the width of its supporting plate 81, Fig. 3 (not shown), and the formation and disposition of both produces a curvature or guide surface, Fig. 2, conforming to the arc of deflection of the indicator fibre 79. The continguous arcuate edges 84, 85, Fig. 2, are conductive and connected to the conductive surface 72 inside the casing 70. The repulsion plate 77 extends both lengthwise directionally, Fig. 2, and laterally directionally, Fig. 1, with respect to the indicator fibre 79 which is by this design feature electrically stabilized so that should overcharge or rough-handling occur the indicator fibre 79 cannot contact and adhere to the conductive surface 72. In an unillustrated modification the construction is such that the functions of the internal conductive surface 72 and repulsion plate 77 are inverted with respect to the indicator fibre 79. Wrist - electroscopes.- In Fig. 7 light is admitted into the casing 90 through a window out of alignment with the optical axis of the viewing lens 107 and comprising a transparent bar 101 integral with upper and lower reflecting prisms 99, 100. The user observes the position of the indicator fibre 104 with respect to scale markings on the upper prism 99 as a magnified virtual image against the illumination transmitted through the bar portion 101 to the prisms 99, 100. The latter together with the bar portion 101 are located in a rectangular slot 98 in the side wall of the casing 90 and one wall of this slot is coincident with a ledge 96, Fig. 5, which is the extremity of a curved surface 97, Fig. 5. The upper prism 99 has a curved surface forming a continuation of the curved surface 97 and the treatment and arrangement is such as to produce a deflection or stabilizing guide surface for the indicator fibre as in the pocket electroscope, Fig. 2. The casing 90 is closed by a lid 108, Fig. 8, on which is mounted a collector disc electrode assembly comprising a flat, wide repulsion plate 138 and the indicator fibre 104, the arrangement being such that in this embodiment also with the repulsion plate 138 lying athwart an angular ionization cavity there is the feature of non-contact by the indicator fibre 104 to any oppositely-charged surface should overcharge or rough usage occur. The lid 108 which carries a charging contact 117 is capable of limited rotation with respect to the casing 90 so that in bulk manufacture of these instruments the indicator fibre 104 in each instrument may be set to the same zero on the scale for a fixed charging voltage of, for example, 100 volts. The collector disc electrode assembly is arranged to accommodate a condenser 140. Design changes may be made appropriate to forming the guide surface on the collector electrode assembly with the indicator fibre and repulsion plate, both secured to the casing, i.e. inversion as proposed for the pocket electroscope. According to a construction disclosed in the first Provisional Specification a separate repulsion plate like 138 is not used and instead the indicator fibre is repelled from a conductive edge in a recess in a disc electrode with, in this case, side shields or a mechanically stabilizing fibre employed to prevent excessive lateral motion of the indicator fibre. The first Provisional Specification also discloses with illustration (1) two methods of fixing the indicator fibres to the collector disc electrode; (2) an instrument with a lid formed with a thin diaphragm so as to produce a wrist-electroscope which may be worn for gamma and beta detection or gamma detection only. Material.-The casing and other parts required to have an electrically-conductive surface may be fabricated in metal, for example aluminium or magnesium, or plastic mouldings with a conductive surface, or graphite, or for fast neutron dosimetry, polythene loaded with graphite or carbon black. Sensitivity to slow neutrons may be obtained by including inside the casing an appropriate amount of amorphous boron or boron carbide. The indicating fibre which may be secured by a conducting cement, electrically conducting shellac or a plastic containing finely-divided graphite may be of fused quartz, gilded or platinized, or of metal or electrically conductive glass. Polythene with a highly polished surface may be used for the parts required to be wholly of insulating material