US3704371A - Radiation detection for soft x-rays - Google Patents

Abstract

The apparatus comprises a proportional counter with a long tubular nose-piece having an end window. The anode is a short wire parallel to the window and mounted on a support so that the support together with the anode is easily removable and replaceable. The support may be axially movable within the long tubular nose-piece, whereby sensitive tuning to specific characteristic X-rays is possible.

Description

United States Patent Cairns et al.

RADIATION DETECTION FOR SOFI X- RAYS Inventors: James Anthony Caims, Didcot; Colin Leonard Desborough, Faringdon, both of England United Kingdom Atomic -Energy Authority, London, England Filed: June 1, 1970 Appl. No.2 42,166

Assignee:

Foreign Application Priority Data May 30, 1969 Great Britain...'. ..27,660/69 US. Cl. ..'.......2s0/s3.6 R, 313/146 Int. Cl ..G01t 1/18 Field of Search.....250/83.6 R, 43.5 D; 313/146,

its] 3,704,371 51 Nov. 28, 1972 [56] References Cited 7 UNITED STATES PATENTS 3,449,573 6/ 1969 Lansiart et al. ...250/83.6 R X 3,110,835 11/1963 Richter et al ..250/83.6 R X 3,091,716 5/1963 Engelmann ..3l3/l46 Primary Examiner-James W. Lawrence Assistant Examiner-Davis L. Willis Attorney-Larson, Taylor and Hinds [57] ABSTRACT The apparatus comprises a proportional counter with a long tubular nose-piece having an end window. The anode is a short wire parallel to the window and mounted on a support so that the support together with the anode is easily removable and replaceable. The support may be axially movable within the long tubular nose-piece, whereby sensitive tuning to specific characteristic X-rays is possible.

14 Claims, 4 Drawing Figures PATENTED NOV 28 m2 3.704.371 sumamz RADIATION DETECTION FOR SOFT X-RAYS BACKGROUND OF INVENTION The invention relates to apparatus. for detecting radiation, more especially X-radiation.

SUMMARY OF THE INVENTION The invention provides apparatus for detecting radiation, more especially X-radiation, which apparatus comprises an enclosure having a window,'that is a region (hereinafter referred to as a window) adapted for transmitting the radiation to be detected, a pair of electrodes one of which comprises an elongated member which extends perpendicularly to the central normal to the window surface, and a support for the said one electrode, the said support together with the said one electrode being removable as a unit from the enclosure.

Preferably the window is plane so that the elongated electrode extends parallel with the plane of the window.

Preferably the other of the said electrodes has a configuration which is symmetrical about the said elongated electrode. Preferably the elongated electrodeis employed in operation as an anode and the said other electrode is employed as the cathode. Preferably the cathode encompasses the anode, the said window being electrically conducting and forming part of the cathode.

Preferably the enclosure is elongated, preferably tubular, and the window is located at an end of the elongated enclosure. Preferably removal and replacement of the combined support and anode unit is effected by extracting the unit from the end of the elongated enclosure remote from the window. In this way, if the vacuum seal into a target chamber is made onto the body of the said elongated enclosure, the anode may be replaced without loss of vacuum.

It is an important feature of the arrangement according to the invention that an electrically conducting element located on the side of the anode remote from the window is in electrical contact with, and forms part of,- the cathode.

Preferably the said electrically conducting element is comprised by the said support for the anode, there being insulating means between the element and the anode.

Preferably means is provided for adjusting the separation of the said one electrode from the window. Preferably the support and the said one electrode are slidable as a unit in the enclosure to effect the adjustment of the said separation.

BRIEF DESCRIPTION OF THE DRAWINGS Specific constructions of apparatus embodying the invention will now be described by way of example and with reference to the accompanying drawings in which:

The apparatus of these examples is similar in the general principles of construction to the type of radiation detector described as a proportional counter. The apparatus will hereinafter be referred to as a counter.

The counter of the example shown in FIG. 1 is primarily intended for the measurement of soft X-rays, that is X-rays of energy less than, say, 1 keV.

Referring to the drawing, the counter comprises a long stainless steel outer tube 11 with an aluminized Mylar window 12 at one end. The window 12 is secured by a threaded ring 13 which traps an O-ring seal 14 to I make a vacuum-tight joint.

An anode wire 15 is supported on stainless steel posts 16 which meet in a Y junction (not visible in the drawing) and are themselves supported by a hollow pin vice 17, one end of which clamps the stem of the Y junction and the other'end 18 of which is secured to an inner tube 19 of stainless steel.

The inner tube 19 extends through the counter to just beyond a locknut, reference 36, where it is connected to polythene tubing. The tubing emerges at 21 where it is connected, when the counter is in operation, to a supply of the counting gas.

The stainless steel posts 16 extend through a spider 22. The spider 22 is metallic and adapted to engage with its fingering against the internal wall of the tube 11. The spider 22 is insulated from the posts 16 by insulating ceramic collars 23. The spider 22, the walls of the outer tube 11 and the window 12 are held at earth potential during operation and enclose the operating zone of the counter within which the anode 15 is situated. Holes 24 in the spider 22 admit gas fromthe inner tube 19 into this zone. The gas passes back along the outer tube 1 1 to an outlet 25.

It will be appreciated that the spider 22 serves to locate the anode 15, to center the inner tube 19 and to provide an earthed screen behind the anode 15. Electrical connection to the anode 15 is made via the inner tube 19.

At the end of the counter tube 11 remote from the window 12, the inner tube 19 extends through and is located by a tubular perspex member 26.

The inner tube 19 and the, perspex member 26 together extend through a rectangular housing 27 with a short hollow cylindrical nose 28 by which it is sealingly joined by union nut 29 to the outer tube 1 1.

The perspex member 26 is fixed to a collar 31 which is grooved to permit passage of gas around the outside of the perspex member 26 to the outlet 25. The collar 31 moves with the perspex member 26 and has a key engaging a groove within the nose 28 to prevent rotation. A bearing 32 secured by union nut 33 where the perspex member 26 enters the rectangular housing 27 has sealing rings 34, 35 to prevent the gas from entering the rectangular housing 27, or to provide a vacuum seal when it is desired to operate the counter with the gas at low pressure by pumping out from 25.

The end of the perspex member 26 remote from the outer tube 1 1 is internally and externally threaded. The internal thread co-operates with an external thread on the inner tube 19 and the perspex member 26 and inner tube 19 are locked together by lock nut 36. The external thread on the perspex member 26 is engaged by an elongated spider 37 fixed to the rotatable portion of a knob'38 mounted on the rectangular housing 27. The knob 38 is calibrated and, when rotated, moves the perspex member 26 together with an inner tube 19 and anode l5 axially.

A head amplifier of thin film/LID construction is mounted on a circuit board 39 which is accommodated within the rectangular housing]. The circuit board 39 is mounted by attachment to a clamp 41 on the perspex member 26. Leads for connections to the amplifier, the anode (via a load resistor 44 on the circuit board and inner tube 19) and the cathode (provided by the earthed body of the outer tube 11) are taken to plug/socket connectors 42.

It will be appreciated that if the knob 38 is appropriately rotated the perspex member 26, carrying with it the inner tube 19 and the anode 15, may be moved axially to adjust the spacing between the anode and the window 12. I

In FIG. 1, the counter is shown with the anode 15 .at the setting closest to the window 12. This limit setting is provided by a collar 43 clamped to the perspex member 26. At the other limit of axial movement of the perspex member 26, the steel inner tube 19 stops short of the end wall of the housing 27 to avoid the introduction of capacitative effects between the inner tube 19 and the body of the knob 38. p

The circuit board 39 moves with the perspex member. This is important for minimizing the length of lead, and hence minimizing the capacity of the lead, in the connection between the anode l5 and the head amplifier.

In this example the window 12 is plane and an important feature of the arrangement of the counter is that the anode 15 comprises an extended or elongated member, the length of which is parallel to the window 12. This geometrical arrangement is desirable for securing good resolution as will be apparent from the following reasoning. Consider a band of X-rays of uniform energy entering the counter through the window 12. The X-rays will all have the same mean path length in the gas of the counter and electrons released by ionization of the gas at the ends of the X-ray paths will travel, under the electrostatic field, to the anode. For good resolution, it is important that all the electrons resulting from X-rays of equal energy should reach the anode at the same time. It will be appreciated that a certain amount of curvature of the window surface may be tolerated. In fact a curvature about the axis defined by the anode might even be desirable but is not very practicable with the dimensions of the counter of this example. However, taking possible curvature into account, the relative orientation of window and anode may be defined in that the anode extends perpendicularly to the central normal to the window surface.

In operation, the long, comparatively narrow tube 1 1 provides a convenient probe by which the sensitive end, that is the window end, may be positioned close to a target within a vacuum system. The vacuum system is sealed onto the outer tube 11 at any convenient point between the union nut 29 and the window end of the outer tube 1 1.

The anode voltage is applied to the inner tube 19 and the surrounding cathode is earthed. Gas is passed through the tube 19 and is guided to the sensitive zone so as to give most effective flushing out of this zone.

X-rays passing through the window 12 ionize the gas and produce current pulses between anode and cathode, these current pulses being detected and amplified by the head amplifier. The amplitude of the pulses isproportional to the energy of the X-rays and the count rate is proportional to the X-ray intensity.

The pressure of the counting gas within the outer tube .1 1 is set so that the gas path length of the X-rays of the energy it is desired to detect is of the right approximate magnitude. The adjustability of the axial position of the anode 15 of the apparatus then'provides for a finer setting to achieve optimum spacing of the anode from the window in relation to the gas path length.

The apparatus of this example does, however, provide for an unexpectedly sensitive tuning of the counter to specific characteristic X-rays. Thus, for example, in a plot of count rate against pulse amplitude, the peaks for the "characteristic X-radiation of silicon-K and copper-L, are very close together and a proportional counter has high resolution if they can be clearly distinguished. It would not be expected that, for such similar peaks, the setting of the anode position relative to the window would produce any differentiation in sensitivity to the X-rays of these two elements. However, it has been found with the apparatus of this example that the counter can be tuned either to the copper-L peak or to the silicon-K peak by adjustment of the knob 38. When set for maximum response to the X-rays from copper- L, the response to silicon-K X-rays was of the order of five-fold less sensitive and vice versa. i

This phenomenon is believed to be due to the. geometry of the arrangement and the relative distribution of the electrostatic field according to whether the spacing between anode (which'extends parallel to the window) and the spider 22 is greater or less than the spacing between the anode and the window. The influence, upon the electric field, of the metal washer 12a on the anode side of the window also has a significant effect upon the distribution of the electrostatic field. It

is important that a cathode screen is positioned behind the anode 15, which screen is provided by spider 22 and moves with the anode 15 so as to have constant spacing therefrom. While the screen has holes 24 for permitting the passage of gas, the number and size of these holes is minimized so that the screen is as complete and uniform as possible.

The above-described phenomenon of exceptionally sensitive selective tuning of the counter may be undesirable or unnecessary in certain applications. Accordingly FIGS. 2 to 4 illustrate a simplified construction in which no provision is made for adjusting the anode position and the window is adapted for mounting in a manner which avoids the sensitive selective tuning mentioned above. The remaining features of the design of FIG. 1 are substantially embodied and improved upon in the apparatus of FIGS. 2 to 4 in a manner which provides an extremely compact counter capable of insertion as a complete unit within a vacuum system.

Referring to FIGS. 2 to 4, a stainless steel outer tube 51 corresponds to the outer tube 11 of FIG. 1, but is ing off the inside walls of the threaded ring and associated washer 53 and sealing ring trapping elements 54. It will be appreciated that such scattered X-rays would be degraded in energy and would spoil the resolution of the counter. v

Further, it has been found that the washer 12a in the FIG. 1 arrangement disturbs the uniformity of the electrostatic field. By securing an indium O ring with adhesive to the Melinex window 55, it has been possible to arrange the components as shown in FIG. 2 and the upper part of FIG. 4 with the window innermost.

The anode unit 56 corresponds closely in construction to the anode unit of FIG. 1 and is located so that the anode wire 57 is equidistant from the window 55 and the cathode screen provided by thesupporting spider illustrated diagrammatically at 58.

This arrangement achieves a particularly uniform electric field and adjustment of the counter for detection of a range of X-ray energies is achieved in the conventional manner by adjusting the operating gas pressure. A degree of additional flexibility is provided by the facility for rearranging the window assembly in the manner illustrated in the lower part of FIG. 4.

The anode unit 56 is secured to a perspex block 59 which is mounted within a cylindrical stainless steel casing 61. Also housed within the casing 61 is a circuit board 62 supporting the head amplifier. Seals are provided at 63, 64, 65 and 66 so that the counter gas chamber and the enclosure of the head amplifier are separate and sealed. Thus, if desired, the whole unit can be placed in a vacuum system. Alternatively, the outer tube 51 only may be received in a seal into the vacuum system. In that case, as with the FIG. 1 arrangement, the anode unit can be replaced without breaking the vacuum.

The gas path comprises inlet connector 67,'bore 68 in the casing, pipe 69, polyvinylchloride connector 71, stainless steel inner tube 72 and then via the anode unit into the counter chamber region within outer tube 51. The return gas path is via grooves 73 in the perspex block, annulus 74 and bore 75 to outlet connector 76 (FIG. 3).

Electrical connector sockets are provided at 77, 78, 79 and connection to the anode is via terminal 81 and the inner tube 72.

The overall length of this example is 4.5 inches (1 1.4 cms) and the diameter of the cylindrical end casing 61 is 3.25 inches (8.3 cms).

It will be appreciated that the modifications in shape and manner of assembly of the window securing components, discussed in relation to the example of FIGS. 2 to 4, may be applied to the example of FIG. 1.

The principal advantageous features of the apparatus of these examples is that the complete anode unit may readily be removed and replaced in the event of damage, without breaking the vacuum in the target chamber (except, of course, when using the apparatus of FIGS. 2 to 4 wholly within the vacuum system). The window 12 can be very thin if desired, e.g. 0.0001 inch aluminized polycarbonate or 0.0002 inch aluminized Mylar, and is adequately supported in vacuum-tight seal without the need for a grid support. Windows can readily be interchanged if desired.

The head amplifier unit and its associated electronic circuitry are built inside the body of the counter to minimize capacitance of the lead to the anode, but can be replaced easily, again generally without breaking the vacuum in the target chamber.

The invention is not restricted to the details of the foregoing examples.

We claim:

1. A radiation detector comprising a first electrode of hollow form, a window for transmitting radiation disposed at one end of the first electrode, a second electrode disposed within the first electrode, said second electrode being of elongated form and extending lengthwise within the first electrode in a direction perpendicular to the central normal to the window surface and adjacent said window, electrically conducting support means for said second electrode slidably supported by the first electrode and in electrical contact therewith, and means insulating said support means from said second electrode, the said support means and said second electrode being removable as a unit from said first electrode.

2. A detector as claimed in claim 1, wherein the window is plane so that said elongated second electrode extends parallel with the plane of the window.

' 3. A detector as claimed in claim 1, wherein, in use, the said elongated second electrode comprises an anode and the said hollow first electrode comprises the cathode, the said window. being electrically conducting and forming part of the cathode.

4. A detector as claimed in claim 3, wherein the said hollow first electrode has a symmetrical configuratio about the anode.

5. Apparatus as claimed in claim 4, wherein the said hollow first electrode is elongated.

6. A detector as claimed in claim 1, provided with a cover demountably connected to the other end of the said first electrode so as to close said other end, and means connecting the unitary second electrode and support means to said cover so that said cover becomes part of the unit.

7. A detector as claimed in claim 3, wherein the said second electrode support means comprises a cathode shield.

8. A detector as claimed in claim 1, wherein means is provided for adjusting the separation of the said second electrode from the said window.

9. A detector as claimed in claim 8, wherein the second electrode support means and the said second electrode are slidable as a unit in the hollow first electrode to effect the adjustment of the said separation.

10. A detector as claimed in claim 8, wherein, in use, the said elongated second electrode comprises an anode and the said hollow first electrode comprises the cathode, the said window being electrically conducting and forming part of the cathode.

11. A detector as claimed in claim 10, wherein the said hollow first electrode is elongated.

12. A detector as claimed in claim 10, wherein the said second electrode support means comprises a cathode shield.

13. Apparatus for detecting radiation comprising an enclosure, a window in the enclosure for transmitting radiation, a pair of electrodes one of which comprises an elongated member which extends perpendicularly to the central normal to the window surface, and a support for the said one electrode, the said support sulating means between the element and the anode, the

element being located, in use, on the side of the anode remote from the window and forming part of the cathode.

14. Apparatus as claimed in claim 13, wherein means is provided for adjusting the separation of the said one electrode from the window.

Claims (14)

1. A radiation detector comprising a first electrode of hollow form, a window for transmitting radiation disposed at one end of the first electrode, a second electrode disposed within the first electrode, said second electrode being of elongated form and extending lengthwise within the first electrode in a direction perpendicular to the central normal to the window surface and adjacent said window, electrically conducting support means for said second electrode slidably supported by the first electrode and in electrical contact therewith, and means insulating said support means from said second electrode, the said support means and said second electrode being removable as a unit from said first electrode.

2. A detector as claimed in claim 1, wherein the window is plane so that said elongated second electrode extends parallel with the plane of the window.

3. A detector as claimed in claim 1, wherein, in use, the said elongated second electrode comprises an anode and the said hollow first electrode comprises the cathode, the said window being electrically conducting and forming part of the cathode.

4. A detector as claimed in claim 3, wherein the said hollow first electrode has a symmetrical configuration about the anode.

5. Apparatus as claimed in claim 4, wherein the said hollow first electrode is elongated.

6. A detector as claimed in claim 1, provided with a cover demountably connected to the other end of the said first electrode so as to close said other end, and means connecting the unitary second electrode and support means to said cover so that said cover becomes part of the unit.

7. A detector as claimed in claim 3, wherein the said second electrode support means comprises a cathode shield.

8. A detector as claimed in claim 1, wherein means is provided for adjusting the separation of the said second electrode from the said window.

9. A detector as claimed in claim 8, wherein the second electrode support means and the said second electrode are slidable as a unit in the hollow first electrode to effect the adjustment of the said separation.

10. A detector as claimed in claim 8, wherein, in use, the said elongated second electrode comprises an anode and the said hollow first electrode comprises the cathode, the said window being electricalLy conducting and forming part of the cathode.

11. A detector as claimed in claim 10, wherein the said hollow first electrode is elongated.

12. A detector as claimed in claim 10, wherein the said second electrode support means comprises a cathode shield.

13. Apparatus for detecting radiation comprising an enclosure, a window in the enclosure for transmitting radiation, a pair of electrodes one of which comprises an elongated member which extends perpendicularly to the central normal to the window surface, and a support for the said one electrode, the said support together with the said one electrode being removable as a unit from the enclosure, the elongated electrode comprising an anode and the said other electrode comprising the cathode, the cathode encompassing the anode, the said window being electrically conducting and forming part of the cathode, said support comprising an electrically conducting element, there being insulating means between the element and the anode, the element being located, in use, on the side of the anode remote from the window and forming part of the cathode.

14. Apparatus as claimed in claim 13, wherein means is provided for adjusting the separation of the said one electrode from the window.

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