GAS-FILLED X-RAY DETECTOR

Abstract: The anode connections in a gas-filled X-ray detector are mounted so as to extend transversely of the anode wire so that a substantial saving as regards space is achieved. In order to reduce field inhomogeneities in the ionization space, there may be provided a field profile bush which partly encloses the anode wire and field bushes may be arranged around the anode connection pins.

Description: The invention relates to an X-ray detector, comprising a gas-filled envelope provided with an entrance window and anode connection pins and in which there is arranged a wire-shaped anode, and also relates to an X-ray analysis apparatus comprising such a detector. In known gas-filled detectors the anode connection pins are situated in the prolongation of the anode wire. In order to prevent field disturbances, the window is substantially smaller, measured in the direction of the anode wire, than the length of the anode wire measured between the anode connection pins. The detector has a circular-cylindrical construction, the anode wire being coincident with the cylinder axis. A detector construction of this kind offers many advantages, for example the symmetry in the ionization space and the absence of further structural parts or connection electrodes in that space. When known gas-filled detectors are used, for example in an X-ray analysis apparatus, however, a drawback exists in that the detectors occupy a comparatively large amount of space in a direction transversely of the propagation direction of the radiation to be measured, the available space being limited in this direction. Moreover, the external anode connection pins often are not readily accessible and the exchange of a detector represents a cumbersome operation. These drawbacks are mitigated by the detector Kusien from US-A- 4,510,391, comprising the features of the precharacterising part of claim 1. However, a further drawback consists in that a suitably uniform ionisation space is not realised around the anode wire. It is the object of the invention to mitigate this drawback; This is achieved by an X-ray detector according to claim 1. The construction of the detector in accordance with the invention enables a substantial reduction of notably the dimension of the detector, measured in the longitudinal direction of the anode wire, i.e. transversely of the direction of the radiation to be measured, ensures that the connections for the anode wire are always readily accessible when the detector is mounted in an X-ray analysis apparatus, end also allows homogenizing the electric field to be applied in the ionization space surrounding the anode wire. In a preferred embodiment, in order to prevent adverse field effects of the anode connection pins, insulated field busches are provided around the anode connection pins. The field profile bush can be arranged on these field bushes and is also in suitable electrical contact with the envelope. In a preferred embodiment in the form of a sealed gas detector, the anode wire is connected to the anode pins by way of burr-free spot welding. In a preferred embodiment of a gas-filled detector in the form of a flowcounter, the anode connection pins are constructed so as to be hollow and the anode wire is detachably connected thereto by means of, for example suitable round head rivets. An entrance window which consists of, for example plastics and which is possibly reinforced with a gauze can then be detachably connected to the entrance window plate by means of an O-ring seal. Some preferred embodiments in accordance with the invention will be described in detail hereinafter with reference to the drawing. Therein: Figure 1 shows a sealed X-ray detector in accordance with the invention, and Figure 2 shows an X-ray detector in accordance with the invention which is constructed as a flowcounter. A detector as shown in Figure 1 comprises an envelope 1 with a window carrier plate 2, a rear plate 4 and a cylindrical side wall 6. The window carrier plate and the side wall can be constructed together as a window cap. In the window carrier plate there is provided a window aperture 8 with, for example a supporting rim 9 on which an entrance window 10 can be mounted. The entrance window is customarily made of beryllium. For connecting the window to the window carrier plate the beryllium disk is covered by a bonding layer 12 over an annular portion to be bonded. Preferably, first an anti-diffusion layer is provided, on which subsequently a layer is provided which can be readily bonded to the carrier rim. For a more detailed description of feasible window bonding techniques, reference is made to US-A- 4,431,703. The effective window surface area amounts to, for example 10 × 12 mm for an anode wire having a length of 15 mm between the anode connection pins. The thickness of the window is dependent of the gas filling which itself is adapted to the wavelength of the radiation to be measured, for example 0.3 mm for a counter filled with xenon, 0.1 mm for a counter filled with krypton, and 0.05 mm for a counter filled with neon. In the rear wall 4 of the envelope there are provided two anode connection pins 12 for an anode wire 14 to be mounted inside the housing. Each of the connection pins comprises a conductor 16 which is accessible on an outer side 18 and whereto the anode wire can be secured on an inner side 19. Via the external connections a detection circuit (not shown) can be connected. For correct mounting and for increased ruggedness of the envelope a mounting plate 20 is provided on the rear wall. This mounting plate also serves as an additional carrier for the feed-through pins. The anode wire is connected to the feed-through pins, for example by soldering or preferably by burr-less welding. In the housing there is arranged a field profile bush 22 whereby a suitably uniform ionization space can be realized around the anode wire. The field profile bush is shaped, for example as a U-profile having a length of, for example 25 mm, a width of 15 mm and a depth of 13.5 mm. The anode wire coincides substantially with a center-line of this U-profile. Around the anode connection pins there are mounted field bushes 24 in order to avoid electrical discharging phenomena. Preferably, the field profile bush is electrically connected to the field bushes as well as to the bottom plate 4 and forms a cylindrical ionization space around the anode wire within the substantially cylindrical space in the envelope. Figure 2 shows a gas detector constructed as a flowcounter which is specifically intended for the measurement of comparatively soft radiation, so for the analysis of light elements. The housing 1 again comprises a window carrier plate 2, a wall section 6 and a bottom plate 4. A window aperture 8 comprises a carrier rim 9 which in this case comprises a sealing O-ring 30 and the window plate which is in this case composed of, for example a plastics film 31 and a carrier grid 32. In the bottom plate 4 there are again provided anode feed-through pins 12 with electrical conductors 16, external connections 18 and internal connections 19, it again being possible to provide field bushes around the connection pins. In the present embodiment the inner connections 19 of the anode connection pins are constructed so as to be hollow. The anode wire 14 is detachably secured in the cavities of the connections by means of round head rivets 34. In order to form a detachable seal in conjunction with the window cap, an O-ring seal 36 is provided between the cylindrical wall portion 6 and the bottom plate 4. In the field profile bush 22 there is arranged a beryllium plate 38 in order to reduce the occurrence of secondary radiation in the ionization space. For gas circulation through the counter an inlet duct 40 and an outlet duct 42 are provided in the bottom plate, which ducts open underneath the field profile bush, viewed from the anode wire, in order to reduce field disturbances. The cylindrical wall portion 6 may form a circular cylinder within which the field profile bush again separates a cylindrical ionization space. The side wall may also have a different shape, for example it may be adapted directly to an elongate shape of the exit window, the anode wire and the field profile bush.

Claims: 1. An X-ray detector, comprising a gas-filled envelope (1) provided with an entrance window (10) and anode connection pins (12), a wire-shaped anode (14) arranged within said envelope between the connection pins, the anode pins being situated in a wall portion (4) opposite the entrance window and being directed transversely to a longitudinal direction of the anode wire, characterized in that a field profile bush (22) at least partly enclosing the anode wire is arranged within the envelope. 2. An X-ray detector as claimed in Claim 1, characterized in that the anode wire is connected to the anode connection pins by burr-free spot welding. 3. An X-ray detector as claimed in any one of the preceding Claims, characterized in that field busches which are situated within the envelope are arranged around the anode connection pins. 4. An X-ray as claimed in Claim 1 or 3, characterized in that the ends of the anode pins which are situated inside the envelope are constructed so as to be hollow, the anode wire being detachably clamped therein. 5. An X-ray detector as claimed in any one of the preceding Claims, characterized in that it is constructed as a flowcounter, the envelope being provided with detachable seals. 6. An X-ray detector as claimed in anyone of the preceding claims, characterized in that the entrance window is composed of a plastics foil and a carrier grid and is connected to the window carrier by way of an O-ring seal. 7. An X-ray analysis apparatus comprising at least one X-ray detectors as claimed in any one of the preceding Claims.

Inventors: VALKONET, LOURENS, VAN DER BORST, JOHANNES, VREUGDENHIL, FRANS GERARD
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H01J47
IPC codes: H01J47, G01T1