US3430086A - Geiger-muller tube with window and internal helix - Google Patents

Description

Feb. 25, 1969 WJMACKINTOSH GEIGER-MULL-ER TUBE WITH WINDOW AND INTERNAL HELIX Filed Feb. 24, 1965 F I G 1 FIG.2.

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United States Patent ()1 8,407/64 US. Cl. 31s 93 4 Claims Int. Cl. H01j 39/26, 39/30;G01t1/18 ABSTRACT OF THE DISCLOSURE A Geiger-Muller tube having a cylindrical metal envelope provided with a thinner wall portion serving as a window to enhance the detection of low-energy radiation. The tube contains on its interior a wire helix of a high atomic number to increase the electron emission eificiency to high-energy radiation to enhance the response thereto.

This invention relates to a Geiger-Muller tube having a cylindrical envelope which forms the cathode of the tube and a wire anode passing through the centre of the tube.

In the detection of gamma-radiation by a Geiger-Muller tube, the action of the tube is influenced by the energy of the incident radiation; two different mechanisms are involved. High-energy radiation, which is not absorbed by the filling gas, gives rise to the emission of electrons from the cathode which, in turn, causes the ionisation of the gas on which the action of the tube depends. With conventional cathode materials the electron emission efficiency falls as the energy of the incident radiation decreases but this effect is much less marked if the cathode material has a high atomic number. The absorption of low-energy radiation by the filling gas causes direct ionisation and provided that the wall of the Geiger-Muller is sufiiciently thin and acts as a window to allow the entry of such low-energy radiation, quite high detection efiiciencies result.

An object of the present invention is to provide an improved Geiger-Muller tube giving a fairly level response curve for the lower and higher energy ranges.

The present invention provides a Geiger-Muller tube comprising a cathode in the form of a cylindrical metal envelope, an anode in the form of a wire extending along the longitudinal axis of the envelope, and in said envelope a window for low-energy radiation, said window being formed by a portion of the envelope having a smaller wall thickness than that of the remainder of the envelope.

If the gas filling contains a small addition of halogen to provide quenching, then the cathode envelope will be of chrome-iron. The ratio of the length of the thin-walled window portion of the total length of the envelope is important for the reasons given below: it is thought that this ratio will be of the order of 1:7. Suitably the window portion is arranged to be in the middle region of the envelope;

In order to provide a cathode having a high atomic number, to increase the electron emission efiiciency, especially with a halogen quenched tube, a tungsten wire helix may be wound around the inside surface of the en velope adjacent the said remainder of the envelope. Tungsten is inert to the halogens. When the window portion is in the middle region of the envelope, then suitably one tungsten helix may be provided which helix is arranged to have a large pitch adjacent the window portion of the envelope.

In order that the invention may be readily carried into 3,430,086 Patented Feb. 25, 1969 "ice effect, one embodiment thereof will now be described, by way of example, with reference to the accompanying diagrammatic drawings wherein:

FIGURE 1 is a longitudinal section of the Geiger- Miiller tube, and

FIGURE 2 is a curve illustrating the sensitivity of a Geiger-Muller tube as a function of the energy of the incident radiation.

In FIGURE 1 a Geiger-Miiller tube comprises a wire anode 1 supported by discs 2 of insulating material within a cylindrical envelope 3 of chrome-iron, the discs 2 serving to close the ends of the cylinder 3. The envelope is filled with a rare gas with a small addition of halogen to provide quenching. A central portion 4 of the cylinder 3 has a smaller wall thickness than the remainder of the cylinder 3, so as to form a window for low-energy radiation. For example, the thickness of the main part of the cylinder may be as much as 5 millimetres, while the window portion may be as thin as 0.05 millimetre, although it is generally more convenient to use a thicker window portion.

A tungsten wire 5 is wound around the inner surface of the cylinder 3, some of the turns being omitted for the sake of clarity. Separate closely-wound helices may be used at each end of the cylinder 3 but preferably the arrangement is as shown with one helix, the helix having a wider pitch in the region of the window portion 4.

During operation of the Geiger-Muller tube the thinwalled portion 4 assists the measurement of low-energy radiation while the remainder of the cylinder 3 serves for the measurement of the high-energy radiation.

In FIGURE 2 curve A represents the response of a standard Geiger-Muller tube having a uniformly thin wall. The maximum in this curve A occurs at a sensitivity of some 500% of the incident radiation and is due to the low-energy radiation acting upon the gas filling, the whole of the envelope serving as a window for the low-energy radiation. In the present invention, most of the envelope is opaque to this low-energy radiation except for a small window portion. The height of the maximum would depend upon the size of the window portion 4 of the tube described in FIGURE 1. Curve B represents the response of the Geiger-Muller tube when the wall of the Geiger- Miiller tube is formed with a window portion as described above with reference to FIGURE 1 and this curve has a maximum at some 140%. In order to present the tube characteristics in a compact form, the scales of S and E in FIGURE 2 are non-linear.

The minimum in curve A is due to the fall-off of the emission efiiciency of the cathode material as the energy of the radiation decreases. This minimum may be increased by using a cathode of high atomic number. Curve C indicates the response when a tungsten wire helix is wound around the inside of the envelope 3. The amount of tungsten will decide the height of the minimum and it can be seen that between energies of Kev. and 1.2 mev. a fairly level response may be obtained.

What is claimed is:

1. A Geiger-Muller tube exhibiting a fairly uniform respouse for the detection of gamma radiation over a wide energy range, comprising a cathode in the form of a cylindrical metal envelope of uniform wall thickness for passing high-energy radiation except for a window portion of the envelope in the cylindrical wall having a reduced wall thickness for passing low-energy radiation, and a closely-wound wire helix of a higher atomic number metal than that of the envelope, said wire helix extending coaxially within the envelope and being positioned against the inside surface of the Wall envelope portions of uniform wall thickness leaving the window portion substantially free of the helix.

2. A Geiger-Muller tube as set forth in claim 1 wherein the helix has a larger pitch adjacent the window portion. 3. A Geiger-Muller tube as set forth in claim 1 wherein the helix is of tungsten.

4. A Geiger-Muller tube as set forth in claim 3 wherein the envelope is of chrome-iron, and the envelope contains a gas filling including a halogen.

References Cited UNITED STATES PATENTS Goldstein 313-93 X Hendee et a1 313-93 Ferre 313-61 X Molins et a1 250-836 Richter et a1 313-93 4 2,552,723 5/1951 Koury 313-93 2,724,779 11/ 1955 McKay 313-93 X OTHER REFERENCES 10 JAMES W. LAWRENCE, Primary Examiner.

P. C. DEMEO, Assistant Examiner.

US. Cl. X.R. 250-836

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