GB2097640A

GB2097640A - Energy filter

Info

Publication number: GB2097640A
Application number: GB8206915A
Authority: GB
Original assignee: Autonnic Research Ltd
Current assignee: Autonnic Research Ltd
Priority date: 1981-04-24
Filing date: 1982-03-09
Publication date: 1982-11-03

Abstract

An energy filter, for enclosing a Geiger Muller tube, comprises a copper sheath (2), open at one end and possessing an aperture (4) at the other, surrounded by a discontinuous jacket (6,7 and 9) of 60-40 tin-lead alloy. The latter, assisted by the copper sheath, reduces those radiant frequencies at which the Geiger Muller tube is most sensitive, enabling the tube to produce a response which is substantially independent of the energy of the incoming radiation. 60-40 tin-lead alloy is a commercial solder of eutectic composition. Consequently the filter is readily constructed by casting the tin-lead parts, either directly onto the copper sheath, or apart from it, for later assembly. The particular configuration of the alloy jacket, comprising two axially spaced rings (6,7) and one end disc (9), and the sheath aperture (4), are chosen to give a circular polar response as nearly as possible. <IMAGE>

Description

SPECIFICATION Energy filter The invention relates to Geiger Muller tubes for detecting ionizing radiations and concerns an improved method of making energy filters used with these tubes.

The response of a Geiger Muller tube is dependent upon the energy of the radioactivity being detected, and typically the sensitivity of the tube at 500-600 key can be about 12 times greater than at 6000 keV.

It is known to be possible to correct this error in the count by enclosing the tube in an energy filter which reduces the number of lower energy photons entering the tube. Such a filter reduces the count at lower energies while leaving the high energy count relatively unaffected and so the sensitivity of the detector tends to level out over an intermediate range of energies.

A known filter comprises a copper sheath for receiving a Geiger Muller tube and placed about this sheath a first jacket and a second thinner jacket of tin and lead respectively, these jackets having apertures to improve the response of the detector. The copper sheath not only supports the jackets but is also part of the energy filter and absorbs a proportion of the additional low energy photons generated when high energy photons are absorbed by the tin and lead jackets.

Conventionally, the components of the filter are machined separately and held together inside a perspex (polymethyl methacrylate) case which encloses the detector. Production of a typical filter by individually machining and assembling these components is an expensive process.

In accordance with the invention there is provided an energy filter for enclosing a Geiger Muller tube, comprising a copper sheath for receiving the tube and a discontinuous jacket of a tin-lead alloy about this sheath.

Conveniently, tin-lead alloys are available as solders which, because of their low eutectic point, can readily be cast about the copper sheath in one operation with sufficientfidelty that no further machining is required. One such alloy is "60/40 tin-lead solder" which contains substantially 60% tin and 40% lead.

This ratio of metals is similar to the proportions of lead and tin present in the combined jackets of known filters and therefore, a jacket of this alloy can be made having the energy absorbing characteristics necessary to level out the sensitivity of the detector over the range of interest.

A copper sheath for enclosing a Geiger Muller tube is generally cylindrical and closed at one end perpendicular to its axis with a central circular aperture in the closed end.

Cast about this sheath is a discontinuous jacket of the tin-lead alloy, comprising two axially spaced rings of alloy cast about the sheath and a disc of alloy cast onto the closed end of the sheath, the axially spaced rings defining a gap in the jacket. This particular configuration of the jackets and the size of the circular aperture in the copper sheath are chosen so that the detector will give a circular polar response as nearly as possible.

Theformer need for individually machined lead and tin components and an outer case to hold these components in place is obviated and the energy absorbing components of the filter can be produced as a single rigid member.

An embodiment of the invention will now be described by way of example with reference to Figure 1 which is a sectional view of a Geiger Muller tube enclosed in an energy filter.

Figure 1 shows a Geiger Muller tube 1 inside a generally cylindrical copper sheath 2 which sheath 2 is closed at one end by a wall 3 perpendicular to its axis. The end wall 3 has a central aperture 4 to improve the circular polar response of the detector.

The Geiger Muller tube possesses a detection head 5.

Two axially spaced rings 6 and 7 of a 60/40 tin-lead alloy are cast about the generally cylindrical sheath 2, these rings 6,7 defining an annular gap 8 therebetween also to improve the response of the detector.

The discontinuous jacket is completed by a disc 9 of the same tin-lead alloy cast onto the outside of the end wall 3.

An alternative method of installing the tin-lead rings is to cast them in isolation from the copper tube, for example, as a single shot moulded set, and then to assemble them around the tube by means of adhesive of any other convenient method.

Preferably the tube 1 is supported inside the sheath 2 by silicone rubber mountings which help to isolate the tube 1 from mechanical shock.

In one embodiment, shown in Figure 1, a disc shaped plug of perspex closes the open end of the sheath 2. In another, the end may be closed by a plug of tin-lead alloy. A conduit 11 carrying electrical connections to the tube 1 projects axially from the tube 1 through a central hole 12 in the plug 10.

1. An energy filter for enclosing a Geiger Muller tube, comprising a copper sheath for receiving the tube and a discontinuous jacket of tin-lead alloy about this sheath.

2. An energy filter as claimed in claim 1 in which the alloy jacket contains substantially 60% tin and 40%lead.

3. An energy filter as claimed in claims 1 or 2 in which the alloy jacket comprises two axially spaced rings attached to the copper sheath and one disc attached to the end of the sheath at the end which houses the detection head of the Geiger Muller tube.

4. An energy filter as claimed in claim 1, in which the copper sheath is substantially cylindrical and possesses an aperture at the end in which the detection head of the Geiger Muller tube is situated.

5. An energy filter as claimed in claim 4 in which the copper sheath is open at the end which does not house the detection head of the Geiger Muller tube.

6. An energy filter as claimed in claim 5 in which the open end is plugged by a disc containing an aperture for an electrical lead.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Energy filter The invention relates to Geiger Muller tubes for detecting ionizing radiations and concerns an improved method of making energy filters used with these tubes. The response of a Geiger Muller tube is dependent upon the energy of the radioactivity being detected, and typically the sensitivity of the tube at 500-600 key can be about 12 times greater than at 6000 keV. It is known to be possible to correct this error in the count by enclosing the tube in an energy filter which reduces the number of lower energy photons entering the tube. Such a filter reduces the count at lower energies while leaving the high energy count relatively unaffected and so the sensitivity of the detector tends to level out over an intermediate range of energies. A known filter comprises a copper sheath for receiving a Geiger Muller tube and placed about this sheath a first jacket and a second thinner jacket of tin and lead respectively, these jackets having apertures to improve the response of the detector. The copper sheath not only supports the jackets but is also part of the energy filter and absorbs a proportion of the additional low energy photons generated when high energy photons are absorbed by the tin and lead jackets. Conventionally, the components of the filter are machined separately and held together inside a perspex (polymethyl methacrylate) case which encloses the detector. Production of a typical filter by individually machining and assembling these components is an expensive process. In accordance with the invention there is provided an energy filter for enclosing a Geiger Muller tube, comprising a copper sheath for receiving the tube and a discontinuous jacket of a tin-lead alloy about this sheath. Conveniently, tin-lead alloys are available as solders which, because of their low eutectic point, can readily be cast about the copper sheath in one operation with sufficientfidelty that no further machining is required. One such alloy is "60/40 tin-lead solder" which contains substantially 60% tin and 40% lead. This ratio of metals is similar to the proportions of lead and tin present in the combined jackets of known filters and therefore, a jacket of this alloy can be made having the energy absorbing characteristics necessary to level out the sensitivity of the detector over the range of interest. A copper sheath for enclosing a Geiger Muller tube is generally cylindrical and closed at one end perpendicular to its axis with a central circular aperture in the closed end. Cast about this sheath is a discontinuous jacket of the tin-lead alloy, comprising two axially spaced rings of alloy cast about the sheath and a disc of alloy cast onto the closed end of the sheath, the axially spaced rings defining a gap in the jacket. This particular configuration of the jackets and the size of the circular aperture in the copper sheath are chosen so that the detector will give a circular polar response as nearly as possible. Theformer need for individually machined lead and tin components and an outer case to hold these components in place is obviated and the energy absorbing components of the filter can be produced as a single rigid member. An embodiment of the invention will now be described by way of example with reference to Figure 1 which is a sectional view of a Geiger Muller tube enclosed in an energy filter. Figure 1 shows a Geiger Muller tube 1 inside a generally cylindrical copper sheath 2 which sheath 2 is closed at one end by a wall 3 perpendicular to its axis. The end wall 3 has a central aperture 4 to improve the circular polar response of the detector. The Geiger Muller tube possesses a detection head 5. Two axially spaced rings 6 and 7 of a 60/40 tin-lead alloy are cast about the generally cylindrical sheath 2, these rings 6,7 defining an annular gap 8 therebetween also to improve the response of the detector. The discontinuous jacket is completed by a disc 9 of the same tin-lead alloy cast onto the outside of the end wall 3. An alternative method of installing the tin-lead rings is to cast them in isolation from the copper tube, for example, as a single shot moulded set, and then to assemble them around the tube by means of adhesive of any other convenient method. Preferably the tube 1 is supported inside the sheath 2 by silicone rubber mountings which help to isolate the tube 1 from mechanical shock. In one embodiment, shown in Figure 1, a disc shaped plug of perspex closes the open end of the sheath 2. In another, the end may be closed by a plug of tin-lead alloy. A conduit 11 carrying electrical connections to the tube 1 projects axially from the tube 1 through a central hole 12 in the plug 10. CLAIMS

7. An energy filter as claimed in claim 6 in which the apertured disc is made of substantially 60-40 tin-lead alloy.

8. A method of making an energy filter as claimed in any of the preceding claims, wherein the discontinuous jacket of tin-lead which is mounted on the copper sheat is made by casting.

9. A method as claimed in claim 8, wherein the discontinuous jacket pieces of tin-lead alloy are made by casting onto the copper sheath.

10. A method as claimed in claim 8 wherein the alloy jacket pieces are cast as a single shot moulded set.

11. A method as claimed in claim 10 in which the alloy jacket pieces are fixed to the tube by means of adhesive.

12. A method as claimed in any of claims 8 to 11, in which the apertured end disc of 60-40 tin-lead alloy, which is fitted to the end of the tube which does not house the detection head, is cast at the same time as the tin-lead jacket pieces.