GB2350718A

GB2350718A - Standard alpha particle source

Info

Publication number: GB2350718A
Application number: GB9912882A
Authority: GB
Inventors: Peter Danyluk
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1999-06-04
Filing date: 1999-06-04
Publication date: 2000-12-06
Also published as: WO2000075692A1; GB0123610D0; GB9912882D0; GB2363673A; GB2363673B; AU5234000A

Abstract

A standard alpha particle source which can be configured so as to provide a pre-set value of emitted alpha energy or configured to give a level of emitted alpha energy which can be varied to suit the required usage of the source. The alpha-emitter 1 is held in a casing 6 and has an air-filled part 3 acting as an absorber. The gap 3 is defined by a membrane 4 and alpha particles are emitted via a collimator 5.

Description

2350718 STANDARD ALPHA PARTICLE SOURCE The present invention relates to

alpha particle sources and in particular to sources used in the calibration of alpha particle detectors.

Known alpha particle sources comprise a sample of radioactive material which is known to emit a well defined alpha energy dependant on the radioactive material used.

A disadvantage of the known sources is that only specific alpha energies can be achieved as the energy is dependent on the radionuclide used. At present there is no standard of variable energy alpha source for setting thresholds on the wide range of alpha detectors and counting systems presently in use, especially within the nuclear and physics research industry.

Depending on where the lower limit of detection (LLD) is set, alpha particle detectors can display significantly varying activity responses (cps per Bq per cm') to degraded alpha sources such as those that occur operationally in fields such as the decommissioning field, and yet give similar responses from ISO reference sources designed for statutory testing with little or no self absorption. If the LLD is set too high, a large proportion of counts will be lost below the LLD threshold for a badly degraded source, with corresponding inability to detect alpha particles.

The aim of the present invention is to provide a standard alpha particle source which can be configured so as to provide a pre-set value of emitted alpha energy or configured to give a level of emitted alpha energy which can be varied to suit the required usage of the source. Amongst other uses this will allow the accurate measurement and setting of the LLD for a wide range of alpha particle detectors both portable and laboratory based.

The present invention utilises an alpha particle energy source the alpha energy from which is varied via energy loss through an absorbing material.

2 Accordingly, the present invention provides a standard alpha particle source configured so as to emit a known level of alpha particle energy comprising a source of alpha particles and an absorbing medium both being mounted in a casing having an area which is substantially transparent to alpha particles through which the alpha particles are emitted wherein the absorbing medium is configured so as to reduce the emitted alpha particle energy to a known level.

The emitted alpha particle energy can be varied linearly or in discrete controlled increments so as to give a set number of specific emissions.

In order to provide a near monoenergetic alpha particle fluence from an alpha source based on energy loss through an.-,,absorber, which is advantageous in the present invention, the distance traversed by each alpha particle from the source, through the absorber to the detector front face is preferably constant. This can be achieved by collimating the source output. As such the standard alpha particle source can advantageously further comprise a collimator.

The collimator can be mounted between the source of alpha particles and the absorbing medium or alternatively the collimator can be mounted such that the absorbing medium is located between the collimator and the source of alpha particles. In the later configuration the external surface of the collimator advantageously provides a robust reference plane for the source.

Great advantage can be achieved by making the alpha energies emitted variable. In order to achieve this a standard alpha particle source can be configured such that the properties of the absorbing medium or the amount of absorbing material through which the alpha particles must pass can be varied so as to controllably produce a variable amount of emitted alpha energy.

The above variability can be achieved by any suitable means however one possibility is the use of interchangeable absorbers which provide different levels of absorption.

3 These interchangeable absorbers can be changed in any suitable way examples which could be used are pulling one out and replacing it with another or the interchangeable absorbers could be provided on a wheel which can be rotated to change the absorbing medium 5 through which the alpha particles must pass.

The use of wheels could be further improved to give more variability by providing a plurality of concentrically stacked wheels which can be rotated in relation to each other to change the total absorbing medium through which the alpha particles must 10 pass.

The absorbing material can also be varied by varying the amount of absorbing material through which the alpha particles must pass this can be done in any suitable way. If a gas such As air is used as the absorbing material, one example would be to move the alpha is source away from the area which is substantially transparent to alpha particles through which the alpha particle energy is emitted, thus varying the thickness of air. Another method would be to increase the pressure of the gas so as to increase the amount of gas between the source and the substantially transparent 20 area.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, wherein Fig 1. Shows a cross-section of a pre-set energy alpha source 25 according to the present invention; Fig 2. Shows an exploded view of a first embodiment of a variable energy alpha source according to the present invention; Fig 3. Shows a cross-section of a second embodiment of a variable energy alpha source according to the present invention; Fig 4. Shows a plan view of the variable energy alpha source shown in figure 3.

Referring to figure 1 a pre-set energy alpha source comprises a source of alpha particles, 1, deposited on a source disc, 2, an 4 absorbing medium in the form of air, 3, a foil, 4, and a collimator 5. The collimator, 5, the source of alpha particles, I and the absorbing medium, 3 & 4, are all mounted in a casing, 6, The collimator, 5, is mounted in the casing, 6, and configured so as to provide a plurality of orifices, 7, through which the alpha particles are emitted. The foil, 4, is mounted away from source, 1, by means of spacers, 8, and is used in combination with the air, 3, to reduce the alpha energy emitted from the standard source to a pre-determined level.

A second embodiment of the present invention is shown in figure 2 which provides a variable source of alpha energy. A source of alpha particles, 21, is mounted on a piston, 22, which in turn is mounted on a spiral thread, 23. The spiral thread, 23, piston, 22 and source, 21, are moveably located within the outer casing, 24.

A collimator, 25, is attached to the one end of the outer casing, 24. An end cap, 26, is attached at the end remote from the collimator, 25, and is attached to the lower half of the spiral thread, 23. The source, 21, can be moved inside the casing, 24, by twisting the end cap, 26. Twisting the end cap, 26, causes the spiral threads, 23, to rotate in relation to each other forcing them together or apart within the casing, 24,dependant on which way the end cap, 26, is twisted. This movement increases or decreases the distance between the source, 21, and the collimator, 25, thus changing the distance the alpha particles have to travel through the absorbing material, in this case the air. The change in distance results in a proportional change in the alpha particle energy emitted from the source.

A further embodiment of the present invention is shown in figure 3 which also provides a variable source of alpha energy however in this case it is in a compact form. The embodiment shown in figure 3 uses two sets of filters, 32 and 33, located between the source, 31, and the collimator, 35, to change the absorbing material through which the alpha particles pass thus changing the alpha particle energy emitted from the source. Referring to figure 3 the variable source comprises a source of alpha particles, 31, a series of 6 fine filters, 32, and a series of 6 coarse filters, 33, mounted on concentrically stacked wheels. The fine and coarse filters give up to 36 possible combinations for the absorbing material and as such 36 possible values in output alpha energy level. Also provided is a shutter, 34, for preventing emission of radiation and a collimator, 35.

Figure 4 shows the arrangement of figure 3 in plan form showing the various orifices, 41, through which the radiation can be emitted. Only one of these orifices will be open at any one time the rest being closed by the shutter, 34, shown in figure 3.

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Claims

1. A standard alpha particle source configured so as to emit a known level of alpha particle energy comprising a source of alpha particles and an absorbing medium both being mounted in a casing having an area which is substantially transparent to alpha particles through which the alpha particles are emitted wherein the absorbing medium is configured so as to reduce the emitted alpha particle energy to a known level.

2. A standard alpha particle source according to claim I wherein the emitted alpha particle energy is varied in discrete increments

3. A standard alpha particle source according to claim 1 further comprising a collimator.

4. A standard alpha particle source according to claim 3 wherein the collimator is mounted between the source of alpha particles and the absorbing medium.

5. A standard alpha particle source according to claim 3 wherein the collimator is mounted such that the absorbing medium is located between the collimator and the source of alpha particles.

6. A standard alpha particle source according to any of the preceding claims wherein the properties of the absorbing medium or the amount of absorbing material through which the alpha particles must pass can be varied so as to controllably produce a variable amount of emitted alpha energy.

7. A standard alpha particle source according to claim 6 wherein the properties of the absorbing medium are varied by using interchangeable absorbers which provide different levels of absorption.

8. A standard alpha particle source according to claim 7 wherein the interchangeable absorbers are provided on a wheel which can be rotated to change the absorbing medium through which the alpha particles must pass.

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9. A standard alpha particle source according to claim 7 wherein the interchangeable absorbers are provided on a plurality of concentrically stacked wheels which can be rotated in relation to each other to change the total absorbing medium through which the 5 alpha

10. A standard alpha particle source according to claim 6 wherein the amount of absorbing material through which the alpha particles must pass can be varied by moving the alpha source away from the area which is substantially transparent to alpha particles through which the alpha particle energy is emitted.

11. A standard alpha particle source according to any of the preceding claims wherein the absorbing material is a gas.

12. A standard alpha particle source according to claim 11 wherein the amount of absorbing material through which the alpha particles must pass can be varied by varying the pressure of the gas.

13. A standard alpha particle source hereinbefore described with reference to the accompanying drawings.