GB1578325A

GB1578325A - Radiation detectors

Info

Publication number: GB1578325A
Application number: GB1013376A
Authority: GB
Original assignee: EMI Ltd
Current assignee: EMI Ltd
Priority date: 1977-03-23
Filing date: 1977-03-23
Publication date: 1980-11-05

Description

(54) IMPROVEMENTS IN OR RELATING TO RADIATION DEThUFORS (71) We, E.M.I. LIMITED, a British company, of Blyth Road, Hayes, Middlesex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to detectors of radiation such as X-radiation. One known form of counter suitable for the detection of such radiation is the gas filled ionisation detector comprising a chamber containing a gas such as Xenon at high pressure, as is required to give effective absorption of X-ray photons.

In such detectors it is known that at least some detection is provided by photo-electric effect at the walls of the chamber with ionisation of the gas being produced by the resultant high energy photo-electrons. It has been proposed (Jeavons et al. IEEE transactions on Nuclear Science NS22 297300, 1975) to make use of this effect by producing a gas-solid hybrid detector. This detector has a chamber containing a solid converter either in the form of a solid block perforated with cylindrical holes parallel to the direction of the radiation to be detected or a plurality of grid plates, having holes which are aligned to achieve the same effect. The incident radiation is converted in the solid to give high energy photo-electrons which enter the holes and provide ionisation of the gas therein.An electric field is applied along the axis of the holes (perpendicular to the grid plates) to cause the ionisation current electrons to travel along the holes out of the solid region to a multi-wire proportional counter region, external to the solid, for counting. Such a detector thus gives the advantage of detection without the use of very high pressures and also allows the gas used to be argon, for which the gas pressure would need to be even higher, in place of the more used but more expensive Xenon. The detector does, however, have disadvantages. First, there is a relatively long drift space across which the ionisation current electrons must travel to be counted, thus resulting in a relatively slow speed of operation for the detector, and also the detector is not as efficient as is possible for a solid detector.

It is an object of the present invention to provide an improved gas-solid hybrid detector.

According to the present invention there is provided a gas-solid hybrid ionisation detector comprising a chamber, having a window of material transmissive to penetrating radiation, and a plurality of substantially parallel electrode plates, spaced from one another and disposed within the chamber in the path of radiation entering at said window so that the radiation is incident obliquely to the surface of the plates, wherein the chamber contains a counting gas which fills the spaces between the plates and wherein the plates include metal the nature and disposition of which is such that the plates produce, and introduce into the spaces therebetween, photoelectrons in response to the incident radiation, the arrangement being such that alternate plates are connected to terminals to which a potential difference can be applied to collect ionisation current generated in the gas by said photoelectrons.

In order that the invention may be clearly understood and readily carried into effect, an example thereof will now be described with reference to the drawings filed with the Provisional Specification, of which: - Figure 1 shows a detector in side and end cross-section, Figure 2 shows the position of a detector plate in accordance with the invention relative to the radiation to be detected, Figure 3 shows a suggested detector configuration for the plate position of Figure 2, Figure 4 illustrates a mode of stacking detectors as shown in Figure 3, and Figure 5 illustrates one example of a mounting for the detector plates.

An example of a detector of the type with which the present invention is concerned is shown in Figure 1 in side and end cross-section. The detector comprises a glass envelope 1 having an end window 2 transmissive to the radiation. A plurality of solid converters in the form of metallic strips or plates 3 and 4 extend across the detector parallel to window 2. Plates 3 are connected to a terminal 5 which is maintained at a positive potential relative to another terminal 6. Terminal 6 is connected to the remaining plates 4 which alternate with plates 3. The detector is, in this example, filled with argon. However, other counting gases such as are known for proportional counters, may be used at an appropriate pressure. For example, the pressure for argon should be 2+ times that for Xenon.

In operation X-rays 7 enter the detector via the window 2. The primary conversion is in the plates 3 and 4 from which high energy photo-electrons are ejected into the intervening gas. Electrons and positive ions created by ionisation in the gas by the photo-electrons drift to plates 3 and 4 respectively under the influence of the electric field therebetween. The drift distance involved for any single interplate gap is relatively small and therefore the drift time is relatively short. Suitable means well known in the art are provided to detect the current thus generated at terminals 5 and 6. It will be understood that the crosssection can be other than circular if desired.

The detector of Figure 1 is shown with nine plates 3, 4 for the purpose of illustration but in a practical arrangement of that form a larger number is required. The choice of a material for the plates is a compromise between the requirements of a low density to allow efficient electron escape and a high atomic number to give good X-ray stopping power. A typical material for which a suitable compromise is obtained is tin.

Considering, therefore, the use of tin electrode plates 2, 4 for, say, 75keV X-ray energies, being chosen to lie in the diagnostic energy range of 30-120keV, the X-ray attenuation coefficient is 25cm-" and the photo-electrons have a range of about 10cm. In order to stop, say, 63010 of the incident X-rays a thickness of 400ym is required for the plates which, if each sheet is 10 m thick, requires forty such plates.

Similar considerations apply for other suitable material.

The gas pressure and electrode spacing are determined primarily by the range in the gas of the photo-electrons. Typically, a 40keV electron has a range of 1 mm in Xenon at 10 atmospheres and 5 mm in Xenon at 2 atmospheres. For argon a range of 5 mm is given in 5 atmospheres of gas.

Electrode spacing similar to the above values gives a maximum gain, i.e. the photo-electron undergoes a maximum number of collisions to give a maximum number of electron-ion pairs. Reduced electrode spacing gives a smaller collection time for the ions, therefore faster response, but results in fewer ionisation events, and therefore less gain, assuming that the operation is in the non-avalanche multiplication region.

For a given voltage, gas and electrode spacing the collection time is proportional to the pressure, while for a given pressure, gas and voltage the collection time is inversely proportional to the square of the electrode spacing.

A suitable high, but not maximum, gain detector using Xenon at 5 atmospheres has an electrode spacing of 0.5 mm with a collection potential of 200 volts. This should give an ion collection time of 0-1 m secs.

As noted hereinbefore, to give maximum gain for such a detector, 20 atmospheres gas pressure would be required, for which the ion collection time would be 0 4 m secs.

Although suitable detectors can be constructed in this manner, it is desirable from constructional considerations if a smaller number of plates can be used. An additional factor to be considered is that the photo-electrons are emitted with a sin2 angular distribution about the direction of incidence of the X-ray photons so that for the arrangement of Figure 1 the majority of photo-electrons are emitted along the planes of the plates with little chance of escape.

A suitable arrangement to allow the reduction of both of the above problems is solved in accordance with the present invention by providing the plates at an angle a to the X-ray beam, where a < < 90 , as shown in Figure 2 for a typical plate 3. It will be seen that, for a plate of thickness t, the effective thickness for X-ray absorption t' is now increased to t/sin a whereas the effective thickness for electron escape, perpendicular to the incident X-rays, is reduced in the plate of the Figure to t/cos a. In a typical arrangement for which a is made 6" the required number of 10,um plates can be reduced from forty to four and yet give a higher probability of electron escape.

One practical means of achieving this with a detector of the type shown in Figure 1 is to allow the X-rays to be incident at an angle as shown at 7', providing a window such as 2 at an appropriate point.

Alternatively, the detector may take the configuration shown schematically in Figure 3 with appropriate supporting structures for plates 3 and 4 and electrical connections as in Figure 1. A detector employing the principle of the Figure 3 arrangement can, of course, have a cross-section similar to that of the Figure 1 detector, if desired.

However, the cross-section indicated in Figure 3 has the additional advantage that a plurality of detectors may be placed in close proximity as shown in Figure 4 to allow detection of X-rays along closely parallel paths.

For any of the electrode configurations described, care should be taken that a firm electrode structure is used to reduce problems caused by microphony. Figure 5 shows an electrode structure which is particularly satisfactory in that respect. In this arrangement electrodes 4, in the form of metal foils, for example lOgam in thickness, are separated by mica spacers 8.

Electrical connections 5 and 6 are provided to the rear while X-rays 7 are incident to the front at angle a as in the Figure 2 and 3 arrangement. The electrodes and spacers have been illustrated in Figure 5 slightly separated to better illustrate the construction.

It will be appreciated that the solid converters may take many other forms provided that they intercept the radiation 7 at an angle, are relatively closely spaced and can have appropriate potentials applied therebetween.

Alternatively, since electrons have shorter ranges in X-ray absorbent material, each plate or strip can be in the form of a plate of good electron transmitting material, possibly plastics, with finely divided heavy metal powder dispersed therein.

WHAT WE CLAIM IS: - 1. A gas-solid hybrid ionisation detector comprising a chamber, having a window of material transmissive to penetrating radiation, and a plurality of substantially parallel electrode plates, spaced from one another and disposed within the chamber in the path of radiation entering at said window so that the radiation is incident obliquely to the surfaces of the plates, wherein the chamber contains a counting gas which fills the spaces between the plates and wherein the plates include metal the nature and disposition of which is such that the plates produce, and introduce into the spaces therebetween, photoelectrons in response to the incident radiation, the arrangement being such that alternate plates are connected to terminals to which a potential difference can be applied to collect ionisation current generated in the gas by said photoelectrons.

2. An ionisation detector according to any of the preceding claims in which the plates are spaced and supported by mica spacers.

3. An ionisation detector according to any preceding claim in which the metal is of atomic number sufficiently high to stop, in said plurality of plates, a substantial proportion of said radiation, while having a density or effective density sufficiently low to allow a substantial proportion of the photoelectrons to pass into the gas.

4. An ionisation detector according to any of the preceding claims in which the metallic plates are plates of electron transmissive material having finely divided heavy metal powder disposed therein.

5. An ionisation detector according to Claim 3 in which the metal is tin.

6. An ionisation detector according to any preceding claim in which the plates are of substantially 10,um thickness.

7. An ionisation detector according to any preceding claim in which the gas pressure and electrode spacing are such that a substantial proportion of the photoelectrons generated cause ionisation in the inter-plate spaces.

8. An ionisation detector according to any preceding claim in which the gas is Xenon.

9. An ionisation detector according to Claim 8 which gas is at a pressure of 7 atmospheres and in which the inter-plate spacing is 10/XB mm.

10. An ionisation detector according to any of Claims 1-7 in which the counting gas is Argon.

11. An ionisation detector according to Claim 10 in which the inter-plate spacing is 5 mm at a gas pressure of 5 atmospheres or in inverse proportion at other pressures.

12. An ionisation detector as claimed in Claim 1 substantially as herein described with reference to the drawings filed with the Provisional Specification.

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

Claims

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

3 with appropriate supporting structures for plates 3 and 4 and electrical connections as in Figure 1. A detector employing the principle of the Figure 3 arrangement can, of course, have a cross-section similar to that of the Figure 1 detector, if desired.

However, the cross-section indicated in Figure 3 has the additional advantage that a plurality of detectors may be placed in close proximity as shown in Figure 4 to allow detection of X-rays along closely parallel paths.

For any of the electrode configurations described, care should be taken that a firm electrode structure is used to reduce problems caused by microphony. Figure 5 shows an electrode structure which is particularly satisfactory in that respect. In this arrangement electrodes 4, in the form of metal foils, for example lOgam in thickness, are separated by mica spacers 8.

Electrical connections 5 and 6 are provided to the rear while X-rays 7 are incident to the front at angle a as in the Figure 2 and 3 arrangement. The electrodes and spacers have been illustrated in Figure 5 slightly separated to better illustrate the construction.

It will be appreciated that the solid converters may take many other forms provided that they intercept the radiation 7 at an angle, are relatively closely spaced and can have appropriate potentials applied therebetween.

Alternatively, since electrons have shorter ranges in X-ray absorbent material, each plate or strip can be in the form of a plate of good electron transmitting material, possibly plastics, with finely divided heavy metal powder dispersed therein.

WHAT WE CLAIM IS: - 1. A gas-solid hybrid ionisation detector comprising a chamber, having a window of material transmissive to penetrating radiation, and a plurality of substantially parallel electrode plates, spaced from one another and disposed within the chamber in the path of radiation entering at said window so that the radiation is incident obliquely to the surfaces of the plates, wherein the chamber contains a counting gas which fills the spaces between the plates and wherein the plates include metal the nature and disposition of which is such that the plates produce, and introduce into the spaces therebetween, photoelectrons in response to the incident radiation, the arrangement being such that alternate plates are connected to terminals to which a potential difference can be applied to collect ionisation current generated in the gas by said photoelectrons.
2. An ionisation detector according to any of the preceding claims in which the plates are spaced and supported by mica spacers.
3. An ionisation detector according to any preceding claim in which the metal is of atomic number sufficiently high to stop, in said plurality of plates, a substantial proportion of said radiation, while having a density or effective density sufficiently low to allow a substantial proportion of the photoelectrons to pass into the gas.
4. An ionisation detector according to any of the preceding claims in which the metallic plates are plates of electron transmissive material having finely divided heavy metal powder disposed therein.
5. An ionisation detector according to Claim 3 in which the metal is tin.
6. An ionisation detector according to any preceding claim in which the plates are of substantially 10,um thickness.
7. An ionisation detector according to any preceding claim in which the gas pressure and electrode spacing are such that a substantial proportion of the photoelectrons generated cause ionisation in the inter-plate spaces.
8. An ionisation detector according to any preceding claim in which the gas is Xenon.
9. An ionisation detector according to Claim 8 which gas is at a pressure of 7 atmospheres and in which the inter-plate spacing is 10/XB mm.
10. An ionisation detector according to any of Claims 1-7 in which the counting gas is Argon.
11. An ionisation detector according to Claim 10 in which the inter-plate spacing is 5 mm at a gas pressure of 5 atmospheres or in inverse proportion at other pressures.
12. An ionisation detector as claimed in Claim 1 substantially as herein described with reference to the drawings filed with the Provisional Specification.