GB2192091A - Improvements in or relating to electron and x-ray detector systems, especially for use with electron microscopes - Google Patents

Abstract

A detector system mounted at the end of a probe includes a semiconductor detector (1) and a shutter located at the detector end of the probe the shutter being open in normal use. The shutter can be closed using either a manual or an automatic control which is situated at some distance from the probe. This protects the detector from over exposure despite the restricted space in which the detector operates. One system controls the shutter by means of a tube mounted round the probe and which rotates to control the shutter. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electron and X-ray detector systems, especially for use with electron microscopes This invention relates to electron and X-ray detector systems, especially for use with electron microcopes.

Figure 1 of the accompanying drawings illustrates diagramatically a known form of such systems. It comprises a semi-conductor detector 1, usually in the form of a crystal of lithium drifted silicon, mounted at the end of a probe 2 which allows the detector 1 to be introduced into an electron microscope in proximity to the specimen under analysis.

When the electron beam of the microscope impinges on the specimen, it knocks out electrons and it also causes the emission of Xrays, having an energy spectrum characteristic of the element struck by the beam and the electrons and X-rays impinge on the detector 1. Adjacent to the detector at the end of the probe is located an FET which is the first stage of a head amplifier 3. The current induced in the detector 1 by the X-rays and electrons is amplified in the head amplifier and passed along the probe to a further amplifier which may be located near the remote end of the probe, though not indicated in the drawings. The detector requires to be cooled during operation and to achieve this cooling the probe 2, which is elbowed at 4, terminates in a heat conductive connection with the inner vessel of a vacuum flask 6.The vessel 6 if filled with liquid nitrogen when the detector is in use, which serves to maintain the detector in the vicinity of - 1800C. To reduce heat loss, the whole of the probe 2 is contained within a tubular envelope 7 forming an extension of the outer evacuated vessel of the flask 6. The end of the envelope 7 at the detector is closed by a beryllium window 8 which maintains the vacuum, when the detector is not inserted in the vacuum of the electron microscope. Reference 9 represents the target chamber of an electron microscope, whilst reference 10 represents the target. Fig. 1 illustrates the detector system in position for use, with the crystal 1 close to the target 10.

When in use the pulse output of the system is fed to an analysing circuit.

One difficulty encountered with systems such as illustrated is that the detector 10 which is in a confined space and inaccessible when in use, is liable to damage because of exposure to excessive radiation. This liability can arise with certain types of microscope, or on the occurrence of certain situations in the microscope. It may also be desirable to use the detector without a beryllium window.

The object of the present invention is to reduce this liability by providing protection from the detector despite the restricted space in which it is located and despite its inaccessibility.

In according with the present invention a detector system of the general kind illustrated in Fig. 1 is provided with shutter means at the detector end of the probe, which is normally withdrawn from the detector, but can be moved to cover the detector, wholly or partly, by control means at distance from the detector.

Preferably the control means may include a tubular member within the envelope for the probe, and surrounding the probe, which tube can be actuated to control the shutter from outside the envelope at a position remote from the detector. In one form of the invention the tubular member is arranged for manual activation, in which case the shutter can be adjusted to vary the exposure of the detector. In another form of the invention, the tubular member is arranged to be actuated automatically, for example by an electromagnet, to close the shutter if the current in the detector exceeds a threshold value.

In order that the invention may be better understood and carried into effect, it will now be described in more detail with reference to Figs. 2 and 10 of the accompanying drawings, Fig. 1 having already been referred to.

In the drawings: Figure 2 a sectional view of the detector end of the envelope for a probe embodying a shutter according to one form of the invention.

Figure 3 is a view from the front of the envelope looking in the direction of the arrow Ill.

Figure 4 illustrates the control mechanism for the shutter shown in Figs. 1 and 2 located at the elbow of the probe, the view being a section looking toward the detector end.

Figure 5 is a sectional view, like Fig. 2, of the detector end of the envelope for a probe embodying a second form of the invention.

Figure 6 is a view, like Fig. 4, of the control mechanism for the shutter shown in Fig. 4.

Figure 7 is a view, also like Fig. 1, showing another form of shutter.

Figure 8 is a view taken in the direction of the arrow VIII in Fig. 7.

Figure 9 is yet another view like Fig. 1 showing a further form of shutter, and Figure 10 is a representation of the acutating mechanism for the Fig. 9 arrangement.

Corresponding parts in the different figures are shown by the same references.

Referring to the drawings, reference 20 in Fig. 2 denotes part of the envelope of the probe near the detector end, the position for the crystal 1 being shown by the dotted outline. A housing 21 for a beryilium window 22 (shown by a dotted line) is fitted to the front end of the envelope 20. This housing has two grooves 23 and 24 cut horizontally (as repre sented in the drawings) in the inner face. One leaf 25 of a shutter had a bar 26 which slides in the groove 23 and a second leaf 27 has a bar 28 which slides in the groove 24. Pins 29 and 30 project from the bars and enter apertures formed in the end wall 31 of a rotatable tube 32 which is a close fit inside the envelope 20. The probe 2, in turn extends inside the tube 32.As can be seen, rotation of the tube 32 causes the leaves 25 and 27 of the shutter to open and close the aperture 33 on the housing 21, so as the expose the crystal 1 to X-rays and electrons, or protect it from them. The apertures in the wall 31 which are entered by the pins 29 and 30 are not visible, but they are sufficiently elongated to allow the tube to rotate.

The tube 32 extends the full length of the envelope 1, to the elbow of the probe. The tube projects into the casing of the elbow, as indicated in Fig. 4, where it is fitted with a clevis 35. The clevis is linked at 36 to a thrust block 37 which slides on one or more pins such as 38 and can be moved axially by a screw 39. The screw is rotatable from outside the casing 34 by a knob 40.

The limits of rotations of the clevis 35 and thus of the tube 32 are adjustably set by studs 41 and 42. The knob 40 is caiibrated for different shutter openings so that the operator of the system can pre-set the shutter to avoid over exposure of the crystal for any particular situation. The clevis is spring loaded toward the open position of the shutter by spring 43.

In the Fig. 5 arrangement, the shutter, instead of being formed by leaves 27 and 28, takes the form of a front cap 50 on the rotatable tube 32. The arrangement of Fig. 5 is intended for use with a transmission microscope and the entrance aperture is inclined and eccentric. A beryllium window 22 is fixed in front of the aperture as before. The front cap 50 also has an eccentric and inclined aperture and this being the case it can be used as a shutter to expose or obscure the crystal 1 merely by rotation.

However, as shown in Fig. 6, the rotation of the tube 32 is performed by an electromagnet having a armature 51 and movable in casing 52 which has the winding 53 of the electromagnet fitted on its outside. The casing 52 is arranged to be vacuum tight, and so the armature 51 moves in the vacuum side of the envelope for the probe, whilst the electromagnet is outside the vacuum. The armature is connected to the clevis by link 54 and the spring 43 rotates the tube 32 to open the shutter 50 when the electromagnet is de-energised. The movement of the clevis to open the shutter is limited by the stop 55.

Figs. 7 and 8 illustrate another example in which the entrance aperture of the system is inclined and eccentric. However, in this case, although the tube is again rotatable the shutter is arranged to slide laterally with respect to the aperture. The shutter slides in a groove 61 on the head of the envelope and is coupled to a forward extension 62 of the tube 32 at 63. The actuating mechanism is not shown for this embodiment, as it is essentially the same as the illustrated in Fig. 6.

In the embodiment illustrated in Figs. 9 and 10, the shutter takes the form of a flap and this form of the invention is especially suitable for detector systems which do not have a beryllium window to protect the crystal. The flap 70 is hinged at 71 and is coupled by link to a projection 73 at the front end of the acutation tube 32. In this form of the invention the tube 32 is not rotatable but is slideable inside the envelope 73 to a sufficient extent to open and close the flap 70 as required. The flap can be closed whilst the front end probe is still in the vacuum of the electron microscope, and the flap is capable of maintaining the vacuum in the probe after withdrawal from the electron microscope, the flap being sealed in the closed position by the "0" ring 74. The actuating mechanism for the tube 32 and flap 70 is merely represented in Fig. 10 by the casing at the elbow in the probe, and it may be of a suitable form but is preferably a bellows sealed gas operated cylinder arranged to produce axial movement of the tube 32.

Where the shutter is activated by an electromagent, gas cylinder or other automatic means, it may be arranged that the shutter is closed automatically if the current in the detector exceeds a threshold value, above which damage to the crystal is likely to occur. Operation of the electromagnet may, however, by manually controlled.

Other variants of the invention may also be made.

It will be appreciated that in the drawings all the figures are not shown to the same scale, and the illustration has been confined to the parts necessary for understanding the invention.

Claims (10)

1. A detector system comprising, a probe, said probe being surrounded by an envelope being capable of being introduced to an electron microscope, a semi conductor detector mounted at one end of said probe, shutter means at the detector end of the probe adapted to cover the detector wholly or partly, and control means for controlling the coverage of said shutter means, located at a position remote from said detector.

2. A detector system as in claim 1 wherein said control means includes a tubular member within the envelope for the probe and surrounding the probe, which tube can be actuated, to control the shutter, from outside the envelope and at a position remote from the detector.

3. A detector system as in claim 2 in which said tubular member is arranged for manual activation, said shutter being adjustable to vary the exposure of the detector.

4. A detector system as in claim 2 in which said tubular member is arranged to be actuated automatically to close the shutter if the current in the detector exceeds a threshold value.

5. A detector as in claim 4 further comprising electromagnet means for actuating the tubular member when the current in the detector exceeds the threshold value.

6. A detector system as in any of the claims 2-5 wherein said shutter comprises a cap on the rotatable tube said cap having an eccentric and inclined aperture.

7. A detector as in claim 1 wherein said control means comprises a slideable tube within the envelope for the probe and the shutter comprises a flap coupled to said tube.

8. A detector as in claim 7 wherein said flap is adapted to seal the probe.

9. A detector as in any of the preceding claims further including means for pre-setting the shutter to avoid over exposure of the detector.

10. A detector substantially as described herein with reference to the accompanying diagrams.