Patent Number: 048903124
Section: description

FIG. 1 shows a rectangular plate 1 of piezoelectric material. The plate is slit from one of the sides 2 by a number of parallel saw cuts which extend in the direction of the opposite side 3, but which terminate at some distance from the side 3. The saw cuts each lie at an equal distance from each other or from the long sides 4, 5 of the plate 1. There is thus formed from the rectangular plate 1 a number (in the embodiment shown 19) of tongues 6 which extend in parallel from a common base which is formed by the unslit part 7 of the rectangular plate. The free ends of the tongues 6 can move in a direction transverse to the surface of the plate 1 with respect to each other. FIG. 2 shows a similar assembly of attenuation tongues formed from a single plate 1' of piezoelectric material. In this embodiment the side 3' is, however, longer than the side 2' and the saw cuts converge when viewed from the longest side 3' in the direction of the shortest side 2', while the long sides 4', 5', corresponding to the long sides 4, 5 of FIG. 1, also converge so that wedge-shaped tongues 6' are obtained which again terminate at some distance from the longest side 3' while leaving free a base 7'. It is pointed out that in FIG. 2 the tongues are not equally long, as a result of which they have a different dynamic behaviour. This possible drawback can be eliminated by, for example, constructing the side 2' of the plate 1' as a circular arc as indicated by broken lines at 2" and making the slits equally long so that the ends of the slits also lie on a circular arc 8. The assemblies of attenuation tongues shown in FIGS. 1 and 2 therefore have the form of a comb. Because the tongues have been formed from a single plate of material and, in addition, continue to form a single whole with the base of the comb (the unslit section 7 or 7') the tongues do not have to be separately mounted and adjusted during assembly in slit radiography equipment. It is possible to make do with mounting and aligning the comb as a whole, which is relatively simple. The replacement of such a tongue assembly can also be performed very simply and rapidly. Because the separation between two tongues is in each case formed by a single saw cut or a slit made in another manner, the gap between two adjacent tongues is automatically equally large over the whole length of the slit. The slits required to form the tongues can be made with a fine small saw. However, it is also possible to make the slits by means of a laser or by means of ultrasonic techniques known per se. In practice, it has proved possible to make very narrow slits with a width in the order of 50 .mu.m by means of ultrasonic techniques. Such a small gap between adjacent tongues cannot, or virtually cannot, be achieved if "loose" tongues are used. FIG. 3 shows in plan view the assembly of FIG. 1 in the assembled state, and FIG. 4 shows a section along the line IV--IV in FIG. 3. The tongue plate 1 provided with slits is mounted in a box-shaped casing 30 with an upper wall 31, a lower wall 32, side walls 33 and 34 and a front wall 35. A back wall is not drawn since it is not necessary. If desired, however, a back wall could indeed be present, which would then have to be provided with a slit. In the front wall 35 a slit 36 has been made. The slit 36, which acts as a slit diaphragm, of the casing faces an x-ray source, the focus of which is indicated by 37. The maximum width and thickness of the fanshaped x-ray beam 38 transmitted through the casing is therefore determined by the dimensions of the slit. Preferably, however, separate adjustment means are further provided in the casing in order to limit the x-ray beam accurately as will be described in further detail below. It is pointed out that hereinbefore and hereinafter the descriptions "upper" and "lower" are used in relation to the orientation of the figure. In reality, the position of the casing depends on the manner in which the slit radiograpjhy equipment is installed. In practical equipment, for example, the wall 34 may be the lower wall and the wall 33 the upper wall, or the wall 31 may be the lower wall and the wall 32 the upper wall. The x-ray source might also be situated on the other side of the casing (i.e. on the right-hand side in FIGS. 3 and 4). In the embodiment shown the non-slit section, the base 7, of the tongue plate 1 is mounted on the lower wall 32 of the casing and its tongues are directed obliquely upwards towards the slit in the front wall. Above the base 7 there is disposed a clamping bracket 39 which is attached by means of bolts 40 or the like to the lower wall of the casing on either side of the tongue plate and thus clamps the tongue plate against a spacer 41 of insulating material. In order to be able to control the piezoelectric tongues separately, the tongues have to be electrically separated from each other at least on one side of the tongue plate. For this purpose, in the exemplary embodiment shown the slits in the plate 1 are continued on the lower side across the base 7 to a depth of about half the thickness of the plate, as is indicated by broken lines 42 in FIG. 3. Since such slits to a depth of half the thickness weaken the tongue plate, the base 7 is preferably provided with a glued reinforcing strip 43 of a suitable material on the non-slit upper surface. Moreover, the ridge 7 is provided with a common electrical connecting point 44 on the upper side. In the spacer 41 there is disposed a cutout in which connector element 45 is placed which brings about separate electrical connections with each tongue 6. For this purpose, the connecting points of the connector element in the exemplary embodiment shown are connected to electrically conducting tracks (not shown) disposed on an insulating plate 46 placed on the lower wall of the casing. One conducting track is provided for each tongue and preferably an additional track is also present which is connected to the common connecting point 44. The conducting tracks are connected to diagrammatically indicated connector pins 47, two of which are visible, which reach through a cutout in the lower wall. The ends of the tongues 6 situated near the slit 36 reach, during operation, to a greater or lesser extent into the x-ray beam 38 and in the quiescent state are situated just outside the x-ray beam or in the x-ray beam, depending on the chosen manner of controlling the position of the tongues. Although the piezoelectric material of the tongues themselves in most cases already attenuates x-ray radiation to an extent which is adequate to influence the x-ray beam in the required manner, if desired, elements 48 of an x-ray radiation-absorbing material, for example lead or tungsten, can be provided on the ends of the tongues. Said elements may advantageously be formed from a strip of material which is provided, for example, by gluing along the edge 2, or 2' of the plate 1, 1' which has not yet been slit. As a result of the slitting the separate absorption elements are then produced simultaneously with the tongues. The use of absorption elements on the free ends of the tongues has the additional advantage that the influencing of the x-ray beam can be made proportional to the angular position of the tongues in a simple manner. In addition, the risk is reduced that unattenuated x-ray radiation is transmitted between two adjacent tongues which have a different angular position, i.e. whose ends reach to a different extent into the x-ray beam. Admittedly, said risk is nevertheless already very small in a tongue assembly according to the invention as a consequence of the very narrow slits between the tongues. In the tongue assembly shown, the risk of transmission of x-ray radiation which cannot be influenced via the slits between the tongues virtually only exists in the case of the slits on either side of the centre tongue (s) because said slits are most in line with the x-ray radiation at that point. This is dependent on the distance between the tongues and the x-ray focus, the dimensions of the x-ray focus and the width of the ends of the tongus. All this can, if desired, be prevented by providing at least the centre tongue(s) with two small elements or horns of absorbing material which reach forward, i.e. in the direction of the x-ray source and which precisely shield slits on either side of the tongue. Such small elements are indicated at 63, 64 for the centre tongue in FIG. 5 which shows the centre tongue 60 and two adjacent tongues 61, 62. If elements 48 are also used, the horns 63, 64 may form a single whole with the element 48 concerned. As an alternative it is possible to provide all the tongues apart from the centre one with absorption elements 48 and to provide the centre tongue with an element 48' (FIG. 6) which reaches forward and which is equally as wide as the tongue itself plus the two slits on either side of the tongue. Depending on the distance between the x-ray focus and the tongues, the width of the ends of the tongues and the dimensions of the x-ray focus, it may be desirable to provide the tongues, at least in the central region, alternately with elements 48 and 48'. The difference in dynamic behaviour of the tongues produced by this can be compensated for by constructing the ends of the tongues provided with elements 48' in a wedge-shaped manner as is indicated by broken lines at 65 in FIG. 6 for the centre tongue. A similar method could in principle be used also in the case of tongue 60 of FIG. 5. It is pointed out that use of additional absorption elements reduces the mechanical resonant frequency of the tongues, as a result of which the response of the tongues becomes slower. This effect can be compensated for by constructing all the tongues with a wedge-shaped end. By choosing the length and/or tapering of the wedgeshaped sections differently for tongues which are provided with an absorption element projecting forwards such s 48' from those chosen for tongues which are provided with an element which does not project forward such as 48, the same resonant frequency can be obtained for every tongue. In general it is of importance in slit radiography equipment provided with attenuation tongues to be able to set the dimensions of the scanning x-ray beam, i.e. the thickness and the width, or rather the angles .rho. (FIG. 3) and .alpha. (FIG. 4) as accurately as possible in order to be certain that the whole of the scanning x-ray beam, or at least a fixed section thereof can also in fact be influenced by the tongues. If a tongue plate according to the invention is used, this is still more important because the maximum deflection of the tongues has preferably to be as small as possible in order to prevent the base 7 from cracking at the position of the junction with the tongues. A relatively small maximum deflection of the tongues requires a relatively large setting accuracy for the dimensions of the x-ray beam. For this purpose, according to the invention, there are disposed on either side of the x-ray beam strongly absorbing, for example lead, elements 49 and 50 respectively which are attached to the side walls 33, 34 of the casing in the exemplary embodiment shown and whose distance from the side walls is adjustable, as is indicated diagrammatically by arrows 51. For the setting, an adjustment screw may, for example, be provided such as is indicated by 52 for the element 50. The thickness of the x-ray beam can be set in a similar manner by an adjustable strip of absorbing material disposed on the upper side of the x-ray beam. The lower side of the x-ray beam is defined in this exemplary embodiment by the lower edge of the slit 36 in the casing 30 which is also manufactured from a material which absorbs x-ray radiation. FIG. 4 shows such a strip of absorbing material 53 which is joined to the upper wall via a spring plate or strip 54 and whose distance from the upper wall can be set by means of one or more adjustment screws 55. It is pointed out that only a few exemplary embodiments of the invention have been described above. In addition to the above various modifications are obvious to those skilled in the art. Thus, for example, the risk of spark flashover between adjacent tongues can be reduced by providing the tongues with a layer of lacquer. Such modifications are considered to fall within the scope of the invention.