X-ray tube for generating soft X-rays

In an X-ray tube, particularly for the detection of elements having a low atomic number by X-ray spectral analysis, an anode support (4) is on the target side provided with a layer (7) of scandium. The scandium layer is provided on the support by adhesion via an intermediate layer (8), preferably of chromium. In a reflection tube, the anode support for the intermediate layer and the scandium layer is preferably made of silver or copper. In a transmission tube, the scandium is provided on a beryllium disc which acts as intermediate layer and as an exit window (6).

BACKGROUND OF THE INVENTION 
The invention relates to an X-ray tube comprising a cathode and an anode 
which are accommodated in an envelope, said anode comprising an anode 
support which is made of a material having comparatively high thermal 
conductivity. 
An X-ray tube of this kind is known from U.S. Pat. No. 4,205,251. For the 
detection of elements having a comparatively low atomic number, for 
example lower than 20, by means of X-ray spectral analysis, known X-ray 
tubes are not ideally suitable, because the X-rays to be generated therein 
for the analysis of light elements contain an insufficient amount of 
long-wave X-rays. 
For generating comparatively long-wave X-rays, it is desirable to use an 
anode material containing an element having a low atomic number. However, 
problems are then encountered because such materials, for example, are 
difficult to machine, have a poor thermal conductivity, and are expensive. 
Moreover, several of these materials frequently also occur in the 
specimens to be analyzed. The cost and particularly the poor thermal 
conductivity usually make it unattractive to manufacture an anode block 
from these materials. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an X-ray tube in which an X-ray 
beam containing a comparatively large amount of long-wave radiation can be 
generated without deterioration of the customary favorable thermal and 
structural properties of the tube and in which the beam contains only a 
small amount of radiation which is characteristic of one of the customary 
elements to be analyzed. An important argument in favour of scandium is 
thus also the fact that scandium occurs only rarely in specimens to be 
analyzed, so that the characteristic radiation of elements in the specimen 
to be analyzed is not disturbed by the characteristic radiation of the 
anode material. To this end, an X-ray tube of the kind set forth in the 
opening paragraph of this specification is characterized in accordance 
with the invention in that a target disc which essentially consists of 
scandium is provided on the anode support by adhesion or galvanic 
deposition, and using an intermediate layer. 
Because a scandium target disc is provided together with an intermediate 
layer in accordance with the invention, a mechanically strong, suitably 
homogeneous bond is achieved between the disc and the block. Moreover, the 
bond is well capable of withstanding large temperature fluctuations and 
elevated temperatures. The occurrence of seriously disturbing X-rays in an 
X-ray beam to be generated by means of this tube is also mitigated. 
In a preferred embodiment the anode support consists mainly of silver or 
copper, the intermediate layer consists of titanium, vanadium or chromium, 
or a mixture of two or more of these elements, which is provided in the 
form of a thin, suitably adhesive layer on a target side of the anode 
block or on an anode target to be bonded thereon. Thanks to the use of 
these materials, the occurrence of disturbing short-wave X-rays in an 
X-ray beam to be generated is avoided. 
The use of chromium for the intermediate layer is particularly attractive. 
Chromium can be provided on the anode block in a suitably adhesive and 
homogeneous manner by electrolysis as a layer having a thickness of, for 
example, 50-100 .mu.m. In a preferred embodiment, a scandium plate having 
a thickness of, for example, approximately 50 .mu.m is bonded to such a 
chromium layer by adhesion at a temperature approximately in the range of 
800.degree.-100.degree. C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An X-ray tube embodying the invention is diagrammatically shown in the 
single FIGURE of the drawing and comprises an evacuated envelope 1 in 
which a cathode 2 with an electron-emissive element 3 and an anode support 
4 with an anode target 5 are accommodated. A voltage can be applied 
between the anode and the cathode so that electrons to be emitted by the 
electron-emissive element are intercepted by the anode in order to 
generate X-rays therein. An X-ray beam emerging through an exit window 6 
can irradiate (if desired via a radiation filter) a monochromator crystal 
or specimen arranged in an X-ray analysis apparatus. 
In accordance with the invention there is provided on the anode target 5 a 
scandium plate 7 which has a thickness of, for example, 50 .mu.m and a 
transverse dimension of, for example, approximately 25 mm. In order to 
generate an X-ray beam containing mainly radiation which is characteristic 
of scandium, the thickness of the scandium plate is chosen so that the 
electrons cannot penetrate into the underlying material or so that, should 
X-rays still be generated therein, they are absorbed by the scandium 
plate. The X-ray beam to be used then remains substantially free of this 
radiation which would have a disturbing effect because of the deviating 
wavelength. Scandium, however, cannot be directly deposited on the anode 
block or the anode target if it consists, for example, of silver or copper 
because comparatively brittle and hence mechanically weak intermetallic 
Ag-Sc or Cu-Sc bonds are then formed. Moreover, particularly because of 
the poor thermal conductivity, it is not attractive to make a complete 
anode block of scandium. In order to enable the use of scandium as the 
target material, which is to be preferred from a point of view of 
temperature resistance, vapour pressure at elevated temperatures, but 
principally because of its favorable radiation spectrum, use is made of an 
intermediate layer 8. From a metallurgical point of view, this layer could 
consist, for example of one of the elements V (23), Nb (41), Ta (73), Cr 
(24), Mo (42) or W (74) or a combination of any two or more thereof. The 
elements of this group which have an atomic number above approximately 40 
have the drawback that disturbing x-rays can be generated therein. In 
order to restrict this possibility, the use of V, Cr, or a combination 
thereof is attractive. Chromium offers the additional advantage that a 
thin, suitably homogeneous and adhesive layer can be provided on the anode 
block by electrolysis in a comparatively simple manner. On a chromium 
layer thus deposited a scandium plate can be provided by adhesion so that 
a suitable bond is obtained. The bonding process is performed, for 
example, in an argon atmosphere at a temperature of approximately from 
800.degree. to 1000.degree. C., the scandium plate being pressed onto the 
chromium layer, for example, 30 minutes with a pressure of, for example, 
100 kg/cm.sup.2. A suitable bond is very desirable for a good X-ray tube. 
Very severe requirements must be imposed notably as regards the 
homogeneity of the bond, which means that the occurrence of local heat 
resistances, inclusions and the like must be avoided, particularly because 
the service life of the tube could otherwise be substantially reduced. A 
suitable bond can also be achieved, for example, by depositing for example 
by a diffusion process a chromium film simultaneously with a scandium film 
on an anode target, preferably in a low-oxygen atmosphere. 
Furthermore, the bond should preferably be capable of withstanding 
temperature fluctuations, for example between 0.degree. C. and 600.degree. 
C., and it should preferably also have a high melting point, for example 
at least 800.degree. C., because of potential further work performed on 
the anode, such as the soldering of a scandium-coated anode target 
structure to a further anode block by means of Cu-Ag solder. It is 
preferable to use materials for the anode construction having a vapour 
pressure of less than about 10.sup.-6 torr at a temperature of 
approximately 600.degree. C., otherwise the service life of the tube could 
also be reduced. 
An X-ray tube embodying the invention may alternatively comprise a rotary 
anode, for example for use in an X-ray analysis apparatus in which a high 
radiation density and a comparatively small emissive surface are required, 
for example for a spectrometer or the like. The scandium disc then is 
substituted, for example, by a ring which corresponds to a focal path 
described on the anode by the electron beam. In order to obtain a high 
radiation density, use can alternatively be made of an X-ray tube as 
described in U.S. Pat. No. 3,914,633 whose stubby anode is provided with a 
scandium disc on its target side. 
In a transmission X-ray tube, that is to say an X-ray tube in which the 
X-rays emerge from the side of the anode remote from the target for the 
electron beam, a scandium coating can be provided on the inner side of a 
Be plate which acts as the anode target carrier as intermediate layer and 
also as the exit window. X-rays generated in the scandium then leave the 
X-ray tube via the Be window which has a thickness, for example, 150-500 
.mu.m. 
It has already been stated that the thickness of the scandium layer should 
preferably be sufficient for comparatively pure scandium radiation to be 
generated, and that the intermediate layer should preferably be chosen 
from elements whose characteristic radiation is absorbed to a high degree 
in the scandium layer. However, it may alternatively be attractive to make 
available the X-rays generated in the intermediate layer, for example by 
application of a higher voltage. A solution in this respect is described 
in our U.S. patent application Ser. No. 613,655 filed on 23 May 1984. A 
double-layer anode tube of this kind, particularly containing scandium as 
one of the anode target materials, can be constructed in exactly the same 
way as the above tube, be it that the scandium layer will be substantially 
thinner in accordance with the desired radiation spectrum. A suitable 
thickness may be from 1 .mu.m to some tens of .mu.m. 
An X-ray tube embodying the invention is particularly suitable for use in 
an X-ray analysis apparatus which serves to demonstrate the presence of 
elements having a low atomic number in a specimen. When use is made of a 
scandium disc, the effective radiation yield is then more than twice as 
great as when chromium is used as the target material.