An X-ray tube is provided with a target that radiates primarily the characteristic X-ray of molybdenum. The target has an electron focal area of molybdenum base alloy containing molybdenum as a major component, and at least one of titanium and a combination of potassium oxide and silicon dioxide.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an X-ray tube, and more particularly to an X-ray 
tube provided with a target radiating the characteristic X-ray of 
molybdenum. 
2. Description of the Prior Art 
In mammography that performs X-ray photography of mammae is performed by 
the use of low energy X-rays from an X-ray tube using a Mo (molybdenum) 
target that radiates X-rays. These X-rays contains wavelength components 
of approximately 0.4 to 0.8 angstrom. In this case, the target (anode) 
acceleration voltage is on the order of 25 to 40 kV. Specifically, to take 
an X-ray photograph having fewer geometric blurs, the focal point should 
be as small as possible. On the other hand, to obtain a sufficient amount 
of X-ray radiation, a tube current of approximately 100 mA or more is 
usually required, so the focal point becoming larger. In general, an X-ray 
photograph is taken using X-ray radiation for a relatively long time, such 
as 1 to 4 seconds, for example. This results in heating electron focal 
area of the anode target to a high temperature, so that the electron focal 
area is susceptible to damage upon repeated operation. Namely, the 
electron focal area frequently exceeds a temperature of 1700.degree. to 
1800.degree. C., i.e., the recrystallization temperature of pure Mo. As a 
result, the metallic crystals of the electron focal area grow large and 
the surface of the focal area becomes rough. When such thermal fatigue 
thereof progresses, the amount of X-ray radiation is reduced, and the 
X-ray radiation quality becomes progressively harder. 
Therefore, in order to alleviate damage to the electron focal area of the 
target, various alloys have been utilized as materials of the electron 
focal area. For example, the use of a Mo-Hf (hafnium) alloy was disclosed 
in Japanese Patent Application Laid-open No. 49-45692. Also, the use of an 
alloy of Mo and one of 25 wt % or less of a metallic element, having an 
atomic number between 39 and 46, for example, Nb (niobium) was disclosed 
in Japanese Patent Application Laid-open No. 49-45693. However, there were 
no notable improvements with these alloys in comparison with a pure Mo 
target. With pure W (tungsten) or an alloy of R (rhenium) and W used as 
the material of the electron focal area, desired characteristic X-rays of 
Mo cannot be obtained. Also, a complex target of pure W or a Re-W alloy 
has been used as a material for the electron focal area, and the 
supporting target base thereof was made of a Mo alloy having large heat 
capacity (disolosed in Japanese Patent laid open No. 60-198045, and 
British Patent No. 1,121,407). Mo characteristic X-rays could not be 
obtained with this structure. 
When a conventional X-ray tube with a target made of pure Mo was subjected 
to a forced operation test, corresponding to repetitive operations of a 
considerably long time. The state of the target surface became 
deteriorated at the end of the test, as shown in FIGS. 10 and 11. FIG. 10 
is a photomicrograph that shows the rotary anode target surface made of 
pure Mo enlarged to 5 times the actual size. FIG. 11 is a photomicrograph 
that shows a portion of the electron focal area enlarged to 30 times the 
actual size. From these photomicrographs, it can be confirmed that the 
crystals of the electron focal area of the pure Mo target became larger, 
and experienced a lot of deep cracks. This test was performed under such 
conditions that the anode acceleration voltage was 40 kV, the tube current 
was 150 mA, and 4-second electron bombardments were made repeatedly 400 
times at 75-second intervals. In addition, another forced operation test 
was performed under such conditions that the anode acceleration voltage 
was 40 kV, the tube current was 260 mA, and 1-second bombardments were 
made repeatedly 5000 times at 50-second intervals. After this test, it was 
confirmed that the amount of X-ray radiation was reduced to a value of 
approximately 46% of the initial amount, as shown in the curve L in FIG. 
2, showing the comparison among X-ray radiation characteristics of the 
invention and the prior arts. 
SUMMARY OF THE INVENTION 
Accordingly, one object of this invention is to provide an X-ray tube 
having a Mo target that can resist roughened and enlargement of crystal 
grains of electron focal area and can maintain the amount of X-ray 
radiation even after long-time repetitive operations. 
Briefly, in accordance with one aspect of this invention, there is provided 
an X-ray tube that comprises a cathode for emitting electrons, and a 
target provided with an electron focal area to receive impact of the 
electrons and for radiating primarily characteristic X-rays of Mo. The 
target comprises a supporting base with the electron focal area disposed 
thereon, and the electron focal area includes a Mo base alloy that 
contains Ti (titanium), K.sub.2 OSiO.sub.2 (combination of potassium oxide 
and silicon dioxide) or mixtures thereof. More preferably, the Mo alloy 
contains Ti of about 0.3 to about 4 wt % or a combination of K.sub.2) of 
about 0.01 to about 0.1 wt % and SiO.sub.2 of about 0.02 to about 0.3 wt 
%. 
According to the invention, the electron focal area having a non-rough 
surface even after repeated operations at heavy loads. Thus, the reduction 
of the amount of X-ray radiation in the desired direction can be 
significantly restricted. This allows the X-ray tube to possess long-life 
properties. 
By the bombardment of electrons, the surface temperature of the electron 
area on the target base reaches a temperature of approximately 
2600.degree. C., which is significantly higher than the surface 
temperature (approximately 1200.degree. C.) of the target base. As a 
result, the thermal influence reaches a depth of approximately 0.1 mm. 
Therefore, the electron focal area should be 0.2 mm in thickness at a 
minimum.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, and more 
particularly to FIG. 1 thereof, one embodiment of present invention now 
will be described. 
FIG. 1 is a schematic configuration diagram illustrating an X-ray tube of 
the present invention adapted to a rotary anode type X-ray tube for use in 
mammography. In FIG. 1, a metallic vacuum envelope 11 is provided with an 
X-ray radiation window 12, which is primarily made of a beryllium thin 
plate and hermetically sealed to a portion of the metallic vacuum envelope 
11. A glass rotor envelope 13 extends in the direction of the tube axis. A 
cathode structure 14 is disposed on the end of the metallic vacuum 
envelope 11 opposing the glass rotor container 13. A rotor 17 is rotatably 
supported by the glass rotor envelope 13. A rotatable disc-shaped anode 
target 15 is supported by a supporting shaft 16 extended from the rotor 
17. A high voltage is applied between the cathode 14 and the anode target 
15 to which a positive potential side of the high voltage is connected. 
When electrons are discharged from the cathode 14, the electrons are 
accelerated and focused into on electron beam which is impinged on an 
electron focal area 18 of the rotatable anode target 15. An X-ray beam is 
produced an radiated outside window 12 in the arrow-marked direction X. 
The rotatable anode target 15 comprises an electron focal area 18 and a 
supporting base 19. Both of area 18 and base 19 are made of a Mo base 
alloy containing major amount of Mo and a small amount of Ti, and 
additionally a small amount of C (carbon) as a deoxidizer. Preferably, the 
Ti content is in a range of 0.3 to 4 wt % and the C content is in a range 
of 50 to 400 ppm (as the aim composition of the target). Namely, FIG. 3 
shows the relationship between the Ti content (wt %) with respect to the 
Mo of the electron focal area and the relative amount of X-ray with the 
number of times of electron bombardment as parameters, wherein the C 
content is determined to be approximately 200 ppm. Here the electron 
bombardment was performed such that a voltage of 40 kV was applied across 
the target 15 and the cathode 14, and 1-second bombardments of electron 
current of 260 mA were made at 50-second intervals. In FIG. 3, the curve A 
represents the values obtained after 1000-times of electron bombardments, 
and the curve B represents the values obtained after 5000-times of 
electron bombardments. From these curves A and B, it can be understood 
that most preferable X-ray radiation amounts may be obtained when Ti 
content is in the range of 0.6 to 2.0 wt %. However, it also can be seen 
that the Ti contents between 0.3 to 4.0 wt % that can secure the X-ray 
radiation amounts of 60% or more even after 5000 times of bombardments can 
be practically acceptable. Moreover, C functions as a deoxidizer, and is 
not absolutely required. However, when present, C remains dispersed 
between the elements of Mo and Ti, and a portion of the C also remains as 
a form of TiC after vacuum sintering, whereby the structure of metallic 
crystals of the electron focal area of the target can be restrained from 
growth. As a result the surface of focal area 18 remains substantially 
flat. 
When the Ti content is excessively small, the Mo-Ti alloy is about the same 
as pure Mo,. but when it is too large, free Ti that does not combine with 
Mo may be present. The free Ti evaporates when the electron focal area 18 
reaches a temperature of 2600.degree. C., and this evaporation of the free 
Ti can be considered to cause unevenness of the area 18. 
Next, the description will be made as to specific examples. 
A mixed powder was prepared such that TiH.sub.2 powder=1 wt %, C powder=100 
ppm and remainder=Mo powder. All the powder was uniformly mixed. Next, the 
mixed powder was formed into pellets, and was heated within a vacuum 
furnace at a temperature of 2000.degree. C. for 2 hours, whereby a sinter 
was obtained. Thereafter, the thus obtained sintered body was formed so as 
to be densified, and further forged into a certain specified shape. Next, 
the thus forged body was machined, and then put into the vacuum furnace 
with a pressure of 1.times.10-5 Torr of less, wherein the machined body 
was heated at a temperature which was below its recrystallization 
temperature (approximately 1400.degree. C.) for 2 hours so that degas 
treatment was performed. As a result, an X-ray tube target was obtained, 
which was assembled into the X-ray tube envelope. 
The X-ray tube thus obtained was put, in the same manner as in the 
above-described prior art, through a forced operation test such that anode 
acceleration voltage=40 kV, tube current=150 mA, and 4-second bombardments 
on the rotating anode were repeatedly performed 400 times at 75-second 
intervals After this test, the electron focal area 18 was examined by the 
photomicrographs thereof such as FIG. 6 of 5.times.magnification and FIG. 
7 of 30.times.magnification. From these observations, it was confirmed 
that although many cracks occurred on the electron focal area 18, the 
crystals thereof were significantly restrained from becoming rough and 
large in comparison with those of pure Mo. 
In addition, another forced operation test was carried out such that anode 
acceleration voltage=40 kV, tube current=260 mA, and 1-second bombardments 
were repeatedly performed 5000 times at 50-second intervals. The result is 
shown by the curve M in FIG. 2, wherein the X-ray radiation amount after 
this test remained at a value of approximately 76% of that in the initial 
period, and this fall became smaller as compared to the fall in the case 
of pure Mo. Moreover, the X-ray radiation quality was substantially the 
same as the X-ray radiation quality of Mo and there was almost no change 
attributable to the test. 
As described above, the X-ray tube according to the present invention 
exhibits superior long-life properties as in X-ray generating source for 
use in mammography. 
Moreover, the target may contain, besides Ti and additional C, extremely 
small amounts of other metal elements as a trace. 
Another embodiment will be described hereinafter. An electron focal area 18 
of a rotatable anode target 15 is made of Mo base alloy containing Mo as a 
major component, and a combination of oxides, i.e., K.sub.2 O and 
SiO.sub.2 as an additive. A supporting base 19 is also made of the Mo 
alloy, the same as the focal area. Preferably, the K.sub.2 O content is in 
a range of 0.02 to 0.3 wt %. More preferably, the K.sub.2 O content is in 
a range of 0.02 to 0.06 wt %, and the SiO.sub.2 content is in a range of 
0.06 to 0.1 wt %. When the contents of K.sub.2 O and SiO.sub.2 are smaller 
than the above-described range, a sufficient restraint effect to restrain 
the electron focal area from becoming crystrallized cannot be obtained. To 
the contrary, when they are greater than the above-described values, these 
surplus metals evaporate during the operation of X-ray tube. The 
evaporation of these metals can readily cause an increase of gases within 
the tube and also cause deterioration of voltage characteristics. 
FIG. 4 shows the relationship between the content of K.sub.2 O--SiO.sub.2, 
i.e., (K.sub.2 O+SiO.sub.2) and the relative amount of X-ray radiation 
after 5000-times bombardment, where the initial X-ray radiation amount is 
defined as 100%. As can be seen from FIG. 4, When the (K.sub.2 
O+SiO.sub.2) content is in the range of 0.03 to 0.4 wt %, the relative 
X-ray radiation amount is maintained at 60% or more, which is a 
practically acceptable range. When (K.sub.2 O+SiO.sub.2) content is in the 
range of 0.07 to 0.2 wt %, the relative X-ray radiation amount is 
maintained at 80% or more, which is a more preferable range. 
Further, when the contents of K.sub.2 O--SiO.sub.2 are excessively greater, 
the anode current Ip becomes unstable and fluctuates, as shown in FIG. 5. 
FIG. 5 shows characteristics of the anode current when a voltage of 40 kV 
was applied between the cathode and target, and the electron focal area 
contained K.sub.2 O of 0.2 wt % and SiO.sub.2 of 0.5 wt %. 
Next, the specific examples will be described. 
First, aqueous solutions of KCl and SiO.sub.2 were added to an Mo 
intermediate oxide powder and mixed such that K.sub.2 O=0.07 wt % and 
SiO.sub.2 =0.10 wt %. Thereafter, the mixture was dried to be dehydrated, 
and then was heated within the hydrogen furnace at a temperature of 
approximately 750.degree. C. for 1 hour so as to be deoxidized. 
Consequently, a doped Mo powder was obtained. Next, the thus obtained 
powder was formed into pellets, and heated within the hydrogen furnace at 
a temperature of approximately 1800.degree. C. for 7 hours. Thus, the 
sinter was obtained. Thereafter, the sintered body was forged to be 
densified, and was further forged into a certain specified shape. Next, 
the thus forged body was machined and then put into the vacuum furnace 
with a pressure of 1.times.1O.sup.31 5 Torr or less to be degassed at a 
temperature which was below its recrystallization temperature 
(approximately 1400.degree. C.) for 2 hours, so an X-ray tube target 
obtained. 
The X-ray tube assembled with the target was put through a forced operation 
test in the same manner as in the above-described embodiment such that 
anode acceleration voltage=40 kV, tube current=150 mA, and 4-second 
bombardments were repeatedly performed 400 times at 75-second intervals. 
After this test, the electron focal area 18 was examined by the 
photomicrographs thereof such as FIG. 8 of 5.times.magnification and FIG. 
9 of 30.times.magnification. From these observations, it was confirmed 
that, although many cracks occurred on the electron focal area 18, the 
crystals thereof were significantly restrained from becoming rough and 
large in comparison with those of pure Mo. 
In addition, another forced operation test was carried out such that the 
anode acceleration voltage=40 kV, the tube current=260 mA, and 1-second 
bombardments were repeatedly performed 5000 times at 50-second intervals. 
The result is shown by the curve N in FIG. 2, wherein the amount of X-ray 
radiation at the end of the test remained at a value of approximately 83% 
of that in the initial amount, and this fall became smaller as compared to 
the fall in the case of pure Mo. 
Moreover, the X-ray radiation quality was substantially the same as the 
X-ray radiation quality of pure Mo, and there was almost no change 
attributable to the test. 
As described above, the X-ray tube according to the present invention 
exhibits superior long-life properties as an X-ray generating source for 
use in mammography. 
As still another embodiment, Ti and K.sub.2 O--SiO.sub.2 of contents which 
are in the range of the abovementioned embodiments may be added to and 
mixed with the major constituent, i.e., Mo. Thus, desired target can be 
obtained. 
Furthermore, in the abovementioned embodiments, the target is an integrated 
electron focal area and supporting base. However, a complex target with 
the supporting base formed of different materials, such as pure Mo and 
Mo-W alloy, can also be utilized. This electron focal area should be 
formed with a thickness of 0.2 mm or more, because cracks of approximately 
0.1 mm in depth caused by the influence of heat generated by the electron 
bombardment may develop. 
Obviously, numerous additional modifications and variations of the present 
invention are possible in light of the above teachings. It is therefore to 
be understood that within the scope of the appended claims, the invention 
may be practiced otherwise than as specifically described herein.