US1690906A - X-ray apparatus - Google Patents

Description

Nov. 6, 1928.

. M. MORRISON X-RAY APPARATUS Filed Deg. 1, 1921 INVENTOR NONTFORD MORRISON ATTORNEY Patented Nov. 6, 1928.

UNITED STATES MONTFORID MORRISON, OF NEW YORK, N. Y.

X-RAY APPARATUS.

Application filed December 1, 1921. Serial No. 519,155.

This invention relates to certain improvements in vacuum tubes and more especially to such tubes operated for the purpose of producing Rontgen or X-rays.

It is an object of the present invention tobprovide a simple self-rectifying vacuum tu e.

Another object of the present invention is the provision of an X-ray device capable of manipulation for the production of X- rays of desired penetrability, with the simultaneous elimination of undesirable rays.

A further objection of the present invention is the provision of a vacuum tube capable of operation with a high input and which will also be self rectifying when operated on a periodically varying voltage.

Other objects will be apparent from a reading of the following specification.

In the operation of modern X-ray devices in which rays of great penetrability are produced, alternating voltage is most conveniently generated to attain the necessary high voltage values. However, the mode of generating X-rays in a vacuum tube requires unidirectional current only, as an alternating current passing through the tube would produce undesired effects and might damage the tube. It is, therefore, necessary to allow current to pass in one direction through the tube and to eliminate the passage of current in the reverse direction. To this end, rectifiers have been disposed in series with the electrodes of an X-ray tube. When thus connected, the rectifier must have sufiicient capacity to carry the full load of the tube.

It is known that the penetrability of X- rays is a function of the voltage impressed on the electrodes of an X-ray device, the penetrability being greater the higher the applied voltage. When, therefore, a voltage continuously varying in value, such as an alternating E. M. F., is employed, the resultant X-rays will continuously vary in penetrative power. At the beginnlng of each half wave of current passing through the tube, when the impressed voltage is low, the X-rays have a low penetrability, or, according to the nomenclature of the art, the rays are soft These soft rays in some cases are not necessary and sometimes even harmful. The soft rays, some of which have such low penetrability that they do not leave the tube, are therefore of no utility and additionally cause an undesirable increase in temperature of the anti-cathode inv the normal operation of the tube. Inasmuch as the heating of the anti-cathode is the limiting feature of the capacity of an X-ray device, it is possible to increase the capacity of the tube by eliminating the non-utilizable soft rays.

According to the present invention, I have found very simple and satisfactory means common for securing a unidirectional current 1n an X-ray device and for obtaining X- rays of a predetermined penetrative powen.

Specific embodiments of my invention are illustrated diagrammaticallyin the drawings in which Fig- 1 shows an X-ray tube with auxiliary apparatus for accomplishing the objects of my invention, and Fig. 2 shows similar alternate means for accomplishing the same results.

Like numerals in the drawings indicate corresponding parts.

Referring to Fig. 1, the X-ra tube 3, is connected to the secondary 4 of a igh-potential transformer by means of conductors 5 and 6. Tube 3 has an envelopev 9 which is preferably of glass. The anode or anti cathode 7 consists preferably of a material. of high atomic Weight, suchas tungsten, and is sealed into the neck 8 of the envelope 9 of the X-ray tube by means of a conductor 10 passing'through stem 11 in the manner well understood in the construction of scaled evacuated glass structures. The plane of the anti-cathode 7, which is also known as the target, being the body in which X-rays are generated by the impingement, of hi velocity electrons thereon, is preferaly placed atan angle to the direction of motion of the electrons. Oppositely disposed to the anode 7 is a cathode 12 consisting preferably of a refractory metal, such as tungsten, which may be wound'into a spiral or helical form and positioned with its axis at an angle of 45 to the face of the anode 7 This cathode is sealed into a neck 14 of the envelope 9 by means of leading-in conductors 15 and 16 passing through a stem 17 and is adapted to be heated electrically by a source of power 18 connected to the lead-in wires 15 and 16.

Interposed between the. anode 7- and the cathode 12, in the path of the electrons which travel from cathode 12 to the target 7, 1s a focusing cup 19 having a neck 20 encircling the cathode 12 and is preferably a hemispherical shell, disposed with its concave surface facing the target 7. The position of the focusing device 19 relative to the cathode 12 may be varied according to whether it is desired to employ primary or secondary electrons to bombard the target 7. If these two elements be so positioned that the primary electrons from the cathode impinge on the neck 20, secondary electrons will be-generated at the latter element and these will be drawn to the target 7. The position of cathode 12 towards the free end of the neck will favor the generation of secondary electrons, while a position near the cup portion will tend to result in primary electrons only reaching the target. The foscusing cup 19 is electrically connected to a terminal of a condenser 21 by means of a conductor 22 which is sealed into the envelope 9 through a stem 24, the other term'nal of the condenser being connected to any desired point on the transformer secondary 4 by means of a conductor 25. The condenser 21 may be constructed so that the leakage current therethrough is such that the best results are secured or an adjustable high resistance may be shunted across the same for this purpose.

During the operation of the tube, the focusing device 19 becomes negatively charged, the magnitude of the charge being dependent largely upon the point of connection of the condenser to the transformer secondary 4. The negative potential of the focusing cup 19 repels the negatively charged particles which have a tendency to be emitted from the hot target 7 during the half cycle when the target 15 negative, the cathode 12 being simultaneously positive. The target 7 has a tendency to emit electrons owing to the elevated temperature imparted to it by the electronic bombardment occurrFng during the half cycle when the cathode 12 is negative and the target 7 positive. The flow of current in the direction from the anode 7 to the cathode 12 which would exist if electrons were permitted to flow from the anode 7, is usually termed the inverse current. This inverse current would not only be of no utility but would damage the cathode 12; the means described are therefore provided to prevent the passage of the inverse current.

As previously stated, it is desirable to avoid the generation of soft X-rays which result from the impingement of low-velocity electrons on the target 7. The comparatively low velocityof a fraction of the electrons passing from the cathode 12 to the anode 7 is due to the low voltage impressedon the electrodes at the beginnng of each half wave of current. The soft rays may, therefore, be obviated bysuppressing the electrons at cathode 12 during the intervals When the applied voltage is low. The low-velocity electrons, being negatively charged particles,

may be suppressed by provlding, on the focusing cup or other element, a negativepotential sufiicient to repel them and thus prevent their passage to the anode 7. Depending on the magnitude of the charge given the focusing means 19, electrons from the cathode 12 will be prevented from flowing until the impressed voltage rises to a predetermined value.

The electron stream from the cathode to the anode will begin when the potential impressed on the electrodes is suflicient to overcome the repellent eifect of the electrostatic field existing between the focusing cup 19 and the cathode 12. It will thus be seen that current will flow through the tube only while the impressed volt-age of the tube is above a predetermined minimum value and thus only X-rays exceeding a desired minimum penetrative power will be, generated.

Fig. 2 shows an X-ray tube the same as described in Fig. 1 and similarly connected to auxil ary apparatus except that a rectifier 27 is substituted for the condenser 21 shown in Fig. 1. The rectifier here shown is of the well known hot-cathode type, comprising an anode 28 and a cathode 29 which are sealed into envelope 33. The anode 28 is electrically connected to the focusing cup 19, and the incandescent cathode 29 is connected by a conductor 31 to a movable arm 32, which may make contact with any desired point on the transformer secondary 4 and is adapted to be heated electrically by a source of power 30. As will be obvious, a means similar to the arm 32 may also be employed with the arrangement illustrated in Fig. 1, so that the condenser 21 may be connected to any desired point on the secondary 4:.

The function of the rectifier 27 is the same as that of the condenser 21 shown in Fig. 1, that is, to keep the focusing cup 19 negatively charged to any desired potential for the purpose of avoiding an inverse current and of regulating the penetrability of the X-rays. The rectifier 27, which is unidirectionally conductive, permits a negative charge to accumulate on the focusing cup 19 during one half the wave of the impressed voltage and acts as an insulator to prevent the loss of this negative charge during the succeeding half wave of t e impressed voltage.

While I have herein illustrated and described certain specific embodiments of my invention, it is apparent that minor modifications therein may be devised which are considered to be within the scope of the appended claims. v

What is claimed is:

1. An electron discharge device comprising an anode and a cathode, a source of alternating current connected to said anode and cathode, and means disposed between said cathode and anode and connected to said source of alternating current for maintaining a, negative electrostatic field adjaing an anode and a cathode, a source of alternating current connected to "said anode and cathode an electrically charged body adjacent said cathode, and means connected to said source and to said body for maintaining the latter at a negative potential with respect to said anode.

3. An electron discharge device comprising an anode and a cathode, a source of alternating current connected to said anode and cathode, an electrically charged body adjacent said cathode, and a condenser con- -nected to said source and to said body.

4. An X-ray tube comprising an anode and a cathode, a source of alternating current connected to said anode and cathode, an electrically charged body disposed adjacent said cathode, and means for maintaining said body at a negative potential with respect to said anode.

5. An X-ray tube comprising an anode and a cathode, a source of alternating current connected to said anode and cathode, an electrically charged body disposed adjacent said cathode, and a condenser connected to said source and to said body.

6. In, an X-ray tube, an envelope, an anode and a cathode mounted therein, a focusing device adjacent said cathode and so disposed with respect thereto as 'to be bombarded by electrons emanating from said cathode so as to produce secondary electrons and means for bombarding said anode by the secondary electrons thus produced.

In testimony whereof, I have hereunto subscribed my name this 28th day of November, 1921.

MONTFORD Monarsoir.

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