Control Circuitry For Preventing Damage To The Target Of A Scanning X-ray Generator

Abstract

An electron beam scans a target for creating a scanning X-ray beam. The rate of travel of the electron beam is detected as an electrical signal and in response to this signal, the electron beam or condenser lens currents, or the deflection coil voltage are controlled to prevent damage to the target.

Description

This invention relates to an X-ray generating apparatus, particularly an X-ray generating apparatus in which the electron beam continuously or flying spot scans an X-ray generating target.

In recent years, continuous or flying spot scanning type X-ray generating apparatus have been developed in which X-rays are generated by the impact of the accelerated electron beam on the target. The X-rays thus generated are passed through a pinhole so as to form an in-line small diameter X-ray beam. The electron beam irradiating the target is continuously or flying spot scanned. Accordingly, as the X-ray generating position on the target varies with time, the direction of the X-ray passing through said pinhole also varies. If the X-ray thus obtained is made to irradiate an object and the X-ray transmitted through said object is detected, and the detected signal supplied to a cathode ray tube synchronized with the electron beam of the X-ray generator or the like, it is possible to obtain a transmission X-ray image based on the continuous or flying spot scanning X-ray.

In the X-ray generating apparatus as described above, since it is impossible to irradiate a small area of the target with a high acceleration, high density electron beam, the intensity of the generated X-ray is weak. Accordingly, the resolution and contrast of the transmission X-ray image are poor; moreover, since the number of frames per second is limited, good pictures cannot be obtained.

If we now consider the relation between the power of the electron beam irradiating the target and the resultant X-ray generation, the X-ray intensity .nu.x is given as follows,

.nu.x = K .sup.. V .sup.. P

Where, V is the accelerating voltage, P is the power of incident electron beam, and K is a constant. As apparent from the formula, to obtain an X-ray beam of strong intensity, the power of the incident electron beam and the related accelerating voltage must be increased. However, in the static state the electrical power of the electron beam irradiating a given area of the target is limited as shown in the following formula.

Wmax = 17.8 (Tm-To) .delta. .sup.. K

Where, Wmax is the maximum allowable target load, Tm is the melting point (.degree.C) of the material constituting the target, To is the temperature (.degree.C) of the target cooling surface, .delta. is the diameter of the electron beam (cm), and K is the heat conductivity of the target at its melting point Tm (Cal/sec.sup.. cm.sup.. .degree.C).

In this formula; when the material of the target is determined, Tm, To and K become constant, so that the maximum allowable input power applied to the target is proportional to the diameter of the electron beam. However, this formula relates to the target in the static state only; if the target is moved at high speed, the heat distribution over the target will vary, so that the maximum allowable load Wmax becomes as follows.

Wmax = 17.0 (Tm-To) .delta. .sup.. K .sup.. .sqroot..delta. .sup.. (.rho. .sup.. c/K) .sup.. v

Where, .rho. is the density of the material constituting the target, C is the specific heat (Cal/g.sup.. .degree.C) of the material constituting the target, and V is the travel rate of the target (cm/sec).

The formula shows that, by increasing the target travel rate, the target input power and the X-ray beam intensity are also increased. For example, if a copper target is used, the diameter of the electron beam is 1mm and the travel rate of the target is 2,000cm/sec, thus allowing an input power 10 times larger than that in the static state.

Regarding the point stated above, the conventional X-ray generating apparatus is designed to give a fairly strong X-ray beam by using a rotating target. However, in the case of a continuous or flying spot type scanning X-ray generating apparatus, since the diameter of the electron beam on the target is sometimes very small (in the order of several to several tens of microns) and moreover, since the electron beam remains at a fixed position for a specific period, if the rotating target referred to above is used, even a minute vibration of the target will adversely effect the X-ray picture displayed on the C. R. T., etc.

In continuous or flying spot type scanning X-ray generating devices, the electron beam is usually deflected by a deflection means and then continuously or flying spot scanned over the target. Moreover, with this type of device, it is theoretically possible to increase the electron beam current according to the increase in scanning speed and thus generate a strong X-ray beam. The contingency to be considered is that for some reason or other, the rate of travel of the electron beam irradiating the target might be reduced or the beam might come to a complete standstill. The heat generated when the electron beam current exceeds a certain limit is sufficient to cause the target to evaporate.

An object of this invention is to provide a continuous or flying spot type scanning X-ray generating apparatus capable of generating an extremely strong X-ray beam.

Another object of this invention is to provide a continuous or flying spot type scanning X-ray generating apparatus capable of preventing the target from being damaged.

Briefly according to this invention, a continuous or flying spot type scanning X-ray generating apparatus has incorporated means for detecting the continuous or flying spot scanning speed of the electron beam and for controlling the beam according to said scanning speed. The scanning speed may be obtained by measuring the distance travelled by the electron beam over the target per unit time, said distance corresponding to the wave amplitude of the deflection signal. Moreover, the travelling time of the electron beam is made available by detecting the continuous or flying spot scanning signal line or step number in terms of frequency. As a result, if the product of the amplitude and frequency of said deflection signal is obtained, the travelling speed of the electron beam irradiating the target will be detected. In one embodiment of this invention, the deflection signal is supplied to a wave-amplitude detector and a frequency detector and the two different output signals from these detectors are supplied to a multiplier circuit so as to obtain the product of both signals. However, it is not absolutely necessary to detect both the wave-amplitude and frequency of the deflection signal. For example, by keeping one parameter constant and varying the other, and by detecting the varied parameter only, the travelling speed of the electron beam on the target can be ascertained.

The advantages of this invention will become more readily apparent by reading the following detailed description in conjunction with the accompanying drawings of which,

FIG. 1 is a block diagram showing one embodiment of this invention, and

FIGS. 2 to 7 are block diagrams showing other embodiments of this invention .

Referring to FIG. 1, an X-ray generating device 1 has at one end an electron gun comprised of a filament 2 and a Wehnelt electrode 3. The electron beam generated by said electron gun is accelerated by an anode 4 and focussed by first and second condenser lenses 5 and 6 on an X-ray generating target 7. Said condenser lenses are energized by an scintillation power source 9, which is controlled by signal from a control unit 8. Electron beam deflection coils 10 and 11 are provided between said condenser lenses 5 and 6, said deflection coils being supplied with deflecting signals by the control unit 8 via an amplifier 12. By means of scintillation electron beam irradiation on said target, an X-ray is generated from said target which passes through a pinhole 14 via a transmission window 13 in order to irradiate an externally located object 15. The X-ray transmitted through said object 15 then enters an X-ray detector 16 such as a scintallation detector where it is detected. The signal detected by the X-ray detector 16, after being amplified by an amplifier 17, is fed into a cathode ray tube 18 to which synchronizing deflection signals are applied from the control unit 8.

In the embodiment as described above, the electron beam generated by the electron gun forming part of the X-ray generating tube 1 is finely focussed on the target 7 by condenser lenses 5 and 6 and deflected by deflection coils 10 and 11. Accordingly, said electron beam continuously or flying spot scans the target in accordance with the deflection signal supplied to said deflection coils. As a result, the X-ray generating position of the target varies with time and since the direction of projection of the X-rays passing through the pinhole 14 varies in accordance with the irradiating position of the electron beam on said target, the object 15 is continuously or flying spot scanned by a beam of X-rays and a continuous or flying spot X-ray transmission image of said object is thereby displayed on the cathode ray tube 18.

The deflecting signal supplied to the deflection coils by the amplifier 12 is also supplied to a wave-amplitude detector 19 and a frequency detector 20, the output signals of which are fed into a multiplier 21 where the product of both signals is obtained. The detector 19 detects the amplitude of the deflecting signal and the detector 20 detects the scanning signal step number or the flying spot signal flying spot number as a frequency. The amplitude of said deflection signal corresponds to the distance travelled by the electron beam over the target and the step number or flying spot number corresponds to the travelling time of said electron beam over said target. Accordingly, the product of the two different signals corresponds to the mean velocity of the electron beam on the target, and the signal corresponding to said velocity is supplied to the electron gun bias power source 22 from the multiplier 21. The bias voltage applied between the filament 2 and the Wehnelt electrode 3 from said bias power source 22 varies in accordance with the signal supplied by said multiplier 21. As a result, when the travelling speed of the electron beam on the target 7 is high, the electron beam current increases and the density of the electron beam increases, thereby increasing the intensity of the X-ray beam generated by said target. On the other hand, when the speed of the electron beam on the target is decreased by the deflection signal supplied to the deflection coils 10 and 11 by the control unit 8, the electron gun bias voltage is increased by the signal supplied to the bias power source 22 by the multiplier 21, thereby decreasing the density of said electron beam.

In this embodiment, it is possible to vary the focal length of the condenser lenses so as to control the spot diameter of the electron beam on the target 7, either at the same time as the bias voltage is controlled or separately therefrom, by feeding the output signal of the multiplier 21 directly into the condenser lens excitation power source 9, as shown by the broken line in FIG. 1.

It is also possible to use the output signal of said multiplier 21 in order to control the electron gun filament heating temperature.

FIG. 2 shows another embodiment of this invention in which electron beam emission is suspended when the scanning speed of the electron beam on the target drops below a certain predetermined value. This is achieved by providing a comparison circuit 23, a standard signal generator 24 and a control signal generator such as a pulse generator 25. The output signal from the multiplier 21 is fed into the comparison circuit 23 together with a standard signal from the standard signal generator 24 and compared. By so doing, when the intensity of the signal from the multiplier drops below the intensity of the standard signal, a pulse signal is generated by the pulse generator 25. This pulse signal is then supplied to the electron gun power source 22 which causes an increase in the bias voltage between the filament 2 and the Wehnelt electrode 3, thereby terminating the outflow of electrons from the electron gun. As a result, even if there is a drop in the scanning speed of the electron beam on the target, damage to the target is prevented.

FIGS. 3, 4 and 5 show variations of the general concept exemplified in the embodiment shown in FIG. 2.

In FIG. 3, a signal from the pulse generator 25 is supplied to the power source 27 of a rapid response, air cored or electrostatic auxiliary lens 26. By so doing, the power source 27 is "switched-on" and current or voltage is supplied to said auxiliary lens 26. As a result, the spot diameter of the electron beam irradiating the target is instantaneously enlarged and the beam density is consequently reduced.

In FIG. 4, an electrostatic or electromagnetic deflection means 28 is arranged between the condenser lens 6 and the target 7, said deflection means 28 being connected to a rapid scanning power source 29. A pulse from the pulse generator 25 is applied to the power source 29. As a result, said power source is triggered, thereby supplying a rapid scanning signal to the deflection means 28. Accordingly, if, for whatever reason, the continuous or flying spot scanning speed of the electron beam irradiating the target is reduced or even if the electron beam comes to a standstill and irradiates just one spot on the surface of the target, the deflection means will operate to shift or deflect the electron beam over the surface of the target at high speed. In this case, it would be a good idea to deflect the beam so as to irradiate a portion of the target surface where the generated X-rays are unable to reach the object.

In FIG. 5, the deflecting means 28 functions so as to deflect the electron beam to an extent such that it ceases to irradiate the target 7. In this case, the power source 30 activated by a pulse from the pulse generator 25 supplies a constant D.C. voltage to said deflecting means. In addition, a secondary or auxiliary target 31 has been provided to prevent the inner wall of the X-ray generating device 1 from becoming pitted due to repeated electron beam impingement.

The embodiment shown in FIG. 6 is a modified version of the embodiment shown in FIG. 1 and is ideally suited when scanning the electron beam continuously. In the figure, 32 is a differential circuit which is connected to the output of the deflection signal amplifier 12 and which serves to detect the rate of variation of the deflection signal. The signal from said differential circuit is supplied to the control unit 33 to control the bias power source 22, the condenser lens excitation power source 9 and/or the filament heating power source. In this case, a differential amplifier of simple construction would serve ideally as a control unit.

In the embodiment shown in FIG. 7, only the deflection signal wave-amplitude detector is connected to the output of the amplifier 12. Moreover, a constant signal generating circuit 34 is used in place of frequency detector. In practice, if the continuous or flying spot scanning signal is generated on the basis of a precise clock pulse, it is not necessary to detect the number of continuously scanned lines or the number of flying spot scanning steps. In this particular case, this embodiment has the advantage that circuit construction is simplified. Also, if the wave amplitude of the deflecting signal is precise, it is possible to provide a constant signal generator in place of the wave-amplitude detector and to connect a frequency detector to the amplifier 12.

Circuit construction can be further simplified by dispensing with the constant signal generating circuit 34 and the multiplier 21 and regulating the detector 19 (or detector 20) output signals. This is possible as the intensity of the signals generated by the circuit 34 is constant.

The embodiments described in FIGS. 6 and 7 can be applied to the embodiments described in FIGS. 2 to 5.

Having thus described the invention with the detail and particularity as required by the Patent Laws what is desired protected by Letters Patent is set forth in the following claims.

Claims

We claim:

1. A scanning X-ray generating apparatus comprising,

a. an electron beam generating source,

b. means for focussing the generated electron beam,

c. a target for generating X-rays by scanning the electron beam thereover,

d. an electron beam deflecting means for scanning the electron beam over said target,

e. a circuit for supplying deflection signals to said electron beam deflecting means.

f. means for obtaining signals indicative of the travelling rate of the electron beam scanning the target by detecting variations of said deflecting signals, and

g. means for controlling the electron beam in response to said obtained signals to prevent damage to the target.

2. An apparatus as set forth in claim 1 in which the electron beam controlling means modulates the electron generating source so that the electron beam current is increased or decreased in response to said signal indicative of the travelling rate.

3. An apparatus as set forth in claim 1 in which the electron beam controlling means modulates the electron beam generating source so as to reduce the electron beam current to zero when the travelling rate of the electron beam on the target drops below a certain predetermined value.

4. An apparatus as set forth in claim 1 in which the electron beam controlling means modulates an electron beam deflection means so that said travelling rate is increased.

5. An apparatus as set forth in claim 1 in which the electron beam controlling means modulates an electron beam deflection means so that said electron beam deviates from the target area.

6. An apparatus as set forth in claim 1 in which the electron beam controlling means modulates a focussing means so that the spot diameter of said electron beam on said target is varied.

7. An apparatus as set forth in claim 1 in which the means for obtaining signals indicative of the travelling rate of the electron beam scanning said target comprises a means for detecting the frequency of the scanning signals, a means for detecting the wave-amplitude of said signals, and a means for obtaining the product of the output signals from said scanning signal frequency and wave amplitude detecting means.

8. An apparatus as set forth in claim 1 in which the means for obtaining the signals corresponding to the travelling rate of the electron beam continuously scanning the target is a differential circuit.

9. An apparatus as set forth in claim 1 in which the means for obtaining signals indicative of the travelling rate of the scanning electron beam comprises means for maintaining a constant scanning frequency and means for measuring the scanning wave-amplitude.

10. An apparatus as set forth in claim 1 in which the means for obtaining signals indicative of the travelling rate of the scanning electron beam comprises means for maintaining the scanning wave-amplitude constant and means for measuring the scanning frequency.