BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A shows the electron-emitting characteristic of an electron gun;
FIG. 1B is a schematic diagram of accelerated electron energy varying with beam load;
FIG. 1C is a schematic diagram of accelerated electron energy varying with fed microwave power;
FIG. 2 is a schematic diagram of the parameter relationship between respective main systems of the accelerator while alternately generating X-rays having different energy levels according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of the variation in accelerated electron energy when simultaneously changing beam load and fed microwave power;
FIG. 4A is a structural schematic diagram of the electron linac according to an embodiment of the present invention;
FIG. 4B is a schematic diagram showing a bipolar electron gun is supplied with power on a time-division base by two small-sized hard limiters, which constitute the pulse modulator 401 as shown in FIG. 4A and output different voltages;
FIG. 5 is a schematic diagram of the energy spectrum difference between X-rays having two different energy levels, which are generated by the electron linac according to an embodiment of the present invention; and
FIG. 6 is a schematic diagram of a discrimination system, which performs non-destructive inspection on container cargo and implements material discrimination by using the electron linac of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereafter an embodiment of the present invention will be explained in detail with reference to the drawings.
FIG. 1A shows the electron-emitting characteristic of an electron gun. FIG. 1B is a schematic diagram of accelerated electron energy varying with beam load. FIG. 1C is a schematic diagram of accelerated electron energy varying with fed microwave power. FIG. 2 is a schematic diagram of the parameter relationship between respective main systems of the accelerator while alternately generating X-rays having different energy levels according to an embodiment of the present invention.
As shown in FIG. 1A, the electron gun has different capabilities of electron emission under different voltage amplitudes. The pulse modulator for driving the electron gun generates high voltages having two different amplitudes, which cause the electron gun to emit electron beams having different current intensities, that is, the obtained electron beams having different beam loads as shown in FIG. 2.
The beam load effect is represented by the formula E=√{square root over (AP)}−BI, where E is the energy of accelerated electrons, I is the beam intensity of accelerated electrons, P is the microwave power fed into the acceleration section, and A and B are preset constants. According to the beam load effect, different electron beams are accelerated to obtain high-energy electron beams having different energy levels.
FIG. 1B is a schematic diagram showing the beam load effect. As shown in Curve 2 of FIG. 1B, the higher the intensity of the beam load, the less the energy obtained by accelerating the beam in the acceleration tube. When the intensity of the pulse electron beam is high, that is, the intensity of the beam load is great, a small microwave power is fed into the acceleration tube, thereby obtaining a electron beam of a relatively low energy level. On the contrary, when the intensity of the beam load of the electron beam is small, a large microwave power is fed, thereby obtaining an electron beam having a relatively high energy level.
FIG. 3 is a schematic diagram of the energy variation in accelerated electrons when simultaneously changing beam load and fed microwave power. As shown in FIG. 3, if the fed microwave power is changed in synchronization with the change in the beam load, the energy finally obtained by accelerating the electron beams will further vary, thereby resulting in greater energy difference between the accelerated electron beams having two different beam loads.
Thus, if the first and the second electron beams having beam loads 10a and 11a respectively are accelerated under the completely same condition, two electron beams having different energy levels will be obtained. Meanwhile, the microwave power fed into the acceleration tube is changed to produce the first and the second microwaves having microwave powers 10b and 11b respectively. As shown in FIG. 2, the power of the first microwave is greater than that of the second one.
Curve 3 of FIG. 1C shows the characteristic of the beam acceleration by the acceleration tube provided with different microwave powers. As can be seen in FIG. 1C, when the beam load is fixed, the greater the fed microwave power is, the higher the energy obtained by accelerating the beam is. If the system operates in the first mode, i.e. the high-energy mode, the first electron beam having the beam load 10a is accelerated with the first microwave having the microwave power 10b to obtain a high-energy electron beam having the electron beam energy 10c. Then the high-energy electron beam is caused to hit a target to generate a high-energy X-ray with its energy being 10d.
If the system operates in the second mode, i.e. the low-energy mode, the second microwave having the power 11b, whose amplitude is smaller than the power 10b of the first microwave, will be fed into the acceleration tube. The second electron beam having the beam load 11a is accelerated to obtain a low-energy electron beam of the electron beam energy 11c. Then the low-energy electron beam is caused to hit the target to generate a low-energy X-ray with its energy being 11d. In this manner, the system alternately changes between the high-energy mode and the low-energy mode so as to generate X-rays with their energy levels alternately changing.
FIG. 4A is a structural schematic diagram of the electron linac according to an embodiment of the present invention. As shown in FIG. 4A, the electron linac 400 of the embodiment is an electron linac alternately generating dual-energy X-rays. The electron linac 400 includes a signal generator 403, a first and a second pulse modulator 401 and 404 connected to the signal generator 403, an electron gun 402 connected to the first pulse modulator 401, a magnetron 405 connected to the second pulse modulator 404, an acceleration tube 406 connected to the electron gun 402 and the magnetron 405, and a target 407 on which the rays generated by the acceleration tube 406 hit.
The signal generator 403 can generate high-level and low-level signals at a fixed frequency depending on predefined parameters. The first pulse modulator 401 and the second pulse modulator 404 generate high voltages having different amplitudes, for example, a first high voltage having the first amplitude and a second high voltage having the second amplitude, based on the signals generated by the signal generator 403. Depending on different systems, the functions of the first pulse modulator 401 and the second pulse modulator 404 can be fulfilled with a single pulse modulator. As shown in FIG. 4A, the first pulse modulator 401 outputs the voltage HV1 or LV1 having different amplitudes to the electron gun 402, and the second pulse modulator 401 outputs the voltage HV2 or LV2 having different amplitudes to the magnetron 405, each based on a synchronization signal. Here, in the case of a grid-controlled electron gun, the first pulse modulator 401 and the second pulse modulator 404 can supply power for the electron gun using a grid-controlled gun power supply with the grid pulse amplitude alternating, or in the case of the electron gun 402 being a bipolar electron gun, power is supplied on a time-division base by two small-sized hard tube pulse modulators (hard limiters) outputting different voltages. The pulse intervals of the pulses generated by the first and the second pulse modulators can be either the same or different from each other.
FIG. 4B is a schematic diagram showing a bipolar electron gun is supplied with power on a time-division base by two small-sized hard tube pulse modulators (hard limiters), which constitute the pulse modulator 401 as shown in FIG. 4A and output different voltages. As shown in FIG. 4B, the outputs of the high-voltage power supplies 1 and 2 are connected to the driving circuit of the electron gun, which is connected to a pulse synchronization and switch control circuit. Based on the external synchronization signals 1 and 2, the pulse synchronization and switch control circuit controls the module 401a and the module 401b to output a high voltage HV1 and a relatively low voltage LV1, respectively. Given different voltage amplitudes, the electron gun 402 emits electron beams with the beam load being 10a or 11a. The second pulse modulator 404, which provides voltage for the magnetron, is similar to the first pulse modulator 401 and includes two modules (not shown) from which voltages HV2 and LV2 having different amplitudes are obtained respectively. That is, the high-voltage power supplies 1 and 2 have their outputs connected to the pulse synchronization and switch control circuit to be controlled to supply power for the magnetron on a time-division base.
The voltages HV2 and LV2 having high and low amplitudes outputted alternately from the second pulse modulator 404 cause the operating current of the magnetron 405 alternating so as to obtain microwaves having different powers alternately, such as the first microwave having the amplitude 10b and the second microwave having the amplitude 11b. In addition, as another embodiment, the magnetron 405 can be controlled such that its magnetic field intensity alternates between high and low in synchronization with its operating current pulse, in order to obtain microwaves having different powers.
Accordingly, microwaves having different powers can be fed in by two methods as follow.
1. The pulse modulator generates pulses having high and low voltage amplitudes and outputs them to the magnetron alternately so as to cause the operating current of the magnetron to alternate between high and low;
2. The magnetic field intensity of the magnetron alternates between high and low in synchronization with its operating current pulses.
By using both or either of the above methods, the alternation of the pulse power can be realized for the microwave outputted by the magnetron.
Based on the synchronization signal, the magnetron 405 feeds microwaves having different powers, such as the first or the second microwaves having microwave powers 10b or 11b, into the acceleration tube 406 by example of traveling wave or standing wave acceleration tube for accelerating the first or the second electron beam having the beam load 10a or 10b. As the synchronized acceleration approach in FIG. 2, the first or the second electron beam is accelerated inside the acceleration tube 406 to obtain accelerated electron beams having different energy levels, for example, the first and the second accelerated electron beams having energy 10c and 11c respectively. Then, the accelerated electron beams having different energy levels are used to alternately hit the efficient target 407 to generate X-ray beams having alternated high and low energy levels, such as the first and the second X-ray beams having energy 10d and 11d.
As a result, the accelerated electron beams having different energy levels can be obtained by changing the beam load intensity of the accelerator. Meanwhile, the energy level difference between two accelerated electron beams can be further widened by synchronously changing the microwave power fed into the acceleration tube. The X-rays, which result from the targeting by the two electron beams having different energy levels, have a large difference between energy levels. There is a great distinction between the energy spectra of the two X-ray beams. FIG. 5 shows X-ray energy spectra when the X-rays having high energy of 9 MeV and low energy of 6 MeV are outputted from the dual-energy accelerator implemented according to the present invention. Curve 51 shows the energy spectrum for low energy of 6 MeV, and Curve 52 shows the energy spectrum for high energy of 9 MeV. It can be seen in FIG. 5 that the energy levels of the two continuous spectra differ greatly from each other.
FIG. 6 is a schematic diagram of an inspection system, which performs non-destructive inspection on container cargo and implements material discrimination by using the electron linac of the present invention. As shown in FIG. 6, the synchronization control part 605 is connected to the accelerator 400 and the detector 603 and provides them with high- and low-level signals 600 generated based on the preset parameters. Based on the synchronization signal, the accelerator 400 alternately generates X-ray beams 606 having high and low energy levels, which results in fan-shaped X-ray beams after passing through the collimator 601.
Next, the fan-shaped X-ray beams having high and low energy levels interact with the inspected object almost at the same position and then collected by the detector 603, which is connected to the image processing and material discrimination system 604 and outputs to them the digital signals. Here, the inspected object moves at certain speed in the direction shown in FIG. 6 so as to ensure the difference between the positions, at which the X-ray beams having high and low energy levels interact with the inspected object respectively, is allowable, that is, they interact at almost the same position. Therefore, the image processing and material discrimination system 604 can obtain the detection signal values D1 and D2 for high and low energy levels after the interaction with the inspected object 602 at the same position.
Then, based on the calibration curve relationship ln(D1/D10)−ln(D2/D20)=f(D1) (where D10 and D20 are the zero load values of the high- and low-energy X-ray beams, respectively) obtained by scanning the substance of known material property, the digital signals, which are collected after the interaction between the inspected object and the dual-energy X-ray beams, are classified so as to finally determine the material property of the inspected object, such as organic matter, light metal, inorganic matter, heavy metal, etc.
The above-mentioned is only the specific embodiments of the present invention, while the scope of the present invention is not limited to it. Any modification or substitution, which is obvious to the skilled in the art within the technical range disclosed in the present invention, should be included in the scope of the present invention, which is thus defined by the claims.
Patent Application
20070269013
