Source for Intra-Pulse Multi-Energy X-Ray Cargo Inspection

Abstract

Methods for generating a multiple-energy X-ray pulse. A beam of electrons is generated with an electron gun and modulated prior to injection into an accelerating structure to achieve at least a first and specified beam current amplitude over the course of respective beam current temporal profiles. A radio frequency field is applied to the accelerating structure with a specified RF field amplitude and a specified RF temporal profile. The first and second specified beam current amplitudes are injected serially, each after a specified delay, in such a manner as to achieve at least two distinct endpoint energies of electrons accelerated within the accelerating structure during a course of a single RF-pulse. The beam of electrons is accelerated by the radio frequency field within the accelerating structure to produce accelerated electrons which impinge upon a target for generating Bremsstrahlung X-rays.

Claims

1. A method for generating a multiple-energy X-ray pulse, the method comprising: a. generating a beam of electrons with an electron gun; b. modulating the beam of electrons prior to injection into an accelerating structure to achieve at least a first specified beam current amplitude and a first specified beam current temporal profile, and a second specified beam current amplitude and a second specified beam current temporal profile, the beam of electrons characterized by an electron beam pulse duration; c. applying to the accelerating structure a radio frequency field with a specified RF field amplitude and a specified RF temporal profile characterized by an RF pulse duration; d. injecting the beam of electrons at a first specified beam current amplitude and then at the second specified beam current amplitude after a specified delay, in such a manner as to achieve at least two distinct endpoint energies of electrons accelerated within the accelerating structure during a course of a single RF-pulse; e. optimizing a coupling coefficient between an RF source and the accelerating structure so as to achieve zero RF power reflection at the specified beam current; f. accelerating the beam of electrons with the radio frequency field within the accelerating structure to produce accelerated electrons; and g. impinging the accelerated electrons upon a target for generating X-rays by Bremsstrahlung.

2. A method in accordance with claim 1, wherein the electron beam pulse duration is shorter than the RF pulse duration by a sum of onset delays defined by a filling time of the accelerating structure.

3. A method in accordance with claim 1, wherein the first specified beam current temporal profile and the specified RF temporal profile begin substantially contemporaneously.

4. A method in accordance with claim 1, wherein the a beam of electrons is characterized by an amplitude of injection current, and wherein the amplitude of injection current decreases during the multiple-energy X-ray pulse.

5. A method in accordance with claim 1, wherein an end-point energy characterizing the beam of electrons increases during the multiple-energy X-ray pulse.

6. A method in accordance with claim 1, wherein applying the RF field includes modulating an RF source.

7. A method in accordance with claim 6, wherein modulating the RF source includes varying at least one of an input voltage and an input current to the RF source.

8. A method in accordance with claim 1, wherein applying the RF field includes modulating an RF modulator disposed between an RF source and the accelerating structure while the RF-source provides a constant level of power at a constant frequency.

9. A method in accordance with claim 1, wherein a high-energy portion of the multiple-energy x-ray pulse is characterized by a lower electron beam flux than a low-energy portion of the multiple-energy X-ray pulse.

10. A method in accordance with claim 1, wherein the accelerating structure includes a standing wave resonator.

11. A method in accordance with claim 10, wherein the RF source provides a constant level of RF-power; wherein a temporal profile of the electron gun is characterized by at least two distinct levels of the amplitude of electron beam I.sub.n; wherein each amplitude of the at least two distinct levels of amplitude is created in temporally descending order; and wherein the coupling coefficient of accelerating resonator .sub.0 is chosen to be optimal at a first level of beam current I.sub.1.

12. A method in accordance with claim 11, wherein breaking points of said at least two distinct levels include breaking points that are dynamically variable.

13. A method in accordance with claim 11, wherein a first portion of an electron beam current pulse starts with an optimum delay (t.sub.b1) relative to a beginning of an RF pulse; wherein said electron beam current pulse ends at a specified time (t.sub.1); and wherein each successive portion of the electron beam current pulse is characterized by a distinct level I.sub.n and starts with a delay $t_{\mathrm{bn}}=.Math.\ln \left(\frac{I_{n-1}}{I_n}\right)$ corresponding to an end of a previous pulse t.sub.n-1, with representing a decay time of the accelerator structure.

14. A method in accordance with claim 12, wherein an end point of each pulse t.sub.n includes a dynamically variable end point.

15. A method in accordance with claim 11, further comprising: a. supplying an RF-power pulse to the accelerating structure at two distinct energy levels, P.sub.L, P.sub.H, corresponding, respectively, to low and high energy portions of the beam of electrons; wherein an electron gun temporal profile is characterized by two distinct levels of the amplitude of electron beam, I.sub.L, I.sub.H, corresponding, respectively, to beam current levels for achieving low and high energy portion of the beam of electrons; and b. optimizing a coupling coefficient of accelerating resonator .sub.0 based upon parameters of the low energy beam.

16. A method in accordance with claim 15, further comprising: c. forming an ascending order of two distinct X-ray energy levels by applying a corresponding ascending sequence of RF-power levels and a descending sequence of injection current pulses; d. delaying a low energy component of the pulse by a low energy current delay t.sub.bL defined by $t_{\mathrm{bL}}=.Math.\ln \left(\frac{2.Math.{}_0}{{}_0-1}\right);$ and e. delaying a high energy component of the pulse by a high energy current delay t.sub.bH a defined by $t_{\mathrm{bH}.Math..Math.\_.Math..Math.a}=.Math.\left[\ln \left(\frac{I_L}{I_H}\right)+\ln \left(1+\frac{2.Math.{}_0}{{}_0-1}.Math.\sqrt{\frac{P_H}{P_L}-1}\right)\right].$

17. A method in accordance with claim 15, further comprising: c. forming a descending order of two energy levels by applying a corresponding descending sequence of RF-power levels and an ascending sequence of injection current pulses; d. delaying a high energy component of the pulse by a high energy current delay t.sub.bH defined by equation $t_{\mathrm{bH}}=.Math.\ln \left(\frac{\sqrt{4.Math.{}_0.Math.{\mathrm{rLP}}_H}}{I_H.Math.\mathrm{rL}}\right);$ and e. delaying a low energy component of the pulse current delay t.sub.bL_d is defined by $t_{\mathrm{bL}.Math..Math.\_.Math..Math.d}=.Math.\left[\ln \left(\frac{I_H}{I_L}\right)+\ln \left(1+\frac{\sqrt{4.Math.{}_0.Math.{\mathrm{rLP}}_H}}{I_H.Math.\mathrm{rL}}.Math.\sqrt{1-\frac{P_L}{P_H}}\right)\right].$

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The foregoing features of the invention will be more readily understood by reference to the following detailed description, taken with reference to the accompanying drawings, in which:

[0049] FIG. 1 depicts a typical high-energy transmission X-ray inspection system, in the context of which embodiments of the present invention are advantageously applied.

[0050] FIG. 2 shows a block diagram of an X-ray source employing an accelerating structure and modulated current injection and RF excitation. A dual-energy pulse is created by applying the I.sub.L and I.sub.H currents to two distinct portion of the single RF pulse.

[0051] FIG. 3 depicts dependence of energy and current within a microwave pulse, in accordance with the same or another embodiment of the present invention.

[0052] FIG. 4 shows a block diagram of an X-ray source employing a standing wave accelerating structure with modulated current injection and RF excitation, in accordance with the present invention.

[0053] FIG. 5 shows a linac implementation with a dual-energy pulse created by applying the I.sub.L and I.sub.H currents with optimal delays, in accordance with an embodiment of the present invention.

[0054] FIG. 6 shows a block-diagram of an intra-pulse dual-energy linac where the energy modulation is achieved by varying both input RF-power and injection current provided into a standing wave accelerating system, in accordance with an embodiment of the present invention.

[0055] FIG. 7 shows an example of creating an ascending order of beam energy steps W.sub.n by applying an ascending order of RF-power levels P.sub.n with corresponding descending order of injection currents I.sub.n, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0056] Definitions. The term multiple-energy shall refer to an X-ray inspection modality in which differential transmission through a medium by X-rays of distinct spectral composition is used to characterize the medium.

[0057] The term pulse duration, denoted t.sub.RF, refers to the duration of time that RF excitation is applied to a linac accelerating structure.

[0058] The term breaking point of a current pulse of duration t.sub.L+t.sub.H has a very specialized meaning herein: It is defined to be the value of t.sub.L/t.sub.H, where t.sub.L and t.sub.H refer, respectively, to durations of the current pulse during which an electron beam emitted from an accelerating structure is characterized by low- and high-energies, respectively.

[0059] In the case of a current pulse having multiple sub-pulses, any pair of sub-pulses may be characterized by a breaking point that is defined as the value of t.sub.L/t.sub.H, where t.sub.L and t.sub.H refer, respectively, to durations of current sub-pulses.

[0060] A breaking point of a set of current pulses or sub-pulses shall be said to be dynamically variable if the breaking point is adapted to be varied from one pair of current pulses or sub-pulses to another pair of current pulses or sub-pulses. Similarly, and end point of a pulse may also be characterized as dynamically variable if the end point is adapted to be varied from one current pulse to another current pulse.

[0061] The term current level, as it pertains to an x-ray source, refers to an average flux of electrons incident upon the target, expressed in milliAmperes (mA), and averaged over a specified duration of time. Unless otherwise indicated, the specified duration over which an average is taken is the duration of a pulse.

[0062] The term current amplitude, as it pertains to an x-ray source, refers to a value of an instantaneous flux of electrons incident upon the target, expressed in mA.

[0063] The term onset delay refers to a period between application of an RF field to an accelerating structure and injection of a pulse or sub-pulse of electron current into the accelerating structure. Where there are multiple sub-pulses of electron current, then the cumulative onset delays of the respective sub-pulses make up a sum of onset delays.

[0064] As used herein, the term Bremsstrahlung may be used to denote X-ray emission produced through impingement of high-energy electrons onto a metallic target, and, additionally, the physical process involved in that phenomenology.

[0065] The systems and methods described herein may be described in terms of X-rays, however the applicability of the teachings to other spectral ranges is clear, and encompasses, within the scope of the invention, all manner of penetrating radiation.

[0066] Various embodiments of the invention described herein employ variation of the spectral content of an X-ray pulse during the course of the pulse to discriminate differences in X-ray transmission of a medium in different energy regimes. Approaches taught in accordance with the present invention are particularly advantageous in cases where high speed of scanning is required, such as train or high-throughput scanners.

[0067] In accordance with embodiments of the present invention, a novel apparatus and novel methods are provided that may advantageously create a pulse profile of a multi-energy beam in such a manner that would improve material discrimination while preserving the highest possible scanning speed and allow optimizing dose to cargo and environment.

[0068] A novel source of penetrating radiation, designated generally by numeral 20, in accordance with an embodiment of the present invention, is now described with reference to FIG. 2. A linac 21, depicted in FIG. 2, includes accelerating structure 22, and an electron gun 23 serving as an injector of electrons emitted by cathode 235. Used in conjunction with linac 21 are an X-ray target 24, an RF-source 25, an RF-source modulator 26, an RF isolator 27 and an electron gun modulator 28. An RF-circuit 29 provides a constant level of microwave power within pulse duration t.sub.RF. Electron gun 23, driven by e-gun modulator 28, provides an electron beam 220 characterized by a two-level injection current pulse 210 (also referred to herein as the pulse) into accelerating system 20 with a total duration t.sub.pt.sub.RF. Injection current pulse 210 may also be referred to herein as injection current, and its amplitude, which, as defined above, corresponds to its instantaneous value of electron flux in mA, denoted I.sub.L.

[0069] The first portion 212 of the pulse 210, characterized by a higher amplitude of the injection current I.sub.L due to higher beam loading, creates a low energy portion of the beam pulse, where the low energy portion is designated by W.sub.L. (For avoidance of ambiguity, it is to be noted that W.sub.L refers both to the low energy portion of the pulse, and to the value of the instantaneous endpoint energy characterizing the low energy portion of the pulse. The same applies, mutatis mutandis, to W.sub.H, the high energy portion of the pulse.) The second portion 214 of the injection current pulse with lower amplitude I.sub.H produces a high energy portion of the beam pulse W.sub.H. The breaking point of the pulse, which, as defined above, has the specialized meaning of the value of t.sub.L/t.sub.H, may be variable, within the scope of the present invention, thereby enabling dynamic control of the dose of emitted X-rays to cargo and environment. In accordance with certain embodiments of the present invention, the breaking point may advantageously be varied from pulse to pulse, rendering it a dynamically variable breaking point.

[0070] Reference is made now to FIG. 3, where dependence of energy and current within a microwave pulse is depicted. Dashed line 32 shows the injection current, while solid line 34 represents the beam energy. Dotted line 30 shows energy dependence at constant current I.sub.H, as previously discussed. As the result of different beam loading effect in the front and rear portion of the pulse, the beam has two distinct energy levels, and, in one embodiment: W.sub.L(0.-1.5)s=3.9 MeV and W.sub.H (1.8-3.3) s=5.8 MeV. As used herein, energy level refers to the instantaneous end-point energy of an ensemble of photons, distributed in energy according to an essentially Bremsstrahlung spectrum of photon energies, or otherwise.

[0071] Optimizing coupling coefficient .sub.0 of the accelerating structure for the parameters of the single energy beam is known in the prior art, and has been described in the Background Section above.

[0072] In FIG. 4, a linac 21 is shown that is similar to the one shown in FIG. 2. The accelerating system 20 is based on a standing wave structure 42 (otherwise referred to herein as a standing wave resonator), the distinction of which with respect to a traveling wave structure has been laid out by Miller (1986). The coupling coefficient .sub.0 is chosen to be optimal at the current I.sub.L, using the algorithm that was laid out in detail above. The value of I.sub.L is chosen to provide energy W.sub.L, and this energy value W.sub.L remains constant over an entire sub-pulse duration if I.sub.L is applied with delay t.sub.bL with respect to the beginning of the RF pulse. Low energy current delay t.sub.bLis defined by Eq. (4) above. At the end of the low energy pulse (after t.sub.L), the current turns off. The value of I.sub.H is chosen to provide W.sub.H, and this energy level remains constant if I.sub.H current is applied with delay t.sub.bH counting from the end of low energy pulse t.sub.L. The high energy pulse delay is defined by equation:

[00011]$\begin{array}{cc}{t}_{\mathrm{bH}}=.Math.\ln \left(\frac{I_L}{I_H}\right).& \left(6\right)\end{array}$

[0073] In accordance with certain embodiments of the present invention, the t.sub.L point may be allowed to vary, thereby allowing the ratio t.sub.L/t.sub.H (defined herein as the breaking point) to be varied, and thus advantageously providing for dynamic control of the X-ray dose to cargo and environment.

[0074] The average current during the lower energy portion of the pulse will be referred to herein as the low energy current, and, mutatis mutandis, the average current during the higher energy portion of the pulse will be referred to herein as the high energy current.

[0075] The energies within each portion of the pulse will remain constant as long as the low energy current begins to be applied after a delay of t.sub.bL, and as long as the high energy current is applied with a delay of t.sub.bH. The constancy of energy within each of the LE and HE portions of the pulse is beneficial for material discrimination: the energy spectrum of X-ray beam remains constant hence no additional calibration point(s) is required.

[0076] A multi-energy pulse configuration, with greater than two distinct energies during the duration of each pulse, may be created in a similar fashion to that described above, using a standing wave accelerating structure. [0077] The coupling coefficient .sub.0 is chosen to be optimal at current I.sub.1, using the design algorithm described in detail above; [0078] The current I.sub.1 has the highest value in the sequence, it is applied first with delay of t.sub.b1 (defined by Eqn. (4)) thus creating the lowest energy of the beam sequence. [0079] The values of next current pulses I.sub.n are preferably created in descending order, thereby providing an ascending order of the beam energy levels. [0080] In preferred embodiments of the invention, after each sub-pulse the current is turned off. [0081] The current I.sub.n for n-th sub-pulse is applied with delay t.sub.bn, given by:

[00012]$\begin{array}{cc}{t}_{\mathrm{bn}}=.Math.\ln \left(\frac{I_{n-1}}{I_n}\right).& \left(7\right)\end{array}$

[0082] Each energy level end point t.sub.n may still be allowed to vary, thereby advantageously providing for dynamic control of the dose to cargo and environment. The energy within each portion will remain constant as long as the lowest I.sub.1 current is applied after a delay of t.sub.b1 (defined by Eqn. (4)) and each successive current step is applied with a delay of t.sub.bn.

[0083] An example of linac implementation with a dual-energy pulse that is created by applying the I.sub.L and I.sub.H currents with optimal delays is shown in FIG. 5. Parameters of the linac are identical to those that have been shown as an example in FIG. 3. Low energy current 51 is applied at an optimal delay of t.sub.bL0.34 s and is turned off after t.sub.L1.45 s. High energy current 52 is applied with delay of t.sub.bH0.36 s at 1.8 s and is turned off at the end of microwave pulse.

[0084] FIG. 6 depicts a block-diagram of an intra-pulse dual-energy linac 60 where energy modulation is achieved by varying both input RF-power P.sub.H and P.sub.L, as well as injection current I.sub.H and I.sub.L provided into the standing wave accelerating structure 42. Low energy current 61 and high energy current 62 are applied with optimum delays. RF-power modulation can be achieved by several known methods such as varying input voltage and current of RF-source 25, or varying the input RF-power to the RF-source by means of RF-source modulator 26, or else by manipulating output power of the RF-source with switches or regulators, generically referred to herein as RF modulator 255.

[0085] In accordance with another embodiment of the present invention, an ascending order of energy levels is created with constant amplitudes. As used herein, the term amplitude refers to an instantaneous flux of electrons within electron beam. In this embodiment. [0086] A corresponding ascending sequence of RF-power levels and descending sequence of injection current pulses is applied. [0087] The coupling coefficient .sub.0 is chosen to be optimal at current I.sub.L, using the design algorithm described in detail above. [0088] A low energy current delay t.sub.bL is as defined by Eqn. (4). [0089] A high energy current delay t.sub.bH_a is as determined using

[00013]$\begin{array}{cc}{t}_{\mathrm{bH}.Math..Math.\_.Math..Math.a}=.Math.\left[\ln \left(\frac{I_L}{I_H}\right)+\ln \left(1+\frac{2.Math.{}_0}{{}_0-1}.Math.\sqrt{\frac{P_H}{P_L}-1}\right)\right].& \left(8\right)\end{array}$

P.sub.H and P.sub.L refer, respectively, to RF power applied during high energy and low energy portions of the beam current pulse.

[0090] While using power modulation, certain benefits may be achieved by creating a descending order of energy levels with constant amplitudes. For implementing such the descending sequence: [0091] A corresponding descending sequence of RF-power levels and ascending sequence of injection current pulses is applied. [0092] .sub.0 is chosen to match the accelerating system with low energy current using design algorithm described in detail above. [0093] A high energy current delay t.sub.bH is determined using Eqn. (3) using I.sub.H and P.sub.H. [0094] A low energy current delay t.sub.bL_d is determined using

[00014]$\begin{array}{cc}{t}_{\mathrm{bL}.Math..Math.\_.Math..Math.d.Math.}=.Math.\left[\ln \left(\frac{I_H}{I_L}\right)+\ln \left(1+\frac{\sqrt{4.Math.{}_0.Math.{\mathrm{rLP}}_H}}{I_H.Math.\mathrm{rL}}.Math.\sqrt{1-\frac{P_L}{P_H}}\right)\right]& \left(9\right)\end{array}$

[0095] The block-diagram of the linac 71 depicted in FIG. 7 demonstrates the option of a linac with multi-energy pulses that is created by modulating both input RF-power and injection current. Linac 71 includes similar subsystems to those presented in FIGS. 2 and 4. FIG. 7 shows an example of creating an ascending order of beam energy steps W.sub.n by applying an ascending order of RF-power levels P.sub.nwith a corresponding descending order of injection currents I.sub.n.

[0096] Embodiments of the invention described above are intended to be merely exemplary; numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in any appended claims.