METHOD FOR OPERATING A LINEAR ACCELERATOR, LINEAR ACCELERATOR, AND MATERIAL-DISCRIMINATING RADIOSCOPY DEVICE

Abstract

A linear accelerator is operated by emitting charged particles from a particle source and accelerating the particles in an accelerator by wayof a high-frequency alternating field in such a way that pulses of charged particles are generated. A high-frequency power is periodically supplied by way of high-frequency pulses to the accelerator in order to generate the high-frequency alternating field. A particle stream emitted by the particle source is varied during a HF pulse length of the high-frequency pulse in such a way that the pulse formed during the HF pulse length has at least two sub-pulses with different mean energies per particle. There is also described a linear accelerator that carries out the method and a material-discriminating radioscopy device with a linear accelerator of this kind.

Claims

1. A method for operating a linear accelerator, the method comprising: emitting charged particles by a particle source; periodically supplying a high-frequency power to an accelerator by way of high-frequency pulses in order to generate a high-frequency alternating field and accelerating the charged particles in the accelerator by the high-frequency alternating field to thereby generate pulses of charged particles; and varying a particle stream emitted by the particle source during an HF pulse length of a high-frequency pulse in such a way that the pulse formed during the HF pulse length has at least two sub-pulses with mutually different mean energies per particle.

2. The method according to claim 1, which comprises generating at least two sub-pulses time-delayed by about 1 s to 3 s by changing a stream strength of a particle stream during the HF pulse length of the high-frequency pulse.

3. The method according to claim 1, wherein the HF pulse length of the high-frequency pulse lies between 2 s and 10 s.

4. The method according to claim 1, wherein a mean energy per particle lies within a range of more than 1 MeV and less than 20 MeV.

5. The method according to claim 1, which comprises injecting a particle stream into the accelerator during an oscillation phase in order to generate one of the at least two sub-pulses.

6. The method according to claim 1, which comprises using a pulse of charged particles containing the at least two sub-pulses for generating X-ray radiation.

7. The method according to claim 6, which comprises generating material-discriminating radioscopic images of an object by way of an X-ray detector that detects the X-ray radiation.

8. The method according to claim 7, which comprises causing the object and the X-ray detector to move relative to each other during acquisition of the radioscopic images.

9. A linear accelerator, comprising: a particle source for emitting a particle stream; an accelerator having a plurality of cavity resonators that are coupled to one another, said accelerator being configured to periodically receive a high-frequency power by way of high-frequency pulses having a HF pulse length in order to generate a high-frequency alternating field; a controller connected to said particle source and configured to vary a particle stream emitted by said particle source during an HF pulse length of the high-frequency pulse in such a way that a pulse of charged particles formed during the HF pulse length has at least two sub-pulses having mutually different mean energies per particle.

10. A material-discriminating radioscopy device, comprising: an X-ray emitter and an X-ray detector disposed to form an intermediate region for introducing an object to be X-rayed between said X-ray emitter and said X-ray detector; said X-ray emitter having a linear accelerator according to claim 9 configured to load a target with pulses of charged particles; and an evaluation device configured to generate radioscopic images from data detected by way of said X-ray detector.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0044] FIG. 1 shows the schematic construction of a material-discriminating radioscopy device having a linear accelerator;

[0045] FIG. 2 shows the progression of a method for the operation of the linear accelerator;

[0046] FIG. 3 shows the acceleration voltage as a function of time in an exemplary embodiment having eight coupled cavity resonators; and

[0047] FIG. 4 shows the acceleration voltage as a function of time in a further exemplary embodiment having 22 coupled cavity resonators.

DETAILED DESCRIPTION OF THE INVENTION

[0048] Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a schematic of the construction of an imaging material-discriminating radioscopy device 100. The device 100 is designed to acquire radioscopic X-ray images of large objects 110, such as, in particular, freight containers, and has for this purpose an X-ray emitter 60 and an X-ray detector 80. The object 110 to be X-rayed is arranged in the intermediate region between the X-ray emitter 60 and X-ray detector 80. The X-ray detector 80, which is designed, for example, as a line detector, detects the X-ray radiation attenuated during penetration through the object 110. In a manner known per se, an evaluation device 81 generates a radioscopic image on the basis of the detected attenuation data.

[0049] The device 100 is designed to provide information about the material composition of the X-rayed object. The X-ray emitter 60 emits for this purpose delayed photons having different energy. Conclusions about the radiographed object can be made from the intensity ratio, detected by the X-ray detector 80, of the attenuation data corresponding to the different radiation energies E.sub.ph. The radiation energy E.sub.ph per emitted photon is, for example, about 4 MeV and about 8 MeV.

[0050] The X-ray emitter 60 has for this purpose a target 61 which is loaded by pulses of charged particles, so bremsstrahlung having the required spectral fractions results. The pulse of charged particlesin the present case these are electronscan be generated by means of the linear accelerator 1, which comprises a particle source 2 and an accelerator 3 having a plurality of coupled cavity resonators 4. An energy supply 5 supplies the accelerator 3 with a high-frequency power P.sub.HF in order to generate a high-frequency alternating field inside the coupled cavity resonators 4 for accelerating a particle stream, which stream is shot or injected by the particle source 2 into the accelerator at specified times.

[0051] The high-frequency power P.sub.HF is supplied periodically, in other words, in the form of high-frequency pulses supplied by the accelerator 3 and which have a HF pulse length t. A controller 6 is connected to the particle source 2 and the energy supply 5 and is designed to synchronize coupling or shooting of the particle streams into the accelerator 3 in respect of the periodically supplied high-frequency power P.sub.HF. The controller 6 and the particle source 2 are designed in particular to introduce at least two particle streams having different stream strengths I into the accelerator 3 during HF pulse length t, which is typically in the range of a few microseconds.

[0052] FIG. 2 schematically illustrates the method for operation of the linear accelerator 1 using a plurality of function graphs which illustrate various physical variables or operating parameters as a function of time t.

[0053] The high-frequency pulse supplied to the accelerator 3 has an HF pulse length t which is between 3 and 5 s. The period length T is in the range of milliseconds, in the illustrated example these are 2 to 3 ms.

[0054] Two sub-pulses of charged particles are generated during the time window specified by the HF pulse length t by injecting two particle streams having different stream strengths I into the accelerator 3. Since in an oscillation phase at the beginning of the high-frequency pulse the maximum acceleration voltage of the oscillated state is not yet available in the resonator structure formed by the coupled cavity resonators 4, the particles contained in the first sub-pulse have a lower mean energy. The X-ray radiation generated thereby accordingly has a lower radiation energy E.sub.ph per photon.

[0055] The stream strengths I of the two particle streams injected during the HF pulse length t are selected such that the deposited dose D is the same for the two sub-pulses. The detector read out A.sub.Det of the low-energy or high-energy sub-pulse accordingly takes place delayed by about 1 to 2 s.

[0056] The conversion of at least two sub-pulses having different mean energies per particle during the HF pulse length t of a high-frequency pulse is based on the property of the accelerator being able to store energy. The change in energy W.sub.B in the resonator structure of the accelerator 3 is given by

[00001]$\frac{{\mathrm{dW}}_B}{\mathrm{dt}}=P_{\mathrm{HF}}-P_{\mathrm{ohm}}-P_{\mathrm{beam}},$

where P.sub.ohm are the ohmic losses of the standing wave in the accelerator 3 and P.sub.beam the beam losses. The acceleration voltage U results from the energy W.sub.B stored in the resonator structure according to

[00002]$W_B=\frac{1}{2}.Math.C_B.Math.U^2.$

[0057] The capacity C.sub.B of the accelerator 3 is the coupling factor between the square of the acceleration voltage U and the stored energy W.sub.B. For the total capacity C.sub.B of the accelerator 3, the following roughly applies

[00003]$C_B=\frac{C_{1.Math.\mathrm{cell}}}{N},$

where C.sub.1cell designates the capacity of a cavity resonator 4.

[0058] The ohmic losses

[00004]$P_{\mathrm{ohm}}=\frac{U^2}{R_S}$

are described by the shunt resistance R.sub.s. The shunt resistance R.sub.S1cell of an accelerator 3 having N coupled cavity resonators 4 is

R.sub.S=N.Math.R.sub.S1cell

[0059] The beam losses P.sub.beam are given by the product of the acceleration voltage U and the stream strength I.

[0060] These assumptions show that the acceleration voltage U is proportional to the root of the number N of coupled cavity resonators. Furthermore, the dependence of the acceleration voltage U on the stream strength I increases as N increases since the shunt resistance decreases.

[0061] FIGS. 3 and 4 show simulation results for accelerators 3, which have 8 (FIG. 3) or 22 coupled cavity resonators 4 (FIG. 4). The course of the acceleration voltage U as a function of time t without injected particle stream is given in both cases by the solid line. The course of the acceleration voltage U as a function of time t with injected particle streams is given by the broken line. In both cases one particle stream is in each case injected at time t.sub.1 and t.sub.2 respectively into the accelerator 3, and this is switched off again at time t.sub.1 or t.sub.2. In both cases the simulation result show that less acceleration voltage U is applied if a particle stream is introduced into the cavity resonators 4.

[0062] The time t.sub.1 is also chosen, moreover, such that it lies within an oscillation phase of the resonator structure formed by the cavity resonators 4. In other words, the acceleration voltage U at this time t1 has still not reached its saturation value, so the particles contained in the first sub-pulse undergo a lower growth in kinetic energy.

[0063] Although the invention has been illustrated and described in detail with reference to the preferred exemplary embodiment, the invention is not limited hereby. A person skilled in the art can derive other variations and combinations herefrom without deviating from the fundamental concept of the invention.