The present invention relates generally to beam energy measurement systems for particle accelerators.
Linear accelerators are used are used in radiotherapy to accelerate particles, typically electrons, protons or heavier ions including helium or carbon ions, up to energies sufficient to allow them to travel to a depth in tissue to irradiate and impart energy to a tumor. In the case of electrons they may alternatively be directed onto a target of material of large atomic number to create high energy X-rays which themselves are then used to treat a tumor at depth.
Typically hadronic and ion particles are generated in a source (for example an Electron Cyclotron Resonance Ion Source (ECRIS) or ion plasma source for protons) and injected into a linear accelerator complex where they are accelerated by high frequency radiofrequency (RF) fields up to a required energy or energies. Acceleration typically proceeds in stages, which may include a pre-accelerator stage, for example a radiofrequency quadrupole (RFQ). The production of a high energy output beam, suitable for radiotherapy treatment or other use, in practice may involve several accelerator sub-units, possibly as many as 10-14, each comprising a sequence of individual accelerator cavities connected to waveguides arranged to couple in the driving RF fields. Typical accelerator stages include drift tube linacs (DTLs), side coupled drift tube linacs (SCDTLs) and coupled cavity linacs (CCLs). The RF fields are typically produced by klystrons or magnetrons.
Typically between the source and pre-accelerator stage is a low energy beam transfer line (LEBT). Typically a medium energy beam transfer line (MEBT) is situated between each accelerator sub-unit or between groups of sub-units. The beam path from beginning to end of the accelerator complex may be many meters long and is typically shielded throughout its length.
In one linear accelerator solution a proton beam is formed into pulses in a proton source injector assembly and these are introduced into a pre-accelerator stage, typically a Radiofrequency Quadrupole (or RFQ) which accelerates the initially drifting pulses up to 5 MeV. During this process of pre-acceleration the pulses gain a bunched structure at 750 MHz as the protons in the pulse start to interact with the accelerating RF field. At the output of the RFQ each pulse is fed into the input of a first linear accelerator stage as a bunched pulse, for eventual acceleration up to medically useful treatment energies. In a particular embodiment a chopper element is arranged to create the beam pulse in a proton source injector assembly. The chopper element, the pre-accelerator stage and the linear accelerator stages operate at a repetition rate of up to 200 Hz. In each subsequent linear accelerator stage applied RF fields couple to the bunches in each pulse and accelerate them to higher and higher energies, while maintaining the structure of the pulse.
The final output energy of the beam from a linear accelerator will be dependent on the number of accelerating structures that are present and the maximum energy is typically equal to the maximum possible energy to which particles can be accelerated. However, it is also possible to vary the energy at output by switching off active units at the end of the linear accelerator gallery. The energy of the emerging beam at output is then equal to the output energy of the last active accelerating unit.
In medical particle accelerators the beam which is produced at the output of the last accelerating sub-unit is transported to the patient through a high energy beam transfer line (HEBT). At the end of the HEBT is a nozzle which is typically arranged to direct, or scan the beam, at the target in the patient, and which nozzle typically also includes a dose delivery system arranged to monitor the dose delivered to the patient.
In a synchrotron based medical accelerator the beam energy necessary for the tumor slice treatment is achieved with a combination of settings in the synchrotron accelerating cavity and in the dipoles' magnetic field, while in cyclotron-based machines it is reached with the insertion of material in the beam by means of the energy degrader in the HEBT. In both cases there is no necessity to accurately measure the energy of the beam directed to the patient, since it can be assumed that it is the same obtained during machine commissioning or quality assurance phase.
Beam profile, beam current and—especially in the Linac-based proton therapy accelerator with variation of energy—beam energy are typically monitored for beam diagnostics or clinical purposes and a beam energy measurement system is typically mandatory under medical device regulations. The HEBT typically has an acceptance in beam energy much higher than the treatment requirements and this could result in a beam with slightly different, or even higher, energy from that which was requested being transported to the patient.
The Dose Delivery System, installed just before the patient, has no means of measuring beam energy of the pulses delivered to the patient. Compliance with medical device regulations may require that the system should incorporate energy measurement for every beam pulse delivered to the patient.
A classical method to measure beam energy is a spectrometer-based system, which includes the use of a bending dipole in the HEBT, in combination with beam position and/or profile detectors upstream and downstream. In case the HEBT is straight, this method cannot be used.
In view of the above, there is a need for a fast response beam energy measurement system. Furthermore, there is a need for a beam energy measurement system which is not affected by the layout of the beam transfer lines.
Accordingly, there is provided a time-of-flight (TOF) measurement system for measuring energy of a pulsed hadron beam, wherein each pulse of the beam is structured into a series of bunches of charged particles, said bunches being repeated according to a repetition rate of the order of magnitude of radiofrequency, said system comprising
a) calculate phase shifts between the output signals of the detectors, and
b) calculate energy of the pulse based on the calculated phase shifts.
According to an embodiment, there is provided a time-of-flight (TOF) measurement system for measuring energy of a pulsed hadron beam, wherein each pulse of the beam is structured into a series of bunches of charged particles, said bunches being repeated according to a repetition rate of the order of magnitude of radiofrequency, said system comprising
a) calculate phase shifts between the output signals of the detectors, and
b) calculate energy of the pulse based on the calculated phase shifts.
The bunches can potentially have a very high repetition rate (up to 3 GHz). The signal strength typically depends on beam intensity as well as beam energy and can vary within a very broad range (for example, more than 3 orders of magnitude). As an example it may vary from −60 dBm to 7 dBm. The system can measure a beam pulse average energy within a range from a minimum energy of, for example, 5 MeV to a maximum energy of, for example, 230 MeV. The system is not interceptive and can be used with any kind of hadron.
Advantageously, the beam energy measurement system according to the invention may be used in a control system of a particle accelerator for radiotherapy, allowing a pulse by pulse control or monitoring of the beam energy; this means that if the energy of a pulse varies from a requested energy by a certain extent (for example by, say, 0.17%) then the next pulse can be prevented or stopped, or mitigated in some way so that it is not delivered to the patient.
More in general, the system of the invention allows a much higher bunches repetition rate than conventional systems. Current systems do not reach 400 MHz; this system can potentially operate up to 1 GHz or higher.
Furthermore, the system allows a very high measurement repetition rate (up to 200 Hz). Moreover, it has an energy detection accuracy, which may in some embodiments be as high as 0.03%, which makes it usable in the Beam Delivery System.
For example, it may be that a medical system (with maximum energy of 230 MeV) is legally required to be able to measure beam E with a resolution of mm water equivalent at maximum energy (230 MeV). This is a challenging case for the ToF system and is equivalent of having 0.15% beam energy resolution. For this reason in an embodiment it was decided to fix the energy resolution requirement to be 5 times better than a possible legal requirement, thus 0.03% across the beam energy range.
The invention is independent of the layout of the transfer lines, and since it does not comprise a spectrometer it can be used in both straight and curved transfer lines, and can detect fast energy changes in both. However it is most advantageously used in a straight transfer line. In fact it may be installed in any straight sector of the machine, and in particular after the pre-accelerator or RFQ which provides bunching.
The invention is advantageously situated in the HEBT where it may be used to measure the output energy of the proton pulses.
Some preferred, but non-limiting, embodiments of the invention will now be described, with reference to the attached drawings, in which:
With reference to
In an embodiment the detectors 1-3 are capacitive pickups and in a specific embodiment these are phase probes. In place of phase probes a beam position monitor, a beam current transformer or a wall current monitor might be used. Alternatively a resonant cavity or an electro-optic crystal may be used. In general any device which measures the electric or magnetic fields of the particle beam is suitable. In an alternative embodiment a beam loss monitor or a device that intercepts part of the beam halo could be used.
Phase probes are capacitive sensors that can be used to detect in a non-interceptive way the passage of a bunch of charged particles. Their main component is a metallic ring, placed around the beam or beam pipe, on which a charge develops when a beam bunch passes inside it. This charge can be collected to get a current proportional to the variation of charge inside the ring.
t12 is the time taken by a particle bunch B to travel the distance L12, which can be used to compute the particles energy:
where E is the kinetic energy of the particle and E0 is the rest energy of the particle (for protons: E0=938.272 MeV); c is the speed of light.
The system is designed to measure the phase shift Δϕ between the output signal of the probes 1-3. To be able to compute t12 from Δϕ12 (i.e. the phase shift between the output signal of the first detector 1 and the output signal of the second detector 2) the relation between the two has to be unambiguous. To this end, the first distance L12 is set out in such a way that the time of flight t12 of a bunch B from the first detector 1 to the second detector 2 is equal to, or lower than a repetition period TRFQ of the bunches B. This poses a limit on the maximum value of L12. For an energy range from 5 MeV to 230 MeV this limit is around 48 mm.
In a particular example the detectors of the invention are situated in a HEBT layout with distance L12=255 mm and distance L13=3595 mm. These distances provide a 0.03% E resolution for beams ranging from 70 up to 230 MeV, as shown in
Given the limit on L12, it is impossible to achieve 0.03% of relative error on the measurement of E with only two close probes, having only one bunch travelling through them, because this would require a precision in the phase shift measurement which is nowadays unachievable. This is the reason behind the use of a third probe. L23 is much greater than L12, so more than one bunch can be positioned along L23. In other words, time of flight T2 of the bunch from the second detector 2 to the third detector 3 is greater than a multiple of the repetition period TRFQ of the bunches. N13 and N23 represent the number of whole bunches present between, respectively, detectors 1 and 3, 2 and 3. In an embodiment, the repetition rate TRFQ is given by the RFQ of the linear accelerator. Note that using only two distant probes is not sufficient as this would not allow an unambiguous energy measurement over the range from 5 MeV to 230 MeV. This is because using the two distant probes only (for example 1 and 3 of
Expressed in another way, if only two detectors were used placed so closely together that only one bunch was present in the inter-detector beamline at any one instant, then the measurement made by both detectors could be interpreted unambiguously as the measurement of the same bunch. However the measurement made, while unambiguous, would be inaccurate because of the phase difference. In fact the real ToF between the two extremely closely spaced detectors would be extremely small and the relative error would be large (calculated from error in measurement=error of instrument/distance between detectors). If we increase the distance between the two detectors so that more than one bunch can now fit simultaneously between the two detectors then the measurement will now result in the detection of two trains of bunches, shifted from each other, and it will not be possible to predict which detected bunch in each train should become the basis of the energy measurement, or in other words how many bunches N should be skipped. To overcome this problem, i.e. to know the correct value of N, it is necessary to already know the energy of the bunches, but calculating energy is the point of the measurement so we face a conundrum.
As an example of this, if the energy of the beam is 100 MeV and:
L12=225 mm,
L13=3595 mm,
delta_L=0.1 mm, and
delta_phi=0.2 deg
implies:
energy_error_12=0.12%,
energy_error_13=0.01%
In an alternative embodiment, if:
L12=40 mm,
L13=1000 mm,
delta_L=0.1 mm, and
delta_phi=0.2 deg
implies:
energy_error_12=0.7%,
energy_error_13=0.025%
However, a worst case occurs if the energy is 230 MeV (a maximum energy in some systems), in which case the corresponding error values are:
in the first case.
energy_error_12 is 0.21% (instead of 0.12%),
energy_error_13 is still 0.01%;
in the second case,
energy_error_12 is 1.16% (instead of 0.7%),
energy_error_13 is 0.05% (instead of 0.025%).
Therefore we measure the approximate beam energy using the signals from detectors 1 and 2 (which allow unambiguous but inaccurate calculation of energy) and use this approximation to calculate the number N of bunches which must be skipped to allow an unambiguous measurement between detectors 1 and 3 (which suffer from ambiguity but provide for a more accurate calculation of energy). By doing this we simultaneously reduce inaccuracy while maintaining unambiguity of calculation. Therefore three detectors are needed to produce a measurement which is both unambiguous and accurate.
This layout greatly improves the energy measurement precision; given the precision in the distance measurement δL and in the phase shift measurement δΔϕ, the relative error on the energy measurement using only detectors 1 and 2 is
While when using also detector 3 it is
which can be reduced by increasing L13 (T13 also increases consequently). The same reasoning can be applied to the opposite situation, i.e. in the case in which they are arranged in ‘reverse order’, and in this case the distance L23 is set out in such a way as that time of flight t23 of the bunch B from the second detector 2 to the third detector 3 is equal to, or lower than a repetition period TRFQ of the bunches B, and then having L12 much greater than L23 such that the time of flight T12 of the bunch from the first detector 1 to the second detector 2 is greater than a multiple of the repetition period TRFQ of the bunches. In such a case N23 will always be 0 and N12 has to be used in its place.
An example of hardware design which allows to measure the phase shifts between the output signals of the phase probes is shown in
The diagrams in
In
Every variable should be set before it is used or alternatively it is set as an a priori known value; the latter is true for constants known from physics and for the following variables: Amin: Minimum amplitude required for the beam pulse to be considered correctly detected. fsampling: Sampling frequency.
A further explanation is required for the correct interpretation of the flow diagrams: Different flows enclosed by black horizontal lines represents operations that can be performed in parallel.
A label put at the top of a parallel branch has to be considered as additional subscript to every variable that appears in that branch, including input and output variables in sub-diagrams. If the label is at the bottom, only output variables gains the subscript.
With reference to
calculated at 102:
N=number of samples in νPP
G=Fast Fourier Transform of νPP.
b=arg maxi|G(i)| between fmin and fmax, wherein fmin and fmax are minimum and maximum values which can be set to constrain the search for the maximum in the Fast Fourier Transform of νPP. They can be used when unknown frequencies are present in the Transform, although this should not be the normal situation.
Then, the frequency fPP of each signal is calculated as
The frequency fg of the signal is then calculated (at 200 in
Then, an I/Q method is performed on each of the output signals νPP (at 300). The I/Q method is shown in detail in
I=Σ
n=0
N-1νPP(n)·sin(2πfgnTs),
Q=Σ
n=0
N-1νPP(n)·cos(2πfgnTs).
where Ts=f1sampling is the sampling period.
Then, phase φPP and amplitude APP of each signal are calculated at 302 as
The signal amplitudes APP are then compared with Amin (at 400 in
Otherwise, the phase shifts Δφ of the output signals are calculated at 500, and subjected to wrapping at 600 (see also
Then, energy values E13 and E23 of the pulse is calculated (at 700) based on time-of-flight measurements between detectors 1 and 3, and between detectors 2 and 3, respectively. This calculation is shown in detail in
The energy E of the pulse is calculated as a mean value between E13 and E23 (at 800).
According to an alternative embodiment, it would be sufficient to use E13 or E23 to provide the beam energy. The mean value between E13 and E23 is used to improve the measurement accuracy. The accuracy might be even further improved using a fourth detector/phase probe or more, but this would add complexity to the system.
The ToF beam energy measurement system allows a high accuracy beam energy measurement to be made at a very high measurement rate (up to 200 Hz) and provides the result typically within 1 ms from the passage of the beam pulse, making it suitable to be used not only as Beam Diagnostics device but also in the Beam Delivery System to monitor each beam pulse average energy, which has been delivered to the patient.
Such a highly responsive system is fast enough to allow the system to take actions to disable generation of the next beam pulse.
The system according to the invention makes no assumptions about the speed at which the beam energy can be changed, thus it poses no restrictions on the energy change rate. This is an improvement over the state-of-the-art because current beam energy measurement systems are either destructive or they do not allow the measurement of fast beam energy changes.
A particular embodiment of the invention is shown in
In a particular embodiment the last accelerating unit may be a CCL.
The detectors (1, 2, 3) share space in the HEBT with other components (902, 903) for example quadrupoles, ACCTs, BPMs, vacuum pumps, etc. The actual components present will depend on the particular HEBT layout, which will depend on the particular geometry of the installation.
The distances between the detectors are:
L12=255 mm and
L13=3595 mm.
These distances allow achievement of 0.03% E resolution for beams ranging from 70 up to 230 MeV, as shown in
After passing through the third detector (3) the proton pulse will continue along the remainder of the HEBT (1000) which leads the beam towards a nozzle and then, finally, the patient placed in a treatment room.
Number | Date | Country | Kind |
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18167210.6 | Apr 2018 | EP | regional |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2019/059376 | 4/12/2019 | WO | 00 |