Patent Publication Number: US-2022236429-A1

Title: Particle energy measuring device and method for determining a beam energy of a particle beam

Description:
Particle accelerators are used in particular in research and medicine to generate a particle beam from particles of a given energy. In medical applications, the particle beam is used to destroy tissue in a targeted manner. To protect nearby tissue, the particle energy must be known to a high degree of accuracy. It is also necessary to know the number of particles—or as an equivalent the overall charge of the particle beam—to be able to calculate the dose applied to the tissue. 
     Accelerators are composed of a number of components. Even small changes to the position of the components relative to each other have a considerable impact on the particle energy. It is therefore necessary to adjust accelerators at least upon commissioning, but generally also at regular intervals. Adjustment not only refers to the purely mechanical alignment, but also to subsequent adjustments, for example to mostly existing focus and deflection electromagnets, phase shifters, RF power controllers and/or the particle source. These all directly affect the energy and power of the beam and must be operated frequently during normal operation. 
     Due to the number of components, whose relative positions to each other are relevant, and the fact that the impact of a change in the position of a component relative to the others is usually unknown, a large number of test measurements must be conducted to achieve an effective alignment of the individual components of the particle accelerator relative to each other. 
     Since a large number of test measurements must be conducted, the duration of each individual test measurement is highly relevant for the overall duration of the adjustment. Particle energy measuring devices should therefore have both a low measurement uncertainty and allow for a short measurement time. 
     The invention aims to improve the measurement of particle energy and the particle beam. 
     The invention solves the problem by way of a particle energy measuring device for determining the energy of a particle beam with (a) at least twenty capacitors that (i) each comprise a first capacitor plate and (ii) a second capacitor plate, and (iii) are arranged one behind the other with respect to a beam incidence direction, (b) a multiplexer that has (i) a multiplexer outlet and (ii) a plurality of multiplexer inputs, each multiplexer input being designed to connect to precisely one capacitor and (iii) that is configured to connect one of the capacitor plates of the respective capacitor to the multiplexer outlet, (c) a total charge measuring device that (i) comprises a total charge measuring device input, which is connected to the second capacitor plates in order to detect a total charge of the charges on all the capacitors, and (ii) a total charge measuring device outlet, and (d) an analysis circuit that is connected to the total charge measuring device and the multiplexer, and is designed to automatically (i) effect a switch from one multiplexer input to another multiplexer input, so that the capacitors are individually discharged in succession and (ii) detect the charge flowing from each capacitor during the discharging process, thereby obtaining charging data from which the particle energy can be calculated. 
     According to a second aspect, the invention solves the problem by way of a method for determining a beam energy of a particle beam, the successive measurement of the charges of the capacitors being achieved by switching the total charge measuring device to detect a total charge of all charges on the capacitors, successively discharging all capacitors and measuring the decrease in the total charge for the discharge of one capacitor at a time for the discharge of all capacitors. The measurement of the decrease in the total charge for the discharge of one capacitor at a time for the discharge of all capacitors is understood particularly to mean that the decrease in the total charge for the discharge of one capacitor is measured and this is conducted for all capacitors. 
     The invention also solves the problem by way of a method for determining a beam energy of a particle beam featuring the steps: (a) allowing the particle beam to fall on a particle energy measuring device that comprises (i) at least twenty capacitors ( 30 . n ), which each comprise a first capacitor plate and a second capacitor plate, and are arranged one behind the other with respect to a beam incidence direction (S), (ii) a multiplexer that has a multiplexer outlet and a plurality of multiplexer inputs, wherein each multiplexer input can be connected to precisely one capacitor and said multiplexer is configured to connect one of the capacitor plates of the respective capacitor to the multiplexer outlet, (iii) a plate charge measuring device that comprises a plate charge measuring device input, which is connected to the second capacitor plates, and a plate charge measuring device outlet, (b) successively measuring the capacitor charges by means of the plate charge measuring device by successively connecting the capacitors to the plate charge measuring device by means of the multiplexer, thereby obtaining charge data, and (c) calculating the particle energy and/or a beam charge from the charge data. 
     The advantage of the solution according to the invention is that a low degree of measurement uncertainty can be achieved. Specifically, it has been proven that a high degree of measurement accuracy can be achieved when a large number of capacitors is provided. For example, the particle energy measuring device preferably comprises 64 capacitors, especially preferably 100 capacitors. It is beneficial if there is a maximum of 1000 capacitors. 
     Another advantage is that very short measuring times can be achieved. In this way, it is possible to measure the particle energy with a measuring time of less than one minute, particularly less than ten seconds, particularly even less than one second. This allows, for example, for a significantly quicker adjustment of a particle accelerator. 
     It is also beneficial that the effort required for the calibration of a particle energy measuring device according to the invention is usually relatively small. This is because only a charge measuring device needs to be calibrated, which can be achieved with a high degree of accuracy. Theoretically, it would be possible to provide a number of charge measuring devices, which could lead to an even shorter measuring time. However, the disadvantage of this is that the calibration of the individual charge measuring devices in this case requires considerable effort. 
     A further advantage is that in most cases a good signal-to-noise ratio can be achieved. The greater the charge that accumulates on a single capacitor, the lower the noise level with which this charge can be measured. It is therefore practical to apply the particle beam to the detector until a sufficiently high number of charges has formed on the capacitors. 
     If no total charge measuring device is used, however, waiting too long could cause the charge accumulated on the capacitors to become so great that the resulting electrical field is above the breakdown threshold, resulting in a breakdown. The capacitor is then destroyed and the particle energy measuring device no longer delivers reliable data. If a total charge measuring device is used, the total charge can be used to determine the ideal time for reading the capacitors, i.e. determining their charge, which represents a preferred embodiment of the method according to the invention. This renders it possible to point the particle beam on the particle energy measuring device until there is a sufficiently high charge on the capacitors, so as to obtain a high signal-to-noise ratio. 
     The total charge measuring device is understood to be a charge integrator, in particular a cyclical charge integrator. The term should indicate that this charge integrator is used to measure the total charge across all capacitors. The total charge measuring device could also be referred to as a first integrator. 
     The plate charge measuring device is also understood to be a charge integrator, in particular a cyclical charge integrator. The term should indicate that this charge integrator is used to measure the charge on the plates of a single capacitor. The plate charge measuring device could also be referred to as a second integrator. 
     The feature that the particle energy measuring device is a particle energy measuring device for determining the energy of a particle beam is understood particularly to mean that the particle energy measuring device can be used to measure the energy and preferably also outputs the energy as a measurement result. It is beneficial if the particle energy measuring device can also output quantities that can be calculated from measured quantities such as charge and energy, for example beam output, absorbed dose and/or fluence. 
     It is beneficial if the capacitors have a solid dielectric. The solid dielectric is preferably an oxide layer applied to one or both capacitor plates. For example, if one capacitor plate is made of aluminium, the dielectric is preferably formed by an anodized layer. The anodized layer could then be metallized, for example with copper, which forms the second capacitor plate. 
     It is noted for all embodiments that the capacitor plates do not have to be separable from each other. It is also possible that the capacitor plates are connected, especially inseparably, to each other. In particular, at least one capacitor plate can be designed as a coating of another object, for example the other capacitor plate. 
     Alternatively, the solid dielectric may be formed by a plastic plate. The plastic plate preferably contains more than  95 % by weight of molecules that do not contain atoms with a nuclear charge number greater than nine. It is especially beneficial if the plastic plates are made of halogen-free plastic. The smaller the nuclear charge number, the smaller the interaction with particle beams, so that processes occurring in the dielectric have only a small impact on measurement uncertainty. Preferably, the plastic plates are made of PET (polyethylene terephthalate). 
     It is especially beneficial if the first capacitor plate, the solid dielectric and the second capacitor plate are bonded together to form capacitors. It is then possible to stack the individual capacitors on top of each other, so that the particle energy measuring device can be produced relatively easily. It is then also possible to replace individual capacitors with little effort if they are defect. 
     According to a preferred embodiment, the particle energy measuring device comprises an analysis circuit that is designed to automatically conduct a method featuring the step of detecting the charge flowing from the respective capacitor by forming the difference between the charge on the total charge measuring device at the beginning and end of the discharging process of the respective capacitor, wherein this step is conducted for each position of the multiplexer and thus for all capacitors. In this way, charge data is obtained. 
     The charge data preferably encode the charge of each capacitor and contain an identifier, by means of which the respective capacitor can be identified. This identifier can also be encoded, for example, in that the charge data is output in a predetermined order and the order in which the capacitors are read is constant and known. 
     Advantageously, the multiplexer outlet is directly connected to earth. The first and second capacitor plates of the corresponding capacitor are thus connected to each other via the multiplexer and the total charge measuring device. In this way, only one total charge measuring device, i.e. an integrator, is necessary, which reduces the effort required for calibration. 
     The analysis circuit is preferably designed to control the multiplexer such that it successively discharges the capacitors when the analysis circuit receives a trigger signal. A trigger signal is understood to mean a signal that encodes the fact that a particle beam is emitted from the accelerator within a predetermined interval around the time of the trigger signal. 
     It is beneficial if the particle energy measuring device (a) has a plate charge measuring device that (i) comprises a plate charge measuring device input, which is connected to the multiplexer outlet, and (ii) a plate charge measuring device outlet, and which (iii) is designed to measure a charge that flows via the multiplexer outlet and the plate charge measuring device input. It is beneficial, but not necessary, if the reference ground of the total charge measuring device outlet is at the same electrical potential as that of the plate charge measuring device outlet. 
     It is favourable for the analysis circuit to be connected to the total charge measuring device and the multiplexer and to be designed to automatically detect the charge flowing from each capacitor by detecting the charge measured by the plate charge measuring device. In this way, charge data is obtained. By means of the plate charge measuring device, the charge on the capacitor plates can be measured with a particularly high degree of accuracy. It should be noted that in this case it possible, but not necessary, that the particle energy is calculated exclusively from the charge data collected by the plate charge measuring device. The total charge measuring device can then be used, for example, to identify the ideal time for a measurement. To this end, the analysis circuit is preferably designed to automatically read the charges of the capacitors in succession when the charge measured by the plate charge measuring device has exceeded a predetermined limit. As described above, this is achieved by successively connecting the capacitors to the plate charge measuring device by means of the multiplexer, so that the charge stored by the respective capacitor is detected by the plate charge measuring device and the capacitor discharged. 
     According to a preferred embodiment, the plate charge measuring device and total charge measuring device are used to determine the charges of the capacitors. The advantage of using both charge measuring devices is that it reduces measurement uncertainty. This is particularly true if more than one measurement per second is conducted and/or the pulse charging of a particle beam is small. The charges on the individual capacitors are considerably smaller than the total charge, but the measurement range of the plate charge measuring device is preferably set to be more sensitive by a factor of  10 . The sum of the individual channel charges provides a second, independent value for the total beam charge, which leads to a redundancy of the charge measurement. 
     It is beneficial if the analysis circuit is designed to automatically discharge the capacitors if no trigger signal is received after a predetermined shut-down period has passed. This minimises the risk of one of the capacitors being charged too much. 
     The effective depth is understood to mean an indication of depth for which it holds that two plates of different densities and/or different thicknesses have exactly the same effect on the particles when the effective thicknesses of the two plates are the same. Given that the strength of the interaction between the particles and the material depends, in good approximation, on the density of the latter, the effective depth is preferably the product of the density and absolute depth. In particular, the effective depth is understood to mean the product of the absolute depth, i.e. a length, and the density of the material through which the beam extends. If, for example, the particle beam extends through aluminium, the effective depth is the product of the density of aluminium and the path covered by the beam in the aluminium. 
     Alternatively or additionally, the analysis circuit is preferably designed to automatically calculate the particle energy from the charge data via the steps (i) determining a width parameter of the charge function and (ii) determining the particle energy from the width parameter. The width parameter is understood to mean a number that indicates how wide the charge function is over a depth, particularly over the effective depth. If the particles exhibit a high particle energy, they can still ionize at greater effective depths. The width of the charge function is therefore a good measure of particle energy. 
     The determination of the width parameter is preferably achieved by numerically determining the integral under the normalized charge function. Normalizing causes the width parameter to become independent of the total charge, i.e. the sum of all charges on the capacitors. For example, normalizing is achieved by dividing by the maximum value of the charge function. It is also possible that the charge function is fitted with a model function (also referred to as data fitting) and, for the purpose of normalizing, the charge function is then divided by the maximum value of the determined balance function. 
     A specific capacity of the capacitors is preferably 30 to 100 Picofarad per square centimeter. Alternatively or additionally, the capacity is at least 1 Nanofarad, especially at least 7 Nanofarad. This allows for a favorable signal-to-noise ratio. The capacity is preferably smaller than 100 Nanofarad. This keeps the effort required to produce the capacitor within a reasonable scope. 
     Due to the specified capacity and/or specific capacity, it is possible to accumulate the charges in the capacitors. This facilitates the sequential reading of the channel charges with only one integrator. With low plate capacities, parallel measurements should preferably be taken, as even small charges build up high voltages. With parallel readings, the charge is permanently discharged from the capacitor. With such a design, however, each integrator would have to be like the other, especially with regard to the respective leakage current, otherwise the measured charge profiles would become noisy and an energy measurement would be afflicted with a high measurement uncertainty. 
     An insulation resistance between the first capacitor plate and the second capacitor plate is preferably at least one gigaohm. Preferably, this applies to at least 90% of the capacitors, particularly for all capacitors. The high resistance between the capacitor plates of a capacitor means that no significant amount of charge is lost during the measurement. It is not essential for the insulation resistance to be greater than 10 gigaohm. 
     To achieve low measurement uncertainty, it is beneficial if a capacity of the capacitors is greater, in particular by a factor of at least 5, than an intermediate capacitor capacity, between capacitor plates of adjacent capacitors. 
     It has been proven to be beneficial if at least a majority of the first capacitor plates are made predominantly of copper and/or aluminium. The first capacitor plate is preferably the beam-side capacitor plate of the corresponding capacitor. 
     The particle energy measuring device preferably has an irradiation side. This irradiation side is, for example, labeled on the particle energy measuring device or specified in an operating manual. It is favorable if an area-related density of the capacitor plate on the irradiated side (with respect to a beam incidence direction S) is greater than an area-related density of the capacitor plate of the capacitor facing away from the beam, preferably by at least a factor of 2, particularly preferably by at least a factor of 3. 
     It is favorable if it holds for at least two, especially for at least a majority of the capacitor plates, that an effective thickness of the capacitor plate facing away from the beam is smaller than an effective thickness of the beam-side capacitor plate. The effective thickness is the actual thickness of the capacitor plate multiplied by the density of the material from which the capacitor plate is made. The effective thickness of the beam-side capacitor plate is preferably at least twice as great as the effective thickness of the capacitor plate facing away from the beam. 
     It is beneficial if the beam-side capacitor plate has a lower density than the capacitor plate facing away from the beam. 
     For example, the effective thickness of the beam-side aluminum capacitor plate is between 0.3 and 3 grams per cubic centimeter and the effective thickness of the capacitor plate facing away from the beam is between 0.15 and 0.35 grams per cubic centimeter. 
     For example, the beam-side capacitor plate is made of graphite or aluminium. A density of the beam-side capacitor plate is preferably at most 3 grams per cubic centimeter. Preferably, the density of the capacitor plate facing away from the beam is also at most 3 grams per cubic centimeter. 
     Preferably, it holds for at least two capacitors, in particular at least 20, that an effective thickness of at least one capacitor plate of the capacitor facing away from the beam is greater than the effective thickness of at least one capacitor plate of the beam-side capacitor. This increases the resolution of the energy in particle energies. 
     It is especially favorable if it holds for at least two capacitors, in particular at least 20, that an effective thickness of the beam-side capacitor plate of the capacitor facing away from the beam is greater than the effective thickness of the beam-side capacitor plate of the beam-side capacitor. 
     A particle energy measuring device according to the invention preferably has a calibration certificate in which the measurement uncertainty for at least one particle energy is stated. In particular, the calibration certification preferably contains (a) the measurement uncertainty for at least one particle energy and/or (b) a width calibration factor, by means of which the particle energy can be calculated from the width parameter. 
     The analysis circuit is preferably designed to automatically effect a discharge of the capacitors, at least if the total charge exceeds a predetermined charge limit. This protects the capacitors from too great a voltage. The charge limit preferably corresponds to a voltage of 10±1 Volt on the total charge measuring device. 
     It is beneficial if the analysis circuit is also designed to automatically effect a discharge of the capacitors if (i) the supply voltage for the electronics is missing, (ii) the trigger signal is missing, (iii) a trigger frequency is too high, so that a pulse overlap can occur, and/or (iv) the integrator signal has a voltage of over 10V. 
     The invention also includes an accelerator system with a particle accelerator, in particular a linear accelerator, for generating a monoenergetic particle beam and a measurement device according to the invention, which is arranged to measure a particle energy of particles of the particle beam and/or a beam charge of the particle beam. The particle accelerator is preferably an electron, proton or ion accelerator. The analysis circuit is preferably configured to automatically conduct a method according to the invention. The particle beam is preferably composed of charged particles, particularly electrons, protons or ions. The particle beam can be continuous. The particle beam is preferably pulsed. 
     A method according to the invention is preferably conducted in such a way that a particle beam made up of particles with an average particle energy is generated and a particle energy measuring device is used whose capacitor plates together are so thick that a total collecting efficiency for the particles with an average particle energy is at least 95%. In other words, this means that a maximum of 5% of all particles cross the particle energy measuring device without being absorbed. 
     The average particle energy is preferably between 2 megaelectronvolts and 1500 megaelectronvolts. 
     The invention also includes a particle energy measuring device for determining a particle beam with (a) at least twenty capacitors that (i) each comprise a first capacitor plate and (ii) a second capacitor plate, and (iii) are arranged one behind the other with respect to a beam incidence direction, (b) a multiplexer that has (i) a multiplexer outlet and (ii) a plurality of multiplexer inputs, each multiplexer input being connected to precisely one capacitor and (iii) that is configured to connect a first capacitor plate of the respective capacitor to the multiplexer outlet, (c) a plate charge measuring device that (i) comprises a plate charge measuring device input, which is connected to the multiplexer outlet, and (ii) a plate charge measuring device outlet, and (iii) is designed to measure a charge flowing via the multiplexer outlet and the plate charge measuring device, (d) an analysis circuit that is connected to the plate charge measuring device and the multiplexer, and is designed to automatically (i) effect a switch from one multiplexer input to another multiplexer input, so that the capacitors are individually discharged in succession and (ii) detect the charge flowing from each capacitor during the discharging process, thereby obtaining charging data from which the particle energy can be calculated. 
     It is possible, but not necessary, that this particle energy measuring device features a total charge measuring device that comprises a total charge measuring device input, which is connected to the second capacitor plate and designed to detect a total charge of the capacitors, and a total charge measuring device outlet. The preferred embodiments of particle energy measuring devices described above also apply for this invention. 
     The invention also includes a particle energy measuring device with (a) at least twenty capacitors that (i) each comprise a first capacitor plate and (ii) a second capacitor plate, and (iii) are arranged one behind the other with respect to a beam incidence direction, (b) a multiplexer that has (i) a multiplexer outlet and (ii) a plurality of multiplexer inputs, each multiplexer input being connected to precisely one capacitor and (iii) that is configured to connect a first capacitor plate of the respective capacitor to the multiplexer outlet, (c) a plate charge measuring device that (i) comprises a plate charge measuring device input, which is connected to the multiplexer outlet, and (ii) a plate charge measuring device outlet, and (iii) is designed to measure a charge flowing via the multiplexer outlet and the plate charge measuring device outlet, and (d) a total charge measuring device that comprises (i) a total charge measuring device input, which is connected to the two capacitor plates for detecting a total charge of the charges on all capacitors, and (ii) a total charge measuring device outlet. 
     It is possible, but not necessary, that this particle energy measuring device comprises an analysis circuit that is configured to (i) effect a switching from one multiplexer input to another multiplexer input, so that the capacitors are individually discharged in succession, and (ii) detect the charge flowing from each capacitor during the discharging process, thereby obtaining charge data from which the particle energy can be calculated. The preferred embodiments of particle energy measuring devices specified above also apply for this invention. 
    
    
     
       The invention will be explained in more detail by way of the attached figures. They show: 
         FIG. 1  an acceleration system according to the invention with a particle energy measuring device according to the invention, 
         FIG. 2 a    a perspective view of a particle energy measuring device according to the invention, 
         FIG. 2 b    a detailed view of a capacitor of the particle energy measuring device according to  FIG. 2 a   , 
         FIG. 3  a schematic circuit diagram of a particle energy measuring device according to the invention in accordance with a first embodiment, 
         FIG. 4  a schematic circuit diagram of a particle energy measuring device according to the invention in accordance with a second embodiment, 
         FIG. 5  a schematic circuit diagram of a particle energy measuring device according to the invention in accordance with a third embodiment, 
         FIG. 6 a    a diagram in which the charge on the capacitors is plotted against the respective effective depth, 
         FIG. 6 b    a diagram in which the width of the curve according to  FIG. 6 a    is plotted against the particle energy, 
         FIG. 7  a block diagram of an evaluation unit of the particle energy measuring device, and 
         FIG. 8  a circuit diagram of the plate charge measuring device. 
     
    
    
       FIG. 1  shows an accelerator system  10  that comprises a particle accelerator  12 , a particle energy measuring device  14  according to the invention and a spectrometer  16 , which is optional. The spectrometer  16  is only used to calibrate the particle energy measuring device  14  and is therefore unnecessary after calibration when the accelerator system  10  is in use. The particle accelerator  12  can be quickly adjusted by means of the particle energy measuring device  14 . Following adjustment, the particle energy measuring device  14  can be removed. The particle accelerator  12  is then ready for use. 
     The particle accelerator  12  is preferably an electron or proton accelerator. The particle accelerator  12  comprises a particle source  18 , for example an electron source, a beam shaper  20  and two accelerator units  22 . 1 ,  22 . 2 . The accelerator units  22 . 1 ,  22 . 2  are subjected to an HF alternating voltage by a klystron  24 , wherein said voltage can have a frequency of F=3 GHz, for example. Frequencies between 1 and 5 GHz are possible. The phase shifters  26 . 1 ,  26 . 2  can be used to impose phase shifts φ 1 , φ 2  on the RF power coupled to the buncher  20  and the first accelerator structure  22 . 1 . An acceleration voltage is, for example, U=50 megavolts. The acceleration voltage is at least as high as the desired particle energy. It is a time-variable quantity and arises at the series resonance of the accelerator structure with the aid of the RF power of the klystron. Here, the resonance quality plays a part, as the following applies: U=Q √(P * Z). A 3 GHz structure achieves a resonance quality Q of up to 20000. Particle accelerators  12  are well-known in the literature and shall therefore not be described in further detail. 
     The particle accelerator  12  generates a particle beam  26 . It is beneficial if the particle accelerator  12  emits a monoenergectic particle beam. To this end, a monoenergetic particle beam is preferably generated from a non-monoenergetic particle beam by selection by means of energy slits after fanning by a dipole magnet. A monoenergetic particle beam is understood to mean a particle beam in which the individual particles barely deviate from the average particle energy. For example, in practice the deviation is at most ±5%, preferably at most ±0.5%. 
       FIG. 2 a    depicts a capacitor unit  28  of the particle energy measuring device  14  according to  FIG. 1 . The capacitor unit  28  comprises a plurality of capacitors  30 . n  (n=1, 2, . . . , N). The number N of capacitors is preferably at least  20 , especially preferably at least  60 . Each individual capacitor  30 . n  has, as shown in  FIG. 2 b   , a first capacitor plate  32 . n , a second capacitor plate  34 . n  and a solid dielectric  36 . n  arranged between them, which is preferably a plastic plate. For example, the plastic plate is made of polyethylene terephthalate. The capacitor plates  32 . n ,  34 . n  and the dielectric  36 . n  are bonded together. It is beneficial if the capacitor  30  has, as in the present case, an insulator  38 . n . which is also formed in the present case by a plastic plate. This plastic plate is bonded to the first capacitor plate  32 . n . The capacitor unit  28  is created by stacking the capacitors  30 . n  on top of each other and subsequent contacting. 
       FIG. 2 b    shows an especially simple way of contacting a capacitor  30 . n . The first capacitor plate  32 . n  is electrically connected to a contact region  40 . n  (through-connection). The reason for this is that aluminum cannot be soldered easily. The problem can be solved by way of a contact region  40 . n  made of a metal that can be effectively soldered, such as copper. The contact region  40 . n  extends in the same plane as the second capacitor plate  34 . n . As a result, the first capacitor plate  32 . n  is electrically connected to the cable  44 . n  and the second capacitor plate  34 . n  with the second cable  42 . n.    
     The first capacitor plate  32 . n  is made of a first material that has a first density ρ n,1 . The second capacitor plate  34 . n  is made of a material with a density) 9   72 , 2 . In principle, it is possible that the densities of different capacitor plates  32 . a ,  32 . b  (a * b) or  34 . a ,  34 . b  (a≠b) differ from each other. However, production is rendered especially easy when the respective materials are the same, so that the densities are also the same. 
     The density ρ n,2  of the second capacitor is preferably greater than the density ♮ n,1  of the first capacitor plate. For example, the first capacitor plate  32 . n  is made of aluminium or an aluminium alloy, the second capacitor plate  34 . n  of copper or a copper alloy. 
     The first capacitor plate has a thickness of d n,1 , the second capacitor plate has a thickness of d n,2 . It is favorable if the thickness d n,1  of the first capacitor plate  32 . n  is greater than the thickness d n,2  of the second capacitor plate  34 . n . Preferably, the first capacitor plate  32  is the capacitor plate facing the beam, meaning that the capacitor plate  32 . n  is in front of the second capacitor plate  34 . n  in terms of a beam incidence direction S. An irradiation side  35  is at the front in the beam incidence direction S. 
     An effective thickness D n,i D=ρ n,1,1 .d n,1  of the first capacitor plate  32 . n  is also greater than the effective thickness d n,2  of the second capacitor plate  34 . n . It is possible, but not necessary, that this applies for all capacitors  30 . n.    
       FIG. 3  depicts a circuit diagram of the particle energy measuring device  14  according to the invention with the capacitors  30 . n . The particle energy measuring device  14  also has a multiplexer  46 , which features a multiplexer outlet  48  and a plurality of multiplexer inputs  50 . n . Each multiplexer input  50 . n  is designed to be connected to precisely one capacitor  30 . n.    
     The particle beam  26  strikes the capacitors  30 . n  and there generates charges Qn. 
     The particle energy measuring device  14  has a total charge measuring device  52 , which has a total charge measuring device input  54  and a total charge measuring device outlet  56 . The total charge measuring device input  54  is connected to the second capacitor plate  34 . n . If the multiplexer input  50 . n  is connected to the respective capacitor  30 . n , charges can flow via the total charge measuring device input  54  and the multiplexer outlet  48 , so that the charge flows from the capacitor  30 . The voltage Un acting on the nth capacitor  30 . n  is thus reduced to zero or close to zero. The fact that the voltage Un is reduced to close to zero should be understood particularly to mean that any remaining voltage is so small that the measurement uncertainty increases by a relative 10%. A capacity C 1  of a capacitor  60  of the total charge measuring device  52  is preferably C 1 =1nF to C 1 =10 μF. 
       FIG. 3  also shows that the particle energy measuring device  14  comprises an analysis circuit  58  that is connected to the multiplexer  56  for switching the multiplexer inputs  50 . n . The analysis circuit  58  is also connected to the total charge measuring device  52  and automatically registers the change in a total charge Q Σ  stored in the total charge measuring device  52 . In particular, the total charge Q Σ  on a capacitor  60  of the total charge measuring device  52  is detected. If a predetermined capacitor  30 . n  is discharged, the total charge Q Σ  changes by a difference ΔQ Σ . This difference ΔQ Σ =Q n  corresponds to the charge Q n  that was stored on the respective capacitor  30 . n.    
     If the analysis circuit  58  receives a trigger signal  61  from the particle accelerator  12  (cf.  FIG. 1 ), the total charge measuring device is reset by short-circuiting the capacitor  60  and detects the next total charge Q Σ  on the total charge measuring device  52  during the waiting time Tw. During the waiting time and until the total charge Q s  is read, the multiplexer input  50  is not connected to any capacitor  30 . n.    
     Once the waiting time Tw has lapsed, the analysis circuit  8  automatically connects the multiplexer input 50.1 in succession to the first capacitor plate  32 . n  of the capacitors  30   n . Following connection to a capacitor  0 . n , the change Q n =ΔQ s , i.e. the change in the total charge Q s  due to the discharging of the respective capacitor  30 . n , is measured. The corresponding charge information represents charge data that encode the respective charges Q n  on the nth capacitor. As explained below, this is used to determine the particle energy. 
       FIG. 5  depicts a second embodiment of a particle energy measuring device  14  according to the invention that features a plate charging measuring device  62 . The plate charge measuring device  62  has a plate charge measuring device input  64 , which can be connected to the multiplexer outlet  48  via the first switch  66 , and a plate charge measuring device outlet  65 . A measurement range capacitor  68  of the plate charge measuring device  62  can be bypassed via a second switch  72  of the plate charge measuring device  62 , so that the capacitor  68  discharges. The capacity C 68  depends on the anticipated plate charge Q n . As a combination of multiple measurement range capacitors, it can be switched freely and is selected in such a way that the voltage U=Q/C at the outlet of the plate charge measuring device  62  does not exceed a limit of 10 Volt. 
     The plate charge measuring device  62  is connected to the analysis circuit  58 , wherein this connection is not depicted for the sake of clarity. If the total charge Q Σ  exceeds a predetermined limit Q z,max , the analysis circuit  58  controls the multiplexer  46  in such a way that it successively connects the capacitors  30 . n  to the plate charge measuring device  62 . 
     When switch  66  is closed and switch  72  is open, a charge balancing occurs on the respective plate capacitor  30 . n . The charge Qn required for the balancing is detected by the plate charge measuring device  62  and appears at the outlet  65  as the voltage equivalent to this charge. This voltage is digitalized by the analysis circuit  58 . 
     Switch  66  is then re-opened, switch  72  closed, and the multiplexer  46  connected to the next plate capacitor  30 . n +1. The cycle repeats N times until the charge on the last plate capacitor  30 .N is balanced and thus measured. Each time, closing the switch  72  discharges the measurement range capacitor  68 . Opening the switch  66  immediately beforehand prevents the return flow of the charge towards the multiplexer  46 . 
     The analysis circuit is preferably designed to automatically close the switches  66 ,  70 ,  72  when the trigger signal  61  does not occur for at least a predetermined shut-off period. 
       FIG. 4  shows a further embodiment of a particle energy measuring device  14  according to the invention that does not have a total charge measuring device  52 , but instead features a single safety circuit  74  that short circuits all capacitors  30 . n  when a limit for the total charge is exceeded. 
       FIG. 6 a    depicts a diagram in which the charge Q n  of the nth capacitor on the y axis is plotted against the respective capacitor  30 . n  on the x axis. In the present case, all capacitors  30 . n  are identical in structure, so that the running index n of the capacitors also represents an effective depth. 
       FIG. 6 a    depicts normalized charge functions K w , which refer to the respective energy W. The charge curves K w  are normalized, meaning that their maximum value is 1. 
       FIG. 6 b    shows the dependency of a width B of the respective charge curve K w  on the (average) particle energy E. The width B is calculated as the numerical integral over the normalized charge curve K w . Using a width calibration factor k in the form of the proportionality factor in the equation W=k*B+b and an axis section b, the particle energy can be calculated from any width B. In the present case, it holds for the width calibration that k=1.2634 and for the axis section that b=0.61105. 
       FIG. 7  depicts a block diagram of the analysis circuit  58 . 
       FIG. 8  shows the circuit diagram of the second charge measuring device  62 . The circuit of the total charge measuring device  52  is in essence identical: only the pins  4 - 5  of component U$2 have been bypassed because the total charge measuring device does not need the input switch. 
     REFERENCE LIST 
       10  accelerator system 
       12  particle accelerator 
       14  particle energy measuring device 
       16  spectrometer 
       18  particle source 
       20  beam shaper 
       22  accelerator unit 
       24  klystron 
       26  particle beam 
       28  capacitor unit 
       30  capacitor 
       32  first capacitor plate 
       34  second capacitor plate 
       35  irradiation side 
       36  dielectric 
       38  insulator 
       40  contact region 
       41  connector 
       42  first cable 
       44  second cable 
       46  multiplexer 
       48  multiplexer outlet 
       50  multiplexer input 
       52  total charge measuring device 
       54  total charge measuring device input 
       56  total charge measuring device outlet 
       58  analysis circuit 
       60  capacitor 
       61  trigger signal 
       62  plate charge measuring device 
       64  plate charge measuring device input 
       65  plate charge measuring device outlet 
       66  switch 
       68  second capacitor 
       70  second switch 
       72  thrid switch 
       74  safety circuit 
     b axis section 
     B width 
     C capacity 
     ΔQ Σ  difference 
     d n,1  thickness of the first capacitor plate of the nth capacitor 
     K w  charge function 
     n running index 
     N number of capacitor 
     Q n  charge on the nth capacitor 
     Q Σ  total charge 
     ρ n,1  density of the first capacitor plate 
     S beam incidence direction 
     T W  waiting time 
     U n  voltage at the nth capacitor 
     E particle energy 
     T W  waiting time 
     U n  voltage at the nth capacitor 
     E particle energy