Particle detector for detecting charged particles

Abstract

The present invention relates to a particle detector (1), comprising a measuring electrode (6) for measuring charged particles (2), a detection device (7) for detecting the charged particles (2) measured by the measuring electrode (6), and a method for calculating the number (Q) of charged particles (2) detected by the detection device (7). N The detection device (7) comprises a charge amplifier (6) for converting a charge signal (Q(t)) generated by the charged particle (2) into a voltage signal (U(t)), and an amplifier (10) for amplifying the voltage signal (U(t)).

Description

[Technical Field]

[0001] The present invention relates to a particle detector comprising a measurement electrode for measuring charged particles, a detection device for detecting the charged particles measured by the measurement electrode, and an evaluation device for determining the number of charged particles detected by the detection device. [Background technology]

[0002] Particle detectors are mainly used in the mass spectrometer area (MS area) to detect or count particles. Charged particles generated and extracted in the micro-area or high vacuum area are accelerated to a suitable measuring or collecting electrode, where they are detected with the help of a detection device. There are various methods to detect charged particles (e.g. ions), and they can be detected by induced charge measurement (non-destructive) and particle collision (destructive) methods.

[0003] Destructive particle detection includes: Current detection of charged particles (e.g. using a Faraday cup) Detection of particles by electron multiplication (using an electron tube) There are two distinct ways of doing this.

[0004] A small dynamic range (up to 3 decades) can be achieved with a Faraday cup. To obtain a larger dynamic range (up to 5-6 decades), so-called active charge multipliers are used, such as dynodes, channel electron multipliers (channeltrons), or secondary electron multipliers (SEMs) and microchannel plates (MCPs) (see, for example, the article "Microchannel plate detectors," JL Wiza, Nuclear Instruments and Methods, Vol. 162, Issues 1-3, 1-15, 1979, pp. 587-601, or "Low power readout electronics for a UV MCP detector with cross strip anode," M. Pfeifer et al., Journal of Instrumentation, Vol. 9, March 2014).

[0005] In general, for the measurement of particle currents, reference is made to the books “Beam instrumentation and diagnostics” by P. Strehl, Springer, Berlin 2006, and “Techniques for Nuclear and Particle Experiments: A How-to Approach,” Springer, New York 1994.

[0006] The problem with charge multipliers is their large drift and degradation during long-term operation. For example, in the case of MCPs, the amplification must be continuously readjusted by continuously increasing the high acceleration voltage due to permanent, irreversible breakdown of the microchannels. Finally, from an application perspective, charge multipliers must be replaced after a short operating time. On the other hand, Faraday cups are very robust and exhibit little measurable drift or degradation, but currently known measurement devices only achieve a small dynamic range.

[0007] In summary, charged particles can be detected by current measurement using a Faraday cup or collecting electrode, which has high robustness and stability, but the sensitivity is lower than when detecting charged particles with a charge multiplier.

[0008] On the other hand, the dynamic range of particle detection using charge multipliers is up to 30 times greater than the detection dynamics using Faraday cups. Additionally, as a result of the current measurement of these charge multipliers, this type of detection differentially reduces sensitivity to high mass particles. The sensitivity of a charge multiplier is inversely proportional to the square root of the mass-to-charge ratio (m / z). [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] “Microchannel plate detectors,” JL Wiza, Nuclear Instruments and Methods, Vol. 162, Issues 1-3, 1-15, 1979, pp. 587-601 [Non-patent document 2] “Low power readout electronics for a UV MCP detector with cross strip anode,” M. Pfeifer et al., Journal of Instrumentation, Vol. 9, March 2014 [Non-patent document 3] “Beam instrumentation and diagnostics” by P. Strehl, Springer, Berlin 2006 [Non-patent document 4] “Techniques for Nuclear and Particle Experiments: A How-to Approach.,” Springer, New York 1994 Summary of the Invention [Problem to be solved by the invention]

[0010] It is an object of the present invention to provide a particle detector which, on the one hand, is highly robust and, on the other hand, has a high dynamic range when detecting charged particles. [Means for solving the problem]

[0011] This object is achieved by a particle detector of the type mentioned above, in which the detection device comprises a charge amplifier for converting a charge signal generated by the charged particle into a voltage signal, and an amplification device for amplifying the voltage signal.

[0012] Unlike conventional particle detectors with measuring electrodes in the form of Faraday cups, the particle detector of the present invention converts the charge signal generated by the charged particles into a voltage signal using a charge amplifier, rather than measuring and evaluating the charged particle current using an electrometer. In this way, the particle detector can be used to detect individual particles with high robustness and stability, just like detectors with measuring electrodes in the form of Faraday cups. Furthermore, to ensure a dynamic range of up to 5-6 decades, the voltage signal is amplified using an amplifier. However, in contrast to charge multipliers, it is not the individual charges that are amplified to create a large dynamic range, but rather the voltage signal generated by the charge amplifier.

[0013] The particle detector according to the invention is capable of detecting individual charged particles and determining the number of particles accelerated to the measurement electrode or their total charge. The detection device is typically designed in the form of an electronic circuit for processing analog signals. The detection device is preferably arranged or attached as close as possible to the measurement electrode.

[0014] The evaluation device is typically an electronic unit designed to digitally process the signal, read the output of the detection device, and evaluate its status or result in order to determine the number of charged particles detected.

[0015] The evaluation device can be designed to control a detection device, for example an amplification device, as will be explained in more detail below.

[0016] In one embodiment, the evaluation device is designed to evaluate the voltage signal in each counting window around the measurement time or impact time of the charged particle at the measurement electrode to determine the number of detected charged particles. As mentioned above, the particle detector described herein does not measure current, but rather measures the individual charge or quantity of particles collected by the measurement electrode at a certain measurement time. In other words, the "impact" of an individual particle or multiple particles at the measurement electrode is detected at the measurement time, which results in a voltage jump in the voltage signal. Each counting window typically has two halves, one before the measurement time of the charged particle and the other after. However, the measurement time does not necessarily have to be in the middle of the counting window. The typical duration of the counting window is on the order of microseconds or less.

[0017] The evaluation device can read the amplified analog voltage signal at the output of the amplifier device and convert it into a digital voltage signal for digital evaluation or further processing. For this purpose, the analog voltage signal is sampled at a suitable sampling frequency f s can be sampled by the evaluation device at f s >>f2 applies, where f2 represents the upper frequency limit of the frequency range of interest (see below).

[0018] In a further development of this embodiment, the detection device comprises a trigger unit, typically in the form of a trigger circuit, for defining a respective counting window. To define the counting window, the trigger circuit switches between two binary states, the switching occurring when the voltage signal exceeds or falls below a threshold. The trigger circuit can comprise a filter, for example in the form of a band-pass filter, for filtering a frequency range relevant for triggering.

[0019] The trigger circuit may comprise an amplifier and a threshold switch, for example in the form of a Schmitt trigger, which switches between two different switching states at two different thresholds of the voltage signal and in this way defines the duration of each counting window, the duration of which may vary slightly as a result of the triggering, i.e. the duration does not have to be constant.

[0020] The trigger circuit is connected downstream of the amplifier of the detection circuit and also supplies an analog trigger signal at its output, which is read by the evaluation device, which generates a pulse for each particle impact or each voltage jump, so that the trigger signal provides information about the number of particle impacts at the measuring electrode.

[0021] Alternatively to the trigger circuit described herein, the trigger can be performed to define a respective counting window on the digitized voltage signal in the evaluation device. If a large number of collisions occur in succession on the measuring electrode, the digital evaluation of the voltage signal may not be fast enough to perform the trigger.

[0022] Preferably, the counting window defined by the trigger device is shifted by the evaluation device to a later time in time so that the counting window is around the respective measurement time or impact time, in particular the counting window is shifted by half its length in order to bring the impact time or measurement time into the center of the counting window.

[0023] In both of the above cases, i.e., with analog and digital triggering, the counting window dT i (dT before and after i / 2) is consequently the impact time or measurement time T i From the counting window, the evaluation device can calculate the amount or number of particles according to an appropriate algorithm (see below). The corresponding algorithm is based on the time of each collision T in order to minimize noise and disturbances when determining the number of charged particles.i a specified time window dT around i ([T i -dT i / 2,T+dT i / 2]).

[0024] In a further development, the evaluation device is designed to filter the voltage signal in each counting window to increase the signal-to-noise ratio. In this development, the voltage signal is digitally filtered within each counting window. The digital filtering can be, for example, a sliding average of the voltage signal. In the case of weak collisions (i.e., collisions of a small number of particles with the measurement electrode), the voltage signal is very noisy, so it can be advantageous to determine a typical shape of the collision signal as a convolution term in the evaluation device in order to apply a known filtering algorithm, for example, a wavelet or Fourier-based algorithm. An improvement in the signal-to-noise ratio can be achieved, which can be, for example, a factor of at least 10 dB or 20 dB, by sliding averaging or another noise-reducing measurement data algorithm that can be used.

[0025] In a further development, the evaluation device measures the voltage difference U in the voltage signals before and after the measurement time in order to determine the number of charged particles detected in the respective counting window. i In each counting window, the difference between the voltage signal before the measurement time (reference level) and the voltage signal after the measurement time is determined. The number of collected particles or total charge Q N The voltage difference U calculated in each counting window is used to determine i The charge Q collected in each counting window is proportional to i Using the charge-to-voltage conversion factor CF of the charge amplifier, which may be, for example, on the order of about 100 nV / As, the following results are obtained: JPEG0007761577000001.jpg2781 where, 10 Nxrepresents the amplification factor of the amplifier device.

[0026] In a further embodiment, the evaluation device measures the sampling times t within each counting interval to determine the voltage difference. s,i It is designed to evaluate the voltage signal after the measurement time at 3 / f0 <t s,i <4.5 / f0 applies, where f0 denotes the (pass) resonant frequency of the charge amplifier. Since the voltage signal decreases slowly after a voltage jump, the sampling time must be chosen not too late to avoid falsifying the measurement result. The sampling time must also be chosen not too late, as additional particles may hit the measurement electrode. However, the sampling time must not be chosen too early, as the charge amplifier has a transient response and it is necessary to wait until the charge amplifier reaches a steady state before measuring the voltage difference. For example, if the pass resonant frequency is around 10 MHz, the sampling time t s,i A suitable value of is, for example, the counting window dT i This occurs between approximately 300 ns and 450 ns after the start of

[0027] In one embodiment, the charge amplifier is a low-noise charge amplifier with a signal-to-noise ratio greater than 10 dB in a predetermined frequency range. The charge amplifier typically corresponds to the first stage of a detection device and is connected to the measurement electrode. The signal-to-noise ratio (SNR) of the low-noise charge amplifier is preferably greater than 10 dB in the frequency range of interest [f1, f2] between the first frequency f1 and the second frequency f2 (i.e., SNR > 3: SNR = 10^(SNR_dB / 20), SNR_dB = 10).

[0028] The signal-to-noise ratio in the frequency range of interest [f1, f2] is: JPEG0007761577000002.jpg2567Here, Δu Q indicates the useful voltage signal (unit: V), and e N 2denotes the noise component (unit: V / Hz) due to the charge amplifier. The lower frequency f1 of the frequency range of interest is typically on the order of kHz, e.g., about 20-100 kHz, and the upper frequency f2 of the frequency range of interest is typically on the order of MHz, e.g., 15 MHz or higher. The charge amplifier may have a pass resonant frequency f0, for example, on the order of about 10 MHz.

[0029] In a further embodiment, the charge amplifier has a phase margin of at least 45°, preferably at least 60°. To keep overshoot low, the phase margin of the charge amplifier should not fall below a value of approximately 45°. This is particularly advantageous since the evaluation of the voltage signal should be performed at a sampling time not too far from the measurement time when the charged particle strikes the measurement electrode.

[0030] In a further embodiment, the amplifier device has an amplification factor that is adjustable by the evaluation device as a function of the voltage signal or its signal level. If the measurement times are far enough apart, the voltage signal or level at the output of the amplifier device returns to the output level or output potential. The voltage signal at the output of the amplifier device increases steadily in the case of closely successive collision times, but the slope is always small and, in the worst case, initially increases almost logarithmically. Therefore, the reference level (baseline) increases more slowly over time. However, the voltage signal at the output of the amplifier always returns to the initial potential in the long term. Therefore, it is advantageous if the amplification factor of the amplifier device is specified or adjusted by the evaluation device as a function of the signal level of the voltage signal. A suitable, non-overdrive amplification factor 10 for the amplifier device as a function of the signal level of the voltage signal is Nx If one selects , a dynamic range of the particle detector spanning four or five decades can be achieved.

[0031] In a further development, the amplification factor of the amplifier device is adjustable over at least 4 decades or more, preferably over at least 5 decades or more. By appropriately adjusting the amplification factor as a function of the value or level of the voltage signal, a dynamic range of about 5 decades of the particle detector can be achieved. To adjust the amplification factor over 4 or 5 decades, the amplifier device typically has a plurality of (e.g., 4 or 5) amplification stages connected in series. For example, each amplification stage may have a 10 1 and the N amplifier stages can have an amplification factor of 10 N The amplifier stage is designed to provide a maximum amplification factor of, for example, 0.5 × 10 1 , 2 × 10 1 It should be understood that it is also possible to have a different amplification factor, such as 10. The evaluation device typically increases the amplification factor by an order of magnitude (10 1 ) increments or decrements. The evaluation device can also perform an automatic offset adjustment of the output of each amplifier stage at the start of each measurement, or reset the individual amplifier stages from overdrive in preparation for a measurement, if desired.

[0032] In a further embodiment, the measurement electrode is designed as a Faraday cup. The Faraday cup is a hollow, electrically conductive body (metal cup). The Faraday cup ensures that all charged particles drawn in through a suitable input aperture are captured by a detection device connected to the Faraday cup with virtually no backscattering.

[0033] Faraday cups can detect particle currents in the fA range, which corresponds to particle currents of approximately 1000 particles / second. Due to the high robustness of Faraday cups, they are often used in corrosive, oxidizing environments, at high particle beam densities, and at high temperatures.

[0034] Furthermore, Faraday cups offer high absolute accuracy, long-term stability, and low drift of the measurement signal. Because the particle charge is measured directly, Faraday cups do not exhibit mass discrimination, i.e., sensitivity, which depends on particle size (including the mass of molecular ions). Faraday cup operation does not require high voltages. This makes them particularly suitable for applications in the high to ultra-high vacuum range, as well as for applications that cannot tolerate strong electric fields. Particle detectors equipped with Faraday cups or Faraday cup-shaped measurement electrodes can be used particularly in the field of mass spectrometry, for example, in residual gas analyzers (RGAs).

[0035] With a measurement electrode in the form of a Faraday cup, destructive measurements are performed by collecting the charged particles. Alternatively, the measurement electrode can be used for non-destructive detection of charged particles without contact with the measurement electrode or for inductive charging measurements. The measurement electrode can be used for non-destructive detection of charged particles in the form of ions, for example, in an ion trap mass spectrometer, but can also be used for destructive detection of particles, similar to the Faraday cup.

[0036] In a further embodiment, the particle detector comprises an extractor for extracting charged particles from the environment of the particle detector. For this purpose, the extractor typically has an opening through which the charged particles can reach the particle detector. The opening is usually aligned with the measurement electrode or with an opening arranged therein, for example, the opening of a Faraday cup. If a source of charged particles is arranged in the environment of the particle detector, the line of sight of the particle detector coincides with this particle source. A line of sight is not absolutely necessary; in a further embodiment, the particle beam can be electromagnetically deflected into the Faraday cup.

[0037] In a further embodiment, the particle detector comprises a particle guide device for guiding the charged particles from the extractor to the measurement electrode. The particle guide device may be, for example, an ion optical system. If the path from the extractor to the measurement electrode is small compared to the mean free path, the particle guide device may not be provided.

[0038] In a further embodiment, the extractor and / or the particle guide device are designed to filter charged particles. Both the particle guide device and the extractor can filter charged particles. For example, the extractor can be designed as an ion filter device in the form of a so-called Nielsen grid with a metal grid structure. The particle guide device can filter charged particles, for example, by means of a Fourier-based filtering method.

[0039] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention and from the claims, with reference to the drawings which show the details essential to the invention. Each of the individual features can be implemented individually or collectively in any combination in an embodiment of the invention.

[0040] Exemplary embodiments are shown in the schematic drawings and explained in the following description. [Brief explanation of the drawings]

[0041] [Figure 1] 1 shows a schematic representation of a particle detector for detecting charged particles, comprising a measuring electrode, a detection device and an evaluation device; [Figure 2] 2 shows a schematic representation of the voltage signal at the output of the amplifier of the detection device and the charge signal of a charged particle with an elementary charge measured by the measurement electrode. [Figure 3] 3 shows a schematic voltage signal in FIG. 2, where multiple particles impinge on the measurement electrode at different measurement times. [Figure 4] 4 is a schematic diagram similar to FIG. 3 showing a trigger signal and a number of counting windows for determining the respective number of charged particles impinging on the measurement electrodes; [Figure 5] 5 shows a schematic representation of the voltage signal in one of the counting windows in FIG. 4, where the voltage difference is determined to determine the number of charged particles. [Figure 6] 5A-5C show schematic diagrams of ideal voltage signals at the output of the charge amplifier of a detector having different phase margins; [Figure 7]1 shows a schematic representation of the signal-to-noise ratio of a charge amplifier in the frequency interval of interest. [Figure 8] 3A-3C show schematic diagrams of voltage signals at the output of an amplifier device with different amplification degrees; [Figure 9] 9 is a schematic diagram similar to FIG. 8, showing the evolution of the voltage signal over a longer period of time. DETAILED DESCRIPTION OF THE INVENTION

[0042] In the following description of the drawings, the same reference numerals are used for identical or functionally identical components.

[0043] FIG. 1 shows a schematic structure of a particle detector 1 for detecting charged particles 2, ions in the illustrated example. The charged particles 2 emerge from a particle source 3, which is located outside the particle detector 1, for example in a chamber not shown in the drawing. The charged particles 2 enter the particle detector 1 from the environment through an opening in an extractor 4. A particle guide device 5 in the form of an ion optics system is used to supply or guide the charged particles 2 from the extractor 4 to a measurement electrode (designed as a Faraday cup 6 in the illustrated example). The charged particles 2 propagate along a linear trajectory from the particle source 3 to the Faraday cup 6, and the charged particles 2 entering the input opening of the Faraday cup 6 can be measured by the Faraday cup 6 substantially without backscattering.

[0044] Both the extractor 4 and the particle guide device 5 can serve to filter the charged particles 2 so that only charged particles 2 with a specific (known) mass-to-charge ratio can enter the Faraday cup 6. For filtering, the extractor 4 can, for example, comprise a Nielsen grid comprising a metal grid structure. The particle guide device 6 can, for example, make it possible to filter the charged particles 2 by means of Fourier-based filtering methods. If the path between the extractor 4 and the measurement electrode in the form of the Faraday cup 6 is small compared to the mean free path of the charged particles 2, the particle guide device 5 can be omitted.

[0045] The particle detector 1 also comprises a detection device 7 for detecting the charged particles 2 detected by a measuring electrode in the form of a Faraday cup 6, and an evaluation device 8 (detection electronic unit) for determining the number of charged particles 2 detected by the detection device 7. In the example shown in FIG. 1, the detection device 7 is configured as an analog circuit and is preferably arranged as close as possible to the Faraday cup 6 .

[0046] The evaluation device 8 is used for digital processing of the analog signal supplied at the output of the detection device 7. For digital processing in the evaluation device 8 (evaluation electronic unit), the analog signal is converted into a digital signal by means of an A / D converter (not shown). The evaluation device 8 can be connected to other digital devices, such as a measuring computer, by means of a digital interface indicated by a double arrow.

[0047] The detector 7 in Figure 1 comprises a charge amplifier 9 for converting a charge signal Q(t) generated by the charged particle 2 into a voltage signal U(t). An amplifier 10 connected downstream of the charge amplifier 9 is for generating an amplified voltage signal U(t) (denoted U(t) in Figure 1) at the output of the amplifier 10.

[0048] Figure 2 shows the current I when an elementary charge (a charged particle 2 with elementary charge) collides with the Faraday cup 6. ion The charge signal Q(t) is expressed as the current curve I(t) shown in Figure 2. ion 2 also shows the amplified voltage signal U(t) at the output of the amplifier device 10, which results from the impact of a charged particle 2 with an elementary charge.

[0049] Figure 3 shows in a diagram similar to Figure 2 the repeated impact of charged particles 2 with elementary charges on a Faraday cup 6. In the example shown in Figure 3, the charged particles 2 impact the Faraday cup 6 with a regular period of 16 microseconds. As can be seen from Figure 3, after the impact of each charged particle 2, a discharge occurs through the electronic unit of the detection device 7, so that the voltage signal U(t) decreases.

[0050] The voltage signal U(t) is evaluated to determine the total charge Q of the detected charged particle 2. N To determine the number of charged particles 2 (or their equivalent, if the charge of the particles 2 is known), the voltage signal U(t) is measured at the Faraday cup 6 with a measurement or collision time T of each charged particle 2 (or multiple charged particles 2) to minimize noise. i Each counting window dT around i It is convenient to evaluate only at (i=1, 2, ...). The collision time T i is typically the time required for each counting window dT i is the center of the time interval, i.e., the counting window dT i is T i -dT i / 2 to T i +dT i / 2. Counting window dT i The definition of can in principle take place in the evaluation device 8 by suitable evaluation of the digitized voltage signal U(t).

[0051] In the example shown in FIG. 1, particle detector 1 has a respective counting window dT i 4 also shows a (binary) trigger signal T(t) present at the output of the trigger device 11. The switching between the two binary states of the trigger signal T(t) occurs when the voltage signal U(t) exceeds or falls below a threshold value. In this way, the trigger signal T(t) determines the impact time T of the charged particle 2 on the Faraday cup 6. i can also be counted.

[0052] As can be seen from FIG. 4, the trigger signal generated during triggering or the corresponding counting window dT' determined by the trigger device 11 i In each case, the measurement time T i or more specifically, before the measurement time T i is the counting window dT' i Therefore, the number of charged particles 2 Q N To determine the counting window dT' i is the counting window (dT i ) is the measurement time T i Preferably, the counting window dT is shifted to a later time by the evaluation device so that it is around i is the measurement time T i is the counting window dT i dT as in the center of i is shifted by / 2.

[0053] The counting window dT shown in Figure 4 i are chosen to be relatively long for clarity of illustration, and the respective counting windows dT i The duration of is typically much shorter than shown in FIG.

[0054] The voltage signal U(t) at the output of the amplifier device 10 is generally very noisy, especially in the case of weak impacts (i.e., when only a small number of particles 2 impact) at the Faraday cup 6. This noise is caused by the electronic unit, e.g., the charge amplifier 9, as will be explained in more detail below.

[0055] Figure 5 shows the counting window dT i the (noisy) voltage signal U(t) between the ideal voltage signal U id (t), and a filtered voltage signal U(t) generated by sliding averaging of the voltage signal U(t) to increase the signal-to-noise ratio. f(t). Instead of sliding averaging, the evaluation device 8 can also implement other noise reduction algorithms. For example, the typical shape of the voltage signal U(t) for a particular particle impact can be determined as a convolution term using known algorithms, such as wavelet or Fourier-based algorithms, to improve the signal-to-noise ratio, for example, by at least 10 dB or 20 dB.

[0056] Each counting window dT i The number of charged particles 2 that collide with the Faraday cup 6 at i To determine the voltage signal U(t), or more precisely the filtered voltage signal U, as shown in Figure 5, f Voltage difference U at (t) i is measured at T i Before and at the measurement time T i The voltage difference U is determined after i is the filtered voltage signal U f (t) is the counting window dT i is determined based on a reference value in the form of a voltage level that the voltage has at the start of the

[0057] Voltage difference U i To determine the time, the voltage signal U(t) in the example shown in Figure 5 is counted over a respective counting window dT around the time of the last particle impact. i Sampling time t s,i Measurement time T i In this respect, 3 / f0 <t s,i <4.5 / f0, where f0 represents the pass resonant frequency of the charge amplifier 9.

[0058] Sampling time t in the time interval specified above s,i are calculated for each time window dT as described below with reference to FIG. i The number of charged particles 2 in Q i It has proven to be convenient for determining

[0059] FIG. 6 shows the voltage signal Xs(t) of an idealized (normalized) voltage jump U0, where the following equation defines the associated voltage signal X S (t) applies. JPEG0007761577000003.jpg20148 where α is the measured phase margin of the charge amplifier 9 (0<α<1), ω represents the pass circular resonant frequency of the charge amplifier 9 (ω=2πf), and f represents the pass resonant frequency of the charge amplifier 9. The pass resonant frequency f can be, for example, on the order of MHz, e.g., f=10 MHz. In FIG. 6, the ideal voltage signal X s (t) is shown as an example for four values ​​of the phase margin α = [0.900, 0.625, 0.425, 0.280]. The sampling time t s,i is on the one hand the steady state of the charge amplifier 9 and on the other hand the measurement time T i The optimum sampling time t s,i For , the intervals specified above have proven to be advantageous, i.e., 1 <t s,i / T0<1.5, T0≒3 / f0.

[0060] To avoid the steady state of the charge amplifier 9 reaching a relatively late point at which additional particles may strike the Faraday cup 6, it has been found to be advantageous for the charge amplifier 9 to have a phase margin of at least 45°, preferably at least 60°, as in the example shown in Figure 5. It has also been found to be advantageous for the charge amplifier 9 to be a low noise charge amplifier 9, having a signal to noise ratio of at least 10 dB in the predetermined frequency interval of interest between a low frequency f1 and a high frequency f2.

[0061] The signal-to-noise ratio is defined as: JPEG0007761577000004.jpg2265Here, Δu Q indicates the useful voltage signal (unit: V), and e N 2denotes the noise component (unit: V / Hz) due to the electronic unit of the charge amplifier 9. The lower frequency f1 of the frequency range of interest is typically on the order of kHz, for example about 20-100 kHz, and the upper frequency f2 of the frequency range of interest is typically on the order of MHz, for example 15 MHz or higher.

[0062] FIG. 7 shows the typical noise behavior of a low-noise charge amplifier 9, or its frequency-dependent noise density eN in the frequency range between f1 / f0 and f2 / f0 normalized to the resonant frequency f0 of the charge amplifier 9, where f0 denotes the pass resonant frequency of the charge amplifier 9.

[0063] The number of colliding particles 2 or the number of (elementary) charges proportional to this number Q N To determine the respective counting windows dT i Charge Q collected on i is formed, which is the voltage difference U calculated in the above way. i is proportional to.

[0064] The charge-to-voltage conversion coefficient CF of the charge amplifier 9, which may be, for example, of the order of about 100 nV / As, allows the N time windows dT i Total charge Q at (1=1,...,N) N The following result is obtained for JPEG0007761577000005.jpg2370 coefficient 10 Nx is the amplification factor of the amplifier device 10.

[0065] The voltage signal U(t) at the output of the amplifier 10 increases steadily in the case of closely successive collisions of charged particles 2, but the steady increase in signal level has a small slope, and the reference level also increases more slowly with increasing time or number of collisions, in the worst case, almost logarithmically. However, the voltage signal U(t) at the output of the amplifier 10 always returns to its initial potential in the long run. Therefore, the amplification factor 10 of the amplifier 10 Nx It is advantageous if the is adjustable.

[0066] In the amplifier device 10 shown in FIG. Nx is adjustable in steps over 5 decades (Nx=1,...5), i.e., 10 1 From 10 5 can be generated in the amplifier device 10. For this purpose, the amplifier device 10 has, for example, five amplifier stages connected in series (not shown in FIG. 1 for clarity). Each amplifier stage has a 1 , which can be individually switched on or off by the evaluation device 8. The amplifier device 10 may have more or fewer amplifier stages, the amplification factor of each amplifier stage not necessarily being one decade (10 1 ) need not be.

[0067] As a function of the signal height (level) of the voltage signal U(t), the evaluation device 8 determines the optimum amplification factor 10 of the amplifier device 10, at which the voltage signal U(t) at the output of the amplifier device 10 is not overdriven. Nx,opt For this purpose, it is advantageous if the evaluation device 8 can read out the output of each of the five amplifier stages of the amplifier device 10. This is also advantageous so that at the start of each measurement, the evaluation device 8 can perform an automatic offset adjustment of the amplifier stages of the amplifier device 10. If necessary, the amplifier device 10 resets the individual amplifier stages from overdrive in preparation for the measurement.

[0068] FIG. 8 shows the case where the number of charged particles 2 is an elementary charge and the elementary charge is about 10 5 (i.e., N x,opt = 4), the optimum non-overdriven amplification factor is 10 4 9 shows that the respective voltage signal U(t) at the output of the amplifier device 10 first increases and then decreases again as the number of collisions increases. With the amplifier device 10 having five amplifier stages, the dynamic range of the particle detector 1 can be about five decades. Thus, the amplifier device 10 essentially takes over the function of a secondary electron multiplier (SEM).

[0069] In summary, the particle detector 1 can be implemented in the above-described manner and is, on the one hand, robust and highly stable thanks to the use of the Faraday cup 6 or measuring electrode, and, on the other hand, can cover a high dynamic range of up to 5 decades. Instead of using the Faraday cup 6, it is also possible to use a separate measuring electrode to measure the charged particles 2. For example, the charged particles 2 can be measured non-destructively by measuring the induced charge with the measuring electrode 6.

Claims

1. A particle detector (1), comprising: a measuring electrode (6) for measuring the charged particles (2); a detection device (7) for detecting the charged particles (2) measured by the measurement electrode (6); The number of the charged particles (2) detected by the detection device (7) (Q N and an evaluation device (8) for determining the particle size, The detection device (7) a charge amplifier (6) for converting a charge signal (Q(t)) generated by the charged particle (2) into a voltage signal (U(t)); an amplifier (10) for amplifying the voltage signal (U(t)); The evaluation device (8) determines the number (Q N ) to determine the measurement time (T i ) around each counting window (dT i ) and evaluate the voltage signal (U(t)) in the respective counting windows (dT i The number of the charged particles (2) detected in N ) to determine the measurement time (T i ) and the measurement time (T i ) after the voltage difference (U i 1. A particle detector characterized in that it is designed to determine a particle size distribution.

2. The detection device (7) detects the respective counting windows (dT i ) a trigger device (11) for determining 10. A particle detector according to claim 1.

3. The counting window (dT i ) is the counting window (dT i ) is the measurement time T i are shifted to later times, so that 3. A particle detector according to claim 2.

4. The evaluation device (8) adjusts the respective counting windows (dT i ) is designed to filter the voltage signal (U(t)) at A particle detector according to any one of claims 1 to 3.

5. The evaluation device (8) measures the voltage difference (U i ) to determine the respective counting windows (dT i ) within the sampling time (t s,i ) at the measurement time (T i ), where 3 / f 0 <t s,i <4.5 / f 0 is applied, and f 0 denotes the resonant frequency of the charge amplifier (6), 5. A particle detector according to claim 4.

6. The charge amplifier (6) detects the frequency at a predetermined interval ([f 1 , f 2 ]), a low noise charge amplifier having a signal-to-noise ratio of greater than 10 dB; A particle detector according to any one of claims 1 to 5.

7. The charge amplifier (6) has a phase margin (α) of at least 45°. A particle detector according to any one of claims 1 to 6.

8. The amplifier device (10) has an amplification factor (10) that is adjustable by the evaluation device (8) as a function of the voltage signal (U(t)). Nx ) A particle detector according to any one of claims 1 to 7.

9. The amplification factor (10 Nx ) is adjustable over at least four decades. A particle detector according to claim 8.

10. The measuring electrode is designed as a Faraday cup (6), A particle detector according to any one of claims 1 to 9.

11. Further comprising an extractor (4) for extracting the charged particles (2), A particle detector according to any one of claims 1 to 10.

12. a particle guiding device (5) for guiding the charged particles (2) from the extracting device (4) to the measuring electrode (6); A particle detector according to claim 11.

13. the extractor (4) and / or the particle guide device (5) are designed to filter the charged particles (2), A particle detector according to claim 12.

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