An induction charge detector with multiple sensing stages has been conceived for use in characterizing sprayed droplets, dust particles, large ionized molecules, and the like. Like related prior single-stage devices, each stage yields a measurement of the electric charge and the time of flight of the particle. In effect, an n-stage sensor yields n independent sets of such measurements from the same particle. The benefit of doing this is to increase the effective signal- to-noise ratio and thereby lower the charge-detection limit and the standard error of the charge measurement.
The sensor includes a set of collinear, equal-diameter, electrically conductive cylindrical tubes (sensing tubes) aligned with the path of incoming particles. The entrance to the sensor is a narrower tube that limits the number of entering particles (ideally to one at a time) and ensures that their trajectories remain close to the cylindrical axis. As a charged particle enters each sensing tube, it induces a charge on the tube nearly equal to its own. Each sensing tube is connected to an operational-amplifier circuit that senses the electric potential associated with the induced charge. The charge of the particle can be calculated from this electric potential and the capacitance of the tube. If the measurement is performed in a vacuum and the particle were accelerated by the use of a known voltage before it entered the sensor, then from (1) the time of flight of the particle as determined from durations of voltage peaks from two consecutive sensing tubes and (2) the known axial distance traversed by the particle, one can calculate the charge-to- mass ratio of the particle. Then from the charge-to-mass ratio and the measured charge, one can calculate the mass of the particle.
The principle of operation of the induction charge detector with multiple sensing stages is best described by reference to the figure, which shows a cutaway drawing of a three-stage prototype. In this case, there are eight electrically conductive tubes. The first and eighth tubes are electrically grounded. The second through seventh tubes are the sensing tubes; they are held in place by electrically insulating supports and are connected to operational amplifiers for measurement of the potentials as described in the next paragraph. Following the eighth tube is a final stopping electrode, which collects the charged particle and is connected to another operational amplifier for measurement of the potential associated with the charge. An electrically grounded housing surrounds all of the aforementioned parts.
The second, fourth, and sixth sensing tubes constitute a three-stage sensing electrode. They are electrically connected to each other, the potential on them is denoted V1, and they are connected to the input terminal of an operational amplifier. Similarly, the third, fifth, and seventh sensing tubes constitute another three-stage sensing electrode; they are electrically connected to each other, the potential on them is denoted V2, and they are connected to the input terminal of another operational amplifier. The potential on the stopping electrode is denoted V3, and this electrode is connected to the input terminal of a third operational amplifier. In operation, V1–V2 is measured as a function of time. As a particle travels along the sequence of six tubes, it induces a three-cycle V1–V2 waveform, each cycle representing the reading from one of the three sensor stages. The charge measurement for each stage can be calculated as the product of (1) the magnitude of the corresponding V1–V2 peak reading and (2) a calibration factor obtained from the V3 reading.
If the readings are analyzed in the time domain, then the use of n detector stages reduces the standard error of the charge measurement, which is proportional to n–1/2. On the other hand, because of its periodicity, the waveform lends itself naturally to analysis in the frequency domain. The minimum detectable charge can be reduced, in the case of frequency-domain analysis, by increasing the number of available waveform cycles and, hence, by increasing the number of stages.
However, increasing the number of stages without limit does not reduce the frequency-domain minimum detectable charge without limit and does not reduce the time-domain standard error without limit. The reason for this is that increasing the number of stages increases the sensor input capacitance, thereby reducing sensitivity. This obstacle can be overcome by the use of a multiblock sensor assembly, recording the output of each block independently. Each block would comprise a multiple-stage sensor as described above, except that the number of stages (not necessarily 3) would be chosen, in conjunction with other design parameters, to optimize performance.