Radiant energy – Invisible radiant energy responsive electric signalling – Including a radiant energy responsive gas discharge device
Reexamination Certificate
2001-02-12
2003-12-09
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Including a radiant energy responsive gas discharge device
C250S389000
Reexamination Certificate
active
06661013
ABSTRACT:
The present invention relates to a device and method for two-dimensional detection of particles or electromagnetic radiation in single event mode.
The present invention relates to a device and method for two-dimensional detection of particles or electromagnetic radiation in single event mode. The present invention relates to a device and method for two-dimensional detection of particles or electromagnetic radiation in single event mode.
Two-dimensional imaging of microscopic particles or electromagnetic radiation is of increasing interest in fundamental and industrial research. For example, in metallurgy and materials science, two-dimensional imaging is used to derive information on the microstructure of a material.
A microscopic particle can be an electron, an atom, a molecule, an ion, or the like. As used herein, the term particle is also intended to encompass electromagnetic radiation (in general electromagnetic radiation behaves as a photon having particle-like properties when detected).
It is necessary for many applications to not only detect whether there is a particle, but also to determine the two-dimensional position of the particle. Photographic film has been used to record two-dimensional images of such particles for many decades, for example in the field of field ion microscopy (FIM). The use of video-cameras including CCD-chips for such two-dimensional imaging is also well-known.
Furthermore, for many applications it is necessary to measure the two-dimensional position and the time when the microscopic particle is detected. So called time-of-flight methods measure the time difference between the time when the primary microscopic particle is created in a reaction or emitted from a substrate and the time when the particle hits the detector. For time-of-flight experiments the primary particle is usually created or emitted from a substrate by a pulsed source, for example a laser pulse or an electromagnetic pulse induced on the substrate. This pulse will be referred to as the initial pulse. The typical flight time of microscopic particles is of the order of several nanoseconds to microseconds and, consequently, a precision of the order of one nanosecond or even less needs to be achieved for the time measurement.
A position-sensitive detector commonly used for two-dimensional imaging and for time-of-flight applications is a delay-line detector. The delay-line detector includes a stack of micro-channel plates (MCP) and a delay-line anode. A MCP or a stack thereof is a position-sensitive secondary electron multiplier. A standard MCP is from about 25 to 100 mm in diameter, about 1 mm thick and comprises hundreds of thousands or even millions of pores. A voltage of about 1000 V is applied over each MCP. When a primary incident particle impinges in one of the pores it starts an avalanche process by secondary electron multiplication in the pore due to the high electric field inside it. As a result of the high voltage necessary for secondary electron multiplication the MCP can only be used in high vacuum at less than about 10
−5
mbar. The primary particle is converted into a cloud of about 1000 to 10000 electrons in this single pore. Several of the MCPs are stacked to increase the amplification factor. Using a stack of two or three is common for single particle detection. The MCPs are preferably mounted directly on to each other. The avalanche distributes to a few pores when reaching the second and third MCP, because the pores of different MCPs are not aligned. But the secondary electron cloud is still spatially very localised in a few of the pores. The electron cloud leaving a stack of three MCPs typically contains 10
6
to 10
7
electrons. The electron leaving the rear side (facing the delay-line anode) of the MCP stack is accelerated onto the anode.
An example of a delay-line anode is a crossed wire anode. A crossed wire anode typically comprises a square metallic substrate acting as a holder, four insulating ceramic rods and two metallic wires. A ceramic rod is mounted on each edge of the square metallic holder and the first wire is wound around the metallic holder on the rods in one direction (x-direction), whereas the second wire is wound perpendicular to the first one (y-direction). All wires are insulated from the metallic holder and from each other by the rods. The metallic wires form a crossed mesh with a wire distance of about 0.5 mm. The electron cloud leaving the rear side of the MCP-stack is collected by the wires being typically 100 V more positive than the rear side of the MCP-stack inducing an electromagnetic signal. The signal is propagating along each wire in both directions. Four analogue amplifiers connected to the four terminating ends of the two wires amplify the signals arriving at the four terminating ends. A timing signal is picked up at the MCP and is amplified by a fifth amplifier. In a pure imaging application, four clocks realised by time-to-digital-converter channels (TDC) are started with the signal from the MCP. Each of the TDC channels is stopped by the signal of one of the other four amplifiers. The TDC directly converts the time difference between a start and a stop signal of each channel to a digital number which can be further processed with a computer. Thus, the method yields the signal propagation times on the two wires from the position where the cloud has hit the crossed wires to the terminating ends in the positive and negative x-directions (tx1 and tx2) and to terminating ends in the positive and negative y-directions (ty1 and ty2). Knowing the mean propagation speed (v) of the signal, the two-dimensional position where the electron cloud has hit the crossed wires can be reconstructed by a simple algorithm. The position in the x-direction (x) is a linear function of the times tx1 or tx2 and the position in the y-direction (y) is a linear function of the times ty1 or ty2. The position in the x-direction is also given by the time difference x=v/2*(tx2−tx1) and the position in the y-direction (y) by the difference y=v/2*(ty2−ty1). The typical position resolution of such a delay-line detector is 0.1 mm and the total propagation time (T) of the signal on a wire from one terminating end to the other is typically from 30 to 100 ns. All this information is stored for each single primary particle by a computer. Thus, a delay-line detector yields the two-dimensional position of each single primary particle and not only an integrated image (as, for example, a CCD-camera does) of many particles. Such a detection method is called single event mode. However, when two particles impinge on the detector within a shorter time difference than T it may occur that a signal of the second particle arrives at a terminating end before the respective signal of the first particle due to different positions. To register such particles at all a so called multi-hit TDC is used. A multi-hit TDC registers more than one stop signal on each channel after each start signal. When first and second primary particles impinge on the detector during T, two stop signals and hence two times are registered at each of the TDC channels following the identical start pulse as before. In principle, therefore, the multi-hit TDC enables two particles to be detected with a shorter time distance than T. However, a typical multi-hit TDC has a dead time for two consecutive stop signals of about 20 ns. When the second signal arrives at a terminating end within this dead time after the first signal the second signal is lost. Whether or not this occurs depends on the time difference between the first and the second particles and on their positions. If all of the four times tx1, tx2, ty1, and ty2 are registered for each of the two particles the position of both particles can be determined uniquely as long as the particles do not arrive at the detector at the same time within the time resolution of the TDC, typically from 0.5 to 1 ns. However, if one of the four times of the second particle is lost due to dead time the position reconstruction for the respecti
Cerezo Alfred
Huang Min
Jagutzki Ottmar
Mergel Volker
Schmidt-Böcking Horst
Hannaher Constantine
Moran Timothy
Roentdek Mandels GmbH
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