Patent Application
20070176089
The present invention relates to a device for detecting particles according to the preamble of claim 1 and to a mass spectrometer according to the preamble of claim 18.
It is known to detect particles by means of spatially resolved detectors which comprise a number of contacted elements on which the particles directly strike and can be electrically detected. In this case the particles may either be charged particles which directly generate an electrical signal owing to their charge, or protons or other uncharged particles which bring about an electrical signal as a result of appropriate interaction with an existing element coating.
US 2004/0222374 A1 describes a spatially resolved detector in which a large number of strip-like, conductive elements is arranged side by side in one plane. The elements are separately contacted by an electronic read-out device. The surfaces of the elements are all located in one plane, wherein gaps of non-contacted material remain between the elements. Particles which strike these gaps cannot be detected as the gaps do not constitute an active detector surface.
U.S. Pat. No. 6,847,036 B1 describes a spatially resolved detector for charged particles which comprises a large number of Faraday cups arranged side by side. The cups each have an electrical contact via which they can be selectably connected by way of a multiplexer circuit to an operational amplifier. During a measuring process impacting charged particles, for example from a mass spectrometer, are collected (integrated) in the Faraday cups until the respective cup is connected via the multiplexer circuit and read out.
The object of the invention is to provide a device for detecting particles which has high detection efficiency and can be easily produced. A further object of the invention is to provide a mass spectrometer with an improved device for detecting particles.
According to the invention this object is realized for a device cited at the outset for detecting particles by the characterizing features of claim 1.
Separation of the adjacent structures by a trough and their geometrical overlapping ensures that the particle beam strikes a detector, the surface of which is fully suited to particle detection, wherein resolution according to the location of the particle beam is also provided.
In a preferred embodiment an aspect ratio is defined by a ratio of a height of the structures to a width of the trough. In this case the aspect ratio is preferably at least 0.5, particularly preferably at least 1 and more preferably at least 5. A high aspect ratio on the one hand allows a relatively flat angle of incidence of the particle beam with complete detection or spatial covering and, on the other hand, a high aspect ratio leads to only a relatively small contact area of the conductive structure with the support element being provided, and this basically leads to low capacitances between structure and support element. In addition, the difference in the capacitances of the structures is reduced hereby, and this leads to particularly good adaptability of the electronic read-out device to the conductive structures. In particular the various structures have high uniformity with respect to their read-out characteristics hereby.
The structures are generally preferably substantially uniformly and equally spaced apart, wherein a spatial periodicity of the structures is given in at least one direction. In a large number of applications uniformity of the individual structures is required as the detection property of the detector surface should be homogeneous. In principle there can, however, also be a non-periodicity of at least some structures, depending on the particular requirements. A period length of the structures is particularly preferably not more than about 500 micrometers, more preferably not more than about 100 micrometers and even more preferably not more than 20 micrometers. Good spatial resolution may be achieved as a result of the appropriately small size of the structures. In addition, with a given number of separate structures, the detector can be configured so as to have a small overall construction, whereby miniaturization of one of the analyzers containing the detector is assisted. The apparatus according to the invention particularly preferably has an overall length of the plurality of structures in the direction of the spatial periodicity of not more than about 15 cm, in particular not more than about 10 cm. The apparatus according to the invention can advantageously then be particularly well combined in conjunction with a mass spectrometer. In principle, and depending on the requirements, a periodicity of the structures in two spatial directions is also possible, for instance analogous to the two-dimensional Faraday cup arrangement in FIG. 2b of U.S. Pat. No. 6,847,036 B1.
In a preferred embodiment the structures are formed as elongated elements having a substantially rectangular cross-section. A cross-section of this kind can be particularly easily produced by known microstructuring processes. Alternatively the structures can also have a different cross-section. Adaptation to the characteristics of the particle beam can take place hereby.
In a particularly preferred embodiment the structures have a surface which is oriented substantially perpendicularly to the beam direction. In this way the variation in the flight time of the various particles over the region of the surface of a structure may be minimized. In particular the overall detector surface can hereby be constructed such that simultaneous striking of all particle beams on one of the respective structures is made possible. The detection efficiency may also be increased as a result of the perpendicular incidence.
The electronic read-out device generally preferably comprises at least one capacitive transimpedance amplifier (CTIA). Amplifiers of this type have proved to be particularly sensitive for reading out small charge quantities. The capacitive transimpedance amplifier particularly preferably has a plurality of different switchable integration capacitors in this case, whereby the detection limit may be increased. Variable transimpedance amplifiers of this type have recently been developed for use with particle detectors—see for example “Array Detectors Impact Modern Chemical Analysis; M. Bonner Denton, University of Arizona, Department of Chemistry, Gary Hieftje, Indiana University, David Kopenaal, PNNL, Bruce McIntosh, Hamilton Sundstrand, Peter Williams, Arizona State University, USA, presented at the 2003 ICSOI in Cozumel, MX, http://www.chem.arizona.edu/icsoi/index.html” and “Advanced Ion Detectors for Ion Mobility Spectrometry; Roger Sperline, University of Arizona, Department of Chemistry, Chris Gresham, Phil Rodacy, Sandia National Laboratories, USA, presented at the 2003 ICSOI in Cozumel, MX, http://www.chem.arizona.edu/icsoi/index.html”.
In a generally preferred embodiment the structures are produced by means of a microprocessing process, in particular a LIGA process. LIGA processes allow large aspect ratios of the structures with relatively simple and inexpensive production.
The object of the invention is realized according to the invention by a mass spectrometer cited at the outset by the characterizing features of claim 18. The apparatus according to the invention for detecting particles is particularly preferably suitable for combination with a mass spectrometer of this type. Mass spectrometers provide the disconnected information as charged particles which can be directly detected in sufficient sensitivity by means of modern, highly sensitive, evaluating electronic devices, so the detector according to the invention can optionally be used without further amplifiers, such as multi-channel plates (MCP), connected upstream. The mass spectrometer is particularly preferably a double focusing spectrometer, in particular of the Mattauch-Herzog type. With spectrometers of this type there is usually a focal plane which is not oriented perpendicularly to the particle beam. Therefore the detector according to the invention is particularly advantageously suitable for combination with a spectrometer of this type owing to the required oblique beam incidence.
Further advantages and features of the apparatus according to the invention can be found in the embodiments described hereinafter and in the dependent claims.
Two preferred embodiments of an apparatus according to the invention for detecting particles will be described hereinafter and explained in more detail with reference to the accompanying drawings, in which:
In the present case the structures 2 are prisms with a rectangular cross-section and are uniform in each case. They each have a length l and a height h. A trough 3, which is not filled with material and has a width b, is located between two of the structures 2 in each case.
Each of the structures 2 is separately electrically connected to an electronic read-out device (not shown). The electrical connection can be produced in any manner known per se. For example separate metallic wires may be provided which in particular are embedded in a protective resin. An integrated contact may also be provided, for example by means of vias, which are formed in the support element 1.
As the schematic plan view of
In an arrangement according to the first embodiment the condition for the existence of 100% spatial covering is:
h/b>tan α
where α is the beam angle between the plane 1a and the particle beams.
In the present example the device has a total extension in the beam direction of about 10 cm, wherein only a small detail is shown. With a period length of the structures of 1+b=0.1 mm a total of about 1,000 structures 2 and therefore 1,000 spatially separate read-out channels would result for simultaneously registering a spectrum. Depending on the resolution of the spectrometer and further requirements with respect to the production costs, detectors with 10,000 channels and more may also be readily provided. The width of the structure perpendicular to the beam direction (perpendicular to the drawing plane according to
In principle any desired combination of cross-sectional form, orientation of faces of the structures and beam angle are possible, wherein a spatial covering of adjacent structures expediently exists to achieve 100% beam covering.
