Fundamentals of Focal Plane Detectors

Abstract

Spatial dispersion of ions in one dimension is a well established means of analysing ion mass and focal plane detectors (FPDs) allow ions of a wide range of masses to be recorded simultaneously. This paper is concerned with the principles governing the performance of FPDs and the types of FPD available. It is focused on magnetic sector mass spectrometry but is relevant to all applications in which spatially dispersed particles can be detected using a microchannel plate electron multiplier, e.g. ions, photons of wavelength AE200 nm, electrons and energetic neutrals.Although it has proved possible to produce mass spectra with a high resolution, this has not been matched by an ability to detect them efficiently. Given that highly resolved spectra are available at the detector but are inaccessible efficiently, it is in the development of high-performance FPDs where there are enormous gains in efficiency to be achieved. Limitations of FPD performance of two fundamental types are discussed : the position of impact of an ion on the FPD cannot be measured exactly, and the upper and lower count rates of the FPD are both restricted. These limitations are not simply characterized but are sometimes determined by the electron multiplier stage, sometimes by the properties of the array and sometimes by the data acquisition system.

Development of a Miniature Double Focusing Mass Spectrograph Using a Focal Plane Detector

“…The active area of the MCP is 55 mmῌ8 mm and the channel diameter is 12 mm. The MCP is composed of two stages and the typical gain is over 10 6 if the voltage applied to the MCP is 2.0 kV. The front surface of the MCP is grounded.…”

“…Quantitative performance, furthermore, can also be improved by simultaneous detection because time-dependent fluctuations can be eliminated. For these reasons, a mass spectrograph that employs a magnetic sector analyzer and a focal plane detector 6) is ideally suited for trace and quantitative analysis. For the same reasons, a mass spectrometer employing multiple collectors 7), 8) is also e#ective.…”

“…17) An MCP is the most often used device as a focal plane detector. 6) Although an MCP can provide a cloud of electrons, a system, which can detect these electrons and retain the spatial information, is necessary for a mass spectrograph. For this reason, an MCP is generally combined with two types of electrondetecting systems: one is a photon detector, e.g., a CCD or a photodiode array with a phosphor screen, and the other is a multi-anode detector.…”

Simultaneous or scanning data acquisition? A theoretical comparison relevant to inductively coupled plasma sector-field mass spectrometers

“…As mentioned above, the multi-collector approach is reserved primarily for applications where a premium is placed on precision. Mass spectrographs using modern detector arrays are not commercially available, probably because of the significant limitations of array detectors suitable for mass spectrometry [11]. Existing array detectors fail to provide an adequate combination of gain, low noise, spatial resolution, and cost, among other figures of merit.…”

“…Omitted from this comparison is the impact of the detector/detection electronics and sample-introduction system on analytical performance. Therefore, the quantitative limitations of present mass spectrographic array detectors [11] will not be allowed to detract from the potential benefits to be realized through multichannel acquisition. Such limitations include restricted linear range, lack of uniform sensitivity among individual array elements, and variable noise characteristics among individual array elements.…”