MicroChannel plates time response

Since the microchannel plate collector records the arrival times of all ions, the resolution depends on the resolution of the TOP instrument and on the response time of the microchannel plate. A microchannel plate with a pore size of 10 pm or less has a very fast response time of less than 2 nsec. The TOP instrument with microchannel plate detector is capable of unit mass resolution beyond m/z 3000. 

MicroChannel plate photomultipliers are preferred to standard photomultipliers, but they are much more expensive. They exhibit faster time responses (10- to 20-fold faster) and do not show a significant color effect (see below). 

With mode-locked lasers and microchannel plate photomultipliers, the instrument response in terms of pulse width is 30-40 ps so that decay times as short as 10-20 ps can be measured. 

The microchannel plate (MCP) remains the detector of choice for many TOF-MS applications because of its large, flat active area high gain and excellent time response. Typical rise times of MCP devices are unparalleled transitions from 10% to 90% of peak intensity are routinely on the order of picoseconds [34]. Often, two microchannel plates are oriented back to back in what is termed a chevron arrangement to achieve gains of 106 or greater, each plate contributing about 103. Mi-... 

The snowball effect within a channel can multiply the number of electrons by 10s. A plate allows an amplification of 102-104, whereas by using several plates the amplification can reach 108. This detector is characterized by a very fast response time because the secondary electron path inside the channel is very short. In consequence, it is well suited to TOF analysers, which need precise arrival times and narrow pulse widths. Furthermore, the large detection area of the microchannel plate allows the detection of large ion beams from the analyser without additional focalization. However, the microchannel plate detectors have some disadvantages. They are fragile, sensitive to air and their large microchannel plates are expensive. 

Fluorescence lifetimes were measured by time-correlated single photon counting using a mode-locked, synchronously pumped, cavity-dumped pyridine I dye laser (343 nm) or Rhodamine 6G dye laser (290 nm). Emissive photons were collected at 90° with respect to the excitation beam and passed through a monochromator to a Hamamatsu Model R2809U microchannel plate. Data analysis was made after deconvolution (18) of the instrument response function (FWHM 80 ps). 

MicroChannel plate detectors are particularly useful in time-of-flight mass spectrometry, as they are flat, minimizing time spread and subsequent mass resolution of homologous ion packets. In addition, they have reasonable gain (104—107 per plate) and fast response time (100-psec time resolution). The major limitation of multichannel plate detectors is the recovery time needed for the detector to rechaige. When a channel is discharged, a recovery time on the order of 10 nsec is typical. This becomes problematic if an ion follows another into a particular... 

A laser system that delivers pulses in the picosecond range with a repetition rate of a few MHz can be considered as an intrinsically modulated source. The harmonic content of the pulse train - which depends on the width of the pulses - extends to several gigahertz. The limitation is due to the detector. For high frequency measurements, it is absolutely necessary to use microchannel plate photomultipliers (that have a much faster response than usual photomultipliers). The highest available frequencies are then about 2 GHz. As for pulse fluorometry, Ti sapphire lasers are most suitable for phase fluorometry, and decay times as short as 10-20 ps can be measured. 

MicroChannel plates are becoming widely used in spectroscopy and two dimensionaj imaging at EUV (100 - 1000 A) and soft X-ray wavelengths (10 - 100 A) for astronomy and microscopy. Although bare microchan-nel plates have low sensitivity (5% - 10% quantum detection efficiency) in this spectral region, use of photocathodes can increase substantially (to 30%-40%) microchannel plate performance. This, combined with the high spatial resolution (<50/rm), fast time response (<300 ps), and large effective area (up to 100 mm diameter) achievable with microchannel plates make the latter a very attractive and versatile tool. We discuss the properties of microchannel plates, photocathode materials, and various microchannel plate detector readout schemes that have been used for soft X-ray and EUV detection. 

The effective resolution of a TCSPC experiment is characterised by its instrument response function (IRF). The IRF contains the pulse shape of the light source used, the temporal dispersion in the optical system, the transit time spread in the detector, and the timing jitter in the recording electronics. With ultrashort laser pulses, the IRF width at half-maximum for TCSPC is typically 25 to 60 ps for microchannel-plate (MCP) PMTs [4, 211, 547], and 150 to 250 ps for conventional short-time PMTs. The IRF width of inexpensive standard PMTs is normally... 

There are also two detector devices that are gaining popularity for fluorescence lifetime studies due to their fast time response the microchannel plate photodetectors and streak cameras. MicroChannel plate photodetectors, similar to PMTs, are based on the use of multiplication of photoelectrons. Instead of using discrete dynodes, continuous semiconductor-coated glass multiplier tubes of 10 pm diameter... 

For fluorescence measurements, by far the most versatile and widely used time-resolved emission technique involves time-correlated single-photon counting [8] in conjunction with mode-locked lasers, a typical mo m apparatus being shown in Figure 15.8. The instrument response time of such an apparatus with microchannel plate detectors is of the order of 70 ps, giving an ultimate capability of measurement of decay times in the region of 7 ps. However, it is the phenomenal sensitivity and accuracy which are the main attractive features of the technique, which is widely used for time-resolved fluorescence decay, time-resolved emission spectra, and time-resolved anisotropy measurements. Below ate described three applkations of such time-resolved measurements on synthetic polymers, derived from recent work by the author s group. 

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