Preamplifiers risetime

The temporal resolution of both methods is limited by the risetime of the IR detectors and preamplifiers, rather than the delay generators (for CS work) or transient recorders (SS) used to acquire the data, and is typically a few hundred nanoseconds. For experiments at low total pressure the time between gas-kinetic collisions is considerably longer, for example, approximately 8 /is for self-collisions of HF at lOmTorr. Nascent rotational and vibrational distributions of excited fragments following photodissociation can thus be obtained from spectra taken at several microseconds delay, subject to adequate SNR at the low pressures used. For products of chemical reactions, the risetime of the IR emission will depend upon the rate constant, and even for a reaction that proceeds at the gas-kinetic rate the intensity may not reach its maximum for tens of microseconds. Although the products may only have suffered one or two collisions, and the vibrational distribution is still the initial one, rotational distributions may be partially relaxed. 

The output pulse of the preamplifier has a fast risetime (of the order of nanoseconds) followed by a slow exponential decay, 100 fis (Fig. 10.37). The useful information in the pulse is its amplitude and its risetime. The risetime is particularly important when the signal is going to be used for timing. The observer should be aware that the risetime increases with external capacitance. The preamplifier pulse is shaped in the amplifier by the methods described in Sec. 10.6. 

A current-sensitive preamplifier is used to transform fast current pulses produced by a photomultiplier into a voltage pulse. The current-sensitive preamplifier is an amplifying instrument. The sensitivity (or gain) of such a unit is expressed as h ut/ in> > mV/mA with typical values of the order of 500 mV/mA. The risetime of the pulse is 1 ns. 

An imaging high-pressure detector can be envisioned from an array of vertically cylindrical ionization chambers, with spatial resolution set by each tube diameter. It may further be possible to segment the collection anode, to derive an azimuthal co-ordinate within each detector and to use signal risetime to get a radial co-ordinate. The precision of such techniques, and the low-energy performance of such detectors is critically dependent upon the preamplifier noise. It may be possible to achieve around 50 electrons rms with modern (optical feedback, or no feedback) amplifiers resulting in an energy resolution of a few percent at 100 keV. 

The second type of event is the absorption of lower energy X-ray photons within the bulk of the silicon photodiode. Here the overall conversion efficiency is determined simply by the energy required to produce an electron-hole pair, which in silicon is 3.6 eV. Thus the OCE for this type of detection is 275 electrons/keV. The signals from the silicon are much faster than those from the scintillator, and will, in practice, be limited by the risetime of the preamplifier used ( 60ns). The basic principle of operation of the hybrid detector is shown in Figure 1. 

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