Coincidences energy resolution

AEc is the effective coincidence energy resolution and the effective coincidence detection efficiency. 

A number of techniques have been used previously for the study of state-selected ion-molecule reactions. In particular, the use of resonance-enhanced multiphoton ionization (REMPI) [21] and threshold photoelectron photoion coincidence (TPEPICO) [22] has allowed the detailed study of effects of vibrational state selection of ions on reaction cross sections. Neither of these methods, however, are intrinsically capable of complete selection of the rotational states of the molecular ions. The TPEPICO technique or related methods do not have sufficient electron energy resolution to achieve this, while REMPI methods are dependent on the selection rules for angular momentum transfer when a well-selected intermediate rotational state is ionized in the most favorable cases only a partial selection of a few ionic rotational states is achieved [23], There can also be problems in REMPI state-selective experiments with vibrational contamination, because the vibrational selectivity is dependent on a combination of energetic restrictions and Franck-Condon factors. 

Comparison between the core-level X-ray absorption spectroscopy (XAS), emission (XES), and X-ray photoemission spectroscopies (XPS) usually shows that the spectral edges rarely coincide with each other and with the Fermi level. It is common practice, however, to place F at the emission threshold which corresponds to a fully relaxed ion core (16). For defining the structure of the edge, an energy resolution of at least 1-2 eV is required in the range of 5-20-keV X-ray photons. This can be achieved with Bonse-Hart channel-cut silicon monochromator crystals. 

The authors have demonstrated that it is possible, by the coincidence technique, to reduce the contribution of noise and background from a PIPS detector by a factor of ten and to improve its energy resolution. Such a detector system could be a useful tool for quality control of low-level, low-energy, pure beta emitters such as Ni from environmental samples. 

We see that for EELS analysis, the resolution is degraded only linearly with foil thickness. Comparison with the values in Table 3.1 shows that if the collector aperture is about 10 mrad, and the energy losses are 200 eV or more, nanometer resolution can be maintained for foils up to about 100 nm thick. This thickness coincides approximately with the mean free path for inelastic scattering, so that spectra from thicker foils suffer degradation of energy resolution anyway. 

For certain measurements, like coincidence-anticoincidence counting or experiments involving accelerators, the time resolution of the signal is also important, in addition to energy resolution. For timing purposes, it is essential to have pulses with constant risetime. 

From 3 to 12 Mev, measurements of the total cross sections in a large number of elements have been made by Nereson and Darden using the neutrons emitted by a fast neutron reactor as a source, and a special energy-selective detector. In that instrument, protons projected in the forward direction from a polythene foil were measured by an ionisation chamber connected in coincidence with a proportional counter located between it and the foil, and in anticoincidence with another counter beyond it. The energy of the protons was measured by the size of the pulses they produced in the chamber the energy resolution was about 10%. 

Time-resolved ionization offers several advantages as a probe of these wavepackets [41, 42, 343, 360]. For example, the ground state of an ion is often better characterized than higher excited states of the neutral molecule, particularly for polyatomics. Ionization is also universal and hence there are no dark states. Furthermore, ionization provides both ions and photoelectrons and, while ion detection provides mass and kinetic-energy resolution in time-resolved studies [508], photoelectron spectra can provide complementary information on the evolution of the wavepacket [22, 63, 78, 132, 201, 270, 271, 362, 363, 377]. Its utihty for real-time probing of molecular dynamics in the femtosecond regime has been nicely demonstrated in studies of wavepackets on excited states of Na2 [22], on the B state of I2 [132], and on the A state of Nal [201]. Femtosecond photoelectron-photoion coincidence imaging studies of photodissociation dynamics have been reported [107]. 

Figure 14. Left Panel The energy resolution at 662 keV is shown a set of crystals of different volumes (from ref [6]). Right Panel Coincidence Resolving Time (CRT) for LaBrs- Ce of different volumes (from ref [3])...

The positron lifetime experiments were carried out with a fast-slow coincidence ORTEC system with a time resolution of about 230 ps full width at half maximum. A 5mCi source of Na was sandwiched between two identical samples, and the total count was one million. The temperature-dependent Doppler broadening energy spectroscopic (DBES) spectra were measured using an HP Ge detector at a counting rate of approximately 800 cps. The energy resolution of the solid-state detector was 1.5 keV at 0.511 MeV (corresponding to positron 2y annihilation peak). The total... 

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