The Time-of-Flight Spectrometer

The time-of-flight (TOF) method, which is also used for the measurement of neutron energy (see Sec. 14.8), has been applied successfully for the determination of the mass of fission fragments and other heavy ions. 

The principle of TOF is simple. A beam of ions is directed along a flight path of length L (Fig. 13.20). The time t it takes the ions to travel the distance L determines their speed V = L/t. This information, combined with the measurement of the energy of the particle, gives the mass (nonrelativistically)  

The errors in determining the mass come from uncertainty in energy, AE, in time. At, and in length of the flight path, AL. The mass resolution is then given by 

Usually, the system is designed in such a way that AL/L is negligible compared to the other two terms of Eq. 13.19. Assuming that this is the case, consider the sources of uncertainty in energy and time. 

The uncertainty AE/E is the resolution of the detector measuring the energy of the ion. The best energy resolution that can be achieved with silicon surface-barrier detectors is about 1.5-2 percent. The resolution can be improved with magnetic or electrostatic analyzers (Dilorio and Wehring achieved 0.3 percent energy resolution using an electrostatic analyzer). 

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The ECAP incorporates an electrostatic lens in the time-of-flight spectrometer in order to improve the mass resolution by compensating for small spreads in the energies of the ions evaporated from the specimen under the pulsed electric field. A lens design by Poschenrieder or a reflectron type of electrostatic lens is used for this purpose, and is standard equipment for metallurgical or materials applications of APFIM. These typically improve the mass resolution at full width half maximum (FWHM) from m/Am 250 to better than 2000. 

The QENS results were obtained at the Institute Laue-Langevin, Grenoble, using the time-of-flight spectrometers INS and IN6 (25-27). The time scale on which the motions can be observed by QENS is determined by the elastic energy resolution, which amounted to 20 and 100 /xeV for the spectrometers INS and 1N6, respectively. Based on an appropriate statistical... 

The incoherent neutron quasi elastic experiments were performed with the time of flight spectrometer INS of the I.L.L.. The scattering angle used corresponded to q values ranging from 0.2 to 1.2 xhe corresponding energy resolution was 18 peV full... 

FIG. 4. Overview of the set-up used in Goteborg for production of clusters with the laser vaporization technique using the arrangement shown in Fig. 5. The clusters produced in section one is transferred through the skimmer to section two, where they are ionized and detected with the time-of-flight spectrometer. This section also contains two reaction cells for studies of the chemical reactivity or sticking probability of the clusters. 

In place of the diffractometer discussed in Section 2.5.3, a spectrometer is used, which allows measurement of the energy spectrum of scattered neutrons at different scattering angles. There are four main types of spectrometers in use today, the tripleaxis spectrometer, the time-of-flight spectrometer, the back-scattering spectrometer, and the spin-echo spectrometer, each of which is briefly described in the following section. 

In the time-of-flight spectrometer, shown schematically in Figure 8.12, the incident neutron beam is converted into pulses and, at the same time, monochromatized by... 

The two usual types of spectrometers are the three-axis spectrometer and the time-of-flight spectrometer. 

In modern static SIMS instruments, the most efficient spectrometer is the time of flight spectrometer. It has typical current densities of the order of 1 nA/cm, which corresponds to approximately lO particles/cm s [23, 24]. The emitted secondary ions are separated according to their mass In this spectrometer. Figure 6 shows schematically the principle of the time of flight measurement. 

The chopper is also used extensively for the me of the total cross sections at different neutron energies. In the time-of-flight spectrometer is superior to the crystal sp "because it can measure the cross sections of a sample at se energies (equal to the number of channels) at the same time. 

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