Chopper spectrometer

Direct geometry instruments use choppers or crystal monochromators to fix the incident energy and they are found on both continuous and pulsed sources. To compensate for the low incident flux resulting from the monochromation process, direct geometry instruments have a large detector area. This makes the instruments expensive, they are generally twice the price of a crystal analyser instrument. At present, they are used infrequently for the study of hydrogenous materials, so we will limit our discussion to a chopper spectrometer at a pulsed source and a crystal monochromator at a continuous source. 

M. Arai, M. Kohgi, M. Itoh, H. Iwasa, N. Watanabe, S. Ikeda Y. Endoh (1989). ICANS X Proceedings of the Tenth International Collaboration On Advanced Neutron Sources Held at Los Alamos 3-7 October 1988, Institute of Physics Conference Series, Institute of Physics, Bristol, 97, 297-308. Development of a chopper spectrometer at KENS... 

S. Itoh, M. Aral M. Kawai (2002). Appl. Phys. A-Mater, 74, Part 1 Suppl. S, SI 98—S200. Improvement of the performance of the chopper spectrometer, INC. 

The full spectral form is seen in the data obtained on a chopper spectrometer, see Figs. 9.18 and 9.19 [68]. The continuum is a very broad response that tracks the unit-mass recoil line and is by far the strongest spectral component. Especially since the unbound scattering cross section of hydrogen, 20 bam ( 2.1), should be used in calculations of this effect. Analysis of this spectmm has proved very difficult because the width of the response. Fig. 9.19, far exceeds conventional predictions. Since neutrons cannot determine the electrical nature of the scatterer directly H" ", H , or H are all possibly present. 

Characteristic parameters of pulsed neutron source time-of-flight chopper spectrometers. 

Three high-resolution inelastic instruments are nearing completion at the NIST Center for Neutron Research (NCNR) (1) a disk chopper time-of-fiight spectrometer (2), a backscattering spectrometer (3), and a neutron spin echo (NSE) spectrometer. All three instruments will be available to researchers through reviewed proposals (4). Both the NSE and Backscattering spectrometers will be operation in early 1999 and the Disk Chopper Spectrometer shortly thereafter. 

Figure 4 Layout of the cold-neutron multi-chopper spectrometer INS at the ILL. Reproduced with permission of Adenine Press from Ferrand M (1997) Neutron instrumentation in studying dynamics of biomoiecuies. in Cusack S, Btittner H, Ferrand M, Langan P and Timmins P (eds). Biological Macromolecular Dynamics. Schenectady Adenine Press.

The preceding theory is a special case of a general quantum-mechanical treatment of neutron cross sections. With high neutron-flux sources (reactors), and the development of velocity selectors (choppers, spectrometers) which permit measurements at chosen velocities, the cross-section values obtained are more accurate, but the methods much more complex. The above theory shows how the first cross-section measurements were made. The presentation of this simple method is further justified by the fact that many cross sections have been measured as averages in the same way, and for some materials this simple method is still very useful. 

Block diagram for molecular phosphorescence spectrometer with inset showing how choppers are used to isolate excitation and emission. 

The design of a conventional atomic absorption spectrometer is relatively simple (Fig. 3.1), consisting of a lamp, a beam chopper, a burner, a grating monochromator, and a photomultiplier detector. The design of each of these is briefly considered. The figure shows both single and double beam operation, as explained below. 

Figure 3.1 Schematic diagram of an AAS spectrometer. A is the light source (hollow cathode lamp), B is the beam chopper (see Fig. 3.2), C is the burner, D the monochromator, E the photomultiplier detector, and F the computer for data analysis. In the single beam instrument, the beam from the lamp is modulated by the beam chopper (to reduce noise) and passes directly through the flame (solid light path). In a double beam instrument the beam chopper is angled and the rear surface reflective, so that part of the beam is passed along the reference beam path (dashed line), and is then recombined with the sample beam by a half-silvered mirror.

As with all other types of spectrometers operating in the UV/visible region of the spectrum, it is advantageous to modulate the primary beam using a mechanical beam chopper, and detect it at the same frequency, to reduce background noise. This is usually done with a rotating beam chopper, shaped... 

Fig. 2. Schematic diagram of a high resolution He time-of-flight spectrometer. N-nozzle beam source, SI, 2-skimmers, Al-5 - apertures, T - sample, G - gas doser, CMA - Auger Spectrometer, IG - ion gun, L - LEED, C -magnetically suspended pseudorandom chopper, QMA-detector, quadrupole mass analyzer with channeltron.

For operational purposes, fluorescence is understood to be short-lived and phosphorescence to be long-lived luminescence. The spectrometer described here uses only a single light chopper 9 and allows simultaneous recording of total luminescence and phosphorescence spectra (Fig. 25). 

In order to determine the dynamics of atoms we have to carry out an inelastic neutron scattering measurement. With a reactor source this can be done with a triple-axis spectrometer, which has an analyzer crystal. Tripleaxis refers to the three axes for the monochromator, sample, and analyzer, all moving independently and controlled by a computer. With a pulsed source we use a mechanical chopper, which is a rotating cylinder with a hole perpendicular to the rotating axis that allows neutrons with a chosen range of velocity to go through. The neutrons scattered by the sample are detected... 

Thus atoms with thermal energy of about 0.02 eV have X = 1 A and can readily diffract from surfaces. A beam of atoms is chopped with a variable frequency chopper before striking the surface. This way, an alternating intensity beam signal is generated at the mass spectrometer detector, that is readily separated from the noise due to helium atoms in the background. 

The operation of a commercial recording IR spectrometer (Fig. 6.5) is as follows. The source radiation is reflected from two mirrors to give two identical beams one beam passes through the sample cell, the other through an empty reference cell (if the sample is in solution, the reference cell is filled with the solvent). A chopper system then causes alternately the... 

Equipment. The spectra were recorded on a Beckman IR12 spectrometer in the absorbance mode, with low amplifier gain and slit widths smaller than 1.6 of the half-band width of the OH or OD bands. Under these conditions the apparent optical density of the OH bands could be reproduced within 0.5%. To avoid errors from sample emission at temperatures higher than 100°C, the spectra were scanned with the chopper between sample and detector disconnected. 

Smaller values of are obtained for interferometers operated in a double-beam mode, since the moveable mirror must be left stationary for a fraction of the cycle time to allow the detector to stabilize each time the beam is switched from the sample to the reference position. With an optical null grating spectrometer the chopper is used not only to modulate the beam but also to alternate the beam between sample and reference channels. Thus, it takes approximately the same time to measure a transmittance spectrum using a double beam optical null spectrometer as it takes to measure a single-beam spectrum with the same S/R. Hence, for this type of spectrometer may be assigned a value of 2. 

Some of the alternative TOF instrument designs involve replacing the beryllium filter with either a crystal or a mechanical chopper to monochromate the incident beam. With this change, the spectrometer can be used with a higher incident neutron energy (typically E 50 meV) so that a smaller momentum transfer Q is possible for 5 the same energy transfer (21,22). With a monochromatic incident beam, a beryllium filter is sometimes substituted for the chopper after the sample in order to increase the scattered intensity but with a sacrifice in the,minimum Q attainable. Energy transfers up to 100 meV (800 cm" ) can be achieved with TOF spectrometers at steady state reactors before the incident neutron flux is limited by the thermal spectrum of the reactor. (With hot moderators such as at the Institut Laue-... 

---