Indirect geometry spectrometer

Indirect geometry spectrometers have no requirement (within the limitations implied by the use of S (Q,a>), 2.5.1) to calibrate detector efficiencies, on either continuous or pulsed sources (compare 3.4.3). Since the final energy of the neutrons never varies the detection efficiency is constant. Variations arising from differing discrimination levels ( 3.3.2) could play a significant role, except that (on low final energy instruments) all detectors follow almost the same path in Q,o ) space ( 3.4.2.3). Occasionally there is a need to calibrate the detected intensity in respect of the sample mass and standard analytical chemical techniques can be readily adapted to this circumstance. 

This is related to similar but subtly different effects observed on indirect geometry spectrometers ( 5.2.1.2). The INS spectrum of Rb2[PtH6], obtained on the low-bandpass spectrometer TOSCA, is given... 

The impact of phonon wings on the spectra observed on direct geometry spectrometers can be as taxing as their effects on indirect geometry spectrometers at the same Q values. The spectrum of Rb2[PtH6]... 

AE/E 3.14 relative energy transfer uncertainty (instrumental resolution of indirect geometry spectrometers) ... 

Figure 9 Principle of operation of the indirect-geometry spectrometer TOSCA at ISIS. Only two of the 10 analyser/detector modules are shown.

For INS spectroscopy there are three main types of spectrometer in use triple axis ( 3.4.1), which is rarely used to study hydrogenous materials more relevant are instruments that fix the final energy which are known as indirect geometry instruments and those that fix the incident energy which are known as direct geometry instruments. Examples of indirect geometry ( 3.4.2, filter and analyser) spectrometers and direct geometry ( 3.4.3, chopper) instruments are discussed in turn. 

Instruments that work with any fixed final energy are conventionally known as indirect geometry instruments. However, we shall limit our consideration to those spectrometers with low final energies. They are remarkably simple to design, relatively cheap to build, easy to operate and have an output, which is both similar to optical spectra and easily compared to calculation. They are ideally suited to exploitation by the chemical and biological scientific communities. 

Fig. 3.26 Trajectories in (Q,a>) space for a direct geometry spectrometer with detectors at angles 3, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120 and 135° and with an incident energy of 4000 cm. The dashed lines are the trajectories of an indirect geometry instrument (low-bandpass) using scattering angles of 45 (long dashes, forward scattering)) and 135° (short dashes, backscattering) and a final energy of 28 cm". ...

Clearly both types of instruments are highly complementary and both have strengths and weaknesses. Ideally, the same sample would be run first on an indirect geometry instrument which would provide a rapid, but still fairly detailed overview of the subject. In many instances this would be sufficient. Subsequent measurements on a direct geometry instrument would allow detailed aspects of the spectroscopy to be probed. Table 3.2 gives a list of INS (excluding triple axis) spectrometers that have recently been in operation, are in operation, or are planned. 

This is a much less serious problem for indirect geometry instruments with low final energies. These spectrometers work close to the maximum... 

The observed INS spectrum of NEUBr is given in Fig. 5.2. The spectrum was taken on the indirect geometry low-bandpass spectrometer TOSCA at ISIS ( 3.4.2.2.2). The spectrum consists of a series of features across the whole spectrum, which sits on a gently rising background. However, bands appear between 500 and 1400 cm, where none were expected and there are no bands about 3000 cm, where the stretches... 

The use of a ratios allows the indirect referencing of and " N chemical shifts through direct referencing to a single, well-determined H standard." The a ratio is independent of both the spectrometer design and the sample geometry and therefore provides an accurate and consistent way to reference and " N chemical shifts. 

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