Direct geometry instruments

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. 

The low final energy has another consequence. In comparison with direct geometry instruments ( 3.4.3), the data from low final energy... 

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. 

Fig. 3.25 Scattering triangles for a direct geometry instrument, (a) Detectors at different angles give different Q at constant energy transfer and (b) an individual detector measures energy transfer at constant Q.

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 simple analysis suggests that the double inelastic event is detrimental to the spectra collected on all instruments, whereas the (elastic + inelastic) case is detrimental only for direct geometry instruments. This is true even for powder samples, since the magnitude of the momentum transfer is also lost. This contamination is most problematic for data obtained from the low scattering angle detectors. The data in these detectors are nominally obtained at low Q but multiple scattering injects high Q information into their data. 

Direct geometry instruments also have advantages in this area. For many elements, the coherent cross section is larger than the incoherent cross section and with ( -resolved data it is possible to see additional structure [29,81] beyond that predicted by the scattering law, Eq. 2.32. These arise from coherent inelastic scattering and offer further possibilities for investigating the dynamics in such systems. 

The MAPS spectrometer at ISIS [2], Fig. 12.1(a), is a third generation instrument that demonstrates the future direction of direct geometry instruments. The principal innovation is the use of large area, position sensitive He detectors ( 3.3.1.1). An area of 16 m. Fig. 12.1(b), of the sample environment tank is covered by 576 detectors that provide ahnost continuous coverage over a large solid angle in the forward scattering direction. 

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". ...

It is stressed that spectral contamination of transition intensities is a common feature of INS data at all energy transfers and on all types of instrument. It arises from the spectral congestion typical of molecules (even those as simple as benzene) and the effects of phonon wings. The effects of congestion can be exacerbated on direct geometry spectrometers if their energy resolution is not good. 

AE/Ei 3.22 relative incident energy uncertainty (instrumental resolution of direct geometry spectrometers) ... 

Surface forces measurement directly determines interaction forces between two surfaces as a function of the surface separation (D) using a simple spring balance. Instruments employed are a surface forces apparatus (SFA), developed by Israelachivili and Tabor [17], and a colloidal probe atomic force microscope introduced by Ducker et al. [18] (Fig. 1). The former utilizes crossed cylinder geometry, and the latter uses the sphere-plate geometry. For both geometries, the measured force (F) normalized by the mean radius (R) of cylinders or a sphere, F/R, is known to be proportional to the interaction energy, Gf, between flat plates (Derjaguin approximation). 

In addition to instrumental improvements, various approaches have been used to improve the purity or geometry of sources of natural samples for gamma spectrometric measurement. For example, improvements in source preparation for " Th measurement in water and sediment samples by gamma spectrometry are discussed in Cochran and Masque (2003). It should be emphasized that one of the main advantages of gamma spectrometry is ease of use, since in many cases samples may be analyzed directly or with significantly reduced sample preparation compared to alpha, beta, or mass spectrometric techniques. 

Local Structure of the Eu2+ Impurity. From the experimental perspective, the doping of lanthanide ions into solid state materials can be probed by different instrumental technics such as nuclear magnetic resonance (NMR),44 extended X-ray absorption fine structure (EXAFS),45,46 or electron paramagnetic resonance (EPR),47 which instead of giving a direct clue of the local geometry offers only data that can be corroborated to it. From the theoretical point of view,... 

We test the hardness of polymers by applying an indenter to their surface with a known force and noting the depth to which the tip penetrates the sample. These tests typically fall into one of two categories. In the first, the depth of penetration is read directly from a dial on the instrument, calibrated in arbitrary hardness units. The farther the tip penetrates the sample, the lower is its hardness. The second type of test involves impressing a pyramidal indenter tip against the sample with a known force and measuring the depth to which it penetrates. In practice we measure the dimensions of the indentation and calculate the depth of penetration and compressive modulus based on the tip geometry. 

In these sensors, the intrinsic absorption of the analyte is measured directly. No indicator chemistry is involved. Thus, it is more a kind of remote spectroscopy, except that the instrument comes to the sample (rather than the sample to the instrument or cuvette). Numerous geometries have been designed for plain fiber chemical sensors, all kinds of spectroscopies (from IR to mid-IR and visible to the UV from Raman to light scatter, and from fluorescence and phosphorescence intensity to the respective decay times) have been exploited, and more sophisticated methods including evanescent wave spectroscopy and surface plasmon resonance have been applied. 

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