Sample Chamber and Detector

Sample Chamber and Detector. The pressure in the sample chamber is typically 10-6torr, although UHY may be required for some experiments. The samples are usually mounted on a five-axis goniometer, so that a series of samples may be loaded and analysed sequentially. The goniometer can tilt and rotate the samples relative to the direction of the incident beam. Comparing spectra obtained at different incident and exit beam angles provides fuller characterization of the sample composition as a function of depth. The samples can be electrical insulators 

The signals from the detectors are amplified to create a voltage pulse with amplitude proportional to the energy of the charged particle. Data acquisition, storage and display is effected by an MCA providing pulse-height analysis. 

---

In order to reduce air absorption in SAXS and USAXS setups, a vacuum tube (cf. Fig. 4.10) is mounted between sample chamber and detector. 

Fig. 2 Top Schematic lay-out of a pin hole small angle neutron diffractometer (SANS). Bottom Photography of one of the two SANS instruments at the research reactor FRJ-2 at Jiilich. Vacuum sample chamber and detector tank are visible. Both Jiihch SANS instruments have been in operation since 1987 and have up until now been seen worldwide to the most powerful instruments after the two SANS instruments at the ILL in Grenohle (France)...

Figure Bl.24.1. Schematic diagram of the target chamber and detectors used in ion beam analysis. The backscattering detector is mounted close to the incident beam and the forward scattering detector is mounted so that, when the target is tilted, hydrogen recoils can be detected at angles of about 30° from the beam direction. The x-ray detector faces the sample and receives x-rays emitted from the sample.

A Fourier transform infrared spectroscopy spectrometer consists of an infrared source, an interference modulator (usually a scanning Michelson interferometer), a sample chamber and an infrared detector. Interference signals measured at the detector are usually amplified and then digitized. A digital computer initially records and then processes the interferogram and also allows the spectral data that results to be manipulated. Permanent records of spectral data are created using a plotter or other peripheral device. 

PIXE is a technique that uses a MeV proton beam to induce inner-shell electrons to be ejected from atoms in the sample. As outer-shell electrons fill the vacancies, characteristic X-rays are emitted and can be used to determine the elemental composition of a sample. Only elements heavier than fluorine can be detected due to absorption of lower-energy X-rays in the window between the sample chamber and the X-ray detector. An advantage of PIXE over electron beam techniques is that there is less charging of the sample from the incoming beam and less emission of secondary and auger electrons from the sample. Another is the speed of analysis and the fact that samples can be analyzed without special preparation. A disadvantage for cosmochemistry is that the technique is not as well quantified as electron beam techniques. PIXE has not been widely used in cosmochemistry. 

The spectrophotometer is used to measure absorbance experimentally. This instrument produces light of a preselected wavelength, directs it through the sample (usually dissolved in a solvent and placed in a cuvette), and measures the intensity of light transmitted by the sample. The major components are shown in Figure 5.5. These consist of a light source, a monochromator (including various filters, slits, and mirrors), a sample chamber, a detector, and a meter or recorder. All of these components are usually under the control of a computer. 

IR radiation is emitted from the electrically modulated light source. The analytically relevant spectral range is transmitted through an interference filter, the sample chamber, and the membrane. This radiation is focused on a thermal detector (Dl), pyroelectrical or thermopile. The reflected radiation from the filter is used as a reference (D2). A comparison of the ATR-, the fiber-, and the transmission-method. Secs. 6.5.2.1, 6.5.4.2, and 6.5.4.4, shows that the ATR method is most versatile for all applications and that the transmission method allows the lowest limit of detection for gases (Hadziladzaru, 1994). The properties of the ATR method by employing wavelength selection with tunable interference filters has been studied by Lebioda (1994). 

Most modern, commercially available mid-IR instrnments are bnilt aronnd an interferometer and are different in this respect from their predecessor, the diffraction IR spectrometer. A modern instrnment consists of a sonrce of IR radiation, an interferometer, a sample chamber, and a detector. Brief comments on varions spectrometer components are provided in the following text. Details of topics related to instrnmentation can be obtained from the literature [66]. 

Old design spectrophotometers work similar with those for UV-Vis domain, i.e. are composed of radiation somce, monochromator designed to select a desired wavelength radiation, the sample chamber and the radiation detector. In IR domain, diffuse radiation presents more serious problems then in ultraviolet and visible domain. Thus, in IR domain. 

A general schematic of a fluorescent spectrometer is shown in Fig. 4.10. The instrument contains the source of UV/VIS radiation, an excitation wavelength selector, an emission wavelength selector, a sample chamber and a detector. Basically this is a single beam instrument. The fluorescence emitted by the sample is usually measured at 90° in order to avoid disturbances by non-absorbed excitation radiation. 

Figure 20 Schematic of X-ray diffraction experimental setup. S is the source of X-rays, M the monochromator, C the sample chamber, and D the detector. 0 is the angle of diffraction.

The HADES is consisted of several sections as shown in Figure 7.3.17 gas handling system, a sample chamber and several process measurements. The nitrogen gas flows into the sample chamber as shown in F igure 7.3.18. The temperature and rate of gas flow are controlled and the gas temperatures before and after the coating sample tray are measured with thermocouples. The coating temperature is measured with thermocouple which is installed at the coating sample tray, and the solvent laden gas flows into the total hydrocarbon analyzer which is equipped with flame ionization detector (FID). The concentration of solvent at the exhaust gas is measured by total hydrocarbon analyzer, which is calibrated with known concentration of solvent vapor (via solvent bubblers). 

Ionization and condensation nuclei detectors alarm at the presence of invisible combustion products. Most industrial ionization smoke detectors are of the dual chamber type. One chamber is a sample chamber and the other chamber is a reference chamber. Combustion products enter an outer chamber of an ionization detector and disturb the balance between the ionization chambers and trip a highly sensitive cold cathode tube that causes the alarm. The ionization air in the chambers is caused by a radioactive source. Smoke particles impede the ionization process and trigger the alarm. Condensation nuclei detectors operate on the cloud chamber principle, which allows invisible particles to be detected by an optical technique. They are more effective on Class A fires (ordinary combustibles) and Class C fires (electrical). 

---