Solid detectors

In the case of X radiation indnced liuninescence (referred to as radioluminescence), the process is a little more complicated. The X-ray irradiation induces the production of electrons with a high energy, which bombard the neighboring atoms, causing electronic transitions and the emission of visible photons by excitation. This physical phenomenon has been known for a long time and is used in several types of X-ray detectors. 

With this type of detector, fluorescence is observed directly. Barium platinocianide, willemite and zinc sulfur are examples of rrtaterials rrsed. It is possible to produce screerts coated with these products. Zinc srrlfur is currently the most used. 

In X-ray diSractiorr, such screens are not used to detect difftacted beams, since their intensity is much too srrtall to yield visible spots. These screens are used to visualize the primary beam while geometric adjustments are made to diffraction apparatus. 

A new application of the fluorescence effect in the visible spectrum has appeared in the past few years. It consists of recording on a digital camera the image created by the light photons emitted by a fluorescent screen. These are CCD (charge coupled device) cameras. The principle of such detectors will not be described in detail in this book. 

These eameras have been known for some time, but their use in X-ray diffraction is reeent [STA 92], They are used to create plane detectors, the main problem being the uniformity in space of the intensity measurement. 

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The tracks of alpha particles and of electrons ejected by X rays were first observed in Wilson chambers. Later the more advanced bubble and spark chambers were designed. Another type of detector, which is widely used for recording particle tracks, is one that fixes the changes in the structure of a medium when treated by certain chemical reagents. These are the photoemulsions and the different types of solid detectors.6... 

Measurements made by using those solid detectors are conducted in three distinct phases recording, reading and erasing. This separation in different... 

Liquid scintillation detectors operate on the principle of interaction of radiations with a special type of scintillating liquid that emits light upon interaction with radiation. The light is then processed in the same manner as in the case of a solid detector, as discussed below. 

Fig. 1 shows the block diagram of the vibrometer, in which the most sensible to small phase variations interferometric scheme is employed. It consists of the microwave and the display units. The display unit consists of the power supply 1, controller 2 of the phase modulator 3, microprocessor unit 9 and low-frequency amplifier 10. The microwave unit contains the electromechanical phase modulator 3, a solid-state microwave oscillator 4, an attenuator 5, a bidirectional coupler 6, a horn antenna 7 and a microwave detector 11. The horn antenna is used for transmitting the microwave and receiving the reflected signal, which is mixed with the reference signal in the bidirectional coupler. In the reference channel the electromechanical phase modulator is used to provide automatic calibration of the instrument. To adjust the antenna beam to the object under test, the microwave unit is placed on the platform which can be shifted in vertical and horizontal planes. 

Figure Al.7.11. Schematic diagram of a generic surface science experiment. Particles, such as photons, electrons, or ions, are mcident onto a solid surface, while the particles emitted from the surface are collected and measured by the detector.

Let the rate of the event under study be R. It will be proportional to the cross section for the process under study, a, the incident electron current, Iq, the target density, n, the length of the target viewed by the detectors,, the solid angles subtended by the detectors, Aoi and A012 the efficiency of the detectors, and... 

In TOF-SARS [9], a low-keV, monoenergetic, mass-selected, pulsed noble gas ion beam is focused onto a sample surface. The velocity distributions of scattered and recoiled particles are measured by standard TOF methods. A chaimel electron multiplier is used to detect fast (>800 eV) neutrals and ions. This type of detector has a small acceptance solid angle. A fixed angle is used between the pulsed ion beam and detector directions with respect to the sample as shown in figure Bl.23.4. The sample has to be rotated to measure ion scattering... 

Figure Bl.24.3. Layout of a scattering experiment. Only primary particles that are scattered within the solid angle O spaimed by the solid state detector are counted.

Ernest O. Lawrence, inventor of the cyclotron) This member of the 5f transition elements (actinide series) was discovered in March 1961 by A. Ghiorso, T. Sikkeland, A.E. Larsh, and R.M. Latimer. A 3-Mg californium target, consisting of a mixture of isotopes of mass number 249, 250, 251, and 252, was bombarded with either lOB or IIB. The electrically charged transmutation nuclei recoiled with an atmosphere of helium and were collected on a thin copper conveyor tape which was then moved to place collected atoms in front of a series of solid-state detectors. The isotope of element 103 produced in this way decayed by emitting an 8.6 MeV alpha particle with a half-life of 8 s. 

Description of Method. Fluoxetine, whose structure is shown in Figure 12.31a, is another name for the antidepressant drug Prozac. The determination of fluoxetine and its metabolite norfluoxetine. Figure 12.31 b, in serum is an important part of monitoring its therapeutic use. The analysis is complicated by the complex matrix of serum samples. A solid-phase extraction followed by an HPLC analysis using a fluorescence detector provides the necessary selectivity and detection limits. 

Caffeine in coffee, tea, and soda is determined by a solid-phase microextraction using an uncoated silica fiber, followed by a GC analysis using a capillary SPB-5 column with an MS detector. Standard solutions are spiked with G3 caffeine as an internal standard. 

Figure 8.28 shows how the X-rays fall on the solid or liquid sample which then emits X-ray fluorescence in the region 0.2-20 A. The fluorescence is dispersed by a flat crystal, often of lithium fluoride, which acts as a diffraction grating (rather like the quartz crystal in the X-ray monochromator in Figure 8.3). The fluorescence may be detected by a scintillation counter, a semiconductor detector or a gas flow proportional detector in which the X-rays ionize a gas such as argon and the resulting ions are counted.

Lead sulfide is used in photoconductive cells, infrared detectors, transistors, humidity sensors in rockets, catalysts for removing mercaptans from petroleum distillates, mirror coatings to limit reflectivity, high temperature solid-film lubricants, and in blue lead pigments (82). 

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