Detectors photographic film

Soft x-rays with wavelengths of 1—10 nm ate used for scanning x-ray microscopy. A zone plate is used to focus the x-ray beam to a diameter of a few tens of nanometers. This parameter fixes and limits the resolution. Holographic x-ray microscopy also utilizes soft x-rays with photoresist as detector. With a strong source of x-rays, eg, synchrotron, resolution is in the 5—20-nm range. Shadow projection x-ray microscopy is a commercially estabflshed method. The sample, a thin film or thin section, is placed very close to a point source of x-rays. The "shadow" is projected onto a detector, usually photographic film. The spot size is usually about 1 ]lni in diameter, hence the resolution cannot be better than that. 

XRD is an excellenr, nondestructive method for identifying phases and characterizing the structural properties of thin films and multilayers. It is inexpensive and easy to implement. The future will see more use of GIXD and depth dependent measurements, since these provide important information and can be carried out on lab-based equipment (rather than requiring synchrotron radiation). Position sensitive detectors will continue to replace counters and photographic film. 

Four other types of radiation detectors are the self-powered neutron detector, wide range fission chamber, flux wire, and photographic film. 

A description of how self-powered neutron detectors, wide range fission chambers, flux wires, and photographic film detect radiation is summarized below. 

The first laser Raman spectra were inherently time-resolved (although no dynamical processes were actually studied) by virtue of the pulsed excitation source (ruby laser) and the simultaneous detection of all Raman frequencies by photographic spectroscopy. The advent of the scanning double monochromator, while a great advance for c.w. spectroscopy, spelled the temporary end of time resolution in Raman spectroscopy. The time-resolved techniques began to be revitalized in 1968 when Bridoux and Delhaye (16) adapted television detectors (analogous to, but faster, more convenient, and more sensitive than, photographic film) to Raman spectroscopy. The advent of the resonance Raman effect provided the sensitivity required to detect the Raman spectra of intrinsically dilute, short-lived chemical species. The development of time-resolved resonance Raman (TR ) techniques (17) in our laboratories and by others (18) has led to the routine TR observation of nanosecond-lived transients (19) and isolated observations of picosecond-timescale events by TR (20-22). A specific example of a TR study will be discussed in a later section. 

Other detection modes in bright CL or BL reactions are multichannel detectors, which provide simultaneous detection of the dispersed radiation and produce a permanent image of a wide area. Photographic films or plates are emulsions that contain silver halide crystals in which incident photons produce stable clusters of silver atoms within the crystals. Internal amplification is provided in the development process by an electron donor that reduces the remaining silver ions to silver atoms within the exposed crystals. A complexing agent is used to remove the... 

It has been found that only a few photons, maybe as little as six, are needed to form the latent image. Photographic film is a very sensitive light detector. The final step in the photographic process, fixing, removes the unreacted silver bromide crystals from the emulsion, thus stabilizing the image (Fig. 2.4). 

X-ray detectors also come in several varieties (1) single-photon counters which yield accurate results but require up to several weeks to acquire the 10,000 -100,000 (lO lO ) reflections necessary to compile a complete data set for a protein crystal (2) image plates that operate much like photographic film but are 10 times more sensitive (3) area detectors, electronic devices that detect X-ray photons on a two-dimensional surface. Both fluorescent-type detectors, image plates and fast area detectors, are more sensitive at the shorter wavelengths of X-ray radiation from synchrotron sources. 

If a single crystal is rotated in a monochromatic X-ray beam, a pattern of spots of reinforced X-rays can be recorded, traditionally on a photographic film placed behind the crystal perpendicular to the primary beam (giving the so-called Laue photographs). Nowadays, X-ray diffractometers use electronic photon counters as detectors. Since, as noted above, different atoms have different X-ray scattering powers, both the positions and... 

Finally, the introduction of new detectors, such as diode arrays and charge-coupled devices (CCDs), has been a boon for Raman spectroscopy. CCDs permit the accumulation of light in the manner of photographic film additionally, their noise level is lower than that of the photomultiplier tube. In addition, by combining CCDs or diode arrays with optical dispersive elements, entire spectra mav be collected in fractions of a second. 

Figure 2.6 A modern CCD diffractometer. Note the circular area detector on the left, which acts as a very sensitive electronic, reusable equivalent of photographic film, allowing many data points to be collected simultaneously. (Photograph courtesy of Nonius.).

Spark chambers (or "streamer chambers") are a set of 5 to 10 metal plates thin enough for high-energy particles to cross them, in a chamber filled with He or Ne when particles cross the plates, a trigger signal sets up a strong electric field, and sparks fly from plate to plate two photographic films mounted perpendicular to the plates and to each other record the spark tracks across the plates. The spark chamber has been superseded by drift chambers and silicon detectors. 

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