Scintillator mechanisms

Figure 18.18 Schematic view of the scintillation mechanism in organic crystals. (From Wang et al., 1975.)...

Since the discovery of luminescence in plastics, the investigation of the action of plastic scintillators has developed rapidly during the last few years. Scintillators based on plastics can be conceived as solid solutions of organic, luminescent substances which are embedded with low concentrations in transparent plastics capable of luminescence. The scintillation mechanism of the system polystyrene— tetraphenylbutadiene was investigated. 

Mat Res Soc Symp Proc, Materials Research Society 348 123-129 Wojtowicz AJ, Wisniewski D, Lempicki A, Boatner LA (1995) Scintillation mechanisms in rare earth orthophosphates. In Radiation Effects and Defects in Solids, Vol. 135. Biersack (ed) Overseas Publishers Assoc., Amsterdam B.V., p. 305-310... 

In our opinion the paper by Moses et al. [36] on the scintillation mechanisms in CeF.3 is a fine example of how scintillators should be studied from a fundamental point of view. A combination of techniques was u.sed, viz. (time-resolved) luminescence spectroscopy, ultraviolet photoelectron spectroscopy, transmission spectroscopy, and the excitation region was extended up to tens of eV by using synchrotron radiation. Further, powders as well as crystals with composition Lai-xCexFy were investigated,... 

These two methods produce different release profiles in vitro. Figure 5 demonstrates the release kinetics of BCNU from wafers loaded with 2.5% BCNU pressed from materials produced using these two methods. The wafers containing tritiated BCNU were placed into beakers containing 200-ml aliquots of 0.1 M phosphate buffer, pH 7.4, which were placed in a shaking water bath maintained at 37 C. The shaking rate was 20 cycles/min to avoid mechanical disruption of the wafers. The supernatant fluid was sampled periodically, and the BCNU released was determined by liquid scintillation spectrometry. The BCNU was completely released from the wafers prepared by the trituration method within the first 72 hr, whereas it took just about twice as long for the BCNU to be released from wafers... 

Recently, a biexcitonic quenching mechanism has been proposed to explain the variation of scintillation intensity in liquid argon (LAr) with the LET and quality of incident radiation (Hitachi et al., 1992). According to this, quenching occurs mainly in the track core due to high-energy deposition density. This... 

Simms et al. [119] employed an integrated approach to determine the mechanism and kinetics of surfactant biodegradation by FAB-MS and liquid scintillation counting. Two compounds, the esterquat A -octadecyl-A - [palmytoyloxyethyl]-A V-dimethyl ammonium chloride and AM2-hydroxyethyl)-AyV-dimethyloctadecyl ammonium chloride,... 

In a conventional logging apparatus, the scintillation material is comprised of a specially formulated organic polymer or plastic. Many conventional plastic scintillators do not exhibit acceptable mechanical and optical properties when used at relatively high temperature of 75-175°C encountered in a borehole. 

The internal standard ratio method for quench correction is tedious and time-consuming and it destroys the sample, so it is not an ideal method. Scintillation counters are equipped with a standard radiation source inside the instrument but outside the scintillation solution. The radiation source, usually a gamma emitter, is mechanically moved into a position next to the vial containing the sample, and the combined system of standard and sample is counted. Gamma rays from the standard excite solvent molecules in the sample, and the scintillation process occurs as previously described. However, the instrument is adjusted to register only scintillations due to y particle collisions with solvent molecules. This method for quench correction, called the external standard method, is fast and precise. 

Liquid Scintillators This detection mechanism is quite similar in principle to the preceding one. Here, however, the radioactive sample and the fluor are the solute in a liquid medium, usually a nonpolar solvent. The energy of nuclear radiation first excites the solvent molecules. This excitation energy eventually appears as photons emitted from the fluor following an intermediate transfer stage. The photons are detected by means of a photomultiplier arrangement. 

These were attached to a fraction collector, whose timing mechanism was altered to allow continuous sampling of 28 separate diffusion cells with increased sample and timing capacity. Radiolabeled drugs were used and were measured in a liquid scintillation counter directly interfaced to a computer network. We describe the details of this system which enable us to conduct reproducible flow-through diffusion experiments, assay the samples, and analyze the data quickly and efficiently. 

Continuous automatic sampling is carried out from these cells into ordinary liquid scintillation vials by an ISCO brand (Lincoln, N.E.) Retriever III fraction collector whose timing mechanism has been altered so that nine timepoints can be sampled in intervals of up to 99.0 hours for 14 diffusion cells. 

Fig. 12. Experimental setup for observation A. (a) - channel scheme p - internal proton beam, Target - target mechanism, Col - collimator, MS - magnetic shield (b) - magnet and detectors M - poles of spectrometer magnet, VC — vacuum chamber, DC - drift chambers, H - scintillation hodoscopes, S,SM - scintillation counters, C -gas Cherenkov counters, Absorber - cast-iron absorber, MC - monitor counters...

Owing to the presence of organic solvent, the mechanism of luminescence for the liquid scintillator is much more complicated than that for a crystal scintillator. The energy successively transfers to generate finally the luminescence, as follows ... 

Electrodes and Instrumentation. Electrodes were mechanically polished with one micron diamond paste (Buehler) until satisfactory background voltammograms were obtained. A disposable, 20 ml scintillation vial served as a convenient, one-compartment electrochemical cell. 

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