Organic Scintillator

To obtain the gamma spectrum from the source, the measured NE 213 spectrum must be unfolded. Results of matrix-inversion unfolding applied to the measured spectrum of Na are shown in Figs. 12.20 and 12.21. The spectrum shown in Fig. 12.21 was obtained by unfolding the spectrum of Fig. 12.20 with the code FORIST, which is a variation of the code FERDOR. Both codes use 

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Organic Scintillators and Liquid Scintillation Counting , Academic Press, NY (1971)... 

Organic scintillation phosphors include naphthalene, stilbene, and anthracene. The decay time of this type of phosphor is approximately 10 nanoseconds. This type of crystal is frequently used in the detection of beta particles. 

Ross HJE, Noakes E, Spalding JD. Liquid Scintillation Counting and Organic Scintillators, Lewis, Chelsea, MI, 1991. 

Hirayama, F. Lipsky, S. Saturated Hydrocarbons as Donors in Electronic Energy Transfer Processes New York. In Organic Scintillators and Liquid Scintillation Counting Horrocks, D.L., Peng, C.-T., Eds. Academic Press, 1971 205 pp. 

There are three common types of organic scintillator. The first type is a pure crystalline material, such as anthracene. The second type, the liquid scintillator, is the solution of an organic scintillator in an organic liquid, such as a solution of p-terphenyl in toluene ( 3 g solute/L solution). The third type is the solution of an organic scintillator, such as p-terphenyl, in a solid plastic, such as polystyrene. 

The scintillation process in inorganic scintillators differs from that in organic scintillators. Consider the structure of an ionic crystal, as shown in Figure 18.19. When an energetic electron passes through the crystal, it may raise valence electrons from the valence band to the conduction band. The electron vacancy in the valence... 

Product purity became a passion at Arapahoe Chemicals, as well as a formidable marketing tool. It became an unwritten trademark in all of Arapahoe s marketed products, including DDQ, organic scintillators, numerous pharmaceutical intermediates, and metallocenes. 

Horrocks, D. L., and Peng, C. (1971). Organic Scintillation and Liquid Scintillation Counting. New York Academic Press. 

D. S. Tinti and M. A. El-Sayed, Organic Scintillators and Liquid Scintillation Counting, Academic Press, New York (1971), p. 563. 

Place 10,000 to 15,000 cpm H-toluene into each of six scintillation vials containing 10 ml organic scintillation fluid. 

E. Schram, R. Lombaert, Organic Scintillation Detectors, Elsevier, Amsterdam, 1963... 

Ross H, Noakes JE, Spaulding JD. Liquid scintillation counting and organic scintillators. New York Lewis Publishers, Inc., 1991 752 pp. 

Details of the neutralization process following radiation-induced primary charge separation may be examined via the medium of ultrafast techniques now employed in studies of luminescence decay processes. As an example, the form of luminescence decay curves of dilute organic scintillator in aliphatic hydrocarbon solution excited by x-ray pulses of about 0.5-1.0 nsec, duration is attributed (in previous papers) to neutralization processes involving ions. The relation, t cc r3, for the time required for neutralization of an ion pair of initial separation r, when applied to such curves, leads to a distribution function of ion-pair separations. A more appropriate and desirable approach involves solution of a diffusion equation (which includes a Coulomb interaction term) for various initial conditions. Such solutions are obtained by computer techniques employed in analogy to corresponding electrical networks. The results indicate that the tocr3 law affords a fair description of the decay if the initial distribution can be assumed to be broad. 

The materials that are efficient organic scintillators belong to the class of aromatic compounds. They consist of planar molecules made up of benzenoid rings. Two examples are toluene and anthracene, having the structures shown in Fig. 6.6. 

Organic scintillators are formed by combining appropriate compounds. They are classified as unitary, binary, ternary, and so on, depending on the number of compounds in the mixture. The substance with the highest concentration is called the solvent. The others are called solutes. A binary scintillator consists of a solvent and a solute, while a ternary scintillator is made of a solvent, a primary solute, and a secondary solute. Table 6.2 lists the most common compounds used. 

One of the important differences between inorganic and organic scintillators is in the response time, which is less than 10 ns for the latter (response time of inorganic scintillators is 1 fis see Table 6.1) and makes them suitable for fast timing measurements (see Chap. 10). Table 6.3 lists important properties of some organic scintillators. 

Anthracene has a density of 1.25 X 10 kg/m and the highest light conversion efficiency of all organic scintillators (see Table 6.3)—which is still only about one-third of the light conversion efficiency of Nal(Tl). Its decay time ( 30 ns) is much shorter than that of inorganic crystals. Anthracene can be obtained in different shapes and sizes. 

The plastic scintillators may be considered as solid solutions of organic scintillators. They have properties similar to those of liquid organic scintillators (Table 6.3), but they have the added advantage, compared to liquids, that they do not need a container. Plastic scintillators can be machined into almost any desirable shape and size, ranging from thin fibers to thin sheets. They are inert to water, air, and many chemicals, and for this reason they can be used in direct contact with the radioactive sample. 

Photons and neutrons. Organic scintillators are not normally used for detection of gammas because of their low efficiency. The liquid scintillators NE 213 is being used for y detection in mixed neutron-gamma fields because of its... 

Figure 9.13 Four examples of response functions (a) 5-MeV Alpha particles detected by a silicon surface barrier detector (Chap. 13), or 20-keV X-rays detected by a Si(Li) reactor (Chap. 12). ib) 1-MeV Gamma ray detected by a NaI(Tl) crystal (Chap. 12). (c) 1-MeV Electrons detected by a plastic scintillator (Chap. 13). (

Pulse-shape discrimination (PSD) is the name given to a process that differentiates pulses produced by different types of particles in the same detector. Although PSD has found many applications, its most common use is to discriminate between pulses generated by neutrons and gammas in organic scintillators (see also Chap. 14), and it is this type of PSD that will be discussed. 

Organic Scintillators Used as Fast-Neutron Spectrometers... 

Stilbene scintillators were used as early as 1957. Stilbene as a crystal is very sensitive to mechanical and thermal shock and shows an anisotropic response to neutrons—i.e., neutrons incident from different directions, with respect to the crystal lattice, produce different light output. Liquid organic scintillators have none of these problems in addition, they have higher H/C ratio, and light production from carbon recoils relatively lower than in stilbene. For all these reasons, liquid organic scintillators are almost exclusively used for detecting fast neutrons. 

The NE series of organic scintillators has been studied in detail and used extensively, in particular NE 213. The NE 213 scintillator, which is most commonly used, consists of xylene, activators, the organic compound POPOP (as a wavelength shifter), and naphthalene, which is added to improve light emission. The density of NE 213 is about 870 kg/m (0.87 g/cm ), and its composition is taken to be CHj 2i-... 

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