Organic Scintillators Used as Fast-Neutron Spectrometers

4 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- 

As the size of an organic scintillator increases, the efficiency increases, the energy resolution deteriorates, and the background increases. The optimum size for MeV neutrons seems to be a scintillator with a volume 10 m (100 cm ), i.e., a cylinder 50 mm in diameter and 50 mm tall. The efficiency of the NE 213 scintillator has been determined by Verbinski et al. using a combination of measurements and Monte Carlo calculations for 20 neutron energies between 0.2 and 22 MeV. 

The response of an organic scintillator to monoenergetic neutrons depends on effects similar to those discussed in the previous section for proportional counters, with the exception of electric field distortions. The most important cause of a response different from the ideal rectangular distribution shown in Fig. 14.9 is the nonlinear relation between the energy of the proton and the amount of light produced by the scintillation process. For organic scintillators. 

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