Study of Organic Scintillators

Abstract

The scintillation counting behavior of a group of fifty-five pure crystalline organic compounds has been extensively studied. The data obtained have been analyzed with the goal of developing a better understanding of the scintillation process and of scintillator behavior. The materials were carefully purified, and massive crystals were grown. Relative scintillation average-pulse-height efficiencies at 30°C and —70°C for cobalt-60 gamma-ray excitation, gamma-ray excited scintillation decay times, and 2537 A ultraviolet-excited reflection and transmission photofluorescence spectra have been determined. A few solutions were also studied for comparison purposes. The purification and properties of the different materials are discussed in detail. The experimental data have been analyzed on the basis of Birks' photon cascade theory of the scintillation process. The ratio of the scintillation efficiency to the integrated photofluorescence intensity is shown to be a measure of Birks' primary photon production efficiency. The primary photon efficiencies vary over a relatively narrow range, so that there is a rough general correlation between the scintillation efficiencies and the integrated photofluorescence intensities. The best phosphors are those with rigid molecular and crystal bonding so that both the primary photon production and photofluorescence efficiencies are reasonably high. There is a tendency for the best scintillators to have the simplest and most sharply defined photofluorescence spectra. The scintillation efficiencies have been correlated with molecular structures and the mobility of the π electrons (or with the resonance interactions) within the molecules. In the simpler cases, consideration of singly charged quinoid structures permits the calculation of parameters which can be directly correlated with the scintillation efficiencies. In more complex cases, steric hindrance, hyperconjugation, ``unshared'' and ``nonbonding'' electron pairs, five-membered aromatic rings, heavy atom (triplet-state) effects, and quinoid (quenching) ground states must be considered. In all cases, the effects of thermal (vibrational) perturbations are present. ``Bond density'' and ``bound valence'' values have proved to be useful in correlating scintillation phenomena and molecular structure effects. Strongly colored compounds are usually poor scintillators. Earlier predictions of high scintillation efficiency for quinquephenyl and sexiphenyl are reaffirmed, and coronene, benzo[ghi]-perylene, and 1,2,5,6-dibenzanthracene are also predicted to be significantly better than the presently known organic scintillators. The further study of compounds containing five-membered aromatic rings appears promising. A generally good correlation has been found between the scintillation efficiencies and molecular diamagnetic anisotropy values. Some relationships between the scintillation decay times and the molecular structures can also be seen. The best crystalline organic scintillators are colorless substances of high melting point possessing molecules of simple structure and low atomic number in which there is extensive resonance conjugation of rings, ethylenic double bonds, and other groups to give extended, rigidly interlocked systems.

Crystal Growth and Optical Properties of Organic Crystals for Neutron Scintillators

“…The single exponential functions were used to fit the decay curves of trans-stilbene and pterphenyl crystals obtaining values of 8.2 and 5.8 ns, respectively. These values are similar to those of previous study, however, in this work the instrumental response function is included [5]. In the future study, we will focus on evaluation of decay time more precisely using a fast decay measurement setup such as a streak camera.…”

Photoconductivity of Some Polynuclear Aromatic Hydrocarbons

“…The first that comes to mind is the presence of an impurity. I t is well lcnown that commercial pyrene has a yello\\-color, and that this color persists after chron~atographic purification through alumilla (9). Chromatography through silica gel, however, yields completely colorless pyrene.…”

The Solid Solutions (Per)2[PtxAu(1−x)(mnt)2]; Alloying Para- and Diamagnetic Anions in Two-Chain Compounds

“…Special care was taken in the purification of the compounds and solvents used. Perylene (Sigma, St. Louis, MO, USA) was purified by multiple recrystallization and silica and alumina chromatography [30], followed by gradient sublimation under reduced pressure at~110 • C. The tetrabutylammonium salts of gold and platinum were synthesized as described [31,32] and recrystallized several times from acetone under argon atmosphere. Dichloromethane was distilled, dried and passed through a column of activated alumina just before use.…”