F. Newton Hayes

LIQUID SCINTILLATORS. III. THE QUENCHING OF LIQUID-SCINTILLATOR SOLUTIONS BY ORGANIC COMPOUNDS

Abstract A study has been made of the quenching characteristics of a large number of selected organic compounds in a few commonly encountered liquid scintillator solutions. From this it should be possible to anticipate the quenching difficulties associated with homogeneous liquid scintillation counting.

Liquid scintillators: Attributes and applications

Abstract Liquid scintillators are organic solutions which emit light in their interaction with ionizing radiation. The spectral distribution of this light is characteristic of the secondary solute, although vital energy degradation and transfer roles are assumed by the solvent and primary solute. In a liquid-scintillation detector the emitted light is collected at the cathode of a photomultiplier. Photoelectric conversion, electron multiplication, pulse amplification, sorting, and data presentation steps follow. Photon collection is a complex process consisting of reflection, scattering, and irreversible absorption. Liquid scintillators are evaluated by measurement of such properties as relative current, relative pulse height, decay-time, fluorescence spectrum, and absorption spectrum. The best solvents are alkylbenzenes and the most efficient solutes are polyarlys such as p-terphenyl, 2,5-diphenyloxazole (PPO), 2-phenyl-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD), and 1,5-di-[2-(5-phenyloxazolyl)]-benzene (POPOP). Liquid scintillators have an array of characteristic properties unmatched by any other variety of scintillator. These include inexpensiveness, ease of preparation, unlimited size or shape, high transparency, short decay-time, and ability to incorporate counting samples or neutron-capture solutes by dissolution or suspension. Liquid-scintillation detectors divide into two basic types: small-volume (with contained sample) and large-volume (with separate sample). Small-volume detectors have fast coincidence circuitry because of their primary application in H3 and C14 determinations, although many more-energetic beta and alpha emitters have been successfully counted. Different solvent systems allow homogeneous counting of a great variety of compounds. Suspension counting provides for the insoluble compounds. An internal standard is used to correct for quenching and to establish the disintegration rate of the sample. Large volume detectors use many photomultipliers but simple circuitry. The external source modification is useful for general considerations of geometry and for high-energy gamma resolution by multiple Compton scattering. Studies of μ mesons, neutrons and neutrinos have been carried out. The internal source modification is oriented to surround the sample, giving almost 4π coverage in studies of body retention of gamma emitters in mice, rats, monkeys, dogs, and humans.

Spectral Effects in the Comparison of Scintillators and Photomultipliers

Photomultiplier evaluation of scintillation performance is considered in the light of its marked dependence on spectral shape effects in the emission spectrum of the scintillator, in photon absorption of cell, reflector and photomultiplier window, and in the photoelectric conversion process. Relative pulse height data indicating gross differences in spectral sensitivity are presented for a group of 132 photomultipliers (types 6292 and 6342). A fluorescent converter for measuring the quantum output of liquid scintillators is described.Six scintillator solutions with fairly evenly spaced mean emission wavelengths from 3360 to 4660 A are evaluated for relative pulse height and relative photon output.Suggestions are presented for desirable methods for measuring the performance of scintillators.

Parameters Affecting the Performance of Large Liquid Scintillation Counters

The performance of large liquid scintillation counters as a function of counter size, photocathode area, solution transparency, and inner wall reflectivity is treated mathematically in an empirically derived equation. The use of the equation allows a degree of quantitation of the light output of the counter, and figures show the effect of varying parameters upon this light output.

Degradation of Nucleic Acid in Aqueous Solution by Ionizing Radiation. I. Loss of Ultra-violet Absorption of Solutions of Thymine or Thymine Derivatives on X-irradiation

SummaryIrradiation of solutions of thymidine or ammonium thymidylate with 250 kV X-rays at neutral pH results in more rapid loss of ultra-violet absorption under inert atmosphere than in air. The opposite behaviour with respect to the presence of oxygen is manifested by thymine solution. The effect of oxygen during irradiation on the ultra-violet absorption of solutions of poly d-T is more like its effect on thymidine or thymidylate solutions than on solutions of thymine.

Degradation of nucleic acid in aqueous solution by ionizing radiation. III. The correlation of radiation damage with change in melting transition--model experiments.

The melting behavior of polydeoxynucleotide double helices of known structure is analyzed in terms of the thermodynamics of helix stability, taking into account separately those contributions to the transition free energy that are proportional to the numbers of polymer molecules and those that are proportional to the numbers of base pairs formed. From the analysis of the melting transitions of helices having an alternating (d-)A.T, G.C base-pair sequence and containing either single-strand nicks or both nicks and damaged thymine bases, the effects of these structural lesions are assessed; it is concluded that, in a moderately long helix of this sequence (400 base pairs), the initial introduction of one mid-chain double-strand break or single-strand break produces respectively some 3.5 or 4 times as much depression in the transition temperature (Tm) as does the destruction of a single internal A.T base pair.

Applications of Liquid Scintillation Counters

The liquid scintillator, in just a few years of development, has shown itself to be an extremely versatile chemical system for radiation detection. Its evolution has been characterized by penetration into almost every phase of experimental science. A survey of the most notable applications of liquid scintillation counting is presented.

THE PHOTO-OXIDATION OF 2,5-DIPHENYLOXAZOLE (PPO)

Abstract The pulse height decay of eighteen selected organic liquid scintillator solutes upon prolonged exposure to ultraviolet light was investigated and the fate of one of these, 2,5-diphenyloxazole (PPO), was studied in detail. The structures of the photo-oxidation products were determined and mechanisms for their formation are proposed.

Degradation of Nucleic Acid in Aqueous Solution by Ionizing Radiation: II. Analysis of the Contribution of Various Water Radicals to the Loss of Ultra-violet Absorption by Thymidylate Solutions during X-irradiation

SummaryExperiments are described in which increasing concentrations of ammonium thymidylate, plus a variety of additional radical-scavenging compounds, are irradiated in aqueous solution with 250-kV X-rays. In addition to hydroxyl radicals, both hydrogen atoms and hydrated electrons can contribute to chromophore destruction. Under some conditions, where an excess of tert-butyl alcohol is present to scavenge hydroxyl radicals, up to 80 per cent of the observed chromophore damage can be attributed to the action of hydrated electrons. On average, about 65 per cent of the dTMP-hydroxyl radical interactions, 75 per cent of the dTMP-hydrogen atom interactions, and 67 per cent of the dTMP-hydrated electron interactions taking place under each set of experimental conditions resulted in chromophore destruction. The mathematical treatment of the data, on which these conclusions are based, is described.

A STEREOCHEMICAL APPROACH TO SELF QUENCHING STUDIES IN SOME BRIDGED p-QUATERPHENYL SYSTEMS*

Abstract The relative pulse height curves for two bridged p -quaterphenyls in both racemic and optically active forms have been measured. Examination of molecular models indicates that the optically active system could be more susceptible to self quenching due to preferential excimer formation. This effect has not been observed in the measurements made thus far. The synthesis of these compounds is also described.