New Liquid Scintillators with Higher Speed and Efficiency

The variation of light intensity with time following excitation by gamma rays has been measured for toluene solutions of quaterphenyl, quinquiphenyl, and sexiphenyl compounds whose solubilities were enhanced by substitution of alkyl groups. The decay curves, measured by use of a light-intensity-sampling technique, were analyzed to yield the mean lives τ1 for energy transfer from solvent to solute, τ2 for decay of the fast component (from the singlet state), and τ3 for decay of the slower component (probably from excimers). Preliminary estimates of τ2 for quaterphenyl, quinquiphenyl, and sexiphenyl are 1.27, 1.22, and 1.02 nsec, respectively. As expected,τ1 appears to vary inversely with solute concentration, and at the highest concentration of quaterphenyl in toluene (~ 65 g/liter, y = 1.12 × 10-2) was < 0.1 nsec. At this concentration, the amount of light in the slower component was only about 1/6 of the total light. This indicates that formation of excimers is inhibited. In addition, the photoelectrons produced in an RCA-8575 photomultiplier per keV of energy lost in the scintillator was measured to be 2.1 for quaterphenyl, 2.2 for quinquiphenyl, and 1.9 for sexiphenyl solutions. The improvement in efficiency and speed is reflected in a corresponding improvement in the time resolution achievable with these scintillators. For a 300-keV energy loss in the quaterphenyl solution, the FWHM was 0.20 nsec and the effective T1 /2 was 0.035 nsec.