Effect of Naphthalene on the Quenching of Liquid Scintillation Solutions

Abstract

IN liquid scintillation counting the passage of β-particles through the phosphor solution results in a complex series of events, the useful ones culminating in the emission of photoelectrons at the photocathode of the photomultiplier. A number of parasitic events occur concomitantly that lead to a lowering of the yield, or total loss of the photoelectrons, that is, quenching. Possible sequences for these events, which occur during a time-interval of approximately 10⁻⁹ sec, have been outlined by several authors^1–3. The addition of naphthalene to some liquid phosphors improves their counting characteristics. The enhancement in counting efficiency has been ascribed by Fust and Kallmann⁴ to the ability of the naphthalene to act as an intermediate solvent that is capable of transmitting the energy from the primary solvent to the scintillators, thereby by-passing the quenching agent. If this is true the series of events leading to photoelectron production could be outlined as follows: (A) emission of β-particle; (B) excitation of primary solvent molecules (toluene); (C) excitation of secondary solvent molecules (naphthalene); (D) excitation of primary solute molecules (PPO); (E) excitation of secondary solute molecules (POPOP); (F) production of photons.