PICOSECOND PULSE RADIOLYSIS STUDIES ON
MECHANISM OF LIQUID SCINTILLATION
Seiichi Tagawa, Yusuke Katsumura,and Yoneho Tabata
Nuclear Engineering Research Lab., Fac. ofEngineering, Univ. of Tokyo
Shirakata Shirane 2-22, Tokai-mura, Ibaraki,
319-ll, Japan

Liquid scintillators, which are widely used for measurementof tritium, carbon-14, other radioisotopes and their labelled
compounds in medical, biological, chemical and fusion fields, offer
the fastest response times and simplest and economical detectors.
A number of original (l) and review (2) papers have been published on the liquid scintillation. However, the studies on themechanism of liquid scintillation have been limited to typicalbinary (and ternary) liquid scintillator systems based on aromaticsolvents (e.g benzene, toluene, or xylene) and containing appreciable concentrations of solutes (and secondary solutes) . The mechanism of liquid scintillation have not been made clear yet.
Recent picosecond pulse radiolysis studies have made the early
events in radiatior-induced reaction in aromatic (3,4) and saturated (3,5,6) hydrocarbon solutions clearer than before.
The present paper mainly describes the picosecond pulse radiolysis studies on mechanism of liquid scintillation, especially on
the roles of solvents. Furst and Kallman (7) reported that solvents could be devided into three classes of scintillation
effeciency; so-called "effective," "moderate" and "poor" solvents.
Kinetic behavior of scintillation emission in various solvents inpica- and nano-second time region is reported in the present
symposium (8) by us and solvents are classified into three classes
from the view point of dinamic behavior of scintillation. These
two classifications agree very well with each other (7,8).
The mechanism of liquid scintillation has been generallyexplained by energy transfer processes so far.
Here, the mechanismof liquid scintillation have been examined based on picosecond
pulse radiolysis experimental results (4,6). The role of charged
species (containing geminate ion pairs, very reactive positiveholes) and excited states have been examined in detail in aromaticand saturated hydrocarbon, and other solvents.
For example, energy transfer processes take most important rolesin the liquid scintillation of aromatic solutions containing appreciable concentrations of solutes at room temperature.
In saturatedhydrocarbon solution, both charged species (geminate ion pairs)
and solvent excited states must be considered. Especially very
reactive positive holes (or very reactive ion-pairs like excitons)
are considered to exist in very short time range (~ 10 ps) in-279-saturated hydrocarbon solvents.
The roles of charged species and solvent excited states inliquid scintillation are strongly dependent on solvents, soluteconcentrations, and temperature.
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J. B. Birks, Photophysics of Aromatic Molecules, WileyInterscience, New York, (1970)(3)

G. Beck and J. K. Thomas, J. Phys. Chern., 76,3359 (1972)(4)

S. Tagawa, Y. Katsumura, T. Ueda, andY. Tabata,Radiat. Phys.
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