PICOSECOND. PULSE RADIOLYSIS STUDIES ON
BEHAVIOR OF SCINTILLATION EMISSION IN VARIOUS
KINDS OF SOLVENTS
Yosuke Katsumura, Seiichi Tagawa and Yoheho Tabata
Nuclear Engineering Research Laboratory,
Faculty of Engineering, Univ. of Tokyo
Abstract
By using picosecond single electron pulse, scintillation
behavior of excited 2,5-diphenyloxazole(PPO ) in various kinds ofsolvents has been observed with 30ps time resolution. The emission
from solute excited states could be devided into two formation processes, faster and slower ones from the view point of formation
speed. From the points of combination of these two processes andemission efficiency, solvents could be classified into threegroups; effective, moderate and poor ones.
Liquid scintillation counting is widely used for the radioassay of biological and other materials labelled with carbon-14,
tritium or other radioisotopes. Since the discovery of liquidorganic solution scintillators in 1949, many combination of solvents and solutes have been tested and a great variety ofscintillators have been prescribed and used. In 1955, Furst andKallmann(l)reported that solvents could be divided into threeclasses of scintillation efficiency; effective, moderate and poor.
Most of the aromatic compounds were included in "effective" solvents. In a second class, so called "moderate" solvents, saturated hydrocarbons are placed with efficiencies within a factor of2"-'4 of aromatic hydrocarbons. The third class of "poor" solvents
included aliphatic alcohols, and ethers with efficiencies almost
two orders of magnitude lower than that of the "effective" solvents. Since most of the studies were based on steady state
photolysis and radiolysis, scintillation behavior with time are
not yet known clearly. To get new scintillation counters having
rapid rise and decay and high efficiency, it is thought thatstudies on the formation processes of solute excited state inliquid hydrocarbon with high time resolution is very important.
The block diagram of picosecond single pulse radiolysis system
is shown in Fig. 1. The present system is a combination of a fast
detection system and a generation of a picosecond electron pulse.
The fast detection system is composed of a streak camera(C979,HTV),
a SIT camera(Cl000-12,HTV), an analyzer(Cl098,HTV) and display
systems. The pulse width of electron beam was estimated to beless than 18ps. Time resolution of the system is about 30ps.
Details of the system was reported elsewhere.(2)In Fig. 2, time profiles of emission from excited 2,5diphenyloxazole in ethylalcohol, cyclohexane and phenylcyclohexane.
In ethylalcohol, growth of emission from the solute excited state
was completed at the end of Cerenkov light pulse induced by elec-277-

tron pulse and the emission decays gradually with decay time of
1.6ns. As a lifetime of excited 2,5-diphenyloxazole in ethylalcohol obtained by photoexcitation was reported to be 1.6ns,(3)above experimental result suggests that most of the solute excitedare already formed immediately after the pulse. This rapid growthof emission has been called the faster formation of solute excitedstate. In phenylcyclohexane, at a lower solute c9ncentration,
terenkov light is clearly seen and at the end of Cerenkov lightpulse, a certain fraction of excited states are already formed anda slow growth is also seen over lns. This slow growth has beencalled the slower formation process. At lOmM 2,5-diphenyloxazole
solution in phenylcyclohexane, ratio of the slower formation to thefaster one is large and the faster one is hardly discriminated. On
the other hand, in cyclohexane, both the slower and the fast formation can be seen, but the growth of the emission corresponding
to the slower formation is faster as compared with those in phenylcyclohexane. Normally above two formation processes can be seenin any kind of solvent. However, contribution of each process isdependent on the kind of solvents and concentration of solute
molecules. Based on above experimental results, solvents could beclassified into three classes from the view points of formation
processes and emission efficiency. Obtained classification is ina good agreement with Kallmann's one.
References
1) M.Furst and H.Kallman; J. Chern. Phys. 23 607 (1955)
2) Y.Tabata, J.Tanaka, S.Tagawa, Y.Katsumura, T.Ueda andK.Hasegawa; J. Fac. Eng. Univ. of Tokyo. 34 619 (1978)
3) I.B.Berlman "Handbook of Fluorescence Spectra of Aromatic
Molecules" Academic Press (1971)SYNC. TRIG
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Time profiles of emission fromlOmM 2,5-diphenyloxazole solution in ethylalcohol; a),cyclohexane; b),and phenylcyclohexane; c).