Radiation Chemistry of Cyclohexane: Ii. Dose Rate Effects on the Formation and Destruction of Cyclohexene

Abstract

Prolonged radiolysis of cyclohesaile leads to a steady-state concentration of cyclohexene where G(cyc1ohexene) = 0. At concentrations greater than this steady concentration, cyclohexene is destroyed. I t is shown that this steady-state concentration is a function of dose rate, varying approxin~ately as the fourth root of the radiation intensity. . A discussion of these and other observations indicates that dose rate and L.E.T. effects are observed in cyclohexane onIy if radical scavengers are present and that cycIohesene, a product of radiolysis, acts as a radical scavenger in irradiated cyclohexane. INTRODUCTIONCyclohexeile and bicyclohexyl are the two major high lnolecular weight products formed in the radiolysis of cyclohexane. Dewhurst and Schuler (1) have shown that the initial G values (molecules formed/100 ev absorbed) for cyclohexene and bicyclohexyl are independent of the L.E.T. (Linear Energy Transfer)* of the irradiation. Dewhurst (2) showed that G(cyc1ohexene) decreases with dose (i.e. with increasing cyclohexene concentration) and, further, that in concentrated solution (5 X lo-.' moles/g), cyclohexene is destroyed. I t follows that prolonged irradiation of cyclohexane or of solutions of cyclohexene in cyclohexane leads to a "steady-state" concentration of cyclohexene where G(cyc1ohexene) = 0. Dewhurst and Schuler (1) found some evidence that G(cyc1ohexene) fell off more rapidly with cyclohexeile concentration a t low dose rates, using a CoGO y-ray source, than a t the higher dose rates obtained with electron and particle beams.We have studied the formation and destruction of cyclohexene as a function of the dose rate, using CoGO y-rays and have shown that the "steady-state" concentration increases slowly with the dose rate. The dependence is less than a proportionality to the square root of radiation intensity and approximates to a fourth root dependence. E X P E R I M E N T A LSamples of cyclohexane (spectroscopic grade from various manufacturers) were irradiated in sealed glass vessels. The liquids were degassed by freezing, pumping, and thawing five times. Two CoGO sources were used (3) having dose rates of 1.4 X 1019 ev/g hr and 9.6X1017 ev/g hr. These dose rates were established using the Friclie dosimeter, G(Fe3+) = 15.5 ions/100 ev. Analysis for cyclohexeile was done by infrared spectrophotometry of the 640 cm-I band of cyclohexene using a Perltin-Elmer, model 21, spectrometer. Path lengths of 0.2-4.0 nlm were used with con~pensating cells containing pure cyclohexane for the thicker cells. The irradiations were made over an extended period of time (irradiations of up to 1400 hours a t the low dose rate). The solutions were stored after irradiation and were all analyzed a t one time together with a series of standard solutions of known concentration. The concentrations were determined by graphical interpolation from the measurements of the standard spectra. At the low 'Manuscript

Radical reactions in liquid cyclohexane. II. The mercury-photosensitized decomposition of cyclohexane

“…CeHu-CeHxo• + CeHn • -> C18H32 (16) In addition to the reactions mentioned so far, reactions 17 and 18 may also contribute to product formation C8H9• + CeHiv -> 2C6H10 (17) CeHu-CeHio• + C8Hu • -> C12H22 + CeHio (18) With this reaction mechanism and using the steadystate approximation, the following relations can be derived d(A + B)/dí ku ^knkujku ku ku) dCi8/di ku kuku(kn + kn) kukujku + ku + ha) [H] .…”