ABSOLUTE YIELDS IN THE Γ-Radiolysis OF GASES

The radiolysis of methylcyclohexane (MCH) vapor was carried out under a variety of conditions. The G-values of the main products a t 110°, extrapolated t o zero dose, are hydrogen (5.2), methylcyclohexene isomers (2.0), ethylene (1.5), methane (1.3), propylene (0.8), and total dimer (0.3). Other products were also measured.The hydrogen yield was reduced t o G = 3.1 b y each of the additives, NzO, SFB, and CC14, and t o G = 1.6 by C2H4. Both DI and ND3 increased the total hydrogen yield above the value in pure MCH. In pure MCH approximately 50% of the ions (G(tota1 ionization) = 4.4) resulted in hydrogen formation, whereas in the presence of D I or NDB, 75y0 of the ions are hydrogen precursors. Thus three different types of positive ions are distinguished in the system: G(M1+) = 2.1,. G(M2+) = 1.3, and G(NC) = 1.0.The average Ion lifetime with respect t o neutralization was s. The ion DI-was therefore stable with respect t o decomposition t o D + I-for a period greater than s under the conditions of the experiments ( N 380 Torr MCH, 110").The yield of methylcyclohexene isomers increased with increasing temperature and increased upon addition of hTD3 or CZH4 to the radiolysis system. The dimer yield was also enhanced by the addition of ND3. This effect was explained by the occurrence of an ionic secondary reaction that destroys methyl~~clohexene and (or) methylcyclohexyl radicals in pure MCH.Approximately 85Yo of the methane is produced by methyl radicals abstracting hydrogen atoms from MCH.

Section: T a B L E I1 Effects O F Additives On G-values Atmentioning

confidence: 79%

Radiolysis of methylcyclohexane. III. Pure vapor phase and effects of additives

Holtslander¹,

Freeman²

1967

“…If we assume t h a t the contribution of the unquenched n~olecular-detaclinlent process in CGD~~-C~~H~?-CGI-IG liiixtures is equal to G1(D), 0.78, then from the hydrogen ).ield a t 50% benzene we find kg/kll -0.16. Now, Schiff and Steacie (12) found the collision yield for reaction [9] to be 1.4X lo-' and Allen, hIelville, and Robb (13) found the collision yield of [l:I.] t o be 1.3X10-'L so that kg/kll -lop3 a t 25" C. As our experiments were carried out a t 125" C, the activation-energy difference would have to be -12 kcal to reconcile these two values.…”

Section: G H G -C~h~~ Il/lixt~~resmentioning

confidence: 99%

“…t o be 1.3X10-'L so that kg/kll -lop3 a t 25" C. As our experiments were carried out a t 125" C, the activation-energy difference would have to be -12 kcal to reconcile these two values. The best estimate for the activation energy of reaction [9] buted to hydrogen-atom scavenging. Our conclusion agrees with that of Ramaradl~ya and Freeman althougl~ our argrunents differ.…”

Section: G H G -C~h~~ Il/lixt~~resmentioning

confidence: 99%

Vapor-Phase Radiolysis of Cyclohexane and Mixtures of Benzene and Cyclohexane

Blachford¹,

Dyne²

1964

T h e vapor-phase radiolysis o f cyclohexane and benzene-cyclohexane mixtures has been studied with isotopic tracer techniques using CsDlr and CGDG. Benzene does not quench the molecular detachment o f hydrogen, CGDI? D! +. CGDIo, but interacts with the secondorder processes giving I-ID in CGD1?-Cr,H1? mixtures. T h~s ~nteraction is not H-atom scavenging and an ionic mechanism is preferred. T h e radiolysis o f all these systenls is very different from their liquid-phase radiolysis.'The low radiolytic yields in liquid benzene-cyclohexane mixtures are a classic example of 'protection'. T h e interactions causing these low yields are physical; they do not involve the ma1;ing or breaking of chemical bonds. This has been shown in two ways: by complete product analysis (1) and by techniques using isotopically labelled hydrocarbons (2). These isotopic techniques have revealed similar interactions in alkane mixtures (3) which are, however, absent in the vapor phase (4). As a result, we expect the vapor-phase radiolysis of benzene-cyclohexane inixtures to show marked differences froin the liquid-phase radiolysis.We have applied our isotopic techniques t o the vapor-phase radiolysis. In the liquid, benzene quenches both the first-order process, which gives D2 by molecular detachment from CGDl?, and the second-order processes which give HD in C G H~~-C G D~~ mixtures. I n the vapor, however, the D2 yield is reduced much more sloivly than would be expected if I-I-atom scavenging were occurring and so we favor an ionic mechanism for this interaction. Physical interactions quenching the first-order processes are absent. 1;isher 'Spectranalyzed' cyclohesane and benzene and Merck benzene-dG and benzene-free cyclohexane-d,z were used as received. T o minimize air contamination the sa~nple vessels (volume about 330 m l ) were evacuated t o 5x10-"mm Hg at 500' C for at least 24 hours before being filled and irradiated. Even so, the gaseous radiolytic products contained about 3% air. T h e vessels were sealed t o a grease-and mercury-free preparation line; the sample (about 2 1111 o f liquid) was degassed b y repeated freezing, pumping, and thawing and was then distilled into the irradiation vessel. During irradiation in our Co60 source, t h e vessels were kept at a constant temperature ( 2~2 ' C ) in a n electric furnace. Using the techniques already described ( 4 ) , we found t h e dose rate t o be 4.S7XlOL6 ev/sec g cyclohexane; the dose in most experiments was 3.61 X 101%v/g organic vapor.After

“…The preparation and purification of hydrocarbon samples for irradiation have been previously described (2,3). Gases were irradiated in cylindrical vessels of 60-mm 0.d.…”

Section: Methodsmentioning

confidence: 99%

“…In practice, the interpretation of the results is very complex, and except for the work of Essex and his collaborators a t Syracuse (I), mostly concerned with a-radiation, the method has not been widely used in radiation chemistry. The present work was undertaken to study the effects oi electric fields on the 7-radiolysis of some simple hydrocarbon gases, using a saturation-current method (2) to measure absolute product yields. In the course of the work, a hitherto unsuspected dependence of hydrogen yield on dose rate was discovered, which appears to be closely related to the action of applied fields on --the radiolysis.…”

mentioning

confidence: 99%

The EFFECT OF ELECTRIC FIELDS, AND a RELATED DEPENDENCE ON DOSE RATE, IN THE Γ-Radiolysis OF HYDROCARBON GASES

Woodward¹,

Back²

1963

Self Cite

The effect of electric fields on the y-radiolysis of ethane, propane, and the butanes has been investigated briefly a t 800 mm pressure, with dose rates between 2X101° and 400X1010 ev/cc sec. Yields of hydrogen were reduced when a saturation field was applied, except with ethane a t low dose rate, where a slight increase in hydrogen yield was observed. With propane and n-butane, the yield of hydrogen in the presence of a saturation field was independent of dose rate, while with ethane, it decreased with decreasing dose rate. A t the same time, a dose rate dependence was discovered in the simple radiolysis, in the absence of any field, of ethane, propane, and n-butane, a decrease in the yield of hydrogen a t low dose rates being observed. An explanation of these observations is suggested in terms of a competition between neutralization of ions in the gas phase and diffusion of ions to the wall. High dose rates should favor the former process, and low dose rates the latter. At sufficiently high dose rates, all ions should be neutralized in the gas phase. A t sufficiently low dose rates, all ions should diffuse t o the wall before neutralization, and it is suggested that the radiolysis under these conditio~ls should closely reseinble that ill the presence of a saturation field a t higher dose rates.The application of an electric field to a gas during irradiation offers a potential means of -distinguishing between products arising from ions and those forined from neutral species. In practice, the interpretation of the results is very complex, and except for the work of Essex and his collaborators a t Syracuse (I), mostly concerned with a-radiation, the method has not been widely used in radiation chemistry. The present work was undertaken to study the effects oi electric fields on the 7-radiolysis of some simple hydrocarbon gases, using a saturation-current method (2) to measure absolute product yields. In the course of the work, a hitherto unsuspected dependence of hydrogen yield on dose rate was discovered, which appears to be closely related to the action of applied fields on --the radiolysis. EXPERIMENTALThe preparation and purification of hydrocarbon samples for irradiation have been previously described (2, 3). Gases were irradiated in cylindrical vessels of 60-mm 0.d. and about 500-to 600-cc volume. Two experiments with propane were done in a smaller vessel of the same design and length, but with about one-half the diameter and one-quarter the \~olume. The measurement of saturation ion current and of hydrogen yield has been described (2). Electric fields were always applied with the central electrode positive, so that the positive ions were drawn to the other electrode, the graphite-coated wall of the cylindrical vessel. Hydrogen was the only product measured, and the extent of decomposition was always small enough that "internal" scavenging was negligible, and the measured yields were very close to their true initial values (3). Ethylene scavenger was added in some experime~lts, in which case the hyd...