RADIOLYSIS OF LIQUIDAROMATIC HYDROCARBONS
1157
Linear Energy Transfer Effects in the Radiolysis of Liquid Aromatic Hydrocarbons1
by J. Y. Yang, J. D. Strong, and J. G. Burr North American Aviation Science Center, Thousand Oaks, California
(Received September 86, 196.4)
Liquid benzene and o-terphenyl have been exposed to radiations of LET values ranging from 0.02 to 25 e.v./A.; G(H2) and G(reactant -+ polymer) values have been determined under these conditi:ns. There is little change in product yields for radiations with LET from 0.02 to 2 e.v./A., but the yields increase sharply with LET above 2 e.v./A. A number of observations cannot be explained by a reaction model based on competition between a unimolecular reaction and a bimolecular reaction involving activated species. It is suggested that the above observations may be explained by thermal spike effects.
Experimental Although linear energy transfer (LET) effects in the radiolysis of water and aqueous solutions are reasonMaterials. Phillips 66 Research Grade benzene and ably well documented,2 comparatively little attention Eastman White Label 0-,m-, and p-terphenyls has been given to such effects in the radiolysis of orwere used without further purification. All reagents ganic liquids. Early investigations by SchUler and COwere analyzed by gas chromatography and found not worker^^-^ indicated that product yields from irradiato contain any significant impurity, ~olonium-210 tions of aliphatic hydrocarbons are independent of LET values, and it was concluded that bimolecular (1) Work supported in part by the U. S. Atomic Energy Commission radical reactions in the ionizing track are UIlimPortant under Contract AT-(ii-l)-GEN-S, at the Atomics International Division of North American Aviation, Inc., and presented at the in such aliphatic systems. Recently, however, evi140th and 148th National Meetings of the American Chemical dentes for a track effect in the Of liquid Society, Chicago, Ill., Sept. 1961 and Sept. 1964, respectively. cyclohexane6" and cyclohexene6bhave been reported. (2) A. 0. Allen. "The Radiation Chemistrv of Water and Aaueous Solutions,'' D. Van Nostrand Company, Inc., New York, N . Y.,- 1961. In the radiolysis of aromatic hydrocarbons, definite, (3) R . H. Schuler and A. 0. Allen, J. Am. Chem. SOC.,77, 507 although small, effects attributed to changes in LET (1955). have been reported by a number of investigator^.^-'^ (4) H. A. Dewhurst and R. H. Schuler, ibid., 81, 3210 (1959). The observed effects cannot be explained by a model (5) R. H. Schuler, J . Phya. Chem., 63, 925 (1959). similar to that proposed for the aqueous system. l 3 (6) (a) J. W. Falconer and M. Burton, ibid., 67, 1743 (1963); (b) W. G. Burns and J. A. Winter, Discussiona Faraday Soc., 36, 124 Intuitive feelings thus far have led to a general belief (1963). in a bimolecular reaction of activated intermediates to (7) W. G. Burns, W. Wild, and T. F. Williams, PTOC.2nd Intern. account for the increased product yield found for very Conf. Peaceful Uses At. Energy, 29, 266 (1959). densely ionizing radiation. Recently, H o ~ h a n a d e l ' ~ ! ~(8) ~ W. G. Burns, Trans. Faraday Soc., 58, 961 (1962). (9) W. G. Burns and C. R. V. Reed, ibid., 59, 101 (1963). has proposed that differences in the decomposition (10) T. Gauman and R. H. Schuler, J. Phys. Chem., 65, 703 (1961). of inorganic salt crystals induced by radiations of vary(11) T. Gauman, Helv. Chim. Acta, 44, 1337 (1961). ing LET can be attributed principally to a tempera(12) J. Y. Yang, F. C. Goodspeed, and J. G. Burr, Chem. Znd. ture effect in the ionizing track. It is logical, therefore, (London), 1018 (1962). to consider also such a thermal spike effect as the cause (13) A. H. Samuel and J. L. Magee, J . Chem. Phys., 21, 1080 (1953). for the LET dependence of radiolysis yields in aromatic (14) C . J. Hochanadel, Radiation Res., 16, 286 (1962). hydrocarbons. (15) C. J. Hochanadel, J . Phys. Chem., 67, 2229 (1963). Volume 69,Number 4
April 1965
1158
J. Y.YANG,J. D. STRONG, AKD J. G. BURR
SIDE VIEW FRONT VIEW sources were purchased from the Mound Laboratory of the Monsanto Chemical Co. Irradiations. The experimental procedure for zlOPo a-radiolysis has been described in our earlier report. l 2 The irradiat,ion apparatus was almost ident,ical with t,hat' described subsequently by Falconer and Burton16 except, that, our zlOPosource housing was construcbed of glass, and the source was positioned by simply raising and lowering the at,tached rod with t'he guiding shaft serving also as the helium gas exit'. Efficient IN THERMOCOUPLE WINDOW z 5 cm o D stirring was essent'ial to avoid polymer deposition on WELL I MIL STAINLESS the 'housing window, and this was accomplished by a STEEL glass-enclosed magnetic stirrer pivot'ing on a sharp point in a small well. The stirring bar enclosure was Figlire 1. He*+arid 11' irradiation cell made disk shaped t'o niinimize the necessary sample size and thus in effect increase the a-dose rate. yields of hydrogen arid cyclohexene from t,he radiolysis Helium ion and deuteron irradiations were carried of cyclohexane under ident,icalconditions. out with t'he 60-in. cyclot,ron at, the Crocker Laboratory, Analysis. The gas yields were obtained by convenUniversity of California (Berkeley). The samples tional volume and pressure measurements. Polymer were irradiat,ed in glass cells with ]-mil stainless steel yields (Clz and higher products) were determined by a foil windows as shown in Figure 1. The windows vacuum sublimation technique. Product analysis for and t8he keeper rings were cemented in place with a irradiated o-terphenyl has been described earlier. one-component, epoxy bonding agent. The tungsten Benzene sublimat'ion was carried out, by initially lead was used as a ground to prevent any charge buildcooling the irradiat,ed sample t.o liquid nitrogen temup in t,he organic liquid. Samples were loaded into perature and then allowing t,he sample t'o warm up the reservoir side of t'he cells and degassed by a conto 0" while being evacuat,ed. The residue was weighed vent.iona1 freeze-thaw technique. ' During irradiation, and redissolved in cyclohexane to allow correction for the samples were st,irred by the use of a shaker assembly ally benzene content detectable by gas chromatography designed by Garrison and co-workers. l6 Benzene (P,P'-oxydipropionit,rile column at 80'). The residue samples were irradiated a t about 35' by cooling wit'h weight, was also corrected for dimeric products carri.ed circulat'ing wat'er. o-Terphenyl samples were preheated over in the original distillation by analysis of the distilto 100" in a steam bat.h and were found to reach an late by gas chromatography using a polyester succinate equilibrium temperatmureof about 150' due to heat column at 180'. generated upon irradiation by the cyclotron beam. Results 7-Radiolyses of t,erphenyl samples were carried out in a cobalt.-60 source of the general Hochanadel-Ghormley The predominant decomposition products from raditype. The dose rate, as measured by ceric dosimetry olysis of aromatic hydrocarbons are always mixtures of and also by G(H2) from benzene, was 1.84 X 10'' high molecular weight substances. Since the come.v.//g.-niin. position and the molecular weight of these polymeric D'osimetry. Dose rate measurements for zlOPoamixtures are riot well defined, the product yields are radiolysis were made by ceric sulfate dosimetry, using often expressed as G(-reactant), the number of ret'he value of 2.88 as G(Ce3+).17 For cyclotron irradiaactant molecules converted per 100 e.v. energy abt,ions, beam currents were measured by a monitoring sorbed. G( -0-terphenyl) values from zlOPo a-radicircuit described by Garrison and c o - w o r k e r ~ ~ ~ ~ ~ ~have been reported in our earlier note12as a funcolysis and the tot.al radiat,ion energy absorbed in each sample tion of temperature. The value at 150' was found to was calculated from range-energy data. The energies be 0.63. Polymer yields from benzene irradiated of the particles accelerat,ed in the cyclotron were 48 1Iev. for helium ions and 24 Nev. .for deuterons, and (16) W.M. Garrison, H. R. Haymond, and B. M. Weeks, Radiation Res., 1, 97 (1954). energies for the att'enuated beams reaching the samples (17) J. Weiss and N . Miller, J . Phys. Chem., 63, 888 (1959). FverE' calculated to be 44 arid 22 Jlev., respectively. (18) W. M .Garrison, et al., Rea. Sci. Instr., 24, 462 (1953). The reliability of the above dose absorption calculation (19) R. M. Garrison, B. M. Weeks, J. 0. Ward. and W.Bennett, was confirmed by nieasurenients of LET independent, J . Chem. p h y s . , 2 7 , 1214 (1957).
T h r Journal o f Physical Chemistry
RADIOLYSIS OF LIQUID AROMATIC HYDROCARBONS
with zlOPoa-particles have now been determined with benzene damage up to 1%, and G(-benzene) was found to be 1.37 a t 25' and 1.43 at 35'. Our data on G(-reactant) as well as G(Hz) values for cyclotron irradiations of benzene and o-terphenyl are summarized in Table I. The results for 6OCo y-radiolysis of 0-, m-, and p-terphenyls are reported in Table 11.
Table I : Cyclotron Irradiation of o-Terphenyl and Benzene
Sample
Particle
o-Terphenyl
He2+ He2+ He2+ D+
Beam curEnergy, rent, iMev. pa.
X lo-*'
C(H1)
actant)
2 2 2 1.5 2 2 2 1.5 1.5 1.5 1.5
4.95 4.95 4.95 4.95 4.95 4.95 4.95 2.50 2.50 2.50 2.50
0.0139 0.0143 0,0141
0.29 0.29 0.29 0.22 0.25 0.26 0.25 1.0 1.1
D+ D+ D+ Benzene
He2+ He2+
D+ D+
44 44 44 22 22 22 22 44 44 22 22
Total dose, e.v.
C( - re-
0.0116
0.061 0.071 0,048 0,044
1159
obtained by Burns* for ionizing particles of intermediate linear energy transfer. There is some discrepancy between our G( - reactant) values for 210Poa-radiolyses of benzene and o-terphenyl and those of G( - benzene) = 2.1 a t 25' and G(-o-terphenyl) = 0.93 at 100" as reported by Burns and Reedg for irradiations with particles from the 'OB(n,a)'Li reaction. The ionizing track densities for both the zlOPoa-particles and the particles from the ' O B ( ~ , O ) ~reaction L~ are expected to be nearly the same, but the condition for pile irradiations is complicated by the necessary presence of organoboron compounds. It may not be entirely valid to compare results from these two sets of experjments. In any case, we are in qualitative agreement that there is little change in product yields for radiations in LET values from 0.02 to 2 e.v./8., and that the yields increase sharply with increasing LET above 2 e . v . / i . Such correlations are presented graphically in Figure 2. Another significant observation is a dependence of the rate of radiation-induced decomposition on the 08 I
Table 11: Initial Yields" of High Boilers from
o-Terphenyl
m-Terphenyl
p-Terphenyl
100 200 300
0.19 0.19 0.24
0.12 0.17 0.17
0 ,034b 0 .074b 0.17
Final HB content about 2-3%. Solid phase.
'
II
1.2
I
I
I
13 I
14
1.5
I
d
]
G( - terphenyl)
Temp., 'C.
1 0 2 2 e.v./g.
IO
1
G SCALE: (-0-TERPHENYL) 0 I - 0 8-fl AT 15OoC
y-Irradiated Terphenyls 7
09
Total doses were 3.7 X
The ionizing track density for a charged particle in liquid hydrocarbons is not far different from that in the aqueous medium. The linear energy transfer value for the 6oCoy-radiation is thus about 0.04 e . v . / k , and that fora 210Po a-particle,attenuated through 0.0003in. stainless steel foil, is near the Bragg ionization peak value of about, 25 e.v./B.20 The LET values for high energy helium ions and deuterons can be calculated by equations given by Schuler and Allenz1; the value for 22-1Iev. deuterons is 0.44 e.v./&, and that for 44-Mev. helium ions is I .75 e . v . / k Our observed hydrogen yields for benzene radiolysis with high energy helium ions and deuterons agree well with those reported by Gauman and Schuler,'O and the G( - benzene) value is essentially the same as that
G(H2IFROM BENZENE SCALE:001-008
d
d
#
I
-I
001
01
02
03
04
05
06
07
08
G (PRODUCTl,MOLECULES/IOO ev
Figure 2. Radiation damages to liquid benzene and o-terphenyl: 0, Gauman and Schuler; I, Burns' data. ~
~
~~~~
(20) E. Collinson, F. S. Dainton, and J. Kroh, Nature, 187, 475
(1960). (21) R. H. Schuler and A. 0. Allen, J . Am. Chem. Soc., 79, 1565 (1957).
Volume 69,h'umber 4
April 1965
1160
J. Y. YANG,J. D. STRONG, AND J. G. BURR
eq. 1, and a unimolecular deactivation of the jntermediate, eq. 2, is suggested specifically by Burns and
M*
ALPHA PARTICLE IRRADIATION AT 2 5 0 O C THIS REPORT I
1 *\ '\
..
,
ELECTRON IRRADIATION AT 400OC (GkTERPHENYL) x 2) HARWELL DATA
I
+ A I * -% radicals and products M * + M kl_ 2,11
(1) (2)
Reed.8v9 I t is not apparent that the above model can account satisfactorily for the observed LET effects. A qualitative analysis based on such a model has to take into consideration not only diffusion of activated species but also complications due to radical diffusion as a result of reaction 3, which is necessary to account
+ M +products
radical
(3)
for the observed high molecular weight products as well as the protective effects of certain additives. I 2 3 4 5 6 7 S 9 IO 1 1 12 13 Simplified calculations based on reactions 1 and 2 will HIGH BOILER CONTENT IN WEIGHT PERCENT account only for a linear increase in yield with LET. Our intuitive estimate on the effect of diffusion G( - t e r p h e n y l ) as functions of t h e polymer content. of radicals and activated species is one of depressed yield a t high LET rather than the observed sharp inpolymer content of the substrate. Figure 3 is a plot crease. A similar conclusion was made recently by of the G ( -0-terphenyl) values (corrected for energy Schuler. 24 absorption in o-terphenyl alone) us. the polymer conSchulerz4has proposed, further, two very interesting tent. Such a marked inhibitory effect of the polymer models, the knock-on model and the charge-displaceon the radiation damage to o-terphenyl indicates an nient model. Both these models can reasonably acimportant conipeti tiori between the product and the count for observed LET effects in liquid aromatic hysubstrate in reactions with active intermediates. drocarbons. However, they are highly speculative Parallel results are observed in irradiations by zlOPo and require large departure from normal considerations a-particles and by fast electrons. Apparently such of radiation-induced processes. scavenging processes are LET independent. All the above models are based on a consideration that the only significant change in experimental conDiscussion ditions due to variations in LET is a change in the Comprehensive analyses of possible mechanisms for track concentration of reactive intermediates. Recent radiation-induced decomposition of aromatic hydroevidences indicate, however, that a temperature effect carbons are given by Burns and Reed8rgand by Burr cannot be neglected for high LET radiations. 14,15,25 and co-workers. zz We shall therefore center our I t may be considered that for relatively low LET radiaattention only to implications of LET effects on the tions the track temperature increase will be small and product yields. the reaction rate increase negligible. For high LET The observed effect is distinctly different from that radiations, however, a sharp product yield increase in the aqueous system and cannot arise from compemay be expected from a arge temperature effect. tition between track recombination and diffusion out Thermal spike calculationsz6based on reactions beof the track by radicals and hydrogen atoms. As we tween activated intermediates and surrounding molehave pointed out in our earlier note,l2 in organic syscules gave results in qualitative agreement with our tems the radicals can react readily with the solvent observations. Such calculations were made, however, molecules and the particle track must be more diffuse than in irradiated aqueous systems. Consequently (22) J. G. Burr, et al., Sucl. Sci. Eng., 11, 218 (1961). bimolecular reactions involving long-lived excited (23) J. M. Scarborough and J. G. Burr, J . Chem. Phys., 3 7 , 1890 (1962). species have been proposed often8-12zz2t23 as most coni(24) R. H. Schuler, Trans. Faraday Soc., 61, 100 (1965). patible with the increase in radiation damage with in(25) J. L. Magee, Discussions Faraday SOC.,36, 232 (1963). creasing LET. A competition between product for(26) T. Wolfram and J. A. Brinkman, private communications. We mation via a bimolecular reaction of excited molecules, will provide details of these calculations on request. ~
The Journal of Physical Chemistry
~
~~
~~
~~
RADIOLYSIS OF LIQUIDAROMATIC HYDROCARBONS
1161
with oversimplified and sometimes unrealistic assumptions. We think, however, the thermal spike modeln should be considered as an alternative explanation for the observed LET effects and further efforts made to bring it to a computational level equivalent to that of the difiusion model.
We are indebted to Mr. John A. Brinkman and Dr. Thomas Wolfram of our laboratory for calculating the radiation damage yield due to a thermal spike, and to Dr. Mark Cher of our laboratory for valuable suggestions. We wish to acknowledge further the generosity of Dr. W. G. Burns of A.E.R.E., Harwell, England, for furnishing us some of his unpublished preliminary reports.
Acknowledgment. We wish to thank Dr. Warren R4. Garrison, 11r. Boyd M. Weeks, and the 60-in. cyclotron staff a t the Crocker Radiation Laboratory, University of California (Berkeley), for their assistance in carrying out helium ion and deuteron irradiations.
(27) F. Seita and J. S. Koehler, “Solid State Physics,” Vol. 11, Academic Press, Inc., New York, N. Y., 1956, p. 351.
Volume 69,Number 4
Apri2 1966
