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Dose Rate Effects in the Steady and Pulse Radiolysis of Liquid Chloroformlv2 by Ned E. Bibler Savannah River Laboratory, E. I . du Pont de h'emours and Company, Aiken, South Carolina (Received April 88, 1971)
29801
Publication costs assisted by the U.8. Atomic Energy Commission
The effect of dose rate on the 100-eV yields of organic products in the radiolysis of chloroform was studied at intensities of 1.6 X 1Ol6and 17 X 10l6eV/g-sec using a 6oCoy-ray source and at maximum instantaneous dose rates of 2.5 X loz4and 6.4 X loz4eV/g-sec using a pulse X-ray generator. The radiolysis products were CHZCI~, C2C14, Cch, sym-CzH2Clr,C2HC4, and CzCle. As the dose rate increased, G(CH2C12)and G(C2C&) decreased, G(CzH2C14) and G(GHC16) increased, and G(C&L) and G(CCl4) remained unchanged. These results are consistent with a free-radical mechanism involving the reaction of C1 and CHCIz radicals with CHCL to produce CCL radicals. The dose rate effects result from a competition between CHCl3 molecules and radicals for the CHClz radicals, with higher dose rates favoring the radical-radical reactions. The observed 100-eV yields in both the steady and pulse radiolysis experiments agreed with those calculated by the application of the previously proposed independent action model. A value of 1.3 f 0.2 M - 1 sec-1 was determined for the rate constant for the reaction of CHClz radicals with CHCla at 25" from the steady radiolysis experiments, and the magnitude of this value was confirmed by the results of the pulse radiolysis experiments. The results of the pulse radiolysis experiments suggest that the rate constant of C1 atom attack on CHCla is lower in the liquid than in the vapor phase. Evidence is also presented that indicates that C1 atoms and CHClz radicals are not radiolytically formed with equal 100-eV yields.
Introduction I n the radiolysis of liquid chloroform, evidence has been ~ i t e d ~ for - - ~the effects of dose rate variations on the 100-eV yields of the organic products (CH2Cl2, CzC14, Cc14, sym-CZHzCl4,CzHCl~, and CzC16). At constant temperature, as the dose rate increases, G(CHzClz) and G(CzC16) decrease, G(CzHzC14)and G(CzHCls) increase, and G(CzC14)and G(CCl4) remain ~ n c h a n g e d . ~ These results are consistent with a free-radical mechanism involving the reaction of C1 and CHClz radicals with CHCL to produce cc1, radicals. The competition between the CHC13 molecules and radicals for the CHCL radicals accounts for the dose rate effects. Abramson and Firestones have calculated this dose rate effect in CHCl, by the application of an independent action model. I n this model two radiolytic mechanisms are proposed to occur simultaneously but independently. One mechanism (the spur mechanism) involves reactions that occur in the tracks and is not affected by the presence of the radical scavenger Brz6 or changes in the dose rate.5 The other (the homogeneous mechanism) involves reactions of CHClz and CC13 radicals distributed homogeneously throughout the solution. The rates of these reactions are dose rate d e ~ e n d e n t . ~By solving for the steady-state concentrations of the radicals in the homogeneous mechanism, Abramson and Firestone calculated values for G(CHzClz), G(CZHzCl,), G(CZHCl,), and G(CzC16) that were in agreement with their experimental results at doseratesintherangeof 1.3 X 1015 to 13 X 1015 eV/ g-sec and temperatures of 0 to 63". Further, their calculations predicted that at very high dose rates, G(CH2ClZ)should approach zero, and the values for The Journal of Physical Chemistry, Vol. 76, No. 16, 1971
G(CzHzC14), G(CzHCl,), and G(CzC16)should be 1.5, 2.0, and 1.2, respectively. We have tested these predictions using an intense 60Co y-ray source for steady radiolysis experiments a t dose rates slightly greater than 10'' eV/g-see and an X-ray generator for pulse radiolysis experiments at dose rates approaching loz5eV/g-sec.
Experimental Section Chloroform from several commercial sources was purified by treating with Bre to remove unsaturates, then washed once with aqueous Na2SO3solution and several times with pure water. It was then triply distilled in an inert atmosphere. The only detectable impurity in the final distillate was ljl-CzH4C1z (-5 X M ) . The oxygen-induced decomposition of the purified CHC1, could be prevented by storing it under distilled water. Samples were prepared by vacuum distiLling CHCla (previously dried by passage through a 5-A molecular sieve attached to the vacuum line) into Pyrex' irradia(1) The information contained in this article was developed during the course of work under Contract AT(07-2)-1 with the U. S. Atomic Energy Commission. This work is being sponsored by the Division of Peaceful Nuclear Explosives of the USAEC. (2) Presented in part at the 153rd National Meeting of the American Chemical Society, Miami Beach, Fla., Apr 9-14, 1967. (3) G. M. Meaburn, Ph.D. Dissertation, The University of Leeds, England, 1959. (4) J. B. Gardner and B. G. Harper, Paper No. 53, 8th Annual Meeting of the Radiation Research Society, San Francisco, Calif., May 9-11, 1960. The results of this study are published in ref 5. (5) F. P. Abramson and R . F. Firestone, J . Phys. Chem., 70, 3596 (1966). (6) H. R . Werner and R. F. Firestone, ibid., 69, 840 (1965). (7) Trademark of Corning Glass Works, Corning, N. Y.
RADIOLYSIS O F LIQUIDCHLOROFORM tion cells, where it was carefully degassed by a t least six freeze-pumpthaw cycles. The cells were then sealed with the sample at liquid nitrogen temperatures under vacuum, For irradiations with the 6OCo sources, the cells were 15-mm 0.d. tubes. For the pulsed irradiations, the cells were 1.7 cm square by 2 cm high. Irradiated solutions mere analyzed with an F & M Nodel 400 gas chromatograph employing flame ionization detection. Three different columns were used isothermally. For CH2C12, a 16-ft, 1/4-in. column (30% silicone 200 on Celites) were used; for CZC14, CsH2C14, CzHC16,and C2C16,a 50-ft, l/s-in. column (2% SE-30 on diatomaceous earth); and for CCh, a 6-ft, 1/4-in. polar column (10% Triton9 x-305 on 60-80 mesh Chromosorb P) followed by a 6-ft, l/d-in. nonpolar column (20% SE-30 on Gas Chrom XIo). On the polar column, CC1, had a shorter retention time than CHC13. The nonpolar portion resolved ccl4 from the l,l-CzH4C12impurity. The gas chromatograph was calibrated using standards prepared from carefully purified solutes each time samples were analyzed. The solute peaks were very narrow; consequently, the concentrations of products were determined by comparing their peak heights with those of the standards. Steady irradiations were performed using 6oCo y-ray sources containing either -240,000 or 22,800 Ci of radioactivity. The more intense source mas calibrated with both LiF and the air-saturated cupric-ferrous dosimeter [G(Fe3+) = 0.661. l l The agreement between the results of the two dosimeters was good and the dose rate in CHC13,after correcting for differences in electron densities, mas 1.7 X lo1' eV/g-sec. The other source was a Gammacell 220 (Atomic Energy of Canada), which was calibrated using the Fricke dosimeter. The dose rate in CHC13 was 1.6 X 1016 eV/g-sec. I n both sources, the temperature of the samples was maintained at 25 f l o ,and the samples were irradiated to doses no larger than 2.40 X 1019eV/g. Pulsed irradiations were performed at ambient temperature (-25") using a Febetron12 Model 705 pulsed electron accelerator. This electron pulse (maximum energy = 2.0 MeV) was impinged on a 20-mil tungsten target, creating an intense X-ray pulse. This pulse was passed through a 5-mil tantalum foil and then through the samples. The total time length of the pulse was -40 nsec with 80% of the total dose delivered within 20 nsec.13 The effective time width (total dose/ maximum instantaneous intensity) of the pulse was 19 nsec. l 3 Samples were positioned behind the tantalum foil so that they were completely within the X-ray beam and irradiated with up to 63 pulses with at least a 2-min recycle time between pulses. The dose/pulse absorbed by the samples were determined using manganese-activated LiF. Samples of this material were irradiated in the cells in positions identical with those of the CHCIB samples. Earlier
2437 studies have shown that the response of LiF is independent of dose rates less than 2 X 10" rads/secI4 and that LiF serves as a good secondary standard for measuring doses from pulsed X-rays.15 (The dose rate in this article was somewhat less than that cited above.) The response of the LiF was calibrated against the Fricke dosimeter using 0.8416 as the ratio of the dose in LiF to that in the Fricke dosimeter. To determine the dose/ pulse received by the CHC13,the dose/pulse in the LiF was multiplied by the ratio of the effective mass energy absorption coefficients for each compound. These two coefficients were calculated by graphical integration of the monoenergetic absorption coefficients for each compoundll over the energy spectrum of the photons in the beam. Since the electrons impinging on the target are not monoenergetic, it is difficult to'determine the exact energy spectrum of the emitted photons. This spectrum has been approximated by CharbonnieP to be closely equivalent to the bremsstrahlung spectrum measured by Tochilin and Goldsteinlg when 2.0-MeV electrons were impinged on a comparable tungsten transmission target. Recently, more elaborate calculations18 correlating the measured electron energy spectrum of the Febetron 705 with the measured photon spectrum from constant-voltage electron accelerat o r ~ ' ~ !substantiated '~ this approximation. I n our experiments, the purpose of the tantalum foil was to "harden" the X-ray spectrum by removing a major portion of the X-rays of energies lower than 70 1teV. Based on the calculations of Charbonnierl8 the energy spectrum of the X-rays irradiating our samples has a peak at 200 keV, with >91% of the photons having energies larger than this. Using this spectrum, the ratio of the effective mass energy absorption coefficients for CHCb to LiF was 1.13. This is only slightly larger than 1.05, the ratio for the coefficients calculated for energies when energy absorption is predominantly by the Compton process. The reproducibility of the dose/pulse for the samples was determined from three or more independent LiF dosimeters. The variance was 8% or less. Small dosimeters placed along the length of a cell indicated (8) Trademark of Johns-.Manville Sales Corp. (9) Trademark of Rohm and Haas Co. (10) Trademark of Applied Science Laboratories. (11) E. Bjergbakke and K. Sebested, Advan. Chem. Ser., 81, 579 (1968). (12) Field Emission Corp., McMinnville, Ore. (13) Field Emission Corp., Technical Bulletin, Vol. 4, No. 1 (1965). (14) N. Goldstein and E. Toohilin, Health Phus., 12, 1705 (1986). (15) E. M . Fielden and E. J. Hart, Advan. Chem. Ser., 81, 585 (1968). (16) A. Brnjolfsson, ibid., 81, 550 (1968). (17) E.Storm and H . I. Israel, "Photon Cross Sections from 0.001 to 100 MeV for Elements 1 through 100," La-3753 (1987). (18) F. M. Charbonnier, Field Emission Corp., McMinnville, Ore., private communication. (19) E. Tochilin and N. Goldstein, USNDRL-TR-939 (1965).
The Journal of Physical Chemistry, Vol. 76,No. 16, 1971
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that the intensity of the radiation varied from the front to the back of a sample by a factor of 2.5 or less. In view of the reproducibility (8%) of the dose/pulse, uncertainties in the dose/pulse for each sample are probably no greater than 15%. The maximum instantaneous dose rate for the samples was calculated by dividing the dose/pulse by the effective time width of the pulse (19 nsec13).
16.0 -
20.0
Results The 100-eV yields of all the organic products at the three dose rates are summarized in Table I.20 At the two lower dose rates, the dose dependence of the yields was always linear up to the maximum dose of 2.40 X 1019 eV/g. Also, the maximum relative difference in the 100-eV yields determined in independent experiments varied by less than 8%. The results at 1.6 X 10I6eV/g-sec agree with those of Abramsonand Firestone determined at 1.3 X lo1*eV/g-sec and26OS5
6.0 -
8.0
Table I: Dose Rate Dependence of the 100-eV Yields in the Radiolysis of Chloroform a t ~ 2 5 ' ,-------
1.6 X lot@
Product
CHzClz cc14 CzCla CzHzCh CzHC16 CZC~S
100-eV yields---------l 1 7 X 1017 2.6-6.4 x 1024
eV/g-seca
eV/g-seca
ev/g-secb
1.9 0.89 0.085 0.73
1.2 0.86
0.2 A 0 . 1 0.94 f. 0.06 0.078 =k 0.08 1.5 f O . l 1.8 f . O . 1 0.85 3Z0.06
1.6
2.0
0.084 1.0 1.9 1.4
a Irradiated at 25 A 1". Uncertainties are estimated a t
