S. C. LIND
ate School with funds given by the Wisconsin Alumni Research Foundation. They are indebted to Professor Robert H. Burris and to the Thomas E. Brittingham Foundation for the use of the mass
spectrometer. The work has been greatly facilitated through the purchase of C14-labeled urea from the Isotopes Division of the Atomic Energy Commission.
DISTRIBUTION OF ALPHA RAY ENERGY IN RADIATION CHEMISTRY BY S. C. LIND Carbide and Carbon C h i c a l s Company, Oak Ridge, Tennessee Received Jdy 93, 196.9
Yield of acetylene polymerization in alpha particle bombardment of mixtures containing inert gases is proportional to the number of ions produced by the irradiation. Such excited atoms or molecules as are primarily produced by the bombardm a t apparently make no significant contribution to the yield.
It is well known that alpha particles expend only part of their energy in ionizing gases in which they are absorbed. Customarily it is assumed that the rest is spent in producing excitation. I n radiation chemistry it has further been postulated that the excited states contribute to chemical action in addition to that brought about by ionization. If in different gases the alpha ray energy divided itself in the same proportion between ionization and excitation it would be difficult to differentiate the two effects, especially since there is no direct method of measuring the number of excited states. But fortunately for our purpose the distribution of energy is not always the same, In the rare gases the excess of energy beyond the ionization potential, which presumably would be available for chemical action caused by excitation, varies from 3.2 e.v. out of a total alpha ray energy of 27.8 e.v. in helium to 8.7 out of 20.8 e.v. for xenon: and in ~.~ .~~~ nitrogen 17 out of 35 e.v. These differencesof energy distribution (Table I) when linked with the discovery‘ that ions of any inert gas mixed with a chemically reactant gas (or gases) produce the same amount of chemical reaction as the ions of the reactant itself, furnish an answer to the question of the contribution made by the excess energy to the chemical yield. For example, if in pure acetylene 20 C2H2 molecules are polymerized per C2H2+ion produced by alpha rays, in a mixture of acetylene and argon the ion yield is again 20 C2H2molecules but only if related to the total of C2H2+ A+ ions. The same is true for a mixture of acetylene with any of the other inert gases and evidently means that the inert gas ions have the same ability to cause polymerization of acetylene as do the C2H2+ions. Here we are not concerned with the mechanism of the reaction but the existing experimental data (Table I) clearly show that correlation is satisfactory for the ion yield, and that there is no relation to the excess energy; where it is highest (N2and A) the ion yield is somewhat low, where excess energy is low (He and Ne) the ion yields are slightly higher. The sixth column of Table I (Energy yield predicted) assumes that the excess energy contributes ~
(I) 5. C. Lind and D. C. Bardwell, Science. 62, 422, 593 (1925); J . Am. Clem. SOC.,18, 1675 (1926); S. C. Lind, “Chemical Effects of Alpha Particles and Electrons,” 1928, p. 189. 63, 310 (1926);
Gas8 He Ne Nt A
TABLE I* RAYPOLYMERIZATION OF ACETYLENE MIXEDWITH RAREGASES Total energy per !on pair (1) 27.8 27.4 35.0 25.4 22.8 20.8
Ioniiation potential (2) 24.68 21.56 15.58 16.76 14.00 12.13
Excess energy (1) (2) 3.2 6.9 18.0 10.1 9.8 8.7
Ion Energy yield predicted found From mixture
(1):(2) 1.13 X 19.8 = 22.4 1.27 X 19.8 25.2 2.24 X 19.8 = 44.3 1.6 X 1 9 . 8 = 3 1 . 8 1.63 X 19.8 = 32.3 1.72X19.8-34.0
19.7 19.2 18.5 18.2 19.5 18.0
Av. 31.4 18.9 Av. %dev. 16.6% 3 . 4 % Yield for CiHs alone 19.8
to chemical action in the same proportion as does the energy of ionization. While this may not be a fair assumption, any other that would bring the predicted energy yields into satisfactory agreement would probably be an ad hoc distribution arbitrarily chosen to fit each inert gas separately and without any experimental or theoretical support. Other gas reactions’ also sensitized by the inert gases under alpha radiation give similar results. Polymerization of acetylene was chosen for the present discussion on account of the wealth of experimental evidence and the perfection of the reaction kinetics. A solid product without vapor pressure leaves the acetylene gas without interfering by-product^.^ Excellent kinetics are easily attained, but only by taking into quantitative account the changing conditions for ionization in the gas mixtures in which the inert gas remains unchanged and consequently becomes relatively enriched as the reaction proceeds. (2) Table I waa included in an unpublished review of theories of radiation chemistry given at the International Congress of Pure and Applied Chemistry, New York, September, 1951. (3) Rutherford, Chadwick and Ellis, “Radiations from Radioactive Substances.” Cambridge University Press. New York, N. Y., 1930, p. 81. (4) Mund and Rosenblum ( J . Pfiua. Chm., 41, 469, 651 (1937), Bull. Soc. Chim. Bdp., 46, 603 (1937)) have found benzene t o be a product of the reaction when carried out in large volume of ficetylene with low intensity of alpha radiation. Under quite diffbrent conditions Lind and Bardwell observed no volatile products and obtained good kinetics from atmospheric pressure to practical exhaustion of CrHr (a few mm.). If benzene is formed under these conditions (small volume and high intensity of radiation) it must quickly be polymerized without diaturbing the kinetics.