RADIATION CHEMISTRY
611
The calorific value and gas-volume constants of a series of precipitated nitrocelluloses of varying nitrogen content have also been directly determined. It has been shown that the calorific value varies linearly with the nitrogen content of the nitrocellulose. The authors are indebted to Dr. H. Thomas for his assistance in preparing the data for this report. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) 10) 11) 12)
BERTHELOT, hl.: Sur la f o r c e des matibres e s p l o s i v e s . Gauthiers-Villars, Paris (1883). H.: Ezplosivestofle, p. 70. G. J. Goschen’sche, Leipzig (1909). BRUNSWIG, ESCALES, R . : Nitroglycerin und D y n a m i t , p. 169. Von Veit, Leipzig (1908). JAHRESRER. X . Chemisch-Techn. Reichsanstalt, p. 94 (1924-1925). KAST,H.: Spreng-und Zundstofle, p. 70. Friedr. Vieweg und Sohn, Braunschweig (1921). MACNAB, W.: Proc. Roy. Soc. (London) 66, 8 (1894). MILUS,P. R . : Ind. Eng. Chem. 29, 492 (1937). XA~UIM P. :, Nitroglycerin und N i l r o g l y c e r i n - S p r e n g s f o ~ ep., 133. J. Springer, Berlin (1924). W. H . : Ind. Eng. Chem. 18, 1196 (1926). RINKENBACH, ROSSINI,F. D . : J. Research Natl. Bur. Standards 22,407 (1939). E. AND VIEILLE,M . : Compt. rend. 9S, 269 (1881). SARRAU, E. W.: J. Research Katl. Bur. Standards 10, 525 (1933). WASHBURN,
RADIATION CHEMISTRY’, MILTON B U R T O P
Clinton Laboratories, Monsanto Chemical Company, Oak Ridge, Tennessee Received August 27, 1046 NATURE OF RADIATIOX CHEMISTRY
In classical chemistry the branch of chemical kinetics concerned with the interaction of ordinary energetic “emanations” and matter is called photo1 Summary of paper presented a t the Symposium on Nuclear Chemistry which was held under the auspices of the Division of Physical and Inorganic Chemistry a t the 109th Meeting of the American Chemical Society, Atlantic City, New Jersey, April 10, 1946. a This paper includes references to the work of many collaborators as well as others, both a t the Metallurgical Laboratory of the University of Chicago and a t Clinton Laboratories, Oak Ridge, Tennessee. S o n e of such work yet appears in the open literature. Material here reported is for the most part qualitative, and there are certain evident hiatuses of subject matter. Aneffort is made, however, t o give due credit by indicationin footnotes of the names of the investigators. Thus, when the literature does become freely available, the references may be identified. The list of names is regretfully incomplete, primarily because not all studies are here reported. It would be futile t o attempt an aoknowledgement of the contributions of D r . James Franck, who was a constant advisor and consultant
612
MILTON BURTON
chemistry. The emanations, photons, are of a type associated with transitions between energy states of the external electrons of atoms and molecules. The wave lengths of effective photons range from the near infrared for some photographic processes (Le., 10,000 A.) to a lover limit in the far ultraviolet (of the order of 1000 A,) for some absorption-spectra studies and for some investigations of ionic crystals. The corresponding energy range is approximately 1.2 to 12 e.v. In these cases, in spite of the fact that details are well understood in only a few of them, the primary processes involved are rather simple. The absorbing molecule is raised to an excited state from which various changes occur in times which are of the order of one vibration period sec.) or considerably longer. Dependent on the molecules, competition favors one or the other of processes which are distinguished in part by the time required for their completion. They are fluorescence,simple rupture, transfer of energy to another molecule or atom physically or chemically (e.g., photosensitization), and internal conversion associated merely with degradation of the energy state to successively lower electronic states. In the field of atomic energy we have a corresponding branch of chemical kinetics, concerned Kith the interaction of extremely energetic “emanations” and matter. In the atomic energy projects it has, for want of a better name, been called radiation chemistry. The “emanations” are of a type or energy ordinarily associated with nuclear transitions in either natural or induced radioactive or fission processes. By extension of this idea, radiation thus includes alpha and beta particles and gamma radiation (found in radioactivity), neutrons and fission recoils, and fast protons, deuterons, electrons, and x-radiation produced by various instrumental means. The energy range for these studies is determined by the interests of the investigator and by the radiation sources available. Some work has been done with 170-kv. cathode rays (11) and there have been studies with x-rays of somewhat higher corresponding energy. This is the lower end of the spectrum of radiations studied in radiation chemistryP The usual particles or radiations studied begin in that energy range for some slow betas and soft gammas, and extended up to 20 m.e.v. in studies of the effects of neutrons, betas, and gammas, and up to 100 m.e.v. for some of the fission recoils. While the particles (Le., photons) of photochemistry cause, in the first instance, only excitation of one molecule, those of radiation chemistry cause not only excitation but ionization of many molecules. The average energy required per ionization act for most substances studied in the gas phase is in the range 30 to 35 e.v. Except in chain reactions, usually no more than a few molecules are chemically converted per ion-pair produced. on most of the work. For the most part, references are to work performed in the Radiation Chemistry Sections a t Chicago and a t Oak Ridge. The locale of other work is indicated. 8 Present Address: Department of Chemistry, University of Kotre Dame, Notre Dame, Indiana. 4 However, investigations a t lower energies, e.g., on chemical effects of accelerated protons of 200-800 e v. (see I. Amdur and H. Perlman: J. Chem. Phys. 8, 7 (1940)) can be very useful for radiation chemistry in the interpretation of details of mechanism.
RADIATIOK CHENISTRY
613
In photochemistry we emphasize the r81e of the photon by reporting results in terms of the quantum yield, 7 . In radiation chemistry, there has been a tendency, somewhat justified in practice, to report yields in terms of the number of molecules converted per ion-pair produced-the so-called X/.V ratio (8). This method of expression neglects the fact that, in condensed systems certainly, the value of iV is essentially an assumed one based on the idea that the average energy, E,,, required per ionization process is precisely that determined for the gas phase, perhaps only of a similar (not even the identical) substance. Also, the X/AJ method of expression introduces a variety of scales of reference, depending on the various substances studied, for E., is different for different substances. Furthermore, its use suggests an idea which remains to be provedthat the ionization process is a t the root of all radiation chemical reactions. hlternative methods of representation of yields have been used in the Radiation Chemistry Sections: either G , the number of molecules converted or produced per 100 e.v., or, a form of expression favored by some, the energy in electron volts required to convert or produce one molecule. RADIATION SOURCES
The magnitude of the Plutonium Project made accessible for its purposes instruments which had simply not been previously available for the work of earlier investigators. Cyclotron sources of high-voltage deuterons were used at the University of Chicago and the University of Michigan, and of high-energy neutrons a t Washington University and the University of California a t Berkeley. Van de Graaff generators a t the Massachusetts Institute of Technology, the University of Chicago, and the University of Yotre Dame were also available; these were used as sources both of high-voltage x-rays and of electron beams. The latter type of instrument is rather interesting because of the monochromatic nature of its beam, and because of the high power levels a t which it is possible to operate. The x-ray instrument a t the Chicago Tumor Institute and the betatron a t the University of Illinois were also employed in a preliminary way. The need for this wide range of instruments will appear in the subsequent discussion. In addition, the pile itself (Le., the atomic energy plant) and products from the pile were both important sources of radioactivity. The flexibility of operations made possible by the availability of these sources is pointed up by the fact that a gram of radium, which only the rarer investigator might afford in the past, in secular equilibrium dissipates energy a t the rate of 0.12 watt, while the piles in actual use for the production of plutonium operate a t many thousands of kilowatts (13). Radioactive fission products from such piles were correspondingly useful, for they could be concentrated to give intensities of radiation from radioactive sources far exceeding any of the past. DETAILED EFFECTS O F HIGH-ENERGY RADIATION
I n the pile itself, all radiations associated nith atomic energy are potentially effective. Figure 1 shows the regions in a graphite pile in which the major
614
MILTON BURTOE;
part of the energy of each particular radiation is dissipated. Obviously, the very energetic fission recoils can conceivably act on the uranium alone, while fast neutrons can be important in many different places. In the subsequent chemical operations, neither fission recoils nor neutrons are present. The major effects, if any, are produced by beta and gamma radiations.
f AND
EB AI JACKET AI WEE a. FILE LATTICE
b. ENERGY DISSIPATION
FIQ.1. Graphite pile
FIQ.2. Possible primary and secondary processes The variety of possible effects produced by very energetic radiations is shown in figure 2.6 Only a few of the effects indicated may be considered outside the province of radiation chemistry. For the main part, the very confusion of the figure illustrates the complexity of the phenomena involved in the subject. 5R.
L.Platzman.
615
RADIATION CHEMISTRY
Casual perusal reveals that all the phenomena of photochemistry are necessarily present. Very energetic radiation produces not only excitation but also ionization and, as in the case of fast neutrons, sometimes simple elastic scattering. The phenomena ensuant on molecular excitation are no different in radiation chemistry from what they are in photochemistry. Since they are subjects which have been extensively, if not exhaustively, discussed in another setting, they will be touched on only lightly here. They are among the lesser complexities of radiation chemistry, knowledge of n hich is necessarily precedent to the understanding of any particular chemical reaction. Thus, while there are perhaps four or five cases of thermal and photochemical reactions whose mechanisms are thought to be tlioroughly understood, the best that can be said for the complicated processes of radiation chemistry is that some of their outlines are apparent. High-energy particles and radiations all produce ionization (as well as escitation) of the molecules or atoms with which they interact.6
AB
--
AB++e
The electron travels several hundred molecular diameters after its liberation before it comes to rest either to discharge a positive ion or to form a negative ion,
M+e+M\\here M may be AB or some other molecule. In water (Le., where M is H20) this may be an intermediate step but the over-all reaction is
e
+ H20 + aq
---f
H
+ OH-aaq
In general, after the primary acts in which electrons are released and trapped a t some remote point, the succeeding processes in which the ions concerned may become involved depend on their nature and their stability in their environment. The nature of the general environmental conditions surrounding ions requires prime consideration. At one time it was thought that the small mobility of gaseous ions in an electric field (unaccountable by simple kinetic theory assuming ordinary molecular diameters) indicated that in reality the diameters were abnormally large and represented a cluster of a central ion and associated neutral molecules held thereto by strong polarization forces. Ultimately, it was established that the phenomenon could be explained in terms of drag exerted on the ions by dipole forces set up in the molecules through whose neighborhood they drift (10). Meanwhile, however, the idea of clustering had given birth to the cluster theory of radiation chemistry. Lind noticed the many instances where M / A r exceeded unity and suggested the idea that energy liberated in neutralization of the ion caused dissociation of the surrounding cluster of molecules (8(b)). The preferred notion now is that small values of M / N inexcess of unity indicate merely the production of a few free atoms or radicals per ionization act (3), as well a$ a possible contribution by excited molecules. The symbol --+ chemistry.
in radiation chemistry has a significance parallel t o
hv
---f
.
in photo-
616
MILTON BURTON
The new point of view does not deny the existence of weakly held clusters (with energy of attachment of the order of thermal motion) (3) nor does it neglect the further possibility of two other significant phenomena. In certain cases, e.g., where AB or M may strongly solvate a dissociation product of AB+, the existence of clusters in liquid or in gas may affect the course of reaction considerably, yielding AB++M+il+*M+B as, for example, in water
+ aq
H20+
-+
H80+
+ OH
instead of the significant reaction where solvation does not occur: namely, AB+
or
AB+
+ e -+A + B
+ M-
-+
h
+B +M
(3)
Furthermore, there may be a slight distinction between a free atom or radical liberated (by neutralization of an ion) when clustering does not occur and when it does. In the latter case the atom or radical escapes through an envelope of oriented molecules. That there is a significance in this fact has not been established. The important point is that reactions of the class of reaction 3 constitute one of the important groups in radiation chemistry. The mechanism in this group of reactions is that radiation causes ionization and that ultimately a positive ion is discharged by an electron or negative ion. The electron moves to the positive ion in a time so short that the constituent atoms remain substantially undisplaced (Franck-Condon principle) ; the energy of neutralization thus appears as excitation of the products. In effect, some of the atoms or radicals are left in positions corresponding to energy states above the dissociation limit for the bond concerned. Dissociation results as in reaction 3. Dissociation of M itself in this process is unlikely. A complete discussion of all types of chemical phenomena associated with radiation-chemical processes is inappropriate here. It is proper to say that usual considerations of free radical chains apply and that, in radiation chemistry, there are phenomena of sensitization (Le., by ions) similar to photosensitization by excited molecules. Recombination of primary products (i.e., free atoms or radicals) plays the usual rBle. However, radiation-induced backreaction involving product molecules plays a more significant rBle in radiationchemical reactions than in other classes of reactions. In photochemistry it is frequently possible to choose radiation of wave length which will not affect the product. In radiation chemistry this procedure is never possible. Thus, a steady state dependent on the variety of circumstances of the investigation is usually to be found (particularly for inorganic systems) in radiation chemistry.
RADIATION CHEMISTRY
617
Energetic heavy-charged particles, such as alphas, deuterons, or protons, rarely make direct nuclear impact but cause ionization of the molecules with which they “collide.” As is generally the case for any incident charged particles, the electrons thus liberated move several hundred molecular diameters from the incident beam. However, such heavy particles cause a relatively large number of iors per unit path length. A deuteron of high energy would ionize about one in flve molecules in its track in water. Thus, for heavy-particle irradiation part of the primary products is concentrated in a restricted locale with an inhomogeneous distribution. An energetic electron or beta, unlike a heavier particle, ionizes about on the average one in five hundred molecules in its track in water. Consequently, the primary effects of fast electrons are diffuse and overlap between tracks, and, unlike the case for heavy particles, the distribution of primary products is essentially homogeneous both along and transverse to the beam. Gamma rays and x-rays interact with molecules to produce ions and energetic electrons. The latter, in turn, are responsible for the major portion of the observed effects. Thus, it is possible to simulate effects of gammas or x-rays by use of an electron beam from a Van de Graaff generator. However, there is an important difference. Whereas the half-thickness for I-m.e.v. x-rays is 12 cm. of mater, and the x-rays give up energy in any conveniently short distance (e.g., 1 cm.) fairly uniformly, an electron of the same energy penetrates only about 0.5 cm. and gives up most of its energy in an element of path somewhat below the surface. Therefore, where homogeneity of effect is essential, an electron beam cannot be used to simulate gamma rays. This consideration is not usually important, but for cases where it is, the Plutonium Project has sufficient radioactive fission product on hand to supply an adequate concentration of gamma or beta rays. Usually, however, instrumental sources suffice. For example, exposures may be made 2 cm. before the target of the Notre Dame generator a t a rate of x-ray dosage of 7 X IOs r./min. If a really high rate of energy dissipation is required, the direct use of the electron beam can increase it by a factor of IO5. Fast neutrons, which are not absorbed in the process, scatter the nuclei with which they collide. The amount of energy transmitted in an elastic scattering process is given by the relation AE/Eo = 2 A / ( A
+ 1)’
where A E / E o is the arithmetical average fractional decrease in energy of the neutron and A is the atomic weight of the element of the atom concerned. The form in which energy appears in an ejected atom depends on the energy and the nature of the incident particle; part may be kinetic and part may be in the form of excitation of its electronic system. In general, two types of impact? we involved in this effect: primary scattering by fast neutrons and secondary scetkring by scattered ions or atoms. The impact of the neutron and the effect of the neutron, unlike that of charged particle radiation, are exclusively on the nucleus. T f the energy of the incident neutron is low enough, the velocity
618
3IILTOK BURTON
of the ejected nucleus will be in turn sufficiently low so that its electvnnjc cloud will remain with it. For sufficiently high velocities of the incident neutron, the velocity of the ejected nucleus mill be such that it leaves one or more electrons behind it. The result may be that in the first case all of the energy of the neutron is transmitted as kinetic energy and, in the second case, the energy may be transmitted to a major degree in the form of electronic excitation or ionization. The threshold value for the latter process depends on the element concerned, and according t o our theory is by no means sharp. The diffuseness of the threshold confuses the effort to calculate a priori the number of atoms displaced by fast neutron bombardment. Resort to experiment is essential. The secondary processes involving the particle ejected by a fast neutron depend strongly on the mass of the particle. Figure 2 indicates the variety of effects produced by the fast ions in passage through a substrate. They may waste their energy in the production of excited molecules or they may produce ions with one or inore of the electrons removed. On the other hand, a fast neutron may possess only sufficient energy t o expel an atom uncharged from its lattice positlion;this atom may in turn by head-on collision displace other atoms from their lattice positions. Such a process involves maximum utilization of t,he kinetic energy of the neutron for production of displaced atoms. The probability of this ideal process decreases with decreasing size of the atoms involved. For small atoms the threshold energy above which a considerable fraction of t,he neutron energy is converted into excitation and ionization is quite small. In consequence, in the case of very light elements or compounds of such elements, the principal effect will be of a type not purely characteristic of fast-neutron effects. In mater, for example, the effect would be nearly like that of an incident proton beam. .4s the atomic \\eight of the element involved increases, the fraction of energy transmitted per collision decreases and the threshold below which the energy is transmitted nearly entirely as kinetic energy increases. The magnitude of the effect will obviously reach a maximum a t intermediate atomic weights which, comparatively speaking, are rather low. We shall return to n discussion of these effectsin connection with solids. DISCOMPOSITION IN SOLIDS'
The effect of fast neutrons on solids was first considered by E. P. Wigner in extenso in 1942. He showed that on theoretical grounds, fast neutrons should cause displacement of relatively light atoms from their lattice positions. Such discomposition is to be expected in the ease of all substances containing elements of lo\\ at,omic weight,. However, the chance of a back-reaction, in which the atom returns to its original lattice position or to a similar lattice position, will depend on the nature of the solid involved. In a compact solid of maximum density, it is possible that an ejected atom may come to rest in an interstitial 7 M . G. Bowman, S. G. Davis, J . Franck, R.I.Goldberger, S. Gordon, 11. C. Hirt, E. J. liochanadel, E . Leaf, R . J. Maurer, A . J. Miller, R . S . Mulliken, T. J. Neubert, A . Novick. It. A. Penneman, F. D. Rossini (Xational Bureau of Standards), J. Royal, F. Seitz, R . T . Schenck, E. Teller, A. R . Van Dgken, P. H . Yuster, and F:. P. Wigner.
R.4DIATION CHEMISTRY
ti 19
position, but that it will be under such great forces in that position that the application of only moderate energy may cause it to move to a more stable trape.g., a hole of a type created in the moment of its ejection. Another phenomenon which may exist in back-reaction is that of instantaneous annealing. We would expect that solids of low melting point, particularly those which are soft to begin with, might not display a detectable discomposition effect, even after prolonged irradiation with fast neutrons. Both the short space allotted and the security restrictions still in force at the time of preparation of this paper prevent discussion of the effects observed in various substances, and the characteristics of those effects.', At this time we can say merely that the effects are real and interesting, and provide a very fruitful field for further investigation. So far as the research work in radiation chemistry is concerned, discomposition is one of the most interesting effects detected. It was found, for example, that the electric resistance, elasticity, and heat conductivity of graphite all change with exposure to intense neutron radiation. COLORATION OF IONIC CRYSTALS
Another interesting effect produced in solids is illustrated in the case of lithium f l ~ o r i d e . ~The LiKisotope undergoes a reaction, 3Li6
+ om1
+ 2He4
+ 1H3
This reaction is by sloxv-neutron bombardment and is exothermal to the extent of 4.6 m.e.v. (9). All this energy is effective in the production of precisely the same types of changes in crystals as is produced by ultraviolet light or by x-rays or by electron bombardment-namely, the coloration of crystals, which was so thoroughly studied before and during the recent war by various groups associated with Hilsch and Pohl, Mott and Gurney, Schneider, Seitz, and others. The interesting thing here is that the existence of this neutron reaction exaggerates the effect produced in the crystal. Thus lithium fluoride, when exposed in the pile, gives a great variety of effects, a t least one of which is completely transient in nearly all other materials. The nature of the spectrum after lithium fluoride bombardment is shown in figure 3. The F and F' designations are according to the terminology of Hilsch and Pohl. The F absorption band is supposed to be characteristic of a single electron trapped a t a negative-ion vacancy, while the visible F' band is attributed to the looser binding of two electrons trapped in such a vacancy. In lithium fluoride, unlike most other alkali halides, the I"' band is fairly stable a t room temperature. The M and R bands are identified according to the terminology of Seite and his coworkers. 3f-centers are believed to be aggregates of two halogen-ion vacancies and one positive-ion vacancy to xhich an electron is attached. R-centers are pairs of halogen-ion vacancies to which one or two electrons are attached. I . Estermann, S . X . Foner, G . I. Kirkland, J . Iioehler, 0. Stern, and others of the Carnegie Institute of Technology. R . A . Penneman.
620 THE SPECIAL CASE OF WATER APiD AQUEOUS SYSTEMS”
In pure water and selected aqueoua solutions we have an illustration of the general class of effects of radiation on inorganic compounds containing mainly covalent bonds. The over-all reactions are :
+ HP HzO + $02
2H20
+ Hz02
Hz02
+
FIG. 3. Absorption spectrum of irradiated lithium fluoride
The situation in the vapor is not greatly different from that in the liquid. Using xenon as sensitizer, Gunther and Holsapfel (6) found M / N for the decomposition by small dosages of x-rays to be as high as 1.0 (i.e., G - 3 ) with clear indication that the yield was greatly decreased by back-reaction. In the liquid, the magnitude of the observable effects depends not only on the radiation but on the method of detection of the effects. When heavy-particle radiation is employed, there is no difficulty in detection of decomposition, but reported data correspond to yields for decomposition, G (decomposition), ranging from 0.07 (deuteron l o A . 0. Allen, M. G. Bowman, J. W. Boyle, W . R . Burns, C . V . Cannon. S. G. Davis, R. Fearing, J. A . Ghormley, S. Gordon, R . C. Hirt, C. J. Hochanadel, J.P. Howe, R . Livingston, A Iirueger, 4 . J. Miller, R . A . Penneman, E . Shapiro, L. Treiman, and M. Tetenbaum.
RADIATIOS CHEMISTRY
621
bombardment) where the products are kept in solution" to 2 3 (alpha bombardment), where the gas is evolved a t low pressure (2). With x-rays most investigators have failed to detect an effect, but values of G as high as 2 are reported when gas is evolved a t low pressure (7). In the Plutonium Project it has been found that G (decomposition) on electron bombardment in a burst of 0.01 sec. is 0.5, based on hydrogen peroxide determination, n i t h no oxygen formed. On the other hand, in continuous bombardment a t low pressure large amounts of gas, including oxygen, are evolved and G (decomposition) is initially 0.3, decreasing to zero a t relatively low gas pressure,'? depending on the intensity of irradiation. The effect of fission recoils on pure water is not readily determinable. Hovever, studies made in uranium solutions indicate an initial G (decomposition) value of 3. All the results are interpretable on the basis of the ideas of the primary effects already set forth. They are summarized by the forward reactions
HzO +HzO+
+ eH30++ OH
HzO+
+ aq
+
(4) (5)
and at a remote point
H20
+ aq + e
H
-+
+ OH-.aq
(6)
+ aq
('7)
as vel1 as the direct reaction in acidic system
HaO+
+e
--f
H
The forward reaction 2H + H, occurs on every collision, but the reaction
(8)
2 0 H -+ HzOz proceeds with some activation energy, E., and consequently conditions for the production of hydrogen peroxide will be most favorable in those special regions where OH exists in unusually high local concentration. This condition occurs most favorably in the ion track of heavy particles. The back-reaction
H
+ OH
+ HzO
(10)
occurs with no E,, and the recombination reaction
+
+
Hz OH + HzO H (11) nith but small E,. Thus conditions which remove product gas instantaneously or favor reaction 9 and disfavor reaction 11 increase G (decomposition). The former favorable effect is seen in those cases of electron and x-radiation where the product gas is rapidly removed. Reaction 9 occurs most favorably on heavy'1 12
A Iirueger. J .4 Chormley
622
?JILTOX BURTOK
particle irradiation. When the exposure is rapidly interrupted, adequate tinir is provided for removal of hydrogen while OH is still a t a lo\\- concentration, thus disfavoring reaction 11. In a detailed discussion of the influence of radiation on water, the effect of H 0 2 , formed by the action of free I T on O?, must be considered. In general, it must be realized that in aqueous systems containing lo\\. concentration of solute or suspended (biological) material, the significant primary effectsobserved on the other component are those of the free atoms and radicals (including HO, when air is present or, after the init,ial stage, when oxygen has been produced). ORGANIC MATERIALS”
Organic materials quite naturally have engaged the interest of many invcstigators far prior to this project. For example, the work of Schoepfle and Fellows with 170-kv. cathode rays (1 1) developed several significant points. In general, TABLE 1 Irvadiation of w r i o i i s hydl.ocarbons wath 170-kv. cathode rays (Schoepfle and Fellows: Ind. Eng. Chem. ‘23,1396 (1931)) CAS OBTAINED IN 30 XIN.
HYDPOC.4PBON
LC.
n-Hexane.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............................................. n-Octane.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . w D c c a n e . ,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 n-Tetradecane, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~
~
~
57.6 51.4 48.3 41 .6 34.9
aromatic compounds yield less gas than do saturated aliphatics, while unsaturated aliphatics yield an intermediate quantity. Data on similar substances of varying molecular veight indicate a “cage effect” of magnitude increasing with molecular size. Escape of large molecules from the cage becomes less probable and consequently there is less decomposition. However, this interpretation of the effect of molecular size must not be accepted too unwarily. An alternative explanation is given by a nev principle involving the ionic discharge equation (equation 3) already discussed. The larger the molecule (AB) the less difference there will be between the atomic configuration of the ion and that of the uncharged molecule and the more likely will be discharge without decomposition. Table 1 illustrates the magnitude of this effect. The effect of molecular configuration on the nature of the products is shown in some similar studies on octanes by the same authors. Table 2 shows a decrease of hydrogen yield with increasing complexity, iyhile the yield of methane increases with number of methyl groups, The decrease in yield of hydrogen and increase in yield of non-volatile substances were not understood. Franck has ,J. W. Burr, R . A. Day, J. V. Flanagan, W. M. Garrison, .4. H. Germany, C. J. Hochanadel, A . J. Miller, and R. Schlegel.
G23
RADIATION CHEHISTRY
now suggested a n explanation based on increased probability of internal conl-ersion with molecular complexity (see introductory remarks on photochemistry). With increased probability of internal conversion there should be less hydrogen production (which takes place largely via free atmomsplits) and greater opportunity for increased yield of large molecules (not radicals) as primary decomposition products in a manner similar to predissociation by rearrangement in photochemistry. Measurement of the yield of gas alone tends to be a deceptive measure of the amount of chemical change in organic compounds. Table 3 summarizes briefly some detailed studies on the effect of 2.5 t,o 2.8 m.e.v. electrons on a selected variety of substances. The gas yield \vas substantially all hydrogen. TABLE 2
-
Irradiation of some octanes with i70-Lu. cathode rays (SchoepAe and Fellows: Ind. Eng. Chem. 23, 1396 (1931)) TOIAL
OCTA>T
LC.
n-Octane. . . . . . . . . . . . 2 , %Dimethylhexane. . . . . . . . 2.2.1-Tritnethylpentane.. . . . . . . . .
H?
G4S
48.3 49.8 50.3
1
~
___~
~
rc.
I
38.8
NON-~OLATILE __._
-
cc
.
1.4
8.1
i
5.8 7.6
23.0 25.1
cc,
I
21.0 17.6
-
CHI
~
TABLE 3 Eflect of fast-electroia irradiation on liquid iiyrlrocarbons* COIIPOUFD
Benzene. . . . . . . . . . . . . . . . . . . . . . TL-Heptane . . . . . . .I Cyclohcsnnr. . . . . . . . . . Cycloherene.. . . . . . . . M e l hylcyclohe~anc . . . . . . . ..I Tolucnc. . . . . . . . . . . . . . . . . ~ ~
~
~
~____
0.04 4,2 4.0 1 .0
4.5 0.09
~
-I
I
0.5 1.7
1.2
I
4.2 4.2 0.i
-
* J . V. Flanagan, C. J. Hochanadel, and R . A . Penneman.
The results are not, altogether startling. ..Is might have been expected, the lon. yield of gas in the aromatic compounds did not completely represent the situation. Polymerization of a considerably greater order was simultaneously occurrent. The result confirms a notion that the low yield of gas in the case of benzene is in part resultant from the frequent cloFely adjacent formation of C&, and CsH, after the process of ionic discharge. The latter radicals may enter into "polymerization reactions" with each other as well as unconverted benzene molecules. The benzene case will be discussed in greater detail in it subsequent paper. In general, it appears from table 3 that unsaturation tends to decreabe the gas yield and to increase the degree of polymerization, vhile introduction of a
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single aliphatic group into the benzene ring boosts the gas yield markedly. Cnsaturated aliphatics tend to give lower gas yields, for the simple reason that unsaturated bonds tend to capture free atoms and radicals. This fact may be used to reduce gas yields in certain instances where hydrogenous materials are required but where gas production might be undesirable. Recently, a group at the Massachusetts Institute of Technology (12) has been studying the effects of radon alphas and high-voltage deuterons on the decomposition of fatty acids. From the point of view of the simple principles with which this paper is concerned, their most important observation \\as the relative simplicity of the products from larger molecules. For example, hydrogen, carbon dioxide, carbon monoxide, and methane are all produced in approzzmately equal amount in the decomposition of acetic acid. When palmitic acid is reached, only the first two are among the major gaseous products; the liquid product is principally n-pentadecane. A naive examination of the problem would have led to the expectation of an increasingly wide variety of products with increase in size and complexity of the parent molecule. The converse phenomenon is, however, to be expected generally in radiation chemistry phenomena. The principle involved in cases such as these is that already suggested in the interpretation of the results of table 1. The larger the molecule AB involved in reactions such as 3, the more closely does the atomic configuration of AB correspond to that of AB+. As a result, although after reaction 3 the molecule AB usually contains sufficient excess energy for rupture of one or more bonds, the energy is insufficiently localized for such rupture to occur within one vibration period. For rupture into free radicals or atoms energy must localize in the bond in a typical predissociation process (l),which may require 10-lo sec. or more. There are two results consequent on this fact. In the first place, the process of relocation of the potential energy of the molecule (and ensuant rearrangement of the atoms) may lead to decomposition to ultimate molecules before rupture can occur. There is an increasing body of evidence that this is a very important process in many cases where free-radical decomposition had been assumed as the only important mechanism (cf. 4, 5). When rearrangement decomposition is concerned, it usually occurs by a preferred path, e.g., to form carbon monoxide and alkanes in decomposition of aliphatic aldehydes and ketones, and, apparently, in the decomposition of fatty acids to form carbon dioxide and the largest possible alkane in the primary act by the straightforward process RCOOH -+ RH
+ COz
Another factor requiring consideration is the effect of liquid state upon the yield. Since internal conversion of energy and the ensuant predissociation process take > 10-'0 sec. and collision in the liquid occurs practically in every vibration period (-10-13 sec.), there is a good probability that energy mill leak from the excited molecule without decomposition. This is an old and wellunderstood phenomenon in reaction kinetics and photochemistry. It plays an important r81e in the radiation chemistry of large molecules because, to a greater extent than in the other cases, the potential energy is not localized in any part
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625
of the molecule during the excitation process. Thus, decomposition yield in radiation-induced reactions of large molecules in the liquid state is considerably reduced and this reduction in yield occurs to a greater extent with increase in molecular size. Consequently, we see well illustrated in radiation chemistry the principle of the effect of molecular size. It leads to the twofold conclusion that increase of molecular size decreases yield and increases the ratio of the relative probabilities of ultimate molecule decomposition by a preferred route and of free-radical decomposition by a variety of (almost indiscriminate) routes. SUXMARY
In the atomic energy pile and in the subsequent processes for separation of plutonium and fission products from parent uranium, quantities and intensities of radiation far exceed those from any previously known natural source. The term “radiation,” as here used, includes also high-energy particles; the radiations whose chemical effects had to be determined in advance of operations included betas, gammas, fast neutrons, and fission recoils as well as others. Sources of radiation used in the work included cyclotrons, Van de Graaff generators, betatrons, x-ray machines, and piles. A new discomposition effect in solids was discovered. Typical results on solids, water, and organic compounds are reported. The first important step (other than simple excitation) in radiation chemistry processes is ionic discharge. The ensuant chemical processes depend on the nature of the medium. There are three principles which seem to govern: the Franck-Condon principle, the principle of increased probability of internal conversion with increase in molecular complexity (Franck), and the principle that the ionic configuration is more nearly like that of the uncharged molecule the greater the size of that molecule. Illustrative examples are cited. REFERESCES (1) Cf.BURTOX, AI., A X D I ~ O L L E F SG. O NK.: , J. Chem. Phys. 6, 416 (1938). W.,AND YCHEUER, 0 . : Le radium 10, 33 (1913). (2) DUAYE, H.S . : J. Chem. Phys. 4, 479, 570 (3) EYRISO,H.. HIRSCHFELDER, J. 0 . . . ~ N D TAYLOR. (1936). (-I) Feldman, h l . II., BURTOX, M.,RICCI,J. E., A X U DAVIS, T. W.:J. Chem. Phys. 13, 440 (1945). ’ (5) GARRISON, w.&I.,A S D BURTOS,AI.: J. Chem. 1’hyS. 10, 730 (1942). (6) G C X T H E R , P., A K D HOLZAPFEL, L . : Z.physik. Chem. B42, 346 (1939). (7) GUNTHER, P . , A N D HOLZAPFEL, L.: Z . physik. Chem. B49,303 (1941). (8) LIKD,S.C . : The Cherriical Eflects of A l p h a Particles and Electrons, 2nd edition, The Chemical Catalog Company, Inc., Kew York (1928): (a) p . 100; (b) Chap. 11. (9) LIVISGSTON, M. s., AXD H O F F M A N , J. G.: Phys. Rev. 60,401 (1936). (10) LOEB,L . B.: Fundamental Processes ofElectrica1 Discharge in Gases, Chap. 1. John Wiley and Sons, Inc., Yew York (1930). (11) SCHOEPFLE, C. S., ASII FELLOXS, C. H.: Ind. Eng. Chem. 23, 1396 (1931). (12) Cf.SHEPPARD, C. W . , . \ X U WHITEHEAD,W . L . : Bull. Am. hssoc. Petroleum Geol. 30, 32 (1946). H. D . : Atomic Energy f o r .WilztarI/ Purposes. 6.41. p . 104. The Princeton (13) SMYTH, University Press. Princeton. S e v Jersey (1045).
