1364
CHARLES E. MCCAULEY AND ROBERT H. SCHULER
Vol. 62
EFFECT OF RECOIL ENERGY ON THE CHEMICAL CONSEQUENCES OF Tu UCLEAR ACTIVATION PROCESSES. ACTIVATION OF SOLUTIONS OF IODINE I N HYDROCARBONS1 BY CHARLES E. MCCAULEY AND ROBERT H. SCHULER Contribution from St. Peter’s College, Jersey City, N . J . , Brookhaven National Laboratory, Upton, N . Y., and Mellon Institute, Pittsburgh, Pa. Received February 8, 1068
Comparative studies of the chemical consequences of activation of the halogens by the (n,?) and (n,2n) processes have been extended to solutions of iodine in hydrocarbons. I n contrast to the alkyl iodide system where no difference in chemistry between the two modes of activation was observed, (n,2n) activation results in this case in an appreciably higher organic yield. The observed increase in organic activity is attributed to the greater local disruption caused by the more energetic recoil atoms.
For a number of years various investigators have considered the possibility that yields of various products from hot atom reactions in organic systems might be dependent on the recoil energy given to the activated nucleus.2 Attempts have been made to determine the extent to which organically bound activity depends on neutron energy in the case of radiative capture by bromine as propyl bromide.3 , 4 Experiments on various organic brom i d e ~and ~ ~ other ~ , ~ substances such as triphenylstibine7 also have been carried out to compare the organic yields for different isotopes produced in the same sample by neutron capture processes. Certain of these experiments3B5Jappear to indicate that differences in the behavior of different isotopes can exist while other^^-^ suggest that such isotope effects are usually not very large. It appears to be extremely doubtful that small changes in the recoil energy of the atoms concerned can have any significant effect on the resulting chemistry and that observed differences are probably due to some other cause. Direct comparison of the chemical effects of different types of activation processes each of which produces a product isotopic with the starting material are few. Such studies include the work of McCallum and eo-workers on the comparison of the copper activityproduced by the ( y - n) and (n - y) processes in copper salicylaldehyde-o-phenylene diamine complexes8 and of chlorine in sodium
~ h l o r a t e ,the ~ work of Melander and Slatis on uraniumlO and the recent work of Suryanarayana and Wolf reported in this symposium.l1 A summary of other early work on the chemistry of (d,p), (y,n) and (n,2n) produced species has been previously given.12 More recent investigations of a similar nature include studies of the (y - n) reactions in carbon13 and (n,2n) reactions in fluorine. l 4 Comparison of the chemical fate of atoms activated by the (n,y) and (n,2n) processes has been the subject of a recent series of investigations carried out in our l a b o r a t o r i e ~ . ~ JIt~ ~has ~ ~ been shown that in the pure alkyl iodides the chemistry is unaffected by the difference in the nuclear processes and in particular by the greatly increased recoil energy available in (n,2n) activation. I n the propyl bromides however significant increases in the organic yields have been shown to accompany the change in nuclear process. Preliminary experimentsla on solutions of iodine in cyclohexane have indicated an analogous increase. A more detailed investigation of the iodine-hydrocarbon systems is the subject of this present contribution where the 25 minute 1 1 2 8 isotope produced by capture of both isothermal and fast neutrons and the 13 day tope produced by the (n,2n) reaction have been studied.
Experimental The hydrocarbons for this investigation were Phillips Research Grade except for a number of experiments with cyclohexane where the samples were prepared from recrystallized stock material. Solutions were made up to known concentrations with iodine and were air saturated. After activation, the solutions were divided into two portions, one of which was extracted with sodium thiosulfate solution. The activity of each of these portions was measured and the organic yield taken as the ratio of the organic to total activity. Appropriate corrections were made for isotope decay and counter dead time loss.
(1) Supported, in part, by the U. 8. Atomic Energy Commission. Presented a t the Symposium on Chemical Effects of Nuclear Tpansformations a t the 132nd Meeting of the American Chemical Society, New York, N. Y., September 9, 1957. (2) For a review of early work, see J. E. Willard, Ann. Rev. Nucl. Sci., 3 , 193 (1953). The term yzeld is used in the present work to represent the fraction of activated atoms which is observed in a particular chemical form. The term retention has been previously used to describe this experimentally measured quantity for bound organic activity. It has of course long been recognized that for most hot atom processes the chemically combined activity does not (9) K. J. McCallum and 0. G. Holmes, Can. J . Chem., 29, 691 represent “true” retention but rather recombination reactions which (1951). ’ result in the formation of stabile products. (10) L. Melander and H. Slatis, Phys. Rev., 74, 709 (1948). (3) P. C. Capron and L. J. Gilly, J . Chem. Phys., 52, 505 (1955); see also previous references to work of Capron and co-workers. 62, 1369 (11) B. Suryanarayana and A. P. Wolf, THIS JOURNAL, (4) J. C. W. Chien and J. E. Willard, J . A m . Ckem. Sac., 76, 4735 (1958). (1954). (12) R. H. Schuler, J . Chem. Phys.,. 22, 2026 (1954). (5) F. S. Rowland and W. F. Libby, J . Chem. Phuls., 21, 1495 (13) L. J. Sharman and K. J. McCallum, J . A m . Chem. Sac., 77, (1953). 2989 (1955). (14) A. H. W. Aten, B. K,och and J. J. Kommrtndeur, ibid., 77, (6) R. H. Schuler and C. E. McCauley, J . A m . Chem. Sac., 79, 821 f 1957). 5498 (1955). (7) R. M. S. Hall and N. Sutin. J . Inarg. Nucl. Chem., 2, 184 (15) C. E. McCauley, G. J. Hilsdorf, P. R. Geisler and R. H. (1956). Sohuler, ibid., 78, 3246 (1956). (8) 0.G.Holmes and H. J. MoCallurn, J . Am. Chem. #DE,,72, 5319 (16) C. E. MoCauley and R. H. Sohuler, J . Chem. Phys., 25, 1080 (1950). (1956).
c
Nov., 1958
ACTIVATION OF SOLUTIONS OF IODINE IN HYDROCARBONS
Thermal neutron activations were carried out in the biological facility'? of the Brookhaven reactor a t a flux of 6.5 X 108 neutrons/cm.a/sec. These neutrons are very highly thermalized (cadmium ratio of 3,000 for a gold activation) and are accompanied by a minimum 7-radiation field (100 r.(h.). Samples were irradiated for from one to twenty-five minutes in quartz ampoules. The available flux permitted sufficient activation to enable measurements t o be made at concentrations down to M iodine. Fast-neutron irradiations were carried out at the Brookhaven BO-inch cyclotron as described previously .12 In these experiments in addition to the I l a 6 and IlZ8activities produced in iodine, a significant yield of 20 minute Cl1 activity results from (n,2n) reactions in carbon. I n general, neutrons having energies up to 24 MeV. were produced by bombarding a beryllium target with a 50 microampere beam of 20 MeV. deuterons. Irradiations were limited to 25 minutes in the experiments where it was desired to make measurements on both 25 minute and 13 day isotopes. Because of the relatively short irradiation period difficulties due to the radiation decomposition of the system such as suggested by Chien and Willard18 which might lead to an isotope effect as an artifact appear to have been eliminated. In experiments where measurements were only on longer irradiations were made (up to 2 hours) in order to increase the activity levels. I n the heptane series several experiments were performed in which external moderator was carefully eliminated from the vicinity of the sample. I n these cases, the sample was exposed to a minimum thermal neutron flux and the fraction of 1128 produced by capture of fast neutrons maximized. By surrounding the target with 3-4 cm. of water i t was possible to increase the 1'" production by a factor of 3. I n these experiments, because the threshold for disintegration of carbon is quite high, the C11 level was reduced by a factor of 2 while the Ila6level, which has a milch lower production threshold, was virtually unaffected. The IlZ8activity produced in the thermal neutron experiments a t the reactor was measured in an annular jacketed Geiger counter. In the case of P a , because of the low activity level, measurements were carried out in the same counter. No interference from the production of other isotopes from iodine is expected since in the experiments on the alkyl iodides it had been shown that a pure 13.1 day activity was observed corresponding to the half-life of Pea. Because of the production of Cl1 in addition to IlZ8in the fast neutron experiments, it was necessary to employ 7-ray spectrometry to distinguish these two isotopes. This was accomplished by using a sodium iodide scintillation detector and single channel pulse-height analyzer in a manner quite similar to that of the previous work on the bromide systems.6 Pulse height spectra for each of the pure isotopes in the region of the photo peaks are illustrated in Fig. 1. These photo peaks correspond to the 0.41 MeV. 7-radiation from 1'" and the 0.51 positron annihilation radiation from C11. In the experimental determination of the organic yields consecutive counts were taken in the two channels indicated in the figure. This data, together with knowledge of the relative counting efficiencies for each of the isotopes in these channels, permitted solution of the necessary equations to give the activity for each component. Appropriate corrections were included in the coefficients of the simultaneous equations in order to account for decay of the isotopes. This pattern of measurement was repeated a number of times for the organic and total activity and the iodine contributions were corrected to a common time. General agreement within the expected statistical fluctuations showed that the peak ascribed to the iodine did indeed decay with the 25 minute half-life. It was found that approximately equal yields of C11 and IlZ8 were produced a t an iodine concentration of 2 X lo-* M in the experiments where external moderator had been eliminated.
Results Thermal Neutron Activation.-The results of the experiments with highly thermalized neutrons are illustrated in Fig. 2. The low radiation level (17) H. J. Curtis, S. R. Person, F. B. Oleson, J. E. Hempel and N. Delihas, Nucleonzcs, 14, No. 2, 26 (1956). (18) J. C. W. Chien and J. E. Willard, J . A m . Chem. Soc., 77, 3441 (1955).
1365
t
Fig. 1.-Pulse
height spectra for 7-radiation from curve, and from C1l, broken curve.
1128,
solid
50 0
e
I
0-5
lo-*
[Iz]
I
I
10-3
10-2
I
I
-Molar,
Fig. 2.-Organic yields for produced by capture of highly thermalized neutrons in iodine. Solutions in research grade cyclohexane, 0 ; he tane, m; 2,2,4-trimethylpentane, A; and recrystallized cycibhexane, 0.
present in these measurements causes only an insignificant increase in organic activity. From the previously measured radiation yields for these systemslg an increase of the order of 0.02y0 is estimated for a 25 minute irradiation of an air saturated solution 10-5M in iodine. I n the case of degassed solutions, however, this increase does become appreciable at low concentrations of iodine (ie., several per cent. a t M). Experiments on the benzene-iodine system gave organic yields in the region of 1520% but with very poor reproducibility. The results varied by as much as 4% at a given concentration and suggest peculiarities similar to those observed in the radiation chemistry of the same system.20 Radiation Effects in Fast Neutron Experiments, -In experiments carried out at the cyclotron, the samples absorb an appreciable amount of energy from the fast neutron beam. An apparent increase in organic yield will therefore result from the reaction of radiation produced radlcals with the iodine which has been made radioactive. Since the radiation chemistry of these iodine-hydrocarbon systems has been examined in detail,lgit is possible to calculate the apparent increase from a knowledge of the radiation dose to which the sample had been (19) R. W. Fessenden and R. H. Sohuler, ibid., 79, 273 (1957). (20) A. T, Fellows and R.H. Schuler, to be published.
CHARLES E. MCCAULEY ANI) ROBERT H. SCHULER
1366
I
I I
0 .
I 1
I
I
4
3
[4]-*M x lot.
Fig. 3.-Increase in organic activity due to radiation induced reactions. These are estimated for a one hour cyclotron exposure of solutions of iodine in cyclohexane.
I .' I
\'
Vol. 62
quite appreciable (up to 19%). In actual experinients, the increase in organically bound activity is less than the observed fractional uptake of iodine in the I i 8 1 experiments due to two significant factors. The first is that the average specific activity of the free iodine at the moment of reaction is only one-half its terminal value and the second is that only 40-60% of the activity produced is in the form of free iodine, the remainder being unavailable for scavenging reactions. These combine to lower the apparent increase in organic yield by a factor of from three to four from that indicated by the Ii31 data. The increase calculated for an exposure of 2.5 X lo1* e.v./g. (corresponding to one hour exposure at maximum intensity) is given in Fig. 3 for both degassed and air-saturated cyclohexane. It is seen that at concentrations above 3 X 10-aM this increase will be less than 0.5% for exposures of 25 minutes or less. At lower concentrations the contribution due to radiation induced reactions becomes quite significant. This unfortunately vitiakes any attempt to increase the irradiation in order to extend the IlZ6measurements to the low concentration levels at which limiting values of organic yields were obtained in the thermal neutron studies. Approximate corrections for the increase in the organic activity have been made where necessary. TABLE I
I
I
I
nz)-; x IO2.
I
1
3
4
Fig. 4.-Organic yields of 1128, solid points, and open points for cyclohexane solutions. Circles represent experiments in which data were obtained on both isotopes. Flagged square a t 1 X lo-* M represents one experiment with neutrons produced in a lithium target.
RADIATION-INDUCED UPTAKEOF IODINE IN CYCLOHEXANE DUETO FASTNEUTRONBOMBARDMENT 11 concn., M x 104
Obsd." reaction, %
Energy input,
Gb
x
e.v./g.
1018C,d
12.3 0.5 2.7 1.4 16.5 1.3 3.4 3.5 7.1 19.5 2.1 5.1 3.8 3.3 1.7 19.4 " For 65 pa. hours bombardment of deuterons on beryllium in geometry used for activation experiments. Solutions airsaturated. P,Assumed: cf. reference 19. Energy input = M X f/G X 100, x N/780 where M is molar concn. of 12; f is observed fractlonal reaction, G is assumed yield, N is Avogadro's number; 780 grams/liter for cyplohexane. Energy input is for 65 pa. h. bombardment. This has been estimated as 3.4 X 10l8e.v./g. from ferrous sulfate dosimetry under similar conditions.
Fast Neutron Experiments.-The results of the experiments carried out at the cyclotron are illustrated by the data of Fig. 4 (cyclohexane), Fig. 5 (heptane and 2,2,4-trimethylpentane) and Table , 30 I 2 3 4 ' I1 (benzene). The solid circles and triangles in the (1,) -M X IO*, figures represent the yields for 1 1 2 6 obtained in Fig. 5.-Organic yields of IL28, solid points, and IlZ8,open measurements where values for 1 1 2 * also were depoints, in heptane, circles and squares, and 2,2,4-trimethyl- termined in the same sample. The indicated pentane, triangles. differences in these measurements are therefore exposed. This dose has been estimated from ex- quite significant. Additional measurements on periments utilizing the Fricke (ferrous sulfate) 1 1 2 6 activity in cyclohexane and heptane solutions dosimeter and also from experiments in which 113' are given by the solid squares. One point at 1 X had been added to cyclohexane before irradiation. 10-2M is given for 1 1 2 6 activity produced in cycloRepresentative data for a number of cyclohexane hexane with the lithium target system used by samples simultaneously irradiated by a 65 pa. h. Suryanarayana and Wolf.ll No difference is obbombardment are given in Table I. A variation by served between this and similar data obtained using a factor of 3 in the exposure at different positions is a beryllium target. I n Figs. 4 and 5 it is seen that except for one exreflected in the results of this table. It will be noted that the fraction of the iodine which reacts at periment with 2,2,4-trimethylpentane the exthe lower concentrations during such an exposure is perimental points obtained with non-thermalized I
L
ACTIVATIONOF SOLUTIONS OF IODINE IN HYDROCARBONS
Nov.? 1958
1367
TABLE I1 Comparative Yields for (n,?) and (n,Zn) Proccomparison of the organic yields of OF IODINE IN BENZENE esses.-Direct ORGANIC YIELDSFOR SOLUTIONS Retention, % '
I? ooncn., il.I x 102
I128
1126
AR/Rin
1.7 3 9 7.7
17.7 19.5 18.6
21.4 22.0 19.8
0.20 .13 .07
neutrons lie above the curve taken from Fig. 2. Since the experiments are, in general, highly reproducible, these differences though small appear to be real. Moderation of the neutron beam to a slight extent makes no apparent difference in the values obtained. This is particularly exemplified by the data for heptane where specific experiments were carried out in the absence of moderator to examine any possible neutron energy effect. In the case of benzene, non-reproducibility similar to that observed in the thermal neutron experiments precludes any comparison of organic yields observed in different experiments. However, internal comparison of the bound 1128 and I l 2 6 activities in the same sample should be significant. The data given in Table I1 indicate that an appreciable difference in organic yield accompanies the change in activation process. Discussion Concentration Dependence of Organic Yield.While a number of investigations of the chemistry following nuclear activation of the halogens in various hydrocarbon media have been carried out,21-28these studies have been largely restricted to relatively high solute concentrations. Little information is available on the concentration dependence of the reactions although Willard and coworkersz6have studied the dependence of the yield of organic activity upon the concentration of added halogen for 0.5 M alkyl halide solutions in hydrocarbons. The dependence observed in the present work is qualitatively of much the same nature as found in the above-mentioned studies and in the work on the pure alkyl iodides.12J7 I n Fig. 3, this concentration dependence is ilhstrated on a logarithmic scale in order to emphasize the approach of the organic yields to a limiting value at low iodine concentrations. Below 10-8 M the scavenging processes apparently are unaffected by the addition of halogen t o the system. Further work is, however, in order since in general the dissolved oxygen in the present studies provides an alternate scavenger which is present at a fixed concentration. I n general for work at low concentrations the iodine-hydrocarbon system possesses considerable advantage over the alkyl halide or alkyl halide-hydrocarbon systems because production of scavenger by radiation decomposition reactions is avoided. (21) A. F. Reid, Phvs. RBU.,69, 530 (1946). (22) L. Friedman and W. F. Libby, J . Chem. Plays.,17, 647 (1949). (23) J. M . Miller and R. W. Dodson, ibid., IS, 865 (1950). (24) S. Goldhaber, R. 9. H. Chiang and J. E. Willard, J . A m . Chem. Soc., 73, 2271 (1951); S. Goldhaber and J. E. Willard, ibid., 74, 318 (1952). (25) J. F. Hornig, G. Levey and J. E. Willard, J . Chem. Plays.,20, 1556 (1952). (26) 6. Aditya and J. E. Willard, J . A m . Chem. Boc., 79, 3367 (1957). (27) G. Levey and J. E. Willard, ibid., 74, 6161 (1952~.
the radioactive species produced both by (n,2n) and by (n,?) reactions in iodine-hydrocarbon systems shows that the values for the (n,2n) process are considerably higher. No such difference was observed to exist for a similar direct comparison in the case of n-propyl iodidela or for the general comparison between the results obtained for the other alkyl iodides.lb The dependence of the 1128 organic yield on iodine concentration is qualitatively of the same nature as that observed for radiative capture of thermal neutrons. The observed difference beactivities tween the organic yields for and 1128 decreases as the halogen concentration is increased. A similar trend of this difference was found in the case of solutions of bromine in n-propyl bromide.s Unfortunately, as mentioned above, the low levels of (n,2n) activation together with complications caused by C1l production make it impossible to carry out significant comparative measurements at halogen concentrations below 5 X low3M. It is not possible therefore to determine whether the IlZ6 yields approach a limit at low concentrations although a logarithmic plot of the data for cyclohexane indicates a trend toward such a limit at a value about 10% higher than observed at 1 X M. It appears therefore that there is a difference of about 15% in organic yields between the (n,2n) and (n,?) reactions in iodine-cyclohexane solutions at very low concentrations. Neutron Energy Eff ects.-Determination of the presence of a neutron energy effect involves comparison of results from different irradiations and is thus inherently open to some suspicion. The reproducibility of the results for iodine-hydrocarbon systems is, however, qnite good and the possibility of significant radiation induced reactions has apparently been eliminated. Impurity effects appear to be minor. It seems that the difference between the results for radiative capture of fast and thermal neutrons indicated in Figs. 4 and 5 is probably real though small. Since in radiative capture only 1/128th of the kinetic energy of the captured neutron is imparted to the activated nucleus it is necessary that this energy be of the order of 10,000 e.v. or greater in order that the energy of the iodine atom be increased significantly above the minimum energy (-100 e.v.) available as recoil from the y-ray. The capture cross section of iodine for 10,000 e.v. neutrons is about 1.5 barns, for 100,000 e.v. neutrons 0.6 barn, and for neutrons above 1 mev. energy less than 0.1 barn.28 These cross sections may be compared to a value of 5.5 barns for neutrons of thermal energies. Since the energy spectrum present at the cyclotron represents a highly non-thermalized flux, an appreciable contribution to the activation comes from neutrons having energies in excess of 10,000 volts. A large fraction of the 112*atoms produced will, therefore, have energies in the region of 100 to 1000 e.v. The low value for the difference between the organic yields observed at the cyclotron and at the (28) D. J. Hughes and J. A. Harvey, Neutron Cross Sections, A.E.C. Document BNL 325, 203, 1955.
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CHARLES E. MCCAULEY AND ROBERT H. SCHULER
Vol. 62
reactor makes it seem unlikely that there is any molecular disruption caused by the more energetic strong dependence of the chemical reactivity of the nuclei will then increase the amount of chemical species produced on recoil energy in the region im- reaction occurring. The difference in behavior mediately above 100 e.v. The rapidly decreasing between the alkyl iodides and the iodine-hydrocross sections at energies above 100,000 e.v. mini- carbon systems can, at least in part, arise from the mize the importance of contributions of (n,?) re- completely different types of collision processes ocactions which are produced by very highly energetic curring in the two media. In the case of hydroneutrons and place an approximate upper limit of carbon solutions only a small fraction of the energy 1000 e.v. on the recoil energy of the activated atom. of the iodine atom can be lost in a particular event, Using the data obtained with recoil atoms having while in the case of the alkyl iodides, at least at energies of the order of 100, 100-1000 and 100,000 high energies where chemical, binding can be nege.v. we obtain a pattern which indicates, as ex- lected, on the average half the recoil energy is lost pected, that the bound activity results predomi- in each collision with an iodine atom. The ennantly from processes occurring a t very low vironment in which the thermalized atom finds itenergies. The one experiment with neutrons pro- self will be completely different in each of these duced in a lithium target shows that further in- systems. I n the iodides the fragments present are crease in energy above 100,000 e.v. has little effect. to a large extent iodine atoms while organic fragThe Chemical Model.-The different ways in ments are the sole products from hydrocarbons. which activation of the same chemical species by I n the former case but not in the latter exdifferent nuclear processes can give rise to variations change of identity between the radioiodine and in resulting chemical behavior of the activated the inactive iodine atoms is possible. The net atom have been enumerated previously.12 Of result of the moderation process in the alkyl iodides these, it seems most likely that the effect is caused is that the reactions leading to the formation of by the increased recoil energy resulting from the organic activity take place in a relatively small different processes produced by the nuclear re- region close to the site of thermalization. In the actions of fast neutrons. The various models pro- case of hydrocarbon media, a much larger diffusion posed to explain the organic activity do not readily zone most probably is involved. We regard the allow a dependence on recoil energy. In particular, observed effect of recoil energy for hydrocarbon the billiard bal120.30J1and epithermal r e a ~ t i o n ~ solutions ~ a ~ ~ and the lack of a similar effect in the alkyl mechanisms predict no such dependence of yield iodides as strong evidence in favor of the contribuon recoil energy provided the recoil energy is above tion, at least in part, of processes caused by the some very low minimum. The billiard ball mech- general chemical disruption of the system in the anism cannot, of course, apply in the case of iodine vicinity of the recoiling atom. At this point a question remains as to why the atoms produced in hydrocarbon substrates since it is not possible for a recoiling atom to simultaneously alkyl bromides appear to behave more like the iobreak a chemical bond and have its energy re- dine-hydrocarbon solutions than the alkyl iodides. duced in a single interaction to a sufficient extent Such an effect might perhaps be caused by the difthat a stable molecule can result. Dependence of ference in the chemistry of iodine and bromine organic yield on recoil energy is possible in the atoms. Hydrogen abstraction reactions are posrandom fragmentation m0de1~3~~ if a sufficiently sible in the case of bromine atoms but not in the high probability exists that as a result of diffusion case of thermalized iodine atoms so that exchange processes the activated atom may react with a reactions such as those postulated above for the fragment produced at some distance from the site iodides would tend to be less important. Reaction of final thermalization. The greater amount of after diffusion to some distance becomes more probable in the case of the alkyl bromides and might (29) W. F. Libby, J . Am. Chem. SOC., 69, 2523 (1947). therefore result in some dependence of yield on re(30) J. M.Miller, J. W. Gryder and R. W. Dodson, J . Chem. Phvs., coil energy. Clarification of this situation will un18, 579 (1950). doubtedly depend on accumulation of data on other (31) P. C. Capron and Y. Oshima, ibid., 20, 1403 (1952). systems. (32) L. Friedman and W. F. Libby, ibid., 17, 647 (1952).
