FISSION RADIOCHEMISTRY (LOW­ENERGY FISSION)!

By L. E. CLENDENIN AND E. P. STEINBERG

Chemistry Division, Argonne National Laboratory, Lemont, Illinois

INTRODUCTION

Radiochemical studies of fission products not only led to the discovery of nuclear fission [Hahn & Strassmann ( 1)] but have since played an important role in the detailed characterization of this phenomenon. Through the measurement of fission yields (probabilities of formation for the various fis­sion products), radiochemical research has resulted in a much more accurate picture of the distribution of mass and nuclear charge in fission than could be obtained by purely physical methods. An extensive investigation of fission yields in low-energy neutron induced fission was made by many workers on the Plutonium Project for U23" U238, and PU239 (2), and also for U233 [Steinberg et al. (3)]. Fission yields in neutron-induced fission of U235 and U233 were in­vestigated on the Canadian project by Crummitt & Wilkinson (4). The results of these investigations have shown that low-energy fission is pre­dominantly asymmetric, i.e., the mass distribution (yield-mass curve) has two maxima (light and heavy group peaks) and a deep central minimum (trough) showing the low frequency of symmetric fission modes. With in­creasing mass of the fissioning nucleus the mass distribution becomes wider, and the light group peak shifts toward higher masses while the position of the heavy group peak remains relatively fixed (with perhaps a slight shift in the direction of lower mass). Some evidence was also obtained in comparing thermal neutron with fast pile neutron induced fission of PU239 for an increase in frequency of symmetric fission modes (trough yields) and very asymmetric modes (wing yields) as the excitation energy of the fissioning nucleus is increased (5). Subsequent investigations, showing the marked increase of trough yields with increasing excitation energy, have been reviewed by Spence & Ford (6).

The radiochemical method is of particular value in the study of the problem of nuclear charge distribution in fission. This problem is concerned with finding the most probable mode of charge division as a function of mass split, and the variation of primary formation (or independent fission yield) with nuclear charge among fission fragments of the same mass number. AI! fission yield data from the Plutonium Project bearing on this question were considered in 1946 by Clendenin, Coryell & Edwards (7) and were shown to be consistent with an empirical hypothesis that the most probable division of nuclear charge (in slow neutron induced fission) is that which leads to equal charge displacements of complementary fission fragments from stabil-

1 The survey of the literature pertaining to this review was concluded in January,

1954.

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70 GLENDENIN AND STEINBERG

ity. This postulate has been of considerable value in predicting many effects in fission product chains, e.g., cases of isomerism and mass assignments, which were later verified. Recent work supporting the equal charge displace­ment hypothesis is discussed below in the section OIl Charge Distribution.

A significant contribution to the radiochemical study of fission products was the introduction by Thode and co-workers (8, 9) of the mass spectromet­ric technique in the determination of relative fission yields. This method is capable of high precision, may be applied to stable as well as radioactive nuclides, and avoids the difficulties involved in {3-counting. Investigation of the relative abundances of stable and long-lived fission-produced isotopes of krypton and xenon by this technique (8, 9) revealed for the first time the existence of "fine structure," i.e., fission yields deviating from a smooth yield-mass curve. Recent mass spectrometric investigations have yielded further evidence for fine structure in mass distribution, and are discussed below in the section on Closed Shell Effects.

A review of radiochemical studies of fission yields prior to 1949 has been given by Way & Dismuke (10) covering mass distribution, charge distribu­tion, and changes that occur as the energy of the particle inducing fission is increased. Spence & Ford (6) have recently reviewed investigations on high­energy fission. Whitehouse (11) has given a general survey of the whole field of nuclear fission covering the literature through 1951. Useful bibliographies (12) on fission have been issued by the Information Office of the British Atomic Energy Research Establishment covering the period from January, 1946 to September, 1952. A survey of spontaneous and slow neutron fission properties of heavy nuclei has been given by Huizenga, Manning & Seaborg (13) covering slow neutron fission cross sections, photofission thresholds, neutron fission thresholds, correlation of slow neutnrn fissionability with fission thresholds and neutron binding energies, and systematics of spon­taneous fission half lives as a function of atomic and mass number.

The present review is limited to the period since 1949 and covers only radiochemical investigations of the low-energy fission process, i.e., spon­taneous fission and fission induced by neutrons ranging in energy from ther­mal to a few Mev. Photofission and high-energy particle induced fission are not included, except for the recent interesting observations on the angular distribution of fission fragments. Investigations primarily concerned with the search for and characterization of individual fission products are not reviewed except insofar as they may relate directly to interpretations of the fission process.

NEUTRON INDUCED FISSION

Mass distribution.-Most of the general features of the low-energy fission yield-mass curves for U233, U235, U238, and PU239 had been fairly well estab­lished by 1949. Work in this field has more recently been directed toward refinements in the measurments to establish absolute fission yields and to examine more detailed features of the mass distribution. In addition, the studies have been extended to some other fissionable nuclides.

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FISSION RADIOCHEMISTRY 71

The relative fission yields of 23 chains in the pile neutron induced fission of Th232 were determined radiochemically by Turkevich & Niday (14). A complete yield-mass curve was constructed from the data and compared with the corresponding curves for U233, U235, U238, and PU239 to illustrate the effect of varying the mass of the fissioning nucleus. Fission of Th232 is similar to the others in being highly asymmetric, and the yield-mass curve

for this nuclide, the lightest mass so far investigated, extends the trends with mass previously noted. The ratio of the most probable yields to yields for symmetrical fission (peak-to-trough ratio) is 110, considerably lower than

the corresponding ratio for thermal fission of U235 (about 600). The average energy of the neutrons effective in inducing fission in Th232 was estimated as 2.6 Mev, appreciably higher than the "threshold" energy of about 1.1 Mev, and the lower peak-to-trough ratio is consistent with the higher energy con­tent of the compound nucleus. The suggestion was made that the observed decrease in the peak-to-trough ratio with increasing energy may be owing to the superposition on the familiar twin-peaked curve of a yield curve with a rather broad maximum at symmetrical fission which increases in magnitude with increasing energy of excitation.

The earlier fission yield-mass curve for pile neutron induced fission of U238 [Engelkemeir et al. (15)] was in doubt in the region of the trough because of a possible cadmium contamination and not very well defined in the region of the heavy group peak. Keller, Steinberg & Glendenin (16) have reinvesti­gated the fission of U238 with pile neutrons (estimated average effective energy of 2.8 Mev) by the determination of the yields of 15 products. The results are in essential agreement with the earlier work except in the region of the trough, the yield of the mass 115 chain being 0.035 ±0.007 per cent compared with the former result of 0.059 per cent. The lower yield of symmetrical modes (peak-to-trough ratio of 200) compared with that for Th232 is con­sistent with the relative excitation energies of the compound nuclei as deter­mined by their neutron fission thresholds and binding energies, and the aver­

age energy of the neutrons effective in inducing fission in the two cases. The

writers note the difficulty of interpreting the effect of energy on the peak­to-trough ratio when fission is induced by a spectrum of neutrons. Compari­sons of the yield-mass curve for U238 with those for Th232 and spontaneous fission of U238 (see below) indicate the expected trends with energy of excita­tion and with mass.

In addition to these rather complete surveys of mass distribution, fission yields for limited mass regions and some individual fission products have been determined utilizing mass spectrometric and improved counting tech­niques. Some of these investigations were primarily concerned with the na­ture of fine structure in the fission yield curves and will be discussed below

in the section on Closed Shell Effects.

Relative fission yields in the mass region 143 to 160 were obtained by Inghram, Hayden & Hess (17) for pile neutron fission of U235 using a mass

spectrometric determination of the relative abundances of isotopes of neo-

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72 GLENDENIN AND STEINBERC

dymium, samarium, europium, and gadolinium. Difficulties of fitting the relative yields of one element to those of another, and the corrections for large neutron absorption cross sections in this region were examined. The results indicate a smooth yield curve with higher yields than those obtained radiochemically (2) above mass 153. The authors pcint out that the irradi­ated sample was a thick uranium slug in which the neutron energy distribu­tion would differ from that occurring in thin sources irradiated with moder­ated pile neutrons as employed in many radiochemical studies. It should be noted, however, that the deviations from the radiochemical curve may also have been attributable to contributions from PU239 fission in which the fission yields of this mass region are considerably higher (2). The fission yields of KrS6, Kr87, and Kr88 in U235 fission were investigated mass spectrometrically by Koch et al. (18) who observed relative values of 33 ± 10, 70 ± 15, and 100, respectively.

Redeterminations of the yields of several fission products in the slow neutron fission of U235 have been made using more accurate techniques for the determination of disintegration rates. Hardwick (19) found yields for RulOS and RU106 of 2.85 ±0.16 per cent and 0.38 ±0.03 per cent, respectively, relative to reference values of 6.2 per cent for BaUO and 6.15 per cent for CS137. These results are significantly lower than the previously accepted values (2) and are more reliable. Bartholomew et al. (20) report a value of 3.1 ±0.1

per cent for the fission yield of 1131 relative to a reference value of 6.1 per cent for Bauo. Sugarman (21) clarified the genetics of the decay chains of mass numbers 77 and 78 and determined the yields of 12-hr. Ge77, 38-hr. As77, 86-min. Ge7S, and total 91-min. As78 to be 2.3XI0-3, 6.7XlO-s, 1.8 X 10-2,

and 2.0 X 10-2 per cent, respectively, relative to a reference value of 6.17 per cent for Bal4O• The values for mass 77 are significantly lower than those previously reported (22) while those for mass 78 are in good agreement. No greater accuracy is claimed for these values than for the earlier ones. Bal40 has served as a fission monitor in many investigations, and its absolute fission yield is therefore of considerable importance. Unfortunately, the decay scheme is complex and determinations of absolute disintegration rates had not been made with high accuracy. The data of Freedman & Engelkemeir (23) have been reevaluated by Steinberg (24), using a half life of 308 hr. with empirical self-absorption and scattering correction factors. A yield of 6.32

per cent was obtained with an estimated error of about 10 per cent. Yaffe & co-workers (25) have determined the absolute yield of Bal40 as 6.26 per cent with an estimated error of about 5 per cent, and Reed & Turkevich (26) ob­tained a value of 6.3 per cent with an estimated error of less than 6 per cent. These results are in good agreement, and a yield of 6.3 ±0.3 per cent is sug­gested as a best value.

For the determination of absolute yields it is, of course, necessary to know the fission rate of the source material as well as the disintegration rate of the product of interest. Accurate fission and {3-counting have been em­ployed in some recent absolute yield determinations in the thermal neutron

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FISSION RADIOCHEMISTRY 73

fission of U2S6. Katcoff & Rubinson (27) determined the absolute yield of 5.27-day Xe133 as 6.62 ±O.l5 per cent and correlated the result with other yields in the mass region 131 to 137. Absolute gas counting and careful fission monitoring were employed in this determination, and the value is probably the best established fission yield yet reported. The absolute yield of M099 has been studied as a function of the neutron energy inducing fission by Terrell et al. (28). A value of 6.14 ±0.16 per cent was obtained for thermal fission. A number of absolute yields were determined by Reed & Turkevich (26) with an estimated accuracy of 6 per cent. Values of 4.78 per cent for Sr89, 5.1 per cent for Sr91, 5.5 per cent for Zr97, 5.98 per cent for M099, 6.4 per cent for Ba139, and 6.3 per cent for Ba140 were obtained. These data are in good agreement with previous yields of lower accuracy, but they are lower in some cases than those which were observed in mass spectrometric deter­minations now being completed (29). The latter investigation encompasses the determination of a rather complete yield-mass curve for the pile neutron fission of U236 by absolute radiometric and mass spectrometric measurements correlated with the results of other investigators.

The possibility of ternary fission into two fragments of about mass 100 and one of about mass 40 was investigated radiochemically on the Plutonium Project [Metcalf et al. (30); Boyd, Larson & Simon (31)]. Positive identifica­tion of fission products in the mass-range 35 to 60 was not made, but upper limits of the order of 10-4 per cent were set for the fission yields of such prod­ucts. Partition into two heavy fragments and one light one in the mass range 4 to 13 had been indicated by cloud chamber and photographic plate studies to occur with a frequency of about 1 per cent of binary fissions. Only a few nuclides in this mass range are suitable for radiochemical investigation, and a search for Be7 was made by Cook (32) who set an upper limit of 10-6 per cent for the yield in uranium fission. A review of the data on such ternary fission was given by de Laboulaye, Tzara & Olkowsky (33), who investigated the short-range particles in a cloud chamber and interpreted them as probably knock-on atoms. The frequency of occurrence was estimated as 1 ±3/1000 binary fissions.

Charge distribution.-The status of the problem of the division of nuclear charge in fission was reviewed by Glendenin (34) in 1949. All pertinent data were shown to be consistent with both the equal charge displacement hy­pothesis (7), and a theory of charge division developed by Present (35) based on a nuclear model with radially nonuniform charge distribution. In an ex­tensive study of fission yields and closed shell effects in fission, Pappas (36) presented a modification of the equal charge displacement hypothesis which takes into account the discontinuities in the stability valley at shell closures. This modified treatment was shown to bring apparently anomalous inde­pendent yields of Nb96 [Cook (37)] and As78 [Sugarman (21)] into better agreement with the charge distribution curve. Additional data of Cook (37) and Brown & Yaffe (38) were also included in a summary of all available information on independent yields in U235, PU239, and U233. The data were

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74 GLENDENIN AND STEINBERG

shown to be well accounted for by a single charge distribution curve, es­sentially identical with that given by Clendenin, Coryell & Edwards (7).

Pappas also points out that Present's theory, which does not consider nu­clear shell structure, cannot be expected to give exact predictions for indi­vidual mass numbers, but is nevertheless in fair agreement with the data. Although no definite choice can yet be made between the two postulates of charge division, the equal charge displacement interpretation has been some­what more successful in the quantitative prediction of independent fission yields.

Some additional independent yield data have been reported. Ford & Stanley (39) found a value of about 2 per cent of the chain yield for La141

in the slow neutron fission of U235, and Turkevich & Niday (14) set an upper limit of about 3 X 10-2 per cent of the total chain for CS136 in the pile neutron fission of Th232. These data are consistent with the charge distribution postu­lates mentioned. Ford (40) has shown that the charge distribution curve for 14 Mev neutron fission of U235 is parallel to that for thermal neutron fission of U235 with the most probable charge for a given mass split shifted toward stability, i.e., a smaller neutron-to-proton ratio for the primary fragments. Steinberg & Clendenin (41) found the independent yield of CS136 in the spon­taneous fission of Cm242 to be in agreement with the prediction of the equal charge displacement hypothesis. Thus, it appears that for all fissile nuclides thus far investigated, a single charge distribution curve is applicable over a wide range of excitation energies (0 to 14 Mev). However, in very high­energy fission, e.g., 190-Mev deuteron fission of Bi [Coeckerman & Perlman (42)], the division of nuclear charge is apparently different, the most probable charge of the primary fragments being that which maintains the same neutron-to-proton ratio as the fissioning nucleus. This may indicate that at very high energies the fission process takes place too rapidly to permit any rearrangement of charge.

Closed shell effects.-The early radiochemical investigations (2) of slow neutron induced fission indicated that the yield distributions were rather smooth functions of mass. In fact, when particular determinations did not fit the smooth curve, errors arising from a failure to achieve interchange, the presence of an isomeric state, or difficulties in the measurement of the dis­integration rate were suspected and generally found. The concept of a smooth relationship between fission yield and mass proved extremely useful in estab­lishing mass assignments, half lives of long-lived fission products, etc. Marked deviations from a smooth curve relationship were definitely estab­lished, however, by the mass spectrometric determinations of the relative abundance of krypton and xenon isotopes produced in U235 fission [Thode and co-workers (8, 9)] and later by the radiochemical determination of the yield of p36 in the fission of U233, U23., and PU239 [Stanley & Katcoff (43)].

A proposal to explain this fine structure in mass distribution was made by Clendenin (34, 44) on the basis of the stability of nuclear closed shells of 50 and 82 neutrons. Nuclei which contain S1 or 83 neutrons, i.e., one neutron

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FISSION RADIOCHEMISTRY 75

more than the closed shell, have abnormally low-binding energies (by ap­proximately 2 to 3 Mev) for the extra neutron. It was postulated that a primary fission product (which has already emitted the usual number of prompt neutrons) containing one neutron in excess of the closed shell may emit this neutron in preference to a ,a-particle or"Y ray. This process of extra neutron emission would result in perturbations in fission yields near a closed shell since the loss in yield from a given chain would not always be exactly compensated by the gain in yield from the chain of one higher mass number. Calculations, based on this mechanism and utilizing the primary yields along fission chains as given by the charge distribution function (discussed in the previous section), indicated a fine structure pattern for the krypton and xenon isotopes and an abnormally low yield for p3G in qualitative agreement with the experimental observations.

Further mass spectrometric investigations of the fission yields of cesium, rubidium, and strontium isotopes were carried out by Wiles et al. (45, 46). In order to account more quantitatively for these results and the earlier in­vestigations of xenon and krypton yields, Wiles (45) suggested that frag­ments containing 82 neutrons are favored (i.e., occur in abnormally high yield) in the fission process, in addition to the postfission effect proposed by Clendenin. Experimental verification of the preference for an 82-neutron fragment in fission was obtained by Clendenin et al. (47), who observed in mass spectrometric studies an abnormally high yield for mass number 100 which is complementary to the 82 neutron containing masses (133 and 134). Relative fission yields for Zr and Mo isotopes, together with those for Xe and Kr (8, 9) and Nd (7), were normalized to give a yield-mass curve dif­fering significantly in shape from the older "smooth" radiochemically­determined curve for U13• fission (2). This investigation has been extended (29) to obtain absolute fission yields for a wide range of mass numbers utiliz­ing the isotope dilution technique, and the results clearly indicate perturba­tions in yield consistent with nuclear structure preference in fission and ex­cessive postfission neutron evaporation.

Radiochemical determinations of fission yields in the regions of fine structure have been made by Pappas (26), Wiles (48), and Yaffe, Day & Creer (49). In general, this work has corroborated the earlier mass spectro­metric observations. Pappas (36) extended the concept of extra evaporation of loosely-bound odd neutrons just outside closed shells to include the third, fifth, and seventh such neutrons on the basis of neutron binding energy systematics, and obtained good agreement with the observed fine structure pattern in the region of the 82-neutron shell. Wiles (48) investigated yields in the mass region 99 to 106 for slow neutron and lS-Mev deuteron fission of U235 and 1S-Mev deuteron and 13-Mev photon fission of U238. Although evidence for fine structure at mass 101 was slight, the results for the deuteron fission of U235 and U238 and the photofission of U238 indicate abnormally high yields at about mass 105. This is ascribed to a selectivity in the fission process for fragments containing 50 prQton� (analogous to the 82-neutron preference),

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76 GLENDENIN AND STEINBERG

mass 105 being the expected complement of the favored fragment. An analy­sis of the expected mass positions for the fine structure peaks attributable to the 82-neutron and 50-proton preferences in the fission process is given for a

number of fissile nuclides. Further supporting evidence for nuclear closed sheIl effects in fission is

given by the observations of fine structure in spontaneous fission of Th232, U238, and Cm242 discussed in the following section. It is interesting to note, for example, that the widely-different mass positions of light group fine structure in U235 fission (mass 100) and Cm242 fission (mass 105) are both complementary to the mass region (around 134) associated with 82 neutrons. The shift of heavy group fine structure from mass 134 in U235 fission to mass 132 in spontaneous fission of Th232 and U238 is also consistent with the ex­pected mass positions of the 82-neutron shell.

SPONTANEOUS FISSION

Information on the mass distribution of the fragments from spontaneous fission was first obtained in ionization chamber studies by Whitehouse & Galbraith (50). It was found that the kinetic energy distributions of the fragments from spontaneous fission of U238 and slow neutron induced fission of U235 are essentially identical within the limitations of resolution inherent in this method. The spontaneous fission of Cm242 was also observed to be asymmetric in similar studies by Shuey (51) and Hanna et at. (52).

More accurate information on the mass distribution in spontaneous fission was obtained by Thode and co-workers (53, 54) and by Wetherill (55), using the highly sensitive mass spectrometric technique. These in­vestigators examined the isotopic composition of the rare gases Kr and Xe produced in various minerals by spontaneous fission of U238 (53, 54, 55) and Th232 (55). The observed abundances of the krypton and xenon isotopes in­dicate that the mass distribution in spontaneous fission has narrower peaks and probably a lower trough than that in slow neutron induced fission. Evidence was also found for an abnormally high yield (fine structure) at mass 132 for both U238 and Th232.

A more complete investigation of the distribution of yields in spontaneous fission was made by Steinberg & Glendenin (41) who determined radiochemi­cally the yields of 2 1 fission products (ranging in mass number from 91 to 140) from the spontaneous fission of Cm242. The yield-mass curve exhibits somewhat higher and narrower peaks than are observed in neutron-induced fission. Although the amount of Cm242 available was not sufficient to produce a measurable activity of fission products in the region of symmetrical fission (mass numbers 116 to 124), the upper limit set at Cdll7 «0.01 per cent) and the slope of the curve indicate a peak-to-trough ratio at least as large as that observed in thermal neutron induced fission of U235, and perhaps con­siderably larger. Rising trough yields and broadening of the mass distribu­tion have been observed with increasing energy of excitation in induced fission (6), and therefore it might be expected that spontaneous fission (no

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FISSION RADIOCHEMISTRY 77

excitation) would exhibit a lower trough and narrower peaks. Prominent fine structure in the mass distribution was also observed in the regions of mass 105 and 134. The effect in the light group is more pronounced than that in neutron induced fission of U236, indicating that nuclear structure preference in fission may diminish with increasing excitation energy of the fissioning nucleus.

ANGULAR DISTRIBUTION OF FISSION FRAGMENTS

Although not strictly within the scope of this review, the recent obser­vations of anisotropic angular distribution of fission fragments in photo­fission and higher energy particle induced fission are of considerable interest. Winhold, Demos & Halpern (56) observed that the fragments from photo­fission of Th232 are emitted preferentially at right angles to the photon beam with an angular distribution of the form a+b sin2 0, and that the amount of anisotropy (ratio bla) decreases with increasing photon energy from 1.2 at 8 Mev to 0.4 at 16 Mev. In further experiments (57), employing radiochemi­cal determinations of individual fission products from 16 Mev photofission, it was found that the anisotropy is a function of mass split with bla increas­ing from zero at a mass ratio of 1.0 to about 0.8 at mass ratio 1.8. Bralley, Dickinson & Henkel (58, 59) investigated the angular distribution of frag­ments from neutron induced fission of U235 for neutron energies between thermal and 20 Mev. It was found that the fission fragments are emitted preferentially in the same direction as the neutron beam with an angular distribution of the form a+b cos2 O+d cos40, and that the anisotropy is a function of neutron energy increasing from zero at thermal energy to a maximum at about 10 Mev and then decreasing at higher energies. The angular distribution of fragments from 22 Mev proton induced fission of

Th232, studied by Cohen et al. (60), is also of the form a+b cos2 O+d cos4 0 with anisotropy greater for asymmetric fission than for symmetric fission as shown by radiochemical determination of individual fission products. Ade­quate theoretical interpretations of these phenomena have not yet been given.

SUMMARY

The radiochemical investigations of the past few years have extended the previously noted trends of the mass distribution with variation in excitation energy and mass of the fissile nucleus, and have established the general ap­plicability of a single charge distribution function. Marked deviations from smooth fission yield-mass curves have been observed and correlated with the influence of nuclear closed shells, giving a plausible picture in terms of pref­erential formation of and neutron emission from particular fragments.

The pronounced asymmetry of low-energy fission is perhaps the most out­standing feature of the process. Many nuclear models have been invoked in attempts to account for this phenomenon, but there has been no general acceptance of any one theoretical development. The most successful treat-

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78 GLENDENIN AND STEINBERG

ments appear to be those of Hill & Wheeler (61) and Fong (62). The former is based on the collective model of the nucleus and, although no quantitative conclusions are drawn, it is stated that the model is not inconsistent with fission asymmetry. Fong's theory is based on the development of a mass equation which takes into account the perturbations attributable to closed shells. The concept of statistical equilibrium is utilized with the probability of formation determined by the number of available quantum states in the excited fragments. Quantitative calculations of the expected mass distribu­tion in slow neutron induced fission of U235 are in excellent agreement with the observed fission yield-mass curve. Some other predictions, however, particularly regarding charge distribution, are not in agreement with obser­vation, and further quantitative tests of the applicability of the theory to other fissile nuclides and other features of the fission process are needed.

LITERATURE CITED

1. Hahn, 0., and Strassmann, F., Naturwiss., 27, 11 (1939) 2. Radiochemical Studies: The Fission Products, Nat'l Nuclear Energy Ser., Div.

IV-9 (Coryell, C. D., and Sugarman, N., Ed., McGraw-Hill Book Co., Inc., New York 18, N. Y., 2086 pp., 1951)

3. Steinberg, E. P., Seiler, J. A., Goldstein, A., and Dudley, A., U. S. Atomic Energy Commission Document, MDDC-J632 (1948)

4. Grummitt, W. E., and Wilkinson, G., Nature, 161, 520 (1948) 5. Steinberg, E. P., and Freedman, M. S., in Radiochemical Studies: The Fission

Products, Nat'l Nuclear Energy Ser., Div. IV-9, Paper 219, 1378 (Coryell, C. D., and Sugarman, N., Ed., McGraw-Hili Book Co., Inc., New York 18, N. Y., 2086 pp., 1951)

6. Spence, R. W., and Ford, G. P., Ann. Rev. Nuclear Sci., 2, 399 (1953) 7. Glendenin, L. E., Coryell, C. D., and Edwards, R. R., in Radiochemical Studies:

The Fission Products, Nat'! Nuclear Energy Ser., Div. IV-9, Paper 52, 489 (Coryell, C. D., and Sugarman, N., Ed., McGraw· Hill Book Co., Inc., New York 18, N. Y., 2086 pp., 1951)

8. Thode, H. G., and Graham, R. L., Can. J. Research, [A125, 1 (1947) 9. Macnamara, ]., Collins, C. B., and Thode, H. G., Phys. Rev., 78, 129 (1950)

10. Way, K., and Dismuke, N., Oak Ridge National Laboratory Report, ORNL-Z80 (1950)

11. Whitehouse, W. J., Prog. Nuclear Phys., 2, 120 (Pergamon Press, Ltd., London, England, 295 pp., 1952)

12. Bibliography on Fission (Atomic Energy Research Establishment, Harwell, Berks., England, Inf./Bib. 76, July, 1951; Inf./Bib. 76, Oct., 1952, Supp!. 1)

13. Huizenga, J. R., Manning, W. M., and Seaborg, G. T., in The Actinide Elements, Nat'l Nuclear Energy Ser., Chapt. 20,839 (Seaborg, G. T., and Katz, J. J., Ed., McGraw-Hill Book Co., Inc., New York 18, N. Y., 870 pp., 1954)

14. Turkevich, A., and Niday, J. B., Phys. Rev., 84, 52 (19,51) 15. Engelkemeir, D. W., Seiler, J. A., Steinberg, E. P., and Winsberg, L., in Radio­

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16. Keller, R. N., Steinberg, E. P., and Glendenin, L. E., Phys. Rev., 94, 969 (1954) 17. Inghram, M. G., Hayden, R. J., and Hess, D. C., Phys. Rev., 79, 271 (1950) 18. Koch, J., Kofoed-Hansen, 0., Kristensen, P., and Drost-Hansen, W., Phys.

Rev., 76, 279 (1949) 19. Hardwick, W. H., Phys. Rev., 92, 1072 (1953) 20. Bartholomew, R. M., Brown, F., Hawkings, R. C., Merritt, W. F., and Yaffe, L.,

Can. J. Chem., 31, 120 (1953) 21. Sugarman, N., Phys. Rev., 89, 570 (1953) 22. Steinberg, E. P., and Engelkemeir, D. W., in Radiochemical Studies: The Fission

Products, Nat'l Nuclear Energy Ser., Div. IV-9, Paper 54, 566 (CoryeJl, C. D., and Sugarman, N., Ed., McGraw-Hill Book Co., Inc., New York 18, N. Y., 2086 pp., 1951)

23. Freedman, M. S., and Engelkemeir, D. W., in Radiochemical Studies: The Fission Products, Nat'l Nuclear Energy Ser., Div. IV-9, Paper 206, 1344 (Coryell, C. D., and Sugarman, N., Ed., McGraw-Hill Book Co., Inc., New York 18, N. Y., 2086 pp., 1951)

24. Steinberg, E. P. (Unpublished data, October, 1952) 25. Yaffe, L. (Personal communication, October, 1952) 26. Reed, G. W., and Turkevich, A., Phys. Rev., 92, 1473 (1953) 27. Katcoff, S., and Rubinson, W., Phys. Rev., 91, 1458 (1953) 28. Terrell, J., Scott, W. E., Gilmore, J. S., and Minkkinen, C. 0., Phys. Rev., 92,

1091 (1953) 29. Glendenin, L. E., Steinberg, E. P., Hayden, R. J., Inghram, M. G., and Flynn,

K. F. (To be published in Phys. Rev., 1954) 30. Metcalf, R. P., Seiler, J. A., Steinberg, E. P., and Winsberg, L., in Radiochemical

Studies: The Fission Products, Nat'\ Nuclear Energy Ser., Papers 47-51, 478-88 (Coryell, C. D., and Sug;lrman, N., Ed., McGraw-Hill Book Co., Inc., New York 18, N. Y., 2086 pp., 1951)

31. Boyd, G. E., Larson, Q. V., and Simon, D. M., Clinton Laboratories (Oak Ridge) Reports MonN-311, 78 (1947); MonN-370, 428 (1947)

32. Cook, G. B., Nature, 169, 622 (1952) 33. de Laboulaye, H., Tzara, c., and Olkowsky, J., Compt. rend., 237, 155 (1953) 34. Glendenin, L. E., Laboratory for Nuclear Science Technical Report No. 35, Massa­

chusetts Institute of Technology (1949) 35. Present, R. D., Phys. Rev., 72, 7 (1947) 36. Pappas, A. C., Laboratory for Nuclear Science Technical Report No. 63, Massa­

chusetts Institute of Technology, (1953); Pappas, A. C., and Coryell, C. D., Phys. Rev., 81, 329 (1951)

37. Cook, G. B. (Personal communication, June, 1953) 38. Brown, F., and Yaffe, L., Can. J. Chem., 31, 242 (1953) 39. Ford, G. P., and Stanley, C. W., U. S. Atomic Energy Commission Document,

AECD-355J (1953) 40. Ford, G. P., U. S. Atomic Energy Commission Document, AECD-3597 (1953) 41. Steinberg, E. P., and Glendenin, L. E., Radiochemical Investigation of the Spon­

taneous Fission of Cm242 (Presented at Gordon Research Conference on Nuclear Chemistry, New Hampton, N. H., June 23, 1953); to be published in Phys. Rev. (July 15, 1954)

42. Goeckerman, R. A., and Perlman, I., Phys. Rev., 73, 1127 (1948) 43. Stanley, C. W., and Katcoff, S., J. Chem. Phys., 17,653 (1949)

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80 GLENDENIN AND STEINBERG

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46. Wiles, D. R., Smith, B. W., Horsley, R., and Thode, H. G., Can. J. Phys., 31 419 (1953)

47. Glendenin, L. E., Steinberg, E. P., Inghram, M. G., and Hess, D. C., Phys. Rev.

84, 860 (1951) 48. Wiles, D. R., A Study of Fission-Yield Fine Structure (Doctoral thesis, Massachu·

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(1950) 52. Hanna, G. C., Harvey, B. G., Moss, N., and Tunnicliffe, P. R., Phys. Rev., 81,

466 (1951) 53. Macnamara, J., and Thode, H. G., Phys. Rev., 80, 471 (1950) 54. Fleming, W. H., and Thode, H. G., Phys. Rev., 92, 378 (1953) 55. Wetherill, G. W., Phys. Rev., 92, 907 (1953) 56. Winhold, E. J., Demos, P. T., and Halpern, 1., Phys. Rev., 87, 1139 (1952) 57. Winhold, E. J., Demos, P. T., Halpern, 1., and Fairhall, A. W., Laboratory f01

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58. Dickinson, W. C., and Brolley, J. E., Jr., Phys. Rev., 90, 388 (1953) 59. Brolley, J. E., Jr., Dickinson, W. C., and Henkel, R. L. (Personal communica·

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Phys. Soc., 29, 21 (1954) 61. Hill, D. L., and Wheeler, J. A., Phys. Rev., 89,1102 (1953) 62. Fong, P., A Theory of Nuclear Fission (Doctoral thesis, University of Chicago,

Chicago, Ill., 1953); Phys. Rev., 89, 332 (1953)

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