Glenn T. Seaborg
and Heavy Ion Nuclear Science

This work was supported by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, Nuclear Physics Division of the U.S Department of Energy under Contract DE-ACO3-76S F00098

Abstract:

Radiochemistry has played a limited but important role in the study of nucleus-nucleus collisions. Many of the important radio chemical studies have taken place in Seaborg' s laboratory or in the laboratories of others who have spent time in Berkeley working with Glenn T. Seaborg. I will discuss studies of low energy deep inelastic reactions with special emphasis on charge equilibration, studies of the properties of heavy residues in intermediate energy nuclear collisions and studies of target fragmentation in relativistic and ultrarelativistic reactions. The emphasis will be on the unique information afforded by radiochemistry and the physical insight derived from radiochemical studies. Future roles of radiochemistry in heavy ion nuclear science also will be discussed.

I. Introduction

Glenn T. Seaborg has had several different careers, any of which would satisfy a lesser person. In this talk, I deal with Seaborg's contributions to science after his return to Berkeley from his position in the Kennedy, Johnson and Nixon administrations. Seaborg set up a new laboratory at Berkeley for radiochemistry, assembled a distinguished group of collaborators and proceeded to re-establish radiochemistry as an important tool for studying nuclear reactions. For the last two decades, Seaborg' s laboratory has served as a training ground for young nuclear chemists and a source of stimulation for visitors. His work has touched the forefronts of modern nuclear science, involving attempts to make superheavy elements and to understand nuclear reactions at energies of tens of MeV to hundreds of GeV.

The Berkeley that Seaborg returned to in 1971 was vastly different from the Berkeley he left in 1961. Among other developments, radiochemistry had ceased to be an important tool for studying nuclear reactions, having been eclipsed by techniques using semiconductor radiation detectors and on-line measurements. Even the rudimentary tools of a radiochemical lab, such as multichannel pulse height analyzers, were not available for Seaborg and his group to use. Undaunted, Seaborg assembled a fine group of younger chemists (Kratz, Norris, Liljenzin et al.) and proceeded to re-establish radiochemistry as a useful tool for the study of nuclear reaction mechanisms.

A schematic representation of typical radiochemi cal techniques for studying reactions is shown in Figure l. The simplest radiochemical method is the thick target-thick catcher method shown in the top portion of figure l. A beam of projectile nuclei impinges on a thick (-50 mg/cm²) target and the reaction products are stopped in the target (T) and catcher foils placed in the forward (F) and backward (B) directions. From off-line analysis¹ of the radionuclides found in T, B and F, one can calculate the cross section for the formation of a product of given Z and A. By careful integration² of these data, correcting for P-decay and non radioactive product nuclei, one can derive mass yield distributions for the reaction (u(A)). A model-dependent analysis³ of the relative amount of radioactivity in F, B and T gives one information about the product energies and the momentum transfer in the reaction.

The middle portion of Figure l shows a typical setup for the measurement of a fragment angular distribution. The projectile beam strikes a thin target (-100 i'g/cm²) and the recoiling product nuclei are stopped in Mylar or aluminum foils that line the walls of a cylindrical scattering chamber. These catcher foils can be cut in any shape and size to achieve the desired resolution in the 4r angular distribution measurement.

The bottom portion of Figure l shows the use of differential range techniques to measure fragment energy spectra. Reaction products recoiling from a thin target impinge on stacks of thin Mylar or aluminum foils placed at a given angle with respect to the incident beam. The measured distribution of product radioactivity in the catcher foil stack can be turned into fragment energy spectra using range-energy relationships.

Radiochemical techniques are, of necessity, off-line techniques for studying nuclear reactions. As such, they suffer because the correlations between many observables in the reaction are lost in radiochemical measurements. This inability to measure the correlations between product characteristics has led to a sharp decline in the use of these techniques. However, the exquisite sensitivity of these techniques (allowing cross section measurements at the picobarn level), the unit Z and A resolution, the simplicity of the apparatus allowing many survey measurements and the lack of many of the detection thresholds that afflict the use of other techniques has assured a continuing interest in the use of these techniques. Seaborg and his collaborators, students, etc. have played a key role in developing and using these techniques.

The first papers to emerge from Seaborg' s newly created laboratory for heavy ion radiochemistry dealt with the fragment mass distribution3-5 from the interaction of near-barrier 40Ar, 84Kr and ¹³⁶Xe with ²³⁸U (Figure 2). In this work, Seaborg and his collaborators showed the utility of radiochemical techniques in revealing the gross characteristics of nuclear reactions. In the fragment mass distribution for the 40Ar + 238U reaction, one can identify the relative importance of quasielastic scattering (E and F) deep inelastic scattering (C), fusion-fission (A) and the fission de-excitation of transfer and deep inelastic products (B). In the Kr and Xe-induced reactions, one saw a feature labelled G (and called the "goldfinger" in honor of a popular novel by Ian Fleming and the later movie). This class of reaction products was unexpected and puzzling until people recognized them as the non-fissioning remnants of the target nucleus. The fission of these nuclei had been inhibited by the high fission barriers in the Au region. Their yield increased as the distributions of the target-like fragments became broader, i.e., with increasing projectile Z and A.

However interesting these studies were, the clear cut primary goal of Seaborg and his collaborators at this time was the synthesis of superheavy nuclei. Figure 3 shows our current understanding of the expected halflives of the heavy and superheavy nuclei. The main feature of our current understanding is that we think there should be a peninsula of long-lived nuclei extending from the region of known nuclei to the superheavy region (rather than an island of superheavy nuclei separated from the known nuclei by a "sea of instability"). Superheavy nuclei are defined as those nuclei whose halflives increase with increasing Z (rather than decrease as expected from the occurrence of spontaneous fission). The halflives of the longest lived superheavy nuclei (Z = 112) are expected to be - 105 sec. No matter how this situation changes in the future, we will always be blessed (cursed) with the allegorical language (islands, peninsulas, seas, mountains, etc.) introduced by Seaborg in his many popular accounts of superheavy nuclei.

The most definitive experiments to synthesize superheavy nuclei involved the 48Ca + ²⁴⁸Cm reaction and were carried out at Berkeley involving Seaborg' 5 group. In Figure 4,I show the upper limit of the superheavy production cross sections that were measured in these reactions. Here the superior sensitivity of radiochemical techniques was used to establish (for the longer-lived nuclei) the lowest upper limit cross sections ( 10-35 cm²). Unfortunately, hindsight has shown use that this failure to synthesize the superheavy nuclei was expected. Using Armbruster' 5 systematics⁸ of s-wave fusion cross sections, we would estimate a fusion cross section for the ⁴⁸Ca + ²⁴⁸Cm reaction of ~3x10³² cm². The excitation energy of the composite species can be calculated⁹ to be -28 MeV, allowing 2-3 chances to fission. A simple-minded, estimate of the energy independent average value of rfl/rf would be - 10-2, resulting in an estimated production cross section for superheavy nuclei of i0-³⁸ -1O-³⁶ cm², well below the experimental upper limit cross sections.

Undaunted by these failures, Seaborg and collaborators, particularly Darleane Hoffman, showed how multinucleon transfer reactions could be used as effective tools to synthesize new heavy nuclei (Figure 5). The upper left panel of Figure 5 shows a typical set of experimental data showing the production of Bk, Cf, Es and Fm nuclei from the Ne + ²⁴⁸Cm reaction. Once again, the exquisite sensitivity of radiochemical techniques is shown in these carefully done measurements of minuscule cross sections. Because of the low production cross sections, few, if any, detailed studies of the mechanism(s) operating in these reactions have been done. However, the work of Hoffman and Hoffman¹⁰ and Magda et al¹¹ has demonstrated a key, unexpected feature of these reactions, i.e., the production of "cold" reaction products (E* < 10 MeV). The excellent agreement between the experimental data and the calculations appears to oniy occur if one assumes E*product < 10 MeV. These reactions promise to be important tools for production of new transuranium nuclei.

The efforts of Seaborg's group were not restricted to the study of low energy nuclear collisions. Instead Seaborg and collaborators brought radiochemical techniques to bear on a wide variety of nuclear reactions. In the late ¹⁹70's, Seaborg and collaborators measured the target fragment mass yield distributions for many relativistic nuclear collisions. Figure 6 shows a sample of this type of data. These data¹² proved not to be very definitive concerning the mechanisms of these reactions as shown in Figure 7. In Figure 7, we compare the data of Figure 6 with predictions of two disparate models for the collisions, the intranuclear cascade model¹³ and the nuclear firestreak model.¹⁴ Despite the significant differences between these two models that emphasize the nucleon-nucleon interaction and collective aspects, respectively, both models seem to do an equally acceptable job of representing the data. This insensitivity of the theoretical models to the results might indicate the property being measured, the fragment mass yield distribution, is largely determined by the collision geometry and the rough distribution of excitation energy in the target residues, i.e., "gross" features of the reactions.

One result from these series of measurements has not been fully appreciated. This is the finding that the fragment N/Z distributions were very narrow, much narrower than one would expect by simple stochastic considerations (Figure 8). Morrissey and co-workers¹⁵ pointed out that the narrowness of these charge distributions reflected an intrinsic correlation between neutrons and protons in excited nuclear matter. These workers made a crude model for this effect in terms of the zero point oscillations of the giant dipole resonance, which fit a large amount of experimental data. Bondorf and collaborators¹⁶ cast these same ideas more general terms involving isosprin correlations in the nuclear ground state. Nifenecker¹⁷ utilized these ideas to explain the shape of the charge distribution in fission. l suspect that these concepts might find further fertile ground in understanding the N/Z distributions in energetic nuclear matter as seen in other heavy ion reactions.

The ability of simple phenomenological models to describe the results of radiochemical studies of intermediate energy and relativistic nuclear collisions has largely been restricted to estimates of the cross sections. As had been established previously for p-nucleus collisions, the momentum transfer to the target nucleus in relativistic nuclear collisions appears to have been grossly overestimated (Figure 9). The work of Seaborg and collaborators¹⁸ has documented this problem thoroughly.

Radiochemistry has also played a role in the latest studies of ultrarelativistic nuclear collisions. Radiochemical techniques¹⁹ have extended the general range of validity of "limiting fragmentation" to the production of intermediate mass fragments (IMFs) in ultrarelativistic nuclear collisions (Figure lOa). Radiochemists20 discovered one of the few surprises of our studies of ultrarelativistic reactions, the finding of very unusual angular distribution for the IMFs produced in these reactions. These workers found that a crude range-weighted measure of the fragment angular distribution, F/B, had a value of 0.85 in the interaction of 14 GeV/nucleon 160 with ¹⁹⁷Au (Figure lOb). This measurement is the first example of an F/B value less than unity and must imply an unusual fragment angular distribution in the cm system. Grabez2¹ has corroborated this finding.

The high intensity accelerator beams available for the study of intermediate energy nuclear collisions have allowed the fullest use of radiochemical techniques for studying nuclear reactions. Once again, Seaborg and his associates have played a central role in these efforts. In a series of measurements involving various heavy targets (154Sm, ¹⁶⁵Ho, ¹⁹⁷Au and ²³⁸U) Seaborg et al. found that as the projectile energy increases, for a given projectile-target combination, the fraction of primary target-like fragments that decay by particle emission relative to fission increases, with heavy residue formation (particle emission) becoming the dominant mode of de-excitation²² (Figure 11). Thus the study of these heavy residues becomes an important aspect of the study of intermediate energy nuclear collisions.

The measurement of heavy residue properties is difficult because of their low energies23 (Figure 12). In reactions induced by 85 MeV/nucleon ions, the residue energies are - 100 keV/nucleon. The threshold-free differential range technique is ideal for measuring these low fragment energies. Coupled with unit Z and A resolution, one has been able to make significant measurements of some features of intermediate energy nuclear collisions. For example, Seaborg et al.²⁴ were able to show the disappearance of the deep inelastic reaction mechanism in Xe-Au collisions as the projectile energy increased from 21 to 45 MeV/nucleon (Figure 13). In deep inelastic reactions, the heavy residue energies result from the Coulomb repulsion of the touching fragments (dashed line). This behavior was observed in the 21 MeV/nucleon ¹²⁹Xe + ¹⁹⁷Au reaction but not the 45 MeV/nucleon ¹²⁹Xe + ¹⁹⁷Au reaction.

Without denigrating the scientific output of Seaborg and his collaborators, perhaps it is fair to say that the most important contribution of Seaborg's group to heavy ion science is Glenn's influence upon the people who worked with him (Figure 14). In Figure 14, I show a list of the co-authors of the papers cited in this review with the names of students being underlined. The list is a subset of a Who's Who of nuclear chemistry. Regrettably I have only had the time to discuss a small subset of all the scientific work done in Glenn Seaborg's research group during this time. To partially rectify this oversight, I include, as on Appendix of this document, a list of all the papers from this group during this period, with the names of participating students being underlined.

As one who has had the pleasure of working with Glenn for over 15 years, I am deeply grateful for his guidance, his enthusiasm, his scientific wisdom and his unique ability to create the opportunity for all of us who worked with him of participating in research at the forefront of nuclear science.

l. DJ. Morrissey, D. Lee, RJ. Otto, and G.T. Seaborg, Nucl. Instr. Meth. 158, 499 (1978).

2. DJ. Morrissey, W. Loveland, M. de Saint-Simon and G.T. Seaborg, Phys. Rev. C21, 1783 (1980).

3. J.V. Kratz, J.O. Liljenzin, A.E. Norris, and G.T. Seaborg, Phys. Rev. C13, 2347 (1976).

4. J.V. Kratz, A.E. Norris, and G.T. Seaborg, Phys. Rev. Lett. 33, 502 (1974).

5. RJ. Otto, M.M. Fowler, D. Lee and G.T. Seaborg, Phys. Rev. Lett. 36, 135 (1976).

6. E.K Hulet et al., Phys. Rev. Lett. 39, 385 (1977); see also RJ. Otto, et A., J. Inorg. Nucl. Chem. 40, 589 (1978).

7. P. Armbruster et al., Phys. Rev. Lett. 54, 406 (1985).

8. P. Armbruster, Ann. Rev. Nucl. Part. Sci. 33, 135 (1985).

9. P. Mšller, W.D. Myers, WJ. Swiatecki and J. Treiner, At. Data Nucl. Data Tables 39 225 (1988).

10. D.C. Hoffman and M.M. Hoffman, Lawrence Berkeley Laboratory Report LBL29502, November, 1990.

11. MT. Magda, A. Pop and A. Scandulescu, J. Phys. G. 13, L127 (1981).

12. P.L. McGaughey, W. Loveland, DJ. Morrissey, K Aleklett and G.T. Seaborg, Phys. Rev. C31 896 (1985).

13. Y. Yariv and Z. Fraenkel, Phys. Rev. C20, 2227 (1979).

14. W.D. Meyers, Nucl. Phys. A296, 177 (1978).

15. DJ. Morrissey, W.R. Marsh, R.J. Otto, W. Loveland, and G.T. Seaborg, Phys. Rev. C18,

1267 (1978).

16. S.P. Bondorf, G. Fai, O.B. Nielsen, Phys. Rev. Lett. 41, 391(1978); Nucl. Phys. A312, 149 (1978).

17. H. Nifenecker, J. Phys. Lett. 41, 47 (1980).

18. W. Loveland, Cheng Luo, P.L. McGaughey, D.J. Morrissey, and G.T. Seaborg, Phys. Rev. C24 464 (1981).

19. K. Aleklett, L. Sihver and W. Lovland, Phys. Lett. B197, 34 (1987).

20. W. Loveland et al Phys. Rev. C37, 1311(1988).

21. B. Grabez, Z. Phys. A335, 111(1990).

22. W. Loveland et al. Phys. Rev. C41, 973 (1989).

23. K Aleklett, M. Johansson, L. Sihver, W. Loveland, H. Groening, P.L. McGaughey and G.T. Seaborg, Nucl. Phys. A149, 591(1989).

24. G.T. Seaborg et A., Phys. Lett B. (submitted for publication).

Glenn T. Seaborg and William R. Corliss

New York: E.P. Dutton and Co., Inc., 1971

Russian translation--Chelovek y Atom (I.G. Pochitalin, translator; B.F. Kuleshova, editor;

Introduction by M.D. Millionshikov). Moscow: World, 1973 Romanian translation--Omul

Si Atomul (Translation directed by Valentin Ceausescu and Mircea Cristu; Introduction by

loan Ursu). Bucharest: Editura Stiintifica, 1974

Polish translation--Czlowiek i atom (Zbigniew Rek, translator; Joanna Szyllejko, editor).

Arabic translation--Dutton Publisher, U.S. al-Wa 'y al- 'Arabi, Publisher, Cairo, Egypt,

March 1980

Glenn T. Seaborg

San Francisco: W.H. Freeman and Company, 1972

Glenn T. Seaborg, Editor

Benchmark Papers in Physical Chemistry and Chemical Physics V.1, Dowden, Hutchinson

& Ross, Inc. Stroudsburg, Pennsylvania, 1978

Edited by Glenn T. Seaborg and Walter Loveland

Benchmark Papers in Physical Chemistry and Chemical Physics V.5, Hutchinson

Ross Publishing Company, Stroudsburg, Pennsylvania, 1982

Edited by J.J. Katz, G.T. Seaborg and L.R. Morss

Vols. l and 2, Chapman and Hall, London, New York, 1986

Glenn T. Seaborg and Walter D. Loveland

New York: John Wiley & Sons, Inc., 1990.

Early History of Heavy Isotope Research at Berkeley: August, 1940 to April, 1942.

Proceedings of the Symposium Commemorating the 25th Anniversary of Elements 97 and

98, held on January 20, 1975.

Glenn T. Seaborg, editor.

History of Met Lab Section C-i: April, 1942 to April, 1943.

History of Met Lab Section C-i: May, 1943 to April, 1944. Vol.II.

History of Met Lab Section C-1: May, 1944 to April, 1945. Vol.III.

History of Met Lab Section C-i: May, 1945 to May, 1946 Vol. IV.

Journal of Glenn T. Seaborg: April 18, 1942 to May 19, 1946. Vol. I-IV. June 1980.

Journal of Glenn T. Seaborg January 1, 1927 to August 10, 1934. October 1982.

Journal of Glenn T. Seaborg: August 11, 1934 to June 30,1939. October 1982.

Journal of Glenn T. Seaborg: July 1, 1939 to April 17, 1942. October 1982.

Journal of Glenn T. Seaborg November 7, 1971 to June 30, 1975. Vol. 1-4a. July 1990.

Journal of Glenn T. Seaborg: May 19,1946 to June 30, 1958. Vol.1-12. July 1990.

"Plutonium Revisited" in Radiobiology of Plutonium, Glenn T. Seaborg, B.J. Stover and W.S.S. Jee, eds., (J.W. Press; Univ. of Utah, Salt Lake City; 1972).

Recent Advances in the Chemistry of Organometallic Compounds of the Actinide Elements, Glenn T. Seaborg, Pure. Appl. Chem. 30, 539 (1972).

Organometallic Transuranium Compounds, Glenn T. Seaborg, Priroda, P.33 (1972).

Recent Advances in the United States on the Transuranium Elements, Glenn T. Seaborg, in Proceedings of the Fourth United Nations International Conference on the Peaceful Uses of Atomic Energy, Geneva, September 6-16, 1971 Vol.1 (United Nations, Geneva, 1972) p.29.

Occurrence of Transuranium Elements in Nature, Glenn T. Seaborg, Contributions to Recent Geochemist'y and Analytical Chemistry (Nauka Publishing Office, Moscow, 1972) p.560.

Transuranium Elements, Glenn T. Seaborg, The Encyclopedia of Chemistry, Third Edition, C.A. Hampel and G.G. Hawley, eds., (Van Nostrand Reinhold, New York, 1973) p. 1123.

Uranium and Compounds, Glenn T. Seaborg, The Encyclopedia of Chemistry, Third Edition, C.A. Hampel and G.G. Hawley, eds., (Van Nostrand Reinhold, New York, 1973) p. 1131.

Transuranium Elements, Glenn T. Seaborg, Encyclopedia Britannica (William Benton, 1973) p.179.

Arnericio, Glenn T. Seaborg, Encyclopedia Della Chimica, Vol. I, USES Utet-Sansoni, Edizioni Scientifiche, Italy, p. 484, 1973.

Attinidi, Glenn T. Seaborg, Encyclopedia Della Chimica, Utet-Sansoni, Edizioni Scientifiche, Italy, Vol.II, P.199 (1973).

Attinidi, Glenn T. Seaborg, Encyclopedia Della Chimica, Utet-Sansoni, Edizioni Scientifiche, Italy, Vol.II, p.207 (1973).

Berkeijo, Glenn T. Seaborg, Encyclopedia Della Chimica, Utet-Sansoni, Edizioni Scientifiche, Italy, Vol. II, p.403 (1973).

Medical Uses: Americium-241; Californium-252, Glenn T. Seaborg, Handbook of Experimental Pharmacology, Vol. XXXVI, H.C. Hodge, J.N. Stannard and J.B. Hursh, eds. (Springer-Verlag, Berlin, 1973), p.929.

Plutonium is Discovered, Glenn T. Seaborg, Explorers Journal, 195, (December, 1973).

New Directions in Development, in Proceedings of the 26th Nobel Symposium, Oslo, September 17-19, 1973, A. Schou and F. Sollie, eds., (Univsersitetsforlaget, Oslo, 1974)

p.67.

Chemistry, Hamburg, Germany. September 4,1973. (Butterworth, London, 1974) p.

1.

Transuranium Elements, Glenn T. Seaborg, Encyclopaedia Britannica, (William Benton,

Los Elementos Superpesados Y Su Ubicacion En La Tabla Periodica (The Superheavy