Heavy-ion Reactions

The study of heavy-ion-induced reactions is a forefront area of nuclear research. By using heavy-ion-induced reactions to make unusual nuclear species, one can explore various aspects of nuclear structure and dynamics at its limits. Another major thrust is to study the dynamics and thermodynamics of the colliding nuclei. 

The use of heavy ions as projectiles has opened up new fields of nuclear reactions, as already mentioned in section 8.3. The general formula of a heavy-ion reaction is 

Different types of interaction are distinguished, as illustrated in Fig. 8.23. (The spherical form is a simplification which is only applicable for nuclei with nuclear spin / = 0.) On path 1 the nuclei are not touching each other elastic scattering and Coulomb excitation are expected. On path 2 the nuclei are coming into contact with each other and nuclear forces become effective inelastic scattering and transfer reactions 

The methods used in the investigation of heavy-ion reactions are similar to those described in section 8.6. The high linear energy transfer (LET) and the relatively short range of heavy ions have to be taken into account. On-line separation of shortlived products is of special importance. 

As an example, the mass distribution of the products obtained by the bombardment of with Ar is plotted in Fig. 8.24. The curve is explained by superposition of the processes described above only few nucleons are transferred by quasielastic reactions (a), and many nucleons by deeply inelastic processes (b). Fusion followed by fission of highly excited products leads to a broad distribution of fission products around l/2(y4i + A2), where A and Ai are the mass numbers of and Ar, respectively (c), and asymmetric fission of heavy products of low excitation energy gives two small maxima (d). 

Quasiatoms containing two nuclei of uranium and a common electron shell may be produced by bombarding uranium with uranium ions. In such a quasiatom an intermediate apparent atomic number Z = 91 + 92= 184 may be obtained. At these high atomic numbers, the K electrons arc near the nuclei and at Z — 184 their mean distance from the nucleus is of the order of its diameter. Therefore, both nuclei must approach each other to distances of the order of the diameter of a nucleus in order that K electrons observe both nuclei as only one. 

---

In principle the density dependence of the SE at higher densities (and further away from N = Z) can be probed by means of heavy-ion reactions using neutron rich radioactive beams. In ref. [35] possible observable effects from the isovector field are considered in terms of the RMF model. Of particular... 

Alhassid, Y., and Iachello, F. (1989), Algebraic Approach to Heavy-ion Reactions, Nucl. Phys. A501, 585. 

In 1994 and 1995 Dr. Darleane Hoffinan of LLNL in Cahfornia and others from Germany used the Separator for Heavy Ion Reaction Products (SHIP) at the GSI laboratory in Darmstadt, Germany, to produce two new isotopes of element 110. 

The Heavy Ion Reaction Separator (SHIP) located in the GSI laboratory in Germany was used to identify elements 107 (bohrium) through element 109 (meitnerium) during the years 1981 through 1984, and it was used again later, between 1994 and 1996, to verify elements 110 (Uun) through element 112 (Uub). 

A systematic decay study of the i9°-200Bi isotopes was carried out by means of the heavy ion reactions natRe(16mg/cm2)[160(<180 MeV), xn] and 181Ta(8mg/cm2)[20Ne(<230 MeV),xn]. Multiscaled a-decay spectra for the masses 190, 192 and 194 were taken with the same a detector as used in the At experiment, together with a-y coincidences. A Ge detector (resolution 2 keV, relative efficiency 22% at 1332.5 keV) was used to detect y rays. In coincidence mode the lower limit on the y detector was set at - 40 keV. 

In summary, the selectivity of certain heavy ion reactions have been used to identify two proton and two neutron states of high spin (both yrast and non-yrast) in Nd nuclei The first direct information about the configurations of some of these states has been obtained and the results suggest simple configurations for some but not all of them At the same time certain members of the neutron 7/2 13/2 multiplet are not seen and comprehensive shell model calculations would be very useful to determine the reason Heavy ion induced transfer reactions, if chosen carefully are valuable spectroscopic tools,... 

The region around 100Sn also offers exciting possibilities. Studies of nearby nuclei have been unable to determine the applicable coupling scheme or the interplay of the nearly symmetric neutron-proton configurations. Furthermore, heavy-ion reaction cross sections pose a severe limit in extending the previous studies in this region. 

The identification of the first transuranium elements was by chemical means. In the early 1960s physical techniques were developed which allowed for detection of nuclei with lifetimes of less than one second at high sensitivity. A further improvement of the physical methods was obtained with the development of recoil separators and large area position sensitive detectors. As a prime example for such instruments, we will describe the velocity filter SHIP (Separator for Heavy-Ion reaction Products) and its detector system, which were developed at the UNILAC. The principle of separation and detection techniques used in the other laboratories is comparable. 

For the synthesis of superheavy nuclei by complete fusion, larger projectiles with at least twice as many protons and neutrons are required. It soon became obvious that in such reactions the deficit in the yields is even larger than can be explained by post-fusion losses. This problem stimulated systematic studies of heavy-ion reactions all the way down from the first touch of two interacting nuclei until final fusion. New types of reactions -... 

Baldo, M., Lanza, E.G. and Rapisarda, A. (1993). Chaotic scattering in heavy-ion reactions. Chaos 3, 691-706. 

Figure 8.23. Heavy ion reactions different types of interaction (schematically) path 1) elastic scattering path 2) quasielastic collision path 3) deeply inelastic collision path 4) frontal collision.

Figure 8.24. Cro.s.s sections (mass yields) for the heavy ion reaction Ar — - experimental values (thick target, chemical separation) a quasiclastic processes b multinuclcon transfer c fusion followed by fission d fis.sion of heavy nuclei produced from by transfer reactions. (According to J. V. KraU, J. O. Liljenzin, A. E. Norris, C], T. Seaborg, Phys. Rev. C 13 2347 (1976).)...

Soft heavy-ion reactions observed somewhat above the Coulomb barrier (about 6McV/u UNILAC, GSI), and hard heavy-ion reactions occurring at relativistic energies of about I GeV/u (STS, GSI) are distinguished. In the case of central collision, the latter proceed in three stages ... 

P. E. Hodgson, Nuclear Heavy Ion Reactions, Clarendon Press, Oxford, 1978 W. U. Schroder, J. R. Huizenga, Damped Heavy Ion Collisions, Annu. Rev. Nucl. Sci. 27, 465 (1977)... 

The energy needed to surmount the Coulomb barrier increases with Z and Z, whereas the cross section decreases. That is why, in general, only small amounts of heavier elements can be produced by heavy-ion reactions. Elements with Z > 106 are often obtained with a yield of only one atom at a time. 

In these experiments, the recoil technique was modified into a double recoil technique by application of a moving belt (Fig. 14.7). The recoiling atoms generated by the heavy-ion reaction (first recoil) are deposited on the belt and transported along a catcher foil on which the recoiling atoms from a decay (second recoil) are collected. From the activity recorded as a function of the distance, the half-life can be calculated. 

In this case, the recoiling products of the heavy-ion reaction were transported on the moving belt to an array of energy-sensitive solid-state detectors. The half-life of Lr was too short to allow chemical separation, and Lr was the first element to be identified by purely instrumental methods. It was named in honour of Lawrence, the inventor of the cyclotron. 

The applicability of heavy-ion reactions to the production of heavy elements increased with the development of efficient heavy-ion accelerators at Berkeley, Dubna and Darmstadt. On the other hand, the importance of instrumental methods... 

---