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Nucleon Charge-Exchange Reactions at Intermediate Energy

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Nucleon Charge-Exchange Reactions at Intermediate Energy AU9716633 Nucleon Charge-Exchange Reactions at Intermediate Energy W.P. Alford University of Western Ontario and TRIUMF B.M. Spicer School of Physics University of Melbourne Nucleon Charge Exchange Reactions at Intermediate Energy Contents Introduction Early Results in the Study of Spin and Isospin Excitations Experimental Facilities Measurement of GT Strength Gamow-Teller Giant Resonance Beyond GT-Strength: Multipole Analysis Spin Dipole and Higher Multipole Transitions QuasiElastic Scattering Summary and Conclusions 1 Introduction For many decades, the Gamow-Teller (GT) or spin-flip, isospin-flip interaction has been central to many important areas of nuclear physics research. First identified as a component of the weak interaction in allowed beta-decay, it plays a critical role in the initial step of the hydrogen fusion reaction leading to nucleosynthesis, and in the electron capture reactions leading to stellar collapse and supernova formation. At higher excitation energies where the GT giant resonance becomes important, its properties have provided critical tests of the limits of the nuclear shell model, or alternatively, have indicated effects which may lie beyond the picture of individual nucleons moving in a mean field. For existence of a spin-flip, isospin-flip component in the nucleon-nucleus effective in- teraction was demonstrated in low energy (p,n) reactions over forty years ago, and the connection between allowed beta-decay rates and (p, n) reaction cross sections was clearly recognised at that time. Interest in this field was high, but until about fifteen years ago there was a very limited data base for comparison with the large body of theoretical spec- ulation. This situation changed with the demonstration at Michigan State University, and soon after more convincingly at the Indiana University Cyclotron Facility (IUCF), that the (p, n) reaction at intermediate energies provided a quantitative tool for the study of GT-transitions corresponding to /?~-decay. Then, in the mid-nineteen eighties, the development of new facilities, first at TRIUMF and soon after at LAMPF, opened up the study of (n,p) reactions, corresponding to allowed /?+-decay, as well as maintaining the possibility of studying (p, n) reactions. Thus it became possible to carry our systematic studies of both GT+ and GT~ giant resonances and to 1 investigate fully the implications of the very powerful GT sum rule. This review describes the intermediate energy charge exchange reaction field at a time when a large body of experimental data has been accumulated and is available for comparison with theoretical models. The presentation reflects an experimentalist's viewpoint; an excellent review of the field from a theoretical viewpoint has recently been given by Osterfeld [Os92]. The two reviews may be regarded as complementary. An historical review of the development of ideas pertaining to Gamow-Teller giant reso- nances is given, and a description of the emergence of techniques for the study of charge exchange reactions - particularly the technical advances which yielded the recent volume of new data. The present status of charge exchange reactions is reviewed and assessed. Evidence is presented from the 14C(p, n) reaction for the dominance of the spin-isospin component of the nucleon-nucleon interaction in intermediate energy reactions. In (p, n) reactions the Gamow-Teller giant resonance dominates the spectra, with higher multipoles contributing. By contrast, in (n,p) reactions in the heavier nuclei, the Gamow-Teller transitions are substantially Pauli-blocked and the spin dipole resonance dominates, with contributions from higher multipoles. Discussions of the multipole decomposition process, used to obtain from the data the contributions of the different multipoles, and the contributions of the multipoles, are given. There is discussion of the nuclear spin-isospin response at large momentum transfer in the quasi-free region of excitation. The study of this region is expected to provide information 2 that is relevant to the nucleon-nucleon interaction in nuclear matter. Finally, a discussion of important open questions and of possible further advances in this field is presented. 2 Early Results in the Study of Spin and Isospin Ex- citations The results to be described involve both the weak beta decay interaction and the strong nuclear interaction. A brief discussion of the essential concepts from these two fields is therefore included here. 2.1 Beta decay The process of allowed beta decay is known to take place by two different modes, the Fermi (F) or Gamow-Teller (GT) modes. In the Fermi mode the transition operator is OF — T*, the isospin raising or lowering operator corresponding to beta decay by positron or electron emission. These transitions occur between isobaric analogue states, the quantum numbers of which differ from one another only in the third component of isobaric spin, Tz = (N — Z)/2. Thus the selection rule for Fermi transitions is AJ = 0, ATT = 0. The comparative half-life for such a transition is given by t* 6162 A ft i = —— seconds , 2 Bp- where the transition strength is defined by [Bo69] Since initial and final wave functions are essentially identical in these transitions, the comparative half-life is short and the transitions are said to be superallowed. The second decay mode, the GT mode, involves the nuclear transition operator OGT = <?T± where & is the usual Pauli spin operator. In this case selection rules are AJ = 0, ±1 4 (0+ •/* 0+), AT = 0, and the transition strength is denned as BGT — The comparative half life is then 6162 7 2 \ (gA/gv) BGT ' where gv,gA are the vector and axial vector weak coupling constants. We finally note that for transitions between isobaric analogue states in odd-A nuclei, both modes may contribute so that 6162 5 2 Bp + {gA/g\') BaT A long-standing puzzle in beta decay studies was posed by the observation that decay rates for GT transitions were always one to two orders of magnitude slower than predicted with the "best" model wave functions. This is the problem of the quenching of GT strength. A great deal of progress in understanding this problem has come from nuclear reaction studies, and some essential ideas in the theory of nuclear reactions are now outlined. 2.2 Direct nuclear reactions The nucleon-nucleon interaction may be described in a variety of ways, but for present purposes its spin and isospin structures are emphasized. It is known to include central, spin- exchange, spin-orbit and tensor components both without and with an isospin-exchange character. Thus the interaction may be written as Vnn = Vijirij) = Vo + Vofo-Wj) + V30(S-L) + VT ST + T, • T}(VT + VOT{ax • a,) + V;0(S • L) + Vf ST) • (3) Here 5 = st + SJ, L = (r\ - r>) x (p, - pj) and ST is the two-body tensor operator. In general, each of the V's is a function of internucleon separation. In a nuclear reaction involving a transition between two states, the cross-section is given in the non-relativistic theory by [Sa65] k (JLX L du ~ \2*V) k, (2Jt + l) i Tj, is the transition amplitude for the reaction, and the sum is taken over magnetic sub- states. The transition amplitude is given by where k, , kj are the momenta of the incoming and outgoing particles, <^, , 4>j &re wave functions of initial and final nuclear states, and V^r,^) is some effective interaction between the incoming projectile and target nucleons. The main focus of interest here will be on nucleon charge exchange ((p, n), (n,p)) reactions at intermediate energies. At energies above about 100 MeV, the impulse approximation [Ke59, Go64] is believed to be applicable, and VeR is the free nucleon-nucleon interaction. With the restriction to charge exchange reactions only the isospin dependent terms will be involved. As an aside it should be noted that in an actual calculation, the wave functions must be antisymmetrized between projectile and target nucleons. This leads to knock-on exchange terms in Tji which make important contributions to the calculated cross section. In order to describe the essential features of the calculated reaction cross section, it is convenient to consider an effective interaction which depends only on the distance between the interacting particles, ven = V(T-JJ). This is not a limitation on the validity of the results, but simplifies the notation in the discussion below. 6 The transition amplitude which is written above in ordinary space can be Fourier trans- formed and written in terms of the momentum transfer in the reaction q = kf — k,. In q space the transition amplitude then can be written T,i(k/,ki) = JD(k}Xq)V(q)p,f(q)dq. In this expression + D(kf,ki,q) = — jX-\kj,rp) exp-{iq-rp) X (kt,rp) drp , (5) is the projectile distortion function. In a distorted wave theory D is evaluated numerically as part of a standard computer code such as DW81. In the absence of distorting potentials the theory reduces to a plane wave Born approximation and D=l. The function is the nuclear transition density and carries the information about the nuclear states in- volved. Since it is usual to consider transitions between states of definite spin and parity, it is convenient to represent p in terms of a multipole expansion. For a given transition ^t —* 4>f only a limited number of terms can contribute, and usually only the lowest allowed multipole need be considered. The specific form of these multipoles will be discussed in the context of their application in later sections. 2.3 Early investigations of charge exchange (isovector) interac- tions A possible connection between nuclear beta decay rates and (p,n) reactions was noted at least as early as 1957 [Vi57], but the first paper to really investigate the potential of charge-exchange reactions for studies of the effective interaction in nuclei was that of Bloom, Glendenning and Mozkowski [B159] entitled "The proton-neutron interaction and the (p,n) reaction in mirror nuclei".
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