Heavy Ion Charge Exchange Reactions Research Articles

Study of Δ excitations in medium-mass nuclei with peripheral heavy ion charge-exchange reactions

Isobaric single charge-exchange reactions, changing nuclear charges by one unit but leaving the mass partitions unaffected, have been for the first time investigated by peripheral collisions of 112Sn ions accelerated up to 1A GeV at the GSI facilities. The high-resolving power of the FRS spectrometer allows us to obtain (p,n)-type isobaric charge-exchange cross sections with an uncertainty of 3.5% and to separate quasi-elastic and inelastic components in the missing-energy spectra of the ejectiles. The inelastic component is associated to the excitation of the Δ(1232) isobar resonance and the emission of pions in s-wave both in the target and projectile nucleus, while the quasi-elastic contribution is associated to the nuclear spin-isospin response of nucleon-hole excitations. An apparent shift of the Δ-resonance peak of ∼63 MeV is observed when comparing the missing-energy spectra obtained from the measurements with proton and carbon targets. A detailed analysis, performed with a theoretical model for the reactions, indicates that this observation can be simply interpreted as a change in the relative magnitude between the contribution of the excitation of the resonance in the target and in the projectile.

Heavy Ion Charge Exchange Reactions as Probes for Beta–Decay

Peripheral heavy ion single and double charge reactions are described by fully quantum mechanical distorted wave methods. A special class of nuclear double charge exchange (DCE) reactions proceeding as a one-step reaction through a two-body process are shown to proceed by nuclear matrix elements of a diagrammatic structure as found also in 0ν2ß decay. These hadronic Majorana-type DCE reactions (MDCE) have to be distinguished from second order DCE reactions, given by double single charge exchange (DSCE) processes, resembling 2ν2ß decay. The theoretical concepts of MDCE are discussed. First results show that ion-ion DCE reactions are the ideal testing grounds for investigations of rare second order nuclear processes, giving insight into nuclear in-medium two-body correlation.

Probing Double Beta-Decay by Heavy Ion Charge Exchange Reactions

A special class of nuclear double charge exchange (DCE) reactions proceeding as a one-step reaction through a two-body process are shown to involve nuclear matrix elements of the same diagrammatic structure as in 0ν2β decay. These hadronic Majorana-type DCE reactions (MDCE) have to be distinguished from second order DCE reactions, given by double single charge exchange (DSCE) processes resembling 2ν2β decay. The theoretical concepts of MDCE are discussed. First results show that ion-ion DCE reactions are the ideal testing grounds for investigations of second order nuclear matrix elements.

Probing beta decay matrix elements through heavy ion charge exchange reactions

To access information on neutrinoless double beta decay (0νββ) nuclear matrix elements, it has been proposed by the NUMEN collaboration to exploit the analogies between double beta decay processes and double charge exchange (DCE) nuclear reactions, looking in particular at the conditions where the corresponding cross section can be factorized into nuclear reaction and nuclear structure terms. DCE reactions can be treated as a convolution of two correlated or uncorrelated single charge exchange (SCE) processes, resembling 0νββ and 2νββ, respectively. Thus it is important to model first SCE processes, to get a deeper insight into the possibility to factorize the corresponding cross section, so one can gain a better understanding of DCE cross section factorization. In this contribution, DCE reactions are discussed in terms of the convolution of two uncorrelated SCE processes, which should allow one to extract information on 2νββ nuclear matrix elements. These theoretical investigations are performed in close synergy with the experimental activity running at INFN-LNS within the NUMEN project.

Electron capture rates in stars studied with heavy ion charge exchange reactions

Indirect methods using nucleus-nucleus reactions at high energies (here, high energies mean ~ 50 MeV/nucleon and higher) are now routinely used to extract information of interest for nuclear astrophysics. This is of extreme relevance as many of the nuclei involved in stellar evolution are short-lived. Therefore, indirect methods became the focus of recent studies carried out in major nuclear physics facilities. Among such methods, heavy ion charge exchange is thought to be a useful tool to infer Gamow-Teller matrix elements needed to describe electron capture rates in stars and also double beta-decay experiments. In this short review, I provide a theoretical guidance based on a simple reaction model for charge exchange reactions.

Coherent pion production in heavy ion charge exchange reactions

We report the first observation of coherent pion production induced by a heavy ion charge exchange reaction. The ( 12 C, 12 N) reaction [D. Bachelier et al., Phys. Lett. B 172 (1986) 23; M. Roy-Stephan et al., Nucl. Phys. A 488 (1988) 178] at 1.1 GeV/nucleon has been used to shine negative (off-shell) pions on nuclei and observe (on-shell) pions, leaving the target nucleus in its ground state. The experiment was performed at the Laboratoire National Saturne with the SPES4- π setup [Laurent Farhi PHD thesis IPNO-T97-12, Universite d'Orsay, 1997; Rasmus Dahl Ph.D., Niels Bohr Institutet, Copenhagen (1999)].

Giant quadrupole resonances and their effects on neutron-rich nuclei

Quadrupole giant resonances (GQR) play a major role in characterizing dynamical properties of both stable and unstable nuclei. In this paper two topics related with GQR are discussed based on recent experimental findings at RIKEN; 1) clear observation of isovector GQR in a heavy ion charge-exchange reaction, ( 13C, 13N) on 60Ni at 100 MeV/nucleon, and 2) E2 effective charges varying with isospin asymmetry as evidenced by the electric quadrupole moments of neutron rich B isotopes.

Heavy-ion charge-exchange reactions and the measurement of Gamow-Teller strength

The applicability of intermediate-energy heavy-ion charge-exchange reactions for measuring Gamow-Teller strength in nuclei is studied. A strong absorption model for such reactions is developed. Within the framework of this model, we define a sensitivity function which determines the important region of the target transition density in momentum space. It is found that the {ital L}=0 cross section provides a relatively model-independent measure of Gamow-Teller strength. An understanding of the large {ital L}=2 cross sections observed in heavy-ion-induced reactions is achieved. Specific application is made to the ({sup 12}C, {sup 12}B) reaction at {ital E}/{ital A}=70 MeV.

Momentum transfer in relativistic heavy ion charge-exchange reactions.

Relativistic heavy ion charge-exchange reactions yield fragments (delta Z = +1) whose longitudinal momentum distributions are downshifted by larger values than those associated with the remaining fragments (delta Z = -1, -2, ...). Kinematics alone cannot account for the observed downshifts; therefore, an additional contribution from collision dynamics must be included. In this work, an optical model description of collision momentum transfer is used to estimate the additional dynamical momentum downshift. Good agreement between theoretical estimates and experimental data is obtained.

ISovector excitations in heavy ion scattering at intermediate energies

A microscopic theory for heavy ion charge exchange reaction is formulated. The approach includes one-step direct charge exchange processes via the isovector NN-interaction and contributons from sequential, proton-neutron transfer processes, which are at least of second order. Nuclear structure is described in microscopic models and a realistic NN-interaction is used in the scattering calculations. Applications of the theory to heavy ion charge exchange reactions at incident energies per nucleon 10 MeV ≲ E/ A ≲ 200 MeV are discussed.

Heavy ion charge exchange reactions to probe the giant electric isovector modes in nuclei

The ( 13C, 13N) reaction has been studied at E/ A = 50 MeV/u on 12C, 40Ca, and 58Ni targets at extremely forward angles including zero degree. Strong excitation of the ΔT z = + 1 analog of the giant dipole resonance is observed, and its angular distribution measured over the range 0°< θ lab<3°.

Excitation of the Δ resonance in heavy ion charge exchange reactions

Results on the Δ excitation by heavy ion charge exchange are presented. 900 MeV per nucleon 12 C, 16 O, 20 Ne and 1100 MeV per nucleon 12 C have been used. The Δ excitation strength depends on the projectile — ejectile nature and on the incident energy. The role of the target mass is also discussed. The peak for the Δ in nuclei is energy shifted from the free Δ peak.

Theory of heavy ion charge exchange scattering at low and intermediate energies

Abstract The theoretical description of heavy ion charge exchange reactions is discussed. The nuclear states are described by quasi-particle and RPA-methods so that transfer and direct charge exchange processes can be analysed in compatible models for a wide class of systems. Results for charge exchange reactions at low and intermediate incident energies E A ≦100 MeV per nucleon are discussed.

Heavy ion reactions as probes for spin strength

Heavy ion charge-exchange reactions have great potential for probing spin-transfer strength in nuclei, provided beams of sufficient energy are used so that the reactions might be expected to be simple (one-step) in nature. Tests of reaction mechanisms have been carried out for two of the most favorable reactions, ( 6 Li, 6 He) and ( 12 C, 12 N). We have shown that the first reaction is mainly one-step in nature at 25 MeV/nucleon and above and have established a calibration curve relating cross sections to Gamow-Teller matrix elements. We have also shown that the second reaction is likely to be mainly one-step above about 50 MeV/nucleon. A ( 12 C, 12 N) measurement on Fe targets was done recently, with the aim of determining astrophysically interesting Gamow-Teller strengths in Fe nuclei.

Excitation of isovector giant resonances in 27Al by heavy ion charge exchange reaction.

Excitation of isovector giant resonances in $^{27}\mathrm{Al}$ was studied by using the $^{27}\mathrm{Al}$${(}^{7}$Li, $^{7}\mathrm{Be}$${)}^{27}$Mg reaction at incident energies of 100, 150, and 180 MeV. We observed the analog of the giant dipole resonance at ${E}_{x}$=14.3 MeV in $^{27}\mathrm{Mg}$. From the observed excitation function, we suggest that a peak at ${E}_{x}$=9.5 MeV is a resonance with a higher multipolarity than one. In the excitation energy region higher than the giant dipole resonance, no resonance was observed.

First observation of the Δ resonance in relativistic heavy-ion charge-exchange reactions

Excitation of the Δ resonance is observed for the first time with heavy ions. It is very strong in the two mirror charge-exchange reactions ( 20 Ne, 20 F) and ( 20 Ne, 20 Na) at 950 MeV per nucleon. With similar incident energies per nucleon, the Δ is weakly excited with a 14 N beam and nearly absent with a 12 C beam. The role of the ejectile bound states is discussed.

Mechanism of the heavy-ion charge exchange reaction 12C(12C,12N)12B at 35 MeV/nucleon.

The heavy-ion charge exchange reaction $^{12}$C${(}^{12}$C,$^{12}$N${)}^{12}$ B has been studied at E${(}^{12}$C)=35 MeV/nucleon. Angular distributions in the range 3.7--12.1\ifmmode^\circ\else\textdegree\fi{} (c.m.) have been measured for the ${1}^{+}$ (0.0 MeV), ${2}^{+}$ (0.95 MeV), ${2}^{\mathrm{\ensuremath{-}}}$ (1.67 MeV), and ${4}^{\mathrm{\ensuremath{-}}}$,${2}^{\mathrm{\ensuremath{-}}}$ (4.5 MeV) states in $^{12}$B. A broad peak at 7.8 MeV excitation was also observed. Microscopic one-step distorted-wave Born approximation calculations yield ${V}_{\ensuremath{\sigma}\ensuremath{\tau}}$ strengths \ensuremath{\ge}2 times those obtained in (p,n) reactions at similar energies per nucleon. Although this is significantly smaller than ${V}_{\ensuremath{\sigma}\ensuremath{\tau}}$ values obtained in other heavy-ion charge exchange studies, mostly carried out at energies less than 10 MeV/nucleon, it is concluded that the reaction mechanism is still dominated by sequential transfer (two-step) processes at 35 MeV/nucleon. A rough estimate indicates that one-step processes will dominate at bombarding energies above about 50 MeV/nucleon.

Excitation of the Δ resonance in relativistic heavy ion charge exchange reactions

Excitation of the Δ resonance has been observed for the first time in a heavy ion reaction. The strength of the Δ signal depends on the projectile nuclear structure. In the same incident energy range, about 900 MeV per nucleon, we observe — a strong Δ excitation in 27Aℓ( 20Ne, 20F) and 27Aℓ( 20Ne, 20Na), — a weak Δ in 12C( 14N, 14C) and — nearly no Δ excitation in ( 12C, 12B) and ( 12C, 12N) whatever the target is among a sample of nuclei from 12C to 89Y.

Heavy-ion charge exchange reactionSi28(O18,F18)Al28

The angular distribution has been measured for the 3/sup +/ and 2/sup +/ ground state doublet in /sup 28/Al with the heavy-ion charge exchange reaction /sup 28/Si(/sup 18/O,/sup 18/F)/sup 28/Al at the incident beam energy of 56 MeV. It has been analyzed in terms of the microscopic one-step direct charge exchange model and the second-order DWBA approach with recoil effect taking into account the two-step successive transfer: one nucleon stripping and pickup, and pickup and stripping process. The general shape of angular distribution has been reproduced by both approaches. The strength of the spin-isospin dependent two-body interaction potential extracted from one-step calculations is close to the upper limit of uncertainties reported in the light-ion charge exchange experiment. The two-step mechanism turns out to be also important and could be a competing process.