Charge-exchange reactions are a unique tool to study the spin-isospin response of nuclei. In contrast to β decay, in which only nuclear states in the limited Q-value window are accessible, charge-exchange reactions probe the entire response function, including the giant resonance region. Gamma-ray tagging in charge-exchange reactions has become an important new experimental tool in recent years, complementing high-resolution particle spectroscopy. A variety of new reaction probes involving the detection of γ rays have been developed to isolate specific spin-isospin excitations in charge-exchange experiments with beams of about 100 MeV/u and above. With rare-isotope beam intensities and γ-ray detection capabilities currently available, the scientific reach of these experiments has been limited: the nuclei studied were light (A<35) and relatively close to stability. With GRETA, the efficiency and quality of these new types of experiments can be vastly improved, and in combination with the availability of intense rare-isotope beams at FRIB, provide exciting opportunities to improve our understanding of the spin-isospin response of nuclei far away from stability.
The (7Li,7Be+γ) reaction in inverse kinematics was used to measure the Gamow-Teller strength distribution in the (n,p) or β+ direction in unstable isotopes [Zeg10,Meh12] with the goal to probe configuration mixing and shell evolution in neutron-rich nuclei. In these experiments, a 429-keV γ line emitted at rest when 7Be is produced in its 1/2- excited state at 429 keV from the 3/2- 7Li ground state, serves as a clean tag for a spin-transfer (ΔS=1) charge-exchange reaction. The excitation energy of the probed rare isotope is determined by tracing the path of the recoiling excited rare isotope in a magnetic spectrometer, while additional information is deduced from detecting in-flight decay γ rays emitted from this recoiled nucleus. Although significant successes were achieved, the technique was limited in part by the ability to detect photons with high efficiency and with sufficient angular resolution to perform Doppler reconstruction of the inflight γ-rays. GRETA will revolutionize these measurements, as both of these issues will be resolved. By placing GRETA around the target station of the planned High-Rigidity Spectrometer (HRS), further improvements can be made by including the detection of neutrons emitted in-flight from the excited rare-isotopes, in particular to reconstruct the Gamow-Teller strength beyond the neutron-decay threshold. Such a development would not only benefit nuclear structure studies, but also be of great importance for testing theoretical models used for estimating electron-capture rates in astrophysical phenomena.
Equally promising is the use of light rare-isotope beams as novel probes to extract detailed information about specific spin-isospin excitations and giant resonances. For example, the (10C,10B+γ) [Sas12] and (10Be,10B+γ) [Sco14] reactions have been employed to seek unambiguous evidence for the elusive isovector giant monopole resonance [Har01,Yos10,Nik13] (IVGMR). The IVGMR is the isovector partner of the isoscalar giant monopole resonance and can be described macroscopically as an out-of-phase breathing mode of the neutron and proton fluids. A detailed knowledge of its properties will complement information about the equation of state of nuclear matter obtained from the properties of the ISGMR [Har01] and further constrain theoretical models used in, for example, the modeling of neutron skins and neutron stars. Because the IVGMR is not associated with spin transfer (ΔS=0), it is usually impossible to isolate its signature in charge-exchange experiments, since spin transfer transitions (ΔS=1) strongly dominate. By impinging unstable 10C or 10Be (both have Jπ=0+) beams on stable targets, and gating on γ rays from the 0+ excited state in 10B (1.022 MeV γ ray from the decay of the 1.74 MeV 0+ state) that are emitted in flight, a clean ΔS=0 filter can be created. The (10C,10B+γ) experiment [Sas12] suffered from a limited Doppler-reconstructed γ-ray energy resolution, which made it difficult to unambiguously characterize background under the 1.022 MeV γ ray and resulted in relative poor signal-to-noise ratios. The (10Be,10B+γ) experiment [Sco14] resolved these issues by using GRETINA. However, given the limited solid-angle coverage of GRETINA, and the relatively low 10Be beam intensities currently available, the heaviest target that could be studied at NSCL was 28Si.
The use of GRETA, in combination with intense rare-isotope beams at FRIB, will provide unprecedented access to details of the IVGMR, for which the only significant information comes from studies that suffered from very significant and poorly-understood backgrounds [Ere86, Iro86, Nak99]. GRETA provides additional opportunities: due to its near 4π solid-angle coverage, high-energy γ rays can be reconstructed by using add-back techniques, which will enable the detection of direct, high-energy decay branches from giant resonances. Moreover, due to the large solid angle coverage, the angular distribution of the emitted γ rays from specific excitations/resonances can be studied to constrain their multipolarities.