E.-W. Grewe

Studies on the double-β decay nucleus Zn64 using the (d,He2) reaction

The (d, 2 He) charge-exchange reaction on the double-β decay (ββ) nucleus 64 Zn has been studied at an incident energy of 183 MeV. The two protons in the 1 S 0 state (indicated as 2 He) were both momentum analyzed and detected simultaneously by the BBS magnetic spectrometer and its position-sensitive detector. 2 He spectra with a resolution of about 115 keV (FWHM) have been obtained allowing identification of many levels in the residual nucleus 64 Cu with high precision. 64 Zn is one of the rare cases undergoing a ββ decay in β + direction. In the experiment presented here, Gamow-Teller (GT + ) transition strengths have been extracted. Together with the GT - transition strengths from 64 Ni( 3 He,t) data to the same intermediate nucleus 64 Cu, the nuclear matrix elements of the ββ decay of 64 Zn have been evaluated. Finally, the GT + distributions are compared with shell-model calculations and a critical assessment is given of the various residual interactions presently employed for the pf shell.

High resolution study of the Gamow–Teller strength distribution starting from the ground state of 14N in the β− and β+ directions

The two charge exchange reactions 14N(d, 2He)14C and 14N(3He, t)14O were studied with high resolution in order to identify the Gamow–Teller (GT) strength distribution starting from the ground state (g.s.) of 14N in the β− and β+ directions. The isospin symmetry in the A = 14 mass system is investigated. An overview of the main features of the experimental investigation is summarized here.

Gamow-Teller transitions studied in the high-resolution {sup 64}Ni({sup 3}He,t){sup 64}Cu reaction

To study the Gamow-Teller (GT) transitions to the pf-shell nucleus Cu-64, the Ni-64(He-3,t)Cu-64 experiment was performed at the Research Center for Nuclear Physics (RCNP) Ring Cyclotron, Osaka, using a He-3 beam of 140 MeV/nucleon. The outgoing tritons were momentum analyzed by the Grand Raiden spectrometer at 0(degrees). A high energy resolution of 32 keV (full width at half-maximum) allowed the separation of individual levels in the excitation-energy region from 0 to 3.5 MeV. In addition to the ground state (gs), known to be a J(pi)=1(+) GT state, many low-lying states showed L=0 nature, suggesting that they are candidates for GT states. Because the GT strength B(GT) for the gs transition is known from the beta-decay measurement, the strengths for the excited states could be determined using the proportionality between the B(GT) and the reaction cross section extrapolated to q=0 momentum transfer. At higher excitation energies, the level density becomes high and the so-called GT giant resonance dominates the spectrum. The lower and the upper limits of the strength contained in this energy region were estimated. Our results show that less than 55% of the strength predicted by the Ikeda sum rule is located in the excitation-energy region from 0 to 17 MeV..

The (d,{sup 2}He) reaction on {sup 76}Se and the double-{beta}-decay matrix elements for A=76

The (d,{sup 2}He) charge-exchange reaction on {sup 76}Se was studied at an incident energy of 183 MeV. The outgoing two protons in the {sup 1}S{sub 0} state, referred to as {sup 2}He, were both momentum analyzed and detected by the same spectrometer and detector. The experiment was performed at KVI, Groningen, using the magnetic spectrometer BBS at three angular positions: 0 deg., 2.5 deg., and 5 deg. Excitation-energy spectra of the residual nucleus {sup 76}As were obtained with an energy resolution of about 120 keV (FWHM). Gamow-Teller (GT{sup +}) transition strengths were extracted up to 5 MeV and compared with those from an (n,p) experiment at low resolution. Together with the GT{sup -} transition strengths from the {sup 76}Ge(p,n) experiment leading to the same intermediate nucleus, the nuclear matrix element of the two-neutrino double-{beta} decay of {sup 76}Ge was evaluated.

The (d,{sup 2}He) reaction on {sup 96}Mo and the double-{beta} decay matrix elements for {sup 96}Zr

The (96)Mo(d,(2)He)(96)Nb charge-exchange reaction was investigated at an incident energy of E(d)=183.5 MeV. An excitation-energy resolution of 110 keV was achieved. The experiment was performed at KVI, Groningen, using the magnetic spectrometer BBS at three angular positions: 0(degrees),2.5(degrees), and 6(degrees). We found that below 6 MeV almost the entire Gamow-Teller (GT(+)) strength is concentrated in a single state at 0.69 MeV excitation energy. As (96)Mo is the daughter of the beta beta decay nucleus (96)Zr, the present result provides information about the nuclear matrix elements active in the 2 nu beta beta decay of (96)Zr.

Low-lying GT{sup +} strength in {sup 64}Co studied via the {sup 64}Ni(d,{sup 2}He){sup 64}Co reaction

The Ni-64(d,He-2)Co-64 reaction was studied at the AGOR cyclotron of KVI, Groningen, with the Big-Bite Spectrometer and the EuroSuperNova detector using a 171-MeV deuteron beam. An energy resolution of about 110 keV was achieved. In addition to the J(pi) = 1(+) ground state, several other 1(+) states could be identified in Co-64 and the strengths of the corresponding Gamow-Teller transitions were determined. The obtained strength distribution was compared with theoretical predictions and former (n,p) experimental results and displayed a good agreement. Due to the good energy resolution, detailed spectroscopic information was obtained, which supplements the data base needed for network calculations for supernova scenarios.

The (t,3He) reaction at 43 MeV/nucleon on 48Ca and 58Ni : Results and microscopic interpretation

We have used the 43 MeV/nucleon primary tritium beam of the AGOR facility with an intensity of 4x10(7) pps and the BBS experimental setup to study the (t,He-3) reaction between 0 degrees and 5 degrees lab angles on C-12, Ca-48, and Ni-58 targets. The standard ray-tracing procedure has allowed us to obtain excitation-energy spectra up to 30 MeV in six angular bins for each residual nucleus, with an average energy resolution of 350 keV. The reaction mechanism has been described in distorted-waves Born approximation (DWBA) using the DWBA98 code. In this approximation, the form factor is treated as a folding of an effective projectile-nucleon interaction with a transition density. The effective projectile-nucleon interaction has been adjusted to reproduce the 0(degrees) cross section of the 1(+) ground state of B-12 populated in the C-12(t,He-3) reaction. We have employed random-phase approximation (RPA) wave functions of excited states to construct the form factor instead of the normal modes wave functions used earlier. This new DWBA+RPA analysis is used to compare calculated and experimental cross sections directly and to discuss the giant resonance excitations in K-48 and Co-58 nuclei.

Distribution of the GT strength starting from the ground state of 14N

Two charge‐exchange reactions were performed at intermediate energies in order to investigate the Gamow‐Teller strength distribution starting from 14Ng.s. in β+ and β− directions: 14N(d,2He)14C and 14N(3He,t)14O. We describe here a few delicate details of the experimental techniques used in these investigations.

Spin‐isospin excitations from the ground‐state of 64Ni

Spin‐isospin (Gamow‐Teller) excitations in 64Cu and 64Co have been studied using (3He,t) and (d,2He) charge‐exchange reactions on 64Ni. As the isospin of the 64Ni ground‐state is T0=4, states with T=3, 4 and 5 in 64Cu are excited via the (3He,t) reaction and states with T=5 in 64Co via (d,2He). If we assume that the nuclear interaction is charge symmetric, the T=5 states in 64Cu should appear at corresponding excitation energies (if corrected for the Coulomb displacement) and with similar strengths as the T=5 states in 64Co. As in the 64Cu spectrum the T=5 states are very weakly excited, only by combining the results of the two complementary experiments one can estimate the Gamow‐Teller strength starting from 64Ni in a consistent way.

Structure of 7He studied with the 7Li(d,2He) reaction

A search for the Jπ = 1/2− spin–orbit partner of the Jπ = 3/2− ground state in 7He has been performed with the 7Li(d,2He) charge-exchange reaction. The results are incompatible with recent claims of such a state at very low excitation energy [Meister M et al 2002 Phys. Rev. Lett. 88 102501] but rather suggest a resonance with parameters Ex = (1.2+0.5 −0.4) MeV, Λ = (1.9+0.8−0.4) MeV. GT strengths deduced for the transitions to the lowest states in 7He are in remarkable agreement with ab initio quantum Monte Carlo calculations.