Search Results (81 total)

The B(E2;0⁺→2⁺) value in ⁶⁸Ni has been measured using Coulomb excitation at safe energies. The ⁶⁸Ni radioactive beam was postaccelerated at the CERN on-line isotope mass separator (ISOLDE) facility to 2.9 MeV/u and directed to a ¹⁰⁸Pd target. The emitted γ rays were detected by the MINIBALL detector array. Not only directly registered but also indirectly deduced information on the nucleus emitting the γ ray was used to perform the Doppler correction, leading to a larger center-of-mass angular range to infer the excitation cross section. The obtained value of 2.8_-1.0^+1.2×10²e² fm⁴ is in good agreement with the value measured at intermediate energy Coulomb excitation, confirming the low 0⁺→2⁺ transition probability.

The reduced transition probabilities, B(E2;0_gs⁺→2₁⁺), have been measured in the radioactive isotopes ^108,106Sn using subbarrier Coulomb excitation at the REX-ISOLDE facility at CERN. Deexcitation γ rays were detected by the highly segmented MINIBALL Ge-detector array. The results, B(E2;0_gs⁺→2₁⁺)=0.222(19)e²b² for ¹⁰⁸Sn and B(E2;0_gs⁺→2₁⁺)=0.195(39)e²b² for ¹⁰⁶Sn were determined relative to a stable ⁵⁸Ni target. The resulting B(E2) values are ∼30% larger than shell-model predictions and deviate from the generalized seniority model. This experimental result may point towards a weakening of the N=Z=50 shell closure.

Collective properties of the low-lying levels in the odd-A ^67–73Cu were investigated by Coulomb excitation with radioactive beams. The beams were produced at ISOLDE and postaccelerated by REX-ISOLDE up to 2.99  MeV/u. In ^67,69Cu, low-lying 1/2^-, 5/2^-, and 7/2^- states were populated. In ^71,73Cu, besides the known transitions deexciting the single-particle-like 5/2^- and core-coupled 7/2^- levels, γ rays of 454 and 135 keV, respectively, were observed for the first time. Based on a reanalysis of β-decay work and comparison with the systematics, a spin 1/2^- is suggested for these excited states. Three B(E2) values were determined in each of the four isotopes. The results indicate a significant change in the structure of the odd-A Cu isotopes beyond N=40 where single-particle-like and collective levels are suggested to coexist at very low excitation energies.

Neutron-rich, radioactive Zn isotopes were investigated at the Radioactive Ion Beam facility REX-ISOLDE (CERN) using low-energy Coulomb excitation. The energy of the 2₁⁺ state in ⁷⁸Zn could be firmly established and for the first time the 2⁺→0₁⁺ transition in ⁸⁰Zn was observed at 1492(1) keV. B(E2,2₁⁺→0₁⁺) values were extracted for ^74,76,78,80Zn and compared to large scale shell model calculations. With only two protons outside the Z=28 proton core, ⁸⁰Zn is the lightest N=50 isotone for which spectroscopic information has been obtained to date. Two sets of advanced shell model calculations reproduce the observed B(E2) systematics. The results for N=50 isotones indicate a good N=50 shell closure and a strong Z=28 proton core polarization. The new results serve as benchmarks to establish theoretical models, predicting the nuclear properties of the doubly magic nucleus ⁷⁸Ni.

Gamma rays deexciting states in the proton emitter ¹⁴⁵Tm were observed using the recoil-decay tagging method. The ¹⁴⁵Tm ground-state rotational band was found to exhibit the properties expected for an h_11/2 proton decoupled band. In addition, coincidences between protons feeding the 2⁺ state in ¹⁴⁴Er and the 2⁺→0⁺ γ-ray transition were detected, the first measurement of this kind, leading to a more precise value for the 2⁺ excitation energy of 329(1) keV. Calculations with the particle-rotor model and the core quasiparticle coupling model indicate that the properties of the πh_11/2 band and the proton-decay rates in ¹⁴⁵Tm are consistent with the presence of triaxiality with an asymmetry parameter γ≈20°.

A study of the level structure of ²⁶Si using in-beam γ-ray spectroscopy is presented. A full level scheme is derived incorporating all states lying below the proton threshold energy. The results are in good agreement with shell model predictions and one-to-one correspondence is found with states in the mirror nucleus ²⁶Mg. Additionally, a γ-decay branch is observed from a state at 5677.0(17) keV, which is assigned to a 1⁺ resonance important in the astrophysical reaction ²⁵Al(p,γ)²⁶Si. The newly derived resonance energy, E_r=159.2(35) keV, has the effect of decreasing the reaction rate at the novae ignition temperature of ≈0.1 GK by a factor of ≈2 when compared with the previous most precise measurement of this state.

The first excited 2⁺ state of the unstable isotope ¹¹⁰Sn has been studied in safe Coulomb excitation at 2.82  MeV/u using the MINIBALL array at the REX-ISOLDE post accelerator at CERN. This is the first measurement of the reduced transition probability of this state using this method for a neutron deficient Sn isotope. The strength of the approach lies in the excellent peak-to-background ratio that is achieved. The extracted reduced transition probability, B(E2:0⁺→2⁺)=0.220±0.022e²b², strengthens the observation of the evolution of the B(E2) values of neutron deficient Sn isotopes that was observed recently in intermediate-energy Coulomb excitation of ¹⁰⁸Sn. It implies that the trend of these reduced transition probabilities in the even-even Sn isotopes is not symmetric with respect to the midshell mass number A=116 as ¹⁰⁰Sn is approached.

We report on the first low-energy Coulomb excitation measurements with radioactive I^π=6^- beams of odd-odd nuclei ^68,70Cu. The beams were produced at ISOLDE, CERN and were post-accelerated by REX-ISOLDE to 2.83  MeV/nucleon. γ rays were detected with the MINIBALL spectrometer. The 6^- beam was used to study the multiplet of states (3^-, 4^-, 5^-, 6^-) arising from the π2p_3/2ν1g_9/2 configuration. The 4^- state of the multiplet was populated via Coulomb excitation and the B(E2;6^-→4^-) value was determined in both nuclei. The results obtained illustrate the fragile stability of the Z=28 shell and N=40 subshell closures. A comparison with large-scale shell-model calculations using the ⁵⁶Ni core shows the importance of the proton excitations across the Z=28 shell gap to the understanding of the nuclear structure in the neutron-rich nuclei with N≈40.

Using a method whereby molecular and atomic ions are independently selected, an isobarically pure beam of ⁷⁰Se ions was postaccelerated to an energy of 206 MeV using REX-ISOLDE. Coulomb-excitation yields for states in the beam and target nuclei were deduced by recording deexcitation γ rays in the highly segmented MINIBALL γ-ray spectrometer in coincidence with scattered particles in a silicon detector. At these energies, the Coulomb-excitation yield for the first 2⁺ state is expected to be strongly sensitive to the sign of the spectroscopic quadrupole moment through the nuclear reorientation effect. Experimental evidence is presented here for a prolate shape for the first 2⁺ state in ⁷⁰Se, reopening the question over whether there are, as reported earlier, deformed oblate shapes near to the ground state in the light selenium isotopes.

The ³⁰P(p,γ)³¹S reaction rate is expected to be the principal determinant for the endpoint of nucleosynthesis in classical novae. To date, the reaction rate has only been estimated through Hauser-Feschbach calculations and is unmeasured experimentally. This paper aims to remedy this situation. Excited states in ³¹S and ³¹P were populated in the ¹²C(²⁰Ne,n) and ¹²C(²⁰Ne,p) reactions, respectively, at a beam energy of 32 MeV, and their resulting γdecay was detected with the Gammasphere array. Around half the relevant proton unbound states in ³¹S corresponding to the Gamow window for the ³⁰P(p,γ)³¹S reaction were identified. The properties of the unobserved states were inferred from mirror symmetry using our extended data on ³¹P. The implications of this new spectroscopic information for the ³⁰P(p,γ)³¹S reaction rate are considered and recommendations for future work with radioactive beams are discussed.

Resonant elastic scattering of a radioactive ²⁰Na beam incident upon protons in a polyethylene target has been used to probe the level structure of ²¹Mg above the proton decay threshold. Three states have been observed, and their properties deduced through analysis based on the R-matrix formalism. The results improve and extend previous studies of this nucleus. An estimate of the ²⁰Na(p,γ)²¹Mg reaction rate, including these new data, suggests this reaction will not play a significant role in explosive hydrogen burning in astrophysical sites such as novae and x-ray bursts.

We have investigated the low-energy states of ¹¹N by the resonant elastic-scattering method in inverse kinematics using a ¹⁰C beam and a (CH₂)_n target at the CYCLONE facility at Louvain-la-Neuve. Recoil protons were detected at laboratory angles θ_lab=5.2^°-18.3^° in a ΔE-E detector system. The absolute ¹⁰C+p elastic cross-section data were analyzed in the R-matrix framework. We found ¹¹N to be unbound with respect to proton emission by 1.54±0.02 MeV, with a decay width of 0.83±0.03 MeV. These results are used to calculate the two-proton decay width of the ¹²O ground state.

Excited states in ³¹S and ³¹P were populated in the ¹²C(²⁰Ne,n) and ¹²C(²⁰Ne,p) reactions, respectively, at a beam energy of 32 MeV. High spin states of positive and negative parity have been observed in ³¹S for the first time, and the yrast scheme of ³¹P has been extended. Large mirror energy differences between the first 9/2^- and 13/2^- states were observed, but only small differences for the first 7/2^- and 11/2^- levels. The significance of these observations is discussed in relation to the electromagnetic spin-orbit effect and the relative binding energy of the levels.

We report on the first radioactive beam experiment performed at the recently commissioned REX-ISOLDE facility at CERN in conjunction with the highly efficient γ spectrometer MINIBALL. Using ³⁰Mg ions accelerated to an energy of 2.25  MeV/u together with a thin ^natNi target, Coulomb excitation of the first excited 2⁺ states of the projectile and target nuclei well below the Coulomb barrier was observed. From the measured relative deexcitation γ-ray yields the B(E2;0_gs⁺→2₁⁺) value of ³⁰Mg was determined to be 241(31)e²   fm⁴. Our result is lower than values obtained at projectile fragmentation facilities using the intermediate-energy Coulomb excitation method, and confirms the theoretical conjecture that the neutron-rich magnesium isotope ³⁰Mg resides outside the “island of inversion.”

The degree to which the (p,γ) and (p,α) reactions destroy ¹⁸F at temperatures (1-4)×10⁸  K is important for understanding the synthesis of nuclei in nova explosions and for using the long-lived radionuclide ¹⁸F, a target of γ-ray astronomy, as a diagnostic of nova mechanisms. The reactions are dominated by low-lying proton resonances near the ¹⁸F+p threshold (E_x=6.411  MeV in ¹⁹Ne). To gain further information about these resonances, we used a radioactive ¹⁸F beam from the Holifield Radioactive Ion Beam Facility to selectively populate corresponding mirror states in ¹⁹F via the inverse ²H(¹⁸F,p)¹⁹F neutron transfer reaction. Neutron spectroscopic factors were measured for states in ¹⁹F in the excitation energy range 0–9  MeV. Widths for corresponding proton resonances in ¹⁹Ne were calculated using a Woods-Saxon potential. The results imply significantly lower ¹⁸F(p,γ)¹⁹Ne and ¹⁸F(p,α)¹⁵O reaction rates than reported previously, thereby increasing the prospect of observing the 511  keV annihilation radiation associated with the decay of ¹⁸F in the ashes ejected from novae.

A multichannel R-matrix formalism was used to fit ²¹Na+ p resonant elastic and inelastic scattering data taken at the TRIUMF UK detector array facility at TRIUMF-ISAC. Five resonances were observed corresponding to states in ²²Mg above the proton threshold. Four of these corresponded to states seen in previous transfer reaction studies where firm spin-parity assignments could not be made. One new resonance, previously unobserved in any reaction, was also seen. Where possible, resonance energies, partial widths, and spin-parity values of these resonances were extracted. The correspondence between these states and possible analogs in the mirror nucleus ²²Ne is discussed, as well as the relation to T=1 states in the nucleus ²²Na.

A study of the reaction ¹²C(e,e^'p)¹¹B in parallel kinematics has been made for the missing-momentum region 250<p_m<310 MeV/c at incident energies of 379 and 585 MeV. A Rosenbluth separation of the cross sections was carried out to extract the longitudinal and transverse structure functions W_L and W_T. Calculations that include meson-exchange currents and isobar currents are seen to describe the data better than those using a one-body current only. For the first time, a comparison between the ¹²C(γ,p) cross section and the transverse part of the ¹²C(e,e^'p) cross section has been made versus an effective momentum that is defined in such a way as to minimize effects due to differences in final-state interactions. This comparison suggests that it may be possible to describe both reactions in one consistent framework if two-body currents are included.

The level structure of ²²Mg has been studied with high-sensitivity γ-ray spectroscopy techniques. A complete level scheme is derived incorporating all subthreshold states and all levels in the energy region relevant for novae burning. The excitation energy of the most important astrophysical resonance is measured with improved accuracy and found to differ from previous values. Combining the present result with a recent resonance energy measurement of this state leads to a derived ²²Mg mass excess of -400.5(13)  keV.

The first results are reported on (⁶He,⁸Be) two-proton pick-up reactions on ¹²C, ¹⁶O, and ¹⁹F nuclei. The measurements were done with an 18  MeV beam on ⁶LiF, ⁷LiF, ⁶Li₂CO₃, and ¹²C targets. The measured angular distributions for the ¹²C(⁶He,⁸Be)¹⁰Be (g.s.) and ¹²C(⁶He,⁸Be)¹⁰Be^*(3.37  MeV) reactions show a clear signature of a direct process. Although the contributions from the ⁶Li(⁶He,⁸Be)⁴H reaction were observed, no clear extraction of the ⁴H data was possible.

The T₂₀ analyzing powers have been measured for the ¹²⁰Sn(⁷Li,⁸Be→2α)¹¹⁹In and ¹²⁰Sn(⁷Li,⁶Li^*→α+d)¹²¹Sn transfer breakup reactions, using a 70  MeV beam. The data exhibit excellent agreement with the results of coupled reaction channels calculations, providing an important test of these calculations when applied to the transfer breakup reaction mechanism.

The proton decay of the highly deformed nucleus ¹³⁵Tb has been observed following bombardment of a ⁹²Mo target with a 310  MeV beam of ⁵⁰Cr ions. This is the first instance of a proton-decaying isotope being produced via the 1p6n fusion-evaporation channel. Evaporation residues were separated in-flight using the Argonne fragment mass analyzer and implanted into a new design double-sided silicon strip detector. ¹³⁵Tb decays by the emission of a proton with energy E_p=1179(7)  keV  [Q_p=1188(7)  keV] and half-life t_1∕2=0.94_−0.22^+0.33ms. The transition is assigned to a highly deformed (β₂∼0.3)  J^π=7∕2⁻ configuration by comparing the proton decay rate with calculations for deformed nuclei.

The reaction ⁶He+⁶⁴Zn was studied in order to investigate the effects of the projectile neutron-halo structure on the reaction mechanism at energies around the Coulomb barrier. Elastic scattering angular distributions, transfer∕breakup angular distributions, and fusion excitation functions have been measured. Due to the low-recoil energy of the evaporation residues and the low intensity of the ⁶He beam, the fusion cross section was measured by detecting off-line the atomic x-ray emission which follows the electron capture decay of the evaporation residues. For comparison the reaction ⁴He+⁶⁴Zn was studied using the same technique. The data for the reaction ⁶He+⁶⁴Zn show that the transfer and breakup mechanisms account for almost 80% of the total reaction cross section, moreover we do not observe an enhancement of the fusion cross section when compared with the ⁴He+⁶⁴Zn reaction.

The proton decay of the highly deformed odd-odd nucleus ¹³⁰Eu has been observed by bombarding a ⁵⁸Ni target with a beam of 425  MeV ⁷⁸Kr ions. Fusion evaporation residues were separated in-flight using the Argonne fragment mass analyzer and implanted into a double-sided silicon strip detector. ¹³⁰Eu decays by the emission of a proton with energy E_p=1020(15)  keV  [Q_p=1028(15)  keV] and half-life t_1∕2=0.90_−0.29^+0.49  ms. The transition is assigned to a highly deformed (β₂∼0.3)J^π=1⁺ ground-state configuration by comparing the decay rate with a new theoretical formalism for deformed odd-odd nuclei. These results clearly demonstrate the sensitivity of the proton decay rate to the spectator neutron configuration.

The ⁶He + ⁴He elastic scattering cross section was measured at E_c.m.=11.6 MeV using a ⁶He radioactive beam and a ⁴He-implanted Al foil as target. The use of the thin implanted target allowed to enlarge the angular range in which the data were collected, with respect to a previous measurement at the same center-of-mass energy. The new and previous data are compared with several calculations. The inclusion of a description for the 2n-transfer process, either with a parity-dependent term, or with a distorted-wave Born approximation transfer amplitude, is crucial to reproduce the increase of the cross section at large angles, thus assessing the contribution of the 2n-transfer process to the reaction. At the present energy, the complicated nature of the transfer process and the possible role played by the coupling to other channels, in particular, the breakup, require sophisticated reaction models in order to extract the relevant information on the peculiar ⁶He nucleus. The quality of the data is such that the differences between ⁶He and the similar ⁶Li system should be identified by such calculations.

The second excited state (J^π=1/2⁺, l=0) of the proton-rich nucleus ¹⁹Na has been observed for the first time using the elastic scattering reaction ¹H(¹⁸Ne,¹H)¹⁸Ne. An intense (4×10⁶ pps) ¹⁸Ne radioactive beam was produced at the CRC-RIB facility at Louvain-la-Neuve to bombard a polyethylene target. The recoil protons were detected at 20 different angles in the LEDA segmented silicon detector. The resulting elastic scattering cross sections have been analyzed using a global R-matrix fit. We find that the second excited state in ¹⁹Na lies at the center-of-mass energy E_c.m.=1.066±0.003 MeV with respect to the ¹⁸Ne+p threshold, and has a proton width Γ_p=101±3 keV. The Coulomb shift between the 1/2⁺ mirror levels in ¹⁹O and ¹⁹Na is 0.73 MeV, among the largest values currently observed in light exotic nuclei.