Intruder features in the island of inversion: The case of33Mg

The Na-33 beta decay was studied online using mass separation techniques and a first description of the level structure of the neutron-rich isotope Mg-33, with N=21, has been obtained. The experiment involved the measurement of beta-gamma, beta-gamma-gamma, and beta -n-gamma coincidences as well as neutron spectra by time-of-flight technique. The first low energy level scheme for the daughter nucleus Mg-33 is given with five bound states. Spin and parity assignments are proposed according to beta feedings and gamma -ray multipolarities, beta -strength distribution is evaluated, taking into account 1n- and 2n-emission channels and it is compared with the calculated GT strength distribution. …

Neutron capture cross section measurements for nuclear astrophyisics at CERN n_TOF

A series of neutron capture cross section measurements of interest to nuclear astrophysics have been recently performed at n_TOF, the neutron spallation source operating at CERN. The low repetition frequency of the proton beam driver, the extremely high instantaneous neutron flux, and the low background conditions in the experimental area are optimal for capture cross section measurements on low-mass or radioactive samples. An overview of the measurements performed during the two experimental campaigns in 2002 and 2003 is presented with special emphasis on the measurement of the capture cross sections of the Os isotopes relevant for the cosmochronology based on the Re/Os clock. http://www.s…

Beta decay of $^{61}$Ga

The β decay of 61Ga to its mirror nucleus 61Zn has been measured for the first time by using on-line mass separation and β-delayed gamma-ray spectroscopy. The observed decay strength to the ground state implies superallowed character in accordance with the systematics of the mirror decays in the sd and fp shell. The β feedings observed to four excited states in 61Zn are consistent with earlier spin-parity assignments based on in-beam experiments. The ground-state spin and parity for 61Ga were determined to be 3/2−.

Spectroscopy of34,35Sibyβdecay:sd−fpshell gap and single-particle states

Beta-delayed neutron decay of 33Na

Abstract Beta-delayed neutron decay of 33 Na has been studied using the on-line mass separator ISOLDE. The delayed neutron spectra were measured by time-of-flight technique using fast scintillators. Two main neutron groups at 800(60) and 1020(80) keV were assigned to the 33 Na decay, showing evidence for strong feeding of states at about 4 MeV in 33 Mg. By simultaneous β–γ–n counting the delayed neutron emission probabilities P 1n =47(6)% and P 2n =13(3)% were determined. The half-life value for 33 Na, T 1/2 =8.0(3) ms , was measured by three different techniques, one employing identifying gamma transitions and two employing beta and neutron counting.

Intruder features in the island of inversion : The 33Mg case

The 33 Na β decay was studied online using mass separation techniques and a first description of the level structure of the neutron-rich isotope 33 Mg , with N = 21 , has been obtained. The experiment involved the measurement of β-γ, β-γ-γ, and β − n − γ coincidences as well as neutron spectra by time-of-flight technique. The first low energy level scheme for the daughter nucleus 33 Mg is given with five bound states. Spin and parity assignments are proposed according to β feedings and γ-ray multipolarities. β-strength distribution is evaluated, taking into account 1 n - and 2 n -emission channels and it is compared with the calculated GT strength distribution. In particular, the 1 p − 1 h …

A step further in the A = 33−35, N ≃ 21, island of inversion: the structure of 33Mg

Experimental indications have been found in the seventies for the deformation of neutron-rich A ≃ 32 nuclei [1]. This could be explained by Hartree-Fock calculations, predicting deformed configurations in the ground state of nuclei in the A = 33−35, N ≃ 21 mass region. This exotic region, called the island of inversion [2], knows a renewed interest since it can be now experimentally accessible for detailed studies.

Studying exotic nuclides close to the N = Z line at the HIGISOL facility

The ion guide [1, 2] for heavy-ion fusion-evaporation reactions (HIGISOL) which was developed by Beraud et al. [3] has been implemented at the IGISOL facility in Jyvaskyla [4]. This system was modified over the past 5 years. Figure 1 shows the present set-up. The HIGISOL takes advantage of the different angular distributions of primary beam and reaction products: the primary beam is stopped in front of the stopping chamber and the reaction products enter the stopping chamber through a thin foil passing the beam stop. This so called “shadow” method removes the plasma effect since the primary beam is not ionising the stopping gas. In order to improve ion optical properties, mainly to reduce t…

Beta-decay half-lives of $^{70}$Kr and $^{74}$Rb

Abstract Beta-decay half-lives of two nuclei close to N = Z line, 70 Kr and 74 Rb, have been measured at the ISOLDE mass-separator facility at CERN. Importance of these half-lives on two ingredients explaining existence and development of the Universe, the astrophysical nucleosynthesis and the Standard Model, are discussed.

Single-Neutron States inS133n

The location of several single-neutron states in ${}^{133}\mathrm{Sn}$ has been identified. The ${p}_{3/2}$, ${h}_{9/2}$, and ${f}_{5/2}$ states were found at 853.7, 1560.9, and 2004.6 keV, respectively, by measuring $\ensuremath{\gamma}$ rays in coincidence with delayed neutrons following the decay of ${}^{134}\mathrm{In}$. Crucial for obtaining the new data were the improved yields at the mass-separator facility ISOLDE-PSB at CERN. A semiempirically adjusted Woods-Saxon calculation, based on parameters from the Pb region and normalized on the mass data at ${}^{132}\mathrm{Sn}$, reproduces the new single particle energies with good precision.

β-decay half-life of70Kr: A bridge nuclide for therpprocess beyondA=70

The -decay half-life of 70 Kr has been measured for the first time at the ISOLDE PSB Facility at CERN. Mass separated 70 Kr ions were produced by 1 GeV proton induced spallation reactions in a Nb foil. The measured half-life is 57(21) ms. This value is consistent with the half-life calculated assuming a pure Fermi decay, but is clearly lower than the value used in a recent rp-process reaction flow calculation. The result shows that the reaction flow via two-proton-capture of 68 Se is 2.5 times faster than previously calculated assuming an astrophysical temperature of 1.5 GK and a density of 10 6 g/cm 3 .

Beta-decay half-lives at the N = 28 shell closure

Abstract Measurements of the beta-decay half-lives of neutron-rich nuclei (MgAr) in the vicinity of the N =28 shell closure are reported. Some 22 half-lives have been determined, 12 of which for the first time. Particular emphasis is placed on the results for the Si isotopes, the half-lives of which have been extended from N =25 to 28. Comparison with QRPA calculations suggests that 42 Si is strongly deformed. This is discussed in the light of a possible weakening of the spin–orbit potential.

Spectroscopy of 34,35Si by beta decay : sd-fp shell gap and single-particle states

The 34,35Al b decays have been studied at the CERN online mass separator ISOLDE by b-g, b-g-g, and b-n-g measurements in order to corroborate the low-level description of 34Si and to obtain the first information on the level structure of the N521 isotope 35Si. Earlier observed g lines in 34Al decay were confirmed and new g transitions following both b decay and b-delayed neutron emission have been established. The first level scheme of 35Si includes three excited states at 910, 974, and 2168 keV. Indication is found for Jp 5(3/2)2 and (3/2)1 for the first two excited states, respectively. Beta-decay half-life of T1/2538.6(4) ms and b-delayed neutron branching value Pn541(13)% were measured …

$\beta$ - decay of the M$_{T}$=-1 nucleus $^{58}$Zn studied by selective laser ionization

$\beta$ - decay of $^{58}$Zn has been studied for the first time. A new laser ion-source concept has been used to produce mass-separated sources for $\beta$ and $\gamma$ - spectroscopy. The half-life of $^{58}$Zn was determined to be 86(18) ms. Comparisons are made with previous data from charge-exchange reactions. Our Gamow-Teller strength to the 1$^{+}$ state at 1051 keV excitation in $^{58}$Cu agrees well with the value extracted from a recent ($^{3}$He, t) study. Extensive shell-model calculations are presented.

Measurement of the n-TOF beam profile with a micromegas detector

A Micromegas detector was used in the neutron Time-Of-Flight (n_TOF) facility at CERN to evaluate the spatial distribution of the neutron beam as a function of its kinetic energy. This was achieved over a large range of neutron energies by using two complementary processes: at low energy by capture of a neutron via the 6Li(n,[alpha])t reaction, and at high energy by elastic scattering of neutrons on gas nuclei (argon+isobutane or helium+isobutane). Data are compared to Monte Carlo simulations and an analytic function fitting the beam profile has been calculated with a sufficient precision to use in neutron capture experiments at the n_TOF facility. http://www.sciencedirect.com/science/artic…

Measurement of the 151Sm n,gamma 152Sm cross section at n_TOF

The 151 Sm(n, γ ) 152 Sm cross section, which is important for the interpretation of the 151 Sm branching as an s -process thermometer, was measured from 1 eV up to 1 MeV at the innovative n_TOF facility at CERN. Based on these data, the Maxwellian-averaged cross section at k T = 30  keV is found to be 3100±160 mb. This value can be used to constrain the thermodynamical conditions in Asymptotic Giant Branch (AGB) stars during He-shell burning.

Isomeric state of 80Y and its role in the rp-process

The HIGISOL facility has been used to investigate carefully the isomeric transition 228.5 keV in 80Y. We have measured the electron internal conversion coefficient for this transition αK = 0.50 ± 0.07 which gives the value for half-life of “bare” isomeric state T 1/2 = 6.8 ± 0.5 s. The isomeric state should play an important role in the rp-process calculations.

Study of the neutron-rich nuclei with $N$ = 21, $^{35}$Si and $^{33}$Mg, by beta decay of $^{35}$Al and $^{33}$Na

Abstract The first information on the level structure of the N =21 nuclei, 35 Si and 33 Mg, has been obtained by the beta decay study of 35 Al and 33 Na, produced by fragmentation of an UC target with 1.4 GeV protons at CERN/ISOLDE. The experimental technique involved β – γ , β – γ – γ , and β –n– γ coincidences, neutron spectra being obtained by time of flight measurements. Gamma detection was made either using large Ge counters or small BaF 2 scintillators (for lifetime measurements). In the case of the 35 Al decay, ( T 1/2 =41.6(2.2) ms), a simple structure has been found for the level scheme of 35 Si ( Z =14, N =21) which has been interpreted with the level sequence : 7/2 − , 3/2 − and …

Beta-decay of 56Cu

Beta-decay studies of proton-rich isotopes near the doubly closed-shell nucleus 56Ni are of interest as (i) nuclei with a few nucleons outside a doubly-magic core are expected to represent comparatively simple configurations and thus be useful for testing nuclear shell-model predictions, and (ii) the large decay-energy window guarantees that a sizeable fraction of the strength of the allowed β-decay can be reached by the experiment. Moreover, nuclear structure properties of proton-rich N ~ Z isotopes are of astrophysical interest, e.g., concerning the EC cooling of supernovae and the astrophysical rp-process.

Measurements of the 90,91,92,94,96 Zr n, gamma cross-sections at n_TOF

Neutron capture cross sections of the 90,91,92,94,96Zr have been measured over the energy range from 1 eV to 1 MeV at the spallation neutron facility n TOF at CERN in 2003. The innovative features of the neutron beam, in particular the high instantaneous flux, the high energy resolution and low background, together with improvements of the neutron sensitivity of the capture detectors make this facility unique for neutron-induced reaction cross section measurements with much improved accuracy. The preliminary results of the Zr measurements show capture resonance strengths generally smaller than in previous measurements. Peer Reviewed

Non-analog β decay of 74Rb

The magnitude of the Coulomb mixing parameter δ 1 has been experimentally deduced, for the first time, for the β decay of 74 Rb. The estimated magnitude is derived from the feeding of the non-analog first excited 0 + state in 74 Kr. The inferred upper limit of 0.07% is small compared to theoretical predictions. The half-life was measured to be 64.90(9) ms.  2001 Published by Elsevier Science B.V.

Isomeric state of $^{80}$Y and its role in the astrophysical rp-process

5 pages, 7 figures.-- PACS nrs: 21.10.Tg; 23.20.Nx; 27.50.+e.