Ch. Miehé

Projectile fission at relativistic velocities: a novel and powerful source of neutron-rich isotopes well suited for in-flight isotopic separation

Abstract Projectile fission of 238U was investigated at a bombarding energy of 750 A·MeV using a Pb target. Forward emitted fragments from 80Zn up to 155Ce were analyzed with the Fragment Separator (FRS) and unambigously identified by their energy-loss and time-of-flight. The magnetic selection of the largest momenta acted as a trigger of the low-energy fission component. More than forty new nuclear species were identified. The related isotopic production cross-sections are presented.

Discovery and cross-section measurement of 58 new fission products in projectile-fission of 750 · A MeV

Abstract The projectile fission of uranium at relativistic energy impinging on a Be target was investigated with the fragment separator, FRS, in order to produce and identify new isotopes and to measure their production yields. Fifty eight new fragments have been observed with a neutron excess of (N - Z)/Z reaching 0.8 and cross sections as small as 0.3 nb.

^{238}

We report the first observation of the doubly magic nucleus78Ni50 and the heavy isotopes77Ni,73,74,75Co,80Cu. The isotopes were produced by nuclear fission in collisions of 750 A·MeV projectiles of238U on Be target nuclei. The fully-stripped fission products were separated in-flight by the fragment separator FRS and identified event-by-event by measuring the magnetic rigidity, the trajectory, the energy deposit, and the time of flight. Production cross-sections and fission yields for the new Ni-isotopes are given.

U

Abstract β -delayed neutron yields from 49–54 K were measured with a high-efficiency liquid scintillator neutron detector. Three new nuclides have been identified; half-lives have been determined for 50–54 K and the 53 Ca daughter, delayed neutron emission probabilities have been measured for 49–53 K.

Production and identification of heavy Ni isotopes: evidence for the doubly magic nucleus 28 78 Ni

Abstract Projectile fission of 238U was investigated at a bombarding energy of 750 A·MeV using Pb and Be targets. The fully stripped forward emitted fragments from Ti to Cs were analyzed with the Fragment Separator (FRS) and unambiguously identified by their energy-loss and time-of-flight. The magnetic selection of the largest momenta acted as a trigger of the low-energy fission component. More than a hundred new nuclear species were identified including the 78Ni, for which a cross-section of 300 pb was measured.

53K, 54K and 53Ca: Three new neutron rich isotopes

Abstract First results on a β-γ decay spectroscopy experiment of nuclei in the region of N=28 are reported. New β half-lives for nuclei from Mg to Ar have been extracted. For the heavier ones, the new periods of 48Ar and 47Cl are of importance in the understanding of solar abundance ratio 48 Ca 46 Ca . The measured half-lives of Si isotopes have been extended from N=25 to N=28 and are discussed in the light of possible deformation occuring in this region. Finally, preliminary spectroscopy of 45Ar is also reported.

Identification of more than a 100 new isotopes from 238U projectile fission and beams of neutron-rich nuclei at BRENDA

Abstract The decay of 49 K has been studied using a mass-separated ion beam by neutron and gamma-ray singles and multiparameter spectroscopic measurements. The 49 K activity was produced by 600 MeV proton fragmentation reactions in a uranium target. The observed beta-strength function displays two resonances centered at about 6.5 MeV and 9.5 MeV in 49 Ca. This structure is discussed in simple shell-model terms.

Beta-decay studies at the N=28 shell closure

Abstract In the decay of the series of T Z =1/2 nuclei, 65 Ge, 69 Se, 73 Kr and 77 Sr, a statistical analysis was applied to the β delayed proton emission to obtain nuclear structure information. New results emerge from this study, in the light of improvements in the experimental techniques and developments of the analysis method. Experimental results are analysed by comparison with a statistical model, in order to extract information on level densities and partial transition widths of the proton emitting nuclei. Limitations of the existing descriptions of processes for nuclei far from stability are pointed out.

The 49K beta decay

Charge distributions of fragments from low energy nuclear fission are investigated in reactions of highly fissile238U projectiles at relativistic energies (750 A·MeV) with a heavy (Pb) and a light (Be) target. The fully stripped fission fragments are separated by the Fragment Separator (FRS). Their high kinetic energies in the laboratory system allow the identification of all atomic numbers by using Multiple-Sampling Ionization Chambers (MUSIC). The elemental distributions of fragments observed at larger magnetic rigidities than the238U projectiles show asymmetric break-up and odd-even effects. They indicate a low energy fission process, induced mainly by dissociation in the electro-magnetic field for the U/Pb-system, or by peripheral nuclear interactions for the U/Be-system.

The delayed proton emission in the A≃65–77 mass region, statistical aspects and structure effects

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.