N. Bijnens

Beams of short lived nuclei produced by selective laser ionization in a gas cell

Abstract An on-line laser ion source has been developed for the production of elemental and isobaric pure beams of radioactive ions. It is based on selective resonant laser ionization of nuclear reaction products thermalized and neutralized in a noble gas at high pressure. The ion source has been tested in a wide range of recoil energies going from 1.3 MeV to ∼ 90 MeV. Efficient schemes of two step laser ionization through autoionizing states have been found for nickel, cobalt and rhodium. Residence times of the reaction products in a gas cell have been measured for helium and argon as buffer gas. Elementally pure beams of 54,55Ni and 54Co, produced in a light-ion induced fusion-evaporation reaction, and of 113Rh, produced in proton-induced fission of 238U, were obtained. An efficiency of the ion source of 6.6% for fusion reactions and of 0.22% for fission reactions has been obtained. A selectivity of the ion source of 300 for fusion and 50 for fission reactions has been achieved.

Alpha decay of [sup 186]Pb and [sup 184]Hg: The influence of mixing of 0[sup +] states on [alpha]-decay transition probabilities

Fine structure in the [alpha] decay of mass-separated [sup 186]Pb and [sup 184]Hg has been studied at the GSI on-line mass separator. Alpha singles spectra as well as [alpha]-x-[ital t] and [alpha]-[ital e]-[ital t] coincidence events were collected. The [alpha] decay of [sup 186]Pb revealed feeding to a low-lying 0[sup +] state at 328(12) keV in [sup 182]Hg. This state can be interpreted as being the bandhead of the deformed rotational band observed previously in in-beam studies. In the [alpha] decay of [sup 184]Hg, feeding towards the first excited 2[sup +] state at 153 keV and the 0[sub 2][sup +] state at 478 keV in [sup 180]Pt was observed. The hindrance factor of the [alpha] decay towards the excited 0[sup +] state gives information about the particle-hole character of the states connected in the [alpha] decay. A two-level mixing calculation is introduced. From the mixing in Pt and the [alpha]-decay hindrance factors, small mixing is deduced fro ground stattes of neutron-deficient Hg and Pb nuclei.

-decay characteristics of neutron-deficient 190, 192Po nuclei and alpha branching ratios of 186, 188Pb isotopes

By using complete fusion reactions and , leading to and compound nuclei, improved -decay characteristics for neutron-deficient isotopes have been obtained. The experiments have been performed at the fragment mass analyzer (Argonne) and at the RITU gas-filled separator (Jyvaskyla). In the l = 0 decay to the excited state in is two times faster than the decay to the ground state. For more precise values of the alpha-decay energy keV and half-life ms were measured. By using correlated -decay chains -branching ratios of 38(9)% and 9.3(8)% were experimentally deduced for the first time for and , respectively. The reduced -width systematics is extended for even Po isotopes, confirming a persistence of the saturation towards the lighter isotopes. Absence of any anomalies in the reduced -widths for the neutron-deficient Pb isotopes is confirmed.

Mono-isotopic beams of exotic nuclei produced by a universal on-line laser ion source

Abstract We report on recent experiments proving the high efficiency and selectivity of a universal and fast on-line laser ion source based on resonance laser ionization in a gas cell. The application for the production of neutron-rich nickel isotopes is discussed, including the first observation of γ-transitions from the decay of 71Ni. We also discuss current efforts to still increase both efficiency and selectivity.

Can the waiting-point nucleus 78Ni be studied at an on-line mass-separator?

Abstract Short-lived nickel isotopes have been studied using a chemically selective Ion Guide Laser Ion Source (IGLIS) based on resonance ionisation of atoms at the Leuven Isotope Separator On-Line (LISOL) separator. The decay properties of different Ni isotopes have been studied using β-γ-coincidences. Experimental production rates of proton induced fission of 238U are obtained for 69,71Ni. These numbers are in a strong disagreement with Silberg-Tsao calculations.

Laser ion source for the Leuven Isotope Separator On‐Line

An on‐line laser ion source has been developed and coupled to an on‐line isotope separator for the production of isotopically and isobarically pure beams of radioactive ions. It is based on element‐selective resonant laser ionization of nuclear reaction products thermalized and neutralized in a high‐pressure noble gas after their recoil out of the target. Pure beams of 54,55Ni and 54Co, produced in a light‐ion‐induced fusion‐evaporation reaction, and of 113Rh, produced in a proton‐induced fission of 238U, were obtained. A preliminary efficiency of the ion source of ∼6% for fusion reactions and of ∼0.2% for fission reactions has been obtained. A selectivity of the ion source of 300 for fusion and 50 for fission reactions limited by the evacuation of non‐neutralized reaction products has been achieved.

Fine-structure in the alpha-decay of even-even nuclei - experimental-evidence for the stability of the z=82 magic shell

The fine structure in the alpha decay of even-even Po, Pb, Hg and Pt isotopes has been studied. Feeding towards low-lying 0+ states in the 196,198Po, 190,192,194Pb, 182,184Hg and 176,178,180Pt isotopes is observed. The energy systematics of the positive parity states in the light Po isotopes indicates the appearance of shape coexistence in these nuclei as well. The hindrance factors of the alpha decay towards the excited 0+ states has been interpreted in a two-level mixing calculation. It offers a probe for the identification of the proton particle-hole character of the 0+ states in this region and strongly supports the persistence of the Z = 82 shell closure in the light Pb isotopes.

Fine structure in the α decay of 192Po

Fine structure in the {alpha} decay of {sup 192}Po has been studied in the reaction of {sup 36}Ar on {sup 160}Dy at 176 MeV. Evaporation residues were selected in-flight using the Argonne Mass Analyzer and implanted into a double sided silicon strip detector. The correlation technique between implants and subsequent decays was used to observe fine structure in the {alpha} decay of {sup 192}Po leading to the identification of an excited 0{sup +} state in {sup 188}Pb at 571(31) keV. The half-life of {sup 192}Po has been determined to be 33.2(14) ms. The observation of a low-lying 0{sup +} state is discussed in terms of proton particle-hole pair excitations across the Z=82 shell gap. The small hindrance factor to the excited 0{sup +} state relative to the ground state supports the picture of shape coexistence in light even-even Po isotopes. {copyright} {ital 1997 American Institute of Physics.}

Element selective laser ion source for on-line mass separator

First results obtained with a new type of an element‐selective laser ion source for an on‐line mass separation are presented. The concept of the ion source is based on laser resonance ionization of nuclear reaction products thermalized in a high‐pressure noble gas. The fast and universal thermalization of reaction products in combination with high selectivity and high efficiency of resonance photo ionization allows to develop an element‐pure source of short lived exotic nuclei.

Gamow-Teller strength in the f7/2-nuclei 54Co and 42Sc studied through the beta decay of 54Ni and 42Ti

At the LISOL facility the neutron-deficient {sup 54}Ni isotope was produced via a {sup 3}He fusion-evaporation reaction on a {sup 54}Fe target positioned in the element-selective Ion-Guide Laser Ion Source (IGLIS). The {beta} decay of {sup 54}Ni was observed for the first time. The Gamow-Teller {beta}-decay strength associated with the (J{sup {pi}}=1{sup +}) level at 937.2 keV in the daughter nucleus {sup 54}Co was deduced. The Gamow-Teller {beta}-decay strength is in agreement with B(GT) obtained from (p,n) reaction studies. For the first time an experiment was performed using a heavy-ion beam in combination with the IGLIS in order to produce {sup 42}Ti.