M. Gaelens

A Direct Measurement of the F-18(p,Alpha)o-15 Reaction

The cross section for the F-18(p,alpha) O-15 reaction has been obtained in reverse kinematics (F-18 beam on CH2 target) between 550 and 740 keV above threshold, i.e. in a region of astrophysical interest. The reaction yield is dominated by a wide resonant state whose spin, parity, total width and partial widths were deduced from the analysis of the alpha-particle and elastic proton data.

Identification of μs-isomers produced in the fragmentation of a 112Sn beam

Abstract Decays of over forty short-lived ( T 1 2 from ≈ 50 ns to 70 μs) isomeric states including a new isomer 66mAs produced in the fragmentation of a 112Sn beam (58 A·MeV, 63 A·MeV) on a natNi target were observed at the final focus of the LISE3 spectrometer at GANIL. Their detection, based on the slow (≈ 10 μs) time correlation of identified ions with the characteristic γ-radiation, represents a novel method to search for new isomers and can be used for unambiguous isotope identification for projectile fragment separator experiments. Isomeric yields and isomer-to-total production ratios were determined.

Two proton emission induced via a resonance reaction

A radioactive beam of N-13 ions was used to bombard a (CH2)(n) target in order to populate a narrow resonance at 7.77 MeV in O-14. A weak two proton decay branch was observed with a width of 125 +/- 20 eV which occurs predominantly by a sequential mechanism. A limit for He-2 emission of Gamma(2He) < 6 eV is compared with calculations.

The 18F(p,α) reaction and its astrophysical implications

F-18 radioactive beams were used to measure the F-18(p,alpha) reaction in the c.m. energy ranges of 265-535 keV and 550-740 keV. In each case, a resonant level was clearly detected, of which the resonant strength and some other properties were deduced. The astrophysical reaction rate was calculated down to the novae peak temperature. Consequences for the hot CNO cycles were drawn

Production of a high purity F-18 radioactive beam

An efficient method for the production and post-acceleration of a pure F-18(T-1/2 = 109.8 min) radioactive beam has been developed. Large amounts of F-18 are produced via the O-18(p,n)F-18 reaction using a 15.5 MeV proton beam on an O-18-enriched water target. After the chemical purification of the target content, the F-18 activity is transferred into [F-18]-fluoromethane. It is then transported to an ECR ion source, where after dissociation of the molecules, the fluorine atoms are ionized. Finally, the F-18(2+) ions are accelerated to 14 MeV in a second cyclotron. The different steps in the production cycle and the characteristics of the final F-18 beam are discussed

New limits for the Ne-19(p,gamma)Na-20 astrophysical reaction rate from direct measurements using radioactive beams

Inexplosive stellar hydrogen burning, the hot CNO cycles and the rp-process are mainly linked by the reaction sequence O-15(alpha,gamma) Ne-19(p,gamma)Na-20. Using intense Ne-19 radioactive beams, both the Ne-19(p,gamma) and the Ne-19(d,n) reaction have been studied. Upper and lower limits for the Ne-19(p,gamma) reaction rate have been deduced, allowing to conclude that the O-15(alpha,gamma) reaction is most likely the bottleneck reaction.

Development of radioactive ion beams of light elements at ARENAS

The development of new radioactive ion beams at the ARENAS(3) facility at Louvain-la-Neuve requires new and specific targets. Crucial target properties that need to be investigated are the yield of the nuclear reaction, the target stoichiometry, the target life time and the release efficiency. To perform these general target studies, we developed a universal setup capable of holding solid, powder or liquid targets. With this setup we have examined targets of BN and B2O3 powders for the production of C-11. Stability under heating and irradiation, pre-treatment tests as well as release studies were performed. For applications on the LISOL isotope separator we also developed a high-temperature graphite target. With the different primary beams available at CYCLONE this target was used for the production of N-13, O-14 and O-15 isotopes. Production yields and release efficiencies for this target in the range 1300-2000 degrees C were measured. Also long-term stability and good temperature control were achieved. Further target development work at Louvain-la-Neuve includes the production of O-15 in LIF targets.

Status report on the electron cyclotron resonance ion sources used for the production of radioactive ion beams at Louvain‐la‐Neuve

Two single‐stage, 6 GHz electron cyclotron resonance (ECR) ion sources are currently in use at the ARENAS project at Louvain‐la‐Neuve. One of them is used to ionize radioactive elements to low charge states (1+ to 4+) before injection in the postaccelerator cyclotron. High intensity beams of 6He, 11C, 13N, 18Ne, and 19Ne are being produced. Recently the source has been adapted for the ionization of 18F and the first experiment with this beam has been performed. A second ECR source, almost identical to the first, is used in combination with the Leuven isotope separator on‐line. It produces stable and radioactive beams at 50*q keV. Mass separated beams of 13N, 14O, and 15O can be produced using a high‐temperature carbon target. An upgrade of this type of ECR source is now under construction. It consists mainly of increased magnetic fields and the capability for insertion of a heated cavity.

The Ne-19(p,gamma)Na-20 astrophysical reaction rate determined from measurements with a radioactive beam

The Ne-19(p,gamma)Na-20 as well as the Ne-19(d,n)Na-20 reaction have been studied in inverse kinematics using Ne-19 radioactive beams. Upper and lower limits for the Ne-19(p,gamma)Na-20 astrophysical reaction rate have been deduced, for the first time on the basis of direct experimental data. It is concluded that the transition from the hot-CNO cycle to the rp-process in explosive hydrogen burning is most likely governed by the preceeding O-15(alpha,gamma)Ne-19 reaction.

Nuclear reactions of astrophysical interest with radioactive beams

The ARENAS(3) facility, coupling the two cyclotrons of Louvain-la-Neuve, is intensively been used over the past years for cross-section measurements of nuclear reactions of astrophysical interest. We will describe the characteristics of the available beams and the experiments going on using these beams. The Ne-19(p,gamma)Na-20 reaction, of considerable importance for the reaction now between the CNO and the NeNa mass region in high-temperature hydrogen burning conditions, will be discussed in particular. The proposed low-energy resonance at 0.447 MeV has been measured in inverse kinematics using novel activation techniques. An upper limit (90% C.L.) of 18 meV is obtained for the strength of this first particle-unbound state of Na-20 together with preliminary values for the strength of the higher excited states.