P. Duhamel

Production, acceleration and use of radioactive ion beams at Louvain-la-Neuve

The production and acceleration of radioactive beams using two cyclotrons coupled by an electron cyclotron resonance ion source is described. Pure beams of N-13 (T1/2 = 9.96 min) and Ne-19 (T1/2 = 17 s) with an energy around 1 MeV/amu are obtained with intensities larger than 50 ppA. As an example, cross section measurements using a N-13 beam on hydrogen and deuteron targets are presented. Finally, the ARENAS3 project, a future plan for the production of radioactive beams in Belgium, is described.

Neutron-induced nuclear reactions and degradation in germanium detectors

We have measured cross sections of neutron-induced nuclear reactions leading to the delayed production of gamma-ray lines similar to the ones of astrophysical interest. Conclusions were drawn concerning the expected background in the Al-26 1809 keV line and the Be-7 478 keV line in SPI. The neutron-induced degradation of Ge detectors was studied vs. the neutron energy, the neutron fluence and the detector temperature. Performance recovery of the detectors was studied for different annealing temperatures. Optimum temperature and times for annealing were determined.

One-step energy scanning of wide low-lying 1− resonances in 13C+p and 13N+p scattering

Thick polyethylene targets have been bombarded with 8.2 MeV C-13 and N-13 beams accelerated by a cyclotron, i.e. having a relatively large energy spread of about 200 keV. Recoil protons have been detected in order to observe the J(pi) = 1- wide resonances at E* = 8.06 and 5.17 MeV in the N-14 and O-14 compound nuclei, respectively. It is shown that accurate values of the energy, width and spin of the resonant states can be obtained under experimental conditions which, at first glance, may seem inappropriate. The method is well suited for resonance studies using radioactive beams. The measured width of the N-14 resonance (GAMMA = 33.7 +/- 1.0 keV) disagrees with a recent measurement (GAMMA = 23 keV). The energy of the O-14 resonance was determined to be E(c.m) = 526 +/- 3 keV instead of 545 +/- 10 keV as quoted in the literature. A new parameter-free microscopic calculation was undertaken to solve an apparent discrepancy between the experimental proton width and a previous calculation.

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.

Gamma-ray Multiplicity Measurements for the Determination of the Initial Angular-momentum Ranges in Normal and Fast Fission Processes

Abstract Gamma-ray multiplicities (first and second moments) have been measured, in the 220 MeV 20 Ne + nat Re and 315 MeV 40 Ar+ 165 Ho reactions, as a function of fission fragment masses and centre-of-mass total kinetic energies. The two reactions lead to the same fusion nucleus, 205 At, at the same excitation energy (167 MeV). The experimental critical angular momentum for the fission process in the Ne + Re system (91 ± 3) ℏ is close to l Bf =0 (~80ℏ) while in the Ar + Ho reaction this critical angular momentum (136 ± 4) ℏ is much larger than the l Bf =0 value, favoring the occurrence of the fast fission process. The observed widths of the fission fragment mass distribution: (42 ± 2) u in the Ne + Re system and (56 ± 4) u in the Ar + Ho reaction strengthen this hypothesis. For both compound nucleus fission and fast fission components in Ar + Ho, the total spin values obtained in absolute magnitude and in their dependence on the mass asymmetry are well described by assuming rigid rotation of the fissioning complex and statistical excitation of some collective rotational modes such as “Bending” and “Wriggling” according to the Schmitt-Pacheco model. These modes, however, are not all fully excited, their degrees of excitation are approximately the same for both fission components. From theoretical estimates of equilibration times, one anticipates the “Tilting” mode to be by far the last to be excited, and from its non-excitation in the present data together with the excitation of bending and wriggling, a time interval of about 10 −21 s to 2 × 10 −20 s can be derived for the reaction time of both normal fission and fast fission. The γ-ray multiplicity as a function of the c.m. total kinetic energy decreases in the Ne + Re system, while it increases in the Ar + Ho system even for symmetric splitting, which indicates experimentally for the first time that fast fission populates the whole mass range. This difference between the two reactions is in agreement with the normal-fission-fast-fission distinction in angular momentum space.

Different aspects of nuclear structure and reaction mechanisms in the collision 13N+11B

The reaction N-13 + B-11 has been investigated at two 13N beam energies: 29.5 and 45 MeV, Charged particles and gamma rays emitted in the reactions have been detected using a large array of silicon strip detectors and BaF2 scintillators, respectively. Several reaction channels have been analysed with different aims. The proton transfer reaction B-11(N-13, C-12)C-12 to various excited states of C-12 has been studied to obtain information on the N-13(g.s.) structure. The excitation energy of the Mg-24 compound nucleus was in the region where several resonances occur in the C-12 + C-12 system. The six-alpha decay of the Mg-24 intermediate system has been studied. Some of these events have been identified as coming from the break-up of two C-12* in the 3(1)(-)-3(1)(-) and 3(1)(-)-0(2)(+) states. The total fusion cross section has also been extracted from the data. In order to extract information on the isospin purity/mixing in Mg-24 at high excitation energy (E-x similar to 47 MeV), the GDR gamma emission from this self-conjugate nucleus has been studied. A comparison of gamma spectra has been undertaken for the reactions N-13 + B-11 and N-14 + B-10

Collective and shell-model excitations in 9745Rh52

Abstract 97Rh has been studied by deexcitation of the compound nucleus formed through the reactions 96Ru(3He,pn) at 20 MeV and 96Ru(2H,n) at 9 MeV. Direct γ-spectra, multidimensional γγ coincidences and angular distribution measurements lead to a proposed level scheme. These levels are compared to calculations based on the interacting boson-fermion approximation IBFA model in which the odd particle moves in the 1 g 9 2 orbital. Evidence for the observation of shell-model states outside the sd boson configuration space in 97Rh is obtained.

Excited states of97Pd

The97Pd nucleus has been studied from the reaction96Ru(3He, 2n)97Pd. From in and out beam directγ-spectra andγ-γ coincidences a partial level scheme is proposed. The observed high spin states can be explained by the coupling of the even proton core with the d5/2 51st neutron.

Measurement of cross-sections for the 9Be(n,3n)7Be and 56Fe(n,p)56Mn reactions producing background lines in γ-ray astrophysics

The reactions 9Be(n,3n)7Be and 56Fe(n,p)56Mn were measured between 28 and 68 MeV and at 28 MeV, respectively. These reactions lead to the emission of γ-rays (478, 847, 1811 keV) that may be disturbing in γ-ray astrophysics missions. Consequences regarding the induced background in detectors are drawn.

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.