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A high-precision measurement of the 14O half-life has been performed using a mass-separated radioactive beam in combination with a germanium detector set-up. This is the first 14O half-life measurement with a contamination-free source. The final result of 70.560 ± 0.049 seconds is in agreement with the generally adopted mean value.

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. (C) 1999 Elsevier Science B.V. All rights reserved.

Levels in ${}^{200,202}\mathrm{Po}$ were studied via the ${\ensuremath{\beta}}^{+}$ and electron-capture decay of mass-separated At isotopes. Multiscaled singles \ensuremath{\gamma}, x, and conversion-electron spectra as well as \ensuremath{\gamma}-\ensuremath{\gamma}-t, $\ensuremath{\gamma}\ensuremath{-}{\mathrm{e}}^{\ensuremath{-}}$-t, and ${\mathrm{x}\ensuremath{-}\mathrm{e}}^{\ensuremath{-}}$-t coincidences were measured. These isotopes contain key information for the presence of \ensuremath{\pi}(4p-2h) intruder configurations in the light Po isotopes.

The population of the Na-20 resonances at 448, 661, 797, and 887 keV above the proton threshold has been studied in measurements of the Ne-19(p,gamma)Na-20 reaction with radioactive Ne-19 beams. The data have been analyzed fully in terms of resonance strengths, enabling a comparison with theoretical estimates and the extraction of upper and lower limits on the resonant part of the Ne-19(p, gamma)Na-20 astrophysical reaction rate. The total cross section of the Ne-19(d,n)Na-20 reaction has been measured from 0.6 to 1.8 MeV (c.m.). On the basis of the results of a subsequent distorted-wave Born approximation analysis the direct component of the Ne-19(p,gamma)Na-20 astrophysical reaction rate has been calculated in a potential model. The astrophysical implications of the resulting total reaction rate have been investigated: the conditions for the breakout from the hot CNO cycle into the rapid-proton capture process are determined by the preceding O-15(alpha, gamma)Ne-19 reaction and the photodisintegration of the produced Na-20 nuclei will not impede the breakout. [S0556-2813(98)05305-9].

The 19Ne(p, γ)20Na as well as the 19Ne(d,n)20Na reaction have been studied in inverse kinematics using 19Ne radioactive beams. Upper and lower limits for the 19Ne(p, γ)20Na 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 proceeding 15O(α,γ)19Ne reaction.

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.

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 cross section for the 18F(p,α) 15O reaction has been obtained in reverse kinematics (18F 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 α-particle and elastic proton data.

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.

The [sup 19]Ne([ital p],[gamma])[sup 20]Na reaction may influence considerably the reaction flow between the CNO and NeNa mass regions in high temperature hydrogen burning conditions. The 447 keV resonance has been studied by exploiting radioactive [sup 19]Ne beams with novel detection techniques to measure [beta][sup +]-delayed [alpha] radioactivity of [sup 20]Na nuclei produced in reactions with polyethylene targets. A 90% C.L. upper limit of 18 meV for the resonance strength has been determined and implications for the spin assignment of the 2.646 MeV state in [sup 20]Na and the stellar reaction are discussed.

Resonances in Na20 above the proton threshold have been studied by scattering radioactive Ne19 beams off thick polyethylene targets. The isobaric F19+p scattering was also examined. The analysis of the data under different theoretical approaches is discussed. The energy, total width, spin, and parity of two resonances in Na20 were assigned unambiguously. The results are compared to those obtained by indirect methods. © 1994 The American Physical Society.

The total cross section for the N-13(d, n)O-14g.s. reaction has been measured with N-13 radioactive beams at laboratory energies of 8.2, 12.0, 16.2, and 28.5 MeV. A spectroscopic factor of 0.9 is obtained from a distorted-wave Born approximation analysis of this transfer reaction. The astrophysical S factor for the N-13(p,gamma)O-14 reaction is calculated using the experimentally known resonance parameters for the resonant contribution and the direct-capture model with the experimental spectroscopic factor for the non-resonant contribution.

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 m) and Ne-19(Tl 2=17 s) with an energy between 0.5 and 4.0 MeV/amu are obtained. The first experiments using these energetic radioactive beams are discussed and the ARENAS3 project is very briefly outlined.

The {sup 206}Fr{r arrow}{sup 202}At{r arrow}{sup 198}Bi, {sup 204}Fr{r arrow}{sup 200}At{r arrow}{sup 196}Bi, and {sup 202}Fr{r arrow}{sup 198}At{r arrow}{sup 194}Bi {ital a}-decay chains have been studied by standard spectroscopic techniques using an on-line isotope separator. All the studied doubly odd isotopes have at least two isomers, which decay by a combination of the following decay modes: {ital a} emission, {beta}{sup +}/EC (electron capture) decay, and internal transition (IT). The internal transition, a highly retarded {ital E}3, is the {ital j}-forbidden transition between the ({pi}{ital h}{sub 9/2}{direct product}{nu}{ital i}{sub 13/2}){sub 10}{sup {minus}} and the ({pi}{ital h}{sub 9/2}{direct product}{nu}{ital f}{sub 5/2}){sub 7}{sup +} states. The {ital B}({ital E}3) values of these IT's together with their energy behavior as a function of the neutron and proton number, compared to the energy difference between the 13/2{sup +}({nu}{ital i}{sub 13/2}) and 5/2{sup {minus}}({nu}{ital f}{sub 5/2}) states in the odd-mass Pb isotones, indicate that these proton-neutron-coupled states have a rather pure shell-model character.

Theα-branching ratios (bα) of192,190,188Pb are measured using mass-separated sources. Different experimental set-ups are used — one detector as well as two detector set-ups — thereby detecting theα particles from the parent and/or viaα decay formed daughter nuclei, theβ-delayed gamma radiation from the parent and/or viaβ decay formed daughter nuclei and the Tl KX rays from electron capture decay. Values forbα of 6.2(6) 10−5 and 4.0(4) 10-3 were found for192,190Pb respectively. For188Pb, limits on thebα values were obtained: 0.03

The mass separator LISOL, on-line to the CYCLONE cyclotron, is described as it is presently configured. Improvements in the cyclotron beams are mentioned, and separator developments to accommodate the various experimental stations are presented. Modifications to the separator to permit ion guide operation are given. Yields of some short-lived isotopes are presented, for heavy-ion induced reactions.

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.

An off-line release study N-13 (T1/2 = 9.96 min) produced by a proton induced reaction on a graphite target (POCO-graphite EDM3, density = 1.84 g/cm3, grain size approximately 3-mu-m) has been performed. The activation energy for the diffusion process is determined to be 6.15(16)x10(5) J/Mol. With this acitivation energy, extraction efficiences for N-13 are obtained at different temperatures and are compared to on-line measured extraction efficiencies.

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.

A review on low‐charge state high‐efficiency ion sources used at on‐line isotope separators (ISOL) is given. These sources for which the efficiency is optimized, can be distinguished from ion sources used at, e.g., accelerators where essentially the output current is optimized. Recent developments conducted to meet the special requirements necessary for on‐line ion sources−high efficiency, selectivity, and short delay times−are presented and new ionization schemes like ECR sources or laser resonant photo ionization, implemented during recent years at ISOL systems, are discussed.

A feasibility study was undertaken for the measurement of the resonance at E(r) = 545 keV in the N-13(p,gamma)O-14 reaction, using intense radioactive ion beams. The analog resonance at E(r) = 512 keV in C-12(p,gamma)N-14 was chosen as a test case, leading to improved knowledge of the resonance parameters. Thin- and thick-target yields were obtained with C-13 beams on an extended gas target and a polyethylene foil, respectively. In addition, thin and thick targets of enriched C-13 were irradiated with proton beams. Normalization problems arising with mixed ion beams (N-13/C-13) are discussed.

In this paper we wish to report on the first observation of the beta decay of the very neutron-rich nucleus 117Rh. It was produced via 23 MeV proton-induced fission of U-238 using the ion guide setup of the LISOL separator. A beta half-life to 0.44 +/- 0.04 s was measured for this nucleus from the decay of the beta-coincident K x rays of Pd. The half-lives of neutron-rich Rh isotopes are compared with the newest model calculations. Three gamma rays of 34.6, 131.7, and 481.6 keV were found to be associated with the decay of 117Rh. Its proposed beta-decay scheme and the decay of the recently discovered 19 ms isomer in 117Pd imply spin and parity of 11-/2 for the isomer fed directly in fission.

The cross section for the astrophysically important $^{13}\mathrm{N}$(p,\ensuremath{\gamma}${)}^{14}$O reaction has been measured directly with an intense (3\ifmmode\times\else\texttimes\fi{}${10}^{8}$ particles/s) and pure (g99%) 8.2-MeV $^{13}\mathrm{N}$ radioactive ion beam. The average value, for the 5.8--8.2-MeV $^{13}\mathrm{N}$ energy range, is 106(30) \ensuremath{\mu}b. The partial \ensuremath{\gamma} width of the resonance which occurs in this reaction at a center-of-mass energy of 0.545 MeV has been deduced to be 3.8(1.2) eV. It is compared with theoretical predictions and indirect determinations.

The alpha-decay of mass separated Bi-190,192,194,196 isotopes has been studied with the LISOL on-line separator. Time sequential alpha-singles spectra were taken and for Bi-190,192,194, alpha-Xt and alpha-gamma-t coincidences were collected. Transition probabilities of levels in Tl-186,188,190 were measured using a fast timing alpha-gamma-t and alpha-e-t coincidence set up. The alpha-decay of 196Bi and the beta-decay of 190Bi are observed for the first time. In all the Bi nuclei presently studied, unhindered alpha-decay of Bi starting from the high- (10-) and low-spin (3+) isomer populating intruder-based states in Tl was observed. The excitation energy of these intruder states is comparable with the intruder states in the neighboring odd-even and even-even nuclei.

Independent and cumulative yields in nearly symmetric fission have been measured for the first time by bombarding $^{238}\mathrm{U}$ with 20-MeV protons and with 18-, 25-, and 41-MeV deuterons. Isobaric charge dispersion has been determined in the A=110, A=112, and A=114 mass chains and, from asymmetric fission, in the A=80 mass chain. The average isobaric charges in the p+U reaction were 44.01\ifmmode\pm\else\textpm\fi{}0.20 (A=110), 45.15\ifmmode\pm\else\textpm\fi{}0.15 (A=112), and 45.74\ifmmode\pm\else\textpm\fi{}0.15 (A=114). The widths (standard deviations) of the charge dispersion curves were 0.82\ifmmode\pm\else\textpm\fi{}0.08 (A=112) and 0.54\ifmmode\pm\else\textpm\fi{}0.08 (A=114). The average charges are roughly 0.3 unit higher than those calculated on the basis of the unchanged charge distribution model. The ground-state branch in the beta decay of $^{80}\mathrm{Ge}$ was determined to be 65%. Altogether some 15 new neutron-rich isotopes have been found, the newest one being $^{114}\mathrm{Ru}$ with a half-life of 0.53\ifmmode\pm\else\textpm\fi{}0.06 s. An ion guide facility was used to separate the isobaric chains on-line from other fission products. Typical cumulative yields in a mass chain were on the order of 1000 atoms/\ensuremath{\mu}C, but for new isotopes independent yields as low as a few atoms/\ensuremath{\mu}C could be observed.

A single stage electron cyclotron resonance (ECR) ion source has been developed for the radioactive ion beam facility in Louvain-la-Neuve. The source was first tested with an off-line test separator using calibrated leaks of neon, nitrogen and carbon dioxide. The lingering time in the source, due to atoms sticking to the walls of the ion source, has been studied. The maximum efficiencies obtained are 52% for Ne-20(1+), 20% for N-15(1+) and 15% for C-12(1+). A N-13(1+) (T1/2 = 9.96 m) efficiency of 8% has been measured on-line.

Theβ-strength function of104Cd is studied in theβ+/EC decay of mass-separated104In. The determination of theβ+ feeding and theβ+/β+EC ratio is the result of the simultaneous measurement of beta and gamma rays in one semiconductor beta-spectrometer. Information on the strength function near theQEC value is obtained from the detection of high-energyγ rays. The total observedβ strength amounts up to 10% of the Extreme Single-Particle Shell Model value. A strong enhancement in theβ+/EC feeding to a group of levels around 5.6 MeV shows the influence of the Gamow-Teller Resonance.

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.

A single‐stage electron cyclotron resonance source was developed for the Radioactive Ion Beam project. The layout of the source and results obtained with an off‐line test separator are presented. Maximum ionization efficiencies obtained for Ne+1, Ne+2, and Ne+3 using He as support gas equal 52%, 15%, and 3%, respectively.

The 19Ne(p,γ)20Na reaction may be of considerable influence for the reaction flow between the CNO and NeNa mass region in high temperature hydrogen burning conditions. The proposed low energy resonance at 0.447 MeV has been measured in inverse kinematics with the Louvain‐la‐Neuve 19Ne beam, using novel activation techniques. An upper limit to the resonance strength has been determined and the implications for the spin assignment of the 2.646 MeV state in 20Na as well as for the stellar reaction rate are discussed.

By studying the β +/EC and α decay of mass—separated neutron—deficient Hg to Fr nuclei, the spectroscopy group at the Leuven Isotope Separator On Line (LISOL) project collected in recent years extensive systematics on shell—model intruder states in the Pb region 1–10. These shell—model intruder states, leading to shape coexistence, have been identified in odd—proton At, Bi and Tl nuclei 3–4, odd—neutron Pb nuclei 8 9, even — even Pb nuclei 1 2 5 7 and odd—odd Tl nuclei 6 10. A remarkable systematic behavior of the excitation energy of the intruder—based states as a function of neutron number is now evident and several theoretical descriptions have been developed to reproduce this specific trend (see the references 11 to 15 and the contributions to this conference by K. Heyde, R. Bengtsson and N. Tajima).

The production of radioactive nuclear beams is a topic of great current interest. Possible applications for such beams can be found in nuclear astrophysics, solid state physics and nuclear structure physics1. Basically one has two schemes for the production of radioactive nuclear beams. A first scheme that is already used for a few years, bombards a target with a high—energy heavy ion beam creating high—energy secondary (radioactive) beams. The kinematics of the reaction makes it possible to separate the recoil products from the primary beam2. In a second scheme the radioactive nuclear beams are produced by post—acceleration of ISOL (Isotope Separator On Line) beams. This scheme has been used, for the first time, at Louvain—la—Neuve (Belgium) by coupling two cyclotrons with an Electron Cyclotron Resonance (ECR) ion source.

The fraction of daughter nuclei that recoil out of the catcher foil after α decay of mass-separator implanted mother nuclei is determined experimentally and compared with calculations. The correction that has to be applied on the α-branching ratio, obtained by comparing the mother and daughter activity in one α detector, is at least 20%. Previously published branching ratios can be wrong due to the underestimation of the correction procedure. As an example, the 202 Rn → 198 Po → 194 Pb decay chain is studied.

An intense beam (1.5*10/sup 8/ particles/sec) of radioactive /sup 13/N/sup 1+/ ions (half-life: /b T//sub 1/2/=10 min) has been produced and accelerated to 0.65 MeV/nucleon, by coupling two cyclotrons with an electron cyclotron resonance ion source. This is the first time a short-lived radioactive ion beam has been produced by this method, at such an energy and with such a high intensity, a result which opens up a wide field in many applications. The first experiment along these lines will be the measurement of the cross section for the nuclear reaction /sup 1/H( /sup 13/N, gamma )/sup 14/O which is the crucial reaction for the operation of the so-called hot CNO cycle in nuclear astrophysics.

The191Bim,g →187T1 →183Au a-decay chain has been studied using mass — separated sources. The a — branching ratios have been measured. Out of the reduced a — decay widths, it is clear that the πs1/2 — πS1/2 and πh9/2 → πh9/2 transitions are not hindered although they involve strong shape changes.

The /sup 104/In/sup g/ and /sup 104/In/sup m/ nuclei have been produced by means of the following reactions: /sup 106/Cd(p,3n), /sup 92/Mo(/sup 14/N,2n), /sup 92/Mo(/sup 16/O,p3n), /sup 92/Mo(/sup 20/Ne,3p5n). Their decay has been investigated after mass separation. From ..beta../sup +/, ..gamma.., and x direct spectra and ..gamma..-..gamma..-T, ..gamma..-x-T, and ..beta..-..gamma..-T coincidences, a /sup 104/Cd level scheme has been constructed. The observation of an intense background of statistical ..gamma.. rays, emitted after strong ..beta.. decay to high energy levels, resolves the problems of previously published work concerning ..beta..-ray intensities, spin determinations, and Q/sub EC/ values.

The fine structure in the α-decay of mass-separated 194,196,198 Po is studied. Feeding to the 0 + ground state and the first excited 0 + intruder state in 190,192,194 Pb is observed. From α-e-t coincidences between the feeding α-ray and the E0 transition, the half life of the 0 + intruder state in 190,192,194 Pb is measured. The mixing between intruder state and ground state is deduced.

The gamma-rays de-exciting the yrast states in neutron deficient nuclei around A-55 have been observed from fusione-vaporation reactions induced by an intense radioactive Ne-19 beam on a Ca-40 target. An array of escape suppressed germanium detectors recorded reaction gamma-rays. Background subtraction of radiation from the decay of the beam was achieved using the timing properties of the pulsed beam and through particle detection using the LEDA charged particle array. These data constitute the first use of an on-line separated and accelerated radioactive beam for studies of near yrast nuclear states using fusion-evaporation reactions. Information on the gamma-ray yields from evaporation residues, relative to yields with a stable beam, has been obtained and the problems associated with performing such experiments in a highly radioactive background environment have been addressed.