W.H. Schulte

A recoil separator for the measurement of radiative capture reactions

A recoil separator in combination with a windowless gas target has been designed for the measurement of the radiative capture reaction p(7Be, γ)8B. The separator consists of momentum and velocity filters and a ΔE−E detector telescope. The setup was tested quantitatively using the p(12C, γ)13N reaction at the effective energy Ecm = 841 keV. Projectile fluxes were measured directly with Faraday cups and indirectly with elastic backscattering into Si detectors, while the 13N recoil flux was measured directly with the ΔE−E telescope. A suppression of the 12C beam particles by a factor 2 × 10−10 was observed when the system was tuned for the recoil 13Ns. Special emphasis was given to the charge state probabilities of the 13N recoils. Possible improvements of the system for the measurement of other capture reactions are discussed.

Improvements in targetry and high voltage stability for high resolution ion beam experiments

Ion beams with high energy resolution are valuable tools for the investigation of a wide range of phenomena in nuclear, atomic, and condensed matter physics. For the study of such effects, involving narrow nuclear resonances, the quality of the target is as important as the quality of the ion beam. Thus, an ultrahigh-vacuum system has been built including such characteristics as in situ target fabrication and variable target temperature. A number of improvements also were made to the newly installed 400 kV Munster accelerator to achieve better resolution for intense ion beams. The thick-target yield curves of narrow resonances in (p, γ) reactions on 23Na, 26Mg, and 27Al nuclides show the Lewis effect clearly. These measurements as well as other resolution tests are discussed.

22Na(p,γ)23Mg resonant reaction at low energies

Abstract The 22 Na( p , γ ) 23 Mg reaction has been investigated in the energy range of E p = 0.20–0.63 MeV, using a 22 Na implanted target. A new resonance has been found at E p = 213 keV. The known resonances at E p = 290 and 613 keV have also been observed and have been used to determine the resonance strength ωγ of the new resonance. A new γ-branch for the E x = 7854 keV state in 23 Mg, corresponding to the E p = 290 keV resonance, has been found. The changes in the stellar reaction rates are discussed.

The S(E) factor of7Li(p, ?)8Be and consequences for S(E) extrapolation in7Be(p, ?0)8B

Excitation functions and forward-backward anisotropies have been measured for the7Li(p, γ)8Be capture reaction over the proton energy rangeEp=100 to 1500 keV, using a 4π summing crystal and Ge(Li) detectors, respectively. The data show at all energies the presence of El and M1 capture amplitudes arising from the direct capture (DC) process and theER=441 and 1030 keV resonances, respectively. Due to the observed DC process, the present data increase significantly the reaction rates (up to a factor of 110) compared to values given in the compilation. The data and their analyses remove the recent criticism on DC model calculations, which had implied a significant reduction in the extrapolated S(E) factor for7Be(p,γ)B and thus in the predicted flux of high-energy solar neutrinos; thus, the solar neutrino problem is still with us.

Resonances in the 22Na(p, γ)23Mg reaction

Abstract The reaction 22 Na(p,γ) 23 Mg has been investigated in the energy range E p = 0.17–1.29 MeV using a 22 Na implanted target, a D 2 O threshold detector, and NaI(Tl) and Ge detectors. Resonances in this reaction have been observed for the first time at E p = 290, 457, 503, 613, 740 and 796 keV. The strengths of these resonances, and upper limits on the strengths for expected resonances in the energy range covered, are given. The stellar reaction rates deduced from the present work are about one order of magnitude lower than previous theoretical estimates; they are however significantly higher than the upper limits from one previous measurement. Some astrophysical consequences are discussed.

Thermonuclear reaction rates of 9Be(p, γ)10B☆

Abstract An excitation function of the 9 Be(p, γ) 10 B capture reaction has been measured over the proton energy range E p = 75 to 1800 keV using a 4π summing crystal. The data are dominated by three broad resonances including interference effects with the direct-capture process. Near temperatures of T 9 = 0.8 the reaction rates are lower by a factor 4 compared to values given in a compilation, while at other temperatures the rates are similar.

Hydrogen depth profiling using18O ions

Nuclear resonant reaction analysis techniques for hydrogen depth profiling in solid materials typically have used15N ion beams at 6.40 MeV and19F ion beams at 6.42 MeV, which require a tandem accelerator. We report a new technique using an18O ion beam at a resonance energy of 2.70 MeV, which requires only a single stage accelerator. Improved values of the nuclear parameters for the 2.70 MeV (18O) and 6.40 MeV (15N) resonances are reported. The beam energy spread was investigated for different ions and ion charge states and found to scale with the charge state. Data obtained using atomic and molecular gas targets reveal the research potential of Doppler spectroscopy. Examples of hydrogen depth profiling in solid materials using15N and18O ion beams are presented.

High energy resolution ion beam techniques for novel investigations in nuclear, atomic and applied physics using narrow nuclear resonances

Abstract The 400 kV accelerator at the Universitat Munster has been improved intense ion beams with high energy resolution. Gas and vapor targets required for high resolution experiments have been built. The energy scanning and data acquisition during the experiment was controlled by a computer. With this system an overall resolution of less than 25 eV for proton beams in the 100 μA range was achieved. The properties of this system are described. Using this system the width of nuclear resonances were determined to values as low as 3 eV. Such narrow resonances were also used as probes to investigate the influence of the nuclear environment on the yield curves. For example, replica resonances corresponding to atomic excitation of the electronic shells of the compound atom were observed, and a strong Lewis effect was measured for both solid and gas targets.

Gamma-ray detection with a 4π NaI spectrometer for material analysis

Abstract A γ-ray detection system with nearly 4π geometry for applications in materials science is described. The energy resolution of the NaI(Tl)-bore hole detector was found to be about 2% for 10 MeV γ-rays. The total detection efficiency has been determined to be above 80% for γ-energies between 0.5 and 20 MeV. A precision of the efficiency calibration of about 2% has been obtained for a wide range of γ-energies. The experimental results were found to be in excellent agreement with computer simulations using the GEANT computer code. The high efficiency of the system combined with its relatively high energy resolution opened a wide range of applications. Examples are the simultaneous detection of light isotopes in thin films by (d,pγ) nuclear reactions, high depth resolution profiling of isotopes by narrow, low-energy resonances in (p,γ) reactions, and hydrogen profiling using the 6.4 MeV resonance in the 1H(15N,αγ)12C reaction.

Atomic effects on α-α scattering to the 8Be ground state

The α-α scattering to the 8Be ground state was investigated in kinematic coincidence for the angle pairs 45‡/45‡ and 30‡/60‡ using the crossing beams technique, with an energy resolution of 26 eV. The nuclear resonance is split into 2 structures, not the 3 structures suggested by earlier work. The atomic physics origin of the splitting, the resonance parameters of the 8Be ground state, and astrophysical implications are discussed.