G. Hammel

Neutron-proton charge exchange scattering between 190 MeV and 590 MeV

Abstract Differential cross sections of the np charge exchange scattering between 190 MeV and 590 MeV have been measured by detecting the scattered protons in an angular range of 0 ° ⩽ θ lab ⩽ 18 ° . The cross section at 0 ° as well as the logarithmic slope of the angular distribution are found to be almost energy independent, in contrast to previous experiments, but in agreement with recent LAMPF data.

Production of positive pions by neutron-proton collisions

Spectra of positive pions from the processn+p→π++n+n have been measured for incident neutron energies from 470 MeV to 590 MeV and for laboratory angles up to 20°. The rather broad pion energy spectra and the pronounced anisotropy of the differential cross sections, both indicate an appreciable non-resonant, isoscalar (T=0) contribution to the pion production.

The γ-decay of25Mg

The reactions24Mg(d,p γ) and27Al(d,α γ) have been used to study the main γ-decay modes of all bound and several unbound levels in25Mg up toEx=8 574 keV. The relevance of the data with respect to the rotational model is discussed.

Evidence for the coexistence of spherical and deformed states in38Ar

The38Ar levels at Ex=5350, 7289 and 9341 keV have been investigated using the35Cl(α,pγ) reaction at Eα=l4 and 14.5 MeV. From particle-γ-ray angular correlations the spin assignments J(5350)=4, J(7289)=6,4 and J(9341)=8,6,4 have been obtained. Life-time measurements using the Doppler-shift attenuation method yielded τ(9341)=106±25 fs and τ(7289)=77±30 fs, while the lifetime τ(5350)=200±50 fs was known previously. All levels have positive parity and decay by enhanced E2 transitions. Hence we propose that they are the Jπ=4+, 6+ and 8+ members, respectively, of a Kπ=0+ rotational band which has the Ex=3377keV, Jπ=0+ and the 3937 keV, Jπ=2+ levels as further members. The enhancement of inband E2 transitions is 30−6+10 W.u. (4→2), 35−14+30 W.u. (6→4) and 20–36 W.u. (8 → 6), respectively, yielding an average intrinsic quadrupole moment Q0=850−125+200 mb. The moment of inertia is given by h22θ=92 keV. The present data are in good agreement with the predictions of a deformed state in38Ar that coexists with the spherical states.

High-spin states in27Al

The24Mg(α, p γ) reaction has been studied atα-particle energies of 13, 15, and 15.2 MeV. Using method II of Litherland and Ferguson spin assignments or restrictions thereof could be made to the27Al levels atEx=5500 keV (I=11/2, 7/2), 6950 keV (I=11/2, 7/2), 7226 keV (I=9/2), 7286 keV (I=9/2–13/2), 7399 keV (I=11/2, 7/2) and 7443 keV (I=13/2, 9/2). Using the Doppler-shift attenuation method a lifetimeτ=20–50 fs was obtained for theEx=7399 keV level and a limitτ<20 fs for the levels atEx=7443, 7286, 7226, 6950, 6718, and 6287 keV, respectively. Further spin assignments in27Al were obtained by a study of the26Mg(p, γ) reaction at the Ep=2174 keV resonance. Fromγ-ray angular distributions the resonance spin was determined as I=9/2 (previous assignmentI=7/2) and the spin of theEx=5959 keV level was determined asI=7/2. Tentative spin assignments were made to the levels atEx=6537 keV (I=7/2) and 6718 keV (I=9/2). From the combined evidence of the24Mg(α, p) and26Mg(p, γ) reactions the spin of the 6287 keV level was found to beI=7/2. The results are compared with shell model calculations and the Nilsson model.

New aspects of the rotational model in25Mg

Spin assignments have been made to the25Mg levels in theEx=5–6 MeV region fromγ-ray angular distributions measured in the22Ne(α, n y) reaction atEα=8.0 and 8.8 MeV and fromp-γ angular correlations measured in the24Mg(d, p γ) reaction atEd=6 MeV. Unique spin assignments ofJ=7/2, 5/2, 5/2, and 9/2 could be made to the levels atEx=5005, 5511, 5851, and 5967 keV, respectively. Ambigious assignments have been made to the levels atEx=5245, 5524, 5785, 6032 keV (J=11/2, 7/2), 5455 keVJ=13/2, 9/2), and 5738 keV (J=3/2, 5/2). The present data confirm previous assignments ofJ=1/2 to the levels atEx=5108 and 5466 keV, respectively. Lifetime estimates have been obtained, using the Doppler-shift attenuation method, for the levels atEx=5245 keV (τ=30–60 fs), 5785 keV (τ=50–100 fs), 5967 keV (τ=50–100 fs), and 5455 keV (τ>1ps). All other levels in theEx=5–6 MeV region haveτ<60 fs. A breakdown of theK-selection rule has been observed in theγ-decay of some high spin states, indicating a deviation from the strong coupling model.

Angular distribution of the reaction np → dπ0☆

Abstract Differential cross sections of the reaction np → d π 0 have been measured for incident neutron energies between 470 MeV and 590 MeV. The angular distributions are fitted by a power series. A significant cos 4 θ contribution is found.

Lifetime measurements in25Mg

The mean lives of25Mg levels have been studied using the Doppler-shift attenuation method in the24Mg(d, pγ) reaction atEd=6 MeV. Except for theEx=6873 keV level, all levels between 2 MeV excitation energy and the neutron binding energy could be investigated, yielding 11 mean lives and 30 limits thereof. A large number of multipole transition rates could be deduced and several parity assignments be made. The relevance of the data with respect to the rotational model is discussed. A serious breakdown of theK-selection rule, which has been proposed earlier for some high-spin states, is confirmed.