D.M. Van Patter

E2 transitions from the second 2+ level of even-even nuclei

Abstract A survey has been made of available data for the γ-ray branching of the second 2+ level of over 50 even nuclei with A > 30, together with data for the E2-M1 mixing of the (2′ → 2) cascade transition. In addition, presently available data for the Coulomb excitation cross sections for the second 2+ level of some 24 even nuclei have been collected. From these data, three ratios of reduced E2 transition probabilities have been calculated, and compared with the predictions of various theories. The comparisons indicate that by far the most successful theory to date for predicting such ratios is the recent asymmetric rotor model of Davydov and Filippov, Perhaps the most striking success of this theory is the prediction for the ratio B(E2; 2′ → 0)/B(E2; 2 → 0) as a function of the energy ratio E2(2+)/E1(2+). However, there are some experimental results which remain unexplained, particularly for the ratio B(E2; 2′ → 2)/B(E2; 2′ → 0).

The (p, p′ γ) reaction in even isotopes of Zn, Ge and Se

Abstract It has been found possible to study the systematic properties of the second 2+ level in medium-weight even nuclei by investigation of gamma-radiation from the (p, p′ γ) reaction. For the mass region A = 60−80, the present method of observing (p, p′ γ) spectra has been successful for nuclei whose (p, n) threshold exceeds about 4 MeV. Measurements of the branching ratio of the second 2+ level have been made for Zn64, Zn66, Zn68, Ge72, Se76, and Se78, which are in reasonable agreement with available results from radioactivity. New levels have been found in Ge70 at 1.71±0.01 and 2.16±0.01 MeV. The 1.71 MeV level is considered to be the second 2+ level with a branching (cross-over/cascade) of 0.9±0.1. Information concerning the excitation curves for various (p, p′ γ) radiations of Zn66 and Ge70 have been obtained for proton energies in the range of 3.0 to 5.2 MeV.

Studies of the low-lying energy levels of Zn64 and Zn66 from measurements of (p, p′γ) radiations

Abstract An experimental investigation of the (p, p′γ) radiations from semi-thick targets of Zn 64 (2.4 mg/cm 2 ) and Zn 66 (2.7 mg/cm 2 ) has been carried out. Angular distributions of the (p, p′γ) radiations and the excitation cross-sections have been measured for proton energies ranging from 4.42 MeV to 5.02 MeV. These results have been compared with theoretical predictions using statistical reaction theory and optical model penetrabilities. Good agreement has been obtained for the angular distribution measurements, and level spins have been assigned on this basis. The measured cross sections, however, show some disagreement with theoretical values, which are particularly pronounced for the first 2 + states in both Zn 64 and Zn 66 . The studies of the stronger γ-rays suggest the following levels and the spins and parities: Zn 64 ; 0.993(2 + ), 1.804(2 + ), 1.904(0 + ), 2.321(4 + ) and 2.618(0 + ); Zn 66 ; 1.037(2 + ), 1.865(2 + ), 2.383(0 + ) and 2.462(4 + ?) MeV. Some additional weaker γ-rays have also been obseerved in both the isotopes. The collective nature of the first 2 + states in Zn 64 and Zn 66 being known, the (2, 0, 4) triplets that are seen in both these isotopes (at an energy roughly twice that of the first 2 + states) suggest that some kind of vibrational motion could account for these low-lying excited states. The present measurements indicate that (p, p′γ) reaction studies provide a promising method for searching for such triplets in medium-weight even nuclei.

Inelastic neutron scattering in Mn55

Abstract Gamma rays excited by inelastic neutron scattering in Mn55 have been studied. Coincidence methods have been used to observe those gamma rays which cascade through the 127 keV first excited state. Branching ratios have been measured for four levels, as well as the cross sections for excitation of each level. Theoretical calculations of the cross section, using a diffuse-edge potential well, give good agreement with the experiment, if appropriate spin values are assigned to each level.

Doppler-shift lifetime measurements in 46Ti and 50Cr

The lifetimes of nine states in 46Ti and fourteen states in 50Cr have been measured through the Doppler-shift attenuation method. The states were populated by the 46Ti(p, p′γ) and 50Cr(p, p′γ) reactions at Ep = 7 MeV. Thick targets (≈ 300 keV) were used to provide adequate averaging over many compound-nucleus levels. Singles γ-ray spectra were obtained with 33 and 55 cm3 Ge(Li) detectors at θγ = 0°, 90° and ≈ 160°, and lifetimes were extracted from the observed centroid shifts. An alternative method of analysis, involving a simulation of profiles of Doppler-shifted peaks with six free parameters gave considerably more accurate results. Both methods were tested by measuring known lifetimes in 58Ni. In addition, the lifetimes of three states in 52Cr were determined. A comparison of the energy spectra and electromagnetic decay properties of these nuclei to shell-model calculations revealed the failure of the simple (f72)6 picture. A startling example of this failure is the fact that the 22+(E2)21+ rate in 46Ti appears to be a factor of ⪆ 25 faster than that in the cross-conjugate nucleus 50Cr.

Gamma transitions in 60Ni following the β+ decay of 60Cu (II)

Abstract The γ-transitions in 60 Ni following the decay of 60 Cu have been investigated using a Ge(Li)NaI(Tl) coincidence-anticoincedence spectrometer. More than 20 previously unreported γ-transitions were observed, and six additional levels at 4007.8, 4318.8, 4760.7, 4843, (4848) and (5046) keV were found to be directly populated. A new 120.5+−0.3 keV branch (3.5%) between the 3 1 + (2626.2 keV) and the 4 1 + (2505.7 keV) states has been identified; it accounts for the major part of the M1 decay of the 3 1 + state. The branching of the 3124.1 keV 2 3 + state to the 0 1 + ground state and to the 2284.9 keV 0 2 + state yields B (E2: 2 3 + → 0 2 + )/ B (E2: 2 3 + → 0 1 + ) = 68+−7. On the basis of available evidence, we tentatively assign J π = 2 + to the 3269.4 and 3393.5 keV levels.

High-resolution study of 70Ge

Abstract Inelastic proton spectra from the 70 Ge(p, p′) reaction at 7.0 MeV have been studied with a 65 cm broad-range magnetic spectrograph. The scattered background was reduced by using 0.4 × 8mm slit targets, and an energy resolution of FWHM = 5.5 keV was achieved. Many closely spaced levels were observed up to 4.1 MeV excitation including a 3.4 keV doublet at 2.16 MeV. Level energies have been determined to ±2 keV or better by combining these precise level separations with γ-ray measurements from the 70 Ge(p, p′γ) reaction and the 70 As(γ) 70 Ge decay obtained with a 20 cm 3 Ge(Li)-NaI(Tl) coincidence-anticoincidence spectrometer. Available information concerning J π assignments for levels up to 3.1 MeV is summarized. Comparison of (p, p′) yields at 50° and 90° combined with relative (p, p′) cross sections provide evidence for probable J = 0 states at 2.31, 2.89 and 3.11 MeV. The octupole 3 1 -state is assigned at 2562 keV on the basis of its prominent yield at E p = 10 MeV. The 2 2 + (E2)0 2 + transition has been observed in 70 Ge with a B (E2) value which is a factor of 14±2 larger than that for the 2 2 + (E2)0 1 + transition. A nuclear temperature T = 0.7±0.1 MeV was derived from the present data.

The E2-M1 mixing ratio for the 2+2 → 2+1 transition in 60,62Ni

Abstract The E2 M1 mixing ratio for the 2 + 2 → 2 + 1 transition in 60,62 Ni has been measured using a reaction-independent method involving ratios of the coefficients of the same order of Legendre polynomial from singles angular distribution data (accuracy ⪅ 1%) for the cascade and crossover γ-ray transitions. Values of δ = −0.68 ± 0.22 and −3.19 ± 0.11 were obtained for 60 Ni and 62 Ni, as well as branching ratios (crossovers/cascades) of 0.161 ± 0.008 and 1.242 ± 0.017 respectively. Weighted averages of available nuclear spectroscopic information yield values of 542 ± 65 ( 58 Ni ), 315 ± 60 ( 60 Ni ) and 25.7 ± 0.4 ( 62 Ni ) for the ratio of the reduced E2 transition rates, B ( E 2: 2 + 2 → 2 + 1 ) / B ( E 2: 2 + 1 → 0 + 1 ). The trend of these values is not well reproduced by any of the various shell-model calculations employing exact configuration mixing or quasiarticles.

58Ni(p, p′γ)58Ni angular correlation measurements

Abstract Angular correlation measurements in a 180° geometry for the 58 Ni(p, p′γ) reaction were used to determine the spins and γ-ray decay properties of states at 2.46, 2.78, 3.04 and 3.26 MeV. The 2.46 MeV state was verified to be spin J = 4, and the states at 2.78, 3.04 and 3.26 MeV were assigned spin J = 2. The (L+1)/L mixing ratio for the 2.78 → 1.46 MeV transition is δ = 1.5 −0.4 +1.5 , strongly suggesting positive parity for this state. Using the available information for the ground state branching of this 2 2 + state, the crossover to cascade ratio of the E2 transition rates is in disagreement by a factor of ≈ 10 4 with the recent shell-model predictions of Cohen et al. For the 3.04 and the 3.26 MeV levels the mixing ratios δ for transitions to the 2 1 + state are −0.1±0.15 and −0.4±0.2 with the corresponding branchings to the ground state of 38±2% and 64±2%, respectively.

Isospin structure of low-lying Ti states

Abstract The 3 1 − state of 46 Ti exhibits unusual γ -decay properties, with E1 decay to the 2 2 + state being highly preferred. A theoretical investigation of the isospin structure of the first two 2 + states in 44, 46, 48 Ti provides a qualitative explanation of this observation.