Evidence for deformed states in 75Br

The excited states in ⁷⁵Br have been studied via the reactions ⁷⁴Se(p, γ), ⁷⁴Se(d, n), ⁷⁴Se(³He, pn) and ⁷⁴Se(α, p2n) by using in-beam γ-ray spectroscopy. In addition to measurements of γ-γ coincidences, excitation functions and angular distributions of γ-rays, ns lifetime measurements have also been carried out. As a result 19 levels have been identified up to spin ($\text{17}\text{2}$) and excitation energies up to 2.6 MeV. The B(E2) value of 88 W.u. derived for the 88.4 keV γ-ray indicates strong collectivity within a positive-parity band. A comparison of the excitation energies of the unique-parity states in ⁷⁵Br and ⁷⁷Br with those in ¹⁵³Tb and ¹⁵⁵Tb reveals that the average deformation increases when going from ⁷⁷Br (N = 42) to ⁷⁵Br (N = 40).     

Der Zerfall des77Kr

⁷⁷Kr (T _1/2=1.2 h) was produced by bombarding⁷⁶Se with 48 MeV-α-particles. After irradiation the radioactive⁷⁷Kr gas was collected in glass ampoules filled with charcoal and cooled by liquid air. The decay of⁷⁷Kr has been investigated using Ge(Li)-counters and a Ge(Li)-Ge(Li)-coincidence circuit. 27 γ-transitions, 13 of them unknown up to the present, have been found and their relative intensities have been determined. A level scheme for⁷⁷Kr→⁷⁷Br having 14 levels is proposed. This accounts for all the observed γ-transitions.     

Levels in 74Se, 78, 80Kr and 84Sr and systematics of quasi-γ bands

Quasi-band structures in ⁷⁴Se, ^{78, 80}Kr and ⁸⁴Sr have been investigated using (p, 2nγ) reactions. The members of the quasi-γ band have been observed up to 5⁺ in ⁷⁴Se, ^{78, 80}Kr and up to 3⁺ in⁸⁴Sr. The analyses of the energy systematics of the quasi-γ bands studied in this mass region as well as in other regions make clear the evolution of the quasi-γ band to the γ-band in well-deformed nuclei. In addition to these positive-parity bands, negative-parity levels were observed in ⁷⁴Se, ⁸⁰Kr and ⁸⁴Sr.     

High spin states in77Se populated by the74Ge(α,n γ) reaction

⁷⁷Se has been investigated by the reaction⁷⁴Ge(α,nγ) at 14 MeV. Gamma singles spectra, gamma angular distributions and gamma-gamma coincidences have been taken. A level scheme has been established, spins and parities have been assigned. States of a rotational band on the 1/2^? ground state, a rotational band on the 5/2^? 249.7 keV state and an anormal band have been identified. Nuclear Reaction ⁷⁴Ge(α,n γ)E _α=14 MeV; measuredE _γ ,I _γ ,γ-γ-Coin.,γ-ang. distr.⁷⁷Se deduced levels,J, π. Enriched target, Ge (Li).     

Level structure of 76Br from the 75As(α,3nγ) reaction

Hitherto unknown states of ⁷⁶Br were excited through the (α, 3nγ) reaction on ⁷⁵As at E_α = 30 to 55 MeV. Excitation functions, γ-ray angular distributions, γ-γ coincidences and the γ-ray emission time with respect to the beam bursts were determined. Levels above the 1^? ground state at energies (in keV) with the following spin-parity assignments are proposed: 141.8, 2; 253.7, 3^?; 491.8, 4; 584.9, 5^?; 1016.3, 6. These levels exhibit the energy spacings of a Coriolis-perturbed rotational band and the branching ratios show collective behavior. The comparison of these levels with those of the $\text{5}\text{2}$⁺ ground-state band recently reported in ⁷⁷Kr show that (a) the Coriolis effects are quite similar and (b) the first two excited levels lie very close in energy in both cases. The possibility that these two facts indicate the validity of the rotation aligned coupling scheme in a doubly odd system is discussed.     

In-beam study of 63 143 Eu80

An in-beam γ-ray study with the¹⁴⁴Sm (α, p4n) reaction has established the high-spin states up to 5 MeV excitation and I=35/2 in the N=80 nucleus¹⁴³Eu. The observed yrast states show the typical irregular pattern of a spherical nucleus, with frequent decay branchings and often dominant dipole de-excitation. The level spectrum is in qualitative accord with the coupling of the h_11/2 proton particle to the known yrast levels of the¹⁴²Sm core nucleus. Only in a few cases specific configuration assignments are made.     

Simultaneousg 9/2 proton and neutron alignments in76Kr

Yrast states with spins as high as 22 ? have been identified in⁷⁶Kr with the reactions⁵⁸Ni(²⁴Mg,α2p) at 110 MeV and⁴⁰Ca (⁴⁰Ca, 4p) at 155 MeV. Simultaneous band crossings due to g_9/2 proton and neutron quasiparticles have been observed in the frequency range 0.60≦?ω≦0.75 MeV. The moment of inertia increases rapidly at the crossing frequency. The systematics of proton and neutron alignments in the Kr isotopes are discussed.     

High spin states in73Se observed by the reaction70Ge(α,nγ)

⁷³Se has been investigated by the reaction⁷⁰Ge(α, nγ). Gamma excitation functions (E _α=10–16 MeV), gamma angular distributions (E _α=18 MeV) and gamma-gamma coincidences (E _α=15 MeV)) have been taken. A level scheme has been established, spins and partities have been assigned. States of a rotational band on the 151.2 keV (5/2^?) state and an anomalous band have been identified. Nuclear Reaction⁷⁰Ge(α,nγ)E _α=15 and 18 MeV; measuredE _γ,I _γ,γ-γ-coin.,γ-angular distribution,γ-excitation function.⁷³Se deduced levels,J, π. Enriched target, Ge(Li).     

Isomeric cross-section ratios in (n,t) reactions on some medium- and heavy-mass nuclei at 14.6 MeV

Cross sections were measured for the first time for the reactions ⁷⁹Br(n, t)^77mSe, ⁸⁸Sr(n, t)(^86mRb, ¹¹³In(n, t)^111mCd, ¹²⁷I(n, t)^125mTe, ¹⁴¹Pr(n, t) ^139mCe, ¹⁶⁹Tm(n, t)^167mEr and ¹⁸¹Ta(n, t)^179mHf at 14.6±0.4 MeV using the activation technique, wherever necessary radiochemical separations, and high-resolution γ-ray spectroscopy. The data lie between 10 and 120 μb. A brief review of isomeric cross-section ratios measured in (n, t) reactions is given, and the ratios are compared qualitatively with those obtained in low-energy (n, γ) reactions as well as in high-energy (n, 2n) processes. The isomeric cross-section ratios are influenced by the reaction energy available and are dependent on the spin of the isomeric state. In (n, t) reactions nuclear states with spins between $\text{3}\text{2}$ and 3 seem to be preferentially populated.     

Collectivity and the role of two-proton and two-neutron excitations in 82Kr

Excited states in ⁸²Kr have been studied in the reaction ⁸⁰Se(α, 2n) by using in-beam γ-ray spectroscopy. Measurements of γγ coincidences, excitation functions, angular distributions and the linear polarization of the γ-rays have been carried out. All together, 38 levels have been identified up to $\text{I = 12}\text{h}\text{?}$ and excitation energies up to 6 MeV. For 23 of these levels the mean lifetime could be determined by Doppler shift and the pulsed-beam γ-timing methods. Most of the levels could be grouped into collective bands according to measured B(E2) values. Two sequences, I^π = 8⁺, 10⁺, 12⁺, are interpreted as rotation-aligned 2q.p. bands built up on $\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ protons and neutrons, respectively. Interaction matrix elements between these bands were deduced as V ? 10 keV. Additional states, I^π = (6⁺), 7⁺, 8⁺, indicate coexistence of rotation-aligned and deformation-aligned 2q.p. excitations. An odd-parity ΔI = 1 band is ascribed to proton excitations of the type $\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{?}\text{P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{f}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$. Several fast M1 transitions with B(M1) = 0.1–1 W.u. have been observe states of equal spin. They are interpreted as being due to configuration mixing.     

High spin states in75Se populated by the72Ge(α,n) and73Ge(α, 2n) reaction

The⁷²Ge(α, n)⁷⁵Se and⁷³Ge(α, 2n)⁷⁵Se reactions have been studied at bombarding energies of 15 MeV and 22.5 MeV respectively using Ge(Li)-detectors.γ-singles spectra,γ-γ-coincidences,γ-angular distributions and excitation functions have been taken. A level scheme has been constructed, which contains a stretched spin and a normal rotational band. Nuclear reactions⁷²Ge(α, n)⁷⁵Se,⁷³Ge(α, 2n)⁷⁵Se; measuredE _γ ,I _γ ,γ-γ-coincidences,γ-angular distribution,γ-excitation function.⁷⁵Se deduced levels,J, π, γ-multipolarity. Enriched target, Ge(Li).     

Spins and parities of low-lying states in 81Kr from the 80kr(d̄, p)81kr reaction and implications for a 81Br solar neutrino detector

The structure of ⁸¹Kr has been investigated via the ⁸⁰Kr(d?, p)⁸¹Kr reaction using an 11.0 MeV vector-polarized deuteron beam. Differential cross sections σ and vector analyzing powers A_y have been measured from 20° to 90° for 17 excited states below 3.0 MeV excitation energy. Comparisons of these distributions to DWBA calculations and empirical shapes were made to extract spectroscopic factors and values of spin and parity for these states. The significance of these measurements to the design of a bromine solar neutrino detector is discussed.     

Evidence for non-axial shapes in81, 82, 83Kr

Excitations up to I=12 have been found in⁸²Kr during an (α, 2n) in-beam experiment. Several levels are interpreted as due to (g9/2)² neutron excitations. A comparison with g9/2 neutron bands in^{81, 83}Kr and with cranked shell model calculations indicates non-axial deformation in connection with a rotation-aligned band starting with an 8⁺ level. Non-collective excitations, 8⁺ (and 6⁺), have been interpreted as deformation-aligned two-neutron states.     

High spin states of 83Sr

The reactions ⁸⁰Kr(α, n)⁸³Sr and ⁸²Kr(α, 3n)⁸³Sr were used to populate excited states of high spin in ⁸³Sr. The de-excitation of these states was studied by in-beam γ-ray spectroscopy. A number of new high-spin states have been observed. The results are discussed within the framework of the Coriolis-coupling model and the cluster-core coupling model.     

The 75Se(n, γ)76Se reaction and the low-lying level structure of 76Se

The ⁷⁵Se(n, γ)⁷⁶Se reaction was studied through consecutive neutron capture with the use of pair and curved crystal spectrometers. The high-resolution data have allowed construction of a very well established level scheme including many new levels above ～ 2.8 MeV excitation energy. The resulting neutron binding energy, 11154.0 ± 0.3 keV, is lower than the value given in the mass table. The ⁷⁵Se(n, γ) cross section was determined to be 330 ± 100 b. The level scheme and branching ratios were compared with results from calculations in the framework of the interacting boson model.     

The excited states of Kr and Rb isotopes in the 78 ≦ A ≦ 85 mass region

The excited states of the ^{78, 80, 81}Kr and ^{80, 81, 83}Rb nuclei were investigated in radio-active decays of the respective Rb and Sr isotopes. The level schemes were established and for a number of levels the spin-parity assignments suggested on the basis of the log ft values and γ-branching ratios, as well as in some cases angular-correlation results. The structures of excited states of the odd Kr, odd Rb, and doubly even Kr isotopes are compared and discussed in the framework of existing theoretical approaches.     

Isomeric cross sections for the (n, 2n) reaction on 82Se, 81Br,and 45Sc at 15.1 MeV

Activation techniques have been used to measure the cross sections at 15.1 MeV neutron energy for the following reactions: ⁸²Se(n, 2n)^81m+gSe, ⁸¹Br(n, 2n)^80m+gBr, and ⁴⁵Sc(n, 2n) ^44m+gSc. Isomeric cross-section ratios were evaluated by applying the method of least squares to the time behavior of γ-ray activity following the ground-state decay of each isomeric pair. The absolute cross section σ_m for the formation of the metastable state was measured by the mixed-powder method with the ²⁷Al(n, α)²⁴Na reaction as the monitor. The cross section σ_g for the formation of the ground state was then determined by using the isomeric cross-section ratio. The sum of σ_m and σ_g for each reaction is compared with the theoretical value obtained from calculations based on the statistical model for the formation of a compound nucleus and its subsequent emission of neutrons.