Scattering and resonance properties of the interaction of slow neutrons with the isotopes of silicon and sulfur

In¹¹⁷Te,¹¹⁹Te and¹²¹Te isomeric states withJ ^π=5/2⁺, 5/2⁺ and 7/2⁺ and half-lives of 19.1(9)ns, 2.2(2) ns and 86(6) ns, respectively, have been identified at low excitation energies using (α, 2n) reactions on enriched^115–119Sn targets. Positive parityΔJ=1 bands built on these isomeric states have been observed up to 17/2⁺. The states are interpreted as members of rotational bands built on deformation driving 5/2⁺ [402] and 7/2⁺ [404] Nilsson orbitals which overcome theN=64 subshell gap. The irregular level spacings and electromagnetic properties of the bands are well explained in Coriolis calculations. The moment of inertia parameter as function of collective angular momentum has been derived from the doubly even Te cores. The hindrance of the band head deexcitation may be caused by shape fluctuations of these transitional nuclei.     

Anomalous shift in crossing frequency for the proton 1/2[541] band in175Ta

High-spin states in¹⁷⁵Ta have been populated in the¹⁶⁰Gd (¹⁹F,4n)¹⁷⁵Ta reaction with beams provided by the HI-13 tandem accelerator at the Institute of Atomic Energy in Beijing. A level scheme was constructed from γ-γ coincidence experiments. Seven decay sequences built on 5/2⁺[402], 1/2^?[541], 7/2⁺[404] and 9/2^? [514] proton Nilsson configuration have been extended to higher spin. An anomalously large delay of the neutron AB crossing built on the h_9/2 proton Nilsson State 1/2^?[541] is discussed.     

Band structures in73Se

Excited states of⁷³Se have been investigated up to spin, 21/2 using techniques of in-beamγ-ray spectroscopy in connection with the⁷⁰Ge(α, n) reaction. Mean lifetimes of 12 levels have been determined applying Doppler-shift andγ-RF-methods. Five different bands have been identified that reflect a variety of different excitation modes. The decoupled 9/2⁺ band is likely to correspond to an oblate deformation while the 5/2⁺ band is interpreted as a strongly coupled prolate band built on the Nilsson configuration [422] 5/2⁺. The 3/2^? band is a strongly coupled band built on the [301] 3/2^} configuration.Nuclear reactions:⁷⁰Ge(α,n),E=14, 16, 18, 19, 20MeV; measuredE _γ,I _γ,σ(E _γ,θ),γγ-coin, linear polarization, DSA,γ(t).⁷⁵Se deduced levels,I, π, τ, δ(E2/M1), B(σλ). Enriched targets, Ge detectors.     

Collective structures in185Hg

Excited states of¹⁸⁵Hg have been investigated via the¹⁶¹Dy(²⁸Si, 4n) reaction at 145 MeV. In-beamγ-ray spectroscopic studies have been performed with the “Château de Cristal” 4π-multidetector array. A level scheme of¹⁸⁵Hg has been established. Shape coexistence, still present in¹⁸⁵Hg like in the neighbouring Hg isotopes, manifests itself through a weakly populated decoupled band built on the 13/2⁺ isomer and three strongly-coupled bands built on the prolate 1/2^? [521], 7/2^? [514], and 9/2⁺ [624] Nilsson states.     

Study of low-lying states in151Eu via (n,n′γ) reaction

The levels of¹⁵¹Eu have been investigated in the (n,n′γ) reaction using nuclear reactor fast neutrons. The energies, intensities and angular distributions of theγ-rays have been measured with the Ge(Li) spectrometer. Four rotational bands with the following band heads and Nilsson configurations have been identified: ground state band, 5/2⁺ [402]; 21.5 keV, 7/2⁺[404]; 196.5 keV, 3/2⁺[411]; 260.5 keV, 5/2⁺[413]. The low spin states at 332.2 and 336.2 keV have been tentatively assigned to the l/2⁺[411] Nilsson orbital, but 522.8, 580.0 and 587.0 keV states to the 1/2⁺[420] Nilsson orbital. The negative parity levels at 353.7, 522.1 and probably 546.2 keV have been proposed basing on theh _11/2 proton state.     

Proton states in193Ir and195Ir populated via the (α) reaction

The nuclear structures of¹⁹³Ir and¹⁹⁵Ir have been studied using the¹⁹⁴Pt(t, α) and¹⁹⁶Pt(t, α) reactions. Levels up to ～2MeV in excitation energy were studied with a resolution of ～13keV (FWHM) and spectroscopic strengths were extracted. The low-lying states in¹⁹³Ir have been interpreted in terms of both the Nilsson and the rotation-vibration models including Coriolis, particle-vibration and rotation-vibrational couplings. The previously assigned 3/2⁺ [402], 11/2^? [505], 1/2⁺[400] and 1/2⁺[411] bands were populated and candidates for the 5/2⁺ [402] and 7/2⁺ [404] bandheads were observed. TheK ₀ +2 gamma vibration based on the ground state was populated because it is mixed with the 7/2⁺ [404] state. Surprisingly, there was no significant difference between the stripping and pick-up strengths for the low-lying states in¹⁹³Ir, suggesting that the equilibrium nuclear shape of¹⁹³Ir has large overlaps with the shapes of both¹⁹²Os and¹⁹⁴Pt. The nuclear level structure of¹⁹⁵Ir appears to be similar to those of the lighter iridium isotopes.     

Signature splitting in two quasiparticle rotational bands of <Superscript>180,182</Superscript>Ta

The signature splittings in K^π = 1 ⁺: 7 /2[404] _π?9 /2[624] _ν, K^π = 0^?: 9 /2[514] _π?9 /2[624] _ν bands of ¹⁸⁰Ta and K^π = 0^?: 7 /2[404] _π?7 /2[503] _ν, K^π = 1^?: 5 /2[402] _π?3 /2[512] _ν, K^π = 1⁺: 7 /2[404] _π?9 /2[624] _ν bands of ¹⁸²Ta are analysed within the framework of two-quasiparticle rotor model. The phase as well as magnitude of the experimentally observed signature splitting in K^π = 1⁺ band of ¹⁸⁰Ta, which could not be explained in earlier calculations, is successfully reproduced. The conflict regarding placement of a 12 ⁺ level in K^π = 1 ⁺: 7 /2 ⁺[404] _π?9 /2 ⁺[624] _ν ground-state rotational band of ¹⁸⁰Ta is resolved and tentative nature of K^π = 0^?: 7 /2[404] _π?7 /2[503] _ν, K^π = 1⁺: 7 /2[404] _π?9 /2[624] _ν bands observed in ¹⁸²Ta is confirmed. As a future prediction for experimentalists, these two-quasiparticle structures observed in ¹⁸⁰Ta and ¹⁸²Ta are extended to higher spins.     

Interaction strength and shape difference for the h9/2 and h11/2 configurations in163Tm

The strongly shape driving πh_9/2[541]l/2^? configuration with α=+1/2 exhibits some anomalous, and so far unexplained, features concerning the crossing frequency, ?ω_c, the aligned angular momentum, i_x, and interaction strength, at the alignment of the first pair of i_13/2 quasineutrons in several odd-Z rare earth-nuclei. The h_9/2[541]1/2^? and h_11/2[523]7/2^? bands have been studied in the stably deformed rare-earth nucleus¹⁶³Tm to investigate these features. A difference in band crossing frequency of ～ 80 keV between the two bands is found. Rotational bands built on these two configurations have been found to cross in the spin range I=25/2–29/2 ?. Theγ-decay pattern between the two bands is established in the crossing region and analysed in terms of a moderate shape difference between them. A theoretical estimate of the size of the interaction strength between the two bands is presented and compared to the experimental value. The observed band structure in¹⁶³Tm is very similar to that of¹⁶⁷Lu which has 2 protons and 2 neutrons in addition. This observation is discussed in relation to the similarity of the yrast bands of the two even-even “core” nuclei¹⁶²Er and¹⁶⁶Yb, for which theγ-transition energies are identical within ～0.2 keV below the vi_13/2 crossing.     

The identical bands in177Ta

The [402]5/2 bands in¹⁷⁷Ta which are identical to the neighboring even-even¹⁷⁶Hf groundstate band have been extended to higher angular momentum. These bands in the two nuclei are seen to diverge from each other in the region of the first i_13/2 neutron alignment. The lower observed crossing frequency for the [402]5/2 bands indicates a lower deformation for these bands compared to¹⁷⁶Hf. Extensions to theh _9/2 [541]1/2 yrast band are also reported.Special thanks to D.C. Radford, It. MacLeod, W.T. Milner and R. Books for their invaluable software support. Thanks also to F.K. McGowan and R. Darlington for their help with the targets. This work was supported by the National Science Foundation and the State of Florida and by the UK Science and Engineering Research Council. MAR and JS acknowledge the receipt of a NATO Collaborative Research Grant.     

Neutroneneinfangs-Gammaspektrum und Niveauschema von Erbium 167

The (n, γ)-spectrum of Er 167 was investigated by means of the Risø bent crystal spectrometer. The energies and intensities of 47 transitions were measured. 22 of them were fitted into a level scheme. The rotational bands of theK=7/2⁺[633],K=1/2^?[521], andK=5/2^?[512] states were found up to the spin valuesI=11/2⁺, 9/2^?, and 7/2^?. Gammavibrational states of these Nilsson orbitals were determined or proposed. The energy of theK=5/2^?[523] state was suggested. Branching ratios of gamma transitions were compared with theoretical predictions.     

Loss of alignment gain in the πh 9/2[541 1/2−] ⊗ (νi 13/2)2 band in 167Tm and neighbouring odd-Z N = 98 nuclei

Theπh_9/2[541 1/2^?] band in ¹⁶⁷Tm has been studied through the ¹²⁴Sn(⁴⁸Ca, p4n)¹⁶⁷Tm reaction at E _beam = 210 MeV. The favoured signature of the πh_9/2[541 1/2^?] band is observed to spin 61/2h?. A remarkable small gain in aligned angular momentum at the AB-band crossing has been found. The crossing frequency and the gain in aligned angular momentum for ¹⁶⁷Tm are compared with data for the N = 98 isotones of Lu, Ta and Re, and contrasted with Cranked Shell Model predictions, based on frequency diabatic configurations. Large deviations are found for the gain in aligned angular momentum.     

A study of the rotational side-bands in 162Dy

Rotational side-bands in ¹⁶²Dy have been studied using the ¹⁶⁰Gd(α, 2nγ)¹⁶²Dy reaction. Seven side-bands are observed, with K^π = 2⁺, 2^?, (0)^?, 0⁺, 5^?, 4⁺ and (6^?). Four of these bands have collective structure at low spin: the K^π = 2⁺γ-vibrational band, the K^π = 0⁺β-vibrational band, and the K^π = 2^? and (0)^? octupole vibrational bands. Of the remaining bands, the 4⁺ band is deformation coupled while the 5^? and (6^?) bands are rotation-aligned. Several bandcrossings are observed in this nucleus. The β and γ-bands are crossed at $\text{I = 6}\text{h}\text{?}\text{and}\text{12}\text{h}\text{?}$, respectively, by a highly aligned ($\text{i}{}_{\mathrm{<mtext>13</mtext><mtext>2</mtext>}}$)² S-band; extrapolation of this S-band to higher spin suggests that it crosses the g.s.b. between $\text{I = 18}\text{h}\text{?}\text{and}\text{20}\text{h}\text{?}$. The 2^? octupole band is crossed by the 5^? band at $\text{I = 9}\text{h}\text{?}$ and again by the (6^?) band at $\text{I = 12}\text{h}\text{?}$. The latter bandcrossings are discussed in terms of two-quasiparticle plus rotor calculations.     

On the level scheme of the159Tm nucleus

Level in¹⁵⁹Tm have been excited in the¹⁵⁰Sm(¹⁴N,5n reaction. The yrast band 7/2^?[523] observed up to spin 45/2^? is built on a isomeric state at 166 keV (T _1/2=37±5ns). Connections between the 7/2^?[523] and 7/2⁺ [404] bands fix the band-head energies.     

Experimental Study of High-Spin States in Deformed Odd-Odd 180 Ir

High spin states in deformed odd-odd ¹⁸⁰Ir have been investigated using the ¹⁵⁴Sm (³¹P, 5nγ)¹⁸⁰Ir reaction through the measurements of excitation functions at 150, 155, 160, 165 and 170 MeV beam energies, K X-γ and γ-γ coincidences at 160 MeV. A new level scheme composed of 5 rotational bands has been established. According to the band structure characteristics and the deduced in band B(M1)/B(E2) ratios, the quasiparticle configurations and spin and parity have been proposed to the observed bands. The neutron AB crossing is observed at hω_c≈0.26 MeV for the π1/2^－[541]⊙ν1/2^－[521] and π1/2^－[541]⊙ν5/2^－[512] bands. This AB crossing frequency is close to that in the ν5/2^－[512] band of ¹⁷⁹Os indicating the loss of intruder nature of the π1/2 - orbit. Gradual alignment gain in both the π9/2^－[514]⊙ν7/2⁺[633] and π9/2^－ν5/2^－[512] bands is observed which is similar to the low spin anomaly in alignment in the πh_11/2,πd_5/2 and πi_13/2 bands of neighboring Ir and Re isotopes. Different alignment properties have been discussed in the framework of cranked shell model, and a larger quadrupole deformation is suggested for the bands with π9/2^－[514] orbit involved.     

The decay of 1.4 min173Er

A 1.4 ± 0.1 min activity which is assigned to β^?-decay of ₆₈ ¹⁷³ Er has been produced with 14–15 MeV neutrons through the reaction¹⁷⁶Yb(n, α)¹⁷³Er. Its decay has been studied with Ge(Li) detectors and several (γ)(γ) as well as (γ)(β) coincidence arrangements. Eight gamma rays with energies 94.2, 116.14, 118.6, 122.40, 192.8, 199.2, 800.8 and 895.2 keV were assigned to the decay of¹⁷³Er. The proposed level scheme of the daughter nuclide ₆₉ ¹⁷³ Tm contains levels at 0, 2.5, 118.6, 124.9, 317.7, 411.9 and 1212.8 keV. The 317.7 keV level is an isomeric state with a measured half-life of 10±3 μs. This 10 μs isomer and the 411.9 keV level are thought to be the band head and 9/2^? rotational member of 7/2^? [523] Nilsson state, respectively. The levels at 0, 2.5, 118.6 and 124.9 keV are interpreted as 1/2⁺, 3/2⁺, 5/2⁺ and 7/2⁺ members of the 1/2⁺ [411] band and the level at 1212.8 keV as the 9/2^? [514] Nilsson state. Some systematic considerations and theoretical transition probability calculations are also included.     

Search for decoupled bands in Odd-Odd 174Re

An effort has been made to search for the decoupled bands in odd-odd ¹⁷⁴Re via the ¹⁵⁹Tb(²⁰Ne,5n γ)¹⁷⁴Re reaction through excitation functions, K x-γ and γ-γ coincidence measurements. The doubly decoupled band and the semidecoupled one have been found in this work. Their quasiparticle configurations are proposed to be π1/2⁻[541] ⊗ν1/2⁻[521] and π1/2⁻[541] ⊗νi _13/2, respectively, according to the characteristics of band structures in this mass region. Another band with strongly coupled character is also reported. Received: 23 November 1999 / Revised version: 5 January 2000     

Intrinsic and rotational excitations in 175Lu

The γ-ray cascades following the ¹⁷⁶Yb(p, 2n)¹⁷⁵Lu reaction are studied with Ge(Li) detectors and interpreted within the level scheme of ¹⁷⁵Lu. According to this interpretation the $\text{1}\text{2}$⁺[411], $\text{1}\text{2}$^?[541], $\text{5}\text{2}$⁺[402], $\text{7}\text{2}$⁺[404] and $\text{9}\text{2}$^?[514] rotat $\text{15}\text{2}$⁺, $\text{21}\text{2}$^?, $\text{13}\text{2}$⁺, $\text{19}\text{2}$⁺ and $\text{19}\text{2}$^? members, respectively. Fu band head is determined to be 626.6 keV. The data for the rotational bands are discussed within the particle-rotor model and compared with the available information on the corresponding bands in the adjacent Lu isotopes.     

Dipole bands and multi-quasiparticle excitations in 193Pb

The nucleus ¹⁹³Pb was populated via the ¹⁶⁸Er(³⁰Si,5n) reaction at a beam energy of 159 MeV and studied with the EUROGAM II spectrometer. Five new dipole Δ I = 1 cascades have been found. These structures have been connected to the level scheme which has been considerably extended up to a spin of ? and an excitation energy of about 8 MeV. Angular distribution coefficients, a₂, have been measured and confirm the dipole character of the in-band transitions. B(M1)/B(E2) ratios have been extracted for the two most intense cascades. The ¹⁹³Pb dipole bands are discussed in comparison with those known in odd lead isotopic series and the structure of the band heads is analyzed in terms of microscopic HF+BCS calculations. The proposed configurations are based on a high-K two quasiproton excitation, π ([505]9/2^t? ? [606]13/2⁺)_K=11^t -, coupled to one or three rotation aligned quasineutrons involving the i_13/2, p _3/2, f_5/2 and/or h{9/2} subshells. The main difference, compared with the heavier lead isotopes, is the presence of large Ω orbitals from the ν (i_13/2) shell near the Fermi surface which are responsible for the increasing band-head spin as A decreases.     

Investigation of the ground state rotational bands in233U and239Pu in the (α, 3n) reactions

The ground state rotational bands in²³³U and²³⁹Pu were investigated in (α, 3n) reactions. Conversion electrons were measured with an iron free orange spectrometer in order to suppress the background from fission. Levels up toI ^π=33/2⁺ of theK=5/2 band in²³³U andI ^π=31/2⁺ of theK=1/2 band in²³⁹Pu were identified ine ^?γ coincidence measurements. The level energies of both rotational bands can be well described up to the highest observed spins by a two-parameter angular velocity expansion. The electromagnetic properties of theK=1/2 band in²³⁹Pu are discussed.