Angular correlation measurements in 131Xe

An experiment is described determining the angular correlations of five γ-γ cascades in ¹³¹Xe appearing in the decay of ¹³¹I. Using a 12-channel goniometer the following results were obtained: 325.8–177.2 keV, A₂ = 0.018(11), A₄ = ?0.003(10); 318.1–404.8 keV, A₂ = ?0.18(4), A₄ = ?0.05(3); 272.5–364.5 keV, A₂ = 0.15(4), A₄ = ?0.03(3); 284.3–80.2 keV, A₂ = ?0.005(7), A₄ = 0.005(15); 318.1–324.6 keV, A₂ = 0.24(5), A₄ = ?0.01(5). The result for the 318.1–324.6 keV cascade has not been corrected for a small influence from the 318.1–404.8 keV cascade. Four E2/M1 mixing ratios are deduced from the angular correlation coefficients (energies in keV): $\text{?0.28 ≦ δ(325.8) ≦}\text{t}\text{?}\text{0.20}$, ?0.19 ≦ δ(318.1) ≦ ?0.035, ?1.5 ≦ δ (324.6) ≦ ?0.05, δ(272.5) = ?0.38(17). Transition strengths and limits to transition strengths are calculated for several transitions. The properties of negative parity states in ¹³¹Xe are investigated theoretically in a model where three neutron holes in the $\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ orbit are coupled to the quadrupole vibrations of the even core. The experimental energies of the lowest negative parity states and the strengths of the transitions between these states are very well accounted for by the calculation.     

Beta decay studies in the vicinity of 13250Sn82: Excited states in 13051Sb, 13052Te and 13252Te

Using the OSIRIS on-line isotope separator facility, the decays of ¹³⁰Sn and ^{130, 132}Sb have been studied. On the basis of singles γ and γ-γ coincidence Ge(Li) spectra and conversion electron Si(Li) measurements, level schemes for ¹³⁰Sb, ¹³⁰Te and ¹³²Te have been constructed. The corresponding half-lives have been measured using multiscaling technique. The 3.8 min ground state of ¹³⁰Sn populates only positive parity states in the πν^?3 nucleus ¹³⁰Sb: the energetically lowest 5⁺ state with the $\text{(π1}\text{g}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{, ν2}\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{)}$ configuration assignment; the $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 3.6 ± 0.3}\text{ns}\text{4}{}^{\mathrm{+}}$ state at 70.0 keV; the 2⁺ state at 262.5 keV; the (0, 1)⁺ state at 697.2 keV; the 3⁺ state at 813.1 keV and the 1⁺ state at 1042.3 keV excitation energy. A 1.7 min isomeric state in ¹³⁰Sn, with the tentative spin assignment (7^?), populates several odd parity levels in ¹³⁰Sb. These arise from the $\text{(π1}\text{g}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{, ν1}\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}{}^{-1}\text{)}$ and/or $\text{(π2}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{, ν1}\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}{}^{-1}\text{)}$ configurations and are located 84.7 keV (6^?), 144.9 keV (7^?), 688.5 keV and 1044.0 keV above the 40 min 8^? β- decaying state. No transitions between odd and even parity states have been observed.The most important excited states in ¹³⁰Te found in the β^? decay of the 6.6 min ¹³⁰Sb 5⁺ state are: 839.4 keV, 2⁺; 1632.8 keV, 4⁺; 1815.1 keV, 6⁺; 2100.8 keV, 5^?.Levels in the π²ν^?2 nucleus ¹³²Te were observed in the β^? decays of the 2.8 min ¹³²Sb (4⁺) and the 4.2 min ¹³²Sb (8^?) states. Unique spin and parity assignments have been given to the following states: 973.9 keV, 2⁺; 1670.7 keV, 4⁺; 1774.1 keV, 6⁺; 1924.7 keV, 7^?; 2053.0 keV, 5^?.     

Short-lived isomers in 211Rn and 210Rn

The following mean lives of excited states in ²¹¹Rn and ²¹⁰Rn have been measured: ²¹¹Rn: 8169 + Δ' keV, (τ = 3.3(3) ns), 7400 + Δ' keV (2,1(5) ns), 6101 + Δ' (41(2) ns), 5247 + Δ keV (5(2) ns), 3244 + Δ keV (3.9(8) ns), 2651 + Δ keV (9.6(4) ns); ²¹⁰Rn: 8555 + Δ keV (2.6(3) ns), 4994 + Δ keV (19(1) ns), 1665 keV (15.0(15) ns). Some theoretical implications of these measurements for the structure of ²¹¹Rn and ²¹⁰Rn are discussed.     

High-spin states and two-quasiparticle structure in 172Hf

Five rotational bands in ¹⁷²Hf have been observed to high spin following the ¹⁶⁰Gd(¹⁶O, 4n)¹⁷²Hf reaction, using γ-γ coincidence techniques. The ground-state band is extended to spin 20⁺ without backbending, in contrast to the neighbouring even-even isotones. The most strongly populated side-band, consisting of separated odd-spin and even-spin sequences, appears to result from the partial decoupling of an i_{$\text{13}\text{2}$} neutron. Three other bands are identified with the following band-heads: an 8^? state at 2006 keV with 163 ns half-life; a 6⁺ state at 1685 keV with a half-life < 16 ns; and a level at 1857 keV with spin 4, 5, 6 or 7 and a half-life < 16 ns. The differences between the bands are discussed in terms of the influence of Coriolis forces and the two-quasiparticle level structure.     

States in 163Dy and 169Er populated in the (t,p) reaction

Angular distributions of protons from the ¹⁶¹Dy(t, p)¹⁶³Dy and ¹⁶⁷Er(t, p)¹⁶⁹Er reactions were studied, using 15 MeV and 17 MeV tritons from the McMaster University tandem Van de Graaff accelerator. The reaction products were analyzed with a magnetic spectrograph and detected with nuclear emulsions. Since the ¹⁶¹Dy target ground state is the $\text{5}\text{2}$⁺[642] orbital, a strong L = 0 transition was observed to the $\text{5}\text{2}$⁺[642] bandhead in ¹⁶³Dy, which was previously assigned at 251 keV. Also transitions to the $\text{7}\text{2}$, $\text{9}\text{2}$ and $\text{11}\text{2}$ band members were observed. Similarly, a strong L = 0 transition was observed to the $\text{7}\text{2}$⁺[633] bandhead at 244 keV in ¹⁶⁹Er, with the other band members only weakly populated. The angular distributions to the various members of these two bands can be described when higher-order reaction processes are taken into account. In ¹⁶³Dy, surprisingly strong L = 0 transitions were observed to levels at 1831 keV, 1937 keV and 2053 keV, with strengths of 23%, 30% and 37% of that for the $\text{5}\text{2}$⁺[642] bandhead. In ¹⁶⁹Er, the 905 keV level was populated with an L = 0 transition that had 31% of the strength observed for the strong L = 0 transition to the 244 keV level. The nature of these states is at present not understood.     

A re-investigation of 19O(β−)19F

Gamma rays observed in¹⁹O(β^?)¹⁹F have been studied with Ge(Li) spectroscopy utilizing activity created via the ¹⁸O(d, p) ¹⁹O reaction. Precision measurements of γ-ray energies and intensities, in conjunction with previous work, define the following excitation energies (in keV) and β-branching ratios (in %) for states of ¹⁹F: 109.894(5), 0.055(13/38); 197.143(4), 45.4(1.5); 1345.67(13), 0.017(2); 1554.038(9), 54.4(1.2); 2779.849(34), < 0.002; 3908.17(20), 0.0081(5); 4377.700(42), 0.0984(30). The corresponding values of logft are 8.34(30/10); 5.384(14); 8.25(5); 4.625(10); > 8.17; 6.133(27); and 3.859(17), respectively. The β-branches to the 1346 and 3908 keV states had not been observed previously. The β-branch for the 4378 keV state is in significantly better accord with theoretical expectations than the previous value of 0.160(12) %.     

Measurement of the parity non-conservation in nuclear forces in 75As, 171Tm,and 175Lu and of the polarized bremsstrahlung from 51Cr, 170Tm, 177Lu,and 198Au

The circular polarization P of γ-rays from unpolarized sources of ⁷⁵Se, ¹⁷¹Er, and ¹⁷⁵Yb of strengths ? 500 Ci has been measured with a Compton polarimeter of the radial transmission type. Eight NaI crystals and a four-fold current integration system were used to simultaneously record the data in four independent channels. The results are: P = ?(1.8 ± 6.0) × 10^?5 for the 401 keV transition in ⁷⁵As (the experimental error is ± 1.5 × 10^?5, the remaining part is due to the uncertainty in the decay scheme of ⁷⁵Se), P = (0.8 ± 1.5) × 10^?4 for the 296 keV and 308 keV transition in ¹⁷¹Tm, and P = (5.7 ± 0.8) × 10^?5 for the 396 keV transition in ¹⁷⁵Lu. The last value confirms the parity non-conservation in nuclear forces. The polarimeter was calibrated with bremsstrahlung from ¹⁷⁰Tm. The correction for polarized bremsstrahlung was given special attention. Correction factors are derived for ⁵¹Cr, ¹⁷⁷Lu, and ¹⁹⁸Au from a comparison of the measured and calculated bremsstrahlung yields.     

High-spin states in the N = 84 nucleus 152Er

High-spin states in ¹⁵²Er have been populated through the ¹⁴⁴Sm(¹²C, 4n)¹⁵²Er reaction. Excitation functions, angular distributions, prompt and delayed coincidences against the beam burst were measured. Three isomeric states at 2183 keV (τ = 1.8 ns), 4519 keV (τ = 1.2 ns) and 4915 keV (τ = 6.6 ns) have been observed. The decay scheme is developed up to 7 MeV and is discussed and compared with that of ¹⁵⁰Dy.     

Levels and transitions in 131Xe studied by the decay of oriented 131I

Sources of ¹³¹I were prepared by implantation in iron foils. Nuclear orientation of these sources was obtained by cooling them to temperatures of 30 to 50 mK using a dilution refrigerator and saturating the iron by an external magnetic field. Anisotropies of angular distributions of eleven γ-transitions in ¹³¹Xe have been measured, yielding the following results: the levels at 405 and 637 keV are $\text{3}\text{2}$⁽⁺⁾ and $\text{7}\text{2}$⁽⁺⁾ states and the assignment $\text{5}\text{2}$⁽⁺⁾ for the level a keV is confirmed. Deduced values of E2/M1 multipole mixing ratios are (energies in keV): ?0.5 ≦ δ(318) ≦ + 0.2; δ(364) = ?4.53±0.12; + 0.2 ≦ δ(405) ≦ +2.0; δ(723) = +0.207 ± 0.005. The allowed β-decay branch to the level at 637 keV is for at least 90% of the Gamow- Teller type.     

Discovery of a very low-lying 0+ state in 98Sr and shape coexistence implication in 98Sr

Using the on-line mass separator OSTIS for studies of short-lived fission products, we found two modes for the β-decay of ⁹⁸Rb with respective half-lives of 96 and 114 ms. The latter decay populates a 0⁺ state in ⁹⁸Sr at the unusually low energy of 215.5 keV, which is by far the lowest 0⁺ excited state observed in even-even nuclei. The half-lives of the 0⁺ and the 2₁⁺ states in ⁹⁸Sr were found to be respectively 25 ns and 4 ns. The transition probabilities, the reduced E0 matrix element and the observed level structure suggest shape coexistence for ⁹⁸Sr: the spheroidal 0₁⁺ ground state shows a rotational band with the 2⁺, 4⁺, (6⁺) levels, whereas the presumably spherical 0₂⁺ excited state may be connected with the 2⁺ level of a vibrational band. A simple model is proposed.     

Three- and five-quasiparticle isomers,rotational bands and residual interactions in 175Hf

Two 3-quasiparticle isomers with spins, parities and half-lives of $\text{19}\text{2}$⁺, 1.1 μs and $\text{23}\text{2}$^?, 1.2 ns have been identified at 1433 and 1766 keV in ¹⁷⁵Hf. A third isomer, possibly $\text{35}\text{2}$^? with a 1.2 μs half-life, is found at 3015 keV. The first two are characterised as a $\text{7}\text{2}$⁺ [633] neutron coupled to the known 6⁺ and 8^? 2-proton isomers of the core nuclei. Rotational bands based on the 3-qp isomers are highly perturbed, due to Coriolis mixing, and their structure is reproduced in a band mixing calculation. The energy depression of the 3-quasiparticle states relative to the 2-quasiproton core states is attributed mainly to the residual proton-neutron interaction, and possibly also to blocking effects through neutron admixtures.     

12C(12C, 8Be)16O angular distributions and excitation functions between 35 and 69 MeV bombarding energy

An array of eight detectors has been developed for identifying the particle unstable ⁸Be nucleus from nuclear reactions with high detection efficiency. Absolute cross sections have been measured for the reaction ¹²C(¹²C, ⁸Be_g.s.)¹⁶O to the ground state and to several excited states in ¹⁶O. Excitation functions at seven angles from 15° to 45° (lab) in 5° steps have been measured for bombarding energies between E¹²_C(lab) = 35 and 69 MeV. Excitation functions were obtained for the following states in the residual nucleus ¹⁶O which were found to be strongly populated: g.s.(0⁺); 6.1 MeV (0⁺, 3^?); 6.9 MeV (2⁺); 10.4 MeV (4⁺); 11.1 MeV (4⁺); 14.7 MeV (6⁺,…) and 16.3 MeV (6⁺,…). The energy range is covered in 250 keV (c.m.) steps; at certain energy ranges in ¹²⁵ keV or 50keV steps. All excitation functions exhibit a strong energy dependence of the cross section; pronounced gross structures with superimposed fine structures, similar to those observed for ¹²C+¹²C elastic and inelastic scattering at these energies, are observed. At 19.3 MeV, where resonant structures were observed in the reactions ¹²C(¹²C, p)²³Na, ¹²C(¹²C, n)²³Mg and ¹²C(¹²C, d)²²Na, no resonance is found for the reaction studied here. At 60, 61 and 63 MeV angular distributions have been measured in 1° and 2.5°(lab) angular steps. The excitation functions have been analyzed in terms of Ericson fluctuations and cross-correlation functions.     

The four quasi-particle 178Hf isomeric state,its excitation energy and multipolarities of deexciting transitions

The decay properties of the 31 y ¹⁷⁸Hf isomeric state have been investigated by means of a Ge(Li)-NaI(Tl) Compton suppression device and a Si(Li) detector, as well as with Ge(Li) and Ge(Li), intrinsic Ge and Si(Li) coincidence arrangements. From the E3 character of the unobserved isomeric transition (11.7 ≦ E_IT ≦ 16.7 keV), deduced from L-subshell and M/L ratios, I^π, K of the isomeric level was determined as 16⁺, 16. The {p[404] ↓ + p1514] ↑ + n[514] ↓ + n[624]↑}_16⁺ + four quasi-particle configuration was assigned to this level at 2447.5 ± 2.5 keV. As proposed earlier the isomeric E3 transition decays to the 13^? level of the band built upon the I^π = 8^? isomer. The |g_K?g_R| values derived from $\text{E}\text{2}\text{M}\text{1}$ ratios of interband transitions show that the 8^? isomer contains about 36 % of the p[404] ↓ + p[514] ↑ configuration and about 64 % of the n[624] ↑ + n[514] ↓ configuration.     

Multipolarities of prompt transitions in 156Dy and spin-parities of the upper band levels

The prompt γ-ray and conversion-electron spectra following the ¹⁵⁹Tb(p, 4n)¹⁵⁶Dy and ¹⁵⁶Gd(α, 4n)¹⁵⁶Dy reactions have been measured with, respectively, a Ge(Li)-NaI(Tl) Compton suppression device and a mini-orange spectrometer. On the basis of the deduced multipolarities, earlier spin-parity assignments for levels in ¹⁵⁶Dy have been confirmed. Furthermore, the recently reported upper band levels could be divided into a negative-parity band up to 11^? or maybe 13^?, and a positive-parity band with tentative K = 0 character and spins 4⁺, 6⁺, 8⁺, 10⁺ and maybe 2⁺. The negative-parity band is described as an aligned-octupole band; the positive-parity band may possibly represent the low-spin extension of the superband responsible for backbending in the β- and ground-state bands in this nucleus.     

Negative-parity states in Pb isotopes: g-factor of the 480 ns,if Jπ = 9− isomer in 200Pb

The g-factor of the 480 ns, 9^? isomer at 2.237 MeV in ²⁰⁰Pb was measured by the time-differential perturbed angular distribution method. The result, g = ?0.0285±0.0011 confirms the rather pure $\text{(}\text{f}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}{}^{\mathrm{?1}}\text{i}{}_{\mathrm{<mtext>13</mtext><mtext>2</mtext>}}{}^{\mathrm{?1}}\text{)}$ quasiparticle structure of this state. Half-lives of 480±20 ns, 43±3 ns and 42±4 ns have been measured for the 2237 keV 9^?, 2154 keV 7^? states in ²⁰⁰Pb and the 2208 keV state in ²⁰²Pb, respectively; E2 transitions and g-factors of negative-parity states in even, neutron-deficient Pb isotopes are discussed.     

Gamma-ray spectroscopy of 66Zn and 67Zn and the role of the g92 orbital

Measurements of γ-ray yield functions, γ-γ coincidence spectra and y-ray angular distributions following the ⁶⁴Ni(α, n) and (α, 2n) reactions have been made. Beam energies of 17 to 25 MeV were used. Among the new levels observed, with their proposed spin-parity assignments, are: in ⁶⁶Zn, 3708 keV, (5)⁺; 3896 keV, 5^?; 4812 keV, (7^?); 5110 keV, (8^?); 6417 keV; (10^?); 7516 keV, (12⁺); and in ⁶⁷Zn, 2937 keV, $\text{13}\text{2}{}^{\mathrm{+}}$; 3491 keV, $\text{17}\text{2}$ or $\text{19}\text{2}$; 3492 keV, ($\text{13}\text{2}$(si+)); 4221 keV, ($\text{19}\text{2}$^?); 4631 keV, ($\text{21}\text{2}{}^{\mathrm{+}}$); 4685 keV, ($\text{21}\text{2}$^?). Comparison between the ground-state sequence, 0⁺ to 6⁺, in ⁶⁶Zn and the first $\text{9}\text{2}$⁺ to $\text{21}\text{2}$⁺ levels in ⁶⁷Zn strongly suggests that the latter be described as a g_{$\text{9}\text{1}$} neutron orbital weakly coupled to the ⁶⁶Zn core. Other sequences of high-spin ⁶⁶Zn states are proposed to be formed from two-quasiparticle states containing the g_{$\text{9}\text{2}$} excitation, coupled to the same ground-state sequence.     

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.     

Multipole mixing ratios of γ-transltions in 182W

γ-γ directional correlation experiments were performed on 14 cascades in ¹⁸²W populated from the β^? decay of ¹⁸²Ta(115 d). Two Ge(Li) detectors were used in a coincidence arrangement, and the ¹⁸²Ta sources were dissolved in HF acid to minimize extranuclear perturbations. For the 1189keV, 2^? → 2⁺ transition, the measured directional correlation coefficients are consistent only with multipole mixing ratios $\text{δ}\text{(}\text{M}\text{2}\text{E}\text{1)}\text{= 0.45 ± 0.03}\text{and}\text{δ}\text{(E3}\text{E1)}\text{= ?0.67 ± 0.07}$. These mixing ratios are discussed and compared with the known conversion coefficients for the 1189keV transition. The E2/M1 multipole mixing ratios determined are (energy in keV): δ(66) = 0.15 ± 0.15, δ(85) = 0.31 ± 0.05, δ(114) = 0.31 ± 0.05, 0.56 ≦ δ(179) ≦ 1.36, δ(1121) = 12⁺²_?1, and δ(1231) = ?(32⁺¹⁴²_?15). The measured M2/E1 mixing ratios are: δ(68) = 0.03 ± 0.02, δ(152) = 0.014 ± 0.013 and δ(156) = ?0.13 ± 0.19.     

Lifetime of the 6+ member of the ground-state band in 22Ne

Mean lifetimes of levels in ²²Ne have been measured by the Doppler-shift attenuation method using the inverse reaction ⁴He(¹⁹F, pγ)²²Ne. The Doppler-broadened lineshapes observed at 0° were analyzed using scaled experimental stopping powers and give the following results for each of the following levels (energy in keV and lifetime in fs): 4457 ( < 15), 5147 (1300 ± 500), 5522 (27 ± 4), 5641 ( < 20), 6311 (70 ± 10), 6345 ( < 15), 6636 ( < 20) and 6817 ( < 15). The lifetime of the 6311 keV 6⁺ level is particularly interesting for testing model predictions. The large basis shell model calculations of Preedom and Wildenthal are in good agreement with the experimental B(E2) values for the 2⁺, 4⁺ and 6⁺ members of the ground-state band.