Spin assignments from the 142Nd(7Li, 6He)143Pm and 144Sm(7Li, 6He)145Eu reactions at 52 MeV

Angular distributions have been measured for transitions to low-lying states in ¹⁴³Pm and ¹⁴⁵Eu populated by the ¹⁴²Nd(⁷Li, ⁶He)¹⁴³ and the ¹⁴⁴Sm(⁷Li, ⁶He)¹⁴⁵Eu reactions at $\text{E(}{}^7\text{Li}\text{) = 52}$ MeV. Elastic scattering of ⁷Li at 52 MeV on ¹⁴²Nd and ¹⁴⁴Sm, and ⁶Li at 46 MeV on ¹⁴²Nd and at 45 MeV on ¹⁴⁴Sm, were measured. Optical-model parameters extracted from fits to the scattering data were used in a finite-range DWBA analysis of the angular distributions for levels below 1.40 MeV excitation energy in ¹⁴³Pm and 1.84 MeV in ¹⁴⁵Eu. The reaction cross sections forward of 6° c.m. allow unambiguous distinction to be made between 2d_{$\text{5}\text{2}$} and 2d_{$\text{3}\text{2}$} final states. Final-state spins have been assigned to d-states in ¹⁴³Pm at 1.40 MeV($\text{3}\text{2}$⁺)and in ¹⁴⁵Eu at 1.042 MeV ($\text{3}\text{2}$⁺). Existing assignments to other levels in both residual nuclei have been confirmed.     

Rotational bands in the odd-odd 152Eu nucleus

The ¹⁵⁰Nd(⁷Li, 5n) reaction has been used to study the high-spin states in the odd-odd nucleus ¹⁵²Eu. Two rotational bands of different behaviour have been identified: a rather regular band based on the I^π = 8^? isomeric state of configuration $\text{[413}\text{5}\text{2}\text{]}{}_{\mathrm{<mtext>p</mtext>}}\text{[505}\text{11}\text{2}\text{]n}$ and a strongly decoupled system belonging to the configuration $\text{[}\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}\text{]}{}_{\mathrm{<mtext>p</mtext>}}\text{[i}{}_{\mathrm{<mtext>13</mtext><mtext>2</mtext>}}\text{]}{}_{\mathrm{<mtext>n</mtext>}}$.It is shown in this work that the aligned angular momentum carried by each two-quasiparticle configuration in ¹⁵²Eu is the sum of the alignment of the odd neutron and odd proton, which indicates a negligible influence of the neutron-proton residual interaction. Particular attention has been focused on the strong deviations of the moment of inertia of the core when different quasiparticle configurations are involved.     

A study of the (n, α) reactions induced by 18 MeV neutrons in 151Eu, 151Eu, 159Tb, 16SHo and 169Tm

The energy spectra of α-particles emitted in the ${}^{151}\text{Eu(n}\text{, α)}{}^{148}\text{Pm}\text{,}{}^{153}\text{Eu(n}\text{, α)}{}^{150}\text{Pm}\text{,}{}^{159}\text{Tb(n}\text{, α)}{}^{156}\text{Eu}\text{,}{}^{165}\text{Ho(n}\text{, a)}{}^{162}\text{Tb and}{}^{169}\text{Tm(n}\text{, α)}{}^{166}\text{Ho}$ reactions have been measured at a neutron energy of 18.15 MeV. For the ${}^{165}\text{Ho(n}\text{, α)}{}^{162}\text{Tb}$ reaction the α-particle angular distribution has been measured as well. The results have been analysed in terms of statistical, pre-equilibrium and knock-on models.     

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.     

Reactions of 40Ar with 159Tb, 142Nd and 144Sm; New α-activities from 189Bi, 173Pt and 177Au

Reactions of ⁴⁰Ar ions with targets of ¹⁵⁹Tb, ¹⁴²Nd, and ¹⁴⁴Sm have been studied at energies below 300 MeV with a helium gas-jet system. Excitation functions for (Ar, xn) reactions, where x = 5–10, were obtained for the radioactive products that decay by α-emission. Based on the characteristics of these excitation functions and on the systematics of α-decay, evidence is presented for the existence of the nuclides ¹⁸⁹Bi with α-particle energy E_α = 6.67±0.01 MeV and half-life < 1.5 sec, and ¹⁷³Pt with E_α = 6.19±0.01 MeV; and for the emission from ¹⁷⁷Au of an α-particle with E_α = 6.15±0.01 MeV.     

The (nth, α) reaction on 147Sm, 151, 153Eu and Yb isotopes

The (n, α) reaction has been studied using the highly pure thermal neutron beam from the 87m curved neutron guide at the Grenoble high flux reactor. The ¹⁴⁷Sm(n, α)¹⁴⁴Nd reaction showed up five lines corresponding to the ground and the first four excited states of the final nucleus. It is shown that ≈53 % of the 581 μb (n, α) cross section comes from the neutron capture by a bound level of the ¹⁴⁸Sm compound nucleus. The 8.7 ± 3 μb cross section of ¹⁵¹Eu(n, α)¹⁴⁸Pm seems to consist principally of at least two lines corresponding to the ground and the second excited states of ¹⁴⁸Pm. The ¹⁵³Eu(n, α)¹⁴⁹Pm cross section for thermal neutrons is ≦ 1 μb. The lower limits of (n, α) thermal neutron cross section values on ytterbium isotopes are ≈ 20 to 40 times lower than the published data.     

The (n, 2n) cross sections of 85Rb, 87Rb and 144Sm

The (n, 2n) cross sections at neutron energies between 14.9 and 17.0 MeV have been measured for ⁸⁵Rb, ⁸⁷Rb and ¹⁴⁴Sm by the mixed-powder method and γ-ray detection by a Ge(Li) spectrometer. Using the ²⁷Al(n, α)²⁴Na reaction for monitoring, the measured cross sections were (in mb): ${}^{85}\text{Rb}\text{(}\text{n}\text{, 2}\text{n}\text{)}{}^{\mathrm{<mtext>84(</mtext><mtext>m+g</mtext><mtext>)</mtext>}}\text{Rb}\text{, 1125±141, 1177±148}\text{and}\text{1235±162}\text{at}\text{15.0±0.4}\text{MeV}\text{, 16.2±0.7}\text{MeV and}\text{17.0±0.9}\text{MeV, respectively}\text{;}{}^{85}\text{Rb}\text{(}\text{n}\text{, 2}\text{n}\text{)}{}^{\mathrm{<mtext>84</mtext><mtext>m</mtext>}}\text{Rb}\text{, 662±83, 688±87}\text{and}\text{765±99}\text{at}\text{15.0±0.4}\text{MeV}\text{, 16.2±0.7}\text{MeV and}\text{17.0±0.9}\text{MeV}\text{,}\text{respectively}\text{;}{}^{87}\text{Rb}\text{(}\text{n}\text{, 2}\text{n}\text{)}{}^{\mathrm{<mtext>86(</mtext><mtext>m+g</mtext><mtext>)</mtext>}}\text{Rb}\text{, 1336±168}\text{and}\text{1301±162}\text{at}\text{15.0±0.4}\text{MeV and}\text{16.2±0.7}\text{MeV respectively}\text{;}{}^{144}\text{Sm}\text{(}\text{n}\text{, 2}\text{n}\text{)}{}^{\mathrm{<mtext>143(</mtext><mtext>m+g</mtext><mtext>)</mtext>}}\text{Sm}\text{, 1202±130, 1300±141, 1516±179}\text{and}\text{1514±179}\text{at}\text{14.9±0.3}\text{MeV}\text{, 15.5±0.3}\text{MeV}\text{, 16.4±0.5}\text{MeV and}\text{16.7±0.2}\text{MeV, respectively}$. The measured values are compared with the statistical model calculations of Pearlstein.     

A new level of 18F

A new $\text{T = 0 level of}{}^{18}\text{F, E}{}_{\mathrm{x}}\text{= 4848.3 ± 0.5}\text{keV}$, has been studied using the ⁶Li(¹⁶O, αγ)¹⁸F reaction. The γ-decay by a 65 ± 4 % branch to the 1121 keV (5⁺) level and 35 ± 4 % to the 3791 keV (3^?) level has been observed and a lifetime limit τ > 2 ps has been given. A tentative assignment J^π = 5^? is proposed which is supported by the α-γ angular correlation measurements. The possible structure of the new level and reasons why it had not been observed in past experiments are discussed.     

Medium-spin levels and the character of the 20.4 ns 132+ isomer in 145Gd

Levels of the N = 81 nucleus ¹⁴⁵Gd have been investigated by in-beam γ-ray and conversion electron spectroscopy with the ¹⁴⁴Sm(³He, 2n) reaction. Fourteen new low- and medium-spin states between 1.0 and 2.4 MeV excitation, the known yrast levels up to spin $\text{21}\text{2}{}^{\mathrm{+}}$, five other high-spin non-yrast states and a new 20.4 ns $\text{13}\text{2}$ isomer at 2200.2 keV in ¹⁴⁵Gd have been observed. The isomer decays via a fast 927.3 keV E3 transition with B(E3) = 48 ± 7 W.u. Another weaker decay branch is a mixed, strongly hindered E1 + M2 + E3 transition to the $\text{v}\text{h}{}^{\mathrm{?1}}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ state. We propose an octupole $\text{v}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{j}{}^{\mathrm{?2}}\text{× 3}{}^{\mathrm{?}}$ main configuration for the isomer, analogous to the 997 keV $\text{13}\text{2}{}^{\mathrm{+}}$ isomer in ¹⁴⁷Gd. The levels of ¹⁴⁵Gd are discussed on the basis of the spherical shell model.     

The 154Eu(t,p) reaction: Some new systematics in the N = 90 spherical-deformed transition region

A radioactive target of ¹⁵⁴Eu(8.3y) has been used to study the ¹⁵⁴Eu(t, p)¹⁵⁶Eu reaction at an incident energy of 17 MeV. The bandhead and one rotational state of the configuration have been identified in ¹⁵⁶Eu. The excitation energy of the 3^? bandhead is determined to be 448 ± 15 keV. The angular distribution of the first excited rotational state is anamolous and may indicate evidence for a strong two-step component in the reaction mechanism. The energy systematics of the Eu-Sm transition region are also investigated. We find that the systematics of $\text{h}\text{?}{}^2\text{2}\text{I}$ suggest that at N = 87 the ¹⁵⁰Eu excited configurations has a significantly more stable deformed structure than the corresponding $\text{11}\text{2}\text{[505]}$ one-quasiparticle structure in ¹⁴⁹Sm.     

Proton states in the N = 88 nuclei 149Pm, 151Eu and 153Tb populated in the (τ, d) and (α, t) reactions

The (τ, d) and (α, t) reaction on targets of ¹⁴⁸Nd, ¹⁵⁰Sm and ¹⁵²Gd have been studied, using beams of 24 MeV ³He and 27 MeV ⁴He from the McMaster University FN tandem Van de Graaff accelerator. The reaction products were analyzed with a magnetic spectrograph and detected with photographic emulsions. The (α, t) spectra were measured at two angles for each target, and the (τ, d) reactions were studied at 8 or 9 angles. The l-values for a number of low-spin states were determined from the (τ, d) angular distributions, and ratios of the (α, t) and (τ, d) cross sections were used to obtain l-values for several other states. There are some striking similarities in the observed structures of the three final nuclei, ¹⁴⁹Pm, ¹⁵¹Eu and ¹⁵³Tb. In each case there are low-lying strongly populated $\text{11}\text{2}$^? states and a higher lying l = 5 level somewhat below 1 MeV of excitation energy. Several states (10 in ¹⁴⁹Pm, 17 in ¹⁵¹Eu and 8 in ¹⁵³Tb) appear to be populated via l = 2 transitions, and there are strongly excited $\text{1}\text{2}$⁺ levels at $\text{≧ 1}\text{MeV}$ of excitation energy in each case. Of particular interest is a $\text{7}\text{2}$^? state located ≦ 50 keV above the lowest $\text{11}\text{2}$^? state in each nuclide. The relatively strong populations of these $\text{7}\text{2}$^? levels in the present experiments are contrary to expectations based on the simple shell model as there are no f_{$\text{7}\text{2}$} states in the 50 < Z < 82 shell.     

The 26Mg(14N, 10B)30Si and 26Mg(14N, 11B)29Si reactions

Reactions induced by ¹⁴N on ²⁶Mg at bombarding energies of 60–95 MeV have been studied. Angular distributions for states populated in ²⁹Si by the (¹⁴N, ¹¹B) reaction and in ³⁰Si by the (¹⁴N, ¹⁰B) reaction have been compared with Hauser-Feshbach and DWBA calculations to determine the reaction mechanism and to deduce spectroscopic information. The cross sections for the states populated in ²⁹Si and ³⁰Si are in poor agreement with statistical model calculations, indicating a non-compound nucleus mechanism.     

A study of the 144Sm(d,p) reaction at 19 MeV

The ¹⁴⁴Sm(d, p) reaction has been studied at an incident deuteron energy of 19 MeV using the injector-tandem accelerator and the multichannel magnetic speetrograph of the University of Oxford. Angular distributions have been measured for transitions to levels of ¹⁴⁵Sm up to an excitation energy of 3.2 MeV. Theoretical (d, p) distributions have been calculated using the program DWUCK. and orbital angular momentum transfers and spectroscopic factors have been deduced by comparing these calculations with the experimental data. The spectroscopic information derived from this study is more complete than that previously reported and many new assignments have been made. The level scheme of ¹⁴⁵Sm has been found to resemble closely the level schemes of the other N = 83 nuclei.     

A study of the 26Mg(12C, 12B)26Al charge-exchange reaction

The reaction ²⁶Mg(¹²C, ¹²B)²⁶A1(5⁺, 3⁺) has been studied using a beam of 102 MeV of ¹²C. Shell-model, microscopic direct model and finite-range coupled reaction channel (CRC) calculations including recoil effects, have been performed, for comparison with the experimental data. DWBA calculations were performed for the intermediate states of interest in the ¹¹B + ²⁷Al and in the ¹³C + ²⁵Mg channels and these results were also compared with the experimental ones. The dominant reaction mechanism for ²⁶Mg(¹²C, ¹²B)²⁶Al(5⁺, 3⁺) appears to be the sequential mode.     

12C1 and 8Be production in 12C + 208Pb collisions

The mechanisms involved in the production of fast α-particles in ¹²C-induced reactions have been studied for the ¹²C + ²⁰⁸Pb system at the bombarding energies of E_12c = 132, 187 and 230 MeV. Absolute cross sections for the reactions ²⁰⁸Pb(¹²C. ¹²C¹→α + ⁸Be), ²⁰⁸Pb(¹²C, ⁸Be(g.s.)) and ²⁰⁸Pb(¹²C, ⁸Be(2.94 MeV)) have been determined by coincidence measurement of two or three correlated α-particles. Inclusive α-particle production cross sections were also measured at E_12c = 187 MeV. It is found that the inelastic process (¹²C, ¹²C¹→α + ⁸Be) does not contribute significantly to fast α-particle production but that the production of ⁸Be by projectile fragmentation is an important source of α-particles. At the highest bombarding energy (230 MeV) it appears that the ¹²C → 3α fragmentation reaction becomes more prominent at the expense of the ¹²C→α + ⁸Be fragmentation channel.     

The 12C(18O, α)26Mg and 12C(18O, 8Be)22Ne reaction

Excitation functions for α-emission leading to the ground and first excited states of ²⁶Mg and ⁸Be emission leading to the ground and first and second excited states of ²²Ne have been measured at several forward angles for E_c.m. = 15 to 22.4 MeV. There is little evidence for correlated structure. The angular distribution at 16.5 MeV for the α + ²⁶Mg(g.s.) channel is rather structureless while that for the ⁸Be+²²Ne(g.s.) channel appears to be dominated by a J = 13 contribution. Statistical model calculations indicate that much of the yield for both the α and ⁸Be exit channel is compound nuclear in origin, with some indication of a larger direct contribution for the ⁸Be channel at the lower end of the bombarding energy range.     

Recoil-distance lifetime measurements in 51Cr, 53Cr, 53Mn and 53Fe

Levels in ⁵¹Cr, ⁵³Cr, ⁵³Mn and ⁵³Fe were excited via (α, n) or (α, p) reactions. Using the recoil-distance method, mean-lives (in ps) have been obtained for excited states (keV) in the residual nuclei: ${}^{51}\text{Cr}\text{(2256) = 66±2,}{}^{53}\text{Cr}\text{(1536) = 21.5±3.5,}{}^{53}\text{Cr}\text{(2173) = 6.7±3.1,}{}^{53}\text{Mn}\text{(2564) = 20}{}^{\mathrm{+8}}{}_{\mathrm{?6}}\text{and}{}^{53}\text{Fe}\text{(1424) = 4.0±1.0}$. Reduced transition probabilities calculated from these values are compared with the available theoretical values.     

Experimental study of excitation functions and isomer ratios for 211, 212Po

Excitation functions have been measured for ground states and isomers populated in the ²⁰⁸Pb(α, n)²¹¹Po, ²⁰⁹Bi(α, np)²¹¹Po, ²⁰⁹Bi(α, p)²¹²Po and ²⁰⁹Bi(α, n)²¹²At reactions for α-beams ranging from 45 MeV to 172.5 MeV. The experimental results have been compared with theoretical calculations assuming direct and preequilibrium reaction mechanisms, respectively. It is found that the experimental excitation functions can be reproduced satisfactorily by calculations in the framework of the preequilibrium model. Isomer ratios have been extracted from the data as well. Their energy dependence can be reproduced by an optical-model calculation for beam energies larger than about 60 MeV.     

The α-transfer reactions 27Al(6Li,d)31P, 29Si(6Li,d)33S and 31P(6Li,d)35Cl at 36 MeV

The α-transfer reactions ²⁷Al(⁶Li, d)³¹P, ²⁹Si(⁶Li, d)³³S and ³¹P(⁶Li, d)³⁵Cl have been studied at a ⁶Li energy of 36 MeV. Absolute cross sections and angular distributions have been measured and an exact finite-range distorted-wave Born approximation analysis assuming a direct cluster transfer has been used to extract from the data α-particle spectroscopic strengths for levels populated in ³¹P, ³³S and ³⁵Cl in the three reactions respectively. The results show that in the case of most of the low-lying excited states of ³¹P, a single value of L of the transferred α-particle contributes, though a multiplicity of L-values are allowed by angular momentum selection rules. It is also found that the α-particle spectroscopic strength of the ground state of ³¹P is a factor of 2 more than the strengths of the ground states of ³³S and ³⁵Cl. The α-spectroscopic strengths of ground states of these, as well as other odd-A s-d shell nuclei, are compared with the presently available shell model calculations.