Beta-decay of148,150,152Pr into levels of148,150,152Nd

Theβ ^?-decay of^148,150,152Pr into levels of^148,150,152Nd has been studied byγ-singles,γγ-coincidence and for¹⁴⁸Pr also byγγ-angular correlation measurements. The praseodymium activities have been obtained by thermal neutron-induced fission of²³⁵U followed by on-line mass separation with the helium-jet coupled mass separator HELIOS. Decay schemes for these heaviest praseodymium nuclides are proposed. From the angular correlation measurements spin values for the seven lowest levels of¹⁴⁸Nd are deduced. Spin and parity assignments for the lower levels in^150,152Nd have been extrapolated from level systematics.     

Nuclearg-factors of theJ π=21/2+isomer in112In and theJ π=6+ 112Sn

Theg-factor of theJ ^π=21/2⁺ isomeric state in¹¹¹In (T _1/2=13.3 ns) and of theJ ^π=6⁺ isomeric state in¹¹²Sn (T _1/2=13.7 ns) were measured using the spin rotation method. The result obtained for theJ ^π=21/2⁺ level in¹¹¹In,g=+0.47 (2), indicates that this state has an almost pure ((πg _9/2)^?1 νg _7/2 νd _5/2) shell model configuration. The experimental valueg=+0.04 (3) for theJ ^π=6⁺ isomer in¹¹²Sn agrees with the theoretical value calculated within the frame of the BCS model.     

Excitation of isomeric 1h 11/2 states in nuclear reactions induced by γ rays and neutrons and in beta decay

Isomeric ratios were measured for N=81 isotones (¹³⁵Xe, ¹³⁷Ba, ¹³⁹Ce, ¹⁴¹Nd, ¹⁴³Sm). In the experiment reported here, μ _ν ≤3±10^? J ^π=11/2^? isomers were excited in (n, γ) and (γ, n) reactions and in the β ⁺ decay of ¹³⁹Pr and ¹⁴¹Pm. In order to determine the reaction yields, use was made of the activation method involving measurement of the gamma-ray spectra of reaction products. It is found that, in the same reactions, isomeric ratios are different for isotones characterized by different atomic numbers Z. Isomeric ratios were calculated with the spectra of low-lying levels and radiative-transition probabilities established on the basis of the quasiparticle-phonon model. Good agreement between the experimental and calculated values of isomeric ratios is obtained for all isotopes invesigated here. The dependence of isomeric ratios on the atomic number Z of a nucleus is explained by the difference of reaction energies, which leads to different probabilities of excitation of activation levels through which the isomers being considered are populated.     

Beta decay energies and nuclear masses of148Ba,148La and151Pr

Theβ-decay energy of the neutron-rich isotopes¹⁴⁸Ba,¹⁴⁸La and¹⁵¹Pr has been measured at the Institute Laue-Langevin (ILL). The¹⁴⁸Ba- and¹⁴⁸La-nuclei were produced using a high-temperature ion source at the OSTIS separator of this institute, whereas samples of¹⁵¹Pr were obtained from the thermal fission of²³⁹Pu, used as a target in the mass separator LOHENGRIN. At both instruments,βγ-coincidences were measured with a plastic scintillator telescope and a Ge(I)-detector, together withβ-singles andγ-singles spectra. Theβ-spectra emitted in coincidence with 44γ-lines in the decay of these nuclei have been evaluated. From their endpoint energies, the followingQ _β -values have been obtained:Q _β (¹⁴⁸Ba)=5115±60keV;Q _β (¹⁴⁸La)=7255±55 keV;Q _β (¹⁵¹Pr)=4170±75 keV. Using these results, the nuclear massesA of these nuclei can be calculated with a relative error ΔA/A?5·10^?7. In addition, the two-neutron separation energies and the mass excesses have been derived; the results are compared with recent predictions of theoretical mass calculations.     

Population of the 236U shape isomer in a photonuclear reaction

A fission isomer with a half-life of 115 ± 5 ns and a yield ratio Y_iso/Y_prompt = (2.02 ± 0.16) × 10^?5 was observed in bremsstrahlung-induced fission of natural uranium. The isomer is ascribed to ²³⁶U populated via a ²³⁸U(γ, 2n) reaction. The integrated cross section for isomeric fission is determined to be σ_int = 32 ± 6 μb MeV. Comparing this value with a calculated total isomer production cross section, a branching ratio of the isomer decay of Γ^γ_II/Γ^f_II ≈ 6 can be deduced.     

Isomeric states in180Os

Two new isomers have been observed in¹⁸⁰Os. A high-K isomer withI, K ≧20 and a half-life ofT _1/2=12+4 ns have been established. It deexcites via two transitions into the 18⁺ level of the yrare band indicating an unusually smallK-hindrance factor. Evidence for an isomer withI, K>16 and a half-life ofT _1/2=41±10 ns was found. A half-life of 17±3 ns was measured for the previously known 7^? state at 1862 keV. The decay scheme of the previously known 7^? isomer at 1928 keV has been extended and a revised version is presented.     

Yield ratio for the two 241Pu fission isomers in the 242(γ, n) reaction

The half-lives and yield ratios for the two ²⁴¹Pu fission isomers have been measured in the ²⁴²Pu(γ, n) reaction. The observed half-life for the long-lived isomer $\text{(T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 20.5 ± 2.2 μs)}$ is in agreement with previous data, and the existence of a short-lived 34 ± 7 ns isomer in ²⁴¹Pu could be confirmed. The measured yield ratios are Y_iso/Y_pr = (9.2 ± 0.8) × 10^?5 and Y_iso/Y_pr = (3.7 ± 0.7) × 10^?5, respectively. From a statistical model analysis of the isomeric fission yield ratio, Y^long_iso/Y^short_iso = 2.5 ± 0.6, a spin assignment for the two isomers is attempted. Possible spin combinations are compared with single-particle shell-model calculations and with available spectroscopic data for the other even-odd Pu isotopes.     

First observation of isomeric transition from <Superscript>148<Emphasis Type="Italic">m</Emphasis></Superscript>Pr and parity assignment for excited levels in <Superscript>148</Superscript>Pr

Internal conversion electrons from the decay of ¹⁴⁸Ce and ^148mPr have been measured with a Si(Li) detector. The radioactive sources were produced by the thermal neutron-induced fission of ²³⁵U, followed by an on-line mass separation. K- and L-conversion electrons of the isomeric transition from ^148mPr were newly observed at 34.6 and 70.8 keV, respectively. The M3 multipolarity was assigned to this transition from the experimental conversion coefficients, and, consequently, the 76.8 keV isomeric level was found to be a 4^- state. Parities of some excited levels in ¹⁴⁸Pr were also deduced.     

Single-particle spin effect on fission fragment angular momentum

Independent isomeric yield ratios (IYR) of ¹²⁸Sb, ¹³⁰Sb, ¹³²Sb, ¹³¹Te, ¹³³Te, ¹³²I, ¹³⁴I, ¹³⁶I, ¹³⁵Xe, and ¹³⁸Cs have been determined in the fast neutron-induced fission of ²⁴³Am using the radiochemical and γ-ray spectrometric technique. From the IYR, fragment angular momenta (J _rms) have been deduced using the spin-dependent statistical model analysis. From the J _rms-values and experimental kinetic energy data deformation parameters (β) have been deduced using the pre-scission bending mode oscillation model and the statistical model. The J _rms- and β-values of fission fragments from the present and earlier work in the odd-Z fissioning systems ( ²³⁸Np ^* , ²⁴²Am ^* and ²⁴⁴Am ^* ) are compared with the literature data in the even-Z fissioning systems ( ^{230, 233}Th ^* , ^{233, 234, 236, 239}U ^* , ^{239, 240, 241, 242}Pu ^* , ²⁴⁴Cm(SF), ^{245, 246}Cm ^* , ²⁵⁰Cf ^* and ²⁵²Cf(SF)) to examine the role of single-particle (proton) spin effect. It was observed that i) in all the fissioning systems J _rms- and β-values of the fragments with spherical 82n shell and even-Z products are lower than the fragments away from the spherical neutron shell and odd-Z products, which indicate the effect of nuclear structure. ii) For both even-Z and odd-Z fission products J _rms-values increase with Z _F ²/A _F due to increase in Coulomb torque. iii) The J _rms- and β-values of even-Z fission products are comparable in all the fissioning systems. However, for odd-Z fission products they are slightly higher in the odd-Z fissioning systems compared to their adjacent even-Z fissioning systems. This is possible due to the contribution of the extra single-particle (proton) spin of the odd-Z fissioning systems to their odd-Z fragments. iv) The yield-weighted fragment angular momentum and elemental yields profile shows an anti-correlation in even-Z fissioning systems but not in the odd-Z fissioning systems.     

Implications on the chiral quark bag from the deuteron asymptoticD-state

Fission yields of^124–132In in the thermal neutron fission of²³⁵U were determined for the first time. Charge displacementsΔZ=Z _p?Z _UCD were calculated for the corresponding mass chains. Both fission yields and charge displacement values were compared with those obtained by systematics by Wahl et al. and Wolfsberg. It was found that the fission yields of the In isotopes obey the gaussian distribution. The displacement function of Wolfsberg seems to give the better representation of the experimental results. Half-lives of^124–131In were determined from beta decay curves.     

Resonances in the isomeric and prompt fission probabilities of240Pu

The prompt and isomeric fission probabilities of²⁴⁰Pu have been studied using the²³⁹Pu(d,p) reaction. A number of resonances are observed in the subbarrier population of the 4 ns fission isomer for excitation energies between 4.0 and 6.2 MeV. Apart from a structure at 4.3 MeV, they do not coincide with transmission resonances found in prompt fission. Calculations with an extended doorway state model which simultaneously reproduce the measured delayed and prompt fission probabilities yield revised fission barrier parameters as well as spectroscopic information on fission and gamma widths of highly excited states in the second minimum.     

The g-factor of the Jπ=12+ isomeric state in188Hg

Using the reaction¹⁷⁵Lu(¹⁹F, 6n) theg-factor of the¹⁸⁸Hg isomeric state (J ^π=12⁺,T _1/2=135 ns) has been measured with the TDPAD method. The experimental value g=?0.168(10) supports the interpretation of an almost pure (vi13/2)^?2 configuration for this isomer.     

Fragment angular momenta in low and medium energy fission of242Pu

Independent isomeric yield ratios of¹²⁸Sb were determined radiochemically in the thermal neutron induced fission of²⁴¹Pu and 34 MeV alpha particle induced fission of²³⁸U, both involving the same compound nucleus (²⁴²Pu). Fragment angular momenta estimated from the measured isomer ratios using the statistical model analysis showed significantly larger fragment angular momenta in the medium energy fissioning system compared to the low energy fissioning system. This has been attributed to the effect of higher excitation energy and angular momentum in the entrance channel leading to increased fragment temperature, moments of inertia and angular velocity. An attempt was made to calculate the fragment angular momentum in the medium energy fission using the Fermi gas model for the fissioning nucleus, taking into account the multichance fission, saddle shapes of the fissioning nuclei and the angular velocity components of the fissioning nuclei both along and orthogonal to the fission axis. The calculated angular momenta agree well with the experimental results.     

Decay of the 21/2− isomer in211Bi and structure of thevg 9 2/2 πh 9/2 levels

Gamma rays and conversion electrons have been measured in²¹¹Bi populated by the²⁰⁹Bi(t,p) reaction, and the near yrast levels were observed up to 21/2^?. The 21/2^? state is isomeric withT _1/2=70(5) ns. No alpha decaying isomer was found in²¹¹Bi. ObservedM1 transitions reveal mixing of the²¹⁰Pb parent states in the levels of²¹¹Bi. The levels and transitions are well reproduced by the shell model with experimental matrix elements and pure configurations.     

Properties of the 13/2+ isomeric decay in201Hg

The isomeric decay of theT _1/2=94 us isomeric state in²⁰¹Hg was studied in beam via the reaction¹⁹⁸Pt(α, n). Gamma-ray and conversion-electron spectra were recorded. From the conversion coefficients, the multipolarities of the two depopulating transitions were determined to beM 2 + 10%E 3 andE 2, respectively. These assignments give the depopulating transitions to be 13/2⁺ → 9/2^? and 9/2^? → 5/2^?, and these thus substantiate the previous suggestion that the isomeric state hasJ ^π=13/2⁺. For the 13/2⁺ → 9/2^? M 2 transition a detailed study ofE 3 andM 4 admixed amplitudes was done. The microscopic properties of the 13/2⁺ and 9/2^? states are discussed, and a comparison to the corresponding ones in^199,203,205Hg is made.     

A new beta decaying isomer of131Sn; partial level scheme of131Sb

Gamma spectrum and half life of¹³¹Sn decay have been measured using the unslowed fission fragment separator Lohengrin at the high flux reactor of the Institute Laue-Langevin, Grenoble. Two beta-decaying isomers are evidenced, with half-lives of 39±2 s and 50±2 s. Additional confirmation of this isomerism is given by fission fragment-γ delayed coincidence, leading to a partial level scheme of¹³¹Sb including an isomeric level of 50±8 μs life-time at 1676.6 keV.     

Particle plus octupoleM2/E3 isomers and high-spin particle-hole states in147Tb and148Tb

The level structures of theN=82 andN=83 nuclei¹⁴⁷Tb and¹⁴⁸Tb have been studied by means of (α, 8n) and (α,7n) reactions induced by 68 to 110 MeVα particle bombardments of¹⁵¹Eu targets. In-beam conversion electron measurements have established that isomers withT _1/2=4.8(6)ns in¹⁴⁷Tb andT _1/2=22(1)ns in¹⁴⁸Tb decay byM2+E3 transitions to the ground states. The measuredB(E3) values show that the isomeric states arise from the coupling of the valence nucleon(s) to the¹⁴⁶Gd core octupole. Particlephonon coupling in these nuclei and in the one-neutron nucleus¹⁴⁷Gd is discussed and compared with well known cases involving the²⁰⁸Pb core. The higher lying yrast states in the two Tb nuclei are described as shell-model particle-hole excitations using empirical single particle energies and nucleon-nucleon interactions.     

Fission fragment angular momentum in ODD-Z fissioning systems

Independent isomeric yield ratios of ¹²⁸Sb, ¹³⁰Sb, ¹³², ¹³¹Te, ¹³³Te, ¹³²I, ¹³⁴I, ¹³⁶I, ¹³⁵Xe and ¹³⁸Cs have been determined in the fast neutron induced fission of ²³⁷Np and ²⁴¹Am using radiochemical and gamma spectrometric technique. From the independent isomeric yield ratios, fragment angular momenta (J_rms) have been deduced using spin-dependent statistical model analysis. Comparison of these data with the literature data for even-Z fissioning systems shows the following important features: (i) Angular momenta for fragments with spherical 82n shell and even-Z products are lower compared to the fragments with out the 82n shell and odd-Z products indicating the effect of nuclear structure. (ii) Angular momentum of even-Z products in all the fissioning systems are comparable where as for odd-Z products it is slightly higher in the odd-Z fissioning systems than in the adjacent even-Z fissioning systems. This indicates the role of single particle on fragment angular momentum in odd-Z fissioning systems. Received: 26 March 1999 / Revised version: 16 October 1999     

The 235 ns 9+ isomer in 63 148 Eu85

From (p, xn) in-beamγ-ray and electron measurements aT _1/2=235(14) ns isomer at 720 keV was identified in ₆₃ ¹⁴⁸ Eu₈₅ and the levels populated in its decay have been characterized. The results determineI ^π=9⁺ for the isomeric state, and we assign it as the (πh _11/2 j ₀ ^?2 ν f ₇ ^2/3 )9⁺ shell model state, analogous to the configuration of the 235 μs isomer in ₆₃ ¹⁴⁶ Eu₈₃.     

The decay of 31 sec98Zr, 2.9 sec98Nb and 51 min98Nb

TheΒ- andγ-radiations of⁹⁸Zr and^{98m, g} Nb have been investigated employing scintillation and semiconductor spectrometers and coincidence techniques. Sources of⁹⁸Nb were produced by the⁹⁸Mo(n, p)⁹⁸Nb reaction, sources of⁹⁸Zr by fission of²³⁵U with thermal neutrons applying chemical separations. For⁹⁸Zr, a half-life of 30.7±0.4 sec and aQ _β-value of 2.3±0.2 MeV were obtained, for the⁹⁸Nb isomers, half-lives of 2.86±0.06 sec and 51.3±0.4 min, andQ _β-values of 4.8±0.2 MeV and 4.5±0.2 MeV, respectively. Noγ-rays were observed in the decay of⁹⁸Zr. The decay of 2.9 sec⁹⁸Nb was found to involve 11γ-ray transitions. In the decay of 51 min⁹⁸Nb, 54γ-transitions were detected. Spin and parity of 1⁺ and 4^? were deduced for the isomeric states of⁹⁸Nb.