The 7.6-h isotope 256mEs was produced from a 2.5-J.Lg/cm2 target of 254Es by the (t,p) reaction. The reaction products were separated radiochemi cally, and the decay properties of 256m Es were determined via /3-1, 'Y-1, and /3-fission correlation techniques. From these measurements we were able to assign 57 1-rays to 26 levels in the daughter 256Fm. An isomeric level was observed at 1425 keV and assigned a spin and parity of 7-. This level has a t112 of (70 ± 5) ns and we observed two /3-delayed fissions with delay times in the proper time range to be associated with fission from this level. This gives a /3-delayed fission probability of 2 x 10-s for this level and a partial fission half-life of o.s~g:~ ms at the 95% confidence level.

We have studied the primary and secondary γ rays (414 in Ni59, 390 in Ni60, and 240 in Ni61) following thermal-neutron capture by the stable Ni58, radioactive Ni59, and stable Ni60 isotopes. Most of these γ rays have been incorporated into the corresponding level schemes consisting of 65 levels in Ni59, 88 levels in Ni60, and 40 levels in Ni61. The measured neutron separation energies (Sn in keV) for Ni59, Ni60, and Ni61 are, respectively, 8999.28±0.05, 11 387.73±0.05, and 7820.11±0.05. The measured thermal-neutron capture cross sections (in barns) for Ni58, Ni59, and Ni60 are, respectively, 4.13±0.05, 73.7±1.8, and 2.34±0.05. In all three cases, primary electric-dipole (E1) transitions account for the bulk of the total capture cross section. We have calculated these E1 partial cross sections (in Ni59 and Ni61) using direct-capture theory and models of compound-nuclear capture. The agreement between theory and experiment is good. The experimental level schemes have been compared with the results from a large-basis shell-model calculation. The agreement was also found to be quite good.

We have studied the primary and secondary $\ensuremath{\gamma}$ rays (414 in $^{59}\mathrm{Ni}$, 390 in $^{60}\mathrm{Ni}$, and 240 in $^{61}\mathrm{Ni}$) following thermal-neutron capture by the stable $^{58}\mathrm{Ni}$, radioactive $^{59}\mathrm{Ni}$, and stable $^{60}\mathrm{Ni}$ isotopes. Most of these $\ensuremath{\gamma}$ rays have been incorporated into the corresponding level schemes consisting of 65 levels in $^{59}\mathrm{Ni}$, 88 levels in $^{60}\mathrm{Ni}$, and 40 levels in $^{61}\mathrm{Ni}$. The measured neutron separation energies (${S}_{n}$ in keV) for $^{59}\mathrm{Ni}$, $^{60}\mathrm{Ni}$, and $^{61}\mathrm{Ni}$ are, respectively, $8999.28\ifmmode\pm\else\textpm\fi{}0.05$, $11 387.73\ifmmode\pm\else\textpm\fi{}0.05$, and $7820.11\ifmmode\pm\else\textpm\fi{}0.05$. The measured thermal-neutron capture cross sections (in barns) for $^{58}\mathrm{Ni}$, $^{59}\mathrm{Ni}$, and $^{60}\mathrm{Ni}$ are, respectively, $4.13\ifmmode\pm\else\textpm\fi{}0.05$, $73.7\ifmmode\pm\else\textpm\fi{}1.8$, and $2.34\ifmmode\pm\else\textpm\fi{}0.05$. In all three cases, primary electric-dipole $(E1)$ transitions account for the bulk of the total capture cross section. We have calculated these $E1$ partial cross sections (in $^{59}\mathrm{Ni}$ and $^{61}\mathrm{Ni}$) using direct-capture theory and models of compound-nuclear capture. The agreement between theory and experiment is good. The experimental level schemes have been compared with the results from a large-basis shell-model calculation. The agreement was also found to be quite good.

The Na-24 nucleus was investigated via ((d) over right arrow ,p) and (n,gamma) reactions. Within the range of excitation energy from 0 to 6.3 MeV 75 levels were observed. The angular distribution of partial cross sections and vector analyzing powers for 70 levels were determined within the framework of distorted-wave Born approximation. Of 240 gamma transitions assigned to Na-24, 234 were placed in the decay scheme. Based on the extensive (n,gamma) data the neutron separation energy was deduced to be 6959.44+/-0.05 keV. Using spectroscopic information from our ((d) over right arrow ,p) measurement we investigated the role of the direct capture mechanism in the Na-23(n,gamma)Na-24 reaction.

Five primary electric-dipole transitions in Si-31 account for the bulk (similar to 98%) of the total thermal-neutron capture cross section (107+/-3 mb) of Si-30. We have recalculated the partial cross sections for these transitions using direct-capture theory and reliable spectroscopic factors for the (d,p) reaction, which have become available recently. There is good agreement now between theory and experiment.

The energies and intensities of 58 gamma rays emitted in thermal-neutron capture by nitrogen (99.63% N-14) have been measured accurately. A major reason was to establish this reaction as a standard for similar measurements on other nuclides. These gamma rays have been placed between 19 known levels (including the ground state and the capturing state) in N-15. The primary gamma rays of both electric dipole (E1) and magnetic dipole (M1) types have been analyzed with existing theories of slow-neutron capture. Unlike many other light nuclides, the cross sections for E1 transitions in N-15 differ drastically from the calculations of pure direct-capture theory. The role of the resonance-capture contribution from the proton-unbound, neutron-bound level at 29+/-2 keV below the neutron separation energy was considered. Some of the properties of this level are quite well known from the C-14(p, gamma) reaction, and others can be derived from an R-matrix analysis of the total cross section as a function of neutron energy. The thermal-neutron capture gamma-ray spectrum is different from the proton-capture gamma-ray spectrum, but if proper account is taken of the interference among the compound-nuclear processes, the valence-neutron mechanism, and potential capture, the data can be satisfactorily explained. In the thermal-neutron reaction, compound-nuclear E1 and direct-capture E1 contributions are of comparable magnitude. Valence-neutron capture forms a significant component of capture by the neutron-bound level at -29 keV. Largely destructive interference between compound-nuclear and valence processes in a few transitions in thermal-neutron capture gives rise to a much smaller total cross section than would be obtained from the compound-nuclear process alone. The M1 transitions also show some evidence of a direct process but not a dominant one. The magnitudes of the compound-nuclear transitions, both E1 and M1, are largely consistent with the values implied by giant resonance theories. The resonance parameters deduced for the -29-keV level are: total radiation width=565+/-24 meV, reduced neutron width=51.6+/-0.3 keV (for a channel radius of 3.5 fm), and proton width=160+/-30 meV.

The energies and intensities of 58 \ensuremath{\gamma} rays emitted in thermal-neutron capture by nitrogen (99.63% ${}^{14}\mathrm{N}$) have been measured accurately. A major reason was to establish this reaction as a standard for similar measurements on other nuclides. These \ensuremath{\gamma} rays have been placed between 19 known levels (including the ground state and the capturing state) in ${}^{15}\mathrm{N}.$ The primary \ensuremath{\gamma} rays of both electric dipole $(E1)$ and magnetic dipole $(M1)$ types have been analyzed with existing theories of slow-neutron capture. Unlike many other light nuclides, the cross sections for $E1$ transitions in ${}^{15}\mathrm{N}$ differ drastically from the calculations of pure direct-capture theory. The role of the resonance-capture contribution from the proton-unbound, neutron-bound level at $29\ifmmode\pm\else\textpm\fi{}2\mathrm{keV}$ below the neutron separation energy was considered. Some of the properties of this level are quite well known from the ${}^{14}\mathrm{C}(p,\ensuremath{\gamma})$ reaction, and others can be derived from an R-matrix analysis of the total cross section as a function of neutron energy. The thermal-neutron capture \ensuremath{\gamma}-ray spectrum is different from the proton-capture \ensuremath{\gamma}-ray spectrum, but if proper account is taken of the interference among the compound-nuclear processes, the valence-neutron mechanism, and potential capture, the data can be satisfactorily explained. In the thermal-neutron reaction, compound-nuclear $E1$ and direct-capture $E1$ contributions are of comparable magnitude. Valence-neutron capture forms a significant component of capture by the neutron-bound level at $\ensuremath{-}29\mathrm{keV}.$ Largely destructive interference between compound-nuclear and valence processes in a few transitions in thermal-neutron capture gives rise to a much smaller total cross section than would be obtained from the compound-nuclear process alone. The $M1$ transitions also show some evidence of a direct process but not a dominant one. The magnitudes of the compound-nuclear transitions, both $E1$ and $M1,$ are largely consistent with the values implied by giant resonance theories. The resonance parameters deduced for the $\ensuremath{-}29\ensuremath{-}\mathrm{keV}$ level are: total radiation $\mathrm{w}\mathrm{i}\mathrm{d}\mathrm{t}\mathrm{h}=565\ifmmode\pm\else\textpm\fi{}24\mathrm{}\mathrm{meV},$ $\mathrm{reduced}\mathrm{}\mathrm{neutron}\mathrm{}\mathrm{w}\mathrm{i}\mathrm{d}\mathrm{t}\mathrm{h}=51.6\ifmmode\pm\else\textpm\fi{}0.3\mathrm{}\mathrm{keV}$ (for a channel radius of 3.5 fm), and proton $\mathrm{w}\mathrm{i}\mathrm{d}\mathrm{t}\mathrm{h}=160\ifmmode\pm\else\textpm\fi{}30\mathrm{}\mathrm{meV}.$

From a study of the F-19(n,gamma) reaction with thermal neutrons incident on a Teflon target, 168 gamma rays have been detected and incorporated into a level scheme of F-20 consisting of 35 previously known levels and a new one at 5939 keV. Two low-energy primary El transitions of energies 584 and 665 keV together account for more than half of the total capture cross section. They populate, respectively, states at 6018 and 5936 keV (both J(pi)=2(-)). These states are also excited strongly in the F-19(d,p) reaction. From each of these states, 17 gamma rays were observed to the lower-lying states. These gamma rays constitute the largest number of branches reported from any nuclear bound state. A weak (6+/-1 mu b)gamma ray of energy 4630.6+/-0.9 keV, placed as a transition between the neutron-capturing state (which is a 0(+) and 1(+) mixture) and the 1971-keV, (3(-)) state, might represent the first observation of a primary M2 transition in the (n,gamma) reaction. The total thermal-neutron-capture cross section of F-19 was measured as 9.51+/-0.09 mb; and the neutron separation energy of F-20 as 6601.35+/-0.04 keV. Estimates of direct neutron capture have been made using physically realistic optical-model parameters. These model estimates are in reasonable agreement with the measured (partial) cross sections. While constructing the (n, gamma) level scheme, the existing data on bound levels in F-20 were critically evaluated. The lifetime values for many levels are poorly known. Therefore, the lifetimes for 25 levels were measured by the Doppler-shift-attenuation method using the inverse reaction H-2(F-19, p gamma) on implanted deuterium targets. The experimental level properties such as excitation energies, J(pi) assignments, branching ratios, and lifetimes have been compared with the results from a large-basis shell-model calculation. The agreement was found to be quite good, but this comparison points out also the need for acquiring new data to give more definitive J(pi) assignments.

A study of the gamma-ray spectrum following thermal-neutron capture by Pb-205 has revealed 54 gamma rays, which have been incorporated into a level scheme consisting of 22 excited states in Pb-206. This study was carried out with an similar to 9 mg lead sample enriched to 78.9% in radioactive Pb-205. The neutron binding energy of Pb-206 was determined to be 8086.67 +/- 0.06 keV, and the thermal-neutron-capture cross section for Pb-206 to be 4.5 +/- 0.2 b. The low-lying portion of the level scheme of Pb-206 and the gamma-ray branchings of positive-parity states have been compared with shell-model predictions. The overall agreement is excellent for the former and reasonably good for the latter.

A study of the \ensuremath{\gamma}-ray spectrum following thermal-neutron capture by $^{205}\mathrm{Pb}$ has revealed 54 \ensuremath{\gamma} rays, which have been incorporated into a level scheme consisting of 22 excited states in $^{206}\mathrm{Pb}$. This study was carried out with an \ensuremath{\sim}9 mg lead sample enriched to 78.9% in radioactive $^{205}\mathrm{Pb}$. The neutron binding energy of $^{206}\mathrm{Pb}$ was determined to be 8086.67\ifmmode\pm\else\textpm\fi{}0.06 keV, and the thermal-neutron-capture cross section for $^{206}\mathrm{Pb}$ to be 4.5\ifmmode\pm\else\textpm\fi{}0.2 b. The low-lying portion of the level scheme of $^{206}\mathrm{Pb}$ and the \ensuremath{\gamma}-ray branchings of positive-parity states have been compared with shell-model predictions. The overall agreement is excellent for the former and reasonably good for the latter. \textcopyright{} 1996 The American Physical Society.

Mean lifetimes of bound states in N-15 were inferred from the Doppler-shift-attenuation (DSA) of gamma rays produced in the inverse reaction H-2(N-14, pgamma) and from the primary gamma-ray-induced Doppler broadening (GRID) of secondary gamma rays iu the thermal-neutron capture reaction N-14(n, gamma). Targets for the DSA measurements were prepared by implanting first neon into a gold backing and then deuterium into the same region such that deuterium was trapped at the neon precipitates. To find out experimentally the initial velocity distributions of the recoiling N-15 nuclei, another target was prepared by implanting deuterium into silicon, which is a slow stopping-power medium. Computer simulations with the Monte Carlo method and experimental stopping powers were used in the DSA analysis of the gamma-ray line shapes. This analysis yielded the following lifetimes for eight bound levels in N-15: 43 +/- 4 fs (5.30 MeV), < 12 fs (6.32 MeV), 11 +/- 2 fs (7.16 MeV), < 3 fs (7.30 MeV), 129 +/- 6 fs (7.57 MeV), < 3 fs (8.31 MeV), < 10 fs (8.57 MeV), and < 4 fs (9.05 MeV). GRID measurements were made with melamine (C3H6N6) and with silicon nitride (Si3N4) as both targets and slowing-down media. Measurements were also made with air (80% nitrogen). The Doppler-broadened gamma-ray line shapes were analyzed by molecular-dynamics simulations of the slowing-down process to obtain the lifetime values of 40 +/- 3 fs (5.30 MeV), < 2 fs (6.32 MeV), < 3 fs (7.30 MeV), and < 3 fs (8.31 MeV) for levels in N-15. The extent to which the mirror symmetry of levels in N-15 and O-15 is valid is examined in some detail.

Mean lifetimes of bound levels in [sup 15]N were inferred from the primary [gamma]-ray-induced Doppler broadening (GRID) of secondary [gamma] rays in the thermal-neutron capture reaction [sup 14]N(n,[gamma]). GRID measurements were made with melamine (C[sub 3]H[sub 6]N[sub 6]) as both target and slowing-down medium. The Doppler-broadened [gamma]-ray line shapes were analyzed by molecular dynamics simulations of the slowing-down process to obtain the lifetime values for four states in [sup 15]N. Preliminary results will be shown.

We have studied the primary and secondary gamma-rays (33 in Mg-25, 212 in Mg-26, and 35 in Mg-27) following thermal-neutron capture by the stable Mg-24, Mg-25, and Mg-26 isotopes. Almost all of these gamma-rays have been incorporated into the corresponding level schemes consisting of 9 excited levels in Mg-25, 55 in Mg-26, and 10 in Mg-27. In each case, the observed gamma-rays account for nearly 100% of all captures. The measured neutron separation energies for Mg-25, Mg-26, and Mg-27 are, respectively, 7330.65 +/- 0.05, 11 093.18 +/- 0.05, and 6443.40 +/- 0.05 keV. The measured thermal-neutron capture cross sections for Mg-24, Mg-25, and Mg-26 are, respectively, 54.1 +/- 1.3, 200 +/- 3, and 39.0 +/- 0.8 mb. In all three cases, primary electric-dipole (E1) transitions account for the bulk of the total capture cross section. We have calculated these E1 partial cross sections using direct-capture theory. We have also speculated on the mechanism responsible for the magnetic-dipole (M1) transitions which are quite strong in Mg-26.

We have studied primary and secondary gamma-rays (46 in Si-29, 107 in Si-30, and 33 in Si-31) following thermal-neutron capture by the stable Si-28, Si-29, and Si-30 isotopes. Almost all of these gamma-rays have been incorporated into corresponding level schemes consisting of 12 excited levels in Si-29, 28 in Si-30, and 9 in Si-31. In each case, the observed gamma-rays account for nearly 100% of all captures. The measured neutron separation energies for Si-29, Si-30, and Si-31 are 8473.56+/-0.04, 10609.24+/-0.05, and 6587.40+/-0.05 keV, respectively. The measured thermal-neutron capture cross sections for Si-28, Si-29, and Si-30 are 169+/-4, 119+/-3, and 107+/-3 mb, respectively. In all three cases, primary electric-dipole (E1) transitions account for the bulk of the total capture cross section. We have calculated these E1 partial cross sections using direct-capture theory. The agreement between theory and experiment is satisfactory.