7Li + 54Fe Single-nucleon stripping reactions at E = 48 MeV

Angular distributions have been measured for the ⁵⁴Fe(⁷Li, ⁶Li/⁶He)⁵⁵Fe, ⁵⁵Co reactions at $\text{E(}{}^7\text{Li) = 48}\text{MeV}$. Exact finite-range DWBA calculations for both reactions, employing Woods-Saxon real and imaginary potentials to generate the distorted waves, reproduce the shape of the f-state angular distributions but not the rate of fall-off with increasing angle for p-states. Using real potentials constructed by double-folding a microscopic nucleon-nucleon potential with a Woods-Saxon shaped imaginary potential to generate the distorted waves did not change the calculations. The forward angle (⁷Li, ⁶He) data allowed the $\text{2}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ states in ⁵⁵Co to be located and the determination of the single-particle centroid energies. The ⁵⁵Co spectroscopic factors are in good agreement with those from the (d, n) reaction but are generally 25 % lower than those from average (³He, d) results. The ⁵⁵Fe spectroscopic factors are in good agreement with (d, p) results.     

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.     

E1 polarization in coulomb excitation of 7Li

Coulomb excitation of the 0.478 MeV $\text{J}{}^{\mathrm{\pi }}\text{=}\text{1}\text{2}{}^{\mathrm{?}}$ state of ⁷Li has been studied by a partiele-γ coincidence technique. From the dependence of the excitation probabilities on bombarding energy and scattering angle a sizeable interference contribution from E1 excitation of continuum states has been determined. General expressions for the size of the E1 polarization effect in Coulomb excitation are given and the observed magnitudes in ^6,7Li are compared with schematic model calculations.     

The (7Li, 6Li) reaction on 28Si and 40Ca at E(7Li) = 45 MeV

Angular distributions for the (⁷Li, ⁶Li) reaction on ²⁸Si and ⁴⁰Ca to ground and excited states of ²⁹Si and ⁴¹Ca have been measured at $\text{E(}{}^7\text{Li}\text{) = 45}\text{MeV}$. The shapes of the angular distributions are well described by exact finite-range DWBA calculations. To test the sensitivity of the calculated angular distributions to the exit channel potential, calculations were made using a strongly absorbing ⁷Li potential for both the entrance and exit channels. This calculation produced as good a fit to the shape of the angular distributions as did calculations with a ⁶Li potential in the exit channel, even though ⁷Li and ⁶Li elastic scattering on these target nuclei are not very similar. However, the magnitudes of the calculated cross sections are sensitive to the choice of potentials, and agreement with light-ion spectroscopic factors is obtained when the more weakly absorbing ⁶Li potentials are used. The spectroscopic factors obtained from the ⁴⁰Ca(⁷Li, ⁶Li) reaction study are in good agreement with the extensive light-ion results showing that the absolute magnitude of the reaction is correctly predicted by exact finite-range DWBA calculations that use optical parameters determined from elastic-scattering data.     

12C(7Li,t)16O and stellar helium fusion

The reaction ¹²C(⁷Li, t)¹⁶O has been studied at $\text{E(}{}^7\text{Li}\text{) = 34}\text{MeV}$ with the LASL tandem accelerator and QDDD magnetic spectrometer. Angular distributions to levels with E_x < 11 MeV have been obtained from 0° to 90°, including 0°. The results have been analyzed with finite-range distorted-wave Born approximation theory. The α-particle spectroscopic factors and reduced widths obtained are compared with those calculated with group theory (SU(3)) and other models. The analysis of data for the 7.1 and 9.6 MeV J^π = 1^? levels, which are of great importance in stellar helium buring, yields a ratio, R, of dimensionless reduced α-widths $\text{θ}{}^2{}_{\mathrm{<mtext>a</mtext>}}\text{(7.1}\text{MeV}\text{)}\text{θ}{}^2{}_{\mathrm{<mtext>a</mtext>}}\text{(9.6}\text{MeV}\text{)}\text{= 0.35b ± 0.13}$. The observed line width of the 9.6 MeV level (Γ_c.m. = 390 ± 60 keV) is less than the accepted value (Γ_c.m. = 510 ± 60 keV) and implies θ²_a(9.6 MeV) ≈ 0.6. These results as well as data for the 6.92 MeV J^π = 2⁺ and 10.35 MeV J^π = 4⁺ “α-cluster” states indicate 0.09 < θ²_a(7.1 MeV) < 0.33 with a mean value θ²_a(7.1 MeV) = 0.14 ± 0.04. The implication for stellar helium burning is discussed.     

High-spin states in 40K investigated with the 26MG + 16O reaction

Yrast levels in ⁴⁰K and ⁴⁰Ar have been investigated with the ²⁶Mg(¹⁶O, pnγ)⁴⁰K and ²⁶Mg(¹⁶O, 2pγ)⁴⁰Ar reactions at a beam energy of 34 MeV. Gamma-ray angular distribution and γ-γ coincidence measurements have been performed with a high-resolution large volume Ge(Li)-NaI(Tl) Compton-suppression spectrometer. Gamma-ray linear polarizations have been measured with a three-crystal Ge(Li) Compton polarimeter. The ⁴⁰K decay scheme involves new high-spin levels at E_tx = 4365.6±0.3, 4875.6±0.4 and 6227.0±0.5 keV with lifetime limits of < 1, < 1 and < 2ps, respectively. Unambiguous spin-parity assignments of $\text{J}{}^{\mathrm{\pi }}\text{= 5}{}^{\mathrm{?}}\text{, 6}{}^{\mathrm{+}}\text{, 8}{}^{\mathrm{+}}\text{, 9}{}^{\mathrm{+}}\text{and}\text{(8, 10)}{}^{\mathrm{?}}$ to the ⁴⁰K levels at E_x = 0.89, 2.88, 4.37, 4.88 and 6.23 and of $\text{J}{}^{\mathrm{\pi }}\text{= 4}{}^{\mathrm{+}}\text{and}\text{6}{}^{\mathrm{+}}\text{to the}{}^{40}\text{Ar}$ levels at E_x = 2.89 and 3.46 MeV, respectively, have been obtained. Branching ratios and multipole mixing ratios are reported.     

β-delayed charged particles from 9Li and 11Li

β-delayed emission of α-particles from ⁹Li and of both α and ⁶He particles from ¹¹Li is observed. Singles energy spectra and two-dimensional energy spectra of coincident particles are measured. A time-of-flight versus energy measurement is used to identify the mass of the particle. New β-branches are observed which populate high-energy levels in the daughter nuclei. The branching ratios are measured and the β-delayed neutron emission probabilities $\text{P}{}_{\mathrm{<mtext>n</mtext>}}\text{for}{}^9\text{Li and}\text{P}{}_{\mathrm{3n}}\text{for}{}^{11}\text{Li}$ are deduced.     

High-spin yrast levels of 38Ar

High-spin states of ³⁸Ar have been studied with the ³⁵Cl(α, pγ)³⁸Ar reaction at E_α = 18 MeV and with the ²⁴Mg(¹⁶O, 2pγ)³⁸Ar reaction at $\text{E(}{}^{16}\text{O}\text{) = 38}\text{and}\text{45}\text{MeV}$. The ³⁸Ar level scheme is obtained with the former reaction from a proton-γ coincidence measurement. Gamma-gamma coincidence, γ-ray angular distribution and linear polarization experiments have been performed with a Ge(Li)-Na(Tl) Compton suppression spectrometer and a three-crystal Ge(Li) Compton polarimeter. Unambiguous spin-parity assignments of $\text{J}{}^{\mathrm{\pi }}\text{= 7}{}^{\mathrm{?}}\text{, 7}{}^{\mathrm{+}}\text{, 8}{}^{\mathrm{+}}\text{, 7}{}^{\mathrm{?}}\text{, 9}{}^{\mathrm{?}}\text{and}\text{11}{}^{\mathrm{?}}$to the ³⁸Ar levels at E_x = 7.51, 8.08, 8.57, 8.97, 10.17 and 11.61 MeV, respectively, are obtained. The 8.57 MeV, 8⁺ level has a mean life below 0.8 ps. Excitation energies, branching ratios, multipole mixing ratios and transition strengths are reported. The experimental results are compared with shell-model calculations.     

Spin-parity assignments to high-spin states of 41K and 41Ca

Yrast states of ⁴¹K and ⁴¹Ca have been investigated with the ²⁶Mg(¹⁸O, p2nγ)⁴¹K and ²⁶Mg(¹⁸O, 3nγ)⁴¹Ca reactions at a beam energy of 34 MeV. Gamma-gamma coincidence, γ-ray angular distribution and linear polarization measurements were performed with a Ge(Li)-NaI(Tl) Compton suppression spectrometer and a three-crystal Ge(Li) Compton polarimeter. Unambiguous spin-parity assignments of $\text{J}{}^{\mathrm{\pi }}\text{=}\text{7}\text{2}{}^{\mathrm{+}}$, $\text{11}\text{2}{}^{\mathrm{+}}$, $\text{11}\text{2}{}^{\mathrm{?}}$, $\text{13}\text{2}{}^{\mathrm{+}}$, $\text{15}\text{2}{}^{\mathrm{?}}$ and $\text{19}\text{2}{}^{\mathrm{?}}$ to the ⁴¹K levels at E_x = 1.68, 2.53, 2.76, 2.77, 4.27 and 4.98 MeV and of $\text{9}\text{2}{}^{\mathrm{+}}\text{,}\text{11}\text{2}{}^{\mathrm{+}}\text{and}\text{15}\text{2}{}^{\mathrm{+}}$to the ⁴¹Ca levels at E_x = 3.20, 3.37 and 3.83 MeV, respectively, have been obtained. Excitation energies, branching ratios, multipole mixing ratios and transition strengths are reported. The main features of the ⁴¹K and ⁴¹Ca level and decay schemes are reproduced in a 2p-1h and 3p-2h shell-model calculation.     

A microscopical DWBA description of the charge-exchange reactions (7Li, 7Be)

The charge-exchange reaction (⁷Li, ⁷Be is described in the frame of a microscopical DWBA with central effective forces. The transition density for ⁷Li-⁷Be is calculated in two ways: with a one-particle model and with a cluster model, taking full account of recoil effects. The formalism is used for the cases of (⁷Li, ⁷Be reactions with ⁶Li and ⁴⁰Ca. Further cross sections for reactions on ¹²C and ¹⁶O with $\text{E(}{}^7\text{Li) = 78 MeV}$ and excitation energies around 4.5 and 6.2 MeV, respectively, are compared to new data obtained at the Kurchatov Institute. Here particle-hole states in an oscillator model were used to characterize the heavy system. Angular distributions are well reproduced, but absolute cross sections are 6–7 times too small. This indicates that the assumption of a direct one-step process is correct, but that tensor forces should be included as a component of the effective nucleon-nucleon force.     

An experimental study of the 9Be(p, π+)10Be and 9Be(p, π−)10C reactions at 185 MeV

A magnetic spectrometer was used for the momentum analysis of pions produced by 185 MeV protons on a ⁹Be target. The obtained energy resolution (0.55 MeV FWHM) made it possible to resolve a number of discrete final states in ¹⁰Be and ¹⁰C. Energy spectra were measured in the angular region 15°–130° (lab). Angular distributions for six peaks in ¹⁰Be and four peaks in ¹⁰C are presented in tables and graphs. A peak corresponding to a not previously reported level in ¹⁰Be was observed at 11.75 ±0.11 MeV excitation energy. The measured $\text{π}{}^{\mathrm{+}}\text{π}{}^{\mathrm{?}}$ ratio for the ground state analogues was found to be angular dependent and varied from 30 at 35° (lab) to 2 at 125° (lab). The results are compared with theoretical predictions and discussed in terms of one- and two-nucleon models.     

The (7Li, 8Li) reaction to the (h92)−1 state in 207Pb

Relative differential cross sections for the reaction ²⁰⁸ P(⁷Li, ⁸Li) leading to the predominantly single-hole states in ²⁰⁷Pb have been analysed using the DWBA to determine the rms radius of the 1h_{$\text{9}\text{2}$} neutron orbital in ²⁰⁸Pb by comparison with known sizes of the 3p_{$\text{1}\text{2}$} and 2f_{$\text{7}\text{2}$} orbits. The experiment was performed at a beam energy of 52 MeV. The insensitivity of the technique to unknown input parameters to the DWBA analysis is demonstrated and a value of 5.94±0.11 fm for the rms radius of the 1h_{$\text{9}\text{2}$} orbit obtained assuming 70% of the hole strength is concentrated on the $\text{9}\text{2}{}^{\mathrm{?}}$, 3.41 MeV state in ²⁰⁷Pb. Effects due to fragmentation of the different hole strengths are examined and a value of 6.00 ± 0.11 fm is extracted as the best value for the rms radius of the neutron excess in ²⁰⁸Pb. The relevance of these data to mean-field calculations of nuclei is discussed.     

Study of 20Ne(3He,n)22Mg at 3He energies between 2.6 and 4.0 MeV

The ²⁰Ne(³He, n) reaction leading to the ground state of ²²Mg has been investigated in the ³He⁺ energy range of 2.6 to 4.0 MeV. Angular distributions were determined with a neutron time-of-flight spectrometer at average incident energies (lab) of 3.27, 3.69, and 4.01 MeV between 0° and 120° (lab). Excitation functions for the energy region were measured at 0° and 80° (lab). The observed differential cross sections are explained by coherent contributions from direct interaction and compound-nucleus formation. A spectroscopic factor was extracted for the DWBA calculation from the absolute cross-section measurements and found to be $\text{? = 0.43±0.21}$. Resonances in the compound-nucleus formation were found at 3.00 and 3.33 MeV (c.m.) with widths of 0.28 and 0.21 MeV and spins of $\text{5}\text{2}{}^{\mathrm{+}}\text{and}\text{1}\text{2}{}^{\mathrm{?}}$, respectively.     

An unambiguous Jπ assignment from γ-rays produced in heavy-ion reactions; The lowest Jπ=6+, 38Ar level

Angular correlation measurements with a Compton-suppression spectrometer in combination with polarization measurements with a three-crystal Ge(Li) Compton polarimeter of γ-rays produced in the ${}^{24}\text{Mg(}{}^{16}\text{O, 2pγ)}{}^{38}\text{Ar}$ reaction result in an unambiguous J^π = 6⁺ assignment to the 6408.3 ± level. It is exclusive decay to the 5^?, 4.59 MeV level is discussed.     

Alpha-deuteron structure of 6Li as predicted by a separable-potential three-body model

Schroedinger's equation with separable n-p (³S₁) and $\text{n}\text{-α (}{}^2\text{P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{)}$ potentials solved to obtain a three-body model of the ⁶Li ground-state wave function. This model predicts the α-n-p binding energy of ⁶Li to be 4.67 MeV [Exp.: 4.53 MeV = 3.697 + 0.834 (Coulomb)], the asymptotic normalization constant of the d-α tail to be 2.39, and the amount of d+α component to be 65%. The ⁶Li→α+d vertex function is slightly more momentum dependent than present experiments suggest.     

On the 18O positive-parity rotational band

Deuteron-alpha angular correlations have been measured for the reaction ${}^{14}\text{C}\text{(}{}^6\text{Li,d}\text{)}{}^{18}\text{O}{}^1\text{→α}{}_0\text{+}{}^{14}\text{C at}\text{E(}{}^6\text{Li}\text{)=34}\text{MeV}$ and θ_d^lab=10°. Transitions involving the 11.69 MeV (6⁺) and the 17.6±0.2 MeV ¹⁸O states have been analyzed. Spin and parity are confirmed for the known 11.69 MeV (6⁺) state and assigned to be 8⁺ for the 17.6 MeV level. This last is suggested to be the fifth member of the positive-parity ¹⁸O rotational band built on the 3.63 MeV (0⁺) level.     

Nucleon-exchange type charge-exchange process and the spin-forbidden (6Li, 6He) transitions

The second order DWBA formalism for the nucleon pick-up and stripping type two-step process is presented for the cases of exact finite-range interaction, no-recoil approximation and zero-range approximation. The several first-order “spin-forbidden” charge-exchange (⁶Li, ⁶He) transitions are investigated by such direct nucleon-exchange mechanisms. They are the analog transitions: ${}^{26}\text{Mg}\text{(0}{}^{\mathrm{+}}\text{) →}{}^{26}\text{Al}\text{(0.226}\text{MeV}\text{0}{}^{\mathrm{+}}\text{)}$ and ${}^{42}\text{Ca}\text{(0}{}^{\mathrm{+}}\text{→}{}^{42}\text{Sc}\text{(}\text{g.s.}\text{0}{}^{\mathrm{+}}\text{)}$, and the natural parity state excitations: ${}^{48}\text{Ca}\text{(0}{}^{\mathrm{+}}\text{) →}{}^{48}\text{Sc(g.s.}\text{6}{}^{\mathrm{+}}$ and 0.253 MeV 4⁺). The spin-flip mechanisms using a spin-independent force through the direct nucleon-exchange process are discussed. This type of two-step approach can provide a satisfactory interpretation for these transitions. It may also indicate a strong nucleon-exchange type charge-exchange mechanism for allowed transitions.