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Register a new userEnergy dependent angular distribution of individual $gamma$-rays in the $^{139}$La($n$, $gamma$)$^{140}$La* reaction
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Neutron energy-dependent angular distributions were observed for individual $gamma$-rays from the 0.74 eV p-wave resonance of $^{139}$La+$n$ to several lower excited states of $^{140}$La. The $gamma$-ray signals were analyzed in a two dimensional histogram of the $gamma$-ray energy, measured with distributed germanium detectors, and neutron energy, determined with the time-of-flight of pulsed neutrons, to identify the neutron energy dependence of the angular distribution for each individual $gamma$-rays. The angular distribution was also found for a photopeak accompanied with a faint p-wave resonance component in the neutron energy spectrum. Our results can be interpreted as interference between s- and p-wave amplitudes which may be used to study discrete symmetries of fundamental interactions.
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Angular distribution of individual $gamma$-rays, emitted from a neutron-induced compound nuclear state via radiative capture reaction of ${}^{139}$La(n,$gamma$) has been studied as a function of incident neutron energy in the epithermal region by using germanium detectors. An asymmetry $A_{mathrm{LH}}$ was defined as $(N_{mathrm L}-N_{mathrm H})/(N_{mathrm L}+N_{mathrm H})$, where $N_{mathrm L}$ and $N_{mathrm H}$ are integrals of low and high energy region of a neutron resonance respectively, and we found that $A_{mathrm{LH}}$ has the angular dependence of $(Acostheta_gamma+B)$, where $theta_gamma$ is emitted angle of $gamma$-rays, with $A= -0.3881pm0.0236$ and $B=-0.0747pm0.0105$ in 0.74 eV p-wave resonance. This angular distribution was analyzed within the framework of interference between s- and p-wave amplitudes in the entrance channel to the compound nuclear state, and it is interpreted as the value of the partial p-wave neutron width corresponding to the total angular momentum of the incident neutron combined with the weak matrix element, in the context of the mechanism of enhanced parity-violating effects. Additionally we used the result to quantify the possible enhancement of the breaking of the time-reversal invariance in the vicinity of the p-wave resonance.
A set of prompt partial $gamma$-ray production cross sections from thermal neutron-capture were measured for the $^{139}$La($n,gamma$) reaction using a guided beam of subthermal (thermal and cold) neutrons incident upon a $^{rm nat}$La$_{2}$O$_{3}$ target at the Prompt Gamma Activation Analysis facility of the Budapest Research Reactor. Absolute $^{140}$La cross sections were determined relative to the well-known comparator $^{35}$Cl($n,gamma$) cross sections from the irradiation of a stoichiometric $^{rm nat}$LaCl$_{3}$ sample. The total cross section for radiative thermal neutron-capture on $^{139}$La from the sum of experimentally measured cross sections observed to directly feed the $^{140}$La ground state was determined to be $sigma_{0}^{rm expt} = 8.58(50)$ b. To assess completeness of the decay scheme and as a consistency check, the measured cross sections for transitions feeding the ground state from levels below a critical energy of $E_{c} = 285$ keV were combined with a modeled contribution accounting for ground-state feeding from the quasicontinuum to arrive at a total cross section of $sigma_{0} = 9.36(74)$ b. In addition, a neutron-separation energy of $S_{n} = 5161.005(21)$ keV was determined from a least-squares fit of the measured primary $gamma$-ray energies to the low-lying levels of the $^{140}$La decay scheme. Furthermore, several nuclear structure improvements are proposed for the decay scheme. The measured cross-section and separation-energy results are comparable to earlier measurements of these quantities.
A correlation term ${{ sigma}_{n} }cdot ({ k_{n}times k_gamma}) $ in the ${}^{139}{rm La}(vec{n},gamma)$ reaction has been studied utilizing epithermal polarized neutrons and germanium detectors. The transverse asymmetry for single $gamma$-ray transition was measured to be $0.60pm0.19$ in the $p$-wave resonance.
A measurement of total cross-section values of the $^{130}$Ba(p,$gamma$)$^{131}$La reaction at low proton energies allows a stringent test of statistical model predictions with different proton+nucleus optical model potentials. Since no experimental data are available for proton-capture reactions in this mass region around A~$approx$~130, this measurement can be an important input to test the global applicability of proton+nucleus optical model potentials. The total reaction cross-section values were measured by means of the activation method. After the irradiation with protons, the reaction yield was determined by use of $gamma$-ray spectroscopy using two clover-type high-purity germanium detectors. In total, cross-section values for eight different proton energies could be determined in the energy range between 3.6 MeV $leq E_p leq$ 5.0 MeV, thus, inside the astrophysically relevant energy region. The measured cross-section values were compared to Hauser-Feshbach calculations using the statistical model codes TALYS and SMARAGD with different proton+nucleus optical model potentials. With the semi-microscopic JLM proton+nucleus optical model potential used in the SMARAGD code, the absolute cross-section values are reproduced well, but the energy dependence is too steep at the lowest energies. The best description is given by a TALYS calculation using the semi-microscopic Bauge proton+nucleus optical model potential using a constant renormalization factor.
The 48Ca({gamma},n) cross section was measured using {gamma}-ray beams of energies between 9.5 and 15.3 MeV generated at the Triangle Universities Nuclear Laboratory (TUNL) high-intensity {gamma}-ray source (HI{gamma}S). Prior to this experiment, no direct measurements had been made with {gamma}-ray beams of sufficiently low energy spread to observe structure in this energy range. The cross sections were measured at thirty-four different {gamma}-ray energies with an enriched 48Ca target. Neutron emission is the dominant decay mechanism in the measured energy range that spans from threshold, across the previously identified M1 strength, and up the low-energy edge of the E1 giant dipole resonance (GDR). This work found B(M 1) = 6.8 pm 0.5 {mu}N2 for the 10.23 MeV resonance, a value greater than previously measured. Structures in the cross section commensurate with extended random-phase approximation (ERPA) calculations have also been observed whose magnitudes are in agreement with existing data.
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