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Register a new userAngular Distribution of $gamma$-rays from Neutron-Induced Compound States of $^{140}$La
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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.
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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.
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.
The intrinsic width of (multiparticle-multihole) compound states is an elusive quantity, of difficult direct access, as it is masked by damping mechanisms which control the collective response of nuclei. Through microscopic cranked shell model calculations, it is found that the strength function associated with two-dimensional gamma-coincidence spectra arising from rotational transitions between states lying at energies >1 MeV above the yrast line, exhibits a two-component structure controlled by the rotational (wide component) and compound (narrow component) damping width. This last component is found to be directly related to the width of the multiparticle-multihole autocorrelation function.
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.
Possibilities of discriminating neutrons and gamma rays in the AGATA gamma-ray tracking spectrometer have been investigated with the aim of reducing the background due to inelastic scattering of neutrons in the high-purity germanium crystals. This background may become a serious problem especially in experiments with neutron-rich radioactive ion beams. Simulations using the Geant4 toolkit and a tracking program based on the forward tracking algorithm were carried out by emitting neutrons and gamma rays from the center of AGATA. Three different methods were developed and tested in order to find fingerprints of the neutron interaction points in the detectors. In a simulation with simultaneous emission of six neutrons with energies in the range 1-5 MeV and ten gamma rays with energies between 150 and 1450 keV, the peak-to-background ratio at a gamma-ray energy of 1.0 MeV was improved by a factor of 2.4 after neutron rejection with a reduction of the photopeak efficiency at 1.0 MeV of only a factor of 1.25.
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