States Of Compound Nucleus Research Articles

CALCULATION OF CROSS SECTIONS FOR RESONANCE NUCLEAR REACTIONS BASED ON AB INITIO COMPUTATIONS OF SPECTRAL CHARACTERISTICS OF LIGHT NUCLEI LEVELS

Based on the ab initio scheme developed by the authors for calculating the asymptotic characteristics of decay channels of nuclear states — the method of orthogonal functions of cluster channels — an approach has been created that makes it possible to directly implement the results of ab initio calculations of Anucleon wave functions, as well as the partial decay widths of resonant states of compound nucleus obtained through their use, into calculations of cross sections for resonance nuclear reactions. Both calculated and reliably measured resonance energies can be employed in these calculations. Using the examples of such a theoretical analysis of the reaction cross sections for 7Li(𝑝, 4He)4He and 7Be(𝑛, 4He)4He, the efficiency of this approach and the wide prospects of its use in nuclear spectroscopy are demonstrated

Microscopic description of induced fission in a configuration interaction approach

Even though more than 80 years have passed since the discovery of fission, its microscopic understanding has still been unclear. To clarify the underlying mechanics of induced fission, we analyze the distribution of a fission width using a miscropic framework based on a configuration-interaction approach. The distribution is known to follow a chi-squared distribution, which is characterized by the effective number of decay channels, ν. We introduce an effective Hamitonian for the space of compound nucleus states and estimate ν from the rank of the imaginary part of the effective Hamiltonian. Applying the model to 235U(n,f), we succesfully reproduce the empirical value of ν = 2.3 ± 1.1. We also find that ν is insensitve to the number of fission channels, which is consistent with an experimental finding.

Generator coordinate method for transition-state dynamics in nuclear fission

Since its beginnings, fission theory has assumed that low-energy induced fission takes place through transition-state channels at the barrier tops. Nevertheless, up to now there is no microscopic theory applicable to those conditions. We suggest that modern reaction theory is suitable for this purpose, and propose a methodology based on a configuration-interaction framework using the generator coordinate method (GCM). Simple reaction-theoretic models are constructed with the Gaussian overlap approximation to parametrize both the dynamics within the channels and their incoherent couplings to states outside the barrier. The physical characteristics of the channels examined here are their effective bandwidths and the quality of the coupling to compound-nucleus states as measured by the transmission factor $T$. We also investigate the spacing of GCM states with respect to their degree of overlap. We find that a rather coarse mesh provides an acceptable accuracy for estimating the bandwidths and transmission factors. The common numerical stability problem in using the GCM is avoided due to the choice of meshes and the finite bandwidths of the channels. The bandwidths of the channels are largely controlled by the zero-point energy with respect to the collective coordinate in the GCM configurations.

Counting Efficiency and Neutron/Gamma Ratio for KDP: Tl+ and UPS-923A Scintillators in a Single Photone Detection Mode

This research related to registration of the fast neutrons with a detector based on the inorganic KDP: TL+ mono crystal (KH2PO4 potassium dihydrogen phosphate) and plastic UPS-923A. The crystal of the KDP: TL+ detector grown from a water solution by the method of lowering the temperature. The high concentration of hydrogen nuclei in the KDP: TL+ crystal grid makes it possible to detect neutron radiation with an efficiency comparable to polystyrene scintillators. KDP: TL+ crystals have a high radiation resistance (up to 1010 neutrons/cm2), which significantly expands the spectrum of their application in high-energy physics applications, intense neutron fields. In this work, we used a technique for recording the detector response in the photon counting mode and pulse filtering mode. Since the detector operates on the principle of detecting gamma quanta from the reactions (n, n 'γ), (n, n' γ)res, (n, γ)cap and others, this makes it possible (in a filtering mode) to isolate the mechanisms of cascade generation processes in the volume of the detector caused by secondary gamma quanta from excited states of compound nuclei. The gamma quanta of the elastic scattering reaction (n, n' γ) for the KDP: TL+ scintillator nuclei are the start of the cascade process of the discharge of excited isomeric states of the input, intermediate, and final nuclei. Measurements of the detection efficiency of fast neutrons were carried out with a KDP: TL+ crystal of size 18x18x42 mm in spherical geometry. The obtained detector reviews in units of impulse / particle for sources and 239Pu-Be and 137Cs were 3.57 and 1.44. In this case, a broadband path with a speed of 7 ns was used. In addition, the counting efficiency of the narrow-band tract measured simultaneously with a processing time of 1 μs and 6.4 μs. The received response from the KDP: TL+ detector (in units of impulse/particle) for both sources 239Pu-Be and 137Cs was 0.09 and 0.00029. The n/γ ratio coefficient was 310. The given measurements of a polystyrene-based scintillator size of 40×40×40 mm. The received response in a single photon-counting mode from the plastic detector (in units of impulse/particle) for both sources 239Pu-Be and 137Cs was 19.4 and 3.9. The n/γ ratio coefficients for detectors are also given: KDP: TL+ - 2.47 and UPS-923A - 4.97 in the 7 ns mode. The statistical error in measurements of the neutron detection efficiency was about ~ 5%.

Reexamining the role of the ( n,γf ) process in the low-energy fission of U235 and Pu239

The $(n,\gamma f)$ process is reviewed in light of modern nuclear reaction calculations in both slow and fast neutron-induced fission reactions on $^{235}$U and $^{239}$Pu. Observed fluctuations of the average prompt fission neutron multiplicity and average total $\gamma$-ray energy below 100 eV incident neutron energy are interpreted in this framework. The surprisingly large contribution of the M1 transitions to the pre-fission $\gamma$-ray spectrum of $^{239}$Pu is explained by the dominant fission probabilities of 0$^+$ and $2^+$ transition states, which can only be accessed from compound nucleus states formed by the interaction of $s$-wave neutrons with the target nucleus in its ground state, and decaying through M1 transitions. The impact of an additional low-lying M1 scissors mode in the photon strength function is analyzed. We review experimental evidence for fission fragment mass and kinetic energy fluctuations in the resonance region and their importance in the interpretation of experimental data on prompt neutron data in this region. Finally, calculations are extended to the fast energy range where $(n,\gamma f)$ corrections can account for up to 3\% of the total fission cross section and about 20\% of the capture cross section.

T-invariance conditions for sequential multistep statistical nuclear reactions

A generalized version of the R-matrix theory is used to determine the amplitudes of sequential n-step statistical nuclear reactions. T-invariance conditions for these amplitudes are analyzed. Within the scope of the unified theory, integral formulas are constructed for the decay amplitudes of the intermediate states of compound nuclei via the matrix elements QHP of the system Hamiltonian H, where Q and P operators perform projections onto resonance and energy-continuous states of the system, respectively.

Calculations of Compound Nucleus Spin-Parity Distributions Populated via the (p,t) Reaction in Support of Surrogate Neutron Capture Measurements

The surrogate reaction method may be used to determine the cross section for neutron induced reactions not accessible through standard experimental techniques. This is achieved by creating the same compound nucleus as would be expected in the desired reaction, but through a different incident channel, generally a direct transfer reaction. So far, the surrogate technique has been applied with reasonable success to determine the fission cross section for a number of actinides, but has been less successful when applied to other reactions, e.g. neutron capture, due to a ‘spin-parity mismatch’. This mismatch, between the spin and parity distributions of the excited levels of the compound nucleus populated in the desired and surrogate channels, leads to differing decay probabilities and hence reduces the validity of using the surrogate method to infer the cross section in the desired channel. A greater theoretical understanding of the expected distribution of levels excited in both the desired and surrogate channels is therefore required in order to attempt to address this mismatch and allow the method to be utilised with greater confidence. Two neutron transfer reactions, e.g. (p,t), which allow the technique to be utilised for isotopes further removed from the line of stability, are the subject of this study. Results are presented for the calculated distribution of compound nucleus states populated in 90Zr, via the 90Zr(p,t)90Zr reaction, and are compared against measured data at an incident proton energy of 28.56 MeV.

Role of dynamical effects in the formation of T-Odd asymmetries for products of polarized-neutron-induced ternary fission of nuclei

Basic dynamical effects that accompany the cold-polarized-neutron-induced binary and ternary fission of actinide nuclei and which determine the properties of T -odd asymmetries in angular distributions of various prescission and evaporated light third particles emitted in true and delayed ternary fission are analyzed on the basis of quantum-mechanical fission theory. It is emphasized that effects associated with the conservation of axial symmetry of the fissioning system under study at all stages of its evolution from the formation of neutron resonance states of the fissile compound nucleus to the separation of its fission fragments, including the appearance of zero wriggling vibrations of the cold compound nucleus in the vicinity of its scission point, are of particular importance, the influence of quantum collective rotation of the polarized fissile system on the asymmetry of the angular distribution of both fission fragments and third particles being taken into account. It is shown that the difference in the behavior of the coefficients characterizing the T -odd asymmetries under analysis for the target nuclei being studied can be explained, upon taking into account the interference between the fission amplitudes for the neutron resonance states of fissile compound nuclei, by the difference in the contributions of even and odd components of the amplitudes of angular distributions of third particles to the coefficients in question.

Resonances Interference Effects on 233, 235U Nuclei

Abstract The asymmetry effects (as forward – backward (FB), left – right (LR) and parity non conservation (PNC)) observed in the fission induced by thermal and resonant neutrons on 233, 235 U nuclei in the frame of the formalism of mixing states of compound nucleus with the same spin and opposite parities were explained. In this work new P resonance parameters from FB and LR effects in order to explain the observed asymmetries and PNC effect have been derived. The present study demonstrates by using the multilevel Breit – Wigner formalism that the resonance states observed in FB and LR measurements are just a result of multilevel interference between S resonances. Only new P resonances, not indicated in nuclear data atlases for neutron resonance parameters, are enough to describe the asymmetry effects.

Radiative strength functions for dipole transitions in 90Zr

Partial cross sections for the (p, γ) reaction on the 89Y nucleus that were measured previously at proton energies between 2.17 and 5.00 MeV and which were averaged over resonances were used to determine the absolute values and the energy distribution of the strength of dipole transitions from compound-nucleus states to low-lying levels of the 90Zr nucleus. The data obtained in this way were compared with the predictions of various models.

Dynamical phase transitions in quantum mechanics

The nucleus is described as an open many-body quantum system with a non- Hermitian Hamilton operator the eigenvalues of which are complex, in general. The eigen- values may cross in the complex plane (exceptional points), the phases of the eigenfunc- tions are not rigid in approaching the crossing points and the widths bifurcate. By varying only one parameter, the eigenvalue trajectories usually avoid crossing and width bifurca- tion occurs at the critical value of avoided crossing. An analog spectroscopic redistribution takes place for discrete states below the particle decay threshold. By this means, a dynam- ical phase transition occurs in the many-level system starting at a critical value of the level density. Hence the properties of the low-lying nuclear states (described well by the shell model) and those of highly excited nuclear states (described by random ensembles) di er fundamentally from one another. The statement of Niels Bohr on the collective features of compound nucleus states at high level density is therefore not in contradiction to the shell-model description of nuclear (and atomic) states at low level density. Dynamical phase transitions are observed experimentally in di erent quantum mechanical systems by varying one or two parameters.

Fission via compound states andJπKA. Bohr's channels: what we can learn from recent studies with slow neutrons

Last data on angular correlations of fission fragments from slow (s-wave) neutron induced binary fission of spin-aligned nuclei 235 U are discussed in the context of Jπ K A. Bohrs channels. Special attention is paid to K = 0 channel. Reasons for its suppression are specified for compound nucleus states of negative parity. A brief overview of recent data on T-odd angular correlations in ternary and binary (with emission of a third particle, a neutron or γ -quantum) fission induced by slow polarized neutrons is presented. On the basis of the developed theoretical approach it is shown that a valuable information on Jπ K fission channels at scission point can be inferred from these T-odd angular correlations.

Dynamical Phase Transitions in Quantum Systems

Many years ago Bohr characterized the fundamental differences between the two extreme cases of quantum mechanical many-body problems known at that time: between the compound states in nuclei at extremely high level density and the shell-model states in atoms at low level density. It is shown in the present paper that the compound nucleus states at high level density are the result of a dynamical phase transition due to which they have lost any spectroscopic relation to the individual states of the nucleus. The last ones are shell-model states which are of the same type as the shell-model states in atoms. Mathematically, dynamical phase transitions are caused by singular (exceptional) points at which the trajectories of the eigenvalues of the non-Hermitian Hamilton operator cross. In the neighborhood of these singular points, the phases of the eigenfunctions are not rigid. It is possible therefore that some eigenfunctions of the system align to the scattering wavefunctions of the environment by decoupling (trapping) the remaining ones from the environment. In the Schrodinger equation, nonlinear terms appear in the neighborhood of the singular points.

Partial radiative strength functions for dipole transitions in the, 49, 51V nuclei

Radiative strength functions for dipole transitions from the compound-nucleus states to excited states of 49V and 51V nuclei are determined from the resonance-averaged intensities of primary γ rays from (p, γ) reactions, measured in the proton-energy range 1.5–3.5 MeV. The obtained data are compared with the predictions of various models. The best description for E1 transitions is obtained on the basis of a model taking into account the dependence of the strength function on the energy of photons, temperature, and the nuclear shell structure.

γ-Decay of the GDR in 147 Eu Compound Nucleus

Results from the measurement of the GDR built on excited states of the 147Eu compound nucleus are presented. Data corresponding to restricted regions of angular momenta and more selected data, associated with different residual nuclei are discussed. The behaviour of the GDR width, measured as a function of nuclear spin and temperature, is well described by the predictions of the thermal shape fluctuation model.

Pre- and post-scission neutron emission in the reaction180Hf(40Ar, fission) atE lab=216 MeV

Neutron and fission fragment spectra following the180Hf(40Ar,f) reaction atElab=216MeV were measured. The following neutron total, pre- and post-scission multiplicities were extracted:Mntot=7.2±1.3,Mnpre=2.7±0.9,Mnpost=4.5±0.9. The average temperature parameters of the neutron spectra areTpre=(1.63±0.16) MeV andTpost=(1.14±0.26)MeV. The mean total kinetic energy of the fission fragments is TKE=(166±10) MeV and the measured width is σTKE=17.3MeV. The width of the fragment mass distribution is σA=18.3u. The same reaction was analyzed using a modified statistical code which includes nuclear dissipation effects and particle and γ-ray emission in the equilibrium compound nucleus state and saddle-to-scission transition. The extracted value of nuclear friction coefficient indicates over-damped large-scale fission motion. The obtained total fission time τfiss=9·10−20s is in agreement with other existing data.

Resonance phenomena near thresholds

The trapping effect is investigated close to the elastic threshold. The nucleus is described as an open quantum mechanical many-body system embedded in the continuum of decay channels. An ensemble of compound nucleus states with states below and above threshold is investigated in an energy-dependent formalism. It is shown that the states below threshold can trap the resonance ones and also that they can directly influence the scattering cross section. \textcopyright{} 1996 The American Physical Society.

PNC effects in neutron-232Th scattering: a window to class II or new states of nuclei?

Abstract The sign correlation of parity-non-conserving ( pnc ) asymmetries observed in neutron 232 Th scattering at the peak of p-wave resonances is examined. Assuming that it is a real effect, the conditions that this implies for a theoretical explanation are determined. It is shown that it has to rely on the existence of two states of opposite parity, which act as “doorway states”, different from those recently considered in the present field. The location of these states, their energy difference and their spreading width are estimated. It is shown that experimental data on 232 Th favor the location of both “doorway states” in the low energy region ( E s 0 , E p 0 ∼0–1 keV). In this case, the asymmetry exhibits a different sign depending on whether the energy of the p-wave resonance is below or above E s 0 . Candidates for such states are then considered, including the well known parity doublet built on a pear-shape nucleus, a more exotic one based on a structure where spins of nucleons align along their momentum ( σ · p correlation), and finally an accidental one in the second well of the potential energy surface. The evidence for a sign correlation in a limited energy range partly depends on the size of the random contribution to the observed pnc effects. The examination of the root mean square value of the pnc matrix element between states of the compound nucleus shows that the range of predictions is quite large and does not provide any constraint at present.

On the width of giant dipole resonance in hot nuclei

An approach describing the width of the giant dipole resonance in hot nuclei is proposed, unifying the microscopic thermal damping of multiquasiparticle states over the compound nucleus state and the quantal damping. The numerical calculations in the schematic model with equally degenerate equidistant shells for a hot nucleus of mass A = 110 carrying no angular momentum are performed which provide a good agreement with the behavior of the recent experimental data.

Estimation of the time scale of last chance alpha emission using an "atomic clock"

The probability of filling a K-vacancy, created on the incoming part of the collision, before \ensuremath{\alpha}-particle emission is used to time the decay of Yb compound nuclei. These nuclei were produced in the fusion reaction 250 MeV $^{60}\mathrm{Ni}$${+}^{100}$Mo. In general the nuclear decays are too fast to be timed by this clock, however, \ensuremath{\alpha}-particle emitting compound nucleus states have lifetimes sufficiently long for this technique to work when both the \ensuremath{\alpha}-particle has low energy and the compound nucleus spin is large. This supports the existence of last, or near last, chance \ensuremath{\alpha}-particle emission in the deexcitation of high spin compound nuclei.