J. P. Svenne

An algebraic solution of the multichannel problem applied to low energy nucleon-nucleus scattering

This is a pre-print version published in Nuclear Physics A

Predicting Narrow States in the Spectrum of a Nucleus beyond the Proton Drip Line

Properties of particle-unstable nuclei lying beyond the proton drip line can be ascertained by considering the (usually known) properties of its mirror neutron-rich system. We have used a multichannel algebraic scattering theory to map the known properties of the neutron-14C system to those of the proton-14O one from which we deduce that the particle-unstable 15F will have a spectrum of two low-lying broad resonances of positive parity and, at higher excitation, three narrow negative-parity ones. A key feature is to use coupling to Pauli-hindered states in the target.

Role of the Pauli principle in collective-model coupled-channel calculations.

A multichannel algebraic scattering theory, to find solutions of coupled-channel scattering problems with interactions determined by collective models, has been structured to ensure that the Pauli principle is not violated. By tracking the results in the zero coupling limit, a correct interpretation of the subthreshold and resonant spectra of the compound system can be made. As an example, the neutron-12C system is studied defining properties of 13C to 10 MeV excitation. Accounting for the Pauli principle in collective coupled-channels models is crucial to the outcome.

Coupled-channel evaluations of cross sections for scattering involving particle-unstable resonances.

How does the scattering cross section change when the colliding bound-state fragments are allowed particle-emitting resonances? This question is explored in the framework of a multichannel algebraic scattering method of determining nucleon-nucleus cross sections at low energies. Two cases are examined, the first being a gedanken investigation in which n + 12C scattering is studied with the target states assigned artificial widths. The second is a study of neutron scattering from 8Be, a nucleus that is particle unstable. Resonance character of the target states markedly varies evaluated cross sections from those obtained assuming stability in the target spectrum.

Constraints on the spectra of 17,19C

Abstract Diverse means are used to investigate the spectra of the radioactive, exotic ions, 17,19 C. First, estimates have been made using a shell model for the systems. Information from those shell model studies were then used in evaluating cross sections of the scattering of 70 A MeV 17,19 C ions from hydrogen. Complementing those studies, a multichannel algebraic scattering (MCAS) theory for n + 16,18 C coupled-channel problems has been used to identify structures of the compound systems. The results show that the shell model structure assumed for these ions is reasonable with little need of effective charges. The conditions that two excited states exist within a few hundred keV of the ground state places some restriction upon the structure models. Other positive parity states are expected in the low-lying spectra of the two nuclei.

Comparison between two methods of solution of coupled equations for low-energy scattering

Cross sections from low-energy neutron-nucleus scattering have been evaluated using a coupled-channel theory of scattering. Both a coordinate-space and a momentum-space formalism of that coupled-channel theory are considered. A simple rotational model of the channel-interaction potentials is used to find results using two relevant codes, ECIS97 and MCAS, so that they may be compared. The very same model is then used in the MCAS approach to quantify the changes that occur when allowance is made for effects of the Pauli principle.

Practical approximation scheme for the pion dynamics in the three-nucleon system

We discuss a working approximation scheme to a recently developed formulation of the coupled piNNN-NNN problem. The approximation scheme is based on the physical assumption that, at low energies, the 2N-subsystem dynamics in the elastic channel is conveniently described by the usual 2N-potential approach, while the explicit pion dynamics describes small, correction-type effects. Using the standard separable-expansion method, we obtain a dynamical equation of the Alt-Grassberger-Sandhas (AGS) type. This is an important result, because the computational techniques used for solving the normal AGS equation can also be used to describe the pion dynamics in the 3N system once the matrix dimension is increased by one component. We have also shown that this approximation scheme treats the conventional 3N problem once the pion degrees of freedom are projected out. Then the 3N system is described with an extended AGS-type equation where the spin-off of the pion dynamics (beyond the 2N potential) is taken into account in additional contributions to the driving term. These new terms are shown to reproduce the diagrams leading to modern 3N-force models. We also recover two sets of irreducible diagrams that are commonly neglected in 3N-force discussions, and conclude that these sets should be further investigated, because a claimed cancellation is questionable.

Nuclear Theory - Nuclear Power

Nuclear Theory - Nuclear Power The results from modern nuclear theory are accurate and reliable enough to be used for practical applications, in particular for scattering that involves few-nucleon systems of importance to nuclear power. Using well-established nucleon-nucleon (NN) interactions that fit well the NN scattering data, and the AGS form of the three-body theory, we have performed precise calculations of low-energy neutron-deuteron (n+d) scattering. We show that three-nucleon force effects that have impact on the low-energy vector analyzing powers have no practical effects on the angular distribution of the n+d cross-section. There appear to be problems for this scattering in the evaluated nuclear data file (ENDF) libraries, at the incident neutron energies less than 3.2 MeV. Supporting experimental data in this energy region are rather old (>25 years), sparse and often inconsistent. Our three-body results at low energies, 50 keV to 10.0 MeV, are compared to the ENDF/B-VII.0 and JENDL (Japanese Evaluated Nuclear Data Library) -3.3 evaluated angular distributions. The impact of these results on the calculated reactivity for various critical systems involving heavy water is shown. Kodolteorija – Kodlenergija Modernās kodolteorijas rezultāti ir pietiekami precīzi un uzticami, lai tos izmantotu praktiskajos pielietojumos, sevišķi kodolenergētikai svarīgu daudznuklonu sistēmu izkliedē. Izmantojot vispāratzītu nuklonu-nuklonu (NN) mijiedarbības teoriju, kas labi apraksta NN izkliedes datus un trīsdaļiņu teorijas AGS formu, mēs veicām precīzus zemas energijas neitronu-deitronu izkliedes aprēķinus. Mēs parādām, ka trīsnuklonu spēku efekti, kas ietekmē zemas energijas vektoru analīzes spēku, praktiski neietekmē n-d izkliedes šķērsgriezuma leņķisko sadalījumu. Šķiet, ka ar šīs izkliedes datiem ENDF novērtēto kodolu datu bibliotēkā pie izejas neitrona energijām mazākam par 3.2 MeV, ir problēmas. Atbalstošie eksperimentālie dati ir diezgan veci (<25 gadiem), reti un bieži pretrunīgi. Mūsu trīsķermeņu problēmas rezultāti pie zemām energijām no 50 keV līdz 10,0 MeV tika salīdzināti ar ENDF. B VII.0 un JENDL-3.3 novērtētajiem leņķiskajiem sadalījumiem. Ir parādīta šo rezultātu ietekme uz dažādu kritisku sistēmu, kas satur smago ūdeni D2O, aprēķināto reaktivitāti.

Spin observables for pion production from pd collisions

We have calculated the proton analyzing power A_{y0} of the pion-production reaction from pd collisions for one energy close to threshold and for another in the region of the

Three-Body Dynamics in One Dimension

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