M. Herman[2]

International Atomic Energy Agency, Vienna, Austria

Lectures given at the
Workshop on Nuclear Reaction Data and
Nuclear Reactors: Physics, Design and Safety
Trieste, 25 February — 28 March 2002

LNS0520002

The status and contents of the Reference Input Parameter Library (RIPL) are summarised. This input library provides an extensive database of model parameters for theoretical calculations of nuclear reactions. It was developed to facilitate use of reaction codes and in­crease the accuracy of theoretical predictions.

1 Introduction

Increased use of nuclear reaction theory for predicting cross sections, spectra, and angular distributions, as required for a large variety of applications, is an important trend in the evaluation of neutron and charged-particle nuclear data. The model codes offer important advantages such as ensuring internal consistency of the data by preserving the energy balance and the coherence of the partial cross sections with the total or the reaction cross sections. These features are essential for transport calculations. In addition, theoretical cal­culations represent the only approach that can fill gaps in the experimental results and predict data for unstable nuclei. Nuclear astrophysics and the design of Accelerator Driven Systems are typical applications that depend strongly on theoretical calculations.

With recent formulation of nuclear reaction models (triple-integral form of the statistical model [1], quantum mechanical Multistep Direct and Mul­tistep Compound [2, 3, 4]) and existing approaches to direct reactions, nu­clear reaction theory is believed to be in a position to meet most of the requirements for practical applications. The major sources of uncertainty are, the input parameters needed to perform theoretical calculations, in­cluding nuclear masses, deformations, nuclear levels and their decay charac­teristics, y-strength functions, neutron resonances and level densities, optical model parameters, and fission barriers. The IAEA has addressed these needs through a Coordinated Research Project on the Reference Input Parameter Library (RIPL), which involves the difficult task of collecting, evaluating and recommending the vast amounts of various nuclear parameters. RIPL is targeted at users of nuclear reaction codes and, in particular, at nuclear data evaluators. The first phase of the project was completed in 1999, with the production of a Starter File and related documentation [5]. A second phase of the project was initiated in 1999 to test the RIPL-1 database and produce interfaces between RIPL and commonly used nuclear reaction codes.

Substantial improvements and extensions to the original database have been made, resulting in a more accurate and reliable library. All files se­lected for RIPL-2 have been prepared in the unified RIPL-2 format, which facilitates their use in the reaction codes. The RIPL-2 library is expected to be released in July 2002. The contents of the RIPL-2 library are out­lined below, with possible improvements that could be made to the current database through new measurements at the SNS.

2 Contents of RIPL-2

2.1 Segment 1: MASSES

The mass segment contains basic ground state properties of nuclei, along with two theoretical predictions of masses and deformations. On the basis of the Hartree-Fock-Bogolubov (HFB) theory, a 10-parameter Skyrme force, along with a 4-parameter delta-function pairing force (with blocking for odd nuclei) and a 3-parameter Wigner term, was fitted to all 1888 measured masses of nuclei with N and Z > 8. The second file contains predictions obtained within the Finite Range Droplet Model (FRDM) [6]. The atomic mass excesses and nuclear ground-state deformations are tabulated for 8979 nuclei ranging from 16O to A=339. These calculations are based on the finite-range droplet macroscopic model and the folded-Yukawa single-particle microscopic model. The most recent evaluated experimental masses by Audi and Wapstra [7] are included as a separate column. A third possibility is provided by a subroutine implementing the Duflo-Zuker formula [8] for nu­clear masses in which, the nuclear Hamiltonian is separated into a monopole term and a residual multipole term. The monopole term is responsible for saturation and single-particle properties, and is fitted phenomenologically while the multipole part is derived from realistic interactions. The latest version of the mass formula made of 10 free parameters reproduces the 1950 experimental masses above 4He with an rms error of 574 keV.

RIPL-2 also provides natural abundancies according to the Wallet Cards and HFB matter densities. The data are necessary for calculation of optical model parameters within the semi-microscopic approach (code MOM) in segment 4.