The Evaluation of Atomic Masses

Abstract

The ensemble of experimental data on the 2830 nuclides which have been observed since the beginning of Nuclear Physics are being evaluated, according to their nature, by different methods and by different groups. The two "horizontal" evaluations in which I am involved: the Atomic Mass Evaluation Ame and the Nubase evaluation belong to the class of "static" nuclear data. In this lecture I will explain and discuss in detail the philosophy, the strategies and the procedures used in the evaluation of atomic masses.Résumé L'évaluation des masses atomiques -Les données expérimentales sur les 2830 nucléides observés depuis les débuts de la Physique Nucléaire sontévaluées, suivant leur nature, par différentes méthodes et par différents groupes. Les deux evaluations "horizontales" dans les-quelles je suis impliqué : l'Évaluation des Masses Atomiques Ame et l'évaluation Nubase appartiennentà la classe des données nucléaires "statiques". Dans ce cours je vais expliquer et discuter de manière approfondie la philosophie, les stratégies et les procédures utilisées dans l'évaluation des masses atomiques. the deformations; all fall in the category of what could be called the "static" nuclear properties.Total and differential (in energy and in angle) reaction cross-sections; reaction mechanisms; and spectroscopic factors could be grouped in the class of "dynamic" nuclear properties.Certainly, one single experiment, for example a nuclear reaction study, can yield data for both 'static' and 'dynamic' properties.It is out of the scope of the present lecture to cover all aspects of nuclear properties and nuclear data. The fine structure of "static" nuclear data will be shortly described and the authors of the various evaluations presented. Then I will center this lecture on the two "horizontal" evaluations in which I am involved: the atomic mass evaluation Ame and the Nubase evaluation, both being strongly related, particularly when considering isomers.2 "Static" nuclear data 2.1 The Nsdd: data for nuclear structureThe amount of data to be considered for nuclear structure is huge. They are represented schematically in Fig. 2 for each nuclide as one column containing all levels from the groundstate at the bottom of that column to the highest known excited state. All the known properties for each of the levels are included. Very early, it was found convenient to Figure 2: Schematic representation of all the available "static" nuclear data (structure, decay, mass, radius, moments,. . . ). Each nuclide is represented as a building with its ground-state at the ground floor. The mass evaluation is represented on the ground floor, across all buildings. It includes also data for upper levels if they represent an energy relation to another nuclide, like a foot-bridge between two buildings that will allow to derive the level difference between their ground floors.organize their evaluation in a network, splitting these data according to the mass of the nuclides, the A-chains. Such a division makes sense, since most of the decay relations among ...

The Nubase2012 evaluation of nuclear properties

“…Detailed discussions can be found in Ref. [8]. Only for 256 Md, we discovered too late, relative to the publication deadline, that the adopted ordering had to be reversed, we only added a remark to 256 Md m (see p. 1278).…”

“…Thus, β + p=23% and β + α=17%, from which the user of the NUBASE2012 table can derive a 60% direct feeding of the ground state of 9 B. In a slightly different example, 8 B decays to only two excited states in 8 Be, which in turn decay by α-and γ-ray emissions, but not to the 8 Be ground state. Thus, one may write β + =100% and β + α=100%, the difference of which leaves 0% for the feeding of the daughter's ground state.…”

Progress in Atomic Mass Evaluation

“…In addition, some quantities may fall at the border of the main interest of such a network. This is the reason why a few 'horizontal' compilations or evaluations have been conducted for the benefit of the whole community (see examples in [2]). …”

“…There are two ways to still be able to observe with some precision the surface of masses: one of them uses the DERIVA-TIVES of this surface, the other is obtained by subtracting a simple function of N and Z from the masses [2].…”

“…After separating the secondary data, and reducing the system of data [2], we are left with a system of 1500 primary data for 800 nuclides to be treated by the least-squares method into which the AME pioneeringly plugged-in, since 1993, the 'Flow-of-Information' matrix [9]. It is a powerful method that allows tracing back, in the least-squares method, the contribution of each individual piece of data to each of the parameters (here the atomic masses).…”