Nuclear Fission Induced by Pi Mesons

Abstract

A range of observables for fission events resulting from irradiation of complex nuclei with beams of charged pi (π) mesons (pions) has been obtained over the last half century, including a campaign of systematic studies using the intense beams from ‘meson factories’ and an efficient detection technique. This effort is now complete. The data arise from a variety of techniques and experimental groups, each with specific features. This review will bring together these data, with comparisons to seek consistency among the data, connections to the special field of pion-nucleus reactions, and comparisons to fission induced by photons and antiprotons. 1.1. Definitions of fission. The collision of an energetic beam particle with a complex nucleus may lead to heavy fragments by several processes. Spallation, with several lighter fragments, may produce a single heavy residue, or an energetic multifragmention reaction may shatter the target nucleus into several massive fragments. Both of these processes need to be distinguished from true fission, in which two (rarely three) fragments of not-too-different mass are formed from the same scission, with a Coulomb repulsion as the main source of their kinetic energy. 1 Some of the experimental methods sense only one of a presumed pair of fragments. Since methods have different sensitivities to fragment properties, intercomparisons of results must be done with care. In this work ‘fission’ is defined to be the detection of one or two fragments, each with near half the target mass, and with energies as appropriate to the Coulomb repulsion of fission. Representative data demonstrating this selection process will be found in Section 3. 1.2. Pion-nucleus reactions. Pi mesons are fields, and may be absorbed into complex nuclei, making available their kinetic plus rest mass (140 MeV) energies and their charge (plus or minus for beams), with little angular momentum due to the low beam mass. Pions must be absorbed onto two or more nucleons in their initial interaction (in order to conserve both energy and momentum), and these absorption cross sections can be a large fraction of the total reaction cross sections on heavy nuclei. 2 Stopped π - may also be captured into a heavy nucleus from atomic orbits, with only the pion rest mass as the energy available for reactions leading to fission. A very complete comparison of theory and data (not including fission) for nuclear reactions following the capture of negative pions (π - ) is found in Reference 3. Since energetic pions interact with free nucleons by a series of important resonances, the energy dependence of the total reaction cross section (σR), of which absorption and fission will be a part, is a starting point for this review. Figure 1 shows reaction cross section data for pion beams of both signs on lead or bismuth up to kinetic energies of 2500 MeV. For comparison, the free negative pion-nucleon total cross sections, summed for the nucleons in lead, are also shown to exhibit the resonances. 10 These structures are severely damped and quenched within a complex nucleus, as discussed in the review of Reference 2. Examples of computed reaction cross sections are shown, using the Distorted Wave Impulse Approximation (DWIA) 8 for both signs up to 300 MeV, showing a strong Coulomb effect, and an eikonal optical model at higher energies. 9 These computed reaction cross sections σR will form the denominators of fission probabilities in Section 7 below.