Pion absorption at rest

The absorption of stopped negative pions in nuclei is interpreted [GE87], [CH81], [AS86], [Wey90] as starting with the absorption of the pion by two or more correlated nucleons; the total energy of the pion is transferred to the absorbing nucleons, which then may leave the nucleus directly, or undergo final-state interactions with the residual nucleus. The remaining nucleus de-excites by evaporation of low energetic particles.

G4PiMinusAbsorptionAtRest generates the primary absorption component of the process through the parameterisation of existing experimental data; the primary absorption component is handled by class G4PiMinusStopAbsorption. In the current implementation only absorption on a nucleon pair is considered, while contributions from absorption on nucleon clusters are neglected; this approximation is supported by experimental results [GE87], [Mac83] showing that it is the dominating contribution.

Several features of stopped pion absorption are known from experimental measurements on various materials [HIE+78], [MVA+82], [SCMZ79], [ODD+80], [PEH+79], [HIP+83], [IZE+83], [eal82]:

the average number of nucleons emitted, as resulting from the primary absorption process;
the ratio of nn vs np as nucleon pairs involved in the absorption process;
the energy spectrum of the resulting nucleons emitted and their opening angle distribution.

The corresponding final state products and related distributions are generated according to a parameterisation of the available experimental measurements listed above. The dependence on the material is handled by a strategy pattern: the features pertaining to material for which experimental data are available are treated in G4PiMinusStopX classes (where X represents an element), inheriting from G4StopMaterial base class. In case of absorption on an element for which experimental data are not available, the experimental distributions for the elements closest in Z are used.

The excitation energy of the residual nucleus is calculated by difference between the initial energy and the energy of the final state products of the primary absorption process.

Another strategy handles the nucleus deexcitation; the current default implementation consists in handling the deexcitatoin component of the process through the evaporation model described elsewhere in this Manual.

Bibliography

AS86: D Ashery and J P Schiffer. Pion absorption in nuclei. Annual Review of Nuclear and Particle Science, 36(1):207–252, dec 1986. URL: https://doi.org/10.1146/annurev.ns.36.120186.001231, doi:10.1146/annurev.ns.36.120186.001231.
CH81: H.C. Chiang and J. Hüfner. Nucleons after pion absorption. Nuclear Physics A, 352(3):442–460, feb 1981. URL: https://doi.org/10.1016/0375-9474(81)90422-X, doi:10.1016/0375-9474(81)90422-x.
eal82: H.P. Isaak et al. Single and coincident neutron emission after the absorption of stopped negative pions in $^6$li, $^7$li, $^12$c, $^59$co and $^197$au. Helvetica Physica Acta, 55:477–500, 1982. URL: https://www.e-periodica.ch/digbib/view?pid=hpa-001:1982:55#479, doi:10.5169/seals-115295.
GE87(1,2): E. Gadioli and E. Gadioli Erba. Nuclear reactions induced by $\pi ^-$ at rest. Physical Review C, 36(2):741–757, aug 1987. URL: https://doi.org/10.1103/PhysRevC.36.741, doi:10.1103/physrevc.36.741.
HIE+78: R. Hartmann, H.P. Isaak, R. Engfer, E.A. Hermes, H.S. Pruys, W. Dey, H.J. Pfeiffer, U. Sennhauser, H.K. Walter, and J. Morgenstern. Spectroscopy of single and correlated neutrons following pion absorption in 12c, 59co and 197au. Nuclear Physics A, 308(3):345–364, oct 1978. URL: https://doi.org/10.1016/0375-9474(78)90556-0, doi:10.1016/0375-9474(78)90556-0.
HIP+83: P. Heusi, H.P. Isaak, H.S. Pruys, R. Engfer, E.A. Hermes, T. Kozlowski, U. Sennhauser, and H.K. Walter. Coincident emission of neutrons and charged particles after $\pi ^-$ absorption in $^6$li, $^7$li, $^12$c, $^59$co and $^197$au. Nuclear Physics A, 407(3):429–459, oct 1983. URL: https://doi.org/10.1016/0375-9474(83)90660-7, doi:10.1016/0375-9474(83)90660-7.
IZE+83: H.P. Isaak, A. Zglinski, R. Engfer, R. Hartmann, E.A. Hermes, H.S. Pruys, F.W. Schlepütz, T. Kozlowski, U. Sennhauser, H.K. Walter, K. Junker, and Nimai C. Mukhopadhyay. Inclusive neutron spectra from the absorption of stopped negative pions in heavy nuclei. Nuclear Physics A, 392(2-3):368–384, jan 1983. URL: https://doi.org/10.1016/0375-9474(83)90133-1, doi:10.1016/0375-9474(83)90133-1.
Mac83: H. Machner. Study of particle emission following $\pi ^-$ absorption at rest. Nuclear Physics A, 395(2):457–470, mar 1983. URL: https://doi.org/10.1016/0375-9474(83)90054-4, doi:10.1016/0375-9474(83)90054-4.
MVA+82: R. Madey, T. Vilaithong, B. D. Anderson, J. N. Knudson, T. R. Witten, A. R. Baldwin, and F. M. Waterman. Neutrons from nuclear capture of negative pions. Physical Review C, 25(6):3050–3067, jun 1982. URL: https://doi.org/10.1103/PhysRevC.25.3050, doi:10.1103/physrevc.25.3050.
ODD+80: C. J. Orth, W. R. Daniels, B. J. Dropesky, R. A. Williams, G. C. Giesler, and J. N. Ginocchio. Products of stopped-pion interactions with cu and ta. Physical Review C, 21(6):2524–2534, jun 1980. URL: https://doi.org/10.1103/PhysRevC.21.2524, doi:10.1103/physrevc.21.2524.
PEH+79: H.S. Pruys, R. Engfer, R. Hartmann, U. Sennhauser, H.-J. Pfeiffer, H.K. Walter, J. Morgenstern, A. Wyttenbach, E. Gadioli, and E. Gadioli-Erba. Absorption of stopped $\pi ^-$ in $^59$co, $^75$as, $^197$au and $^209$bi investigated by in-beam and activation $\gamma -$ray spectroscopy. Nuclear Physics A, 316(3):365–388, mar 1979. URL: https://doi.org/10.1016/0375-9474(79)90043-5, doi:10.1016/0375-9474(79)90043-5.
SCMZ79: F. W. Schlepütz, J. C. Comiso, T. C. Meyer, and K. O. H. Ziock. Emission of low-energy charged particles following negative-pion capture from rest. Physical Review C, 19(1):135–141, jan 1979. URL: https://doi.org/10.1103/PhysRevC.19.135, doi:10.1103/physrevc.19.135.
Wey90: Heinz J Weyer. Pion absorption in light nuclei. Physics Reports, 195(6):295–367, nov 1990. URL: https://doi.org/10.1016/0370-1573(90)90076-E, doi:10.1016/0370-1573(90)90076-e.