Complementary parameterised and theoretical treatment

Absorption of negative pions and kaons at rest from a nucleus is described in literature [1], [2], [3], [4] as consisting of two main components:

• a primary absorption process, involving the interaction of the incident stopped hadron with one or more nucleons of the target nucleus;
• the deexcitation of the remnant nucleus, left in an excitated state as a result of the occurrence of the primary absorption process.

This interpretation is supported by several experiments [5], [6], [7], [8], [9], [10], [11], that have measured various features characterizing these processes. In many cases the experimental measurements are capable to distinguish the final products originating from the primary absorption process and those resulting from the nuclear deexcitation component.

A set of stopped particle absorption processes is implemented in GEANT4, based on this two-component model (PiMinusAbsorptionAtRest and KaonMinusAbsorptionAtRest classes, for $\pi^{-}$ and $K^{-}$ respectively. Both implementations adopt the same approach: the primary absorption component of the process is parameterised, based on available experimental data; the nuclear deexcitation component is handled through the theoretical models described elsewhere in this Manual.

Pion absorption at rest

The absorption of stopped negative pions in nuclei is interpreted [1], [2], [3], [4] 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 [1], [13] showing that it is the dominating contribution.

Several features of stopped pion absorption are known from experimental measurements on various materials [5], [6], [7], [8], [9], [10], [11], [12]:

• 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

1. E. Gadioli and E. Gadioli Erba Phys. Rev. C 36 741 (1987)

2. H.C. Chiang and J. Hufner Nucl. Phys. A352 442 (1981)

3. D. Ashery and J. P. Schiffer Ann. Rev. Nucl. Part. Sci. 36 207 (1986)

4. H. J. Weyer Phys. Rep. 195 295 (1990)

5. R. Hartmann et al., Nucl. Phys. A300 345 (1978)

6. R. Madley et al., Phys. Rev. C 25 3050 (1982)

7. F. W. Schleputz et al., Phys. Rev. C 19 135 (1979)

8. C.J. Orth et al., Phys. Rev. C 21 2524 (1980)

9. H.S. Pruys et al., Nucl. Phys. A316 365 (1979)

10. P. Heusi et al., Nucl. Phys. A407 429 (1983)

11. H.P. Isaak et al., Nucl. Phys. A392 368 (1983)

12. H.P. Isaak et al., Helvetica Physica Acta 55 477 (1982)

13. H. Machner Nucl. Phys. A395 457 (1983)