Fermi break-up simulation for light nuclei
For light nuclei the values of excitation energy per nucleon are often comparable with nucleon binding energy. Thus a light excited nucleus breaks into two or more fragments with branching given by available phase space. To describe a process of nuclear disassembling the so-called Fermi break-up model is formulated [Fer50], [Kre61], [EGKR69, EpherreG67], [BBI+95]. This statistical approach was first used by Fermi [Fer50] to describe the multiple production in high energy nucleon collision. The Geant4 Fermi break-up model is capable to predict final states as result of an excited nucleus with \(Z < 9\) and \(A < 17\) statistical break-up.
Allowed channels
The channel will be allowed for decay, if the total kinetic energy \(E_{kin}\) of all fragments of the given channel at the moment of break-up is positive. This energy can be calculated according to equation:
\(U\) is primary fragment excitation, \(m_{b}\) and \(\epsilon_{b}\) are masses and excitation energies of fragments, respectively, \(E_{Coulomb}\) is the Coulomb barrier for a given channel. It is approximated by
where \(V_0\) is the volume of the system corresponding to the normal nuclear matter density
where \(r_{0} = 1.3\) fm is used. Free parameter of the model is the ratio of the effective volume \(V\) to the normal volume, currently
Break-up probability
The total probability for nucleus to break-up into \(n\) componets (nucleons, deutrons, tritons, alphas etc) in the final state is given by
where \(\rho_{n}(E)\) is the density of a number of final states, \(\Omega = (2\pi \hbar)^{3}\) is the normalization volume. The density \(\rho_{n}(E)\) can be defined as a product of three factors:
The first one is the phase space factor defined as
where \({\bf p_b}\) is fragment \(b\) momentum. The second one is the spin factor
which gives the number of states with different spin orientations. The last one is the permutation factor
which takes into account identity of components in final state. \(n_j\) is a number of components of \(j\)- type particles and \(k\) is defined by \(n = \sum_{j=1}^{k}n_{j}\)).
In non-relativistic case (Eq. (261) the integration in Eq. (259) can be evaluated analiticaly (see e. g. [BaravsenkovBarbavsevB58]). The probability for a nucleus with energy \(E\) disassembling into \(n\) fragments with masses \(m_b\), where \(b = 1,2,3,...,n\) equals
where \(\Gamma(x)\) is the gamma function.
Fragment characteristics
We take into account the formation of fragments in their ground and low-lying excited states, which are stable for nucleon emission. However, several unstable fragments with large lifetimes: 5He, 5Li, 8Be, 9B etc. are also considered. Fragment characteristics \(A_b\), \(Z_b\), \(s_b\) and \(\epsilon_b\) are taken from [AS81, AS82, AS83, AS84, AS85, Err83, Err84]. Recently nuclear level energies were changed to be identical with nuclear levels in the gamma evaporation database (see Section Photon evaporation).
Sampling procedure
The nucleus break-up is described by the Monte Carlo (MC) procedure. We randomly (according to probability Eq. (260) and condition Eq. (258) select decay channel. Then for given channel we calculate kinematical quantities of each fragment according to \(n\)-body phase space distribution:
The Kopylov’s sampling procedure [I70, Kop85, Kop73] is applied. The angular distributions for emitted fragments are considered to be isotropical.
Bibliography
- AS81
F. Ajenberg-Selove. Energy levels of light nuclei a = 13–15. Nuclear Physics A, 360(1):1–186, May 1981. URL: https://doi.org/10.1016/0375-9474(81)90510-8, doi:10.1016/0375-9474(81)90510-8.
- AS82
F. Ajzenberg-Selove. Energy levels of light nuclei a = 16–17. Nuclear Physics A, 375(1):1–168, Feb 1982. URL: https://doi.org/10.1016/0375-9474(82)90538-3, doi:10.1016/0375-9474(82)90538-3.
- AS83
F. Ajzenberg-Selove. Energy levels of light nuclei a = 18–20. Nuclear Physics A, 392(1):1–184, Jan 1983. URL: https://doi.org/10.1016/0375-9474(83)90180-X, doi:10.1016/0375-9474(83)90180-x.
- AS84
F. Ajzenberg-Selove. Energy levels of light nuclei a = 5–10. Nuclear Physics A, 413(1):1–168, Jan 1984. URL: https://doi.org/10.1016/0375-9474(84)90650-X, doi:10.1016/0375-9474(84)90650-x.
- AS85
F. Ajzenberg-Selove. Energy levels of light nuclei a = 11–12. Nuclear Physics A, 433(1):1–157, Jan 1985. URL: https://doi.org/10.1016/0375-9474(85)90484-1, doi:10.1016/0375-9474(85)90484-1.
- BaravsenkovBarbavsevB58
V. S. Barašenkov, B. M. Barbašev, and E. G. Bubelev. Statistical theory of particle multiple production in high energy nucleon collisions. Il Nuovo Cimento, 7(S1):117–128, Jan 1958. URL: https://doi.org/10.1007/BF02725091, doi:10.1007/bf02725091.
- BBI+95
J.P. Bondorf, A.S. Botvina, A.S. Iljinov, I.N. Mishustin, and K. Sneppen. Statistical multifragmentation of nuclei. Physics Reports, 257(3):133–221, Jun 1995. URL: https://doi.org/10.1016/0370-1573(94)00097-M, doi:10.1016/0370-1573(94)00097-m.
- EGKR69
Marcelle Epherre, Eli Gradsztajn, Robert Klapisch, and Hubert Reeves. Comparison between the evaporation and the break-up models of nuclear de-excitation. Nuclear Physics A, 139(3):545–553, Dec 1969. URL: https://doi.org/10.1016/0375-9474(69)90278-4, doi:10.1016/0375-9474(69)90278-4.
- Err83
Errata. Energy levels of light nuclei, a = 16–17. Nuclear Physics A, 392(1):185–216, Jan 1983. URL: https://doi.org/10.1016/0375-9474(83)90181-1, doi:10.1016/0375-9474(83)90181-1.
- Err84
Errata. Energy levels of light nuclei, a = 18–20. Nuclear Physics A, 413(1):168–214, Jan 1984. URL: https://doi.org/10.1016/0375-9474(84)90651-1, doi:10.1016/0375-9474(84)90651-1.
- Fer50(1,2)
E. Fermi. High energy nuclear events. Progress of Theoretical Physics, 5(4):570–583, Jul 1950. URL: https://doi.org/10.1143/ptp/5.4.570, doi:10.1143/ptp/5.4.570.
- I70
Kopylov G. I. Principles of resonance kinematics (in Russian). Moskva : "Nauka, " Glav. red. fiziko-matematicheskoi, 1970.
- Kop85
G. I. Kopylov. ELEMENTARY KINEMATICS OF ELEMENTARY PARTICLES. Moscow, USSR: Mir Publ. ( 1983) 270p, 1985.
- Kop73
G.I. Kopylov. The kinematics of inclusive experiments with unstable particles. Nuclear Physics B, 52(1):126–140, Jan 1973. URL: https://doi.org/10.1016/0550-3213(73)90090-4, doi:10.1016/0550-3213(73)90090-4.
- Kre61
M Kretzschmar. Statistical methods in high-energy physics. Annual Review of Nuclear Science, 11(1):1–38, Dec 1961. URL: https://doi.org/10.1146/annurev.ns.11.120161.000245, doi:10.1146/annurev.ns.11.120161.000245.
- EpherreG67
Marcelle Épherre and Élie Gradsztajn. Calcul de la spallation de 12c et 16o par des protons de 70 à 200 MeV. Journal de Physique, 28(10):745–751, 1967. URL: https://doi.org/10.1051/jphys:019670028010074500, doi:10.1051/jphys:019670028010074500.