Foreword
Perface
- Monte Carlo simulation. Basic concepts.
- Photon interactions
- Electron and Positron interactions
- Electron/Positron transport mechanics
- Constructive quadric geometry
- Structure and operation of the code system
- knowledge of radiation transport properties is needed for quantitative analysis in surface electron spectroscopies,(Jablonski, 1987; Tofterup, 1986).
- position surface spectroscopy, (Schultz and Lynn, 1988),
- electron microscopy, (Reimer, 1985),
- electron energy loss spectroscpy (Reimer et al., 1992),
- electron probe microanalysis (Heinrich and Newbury, 1991), etc.
The study of radiation transport problems was initially attempted on the basis of the Boltzmann transport equation.
- However, this procedure comes up against considerable difficulties when applied to limited geometries, with the result that numerical methods based on the transport equation have only had certain success in simple geometrics, mainly for unlimited and semi-infinite media (see, e.g.,Zheng-Ming and Brahme, 1993).
- At the end of the 1950s, with the availability of computers, Monte Carlo simulation methods were developed as a powerful alternative to deal with transport problems.
- Basically, the evolution of an electron-photon shower is of a random nature, so that is a process that is particularly amenable to Monte Carlo simulation.
- Detailed simulation, where all the interactions experienced by a paricle are simulated in chronological succession, is exact, i.e., it yields the same results as the rigorous solution of the transport equation (apart from the inherent statistical uncertainties).
The simulation of photon transport is straightforward since the mean number of events in each history is fairly small.
Indeed, the photon is effectively absorbed after single photoelectric or pair-production interaction or after a few C0mpton interations (say, of the order of 10).
With present-day computational facilities, detailed simulation of photon transport is a simple routine task.
The simulation of electron and positron transport is much more difficult than that of photons.
The main reason is that the average energy loss of an electron in a single interactions is very small (of the order of a few tens of eV).
As a consequence, high-energy electrons suffer a large number of interactions before being effectively absorbed in the medium.
In practice, detailed simulation is feasible only when the average number of collisions per track is not too large (say, up to a few hundred).
Experimental situations which are amenable to detailed simulation are those involving either electron sources with low initial kinetic energies (up to about 100keV) or special geometries such as electron beams impinging on thin foils.
For larger initial energies, and thick geometrics, the average number of collisions experienced by an electron until it is effectively stopped becomes very large, and detailed simulation is very inefficient.
For high-energy electrons and positrons, most of the Monte Carlo codes currently available
- [e.g., ETRAN (Berger and Seltzer, 1988),
- ITS3 (Halbleib et al., 1992),
- EGS4 (Nelson et al., 1985),
- GEANT3 (Brun et al., 1986),
- EGSnrc (Kawrakow and Rogers, 2001),
- MCNP (X-5 Monte CarloTeam, 2003),
- GEANT4 (Agostinelli et al., 2003; Allison et al., 2006),
- FLUKA (Ferrari et al., 2005),
- EGS5 (Hirayama et al., 2005), …]
Following Berger (1963), these simulation produres will be refered to as "condensed"簡明 Monte Carlo methods.
The multiple-scattering theories implemented in condensed simulation algorithms are only approximate and may lead to systematic errors, which can be made evident顯然 by the dependence of the simulation results on the adopted step length (Bielajew and Rogers, 1987).
