PENELOPE 2003 - OECD Nuclear Energy Agency

PREFACE
Radiation transport in matter has been a subject of intense work since the beginning of the
20 th century. Today, we know that high-energy photons, electrons and positrons penetrating matter
suffer multiple interactions by which energy is transferred to the atoms and molecules of the material
and secondary particles are produced. 1 By repeated interaction with the medium, a high-energy particle
originates a cascade of particles which is usually referred to as a shower. After each interaction of a
particle, its energy is reduced and further particles may be generated so that the evolution of the shower
represents an effective degradation in energy. As time goes on, the initial energy is progressively
deposited into the medium, while that remaining is shared by an increasingly larger number of particles.
A reliable description of shower evolution is required in a number of fields. Thus, knowledge of
radiation transport properties is needed for quantitative analysis in surface electron spectroscopies
(Jablonski, 1987; Tofterup, 1986), positron surface spectroscopy (Schultz and Lynn, 1988), electron
microscopy (Reimer, 1985), electron energy loss spectroscopy (Reimer, et al., 1992), electron probe
microanalysis (Heinrich and Newbury, 1991), etc. Detailed information on shower evolution is also
required for the design and quantitative use of radiation detectors (Titus, 1970; Berger and Seltzer,
1972). A field where radiation transport studies play an important sociological role is that of radiation
dosimetry and radiotherapy (Andreo, 1991).
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 a certain success in simple geometries, 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 this is a
process particularly amenable to Monte Carlo simulation. Detailed simulation, where all the
interactions experienced by a particle 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).
To our knowledge, the first numerical Monte Carlo simulation of photon transport is that of
Hayward and Hubbell (1954) who generated 67 photon histories using a desk calculator. 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 a single photoelectric or pair-production interaction or
after a few Compton interactions (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 interaction 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
1 In this report, the term particle will be used to designate either photons, electrons or positrons.
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