5.3.19. Definition of energy distribution

When defining source particles with an energy distribution, use the e-type subsection instead of the e0 parameter. In that case, e0 should be left undefined, for example by commenting it out. The energy unit is MeV for non-nuclei and MeV/n for nuclei. In the explanation below, only the case where the energy unit is MeV is shown, so for nuclei please read it as MeV/n as appropriate.

Table 5.3.218 summarizes the available types of source energy distributions. Except for several special cases, the distribution types are broadly divided into integral type and differential type. The integral type defines the intensity integrated over energy, while the differential type defines the source spectrum as an energy differential value, that is, with units of [particles/MeV]. There are also two source generation methods. One adjusts the number of generated particles for a given intensity, and the other adjusts the particle weight. The former statistically adjusts the number of particles generated in each energy bin so that the distribution is reproduced. The latter reproduces the distribution by generating the same number of particles in every bin and changing the particle weight according to the intensity.

Table 5.3.218 Types of source energy distributions. The generation method is divided into the case where particle weights are kept constant and the number of generated particles is adjusted in each bin, and the case where the number of particles is kept constant and particle weights are changed in each bin. By specifying the number in parentheses for e-type, energy points can be given in wavelength, angstrom.

Type of energy distribution

Generation method: e-type

Explanation

Integral-type continuous energy distribution

Number of particles: 1, (11), weight: 4, (14)

A continuous energy distribution is defined by giving the lower bound of each energy bin and the integrated value of the source particle generation probability in that bin.

Differential-type continuous energy distribution

Number of particles: 21, (31), weight: 24, (34)

A continuous energy distribution is defined by giving the lower bound of each energy bin and the differential value of the source particle generation probability in that bin.

Continuous energy distribution with double (energy & angular) differential type

Number of particles: 41, (51), weight: 42, (52)

Similar format as the continuous-energy distribution with differential type above, but specifies angle-dependent energy spectra (/MeV/sr) in matrix form. The double-differential intensity is specified in a matrix format, where each row corresponds to an energy bin and each column corresponds to an angular bin. When using this format, you must have previously set dir = data and a-type = 1, 4, 11, or 14.

Discrete energy distribution

Number of particles: 8, (18), weight: 9, (19)

A discrete energy distribution is defined by giving each energy point and the source particle generation probability at that point.

Discrete plus integral-type continuous energy distribution

Number of particles: 22, (32), weight: 23, (33)

An arbitrary energy distribution is defined by giving the lower and upper bounds of each energy bin and the integrated value of the source particle generation probability in that bin.

Differential-type Gaussian distribution

Number of particles: 2, (12)

A differential source spectrum is given as a Gaussian distribution by specifying the central value and the full width at half maximum.

Differential-type Maxwell distribution

Number of particles: 3, weight: 7

A differential source spectrum is given as a Maxwell distribution by specifying the temperature parameter.

Differential-type arbitrary function distribution

Number of particles: 5, (15), weight: 6, (16)

A differential source spectrum is given by an arbitrary function.

Energy distribution from an RI source

Number of particles: 28, weight: 29

By giving the radionuclide and its radioactivity, radiation emitted from the decay of that nuclide is used as the source.

Energy distribution using tally results

Number of particles: 20

An energy distribution obtained by tally is specified as the source distribution.

Energy and angular distribution of cosmic-ray flux

Number of particles: 25, weight: 26

A source that reproduces the energy and angular distributions of cosmic-ray flux in space and in the atmosphere, including the absolute value.