PenNuc: Monte Carlo simulation of the decay of radionuclides

The Fortran subroutine package PenNuc that performs Monte Carlo simulation of the decay of radioactive nuclides is described. The adopted nuclear decay characteristics (possible disintegration modes and branching ratios, energies and half-lives of nuclear energy levels, types and energies of emitted particles) are from the NUCLEIDE evaluated database ( http://www.nucleide.org/DDEP_WG/DDEPdata.htm) in the specific format PenNuc. The initial energies of electrons and positrons emitted in beta disintegrations are sampled from their continuous spectra by means of the RITA (rational inverse transform with aliasing) algorithm, which is described in the Appendix. Electronic relaxation of the residual ion after electron capture and internal conversion is simulated by using the data and sampling methods of the PENELOPE code system. At each call the subroutines deliver a list of state variables of the particles (photons, electrons, positrons and alphas) emitted in a random decay path of the considered radionuclide down to a metastable level or to the ground state of the daughter nucleus. The distribution package includes a main program that simulates the emission spectrum of a radionuclide and, optionally, produces a list of particles emitted in a prescribed number of decays.

本文详述了一款用于放射性核素衰变蒙特卡洛模拟的Fortran子程序包PenNuc。
该包所采用的核衰变特性（包括可能的衰变模式与分支比、核能级的能量与半衰期、出射粒子的类型与能量）源自以PenNuc专用格式存储的NUCLEIDE评价数据库（网址：http://www.nucleide.org/DDEP_WG/DDEPdata.htm）。
β衰变过程中产生的电子与正电子的初始能量，通过附录中详述的RITA（混叠有理逆变换，rational inverse transform with aliasing）算法从其连续能谱中采样得到。
电子俘获与内转换过程后残留离子的电子弛豫过程，通过调用PENELOPE代码系统的数据集与采样方法进行模拟。
每次调用该子程序时，程序将输出光子、电子、正电子与α粒子等出射粒子的状态变量列表，这些粒子由所考虑的放射性核素沿随机衰变路径衰变至子核的亚稳能级或基态的过程中产生。
该分发包附带一款主程序，可模拟放射性核素的发射谱，还可根据需求生成指定衰变次数下的出射粒子列表。