4. METHOD OF SOLUTION
(A). ELMC employs the Monte Carlo method with angular scattering treated by the method of Leiss, Penner, and Robinson. Energy loss is treated by the continuous slowing down approximation, and energy straggling is not treated. The energy dose and angular deflections are calculated in path length segments of delta x, where delta x can be adjusted by input data and made proportional to particle energy if desired.
(B). The EPEN code calculates the absorbed dose at a point of interest caused by electrons penetrating a shielding system. The penetrating electron energy spectrum is also calculated. Multilayer shields can be treated.
(C). The bremsstrahlung differential energy spectrum produced in a material is estimated by an expression given by Wyard.
The photon energy spectrum is then transported through the remaining shielding material by the use of ray theory plus buildup factors.
Two basic calculational modes are available. In the surface production option, the bremsstrahlung is all produced at the surface of the shield. In the volume production option, the attenuation of the electron spectrum is considered, and the bremsstrahlung source is volume distributed. (BREMS).
(D). The straight ahead approximation is used in HEVPART and nuclear interactions are neglected to provide a rapid solution of the heavy ion transport problem. The range-energy and stopping power tables of Barkas and Berger are used. Low energy correction factors are employed to describe the changes in stopping power resulting from electron capture.
(E). The first collision approximation and the straight ahead approximation are employed in SECPRO to simplify the cascade transport problem. Neutron induced protons are also calculated to refine the neutron dose estimate. The code employs the tabulated Barkas and Berger range energy data and the secondary particle production data of Bertini for numerical integration of the primary and secondary particle fluxes.
(F). The user supplies to TANDE description of a vehicle trajectory and radiation-environment data. The program calculates electron or proton flux rate and time-integrated flux along the trajectory.
The general then compute radiation flux at these points.
Given a description of the orbit and the point of injection, subject trajectory points are calculated as a function of time, using orbital flight equations. The trajectory points are converted to McIlwain's geomagnetic coordinates (B,L, and R, lamda).
Proton or electron flux at each point is determined by a table lookup and interpolation. Numerical integration (in conjunction with an interpolation scheme on B and L) gives a time-integrated flux for each point. A table lookup and interpolation on an array of spectral coefficients determines the spectral coefficients for the point. The flux at the point, dose-conversion factors, and the spectral coefficients are then used to determine dose rate and total dose at the point.
Angular distribution is determined for each trajectory point by solution of a pitch angle distribution function.
The code is designed so that new experimental data on the radiation environment and on the interaction of radiation with matter can be accepted.
The following general methods are followed:
1. calculation of the spacecraft trajectory in B, L, and t coordinates,
2. devising a mathematical representation of the space-radiation environment, including geomagnetically trapped radiation (Van Allen belts), solar particle event radiation, and galactic cosmic radiation;
3. determination of the radiation flux and energy spectra encountered in a given space mission.