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Ionising dose model SHIELDOSE

Overview

SHIELDOSE [Seltzer, 1980] is a computer code for space-shielding radiation dose calculations. It determines the absorbed dose as a function of depth in aluminium shielding material of spacecraft, given the electron and proton fluences encountered in orbit. The code makes use of precalculated, mono-energetic depth-dose data for an isotropic, broad-beam fluence of radiation incident on uniform aluminium plane media. Such data are particularly suitable for routine dose predictions in situations where the geometrical and compositional complexities of the spacecraft are not known. Furthermore, the restriction to these rather simple geometries has allowed for the development of accurate electron and electron-bremsstrahlung data sets based on detailed transport calculations rather than on more approximate methods.

SHIELDOSE Calculates, for arbitrary proton and electron incident spectra, the dose absorbed in small volumes of different detector materials for the following aluminium shield geometries:

  1. in a semi-infinite plane medium, as a function of depth; irradiation is from one side only (the assumed infinite backing effectively insures this).
  2. at the transmission surface of a plane slab, as a function of slab thickness; irradiation is from one side only.
  3. at the centre of a solid sphere, as a function of sphere radius; irradiation is from all directions.
The shield geometries are illustrated in Fig. 1.

Shielding configurations
Figure 1. Shielding configurations.

A set of depth-dose distributions for aluminium shields were generated for mono-energetic sources. The distributions were smoothed in depth and incident energy. The overall uncertainty of the final results, including the uncertainties in cross sections used and those due to statistical fluctuations and smoothing of the Monte Carlo data, is estimated to be 10%.

The mono-energetic source results were incorporated into the computer code SHIELDOSE which performs the necessary interpolation and integration for any incident spectrum.

The aluminium dose expressed in rads, as a function of incident energy for fixed depths, is shown for a semi-infinite medium in Fig. 2 for protons (in the straight-ahead approximation, the absorbed dose in a semi-infinite plane medium is equal to the dose at the transmission surface of a plane slab) and in Fig. 3 for electrons.

Proton doses
Figure 2. Proton dose (per unit 2pi isotropic incident fluence) for fixed depth z in a semi-infinite aluminium medium, as a function of incident energy.

Electron doses
Figure 3. Electron dose (per unit 2pi isotropic incident fluence) for fixed depth z in a semi-infinite aluminium medium, as a function of incident energy. The dashed curve is the zero-depth dose based only on the stopping power and neglects backscattering from greater depths.

Method of calculation

Electrons and bremsstrahlung

The electron calculations made use of the Monte Carlo code ETRAN which treats the following processes:
  1. electron energy loss, including energy loss straggling (fluctuations) due both to multiple inelastic scattering by atomic electrons and to the emission of bremsstrahlung photons;
  2. angular deflections of electrons due to multiple elastic scattering by atoms;
  3. penetration and diffusion of the secondary bremsstrahlung photons;
  4. penetration and diffusion of energetic secondary electrons produced in electron-electron knock-on collisions (delta rays) and in the interaction of bremsstrahlung photons with the medium (pair, Compton, and photoelectrons).

Details of the Monte Carlo model and of the cross sections used can be found in Berger and Seltzer [1968a, 1968b, 1970, 1974], along with numerous comparisons with experimental results. The ETRAN bremsstrahlung results, based on the use of a set of empirically corrected Bethe-Heitler bremsstrahlung cross sections, were adjusted to reflect the exact calculations of the bremsstrahlung production cross section of Pratt et al. [1977]. Comparisons , using both the ETRAN and Pratt cross section data sets for aluminium, of the electron mean energy loss per unit pathlength due to the emission of bremsstrahlung and of the total energy radiated in the course of slowing down (in the continuous-slowing-down approximation) give a multiplicative correction factor for the ETRAN results that is very close to unity for electron source energies above 1 MeV, and is equal to 0.96, 0.94, 0.92, 0.89, and 0.82 at 0.5, 0.2, 0.1, 0.05, and 0.02 MeV, respectively.

Protons

The treatment of protons was limited to Coulomb interactions but neglected nuclear interactions. The error incurred by this simplification is generally no greater than 10-20% for shields up to ~30 g cm-2.

The proton calculations were done in the straight-ahead, continuous-slowing-down approximation using the stopping power and range data of Barkas and Berger [1964]. Alsmiller et al. [1969] have shown that the neglect of angular deflections and range straggling is negligible in spare-shielding calculations.

Implementation in SPENVIS

Two versions of SHIELDOSE are implemented in SPENVIS: SHIELDOSE-2 Differs from SHIELDOSE mainly in that it contains new cross sections and supports several new detector materials, and has a better treatment of proton nuclear interactions.

Availability

SHIELDOSE Can be retrieved from NSSDC's anonymous FTP site, and SHIELDOSE-2 from the NIST FTP site.

References

Alsmiller, R. G., J. Barish, and W. W. Scott, Nucl. Sci. and Enrg., 35, 1969.

Barkas, W. H., and M. J. Berger, NASA Publ. SP-3013, 1964.

Berger, M. J., and S. M. Seltzer, NASA Publ. SP-169, 1968a.

Berger, M. J., and S. M. Seltzer, Computer Code Collection 107, Oak Ridge Shielding Information Center, 1968b.

Berger, M. J., and S. M. Seltzer, Phys. Rev., C2, 621, 1970.

Berger, M. J., and S. M. Seltzer, Nucl. Instr. and Meth., 119, 157, 1974; Seltzer, S. M., National Bureau of Standards Publ. NBS-IR 74457, 1974.

Pratt, R. H., H. K. Tseng, C. M. Lee, and L. Kissel, At. Data and Nucl. Data Tables, 20, 175, 1977.

Seltzer, S. M., SHIELDOSE, A Computer Code for Space-Shielding Radiation Dose Calculations, National Bureau of Standards, NBS Technical Note 1116, U.S. Government Printing Office, Washington, D.C., 1980.

Seltzer, S. M., Electron, Electron-Bremsstrahlung, and Proton Depth-Dose Data for Space-Shielding Applications, IEEE Trans. Nuclear Sci., 26, 4896, 1979.

IEEE Trans. Nucl. Sci., 33, 1292, 1986.

Seltzer, S. M., Updated calculations for routine space-shielding radiation dose estimates: SHIELDOSE-2, NIST Publication NISTIR 5477, Gaithersburg, MD., December 1994.

Last update: Mon, 12 Mar 2018