NBSIR 86-3429

Transport of Electrons and

Associated Bremsstralung Through

A Composite Aluminum-Lead

Shield, With Applications to

Spacecraft Shielding

Gideon Barnea, Stephen M. Seltzer and Martin J. Berger

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Radiation Research

Gaithersburg, MD 20899

July 1986

Prepared for: Office of Health and Environmental Research

Department of Energy

Washington, DC 20545

and

ice of Naval Research

QC ington, Virginia 22217

100

.U56

NO. 86-3429

1986 c.,%.

NBSIR 86-3429

TRANSPORT OF ELECTRONS AND

ASSOCIATED BREMSSTRALUNG THROUGH

A COMPOSITE ALUMINUM-LEAD

SHIELD, WITH APPLICATIONS TO

SPACECRAFT SHIELDING

Gideon Barnea, Stephen M. Seltzer and Martin J. Berger

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Radiation Research

Gaithersburg, MD 20899

July 1986

Prepared for: Office of Health and Environmental Research

Department of Energy

Washington, DC 20545

and

Office of Naval Research

Arlington, Virginia 22217

U S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director

TRANSPORT OF ELECTRONS AND ASSOCIATED BREMSSTRAHLUNG THROUGH A COMPOSITE

ALUMINUM-LEAD SHIELD, WITH APPLICATIONS TO SPACECRAFT SHIELDING

if if Gideon Barnea, Stephen M. Seltzer and Martin J. Berger

Center for Radiation Research

National Bureau of Standards

Gaithersburg, Maryland 20899

ABSTRACT

Monte Carlo calculations have been made of the stopping of

electrons and the penetration of secondary bremsstrahl ung through

layered aluminum-lead spacecraft walls. The results are presented

in terms of the resultant radiation dose to objects inside. Dose

values for monoenergetic incident electrons are given as a function

of the aluminum/lead thickness ratio. These data, integrated over a few typical Earth-orbit electron spectra, demonstrate the

substantial reduction in radiation dose that can be achieved by

replacing a portion of an aluminum shield with an inner layer of

lead. The main results were obtained by applying a comp! ex -geometry

code to spherical -shell configurations. It was found that these

results could be reasonably well approximated by an alternative and more economical approach, involving the use of slab-geometry

transport results.

KEYWORDS: absorbed dose; bremsstrahl ung ; composite shield; electrons; Monte

Carlo calculation; space shielding;

* , This work was carried out with support from the Office of Naval Research,

and the Department of Energy (Office of Health and Environmental Research).

**

On sabbatical leave from Raphael Laboratories, Haifa, Israel.

1. INTRODUCTION

In a recent paper [1],^ we reported on calculations of the penetration of

electrons and their bremsstrahl ung through spacecraft walls and the resultant

radiation dose to objects inside. These calculations were done with a general -geometry , electron/photon transport Monte Carlo code. Results were presented for four incident electron spectra, typical of those that might be

encountered in Earth orbit. The present report extends this work in two

respects: a) results are given for monoenergeti c electron beams, which can be

used to obtain results for any incident flux spectrum, and b) some comparisons

are made with results from a more-efficient, approximate method of

cal cul ati on

The calculations were done for application to the near-Earth radiation

environment which contains electrons with kinetic energies less than about

6 MeV. We consider an omni-directional flux of electrons incident on spacecraft walls, typically made of aluminum. The mass thicknesses of the

wall (shield) assumed here, 3 and 5 g/cm 2

, are such that nearly all the

incident electrons are stopped in the wall. However, part of the electron

energy is converted into bremsstrahl ung photons which can penetrate the

shield. The efficiency for the conversion of electron energy to bremsstrahlung energy is roughly proportional to the atomic number Z of the medium.

Even though this efficiency is relatively low in aluminum, the bremsstrahl ung

photons may nevertheless constitute a significant radiation hazard. The

penetration of this secondary radiation could be reduced by merely increasing

the mass thickness of the wall, but this would result in heavier spacecraft.

Instead, our calculations were done to determine the extent to which the

radiation dose from transmitted bremsstrahl ung can be reduced by replacing the

aluminum shield with a two-layer aluminum/lead shield, without an increase in

the total mass thickness of the shield. In this case, the outer aluminum

(low-Z) layer serves to slow down the incident electrons, while limiting the

production of bremsstrahl ung photons to relatively low levels. The inner lead

(high-Z) layer then serves to absorb the bremsstrahl ung photons as much as possible. For the purpose of this study, we calculate the absorbed dose

(energy deposited divided by the mass) in a 40-cm diameter water sphere

located inside relatively simple shell structures used to represent the

spacecraft.

The primary aim of this work was to develop quantitative information on the ability to reduce the bremsstrahl ung dose inside the spacecraft by

replacing an inner layer of the aluminum wall with the same mass thickness of

lead, assuming a total mass thickness for the wall of either 3 or 5 g/cm 2

. For this purpose we used a state-of-the-art computer code which accurately

calculates the transport of electrons and bremsstrahl ung through matter and

which handles the general problem of multi -material , three-dimensional

yeometries. Such a code requires substantial computing time not only because

it must accurately treat the complex radiation transport processes involved,

but also because it must handle the complicated geometry. Therefore, a secondary purpose was to see to what accuracy correspondi ng results obtained

in simple slab geometry, using the same transport model, could be applied to

these three-dimensional cases, with much less computational effort.

^Numbers in brackets indicate the literature references at the end of this

document

1
The methods of calculation are indicated in section 2. Section 3 gives

results of test calculations which indicate the sensitivity of the dose to the

shape and inner dimensions of the shield. In section 4, the dose for mono- energetic fluxes of electrons with incident energies of 0.2, 0.5, 1, 2, 3, 4,

5, and 6 MeV is given, as a function of the fraction of the shield mass thickness that is lead, for a total mass thickness of the shield of both 3 and

5 g/cm2

. In section 5, dose rates are presented in a similar fashion for some typical space missions, obtained by integrating the monoenergetic results over

the appropriate orbit- averaged flux spectra. Throughout, we also offer for

comparison some corresponding results obtained by transforming simple slab

results to apply to the same geometric configuration used in the direct

cal cul ati ons

.

2. METHODS OF CALCULATION

All calculations were made using codes based on the ETRAN Monte Carlo

electron/photon transport model [2,3]. This model has previously been used in

space-shielding calculations done for simple, uniform slabs [4,5]. The model

treats in detail the penetration, diffusion and slowing down of the electrons,

the production of bremsstrahl ung in successive Coulomb collisions, and the

further penetration of the secondary bremsstrahl ung photons. The direct calculations for the three-dimensional, multi -material configurations were done

with the ACCEPT code [6], which couples the ETRAN model to a combinatorial

geometry program. The use of combinatorial geometry enables ACCEPT to handle

quite complicated configurations. The present calculations were restricted to

spherical and cylindrical shells, representing the spacecraft walls, surrounding the 40-cm diameter water target sphere.

The calculations in simple, multi-layer slab geometry were done using the

ETRAN-based ZTRAN Monte Carlo code [7], a one-dimensional calculation which

takes into account only the spatial variable measured normal to the plane

boundary interfaces. The transformation of the slab results to the threedimensional case was accomplished using a procedure whose details are given

elsewhere [8]. Briefly, this procedure is based on the assumption of (i) a uniform, isotropic flux of radiation incident on a homogeneous target, either

a semi-infinite slab or a solid sphere, and (ii) the existence of a function,

applicable to both the slab and the sphere, that describes the energy

deposited by the electrons and secondary bremsstrahl ung as if the incident

radiation penetrates along the direction of incidence. As applied to the

geometry considered here, the energy deposited in the water target sphere is

given in terms of an appropriate integral over the depth-dose distribution in

a slab composed of the layered shield followed by a thick water layer. The

integral is multiplied by a geometric factor which is introduced to take into

account the void region that is present in our three-dimensional cases.

3. DEPENDENCE ON THE SPACECRAFT SHAPE AND INNER DIMENSIONS

To study the sensitivity of the dose inside the spacecraft to its shape

and inner dimensions, several calculations were made for various size

spherical and cylindrical shells. In all cases the walls were assumed to be

1 cm (2.7 g/cm 2

) thick aluminum, irradiated by 2-MeV electrons. In the Monte

Carlo calculations, cut-off energies (below which histories are no longer

followed) were chosen to be 0.1 MeV for electrons and 0.01 MeV for photons.

2
Table 1 gives the directly-calculated energy deposition (normalized to an incident fluence of one electron per cm 2

) in the 40-cm diameter water sphere

located at the center of the shell. The results indicate that the dose is

rather insensitive to the shape and inner dimensions of the shell: a change of

nearly an order in magnitude in the inner dimensions causes a change of less

than 1U percent in the dose. Such a finding tends to confirm the expectation

that the isotropic flux of electrons incident on the outside surface of the

uniformly-thick shell produces a flux of bremsstrahl ung photons emerging from

the inside surface of the shell that is effectively isotropic, so that the

1/r 2 decrease in the dose at the center of the shell is largely compensated

for by the increase in the shell surface area as the radius of the shell is

made larger. Moreover, if this is the case, one can consider the problem as a detector sphere located in a region of isotropic photon flux, so that the dose

is also independent of its position in that region. This was investigated by means of a few calculations which vary the position of the water sphere within

a shell. The results, given in Table 2, indeed show little sensitivity to the

position of the water sphere.

The results from the transformation of simple slab results are also given

in Table 1, and agree fairly well with those from the direct calculations.

The agreement is in good part due to the fact that the high degree of isotropy

of the generated photon flux mirrors the isotropy of the incident electron

flux, which conforms well with the type of straight-ahead approximation underlying the transformation procedure.

Because the results are relatively independent of the inner dimension of

the shell and of the position of the water sphere within, all subsequent

calculations were made for a spherical shield with an outer diameter of

1 meter, concentric with the 40-cm water target sphere (see fig. 1). This

configuration was adopted because it takes into account the realistic case of

a void region between the shield and the target, while keeping the Monte Carlo

calculation statistically efficient (i.e., making it sufficiently likely that

the sampled bremsstrahlung photon trajectories actually traverse the water

sphere)

4. MONOENERGETIC INCIDENT ELECTRONS

The energy deposited in the water target sphere was calculated for monoenergetic, omni-directional incident electrons as a function of the fraction

of the shield mass thickness that is lead. The total mass thickness of the

shield was kept constant at 3 g/cm 2 or 5 g/cm 2

, and the mass thickness of the

outer aluminum layer was varied from the total mass thickness down to

0.1 g/cm 2. Calculations were done for incident electron kinetic energies of

6, 5, 4, 3, 2, 1, 0.5, and 0.2 MeV. Lower incident electron energies were not

considered because, for the spectra likely to be encountered in near-Earth

orbit, the contribution to the dose by electrons with energies below 0.2 MeV

is negligible for our geometry. The cut-off energies were chosen to be either

5 percent of the incident energy or 0.2 MeV, whichever was lowest. Each of

the ACCEPT calculations was based on a sample of from 5000 to 10000 incident

electron histories and on from 125,000 to 250,000 generated bremsstrahl ung

photon histories. The statistical uncertainty of the calculated energy

deposited in the water target sphere is estimated to be 5-10 percent.

3
The results of the calculations are given in figures 2a and b for the

shield with a total mass thickness of 3 g/cm 2

, and in figure 3 for 5 g/cm 2

. The small vertical arrows in figures 2 and 3 indicate that fractional mass thickness for which the thickness of the outer aluminum layer is equal to the

incident electron range (in the conti nuous-sl owi ng-down approximation). In most cases, an increase in the fractional mass thickness of the lead layer

produces a similar effect. At first there is a gradual decrease in the energy

deposited, due to an increase in the attenuation of the bremsstrahl ung by the

lead. This decrease slows at intermediate fractional thicknesses, until at

large fractions — where the aluminum thickness is less than the electron range — a sharp increase in the dose occurs. This increase is due to the fact that

for such thin aluminum layers, a substantial number of electrons penetrate

into the lead layer which is a much more efficient bremsstrahl ung radiator

than is aluminum. In the case of 0.2-MeV incident electrons, no such increase

is observed because even the thinnest aluminum layer considered (0.1 g/cm 2

) is

sufficient to stop all the electrons.

For a number of cases in which the total mass thickness of the shield is

3 g/cm 2

, depth-dose distributions obtained from simple slab calculations were transformed to the spherical geometry. These results are shown as the open

circles in figures 2a and b. The agreement with the results from the direct

calculations is reasonably good (typically 10-20 percent), both in respect to

the absolute value of the dose in the three-dimensional geometry and in

respect to the relative trends of the curves.

5. RESULTS FOR SOME REALISTIC INCIDENT ELECTRON SPECTRA

The foregoing monoenergetic results can be applied to any electron

spectrum of interest. Let F(E) be the energy deposited in the water sphere by

electrons incident with kinetic energy E, per unit incident electron fluence,

with F(E) in units of keV/ (el ectron cm* 2

) (see Figs. 2 and 3 ). Let <j>(E) be

the full-space (4n) incident electron flux, differential in incident electron

kinetic energy E, in units of electron keV* 1 cm* 2 sec -1. Let M be the mass in

grams of the water target sphere (M = 33510 g). Then, for the flux spectrum

4>(E), the absorbed dose D[<j»(E)] in the water sphere, in units of rads/sec is

given by

0U(E)] = £ / »(E)F(E)dE , (1)

where the constant K = 1.6022 x 10" 11 rads/(keV g

-1

).

We have evaluated Eq. (1) for spectra associated with three space shuttle

orbits (inclinations of 28°, 57° and 90°, altitude of 300 km), and one geosynchronous orbit (0° inclination, 160° parking longitude, and 35790 km

altitude). The orbit-averaged spectra are shown in figure 4, and were kindly

provided by E. G. Stassinopoulos of NASA/GSFC, based on current radiationenvironment models and orbit-integration codes [9,10]. The integral in

Eq. (1) was evaluated numerically using Simpson's rule for a fine grid of

energies, and using cubic-spline interpolation to obtain values of $(E) and

F(E) on the energy grid.

*Note that Figs. 2 and 3 give the energy deposited per unit incident electron.

Therefore, F(E) is obtained by dividing those results by 4 (see, e.g..

Ref. [4]).

4
The results for each of the orbits considered are given in terms of the

average dose rate to the water target sphere as a function of the fraction of

the shield mass thickness that is lead. Figure 5 gives the results for a shield with a total mass thickness of 3 g/cm 2

, and figure 6 for 5 g/cm 2

. There are fairly broad minima in the dose-rate curves which indicate, for each

of the total mass thicknesses, those fractions of lead that furnish the

maximum shielding. For all the cases considered here, these results indicate

an optimal thickness for the aluminum layer of about 1 g/cm 2 (actually from

0.7 to 1.3 g/cm 2

)

The relative effectiveness of the lead is shown in figure 7a for the

3-g/cm 2 shield, and in figure 7b for 5 g/cm 2

, which give the dose reduction

with respect to an all-aluminum shield, as a function of the thickness of the

lead layer. It can be seen that replacing a portion of aluminum by a layer of

lead only 1-mm thick can reduce the dose by from 30 to 60 percent, depending

on the orbit and on the total mass thickness of the shield. A thicker lead

layer can reduce the dose by as much as 75 percent without an increase in the

total mass thickness of the shield.

The results obtained from the transformation of slab results for mono- energetic electrons were too few to be able to integrate numerically for the

various incident spectra. However, the integration could be carried out

implicitly within a single ZTRAN calculation. This was done for the geosynchronous-orbit spectrum in two cases: a shield of 3-g/cm 2 aluminum and a twolayer shield of 1.5-g/cm 2 aluminum followed by 1.5-g/cm 2 lead. The result for

3 g/cm 2 of aluminum is 259 mrad/day, only about 16 percent higher than the

dose of 223 mrad/day obtained from the direct calculations. For the two-layer

shield, the transformed ZTRAN dose is 84 mrad/day which is 9 percent higher

than the 77 mrad/day from the ACCEPT calculations (see fig. 5c). The slab

results thus predict for this two-layer shield a relative dose reduction of

68 percent, which is in good agreement with the reduction of 65 percent

obtained from the direct calculations.

6. CONCLUSION

Our calculations demonstrate that the bremsstrahl ung dose inside

spacecraft can be substantially reduced through the use of graded shields,

without an increase in the shield mass. Such a notion is not original [11].

However, the present results, from a state-of-the-art Monte Carlo code which

accurately treats the radiation transport processes and the geometry, provide

a firm quantitative foundation. These data, in addition to being directly

applicable to the configurations considered here, are valuable also as benchmark data against which other more-approximate space-shielding

calculations can be checked. Thus, we have found for the class of problems

considered here that the use of more-computational ly-efficient slab

calculations can give results which agree with these direct calculations to

within 10-20 percent.

ACKNOWLEDGEMENTS

One of us (G.B.) wishes to thank the National Bureau of Standards for the

hospitality afforded during the course of this work.

5
REFERENCES

[1] G. Barnea, M.J. Berger and S.M. Seltzer, J. Spacecraft and Rockets, in

press

.

[2] M.J. Berger and S.M. Seltzer, Computer Code Collection CCC-107, Radiation

Shielding Information Center, Oak Ridge National Laboratory, Oak Ridge,

TN (1968).

[3] M.J. Berger, Methods in Computational Physics, Vol . 1 (Academic Press,

New York, 1963) 135.

[4] S.M. Seltzer, IEEE Trans. Nucl . Sci . NS-26 , 21 (1979).

[5] S.M. Seltzer, National Bureau of Standards Technical Note 1116 (1980).

[6] J.A. Halbleib, Sandia National Laboratories Report SANO 79-0415 (1979).

[7] S.M. Seltzer and M.J. Berger, National Bureau of Standards Report

NBSIR 84-2931 (1984).

[8] S.M. Seltzer, IEEE Conf. on Nuclear and Space Radiation Effects,

Providence, RI (1986); submitted to IEEE Trans. Nucl. Sci. (1986).

[9] E.G. Stassinopoul os, J. of Spacecraft and Rockets, T7, 145 (1980).

[10] E.G. Stassinopoulos and J.M. Barth, to be submitted to J. of Spacecraft

and Rockets ( 1986) .

[11] See, e.g., M.L. Rossi and M.C. Stauber, IEEE Trans Nucl. Sci. NS-2 4 (1977) 2248.

6
Table 1. Dependence of the energy deposited in the water target sphere on the

shape and dimensions of the aluminum shield. The results pertain to

an isotropic flux of 2-MeV electrons incident on either spherical or cylindrical aluminum shells 1 cm thick, with the 40-cm diameter

water target sphere located at the center of the shell. The

directly-calculated results were obtained with the ACCEPT code; the

slab results, converted to apply approximately to the threedimensional configuration, were obtained with the ZTRAN code.

Energy deposited per unit

incident electron flux

(MeV cm 2 elec -1

)

Shape and From

Shield Outside Dimension(s) ACCEPT ZTRAN

1 sphere, 42-cm di am. 10.1 11.7

2 sphere, 100-cm diam. 9.90 11.6

3 sphere, 200-cm diam. 9.75 11.4

4 sphere, 300-cm diam. 9.68 11.4

5 sphere, 400-cm diam. 9.53 11.3

6 cylinder 9

, 126.5-cm diam.

253.0-cm height 9.83 11.2

7 cylinder^, 253-cm diam.

506-cm height 9.25 11.4

a Ratio of height to diameter is 2; same outside surface area as shield

b Ratio of height to diameter is 2; same outside surface area as shield 5.

7
Table 2. Dependence of the energy deposited in the 40-cm diameter water

target sphere on the position of the target within the shield. The

results were calculated directly with the ACCEPT code and pertain to

an isotropic flux of 2-MeV electrons incident on either a spherical

or cylindrical aluminum shell 1 cm thick.

A. Spherical shell: outside diameter 300 cm

Distance from center of shell

to center of target sphere

0

125 cm

Energy deposited per unit

incident electron flux

(MeV cm 2 elec -1

)

9.68

10.8

B. Cylindrical shell: outside diameter 253 cm, height 506 cm

Distance along cylinder axis Energy deposited per unit

from center of shell to incident electron flux

center of target sphere (MeV cm 2 elec -1

)

0

100 cm

232 cm

9.25

9.45

10.3

8
FIGURE CAPTIONS

Fig. 1. Schemati zation of spacecraft for shielding calculations.

Fig. 2. Average energy deposited in the central water sphere due to an isotropic flux of monoenergetic electrons incident on an outer shield of mass thickness 3 g/cm 2

. The results are given as a function of the

fractional mass thickness of the lead layer in the shield, and are normalized to one incident electron. The solid points, fit by the

solid curves, represent results from the ACCEPT code. The open

circles represent slab results from the ZTRAN code, converted to apply

approximately to the three-dimensional configuration. The arrows indicate the fractional thicknesses which correspond to the

continuous-slowing-down-approximation values of the electron range in

al umi num.

a. Incident electron energies: 0.2 and 0.5 MeV.

b. Incident electron energies: 1, 2, 3, 4, 5, and 6 MeV.

Fig. 3. Average energy deposited in the central water sphere due to an isotropic flux of monoenergetic electrons incident on an outer shield of mass thickness 5 g/cm 2

. The results are given as a function of the

fractional mass thickness of the lead layer in the shield, and are normalized to one incident electron. The points (solid and open), fit

by the solid curves, represent results from the ACCEPT code. The arrows indicate the fractional thicknesses which correspond to the

continuous-slowing-down-approximation values of the electron range in

al umi num.

Fig. 4. Average differential flux of incident electrons for selected shuttle

and geosynchronous orbits. Orbit parameters are indicated in the

fi gure.

Fig. 5. Average dose rates in the central water sphere due to various spectra

of electrons incident on an outer shield of mass thickness 3 g/cm 2

. The results, obtained with the ACCEPT code, are given as a function of

the fractional mass thickness of the lead layer in the shield.

a. Shuttle orbits: 300 km altitude and inclinations of 57° and

90°.

b. Shuttle orbit: 300 km altitude and 28° inclination.

c. Geosynchronous orbit: 35790 km altitude, 0° inclination, and

160°W parking longitude. The single open circle represents

slab results from the ZTRAN code, converted to apply approximately to the three-dimensional configuration.

Fig. 6. Average dose rates in the central water sphere due to various spectra

of electrons incident on an outer shield of mass thickness 5 g/cm 2

. The results, obtained with the ACCEPT code, are given as a function of

the fractional mass thickness of the lead layer in the shield.

a. Shuttle orbits: 300 km altitude and inclinations of 57° and

90°.

b. Shuttle orbit: 300 km altitude and 28° inclination.

c. Geosynchronous orbit: 35790 km altitude, 0° inclination, and

160°W parking longitude.

9
Fig. 7. Percent dose reduction relative to the case of an all- aluminum

shield. The reduction of the dose to the central water sphere is

given as a function of the mass thickness of the lead layer in the

aluminum/lead shield whose total mass thickness is kept constant.

Curves A - Shuttle orbit: 300 km altitude and 57° inclination.

Curves B - Curves C - Curves D -Shuttle orbit: 300 km altitude and 90° inclination.

Shuttle orbit: 300 km altitude and 28° inclination.

Geosynchronous orbit: 35790 km altitude, 0° inclination,

and 160°W parking longitude.

a. Total

b. Total mass thickness of the shield is 3 g/cm 2

. mass thickness of the shield is 5 g/cm 2

.

10
LEAD

Fig. 1
DOSE

IN

WATER

PHANTOM,

keV•

electron

Fig. 2a

lill

I... IIIII

Mil
DOSE

IN

WATER

PHANTOM,

keVelectron

Fig. 2b

DOSE

IN

WATER

PHANTOM,

keV

•

electron”

Fig. 3
AVERAGED

DIFFERENTIAL

FLUX

ernes'

keV

Fig. 4
LU

H

<

OC

LU

COO

O

01 l I l I I I I I I L_

0 0.2 0.4 0.6 0.8 1.0

FRACTIONAL MASS THICKNESS

Fig. 5a
FRACTIONAL MASS THICKNESS

Fig. 5b
FRACTIONAL MASS THICKNESS

Fig. 5c
DOSE

RATE

[m

rad

(water

)/day]

FRACTIONAL MASS THICKNESS

Fig. 6a
FRACTIONAL MASS THICKNESS

Fig. 6b
DOSE

RATE

[mrad

(water

)/day]

Fig. 6c
80

60

40

20

0

A, B

1 mm 2 mm

I 2

LEAD MASS THICKNESS (g/cm 2

)

Fig. 7a
PERCENT

DOSE

REDUCTION

80

LEAD MASS THICKNESS (g/cm 2

)

Fig. 7b
NBS-114A (REV. 2-BC) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) NBSIR 86-3429 AUGUST 1986

4. TITLE AND SUBTITLE

TRANSPORT OF ELECTRONS AND ASSOCIATED BREMSSTRAHLUNG THROUGH A COMPOSITE

ALUMINUM-LEAD SHIELD WITH APPLICATIONS TO SPACECRAFT SHIELDING

5. AUTHOR(S)

Gideon Barnea, Stephen M. Seltzer and Martin J. Berger

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

10. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT ( A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Monte Carlo calculations have been made of the stopping of electrons and

the penetration of secondary bremsstrahl ung through layered aluminum-lead

spacecraft walls. The results are presented in terms of the resultant

radiation dose to objects inside. Dose values for monoenergetic incident

electrons are given as a function of the aluminum/lead thickness ratio. These

data, integrated over a few typical Earth-orbit electron spectra, demonstrate

the substantial reduction in radiation dose that can be achieved by replacing

a portion of an aluminum shield with an inner layer of lead. The main results

were obtained by applying a complex-geometry code to spheri cal -shell

configurations. It was found that these results could be reasonably well

approximated by an alternative and more economical approach, involving the use

of slab-geometry transport results.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) absorbed dose; bremsstrahl ung ; composite shield; electrons; Monte Carlo

calculation; space shielding;

13. AVAILABILITY 14. NO. OF

Uni imited PRINTED PAGES

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