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1、, b95534 0003220 57T U Special Copvright Notice o I999 by the American Institute of Aeronautics and Astronautics. All rights reserved. Copyright American Institute of Aeronautics and Astronautics Provided by IHS under license with AIAA Licensee=IHS Employees/1111111001, User=Wing, Bernie Not for Res
2、ale, 04/18/2007 03:27:31 MDTNo reproduction or networking permitted without license from IHS -,-,- AIAA G-083-1999 Guide to Modeling Earths Trapped Radiation Environment Copyright American Institute of Aeronautics and Astronautics Provided by IHS under license with AIAA Licensee=IHS Employees/111111
3、1001, User=Wing, Bernie Not for Resale, 04/18/2007 03:27:31 MDTNo reproduction or networking permitted without license from IHS -,-,- AIAA G-083-1999 Guide Guide To Modeling Earths Trapped Radiation Environment Sponsor American Institute of Aeronautics and Astronautics Abstract This Guide serves as
4、both an introduction to the phenomena of radiation in the space environment and the product engineering issues facing spacecraft designers. Emphasis is on the trapped radiation environment of the Earth which is known as the Van Allen Belts. The leading empirical models are described and the problems
5、 in using them are identified. Current radiation modeling efforts are also discussed, along with shielding design and optimization. The Guide is intended for students, designers, mission planners, and others who need a ready understanding of this critical issue affecting spacecraft performance in Ea
6、rth orbit. Copyright American Institute of Aeronautics and Astronautics Provided by IHS under license with AIAA Licensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 04/18/2007 03:27:31 MDTNo reproduction or networking permitted without license from IHS -,-,- AIAA G-083-1999 Library o
7、f Congress Cataloging-in-Publication AIAA guide to modeling earths trapped radiation environment/sponsor, American Institute of Aeronautics and Astronautics p. cm. “AIAA G-083-1999” Includes bibliographical references ISBN 1-56347-349-6 (softcover), 1-56347-367-4 (electronic) 1. Van Allen radiation
8、belts-Mathematical models. 2. Magnetohydrodynamics-Mathematical models. I. American Institute of Aeronautics and Astronautics. QC809.V3G85 1999 538 ,766-dc21 99-35575 CI P Published by American Institute of Astronautics and Aeronautics 1801 Alexander Bell Drive, Suite 500, Reston, VA 20191 Copyright
9、 O 1999 American Institute of Aeronautics and Astronautics All rights reserved. No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without prior written permission of the publisher. Printed in the United States of America. II Copyright American
10、 Institute of Aeronautics and Astronautics Provided by IHS under license with AIAA Licensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 04/18/2007 03:27:31 MDTNo reproduction or networking permitted without license from IHS -,-,- AIAA G-083-1999 Contents Foreword . v 1. Introduction
11、2. The Space Radiation Environment: Basic Concepts 2 3. The Trapped Radiation Environment 7 3.1 Overview . 7 3.2 Geomagnetic Field 3.3 Magnetic and 3.3.1 Basic Particle Motion 3.3.2 Invariants of the Particle Motion 17 4. AE8 and AP8 Models . 20 5. Problems with AE and AP . 23 5.1 Solar Cycle E 5.2
12、Examples: Lo 5.3 Coverage Limitations 5.4 AE8/AP for shorter missions, factors approaching 10-1 O0 are easily possible). Even given an accurate ?average? description of the environment, short-term variations of several orders of magnitude in dosage and single event upset (SEU) rates have been seen i
13、n the span of hours (e.g., the 1989 solar proton events). Complicating the practical application of the radiation environment to spacecraft design, radiation transport codes and estimates of the effects of radiation damage are often inaccurate. Comparisons between ground tests and in situ measuremen
14、ts have shown significant disagreement. Furthermore, the parts used on the spacecraft can show variations in sensitivity of factors of 2-10, even within the same parts lot. Often, how a system is actually used can mask, or hopefully limit, the effects of radiation damage. Thus, to a degree, mitigati
15、ng radiation effects is a black art and, increasingly, a very expensive art for which any imprecision in the knowledge of the trapped radiation environment becomes a critical component. However, the ultimate solution is a comprehensive process that treats all uncertainties. 1 Copyright American Inst
16、itute of Aeronautics and Astronautics Provided by IHS under license with AIAA Licensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 04/18/2007 03:27:31 MDTNo reproduction or networking permitted without license from IHS -,-,- AIAA G-083-1999 2. The Space Radiation Environment: Basic C
17、oncepts This section provides an overview of the basic physical concepts and definitions that will be used throughout the guide. In particular, the concepts of energy, flux, fluence, and dosage will be briefly described. The reader is referred to the many excellent texts on space physics or astronom
18、y for more detailed explanations. !2 First consider the concept of energy. In the case of particles that have a rest mass, the fundamental equation relating particle mass and velocity to kinetic energy is: E = (y - i)moc2 Relativistically (1 1 Non-Relativistically 1 2 = -moV2 where m = particle rest
19、 mass V = particle velocity c = speed of light E = particle kinetic energy For photons (which have no rest mass), the equivalent equation is: E= hv where h = Plancks constant v = frequency of the light Closely coupled to the concept of energy is that of dose. Simply put, dose is the total energy acc
20、umulated in a given volume element of a specific material due to incident radiation. It is typically given in units of rads or “radiation absorbed dose” for a particular material (the material must be specified because energy absorption is dependent on the material). As an example, for silicon, 1 ra
21、d (Si) = lo- J/kg (Si). It must be emphasized that, for the same incident flux, different materials will be affected differently depending on the composition of the radiation and the composition of the absorbing material. In addition to the energy and composition of a particle or photon, it is also
22、necessary to describe how many of them there are. This is usually done in terms of intensity or flux and, when speaking in terms of a time interval, fluence. Confusion arises over the concepts of intensity/flux and fluence because there are many different ways to define these quantities. Here, we wi
23、ll define the quantity “unidirectional differential intensity” j( E, O, t) as : The flux (number of particles or photons per unit time) of a given energy per unit energy interval dE in a unit solid angle (di2 =27ccos OdOd) about the direction of observation (in the O,direction), incident on unit of
24、surface area (dA) perpendicular to the direction of observation. 2 Copyright American Institute of Aeronautics and Astronautics Provided by IHS under license with AIAA Licensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 04/18/2007 03:27:31 MDTNo reproduction or networking permitted
25、without license from IHS -,-,- AIAA G-083-1999 This is illustrated in Fig. 1 .2 Typical units are particlescm-2si .sri .keV-l for protons or electrons and particles.m-2si .sri .(MeVp-l)-l for heavy ions (where ,u is nucleon). A typical spectrum for iron cosmic rays is presented in Fig. 2.3 In the fi
26、gure, the solid curves are for solar maximum (lower) and solar minimum (upper). The dashed curve is the 90% worst case iron spectrum, which is implied by comparison with the cosmic ray helium spectrum. The “unidirectional integral intensity” (or flux) is defined as the intensity of all particles wit
27、h energy greater than or equal to a threshold energy E: (4) with units of particles cm-2s-isri. We define the “omnidirectional flux” Jas: J = / j d Q 4z Fluence / is the integral of the flux over a given time interval (e.g., one hour, one year): I = / j d t (5) 6t Here, when we refer to omnidirectio
28、nal fluence /(E), we will normally mean the “omnidirectional integral (in energy) fluence” such that: I, = jrn E dE/dQ/ 4z 6t jdt The units of this quantity are particlescm-2 for some specified (6) threshold energy E (typically 1 MeV or higher for radiation effects) and for a specified time interval
29、 (often one year). FLUX Figure 1 - The flux of a given energy per unit energy interval din a unit solid angle about the direction of observation (Copyright by and used by permission of Springer-Verlag, New York) 3 Copyright American Institute of Aeronautics and Astronautics Provided by IHS under lic
30、ense with AIAA Licensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 04/18/2007 03:27:31 MDTNo reproduction or networking permitted without license from IHS -,-,- AIAA G-083-1999 10 n 3 % 10 I -1 U J 10-71 I I I IlIlII I I I 1 1 1 1 1 I 10-8“ , 10 IO2 io3 u io4 io5 KINETIC ENERGY (MeV
31、/u) Figure 2- The iron cosmic ray spectrum To allow comparisons among different energies, particle types, and dosages, it is common practice to talk in terms of “1 -MeV equivalent“ (typically 1 -MeV electrons or 1 -MeV neutrons in silicon). First, the energy dependence of the damage and energy conte
32、nt of the spectra for the environment to be considered are used to determine what fluence of 1-MeV particles (electrons or neutrons) would produce the same amount of damage or dose in the material (typically silicon or aluminum). A curve for neutrons, in units of MeV-mb (where b is a barn or cm2 and
33、 the relative displacement damage is defined in terms of the cross section times the energy of the incident particle), is given in Fig. 3.4 As an illustration, for 14 MeV neutrons, the 1-MeV neutron dose damage equivalent is given by multiplying the 14 MeV dose by 2.5 (obtained from Ref. 4). (Note:
34、because of variations in the damage parameter with material and property, it should always be kept in mind that the use of a damage equivalent is not exact but an approximation for comparison purposes.) 4 Copyright American Institute of Aeronautics and Astronautics Provided by IHS under license with
35、 AIAA Licensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 04/18/2007 03:27:31 MDTNo reproduction or networking permitted without license from IHS -,-,- AIAA G-083-1999 n E 1 o-1 1 oo 1 o1 I O 2 INCIDENT NEUTRON ENERGY (MeV) Figure 3- Neutron displacement damage equivalence curve (Co
36、pyright by and used by permission of Institute of Electrical and Electronics Engineers) A final quantity related to energy absorption and flux is the Linear Energy Transfer (LET). The LET is the energy transferred by radiation per unit length of absorbing material. That is, LET is proportional to dE
37、/dx (note: there is in fact a slight difference between “energy transferred” and “energy lost per unit length” but that will be ignored here). For ionization and excitation effects, LET is often expressed in MeV/pm of the primary particle track length or, if the density of the material is known, MeV
38、cm2.rng-l (this is typically the unit when the reference is to an LET between 1 and 30 and is given by: -). 1 dE P d X The concept of LET is particularly important when discussing single event upsets (SEUS) or “soft errors.” These occur when a particle, typically an ionized, high energy atomic nucle
39、us, deposits enough energy in the sensitive region of an electronic device to cause a change in the logic state of the device. Upsets occur only when the energy deposited exceeds a critical level in the sensitive region of the device. This is often computed in terms of LET. When viewed as a function
40、 of LET, the probability of upset is, in its simplest form, a threshold phenomenon: any particle with a minimum LET of Lo or greater will cause an upset. This behavior is illustrated in Fig. 4 where the energy deposited per unit length (LET) is plotted vs incident particle energy-note how the curve
41、has a peak rate. Lo corresponds to a constant value of LET. As illustrated, there can be multiple values of energy (E, and E, here) that correspond to the same value of LET. A useful way of presenting the environment in terms of LET is the Heinrich curve. The Heinrich curve gives the integral flux a
42、s a function of LET rather than particle energy. The Heinrich flux FH is the flux of particles for a single species with a (threshold) LET of LETo or greater: E, Copyright American Institute of Aeronautics and Astronautics Provided by IHS under license with AIAA Licensee=IHS Employees/1111111001, Us
43、er=Wing, Bernie Not for Resale, 04/18/2007 03:27:31 MDTNo reproduction or networking permitted without license from IHS -,-,- AIAA G-083-1999 where fi is the particle flux for the species i as a function of energy and E, and E2 are the energies between which the LET is greater than or equal to the t
44、hreshold LET, (a representative integral Heinrich curve for iron is plotted in Fig. 55). The LET depends not only on particle energy, but on the target material as well because the LET vs energy curve will be different for all particle species. Experiments have shown, however, that to the first orde
45、r it is the LET that is important for determining upsets and not the particle energy or its species. The Heinrich flux vs LET plot is the principal means of presenting radiation data for use in SEU calculations just as the particle flux vs energy is the main means of presenting radiation data for us
46、e in dosage calculations. Energy Figure 4- Linear Energy Transfer Function (LET) vs Energy lo2J t IRON I I I I IIIII I I I IIIII 1 o5 1 o-6 lo3 lo4 MeV cm /g Figure 5- Heinrich curves for iron cosmic rays 6 Copyright American Institute of Aeronautics and Astronautics Provided by IHS under license wi
47、th AIAA Licensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 04/18/2007 03:27:31 MDTNo reproduction or networking permitted without license from IHS -,-,- AIAA G-083-1999 To summarize, this section has defined the basic terminology used to describe the radiation environment-dose, flu
48、x/intensity, fluence, LET, and 1 -MeV equivalent. The reader is referred to books and articles by Roederer2 and others for more complete descriptions of these concepts. 3. The Trapped Radiation Environment 3.1 Overview By definition, the high energy particle radiation environment in space consists of electrons with energies greater than 40 KeV, protons or neutrons with energies greater than 1 MeV, and heavy ions with energies abov