N A S A C O N T R A C T O R

R E P O R T

STUDY OF RADIATION HAZARDS TO MAN O N EXTENDED

by S. €3. Curtis dnd M . C. WiZkinson

a Prepared by THE BOEING COMPANY Seattle, Wash.

for

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

MISSIONS

WASHIN

"

TECH LIBRARY KAFB, NM

OOb0402 NASA CR- 1037

STUDY OF RADIATION HAZARDS TO MAN ON EXTENDED MISSIONS

By S. B. Curtis* and M. C. Wilkinson

Distribution of this report is provided in the interest of information exchange. Responsibility for the contents resides in the author or organization that prepared it.

* Consultant, Lawrence Radiation Laboratory

Prepared under Contract No. NASw- 1362 by THE BOEING COMPANY

Seattle, Wash.

for

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

For sale by the Clearinghouse for Federal Scientific and Technical Information Springfield, Virginia 22151 - CFSTI price $3.00

I

ABSTRACT

This study attempts to identify the particle types and energies which are important

i n the evaluation of the radiation hazard on long manned missions outside the

magnetosphere. Important areas where information is presently lacking are

identified. Spectra of the various components of the galact ic cosmic radiation

have been compiled from experimental d a t a gathered d u r i n g the period of minimum

so la r ac t iv i ty . These spectra, as we1 1 as typical solar par t ic le spectra , have

been used t o determine depth-dose curves and d i f fe ren t ia l par t ic le and dose

spectra behind typical shielding thicknesses. From this analysis, the important

energies for var ious thicknesses and various spectra have been determined. In

general low energy par t ic les (below 100 MeV) appear t o be the most important

a t the dose p o i n t in solar par t ic le events . From the biological standpoint

very low energ.y particles are important only for very steep spectra. The high

energy heavy component o f the galact ic cosmic rays i s of considerable

importance for thin shielding and i t appears t h a t low energy heavy par t ic les

may continue to be important a t thicker shielding, a l though secondary

production d a t a are n o t ava i lab le for a thorough analysis.

iii

FOREWORD

T h i s r e p o r t i s s u b m i t t e d t o t h e N a t i o n a l A e r o n a u t i c s and Space A d m i n i s t r a t i o n ,

Headquarters Branch, Washington, D . C., i n accordance w i th the requ i rements se t

f o r t h i n NASA c o n t r a c t NASw-1362. The work here in repor ted was performed by

M. C. Wi lk inson o f the Space Physics Group, Space D i v i s i o n , The Boeing Company,

and by D r . S tan ley B. C u r t i s o f t h e Lawrence Radiat ion Laboratory , consul tant

t o The Boeing Company. This work was done under the supervis ion o f J . A. Bar ton

and D r . R. V . Hanks.

V

Introduction

CONTENTS Page

1

The Natural Radiation Environment 2 Galactic Cosmic Rays 2

Time Variations of the GCR 3 Heliocentric Intensity Gradients o f the GCR a t S o l a r Minimum 5

Solar Par t ic le Events General Considerations Solar Cosmic Rays Energetic Storm Parti cl es Recurrent Energetic Storm Part ic les Summary of Solar Par t ic le Act ivi ty

6 6 8 9 9

10

Energy Deposition Studies 11

General Considerations 11

Galactic Cosmic Rays Dose from Galactic Protons Dose from Heavy Ions

13 14 15

Solar Cosmic-Ray Dose Calculations 16 Depth-Dose Characterist ics 16 Importance o f Various Par t ic le Energies 17 Importance of Various Proton Energies i n Producing Secondary Doses 19 Conclusions 20

Energetic Storm Par t ic le Doses 22

Biological Considerations 22

Biologically Weighted LET Spectra 25

Concl usi ons 27

Acknowledgements 32

References 33

vii

I

I

LIST OF FIGURES

1. D i f f e r e n t i a l Energy Spectrum o f Ga lac t i c P ro tons 2. D i f f e r e n t i a l Energy Spectrum o f G a l a c t i c He Nuclei

3. D i f f e r e n t i a l Energy Spectrum o f Ga lac t i c L -Nuc le i 4. D i f f e r e n t i a l Energy Spectrum o f Ga lac t ic "Nuc le i 5. D i f f e r e n t i a l Energy Spectrum o f Ga lac t ic LH-Nuc le i

6. D i f f e r e n t i a l Energy Spectrum o f G a l a c t i c VH-Nuclei 7. D i f f e r e n t i a l S p e c t r a o f P r o t o n s a t D i f f e r e n t Times in t he So la r Cyc le

8. 9 .

10.

11. 12.

13. 14.

15. 16.

17.

18.

19.

D i f f e r e n t i a l S p e c t r a o f H e l i u m N u c l e i a t D i f f e r e n t Times in t he So la r Cyc le Comparison o f Boeing Secondary Code and Oak Ridge NTC Pro tons w i th Inc ident Exponen t ia l R ig id i t y Spect rum Having Character is t ic Rig id i ty 100 MV. Galact ic Cosmic-Ray Protons i n Aluminum - Depth Dose Curves

Heavy P a r t i c l e G a l a c t i c Depth Dose Curves 40 MV Proton Spectrum - Depth Dose

100 MV Proton Spectrum - Depth Dose 160 MV Proton Spectrum - Depth Dose Depth-Dose P l o t s f o r 40, 100, and 160 MV R i g i d i t y I n c i d e n t Alpha Spectrum D i f f e r e n t i a l Dose D i s t r i b u t i o n s f o r 100 MV Rig id i ty Spect ra Protons I n c i d e n t on A1 uminum D i f f e r e n t i a l Dose D i s t r i b u t i o n s f o r 40 and 160 MV Rig id i ty Spect ra Protons I n c i d e n t on Aluminum

D i f f e r e n t i a l Dose D i s t r i b u t i o n s f o r G a l a c t i c Cosmic-Ray Protons Dose Per Un i t Logar i t hm ic I n te rva l f o r He1 ium, Ni t rogen (M) , Magnesium (LH) and Cobalt (VH)

20. Penetrat ing Proton Number - Energy Spectrum i n Various Depths o f Aluminum -

21. Penetrat ing Proton Number - Energy Spectrum i n Various Depths o f Aluminum - 22. Penetrat ing Proton Number - LET Spectrum - 40 MV Spectrum I n c i d e n t

23. Penetrat ins Proton Number - LET Spectrum - 160 MV Spectrum I n c i d e n t 24. Number - LET Spectrum - 100 MV Alpha Spectrum 25. Number - LET Spectrum - 160 MV Alpha Spectrum

40 MV Spectrum I n c i d e n t

160 MV Spectrum I n c i d e n t

ix

26. 27. 28. 29. 30. 31 . 32. 33, 34 * 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.

45.

46.

47.

48.

Primary Dose - 1 gm/cm AL Primary Dose - 5 gm/cm AL Primary Dose - 10 gm/crn AL Secondary Pro ton Dose - 1 gm/cm AL Secondary Proton Dose - 5 gm/cm AL Secondary Proton Dose - 10 gm/cm AL Secondary Proton Dose - 20 gm/cm AL Neutron Dose - 1 gm/cm AL Neutron Dose - 5 gm/cm A L Neutron Dose - 10 gm/cm AL Neutron Dose - 20 gm/cm AL Neutron Dose - 30 gm/cm AL Neutron Dose - 50 gm/cm AL Differential Secondary Proton Dose Per U n i t Incident Proton Energy Differential Neutron Dose Per Unit Incident Proton Energy Depth-Dose Profi les for 10 , 20, and 30 Mv Rigidity Events Penetrating Proton Number - LET Spectrum 20 Mv Spectrum Incident LET Spectrum for Galactic Cosmic-Rays Under 0 .2 g/cm Water Shielding

2 2

2 2 2

2 2

2 2

2 2 2 2

2 -. Lower Limits for the Number of Thin-Down Hits Per cmj-day Neglecting Secondaries Solar Particle Event LET Spectrum for Penetrating Protons Po=20Mv for Various Thicknesses of Aluminum Solar Particle Event LET Spectrum for Penetrating Protons Po=160MV for Various Thicknesses of A l u m i n u m Solar Particle Events LET Spectrum for Penetrating He Ions P0=160Mv for Two Thicknesses o f Aluminum S o l a r Par t ic le Event LET Spectrum for Penetrating He Ions Po=lOOMv for Two Thicknesses of Aluminum

X

LIST OF TABLES

1. Radial Intensity Gradients From Mariner I1 Data

2. Characterist ics o f Different Types and Classes o f Solar-Particle Events

3 . Free Space Dose Rates From the Various Components o f the Galactic Cosmic Rays a t Solar Min imum

Xi

I

INTRODUCTION

As the qoals o f manned space missions expand beyond the Apollo lunar landing,

the astronaut will f i n d himself on extended earth-orbital t r ips , extended

vis i ts t o the moon, and eventually on missions t o Mars and Venus, lasing

on the order of a year or longer. The problem of radiation and i t s e f fec ts

on performance and health w i 11 grow i n importance as the total dose and the

probabili ty for encountering large solar particle f luxes increase. The purpose

of this study is f i r s t t o summarize our knowledge to da te (ear ly 1967) on the

natural radiation environment beyond our magnetosphere. Secondly, the

radiation spectra will be used to ca lcu la te the spectra of various components

of the radiation dose behind a variety of shielding thicknesses. Thirdly, we

will at tempt to evaluate, w i t h the available know1 edge of biological effects,

the re la t ive importance of the energies and types of particles found i n space.

I t i s hoped tha t this evaluation and the specification of the l imitat ions i n

our knowledge will be helpful i n directing future experimental e f fo r t s so tha t

the necessary information for a more accurate evaluation will eventually

become available.

1

THE NATURAL RADIATION ENVIRONMENT

I t i s convenient to divide the r a d i a t i o n environment outside the magnetosphere

into two par t s : tha t a r i s ing from the galact ic cosmic rays (GCR) and that,

ar is ing from the solar emissions, the solar cosmic rays (SCR) and energetic

storm par t ic les ( E S P ) . The GCR originate outside our so la r system and are

characterized by ra ther low, fairly steady isotropic f luxes which vary Over

the eleven-year solar cycle by abou t a fac tor of three o r fou r and reach

maximum during or soon a f t e r t he period of m i n i m u m so l a r ac t iv i ty . The solar

act ivi ty cycle modulates the lower energy portion of the GCR spectra. The

SCR occur in association with solar flares, and such fluxes can be many

orders of magnitude higher t h a n the GCR fluxes d u r i n g the occurrence of a

g i a n t solar particle event. Typically, such events l a s t two or three days,

w i t h the f lux r is ing t o a maximum within 24 hours a n d then decaying more

slowly through the res t of the event. The ESP ar r ive i n the vicini ty of the

earth a b o u t 24 hours after the onset of the so la r f la re . These low energy

par t ic les a re of concern o n l y for EVA ac t iv i ty , or very thinly shielded space-

c r a f t . A thorough discussion of the physical characteristics o f so lar par t ic le

events has appeared in the l i t e ra ture (1,2), and only a synopsis of these

resul ts wi l l be presented here.

GALACTIC COSMIC RAYS

Recent we1 1-instrumented experiments on sate1 1 i tes are yielding good d a t a on

the ident i ty and energy spectra of the various components of the GCR.

Spectra have been measured of par t ic les with atomic numbers ( i . e . , charges)

up through z=28. There i s a def in i te preponderance o f par t ic les with even z

over those w i t h odd z . I t i s convenient to divide the heavier ions into groups

2

w i t h similar charge. Our designation will be:

L-particles: 3 5 z 5 5 LH-particles: l o < z < 14 "particles: 6 5 z_C 9 MH-particles: 15 z 5 25

VH-particles: 26 5 z < - 28

-

A compilation of GCR d a t a obtained d u r i n g the most recent period of the quiet sun

(1964-1965) is given in Figures 1-6 for protons , He ions , and the particle

groups 1 isted above (3-9). One group, the MH-particl es , has been omitted because

good d a t a on the spectral shape are n o t .presently a v a i 1 able. I t i s known , however , t h a t the flux of these particles i s low compared w i t h t h a t of the LH- and VH-

pa r t ic les , and their exclusion will n o t affect the conclusions t o be discussed

bel ow.

Time .. - V a r i a t i o n of the G C R

Solar activity and the resultant solar w i n d emission and disturbances in the

solar magnetic field configuration cause a decrease i n the GCR fluxes during

the height of the activity cycle. The decrease occurs only in the lower energy

portions o f the GCR spectra. Recent comparisons o f d a t a obtained near the

minimum of the solar cycle (6 , 10) lead t o the conclusion t h a t for par t ic les

w i t h energies greater t h a n 100 MeV/nucleon, the modulation mechanism has a

r ig id i ty dependence exp(-K/Pp) where K i s constant for any given time and P

i s the r ig id i ty and /3 the velocity relative to the velocity of l i g h t . This

means t h a t the spectrum near the o r b i t of the Earth can be written

- = exp ( - tl(t)/PP) for T > 100 MeV/nucleon* dJ dT dT cb

*Note added: The res t r ic t ion t o higher ener ies for this analytic form has been suggested by very recent work 9 G . Gloeckler and J . R . Jokipi i , Astro. J . - 148, L41 (1967)).

3

where 31 i s t h e spectrum outside the influence of the solar magnetic f i e ld

and r\ ( t ) depends on time b u t n o t on pa r t i c l e r i g id i ty or ve loc i ty . I t appears

t h a t d u r i n g so la r m i n i m u m , the low energy p o r t i o n of the spectrum varies con-

siderably over a few months ' time. Such variations will cause variations in the

free space (no shielding) G C R dose, and thus the accuracy of f r ee space dose

calculations is l imited by our lack of knowledge of the very low energy portion

of the spectra.

dT a0

There have appeared in the l i t e r a tu re s tud ie s of the correlation o f spectral

shape wi.th g round level neutron monitor readings. The cor re la t ion - i s good

enough so t h a t the shape of the GCR spectra i n the rigidity region around 1 G V

can be roughly inferred from a knowledge of ground-level neutron fluxes. Thus

a rough idea of the shape of the GCR spectrum can be obtained a t any time during

the solar cycle if the ground-level neutron rate is known. Results o f two

studies (11 , 1 2 ) are shown i n Figures 7 and 8. In the two s tud ies , d i f fe ren t

values of the Mt. Washington neutron monitor counting r a t e were used as the

baseline value. To find the shape of the proton or He ion spectrum for any

time during the solar cycle ( i .e . , fo r any neutron counting r a t e ) , the

percentage difference between the neutron r a t e a t the time of i n t e re s t and the

base1 ine value (label led N in the graphs) is calculated. Then the desired

curve i s found by interpolation on the appropriate g r a p h . This i s admittedly a

crude estimate and qives no information on t he c r i t i ca l low energy portion.

Other more accurate descriptions of the variation of the spectral shapes with

time wi 11 undoubtedly be avai lab1 e soon as more i s learned abou t the modulation

mechanism and i t s time dependence.

4

Heliocentric "______ Intensity Gradients - - - - " of the GCR a t S o l a r Minimum

The dependence of the GCR f l u x on distance from the sun is of interest fo r

missions t o Mars and Venus. Recent data from Mariner IV (10) ind ica te tha t

the radial dependence i s measurable between the Earth and Mars. The gradient

appears t o be 1 i near and is g i v e n i n Table I for protons and He ions o f various

energies as measured on Mariner IV between 1 .O and 1.6 a.u. (astronomical units) .

The overall proton f l u x i s about 6% higher a t Mars (1.56 a . u .) than a t the

Earth d u r i n g so la r m i n i m u m . Also, the He ion flux between 100 and 300 MeV/nucleon

i s increased 36% and between 300 and 420 MeV/nucleon is increased 31%. The

proton flux between 1 and 15 MeV rises by less than a fac tor of three.

TABLE I (From Ref. 10)

Par t ic le

Protons

Protons

He Ions

He Ions

Kinetic Energy (per cent/a. u . ) (MeV/nucleon)

Radial Intensity Gradient

~ i. .. I I

Mean energy - 6000 + 9.6 _+ 0.9

1-15 I + 500

100-300

300-420

+ 6 5 f 8

+ 5 5 _ + 5

5

SOLAR PART1 CLE EVENTS

General Considerations

A1 t h o u g h the character is t ics of the galact ic cosmic r a d i a t i o n have been recognized

and investigated for many years, only in the past ten years have the particulate

emissions of the sun been studied i n de t a i l . The recognition t h a t these emissions,

or solar particle events, occurred more frequently t h a n previously supposed

focused at tent ion on the radiation hazard they might pose t o manned space t rave l .

The aspects of solar par t ic le events which are of grea tes t importance in deter-

mining their potential radiation hazard are:

1 ) Particle type (or charge composition) o f the ejected particles.

2 ) The time dependent intensi ty and energy spectra of the emitted par t ic les .

3) The a n g u l a r d is t r ibut ion of the par t ic les a t the spatial regions of i n t e re s t .

4 ) The predic tab i l i ty of the particle events.

Conclusions From Solar Cycle 19 Data

I t was soon recognized t h a t protons formed the b u l k of the particulate emissions,

a l t h o u g h s ignif icant f ract ions o f helium and heavy ions were occasionally present.

The determination of the time dependent energy spectrum of the solar particle

events proved a major problem, and n o t until the exponential rigidity spectrum

representation was proposed by P. S . Freier and W . R . Webber ( 1 3) did one have

available a reasonably accurate analytic representation of the solar par t ic le

energy spectrum which could be used over an energy range suff ic ient ly large

(30-100 MeV) t o be useful for shielding studies.

6

The ground-based observational techniques available d u r i n g solar cycle 19

indicated t h a t the solar par t ic les were nearly isotropic i n angular distribution

i n the vicinity of the earth. Marked anisotropies were f e l t t o be associated

w i t h the high-energy particles and w i t h the effects of the ear th 's magnetic

f ie ld . Pred ic tab i l i ty of the large solar particle events was unreliable.

Anticipated Results from Cycle 20

Due t o advances i n observational techniques, chiefly from s a t e l l i t e s , we can

expect a great deal of improvement i n our information on the solar particle

events. We can expect t o o b t a i n more accurate time dependent spectral a n d

intensi ty measurements, extending over a wider energy range than were available

in cycle 19. The angular dependences of the particle f luxes are being measured

d i rec t ly . The limited results available a t th i s time (7 ,14) indicate t h a t there

are strong angular dependences in the proton fluxes, and t h a t this anisotropy

i s important for low energy protons, t h a t i s for energies less t h a n 80 MeV.

The major questions t o be answered d u r i n g solar cycle 20 are:

1 ) Is the exponential rigidity representation of the particle energy

spectra adequate, or must i t be modified?

2 ) How strong are the angular dependences of the proton events, and

can th i s dependence be predicted in advance from a knowledge of the

interplanetary magnetic field configuration?

3 ) What will be the intensity of solar proton ac t iv i ty d u r i n g cycle 20,

and how will the characterist ics of the events observed in cycle 20

compare w i t h cycle 19?

7

4 ) How well can the larger particle events be predicted?

These points all bear heavily on our abi l i ty to predict the radiat ion hazards

t o man on extended missions.

SOLAR COSMIC RAYS (SCR)

This c lass of par t ic les is character ized by spectra in which an appreciable

f ract ion of the par t ic les have energies above 20 t o 30 MeV. The time dependence

of the SCR in tens i ty can be expressed as a rapid quasi-exponential rise from

onset. The character is t ic onset time depends on the location of the source

f l a r e on the solar disk and on the condition of the interplanetary magnetic f i e l d .

Typical onset times range from 2 times the rectilinear earth-sun travel time

fo r west-limb f l a re s t o 10 times for east-l imb flares.

After maximum intensity is reached, for any given energy, the particle flux

decays slowly in an exponential-like fashion. The typical decay times may be

greater t h a n the onset times by factors of from 10 t o 100. D u r i n g this decay

time, the spectrum typically steepens considerably; i . e . , the percentage of

high energy particles decreases. The incidence of these SCR events has been

relatively infrequent in the recent sunspot minimum, b u t d u r i n g the past several

months, a number of larger SCR events have occurred. These recent events,

corresponding to the increasing solar activity of cycle 20 , were observed by a

number of s a t e l l i t e s and should provide more information on the SCR events.

The available d a t a on cycle 19 has been tabu1 ated by Webber (1 , 2 ) .

8

ENERGETIC STORM PARTICLES (ESP)

Beginning about 1962, nearly continuous measurements of particle f luxes down t o

energies of about 1 MeV confirmed the presence of a large low-energy flux of

particles associated w i t h the solar par t ic le events . This population of par t ic les

does n o t begin t o arrive in appreciable numbers unt i l about 24 hours a f te r the

onset o f the solar cosmic rays. I t is found t h a t most SCR events are accompanied

by energetic storm particles ( E S P ) , although the correlation between the s ize of

the SCR and i ts associated ESP i s n o t understood.

The ESP p a r t

t h a n the SCR

19 are given

icle population is characterized by a much steeper energy spectrum

. Best estimates for the ESP characterist ics associated with cycle

in Reference 2 .

RECURRENT ENERGETIC STORM PARTICLES

These par t ic les const i tute t h a t phase o f an SCR event which tends t o recur with

the 27-day solar rotation period. This series is s ta r ted by a so la r par t ic le

event and continues with a succession of ESP events. The duration of the indi-

vidual events i s 1 t o 2 days with the intensity increasing and decreasing more

o r less symmetrically in time about the time of peak in tens i ty . The par t ic les

causing the event are presumably contained i n a ra ther h i g h l y ordered magnetic

f ie ld "bot t le" t h a t sweeps through interplanetary space, anchored t o the act ive

region on the sun. These events have the typically steep spectra of the prompt

ESP events b u t the in tens i ty i s down by a fac tor of from 1000 t o 10,000 from

the original ESP event.

9

SUMMARY OF SOLAR PARTICLE ACTIVITY

I n summary, the integral spectrum o f each class of solar par t ic le event ( i .e . ,

SCR or ESP) i s t h o u g h t t o be best described a t the present time by an exponential

form in r ig id i ty : J (P) = Jo exp(-P/Po) , where the steepness

determined by the charac te r i s t ic r ig id i ty , P o , and the total

of a par t icular c lass i s determined by Jo. The behavior of Po

of the spectrum i s

intensi ty o f par t ic

a n d Jo during

1 es

the course of a typical solar particle event i s shown in Figure 1 of Reference 2 .

Typical values of these parameters are shown in Table 2 .

TABLE 2

CHARACTERISTICS OF DIFFERENT CLASSES OF SOLAR-PARTICLE EVENTS

C 1 ass

Sol a r Cosmic 40-200 lo2 to l o 4 Energetic Storm Part ic les 5 t o 20 lo5 to lo7 Recurrent Events 5 t o 20 10 to l o 3 Thermal P1 asma 1 t o 2 lo8 to l o 9

The thermal plasma, or solar wind, i s included for completeness.

As will be shown i n the shielding analysis, some SCR par t ic les are of suf f ic ien t

energy t o penetrate any reasonable space-vehicle shielding, while the ESP events

present a threat only t o very thinly shielded vehicles, or during extra-vehicul a r

ac t iv i ty .

10

ENERGY DEPOSITION STUDIES

GENERAL CONSIDERATIONS

When the incident particle types and energy spectra have been determined o r

specified i n some manner, the next step i n the r a d i a t i o n hazard evaluation process

i s t o determine the energy deposition distribution i n the body of the space

t raveler . The logical program t o follow i n making this ca lcu la t ion i s f i r s t t o

determine the particle energy spectrum a t each dose point of in te res t and then

determine the energy deposited a t the dose p o i n t . T h i s straight-forward

approach i s complicated by several major d i f f icu l t ies , the most severe of which

are the complexity o f the complete angle and energy dependent transport equations,

the multiplicity of particles result ing from the interactions of high-energy

par t ic les with nuclei , and the 1 ack of accurate and complete nuclear production

cross section d a t a for the complete range of particle types and energy regions

of in te res t .

In an e f for t t o advance the dose calculational capability for the high energy

protons encountered i n space travel, the Neutron Physics Division of the Oak

Ridge National Laboratory has engaged for several years in research efforts t o

provide the cross section d a t a needed for such work. In addition, a ser ies of

Monte Carlo transport codes uti1 izing these d a t a has been developed. These

computer codes provide the most accurate theoretical calculations available of

the dose dis t r ibut ions from incident protons. The Boeing Company has developed

shielding codes for the determination of the radiation dose received by space

travelers i n the complex shielding configurations of actual space vehicles.

In order to make these calculations of manageable length, i t is necessary to

simplify the calculations as much as possible. The Boeing codes which evaluate

11

the dose from incident protons and heavier charged par t ic les have been described

in detail (15) , and a complete description of the theory and assumptions will n o t

be presented here. The major assumptions made in the calculations, however, will

be br ie f ly summarized below.

I n the proton secondary dose code:

1 ) The range-energy and stopping power d a t a of Barkas and Berger (16) are used, augmented a t low energies by the work of Northcl i f f e ( 1 7 ) .

2 ) Range and p a t h length of the par t ic les a re assumed the same, and multiple coulomb scat ter ing and straggling are neglected.

3 ) Nuclear interact ion d a t a o f Bertini and Dresner (18) are used.

4 ) The straight-ahead approximation i s used th roughou t .

5) Only first-generation cascade and evaporation particles are treated.

6 ) The neutron transport is estimated by removal theory, a n d the dose from the h i g h energy neutrons is calculated by the energy removal cross sections of W . Gibson ( 1 9 ) .

In the heavy pa r t i c l e dose code , nuclear interactions are neglected. In these

computer codes and t h r o u g h o u t th i s repor t , the doses calculated are i n t i s sue

rads. The primary pro ton dose i s defined as the ionization dose resul t ing from

incident protons which have n o t been involved in nuclear interactions. I t i s

assumed t h a t the ra te a t which energy i s being l o s t by the proton through ioni-

zation of the atoms in the tissue a t a point is the sole contributor to the

dose a t t h a t point. The secondary pro ton dose i s defined i n the same way as the

primary proton dose, b u t nuclear interactions are n o t assumed t o attenuate the

secondary proton f lux. The neutron dose is calculated by assuming t h a t for h i g h

neutron energies , a1 1 the neutron energy involved i n nuclear interaction is

12

deposited a t the point of the interaction. A t low energies , the f i rs t col l is ion

dose-conversion values of Snyder and Neufel d are used. The nuclear recoi 1 dose

i s defined as the ionization dose result ing from the recoiling nuclei caused by

the nuclear interactions of the incident proton flux. For the alpha and heavier

particles, secondaries from nuclear interactions are neglected and only the

primary ionization dose is calculated.

To check the various approximations used.

between the Boeing code and the Oak Ridge

shown in Figure 9 . I t i s f e l t t h a t these

ison was made

results are

agreement.

Significant differences were noted, i n a previous comparison before the nuclear

cross section data of Bertini were incorporated in the Boeing code (20) .

i n these codes , a compar

Monte Carlo codes. The

results indicate a good

GALACTIC COSMIC RAY DOSE CALCULATIONS

The character is t ics o f the galactic cosmic ray spectra cause several problems

in determining their ionization dose. First of a l l , the galactic cosmic-ray

energy spectra extend t o a t l eas t lo2' electron volts, and the resulting nuclear

and electromagnetic interactions are of suff ic ient complexity t o occupy the

attention of a sizeable p o r t i o n of the physics community. As the nuclear cascade

calculations available t o us a t the present time are restricted t o the energy

region between 25 and 400 MeV, no estimate is made of the secondary dose result ing

from incident protons of greater t h a n 400 MeV in our calculations. In addition,

the nuclear cascades which resul t from the incident he1 ium and heavier nuclei

present in the galactic particles are n o t amenable t o the same theoretical

treatment as is used f o r incident protons. Consequently, no estimate is made i n

th is report of t he i r secondary dose contributions. In future work i t i s hoped

13

t o extend our estimates o f the secondary dose t o higher energy protons as the

Oak Ridge work i n the energy region from 400 t o 2000 MeV i s made available.

DOSE RATE FROM GALACTIC PROTONS

Figure 10 shows the dose r a t e t h a t resu l t s from the galact ic cosmic-ray proton

spectrum normally incident on a slab shield of aluminum. The primary proton

dose i s defined as the dose resul t ing from proton-electron collisions in a

small volume of t i s sue . The fur ther t ransport of energy by delta rays is

neglected, and the electrons are assumed t o deposi t their energy local ly . The

dose presented i s from protons of energies between 1 MeV and 10 GeV a t the

dose point. On the assumption t h a t the actual galactic cosmic-ray proton spectrum

can be extrapolated t o higher energies by t a k i n g the d i f fe ren t ia l energy spectrum,

dJ/dT, t o be proportional t o 1 / (T+m )5’2 , i t i s found tha t protons above 10 GeV

contribute approximately an additional 10% t o the t o t a l primary ionization dose. P

The secondary dose components shown are the secondary p ro ton dose, the neutron

dose, and the dose resul t ing from recoiling nuclei . These secondary dose com-

ponents are calculated only for interacting primary protons w i t h l ess than 400

MeV energy. The secondary dose components are shown by dotted lines t o indicate

t h a t they include only p a r t of the total secondary dose. An es t imate is made of

the nuclear recoi 1 dose resul t ing from pro tons of u p t o 10,000 MeV, and t h i s

curve i s shown as sol i d . The secondary pro ton dose is calculated in the same

manner as the primary pro ton dose except t h a t nuclear interactions are neglected.

The neutron dose is calculated using removal theory and the dose conversion

values o f Gibson (19). The nuclear recoi 1 dose is calculated by the use of the

Monte Carlo results of the Oak Ridge nucleon transport code.

14

DOSE RATES FROM INCIDENT HEAVY IONS

In Figure 11 a re shown the dose rates result ing from incident He, M y LH and

V H ions incident on a slab shield of aluminum. In these calculations, "particles

were a1 1 assumed t o have Z=7, LH-particl es , Z=12 , and VH-particl es , Z=27.

Only the primary ionization dose from these ions is calculated. The sol id l ines

show the dose rate estimates neglecting nuclear interactions entirely. The dashed

l ines indicate the dose r a t e from those ions surviving t o the dose p o i n t without

undergoing a nuclear interaction, assuming interaction mean free paths independent

o f ion energy. The true depth-dose rate curves will f a l l somewhere between these

curves.

Clearly, the dose from the secondary particles resulting from nuclear interactions

i s important, and i t would be highly desirable t o evaluate this dose c o n t r i b u t i o n .

A t zero depth, the dose rates from the various components can be read from the

figures and are as accurate as the spectral measurements allow. Converted into

rads/yr, they are presented in Table 3 .

T A B L E 3

F R E E SPACE DOSE RATES FROM THE VARIOUS COMPONENTS OF THE GALACTIC COSMIC RAYS AT SOLAR MINIMUM ( < 10 GeV)

Part ic le

Protons ( Z = 1 ) He Ions ( Z = 2 ) M Ions (6 S Z 3 9 ) LH Ions (10 < Z < 14) V H Ions (26 I Z 5 28)

.___- Dose Rate (Rads/Yr)

4.6 3.5 1 .9 1.3 1.3

Total 12.6 rads/yr

15

r i gi di t y

pa rame tr

r ig id i ty

SOLAR COSMIC-RAY DOSE CALCULATIONS

Depth-Dose Characterist ics

The representation of the so la r cosmic-ray particle spectra as exponential

allows a convenient method of presenting the resulting depth-dose

i ca l ly . Figures 12 t h r o u g h 14 present depth-dose profiles for inc

spectra with characterist ic r igidit ies of 40, 100 and 160 M V . I n

case, the primary ionization dose from protons of between 1 and 400 MeV i s

i n

prof i 1 es

i dent

each

calculated, assuming the incident pro ton flux is at tenuated by nuclear interactions.

The neutron, secondary p ro ton , and nuclear recoil doses are calculated w i t h the

same assumptions as in the galactic cosmic ray pro ton dose calculation. As the

energy spectra fa1 1 off much more rapidly t h a n the GCR spectra , the 400 MeV

cutoff is of less importance, particularly for the low rigidity event. Several

observations on the re la t ive importance of the various dose components are of

interest . Firs t , the ionizat ion dose from the primary protons i s the dominant

dose contribution t o thicknesses of several tens of g/cm of A1 shielding. The

thickness a t which the secondary radiation rivals the primary dose i s dependent

on the charac te r i s t ic r ig id i ty of the incident pro ton spectrum, with the lower

r ig id i t ies resu l t ing in a secondary dose contribution which becomes s ignif icant

for thinner shielding. The rad dose from the recoiling nuclei remains low

re la t ive t o the other dose components a t all shielding thicknesses. I t should

be noted t h a t the neutron dose, calculated by the energy removal cross section

of Gibson actually contains some fract ion of the total nuclear recoil dose.

Figure 15 presents the depth-dose profiles of incident alpha particles. Nuclear

interactions are neglected.

2

16

A word of caution i s i n order when us ing the rigidity representation for thick

shields. The resu l t s shown depend strongly on the validity of the exponential

rigidity representation above pro ton energies t h a t can penetrate t o the dose

point of in te res t . For large shielding thicknesses, greater t h a n 10 g/cm2, i t

may well be t h a t the prompt spectrum from certain particle events determines the

spectral shape a t h i g h energies, and the average rigidity for the entire event

may not represent this energy region adequately. This is cer ta inly t rue of the

great event of February 23, 1956. I t s prompt spectrum seems best f i t by a much

higher characteristic rigidity (perhaps near 700 M V ) t h a n is obtained by con-

sideration of the total fluence only. This will strongly affect the dose rates

a t great shielding thicknesses.

- Imprtance -. . - - - - of Various Part ic le Energies

I f we f ix a t tent ion on any particular dose point i n the aluminum slab, we find

an energy distribution of proton and neutrons which are producing dose a t the

p o i n t i n question. The i o n i z a t i o n dose from the protons can be expressed i n the

approximations used as:

D =

where: p !2 ( E ) i s the a t the

S ( E ) i s t he

K i s t he

The different ia l dose

dD = K pp (E)S(E)dE .

K p ; S c E , d E E max

proton different ia l energy spectrum in protons (cm2 - MeV) point ;

stopping power, i n MeV cm /gm;

conversion factor to convert the dose t o rads.

2

result ing from protons of energy between E and E+dE is

17

Thus the importance o f protons w i t h energies between E and E+dE in providing dose

can be evaluated from a knowledge of the proton energy spectrum a t the p o i n t and

the stopping power. We f ind, as a typical resul t , t h a t the lowest energy protons

a t the dose point are the most impor tan t i n producing dose, when judged in a dose

per unit energy basis. This result holds generally for a1 1 solar par t ic le spectra

and shield thicknesses. This result can be deceptive, however, unless one

real izes t h a t the fraction of the total dose deposited by low energy protons i s

n o t large. Perhaps a c learer way t o present the importance of the various pro ton

energies is t o p lo t the d i f fe ren t ia l dose per unit logarithmic interval.

The reason for presenting the curves i n t h i s way i s because the energy range o f

interest extends over several orders of magnitude. Since i t i s convenient t o

p l o t the results using a logarithmic scale i n energy, i t i s i n s t r u c t i v e t o plot

the dose d is t r ibu t ions for l inear in te rva ls along the absc issa , i . e . , per

logarithmic energy interval. In t h i s way, equal distances along the abscissa

have equal weights for the evaluation of the importance of different energy ranges

in contributing t o the dose. Thus, we wri te the different ia l dose element

dD = Kg (E)s(E)Ed(lnE). The quantity plotted in this case is K !J ( E ) S ( E ) E . I n P P

t h i s way the importance of the higher energy p ro tons , which contribute less dose

per unit energy b u t span a much larger energy range i s more clearly presented.

We see in Figures 16 and 17 t h a t i t i s in fact these higher proton energies ( u p

t o 100 MeV) which can contribute a large f ract ion o f the total dose. In Figures

18 and 19 the different ia l dose distributions for the galactic particles are also

shown .

18

One additional way to view the importance of the various particle energies

i s t o examine the different ia l par t ic le energy spectrum a t the dose p o i n t .

Figures 20 and 21 show the number-energy spectrum of protons present from

proton spectra w i t h charac te r i s t ic r ig id i t ies of 40 and 160 MV a t various

depths in aluminum. One can also show the number of particles present as a

function of the s t o p p i n g power, or LET, of the par t ic le , and Figures 22

throuqh 25 show the results of typical pro ton and alpha particle r igidity

spectra incident on aluminum.

I t i s a l so of in te res t t o examine the importance of the various incident p r o t o n

energies i R producing dose a t a r i n t e r i o r p o i n t . I f we examine f i r s t t h e dose

from penetrating protons, i t i s apparent t h a t f o r any given shield thickness,

x , only those incident protons with range equal or greater than x can deliver

dose a t the depth x . Thus we have an energy cutoff , E o , below which the

protons cannot reach the dose point. In Figures 26 t h r o u g h 28 we show the

fraction o f the total dose delivered by protons o f incident energy greater t h a n

E a t various thicknesses o f aluminum for three typical solar pro ton r ig id i ty

spectra. We see t h a t the higher characterist ic r igidity spectra cause the

curves t o fa l l off more slowly w i t h increasing incident proton energy t h a n do

the lower rigidity events. This is another way of s ta t ing the fact t h a t h i g h

incident energies are more impor tan t f o r f l a t spec t r a than for steep spectra.

Importance of Various Proton Energies i n Producing Secondary Doses

Next, l e t us examine the secondary doses from the incident protons. In

Figures 29 through 38, we present the fraction o f the secondary pro ton and

neutron dose delivered as a resu l t of nuclear interactions which involve

19

incident pro tons of greater t h a n i n i t i a l energy E . For the secondary protons,

we have a similar cut-off effect as w i t h the primary dose since the secondary

protons are less t h a n or equal t o the energy of the primary interacting pro ton .

I n addition, i t i s of i n t e re s t t o examine the importance of the nuclear

in te rac t ions in i t ia ted by protons above particular energies i n producing neutron

and secondary pro ton dose. This gives one a feel ing for the importance of an

accurate knowledge of these cross sections a t the various energies of i n t e re s t .

These resu l t s a re shown as dashed lines in Figures 29 t h r o u g h 38.

Finally, we present the normalized d i f fe ren t ia l dose d is t r ibu t ions of the

secondary pro ton and neutron dose as a function of incident pro ton energy in

Figures 39 and 40. Results are presented for three typical characteristic

r i g i d i t i e s a t a depth i n aluminum of 10 g/cm . 2

Conclusions

Certain general conclusions can be drawn from the d a t a presented. We note t h a t

for any given shielding thickness, the fraction of the total secondary dose

result ing from protons of i n i t i a l energy greater than E increases with increasing

character is t ic r iqidi ty . Also, we find as the shielding thicknesses increase,

the lower incident pro ton energies contribute less t o the t o t a l secondary dose.

When we compare the incident pro ton energy cutoff curves with the reacting

energy cutoff curve for a given spectral shape, we f ind t h a t they gradually

approach a t higher energies. We now present three specific i l lustrations of

the use of the d a t a . First, what incident pro ton energies are most important

i n producing secondary pro ton and neutron dose under 10 g/cm of aluminum?

Figure 39 shows t h a t fo r secondary proton dose, they are those protons t h a t

2

20

have energies just s l igh t ly g rea te r t h a n necessary t o reach the dose p o i n t . Only

the 160 MV d i f fe ren t ia l dose spectrum peaks a t a sl ightly higher energy (130 MeV)

than the cutoff energy for this case (100 MeV). The neutron dose, on the other

hand, shows a strong dependence on the incident spectral character is t ic r igidi ty .

The 40 MV d i f fe ren t ia l dose plot shown i n Fiqure 40 peaks a t an incident proton

energy of 35 MeV, the 100 MV spectrum peaks a t 65 MeV, while the 160 MV

spectrum shows a very broad peak from 150 t o 260 MeV. Second, 1,et us estimate

the importance of the low energy cross sections used i n the calculations. A t

10 g/cm' in a luminum, we note from Figure 35 t h a t 30 percent of the neutron

dose resul t ing from the 40 MV spectrum is contributed by reactions involving

protons of less t h a n 25 MeV. Cross sections i n t h i s region are extrapolations

of Bertini ' s d a t a and could contain 1 arge errors. The 100 and 160 MV spectra , however, show t h a t less t h a n 10 and 5 percent, respectively, of the neutron

dose comes from the low energy p ro ton i n t e rac t ions . I t appears unlikely t h a t

a knowledge of the pro ton reaction cross sections bel ow 25 MeV i s c r i t i c a l

for evaluating the neutron doses resulting from most SCR events. Thirdly,

assuming some type of exterior cutoff in proton energy is operat ing, such as

a magnetic shield or the ea r th ' s magnetosphere, how does the secondary dose

under 10 g/cm of aluminum compare w i t h t h a t which would be received from the

complete spectrum? Figure 35 t e l l s us, assuming a 100 MeV cutof f , t h a t 65%,

45% and 4% of the neutron dose would be received from 160 M V , 100 MV and

40MV rigidity spectra, respectively.

2

21

L

ENERGETIC STORM PARTICLES DOSES

In Figure 41, we present depth-dose plots for three characteristic rig

t h o u g h t t o be representative of the energetic storm particles: 10, 20,

No secondary dose estimates are given here, as the results depend comp

i d i t i e s

and 30 M V .

l e t e ly on

the low energy pro ton reaction cross sections below 25 MeV, which are n o t

adequately known. Figure 42 presents the LET spectrum associated with these

typ ica l charac te r i s t ic r ig id i t ies . Tables 131 a n d 132 of Reference 2 give the

t o t a l dose estimates which have resulted from the events of cycle 19 . For the

shielding thickness shown, the secondary doses should be negligible.

BIOLOGICAL CONSIDERATIONS

A t th is point , i t i s considerably easier t o identify the important problems t o

be tackled in radiobiology in order t o better evaluate the radiation h a z a r d

e f fec ts and enumerate which

e f f e c t . I t appears t h a t two

i r s t , we consider the problem of

. Here the danger wil l d i f fer

considerably depending on whether the astronaut is caught in extra-vehicular

ac t iv i ty ( t h a t i s , in a low shielding si tuation) or has the time and shielding

available t o r e t i r e t o an area where say 10 t o 15 g/cm or more of shielding

i s between h i m and the incident f lux. In the f i r s t c a se , r e l a t ive ly low fluxes

may const i tute a hazard and the most cri t ical area will be the skin and perhaps

the eyes i f they are n o t well shielded. The reason f o r t h i s i s t h a t the body

i t s e l f provides considerable shielding t o the internal organs for the rather

steep spectra which typically occur in solar particle events. The shielding

2

t h a n i t i s t o speculate on the various important

par t ic les and energies are important for a given

d i s t inc t kinds of hazards present themselves. F

a massive dose from a giant solar particle event

22

I

provided by the Lunar Module i n which the as t ronauts wil l f i rs t touch down on

the moon i s i n the category of t h i n shielding, where the dose to the skin will

be the impor tan t consideration. The most important particles here will be the

ESP and low energy SCR particles. If the anisotropy recently measured (7,14)

i s a typical characterist ic of large events with steep spectra i t would

def ini te ly be of help t o or ient the craf t so that the area of thinnest shielding

i s pointed i n the minimum flux direction. According t o d a t a ( 7 ) received on

Pioneer 6 from the solar event of 30 December 1965 , low energy protons (1 3-20

MeV) were highly anisotropic over the period of peak flux. The direction of

minimum flux ( for a 2-hour period) was around 150" from the sun-spacecraft line.

The r a t io of maximum t o minimum flux i n opposite directions was a b o u t 3 t o 1

over this per iod. I t was also noted t h a t this direction of peak intensity was

highly variable over periods of 10 minutes or so.

I f the astronaut has with h i m enough shielding t o effect ively absorb the low

energy component, then the event must be a large one for h i m t o be i n serious

danger. In this case the skin ceases t o be a c r i t i ca l organ since the body

self-shielding is less effect ive and the doses t o the intest inal t ract and

marrow become important. The prodromal syndrome of nausea, vomi t ing , anorexia

and fatigue could cause trouble inside a spacecraft. The par t ic les most

responsible for this have init ial energies whose range is the order o f the

shielding thickness available. Thus events with f l a t spec t r a ( h i g h Po's)

become important. The most impor tan t par t ic les would be the penetrating

protons and the secondaries caused by those which interacted. Alpha par t ic les

and the heavier components would n o t contribute substantially to the dose.

23

I

Recovery i s an important consideration here since i t has been noted t h a t t he re i s

a tendency for several giant events to occur w i t h i n several days. Thus i f

recovery and repair from t h e f i r s t event are n o t complete when the second event

occurs, the resultant biological damage a t the end of the second event could be

considerably greater t h a n i f i t had been an isolated one. Unfortunately, repair

mechanisms in humans and the recovery capacity of the body involve complex

processes. I t i s known t h a t repair i s l e s s l i ke ly fo r damage caused by the

highly ionizing components - recoil protons and heavy recoi l s . Thus i t i s

possible t h a t damage t o certain organs i n which ce l l s a r e no t replaced, such as

the re t ina , might accumulate over a period of time and eventually cause a problem.

I n this regard, the GCR may play a significant role. Figure 43 shows the LET

spectrum of the GCR a t a depth of 0 . 2 g/cm of water. The upper curves give

the L E T spectra multiplied by QF (see next section). Because of the high

energies involved, this basic spectrum does n o t change significantly as the

shielding is increased, except t h a t the higher dE/dx components are attenuated

f a s t e r t h a n the p r o t o n component.

2

One glaring gap in our know1 edge i s the lack of information on the secondary

production cross sections from He and heavier ions. I t i s of i n t e re s t , however,

t o calculate the number o f stopping GCR heavy ions (sometimes called "thin-down

h i t s " ) per cm3-day as a function of depth assuming only pa r t i c l e removal from

the flux due t o nuclear interactions , b u t neglecting the secondaries caused by

the fragmentation of heavier ions. The resu l t s of such a calculation are shown

i n Figure 44 for the M-, L H - , and VH-particles. The VH-particle calculation

i s probably quite accurate (assuming a correct in teract ion mean f r ee p a t h ) ;

24

b u t the LH- and "particle curves are certainly lower 1 imi t s , since secondaries

i n those classes t h a t a r i s e from interactions o f heavier ions are neglected

ent i re ly . Even so, i t i s i n t e re s t ing t o note t h a t the number of "particle

th in-down h i t s a t 15 cm i s s t i l l 30% of t h a t a t 1 cm, and the number of LH-

par t ic le thin-down h i t s a t 15 cm is 25% t h a t a t 1 cm.

B-IOLOGICALLY WEIGHTED LET SPECTRA

In general, large uncertainties are involved in making quantitative calculations

using biological effectiveness factors (RBE's) t o o b t a i n "rem" values which

m i g h t o r might n o t have significance t o the spacecraft situation of mixed

radiations, low dose rates and steep depth-dose curves. Therefore, only crude

estimations have been made t o indicate trends. A few LET spectra under various

shielding thicknesses for protons and He ions i n typical solar particle events

and for the galactic cosmic radiation behind t h i n shielding have been calculated

and are shown in Figures 43 and 45 through 47. In a d d i t i o n , these spectra

were multiplied by the QF recommended by the ICRP ( 2 1 ) as the biological

effectiveness factor for use w i t h radiations o f vary ing dE/dx t o give a rough

idea of the biological importance of various dE/dx's in a hazard evaluation.

As stated by the Commission, such a f a c t o r i s probably conservatively high and

so the areas between the two curves are shaded i n the figures. All t h a t can be

concluded a t present is that the ' ' t rue ' ' curve l ies somewhere between these two

extremes. The curves will vary for different shielding thicknesses and body

depths.

25

It i s c l e a r t h a t t h e h i g h dE/dx component o f t h e GCR i s t h e m o s t b i o l o g i c a l l y

i m p o r t a n t p o r t i o n f o r t h i n s h i e l d i n q . A t t h e p r e s e n t t i m e i t i s i m p o s s i b l e

t o t r e a t a c c u r a t e l y t h e s i t u a t i o n f o r t h i c k e r s h i e l d i n g because o f t h e l a c k o f

data on secondary product ion by the heavier ions and by protons above 400 MeV.

26

CONCLUSIONS

From the above considerations i t has been possible t o make several conclusions

concerning the nature of the radiation, the properties o f the r a d i a t i o n behind

various shielding thicknesses, the areas where

are lacking, and the problems which need immed

improve our evaluation capability. Several of

discussed i n the text , b u t i t might be helpful

here.

1 .

physical and biologica 1 d a t a

ia te a t tent ion i n order t o

these conclusions have been

t o l i s t the major ones again

Experimental d a t a on the r a d i a t i o n environment are being supplied by well-

instrumented s a t e l l i t e s . There are several areas, however, in which more

detailed information is needed. Experimental errors on the d a t a points

for spectra of the heavier ions i n the GCR a r e s t i l l l a r g e , and for

completeness the shape o f the MH-particle spectrum should be obtained.

The spectra a t very low energies are impor t an t in determining the free

space dose a t solar minimum; therefore, the accuracy of f ree space dose

calculations suffer from the lack of information in th i s low energy region.

The rather r a p i d variation of the low energy p o r t i o n of the pro ton a n d

He ion spectra near solar minimum implies that the free space GCR dose

also varies throughout this period. An expl ic i t form for the temporal

and spatial dependence o f the modulation mechanism i s necessary for a

complete description of the spectra and therefore for an accurate

estimation of the dose.

27

2. Secondary production d a t a f o r protons o f energy greater t h a n 400 MeV are

necessary fo r a proper treatment of the dose behind typical shielding

thicknesses from the galactic protons, and a lso from solar par t ic le spectra

with la rge charac te r i s t ic r ig id i t ies ( 2 160 M V ) . Secondaries from

protons in this energy region were omitted i n the present treatment and

the depth-dose curves for the GCR are consequently too low. Data in this

energy region w i 11 soon be ava

3 . Secondary production from heav

L i t t l e i s known abou t the mean

i l ab le from Oak Ridge National Laboratory.

i e r ions was neglected i n a1 1 energy regions.

free paths or production cross sections for

h i g h energy heavy ions in light elements, and one may have t o await the

construction of a high-energy heavy-ion accelerator t o obtain the necessary

d a t a for an accurate evaluation. I t may be possible, however, t o arr ive

a t crude estimates by making certain simplifying assumptions as t o the

character of the primary interact ions between the heavy ion and a complex

nucleus. The proper inclusion of all secondaries from the various heavy

i o n nuclear interactions i n a n absorber for the different spectra of the

GCR heavy par t ic les and the resultant calculation o f a dose would undoubtedly

involve the development of a computer code of some complexity.

4. The rigidity representation for the solar particle spectra should be

cr i t ical ly evaluated as the d a t a on particle events accumulate in the

present cycle t o ascer ta in whether a1 1 large events can be adequately

described in this manner. The presence of low energy anisotropies a t the

time of peak f l u x i s an important new experimental r e su l t and has an obvious

bearing on spacecraft operational procedures d u r i n g the course of an event.

28

5. Regarding the calculations, reasonable agreement has been obtained between

the resul ts of the code used i n this study and the more comprehensive codes

developed a t Oak Ridge National Laboratory. Excellent agreement was obtained

for the primary doses. Somewhat less good agreement was obtained for the

secondary doses due to the approximations involved i n the present code.

The agreement i s f e l t t o be adequate for meaningful conclusions to be drawn.

6. The detailed calculations show t h a t , i n general, protons of energies between

10 and 100 MeV a t the dose p o i n t are the most impor t an t i n contributing

dose from solar particle events. T h u s , the most important incident energy

of the primary p ro ton i s t h a t energy such t h a t the pro ton arrives a t the

dose p o i n t in the above energy range. For instance, behind 1 g/cm , protons of i n i t i a l energy between 28 MeV (the cutoff energy) and 40 MeV

account f o r 85%, 65% and 57% of the primary dose for spectra with character-

i s t i c r i g i d i t i e s of 40 M V , 100 MV and 160 M V , respectively. One consequence

of t h i s is t h a t i f another type of shielding (such as a magnetic f i e ld or

plasma sh ie ld ) i s used in conjunction with static shielding, the energies

most important t o be swept away are those just above the s ta t ic shielding

cut-off energy ( the energy of a par t ic le with a range equal to the shielding

thickness). Higher pro ton energies become more important for f latter

spectra and for thicker shielding. For secondary protons the situation i s

not so clear cut . Behind 1 g/cm2, protons of i n i t i a l energy between 2 8 MeV

and 40 MeV account for 50%, 20% and 15% of the secondary proton dose fo r

spectra w i t h charac te r i s t ic r ig id i t ies of 40 M V , 100 MV and 160 M V ,

respectively. In the case o f secondary neutrons, proton energies below

2

29

the cutoff energy are important. The percentages of the dose from protons

w i t h init ial energies below 40 MeV are 70%, 30%, and 20% for the three

spectral shapes. As another example for the secondary par t ic les , a t

10 g/cm of a luminum shielding the most important i n i t i a l proton energy

for the secondary p ro ton dose i s s l igh t ly g rea te r t h a n 100 MeV (the cut-off

energy) for the spectra with characteristic rigidities of 40 M V and 100 M V .

The most impor t an t energy moves o u t t o 130 MeV for the spectrum of 160 MV

charac te r i s t ic r ig id i ty . For the cases of the neutron dose a t the same

depth, the most important i n i t i a l pro ton energies are 35 MeV for the spectrum

with character is t ic r igidi ty of 40 M V , and 65 MeV for the spectrum with

character is t ic r igidi ty of 100 M V . For the spectrum w i t h 160 M V characteris-

t i c r i g id i ty , t he maximum i s very broad and i n i t i a l proton energies between

100 and 300 MeV are important. A l s o , since the calculation requires this

curve t o go t o zero a t 400 MeV, the lack of secondary d a t a for primaries

greater t h a n 430 MeV is affect ing the resul ts for this spectrum.

2

Finally, the curves indicating the fraction of the dose produced by protons

greater than various energies show t h a t i n events with low character is t ic

r ig id i ty , a s ignif icant f ract ion of the dose i s produced by nuclear

reactions involving protons with energies less t h a n 25 MeV where the neutron

production cross sections can only be roughly estimated since few experi-

mental d a t a are available. Thus, i n these cases the conclusions must be

treated with caution.

30

7. Although biological effects are diff icul t t o quantify a t th i s s tage ,

certain general conclusions can be made concerning the biological

importance of the var ious par t ic les . For the solar particle events, i t

i s c l e a r from the LET spectra t h a t the low energy par t ic les a t the dose

p o i n t ( i- 10 MeV) are the most important fo r l ow charac te r i s t ic r ig id i ty

events behind t h i n shielding. Higher energies become increasingly

impor tan t as the character is t ic r igidi ty increases . However, for the

G C R , a t l e a s t f o r low shielding, there is a n interest ing reversal .

A1 t hough the high dE/dx components are particularly important biological ly ,

the higher energy heavy pa r t i c l e s , i . e . , t he ones near minimum ionization,

are the most impor tan t . This i s due, of course, t o the very d i f fe ren t

shapes of the GCR and SCR energy spectra. The GCR spectra al l have

maxima i n the range of several hundred MeV/nucleon, while the SCR spectra

peak a t very low energies! Also of i n t e re s t i s t he slow drop-off of t h i n -

down hits/cm -day as a function of shielding thickness. Certainly, 3

i n t e r io r

t h r o u g h

1 eve1

research i s necessary on the effects of heavy ions t o

b o t h b.y stopping ions as well as by those penetrating

ina l ly , work on recovery and repair from chronic, low-

considerable

body organs ,

the body. F

radiation with special emphasis on accumulated damage from multiple exposures

t o b o t h h i g h and l ow dE/dx pa r t i c l e s i s impor tan t t o be able t o improve our

evaluation capabili ty for extended missions.

31

ACKNOWLEDGEMENT

Dr. R. G . Alsmiller, J r . , of the.0ak Ridge National Laboratory, assisted

US great ly by providing, before publication, much of the nuclear d a t a

used in our calculations and the resul ts of the Oak Ridge Nucleon Transport

Code. Dr. W . R . Webber, a consultant t o Boeing from the University of

Minnesota, provided much of the information on solar par t ic le events

presented in this report. His assistance i n defining the galactic cosmic-

ray environment was a1 so most helpful.

32

REFERENCES

1 . W . R . Webber, "An Evaluation o f the Radiation Hazard due t o Solar-Part ic le Events ' ' , Boeing Document D2-90469 , December 1963

2. W . R . Webber, "An Evaluation o f Solar-Cosmic-Ray Events During Solar Minimum" , Boeing Document D2-84274-1 , June 1966

3. F . B. McDonald and G . H . Ludwig, "Measurement of Low-Energy Primary Cosmic-Ray Protons on IMP-1 S a t e l l i t e " , Phys. Rev. Lett. - 13, 783 (1964)

4. P . S . Freier and C . J . Waddington, "Electrons, Hydrogen Nuclei , and Helium Nuclei Observed i n the Primary Cosmic Radiation Dur ing 1963", J . Geophys. Res. - 70, 5753 (1965)

5. C . Y . Fan, G . Gloeckler and J . A . Simpson, "Cosmic Radiation Helium Spectrum Below 90 MeV per Nucleon Measured on IMP-1 S a t e l l i t e " , J . Geophys. Res. - 70, 3515 (1965)

6 . G . Gloeckler and J . R . J o k i p i i , "Low-Energy Cosmic-Ray Modulation Re la t ed t o Observed In te rp lane tary Magnet ic F ie ld I r regular i t ies" , Phys. Rev. Lett. - 17, 203 (1966)

7. C . Y . Fan, J . E . Lamport, J . A . Simpson and D. R . Smith, "Anisotropy and F1 uc tua t ions o f Solar Proton F1 uxes o f Enerqi es 0.6-100 MeV Measured on the Pioneer 6 Space Probe", J . Geophys. Res. 71, 3289 (1 966)

-

8. S . M . Comstock, C . Y . Fan and J . A . Sirnpson, "Abundances and Energy Spectra o f Galac t ic Cosmic-Ray Nuclei Above 20 MeV/Nucleon i n the Nuclear Charge Range 2 I Z 5 26" , Astrophys . Journ. 146, 51 (1966)

9 . W . R . Webber, t o be published

10. J . J . O'Gallagher and J . A. Sirnpson, "The He l iocen t r i c In t ens i ty Gradient o f Cosmic-Ray Protons and Helium During Min imum Solar Modulation" , Astrophys. Journ. 147, 819 (1967)

11. W . R . Webber, "Time Variat ions o f Low-Rigidity Cosmic-Rays D u r i n g the Recent Sunspot Cycle", Progress i n Elementary Par t ic le and Cosmic Ray Physics , Vol . VI , 75 (1 962)

12. P. S. Frei er and C . J . Waddington , "The He1 i um Nuclei o f the Primary Cosmic Radiation as Studied Over a Solar Cycle o f A c t i v i t y I n t e r - preted i n Terms o f Electric Field Modulation", Space Science Reviews - 4, 313 (1965)

33

13.

14.

15.

16.

17.

18.

19.

20.

21 .

P . S. Freier and W . R . Webber, "Exponent ia l Rigidity Spectrums f o r Solar Flare Cosmic-Rays", J . Geophys. Res. - 68, 1605 (1963)

W . C . Bartley, R . P . Bukata, K . G . McCracken and U . R . Rao, "Anisotropic Cosmic Radiation Fluxes o f Solar O r i g i n " , J . Geophvs. Res. - 71, 3297 (1966)

J . A . Barton and B . W . Mar, "Computer Codes for Space Radiation Environment and Shielding", Technical Documentary Report No. WL TDR-64-71 , Vol . I . Air Force Weapons Laboratory, August 1964

W . H. Barkas and M . J . Berger, "Tab1 es o f Energy Losses and Ranges o f Heavy Charged Part ic les" , NASA SP-3013, 1964

L . C . Northcliffe, "Passage o f Heavy Ions Through Matter", Annual Review o f Nuclear Science, Vol . - 13, p . 67 (1963)

R . G . Alsmil ler ,Jr . , M . Leimdorfer, and J. Barish, "Analytic Representation o f Nonelastic Cross Sections and Particle- Emission Spectra From Nucleon-Nucleus Collisions i n the Energy Range 25 t o 400 MeV", Oak Ridge National Laboratory Report ORNL-4046, Apri 1 1967

W . A . Gibson, "Energy Removed From Primary Pro ton and Neutron Beams by Tissue", Oak Ridge National Laboratory Report ORNL-3260, 1962

W . Wayne Scott and R . G . A1 m i l l e r , J r . , "Comparison of Results Obtained w i t h Several Proton Penetration Codes", t o be pub1 ished.

"Report o f the R B E Committee to the ICRP and ICRU", Health Physics - 9, 357 (1963)

34

10

>

2 1.0

I

PIONEER 6 - C.Y. FAN, J.E. LAMPORT, J.A.SIMPSON AND D.R.SMITH: JOURN. GEOPHYS. RES.71, 3289 (1966)

RES. 69, 3289 (1964) & V. K. BALASUBRAHMANYAN AND F. B. McDONALD; JOURN. GEOPHYS.

$ P. S. FREIER AND C. J. WADD INGTON; JOURN. GEOPHYS. RES. 70, 5753 (1965) I M P - I - F. B. MCDONALD AND G.H. LUDWIG; PHYS. REV. LET. 13, B, 783 (1964) V.K. BALASUBRAHMANYAN, D. E. HAGGE, G.H. LUDWIG AND F. B. McDONALD;

PROC. INT. CONF. COSMIC RAYS 1965, 417 (1966)

D I F F E R E N T I A L E N E R G Y S P E C T R U M O F G A L A C T I C P R O T O N S ( S O L A R M I N . )

T

T, M e V

FIGURE 1

35

I PU : PHYS. REV. LETT. l7, )+I I M P I I I - G. GLOECKLER AND J. R. JOK

1 V. K. BALASUBRAHMANYAN, D. E. HAGC 203 (1966)

.r McDONALD: PROC. INT. CONF. ;E, G. H. LUDIMG AND F. B.

COSMl C RAYS 1965, 427 (1966) f P. S. FREIER AND C. J. WADDINGTEN: JOURN. GEOPHYS. RES. 70,

5753 (1965)

GEOPHYS. RES. 70, 3515 (1965) +f$+ I M P I - C. Y. FAN, G. GLOECKLER AND J. A. SIMPSON: JOURN.

D I F F E R E N T I A L E N E R G Y S P E C T R U M O F G A L A C T I C H e N U C L E I ( S O L A R M I N I

NO DATA

T , Me VlNUCLEON

36

FIGURE 2

S. M. COMSTOCK, C. Y. FAN AND J. A. SIMPSON, ASTROPHYS ICs B I SWAS

JOU RN. 146, 51 (1966)

8 FICHTEL AND REAMES

D I F F E R E N T I A L E N E R G Y S P E C T R U M O F G A L A C T I C

T, M e VlNUCLEON

FIGURE 3

37

G. M. COMSTOCK C. Y. FAN AND J. A. SIMPSON; ASTROPHYS. JOU RN. 146 51 (1966)

+ C. F. FICHTEL D. E. GUSS K. A. NEEUKANTAN AND D. V. REAMES: PROC. INT. CONF. COSMIC RAYS 1965 400 (1966)

'+ Y. K. LIM AND K. FUKUI ; JOURN. GEOPHYS. RES. 70 4965 (1965)

$ D. E. EVANS: NUOVO CIM. 37 394 (1963) + S. B I SWAS AS QUOTED BY W. R. WEBBER

D I F F E R E N T I A L E N E R G Y S P E C T R U M O F G A L A C T I C M - N U C L E I ( 6 L Z 5 9 1 ( S O L A R M I N . )

/' (UNPUBLISHED)

\

T Me VlNUCLEON

FIGURE 4

38

G. M. COMSTOCK, C. Y. FAN AND J. A. SIMPSON: PROC. I N T . CONF. COSM I C RAYS 1965, 383 (1966) Y. K. L I M AND K. FUKUl

B I SlnlAS

/

D I F F E R E N T I A L E N E R G Y S P E C T R U M O F G A L A C T I C L H - N U C L E I ( 10 5 Z 5 1 4 ) ( S O L A R M I N . )

/

T, M e VlNUCLEON

FIGURE 5

39

z s

I

I

w

I

I >

.-E G. M. COMSTOCK, C. Y. FAN AND J. A. SIMPSON; ASTROPHYS. JOURNAL 146 51, (1966)

D I F F E R E N T I A L E N E R G Y S P E C T R U M O F G A L A C T I C V H - N U C L E I ( 2 6 5 Z 5 2 8 1 ( S O L A R M I N . 1

- - - - /' A N ARROW INDICATES

THAT THE TRUE VALUE COULD BE GREATER

- - /

; / NO DATA

lom4 - - / - - - - - -

I I I I 1 1 I l l I I I I I I l l 1 I I ~ I I "

10 lo2 lo3 lo4 T, Me VlNUCLEON

FIGURE 6

40

I

V I-

S a c3

D I F F E R E N T I A L R I G I D I T Y S P E C T R A O F G A L A C T I C P R O T O N S A T D I F F E R E N T T I M E S D U R I N G T H E S O L A R C Y C L E ( F R O M R E F . 11)

c MT. WASHINGTON NEUTRON RATE 2330 = N CURVE NUMBER PERCENT CHANGE IN MT. WASH. NEUTRON RATE

- (1) - 5% ( 2) - 10% ( 3) - 15%

- (4) [SOLAR MAX.] - 20% (5) + 5 % (6) [SOLAR M I N . ] + 10%

10-1 1.0 P, R I G I D I T Y I N GV

FIGURE 7

41

D I F F E R E N T I A L R I G I D I T Y S P E C T R A O F G A L A C T I C H E L I U M I O N S A T D I F F E R E N T T I M E S D U R I N G T H E S O L A R C Y C L E ( F R O M R E F . 12 )

RATE

10 P R I G I D I T Y IN GV

FIGURE 8

42

O A K R I D G E N T C

I 10

AL

SOLID LINES BOEING CODE " - NTC HISTOGRAMS

+L PRIMARY PROTON

\"" SECONDARY PROTON "

0 10 20 30 FIGURE 9

43

F G A L A C T I C C O S M I C R A Y P R O T O N S I N A L U M I N U M

n a E

- - " SECONDARY PROTON 5 400 M e V ""

NUCLEAR RECOl L 5 10 GEV

- - - / - NEUTRON 5 400 M e V

- /

\ 1 1 - NUCLEAR RECOl L L 400 MeV

GMICM* - AL FIGURE 10

44

U

\ I

a d

H E A V Y P A R T I C L E G A L A C T I C D O S E S

He , N , Mg , Co

CO - (VH)

N - (MI

\ \ \ \

I I I I I I I I I I I 0 10 20 30 40 50

GMICM* - AL FIGURE 11

45

I

40 MV P R O T O N S P E C T R U M DEPTH DOSE

0 10 20 30 40 50

GMKM' ALUMINUM FIGURE 12

46

0 10 20 30 40 50

GMKM* ALUMINUM

FIGURE 13

47

u VI w

I

I

160 MV PROTON SPECTRUM DEPTH DOSE

0 10 20 30 40 50

GMICM~ - ALUMINUM FIGURE 14

48

49

D I F F E R E N T I A L D O S E

D I S T R I B U T I O N S

F O R 100 M E V

R I G I D I T Y S P E C T R A

P R O T O N S I N C I D E N T

O N A L U M 1 N U M

FIGURE 16

D I F F E R E N T I A L D O S E D I S T R I B U T I O N F O R 40 A N D 160 M V R I G I D I T Y S P E C T R A P R O T O N S I N C I D E N T O N A L U M I N U M

1 10

E ( M E V ) - I N T E R

F1 GU RE

51

I

D I F F E R E N T I A L D O S E D I S T R I B U T I O N F O R G A L A C T I C

C O S M I C - R A Y P R O T O N S

FIGURE 18

D O S E P E R U N I T L O G I N T E R V A L F O R H E L I U M , N I T R O G E N ( M ) , M A G N E S I U M (LH), A N D C O B A L T ( V H )

Y \ 4

lo2

10

1

10 GM/CM* AL

T - MEVlNUCLEON INTERIOR ENERGY FIGURE 19

e E I

lo+

10-8

P E N E T R A T I N G P R O T O N N U M B E R - E N E R G Y S P E C T R U M I N V A R I O U S D E P T H S OF A L U M I N U M

40 MV S P E C T R U M I N C I D E N T

1 1 I . d

1 10 lo2 lo3 E - MEV INTER 10R ENERGY

FIGURE 20

54

P E N E T R A T I N G P R O T O N N U M B E R - E N E R G Y S P E C T R U M I N V A R I O U S D E P T H S O F A L U M I N U M

160 M V S P E C T R U M I N C I D E N T

I

1 10 lo2 lo3

E - M N INTER IOR ENERGY FIGURE 21

55

P E N E T R A T I N G P R O T O N N U M B E R - L E T S P E C T R U M 40 M V S P E C T R U M I N C I D E N T

I 1

o GMICM’

10 lo2

FIGURE 22 56

P E N E T R A T I O N P R O T O N N U M B E R - L E T S P E C T R U M 160 M V S P E C T R U M I N C I D E N T

MEWGMICM'

FIGURE 23

57

100

1 10

N U M B E R - L E T S P E C T R U M M V A L P H A S P E C T R U M

MEV CM*/GM FIGURE 24

58

I . . ..

L

1

N U M B E R - L E T S P E C T R U M 160 M V A L P H A S P E C T R U M

1

10 lo2 - M N CM!

GM

FIGURE 25

59

P R I M A R Y D O S E - 1 g m / c m * A L 10

I I\

- " "

" .

FIGURE 26

60

P R I M A R Y P R O T O N

10

D O S E - 5 g m l c m 2 A L

lo2

EXTERIOR CUTOFF ENERGY - MEV FIGURE 27

61

PRIMARY PROTON DOSE - 10 g m / c m 2

10

4

EXTERIOR CI

lo2

JTOF

c

F ENERGY - MEV FIGURE 28

62

SECONDARY PROTON DOSE - 1 GMICM'

100 MV

40

- EXTERIOR ENERGY CUTOFF

" REACTION ENERGY CUTOFF

l " L 1 I I I l l I I 1 I I 1 1 1

10 lo2 lo3 E X T E R I O R A N D R E A C T I O N C U T O F F E N E R G Y - M E V

FIGURE 29

63

SECONDARY PROTON DOSE - 5 GMICM~

64

SECONDARY PROTON DOSE - 10 GMICM* AL

10-3 1 .. I I I I I I I I I ~ I I I I I I I I J

10 IO2 lo3 E X T E R I O R A N D R E A C T I O N E N E R G Y C U T O F F - MEV

FIGURE 31

65

1

SECONDARY PROTON DOSE - 20 GMICM’

10 lo2 lo3 E X T E R I O R A N D R E A C T I O N E N E R G Y C U T O F F - M E V

FIGURE 32

66

NEUTRON DOSE - 1 GM/CM*

I I I I I I I l l I I I 1 I I I l l

10 lo2 lo3 E X T E R I O R A N D R E A C T I O N C U T O F F E N E R G Y - M E V

FIGURE 33 67

I

NEUTRON DOSE - 5 GMICM~ AL

-

-

-

-

w v)

n 0

d s 2

0

0 i=

I&

- - - - - - -

lo-;() 1 I I I 1 I l l 1 I I 1 1 I I I l l

lo2 lo3 E X T E R I O R A N D R E A C T I O N C U T O F F E N E R G Y - MEV

FIGURE 34

68

I

NEUTRON DOSE - 10 GMKM’ - - - - - - - -

-

- - - - - - - -

-

- - - - - - - - -

I 1 I 1 I I I l l 1 I 1 I I I l l 1

10 10‘ lo3 E X T E R I O R A N D R E A C T I O N C U T O F F E N E R G Y - M E V

FIGURE 35

10-

I- o

lo-*

69

I

10-

m W

0 n

a 2 z 0 I- O - s LL

lo-*

10

-

100

NEUTRON DOSE - 20 GMICM’ AL

,160 MV

\ 40 MV

LL I lo2

I I 1 1 I l k 1

lo3 E X T E R I O R A N D R E A C T I O N E N E R G Y C U T O F F - M E V

FIGURE 36

70

n

10 lo2 lo3 E X T E R I O R E N E R G Y A N D R E A C T I O N E N E R G Y C U T O F F - M E V

FIGURE 37

71

I

NEUTRON DOSE - 50 GM/CM* AL

-

I I I I I I I l l I I I I I I l l

10 lo2 lo3 E X T E R I O R A N D R E A C T I O N E N E R G Y CUTOFF - M E V

FIGURE 38

72

4 W

0.03

0.02

0.01

0

D I F F E R E N T I A L S E C O N D A R Y P R O T O N D O S E P E R U N I T I N C I D E N T P R O T O N E N E R G Y 1 0 g m / c m * AL

h 7: ~

1 I i ! I ~

, I

I i ,

i

400

i I 1 I

100 200 300 E - INCIDENT PROTON ENERGY

FIGURE 39

D I F F E R E N T I A L N E U T R O N D O S E P E R U N I T I N C I D E N T P R O T O N E N E R G Y 4 0 , 100, 160 M V P R O T O N S P E C T R U M I N C I D E N T

0. O?

2 0 0.0;

0.01

0

10 g m / c m 2 A L U M I N U M I

40 M V I i i

I

I -5 - 100 200 300 400

E - INCIDENT PROTON ENERGY FIGURE 40

GM/CM* AL F I G U R E 4 1

P E N E T R A T I N G P R O T O N N U M B E R - L E T S P E C T R U M 20 M V S P E C T R U M INCIDENT

CM2 MEV - GM FIGURE 42

4 4

L E T S P E C T R U M F O R G A L A C T I C C O S M I C R A Y S U N D E R

0.2 GICM* W A T E R S H I E L D I N G

UPPER CURVE IS LET SPECTRUM MULTIPLIED BY Q F

LH - h VH - PARTICLES

1 1 10 lo2 lo3 lo4

dEldX, M e V CM21G

FIGURE 43

I

50

IO

I

0. I

r

I I I I I 1 7 -

-

- - - - - - - -

LOWER L I M I T S FOR THE NUMBER OF TH IN-DOWN HITS PER CM3 DAY

I- NEGLECT1 NG SECONDAR I ES 1

CENTIMETERS OF WATER FIGURE 44

78

S O L A R P A R T I C L E E V E N T L E T S P E C T R U M F O R P E N E T R A T I N G

P R O T O N S P o = 20 M V F O R V A R I O U S T H I C K N E S S E S O F

A L U M 1 N U M UPPER CURVE FOR EACH THICKNESS IS LET SPECTRUM MULTIPLIED BY Q F

PROTON E I N M e V

:NERGY I I I I I I

20 10 4 2 1 . 4 . 2 . 1

10-3 I I I I I I 1 I I I I I I I I I I 1 I l l

1 10 lo2 lo3

dEldX M e V C M 2 E

79

S O L A R P A R T I C L E E V E N T L E T S P E C T R U M F O R

P E N E T R A T I N G P R O T O N S Po = 1 6 0 M V F O R V A R I O U S

T H I C K N E S S E S O F A L U M I N U M

I

1.0 10 lo2 lo3

$fi dE/dX, M e V CM2/G FIGURE 46

80

. n W t VI

2 w n

lo2

10 I S O L A R P A R T I C L E E V E N T L E T S P E C T R U M F O R P E N E T R A T I N G H E I O N S P o = 160 M V F O R T W O T H I C K N E S S E S O F A L U M 1 N U M

1.0

10-

- - UPPER CURVE FOR EACH THICKNESS IS THE LET

- SPECTRUM MULTI PLIED BY Q F

- - - - - - - - -

1 - c - - - - - - -

- Me VlNUCLEON I I I I I I I I 1 I I

- 100 5040 30 20 108 4 2

I I I I I I l l 1 I I I I I I l l 1 I I I I l l l l l

I 10 IO* 103

dEldx, MeV CM21g

FIGURE 47

81

IO

I. 0

io-'

10-2

SOLAR PARTICLE EVENT LET SPECTRUM FOR PENETRATING He IONS PO= 1 0 0 MV FOR TWO THICKNESSES OF ALUMINUM UPPER CURVE FOR EACH THICKNESS IS THE LET SPECTRUM MULTIPLIED BY QF

MeV /NUCLEON I I I I I I I I I I I 100 5040 252015 108 4 2

J 1 I I I I l l l 1 I I I l l l l l I I I I I l l

IO I02 103

dE/dx , MeV CMZ/g

FI GURE 48

82 NASA-Langley, 1968 - 29 CR-1037