The Space Environment and Its Effects on Materials and Component Parts

24 IRE TRANSACTIoNS ON RELIABILITY AND QUALITY CONTROL August

The Space Environment and Its Effects on Materials and Component Parts*

S. N. LEHRt, Senior Member, IRE and V. J. TRONOLONEt

Summary-The best available preliminary in- space vehicle is decided on the basis of the rela-formation has been gathered on what materials can tive importance of each piece of equipment inbe used successfully and how these materials re- terms of program objectives. At the same time,act in various space environments. Such informa- it is important that reliability objectives receivetion is necessary as a guide to space vehicle de- a priority in equipment design because of thesign engineers. great expenditure of time, effort, and resources

In addition to the factors presented here, such involved in any space program. To achieve theitems must be considered as: the exact nature of lowest possible failure rate and the greatest in-the missile of a space vehicle, the type of orbit, herent reliability, materials and electronic partsthe length of time the vehicle is expected to func- must be selected with utmost care.tion, the reliability objective, and similar goals, Although data are rapidly accumulating on thealthough regardless of the mission, certain general nature and effects of space environment, on ma-effects of the space environment present problems terials and electronic parts, and on techniqueswhich must be met in the design itself. Data have suitable for space equipment design, much re-been gathered on these general effects, which in- mains to be learned. Major work currently include high vacuum, magnetic fields, gravitational progress will undoubtedly result in the additionfields, micrometeorites, cosmic rays, neutrons, of specific information which can be used success-trapped charged particles, and electromagnetic fully by the design engineers for future space ap-radiation, including ultraviolet light, X rays, and plications.gamma rays. This information is summarized inTable 1.

THE SPACE ENVIRONMENT

INTRODUCTION The space environment is defined as the char-acteristics of space outside the earth's atmos-

Selection of materials and electronic parts for phere, or that region beyond 100 miles above thespace applications requires a knowledge of both earth. The environment includes:the known and the calculated or estimated charac- 1) High vacuumteristics of space environment. Both types of in- 2) Magnetic fieldsformation are presented in this document, derived 3) Gravitational fieldsfrom experience and literature surveys. 4) Micrometeorites

Since design requirements are based upon spe- 5) Cosmic rayscific mission objectives, the capabilities of the 6) Electromagnetic radiationlaunching vehicle, and capabilities of related fa- a) Ultraviolet rayscilities, information on environment must be con- b) X rayssidered in the light of its relationship to consider- c) Gamma raysations of the vehicle's size, weight, reliability, 7) Neutronsperformance and mission. For example, space 8) Charged electron and proton particles.vehicle design is limited by the capabilities of the A literature search indicates that many experi-launching vehicle; therefore the requirements of menters have gathered data on this environmentlight weight and minimum size become paramount. for many years. The following summarizes someAlso, allotment of space and weight within the of these data.

*This paper was presented at the Joint Reliability Semi- High Vacuumnar, Los Angeles Section, IRE, under the sponsorship ofthe PGRQC, PGCP, AND PGED, Los Angeles, Calif., De- The pressure at different altitudes is calcu-cember 5, 1960. lated from density measurements, assuming thattSpace Technology Labs ., Los Angeles, Calif. the perfect gas law holds. Data from rocket

1961 LEHR AND TRONOLONE: SPACE ENVIRONMENT; EFFECTS ON MATERIALS AND PARTS 25

TABLE I

Environment Effects Design Factors

Temperature Thermal energy within vehicle produced by Design for temperature control by means ofsolar radiation, earthshine, earth radiation, absorptive and reflective surfaced (ce ratio)and internal heating. No convection heating with heat control servo circuits. Isolate inter-outside earth atmosphere. nal equipment thermally for temperatures be-

tween 0' and 60'C depending on requirements.For heat transfer, use radiation and conductionheat sinks. Spin space vehicle to eliminatetemperature gradients around surface

Vibration Unimportant in space (except for launch Any vibration, acceleration, or shock levels inenvironment). No acoustic, frictional, or space which may occur would be very smallcombustion vibration problems except for compared with those during the boost phase.special applications. The equipment must be designed to withstand

the levels during boost, and therefore the lowerAcceleration Unimportant in space except for special levels encountered in space should pose no

applications. problems.

Shock Unimportant in space except for meteoriteand micrometeorite impacts.

High Vacuum Sublimation and evaporation of materials Use materials with low sublimation rates. Al-occur in high vacuum. low sufficient thickness for sublimation and

evaporation over expected operating life.

Chemical atmosphere produced by outgas- Select material with care to avoid hazardoussing and sublimation may have corrosive, conditions.plating, or chemical effects.

Electrical arc-over or corona discharge. Provide adequate insulation material and insu-lation paths.

Magnetic fields No effects except for fine instrumentation. Avoid use of instruments not shielded againstvariations outside earth's magnetic fields.

Gravitational fields No effect on materials or parts. None for materials or parts. For mannedvehicles, physiological considerations are in-volved.

Meteorites and micrometeorites Collisions with particles of varying sizes Statistically calculated risk is involved. Useoccur. preroughened surfaces or oxide finishes to

minimize changes in a/c ratio. Use sufficientouter skin thickness or secondary outer shellto provide protection against small particles.

Ultraviolet light Increases sublimation rates in high vacuum Minimize sublimation by selection of materialsand by providing sufficient material thicknessallowances.

X rays and gamma rays Ionization of material occurs, possibly Intensity of primary radiation is negligible butcausing atomic displacements which pro- shielding with heavy material may be con-duce changes in material characteristics or sidered for secondary ionizing radiationcomposition. effects.

Neutrons Intensity too low to require consideration of No special precautions necessary because ofatomic displacement effects. low intensity.

Trapped electrons Ionizing radiation occurs primarily in Van Protection required for externally mountedAllen belts, possibly causing atomic equipment such as solar cells. Space vehicledisplacements which produce changes in shell normally provides protection for internalmaterial characteristics or composition. equipment.

Trapped protons Ionizing radiation occurs primarily in Van For low-energy protons in outer Van AllenAllen belts, possibly causing atomic dis- belts, protection requirements are similar toplacements which produce changes in mate- those for trapped electrons. For high-energyrial characteristics or composition. protons in inner Van Allen belts, there is no

known adequate protection..

Biological organisms Contamination of space environment. Sterilization of vehicle and contents now re-quired by international agreement to preventpossible biological contamination of lunar orplanetary environment.

26 IRE TRANSACTIONS ON RELIABILITY AND QUALITY CONTROL August

flights up to 700 km (425 miles) have been reduced Results of Explorer VI and Pioneer V experi-and several models have been assumed from the ments verify that a region of ionized particlesaverage molecular weight of the atmosphere. Be- exists at an altitude of 5 to 7 earth radii and thatcause of the inability to measure pressures in the another disturbed region of the magnetic fieldrange of interest, direct measurements are not exists at 10 to 14 earth radii.possible. Readings in the 5 to 7 earth radii area varied

There have been a number of calculations of the from 150 to 10 microgauss. In the 10 to 14 earthambient pressures at various altitudes (Fig. 1). radii area, the magnetic field is irregular due to

reactions between the earth's magnetic field ando-^ the solar winds. Beyond this distance, there is

evidence of a magnetic field greater than 101o5 microgauss[8]

6 __l_ Gravitational Field7__ \ \ .The acceleration of gravity due to the earth's

AI 10 Xgravitational field is given by

\ARDC 1956 where g0 is the acceleration of gravity at a dis-o~~tl---~~~~ | 100 tanlce r0 from the center of the earth, and Z is the0 Z00 400 60 800 100 altitude above r . At the surface of the earth r0

V)8 ~ ATTD (TTT MLS

ALTITUDER(SATUTE:nMILES)d= 6,357,000 meters (3951 miles), and gr = 9.807Fig. 1. Pressure vs altitude (for several meters/sec2 (32.17 ft/sec2) at 45°N latitude[ 1].

atmospheric models).M icrometeor ites

The ARDC 19A56model atmosphere estimates dataup to 300 miles[ 11. Sicinski artd is associates Clouds of particles or micrometeorites ap-have made further calculations . Two possible preciably larger than molecular size exist in themodels of the upper atmosphere have resulted outer space. They are presumably the debrisfrom studies published by the RAND Corporation from other bodies in the solar system and for thein 1958[=3]. most part are very small. These particles are

The latest and probably the most authoritative observed by reflected light in the outer solarmodel of the atmosphere is the result of ARDC corona and from zodiacal light in the region of the1959 studies[ 4] This latest model gives densities earth, and the observations indicate that they are20 times higher than the 1956 model at 600 km and generally larger than one micron and in the regiononly half that of the 1956 model at 120 km. The of the earth's orbit may be larger thanexis.data are extrapolated beyond 700 km (425 miles). The velocity of these particles relative to theI free space, remote from the solar system, the earth varies from 11 to 73 km/sec and the aver-pressure may be as low as 10-16 mm of mercury, age density is approximately 3.4 g/cm3[9], al-which corresponds to one hydrogen atom/cm3 of though several experimenters believe that adustspace. balls" may have densities as low as 0.05

g/cm3[9]-[12].Magnetic Fields The best fit to the data obtained indicates the

following particle flux, which is plotted on Fig. 2.The maximum horizontal component of the

earth's mal etic field is 0.3 gauss at the magnetic N = number of particles larger than R/cm2secequator[ 5] ,[L6] . Theoretically, this field variesinversely as the cube of the distance from the cen- = exponent [-iof (s 4 l+i toter of the earth. This is true up to an altitude of Lvarie+ log R]about 300,000 feet (60 miles).) Above 300,000 feeta sharp decrease is noted[6a ,[ 7] in the measured where R - particle radius in centimeters.field intensity, which is associated with the pres- Mayo 13e] calculated the hours between hits onence of ionized particles, a 3-in-diameter vehicle vs meteorite diameter.


TABLE lloa RELATIVE PERCENTAGE OF COSMIC

FLUX CONSTITUENTS10° \_

20 -- - ~~~~~~~~~~~~~~~~~~~~~~RelativeNumberD: f f X X L X Particle PercentagelotNceons

___ ___ --___ ___ ~~~~~~~~Protons 80 0.4650Alpha 19 0.4419

___ ___ Li, Be, B Negligible NegligibleC, N, 0 0.66 0.0535Na, Mg, Al, Si 0.12 0.0186

I, 5~~~~~~~ ~ ~~~~~~~~~,A,Ca0.04 0.0081Fe 0.02 0.0063

111-z IO I-FAl.62*) la-I 10-S 10 n-,

-10-30

Fig. 2. Integrated flux density (number of particles Z - ----- .larger than indicated size/cm2/sec. 0 Pe 6

ti JO 1.0 1o .5 .2 I) 2.s 3.0 3.5

H ALTITUDNF 1N EARTH RADII

X 1 nel \ Fig. 4. Maximum ionization in 24 hours at severalgeomagnetic latitudes.

o 0~~~~~~~~~~~~~~~~~~~~~~~~~~~~~0

X \ ~~~~~~~~~~~Dataon extensive experiments covering theo lo2 performance of complex electronic equipment ato I \ high altitudes are already available[14]h [ 16] -[18]

X too~~~~~~~ \ ~~In no case has equipment failure been competentlyO \ ~~~~~~~~~~~~attributedto effects of cosmic radiation.

11

MET'F.OR[rEi IJ!'AMErER (INCHsES) Ultraviolet Light and X Rays: The total radia-tion from the sun in the several-thousand-mile

Fig. 3. Frequency of hits by meteorites of various range above the earth is 7.38 Btu/ft2-min[ i],[ 19],*diameters on a 3-in-diameter vehicle. This energy could be divided into four ranges of

radiation, each with the following percentage of theThese data are presented in Fig. 3. total energy:

Cosmic Rays Infrared 7000 A andiup 51.0 per centVisible 3800 A to 7000 A 41.0 per cent

The primary cosmic flux consists of particles t e

whose energy ranges from a few Mev upwards of Near ultraviolet 2000 A to 3800 A 7.5 per cent109 Bev. Excluded from this category are thermal, Far ultraviolet 1 A to 2000 A 0.2 per centX-ray, and optical radiations. The relative per- L 8centage of cosmic flulx constituents is given in (A = angstrom units - 10-8 cm).Table 11[E14]-[N17]E.

The maximum particle intensity is about two The total energy in the ultraviolet region isparticlescm2seec, and is a function of the geomag- about 0.5 Btu/ft2-mhinsThe energy in the X-raynetic latitude[ 14r region (o1 A to l00 A) is negligible.The maximum ionization in any material due to Gamma Rays: Observations of cosmic ray

cosmic rays is one milliroentgen per hour. Fig 4 fluctuations associated with solar flares accom-plots the maximum ionization in 24 hours as a panied by intense geomagnetic storms indicatefunction of altitude and geomagnetic latitude[ 14]. that intense pulses of gamma rays are injected

28 IRE TRANSACTIONS ON RELIABILITY AND QUALITY CONTROL Augustinto space[20]. These pulses are difficult to de- 16tect due to the shielding effects of the atmosphere.

There is no evidence that these events occuroften enough to be significant, and it could be as-sumed that the gamma-ray contribution to radiation R-intensity is negligible. They would be significant 8in communication, however, because of the bursts ,14of radio noise. ° 4 7

Neutrons 7r(0 X i, 0(0 ?2, 50OD)I'TANC;E FROM I'JIE'. FURFACE OE' THE EARTrH-j (kin)

Apparently there are no neutrons in the primarycosmic flux[ 17]. This is consistent with the 12- Fig. 5. Ionization radiation measurement of theminute half-life of the neutron. Any neutrons radiation zone.emitted by a source would have adequate time todecay to a proton and electron before reaching theearth's field. The neutrons present in the vicinityunder consideration are due to the upward moving 0 2(albedo) components of the neutrons formed by re- Mactions of primary cosmic radiation with the upper u. f _atmosphere. Z

The cosmic radiation intensity is roughly twoparticles/cm2 sec. The intensity of albedo neu- 0,4trons probably does not exceed this quantity be- ,cause the cross section of the cosmic particle- 226 Z8 30neutron reaction is not high[ 21]. The albedo DEGRIEES NOR'rTl LATI1'IJDEneutron flux will decrease inversely as the squareof the distance above the atmosphere. It is safe to Fig. 6. Variation of radiation with latitude.assume that the neutron flux above the atmospherewould not exceed one neutron/cm2 sec. Since the not be exact, the general tendency of decrease inthreshold of neutron damage to any material or ionization with increasing latitude is still ex-component is at least 1011 NVT (total integrated pected.neutron flux), even after years of exposure to this Pioneers m and IV gave the first indicationlevel (3.5 x 107 NVT each year), the total inte- that two or more ionized regions exist. Pioneergrated flux due to the albedo neutron in any equip- IV showed a large increase in radiation in thement should be insignificant. outer zone over the values measured by Pioneer

III without a corresponding increase in the innerCharged Particles-Electrons and Protons zone[28],[29]. This suggested the possibility of

a different origin of the particles in the two zones.The first experimental evidence of trapped The orbit of Explorer VI provided excellent

radiation around the earth resulted from the ex- opportunity for observing the changes in the twoperiment of Van Allen and his associates on Ex- regions over a period of time. The data con-plorer L Additional data were received from firmed the relative stability of the inner zone andExplorers M, IV, and VI and Pioneers I, II, III, the large time variation of the outer zone[ 28].and IV[22]-[28]. Pioneer I traversed the radia- Fig. 7 plots Explorer VI isointensity contours.tion zone radially, and the ionization radiation From these data, it is reasonable to deducemeasured as a function of distance above the sur- that there are two sources for the trapped par-face of the earth is plotted in Fig. 5. This curve ticles[ 30] ,[ 31]. The inner belt, containing large-indicates that the level exceeds two roentgens per ly high-energy protons, probably results from thehour between 4000 and 24,000 km altitude and be- decay of the albedo neutrons, while the outer belt,tween 30°N latitude and 100N latitude. It peaks at containing largely low-energy electrons, is prob-about 10 roentgens/hr at 10,000 km and 20°N lati- ably of solar origin. This would explain the timetude. variance of the outer zone as a function of solar

Pioneer Jl indicated the variation of the field storms.with latitude at an altitude of 1525 km. This is Summarizing the data reported by Van Allen,shown in Fig. 6. Rosen, and others, an approximnate plot of elec-

Although at higher altitudes this curve would tron intensity above 20 Key can be deduced for an


Il)NIA ('NJ- 1C

AXX;S |(

EARTH4

500k 100k Ok 1000 100 t()01kIo

/~~~~~~~~~~~~~(Contaurs inagtcateparticle countintensity.) u

4a1

Fig. 7. Explorer VI isointensity contours. Enere s

IA Xinten r fromta el

UAI8

The_intensity_at_the_20-mev_$hreshold_is_estimated Fig. 9. Proton intensity at the geomagnetic equator.0 10k 20k

ALTITUICE: EnIi) radiation sources change with distance from the

Fig. 8. Electron intensity above 20 Key (equatorial orbit). earth. For example, a few hundred mles abovethe earth the instantaneous power incident on a20-inch spherical satellite is approximately 67

equatorial orbit (Fig. 8). cal/sec from direct sunlight, from 0 to 25 cal/secAn approximate plot of the proton intensity for from earthshine, and of the order of 12 cal/sec

several threshold energies is presented in Fig. 9. from direct earth radiation. Contributions fromThe intensity at the 20-meve ries, dgaiestimated earthshine and from direct earth radiation fall offfrom the best available dat[L 17,L[20],L 30] 31]. with altitude above the earth. For instance, the

contribution at 4000 miles is only 20 per cent ofthat at 200 miles. The contribution from direct

EFFECTS OF THE SPACE ENVIRONMENT solar radiation does not change significantly forON MATERIALS AND COMPONENT PARTS any earth satellite except for periods of eclipse

by the earth, but may change drastically for spaceEffects Other than Radiation probes whose distance from the sun may differ

appreciably from that of the earth.Besides radiation, such factors as temperature', Two techniques for thermal control of elec-

high vacuum, micrometeorites, and gravitational tronic packages in space vehicles are used, pas-and magnetic fields affect missile and space sive temperatinlre control and active temperaturevehicle design. control.

Temperature Effects and Control: Tempera- Passive temperature control relies on selec-ture effects in space environment are the result of tion of a package surface material with the properthermal radiation from direct sunlight., from sun- ratio of solar heat absorptivity to infrared emis-


with the space vehicle skin, and heat transfer from where G is the rate of loss per unit area of ex-electronic equipment, Over a period of time, the posed surface, M is the molecular weight of thespace vehicle surface will be sufficiently changed material, T is its absolute temperature, P is itsby the effects of micrometeorite sputtering, ultra- vapor pressure at temrper ture T, and R is theviolet radiation, and particle radiation so that the universal gas constantL 35],absorptivity-to-emissivity ratio will be altered By applying this formula to elemental metals,significantly, and must be compensated for in it will be seen that in no case is the loss of struc-equipment design. tural significance except at highly elevated tem-

The second technique, active temperature con- peratures (which are not likely to be encounteredtrol, relies on controlled heaters or thermoelec- by spacecraft). However, some metals (e.g.,tric coolers to maintain the desired temperatures. cadmium, zinc, and magnesium) sublime enoughHeat is transferred from electronic components to at relatively low temperature to warrant attentionradiating surfaces through preselected conduction when used for nonstructural applications such aspaths. Heat gains or losses from other equipment platings or optical films.surfaces not selected as radiators are virtually The high-temperature, creep-rupture, andeliminated by use of a multilayer aluminum foil fatigue properties of metals are appreciably af-thermal radiation shield with an effective thermal fected under vacuum tests[ 36]. At high tempera-conductivity in vacuum of 2.5 x 10-5 Btu/hr-ft2-°F. tures and low stresses, creep-rupture specimensAn alternative type of active temperature control are stronger in air than in vacuum, but at lowuses mechanically operated temperature-sensor- temperatures and high stresses there is a rever-controlled radiation louvers which balance the sal and they are stronger in vacuum. Althoughabsorptivity-to-emissivity ratio. limited fatigue data show a similar behavior,

High-Vacuum Effects: Two important effects on tests are not sufficiently complete to be conclu-solid materials result from high vacuum. First, sive. A mechanism to explain this reversal in-sublimation and evaporation are enhanced by the volves two competing processes,: 1) strengtheningabsence of an atmosphere because molecules leav- by oxidation, and 2) reduction in strength by low-ing the surface of a material do not make collisions ering of surface energy. By definition, surfacethat return them to the surface. Therefore, if a energy is the work required to produce a unitspace vehicle is high enough so that the mean free area-of-fracture surface in a brittle materialpath of the molecules is long compared to the size with no plastic deformation.of the craft, any molecule that leaves the surface The higher the vapor pressures of a material,can be assumed not to return. Second, a partial or the more rapidly it sublimesf 37], A plot of vaporcomplete removal of the surface film of gas which pressure against temperature (Fig. 10) indicatescovers all material in the sea level atmosphere is the severity of the problem for a number of ma-affected. This depletion of surface gas layers has terials. The metals on the riglt portion of thean effect on the properties of materials[37]. figure, including the lightweight structural metals

Plastics: The effects of high vacuum on plastic titanium, beryllium, aluminum, and gold, wouldmaterials are varied. In general, it can be said not sublime at significant rates at their expectedthat the basic polymer of the plastic is not likelyto have a high enough vapor pressure to cause asignificant loss of material. However, plasticizers TEMPERAT00RE00C0used in many plastics have relatively high vapor IGpressures and therefore may cause the loss oflarge amounts of material. As can be expected, 10-4

the rate of diffusion of the plasticizer within the 250 Dplastic plays an important part in the over-all loss. IG '- BUTYL ROBO f

Tests conducted on several samples indicate 0 OOOOOOR /that exposure does not have significant effect onthe weight loss and flexural strength for periods ,/ /up to 500 hours, to ultraviolet in the range of 2000 M LARto 6000 A, and pressures in the 10-6 mm Hg range, POLYETHYLENE /

Metals: The effects of high vacuum on metals 0a OOJOL fj / iIican be calculated from kinetic theory / I ji /111 / ll

PM 200) 400 SOO 800 1000 2000

XJ ~~~~~~~Fig.10. Sublimation of metals in space (F. J. Clauss).


use temperatures. These materials should not Vehicle puncture is, therefore, a statisticalpresent a problem in high vacuum due to their low- probability and the ouiter shell thicknesses or theer vapor pressures. Metals which have higher va- possibility of using a secondary outer shell shouldpor pressures would be a potential source of be a function of this probability.trouble. For example, magnesium and also lithi- Erosion and general roughening of the outerumr, with which magnesium is sometimes alloyed, shell will effect the thermal considerations, andcould lose appreciable amounts. Zinc and cad- these effects should be taken into account in themium, both often used for electroplating parts, thermal design.have higher vapor pressures than magnesium. Other Environmental Factors: Several factorsTheir use within satellites could cause short- in the space environment require considerationcircuiting problems, due to evaporation and then for special applications but do not generally affectplating out on exposed electrical surfaces. A space the design of equipment.vehicle thus can actually be thought of as a vacuum Vibration, acceleration, and shock levels en-plating chamber, since any volatile material would countered by an orbiting space vehicle or a spaceleave the warm surfaces and plate out on the cooler probe are very small in comparison with thosesurfaces. experienced during launching or boost phases.

The phenomenon of sublimation from metals can Since the equipment must be designed to withstandbe retarded by inorganic surface coatings, such as the levels during launching and boost, the loweroxides, which often have lower vapor pressures levels encountered in space should pose no prob-than the metals from which they are formed. lems. For other space vehicle applications in-

Vapor pressures for plastic and elastomer ma- volving re-entry, recovery, landings, or other re-terials at room temperature (the left portion of lated missions, the effects of vibration,Fig. 10) are quite high in comparison to the acceleration,\l and shock may become major con-metals. siderations in equipment design.

Micrometeorite Effects: Micrometeorite par- Gravitational fields and magnetic fields to dateticle damage to a spherical space vehicle with a have required little consideration in the selectiondiameter of 3 m near the earth can be estimated of parts or materials for space vehicles. Thefrom the following curve. decreased gravitational fields encountered with

the increase in distance from the earth, however,do have effects which would require consideration

o in the case of manned space vehicles.10; \ Magnetic effects, which are unimportant forProbability 10 the design of space equipment at present, mightof a hit 0n 10 \ become important in space vehicles designed for

sphere 10-3 advanced applications. At present, the only seri-(per hour) 10-4 ous consideration of magnetic effects is for fine

10-5 \ B instrumentation which may need shielding to pre-io-6 \ D vent excessive variations outside the earth's10-7 magnetic fields.

110-3 l-2 l-l 10 Effects of Radiation Environment in Space10 10 10

Particle Diameter (cm)Background and Def initions: Both electromag-

Curves A and C represent the upper and lower netic (zero rest mass) and particulate (finitebounds of the estimates from[ 39]: curves C and D rest mass) radiation will be considered. Electro-the upper and lower bounds from [ 38]. Curve B magnetic radiation includes ultraviolet light, Xis probably the best estimate to date and summa- rays, and gamma rays (photons). Particulaterizes the latest available Whipple data. Using this radiation consists of electrons, protons, neutrons,curve, the best estimate of particle damage to a alpha particles, and a small amount of higher3-m sphere is shown below. atomic number particles[ 40] ,[ 41] -[ 43].

Radiation may also be classified as ionizing orPenetration of nonionizing. Ionizing radiation is capable of pro-Aluminum Frequency of Penetration ducing ions in the material through which it

nO5passes. Of the types of radiation in which we are

0.12cm( /in Once in 500 days interested, only neutrons are nonionizing in their1.0 cm Once in 100 years In addition to ionization, which is the loss of an

32 IRE TRANSACTIONS ON RELIABILITY AND QUALITY CONTROL Augustelectron by an atom or molecule when ionizing atomic nuclei, rather than by electron interactionradiation passes through material, excitation also as is the case with gamma radiation, a heavy ma-results. Excitation is the process in which an atom terial such as lead does not very effectivelyor molecule gains energy without being ejected, shield against fast neutrons. Very little energyand in many cases, is as important in producing loss occurs in collisions of neutrons with leadsecondary radiation. There is also the effect in atoms. Hydrogenous materials such as water orwhich a particle radiates part of its energy in the plastics are most effective in shielding againstform of electromagnetic waves when it interacts fast neutrons, since hydrogen nuclei and neutronswith matter. This process is known as brem- have the same mass.strahlung and is usually not an important radiation For charged particles, the penetration rangesource for material damage. These processes al- listed in Table III is the thickness required to re-ways occur simultaneously when ionization radia- duce the intensity essentially to zero. For gam-tion interacts with matter. ma rays and neutrons, the thickness is that re-

The basic assumption in radiation effects quired to reduce the intensity to half the incidentstudies is that changes to a material under irradi- value.ation result only from the transfer of energy to thematerial. Radiation which passes through a ma- TABLE III,terial without transferring any energy has no effect COMPARATIVE PENETRATING POWERon the material. Total radiation affecting a mate- OF TYPES OF RADIATIONrial is described by the energy flux, number flux,or exposed dose. Energy absorbed is describedby the absorbed dose. Radiation Energy Penetrating Range

The absorbed dose is useful in estimating radi- Particles (mev) (inches)ation tolerance, without regard to the kind of radi- Water Aluminumation, for materials such as organic compounds Alpha 1 0.002 0.001ation, ~~~ ~~~~~ ~~~~~~~~~100.010.004(plastics, elastomers, oils, greases) and many in- 100 0.4 0.14organic compounds. Proton 1 0.001 0.0004

For other materials (metals, ceramics, or 10 0.04 0.014semiconductors, for example), the radiation ef- 100 2.3 0.74fects are proportional to the exposure dose rather 300 24.0 7.9than absorbed dose. For a metal in a reactor, the Electrons 1 0.14 0.055most damaging component of the radiation field is 3 0.58 0.21the fast neutron; the effect is proportional to the Gamma Ray 1 4.5* 1.7*time-integrated fast neutron flux (neutrons/cm2). 5 9.1* 3.7*Different units are therefore used for different Neutrons 2 3.0* 3.5*materials. *Thickness necessary to reduce the intensity

Penetration of Radiation in Material: An impor- by 0.5.tant effect of radiation on material involves rangeor penetrating power. Since an alpha particle is Radiation Damage Mechanisms: Changes inrelatively massive and highly charged, it interacts properties of materials resulting from high-strongly and penetrates only a small distance be- energy radiation may be interpreted in terms offore stopping completely. In this distance it gives several types of defects produced in the materialup a large amount of energy, predominantly by in- by the radiation. These defects are: vacancies,teracting with and scattering electrons, resulting interstitial atoms, impurity atoms, replacementin a very high density of ionization. Similar con- collisions, thermal and displacement spikes, andsiderations hold true for protons. Electrons, al- ionization effects.though charged, are less massive and penetrate Effects on Metals: When a metal is exposed tofurther. ionizing radiation, the energy absorbed in ioniza-

Electromagnetic radiation, on the other hand, tion and excitation appears as heat and results inis highly penetrating. Similarly, a neutron, being temperature rise in the metal.electrically neutral, can travel relatively far be- When the radiation causes atomic displace-fore slowing down. Neutrons, X rays, and gamma ments, vacancies created in the lattice structureradiation are attenuated exponentially in passing are more serious. Secondary collisions may pro-through matter, and it is impossible to fix a defi- duce as many as 1000 displaced atoms per inci-nite distance of penetrationl, as is possible for the dent particle with 2 Mev neutrons141] .charged particles. Radiation canl cause: 1) stabilization of phases

Since neutrons are attenuated by collisions with in a temperature region which lies outside the


normal stability region of the particular phase, 2) Typical of the ionic-covalent bond are ceram-formation of metastable phase in supersaturated ics such as beryllium oxide and aluminum oxide,alloys, 3) formation of Frenkel defects (vacancy glasses such as the silicates and borates, andand interstitial) which influence diffusion- certain minerals such as mica. The principalcontrolled phenomena 4) thermal spikes, and damage mechanism occurs by atom displacement,5) transmutations[ 451. with changes in density, thermal conductivity, and

The following general summations of expected electrical conductivity resulting from the in-radiation effects, although not a complete quantita- creased lattice disorder.tive evaluation for all metals and metal systems, Typical inorganic materials which are essen-summarize many of the effects of radiation on tially covalent in structure are the elements car-metals. bon, silicon, and germanium, and compounds such

Gamma or X-ray radiation has primarily an as silicon carbide, indium antimonide, and zincionizing effect which does not change the metal oxide. Both the electrical and thermal conduc-properties. High-energy secondary radiation tivity of graphite decrease upon irradiation.(electrons), however, may have additional effects. Silicon and germanium, along with compoundsElectrons (or beta particles) do not have suffi- like indium antimonide, are well-known semicon-cient mass to cause any appreciable effects, since ductors. Irradiation with gamma rays results inthe energy rarely exceeds 1 Mev. Fast neutrons appreciable leakage currents; this effect quicklyappear to have the greatest effects on the physical disappears upon removal from the radiation field.and mechanical properties of structural metals. Permanent effects, however, are produced throughHigh-energy protons and the heavier charged par- the process of atom displacement.ticles are both similar in effects with damage Effects on Organic Materials: Organic mate-mechanisms mainly in the form of atomic displace- rials are susceptible to damage from all types ofments. In addition, protons may come to rest as nuclear radiation.interstitial hydrogen which may effect metal pro- Radiation damage in organic materials resultsperties. from the formation of foreign compounds. As

Neutron effects on various metals have been nuclear particles traverse a material, energy isstudied at integrated fluxes between 1015 and 1022 transferred to the electrons and nuclei of indi-fast NVT, primarily at temperatures between 30 vidual atoms in quantities sufficient to break theand 400TC. Properties of metals can be further bonds or linkages which bind the atoms into mole-changed either by increasing the neutron dose or cular groups. Afterpassage of the radiationby decreasing the irradiation temperature. Simi- particle, fragments of the disrupted moleculeslarities between cold work and radiation damage react chemically to form new compounds. Con-have been noted. In generalY the properties of centration of these impurity compounds increasesmetals such as thermal conductivity, electrical with increasing radiation and results in corres-resistivity, and density, do not appear to undergo pondingly greater changes in physical and me-any appreciable changes upon irradiation, even chanical properties.with fast neutrons. However, those properties of Elastomers as a class of materials are amongmetals which are structure-sensitive (yield those most susceptible to radiation damage. En-strength, hardness, ductility, etc.) usually experi- ergy absorbed by elastomers from ionizing radi-ence a considerable change upon prolonged expo- ations disrupts the bonds between the atoms andsure to fast neutrons. destroys the balance between the inherent free-

Effects on Inorganic Materials: Inorganic ma- dom of motion of the chain-like molecule and theterials include bond types ranging from purely degree of cross-linking or chemical bonding be-ionic through ionic-covalent (ceramics, glasses, tween the individual chains. As a result, hard-mica) to pure covalent (silicon, germanium, car- ness and tensile strength are increased and com-bon). pressibility and elongation are decreased.

From an atomic point of view, a typical exam- Most elastomeric materials are not satisfac-ple of a purely ionic compound (a salt such as tory for use beyond a gamma dosage of 108 ergssodium chloride) consists of a fixed network of gm-1 (based upon carbon) and a neutron flux ofpositively and negatively charged ions held together 1015 neutrons cm-2 sec41. Natural rubber is theby strong electrostatic forces. Fast neutrons pro- most radiation-resistant of the elastomers.duce atomic displacements by collisions with the Styrene-butadiene rubber is the most resistant oflattice ions, resulting in the interstitial-vacancy the synthetic elastomers. Silicones and fluorine-type of radiation damage. Also, electrons ejected based polymers are below average in radiationduring ionization processes may become trapped resistance.interstitially, resulting in "coloration cen- Plastic materials likewise are susceptible totersD[ 41] ,[ 461.

34 IRE TRANSACTIONS ON RELIABILITY AND QUALITY CONTROL Augustdamage by all types of radiation because of the .--

ease with which molecular structure can be re- --=oriented. Since plastics are also polymers, thereorientation consists of the formation of newbonds between chains, the breaking of chalns, the -20evolution of gases, the reaction with the environ-

2

ment, such as the absorption of oxygen, all result- t' 0 to 3

ing in changes in physical and mechanical proper-ties. Fig. 11. Conductivity of insulations as function of dose

Among the plastics, materials such as mineral- rates.filled polyester, mineral-filled phenolics, polyethy-lene, polyethylene teraphthalate (Mylar), polysty- dose rate is shown in Fig. 11. Obviously, higherrene, polyvinyl chloride, and polyvinyl formal will dose rates increase the leakage of the materi-perform satisfactorily to 109 ergs gm-1 gamma als[ 48],[49],flux (based upon carbon) and 1018 neutrons cm-2 Primary atomic displacements as well as dis-sec-1. Some unfilled polyesters are below aver- placements due to secondary radiation usually re-age in radiation resistance and are unsatisfactory sult in permanent damage.for most radiation applications. Electronic parts which contain organic mate-

Phenomena of interest in many organic poly- rials are damaged by radiation proportional tomers are cross-linking and scission. Cross- the exposure dose. For cables and wire leads,linking is the formation of bonds across polymer connectors, and transformers, which use organicchains due to displacements of hydrogen or other insulating materials, the threshold of damage toatoms outside the carbon chain. Scission is the insulation is much lower than any effect on thebreaking of a carbon chain. A material which metal.cross-links strongly will increase in strength with Most electronic parts are damaged by radia-radiation up to a certain point. One which scis- tion proportional to exposure dose, which is. asions strongly will decrease in strength with radi- function of the incident radiation. The thresholdsation. of damage for the most commonly-used compon-

Various organic insulating materials are listed ents under fast neutron and gamma radiation arebelow in decreasing order of effective lifetimes listed in Table IV[ 50] [51].under radiation[ 43] [47'].

1) Polyethylene TABLE IV2) Silicone Rubber3) Mylar RADIATION DAMAGE THRESHOLDS4) Polystyrene FOR ELECTRONIC PARTS5) Polyester

6) Eploxie Electronic Part Types |Fast Neutrons Gamma71) Nylon (NVT) (Roentgens)8) Polypropylene Resistors 1o8 1099) Neoprene 1015 to io18 106 to 90910) Butyl R-ubber Capacitors 0t1 0to0

11) Vinylidene Chlorides Transformers 1O18 10912) Kel-F Vacuum Tube 5 x 10l5 to 1018 106 to 10913) Teflon. Rectifiers (thin base Si) 1.5 to 1015 106Polyethylene, which cross-links strongly, is the Computer Diodes (Si, Ge) iol3 104

best and Teflon, which scissions strongly, the Transistors (Si) 101°to 12 103poorest. 11 14 4Effects on Electronic Parts: Electronic parts Transistors (Ge) 10 to 10 10

reflect radiation effects on the material used in Dry Cells 1015 5 x 1016their construction. In addition, other effects are a Lead-Lead Oxide Battery 1o15* 3 x 106function of the electronic circuitry. In general, Alnico V Magnets iol7* 7 x 108*digital is more affected by transients than analogcircuitry, while the latter has a lower total dose *No significant damage.tolerance. The variation in small-signal B (current gain)

Transient effects usually result from ionizing for several transistor types has been measured.radiation and are a function of the dose rate. The One of these, type T1075, showZed no variation inconductivrity of several insulations as a function of gain for exposure to 1014 NVT. The curve


1.0 a T1075-03TASTOa DIODE (POiNT CONTACT) F TRANSISTOR

iL5405, T1166 -to 4ACUUM TUBE

\ TtO 905 2N247 RESIS TEFLONa

1 39 OI~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~PLEHLEN- 10 9

NYLOLa PRENOLICe OREASE =

HA500312.Neutronradiationeffects on currentgainoRUfER C3HA5003 2N94A BUNA-NN

T-n900 2N247 RESISTOR (CARBON) s-iST33sr, E2NI74, 2N176 POIDETHLENE0DB

H7 CAPACITOR (CERAMlIC -MYLAR)-E

0.p- . I(d I I CAPACITOR PowCA gp

0.~~~~~~ INERAE DOS IRCA)

10~~~~1 3 14 FORMED10 1 0 10 POLYSTYRENE -[IOFAST NEUTRONS (NVT) CARBON

CONCRETE 0I12

Fig. 12. Neutron radiation effects on current gain of CERAMIC 1013CPE

several selected transistors. (a) HF germanium ALAMINUM ol4BP-nLp. (b) HF silicon p--p (c) HF siliconnppjn. (d) Power germanium p-n-p. (e) General- INTEBRATED DOSE (RABBIpurpose germanium p-n-. (f) Switching siliconn-p-n. Fig. 13. Functional radiation dose thresholds.

(Fig. 12) indicates clearly that germanium tran- BIBLIOGRAPHYsistors are more radiation resistant than silicon [1] R. A. Minzer and W. S. Ripley, "The ARDC Modeland that HF narrow-base devices are be ter under Atmosphere, 1956," AF Cambridge Res. Ctr.,radiation than LF thick-base devices[52]v ARDC, Bedford, Mass., ASTIA 110233; December,

Little information is available on the effects of 1956.protons on these electronic parts. Bombarding [2] H. S. Sicinski, N. W. Spencer, and R. L. Boggis,bulk germanium and silicon with deuterons has re- "Pressure and density measurement through par-sulted in large changes in resistivity and in the tial pressures of atmospheric component at mini-mum satellite atmospheres," in "Scientific Uses ofcase of gprmanium, a change from n- to p-type Earth Satellites," J. A. Van Allen, Ed., University[50],[ 51] . Empirical calculations have been of Michigan Press, Ann Arbor, pp. 109-118; 1956.made[ 53] on the number of displacements/cm3 [3] H. K. Kallmann and M. L. Juncosa, "A Preliminaryper incident proton with energies below and above Model Atmosphere Based on Rocket and Satellite100 Mev. Lark-Horovitz[40] believes that the Data," The RAND Corp., Santa Monica, Calif.,threshold energy for displacement in silicon and ASTIA AD 207752; October 30, 1958.germanium is 30 evt resultig inapproximately [4] R. A. Minzer, K. S. W. Champion, and H. L. Pond,1000 displacements/cm3 per incident proton. 'The ARDC Model Atmosphere, 1959," AF Cam-

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The Space Environment and Its Effects on Materials and Component Parts