first we will briefly review some of the fundamentals of radiation physics and the interaction of...

First we will briefly review some of the fundamentals of radiation physics and the interaction of radiation with matter.

Radiation Effects on Space SystemsKnowledge of the radiation environment is relevant to the design of all types of spacecraft, manned and unmanned.

ASEN-5335 Aerospace Environments -- Radiation Effects on Space Systems 2

Radiation and Spacecraft DesignThere are 3 primary considerations in the design of spacecraft for radiation

effects:

1. Specification/prediction of the radiation environment, and its variability

2. Determination of how that radiation propagates through material to the

target area (Note: this involves generation of many secondary particles.)

3. Knowing how radiation at the target affects specific spacecraft

components (circuitry, solar cells, etc.)

From the standpoint of radiation interactions with matter, 3 types of radiation need to be considered:

1. Photons (primarily EUV, X-rays, & gamma rays)

2. Charged particles (protons, electrons & heavy ions)

3. Neutrons -- produced as secondary particles as

GCRs interact with Aluminum

Principal physical characteristics of interest:

Impacting Particles: mass, charge and K.E.Target Material: mass and density

Monte Carlo techniques are often used to estimate gross effects of the cascade process. In the simplest models, analytical/empirical expressions are fit to these results or actual measurements to give estimates of the end products produced as a function of depth in the shielding material.


BASIC RADIATION PHYSICS

If the electron is moving too slowly, it lacks the K.E. necessary to create ionizations.

If the electron moves too fast, it "passes through" without effectively interacting with the ambient atoms and molecules.

Consequently, radiation damage to materials is dependent not only on the nature of the radiation, but on the energy of the radiation and the nature of the material itself.

As an example of the interaction between radiation and matter, an energetic electron passing through air will leave a trail of ionized particles:


Photon Interactions

Photoelectric Effect

Photon completely absorbed by the emitted L-(outer) shell or K-shell electron

Compton Scattering

The incident is so energetic that it is not completely absorbed. Part of the energy goes into scattering an electron (“Compton electron”), and the rest into a scattered lower-energy photon.

Pair Production

Photons with sufficiently high energies are fully absorbed by a sufficiently high-Z material. A positron-electron pair is then formed

Note: The products of these interactions further interact with the target material, producing a complex cascade of electrons and photons.


UV photons interacting with complex organic polymers can cause material degradation and change the properties of materials.

Flight data from the SCATHA Satellite indicating that the solar absorptance of thermal control coatings increases under UV radiation.

Temperature of a spacecraft is proportional to the solar absorptance, and too great a temperature increase can lead to loss of the spacecraft.

The absorption of UV radiation by organic molecules leads to highly reactive excited states that react on a molecular level and lead to macroscopic changes in the surface properties.

Careful selection of radiation-resistant, external surface materials can be an important consideration in spacecraft design.

SilverTeflon


Charged-Particle Interactions

Rutherford (Coulomb) Scattering Nuclear Interactions

• Usually dominates

• Charged particle interacts with electric field of target atom

• Results in both excitation and ionization of atomic electrons

• For sufficiently energetic impacts, can displace atoms within the lattice structure

• Impacting particle actually interacts with the atomic nucleus

• Can result in transmutation (conversion of on element or isotope to another)

Recoil atoms from displacement can cause significant damage

Long-term exposure can make spacecraft material radioactive


UNITS

1 Roentgen = amount of gamma radiation or X-rays that will produce one electrostatic unit of charge of either sign (2.08 x 109 ion pairs) in one cubic centimeter of air at standard temperature and pressure.

1 Rad = radiation which deposits .01 J/Kg of material

You may be more familiar with the Rad (radiation absorbed dose) or Roentgen:

1 Gray = radiation which deposits 1 J/Kg of material

The official SI unit of radiation is the Gray (Gy)


Radiation dose is the amount of energy deposited in a material.

Two major factors in the determination of radiation damages:

(1) total dose over the life of a material; and

(2) dose rate, the rate at which energy is deposited.

Aluminum

X particles cm-2 s-1

= Rad s-1


Different materials have different susceptibilities to damage:

Radiation dose is dependent on the type of radiation and its energy in addition to the material itself.

This is why metals are used toprotect the other materials.


By considering the electrostatic forces involved, one can calculate the KE lost by a charged particle traversing matter, leading to the definition of stopping power. Experimental data also exist.

€

R = dx=−dKEnσ stopKE

0∫∫

The range of the particle in the absorbing material can be calculated by integrating the above to give:

Stopping power is the amount of energy lost by a particle per unit length of path through the material. This is sometimes called linear energy transfer (LET):

€

−dKE

dx= nσ stop where n is the

material density (cm-3).

Note: “stopping cross-section”, not in units of area.

STOPPING CHARGED PARTICLES


Sample Stopping Power Curves forIons and Electrons in Silicon

Note units

€

1

ρ

dE

dx


Sometimes Penetration Depth is used as a Measure

Penetration depth is a rough estimate of the minimum cutoff energy for a given thickness of spacecraft shielding, and hence the effectiveness of this shielding.

PenetrationDepth:

maximum distance a particle of a given energy can penetrate

Typical shielding thickness 40-80 mils (0.1-0.2 cm

Note that:A 1-MeV electronpenetrates over 100 times more shielding(≈0.2 cm) than a 1-MeV proton (≈0.0015 cm)


Sometimes Fraction of Fluence orFlux Transmitted as Function of Depth (of Shielding

Material) is used as a Measure

i.e., 40 mils Al ineffective in stopping 5 MeV electron

Density of aluminum is used as standard reference

To first order the amount of shielding scales with the density of the material

Electrons above a few hundred keV can penetrate a significant amount of material

For very high-energy (relativistic) electrons, Brehmsstrahlung radiation produces additional damage in materials.

1 mil = .001 inches


Information on satellite anomalies is not widely shared within the industry or with the public. From the July 11, 2002 Space Daily:

“Satellite Anomalies Pushing Insurance Rates Up”

Satellite AnomaliesMost Satellite Anomalies are Unrelated to the Space Environment

On-orbit anomaly records (guidance, navigation and control only) for satellites launched between 1990 and 2001


MISR - Multi-Angle Imaging Spectro-

Radiometer

Satellite Anomaly Example:TERRA – MISR Data Before

Shutter Opening3-16 Feb 2000

Besides light, energetic particles can generate

a signal in a CCD.


Geomagnetic field contours near 1,300 Km altitude and superposed anomaly locations for TOPEX (red dots) and TERRA-MODIS temporary failure location (star) in June 2001.

Satellite Anomaly Examples:TOPEX – 1992-1998 and TERRA-MODIS 2001


Tracking and Data Relay Satellite (TDRS) is one of a network of communications satellites of the Tracking and Data Relay Satellite System (TDRSS) used by NASA and agencies for communication to satellites or the International Space Station. The system was designed to replace an existing network of ground stations that had supported all of NASA's manned flight missions. The prime design goal was to increase the time spacecraft were in communication with the ground and improve the amount of data that could be transferred.

In the mid-1980s, NASA’s TDRS-1 satellite was launched into GEO to begin replacing the global tracking antenna network with an array of satellites to relay commands to LEO satellites and Shuttles. Eventually there were to be 4 such satellites operating in a constellation to provide uninterrupted communications. However, experiences with the first satellite of this series (see next two slides) were not good. Eventually convincing evidence was produced that the many anomalous events on TDRS-1 were a product of susceptibility to galactic cosmic rays and to solar energetic protons and heavier ions. One of the engineers involved in Albuquerque with operating TDRS-1 characterized the 93L422 RAM chip as “a flying solid state cosmic ray detector”. At some expense the chips were replaced in TDRSS-2 to –4 and greatly reduced the number of anomaly events (but did not eliminate all of them).

NASA continued using the RAM chip at least through the construction and launch of the Hubble Space Telescope (HST) which has problems similar to those of TDRS-1 when HST passes through the SAA.

Slide Credit: Joe Allen, NOAA/NGDC -- http://www.scostep.ucar.edu/archives/gomac_2002/GOMACTalk.html

In the mid-1980s, NASA’s TDRS-1 satellite was launched into GEO to begin replacing the global tracking antenna network with an array of satellites to relay commands to LEO satellites and Shuttles. Eventually there were to be 4 such satellites operating in a constellation to provide uninterrupted communications. However, experiences with the first satellite of this series (see next two slides) were not good. Eventually convincing evidence was produced that the many anomalous events on TDRS-1 were a product of susceptibility to galactic cosmic rays and to solar energetic protons and heavier ions. One of the engineers involved in Albuquerque with operating TDRS-1 characterized the 93L422 RAM chip as “a flying solid state cosmic ray detector”. At some expense the chips were replaced in TDRSS-2 to –4 and greatly reduced the number of anomaly events (but did not eliminate all of them).

NASA continued using the RAM chip at least through the construction and launch of the Hubble Space Telescope (HST) which has problems similar to those of TDRS-1 when HST passes through the SAA.


Satellite Anomaly Example: TDRS


TDRS-1 anomalies during one of several hard-spectrum proton flare/CME events in October 1989.

TDRS-1 anomalies during one of several hard-spectrum proton flare/CME events in October 1989.

Slide Credit: Dan Wilkinson and Joe Allen, NOAA/NGDC -- http://www.scostep.ucar.edu/archives/gomac_2002/GOMACTalk.html


The 5-year time-series of TDRS-1 SEU events shows both a susceptibility to galactic cosmic rays (upper panel - long period variation out of phase with the sunspot cycle) and to solar energetic particle events (many spikes in 1989).

The 5-year time-series of TDRS-1 SEU events shows both a susceptibility to galactic cosmic rays (upper panel - long period variation out of phase with the sunspot cycle) and to solar energetic particle events (many spikes in 1989).

TDRS-1 was an unintended probe of the energetic particle environment at GEO altitude



CALCULATING RADIATION DOSE

A crude measure of damage by penetrating energetic particles is the radiation dosage measured in rads.

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

1. in a semi-infinite plane medium, as a function of depth; irradiation is from one side only (the assumed infinite backing effectively insures this).

2. at the transmission surface of a plane slab, as a function of slab thickness; irradiation is from one side only.

3. at the center of a solid sphere, as a function of sphere radius; irradiation is from all directions.

Computer codes exist that will convert incident energetic particle fluxes into radiation dose behind a specified shield thickness, i.e., SHIELDOSE

http://www.spenvis.oma.be/spenvis/help/background/shieldose/shieldose.html


Effects ofatmosphericshielding

Accumulated doseover 5-yr missiondue to radiationbelt particles

“hard”

“soft”

Range of total dosevariability for variousmicroelectronic devices

In many cases a tradeoff exists between orbit choice and system lifetime.

.1 inch = 100 mils


Total accumulated dose depends on orbit altitude, orientation and time. Total ionizing dose (TID) for satellites typically ranges between 10 and 100 krad (Si).

4mm Alshielding

* total


Optimal Design Point

Radiation Dose For Different Al Shield Thicknesses, LEO

Note: Protons dominate in general


Geosynchronous Orbit Dose for Different Al Shield Thicknesses (8-Year Mission)

102

103

104

105

106

107

108

10 100 1000 10000

Rads (8 Years)

Shield thickness, mils aluminum

Electrons

Total

ProtonsBrems

Solar

Geosynchronous orbit

Optimal Design Point

R

ads/

year

Note: electrons dominate

for thinnest shields; also, higher doses

than LEO


The CRRESRAD Model

Differences between CRRESRAD and equivalent results for AP8 and AE8 can sometimes be substantial.

PC-based software program that provides estimates of the dose behind 4 shielding thicknesses of hemispherical aluminum shielding (0.57, 1.59, 3.14, and 6.08 gm/cm2) --- corresponding to electron energies > 1, 2.5, 5, and 10 MeV and proton energies > 20, 35, 52, and 75 MeV. Different levels of magnetic activity are included.

-- complete complement of radiation environment sensors (dosimeter, e- and H+ monitors, etc.)

-- July 25, 1990 - 12 October 1991; 18.1°, 350 km x 33,000 km orbit

Based on the CRRES (Combined Release and Radiation Effects Satellite) Mission.

See http://www.kirtland.af.mil/library/factsheets/factsheet.asp?id=7899 for a complete list of available “AF-GEOSPACE” user-friendly, graphics-intensive software program bringing together many of the space environment models, applications, and data visualization products developed by the Air Force Research Laboratory

http://www.kirtland.af.mil/library/factsheets/factsheet.asp?id=7899


Some Additional Slides for Reference


fo(E) ambient flux

shielding

target

€

D(E) = dose particle−1

=energy

mass=δE

M=δτ

M

dE

dx E

€

D(E) = 1

ρ

dE

dx E

1

N(E)

⎡

⎣ ⎢

⎤

⎦ ⎥N(E)

δA

Total dose at energy E

€

DT (E) = N(E)D(E) = 1

ρ

dE

dx E f (E)(rads MeV-1)

f(E) shielding-attenuated flux

€

=N(E)

δAparticles cm-2MeV-1

unit area A

€

τ energy depositedin target for particleof initial energy E

€

=E = δτdE

dx E

Jparticle-1MeV-1

€

M = ρδτδA

€

= f (E)

Radiation-Dose Environment Within a Spacecraft:Basic Mathematical Steps

flux to doseconversion factor(see homework)

Total doseover allenergies

€

= 1

ρ

dE

dxEo

∞

∫ E f (E)dE

One then needs to repeat the above process over different parts of the spacecraft, and integrate over the expected radiation conditions for the expected spacecraft orbit during the period of the desired orbital operations (mission lifetime).


Case Study: The Clementine SpacecraftLunar Transfer Orbit

• Clementine was a DoD/NASA mission designed to map the moon and an Earth orbit-crossing asteroid.

• Also tested the effects of the radiation environment on a number of microelectronic systems.

• Clementine left behind its lunar transfer stage in a unique, highly elliptical orbit that passed repeatedly through the trapped radiation belts.

• This interstage and Clementine were both instrumented with radiation dosage and SEU detectors. They carried boxes of microelectronics components for direct exposure to the radiation environment.

• A detailed radiation environment prediction was required to allow identification of radiation sensitive parts and to determine appropriate replacement parts or provide enhanced protective measures.

• It was also desired to predict the performance of the systems and test components in the radiation environment.

• These are all typical requirements for a space mission and illustrate the wide range of potential requirements that a trapped radiation model might be expected to address.


Five geometrieswere considered:

• B and L coordinates were calculated for the orbit.

• Particle integral flux as a function of energy was computed from the AE8/AP8 models, and summed over mission lifetime to give the total integral fluence spectrum in terms of energy and particle species.

• These spectra were inserted into the commercially-available NOVICE code to compute total energy deposited at a point as a function of shield thickness, composition (Aluminum for shield and Si for dose site) and geometry.

Steps for Obtaining Clementine Dose Estimates

Usually mostappropriate


• A set of orbital data for the portions of the Clementine orbitwithin 11 RE was assembled. (AE8 models are only usefulinside ~11 RE ; ~6 RE for AP8):

Examples follow

• Radiation design for Clementine used a radiation design margin (RDM) = 4 (i.e., estimated dose values were multiplied by 4 to provide a worst case design requirement for parts).

• A range of doses was calculated for the different geometrical considerations, SSMIN & SSMAX, shielding thickness and particle type.

1) 1994/01/24 (01:48)—1994/01/26 (01:00) (initial parking orbit).2) 1994/01/26 (01:00)—1994/01/30 (06:44) (Earth–moon transfer injection).3) 1994/01/30 (07:00)—1994/02/09 (12:00) (Earth–moon transfer orbit).4) 1994/02/09 (12:00)—1994/02/21 (01:09) (Earth–moon transfer orbit).5) 1994/05/03 (13:17)—1994/05/15 (12:00) (Earth gravity-assist flyby).6) 1994/05/15 (12:00)—1994/05/27 (18:08) (Earth gravity-assist flyby).


Radiation dosage from the trapped proton environment (AP8) for solar quietconditions (worst-case condition for protons) and for the first Earth-moon transfer orbit.

nominal


Radiation dosage from the trapped electron environment (AP8) for solar activeconditions (worst-case condition for electrons) and for the first Earth-moon transfer orbit.

nominal

Note widervariation inresults for electrons.


Mission summary plot for the total radiation dose for the Clementine interstage.This assumes a 450-day mission and 95% confidence flare environment.

first we will briefly review some of the fundamentals of radiation physics and the interaction of...

Documents

radiation effects

types of radiation

radiation environment

interaction of radiation

radiation damage

absorption of uv radiation

careful selection of

aerospace environments