Radiation Effects in Materials, with Emphasis on ...Origins of Radiation Effects in Materials...

Radiation Effects in Materials, with Emphasis on Insulators for

Couplers

S.J. Zinkle and L.K. Mansur

Metals and Ceramics Division, Oak Ridge National Laboratory

Workshop on High-Power Couplers for Superconducting AcceleratorsJefferson Laboratory

October 30-November 1, 2002

Outline• Radiation effects in materials• Radiation effects on the electric and dielectric

properties of inorganic insulators (focus on Al2O3)–Postirradiation (accumulated dose) and in-situ

(prompt) effects–Accumulated damage effects are generally

insignificant for doses below ~1 GGy in materials at do not experience radiolysis

• Radiation effects in polymers–Dose range for degradation of polymers and

comparison with anticipated doses in SNS couplers–Sensitivity to radiation for significant degradation

ranges from ~1 kGy to /103 kGy

Radiation Effects on Properties of Materials

• Short answer–Virtually every property can be changed

• Long answer–Dimensions–Mechanical properties–Electrical properties–Optical properties– • • •

• Underlying these changes are the production of defects and defect clusters, alterations in microstructure (e.g., dislocations, voids, precipitates) compositional segregation, electronic ionization and excitation …

Historical Perspective on Radiation Effects

• Some radiation effects were observed in minerals in the 19th century, but their origin was not understood

• E. P. Wigner, 1946, Journal of Applied Physics 17–“The matter has great scientific interest because pile

irradiation should permit the artificial formation of displacements in definite numbers and a study of the effect of these on thermal and electrical conductivity, tensile strength, ductility, etc. as demanded by the theory.”

• The full scope of radiation effects in materials was only appreciated after fast spectrum reactors were operated in the 1950’s and 1960’s

Origins of Radiation Effects in Materials

• Displacement of atoms (nuclear stopping)–Dominant damage process for metals–Can be important for ceramics and polymers–The unit of damage is the displacement per atom, dpa–One dpa is the dose at which on average every atom in

the material has been energetically displaced once• Ionization and excitation (electronic stopping)

–Generally negligible for metals–Important for ceramics and polymers–The unit of damage is the Gray, Gy–One Gy is the dose at which the material has absorbed

1 J/Kg

Origins of Radiation Effects in Materials

• Transmutation reactions–Transmutation products, especially helium and

hydrogen from proton- and neutron-induced reactions, exacerbate damage

–Customary unit of measure is appm transmutant per dpa, e.g., appm He/dpa

• Typical highest damage rates (10-6 dpa/s, >103 Gy/s, 100 appm He/dpa)–High power spallation target –High flux reactor core–Fusion reactor first wall

Hierarchy of Reactions Leading to Property Changes by Displacement Damage

Swelling, Irradiation Creep, Embrittlement

Time and Energy Scales for Radiation Effects by Displacement Damage

Time

Cascade Creation10-13 s

Unstable Matrix10-11 s

Interstitial Diffusion10-6 s

Vacancy Diffusion100 s

Microstructural

Energy

Neutron or Proton106 - 109 eV

Primary Knock-on104 - 105 eV

Displaced Secondary102 - 103 eV

Unstable Matrix100 eV

Thermal DiffusionEvolution106 s kT

Ionizing and displacive radiation doses for SNS couplers—Coupler calculations courtesy of Franz

Gallmeier, ORNL

36 dpaSpallation target container peak displacement dose (1 y)

~ 10-6 dpa/s~ 2/3 n, 1/3 p

Spallation target container peak displacement dose rate

95 kGyCoupler peakionizing dose(10 y)

~ 3 x 10-4 Gy/sCoupler peakIonizing dose rate

6.3 x 10–8 dpaCoupler peakdisplacement dose (10 y)

~ 2 x 10-16 dpa/s~ 3/4 n, 1/4 p

Coupler peakdisplacement dose rate

Radiation can alter the properties of ceramics via three general mechanisms

• Permanent defect production by knock-on collisions and nuclear reactions–Displacement damage–Transmutations

• Displacement production via ionization (radiolysis) processes–Occurs in SiO2, alkali halides, etc. –Does not occur in Al2O3, BeO, AlN

• Radiation-induced conductivity (RIC)–Transient excitation of valence electrons into

conduction band

Effect of irradiation on loss tangent

• Power absorbed by dielectric feedthrough is given by

P = ωε' E 2 tanδ

tanδ =σDC

ωε '+

χε ' /εo

Can be increased by radiation-induced defects(permanent displacement damage effect)

Can be increased by RIC (prompt effect):

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radiation

valence band

shallow trap

conduction band

Radiation Induced Conductivity in Insulators

Summary of RIC data for oxide ceramics

dRIC Kφσσ += 0

SNS Coupler

d~1.0

K depends on electron trap concentration

Loss tangent of alumina depends on type and chemical bonding behavior of impurities

Three 9’s +

Two 9’s +

Three 9’s +

Example: three grades of nominally high-purity Al2O3

Effect of neutron damage on the loss tangent of Al2O3 irradiated near room temperature

0.01 dpaSNS Coupler

University of Illinois TRIGA Reactor Facility

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Beam Port Plugs

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Coaxial line to network analyzer

vaccum line

15 cm

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Water

Reactor core

Resonant cavity with sample

Thru-port

For most materials tested increase in loss tangentis proportional to the ionizing dose rate

• Example shows overlayed profiles of ∆tanδ and ionizing dose rate for Al998 (both normalized to 1)

• Indicates τtrapping << pulse rise and fall times

0

0 .2

0 .4

0 .6

0 .8

1

∆ta

nδ,

gam

ma

do

se

rt.

(no

rma

lize

d)

1 . 2

0 .1 0 .12 0 .14 0 .16Time (s)

∆tanδγ dose r a t e

Pressure effects

• Gamma flux ionizes residual gas in the cavity,• Causes spurious losses and frequency shifts which cannot be distinguished from changes in the dielectric properties of theceramic• Consistent with a model of the ionized gas as a dielectric with permittivity

ε = ε 0 − nee2

m ν 2 + ω2( ) − jnee

2νωm ν 2 + ω2( )

ne = electron density, ω = applied frequency, ν = collision frequency

• Maximum effect seen experimentally at p ~ 0.1 torr, little effect for p≤ 10-4 torr

10-5

0.001

0.1

0 0.0001

tanδ

appa

rent

0.01 1 100

before pulse

Wesgo AL998

Pressure (Torr)

Sapphire

Change in measured loss tangent

102

103

104

105

106

107

–∆f

(H

z)

0 0.0001 0.01 1 100Pressure (Torr)

Shift in cavity resonant frequency

Wesgo AL998Sapphire

0

3

6

Wesgo Al998Wesgo Al300Si

3N

4

Sapphire

Tan

δ ×

104 a)

0

30

60

0 10 20 30 40 50

AlNTan

δ ×

104

Ionizing dose rate (kGy/s)

b)

SNS Coupler3 x 10-4 Gy/s

Basics of Radiation Effects on Polymers

• Comparatively low doses can produce changes in properties

• Why? Because of typically very high molecular weight, a large fraction (tens of percent) of the molecules can suffer at least one event in doses of order 10 kGy

• Predominant changes can be described as chain scission and cross-linking (other changes: release of small molecules, i.e., gas formation, modification in types of bonding, …)

• For a given polymer, radiation type and temperature, either cross-linking or scission will usually dominate

• Cross-linking increases molecular mass, lowers solubility and improves mechanical properties

• Scission generally degrades properties

Basics of Radiation Effects on Polymers

• While polymers are often classed as cross-linking type or degrading (scission) type, our research has shown that the ratio of cross-links to scissions depends strongly on LET. Energetic heavy ions cause much more cross-linking than ( or e- and can lead to reclassification of material from scission-type to cross-linking type

• Range of sensitivity for producing significant degradation spans more than three orders of magnitude in dose, for example, for reduction in uniform elongation by 25%:/1kGy PTFE (Teflon)/103 kGy PI, PS (Polyimide, Polystyrene)

• Sensitivity also depends on irradiation conditions and environment. Irradiation in vacuum can improve dose endurance over air by an order of magnitude. Irradiation at higher temperatures can give improvement.

Mechanical Propertiesof Polymers

(dose to reduceelongation by 25%)

K. J. Hemmerich, Med. Dev.& Diag. Ind. Magazine, Feb. 2000

Decrease in Elongation of Viton ElastomerIrradiated at Various Temperatures

SNS Coupler

M. Ito, Radiat. Phys. Chem.47(1996)607-610

Conclusions• Radiation effects should not produce significant

degradation of the performance of the ceramic insulators in RF power couplers in SNS and other advanced accelerator systems–The leading candidate inorganic insulators (Al2O3,

BeO, AlN) are not susceptible to radiolysis, and the displacement damage associated with protons and neutrons are well below the threshold for causing permanent deterioration of loss tangent

–The anticipated ionizing radiation fluxes are too low to affect the electrical resistivity or loss tangent performance of the ceramic insulators

Conclusions

• Radiation effects in polymers become significant over the range from ~ 1 kGy to /103 kGy, depending on the material–At the anticipated dose levels of SNS couplers, acetal,

polypropylene, and PTFE (teflon) should be avoided–Top performers are PI (polyimide) and PS

(polystyrene), which sould easily withstand the anticipated doses with little degradation

–High performance fluoropolymers like Viton are in an intermediate range and should perform satisfactorily. However, “Viton” is a general name for entirely different formulations. Specific data for the precise formulation should be consulted.

The “Radiation-Induced Electrical Degradation” Controversy

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-6 10-5 10-4 10-3 10-2

RIED studies on single crystal alumina at 450-530ÞC

Patuwathavithane et al 1995Hodgson 1991,1994Zong et al 1994Shiiyama et al 1996

Elec

trica

l Con

duct

ivity

(S/m

)

Dose (dpa)

1.8 MeV e-

2 MeV H+

527ÞC1 MeV e-

450ÞC

500ÞC450ÞC

500ÞC

E>100 V/mm during irradiation

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��Upper Limit for Fusion RF Heating Systems

• **also give corresponding ionizing radiation dose levels• Observed in several electron irradiation studies

Schematic of fission reactor irradiation capsule for in-situ electrical conductivity measurements

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Copper Ring

Copper Feedthrough

Alumina Cap

Sample

Nickel Hi-Side Supply

Alumina Pedestal

Vanadium Heat Sinkthermocouples coax

power lead

triax center and guard leads

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8.5 mm

0.75 mm

4 mm

6 mm

RESISTIVITY EXPERIMENTS ON CERAMIC INSULATORS

• Several recent high-dose fission reactor studies have not observed RIED

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-6 10-5 10-4 10-3 10-2 10-1 100 101

RIED studies on single crystal alumina at 450-530ÞC

Patuwathavithane et al 1995Hodgson 1991,1994Zong et al 1994Shiiyama et al 1996HFIR TRIST-ER1El

ectri

cal C

ondu

ctiv

ity (S

/m)

Dose (dpa)

1.8 MeV e-

2 MeV H+

527ÞC1 MeV e-

450ÞC

500ÞC

450ÞC

500ÞC

fission neutrons450ÞC (incl. RIC)

E>100 V/mm during irradiation

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Upper Limit for Fusion RF Heating Systems

Summary of Radiation-induced conductivity data for Al2O3irradiated near room temperature

σ RIC = σ 0 + Kφ d

d~1.0

K depends on electron trapconcentration

University of Illinois TRIGA Reactor Facility

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Water

Reactor core

Resonant cavity with sample

Thru-port

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micrometer

single fold bellows

Material: OFHC Copper

6.00 in

18.00 in

1.33 in

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ceramic sample:6mm dia. x 1mm thick

High Resolution Loss Tangent Measurement Cavity

1.00 in

1.00 in

Fabricate From 6 in. Coax Outer Conductor ?

Fab from 3 in. Coax Inner Conductor ?

Analysis of Results

• Measured in-situ Al2O3 loss tangent results are consistent with publishedradiation induced conductivity data assuming increased tanδ is due toincreased σDC , where

tanδ =σDC

ωε '+

χε ' /εo

• Neutron displacement damage effects are insignificant due to low accumulateddamage in the reactor pulse (<10-8 dpa)

∆ tanδ )dpa =n (Zea )2 (ε ∞

' + 2)2

18εokTε ∞' < 10−5

Early Reports of Radiation Effects in Materials Caused by Displacement of Atoms

• Embrittlement–fcc alloys 1959 –bcc alloys 1950’s

• Irradiation creep–Uranium 1955–Stainless Steel 1959, 1962

• Swelling–Stainless Steel 1967

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Radiation Effects in Materials, with Emphasis on ...Origins of Radiation Effects in Materials...

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