Radiation Effects in Materials, with Emphasis on Insulators for Couplers

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1 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 Accelerators Jefferson Laboratory October 30-November 1, 2002

2 Outline Radiation effects in materials Radiation effects on the electric and dielectric properties of inorganic insulators (focus on Al 2 O 3 ) 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 /10 3 kgy

3 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

4 Historical Perspective on Radiation Effects Some radiation effects were observed in minerals in the 19 th 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

5 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

6 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, >10 3 Gy/s, 100 appm He/dpa) High power spallation target High flux reactor core Fusion reactor first wall

7 Hierarchy of Reactions Leading to Property Changes by Displacement Damage Swelling, Irradiation Creep, Embrittlement

8 Time and Energy Scales for Radiation Effects by Displacement Damage Time Cascade Creation s Unstable Matrix s Interstitial Diffusion 10-6 s Energy Neutron or Proton ev Primary Knock-on ev Displaced Secondary ev Vacancy Diffusion Unstable Matrix 10 0 s 10 0 ev Microstructural Evolution Thermal Diffusion 10 6 s kt

9 Ionizing and displacive radiation doses for SNS couplers Coupler calculations courtesy of Franz Gallmeier, ORNL Coupler peak displacement dose rate ~ 2 x dpa/s ~ 3/4 n, 1/4 p Coupler peak displacement dose (10 y) 6.3 x 10 8 dpa Coupler peak Ionizing dose rate ~ 3 x 10-4 Gy/s Coupler peak ionizing dose (10 y) 95 kgy Spallation target container peak displacement dose rate ~ 10-6 dpa/s ~ 2/3 n, 1/3 p Spallation target container peak displacement dose (1 y) 36 dpa

10 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 SiO 2, alkali halides, etc. Does not occur in Al 2 O 3, BeO, AlN Radiation-induced conductivity (RIC) Transient excitation of valence electrons into conduction band

11 Effect of irradiation on loss tangent Power absorbed by dielectric feedthrough is given by P =ωε' E 2 tanδ tanδ = σdc ωε ' + χ ε '/εo Can be increased by RIC (prompt effect): Can be increased by radiation-induced defects (permanent displacement damage effect)

12 radiation valence band shallow trap conduction band Radiation Induced Conductivity in Insulators

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14 Summary of RIC data for oxide ceramics σ σ + RIC = 0 Kφ d d~1.0 SNS Coupler K depends on electron trap concentration

15 Loss tangent of alumina depends on type and chemical bonding behavior of impurities Example: three grades of nominally high-purity Al 2 O 3 Three 9 s + Two 9 s + Three 9 s +

16 Effect of neutron damage on the loss tangent of Al 2 O 3 irradiated near room temperature SNS Coupler 0.01 dpa

17 University of Illinois TRIGA Reactor Facility Concrete Shield Water Thru-port 15 cm 6.71 m Reactor core ~54 cm Resonant cavity with sample Coaxial line to network analyzer Beam Port Plugs vaccum line

18 For most materials tested increase in loss tangent is 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 tanδ, gamma dose rt. (normalized) Time (s) tanδ γ dose rate

19 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 the ceramic Consistent with a model of the ionized gas as a dielectric with permittivity n ε = ε 0 e e 2 m ν 2 + ω 2 ne = electron density, ω = applied frequency, ν = collision frequency ( ) j n e e 2 ν ωm( ν 2 + ω 2 ) Maximum effect seen experimentally at p ~ 0.1 torr, little effect for p 10-4 torr Change in measured loss tangent 10 7 Shift in cavity resonant frequency 0.1 Wesgo AL998 Sapphire 10 6 tanδ apparent before pulse Pressure (Torr) f (Hz) Wesgo AL998 Sapphire Pressure (Torr)

20 6 Tan δ a) Wesgo Al998 Wesgo Al300 Si 3 N 4 0 Sapphire Tan δ b) AlN Ionizing dose rate (kgy/s)

21 SNS Coupler 3 x 10-4 Gy/s

22 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

23 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 crosslinking 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) /10 3 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.

24 Mechanical Properties of Polymers (dose to reduce elongation by 25%) K. J. Hemmerich, Med. Dev. & Diag. Ind. Magazine, Feb. 2000

25 Decrease in Elongation of Viton Elastomer Irradiated at Various Temperatures SNS Coupler M. Ito, Radiat. Phys. Chem. 47(1996)

26 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 (Al 2 O 3, 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

27 Conclusions Radiation effects in polymers become significant over the range from ~ 1 kgy to /10 3 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.

28 The Radiation-Induced Electrical Degradation Controversy Electrical Conductivity (S/m) RIED studies on single crystal alumina at ÞC Upper Limit for Fusion RF Heating Systems 1 MeV e - 450ÞC 450ÞC 1.8 MeV e - 500ÞC E>100 V/mm during irradiation 500ÞC Patuwathavithane et al 1995 Hodgson 1991,1994 Zong et al 1994 Shiiyama et al MeV H + 527ÞC Dose (dpa) **also give corresponding ionizing radiation dose levels Observed in several electron irradiation studies

29 Schematic of fission reactor irradiation capsule for in-situ electrical conductivity measurements Copper Ring Copper Feedthrough Alumina Cap triax center and guard leads Sample Nickel Hi-Side Supply Alumina Pedestal thermocouples coax power lead Vanadium Heat Sink

30 RESISTIVITY EXPERIMENTS ON CERAMIC INSULATORS 6 mm 4 mm 0.75 mm 8.5 mm

31 Electrical Conductivity (S/m) RIED studies on single crystal alumina at ÞC 1 MeV e - 450ÞC Upper Limit for Fusion RF Heating Systems 450ÞC 1.8 MeV e - 500ÞC 500ÞC 2 MeV H + 527ÞC fission neutrons 450ÞC (incl. RIC) E>100 V/mm during irradiation Patuwathavithane et al 1995 Hodgson 1991,1994 Zong et al 1994 Shiiyama et al 1996 HFIR TRIST-ER Dose (dpa) Several recent high-dose fission reactor studies have not observed RIED

32 Summary of Radiation-induced conductivity data for Al 2 O 3 irradiated near room temperature σ RIC =σ 0 + Kφ d d~1.0 K depends on electron trap concentration

33 University of Illinois TRIGA Reactor Facility Concrete Shield Water Thru-port 15 cm 6.71 m Reactor core ~54 cm Resonant cavity with sample Coaxial line to network analyzer Beam Port Plugs vaccum line

34 High Resolution Loss Tangent Measurement Cavity ceramic sample: 6mm dia. x 1mm thick Fabricate From 6 in. Coax Outer Conductor? 1.00 in 1.00 in Fab from 3 in. Coax Inner Conductor? micrometer 6.00 in 1.33 in single fold bellows Material: OFHC Copper in

35 Analysis of Results Measured in-situ Al 2 O 3 loss tangent results are consistent with published radiation induced conductivity data assuming increased tanδ is due to increased σ DC, where tanδ = σ DC ωε' + χ ε'/ε o Neutron displacement damage effects are insignificant due to low accumulated damage in the reactor pulse (<10-8 dpa) tanδ ) dpa = n(zea)2 (ε ' + 2) 2 18ε o ktε ' < 10 5

36 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