Romanenko - Elastic Recoil Detection and Positron Annihilation Studies of the Mild Baking Effect
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This document discusses the historical context of the "Stage III" phenomenon observed in metals like niobium and molybdenum, where resistivity suddenly decreases upon heating around 100-120°C. Previous studies using techniques like positron annihilation and elastic recoil detection showed this is due to the recovery of lattice defects like vacancies. More recent work suggests the presence of hydrogen plays a key role by binding to vacancies up to 100-120°C, above which the vacancies dissociate from hydrogen. The document then describes an elastic recoil detection study of the near-surface hydrogen concentration in niobium samples subjected to different baking treatments.
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Romanenko - Elastic Recoil Detection and Positron Annihilation Studies of the Mild Baking Effect
- 1. Elas%c Recoil Detec%on and Positron Annihila%on Studies of the Mild Baking Effect A. Romanenko Fermilab L. Goncharova, P. Simpson Univ. of Western Ontario D. Gidley UMich
- 2. Different mild baking mechanisms • Models previously considered – Inters%%al oxygen-‐based models – Natural oxide modifica%on • Inters%%al hydrogen in the near-‐surface region • LaJce defects – Local misorienta%on (disloca%on density) reduc%on with baking revealed by EBSD studies
- 3. Historical Prospec%ve • “Stage III controversy” • Origin -‐ observa%ons of resis%vity recovery (strong change) in different group V metals (tungsten, molybdenum, niobium) aSer either deforma%on or irradia%on in 1960-‐70s • Controversy essence -‐ is it inters%%al impuri%es or laJce defects, which are changing in Stage III • Stage III happens in niobium around -‐50C without any hydrogen and at around 120C with hydrogen presence • Near-‐surface niobium is exactly that -‐ niobium with some hydrogen
- 4. D. E. Peacock, A. A. Johnson, Philosophical Magazine, Volume 8, Issue 88 April 1963 , pages 563 -‐ 577 A clear resis%vity recovery stage in neutron irradiated niobium iden%fied at around 100-‐120C • Radia%on damage – laJce defects – mostly vacancies and disloca%on loops • Degree of recovery depends on the amount of damage – the “recovering” en%ty is laJce defects • Similar stage found in Mo
- 5. L. Stals and J. Nihoul, Phys. Stat. Sol. 8, 785, 1965 Same resis%vity recovery stage in heavily cold worked niobium iden%fied at around 100-‐120C • Heavy cold work – laJce defects – mostly disloca%ons and vacancies • From the analysis of recovery at different temperatures – driving process most likely bimolecular process – vacancies annihila%ng with self-‐ inters%%als • Abributed to the recovery of point defects
- 6. P. Hautojarvi et al, Phys. Rev. B, Vol. 32, Num. 7, 1985 Positron annihila%on – studies of open volume defects (vacancies) • Temperature of the Stage III recovery depends on the hydrogen presence -‐ vacancies are bound by hydrogen up to 100-‐120C • Similar effect found in Ta
- 7. Physica Scripta. Vol. 20,683-‐684, 1979 Annealing of Defects in Irradiated Niobium 0. K. Alekseeva et al. Positron annihila%on – studies of open volume defects (vacancies) • Clear decrease in open volume defects (i.e. vacancies) starts at around 120C
- 8. Hydrogen-‐induced defects • Hydrogen can cause laJce defects – vacancies and disloca%ons depending on the concentra%on – equivalent to heavy cold-‐ work • Superabundant Vacancies (SAVs) – general phenomenon recently uncovered for M-‐H systems – emerges when surface chemisorp%on is preferable to inters%%al solu%on For review – A. Pundt and R. Kirchheim, Annu. Rev. Mater. Res. 2006. 36:555–608 29 orders of magnitude higher concentra%on of vacancies in the presence of hydrogen as compared to thermal equilibrium
- 9. Inves%ga%on of near-‐surface hydrogen • Mo%vated by the possible driving mechanism for the mild baking effect – Vac-‐H complexes dissocia%on occuring around 100-‐120C • Leading to the elimina%on of the HFQS by – LaJce defect density reduc%on in the near-‐ surface layer? [A Romanenko and H Padamsee 2010 Supercond. Sci. Technol. 23 045008] – Or hydrogen concentra%on decrease? -‐ inves%gated by Elas%c Recoil Detec%on (ERD)
- 10. Elas%c Recoil Detec%on Sample He+ Incident energy = 1.6MeV He+ Incident angle = 75o Scabering Angle = 29o H+ Dose: normalized to 1µC Facility at the Univ. of Western Ontario (Prof. L. Goncharova) • Based on the detec%on of recoiled H ions • Sensi%vity of order 1 at.% • Depth resolu%on achievable ~ 1 nm • Depth profile is reconstructed from energy spectrum of ions
- 11. Hydrogen Concentration profiles obtained from energy spectra simulations • Area under each peak corresponds to the concentration of the element in a 1nm slab • Peak shapes and positions come from energy loss, energy straggling and instrumental resolution. • The sum of the contributions of the different layers describes the depth profile.
- 12. Samples inves%gated with ERD Sample Origin Treatment HA-‐1 Single grain Nb BCP 150 um HA-‐2 Single grain Nb BCP 150 um + 800C 4 hrs HA-‐3 Single grain Nb BCP 150 um + 800C 4 hrs + 110C 74 hrs HA-‐4 Single grain Nb BCP 150 um + 800C 4 hrs + 110C 74 hrs + HF rinse 10 min HA-‐5 Single grain Nb BCP 150 um + 600C 10 hrs HA-‐6 Single grain Nb BCP 150 um + 600C 10 hrs + 110C 54 hrs LE1-‐37 hot spot Large grain Nb cavity BCP 200 um cutout TE1AES004 cold spot Fine grain Nb EP cavity EP 100 um + 120C 48 hrs cutout
- 13. Experimental data (Overview) Incident energy = 1.6MeV He+ Incident angle = 75o He+ Scabering Angle = 29o Dose: normalized to 1µC H+
- 14. Experimental data (vs Energy)
- 15. Different posi%ons at the surface BCP+800C 2 hrs BCP ERD results for different posi%ons on the surface are shown; integrated intensi%es for bulk (ch.100-‐240) and surface (ch.240-‐320) hydrogen yield are listed below • difference between different spots is noted in the table Sample Spot Integrated Yield, ch 100-240 Integrated Yield, ch 240-310 HA-1 1 566 1038 2 513 925 HA-2 1 383 761 2 347 756 3 347 846
- 16. Different posi%ons at the surface ERD results for different posi%ons on the surface are shown; integrated intensi%es for bulk (ch.100-‐240) and surface (ch.240-‐320) hydrogen yield are listed below • difference between different spots is noted in the table Sample Spot Integrated Yield, ch 100-240 Integrated Yield, ch 240-310 HA-3 1 355 860 2 365 882 3 354 918 HA-4 1 472 1041 2 521 1136 3 533 1102
- 17. Different posi%ons at the surface ERD results for different posi%ons on the surface are shown; integrated intensi%es for bulk (ch.100-‐240) and surface (ch.240-‐320) hydrogen yield are listed below • difference between different spots is noted in the table Sample Spot Integrated Yield, ch 100-240 Integrated Yield, ch 240-310 HA-5 1 393 1220 2 417 1045 HA-6 1 375 855 2 347 696
- 18. HA-‐X ERD Summary Sample Treatment Surface Content Bulk Content # HA-‐1 BCP 53Å Nb0.79H0.21 Nb0.994H0.008 HA-‐2 BCP + 800C 4hrs 48Å Nb0.80H0.20 Nb0.994H0.006 HA-‐3 BCP + 800C 4 hrs + 110C 54 hrs 53Å Nb0.80H0.20 Nb0.994H0.006 HA-‐4 BCP + 800C 4 hrs + 110C 54 hrs + HF 10 110ÅNb0.91H0.09/ Nb0.992H0.008 min 170Å Nb0.96H0.04 HA-‐5 BCP + 600C 10 hrs 62Å Nb0.77H0.23 Nb0.994H0.006 HA-‐6 BCP + 600C 10 hrs + 110C 72 hrs 65Å Nb0.85H0.15 Nb0.994H0.006
- 19. Data on cutout samples • Used samples, which were cut out of real RF cavi%es characterized with thermometry during the tests • One sample from the “hotspot” in Cornell high field Q-‐slope limited large grain BCP cavity • One sample from FNAL baked fine grain EP cavity – no high field Q-‐slope, cavity limited by local quench at around 150 mT at the other loca%on
- 20. Cutouts Data Incident energy = 1.6MeV He+ Incident angle = 75o He+ Scabering Angle = 29o Dose = 4µC H+
- 21. Large grain BCP hot spot Fine grain EP baked cutout Sample Spot Integrated Yield, ch 100-240 Integrated Yield, ch 240-310 Large grain BCP cutout 1 464 1666 2 528 1877 3 511 2075 4 506 2082 EP baked cutout 1 579 1829 2 596 2292 3 558 2279 4 636 2121
- 22. Sample 2 from baked EP cavity – no high field Q-‐slope, losses negligible Sample 1 – Hot Spot in the high field Q-‐ slope of large grain BCP cavity – strong dissipa%on detected by thermometry But – hydrogen profile is the same! Sample Surface Content Bulk Content # 1 7.6 nm Nb0.78H0.22 Nb0.994H0.006 2 7.5 nm Nb0.77H0.23 Nb0.994H0.006
- 23. Positron Annihila%on Doppler Broadening Spectroscopy • Positron life%me depends on the electron density – lives longer at open volume defects (i.e. vacancies) • Width of the spectra of gamma quants produced on annihila%on depends on the local electron density and momenta • Characterized by S-‐ parameter – roughly the higher S the larger the concentra%on of open volume defects • Varying positron energy – non-‐ destruc%ve depth profiling
- 24. Doppler broadening spectroscopy – preliminary results UMich/NCSU data UWO data Baking 120C in situ Baked/unbaked Decrease in the density of vacancies detected Life%me spectra in both cases
- 25. Conclusions • Hydrogen seems to be uncorrelated with the mild baking improvement in the HFQS – Same H content with/without HFQS • HF rinsing results in the smearing of H-‐profile • Preliminary positron annihila%on data – decrease in near-‐surface laJce defects during mild baking • Same samples from ERD are going to be used for further positron annihila%on studies