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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
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
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
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
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
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
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
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
29 orders of magnitude higher concentra%on of vacancies in the presence of hydrogen as compared to thermal equilibrium
For review – A. Pundt and R. Kirchheim, Annu. Rev. Mater. Res. 2006. 36:555–608
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)
Elas%c Recoil Detec%on
• 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
He+
H+
Sample Incident energy = 1.6MeV He+
Incident angle = 75o
Scabering Angle = 29o
Dose: normalized to 1µC
Facility at the Univ. of Western Ontario (Prof. L. Goncharova)
• 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.
Hydrogen Concentration profiles obtained from energy spectra simulations
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 cutout
BCP 200 um
TE1AES004 cold spot Fine grain Nb EP cavity cutout
EP 100 um + 120C 48 hrs
Experimental data (Overview)
Incident energy = 1.6MeV He+
Incident angle = 75o
Scabering Angle = 29o
Dose: normalized to 1µC
He+
H+
Experimental data (vs Energy)
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-1 1 566 1038
2 513 925
HA-2 1 383 761
2 347 756
3 347 846
BCP BCP+800C 2 hrs
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
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
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 min
110ÅNb0.91H0.09/ 170Å Nb0.96H0.04
Nb0.992H0.008
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
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
Cutouts Data
Incident energy = 1.6MeV He+
Incident angle = 75o
Scabering Angle = 29o
Dose = 4µC
He+
H+
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
Large grain BCP hot spot Fine grain EP baked cutout
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
Sample 1 – Hot Spot in the high field Q-‐slope of large grain BCP cavity – strong dissipa%on detected by thermometry
Sample 2 from baked EP cavity – no high field Q-‐slope, losses negligible
But – hydrogen profile is the same!
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
Doppler broadening spectroscopy – preliminary results
UMich/NCSU data UWO data
Baking 120C in situ
Baked/unbaked
Decrease in the density of vacancies detected in both cases Life%me spectra
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