Romanenko - Elastic Recoil Detection and Positron Annihilation Studies of the Mild Baking Effect

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+



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





HA-3 1 355 860

2 365 882

3 354 918

HA-4 1 472 1041

2 521 1136

3 533 1102





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+



Dose = 4µC

He+

H+


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

Romanenko - Elastic Recoil Detection and Positron Annihilation Studies of the Mild Baking Effect

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