Addition of Hydrogen to Ni-Ti Multilayers: Implications for Neutron Monochromator and Supermirror Performance Brent J. Heuser Dept. Nuclear, Plasma, & Radiological Engineering University of Illinois at Urbana-Champaign Outline Introduction—neutron monochromators, supermirrors, & guides Sample preparation—magnetron sputtering (Ar or Ar+H2 gas) Experimental results—NR, XRD, AFM, AES, TPD analysis Discussion—effect of H; correlation between NR, AFM, & AES Supported by the DOE INIE Program

Acknowledgements Hyunsu Ju (NPRE UIUC) Sungkyun Park (IPNS ANL) Rick Goyette (SNS ORNL) UIUC FS-MRL staff: Tony Banks Nancy Finnegan Scott MacLaren Vania Petrova Mauro Sardela

Cold Neutron Guide Halls Orphée Reactor & Guide Hall at the LLB, Saclay Neutron guides transport long wavelength neutrons far from reactor containment where neutron and gamma-ray backgrounds are much lower. Guides are based on total external reflection and must be very efficient. NIST Reactor & Guide Hall

Evacuated channels with coatings on top, bottom, and sides Neutron Guides Evacuated channels with coatings on top, bottom, and sides that reflect neutrons. Scattering Instrument Neutron Guides Cold Source LH2 or CH4 ~4-25 K Vacuum Reflective coatings

Review of Basic Neutron Optical Elements Single-layer films R lc l q fixed R l fixed q or Q qc or Qc Interdiffusion barrier Ti (Nb = -1.95 x 10-6 A-2) Ni (Nb = 9.40 x 10-6 A-2) Multilayer films R Q d

Substrate + Nickel Coating Bare Si Substrate q Substrate + Nickel Coating Substrate + Ni-58 Coating Substrate + Ni-Ti Multilayer Coating Review of Basic Neutron Optical Elements Total External Reflection Qc guide monochromator

Ni-Ti Supermirror (Gain~m2) Review of Basic Neutron Optical Elements Hino et al., NIMB, 529 (2004) 54. Supermirror films R Q Continuous distribution of d-spacing values extends critical edge l=8.8Å NiC-Ti Neutron guides Internal coating Natural Ni Ni-58 (Gain~1.5) Ni-Ti Supermirror (Gain~m2) Must be able to accept larger angular divergence or use shorter wavelength neutrons

Fabrication of Ni-Ti Multilayer Films Using Magnetron Sputtering Samples ~500 Å Ni ~500 Å Ti 1 Ni-Ti BL 2 Ni-Ti BL 4 Ni-Ti BL 6 Ni-Ti BL 10 Ni-Ti BL 15 Ni-Ti BL 20 Ni-Ti BL 40 Ni-Ti BL Substrate @ RT Sputter Gas 2.7 mT Ar 2.7 mT Ar + 0.3 mT H2 Neutral sputtered atoms Two separate targets: Ti or Ni plasma magnetic field lines -V Growth rate: 0.4 Å /sec Bi-layer spacing: ~80 Å

Neutron Reflectivity Measurements—POSY 2 @ IPNS-ANL 40 BL 20 BL 6 BL 2 BL 10 BL 4 BL R vs. Q—measurements and fits w/o H w/H R vs. BL Number Reflectivity Ratio

Fits to the Neutron Reflectivity Measurements 6 BL Fits not unique!

Atomic Force Microscopy Measurements of Surface Roughness 2 BL w/o H 4 BL w/o H 6 BL w/o H 40 BL w/o H 20 BL w/o H 10 BL w/o H 500 Å Ni 500 Å Ti 500 Å TiH2 Ra=1.4 Å Ra=1.6 Å Ra=1.8 Å Ra=5 Å Ra=7 Å Ra=9 Å Ra=15 Å Ra=4 Å Ra=11 Å Gradual increase in roughness of top surface is observed that is consistent with degradation of reflectivity for BL > 6.

Auger Electron Spectroscopy Measurements of Atomic Concentration 20 ML w/o H (new Ti target) 20 ML w/o H (old Ti target) 20 ML w/H Oxygen content at noise level; oscillations in oxygen signal in 20 w/H ML sample have same period as Ti and Ni oscillations, but correlated to Ti. Concentration profiles for Ti in the with-hydrogen ML samples are flat indicating uniform hydrogen concentration within Ti layers. Ti and Ni signal oscillations in ML samples dampen away from the film-substrate inter- face, consistent with increase surface roughness for high BL number observed with AFM. 20 ML w/o H (old Ti target) 20 ML w/H

Correction to Theoretical 1st Order Peak Reflectivity

Temperature Programmed Desorption Measurements of Hydrogen Concentration TiH2 powder 500 A Ti 40 ML 2 ML Concentration of hydrogen proportional to area under curve. Sample [H]/[Ti] 500 A 2.0 40 ML w/H 20 ML w/H 2.2 10 ML w/H 1.5 6 ML w/H 2.5 4 ML w/H 1.7 2 ML w/H 40 ML w/o H 0.7 20 ML w/o H 0.8 10 ML w/o H 1.0 6 ML w/o H 4 ML w/o H 0.9 2 ML w/o H 1.2 0.4 0.3

Conclusion Addition of hydrogen to Ti works—increase in 1st order diffraction peak reflectivity observed. Gains in on-sample intensity of 2-3 should be possible without too much effort. Degradation in 1st order peak reflectivity with BL value consistent with surface roughening observed with AFM. Larger interfacial roughness as BL value increases was observed with AES, consistent with AFM. Munter et al., Physica B 221 (1996) 500. Substitution of Be for Ni + Addition of H to Ti

NBS Reactor LH2 Cold Source Orphee Reactor—LLB Saclay NBS Reactor LH2 Cold Source NIST

Auger Electron Spectroscopy Measurements of Atomic Concentration 40 ML w/H Oxygen content at noise level; oscillations in oxygen signal in 20 w/H ML sample have same period as Ti and Ni oscillations, but correlated to Ti. Concentration profiles for Ti in the with-hydrogen ML samples are flat indicating uniform hydrogen concentration within Ti layers. Ti and Ni signal oscillations in ML samples dampen away from the film-substrate inter- face, consistent with increase surface roughness for high BL number observed with AFM. Bulk Ni 20 ML w/H Bulk Ti 20 ML w/o H 10 ML w/H

Addition of Hydrogen to Ni-Ti Multilayers: Implications for Neutron Supermirror Performance Brent J. Heuser, UIUC Hyunsu Ju, UIUC Sungkyun Park (ANL), Rick Goyette (ANL), Tony Banks (UIUC), Nancy Finnegan (UIUC), Scott MacLaren (UIUC), Vania Petrova (UIUC) Mauro Sardela (UIUC) Neutron optics—monochromators and supermirrors Sample preparation. Experimental results—NR, XRD, AFM, AES, TPD analysis Supported by the DOE INIE Program