NUCLEAR REACTION ANALYSIS OF HYDROGEN SURFACE CONTAMINATION OF Al(Li) ALLOYS

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NUCLEAR REACTION ANALYSIS OF HYDROGEN SURFACE CONTAMINATION OF Al(Li) ALLOYS

J. Thomas, M. Fallavier, G. Beurton, G. Berlioux

To cite this version:

J. Thomas, M. Fallavier, G. Beurton, G. Berlioux. NUCLEAR REACTION ANALYSIS OF HYDRO-

GEN SURFACE CONTAMINATION OF Al(Li) ALLOYS. Journal de Physique Colloques, 1987, 48

(C3), pp.C3-527-C3-533. �10.1051/jphyscol:1987361�. �jpa-00226592�

NUCLEAR REACTION ANALYSIS OF HYDROGEN SURFACE CONTAMINATION OF Al(Li) ALLOYS

J.P. THOMAS, M. FALLAVIER, G. BEURTON* and G. BERLIOUX*

Institut de Physique Iiuclgaire (et IN2P3), Universite Claude Bernard, Lyon I , 43, Boulevard du 11 novembre 1918,

F-69622 Villeurbanne Cedex. France

" ~ ~ g e d u r - ~ e c h i n e y , Centre de Recherches et D6veloppernent, B.P. 27, F-38340 Voreppe, France

ABSTRACT - The zebonant nuctem zeaction ' ~ [ ' ~ ~ , a y ) has been used to nun desttuctively depth-pzo6Ze hydzogen neaz the suz6ace

AP.[Li) binazy d o y b . Lithium is one

the zaze intet6ezence 06 thih technique but we show how to extzact its contzibution. In otdez t o zeduce oz eUminate the hydzogen buzdace contamination [addecting the b ~ d k content detet- minationl vauouh tzeatments [machining-anneding] have been invehtigated.

I . INTRODUCTION

Hydrogen can be found i n aluminium i n various forms : inclusions, solid solutions hydroxides and hydrocarbons adsorbed a t the surface. I n the determination o f bulk content using the technique

fusion under i n e r t gas, the superficial contamination has to

estimated i n order to be reduced or eliminated.

A m o n g the non d e s t r u c t i v e a n a l y t i c a l methods able t o p e r f o r m a q u a n t i t a t i v e determination o f the hydrogen content i n the near surface r e ion nuclear reactions can be proposed such as ' H ( ~ L ~ , ~ ) c ~ c ~ , 'H( l ~ , a ) a a , ' ~ ( l 9 ~ , a y ) 1 6 0 and H ( " N , ~ ~ ) ' ~ c 9 [I]. Even i f the last one has been proved to be the most performing as regard depth resolution 121 and detection l i m i t [3], l i k e f o r the other ones, l i t h i u m is one o f the rare elements leading t o interfering reactions. One aim of this paper is t o demonstrate how t o eliminate the parasitic contribution of L i f r o m AR(Li) binary alloys when using the reaction.

Dealing w i t h samples o f cylindrical shape, base and generatrix have been differen- tiated in the analysis and various machining procedures (tool-lubricant) have been investiga- ted i n this respect.

A t last, since thermal annealing is p a r t o f the fusion technique procedure, such t r e a t m e n t s w e r e r e p e a t e d i n U H V c o n d i t i o n s and p r e l i m i n a r y r e s u l t s a r e r e p o r t e d .

11. EXPERIMENTAL PROCEDURE

11.1 - The analytical problem

The resonant nuclear reaction 15N('H,ay) is characterized by a y emission (4.43 MeV) showing a n a r r o w isolated resonance a t E = 6.384 MeV. The profiling method consists o f varying the incident energy o f "N2+ i o s according to the sketch shown fig. 1. A t the resonance energy the reaction takes place a t the surface and when increasing the energy, various depths can be probed according t o the slowing down o f the particles i n the material.

With the experimental values given fig. 1 f o r the resonance, the depth resolution correspon- ding to the FWHM of the resonance is about - 8 n m a t the surface f o r AR(Li 1.5%). The straggling e f f e c t is responsible f o r the degradation o f this parameter when the probed depth increases. A typical value of 30 n m can be reported a t a 400 n m depth.

F r o m the 4MV Van de Graaff accelerator o f the I n s t i t u t de Physique Nuclgaire de Lyon, doubly charged

ions can be accelerated allowing to cover an energy range f r o m

6.384 MeV t o 8 MeV. This ensures an analysable depth o f the order o f 1 pm f o r the aluminium alloys.

The samples can be placed either into a target chamber described elsewhere [21 where a translational displacement is allowed f o r the irradiation of the various samples, or into an UHV chamber [4] where the beam spot can be moved along the x and y axes and the sample can be heated up t o 400°C. Both chambers are designed to ensure a maximum efficiency for the y-ray detection performed w i t h a well shielded NaI(TR) detector 150 m m long and w i t h

106.5 rnrn diameter.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987361

C3-528 JOURNAL DE PHYSIQUE

The c o u n t i n g o f the y-rays r e s u l t i n g f r o m the previous r e a c t i o n is done f r o m a selected window including the 4.43 MeV and i t s t w o escape peaks. When i r r a d i a t i n g a L i F sample, a large background c a n be d e t e c t e d due t o a ~ i ( " ~ , y ) reaction. The t w o c o n t r i b u - tions are hardly discernible f o r r e a l i s t i c countings ( s t a t i s t i c a l l y s i g n i f i c a n t ) or would r e q u i r e p r o h i b i t i v e t i m e t o be observed b u t n o t d i f f e r e n t i a t e d . This p a r a s i t i c c o n t r i b u t i o n c a n lead t o erroneous i n t e r p r e t a t i o n of hydrogen c o n c e n t r a t i o n p r o f i l e s : a l l the AR(Li) samples i n v e s t i g a t e d ( f r o m 0.7 t o 5%) were showing "raw" p r o f i l e s w i t h apparent increase o f hydrogen as a f u n c t i o n o f the energy (depth). This is i n disagreement w i t h the hydrogen p r o f i l e i n a 2214 l i t h i u m - f r e e sample (fig. 2). Such p r o f i l e s corresponds t o stable hydrogen under beam impact, w h ~ c h w i l l be explained l a t e r on.

The i n t e g r a t e d e x c i t a t i o n f u n c t i o n o f t h e ~ i ( " ~ , y ) r e a c t i o n is shown on fig. 3 as obtained f r o m t h e v a r i a t i o n o f t h e yield o f the y-rays e m i t t e d i n the window previously defined. The presence o f surface hydrogen c a n be observed f r o m the peak a t the resonance energy o f 6.384 MeV.

'"I ^;*.

2214 Li

-

-.-.-.-.-.A-

.-.-.- .-.-'- I

I I I

6.4 7 7.5 8

E (MeV)

F i g . 1 - P~inciple 06 ^the

hydwgen depth-p~odiling.

Exnczimenta! shape

the tebonance 6 ~ o m a pol Pished sample od u l t ~ a - puqe 4iPic0 n.

F i g .

- "Raww p.rog2e.s

dram a Li-61ee 2 2 1 4

sample and an A Q ( L i

0.7 % j one. Stable pzodiPes

undez beam impact (see

t e x t ] .

F i . 3 - $.liation 06 the y-zay emission 6zom the Lill 5N,y teaction a a 6unction e N enetgy [LiF b a m p k - same window as pzevious expezimentj.

11.2

- Estimation o f the parasitic contribution due t o l i t h i u m

The number o f emitted y-rays f r o m the reaction on Li, a t an energy E, is given by :

P r

N(E) = K.n.

I' 'threshold ^{a [E(x) I d~ldx dE}

where K includes the geometric factor and the detection efficiency ; u [E(x)] is the cross-section a t a depth x where the energy is E(x) ; dE corresponds _to the energy interval defined by dx a t an energy E(x) ; dE/dx is the stopping power (keV pg 'cm2) of the incident particles a t an energy E(x) (in keV) ; n is expressed i n a t / g (the l i t h i u m content o f the considered medium). The only possibility of establishing a simple r a t i o between N(E) determined f o r a given medium (LiF) and another medium [AR(Li)] is to f i n d a simple relationship between the respective stopping powers. This occurs i n our own case i n which f o r the energy range between 6 and 8 MeV, the stopping powers of "N++ i n L i F and AR(Li) obtained f r o m Ziegler's tables [5] have

nearly constant value. For example the 15N stopping powers r a t i o in L i F and AR(Li 3%) is 1.198 2% over the 6-8 MeV range. One can therefore w r i t e :

n,(at/g L i ) o f AR(Li) N(E) AI(Li 3%)

n2(at/g L i ) o f L i F x 1.198 x N(E) L i F

This multiplying coefficient can be determined for d i f f e r e n t percentage contents of Li i n each alloy as long as the considered conditions apply. Then, f r o m the Li profile obtained w i t h L i F (fig. 3) the L i contribution can be f i t t e d on spectra corresponding t o various L i content in AI(Li) as f o r the example of fig. 4. One should notice that the remaining difference between the "raw" profile and the l i t h i u m contribution a t the highest energy ( l a r g e s t d e p t h ) corresponds t o the e x p e r i m e n t a l b a c k g r o u n d o f ' H ( ' ' N , ~ ~ ) r e a c t i o n .

Fig. 4 - "Raw" pzo6iees 06

hydiogen 6zom t h t AQlLi 0.3 % j sample and lithium contzibution adjustment.

-

6.4 7 7.5 8

E

C3-530 JOURNAL DE PHYSIQUE

The l i m i t a t i o n of our procedure lies on the f a c t that the l i t h i u m concentration i n the investigated zone is well known and remains constant. This absolute uncertainty can only be removed if the real l i t h i u m p r o f i l e i n the surface region can be obtained. This has been the case f o r a few SIMS experiments performed a t the Centre de Recherches CEGEDUR- PECHINEY .de VOREPPE. Excepted f o r the 5% L i content where the poor agreement of our f i t can be explained by a very inhomogeneous Li distribution upon - 300 nm, a typical situation seems t o be a Li segregation l i m i t e d t o - 20 n m w i t h an increase f r o m 2.5% up t o

- ^{2% (2.5% Li sample). In} such a case, the estimated incidence on the y contribution does n o t exceed 5%. As a consequence of this " a r t i f i c i a l " background, the detection l i m i t (at.%) w i l l be degraded as a function of depth. Typical values f o r three increasing depths (100, 500 and 1000 nm) are, f o r example, respectively 0.09, 0.14 and 0.25% for AR(Li 2.5%) sample, but 0.11, 0.18 and 0.34% f o r a AR(Li 5%) sample. These values include the background o f the

reactlon on hydrogen which sets a constant detection l i m i t of 0.06% (integrated

dose : 10 PC).

Formula giving the H content ( a t % of AR) f r o m the y-couting as a function of energy (depth) as well as the number of a t / c m 2 deduced f r o m the area under the p r o f i l e have been detailed elsewhere [6] and w i l l n o t be explicited here. Each depth p r o f i l e w i l l associate to the number of counts as a function o f the incident energy, a concentration scale as a function of depth and the number o f H a t / c m 2 corresponding to the integrated profile.

111.

OF THE

Two types o f depth profiles w i l l be presented : - I) zero dose profile : a l l the samples present an important hydrogen effusion a t the surface [7] and sometimes along the probed depth. Then the "true" concentration requires the extrapolation o f the curve giving the variation o f the number of characteristic y-rays as a function of the integrated ion dose.

Therefore each point o f such profile must be obtained f r o m "virgin" sample zones, which can be o f l i m i t e d number according t o the spot size (- 1 m m 2 ) and the dimensions o f the sample (0 8.7 mm). O f course, such profiles can be subject t o lateral inhomogeneities. As i t w i l l be shown, a t the very surface, the l i t h i u m contribution is generally negligible.

- 2) "Stable" p r o f i l e : obtained a t a given ion dose f o r which the hydrogen is no longer effusing under beam impact. Although depending of the irradiation conditions (dynamical equilibrium state) such p r o f i l e is rather well indicative o f the bound hydrogen quantity. A t t h i s stage, the l i t h i u m c o n t r i b u t i o n i s a l w a y s s i g n i f i c a n t and m u s t be deduced.

F r o m a rapid survey of the profiles obtained f r o m the base o f samples w i t h l i t h i u m content ranging between 0.7 and 5%, w i t h the exception o f the 5% case, already mentioned, the parasitic y contribution of the l i t h i u m seems correctly estimated. N o noticeable differences w i t h the l i t h i u m free sample can be reported. Detection l i m i t s are reached a t low depth (200-300 nm) but as expected they are much more higher than the bulk content (- 50 ppm against 1 t o 5 ppm). The l i t h i u m contribution is negligible as concerns the superficial content a t zero dose (several 1016 ~ / c m ' ) b u t such values w i l l induce a large uncertaint a t the 0.1 ppm level (- 33% f o r a disc o f 8.7 m m diameter and 40 m m thick f o r which 'lo1' a t / c m 2 are measured on the base as w e l l along the generatrix).

111.1 - Residual hydroqen from machininq procedures

Two c u t t i n g tools using natural or synthetic diamond have been tested using two lubricants acetone and methanol, on AR(Li 2.5%) samples.

As an example o f the profiles obtained ( L i contribution substracted), the results f r o m

a sample c u t w i t h natural diamond using an acetone lubricant are reported on fig. 5. As i t

can be seen, noticeable differences can be observed both on i n i t i a l and stable hydrogen

between base and generatrix. Surface concentrations range between I O ~ ~ - I O ~ ' H a t / c m 2 f o r

i n i t i a l hydrogen (zero dose profiles) while they are of the orher of l o i 5 H a t / c m 2 f o r stable

hydrogen. Such variations shown on fig. 5 are a general trend observed f o r a l l the samples

b u t w i t h a given tool, methanol leads to a higher hydrogen content f o r the base compared

to the generatrix while the opposite is observed for acetone. F o r either acetone or

methanol, the trend is f o r a decrease o f the residual hydrogen when natural diamond is used

instead o f the synthetic material. The differences range between 20% and ZOO%, significant

enough f o r ascertain a trend, but requiring a larger sampling t o make quantitative

conclusions. As a m a t t e r of f a c t the reproducibility between two series of each association

l u b r i c a n t - t o o l c a n e x c e e d 20% i n the w o r s t case ( f r o m t h e zero-dose p r o f i l e s ) .

60% babe and genezutzix bampt~b.

Condition5 : methanol, natuzd diamond.

E

soot

BASE (3.9 x10'5 ~ / c r n *

-

s

2,

0

-

B KG

-

Fig. 516) - Stable pzodieeb 6oz the

I I , , , I t ~ ~

p'~eviou5 .)ample&.

6.4

7

E

111.2

-

Thermal annealing under UHV conditions h a s been performed a t t e m p e r a t u r e values corresponding t o given s t e p s in t h e fusion process used f o r hydrogen analysis a t t h e CRV laboratory. F o r this s t u d y t w o s e r i e s of s a m p l e s corresponding t o methanol and a c e t o n e lubricants have been considered. An AR(Li 2.5%) s a m p l e was analyzed only along i t s generatrix.

Although qualitatively similar t o t h e results obtained with methanol those concerning a c e t o n e wul n o t b'e r e p o r t e d h e r e s i n c e high initial c o n c e n t r a t i o n s w e r e d e t e c t e d on these s a m p l e s ( c o m p a r e d t o t h e previous results). As shown on

6 a 250°C annealing f o r 90 m n l e a d s t o a n i n c r e a s e of t h e superficial hydrogen c o n t e n t e i t h e r f o r t h e z e r o dose profile a s f o r t h e s t a b l e one. A t 400°C, 1 H annealing, t h e hydrogen c o n t e n t d e c r e a s e is l i m i t e d t o 20%, a s also observed f r o m both type of profile.

Considering t h e hydrogen p r e s e n t a t t h e very s u r f a c e (measured a t t h e resonance

energy) t w o d i f f e r e n t behaviours a r e observed f o r i t s evolution a s a function of t h e

annealing t i m e a t these t e m p e r a t u r e s . A t

e x c e p t a t t h e very beginning t h e hydrogen

c o n t e n t is continuously increasing reaching in a b o u t 1 H a s t a b l e value a l m o s t t w o t i m e s

g r e a t e r than t h e initial value. According t o t h e profile of

6 a fairly l a r g e thickness

(- 3 0 0 nm) s e e m s t o b e c o n c e r n e d by this accumulation. A t 400°C, a n i n c r e a s e of t h e

hydrogen c o n t e n t is f i r s t observed, f o r t h e f i r s t 2 0 n m followed by a rapid d e c r e a s e

down to a quasi s t a b l e value r e a c h e d a f t e r 25 mn. This value is lower than t h e initial value

7). To observe a c o n s i s t a n t d e c r e a s e of t h e hydrogen c o n t e n t , 1 3 H annealing a t 400°C

a r e required leading t o a - 6 0 % reduction of t h e initial value. This is f a r t o be useful for an

e f f i c i e n t b u t realistic p r e t r e a t m e n t in t h e o p e r a t i n g mode of analysis by t h e fusion process.

JOURNAL DE PHYSIQUE

6.4 6.5 6.6 6.7

E

F i g - . 6

fnitial (ze.ro dose) pzog2ea 60% the "methanoP" sample taken at R.T., and adtez 250°C and 400°C annealing.

,100

0 P

-. _o

-

5

8

F i g . iL - Evolution 06 the butdace hydzogen content ('te4onance enezgy) dot the "methanol" sample, a4 a dunction 06 annealing time (250°C and 40O0CJ.

CONCLUSION

-

lH

-

R.T. (4.6 x 10 16 )

-

1

H

--- --- - ^{.- .-.- .-.-.-.- -=---} -

I 1 I

Non d e s t r u c t i v e hydrogen d e p t h profiling c a n be p e r f o r m e d in AR(Li) a l l o y s using t h e r e s o n a n t n u c l e a r r e a c t i o n ' H ( ' ~ N , C ~ ~ ) . In t h i s p a r t i c u l a r c a s e t h e i n t e r f e r i n g r e a c t i o n w i t h l i t h i u m c a n be f a i r l y well e s t i m a t e d provided t h a t t h e Li c o n t e n t is well known and d o e s n o t show too l a r g e d e v i a t i o n f r o m a c o n s t a n t value. Although t h e p r o c e d u r e l e a d s t o a l a c k in bulk s e n s i t i v i t y , t h e r e p o r t e d values a t t h e s u r f a c e a r e l a r g e enough t o n e g l e c t t h e parasitic c o n t r i b u t i o n ( 1 0 ' ~ - 10"). T h e m e t h o d h a s b e e n a b l e t o show s i g n i f i c a n t d i f f e r e n c e s b e t w e e n machining p r o c e d u r e s (tool - l u b r i c a n t ) which a r e n o t y e t e f f i c i e n t enough t o avoid u n c e r t a i n t i e s in bulk c o n t e n t d e t e r m i n a t i o n using a fusion technique. Nor t h e i n t e r m e d i a t e s t e p s of t h e r m a l a n n e a l i n g a t 250°C a n d 400°C a r e a b l e t o r e m o v e t h e s u p e r f i c i a l hydrogen.

As m o r e c o n c l u s i v e e x p e r i m e n t s , ion e t c h i n g u n d e r UHV c o n d i t i o n s is r e a d i l y a p p l i c a b l e in o u r s e t - u p o r o t h e r t e c h n i q u e s c a n be possibly a d a p t e d like glow discharge.

20

- s .-

15

-

7

-I0 I 5

Wotk dinanced by CEGEDUR-Pe'chiney and the Minihtkze de !a Rechezche e t de

PIEnseignement Supe'tieui.

[I] ZIEGLER J.F. e t a]., Nucl. Instr. and Meth., 149 (1978) 19

[21 THOMAS J.P., PIJOLAT C. and F A L L A V I E R M., Rev. de Phys. Appl., 13 (1978) 433 [3] D A M J A N T S C H I T S C H H., WEISER M., HEUSSER H., K A L B I T Z E R S. and M A N N S P E R G E R H., N u c l . I n s t r . and Meth. i n Phys. Res., 218 (1983) 129 [41 FALLAVIER M., CHARTOIRE M.Y. and THOMAS J.P., Nucl. Instr. Meth. i n Phys. Res.

B-15 (1986) 712

[5] "Handbook o f stopping cross-sections f o r energetic ions i n a l l elements"

ZIEGLER J.F., ed. (Pergarnon Press, New York), 1980

[7] THOMAS J.P., F A L L A V I E R M., PIJOLAT C. and TOUSSET J., Radiation Eff., 61

(1982) 207