Low energy p-Be nuclear reactions for depth-profiling Be in alloys

N U C L E A R INSTRUMENTS

AND METHODS

149 ( 1 9 7 8 )

77-82;

©

NORTH-HOLLAND

PUBLISHING CO.

LOW ENERGY p-Be NUCLEAR REACTIONS FOR DEPTH-PROFILING Be IN ALLOYS* P. P. PRONKO, P. R. OKAMOTO and H. W1EDERSICH

Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A.

Beryllium distributions within the first micron of the surface of nickel- or copper-based alloys were investigated with a 300 keV proton probe utilizing low energy nuclear reactions. In the case of the Ni specimens, segregation of Be was induced by point defect flows to the surface of the specimen during Ni-ion bombardment at elevated temperatures. The nuclear reactions used for the detection of Be are 9Be(p, d)SBe with a Q value of 0.56 MeV and 9Be(p, ~)6Li with a Q value of 2.12 MeV. The deuteron and alpha groups are simultaneously observable using a standard surface barrier detector. Observations were made at a 150° scattering angle with and without a 2.5/xm mylar filter in front of the detector. This filter was found necessary for observing the deuteron yields since pulse pile-up from backscattered protons would otherwise mask the deuteron signal. The alpha group may be observed with or without the filter depending on whether counting statistics or energy resolution are the more important constraints. Significant Be segregation toward the surface was observed in specimens after irradiation at 625°C to 23 dpa with 3.2 MeV Ni ions. Concentrations of Be were nearly doubled within 500 A of the surface and a region depleted of Be extended below the surface layer to a depth of about 3000 A. These results are in reasonable agreement with predictions from computer calculations of alloy segregation induced by defect fluxes to free surfaces during irradiation at elevated temperatures.

1. Introduction Radiation-induced segregation of alloying elements during elevated temperature irradiation has recently received considerable attention1,2). The radiation-induced segregation phenomenon has its origin in the coupling between defect fluxes and fluxes of alloying elements. Energetic irradiation produces point defects and defect clusters in an approximate random distribution throughout the material. Those defects that are mobile and escape recombination are reincoporated into the crystal structure at free surfaces and internal sinks such as dislocations and grain boundaries. Hence, irradiation induces defect fluxes from the interior of the grains to spatially discrete sinks. Since the motion of defects is caused by motion of atoms, fluxes of atoms are associated with defect fluxes. Any preferential association of defect with a particular alloying component and/or preferential participation of a component in defect diffusion will couple a net flux of the alloying element to the defect fluxes. The flux of an element causes the build-up or depletion of solutes in the vicinity of defect sinks and, therefore, concentration gradients in initially homogeneous alloy phases. Experimental evidence and theoretical analysis of alloy element segregation during irradiation indicates that undersize elements will migrate to Work supported by the U.S. Energy Research and Development Administration.

free surfaces and internal sinks in the presence of radiation-induced defect fluxes2-3). Beryllium is an undersized alloying element in many alloy systems and a method is needed to measure, nondestructively, depth profiles of Be in the first micrometer of the surface before and after irradiation.

2. Experimental method and results Since Be is a low-Z element, it is possible to use low energy proton-induced nuclear reactions as a depth profiling probe. Information on the energy level diagrams and cross sections for two of these reactions is presented in fig. 1 for the case of a 300 keV proton probe. These reactions are the 9Be(p, d)SBe and 9Be(p, a)6Li. It is seen in the figure that the p, d reaction has a direct transition to the ground state of 8Be with a Q value of 0.56 MeV. The p, a reaction is somewhat more complicated but still very attractive as a depth probe. In this case the excited 9Be nuclei undergo transitions both to the ground state and the first excited state of 6Li. Each of these transitions gives off a-particles. The 6Li* first excited state emits either a y-ray in its transition to the ground state or breaks up into an a and a deuteron. Also presented in fig. 1 are the published cross sections for these reactions as a function of incident energy4). These are seen to peak between 300 and 400 keV at values of 20-30 mb/sr. It should be possible therefore, using a proton beam of 300 keV, to ob1. LIGHT ELEMENT P R O F I L I N G

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serve concentrations of Be in excess of 0.1 atomic percent. From the energy dependence of the cross sections, it is expected that a uniform distribution of Be will result in a decreasing yield with depth due to the rapidly decreasing cross section resulting from deceleration of the proton by electronic stopping. The various energies of the particles coming from the p,d and p , ~ reactions must be considered with respect to the reaction product particles of interest in order to assess possible difficulties with background effects. We have used a Be

target for this purpose. The spectral patterns ob-

served are presented in fig. 2. Since the incident protons are of such low energy, the spectra are not affected by carbon or oxygen contamination. Lithium is the only impurity that could be of significance. Distinct groups of particles are observed in fig. 2 at energies well separated from the edge of the elastically backscattered protons. The particle groups are easily identified through the kinematics of the reaction. At 150° the deuteron group has an energy of 0.56 MeV for reactions taking place at the surface of the specimen. Similarly, the ~ group

p-Be N U C L E A R REACTIONS

79

has a surface edge at an energy of 1.22MeV. (Cl0HsO4) was used as a filter in front of the deThese two groups of particles are well displaced tector to absorb the backscattered protons. Only from the low energy elastic scattering edge. The the highest energy elastically scattered protons will only background interference is seen at an energy get through the foil and significant dead time and just above the deuteron surface edge. The particles pile-up is avoided. The foil shifts the deuteron and in this group result most likely from the break-up ~z peaks to lower energy and introduces a degradaof the 6Li* first excited state into an ~ and a deu- tion in energy resolution due to straggeling in the teron. The energies in this group are close to those foil. expected for the break-up reaction. The initial ~z The energy spectrum of the reaction product given off in the formation of the 6Li* state will be particles contains, in its energy scale, direct inforat energies of - 1 8 0 keV or less which is below mation on the depth at which the reaction octhe elastic edge. The y-ray coming from the alter- curred. Under the assumption of a constant stopnate path of the deexcitation of the 6Li* state to ping power, the depth z is obtained through the the ground state will not be observable in the equation: 100 # m depth surface barrier detector. EkIz=o -- Ea,~ In changing from a Be target to a Ni-0.7% Be Z (~E~/~Ep) o (dEp/dz) + (dEa,jdz)/cos O" alloy one gets a shift of the elastic edge from 275 keV to 295 keV when using 300 keV protons. ERIz~0 is the reaction product energy at the surface This elastic edge for a Ni substrate is still well be- of the specimen and Ed,, is the energy of the delow the energy of the deuteron and ~ groups; tected particles (either deuterons or ~z's). The term however, it is observed that the pile-up from the 8ER/8Ep describes the change in the energy of the Ni elastic scattering completely masks the low reaction product as a function of the proton enerlevel deuteron signal from the 0.7% Be concentra- gy. This term accounts for the change in energy tion. The ~ signal however is still clearly observ- of the reaction (in the laboratory coordinate sysable. Before studying the ~ group alone, it was felt tem) as the incident proton is slowed in its inward that efforts to recover the deuteron signal should penetration of the material due to the proton stopbe made in order to take advantage of its large ping power dEp/dz. The term clEd,~/dz accounts depth analysis potential as well as the higher for the stopping power of the exiting particle and ( - 3 ×) cross section as compared to the ~ signal. cos 0 is a geometric correction for the detector poIn order to accomplish this a 2.5 # m mylar foil sition with regard to the specimen normal. Values EN ERGY 240 200

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CHANNEL Fig. 3. Observed spectra for irradiated (upper curve) and nonirradiated (lower curve) specimens of Ni-0.7% Be alloy. These data are taken with a 2.5 # m mylar filter in front of the detector. Depth resolutions of 600 A. for deuterons and 350 A for ~z's are depicted in the upper curve. In both cases the specimens were held at 625°C.

I. LIGHT ELEMENT P R O F I L I N G

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PRONKO

for (OER/OEp)may be obtained by plotting ER vs Ep, in the energy range of interest, as obtained from the nuclear kinematic relations. It is found, in the present case ( 0 = 30°), that a simple linear relation is obtained with OER/OEp for deuterons being 0.481 and cgER/oIEp for a's being 0.123. All necessary stopping powers can be obtained from standard tablesS). Fig. 3 shows the energy spectra obtained at normal incidence of protons and a detector angle 0 = 3 0 ° from the specimen normal for two Ni-0.7% Be specimens. One was irradiated to 23 dpa (displacements per atom) at 625°C (top) and the other was held at 625°C next to the irradiated specimen but shielded from the nickel ion beam (bottom). The control specimen was used to determine whether significant thermal segregation occurs independent of the irradiation-induced process. It is seen that the shape of the spectrum of the control specimen (fig. 3, bottom) is similar to that observed in fig. 2 except that the a group is now shifted to a substantially lower energy. This is caused by the loss, on average, of 725 keV when the ~z's pass through the mylar filter. The deuteron group is also shifted downward, but these particles lose only 235keV in passing through the foil. These energy losses are calculated values based on the Northcliffe and Schilling s) stopping power tables. The energy scale in fig. 3 is based on the difference in energy expected for the ~ and deuteron surface edges after the particles pass through the foil. It is clear from the results shown in the lower part of fig. 3 that complete recovery of the deuteron signal is possible with the 2.5 # m mylar filter. An additional advantage of the filter is that one can collect data at much

et al.

higher counting rates using increased beam current (300 nA) since the-backscattered protons are suppressed. Disadvantages are introduced by way of lower depth resolution and interference of the ~z group with the deuteron group. This results from the higher stopping power of the a's compared to the deuterons. The upper part of fig. 3 shows results obtained from the specimen which was irradiated at 625°C to a total fluence of 23 dpa. The segregation of the Be from a subsurface region to the surface is significant as can be deduced from the strong surface peak and the associated reduced count rate of particles originating from the subsurface region. These features are observed both in the deuteron and the ~z spectra. The deuteron group shows that the depletion occurs in a region from 500-3000 ~ below the surface. It is observed in both spectra that the near surface concentration is tmilt up in a depth region extending about 500 ,X, from the surface. Because of the energy straggeling in the mylar foil the resolution of the observed data is about 350/~ fwhm for the ~z's and 600/~ fwhm for the deuterons. These. values are obtained from the shape of the surface edge in the lower part of fig. 3 for the nonsegregated specimen. Information about the concentration profiles of the radiation-induced segregation is obtained most simply by overlaying the spectra taken for corresponding specimens with and without irradiation and calculating a point-by-point ratio of the two. This normalizes out the effect of the cross section which changes as a function of depth. Furthermore, any contributions from a possible thermal segregation of Be to the surface are divided out to

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ENERGY (keY) Fig. 5. Observed yields from C u - 7 % Be alloys that were held at temperature b u t not irradiated. T h e solid curve is from a speci m e n that was annealed at 525°C and t h e dashed curve from one annealed at 885°C. T h e s e data were taken u s i n g the ~ group without t h e mylar filter.

a first order. The results of applying such a procedure to the data in fig. 3 are presented in fig. 4. It can be seen in fig. 4 that the concentration of Be remains unchanged for a depth of >3000 from the surface. A Be depleted region is evident at a depth between -500-3000 A. The excess Be concentration near the surface is seen to extend to a depth of about 500 .~. The width of the surface peak appears to be dominated by the resolution function of the mylar-detector system. It is clear from the above results that the advantages obtained by use of the mylar foil can, in some cases, be outweighed by disadvantages associated with loss of depth resolution. If the Be concentration is large enough to be observable with the o~ group, one can obtain enhanced depth resolution by eliminating the foil and taking advantage of the higher stopping power for the o~-particles. A lower level discriminator and moderate beam currents (100 nA) are required to keep dead time and pile up to tolerable levels. Adequate counts can be accumulated in feasible times. Fig. 5 shows results obtained in the ~z mode with no filter for two specimens of Cu-7% Be that were held in vacuum at temperatures of 525 and 885°C, respectively, but not irradiated. The data presented in fig. 5 are energy spectra as obtained

with the pulse height analyzer. A uniform concentration of Be gives a decreasing yield with depth due to the changing cross section. The depth resolution is nominally 150 A. It is clearly evident from the data of fig. 5 that a large increase in the surface concentration (N 3.6 ×) of Be occurs due to annealing at 885°C, relative to the specimen annealed at 525°C. The segregation observed here is not associated with an irradiation process but is caused by annealing. The increase of Be concentration could be due to preferential oxidation of Be on the surface. It is worth noting that the fullwidth at half-maximum of the segregation peak is of the same magnitude as the depth resolution for the o~'s. This suggests that resolution enhancement through an increase in the detector angle 0 with respect to the specimen or tilting the specimen with respect to the incident beam would be advantageous. 3. Conclusion It has been determined that the 9Be(p,d)SBe and 9Be(p, o~)6Li reactions can be used to analyze, nondestructively, the segregation profiles of Be in metals. The deuteron reaction product is of particular advantage in moderately deep profiles (to 1/,zm in, e.g., Ni or Cu) with reduced resolution I. L I G H T

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PROFILING

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PRONKO et al.

and the ~ reaction product is useful in cases where high resolution, near surface analysis is required. The authors would like to thank L. J. Thomp'son for technical assistance with the accelerator and J. G. Pronko for discussions and calculations on the nuclear kinematics.

2)

3) 4)

References 1) p. R. Okamoto and H. Wiedersich, J. Nucl. Mat. 53 (1974) 336; A. Barbu and A. J. Ardell, Scripta Met. 9 (1975) 1233: E. A. Kenick, Scripta Met. 10 (1976) 733; K. Farrell, J. Bent-

5)

ley and N. D. Braski, Scripta Met. 11 (1977) 243; N. Q. Lam, P. R. Okamoto, H. Wiedersich and A. Taylor, Met. Trans., in press. D. Potter, L. E. Rehn, P. R. Okamoto and H. Wiedersich, Proc. lnt. Conf. on Radiation effects in breeder reactor structural materials, Scottsdale, AZ (1977); and Wiedersich, H. Okamoto, P. R. and N. Q Lain, ibid. A. R. Johnson and N. Q. Lain, Phys. Rev. BI3 (1976) 4346 and 15 (1977) 1794. Charged particle cross sections, Los Alamos Scientific Laboratories, University o f California Report No. LA-2014 (1956). L. C. Northcliffe and R. F. Schilling, Nucl. Data Tables 7 (1970) 233.