High resolution perturbed-angular-correlation measurements on substitutional 111In in α-iron

Nuclear Instruments and Methods in Physics Research A244 (1986) 509-512 North-Holland, Amsterdam

509

HIGH R E S O L U T I O N PERTURBED-ANGULAR-CORRELATION M E A S U R E M E N T S ON S U B S T I T U T I O N A L Ulln IN a-IRON J. GOLCZEWSKI and K. M A I E R Max-Planck-lnstitut fiir Metallforschung, lnstitut fiir Physik, and Uni~)ersiti~t Stuttgart, lnstitut fiir Theoretische und A ngewandte Physik, Postfach 800665, D-7000 Stuttgart 80, FRG Received 25 October 1985

With the aid of BaF2 fast scintillators a resolution time 2% = 0.8 ns for the 171-245 keV 7-Y cascade in l~lCd has been obtained. As a new technique for the dissolution of ~1~In probe atoms in a-iron the electroplating of mixed Feln layers combined with short thermal treatments has been developed. By means of PAC measurements it is demonstrated that this technique permits a virtually complete incorporation of the rain probe atoms on substitutional sites in bcc Fe.

1. Introduction In recent years the perturbed angular correlation (PAC) of gamma rays has found widespread application in solid state physics, particularly in the study of lattice defects in metals [1]. In this application the technique usually employs the electric field gradient produced at the site of a substitutionally dissolved PAC nucleus (e.g. n l C d or lSlTa) by a lattice defect trapped by the PAC mother probe (in the above mentioned examples: r a i n or 181Hf). The interaction of the electric field gradient with the electric quadrupole moment of the PAC nucleus gives rise to a quadrupolar perturbation frequency VQ which is, together with the symmetry of the field-gradient tensor, characteristic for the probe-defect complex. Within the range 20_
lattice defects, e.g., with vacancies or self-interstitials, and to determine quantitatively the fraction f0 of the unperturbed PAC nuclei. The possibility to obtain the fraction f0 of PAC probes in undisturbed environments is particularly valuable for the study of defect-antidefect reactions, e.g., fo should increase if self-interstitials affecting nearby PAC probes are annihilated by migrating vacancies. The application of this idea to c~-Fe is very attractive, since here the temperature range of vacancy migration is the subject of a long-standing controversy ]2-5]. Two practical difficulties have to be overcome, however: (1) a t method has to be found that allows the PAC probes to be incorporated in ~-Fe virtually completely in a substitutional solid solution; (2) the time resolution 2% of the gamma lifetime spectrometer must be small compared to the period 2~r/oa c of the modulation of the PAC pattern by the magnetic hyperfine interaction at the PAC nucleus ( , % = spin precession frequency). In the case of u l C d in a-Fe we have 2¢r/~0 L -= 11 ns (at room temperature or below). This means that for the type of experiment outlined above the resolution time 2% should be about 1 ns or less. The need to overcome the above mentioned difficulties is fairly obvious from a recent attempt by Pleiter et al. [6] to investigate the recovery of radiation damage in iron by means of PAC measurements on naInFe. The n l I n nuclei were introduced by implantation.7-t was found that only about 50% of the l U l n atoms were substitutionally dissolved. The time resolution of 2 % = 3 ns for the 171-245 keV Cd cascade of the conventional lifetime gamma spectrometer with NaI(TI) crystals leads to a strong attenuation of the PAC anisotropy. The present work reports on a gamma lifetime spectrometer using BaF 2 scintillators and on a novel electro-

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J. Golczewski, K. Maier / PA C measurements in substitutional H/In in e~-Fe

polating procedure for obtaining substitutional l~lIn solutions in a-Fe. By the combination of these two techniques the limitations of the earlier experiments could be almost completely removed. The application to the study of vacancies and self-interstitials in a-Fe will be reported in a subsequent paper.

2. BaF2 fast gamma-gamma lifetime spectrometer The recently discovered ultraviolet fluorescence of the inorganic scintillator BaF 2 combines fast time response with good energy resolution and high efficiency for gamma-ray detection. The density of BaF 2 is 4.88 g / c m 3, its gamma detection efficiency hence even somewhat better than that of NaI(TI) scintillators. The energy resolution exceeds that usually observed with NaI(TI) crystals by 20-30%. Nevertheless, the time resolution is equivalent to that of the fastest plastic scintillators, e.g., Ne 111 or Pilot U [7]. An otherwise conventional lifetime gamma spec-

N a 22

7~2&5 KeY

7 171 KeV

o)

Am×P(t ) .15

.10 .05 .0 -.05 - .10 I

l

I

I

I

I

I

I

I

10 20 30 40 50 60 70 80 90 100 t Ens'l

Fig. 2. Experimentally observed PAC anisotropy and fitted curve [O.136foG~agn(t)] (see text).

trometer was built of 1 " × 1" BaF 2 crystals with XP 2020 Q photomultipliers. Similar to our positron lifetime spectrometer [8] the timing signal is an inverted dynode pulse connected with differential constant fraction discriminators (CFD). The output signal of a timeto-pulse height converter (TAC) was stored in a standard multichannel analyzer (see fig. la). For energy settings at 171 keV and 245 keV the prompt curve was measured with a 22Na source. The fwhm time resolution was 2% = 0.8 ns (see fig. 2b). The superiority of the BaF 2 lifetime gamma spectrometer becomes evident from a comparison with 2% = 3 ns as usually measured with conventional NaI(T1) scintillators for 11~Cd 7 - y cascades. The attenuation of the PAC anisotropy due to the time resolution in our ~11InFe samples is only 5%.

N (t)

3. Sample preparation

N (t)

log

o~o FWHM / / 2To = 0"Sns

1000

o o

o

500 o 0

0

/

b)

o

100

0

I

I

I

I

2

3

I

4 t Ens.3

I

I

5

6 ~---

Fig. I. (a) Schematic diagram of experimental setup. (b) Time resolution curve measured with 22Na source and 171-245 keV gamma energy setting.

In quite a number of metals the implantation of 111In atoms, followed by heat treatment, permits the preparation of reasonably good PAC samples [9,10]. Iron, however, shows a much poorer In solubility than all other metals so far investigated. Introduction of t h i n by implantation [6] or by electropolating [1t] an In layer onto Fe followed by annealing lead to strongly attenuated PAC signals. This suggests that a substantial fraction of the HlIn atoms are not dissolved on substitutional sites in an otherwise perfect Fe environment. In the present work electropolating of mixed FeIn layers was used for the first time for the preparation of H l I n F e samples. As starting material we employed high-purity iron with a residual resistivity ratio FRR R = 3000. Foils of 200 /~m thickness were annealed for 60 h in an H 2 atmosphere and polished chemically. Samples of 10 mm × 5 mm and 200/xm thick were then mounted as cathodes in an electrochemical cell. In order to keep

J. Golczewski, K. Maier / PAC measurementsin substitutional/llln in a-Fe

511

the electrolyte clean, pure iron of the same quality was used as anode material. The electrolyte consisted of a mixture of an aqueous solution of FeSO 4 and a commercially available carrier-free mlnC13 solution. In the electrolyte a ratio of Fe ions to r a i n ions of the order of 10 6 w a s maintained. Typically the electrolytic current density was between 20 and 50 A / m 2 and the electropolated amount of Fe between 1 and 5 rag. The procedure just described leads to the incorporation of 1013-1014 In atoms into Fe layers of a thickness of a few ~tm. The samples were put inside a thick-wall Ta cylinder which was placed in a quartz capsule. After evacuation and sealing of the capsule the samples were heated for about 2 h at 1100-1150 K in an hf inductions furnace. This resulted in homogenisation of the electropolated m l n F e layer and substitutional dissolution of r a i n in the Fe matrix, as tested by means of PAC measurements. During the heating Ta acts as a gettering material for gaseous impurities.

in the unpolarised ferromagnetic sample, was taken as [13]

4. PAC measurements and results

5. Conclusions

For the PAC measurements a simple two-counter setup with BaF 2 scintillators was used. The coincidence spectra

(1) The use of fast BaF 2 scintillators improves the time resolution 2% of PAC measurements on the m C d gamma cascade over that achieved with conventional NaI(T1) crystals by a factor of 3. (2) Electroplating of mixed FeIn layers followed by thermal treatment leads to a virtually complete substitutional incorporation of In atoms into the Fe lattice. (3) The improvements in the measuring technique and in the sample preparation permit detailed PAC studies on point defects in a-iron by means of PAC measurements.

U( O, t) = Uo e-roW(O, t) were measured at room temperature at 0 = 180 ° and 90 °. Here N o is a constant connected with the decay rate of l n l n nuclei, X is the decay constant of the 245 keV excited state in m C d and W(O, t) is the PAC function for the 171-245 keV 7 - 7 cascade. The experimental PAC anisotropy was derived from N(O, t) (fig. 2.)

AeXp(t ) = U ( 9 0 °, t ) - N(180 °, t)

(1)

S ( 9 0 °, t) + ½N(180 °, t ) ' with W( O, t) = 1 + A2G2( t )P2(cos O) [12] AeXp = -A~°r~Gz(t),

(2)

where A~°'r is the correlation coefficient corrected for source dimension and measurement geometry. For the 171-245 keV Cd cascade measured A~°rr = - 0 . 1 3 6 [12]. The PAC perturbation factor G2(t ) contains information about the interaction between nuclear moments and extranuclear hyperfine fields in the crystal lattice. The following expression for PAC anisotropy can be obtained: Ae~P(t) = 0.136[foG~ag(t) + (1 --fo)G~°mb(t)].

(3)

Here f0 denotes the fraction of In atoms substitutionally dissolved in the Fe lattice. The first term of the perturbation factor, G~ag(t), due to the magnetic field

G ~ a g ( t ) = l / 5 [ 1 + 2 COS(~Lt) + 2 COS(2WLt)].

(4)

The second term in eq. (3) represents perturbation due to possible combined quadrupole and magnetic interaction affecting the nonsubstitutional fraction of the ~HIn probe atoms. Assuming f0 = 1, we neglected the second term in eq. (3) when eq. (3) was fitted to the experimental data Aexp(t) and the parameters 6.}L and fo were estimated. Ae~p(t) and the fitted curve are shown in fig. 2. In this way w L = (572 + 1) M H z was found. With this value of o~L, the internal magnetic field acting on Cd atoms, Hin t : C 0 L h / ~ ( / t is the nuclear magnetic moment of the 245 keV excited m C d state), was calculated as (39.01 + 0.14) T, which is in excellent agreement with the literature value of Him(CdFe) [14]. The fraction f0 = 0.95 _+ 0.07 was estimated, i.e. the overwhelming majority of the In atoms occupy substitutional sites in the bcc a-iron lattices.

Acknowledgments The authors wish to thank Prof. A. Seeger and Prof. W. Frank for their permanent interest in this work and for their critical reading of the manuscript. The technical assistance of Mr. R. Hennes, Mr. F. Klopfer, and Mr. W. Bauer is very kindly acknowledged.

References [1] E. Recknagel, G. Schatz and T. Wichert, in: Topics in Current Physics, vol. 31, ed., J. Christiansen (Springer, Berlin, 1983). [2] H.E. Schaefer, K. Maier, M. Weller, D. Herlach, A. Seeger and J. Diehl, Scr. Met. 11 (1977) 803. [3] W. Decker, J. Diehl, A. Dunlop, W. Frank, H. Kronmi~ller, W. Mensch, H,E. Schaefer, B. Schwendemann, A. Seeger, H.P. Stark, F. Walz and M. Weller, Phys. Stat. Sol. (a)52 (1979) 239.

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J. Golczewski, K. M a i e r / P A C measurements in substitutional 1/ l l n in a - F e

[4] P. Hautoj~rvi, T. Judin, A. Vehanen, J. Yli-Kauppilo, J. Johansson, J. Verdove and P. Moser, Solid State Commun. 29 (1974) 855. [5] S. Takaki, J. Fuss, H. Kugler, U. Dedek and H. Schultz, Radiat. Eft. 79 (1983) 87. [6] F. Pleiter, C. Hohenemser and A.R. Arends, Hyp. Int. 10 (1981) 691. [7] M. Laval, M. Moszynski, R. Allemand, E. Cormoreche, R, Ordu and J. Vacher, Nucl. Instr. and Meth. 206 (1983) 169. [8] W. Bauer, W. Weiler, J. Major and H.E. Schaefer, Proc. 7th Int. Conf. on Positron Annihilation, New Dehli (1985) (World Sc. Publ. Co, Singapore, 1985).

[9] O. Echt, E. Recknagel, A. Weidinger and Th. Wichert, Z. Phys. B32 (1978) 59. [10] E. Lindgren, E. Karlsson and B. Jonsson, Hyperfine Interactions 1 (1976) 505. [11] J.I. Cisneros, G. Liljegren, T. Lindquist and A.L. Garcia, Ark. Fys. 38 (1968) 363. [12] H. Frauenfelder and R.M. Steffen, in: Alpha-, Beta- and Gamma Ray Spectroscopy, ed., K. Siegbahn (North-Holland, Amsterdam, 1968). [13] E, Matthias, S.S. Rosenblum and D.A. Shirley, Phys. Rev. Lett. 14 (1965) 46. [14] K.S. Krane, Hyperfine Interactions 15/16 (1983) 1069.