DEFECT 119Sn ATOMS AFTER NUCLEAR DECAYS AND REACTION IN SnSb AND SnTe

The lattice position of 119Sn atoms after EC decays and a proton-reaction was studied in SnSb and SnTe by emission spectroscopy. In Sn1 l9Sb and Sn 1 lgmTe, defect ll9Sn atoms were found in the Sb or Te site of the matrices. The 119Sb recoil atoms after the proton reaction were found only in the Sb site of SnSb, but they were distributed between both the Sn and Te sites of SnTe. The defect structures of '19Sn atoms produced by nuclear decays and reaction have been investigated in the matrices of SnSb and SnTe as a part of Mossbauer emission studies on l19Sn with l19Sb as the source nuclide [l-41. The nuclear processes studied were the following ones characterized by different recoil energies (E,) relative to the displacement energy in solid (ED z 25 eV) : 119 E C 1 1 9 (a) S b , Sn The Mossbauer emission spectra of 'l9Sn arising from l19Sb were measured in (a) and (b) on the SnSb and SnTe sources labeled with l19Sb or 119m~e, and in (c) on the samples irradiated by protons. A preliminary report on the proton-irradiated SnTe has been already made [5]. Tin metal depleted in 119Sn(120Sn 98.39 %, l19Sn 0.39 %, referred to hereafter as Iz0Sn) was employed in preparing the sources to minimize the resonant selfabsorption of the Mossbauer y-rays. Irradiation of the 120SnSb and 120SnTe samples were carried out with BaSnO, absorber (0.9 mg 119Sn/cm2) at the same temperature. Measurements on the 120Sn119mTe samples were started after a radioactive equilibrium has been established between ' l 9 " ~ e and l19Sb. The 1Z0Sn119Sb sources gave an emission line with an isomer shift of 2.43 , 0.03 mm/s relative to BaSnO, at 78 K, as may be seen in figure la. Comparison of the isomer shift with that of the absorption line of the same compound (2.79 + 0.03 mm/s) indicates that the l19Sn atoms arising from '19Sb in the SnSb matrix are in an electronic state different from that of the tin atoms in the normal Sn site of SnSb. Since the recoil energy associated with the EC decay of l19Sb to '19Sn is much smaller (1.4 eV) than the displacement energy, no displacement of the l19Sn atoms from the original site of l19Sb is expected. Therefore, the observed emission line is attributed to the defect l19Sn atoms in the Sb site of the SnSb matrix. EC As the recoil energy of the 'lgmTe -t 'l9Sb is estimated to have a distribution with a minimum value below the displacement energy and a maximum exceeding it, partial displacement of the decaying atoms is expected in the successive decays, EC EC llsrnTe _t l19Sb + l19Sn. In fact, two lines with 15 MeV protons on a water-cooled aluminum plate isomer shifts of 2.24 f 0.05 mm/s and 3.3 ) 0.2 mm/s under a helium atmosphere. The irradiated powder were observed in the emission spectra of 120Sn119mTe samples gave no visible indication of melting. The (Fig. Ib). The latter line is attributed to 119Sn in the Sn emission spectra were measured at 78 K against a site of SnTe, since the isomer shift is close to that of the absorption line of the same compound (*) Present address : FB8 Kernchemie, Technische Hoch(3.54 * 0-05 mm/s)The f0I3ner line is consequently schule Darmstadt, 61 Darmstadt, West Germany. attributed to l19Sn resting in the Te site. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19766198 C6924 F. AMBE AND S. AMBE L I -4 0 4 8 Relative Velocity (m/s vs BaSn03 at 78K) FIG. 1. IlgSn-Mossbauer emission spectra of (a) 120SnllgSb and (b) l2oSn 119mTe against BaSnOs at 78K. The curves are the least-squares fit of the experimental points with Lorentzians. On the basis of these results together with the data reported previously [2,3], a systematics was established between the isomer shift of defect and normal li9Sn atoms and the electronegativity of the nearest neighbor atoms in metals and binary compounds of Sn, Sb and Te. In these systems the isomer shift is determined to the first approximation by the nearest-neighbor atoms and increases with an increase in the electronegativity of the nearest-neighbor atoms. From the isotopic composition of tin and the energy of protons the main reaction leading to '19Sb in the proton-irradiation is estimated to be the lZ0Sn(p, 2 n)l19Sb. The accompanying recoil energy is so large (10' keV) that all the '19Sb atoms are knocked out from the original site of lZOSn. Since the following EC decay of '19sb to l19Sn brings about no atomic dispiacement, the lattice position of l19Sb is considered to have been the same as that of its daughter '19Sn, which is estimated from the emission spectra. As may be seen in figure 2a, the '19Sn atoms arising from 'l9Sb recoiled by the proton-reaction in 120SnSb gave an emission line with an isomer shift of 2.42 _+ 0.03 mm/s. The isomer shift is essentially the same as that of the labeled source 120Sn119Sb. This shows that the '19Sn atoms and, accordingly, also the Il9Sb atoms were in the Sb site of the matrix. The emission spectra of proton-irradiated lZ0SnTe were composed of two lines with isomer shifts of 2.3 f 0.1 mm/s and 3.48 9 0.05 mm/s (Fig. 2b), indicating that the recoil '19Sb atoms came to rest in two different states. The latter line is attributed to l19sn in the normal lattice position of Sn in SnTe, because its isomer shift is the same as that of the absorption line of SnTe within the experimental errors. The former line is attributed to 'I9Sn in the Te site (possibly accompanied with some near-by defects), referring to the dominant emission line of the labeled source 120Sn119m Te. l # I I I I -4 0 4 8 Relative Velocity (mm/s vs BaSn03 a t 78K) FIG. 2. IlgSn-Mossbauer emission spectra of proton-irradiated (a) 120SnSb and (b) 120SnTe against BaSn03 at 78K. The curves are the least-squares fit of the experimental points with Lorentzians. The absorption spectra of the irradiated l2OSnSb and lZ0SnTe revealed no radiation damage of the matrix by protons, giving only one line corresponding to l19Sn in the normal lattice site. This confirms that the emission spectra described above do not reflect the macroscopic radiation effects of protons, but reveal the consequences of the proton reaction leading to '19Sb. In the proton-irradiated '"SnSb, one cannot disregard the possibility that the observed distribution of '19Sn is the result of local melting of the matrix along the recoil track. In 12'Sn~e, however, the idea is not compatible with the following observations. Firstly, the distribution shows a marked change by thermal annealing after irradiation. Secondly, the I2OSnSb samples melted once after irradiation give a different spectrum. It may be noteworthy that the '19sb atoms were found in the two distinct lattice positions of l Z 0 S n ~ e after recoil by the proton reaction. This shows that DEFECT 119Sn ATOMS AFTER NUCLEAR DECAYS AND REACTION IN SnSb AND SnTe C6-925 the SnTe lattice is preserved to a large extent in the with a rather unperturbed environment as the result of vicinity of the '19Sb atoms and suggests that the l19Sb a replacement collision and the following annealing atoms have been stabilized at one of the lattice points process.