Volume 100A, number 4
23 January 1984
POSITRON ANNIHILATION IN UO 2 T. TROEV, I. PENEV and H. PROTOCHRISTOV Institute of Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, Sofia 1184, Bulgaria Received 22 August 1983
Positron lifetime and Doppler broadening of the annihilation gamma quanta were measured as a function of uranium concentration at room temperature. The measurements showed that the positron lifetime spectra exhibit three components. The relative intensity of the parabolic component of the Doppler broadened spectra correlate non-linearly with the uranium concentration. From these measurements it is concluded that positronium forms m UO2 and is strongly influenced by uranium content.
The investigation of solids by positron lifetime, angular correlation and Doppler broadening of annihilation gamma-quanta is well known [ 1 - 3 ] . Studies of defects in metallic 238U as a function of temperature by measurements of Doppler broadening of the annihilation gamma-line [4,5], and of phase transitions by measurements of angular correlation of annihilation gamma-quanta in the peak at 0 = 0  have received much interest. Rothman et al.  have measured the chemical diffusion coefficient of Ta in ?-U. They observed formation of porosity when uranium is cycled from the a to the 3' phase. In recent years it has been established that when a positron enters into an oxide it can take a Bloch state, a localized state at a defect or a positronium state. The trapping process of a positron at cation defects in oxides and positronlum formation is not clear. Up to now there have been two positron investigations of UO 2: Coussot and Paulin  measured angular correlation of the annihilation gamma-quanta in a fcc UO 2 single crystal and they observed an anomaly in the correlation curve corresponding to the limits of the first Brillouin zone. Upadhyaya et al.  found that positron lifetime spectra in uranium dioxide powders have three components which depend on the particle size of the oxide. Our purpose was an investigation of the influence of the physical properties of nuclear, fissile materials on positron annihilation spectra by an examination of the positron lifetime and Doppler broadening of the 0.375-9601/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
(1). Doppler broadening measurements. The energy distribution E.~ was measured with a stabilized 1 cm 3 Ge detector. The energy resolution FWHM of the Ge detector for the 514 keV gamma-line of 85Sr using an "Ortec" amplifier was 1.15 keV. Doppler broadened annihilation spectra were analyzed by the computer program "Doppfit" . (2). Positron lifetime measurements. The positron lifetime was measured by a fast-slow coincidence spectrometer. The photomultiplier of the detectors were XP2230 with plastic scintillator NE111. The spectrometer resolution was 300 ps for a 60Co source. One channel of the multichannel analyzer corresponds to 52 ps. The positron lifetime spectra were recorded at room temperature and about 3.2 X 104 coincidence counts in the peak channel were obtained. We used a 22Na 10/aCi positron source deposited on a Mylar foil (0.5 mg/cm2). The analysis of the experimental data was performed using the "Positronfitextended" computer programm . The specimens were prepared from uranium dioxide powders, characterized with one and the same specific grain size. Specimen C contained 87% U, 21 ppm Fe, 20 ppm N and 80 ppmC. Specimen D contained 85.2% U, 80 ppm Fe, 250 ppm N and 210 ppm C. Specimens A and B were prepared by pulverizing the initial uranium dioxide materials and were sintered in an H 2 221
Volume 100A, number 4
23 January 1984
Table 1 Specimen
A B C D G
8.4 89 8.0 93 9.8
2.7 2.5 3.1 2.2 2.1
atmosphere at 1300°C and 1150°C, respectively. Specimen G was prepared by annealing the initial UO 2 of specimem D material in air under normal conditions. Pellets with various uranium contents were prepared at a compacting pressure of 2.5 Mp/cm 2 using compacts of 10 mm diameter and were then sintered. While the specimens A and B were sintered in a hydrogen atmosphere the remaining specimens C, D and G were sintered simultaneously at 1100°C to 1150°C in closed, slightly evacuated silica glass ampoules. The specimens A and B were sintered in gazeous atmosphere with the purpose to obtain high uranium content in them. The pellets decrease in uranium content at air oxydation and moisture. All materials were natural uranium oxides and were delivered by IAEA. The informative parameters are shown in table 1. The positron annihilation measurements on uranium dioxides were done by employing pellets of approximately 10 mm in diameter. The thickness of the uranium dioxides specimens was sufficient to stop all positrons. X-ray fluorescence analysis and chemical titrations were performed to check the uranium contents of these dioxides. The pellets were homogeneous in uranium concentration. The typical experimentally obtained spectrum of the positron lifetime in uranium dioxide at room temperature consists of three components after separating the r s source component. The lifetime spectra stored in a multi-channel analyzer were analyzed with the computer EC 1020 of the Institute of Nuclear Research, as three exponential decays. Similar results were obtained by Upadhyaya et al.  in an investigation of positron lifetimes in ThO 2 powders. Fig. 1 presents the values of positron lifetime components as a function of uranium concentration in uranium dioxides. It is seen in the figure that the r 1 component decreases with increase of the uranium concentration. The 7-2 component also decreases about 222
88.2 87.8 87.0 85.2 84.4
1300 1150 1100 1110 1140
29% for uranium concentration between 85.2%-87%. At uranium contents between 87% and 87.8% the r 2 component increases again. It was observed that the long-lived component r 3 also changes with increase of uranium content. The largest change in r 3 is observed for specimens G and D, about 12.7%. The change of positron lifetime in different uranium dioxides specimens is marked in fig. 1 in percent. The 7"2 and r 3 positron lifetime components in uranium dioxide non-linearly correlate with increase in uranium content. Paulin and Ambrosino  reported that positronium is formed efficiently when positrons are stopped in metal oxide powders. In our measurements the shortest lifetime component corresponds to the free positron annihilation in UO 2. In uranium dioxide powders positrons can form for short-time states like positronium when the positron kinetic energy is higher than the ionization energy of the matrix atoms. The r 1 component can be also connected with para-posi-
26 22. u 20"
~__ 1 2 N E 7
2 -] --m . . . .
1 G 84
Fig. 1. Change in positron lifetime at different uranium concentration in UO2 specimens; Intensity of the lifetime components 11 = 78.8%; 12 = 13.4%; 13 = 7.8% for specimen A.
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tronium (p-Ps) formation. The lifetime of p-Ps, rp.Ps is 125 ps. The r 2 component in ionic solids is generally attributed to pick-off annihilation of orthopositronlum (o-Ps) . The r 2 component observed in UO 2 for specimens A, B and C is difficult to explain. The lifetime r 2 is equal to 500-600 ps and can be associated with the trapping of positrons in UO 2, due to differences in defect concentration. We treated the r 3 component as the value for annihilation of o-Ps. The probability of o-Ps decay is equal to h 0 --- 71 X 10 - 7 ps -1. The hfetime of o-Ps r 0 = I/2~0 = 1.4 X 105 ps. The pick-off annihilation process at the moment of annihilation sharply decreases the o-Ps lifetmae in UO 2, 7-o_ UO~=~ 2 X 1 0 - 2 r 0 = 2.8 X 103 ps. The r3 component is significantly larger than is usually observed for o-Ps. This hfetime component is due to o-Ps formation near the surface of the UO 2 specimens. Probably positronium forms within a uranium oxide grain and diffuses to the surface. We explain the decrease of r 3 as the result of increased trapping rate of o-Ps near the surface. The annihilation lineshapes spectra obtained for specimens of the UO 2 oxides have been normalized to equal area. It was found experimentally that the observed spectra could be described as a sum of an inverted parabola with intensity Ip and a gaussian with intensity Ig = 1 - Ip. The inverted parabola and gaussmn are convoluted with the resolution function determined by measurements of the 514 keV gammaline of 85Sr. The spectra are not fully symetric due to incomplete charge collection. The results of the analysis of the Doppler broadening of annihilation gamma-spectra in UO 2 dioxides are shown in fig. 2. The main changes of the annihilation parameters in UO 2 are in the parabolic part rather than in the gaussian part. As it is seen in fig. 2, Ip decreases with increase in the uramum concentration, which is evidence that positronium forms with higher probability at lower uramum concentration in UO 2 dioxides. The relation between the H-parmneter as defined in ref.  and the uranium concentration in UO 2 is gwen in fig. 2. The most notable feature is a decrease of the H-parameter value at about 8% for D and C specimens. This result is consistent with lifetime experiments. The interesting result in fig. 2 is the nonlinearity o f l p for all the five UO 2 specimens. This shows that it is impossible to determine the trapping cross section for positrons. The variations in the in-
23 January 1984 /p% u oe
"~" " 2O
, q 85
URANIUM CONCENTRATION %
Fig. 2 H-parameter and intensity of the parabolic c o m p o n e n t as a function of uranium concentration in UO 2.
tensity of the parabolic component Ip, H-parameter and lifetime component for different uranium concentration shed light on the annihilation mechanism of positrons in uranium dioxides. We come to the following conclusions: (1). Positron lifetime and Doppler broadening measurements have shown that the positronium formation and its decay in the matrix and at the surface ~ in UO 2 can be studied. The positron measurements give evidence of the strong influence of uranium concentration on the positronium formation in UO 2 oxides. (2). Positron lifetime spectra of UO 2 oxides at different uranium concentration exhibit three components. The r 2 component is attributed to trapping of positrons by defects. The longlived r 3 component is due to positronium formation. (3). The relative intensity Ip of the Doppler broadened spectra correlates non-linearly with the uranium concentration in UO 2. The authors are grateful to Professor Dr. Zh. Zhelev and Dr. V. Andreicheff for their interest and encouragements. We also wish to thank Dr. R. Tsolov and Mr. P. Doichev for their assistance throughout the course of this work.
References  A. Seeger, J. Phys. F3 (1973) 248.  M. Doyama and R.R. Hasiguti, Cryst. Lattice Defects 4 (1973) 139. 223
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[ 3 ] T. Troev, M. Marinov, S. Darnianov and T. Diakov, Bull. Inst. Phys. Reeh. Atomique, Bulg. Acad. Sei. 18 (1969) 85.  T. Troev and I. Penev, in: Positron annihilation, eds. R.R. Hasiguti and K. Fujiwara (Japan Inst. Met., Sendal, 1979); Bulg. J. Phys. 8 (1981) 577.  G. K6gel, P. Sperr and W. Trfftshauser, Fruhjahrstagung der Deutsch. Phys. C-es. (Freudenstadt, 1983) M160, 787.  H. Matter, J. Winter and W. Trfftsh~user, J. Nucl. Mater. 88 (1980) 273.  S.J. Rothman et al., J. Nucl. Mater. 110 (1982) 55.  G. Caussot and R. Paulin, J Phys. C3 (1970) L100.
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 D.D. Upadhyaya, R.V. Muraleedharan and B.D. Shatma, J. Nucl. Mater. 105 (1982) 219.  S. Dannefaer and D.F. Kerr, Nucl. Instrum. Methods 131 (1975) 119. [ 11 ] P. Kierkegaard and M. Eldrup, Computer Phys. Commun. 7 (1974) 401.  D.D. Upadhyaya, R.V. Muraleedharan and B.D Sharma, Philos. Mag. A 45 (1982) 509.  R. Paulin and G Ambrosino, J. Phys. (Paris) 29 (1968) 263.  R.N. West, Adv. Phys. 22 (1973) 263.