Variable energy positron beam study of Xe-implanted uranium oxide

Journal of Nuclear Materials 432 (2013) 287–293

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Variable energy positron beam study of Xe-implanted uranium oxide Nikolay Djourelov a,⇑, Benoît Marchand b,c, Hristo Marinov a, Nathalie Moncoffre b, Yves Pipon b,e, Patrick Nédélec b, Nelly Toulhoat b,d, Daniel Sillou f a

Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., BG-1784 Sofia, Bulgaria Université de Lyon, Laboratoire IPNL, UCB Lyon 1, Bâtiment Paul Dirac 4, rue Enrico Fermi, 69622 Villeurbanne Cedex, France c AREVA, AREVA NP, 10 rue Juliette Récamier, 69 456 Lyon, France d Commissariat à l’Energie Atomique CEA/DEN, Centre de Saclay, 91191 Gif sur Yvette Cedex, France e Institut Universitaire de Technologie (IUT), Université Claude Bernard Lyon 1, 94 Bd. Niels Bohr, 69622 Villeurbanne Cedex, France f LMOPS, University of Savoie, Bâtiment IUT, 73376 Le Bourget-du-lac Cedex, France b

a r t i c l e

i n f o

Article history: Received 19 September 2011 Accepted 23 July 2012 Available online 31 July 2012

a b s t r a c t Doppler broadening of annihilation gamma-line combined with a slow positron beam was used to measure the momentum density distribution of annihilating pair in a set of sintered UO2 samples. The influence of surface polishing, of implantation with 800-keV 136Xe2+ at fluences of 1  1015 and 1  1016 Xe cm2, and of annealing were studied by following the changes of the momentum distribution shape by means of S and W parameters. The program used for this purpose was VEPFIT. At the two fluences in the stoichiometric as-implanted UO2, formation of Xe bubbles was not detected. The post-implantation annealing and over-stoichiometry in the as-implanted sample caused Xe precipitation and formation of Xe bubbles. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The usual fuel used in pressurized water nuclear reactors is uranium oxide (UO2). About 25% of the fission products are gaseous. These are mainly noble gases: xenon, up to 90% of the gaseous products, and krypton. In both modes, operational and spent fuel storage the understanding of the retention of these fission gases is of great importance. These noble fission gases have a low solubility in UO2 and do not interact with it or with any other fission product, and, as a result, Xe segregates and forms bubbles [1,2], which have been observed as well in other materials [3–6]. The nucleation and diffusion of Xe bubbles in UO2 have been studied by different experimental methods such as Electron Probe Micro Analysis (EPMA) [7], Secondary Ions Mass Spectroscopy (SIMS) [8], X-ray absorption spectroscopy (XAS) [9], and Transmission Electron Microscopy (TEM) [10–12]. One of the processes considered in modeling the fission gas release in UO2 consists in intragranular bubble diffusion to the grain boundaries taking into account the precipitation of single atoms into intragranular bubbles accompanied by resolution of gas atoms. During postirradiation annealing it is suggested that a thermal vacancies gradient may be responsible for the intragranular bubble diffusion towards the grain boundaries [13]. In order to study diffusion processes, the technique of ion implantation is widely used to simulate the accumulation of fission ⇑ Corresponding author. E-mail address: [email protected] (N. Djourelov). 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.07.035

products, however, the simultaneous formation of near surface defects must be well-understood for proper modeling achievement. The power of positron annihilation spectroscopy to study defects (e.g. vacancies, interstitials, pores, gas bubbles) is well-known [14,15]. Moreover, the slow positron beam (SPB) technique (for a review see Ref. [16]) allows to perform depth profiling of defects. A great number of studies of gas ions implantation in various materials by SPB have been performed. They are based on the fact that in gas bubbles positronium (Ps) is formed and annihilate sharpening the momentum distribution of the annihilating electron– positron pairs. However, to our knowledge, only a few SPB studies on UO2 have been published studying the effect of electron irradiation, helium, and krypton implantation [17–20]. The aim of the present work is to study the defects in sintered and polished UO2 and the effect of post polishing annealing, Xe implantation, and post implantation annealing by means of Doppler broadening of the annihilation gamma-line combined with SPB. 2. Experimental 2.1. Sample preparation Ten depleted (0.3 at.% 235U) UO2 pellets, provided by AREVA NP, have been prepared for positron spectroscopy analysis. One of the pellets, named LI, was sintered at 2043 K with 150 min soaking time which resulted in 22 lm grain size and density of (97.8 ± 0.3)% of the theoretical density. The rest of the samples

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were sintered at 1973 K resulting in an average grain size of 7 lm and a density of (94.9 ± 0.3)% of the theoretical density. The samples were polished on one-side at the CSNSM-Orsay (Centre de Spectroscopie Nucléaire et de Spectroscopie de Masse) on an EcoMet 3000Ò polishing machine inside a glove box. To analyze the surface texture after polishing, interferometry measurements were performed at INSA, Lyon (Institut National des Sciences Appliquées) on a non-contact 3D Optical Profiler (Neox) of SensofarÒ. The measured mean roughness was 4 nm. After polishing all samples were annealed at 1273 K for 10 h in a PECKLYÓ tubular furnace in high vacuum (107 mbar) in order to degas particles adsorbed on the surface. An additional high temperature post-polishing annealing at 1873 K during 4 h in a

(a) Count (A.U.)

P; as polished I8; 10 Xe/cm2; 8h at 1673K I8H; 10 Xe/cm2; 8h at 1873K OI; over-stoichiometric; 10 Xe/cm2

1000

2000

3000

4000

5000

6000

7000

Backscatterd α energy (keV) 2.4

(b)

O/U ratio (U.A.)

2.3

I; 10 Xe/cm2 OI; over-stoechiometric;10 Xe/cm2 I8H; 10 Xe/cm2; 8h at 1873K

2.2

2.1

2.0 0

500

1000

1500

2000

reducing atmosphere (Ar + 2% H2) was performed in a NABERTHERMÒ tubular furnace to anneal the defects created during polishing. This last treatment was performed for all samples except one, named P, to check the effect of the post-polishing annealing. The stoichiometry of the sample P was checked by XPS in order to use this sample as a reference of non-oxidized sample. The sample named OI was oxidized up to the O/U ratio of 2.27. 2.2. Implantation and annealing The samples have a diameter of 8 mm and were implanted with 800-keV 136Xe2+ on the sample central part of 6 mm in diameter at IPNL (Institut de Physique Nucléaire of Lyon) using the IMIO400+ accelerator facility. Two fluencies have been reached: 1  1015 Xe cm2 (I0 sample) and 1  1016 Xe cm2 (I, I8, I16, I3H, I8H, OI and LI samples). Simulation of the Xe implantation has been performed with the SRIM.2008.4 software [21]. In the full cascade mode, SRIM software calculated the creation of 7 and 70 dpa (displacements per atom) for the two fluences with a distribution peaking at Rd = 80 nm. The corresponding maximum Xe concentrations were calculated as being respectively 0.1 at.% and 1 at.% at the depth of projected range RXe = 148 nm. After implantation, the samples were annealed up to 16 h in a NABERTHERM furnace at 1673 K (I8 and I16 samples) and at 1873 K (I3H and I8H samples) under reducing atmosphere (Ar + 2% H2). The stoichiometry was controlled for each sample by Nuclear Backscattering Spectrometry (NBS) experiments performed at IPNL using a 4-MV Van De Graaff accelerator. The 16O(a,a)16O Nuclear resonant reaction was used with 7.5-MeV incident He2+ ions [22]. A depth resolution of around 30 nm has been found by RESOLNRA [23]. NBS spectra of four representative samples are presented in Fig. 1a. The spectra represent the superimposition of two contributions: Oxygen signal (up to 2700 keV) and Uranium (up to 7000 keV). After treatment and comparison with the non-oxidized sample P, the O/U ratios vs. depth have been plotted in Fig. 1b. Fig. 1b shows that OI sample is homogeneously oxidized over a large depth (>2 lm). This sample displays a higher oxygen component compared to other samples. This figure also shows that the I8H is oxidized in the first lm and that I8 is comparable to the reference (non-oxidized P sample). The sample characteristics, the pre- and post-implantation annealing, Xe implantation fluence, grain size and stoichiometric ratio are summarized in Table 1.

Depth (nm) 2.3. Positron beam setup Fig. 1. (a) NBS spectra obtained on P, I8, I8H and OI samples performed using 7500keV incidental He2+. Uranium signal is observed in the range 2700–7000 keV. In the range 1000–2700 keV a superimposition of Uranium and Oxygen signals is observed. (b) O/U ratio determined by NBS using the P spectrum as a reference of non oxidized sample.

The Doppler broadening of the annihilation gamma-line measurements were carried out with a Canberra high purity Ge detector (HPGe) with a resolution G = 1.17 keV (FWHM) at the 514 keV line of 85Sr on the direct current SPB in Sofia (the former Ghent

Table 1 Sample, implantation fluence, pre- and post-implantation treatment, stoichiometric ratio O/U. Sample

P A I I8 I16 I3H I8H I0 OI LI

Grain size (lm)

7 7 7 7 7 7 7 7 7 22

Post-polishing annealing at 1873 K

+ + + + + + + + +

Implantation fluence (Xe cm2)

Post-impl. annealing time (h)

O/U ± 0.04

at 1673 K

At the surface

At 300 nm

2.00 2.00 2.00 2.04 2.08 2.15 2.17 2.00 2.21 2.00

2.00 2.00 2.00 2.00 2.00 2.08 2.15 2.00 2.27 2.00

at 1873 K

16

1  10 1  1016 1  1016 1  1016 1  1016 1  1015 1  1016 1  1016

8 16 3 8

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289

Helmholtz coils Accelerator 0V sample Internal tube at Negative High Voltage

HPGe

e+

Pb

Fig. 2. Schematic arrangement of the target part of the e+ beam line with HPGe detector and led shielding (2 cm thick).

SPB). The HPGe detector was set up at the sample position perpendicular to the beam axis at a distance of 4 cm from the sample. At the standard value of the magnetic field of 70 Gauss the e+ spot diameter is 6 mm however a positron energy dependent shift of 1 mm is usually observed for the whole energy range (E = 0.3– 28 keV). In order to reduce the shift influence on the measurements we decreased the strength of the magnetic field in the acceleration and target region down to 40 Gauss which resulted in a beam spot size of 10 mm. Thus the positron distribution on the sample is almost uniform. Special arrangements as shown in Fig. 2 were made to ensure that e+, which did not hit the sample, annihilate far (45 cm) from the detector at the end of the oneside-closed internal tube. The sample holder was a cross of thin wires to spread the negative high voltage to the sample and at the same time to have negligible cross-section with the e+ which miss the sample. The long distance from the detector in combination with the used 1.5-cm-thick lead shielding reduced significantly (1500 times) the efficiency of the detection of the annihilation gammas of that e+ which have missed the sample. The HPGe signals were read by a digital signal processor unit, model 2060 from Canberra. The energy spectra were collected during 4 h each (statistics of 6  105 counts in the 511-keV peak region).

2.4. Description of the model used for DBS data treatment The Doppler broadened 511-keV peak after subtraction of the stepwise background was characterized by a S parameter defined as the ratio of the central region counts (|D| < Es = 0.9 keV, where D is the shift from 511 keV) over the integrated counts in the peak (511 ± 9 keV) and the W parameter was defined as the ratio of the peak wings counts (2.9 < |D| < 8.1 keV) over the peak counts. The positron implantation profile was taken as P(z,E) = 2(z/ hzi)exp(-(z/hzi)2), where E is the incident e+ energy in keV, z in nm is the depth, and the mean penetration depth is determined by hzi = (36/q)E1.62, the density q is in g cm3 [24]. The S(E) and W(E) profiles were fitted by the VEPFIT program which can model the defect concentration by either a Gaussian or a step function taking into account the positron implantation profile and diffusion [25]. In a preliminary analysis of the data we first tried a two steps function model (surface, damaged layer starting from the surface and the bulk) which did not lead to acceptable fits. The three steps model (surface, two damaged layers and the bulk) also was used. It gave acceptable fits but the uncertainties in the fitting parameters were too large. That is why we choose the modeling with a Gaussian distribution. The samples were finally analyzed with a three contribution model: surface contribution, bulk contribution (assuming no defects) and defects contribution (the defects being introduced by polishing or implan-

Fig. 3. The fraction of positrons annihilating in each state as a function of incident positron energy resulting from the best fit by VEPFIT of sample I8H.

tation) which follows a Gaussian depth distribution truncated at the surface whenever necessary. The fit parameters are: the centroid, d, and FWHM of the defects concentration depth profile, the Sd and Wd characteristic parameters for the defects, Sbulk (S parameter) and Wbulk (W parameter) and Lbulk (e+ diffusion length in nm) for the undamaged (bulk) material. To take into account the fact that only the central part of the sample was implanted, a Sd parameter is introduced defined as Sd = (SdSbulk)/k + Saverage , where k is a coefficient with two posbulk sible values; (i) k = 0.5626, representing the ratio of the implanted area over total surface area for implanted samples; (ii) k = 1 for non-implanted samples. Saverage , is the average value of Sbulk on bulk all the analyzed samples. Fig. 3 shows the fraction of positrons annihilating in each state as a function of incident positron energy E, resulting from the best fit by VEPFIT of S(E) data for sample I8H. The fractions for the surface are representative for all the samples and it is clearly seen that the surface influence can be completely neglected above E = 7 keV and positrons annihilate from the bulk state and from the defects. 3. Results and discussion The best fit parameters for the ten samples are gathered in Table 2. The R parameter will be discussed bellow in the Section 3.4. The results of the VEPFIT analysis for the bulk layer (see Table 2) were in average (Saverage , W average ) = (0.4993, 0.0478) with standard bulk bulk deviations (STDs) = (0.0033, 0.0006). The STDs of averaged param-

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Table 2 Depth position d, and width FWHM of the defects profile and corrected parameters for the defects Sd and W d , and Sbulk and Lbulk as obtained by VEPFIT. Sample

Defects

P A I I8 I16 I3H I8H I0 OI LI

Bulk

d (nm)

FWHM (nm)

Sd

W d

47 ± 6

60 ± 7

0.513

0.0434

41 ± 6 49 ± 3 42 ± 2 62 ± 2 81 ± 2 72 ± 4 40 ± 5 40 ± 2

68 ± 10 62 ± 4 56 ± 2 64 ± 2 68 ± 2 67 ± 4 76 ± 8 60 ± 2

0.532 0.571 0.569 0.552 0.566 0.532 0.546 0.534

0.0373 0.0276 0.0276 0.0312 0.0300 0.0362 0.0333 0.0362

± 0.001

± 0.0001

eters are 6–8 times higher than the VEPFIT error for the determination of individual sample Sbulk. This indicates small differences in the bulk of the samples. The origin of the differences could be attributed to the sintering process, post-polishing annealing, additional grain growth as result of post-implantation annealing, or stoichiometric ratio. However, we could not find any correlation between Sbulk or Wbulk and the above mentioned processes, and the experimental results are displayed in this work as a ratio to bulk values in order to emphasize in the figures the visual effect of implantation and post-implantation annealing treatments. For the following sections, the W(E) data are not graphically presented as they have similar inverted trends compared to the S(E) ones, but analogous corrections were applied.

3.1. Post-polishing annealing and Xe implantation The normalized parameter S/Sbulk versus E for samples P, A, I0 and I are plotted in Fig. 4 (the secondary x-axis scale indicates the calculated mean positron depth). The S/Sbulk starts at relatively high value of 1.08 for low E and tends to 1 for high E. The value of 1.08 is a characteristic of the surface as a complex contribution of thermal positrons, surface positrons and positronium. An enhanced S/Sbulk at low E is typical for materials in which positronium (Ps) is formed dominantly by surface electron capture [26] and Ps formation occurs also if voids exist in these materials [27–29]. Since the surface state is out of the scope of the study, this aspect will not be discussed further. 10

50

100

200

R ± 0.02

Sbulk ± 0.0004

Wbulk ± 0.0001

Lbulk (nm)

0.4985 0.4975 0.5044 0.4970 0.4983 0.5009 0.4982 0.5002 0.4933 0.5031

0.0478 0.0474 0.0469 0.0488 0.0476 0.0478 0.0479 0.0479 0.0488 0.0474

249 ± 45 223 ± 30 193 ± 26 205 ± 12 206 ± 13 103 ± 22 46 ± 9 228 ± 33 93 ± 20 241 ± 26

600 (nm)

400

I; 101 Xe/cm2 (defects, VEPFIT) I'; low fluence 101 Xe/cm2 (defects, VEPFIT) P; as polished (defects, VEPFIT) dpa (SRIM) Xe (SRIM)

P; as polished A; post-polishing annealed

1.07

I'; low fluence 101 Xe/cm2 I; 101 Xe/cm2

1.05

arb. units

S /S bulk

1.06

0.29 0.33 0.31 0.37 0.36 0.27 0.34 0.27

The reconstruction of the defects profiles for samples P, I0 , I from the best fit VEPFIT data (given in Table 2) together with the Xe distribution and displacements per atom (dpa) are shown in Fig. 5. The low Sd = 0.513 for sample P indicates that the polishing introduces small point defects and/or dislocations in the near surface region (few tens of nm) in consistency with reference [30]. Also, the low temperature 1273 K post-polishing annealing is not efficient in annealing these defects. For sample A, i.e. after the high temperature post-polishing annealing, S/Sbulk sharply drops as the positron energy increases up to 1–2 keV and levels off at 1 for deeper probing (see Fig. 4). The analysis by VEPFIT showed that this data can be fitted by a single uniform layer (plus the surface state). The last indicates that the chosen second step of the postpolishing thermal treatment is efficient in defects annealing providing good reference samples to study influence of Xe implantation and post-implantation annealing. The created defects as an effect of the implantation at fluence of 1015 Xe cm2 (sample I0 ) is revealed by VEPFIT as a Gaussian distribution with depth position at d = 72 nm and FWHM = 67 nm. This depth position is much less than the Xe projected range RXe = 148 nm but it is in agreement with the peak position of dpa Rd = 72 nm. For a material with covalent bonds (ZrC) we have shown that positrons sensitivity is negligible to Xe interstitials caused by Xe-implantation [5]. The ionic bonds in UO2 determine defects (interstitials, Frenkel pairs, Schotky) most of them charged and positrons due to their positive charge will be sensitive to negatively charged defects, i.e. related to uranium vacancies or oxygen interstitials, but not to neutral or positively charged defects. Thus,

1.09

1.08

0.19

1.04 1.03

1.02 1.01

1.00 0.99

0

5

10

15

20

25

Incident positron energy (keV)

0

50

100

150

200

250

300

350

Depth (nm) Fig. 4. Normalized parameter S/Sbulk as a function of the incident positron energy, E, for as-polished sample, P, post-polishing annealed, A, as-implanted at 1  1015 Xe cm2, I0 , and as-implanted at 1  1016 Xe cm2, I. The curves are the best fits obtained by VEPFIT. The errors are of the order of the point sizes.

Fig. 5. Depth distribution of implanted Xe and displacement per atom as obtained by SRIM, together with depth relative defect concentration for samples P, I0 , and I as obtained by VEPFIT.

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3.2. Post-implantation annealing The specific S parameter for Ps annihilation in voids, SPs, can be calculated as well as the contributions of Ps self and pick-off annihilation. The self annihilation mainly comes from p–Ps (the singlet state of Ps) annihilation with high S parameter (starts at 0.84 for void radius of 0.3 nm and saturates at 0.93 for radius >1 nm) for the chosen ES and taking into account the Ge detector resolution, while the pick-off is greatly dominated by o-Ps (the triplet form of Ps) annihilation. Good approximation for Spick-off is the value of Saverage , and assuming p-Ps to o-Ps statistical ratio as 1:3 we estibulk mate 0.586 < SPs < 0.607. The same estimation cannot be transferred directly for Ps annihilation in Xe bubbles. In agreement with the conclusions in references [31,32], in a study of Xe implantation in zirconium carbide we have found that Xe is efficient for oPs into p-Ps conversion [5]. The o-Ps into p-Ps conversion significantly increases the resulting SPs, thus, the estimation SPs = 0.586 (or SPs =Saverage = 1.174) can be used only as the lower limit in the bulk case of Ps in Xe bubbles. Martin et al. [9] have concluded from Xray absorption spectroscopy study in UO2 implanted with 1017 Xe/cm2 at 800 keV that Xe bubbles remain highly pressurized after annealing at temperatures up to 1073 K while at higher temperature (1663 K and 1773 K) larger unpressurised bubbles are present as a result of trapping migrating vacancies. Such a scenario when bubbles grow but the Xe content stays the same is not expected to have big impact on the S parameter resulting from Ps annihilation because the o-Ps to p-Ps conversion rate in a grown pore probably will stay the same. Fig. 6 shows the S/Sbulk versus E for the post-implantation annealed samples I8, I16, I3H, I8H together with the best fit curves for sample I and A for comparison. The post-implantation annealing leads to a clearly pronounced peak in S(E) at position 40– 80 nm (5–8 keV). In general, a high S parameter can be due to e+ annihilation in small or medium vacancy clusters decorated or not with Xe. We have shown in Section 3.1 that the annealing at 1873 K successfully removes small defects introduced by the polishing. The effect of the post-implantation annealing at the same temperature is expected to be similar, i.e. to work in direction of

10

50

100

200

600 (nm)

400

1.09

I8; 101 Xe/cm2; 8h at 1673K

1.08

I16; 101 Xe/cm2; 16h at 1673K 1.07

I3H; 101 Xe/cm2; 3h at 1873K

1.06

S /S bulk

lack of sensitivity to Xe interstitials in UO2 is also naturally to expect. As can be seen in Table 2, the Sd parameter for I0 and I, for which the Xe concentration defers by factor of 10, is almost the same 0.53 which confirm the last expectation. As seen in Table 2, the defects distribution at higher fluence of 1016 Xe cm2 (sample I) is situated closer to the surface at d = 41 nm and this effect of Xe fluence is similar to what is observed in Xe-implanted zirconium carbide [5]. This fact together with the rather narrow widths of the defects distributions for I0 and I is in discrepancy with the dpa profile calculated by SRIM. It has to be mentioned that neither clustering nor recombination of defects is taken into account by SRIM. An explanation for the observed discrepancy could be formation of more efficient positron traps which occurs at a definite dpa threshold as a result of clustering of open volume defects. As can be seen in Table 2, the Sd parameter for I0 and I is almost the same 0.53 but higher that the one for P (0.513). The higher value can be explained either as due to annihilation of positrons trapped in bigger open volume defects as compared to these introduced by the polishing or to positronium annihilation either in Xe bubbles [1] or in voids, and which process prevails is impossible to distinguish from the analysis of S(E) profiles. However, in 1-MeV He implanted UO2 at fluence of 5  1015 He cm2 Labrim et al. have measured a characteristic for the damaged ion track layer normalized S/Sref parameter of 1.063 and associated this value to uranium vacancies, VU [17]. This value transformed to our case corresponds to SV U = 0.531 which implies that in I and I0 samples most probably Ps annihilation does not occur, i.e. Xe bubbles are not formed.

I8H; 101 Xe/cm2; 8h at 1873K

1.05

1.04 1.03

I fit

1.02 1.01

1.00 A fit 0.99

0

5

10

15

20

25

Incident positron energy (keV) Fig. 6. Normalized parameter S/Sbulk as a function of the incident positron energy, E, for post-implantation annealed samples I8, I16 annealed at 1673 K, and I3H, I8H annealed at 1873 K. The curves are the best fits obtained by VEPFIT. The errors are of the order of the point sizes. The fit curves of A and I samples are repeated from Fig. 4 for comparison reasons.

annealing open volume defects. The model calculation of electron momentum distribution of vacancy clusters in ZrC decorated with Xe leaded to decrease of S parameter for e+ annihilation [5]. That is why the high Sd parameter (0.55–0.57, see Table 2) implies a contribution from Ps annihilation either in voids or in Xe bubbles. Ps formation yield in voids in UO2 is limited to <30% [28,29]. Consequently, the observed high value for Sd of 0.55–0.57 can be explained only by contribution of Ps formation in Xe bubbles (accompanied with o-Ps into p-Ps conversion). The VEPFIT results in Table 2 show that Sd of the post-implantation annealed at 1873 K sample I8H is higher compared to I3H indicating that the process of Xe bubble formation and/or Xe content in the bubbles is in evolution. On the other hand, Sd for I8H and for the post-implantation annealed at 1673 K samples I8 and I16 are very close which indicates that at 8 h of annealing at these temperatures Xe bubbles are formed and there is no significant evolution in terms of Xe content in the bubbles after up to 16 h. This is in agreement with the SIMS results showing absence of Xe release afterwards up to 32 h annealing at 1673 K [33]. There is a marked difference in depth position of the defects profiles for samples annealed at 1673 K and 1873 K (Table 2). For samples annealed at 1873 K d increases. In addition, Lbulk for samples annealed at 1873 K is greatly reduced to 103 and 46 nm for I3H and I8H, correspondingly, which will be correlated to oxygen interstitials in the Section 3.3 below. The S(E) peak positions for I8 and I16 are not easily determinable by eye as can be seen in Fig. 6. It appears that values reach a plateau, between 30 and 80 nm. The plateau could be explained by high enough defect concentration to have positron trapping at saturation. In favor of this suggestion is the fact that high fluence as-implanted sample I shows very similar S(E) curve as compared to the low fluence as-implanted sample I0 despite of the one order of magnitude higher defect concentration in the damaged layer. The S(E) peak positions for I3H and I8H can be determined easier by eye and they are situated deeper compared to those for I8 and I16. The annealing at 1873 K is efficient in small defects annealing (see Section 3.1 above), consequently, the post-implantation annealing at the same temperature most probably reduces the small defect concentration in the damaged region. Therefore the Xe bubbles are detected with less distortion. This leads to damaged layer Gaussian deeper and closer to the Xe projected range for the samples I3H and I8H. About 17% Xe release from I8H, post-implantation annealed at 1873 K, has been observed by SIMS. The Sd value of 0.566 for I8H

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is very close to these parameters for I8 and I16 which does not allow us to interpret the S parameter change as an indication of Xe release from the bubbles. However, it is essential mentioning that SIMS detects the total Xe release out of the sample while the S parameter is sensitive to Xe in the bubbles but not to the interstitial Xe. Thus, the observed 17% Xe release may be due to Xe interstitials released out of the sample without significant impact on the Xe in the bubbles. 3.3. O/U ratio and grain size effect As seen in Table 2, except for the samples OI, I3H and I8H, Lbulk is long 200 nm, a typical value for defect free materials. The lower Lbulk for the mentioned exceptions indicates presence of defects. At the same time we see in Table 2 that these defects do not contribute to any increase in Sbulk or decrease in Wbulk and the lower Lbulk correlate with O/U ratio at 300 nm greater than 2 (Table 1), i.e. to interstitial oxygen in the region seen by positrons as bulk. Consequently, the interstitial oxygen traps positrons reducing their diffusion length but has a characteristic S and W parameters close to the perfect lattice values. The effect of oxidation to O/U = 2.27 (sample OI) and larger grain size (LI) on the implantation can be seen in Fig. 7 which shows S/Sbulk versus E. The difference due to the grain size is hardly seen in agreement with the large ratio of grain size over maximum positron penetration depth. So, the possible effect of intergranular Xe bubbles can be neglected and we may accept the results in the present study as valid for intragranular ones. The oxidation of UO2 to UO2.27 facilitates clustering of open volume defects or Xe bubble formation as can be deduced from the higher Sd = 0.546 for OI compared to 0.532 for I. In the three cases I, OI and LI the depth position of the defect profiles is the same 40 nm (see Table 2) the only variation concerns the broadening of the distribution for OI. Comparing the data for the defect profiles between samples OI and I8H which are over-stoichiometric we conclude that the shift to deeper position does not result on the presence of oxygen interstitials but rather to temperature effect. 3.4. S as a function of W parameter

the perfect lattice) of the linear fit is a characteristic feature of the defect type [34,35]. Both S and W parameters obey to the superposition rule and a generalization is easily made for a number of annihilation states. Namely, linear relationship has to be observed when positrons annihilate from a number of different states and the contribution of one of the state is changing at the expense of the others but keeping the relative ratios between the others. In positron beam studies the S(W) plot is a commonly accepted way for mapping the sample layers [16]. Due to the surface contribution to S and W at E < 7 keV (see Fig. 3) the S(W) change with running parameter E plotted in Fig. 8 is rather complicated. However, at E > 7 keV the annihilation occurs from two states: (i) bulk and (ii) defects (The annihilation from the defects is a complex state, i.e. a combination of few different annihilation channels which contribution to the complex state stays unchanged for a specific sample.). In other words the generalization given in the previous paragraph is fulfilled, therefore, a linear relationship has to be observed. This is demonstrated in Fig. 8, where the thick lines represent linear fits of the (W/Wbulk, S/Sbulk) data for E > 7 keV and the corresponding slopes R are given in Table 2. The slope R for the polished sample is clearly distinguishable among the others. Despite the relatively large error in R, differences are seen also for the Xe-implanted samples. These differences indicate that the positron annihilation defect state is not a pure state (only Ps annihilation in Xe bubbles) but a complex one with contribution at different levels most probably from uranium vacancy related defects, because otherwise the slopes should coincide. The characteristic points (W d , Sd ) for the defect state in the different samples are shown in Fig. 9 together with (Wbulk, Sbulk) for sample A and the averaged bulk values (W average , Saverage ). The points bulk bulk for the Xe-implanted samples are well aligned along a straight line. This indicates that the combination of defect types do not change for these samples and what varies is only the contribution from the Ps annihilation in Xe bubbles. The extrapolation of the fit obviously does not pass the (W average , Saverage ) point confirming the bulk bulk statement made in the end of the previous paragraph. The horizontal line indicates the level of SV U as obtained in reference [19]. Points corresponding to Xe implanted but unannealed samples (I,

A linear relationship between S and W occurs when in metals or semiconductors exists only one defect type with variable concentration. The slope R = (SSbulk)/(WbulkW) (here ‘‘bulk’’ stands for 10

50

100

200

1.09

1.07

600 (nm)

400

1.09

LI; grain size 22 µm; 101 Xe/cm2

1.06 1.05

1.05

S /S bulk

1.08

S /S bulk

1.06

OI; over-stoihiometric; 101 Xe/cm2

1.07

A; post-polishing annealed P; as polished I; 101 Xe/cm2 I8; 101 Xe/cm2; 8h at 1673K I3H; 101 Xe/cm2; 3h at 1873K

1.08

1.04 1.03

1.04

1.02 typycal errors

1.03

1.01

1.02

I fit

1.00

1.01

1.00

0.99

0.70

0.99

0

5

10

15

20

25

0.75

0.80

0.85

0.90

0.95

1.00

W /W bulk

Incident positron energy (keV) Fig. 7. Normalized parameter S/Sbulk as a function of the incident positron energy, E, for samples OI (O/U = 2.27) and LI (grain size 22 lm) as-implanted at 1  1016 Xe cm2. The curves are the best fits obtained by VEPFIT. The errors are of the order of the point sizes. The best fit curve for sample I (grain size 7 lm) is repeated for comparison.

Fig. 8. Normalized parameter S/Sbulk as a function of the normalized parameter W/ Wbulk for selected samples. The curves are constructed from S(E) and W(E) best fits by VEPFIT. The thick lines are linear fits of the (W/Wbulk, S/Sbulk) data for E > 7 keV. The data for samples (OI, I0 , LI) and (I16, I8H) showed similar patterns as for sample I and I8, correspondingly, and are omitted for clearness. The arrows are to guide the eye with increasing the incident positron energy.

N. Djourelov et al. / Journal of Nuclear Materials 432 (2013) 287–293

Acknowledgements

0.58

Xe implanted

I8

0.57

I16

average bulk

I8H

0.56

P; as polished

S d* parameter

I3H 0.55

A; post-polishing annealed

OI

0.54

LI I

0.53

293

I'

S VU

0.52

P

References

0.51 0.50 0.49 0.025

A bulk 0.030

0.035

0.040

We acknowledge the support by the Bulgarian/French program Rila-5/PHC, CNRS-Bulgarian Academy of Sciences agreement which have made possible the development of this collaboration, as well as the AREVA NP support. The authors are thankful to FBFC, AREVA for the samples preparation and to Frederico Garrido, and all the team ‘‘Physico-chimie de l’irradiation’’ at the CSNSM for fruitful discussions and their help in the polishing procedure. We wish also to thank Angela Perrat-Mabilon who performed the ion implantations and Marie-France Barthe for fruitful discussions.

0.045

W d* parameter Fig. 9. Characteristic points (W d , Sd ) for the defects region in the samples together with (Wbulk, Sbulk) for sample A and (W average ; Saverage ). The straight solid line fits bulk bulk (v2 = 0.982) the points for the Xe-implanted samples. The straight dash line fits (v2 = 0.989) the points for samples P, I, I0 , and Li, (Wbulk, Sbulk) for A, and (W average bulk Saverage ). The horizontal (dash-dot) line indicates the level of SV U as obtained in Ref. bulk [19].

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

I0 and LI) are just slightly above the SV U level. It is worth mentioning that the three points can also be aligned well with the points corresponding to P, Abulk and (W average , Saverage ). This indicates absence bulk bulk of any additional annihilation state compared with the damaged layer in P or if additional annihilation state, due to Ps formation in the damaged layer, is present in I, I0 and LI it is with negligibly low population. That is why we may accept that above this level we detect Ps formation and annihilation in Xe bubbles.

[12] [13] [14]

[15] [16] [17] [18]

4. Conclusions The momentum distribution of annihilating positron–electron pairs has been measured by Doppler broadening combined with a SPB in a series of sintered UO2 samples, polished, implanted with 800-keV 136Xe2+ ions, and annealed in different conditions. The momentum distribution shape parameters S and W as a function of the incident positron energy E have been analysed by the help of VEPFIT program. This analysis together with the analysis of the S–W plots allows drawing few conclusions. The implantation at the two fluences 1  1015 and 1  1016 Xe cm2 in the stoichiometric UO2 has not led to detectable concentration of Xe bubbles and point defects as vacancies, interstitials, and substitutions are predominant. The higher implantation fluence has led to thinner damaged layer. The post-implantation annealing lead to Xe precipitation and forming of Xe bubbles as revealed by the high S parameter. In contrary to stoichiometric UO2, the over-stoichiometry causes formation of Xe bubbles in the asimplanted samples. The process of Xe bubble formation has not been completed after 3 h of post-implantation annealing at 1873 K. The amount of Xe caught in bubbles has seemed to reach a maximum after 8 h of post-implantation annealing at both temperatures 1673 and 1873 K. It has been found a good Xe retention in the bubbles between 8 and 16 h of post-implantation annealing at 1673 K.

[19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]

[34] [35]

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