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Low temperature uranium fission fragment irradiations of α-iron
706
LOW
Nuclear Instruments and Methods in Physics Research B33 (1988) 706-709 North-Holland, Amsterdam
TEMPERATURE
A. DUNLOP
URANIUM
If, N. LORENZELLI
FISSION
FRAGMENT
‘) and W. MANSEL
IRRADL4TIONS
OF a-IRON
*
‘)
I) DTech /SESI, Laboratoire des Solides Irradiis, Eeole Polytechnique, Route de Saclay, 91128 Palaiseau Cedex, France ‘) Physik Department E21, Technische Uniuersittit, Miinchen, D 8046, FRG
We present here a study of 5 K irradiation damage in iron: a high purity iron ribbon and various dilute Fe -235U alloys (0.01 to 0.5 at.% ura~um) are introduced in the liquid helium irradiation loop of the Garching reactor. The sampte holder is set in a zone of intense
thermal
neutron
flux: the alloys are consequently
submitted
to uranium
fission fragments
irradiations.
Electrical resist&&y increase measurements allow us to follow the damage processes and to explain why they differ from those occurring during neutron or light ion irradiations.
1. Introduction This study of 5 K uranium fission fragments (F.F.) irradiation of cu-iron is motivated by the fact that iron behaves differently when irradiated with fission neutrons and with F.F. [I]. The F.F. irradiation reported in [l] was obtained by introducing in a thermal neutron flux 8 to 12 pm thick ribbons of a-iron which were coated on both sides with enriched uranium. The F.F. damage inhomogeneously the target [2] and have a range of 7.5 pm in iron. The facts: (i) that the ribbons are irradiated from both sides; and (ii) that the induced damages overlap in the middle of the sample, partly correct the damage i~omogeneity, but there was a controversy concerning the reliability of the results [3,4]. We thus decided to irradiate at 5 K dilute Fe-“‘U alloys in the thermal neutron flux of the Munich neutron reactor. The uranium concentrations varied from 0.01 to 0.5 at.% uranium, which allowed us to study in detail and for reasonable irradiation times the whole defect production curve and to see if impurity effects become important when the uranium concentration increases.
with 90% 235U. The elaboration was made in order to obtain uranium concentrations of 0.01 to 0.5 at.%. Prior to irradiation: (i) the effective uranium content of each type of alloy was determined by a spectrometry technique: sample 2 (0.01 at.% U), sample 3 (0.053 at.48 U), samples 4 and 5 (0.10 at.% U), sample 6 (0.49 at.% U). (ii) We irradiated for one hour a piece of the 0.49 at.% U alloy in the exact irradiation position that would be used later and performed y-ray spectroscopy a few months after irradiation. We cm deduce the dose, i.e. the number of fissions per second and per cm3 that occurred for this U concentration in the exact irradiation position. As the thermal neutron fiux is continuously measured, from the recorded neutron fluence and from the
2. Experimental Six ribbons (width 1 mm, thickness 50 to 60 pm) on which platinum leads were soldered in order to make electrical resistivity measurements, were mounted on the same sample holder and irradiated by thermal neutrons. The first sample was a pure iron ribbon, the five others were alloys which were prepared in a levitation furnace from high purity iron and uranium enriched
* I~adiations performed Miinchen, FRG.
in the TTB faci~ty/Garc~~g-
0168-583X/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
il w
Fe- .49at%U
ii! 0
4X++
2 JO’4
IRRADIATION FLUENCEfi%md2) Fig. 1. Electrical resistivity variations of a Fe-23’U alloy (sample 6: 0.49 at.% U) during an irradiation at 5 K with thermal
neutrons.
A. Dunlop et al. / Low temperature irradiation of a-iron
707
2:.Olat%U
3: .053at%U 45.1 at%U 6:.L9at%U
T=SK
IRRADIATION FLUENCE (Ffcrn-2) Fig. 2. Electrical
resistivity
variations
of Fe- 235U alloys [samples 2 (0.01 at.% U), 3 (0.053 at.% U), 4 and 5 (0.1 at.% U), 6 (0.49 at.% U)] during an irradiation at 5 K with thermal neutrons.
above calibration, we can calculate the exact F.F. dose corresponding to each target. (iii) We determine the form factor of the samples (with a precision of & 5%) in order to deduce the electrical resistivity increase of each target from its measured resistance. The dose deduced from point (ii) above is expressed in F.F. cmm3 and corresponds to our homogeneous irradiation of the whole volume of the sample (uranium is homogeneously distributed in the target; F.F. are isotropically emitted at each fission). In order to use the conventional units (ions cm-*) to describe ion irradiations, we can calculate the conversion factor to be
applied to our doses taking into account the range of F.F. in iron. These units are used in figs. 1 and 2 on which we plot the electrical resistivity variations measured on all the alloys as a function of the F.F. fluence: (i) Sample 2 (0.01 at.% U) shows a constant resistivity increase. (ii) The curves relative to samples 3 (0.053 at.% U), 4 and 5 (0.1 at.% U) reach a maximum resistivity increase and show then a slight decrease as the irradiation goes on. (iii) The electrical resistivity of sample 6 (0.49 at.% U)
. .land .49 at%U
T,SK ’
4
a ,053 at%U ,
’ .
. 41 at%U
’
3
3
ELECTRICALRESISTIVITY INCREASE&cm) Fig. 3. Differential defect production curves of various Fe- 235Ualloys simultaneouslyirradiated at 5 K in a thermal neutron flux. X. RADIATION DAMAGE
708
A. Dunlop et al. / Low temperature irradiation of a-iron
increases, reaches a maximum, then shows a slight decrease but starts increasing again at very high F.F. fluences. The production curves relative to the various samples are not surperposed. We see two reasons for that: (i) there may be some impurity effects, (ii) there are uncertainties in the U concentration determinations (i.e. relative fluences) and in the form factors (i.e. resistivity increases). The maximum resistivity increases measured in the various alloys are of (3.0 + 0.2) ps2 cm. We shall apply to the curves of fig. 2 an affinity on both axes in order to try to compensate for uncertainties (ii). These affinities are made in such a way that the curves comcide at a fluence of about 1.5 X 1013 F.F. cm-* and correspond to a maximum resistivity increase of 3 PFLPcm. The obtained differential defect production curves (DDPC) are plotted in fig. 3: (i) the DDPC of the different alloys are exactly superposed, so that we are sure that the impurities do not affect the damage creation processes; (ii) an insert in fig. 3 details the approach to saturation.
3. Interpretation and discussion Let us first recall that DDPC in iron during low-temperature fast neutron irradiations show a negative curvature [1,5], which is interpreted by supposing that: (i) the initial state of neutron damage consists of clustered defects (which are in a me&table configuration); (ii) these damaged regions collapse to dislocation loops (stable configuration) when there is a spatial overlap with a newly created cascade; (iii) when the defects collapse to dislocation loops their contribution to the resistivity changes from pF (Frenkel pair resistivity, which we suppose to also represent the resistivity contribution of uncollapsed damage created during neutron irradiation) to 0.4~~. This last number is deduced from an adjustment of a theoretical modelisation [6] to the experimental DDPC. During F.F. irradiation, the behaviour is obviously different: the DDPC shows a very strong positive curvature, the defect production rate (DPR) decreases, vanishes, becomes negative, vanishes again and then becomes positive. We thus confirm our previous observation [l] of a positive curvature of DDPC observed during low-temperature F.F. irradiation, and propose the following interpretation: (i) At the very beginning of the irradiation, the cascades induced by the F.F. (masses 70 to 160, energies 40 to 100 MeV) consist of damage cores containing high vacancy concentrations, the interstitials being located at the periphery. The vacancy core might consist of clustered vacancies or of vacancy loops: in the first case the irradiation fluence necessary to induce the collapse of
the cluster by an overlap phenomenon is so small, that we cannot experimentally observe this part of the damage creation. In either case, to interpret our DDPC curve we start with a configuration in which most of the vacancies belong to dislocation loops. The corresponding average resistivity per defect is 0.4~. [6]. (ii) As the irradiation goes on, the probability of overlap of the interstitial rich zones increases. When there is a sufficient overlap, the clustered interstitials agglomerate into dislocation loops, which induces a new decrease of the resistivity per defect. (iii) This model [6,7] allows us to obtain DDPC having exactly the same behaviour as the experimental ones (even near the saturation resistivity) and thus to confirm our interpretation. (iv) The behaviour of the DDPC when the resistivity increase approaches the saturation value is explained by the competition of two phenomena: _ defect production during irradiation (resistivity increase); _ agglomeration of defects into loops (resistivity decrease). (v) A last confirmation of this interpretation comes from a measurement of the dechanneling of a-particles by F.F. irradiated iron ribbons [7]: the dechanneling coefficient increases very rapidly and saturates at F.F. fluences of about 1015 F.F. cm-* whereas the maximum resistivity increase is obtained as early as = 1014 F.F. cm-*. This is explained by the high dechanneling effect of the interstitial dislocation loops [S-10] that are formed. (vi) If we can affirm that the F.F. damage takes place as described above (vacancy loops at the very beginning of the irradiation, interstitial loops appearing later as a consequence of overlap phenomena), we cannot exclude a contribution to these defect structure evolutions of the numerous underthreshold events (the repartition of transferred energies T follows a l/T* law) or of some transfer of energy from electronic excitations to lattice atoms [11,12] as suggested in [l].
4. Conclusion Our results on various Fe- 235U alloys allow us to describe the creation of damage in low-temperature F.F. irradiated iron: vacancy loops exist from the very beginning of the irradiation, whereas interstitial loops are formed when there is a sufficient overlap of the interstitial rich zones. This interstitial loop formation leads to a decrease of the resisitivity contribution of the corresponding defects and allows us to explain the experimental DDPC with the help of a theoretical model. It is a great pleasure to thank J. Bigot for the elaboration of the alloys, W. Week and J. Marangos for
A. Dunlop et al. / Law temperature
their help during the irradiation and D. Lesueur for his constant
interest
in our work.
References
PI A. Dunlop,
N. Lorenzelh and J.C. Jousset, Phys. Status SoIidi A51 (1979) 479. PI J. Leteurtre, J.L. Pouchou, J. Soullard and L. Zuppiroh, J. Nucl. Mater. 54 (1974) 254. [31 R.S. Averback, R.C. Birtcher and L.J. Thompson, J. Nucl. Mater. 92 (1980) 144. [41 A. Dunlop, N. LorenzeUi and J.C. Jousset, J. Nucl. Mater 92 (1980) 147.
irradiation of a-iron
709
[5] A. Dunlop, B.M. Pande, K. Boning, P. Rosner and H.E. Schaefer, J. Nucl. Mater. 108/109 (1982) 83. [6] A. Dunlop, CEA Report R-5053 (1981). [7] A. DunIop and N. Lorenzelli, to be published. [8] Y. Qutrk, Phys. Status Solidi 30 (1968) 713. [9] J. Mory and Y. Quere, Radiat. Eff. 13 (1972) 57. [lo] K. Kimura, T. Oshiyama and M. Mannami, Jpn J. Appl. Phys. 21 (1982) 1222. [ll] A. Dunlop, D. Lesueur, G. Jaskierowicz and J. Schildknecht, Communication at the Int. Symp. on Applications of Ions Beams Produced by Small Accelerators, Jinan, China (October 1987) to be published in Vacuum (1988). [12] A. Iwase, S. Sasaki, T. Iwata and T. Nihira, Phys. Rev. Lett. 58 (1987) 2450.
X. RADIATION
DAMAGE
LOW
Nuclear Instruments and Methods in Physics Research B33 (1988) 706-709 North-Holland, Amsterdam
TEMPERATURE
A. DUNLOP
URANIUM
If, N. LORENZELLI
FISSION
FRAGMENT
‘) and W. MANSEL
IRRADL4TIONS
OF a-IRON
*
‘)
I) DTech /SESI, Laboratoire des Solides Irradiis, Eeole Polytechnique, Route de Saclay, 91128 Palaiseau Cedex, France ‘) Physik Department E21, Technische Uniuersittit, Miinchen, D 8046, FRG
We present here a study of 5 K irradiation damage in iron: a high purity iron ribbon and various dilute Fe -235U alloys (0.01 to 0.5 at.% ura~um) are introduced in the liquid helium irradiation loop of the Garching reactor. The sampte holder is set in a zone of intense
thermal
neutron
flux: the alloys are consequently
submitted
to uranium
fission fragments
irradiations.
Electrical resist&&y increase measurements allow us to follow the damage processes and to explain why they differ from those occurring during neutron or light ion irradiations.
1. Introduction This study of 5 K uranium fission fragments (F.F.) irradiation of cu-iron is motivated by the fact that iron behaves differently when irradiated with fission neutrons and with F.F. [I]. The F.F. irradiation reported in [l] was obtained by introducing in a thermal neutron flux 8 to 12 pm thick ribbons of a-iron which were coated on both sides with enriched uranium. The F.F. damage inhomogeneously the target [2] and have a range of 7.5 pm in iron. The facts: (i) that the ribbons are irradiated from both sides; and (ii) that the induced damages overlap in the middle of the sample, partly correct the damage i~omogeneity, but there was a controversy concerning the reliability of the results [3,4]. We thus decided to irradiate at 5 K dilute Fe-“‘U alloys in the thermal neutron flux of the Munich neutron reactor. The uranium concentrations varied from 0.01 to 0.5 at.% uranium, which allowed us to study in detail and for reasonable irradiation times the whole defect production curve and to see if impurity effects become important when the uranium concentration increases.
with 90% 235U. The elaboration was made in order to obtain uranium concentrations of 0.01 to 0.5 at.%. Prior to irradiation: (i) the effective uranium content of each type of alloy was determined by a spectrometry technique: sample 2 (0.01 at.% U), sample 3 (0.053 at.48 U), samples 4 and 5 (0.10 at.% U), sample 6 (0.49 at.% U). (ii) We irradiated for one hour a piece of the 0.49 at.% U alloy in the exact irradiation position that would be used later and performed y-ray spectroscopy a few months after irradiation. We cm deduce the dose, i.e. the number of fissions per second and per cm3 that occurred for this U concentration in the exact irradiation position. As the thermal neutron fiux is continuously measured, from the recorded neutron fluence and from the
2. Experimental Six ribbons (width 1 mm, thickness 50 to 60 pm) on which platinum leads were soldered in order to make electrical resistivity measurements, were mounted on the same sample holder and irradiated by thermal neutrons. The first sample was a pure iron ribbon, the five others were alloys which were prepared in a levitation furnace from high purity iron and uranium enriched
* I~adiations performed Miinchen, FRG.
in the TTB faci~ty/Garc~~g-
0168-583X/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
il w
Fe- .49at%U
ii! 0
4X++
2 JO’4
IRRADIATION FLUENCEfi%md2) Fig. 1. Electrical resistivity variations of a Fe-23’U alloy (sample 6: 0.49 at.% U) during an irradiation at 5 K with thermal
neutrons.
A. Dunlop et al. / Low temperature irradiation of a-iron
707
2:.Olat%U
3: .053at%U 45.1 at%U 6:.L9at%U
T=SK
IRRADIATION FLUENCE (Ffcrn-2) Fig. 2. Electrical
resistivity
variations
of Fe- 235U alloys [samples 2 (0.01 at.% U), 3 (0.053 at.% U), 4 and 5 (0.1 at.% U), 6 (0.49 at.% U)] during an irradiation at 5 K with thermal neutrons.
above calibration, we can calculate the exact F.F. dose corresponding to each target. (iii) We determine the form factor of the samples (with a precision of & 5%) in order to deduce the electrical resistivity increase of each target from its measured resistance. The dose deduced from point (ii) above is expressed in F.F. cmm3 and corresponds to our homogeneous irradiation of the whole volume of the sample (uranium is homogeneously distributed in the target; F.F. are isotropically emitted at each fission). In order to use the conventional units (ions cm-*) to describe ion irradiations, we can calculate the conversion factor to be
applied to our doses taking into account the range of F.F. in iron. These units are used in figs. 1 and 2 on which we plot the electrical resistivity variations measured on all the alloys as a function of the F.F. fluence: (i) Sample 2 (0.01 at.% U) shows a constant resistivity increase. (ii) The curves relative to samples 3 (0.053 at.% U), 4 and 5 (0.1 at.% U) reach a maximum resistivity increase and show then a slight decrease as the irradiation goes on. (iii) The electrical resistivity of sample 6 (0.49 at.% U)
. .land .49 at%U
T,SK ’
4
a ,053 at%U ,
’ .
. 41 at%U
’
3
3
ELECTRICALRESISTIVITY INCREASE&cm) Fig. 3. Differential defect production curves of various Fe- 235Ualloys simultaneouslyirradiated at 5 K in a thermal neutron flux. X. RADIATION DAMAGE
708
A. Dunlop et al. / Low temperature irradiation of a-iron
increases, reaches a maximum, then shows a slight decrease but starts increasing again at very high F.F. fluences. The production curves relative to the various samples are not surperposed. We see two reasons for that: (i) there may be some impurity effects, (ii) there are uncertainties in the U concentration determinations (i.e. relative fluences) and in the form factors (i.e. resistivity increases). The maximum resistivity increases measured in the various alloys are of (3.0 + 0.2) ps2 cm. We shall apply to the curves of fig. 2 an affinity on both axes in order to try to compensate for uncertainties (ii). These affinities are made in such a way that the curves comcide at a fluence of about 1.5 X 1013 F.F. cm-* and correspond to a maximum resistivity increase of 3 PFLPcm. The obtained differential defect production curves (DDPC) are plotted in fig. 3: (i) the DDPC of the different alloys are exactly superposed, so that we are sure that the impurities do not affect the damage creation processes; (ii) an insert in fig. 3 details the approach to saturation.
3. Interpretation and discussion Let us first recall that DDPC in iron during low-temperature fast neutron irradiations show a negative curvature [1,5], which is interpreted by supposing that: (i) the initial state of neutron damage consists of clustered defects (which are in a me&table configuration); (ii) these damaged regions collapse to dislocation loops (stable configuration) when there is a spatial overlap with a newly created cascade; (iii) when the defects collapse to dislocation loops their contribution to the resistivity changes from pF (Frenkel pair resistivity, which we suppose to also represent the resistivity contribution of uncollapsed damage created during neutron irradiation) to 0.4~~. This last number is deduced from an adjustment of a theoretical modelisation [6] to the experimental DDPC. During F.F. irradiation, the behaviour is obviously different: the DDPC shows a very strong positive curvature, the defect production rate (DPR) decreases, vanishes, becomes negative, vanishes again and then becomes positive. We thus confirm our previous observation [l] of a positive curvature of DDPC observed during low-temperature F.F. irradiation, and propose the following interpretation: (i) At the very beginning of the irradiation, the cascades induced by the F.F. (masses 70 to 160, energies 40 to 100 MeV) consist of damage cores containing high vacancy concentrations, the interstitials being located at the periphery. The vacancy core might consist of clustered vacancies or of vacancy loops: in the first case the irradiation fluence necessary to induce the collapse of
the cluster by an overlap phenomenon is so small, that we cannot experimentally observe this part of the damage creation. In either case, to interpret our DDPC curve we start with a configuration in which most of the vacancies belong to dislocation loops. The corresponding average resistivity per defect is 0.4~. [6]. (ii) As the irradiation goes on, the probability of overlap of the interstitial rich zones increases. When there is a sufficient overlap, the clustered interstitials agglomerate into dislocation loops, which induces a new decrease of the resistivity per defect. (iii) This model [6,7] allows us to obtain DDPC having exactly the same behaviour as the experimental ones (even near the saturation resistivity) and thus to confirm our interpretation. (iv) The behaviour of the DDPC when the resistivity increase approaches the saturation value is explained by the competition of two phenomena: _ defect production during irradiation (resistivity increase); _ agglomeration of defects into loops (resistivity decrease). (v) A last confirmation of this interpretation comes from a measurement of the dechanneling of a-particles by F.F. irradiated iron ribbons [7]: the dechanneling coefficient increases very rapidly and saturates at F.F. fluences of about 1015 F.F. cm-* whereas the maximum resistivity increase is obtained as early as = 1014 F.F. cm-*. This is explained by the high dechanneling effect of the interstitial dislocation loops [S-10] that are formed. (vi) If we can affirm that the F.F. damage takes place as described above (vacancy loops at the very beginning of the irradiation, interstitial loops appearing later as a consequence of overlap phenomena), we cannot exclude a contribution to these defect structure evolutions of the numerous underthreshold events (the repartition of transferred energies T follows a l/T* law) or of some transfer of energy from electronic excitations to lattice atoms [11,12] as suggested in [l].
4. Conclusion Our results on various Fe- 235U alloys allow us to describe the creation of damage in low-temperature F.F. irradiated iron: vacancy loops exist from the very beginning of the irradiation, whereas interstitial loops are formed when there is a sufficient overlap of the interstitial rich zones. This interstitial loop formation leads to a decrease of the resisitivity contribution of the corresponding defects and allows us to explain the experimental DDPC with the help of a theoretical model. It is a great pleasure to thank J. Bigot for the elaboration of the alloys, W. Week and J. Marangos for
A. Dunlop et al. / Law temperature
their help during the irradiation and D. Lesueur for his constant
interest
in our work.
References
PI A. Dunlop,
N. Lorenzelh and J.C. Jousset, Phys. Status SoIidi A51 (1979) 479. PI J. Leteurtre, J.L. Pouchou, J. Soullard and L. Zuppiroh, J. Nucl. Mater. 54 (1974) 254. [31 R.S. Averback, R.C. Birtcher and L.J. Thompson, J. Nucl. Mater. 92 (1980) 144. [41 A. Dunlop, N. LorenzeUi and J.C. Jousset, J. Nucl. Mater 92 (1980) 147.
irradiation of a-iron
709
[5] A. Dunlop, B.M. Pande, K. Boning, P. Rosner and H.E. Schaefer, J. Nucl. Mater. 108/109 (1982) 83. [6] A. Dunlop, CEA Report R-5053 (1981). [7] A. DunIop and N. Lorenzelli, to be published. [8] Y. Qutrk, Phys. Status Solidi 30 (1968) 713. [9] J. Mory and Y. Quere, Radiat. Eff. 13 (1972) 57. [lo] K. Kimura, T. Oshiyama and M. Mannami, Jpn J. Appl. Phys. 21 (1982) 1222. [ll] A. Dunlop, D. Lesueur, G. Jaskierowicz and J. Schildknecht, Communication at the Int. Symp. on Applications of Ions Beams Produced by Small Accelerators, Jinan, China (October 1987) to be published in Vacuum (1988). [12] A. Iwase, S. Sasaki, T. Iwata and T. Nihira, Phys. Rev. Lett. 58 (1987) 2450.
X. RADIATION
DAMAGE