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Radiation defects in stainless steel and high-nickel alloy at hydrogen and helium ion bombardment
Nuclear Instruments and Methods in Physics Research B 115 ( 1996) W-543
NOMB
Beam Interactions with Materials 8 Atoms
Radiation defects in stainless steel and high-nickel alloy at hydrogen and helium ion bombardment A.A. Pisarev a, B.A. Gurovich b a Moscow Engineering Physics Institute, Moscow 115409, Russiun Federation b Kurchutoo Research Centre. Moscow 123182, Russian Federation
Abstract Radiation defects in a stainless steel and a high-Ni alloy are investigated after D+ and He+ ion bombardment at 400°C.
Development of dislocation loops and pores with ion fluence is analyzed. Unusual fractal-like dislocation loops were observed after D+ bombardment. Influence of gaseous impurities in defect structure formation is discussed.
1. Introduction Radiation damage in atomic collisions in solids is of interest for many applications. Defect structure depends not only on the displacement conditions but also on impurities in solids. Typical gaseous impurities are hydrogen and helium. Usually, it is assumed that inert gases but not hydrogen isotopes influence radiation damage because hydrogen-to-defect binding energy is much less than that for helium. However there are some experimental observations reviewed in Ref. [l] indicating that hydrogen can change the mobility of defects in metals and in this way change the defect structure. We used Dl and He+ ion implantation technique to introduce gas atoms and primary defects simultaneously. A review of kinetic processes leading to radiation damage during ion bombardment is given in Ref. [2] but many features are still not described and many questions still need to be answered.
2. Experimental
details
Samples of austenitic chrome-nickel stainless steel (Cr 16, Ni 15, MO 3, B 1%) and a high-nickel alloy (Cr 19, Ni 41, MO 4, B 1%) were used. The main difference was in Ni content (15 and 41% respectively). Samples were preliminarily prepared for the transmission electron microscopy and then irradiated at 400°C by Dl and He+ ions with energy of 1.5 keV to three fluences: 5 X 1016, 3 X IO” and 2 X lOI ions/cm*. The maximum of the ion stopping profile was approximately in the middle plane of the TEM analysed area.
Fig. 1. Fmctal dislocation loops in the high-Ni alloy after irradiation by deuterium ions at a fluence of 5 X lOI cm-*. Dark field image in the weak beam.
0168-583X/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PII SOl68-583X(96)00161-9
AA. Pisarev, BA. Gurovich / Nucl. Instr. and Meth. in Phys. Res. B 115 (1996) 540-543
541
Fig. 4. Pores in the high-Ni alloy after irradiation by deuterium ions at a fluence of 3 X lOI cm-‘.
Fig. 2. Dislocation loops in the stainless steel after irradiation by deuterium ions at a fluence of 5 X lO”j cm-‘; (a) dark field image in the weak beam, (b) light field image.
3. Experimental results The typical defects after Dl irradiation were pores and interstitial dislocation loops. Many of the loops were with
Fig. 3. Pores in the stainless steel after irradiation by deuterium ions at a fluence of 5X lOi cm-‘.
the stacking fault. The loops were mainly in the plane (111) and had the Burgers vector b= l/34111). For a low ion fluence (5 X lOi cm-*) the typical defects were dislocation loops. Many of them were exotic in form (Fig. 1) having many long branch pieces. They were very large in size: 200-400 nm. Besides, smaller (up to 100 nm) loops of the usual form were also seen. Under similar conditions, the loop dimensions were larger in the high-nickel alloy. The branches of loops were also more strongly expressed. For example, the loops in the steel are shown in Fig. 2. Void formation at the low fluence of 5 X 10 I6 cm -* was observed only in the steel having a relatively low nickel content (Fig. 3). The pore sizes were around 10 nm. The voids had indications of facets and did not interact with sinks. With increase in D; fluence to 3 X 10” cm-*, the density of the loops increased and apparent decrease in their size was seen. In reality, the loops did not diminish but grew in size, but during loop growth, the largest loops either reached the surface or interacted forming a grid of dislocations with large curvature, and for this mason could not be detected. The branched shape of the loops was smoothed there. Many loops contained stacking fault. Pore formation at these doses was observed in both materials (Fig. 4). At the highest Dz fluence (2 X 10 ‘* cm- *) pores were the basic observable defects. Their sizes considerably increased up to 50 nm but density decreased, the pore faceting was evident (Fig. 5). The latter indicates that the pores were of vacancy type and contained practically no gas. Investigation of the gas desorption during and after implantation had shown that practically all implanted deuterium (at least 99.9%) desorbed into vacuum. The voids were uniformly distributed over the volume and interacted only slightly with sinks. VI. PROJECTILE INDUCED/ASSISTED PROCESSES
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Instr. and Meth. in Phys. Res. B 115 (1996) 540-543
Fig. 5. Pores in the stainless steel after irradiation by deuterium ioas at a fluence of 2X 10’s cm-‘.
After helium irradiation, pores at the low fluence were observed in both materials, and there was no faceting which meant that the pores are helium gas bubbles of high pressure. There was a strong interaction of the pores with sinks, first of all with grain boundaries where the pore dimensions were much greater than in the body of grains (Fig. 6). Pore growth at the boundary was more strongly expressed if disorientation of the grains on both sides of the boundary was greater. Dislocation loops after He+ irradiation were usually rounded in form, their density was higher while their size smaller than after D+ irradiation.
4. Discussion The temperature in these experiments was rather high, therefore both self-interstitial atoms (SIAs) and vacancies produced in collisions were mobile. According to Ref. [2], this can lead to interstitial loops and pore growth. The
Fig. 6. Pores in the stainless steel after irradiation by helium ions at a fluence of 3 X 10” cm-‘.
same picture was observed in these experiments. At the respectively low temperature (300 K), only clusters and dislocation loops but not voids were observed in Ni and SS after D+ ion bombardment [3,4]. After He+ bombardment in Ref. [4], helium bubbles were visible at elevated temperatures. Their radius was of order of 1 nm at 610 K, but at 770 K it rapidly increased to 20 ML In our experiments at intermediate temperature of 670 K, the bubble radius was about 2-3 nm. Estimations of the number of atoms and vacancies in visible defects in TEM pictures gave about 7 X 10e3 atoms and 1.5 X 10e3 vacancies per every bombarding deuteron at the lowest fluence of 5 X lOI cm-*. These values are many orders of magnitude less than the rate of primary SIA-vacancy pair production, which can be estimated as N 10 pairs/deuteron [.5] and much less than the number of primary defects surviving due to mutual annihilation. That means that the majority of interstitial atoms and vacancies surviving after annihilation diffuse to the surface of the sample which is the most nearest sink. Pores appeared after DT and He+ bombardment are different in shape, dimensions, densities and in interaction with grain boundaries. The temperature was high enough for desorption of implanted deuterium, but low enough to prevent re-emission of helium. This is why the pores on D: bombardment had the faceted form typical for vacancy voids, while on He+ implantation, pores were of the gas bubble type. Immobile helium in the grain body gives many precursor sites for further bubble growth, therefore the concentration of helium bubbles was much higher but dimensions much less than those for vacancy voids at D+ bombardment. Helium atoms seem to be important also at the grain boundaries. At D+ bombardment there were no pores on boundaries because vacancies and interstitial atoms either annihilate there or diffuse along them to the outer surface. But at He+ bombardment, helium-vacancy complexes did not migrate being precursors of bubbles at the boundary as they were in the grain bulk. So, one should conclude from comparison of D and He experiments that He-vacancy complexes are much less mobile than vacancies. One of the most interesting results obtained in this work was the particular character of the dislocation loops on irradiation by deuterons: they had stacking faults, very large dimensions, and unusual forms. Under neutron irradiation of analogous metals in similar conditions (temperature, displacement level), dislocation loops do not have visible stacking fault, are rounded in form, are no larger than 20-40 nm in size, as a rule, and have a higher density. In addition, during neutron irradiation, a large quantity of small dislocation loops (so-called black-point defects) are seen; these were completely absent after deuterium ion irradiation in the given conditions. A feature of irradiation by deuterium ions is a high rate of primary displacements (0.05-0.1 dpa/min in these experiments). However, this factor is not important, since
AA. Pisnrev. B.A. Gurovich/Nucl.
Instr. andMeth.
the defects observed after irradiation by heavy ions and electrons where the rate of defect production is also high, are of ordinary type. The second factor is the local density of primary pairs. This value is very high during neutron irradiation because point defects are created mainly in cascades. Therefore, a large number of vacancy loops of cascade origin in depleted zones and interstitial loops of diffusional origin appear. In our experiments, there were no cascade loops. That may serve as conflation that dense cascades do not develop during i~adiation by light ions and that the dominant role in the dislocation loop growth during D’ ion bombardment belongs to the diffusion mechanism. The third feature of D+ irradiation is the simultaneous introduction of gas. During low flux neutron irradiation, not much gas is produced and stacking faults are rare. But at high neutron fluxes, many products of (n, p) and (n, cr> reactions appear and stacking faults are common. This agrees with the assumption about influence of gas impurities. Irradiation by helium ions does not facilitate but rather hinders growth of large loops, i.e., the presence of hydrogen is perhaps an important factor in formation of large disl~ation loops with stacking faults. Though deuterium is not retained in the sample at 4OO”C, its concentration at any time is approximately the same as the concentrations of free movable vacancies and self-interstitial atoms, because about one pair of defects per deuteron, or so, ‘survives’ after annihilation. Thus, deuterium can influence either at the stage of diffusion of defects or at the stage of loop nucleation or at the stage of adhesion of new atoms. It is possible that in the presence of a hydrogen atom, the energy of stacking fault formation decreases and loops with stacking faults grow. The hydrogen atoms can either be trapped at the loop or leave it but that does not change the mutual position of the metal atoms. One of the most interesting features of the loops is their shape at low fluences. Loops of an unusual shape with several branches were observed also at other forms of irradiation, as reported in Ref. 161.But they were small in size and looked like some of the small loops in Fig. 1 which can be considered as loops at the initial stage of growth. The branched shape was ascribed in Ref. [6] to cutting of loops by some microscopic inclusions during loop growth. This explanation, however, seems not acceptable at least for large loops having many branches growing close to each other without meeting any additional obstacles. We think that the loops observed here belong to the class of objects called ‘fractals’. Fractals are very different objects having a branched shape [7]. Their growth can be described by the model of aggregation limited by diffusion. Two conditions are important in the model: first, me particles arrive to the object as a result of random motion,
in Phys. Res. B II.5 (19961540-543
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and second, the particle sticks to the object at the point of the first contact with the object boundary. Under these conditions the object does not tend to minimization of its surface energy and grows with many branches. The fractal structure becomes smoother or even disappears if the particles may detrap from or diffuse along the boundary, penetrating deeper between branches. Smoothing of the branched shape in conditions of the present experiments can be connected also with arrival of atoms to the loop boundary not only from the plane where the loop lies but also from an arbitrary plane of the crystal. In this case the atoms can easily reach any interior point of the loop and stick there. At high magnification in Fig. 1, it is seen that there are small cavities in the loop, formed in the course of the growth of internal lateral branches, their gradual coalescence and filling of gaps between them.
5. Conclusion Irradiation of austenitic stainless steel and high-nickel alloy by 15 keV D,+ ions at 400°C lead to fo~ation of interstitial dislocation loops and pores. Loops of anomalously large size (up to 400 nml having the shape of fractals and stacking faults were observed. Faceted pores grew to large dimensions (up to 50 nm), and did not interact with the grain boundaries. In the high-nickel alloy the loop size was larger, the fractal shape was expressed more clearly, and pore formation was less intensive. Irradiation by He + ions leads to formation of small interstitial loops and gas bubbles of high concentration and small dimensions. The bubbles strongly interact with the grain boundaries.
Acknowledgement
This work was made possible in part by Grant MJG 300 from the International Science Foundation and Russian Government.
References [I] A.A. Pisarev, At. Energy 62 (1987) 109. [2] H. Wiedersich, Nucl. Instr. and Meth. B 7/8 (1985) 1. [3] G.J, Thomas and K.L. Wilson, J. Nucl. Mater. 76/77 (1978) 332. [4] W. Jtger and J. Roth, J. Nucl. Mater. 93/94 (1980) 756. [s] W. MBller, Nucl. Instr. and Meth. 209/210 (1983) 773. [6] F.A. Garner and D.S. Gelles, J. Nucl. Mater. 159 (1988) 286. [7] Ya.B. Zel’dovich and D.D. Semenov, Usp. Fiz. Nauk 146 ( 1985) 493.
Vi. PROJECTILEINDUCED/ASSISTED PROCESSES
NOMB
Beam Interactions with Materials 8 Atoms
Radiation defects in stainless steel and high-nickel alloy at hydrogen and helium ion bombardment A.A. Pisarev a, B.A. Gurovich b a Moscow Engineering Physics Institute, Moscow 115409, Russiun Federation b Kurchutoo Research Centre. Moscow 123182, Russian Federation
Abstract Radiation defects in a stainless steel and a high-Ni alloy are investigated after D+ and He+ ion bombardment at 400°C.
Development of dislocation loops and pores with ion fluence is analyzed. Unusual fractal-like dislocation loops were observed after D+ bombardment. Influence of gaseous impurities in defect structure formation is discussed.
1. Introduction Radiation damage in atomic collisions in solids is of interest for many applications. Defect structure depends not only on the displacement conditions but also on impurities in solids. Typical gaseous impurities are hydrogen and helium. Usually, it is assumed that inert gases but not hydrogen isotopes influence radiation damage because hydrogen-to-defect binding energy is much less than that for helium. However there are some experimental observations reviewed in Ref. [l] indicating that hydrogen can change the mobility of defects in metals and in this way change the defect structure. We used Dl and He+ ion implantation technique to introduce gas atoms and primary defects simultaneously. A review of kinetic processes leading to radiation damage during ion bombardment is given in Ref. [2] but many features are still not described and many questions still need to be answered.
2. Experimental
details
Samples of austenitic chrome-nickel stainless steel (Cr 16, Ni 15, MO 3, B 1%) and a high-nickel alloy (Cr 19, Ni 41, MO 4, B 1%) were used. The main difference was in Ni content (15 and 41% respectively). Samples were preliminarily prepared for the transmission electron microscopy and then irradiated at 400°C by Dl and He+ ions with energy of 1.5 keV to three fluences: 5 X 1016, 3 X IO” and 2 X lOI ions/cm*. The maximum of the ion stopping profile was approximately in the middle plane of the TEM analysed area.
Fig. 1. Fmctal dislocation loops in the high-Ni alloy after irradiation by deuterium ions at a fluence of 5 X lOI cm-*. Dark field image in the weak beam.
0168-583X/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PII SOl68-583X(96)00161-9
AA. Pisarev, BA. Gurovich / Nucl. Instr. and Meth. in Phys. Res. B 115 (1996) 540-543
541
Fig. 4. Pores in the high-Ni alloy after irradiation by deuterium ions at a fluence of 3 X lOI cm-‘.
Fig. 2. Dislocation loops in the stainless steel after irradiation by deuterium ions at a fluence of 5 X lO”j cm-‘; (a) dark field image in the weak beam, (b) light field image.
3. Experimental results The typical defects after Dl irradiation were pores and interstitial dislocation loops. Many of the loops were with
Fig. 3. Pores in the stainless steel after irradiation by deuterium ions at a fluence of 5X lOi cm-‘.
the stacking fault. The loops were mainly in the plane (111) and had the Burgers vector b= l/34111). For a low ion fluence (5 X lOi cm-*) the typical defects were dislocation loops. Many of them were exotic in form (Fig. 1) having many long branch pieces. They were very large in size: 200-400 nm. Besides, smaller (up to 100 nm) loops of the usual form were also seen. Under similar conditions, the loop dimensions were larger in the high-nickel alloy. The branches of loops were also more strongly expressed. For example, the loops in the steel are shown in Fig. 2. Void formation at the low fluence of 5 X 10 I6 cm -* was observed only in the steel having a relatively low nickel content (Fig. 3). The pore sizes were around 10 nm. The voids had indications of facets and did not interact with sinks. With increase in D; fluence to 3 X 10” cm-*, the density of the loops increased and apparent decrease in their size was seen. In reality, the loops did not diminish but grew in size, but during loop growth, the largest loops either reached the surface or interacted forming a grid of dislocations with large curvature, and for this mason could not be detected. The branched shape of the loops was smoothed there. Many loops contained stacking fault. Pore formation at these doses was observed in both materials (Fig. 4). At the highest Dz fluence (2 X 10 ‘* cm- *) pores were the basic observable defects. Their sizes considerably increased up to 50 nm but density decreased, the pore faceting was evident (Fig. 5). The latter indicates that the pores were of vacancy type and contained practically no gas. Investigation of the gas desorption during and after implantation had shown that practically all implanted deuterium (at least 99.9%) desorbed into vacuum. The voids were uniformly distributed over the volume and interacted only slightly with sinks. VI. PROJECTILE INDUCED/ASSISTED PROCESSES
542
AA. Pisarev, EA. Gurovich/Nucl.
Instr. and Meth. in Phys. Res. B 115 (1996) 540-543
Fig. 5. Pores in the stainless steel after irradiation by deuterium ioas at a fluence of 2X 10’s cm-‘.
After helium irradiation, pores at the low fluence were observed in both materials, and there was no faceting which meant that the pores are helium gas bubbles of high pressure. There was a strong interaction of the pores with sinks, first of all with grain boundaries where the pore dimensions were much greater than in the body of grains (Fig. 6). Pore growth at the boundary was more strongly expressed if disorientation of the grains on both sides of the boundary was greater. Dislocation loops after He+ irradiation were usually rounded in form, their density was higher while their size smaller than after D+ irradiation.
4. Discussion The temperature in these experiments was rather high, therefore both self-interstitial atoms (SIAs) and vacancies produced in collisions were mobile. According to Ref. [2], this can lead to interstitial loops and pore growth. The
Fig. 6. Pores in the stainless steel after irradiation by helium ions at a fluence of 3 X 10” cm-‘.
same picture was observed in these experiments. At the respectively low temperature (300 K), only clusters and dislocation loops but not voids were observed in Ni and SS after D+ ion bombardment [3,4]. After He+ bombardment in Ref. [4], helium bubbles were visible at elevated temperatures. Their radius was of order of 1 nm at 610 K, but at 770 K it rapidly increased to 20 ML In our experiments at intermediate temperature of 670 K, the bubble radius was about 2-3 nm. Estimations of the number of atoms and vacancies in visible defects in TEM pictures gave about 7 X 10e3 atoms and 1.5 X 10e3 vacancies per every bombarding deuteron at the lowest fluence of 5 X lOI cm-*. These values are many orders of magnitude less than the rate of primary SIA-vacancy pair production, which can be estimated as N 10 pairs/deuteron [.5] and much less than the number of primary defects surviving due to mutual annihilation. That means that the majority of interstitial atoms and vacancies surviving after annihilation diffuse to the surface of the sample which is the most nearest sink. Pores appeared after DT and He+ bombardment are different in shape, dimensions, densities and in interaction with grain boundaries. The temperature was high enough for desorption of implanted deuterium, but low enough to prevent re-emission of helium. This is why the pores on D: bombardment had the faceted form typical for vacancy voids, while on He+ implantation, pores were of the gas bubble type. Immobile helium in the grain body gives many precursor sites for further bubble growth, therefore the concentration of helium bubbles was much higher but dimensions much less than those for vacancy voids at D+ bombardment. Helium atoms seem to be important also at the grain boundaries. At D+ bombardment there were no pores on boundaries because vacancies and interstitial atoms either annihilate there or diffuse along them to the outer surface. But at He+ bombardment, helium-vacancy complexes did not migrate being precursors of bubbles at the boundary as they were in the grain bulk. So, one should conclude from comparison of D and He experiments that He-vacancy complexes are much less mobile than vacancies. One of the most interesting results obtained in this work was the particular character of the dislocation loops on irradiation by deuterons: they had stacking faults, very large dimensions, and unusual forms. Under neutron irradiation of analogous metals in similar conditions (temperature, displacement level), dislocation loops do not have visible stacking fault, are rounded in form, are no larger than 20-40 nm in size, as a rule, and have a higher density. In addition, during neutron irradiation, a large quantity of small dislocation loops (so-called black-point defects) are seen; these were completely absent after deuterium ion irradiation in the given conditions. A feature of irradiation by deuterium ions is a high rate of primary displacements (0.05-0.1 dpa/min in these experiments). However, this factor is not important, since
AA. Pisnrev. B.A. Gurovich/Nucl.
Instr. andMeth.
the defects observed after irradiation by heavy ions and electrons where the rate of defect production is also high, are of ordinary type. The second factor is the local density of primary pairs. This value is very high during neutron irradiation because point defects are created mainly in cascades. Therefore, a large number of vacancy loops of cascade origin in depleted zones and interstitial loops of diffusional origin appear. In our experiments, there were no cascade loops. That may serve as conflation that dense cascades do not develop during i~adiation by light ions and that the dominant role in the dislocation loop growth during D’ ion bombardment belongs to the diffusion mechanism. The third feature of D+ irradiation is the simultaneous introduction of gas. During low flux neutron irradiation, not much gas is produced and stacking faults are rare. But at high neutron fluxes, many products of (n, p) and (n, cr> reactions appear and stacking faults are common. This agrees with the assumption about influence of gas impurities. Irradiation by helium ions does not facilitate but rather hinders growth of large loops, i.e., the presence of hydrogen is perhaps an important factor in formation of large disl~ation loops with stacking faults. Though deuterium is not retained in the sample at 4OO”C, its concentration at any time is approximately the same as the concentrations of free movable vacancies and self-interstitial atoms, because about one pair of defects per deuteron, or so, ‘survives’ after annihilation. Thus, deuterium can influence either at the stage of diffusion of defects or at the stage of loop nucleation or at the stage of adhesion of new atoms. It is possible that in the presence of a hydrogen atom, the energy of stacking fault formation decreases and loops with stacking faults grow. The hydrogen atoms can either be trapped at the loop or leave it but that does not change the mutual position of the metal atoms. One of the most interesting features of the loops is their shape at low fluences. Loops of an unusual shape with several branches were observed also at other forms of irradiation, as reported in Ref. 161.But they were small in size and looked like some of the small loops in Fig. 1 which can be considered as loops at the initial stage of growth. The branched shape was ascribed in Ref. [6] to cutting of loops by some microscopic inclusions during loop growth. This explanation, however, seems not acceptable at least for large loops having many branches growing close to each other without meeting any additional obstacles. We think that the loops observed here belong to the class of objects called ‘fractals’. Fractals are very different objects having a branched shape [7]. Their growth can be described by the model of aggregation limited by diffusion. Two conditions are important in the model: first, me particles arrive to the object as a result of random motion,
in Phys. Res. B II.5 (19961540-543
543
and second, the particle sticks to the object at the point of the first contact with the object boundary. Under these conditions the object does not tend to minimization of its surface energy and grows with many branches. The fractal structure becomes smoother or even disappears if the particles may detrap from or diffuse along the boundary, penetrating deeper between branches. Smoothing of the branched shape in conditions of the present experiments can be connected also with arrival of atoms to the loop boundary not only from the plane where the loop lies but also from an arbitrary plane of the crystal. In this case the atoms can easily reach any interior point of the loop and stick there. At high magnification in Fig. 1, it is seen that there are small cavities in the loop, formed in the course of the growth of internal lateral branches, their gradual coalescence and filling of gaps between them.
5. Conclusion Irradiation of austenitic stainless steel and high-nickel alloy by 15 keV D,+ ions at 400°C lead to fo~ation of interstitial dislocation loops and pores. Loops of anomalously large size (up to 400 nml having the shape of fractals and stacking faults were observed. Faceted pores grew to large dimensions (up to 50 nm), and did not interact with the grain boundaries. In the high-nickel alloy the loop size was larger, the fractal shape was expressed more clearly, and pore formation was less intensive. Irradiation by He + ions leads to formation of small interstitial loops and gas bubbles of high concentration and small dimensions. The bubbles strongly interact with the grain boundaries.
Acknowledgement
This work was made possible in part by Grant MJG 300 from the International Science Foundation and Russian Government.
References [I] A.A. Pisarev, At. Energy 62 (1987) 109. [2] H. Wiedersich, Nucl. Instr. and Meth. B 7/8 (1985) 1. [3] G.J, Thomas and K.L. Wilson, J. Nucl. Mater. 76/77 (1978) 332. [4] W. Jtger and J. Roth, J. Nucl. Mater. 93/94 (1980) 756. [s] W. MBller, Nucl. Instr. and Meth. 209/210 (1983) 773. [6] F.A. Garner and D.S. Gelles, J. Nucl. Mater. 159 (1988) 286. [7] Ya.B. Zel’dovich and D.D. Semenov, Usp. Fiz. Nauk 146 ( 1985) 493.
Vi. PROJECTILEINDUCED/ASSISTED PROCESSES