In-Situ Observation of Radiation Damage Using a Combined Facility of Electron Microscope and Heavy-Ion Accelerator(S) ®History and Future Prospectives®

In-Situ Observation of Radiation Damage Using a Combined Facility of Electron Microscope and Heavy-Ion Accelerator(s) ®History and Future Prospectives®

Shiori Ishino+ The University of Tokyo, Tokyo 113-8656, Japan

Radiation effects in materials for ﬁssion and fusion reactors are caused by fast neutrons which produce cascade damage, which has been known to be considerably different from simple Frenkel pairs produced by relativistic electrons. In-situ observation of radiation damage using a combined facility of electron microscope and heavy-ion accelerators has been a powerful tool to investigate the nature of the cascade damage. In this overview, brief historical survey of this experimental technique will be given, followed by the results and discussions on cascade damage mainly in gold will be reviewed. Careful considerations will be required to generalize the results to other metals as well as to other experimental conditions. Finally, there are a number of points to be developed for better understanding of cascade damage and for extending the “in-situ” techniques to other materials as non-metallic solids and to other intriguing phenomena. [doi:10.2320/matertrans.MD201304]

(Received September 2, 2013; Accepted December 4, 2013; Published February 25, 2014) Keywords: in-situ observation, cascade, heavy ion irradiation, electron microscope-accelerator link, defect clusters, cascade collapse, subcascade, production rate, annihilation rate

1. Introduction The cascade damage has been of great concern for many years, especially since the development of fast breeder and Most fundamental process of radiation damage in fusion reactors were started around 1970. In particular, crystalline materials has been thought to be creation of the great interest has aroused after the discovery of void Frenkel pairs as a result of transfer of energy from energetic swelling in fast reactor irradiated stainless steel in 1968.2) incident particles to target atoms by collision. When the Utilization of heavy ion accelerators for simulation of the transferred energy exceeds a certain value, called displace- effect of neutron irradiations to a high dose was initiated in ment energy, the atom is displaced from the normal lattice a number of laboratories because it is easy to obtain very site to an interstitial position, thus creating a pair of single high dose rate in terms of dpa with energetic heavy ions. vacancy and an interstitial atom. Soon later however, it was recognized that “the simulation” For transmission electron microscopic studies, accelerating was very difficult. Particularly in U.S.A. use of heavy ion voltage of the electron microscopes is of the order of 100 kV accelerators to simulate high fluence neutron radiation to a few MV. The electrons in the range of such energies can effects was quickly declined. Mechanistic study of cascade create simple Frenkel defects. In particular, high voltage damage was one of the big remaining issues. As a tool to electron microscope, HVEM with the accelerating voltage study such fundamental problems as clustering of point exceeding 1 MeV has provided a very useful platform to defects under cascade damage conditions, accelerator- study the nature of simple point defects, vacancies and electron microscope link facilities became utilized around interstitials by observing the behavior of their clustered form, 1980. In an early paper by the authors,3) 17 in-situ dislocations, and thereby fundamental information on point observation facilities constructed before 1993 are listed up. defects has been obtained.1) Each of these facilities has its own major objectives. In However, reactor core structural materials or first wall and recent years, richness of information obtained from in-situ blanket structural materials of a fusion reactor are exposed to TEM observation of damage caused by electrons and ions high energy neutrons and the transferred energies to the target on one hand and decreasing number of research reactors on atoms will become much higher than that of electrons in an the other hand, the number of in-situ TEM facilities is HVEM, thus the defects are created in a collision cascades. increasing and a few new facilities have been constructed The cascade had been thought to have central vacancy-rich or being under construction. For interested readers, some core and interstitials in the periphery and such special comprehensive review papers are cited here,4,5) together with inhomogeneity of vacancy-interstitial distribution and local a few papers describing the details of the new facilities high density of point defect generation would produce (JANNuS,6) MIAMI7) and I3TEM8)). different radiation effects from those of more-or-less In the present paper, I will concentrate on the elucidation homogeneous production of Frenkel pairs by MeV-electrons. of cascade damage in metals, summarizing major findings (It should also be recognized that there is a strong spatial obtained using a facility installed in the Tokai campus of the correlation of vacancy-interstitial pair in the case of electron University of Tokyo (Fusion Blanket Engineering Research irradiation.) Facility: UTBLT). Unfortunately, the facility has already been decommissioned but for those who might be interested +Corresponding author, E-mail: [email protected]. Professor Emeritus in the facility, brief description of the facility with the design of the University of Tokyo principles, capability and major experimental conditions will In-Situ Observation of Cascade Damage 397 be briefly described first in Chapter 2, followed by brief 2.3 Experimental conditions summary of the results obtained on cascade damage in The accelerator could produce ions of various kinds of Chapter 3. In the course of revisiting the old data, we have elements and in fact, we had accelerated various ions as found a number of problems in the interpretation of the H, He, C, Al, Ar, V, Cu, Ni, Fe, Xe and Au. Specimens results, which will be discussed in Chapter 4. Finally, examined ranged from aluminum and aluminum alloy, prosperous research areas with in-situ observation facilities vanadium, copper, nickel Austenitic stainless steels, gold in the future will be discussed in Chapter 5. and so on. However, the vacuum condition in the accelerator- electron microscope system was not good enough to keep 2. Brief Description of the Facility Used at the the specimen surface free from oxidation or contamination University of Tokyo (UTBLT) especially at high irradiation temperatures. Therefore, most of the results presented here are for pure gold. Some additional 2.1 Design principle results for aluminum-alloys, copper, nickel, Austenitic The facility we had used was built in the Fusion Blanket stainless steels will also be mentioned as required. Specimen Engineering Research Facility of the University of Tokyo temperature can be varied from 120 to 870 K by using (Tokai campus) in 1979, hereafter abbreviated UTBLT. The cooling and heating goniometer stages. facility has already been decommissioned for more than 10 years. It was designed to study radiation damage of materials 3. Highlight of the Experimental Results of In-Situ Ion due to DT fusion neutrons, typically with the energy of Irradiation 14 MeV, causing cascade damage in comparison with electron damage, producing simple Frenkel pairs. Since the 3.1 Cascade damage in gold primary knock-on atom (PKA) energies for medium heavy In our facility, characteristic properties of cascade damage elements caused by the 14 MeV neutrons are around several can be obtained by observing mostly vacancy clusters, which hundred keV9) and the ranges of heavy ions, simulating the are the remnant of damage following the displacement PKAs, are around a few hundreds nm, we selected the cascade event. In the case of gold, interstitial components as combination of 400 kV heavy ion accelerator and 200 kV self interstitials (SIAs), SIA clusters and SIA loops which are electron microscope. thought to have high mobility, effectively disappear to the It is thought to be desirable to select the ion energy and surface or to other sinks, leaving the vacancy components as accelerating voltage of the electron microscope according the remnant clusters. This statement may depend on the to the kind of the specimen, specimen thickness, specific experimental conditions, such as the kind of specimens, experimental objectives and so on. How the results are specimen thickness, temperature and irradiation history. This affected by these experimental parameters is one of the major point will be discussed later. topics of this paper, which will be discussed in the later The vacancy clusters always shrink after sudden appear- section. ance during irradiation. How the point defect clusters change their sizes are either they absorb or emit vacancies or 2.2 Outline of the facility interstitials and their clusters in the simplest cases as shown The facility comprised a Cockcroft-Walton type 400 kV in Table 1. It seems very difficult for the clusters to emit heavy ion accelerator equipped with Danfysik 911A heavy interstitials or their clusters because of their large formation ion source capable of producing any ions of elements having energies. Therefore, the clusters we are observing are those vapour pressure exceeding about 1300 Pa at 2000 K, and the which absorb interstitials or emit vacancies, i.e., the clusters JEM200C electron microscope, started operation in 1979. are of vacancy type. There are always exceptions. In a very The ion beams were introduced into the specimen chamber of rare case, the cluster suddenly increases the size during the electron microscope with an angle of 45 degree to the irradiation.13) This may be due to the occurrence of another electron beam axis of the electron microscope. (By using a cascade near the existing cluster. It is very difficult to identify tilting goniometer, normal incidence of the bombarding ion the nature of the clusters or loops during ion irradiation. beams to the specimen surface could be possible.) The However, there are a few evidences that interstitial loops electron-microscopic image formed on a fluorescent screen do exist in gold by diffraction analyses and analysis of was converted to a video signal by a silicon vidicon, which movement of clusters with respect to crystallographic also acted as an image intensifier, enabling us to observe directions.14) thicker specimens. One image was formed in 30 msec. Figure 1 is reiterated to show how the cascade damage Typically, we used the magnification of ©100,000 with the evolves during irradiation.3) This shows six frames of image size of 256 nm © 329 nm. Minimum beam current microstructure in gold taken just after the onset of irradiation was 8.7 © 1013 ions/m2/s, which corresponded to 7 ions/s with 400 keV Ar+ ions at 673 K with the beam current of on the image size given above. The beam current was 7 © 1015 ions/m2/s.3) Time difference between adjacent measured by a long cylindrical Faraday cup with a hole of frames are shown in the figure. Between the frames of 3 mm in diameter, which also served as a beam defining (1) and (2), a subcascade with three subclusters is formed window. Although there was some residual image effect of suddenly, gradually diminishing the size, eventually dis- one frame or two, we could obtain the microscopic changes appearing within less than 4 s. By frame-by-frame analyses of produced by a single impinging ion by analyzing the images these continuous video recordings, cascade characteristics as frame by frame. The detailed account of the facility is given cascade size, number of subcascades, number of subclusters in references.1012) in a group as a function of incident ion mass and energy and 398 S. Ishino

Table 1 Change of size of point defect clusters by absorbing or emitting point defects or their clusters.

Vacancy clusters or Vacancy loops Interstitial clusters or Interstitial loops (1) Absorption of vacancies or vacancy clusters Size increases Size decreases or disappear (2) Absorption of interstitials or interstitial clusters Size decreases or disappear Size increases (3) Emission of vacancies or Vacancy clusters Size decreases Size increases (4) Emission of interstitials or Interstitial clusters (Difﬁcult to occur) Size increases Size decreases Other possible mechanisms: (a) Change in imaging condition. (Latent image to visible image.) (b) Change in strain ﬁeld around the defect cluster. (c) Appearance or disappearance of image by rotation. (d) Disappearance by sessile-glissile conversion to surface or moving out from the image by one dimensional motion.

B C

Fig. 1 Series of video recordings showing sudden appearance and gradual decay of vacancy clusters under irradiation with 400 keV Ar+ ions at 673 K. Ion ﬂux was 7 © 1015 ions/m2/s. Time intervals between frames are also shown Ref. 3).

so on are obtained.15) Cascade size produced by 400 keV Xe 3.2 Production, annihilation and lifetime of vacancy ions, which are assumed to simulate PKA of gold, is about clusters 30 nm. (More detailed discussion will be given later.) The In an ordinary radiation damage studies, defects are number of subcascades may be modeled by NRT (Norgett, measured after irradiation. Their production rate as a function Robinson, Torrens) model16)-like formulae with the sub- of dose, for example, is essentially production rate minus cascade energy17) being 1020 keV for many metals. annihilation rate. However, in-situ observation can trace the Sekimura et al. have succeeded to carry out in-situ whole life of individual defect. Therefore, production rate observation of gold under irradiation using self ions of gold, and annihilation rate can be measured separately.3) This is obtaining renewed equations for production efficiencies and particularly important for some materials, in which disap- probability functions for the number of subclusters.18) In this pearance of some species does not necessarily mean to be real model, damage energy distribution with the depth from the annihilation but conversion to other kind of species. Such surface was duly taking into account. Furthermore, they phenomena were fairy commonly known, for example, in the carried out fast neutron irradiation with YAYOI19) reactor and annealing studies of electron-irradiated elemental semicon- high energy (21 MeV) self ion irradiation using a Tandem ductors in 1960s.21) heavy ion accelerator at TIARA (Takasaki Ion Accelerators Important point to note in the in-situ observation is that for Advanced for Advanced Radiation Applications, JAEA- the microstructure is vigorously changing under irradiation. Takasaki). They analyzed the PKA spectrum for these Even for the lowest ion flux of less than 1 © 1014 ions/m2/s, neutron and heavy ion irradiations and predicted the cascade clusters are moving or vibrating by absorbing or emitting damage characteristics, which were consistent with exper- point defects and their clusters. Once the beam current is off, imental observations.20) This seems to be an important the microstructure becomes still instantaneously. The lifetime contribution to irradiation correlation. (Here, the irradiation of the defect clusters depends strongly on the flux. Lifetime correlation means, predicting radiation effects under a certain of the vacancy clusters in gold at 673 K during 400 keV Xe irradiation environment, e.g., fusion neutron irradiation ion irradiation is typically less than 30 s regardless of their environment, from the data obtained under different irradi- size. However, the lifetime of stacking fault tetrahedra (SFT) ation environment, e.g., iron irradiation.) seem to have longer lifetime than those having loop-like or In-Situ Observation of Cascade Damage 399 obscured images. When the beam stops, the lifetime becomes (a) much longer, at least two orders of magnitude. Moreover, there are clearly two components, the shorter one for loops and longer one for SFTs. More recent study by Abe et al. has reported that there is still one more component, having much shorter lifetime.14) They attribute this to the lifetime of interstitial clusters. Annihilation process of vacancy clusters in various metals has been discussed by many researchers. It depends on various conditions, kind of specimens, specimen thickness, i.e., existence of strong surface sinks, temperature, spatial distribution of damage energy deposition, fluence, or in other words, amount of defects already accumulated, and so on. In our experimental conditions for gold specimens, specimen thickness is thought to be, typically, 50200 nm, implying the specimens are in strong surface sink effect. Therefore, in early stage of irradiation, vacancy rich environment will be realized. Moreover, vacancy rich core of the cascade will (b) collapse to form vacancy clusters. Once formed, the vacancy clusters diminish the size either by absorbing interstitials or emitting vacancies. The size reduction process is always jerky, probably reflecting the number of interstitials in a group or in a bundle of interstitials.22) Annihilation of existing vacancy loops by ion irradiation is quite different from that by electrons. Muroga has devised an ingenious method of utilizing a 200 kV electron microscope as a kind of “high” voltage electron microscope for aluminum dilute alloy (because displacement damage is introduced in light metals as aluminum by 200 keV electrons) and compare annihilation process of quench introduced vacancy loops in a Fig. 2 (a) Use of wedge-shaped specimen for in-situ observation with same crystal grain from the same incident direction of Al+ HVEM. (b) Use of wedge-shaped specimen for in-situ observation with ion and electron beams.2325) The result is striking; in the case TEM heavy ion accelerator link. Complex situation is expected due to damage distribution in relation to specimen thickness. of electron irradiation, the vacancy loops disappear smoothly, while in the case of Al ion irradiation, small interstitial loops decorate the vacancy loop and the original vacancy loops eventually disappear catastrophically. the whole thickness of the specimen. Surface denuded zone As a mechanism of annihilation of vacancy loops, loop and surface segregation can be evaluated, and volume density movement assisted by image force has been discussed.26) As of defect clusters can be derived, provided the thickness can is well known, the image force is exerted on the defects with be evaluated precisely. In the case of TEM-ion accelerator strain field near the surface from mirror image defects with link facility, the situation is more complex because the opposite character in order to realize the force-free condition damage rate is dependent on the distance from the surface of the surface. and because transport of defects as well as chemical elements to the thickness directions are expected. Even though the 3.3 Effect of specimen thickness and use of wedge- situation is complex, the use of the wedge-shaped specimens shaped specimen may still have some advantages. Elucidating the complex As already discussed, the presence of surface has a mechanisms of defect structure evolution in nickel is shown profound effect on microstructure evolution. Other factors, as schematically28,29) using a figure similar to Fig. 2. The irradiation temperature, species and energy of impinging essential point is the balance between vacancy defects and ions, ion flux, ion fluence are also important. The gold interstitials, both affecting each other according to the locally specimen we have used is appropriate for observing vacancy determined production rates and the evolution of defect sinks, clusters particularly for low fluence. We have done similar which is a function of fluence. Thus, in experiment using experiment in copper and nickel. In Ni, we used wedge copper14,30) and nickel27,28) specimens, both vacancy and shaped specimen27) and irradiation was performed with interstitial clusters are observed, whereas in gold, vacancy 400 keV Ar+ ions at 673 K up to a rather high fluence of clusters are dominant defects observed.29) about 1016 ions/m2. The microstructural evolution depends on the thickness, i.e., distance from surface, as well as 3.4 General trend of microstructure evolution to high fluence.27,28) Using a wedge-shaped specimen is useful for fluence extracting more general information in analyzing the data. Since the mission of the UTBLT facility was to study high The technique is particularly useful for in-situ observation fluence neutron radiation effects in fusion materials, micro- with HVEM, because the damage rate is almost constant over structural evolution of various materials (mostly metallic 400 S. Ishino materials) are examined in-situ in the microscope. To reach reported that there are two defect clusters of more than high fluence region, we used high flux ion beams with the 100 nm apart, which are generated at the same time by frame- damage rate of the order of 10¹410¹3 dpa/s.31,32) The defect by-frame synchronism analysis.33,34) In fact, presence of accumulation started quickly just after the onset of distant clusters from one cascade has also been suggested irradiation. In most metals, the black dot defects first from computer simulation of collision cascade. Classical appeared quickly, growing to interstitial loops, interacting binary collision cascade calculations for 100 keV cascade in with each other, forming dislocation tangles. Then, a kind of gold show that channeling-like elongated lobes of subcas- self organization process occurred, forming aligned disloca- cades often appear.35) The length often exceeds 100 nm. In tion structure. This evolution process was common to nearly the case of large scale molecular dynamics calculation of all materials.31) pure iron, about 15% among 100 cases of 50 keV cascade calculations were grouped into a channeling-like mode, 4. Problems Associated with In-Situ Heavy Ion the length of the elongated lobes exceeding 100 nm.36) Irradiation Techniques Possibility remains that the distant cluster might be of an interstitial type which has moved long distance by one There are a number of differences between electron dimensional motion. From these information, it seems also radiation effects and those by energetic heavy particles as difficult to examine the whole volume of large cascade fast neutrons and heavy ions. The relativistic electrons initiated from high energy PKAs because of the limitation of produce Frenkel pairs, while the latter produces cascade, a specimen volume in electron microscopy. region of dense atomic displacements, the elucidation of It should be mentioned here that recent striking finding which is one of the main objectives of the in-situ heavy ion of the one dimensional motion of deformation-introduced irradiation experiments. In this technique, the incident heavy vacancy clusters in gold37) might have important implication ion is regarded as a primary knock-on atom, by which a on the formation and structure of cascade damage. series of atomic collisions in the cascade are started. Electron (4) Presence of specimen surface in in-situ observation by microscope is usually the means of observing the defects. TEM has an inherent problem. As mentioned above, the thin Application of other measuring techniques will be discussed foil specimens will limit the observable cascade volume. The in the next chapter. surface will act as sinks for point defects and their clusters The in-situ observation technique using TEM-ion accel- created by the heavy ion bombardment will also act as sinks. erator link might seem to be a straight forward means to These will disturb the defect balance and, hence, the spatial study cascade damage. However, there are a number of distribution of clustered defects. The surface will also act as factors which need be carefully analyzed or artifacts to be a site for solute segregation, affecting the spatial distribution eliminated or avoided. of defects in the thin foil specimens.38) These points will be discussed first. (5) To correlate the in-situ observation results to bulk (1) In the case of in-situ HVEM observations, most of the radiation effect, it is necessary to convert the areal number electron energies are dissipated by electronic excitation and density of defects to volumetric density. In the case of in-situ very few of them are elastically displacing atoms. While observation using HVEM, it is possible to use a wedge- in the case of heavy ion irradiations with energies relevant shaped specimen as discussed in Section 3.3 above (see also to PKA for fission and fusion reactors, kinetic energy of Fig. 2(a)). However, in the case of in-situ observation of impinging ions is effectively transferred to the target atoms heavy-ion radiation damage, distribution of defects depends and the energy lost to the electronic excitation is roughly half on various parameters as ion species and energy, position or less, unless the ion energies exceed MeV range. within the specimen (as distance from surface) etc. and (2) Whether the impinging heavy ion can simulate the PKA converting the areal density to volumetric density has less which produces the similar cascade damage as starting from important meaning (Fig. 2(b)). It might be more meaningful the PKA in neutron irradiation is a difficult question. It if the location of each cluster within the specimen is depends on the experimental conditions and also needs identified, rather than to convert the data to volumetric careful analyses of the data. As discussed in a previous defect density. However, to determine the position of clusters section 3.3, different form of defect clusters appears depend- within the specimen will require a rather tedious procedure. ing on the specimen, temperature, thickness of the specimen, (6) Heavy ion irradiation produces spatial distribution of depth from surface and so on. Thus, for example, in nickel damage energy, or more generally elastic and electronic with thickness of less than 100150 nm, irradiated with energy deposition. As a result of this, vacancy and interstitial 300 keV Ar+ ions at 773 K, interstitial loops first appear, defects distribution is space-dependent, and more displace- growing to a certain size with increasing fluence, then ments are generated near the end of the impinging ions. becoming unstable, diminishing their size and eventually Hence, the defect interactions are also strongly space- disappearing. The microstructure is then replaced by cavities, dependent. Implanted ion distribution (= range distribution) a kind of vacancy cluster, probably containing argon and damage energy deposition distribution are different. atoms.27) Therefore, simple interpretation of microstructure Implanted ions are regarded as implanted interstitials, evolution will lead to erroneous results. which will not be neglected particularly for high fluence (3) From our experimental observation, typical diameter of irradiations. cascade volume for 400 keV cascade is around 30 nm in (7) Relaxation of deposited energy by energetic particles is a gold.29) However, there is an argument that cascade volume major concern for radiation damage studies. In this context, may be more extended because Abe and others14) have fate of electronic energy loss should not be discarded. For In-Situ Observation of Cascade Damage 401 low energy heavy ions (PKAs) of less than 10 keV, the effect where the electronic loss becomes more than a thousand of electronic energy loss may not be large. However, for ions times more than the elastic energy loss. It has been known with the energies exceeding ca. 100 keV, the electronic loss that swift heavy ions produce rather simple point defects might have important effects, e.g., local heating around the (³Frenkel pairs).41) cascade region. During irradiation, the TEM images seem to Recently, the authors have been interested in viewing the show slight vibration depending on the beam current, radiation effects as the phenomena, in which energy in the instantaneously becoming still when the beam is switched form of radiation is injected in the material, creating some off. There may be also an effect of the presence of mobile excited state in a broad sense. And then, it is interesting to point defects during irradiation. As already mentioned in the see how the injected energy or energy stored as excited state previous chapter, and also it depends on irradiation temper- shows relaxation to more stable states. Thus, heavy charged ature, the lifetime of the defect clusters is short during beam- particles carry kinetic energy which will be imparted to on, becoming a few to several orders of magnitude longer for bombarded atom by elastic process. The charged particle will beam-off condition. also collide with electrons, producing excitations (level (8) It is very difficult to precisely analyze the nature of promotion) and ionization. The electric field associated with defects during irradiation. For the analysis of the nature of moving charged particle will also have long range inter- the defect clusters, temperature must be low and the beam actions and some energy is lost by dielectric friction. It is should be switched off. One should be aware of the interesting to see the time structure of these processes.42) If possibility that “in vivo” microstructure might be different the time is longer than 0.1 ps, energy imparted to electronic from “post mortem” microstructure. system may have a chance to come back to the atomic (9) The current facility is capable of obtaining information on system by electronphonon interaction. These kinds of the effect of single impinging ion. This renders us a unique relaxation processes may have influences on the fate of possibility. However, there are a number of limitations as cascade such as cascade cooling or cascade collapse discussed in the previous chapter. For example, single event processes. must occur within a small volume in a TEM specimen. Therefore, the initial process of cascade relaxation is Multiscale nature of both in temporal and spatial dimensions interesting but not well examined in metals. Studies along must be clearly defined. To enhance the capability along this these lines have been pursued in molecules and non-metallic line, high speed recording of TEM images or, more generally, solids in particular those in which optical means can be high speed acquisition of defect characteristics should be applied. For metallic materials, single particle event can be developed. It is also desirable to develop a method of more identified. But very early stage of radiation effects as cascade sensitive ion beam counting. melting and cooling phases have not been explored yet.42) Elucidating the very early stage of cascade damage formation 5. Future Prospectives by experiment is an exciting challenge. Along this line, a trial has been reported to demonstrate ultra-fast electron diffrac- I have discussed in-situ observation of heavy ion radiation tion by using electrons accelerated in plasma produced by damage in metals from the stand point of the study of cascade intense short pulse laser.43) This is a snap shot by single pulse damage, because the cascade damage is very much different of ³500 fs. from radiation damage caused by Frenkel pairs in various Associated with the technical development, new develop- respects. I have explained typical results obtained by this ment of computational physics should be accompanied. technique and made some remarks to correlate the results Classical molecular dynamics assumes the system to be to bulk radiation damage results as obtained by neutron electronically in a ground state (BornOppenheimer approx- irradiation. imation), but this assumption clearly does not hold for I have not so far touched upon radiation effects in non- radiation damage calculation. Non adiabatic computations metallic materials. The area contains a large variety of and fully first principle computations are sought for. materials and phenomena, lots of complexities and prospects Many years ago, the authors have proposed to develop a but it is outside the scope of the present paper. For those concept of “micro radiation laboratory”28) because to go one who might be interested in this field, I will refer two papers step further, we need a considerable innovation. The idea is to related to in situ TEM observation of non-metallic solids, for convert the specimen chamber of an electron microscope intermetallics39) and complex ceramics.40) a kind of laboratory with three major parts: (I) giving Apart from heavy ion radiation damage, there are specimens a various kinds of excitations as given by numerous interesting phenomena other than cascade damage. electrons, heavy ions, X-rays, laser beams, mechanical There are many unexplored area in that respect, which might stresses etc.; (II) providing various means of measurements become very interesting and important in the future. In this as electrons, X-rays, light, property measurements as chapter, I will discuss some of these subjects. resistivity etc.; (III) providing means to give well defined One of the problems associated with radiation effects by environment for materials study as, controlled temperature, energetic heavy particles in metals is that electronic energy vacuum, or gas environment or pressure etc. with control and loss has long been neglected. The amount of this energy is data logging facilities. The facility may be effectively placed far from being neglected particularly for high PKAs as near research reactor or synchrotron orbit radiation facility. encountered in fusion structural materials as several hundred I believe the micro radiation laboratory is an advanced form keV. For higher energies, the electronic loss will become of in-situ observation facility, the need and expectation of overwhelming up to swift heavy ions (of GeV energy range) which will be increasing in the near future. 402 S. Ishino

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