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The bombardment of energetic ions on material produces a variety of defects in the material. The nature of defects and their density distribution within the material are dependent on many parameters like, energy of ion, mass, charged state and flux of the ion beam as well as on the temperature, chemical composition, density and thickness of the target material. Among them few kind of defects are short lived while some are of permanent nature responsible for the modification in the structure and properties of the target material. Identification, characterization and estimation of defects density distribution is very important aspect of material science research when ion beam interacts with materials. The application of TRIM software reveals qualitative features of the process of ion beam interaction with in the solids. The contribution of energy loss of ion due to electronic and nuclear stopping in the material can be estimated. This energy loss deposited by ion is responsible to create different defect distribution in the material. The selective and homogenous irradiation can be achieved by beam moderation. The information pertaining to the nature, concentration and spatial distribution of defects present at the surface as well as in the bulk can be obtained using positron annihilate spectroscopy (PAS). Positron injected into the sample gets thermalized and annihilates with an electron of the medium giving rise to annihilation gamma rays. The annihilation parameters have high sensitivity and selectivity to defects and their concentration up to 10-2 to 10-4. In conventional PAS, the positrons are obtained from radioactive source having energy 0-0.5 MeV sampling region from surface to 0.2mm. The development of Low Energy Positron Beam (LEPB) is useful for the investigation of defects at the surface and various depths in the near surface layer where heavy ion effects are dominant over light ion induced effects.

To study the radiation induced defects density distribution and characterization using Positron Annihilation techniques.

It is reported that for semiconductors their properties are very sensitive to atomic defects and micro structures [1]. Other kind of induced defects are very important from the engineering (ED) aspects of material processing like casting, cutting and polishing, quenching, annealing, alloying and diffusion. Small change in processing parameter changes the defects configuration significantly [2].

The radiation induced defects (RID) are another class of material degration defects observed in the nuclear reactor technology. At typical reactor operating temperatures, a whole range of mostly irreversible effects like creep, swelling, embrittlement, spitting etc., may occur in reactor components leading to serious material degradation in properties and impose severe limitation on reactor design and material choice.

All the three kind of defects ID, ED and RID are uniformly distributed in the material. The information on such distributions is generally obtained by careful measurement on the changes in bulk properties of the material and its dependence on the process it had followed. The information on the micro structure is obtained by X-ray, Neutron and Particle Scatting or by imaging technique like transmission electron microscopy (TEM), field ion microscopy, scanning electron microscopy (SEM). The Positron Annihilation Technique (PAT) is another important tool to investigate the defect states and micro structure. The information pertaining to micro structure even in case of well annealed pure samples could be systematically obtained by PAT, for example, the mean life time data for different elements obtained from review by Meckenzi [3] are shown in Figure 1. This represents sensitivity of positron to probe micro structure in time scale as mean life time parameter. It varies systematically having high value for alkali metals and decreasing to steady average value for other elements in the same period.

Figure 1: Variation of Positron mean lifetime with Atomic Number

In contrast, the heavy ion induced defects (HID) in the material depend upon many parameters like ion energy, mass, relative size, charge state, intensity, exposure time etc. The defect characteristic and density distribution due to (HID) is also depending upon parameters of target material like, density, chemical constituents, purity, thermal properties, orientation with reference to beam etc. A typical estimate of the maximum projected range R_P, of 100 MeV B⁺⁵ ion in different target elements show a systematic variation using TRIM 95 program [5], as shown in Figure 2.

Figure 2: Effective range of Boron Ion of 100MeV with Atomic Number 

This also incidentally reveals similar behavior as variation of time parameter. The only difference is that the effect of interaction in estimated in length scale. One can conclude that the projectile host interaction is responsible to modify the micro structure. It is very much appropriate to call these variations in micro structure as Tailored Defects (TD), whose density distribution and nature is to some extent controlled. The experimental parameters can be varied in a systematic way to get reproducible effects. Heavy ion irradiations predominantly in conjunction with microscopy techniques have made major contribution to the understanding of fundamental RID mechanism and simulation of fast neutron damage.

The fast ion moving through condensed matter with its energy more than its range in that material can produce variety of effects before it comes to rest in the host or passes through thin films or foils. These may include excitation, ionization and modification in the host micro structure. Creation and annealing of defects due to heavy ion energy locked into electronic excitation needs to be understood and examined by appropriate technique. These effects may modify or alter surface properties and structure. However, the buried projectile into the host lattice will alter the interior micro structure of the host and alterations in the neighborhood.

The heavy ion when passing through a material loses energy by two independent means: (i) elastic collision with the nuclei of the target giving rise to nuclear stopping power  and inelastic collision with electrons of material represented by electronic stopping power . This can be estimated for different ions at different maximum energy and in different targets using TRIM. This is typically represented by a Braggs curve as shown in Figure 3. The electronic stopping is predominant at higher energies of ion and nuclear stopping is effective only at the end of the range of ion in that material. Thus, defects introduced due to cascade of atomic collision are highly localized at the end of range of projectile in the given material. The natures of defects due to nuclear stopping are mainly Frankel pairs [4].

Figure 3:  Stopping of heavy ion in material

However, possibility of forming other kind of defects is still an open field of investigations depending upon sensitivity of the experimental technique used to probe them. On the other hand the effect of electronic stopping are due to uniform and high value of S_e from surface to maximum depth of projectile will produce defect distribution uniformly into the target. The dominant effects such as latent tracks and columnar defects are observed in several materials. The surface of the target also gets modified due to heavy ion irradiation. Variation on reflectivity modification of surface hardness has been observed on SS-304 coated with Cr and bombarded with Ni ions [4]. This paper describes the method of generation of tailored defects (TD) using moderated heavy ion beam and their characterization using Position Annihilation Technique (PAT).

The computer simulation program TRIM [5] has been used to calculate the nuclear and electronic energy loss distribution, projected range and damage profiles for boron ions in different host like Cd, Bi and Se. A typical distribution of energy losses due to electronic and nuclear stopping of boron ions in Bi, Cd and Se targets is shown in Figure 4. The implantation profile indicates that about 80-90% of injected ions are confined within the 10% of the end of projected range.

Figure 4: Energy Losses due to electronic and nuclear stopping of boron ions in Bi, Cd and Se targets

In order to achieve a uniform implantation profile or uniform damage distribution due to nuclear stopping, the ion beam energy is moderated using stopper foils. When the mono-energetic beam passes through foils of different thickness less than the projected range, the transmitted ion beam energy is reduced considerably.

The estimated values for energy boron ion of 60 MeV passing through foils of different thickness are mounted on a disc of diameter 8" having slots of 1" x 1". The disc is mounted on a low speed motor and kept before the sample for exposure. Using about 10-12 foils of varying thickness, one can achieve very uniform implantations. We have made choice of boron ions because of their range in these hosts is of the order of 100 μm. It may be convenient to distinguish the effects of TD at the depth of about 90 µm from the top of the surface where electronic effects are there and after 90 µm where nuclear effects become significant.

Positron Annihilation Technique (PAT) for defects characterization- In PAT positrons from a radioactive source are used as a probe for medium under study. Positrons injected into the sample get thermalized and annihilate with electrons of the medium, giving rise to annihilation gamma rays. The information pertaining to the electron momentum distribution and local electron density of the medium under study can be obtained by Positron Annihilation Spectroscopy (PAS) measurements. It has been widely used for a variety of investigations [6-10] related to electronic micro structure and defects in metals, semi-conductors, super-conductors and insulators. Defects concentration ranging from 10-7 to 10-4 can be probed using PAT, which has high sensitivity and selectivity to variety of defects like vacancy, interstitial, free volume cavities, charged defects etc. The technique provides information about the bulk state of the material, averaged over depths starting from surface to 100 µm inside. Therefore, different layers cannot be probed by this method. On the other hand some comparative information regarding generated TD using (a) mono energetic beam and (b) homogenous irradiation with beam moderation could be obtained by this method. Positron life time measurements have been made on the pure selenium and bismuth the target bombarded with boron ions of 60 MeV It is found that in case of selenium the PAS parameters changed with mono-energetic irradiation and not much variation due to homogeneous irradiation, indicating the kind of defects generated due to electronic excitation are responsible to vary the electron density distribution. The value of the increased life time component is an indication of generation of C_1^- type point changed defects (PCD) due to electronic excitations. In case of homogenous irradiation the random distribution of boron in selenium neutralize the PCD resulting in no variation in the life time value. The PAS life time data of similar experiment on bismuth do not show any variation due to mono-energetic radiation. This indicates that the electronic energy loss in bismuth do not change the defect configuration, however, the boron induced effects have been responsible for the change in life time parameters due to homogeneous irradiation. The change in life time value is from 258ps to 280ps indicating presence of some shallow traps less than mono vacancies are generated in bismuth due to boron plantation.

The inherent defects (ID) and engineering defects (ED) are uniformly distributed in the material and are responsible for several properties of materials. The swift heavy ion produced defects or TD could be produced in a controlled and reproducible manner at the surface, deep layer or uniformly (using beam moderation) in the sample. PAT could be used to study defects distribution and characterization.

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