Observations Of Microsized Craters On Metal Targets

Does the difference of target materials cause the difference in ion production from a projectile and a target? To find a clue to elucidate what caused such difference in detecting ions, we analyzed impact craters on these metal targets with SEM and STM. The representative three craters on a target are shown in Figure 1, where the projectiles were impacted from the upper side with the incident angle of 45 degrees. The distributions of Ag residues are shown below the SEM images of the craters.

Impact craters are roughly divided into two types in shape. One is a ripple-shaped crater, which is created when an Ag projectile bombarded onto the target with higher velocity and the projectile was evaporated completely. The other is the case that an Ag projectile did not have enough velocity to vaporize itself and it remained in/around the crater as residues. The distributions and the amounts of Ag residues are different on the respective targets.

The craters on Al target have two characteristics. One is the quantity of Ag residues; the craters have larger quantities of Ag residues inside the craters but not around the craters (a-1 and a-2). Another characteristic is that more than 80% of craters on Al target have a widespread melting trace around the crater (a-3). The spread of melting trace is more than a few times as large as the diameter of crater. The craters with the melting trace are larger than the other craters without it.

On Mo target, more ripple-shaped craters (b-1) are observed compared to the craters with Ag residues inside the crater or/and on the rim (b-2 and b-3). Among 217 craters we analyzed, the ripple-shaped craters and the craters with Ag residues are about 9.7% and 84%, respectively. As can be seen in Table 1, the mean diameter of ripple-shaped craters is larger than that of craters with Ag residues. The SEM images show that the craters on Mo target are shallower than ones on other targets.

The craters on Au target have the largest mean diameter of 3.05(im (Figure lc, Table 1). However, there is a clear distinction between the mean diameters of the ripple-shaped crater and the craters with Ag residues. Most of Ag residues distribute outside the craters in all directions, and they distribute broader than other targets. It is also noticed that the existence probability of ripple-shaped crater of 43% is higher than other targets.

Crater depth is one of the most important factors when we discuss the relation between the crater shape and the impact velocity. According to the SEM images, the crater depth of Al target is deeper than that of other targets, which are confirmed by stereo pairs of SEM images. Since the absolute value of crater depth cannot be measured with SEM, we observed some craters on Al and Au targets with a laser microscope and STM. The crater depth is about 5-6|am on Al target which is measured with a laser microscope, and the STM image shows that the depth is about 0.8^m on Au target.

Analysis of micro-craters on metal targets formed by hyper velocity impacts

Figure 1. The SEM images of impact craters.

Silver projectiles impacted from the upper direction with the incident angle of 45 degrees. The distributions of Ag residues are shown below the crater images.

Figure 1. The SEM images of impact craters.

Silver projectiles impacted from the upper direction with the incident angle of 45 degrees. The distributions of Ag residues are shown below the crater images.

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