Crater depth, profiling

TOF-SIMS can be applied to identify a variety of molecular fragments, originating from various molecular surface contaminations. It also can be used to determine metal trace concentrations at the surface. The use of an additional high current sputter ion source allows the fast erosion of the sample. By continuously probing the surface composition at the actual crater bottom by the analytical primary ion beam, multi element depth profiles in well defined surface areas can be determined. TOF-SIMS has become an indispensable analytical technique in modem microelectronics, in particular for elemental and molecular surface mapping and for multielement shallow depth profiling. 

Depth scale calibration of an SIMS depth profile requires the determination of the sputter rate used for the analysis from the crater depth measurement. An analytical technique for depth scale calibration of SIMS depth profiles via an online crater depth measurement was developed by De Chambost and co-workers.103 The authors proposed an in situ crater depth measurement system based on a heterodyne laser interferometer mounted onto the CAMECA IMS Wf instrument. It was demonstrated that crater depths can be measured from the nm to p,m range with accuracy better than 5 % in different matrices whereas the reproducibility was determined as 1 %.103 SIMS depth profiling of CdTe based solar cells (with the CdTe/CdS/TCO structure) is utilized for growing studies of several matrix elements and impurities (Br, F, Na, Si, Sn, In, O, Cl, S and ) on sapphire substrates.104 The Sn02 layer was found to play an important role in preventing the diffusion of indium from the indium containing TCO layer. 

Interpretation of Analytical Results—Tritium. The locations of transects in the Sedan ejecta field at which samples were collected are shown in Figure 1. The distribution of tritium with depth at five sites on Sedan crater lip is shown on Figure 2. Except for the 9A area, the various sectors of the crater lip have very similar tritium depth profiles. The 9A area is a unique sector of the crater lip. A large mass of earth lifted by the detonation fell back to the crater in the 9A area earlier than the rest of the crater ejecta. Part of this material slumped into the crater, and the rest remained on the crater lip forming a prominence on the crater profile. Missile ejecta is thinner on this high point, and open-field radiation levels are lower. Tritium concentrations in the ejecta or slumped material at 9A are lower than in the rest of the crater lip mass. 

Figure 3 shows the pattern of tritium distribution on the 18A (southeastern) transect. The concentrations of tritium increase sharply with depth at all stations on the transect, and the peak tritium concentration occurs at a depth of 3 to 5 feet at all stations regardless of ejecta depth. In another transect of the ejecta field on the west side of the crater at 20A, shown in Figure 4, the tritium depth profiles show a similar pattern, and maximum tritium concentrations occur in the same zone, 2-5 feet. At a distance of 2000 feet from the crater lip, the depth of ejecta was 2-3 inches. The maximum concentration of tritium in soil water was at 3 feet at that site. 

Figure 9. Gamma-emitting radionuclide depth profiles at Sedan crater lip, 16A site...

The ejecta layer was a shallow surface stratum at distances between 1000 and 3000 feet from the crater lip. The radionuclide depth profiles at 1500 feet were studied in detail to determine whether or not any radionuclide had been leached from the surface stratum of radioactivity. At... 

Figure 13. 137Cs/SAMn ratios and tritium depth profiles at two crater lip sites...

If the Cs/Mn ratios are calculated for the depth profiles at the crater lip stations, as was done for the 20A—1500 foot station, a similar condition is seen. Figure 12 shows the Cs/Mn ratios for ejecta samples collected at several crater lip stations. An increase in 137Cs concentration relative to 54Mn with depth is seen at each crater station. Increases in the ratio with depth represent enrichment of Cs with respect to Mn by rainfall leaching. Cs/Mn ratios of 0.01 to 0.02 are characteristic of the... 

Figure 13 shows a comparison of the tritium depth profiles with Cs/Mn ratios for two crater lip sites. A maximum Cs/Mn ratio occurs at approximately 4 feet where peak tritium specific activity is also found. The use of the Cs/Mn ratio has therefore demonstrated the similarity of mechanisms for cesium and tritium movement in the postshot environment. 

In another review, Magee and Honig [24] discuss three important aspects of depth profiling by SIMS depth resolution, dynamic range and sensitivity. First, the depth resolution is a measure of the profile quality. They point out that the depth resolution is limited by atomic mixing effects and the flatness of the sputtered crater within the analyzed area. Second, the dynamic range of depth profiles is limited by crater edge... 

---