tailieunhanh - ENCYCLOPEDIA OF MATERIALS CHARACTERIZATIONC phần 10

Nguyên tử được sputtered nonselectively fiom bề mặt mẫu bởi các ion tăng tốc từ huyết tương vào surfice điện áp cực âm. Các nguyên tử sputtered khuếch tán vào huyết tương và chủ yếu là ion hóa do va chạm với các nguyên tử khí xả siêu bền (cái gọi là Penning ion hóa) mà còn một phần nhỏ bởi tác động điện tử. | to what sensitivities for NRA will be without considering the specific reactions and sample materials involved in each case. However sensitivities on the order of 10 100 ppm are common. Other Considerations Sample Requirements The maximum sample size is limited only by the design of the sample chamber. Typically samples up to several cm in diameter can be accommodated. A diameter of a few mm is generally the lower limit because high-energy ion beams focused through standard beam optics are on the order of a few mm in diameter however microbeam setups permit the use of samples an order of magnitude smaller. Nonconducting samples require special consideration. The incident ion beam causes a buildup of positive charge on the sample surface. Discharging of the sample may create noise in the spectrum collected by surface barrier detectors. In addition the presence of accumulated positive charge on the sample may affect the accuracy of current integration systems making it difficult to determine the exact beam dose delivered to the target. This problem may be obviated by flooding the sample surface with electrons to compensate for the buildup of positive charge or by depositing a thin layer of conducting material on the sample surface. If the latter option is chosen the slowing down of ions in this layer must be considered when calculating depth scales. In addition care must be taken to select a material that will not experience nuclear reactions that could interfere with those of the species of interest. Accidental Channeling Effects When analyzing single-crystal samples the experimenter should be aware that accidental channeling may occur. This happens when the sample is oriented such that the ion beam is directed between rows or planes of atoms in the crystal and generally results in reduced yields from reactions and scattering from lattice atoms. Such effects may be minimized by rotating the target in such a way to make the direction of the beam on the target more .