Straggling

Straggling. The essence of RBS is to measure the energy of the scattered beam and to calculate thereby the depth and/or mass from which scattering occurs. Any uncertainty in particle energy leads to a reduction in the precision with which mass and depth analysis can be achieved. 

Straggling thus limits the depth and mass resolution for features buried within the target material. The depth resolution, Az depends on the stopping power, dE/dz, the detector resolution, AEdet and the beam energy spread, AEbeam of the incident particles  

Usually the beam spread can be neglected, since it is much smaller than the energy resolution of the detectors. 

Channelling. When the incoming beam is aligned with any low-index axis or plane in a crystal it can be channelled, so that the probe atoms are steered down the channels. Under these conditions the backscattering yield will decrease to a few percent of its original non-oriented value (the random yield ). 

Channelling effects can provide two types of information in RBS experiments. If a detector is adjusted to have an energy window corresponding to a chosen atomic species, a specimen tilt-through over a channelled direction brings information on the perfection of crystallinity of the target and also on the lattice location of dopants or impurities. The yields vs. tilt-through curve has a minimum in the channelled direction, and the smaller this minimum yield, the more perfect is the crystal. 

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There is a relatively large number of alkaloids which maybe considered as simple phenethyl amine [64-04-0] (59, R = H), CgH N, or tyramine [51-67-2] (59, R = OH), CgH NO, derivatives. These iaclude mescaline (61) from the small wooly peyotyl cactus l ophophora mlliamsii (L emaire) Coult. anhalamine (62) and lophocerine (63) from other Cactaceae, and the important antamebic alkaloids (—)-protoemetiae (64), (—)-ipecoside (65), and (—)-emetine (66) from the South American straggling bush Cephaelis ipecacuanha (Brotero) Rich. AH of these bases appear to be derived from tyrosiae (25,... 

An estimate of AR ia alloys can be made usiug the empirical expression (12) of equation 10 where the average alloy reduced energy, is defined by equation 11, where C (i = 1, 2,... , n) is the elemental atomic fraction of the /th element, and is the elemental reduced energy defined iu equation 3. Using this formulation, the projected range straggling iu compounds can be calculated to within 20%. 

Figure 7 shows a contribution from ion energy straggling in the sample. This, of course, is zero for near-surface layers and gets rapidly worse for layers several thousand A deep, or for (a, <[)) in grazing configurations. 

Determination of concentration profiles from the raw data can be more complicated when protons are used as the incident particles. The energy loss ( dE/ die) is smaller for protons and straggling effects are more important. The observed profile A (AJ)) is a convolution of the actual concentration profile C x with a depth resolution function (x, Eq), which broadens with increasing x roughly as Jx- Hence, resolution deteriorates with depth. However, near-surface resolution for resonant profiling may be on the order of tens of A. 

Depth resolution in NRA is influenced by a number of factors. These include energy loss per unit depth in the material, straggling effects as the ions travel through the sample, and the energy resolution of the detection system. 

Straggling effects become more dominant further into the sample. They are most pronounced with proton beams, because the ratio of energy straggling to energy loss decreases with increasing ion mass. For protons, these effects may be quite substantial for example, depth resolutions in excess of 1000 A are typical for 1-MeV protons a few pm into a material. 

The depth resolution of ERDA is mainly determined by the energy resolution of the detector system, the scattering geometry, and the type of projectiles and recoils. The depth resolution also depends on the depth analyzed, because of energy straggling and multiple scattering. The relative importance of different contributions to the depth resolution were studied for some specific ERDA arrangements [3.161, 3.163]. 

When, in NRA, energy spectra of emitted particles are analyzed, a sufficiently thick foil in front of the detector is usually used to absorb the scattered projectiles. This reduces the depth resolution of NRA, because of energy loss straggling of the reaction products in the foil. 

As a continuous support to an existing program that is straggling to maintain overall effectiveness. Here a straight chelant formulation may be used. 

The irradiation conditions were chosen to avoid overlap of the implanted Ne atoms with the already formed clusters the Rp of the Ne ions is indeed about 410 nm with a straggling of about 100 nm. Therefore irradiating ions release part of their energy crossing the region in which the bimetallic nanoclusters are present (centered around a depth of 70 nm). 

Gaussian-shaped depth profiles of P with three parameters of maximum concentration (Cmax), projected range (Rp) and range straggling (ARp). The energy loss (dE/dx) and energy straggling ( 2 square root of the variance) of the a beam in the Si layer were taken into account ... 

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