Measurement self-absorption

Equation 8 is true in the limit that the transit time for light in the plate Is short compared to the lifetime, and that self— absorption in the critical cones is negligible (i.e., r of the critical cone, r, is zero). Thus Equation 8, which can be easily rewritten to Include r, is not valid for large values of P. Note that when T-To, r-0, and when (1 — P) increases both, r and T will change. The smallest error bars in Figure 7 show the measured self—absorption rates for rhodamine-575 in methanol as a... 

The PL spectrum and onset of the absorption spectrum of poly(2,5-dioctyloxy-para-phenylene vinylene) (DOO-PPV) are shown in Figure 7-8b. The PL spectrum exhibits several phonon replica at 1.8, 1.98, and 2.15 eV. The PL spectrum is not corrected for the system spectral response or self-absorption. These corrections would affect the relative intensities of the peaks, but not their positions. The highest energy peak is taken as the zero-phonon (0-0) transition and the two lower peaks correspond to one- and two-phonon transitions (1-0 and 2-0, respectively). The 2-0 transition is significantly broader than the 0-0 transition. This could be explained by the existence of several unresolved phonon modes which couple to electronic transitions. In this section we concentrate on films and dilute solutions of DOO-PPV, though similar measurements have been carried out on MEH-PPV [23]. Fresh DOO-PPV thin films were cast from chloroform solutions of 5% molar concentration onto quartz substrates the films were kept under constant vacuum. 

Figure 6. Tempcraiure dependence of the fluorescence lifetime of BMPC in 1 1 ethanol-mcihanol. Measurements were carried out at the LENS laboratory of Florence by a picosecond apparatus using as an excitation source (at 380 nm) a dye laser pumped by a frequency-doubled cw Nd-YAG laser and recording the fluorescence time jirofiles by a streak camera. Since the overall insuumental response time was 75-80 ps, decays with t>200 ps, observed at T<130 K, were analyzed without deconvolution. At 177, 178 and 193 K, the lifetimes were roughly estimated as i=(FWHM -77 ), where FWHM was the width at half maximum of the decay. Because of the rather high sample absorbances (An,x=2), self absorption may have reduced the lifetimes to some extent.

One of the limitations of the portable field survey instruments in the measurement of americium is that their quantitative accuracy depends on how well the lateral and vertical distribution of americium in the soil compares with the calibration parameters used. These methods can provide a rapid assessment of americium levels on or below surfaces in a particular environment however, laboratory-based analyses of samples procured from these environmental surfaces must be performed in order to ensure accurate quantification of americium (and other radionuclides). This is due, in part, to the strong self absorption of the 59.5 keV gamma-ray by environmental media, such as soil. Consequently, the uncertainty in the depth distribution of americium and the density of the environmental media may contribute to a >30% error in the field survey measurements. Currently, refinements in calibration strategies are being developed to improve both the precision and accuracy (10%) of gamma-ray spectroscopy measurements of americium within contaminated soils (Fong and Alvarez 1997). 

Since the substrate hematite contains Fe-57, a certain Mossbauer self-absorption is inevitable in the case of measurements on Co-57. But, the effect of the absorption is considered to be not important as far as the pH dependence of the spectra is concerned, since the amount of hematite was kept constant and the adsorbed divalent Co-57 was dispersed uniformly in it. 

The RQDO /-values conform with the recommended values by Wiese et al. [18] for the 3p ( P) 4s-3p ( P) 4p transitions in C//better than those of the much more complex theoretical procedure of Ojha and Hibbert [17], who used large multiconfiguration expansions in the atomic structure code CIVS. On the other hand, the discrepancies between the length and velocity CIVS oscillator strengths are not negligible. Wiese et al. [18] remark that, for the case of the 4s " P-4p multiplet, the stronger lines measured by Bengtson et al. [16] seem to be affected by self-absorption (Table 2). 

The relation between fluorescence intensity and structure was studied by Goldzieher et al. [113]. For this work, 0.2 mL of an ethanolic steroid solution was added to 1 mL of 90% (v/v) sulfuric acid, the mixture heated at 80°C for 10 minutes, and finally diluted with 4.0 mL of 65% (v/v) sulfuric acid. The resultant solution contained 25-250 pg/ml, and was measured at an excitation wavelength of 436 nm. No correction was made for self-absorption of the solutions. 

The X-ray florescence is proportional to the X-rays absorbed by the sample and therefore can be used to measure the FXAFS oscillations. In general, this technique has a poorer signal-to-noise ratio than transmission FXAFS and there are problems with self-absorption effects, requiring dilute or thin sample sizes. 

The adsorption of ATP-14C to surface films of stearic acid and brain lipid was examined over an extended period of time under various conditions (Table I and Figure 6B). Table I shows the short-term results, where adsorption was studied during the first 30 minutes, and evaporation was not a factor. Upon adding stearic acid or brain lipid the measurable radioactivity decreased, probably as a result of displacement of ATP-14C from the surface layer and self-absorption of the beta particles by the lipid film. When PMCG was present, there was a slight but significant increase in the surface adsorption of ATP. The amount of ATP adsorbed was 4 X 10 10 moles/sq. cm. for stearic acid and 2.5 X 10"10 moles/sq. cm. for brain lipid. If the lipid concentration in the surface is assumed to be about 8 X 10 10M (as phospholipid in the case of brain lipid), the molar ratio of ATP to lipid would be about 0.5 for stearic acid and 0.3 for brain lipid. 

A crucial step in both emission and absorption studies is the conversion of intensities of bands or lines to concentrations of molecules. This is not easy. The transition probabilities for bands connecting excited states are not often known to any reasonable precision. Also, one further difficulty plagues the emission studies. In contrast to absorption studies the population of molecules in the lowest state, v = 0, is not measurable. Relative populations are usually reported. Self absorption measurements can be used to overcome this difficulty43. ... 

This experiment examines the count rate as a function of sample thickness. All other variables are held constant (except for a small change in source-detector distance). As the sample becomes thicker, more of the beta particles are absorbed in the sample itself. This is called self-absorption, and is shown in Figure 4.1. In thin samples, self-absorption is relatively small or negligible, but in thick samples it is measurable and must be considered when calculating the counting efficiency. 

Attempt to fit a better curve of the self-absorption factor f through the measured values. Record the best values of R0, p, and e0 ... 

You are preparing to count beta particles from samples that contains low levels of "Mo. The final counting form is Mo03. Outline an experiment to prepare a "Mo beta self-absorption curve. How would the self-absorption be used to correct the observed counting activity in actual measurements (consider Tc-99m ingrowth) ... 

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