Beta self-absorption

If 40K is counted as potassium hexachloroplatinate (K2PtCl6) rather than KC1, would you expect beta self-absorption to be greater or less per mg/cm2 of precipitate Explain. 

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) ... 

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. 

For samples that emit beta particles, the sample must be evenly distributed, with defined and uniform thickness. Quantifying geometry and self-absorption of beta particles is unreliable for an unevenly deposited source. 

Preparation of a Beta-particle Self-absorption Curve for 40K... 

To quantify beta-particle self-absorption in KC1 samples of varying thicknesses. 

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. 

This experiment examines the self-absorption of the beta particles emitted by 40K (t1/2 = 1.28 x 109 a, Emax = 1.311 MeV, 89.3%), a naturally-occurring isotope of potassium (abundance = 0.0117%), in potassium chloride (KC1) salt. The degree of beta-particle self-absorption (including self-scattering) depends on the energy of the beta particles, the sample matrix elemental... 

Beta particle self-absorption can be approximated as in Equation 4.1 ... 

The formula for the self-absorption factor is exact for gamma rays (see Experiment 3) but approximate for beta particles. That it is applicable at all is due to the near-linear decrease of the logarithm of the count rate with absorber thickness of a beta-particle group (see Figure 2.6 in the Radioanalytical Chemistry textbook). The obvious deviation is that this relation ends at the range of the maximum-energy beta particle, whereas it continues indefinitely for gamma rays. 

The simplest and most direct reliable way to characterize the self-absorption of beta particles in a sample is to obtain a self-absorption curve by counting the same radionuclide in aliquots of a solid sample over a range of sample thickness. Aliquots can be prepared with the same specific activity (activity per unit mass) but with different masses, in identical planchets. The sample aliquots are counted and the results are presented in graphic form for interpolating the self-absorption factor for the sample mass of interest. 

Use calculations as shown in the beta-particle self-absorption experiment ( 4) or a comparison of the two sets of efficiency values with and without added mass in Experiment 2 to suggest that the self-absorption of 90Y beta particles is small for this experiment. 

The scattering of the results depends upon the soft beta counting of which is sensitive to the self absorption of the source the spectro-photometric determination of unstable cerium (IV) and the great number of experimental determinations needed for calculating the potential. 

Since the radioactive sample material from most methods of sample preparation is in intimate contact or in actual solution with the phosphor, the detection of emitted particles or radiation is highly efficient and may even approach 100%. Problems of self-absorption of the emissions are thus absent, or considerably smaller than those associated with planchette counting of solid samples. This is of particular importance for the measurement of low energy beta emitters such as tritium and carbon-14. On the other hand, the measurement method has intrinsic drawbacks such as quenching and chemiluminescence. 

An approximate correction for self-absorption can be obtained if the source emits particles following a known attenuation law. As an example, consider a source with thickness / (Fig. 8.14) that has a uniform deposit of a radioisotope emitting /3 particles. Assume that the source gives S betas per second in the direction of the positive x axis. If self-absorption is absent, 5 betas per second... 

Figure 8.14 Diagram used for the calculation of the source self-absorption factor for betas.

The self-absorption of the radiation emitted by the sample. This is particularly important if the radiation detected is betas or soft X-rays. 

On the other hand, the low concentration of a radionuclide provides opportunities for use as tracer in chemical and physical studies. In radioanalytical chemistry, one benefit is that the addition of a stable-element carrier permits analysis without the requirement of quantitative analyte recovery. Another benefit is the opportunity to deposit very thin sources that minimize self-absorption in a source of alpha-and beta-particle radiation. 

Fe beta particles are counted with a proportional detector or its gamma rays are analyzed with a Ge detector and spectrometer. The sample is then measured for Fe content with a thin Ge detector and spectrometer or xenon-filled X-ray proportional detector with a thin (e.g., 140 mg cm ) beryllium absorber. The Fe count rate is adjusted for background, the Fe contribution, self-absorption in the plated sample, and the chemical yield, and converted to the disintegration rate. The activity of both radioisotopes is corrected for radioactive decay from the sampling date. 

Figure 7.1. Alpha- and beta-particle efficiency self-absorption curves for " Am and Cs, respectively, on 5-cm-diameter planchet measured with proportional counter.

Figure 7.2. Beta-particle efficiency self-absorption cnrves for Sr, °Sr, and C as 1.6-cm-diameter ring-and-disk sonrce, calcnlated by Monte Carlo simnlation. (from Nichols 2006).

An aqueous sample may be added to the cocktail directly, after minor prior processing, or at the end of a radiochemical separation procedure. Direct addition is the equivalent of gross activity counting discussed in Section 7.2.4 except that some spectral analysis may be possible. Alpha particles can be differentiated from beta particles by deposited energy, pulse shape, and decay time. Self-absorption is of no concern. Quenching and luminescence, discussed in Section 8.3.2, often occur. Identification by maximum beta-particle energy is approximate, and requires comparison to radionuclide standards. 

Although gamma rays are much less subject to attenuation than alpha and beta particles, a density correction is needed if the density of the sample deviates significantly from the density of the calibration standards. The effect of density on self-absorption for both the standard and the sample is estimated by Eq. (7.2) [x for this purpose is the photon attenuation coefficient in cm /g and x is the sample area density in g/cm. Values for ix in some common materials are listed in Table 2.2 and in its cited reference. If a large set of samples with consistent density is analyzed, it may be possible to prepare radioactivity standards at the same density to avoid the need for correction. Interpolating efficiency values as a function of density is feasible at energies above 0.1 MeV because the effect of minor density difference on counting efficiency is small. 

Reducing the sample mass and the amount of materials between the sample and the detector reduces radiation attenuation 1. This effort is most important for alpha particles and least important for energetic gamma rays, as discussed in Chapter 7. The fractional self-absorption within the sample can be estimated for beta particles and gamma rays by using Eq. (7.2). The fractional attenuation of gamma rays in... 

The method used was to irradiate them inside a vessel having thick walls of UsOs, so only those neutrons having the property of not being easily absorbed (hence, not suffering self-absorption) would induce the calibrating activity. The samples were thin enough so the beta absorption was negligible. 

Phosphorous. The presence of trace amounts of phosphorus in metals and semiconductors is known to affect material properties. The produced in the (n,y) reaction is a pure emitter and has to be separated and rigorously purified. Paul (1998,2000) developed a method for P determination in steels and other high-temperature refractory alloys of interest to the aircraft industry. Irradiated samples were dissolved passed through cation-exchange columns to remove Co, Ni, and Cr followed by repeated precipitations of magnesium ammonium phosphate and ammonium phosphomolybdate. One of the major advantages of this technique is the determination of the chemical yield by gravimetry. Phosphorus was determined by INAA in matrices other than metals, e.g., polymers. In this case, the beta spectrum was corrected for interferences and self-absorption (St-Pierre and Kennedy 1998). A modified version of this procedure has been used to certify implanted phosphorus in silicon (Paul et al. 2003). 

Beta rays can be attenuated or even completely stopped by the sample itself (self-absorption), by the wall of the sample vessel, by the air layer between the sample and the counter, and by the window material of the counter. On the other hand, P rays can be detected with higher eflSciency and lower background than y rays. Various types of measurement of P rays are described below. 

N6 - W.E. Narvik and P. C. Stevenson, "Self-Scattering and Self-Absorption of Betas by Moderately Thick Samples", Nucleonics... 

Self-absorption. Beta particles have a continuous energy spectrum extending from zero energy up to a maximum energy max. K a flat beta source is measured with a thin absorber of thickness L between the source and detector, the count rate / is very closely represented by... 

Solid samples suffer from self-absorption of the beta particles. This problem must be addressed by using thin samples and extrapolating to zero thickness for the maximum range of the beta particles in a particular sample material. Calibration soiu-ces should resemble the samples in thickness and absorption properties. 

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