Spectral analysis alpha

Carrier or Tracer Addition. To quantify the purified final sample that will be measured by a radiation detection instrument (as compared to a mass spectrometer), a carrier or tracer is added to the sample. The carrier usually is the same element as the radioanalyte ( isotopic carrier ) and is standardized, typically at 5-20 mg/mL concentration. The carrier serves two purposes to provide macro quantities so that certain chemical steps (such as precipitation) may be performed on the sample, and to determine the chemical yield, usually by weight. A tracer serves only to determine the chemical yield of the process its nanogram quantities or less, comparable to the radioanalyte in the sample, prevent use as carrier. The tracer is measured by its characteristic radiation at the same time as the radioanalyte. An advantage in alpha-particle spectral analysis is that the activity of the analyte can be calculated from the activity of the tracer without knowledge of the detector counting efficiency, as discussed below. 

Re-analysis with better purification should be considered if the count rate is unexpectedly high or the observed half life and radiation energies are not those of the radioanalyte. Contaminant radionuclides may be tolerated if they do not interfere with counting the radioanalyte, or can be subtracted from the count rate with only a minor increase in detection uncertainty. In spectral analysis of alpha particles and gamma rays, for example, contaminant radionuclides are tolerated in the sample if they do not interfere with counting the characteristic spectral peaks of the analyte. 

Count the alpha particles in each tracer sample for a time period sufficient to accumulate at least 1000 counts. An initial estimate of the sample counting period is based on the activity of the tracer and the known counting efficiency. Count all disks for the same period of time. The samples may be counted more than once. Count the spectral analysis background for approximately 200,000 s and the proportional-counter alpha-particle background at least 30,000 s. Record data in Data Table 6.2. 

Determine counting efficiency of the proportional detector in Step 5 for three 3,000-s periods to measure alpha particles and beta particles. Record in Data Table 7.2. Also perform overnight count (50,000 s) for alpha-particle spectral analysis of the planchet to identify the uranium isotopes and any other radionuclides and to determine their relative amounts from their alpha-particle energy spectra and record results in Data Table 7.2. Count alpha- and beta-particle background in proportional counter and alpha-particle spectral background in spectrometer for at least the same periods. 

Earlier methods used in the analysis of radium isotopes in water required labor-intensive radiochemical separations and subsequent measurement of alpha particles for 226Ra and beta particles for 228Ra. The method used in this experiment applies simpler gamma-ray spectral analysis of the progeny of both 226Ra and 228Ra. 

Proper preparation of biological solids for radiochemical analysis is essential for obtaining valid radioanalytical chemistry results. The samples often must be large because the radioactivity levels are low. Gamma-ray spectral analysis is the preferred method of radiation measurement because it requires little preparation. If gamma-ray spectral analysis of the untreated sample is not feasible because few or no gamma rays are emitted, the sample must be dissolved. Dissolution is almost always required for alpha- and beta-particle analysis. The first step usually reduces the mass of the solid sample and prepares it for dissolution. 

The measured recovery of added 90Sr tracer in the QC sample is then taken to be the yield for all samples in a batch. If the 90Sr tracer is from a standard solution, then the QC sample measurements provide the combined yield and counting efficiency. (Note This is described in the alpha-particle spectral analysis in Experiment 15.) The 90Sr activity in each sample is its net count rate multiplied by the ratio of the QC sample activity (in Bq or pCi) to the average QC sample count rate. 

Plutonium is electrodeposited onto a stainless steel disk to obtain a thin and uniform source for counting alpha particles. Counting is by spectral analysis to identify the plutonium alpha particles by peak energy and determine their activity by the integral of the count rate at the peak. 

Uranium in nature may be measured either radiometrically or chemically because the main isotope - 238U - has a very long half life (i.e., relatively few of its radioactive atoms decay in a year). Its isotopes in water and urine samples usually are at low concentrations, for which popular analytical methods are (1) radiochemical purification plus alpha-particle spectral analysis, (2) neutron activation analysis, (3) fluorimetry, and (4) mass spectrometry. The radiochemical analysis method is similar in principle to that of the measurement of plutonium isotopes in water samples (Experiments 15 and 16). Mass spectrometric measurement involves ionization of the individual atoms of the uranium analyte, separation of the ions by isotopic mass, and measurement of the number of separated isotopic ions (see Chapter 17 of Radioanalytical Chemistry text). 

Most metals except the alkalis and alkaline earths can be electroplated at the cathode with suitable applied voltage from acid solutions. Relatively early experience with electrodeposition of various metals is summarized in Fig. 3.9. The process typically is applied for carrier-free or low-concentration samples to prepare sources for alpha-particle spectral analysis. It is also useful for depositing thin sources for counting radionuclides that emit beta-particles with low maximum energy. 

Some elements are not suitable for electrodeposition from aqueous solution as the metal. Among these are the radionuclides plutonium, uranium, and thorium, which are prepared for alpha-particle spectral analysis by deposition of oxides. Other metals, such as lead, can also be deposited as oxides under empirically derived conditions (Laitinen and Watkins 1975). 

The tracer and its radiation detector must be selected to avoid cross-talk between their radiations in measuring the two radionuclides. A tracer must be selected that emits different radiations than the radionuclide of interest, or emits the same type of radiation at an energy sufficiently different for resolution by spectral analysis. Use of radioactive tracer is common for actinides that are measured by alpha-particle spectral analysis. A correction factor may be applied if cross-talk cannot be entirely avoided but is small enough to maintain the reliability of the activity calculated for the radionuclide of interest. 

Air filters and gummed deposition collection films first are measured by gross alpha- and beta-particle counting and gamma-ray spectral analysis. Usually, 1/2 of the sample then is dissolved to perform radiochemical analysis of the deposited radionuclides. The filter can be dry ashed, and then totally dissolved with an HNO3-HF treatment. 

The sample may arrive unliltered or separated as filtered water and the filter that contains the solids. The water sample is preserved with dilute acid or a preservative suitable for a radionuclide such as 1 that may be lost from an acid solution. Water without suspended solids is ready for evaporation to measure the gross alpha- and beta-particle activity, measure gamma rays by spectral analysis, and perform radiochemical analysis. The solids usually are counted similarly and then processed for dissolution as described in Section 6.2.1 for subsequent radionuclide analysis. 

Urine samples are measured directly by spectral analysis for radionuclides that emit gamma rays. To measure radionuclides that emit only alpha or beta particles, a general approach for urine pretreatment is boiling to dryness and then ashing. The ash is dissolved in mineral acid to prepare the aqueous solution for analysis (Chieco 1997). Other bioassay samples are pretreated similarly. 

Paper or cloth smears are used, often in response to regulations, to wipe surfaces of specified area (e.g., 100 cm ) to check for removable radionuclides. The smears are counted directly by gamma-ray spectral analysis. For gross alpha- or beta-particle measurements, thin smears are counted in a proportional counter or immersed in a cocktail for LS counting. Further analysis for a radionuclide of interest that emits... 

Although isotopic tracers are preferred, application of nonisotopic tracers provides more choices for selecting a radionuclide with suitable radiation, e.g., Ba tracer by gamma-ray spectral analysis for Ra by counting alpha particles. As stated in the preceding section, tests must be performed to determine whether yields for the two nonisotopic radionuclides are identical or have a constant ratio. 

For tracing plutonium yield, Pu is available. The plutonium isotope peaks are measured with a Si diode and alpha-particle spectral analysis. The Pu activity is calculated from the product of the Pu activity and the Pu peak area ratio. The Pu activity includes any contribution from Pu because the energies of their peaks are almost identical. Other plutonium isotopes (except " Pu) are measured at the same time in terms of characteristic peak areas at the energies listed in Table 6.3. A mass spectrometer is used to separate Pu from " °Pu if they must be reported separately. As indicated in Table 6.3, the emitted gamma rays can be used for intense sources but are too weak for sensitive measurements. 

Both conventionally are measured with a silicon diode by alpha-particle spectral analysis. The disintegration rate is calculated as discussed above for plutonium. 

The radionuclides in this category that emit beta particles also emit gamma rays that can be detected by spectral analysis. Short-lived radionuclides that emit alpha particles occur in the natural decay chains and usually are identified by other members of the decay chain that emit gamma rays. One caution to consider is that air filters and other surfaces in the environment collect particulate progeny of °Rn and Rn that emit alpha particles, beta particles, and gamma rays with half-lives of minutes to hours. Observation of such emissions and decays has misled unprepared observers into attributing these radiations to man-made radionuclides. 

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. 

The relatively large pulses from alpha particles are measured with a Frisch grid chamber (Knoll 1989) with a resolution of about 40 keV. Its advantage is the capability to measure and perform spectral analysis for samples with relatively large areas, e.g., 500 cm. ... 

Most modern alpha-particle spectral analysis is performed with a semiconductor such as a surface barrier detector with a very thin dead region in front of the active... 

Alpha particles are also measured by spectral analysis in Frisch grid chambers (see Section 8.3.1) and specially designed LS counting systems (see Section 7.3.3). Although solid-state detectors are most commonly used, the other detection systems, if available, can simplify the processing of certain samples. 

Use of the LS counter for alpha-particle spectral analysis is discussed in Section 8.3.2. Source preparation is simpler, but energy resolution is worse than with the solid-state detector. Special source preparation and electronic pulse-shape selection can improve resolution. 

For some radionuclide mixtures, a group separation, e.g., for actinides, is satisfactory for measuring its component radionuclides by alpha-particle spectral analysis. As discussed in Section 6.4.1, further chemical separation is needed for radionuclides that emit alpha particles of almost the same energies, or even a mass spectrometer for radioisotopes of the same element with almost identical alpha-particle energies such as Pu and Pu. 

Radionuclides are confirmed by applying redundant processes. A radionuclide is identified by gamma-ray spectral analysis and checked by chemical separation followed by a second spectral analysis measurement. Different analysts are assigned to analyzing the same sample by different chemical separations. The expected absence of gamma rays is confirmed by gamma-ray spectral analysis after the usual measurement for alpha- or beta-particle activity. Measurements are repeated to determine the half-life or a parent-daughter relation. Tabulations of... 

If the gross alpha-particle activity is sufficiently high, a thin sample—of the order of 1 mg/cm —is prepared for alpha-particle spectral analysis with a Si detector plus spectrometer. The energy range of interest usually is 4-10 MeV. 

For gamma-ray spectral analysis, a set of bulk samples can be placed on a rotating tray that moves each sample in turn next to the massive shield that encases the detector (see Fig. 15.3). The door in the shield opens and a mechanical arm places the sample on top of the detector. After counting, the sample is lifted and returned to the tray. Alpha-particle spectral analysis generally uses no automation because the samples are counted for a long time. 

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