Alpha particle Analysis

The use of nuclear techniques allows the determination of C, N, H, O, and heavier contaminants relative fractions with great accuracy, and of the elements depth profile with moderate resolution (typically 10 nm). Rutherford backscattering spectroscopy (RBS) of light ions (like alpha particles) is used for the determination of carbon and heavier elements. Hydrogen contents are measured by forward scattering of protons by incident alpha particles (ERDA) elastic recoil detection analysis [44,47]. 

Marechal CN, Telouk P, Alberede F (1999) Precise analysis of copper and zinc isotopic compositiorrs by plasma-source mass spectrometry. Chem Geol 156 251-273 Martin P, Hancock GJ, Paulka S, Akber RA (1995) Determination of Ac-227 by alpha-particle spectrometry. Appl Radiat Isot 46 1065-1070... 

Sample preparation is rather involved. A sample of urine or fecal matter is obtained and treated with calcium phosphate to precipitate the plutonium from solution. This mixture is then centrifuged, and the solids that separate are dissolved in 8 M nitric acid and heated to convert the plutonium to the +4 oxidation state. This nitric acid solution is passed through an anion exchange column, and the plutonium is eluted from the column with a hydrochloric-hydroiodic acid solution. The solution is evaporated to dryness, and the sample is redissolved in a sodium sulfate solution and electroplated onto a stainless steel planchette. The alpha particles emitted from this electroplated material are measured by the alpha spectroscopy system, and the quantity of radioactive plutonium ingested is calculated. Approximately 2000 samples per year are prepared for alpha spectroscopy analysis. The work is performed in a clean room environment like that described in Workplace Scene 1.2. 

Radioactivity of uranium can be measured by alpha counters. The metal is digested in nitric acid. Alpha activity is measured by a counting instrument, such as an alpha scintillation counter or gas-flow proportional counter. Uranium may be separated from the other radioactive substances by radiochemical methods. The metal or its compound(s) is first dissolved. Uranium is coprecipitated with ferric hydroxide. Precipitate is dissolved in an acid and the solution passed through an anion exchange column. Uranium is eluted with dilute hydrochloric acid. The solution is evaporated to near dryness. Uranium is converted to its nitrate and alpha activity is counted. Alternatively, uranium is separated and electrodeposited onto a stainless steel disk and alpha particles counted by alpha pulse height analysis using a silicon surface barrier detector, a semiconductor particle-type detector. 

Table 5 provides calculation of strand breakage with the additional information on the number of base damages for 3.2 MeV alpha particles. The left-hand side of Table 5 presents frequencies of strand breaks containing none or at least one or more base damage on purine or pyrimidine bases. The right-hand side of the table shows a more detailed analysis of the frequencies of breaks in italic-bold. For example, 12.5% of 58% DNA segments with no strand breaks contains two base damages. Of these, 3.6% (1.7 -I-1.9) are located within 3 bp of each other, while 8.9% (4.3 + 4.3) are located at distances >3 bp from each other. 

Elemental mass distribution - The aerosol sampled by the LPI for elemental analysis was impacted on coated mylar films affixed to 25 mm glass discs. The mylar had been coated with Apiezon L vacuum grease to prevent particle bound. The LPI samples were sent to Crocker Nuclear Laboratory for elemental analysis by PIXE using a focused alpha particle beam of 3 to 4 mm diameter. Nanogram sensitivities for most elements were achieved with the focused beam. A detailed description of the PIXE focused beam technique applied to LPI samples can be found in Ouimette (13). Based upon repeated measurements of field samples, the estimated measurement error was about 15-20% or twice the minimum detection limit, whichever was larger. 

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. 

In this experiment, a known activity of 242Pu is diluted for use as tracer and its activity is checked with a detector that was previously calibrated for alpha-particle counting. This tracer will be used in Experiments 14 and 15 for the analysis of mPu... 

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). 

A water sample that you submitted for analysis is found to have a 235U 238U atom/atom ratio of 1 146 by radiochemical analysis (alpha-particle spectrometry), but when measured by ICP-MS has a 235U 238U atom/atom ratio of 1 62. What do you suspect happened regarding these analyses Be specific. 

Thermal neutrons in the reactor are efficient in producing ( , y) neutron capture reactions e.g. Fe (n, y) f< Fe. The products of these reactions will have an excess of neutrons and generally decay by (/ ", y) emission. The major disadvantage is that the radioactive atoms will always be diluted with many -non-radioactive atoms and chemical separation is not possible, (n, y) reactions are however usefully exploited in neutron activation analysis (p. 471). With fast neutrons, proton, deuteron or alpha particle bombardment a change in atomic number accompanies the.reaction and chemical separation of the carrier free radiotracer becomes possible,... 

SIMS analysis of electrodeposited Th alpha-particle sources gives rise to higher signals for the ThO and ThO/ than for Th. This leads to difficulties in quantitation as the oxide to atomic ion ratios will be sensitive to local oxygen concentrations. Isobaric... 

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