| Themes > Science > Chemistry > Analytical Chemistry > Methods and Instrumentation > Radioactivity Analysis | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
A variety of radiochemical techniques are employed in the determination of natural radionuclides, transurancs, and fission products present in water, soil, ores, tailings, vegetation, biological tissue, filters, resins, etc. at low levels. Sample breakdown and radionucleide solubilization is accomplished by wet-ashing or dry-ashing in a muffle furnace, depending on the volatility of the radionuclide(s) of interest. Because of the low-level nature of these samples, subsequent radiochemical separators are generally done sequentially from a single sample. Radiochemical operations include coprecipitation, ion exchange, and solvent extraction. Each sample is spiked with the appropriate carriers and yield tracers prior to radiochemical analysis. Following separation, each radionuclide fraction is converted to a suitable form for counting using precipiation or electrodeposition. This source is counted for alpha, beta, or gamma using alpha spectrometry, gas proportional counting, liquid scintillation counting, or gamma spectroscopy. The preparation of a counting source for alpha spectrometry is one of the most critical steps in the radiochemical analysis of environmental samples, waste effluents, uranium tailings, spent fuel, etc. for long-lived emitters. An acceptable method must produce thin, uniform radionuclide deposits in high yields. Minimization of plate thickness is of considerable importance, as lack of a thinly plated surface leads to self-absorption of alphas in the electrodeposition matrix, which in turn results in poor resolution of alpha peaks in the ultimate spectroscopic analysis. Following sample breakdown, radionuclide separation and electrodeposition, the plated sample is counted using alpha spectrometry. The sample disc is placed in a chamber to within one centimeter of a vertically mounted silicon surface barrier detector. All counting is done with the chamber evacuated to eliminate the absorption of alpha particles in air. The signal is amplified and fed to multi-channel analyzer which processes and displays it as a spectrum. Liquid scintillation counting is a method for the detection of both alpha and beta emitters which has been widely applied in the fields of biological research, dating, health physics, particle physics, and radiochemistry. The technique involves dissolving a sample in a small volume of solvent (commonly toluene) which contains a low concentration of solute (scintillator), and often, a secondary solute (wavelength shifter). The ionizing alpha or beta particles in the sample cause excitation of some of the pi electrons of the solvent from their ground state to singlet states. Subsequently, an energy transfer from the solvent to the solute and then to the secondary solute takes place. The secondary solute then loses this excitation energy by a fluorescence which can be detected by two bialkali photomultiplier tubes which are connected in coincidence to a summing amplifier, three pulse height analyzers, and three scalers.
|
| Radionuclide | Method of Analysis | Detection Limit (pCi) | |||
| H-3 | Combustion/Liquid Scintillation | 7.5 | |||
| C-14 | Combustion/Liquid Scintillation | 12.5 | |||
| Sr-90 | Coprecipitation/Cherenkov Liquid Scintillation | 1.5 | |||
| Cs-137 | Ion Exchange/Liquid Scintillation | 1.5 | |||
| Po-210 | Spontaneous Deposition/Alpha Spectroscopy | 0.05 | |||
| Pb-210 | Solvent Extraction/Cherenkov Liquid Scintillation | 3.0 | |||
| Ra-226 | Rn De-Emanation/Scintillation Counting | 0.05 | |||
| Th-228, Th-230, Th-232 | Ion Exchange/Alpha Spectroscopy | 0.05 | |||
| U-238 | Solvent Extraction/Ion Exchange/Alpha Spectroscopy | 0.05 | |||
| Pu-239, Pu-240 | Ion Exchange/Alpha Spectroscopy | 0.05 |
| Soil | Coals | ||||
| Vegetation | Chemicals | ||||
| Tissues | Silicon | ||||
| Tobacco | Water | ||||
| Food Products | Air Particulates | ||||
| Mining Tailings | Ores | ||||
| Waste Effluents | Bomb Testing Sites |
| Facility | Use | ||
| Liquid Scintillation Counter | Beta-emitting radionuclides (and some alpha) including H-3, C-14, Tc-99, I-129, Ni-63, Sr-89, Sr-90, etc. | ||
| Oxidizer Unit | Combustion of samples and conversion to liquid for liquid scintillation counting | ||
| Alpha Spectrometry System (minimum six detectors, with electronics, chambers, and vacuum system | Analysis for Np-237, Pu-238, Pu-239, Pu-240, Pu-242, Am-241, Cm-242, Cm-243, Cm-244, Th-228, Th-230, Th-232, U-234, U-235, U-238, Po-210, etc. | ||
| 15%, 21%, 23%, 24%, 26%, 35%, 38%, and 42% Ge(Li) detectors | Analysis for NAA products and fission products; use in yield tracer (gamma) determination for radiochemical procedures; analysis of U and Th series radionuclides. | ||
| Ortec Omnigam-N 918A Gamma Spectroscopy System and Ortec Omnigam-N 919 Gamma Spectroscopy System | To accommodate signal from Ge(Li) and alpha spectrometer systems | ||
| Gas Proportional Counting | Alpha, Beta (gross counting); Beta tracer determination for radiochemistry procedures. | ||
| Radon Transfer System | Ra-226 Analysis | ||
| ZnS Scintillation Cell (Lucas cell) Counting System |
Ra-226 Analysis Rn-226 Analysis |
||
| Large Muffle Furnace Platinum Dishes |
Sample ashing Sample ashing |
||
| 0 - 10 amp DC Power Supply | Electrodeposition | ||
| Platinum Wire | Electrodes - electrodeposition |
Information provided by: http://www.ne.ncsu.edu