Gamma-ray spectrometry losses through pulse processing dead time and pile-up are best assayed with an external pulse technique. In this work, the virtual pulse generator technique as implemented commercially with the Westphal loss free counting (LFC) module is set up and tested with four high resolution gamma-ray spectrometers. Dual source calibration and decaying source techniques are used in the evaluation of the accuracy of the correction technique. Results demonstrate the reliability of the LFC with a standardized conventional pulse processing system. The accurate correction during high rate counting, including during rapid decay of short lived activities, has been the basis for highly precise determinations in reference materials studies.
Instrumental neutron activation analysis (INAA) procedures were optimized for the analysis of small (1 mg) solid samples. This process included sample handling as well as detailed evaluation of high rate counting techniques, also in conjunction with rapidly decaying sources, to establish the necessary analysis environment for the determination of large numbers of samples with high reproducibility. The investigated materials included biological materials such as peach leaves (SRM 1547), lichen, and bovine liver, as well as environmental materials such as urban particulate matter (SRM 1648), and deep ocean sediment. The analytical data obtained with the INAA procedures were used to determine homogeneity values.
Chromium is one of the most difficult elements to accurately determine at the naturally occuring, ultratrace levels normally found in uncontaminated biological samples. In view of the importance of Cr, both as an essential and as a toxic element, efforts have focused on developing a simple, yet reliable, radiochemical procedure for Cr determination using neutron activation analysis. A number of problem areas have been identified in earlier methods, and an improved radiochemical separation procedure, based upon the liquid/liquid extraction of Cr(VI) into a solution of tribenzylamine/chloroform, has been developed. The fast neutron interference from Fe has been evaluated for the highly thermal RT-4 facility of the NBS Research Reactor, and Cr concentrations have been determined in samples of whole human blood collected under clean conditions and in two certified reference materials.
A neutron activation analysis scheme based upon a radiochemical separation of the activation products has been developed.
The method utilizes the inherent sensitivity of the activation reaction198Pt(n, γ)199Pt and counting of the daughter nuclide199Au. This nuclide is radiochemically separated from interfering activities by homogeneous precipitation as elemental gold.
The remaining interference of the secondary reaction197Au(n,γ)198 Au(n,γ)199Au from gold in the samples is quantitatively assessed and corrected. During this process accurate gold concentrations in
the samples are obtained at ultratrace levels. The analysis scheme is applied to gold and platinum determinations in biological
Standard Reference Materials and human liver specimens. Gold and platinum are determined at concentrations of 5·10−11 g/g, and at higher levels.
A fast (14-MeV) neutron activation analysis procedure was employed to directly measure mass fractions of oxygen in coal samples. The procedure demonstrated sufficient precision and accuracy to determine a relative change of about 5% or more, in the oxygen mass fraction of about 12%. The procedure was applied to test samples of the newly developed SRM 1632c Trace Elements in Coal (Bituminous). The samples had been stored at three conditions: in liquid nitrogen vapor and at room temperature in the original containers packaged under argon, and for accelerated aging at 50 °C open to air. Following six months storage increments, duplicate samples of each of twelve bottles of the SRM were measured for each storage condition. In addition each sample was processed through ten separate analytical runs yielding a total of some 720 measurements per storage duration. Oxygen was determined by comparison to a primary standard potassium dichromate and the accuracy of the method was assessed through the analysis of replicate samples of three reference materials that are certified for oxygen content. The initial 6-months test period indicated uptake of oxygen in the open-air storage.
Possible relationships between insufficient blood volume increases during pregnancy and infant mortality could be established with an adequate measurement procedure. An accurate and precise technique for blood volume measurements has been found in the isotope dilution technique using chromium-51 as a label for red blood cells. However, in a study involving pregnant women, only stable isotopes can be used for labeling. Stable chromium-50 can be determined in total blood samples before and after dilution experiments by neutron activation analysis (NAA) or mass spectrometry. However, both techniques may be affected by insufficient sensitivity and contamination problems at the inherently low natural chromium concentrations to be measured in the blood. NAA procedures involving irradiations with highly thermalized neutrons at a fluence rate of 2·1013 n·cm–2·s–1 and low background gamma spectrometry are applied to the analysis of total blood. Natural levels of chromium-50 in human and animal blood have been found to be <0.1 ng/ml; i.e., total chromium levels of <3 ng/ml. Based on the NAA procedure, a new approach to the blood volume measurement via chromium-50 isotope dilution has been developed which utilizes the ratio of the induced activities of chromium-51 to the iron-59 in three blood samples taken from each individual, namely blank, labeled and diluted labeled blood.
At the end of the 1991 Gulf War the U.N. Security Council Resolution called upon IAEA, assisted by the U.N. Special Commission, to carry out inspections of all Iraqi nuclear installations. The IAEA Action Team succeeded in implementing, on very short notice, a comprehensive system of inspection activities, including sampling and analysis at the Agency's Laboratories and other laboratories in Member States. The Agency's Laboratories developed and implemented an analytical strategy with the aim to rapidly and accurately obtain the information necessary for verifying the Iraqi declarations. The analyses ranged from screening for - and /-emitters to accurate determinations of the amounts and isotopic composition of the radionuclides and associated trace elements and compounds. The arsenal of methods included ultra-sensitive radiometric methods, mass spectrometry, neutron activation, X-ray fluorescence and inductively coupled plasma emission spectrometry. Selected results include the detection of uranium chloride compounds, special composition steels, and quantitative accounting of uranium and plutonium production. The selectivity, sensitivity and reliability of the applied analytical techniques in conjunction with validated sampling procedures are essential components of an analytical measurements system that can provide credible results.
The optimization of neutron activation analysis with regard to detection limit and uncertainty of measurement using physical
and chemical means is reviewed. Using selected examples it is demonstrated that radiochemical separation is the most effective
means of optimization, especially in neutron activation analysis, because it yields the lowest detection limits and uncertainties.
A new radiochemical neutron activation analysis (RNAA) method has been developed for low-level determination of Si in biological materials, which is based on the 30Si(n,γ)31Si nuclear reaction with thermal neutrons. The radiochemical separation consists of an alkaline-oxidative decomposition followed by distillation of SiF4. Nuclear interferences, namely that of the 31P(n,p)31Si with fast neutrons, have been examined and found negligible only when irradiation is carried out in an extremely well-thermalized neutron spectrum, such as available at the NIST reactor. The RNAA procedure yields excellent radiochemical purity of the separated fractions, which allows the measurement of the β--activity of the 31Si by liquid scintillation counting. Results for several reference materials, namely Bowen’s Kale, Bovine Liver (NIST SRM 1577b), Non-Fat Milk Powder (NIST SRM 1549) and several intercomparison samples, Pork Liver-1, Pork Liver-2 and Cellulose Avicel, are presented and compared with literature values.
On a étudié par analyse par activation le comportement des éléments de l'acier V4A dans des tissus intoxiqués par les métaux.
V4A: Fe, Cr, Ni, Mo, Ag et Ta comme métaux purs implantés, et Co, Zn et Sb comme oligoéléments. Pour la recherche du Mn, le24Na a été séparé radiochimiquement. Les limites de détection se situent entre 0,005 ppm (Ta) et 5 ppm (Fe). D'après les résultats
des analyses, on peut déduire un processus de corrosion intercellulaire transcristallin du matériau implanté suivi d'une disparition
rapide des constituants simples. Seul le Fe est accumulé dans les cellules sous la forme d'un composé biologique qui se dégrade
lentement. La teneur en Zn, oligoélément essentiel, est remarquablement diminuée dans le tissu. Cela peut être la conséquence
du déplacement du zinc lié aux enzymes par les constituants implantés, provoquant des changements dans les processus enzymatiques
qui jouent un rôle déterminant dans le développement de l'intoxication métallique.