137Cs and to a lesser extent, 210Pb have been widely used for soil erosion and sedimentation studies. The accuracy of their quantification is greatly dependent on the proper handling of the sources of error that may affect the results. These include ambient background variation, attenuation for the 210Pb 45 keV gamma-photons in the sample, efficiency calibration and large statistical errors due to low levels of radioactivities and small sample quantities combined with Compton scattering of high energy photons during acquisition. In this study, we investigated the distribution of both 210Pb and 137Cs in three soil cores from Konza and Fort Riley, Kansas. Special attention was given to the analytical treatment of the spectra and the uncertainty budget. We show here that the ambient background is a major factor in the results. If it is not well monitored, errors of as high as 25& can result. We investigated the advantages of using Compton suppression with gamma-ray spectrometry for 137Cs and 210Pb quantification. Natural matrix certified reference materials (CRM) were used for efficiency calibration and quality assurance.
An overview of the nuclear analytical methods oppurtunities at the University of Illinois at Urbana-Champaign is given. Undergraduate and graduate level teaching and research are highlighted. The TRIGA reactor and neutron activation analysis facilities are described in the context of this role within an inter-disciplinary environment.
An overview of Compton suppression neutron activation analysis (NAA) is given. The basic theory of the technique, its experimental design, uniqueness, and limitations are discussed. Experimental data showing its usefulness for the low level determination of several key elements in environmental samples that can not be obtained using conventional non-destructive neutron activation is deliberated.
Non-destructive neutron activation analysis was employed to determine zinc in ten biological standard reference materials from the National Bureau of Standards and the National Research Council of Canada. The use of a 4 h. irradiation at a medium neutron flux allows excellent accuracies, precision and sensitivities to be attained for all the samples analyzed. It is suggested that neutron activation analysis be one of the benchmark methods for the determination of zinc in biological reference materials for concentrations as low as the one part per million level.
Aerosol samples were collected on Whatman 41 filters at two sites near Lake Huron and one site near Lake Ontario. These samples were then analyzed by instrumental neutron activation analysis (NAA) at the University of Illinois. The detection limits for certain trace elements were enhanced by irradiation with both thermal and epithermal neutrons and also by counting with Compton suppression techniques. The sample was divided in half to allow for four irradiations. Short-lived thermal NAA resulted in the determination of Al, Br, Ca, Cl, Cu, K, Mn, Na, Ti, and V. A short epithermal irradiation was used to determine Cu, I, In, Si, Sn and U. A one and one-half hour epithermal irradiation was utilized for the determination of As, Au, Br, Sm, Sb, and W. The elements Cr, Cs, Fe, Hf, Ni, Sc, Se, Th. Zn, and several rare earths were determined with a long thermal irradiation. Utilizing a Compton suppression gamma-ray counting system reduced the background and enhanced the detection of several isotopes which primarily emit only a single gamma-ray upon decay. Counting was simultaneously performed with a normal counting mode so that the detection of isotopes with multiple decay gamma-rays was not impaired.
Instrumental neutron activation analysis in conjunction with Compton suppression methods has been used to determine chromium concentrations in 13 biological reference materials. Special attention was focussed on the interferences and the ways of lowering their contribution. Separate data was obtained for normal and Compton suppressed modes of counting so that a comparison could be performed. The results for chromium as compared to the certified or compilation values were in excellent agreement.
Bromine can be determined by neutron activation analysis (NNA) through either the activation of79Br (50.69% naturally abundant) or81Br (49.31% naturally abundant). When79Br is activated, both80mBr and80Br are produced.80mBr (4.42 h) decays to80Br (17.66 m) which then beta decays to either80Se or80Kr. If one would like to determine bromine concentrations with short lived NAA, special equations must be used to account for the parent daughter relationship between80mBr after their simultaneous production. The development of the equations needed to calculate bromine concentrations with short-lived NAA is described including variations on irradiation times, decay times, and count times.
Biological materials containing trace amounts of mercury and selenium were examined using neutron activation analysis. They
were analyzed using Compton suppression and γ–γ coincidence counting. The 279 keV photopeak of activated mercury (203Hg) was analyzed in order to observe the mercury content in these samples. Selenium, an element found in many biological samples,
interferes with the analysis of 203Hg when activated (75Se). Because the selenium interference comes from a cascading emission, Compton suppression was utilized to reduce this interference.
In order to fully characterize the selenium content in the samples, γ–γ coincidence was used which reduced the background
and eliminated bremsstrahlung interference produced from neutron activated phosphorous through the 31P(n, γ)32P reaction which is a pure beta emitter. As a result, we determined the mercury and selenium concentrations in three standard
reference materials, which contain varying ratios of mercury to selenium concentrations. This study also showed that these
types of concentrations can be determined from small (<500 mg) sample masses. Further work needs to be done on wet samples
that require dehydration, as mercury can be lost through this process.
In this work a review of the development of compton suppression is presented. It was shown that the application of Compton-suppression counting in instrumental NAA reduces the detection limits and improves the accuracy for a list of elements by substantial reduction of the background of the -spectroscopy. Results for certified reference materials obtained through the use of Compton suppression are normally more accurate and in agreement with the published values. Compton suppression is particularly helpful for low level concentrations in environmental samples to those elements which exhibit severe special interferences in the normal NAA counting. A list of the elements with isotopes having single or close to single -ray decay schemes and which could benefit from Compton-suppression counting is presented. Also, evaluation is made regarding the reliability of Compton suppression with increase in the overall dead-time of the counting. It was concluded that this method does not provide accurate quantification of the isotopes when the overall dead-time exceeds the 10% range. Investigation of the natural background was performed with Compton suppression for the purpose of neutron activation analysis application. The method presented proves to broaden the application of NAA and helps in its competition for simplicity, accuracy and reliability with the modern methods of elemental analysis. Future application of coincidence spectrometry in activation analysis should include better enclosing of the primary detector, utiliza5tion of x-ray and well type detectors, -, -, and -- coincidence techniques.