Neutron activation analysis is attractive for trace-element determinations in large samples. Facilities for reactor irradiation and -ray spectrometry of kilogram-size cylindrical samples are described. The thermal neutron flux is ca. 5·1012m–2·s–1 with ath/epi>104, so neutron self-thermalization can be neglected. The correction for the neutron attenuation within the sample is derived from measurement of the neutron flux depression just outside the sample. Correction for -attenuation in the sample is performed via linear attenuation coefficients derived via transmission measurements. Also the natural radioactivity in the sample is taken into account. Examples are given of materials to which large sample INAA has been applied successfully, and further lines of development and exploration are indicated.
Profiles of thermal flux, epithermal flux, f and have been determined for a multisample irradiation container in use with k0-based NAA. It could be concluded that for accurate NAA with the IRI-reactor, samples should be surrounded by at least two thermal flux monitors. Since f and values were found to vary considerably between irradiations as compared to the variations within one container, the possibility of determining f and just once for the irradiation facility used, and monitoring only the thermal flux in future irradiations, was ruled out. The use of average values for f and within a container was shown to yield unacceptable inaccuracies mainly for Zr and U determinations.
Authors:M. Koster-Ammerlaan, P. Bode, and A. Winkelman
The assumption that the shape of the epithermal neutron spectrum can be described, in any research reactor, by the 1/E1+α function is a fundamental starting point of the k0 standardization. This assumption may be questioned from a reactor physics viewpoint. The type of moderator, the existence
of neutron reflectors, the additional production of (γ, n) neutrons and resonance capture by construction materials may be different for each reactor, with consequences for the shape
of the neutron spectrum. This dependency may explain that various practitioners reported contradicting experiences with the
use of Zr–Au flux monitors for the determination of the α-parameter. An objective view on the influence of the design of the
reactor and irradiation facility on the shape of the neutron spectrum can be obtained by modeling. This has been applied in
the Reactor Institute Delft for reactor configurations in which the irradiation facilities face the fuel elements with the
presence of beryllium reflector elements. The Monte Carlo calculations indicate a distortion of the 1/E1+α relationship at the higher energy edge of the epithermal neutron spectrum. This distortion is attributed to the formation
and thermalisation of both photoneutrons and (n, 2n) produced fast neutrons in the beryllium, and has a direct impact on the resonance activation of 95Zr, other than represented by the 1/E1+α function. The obtained relationship between neutron flux and neutron energy was also used for estimating the f-value and compared with the value obtained by the Delft Cr–Mo–Au flux monitor.
The CITATION code based on neutron diffusion theory is used for flux calculation inside voluminous sample in prompt gamma
activation analysis with an isotopic neutron source (241Am-Be). The code used the specific parameters related to energy spectrum source, irradiation system materials (shielding,
reflector, etc.), geometry and elemental composition of the sample. The flux distribution (thermal and fast) was calculated
on three-dimensional geometry for the system: source, air, and polyethylene and water cylindrical sample of 125 liters. The
thermal flux was calculated in series of points inside the sample, and agreed with the results obtained by measurements with
good statistical uncertainty. The maximum thermal flux was measured at distance of 4.1 cm and calculated at 4.3 cm by the
CITATION code. Beyond a depth of 7.2 cm, the ratio of thermal flux to fast flux increases up to twice and allows us the optimization
of the detection system in the scope of in-situ PGNAA.
Large-volume instrumental photon activation analysis (IPAA) was used for the investigation of shredded electronic waste material.
Sample masses from 1 to 150 grams were analyzed to obtain an estimate of the minimum sample size to be taken to achieve a
representativeness of the results which is satisfactory for a defined investigation task. Furthermore, the influence of irradiation
and measurement parameters upon the quality of the analytical results were studied. Finally, the analytical data obtained
from IPAA and instrumental neutron activation analysis (INAA), both carried out in a large-volume mode, were compared. Only
parts of the values were found in satisfactory agreement.
The goal of this study was to evaluate the selenium mass fraction in toenail clippings taken from random inhabitants living
in various areas of the Pomeranian (Northern Poland) and Lubuskie (Western Poland) Districts. Toenail clippings were analyzed
by instrumental neutron activation analysis (INAA) giving means of 0.57±0.10 and 0.60±0.16 mg·kg−1 for the two areas, respectively, but the difference was statistically not significant. In additional, it was found that gender,
age, body mass index (BMI), smoking, and selenium supplementation are factors with apparent effects to the selenium levels
in toenail clippings.
Phosphogypsum discharges by phosphate-ore processing industries pollute sediments of the Rotterdam harbour area with226Ra. Direct measurement of this radionuclide in sediments does not provide a reliable indication of the elevation of the levels,
since226Ra levels in sediments depend on the particle size. To eliminate the size effect, the226Ra/Ba ratio in sediments was tested as a possible indicator for the226Ra in the discharges. The results indicate almost a doubling of the226Ra levels in sediment samples due to phosphogypsum discharges. The contribution of phosphogypsum to the sediment mass was
calculated in the order of a few percent equivalent.
Authors:P. C. Favaro, P. Bode, and E. A. De Nadai Fernandes
Trace elements were determined in 27 brands of nail polish using INAA so as to estimate the impact of nail polish remainders when using (finger, toe) nail clippings as a bioindicator for epidemiological studies. Fe was found to be present up to almost 1.7% in one brand. No unambiguous correlation between trace elements and color was established, though transparent polish has the lowest trace element concentrations. An estimate of the maximum effect (no removal of the paint) indicates that over-estimates of some trace element levels in nail with more than 200.000% may occur. This study showed that if nail clippings will be used to study trace elements, very high demands have to be set to the efficiency and selectivity of the cleaning procedure.
Authors:M. Bacchi, E. Fernandes, E. França, and P. Bode
This work is focused at the demonstration of the overall adequacy of the procedures introduced in a Brazilian laboratory for conducting the k0-standardized NAA. Concentrations of 29 elements determined in certified reference materials of plants and geological matrices are evaluated. Most of the experimentally determined values agreed well with the reference data and no consistent bias could be attributed to the analytical system adopted.
Authors:F. Tagliaferro, E. De Nadai Fernandes, P. Bode, and H. Baas
The degree of homogeneity is normally assessed by the variability of the results of independent analyses of several (e.g.,
15) normal-scale replicates. Large sample instrumental neutron activation analysis (LS-INAA) with a collimated Ge detector
allows inspecting the degree of homogeneity of the initial batch material, using a kilogram-size sample. The test is based
on the spatial distributions of induced radioactivity. Such test was applied to samples of Brazilian whole (green) coffee
beans (Coffea arabica and Coffea canephora) of approximately 1 kg in the frame of development of a coffee reference material. Results indicated that the material do
not contain significant element composition inhomogeneities between batches of approximately 30–50 g, masses typically forming
the starting base of a reference material.