Experimental validation of some thermal neutron self-shielding calculation methods for cylindrical samples in INAA

Neutron self-shielding has been a factor of concern in the history of Instrumental Neutron Activation Analysis. When the sample to be analyzed cannot be made small enough in size and/or sufficiently diluted, this undesired phenomenon must be accounted for. Several analytical, semi-empirical and computational methods for estimating the thermal neutron self-shielding effects have been extensively discussed in the literature and this work aims at the experimental validation of some of these methods by neutron irradiation of cylindrical samples containing strong thermal neutron absorbers. The accuracy and the relative differences in the results between these methods are discussed for cylindrical samples with up to 40% thermal self-shielding, showing that a semi-empirical sigmoidal function can be more accurate in modeling this effect than other exact algorithms, where a maximum 2% relative difference to the experimental values was obtained

Using k0-UNAA for the determination of depleted uranium in the moss biomonitoring technique

The use of ammunition containing depleted uranium (DU) in several military actions is of growing health concern to the population related to the risk arising from contamination of the environment with DU penetrators and dust. Environmental monitoring of uranium and its isotopic ratios therefore become important parameters since they allow for source identification. Neutron activation analysis according to the k0-standardisation not only allows for quantification of trace elements including uranium, but it can also provide information about the 235U/238U isotope ratio. The application of this method as a screening technique to moss samples, as a bio-indicator for this atmospheric pollution, was evaluated for samples spiked with different 235U/238U isotope ratios. Detection limits of about 0.6 ng/g and 6 ng/g were obtained for 235U and 238U, respectively. The concentration of natural uranium found in moss samples is however the limiting factor, reducing the quantification limit for DU to 400 ng/g

Experimental determination of Q0 factors and effective resonance energies with a multi-channel approach: the α-vector method

Some of the Q 0 and Ē r factors from the 2003 to 2012 recommended k 0-datasets were adopted from older literature (Lit; before 1984), at the time of the launch of the k 0-method and have associated uncertainties of 10 %. The Ē r factors on the other hand, were derived analytically in 1979 and 1987 by employing the Breit–Wigner resonance representation, but in 1984 a multi-channel method was proposed for simultaneous Q 0 and Ē r experimental determination. In this work we assumed the α-dependence of the Ē r parameter proposed later in 1987 for a generalization of this method, which was then employed in the experimental determination of Q 0 and Ē r factors for 17 (n,γ) target isotopes on up to three channels of the BR1 reactor at the SCK-CEN (Mol, Belgium). Retrospectively, this “α-vector method” can be employed for α-calibration and it provides a practical way of averaging the experimental Q 0 (and Ē r) results from several laboratories. Our Q 0 results have 2–3 % uncertainty and are compared with the Lit elsewhere

Experimental determination of k0, Q0, Ēr factors and neutron cross-sections for 41 isotopes of interest in Neutron Activation Analysis

The k0 -literature has been reviewed every decade but some of its nuclear data is still more than 30 years old. Sometimes Q0 values were adopted from the nuclear data at that time or were experimentally determined by only 1 laboratory. Other isotopes were listed with accurate pairs of (k0, Q0) values but were also quoted as candidates for redetermination for different reasons (i.e. imprecise cadmium transmission factors, half-lives). In this work we aim at the experimental re-determination of k0 and Q0 values for 41 isotopes of analytical interest while introducing a methodology for simultaneous - Er and Q0 determination employing N irradiation channels. In order to satisfy the metrological level required, up to 12 repeats per standard were irradiated in up to 4 irradiation channels of the Belgian Reactor 1 (BR1, SCK∙CEN) having a wide spread in neutron characteristics. Our relative percentile differences to the literature values were usually ≤10 % for Q0 factors, ≥25 % for - Er values and ≤4 % for k0 values. Our precision and accuracy are discussed thoroughl

Experimental determination of k0, Q0 factors, effective resonance energies and neutron cross-sections for 37 isotopes of interest in NAA

The recommended k0 nuclear data from 2003 has been re-investigated by some authors during the last decade, motivated by some discrepancies that were systematically observed during the analysis of reference materials. Their significant findings have not been included (yet) on a newer compilation, as it is difficult to draw conclusions on the accuracy of k0 and Q0 factors when the statistical population of independent experimental values is quite scarce. In some cases, a strong correlation to the adopted Q0 factor means that a direct comparison between the results of different authors is not possible if the data required for a proper renormalization was not provided. At the SCK-CEN and UGent (Belgium) we would like to continue with the experimental k0 determination exercises performed during the last years and to supply to the k0-community with the nuclear data of 37 additional target isotopes isotopes, for a total of 77 isotopes investigated since 2012. The isotopes were investigated on up to 4 channels of the BR1 reactor at the SCK-CEN, obtaining values with <2.6% uncertainty. Our results are discussed and compared to the literature elsewhere

The impact of polyethylene vials on reactor channel characterization in k0-NAA

Reactor channel characterization is commonly performed by irradiating bare and cadmium-covered ‘‘fluence rate monitors’’, avoiding as much as possible the use of irradiation vials and spacers to position the monitors inside the channel. However, in routine k0-Neutron Activation Analysis often samples are packed in small polyethylene vials prior to irradiation. This work aims at studying the impact of typical NAA polyethylene vials (~1 mm wall thickness) on the determination of the f and a channel-specific parameters through the ‘‘Bare’’, ‘‘Cadmium-Covered’’ and ‘‘Cadmium- Ratio’’ methods. The impact of these vials on each method was studied for 3 irradiation channels of the Belgian Reactor 1 at SCK-CEN (Mol, Belgium) with low to high f and alpha-values. The net impact was 1% on each parameter. Inconsistencies between the different methods were found when the impact of the polyethylene was neglected, implying that all determination methods must be pooled and thin monitors should be used for an accurate channel characterization

Experimental k0 and k0-fission factors for the determination of the n(235U)/n(238U) enrichment levels and correction for 235U fission interferences in samples containing uranium

In 2010 we investigated the applicability of the current k (0) and k (0)-fission factors for the determination of the n(U-235)/n(U-238) isotopic ratio in multi-elemental samples containing uranium. An overestimation 3-4 % was observed in our determinations when employing the recommended 2003 k (0)-literature. After a recalibration of all our laboratory instruments, a 3 % overestimation was still observed in this work when employing this nuclear data. Therefore we aimed at the experimental re-determination of these composite nuclear constants in order to enhance the reliability of the isotopic ratio determination method and the accuracy of our data-filtering algorithms. New k (0)-fission factors are given for 7 nuclides that are not currently present in the 2012 k (0)-database. Several additional k (0) factors are introduced for some nuclides in this library. Our k (0) results are also compared with those recently reported by Blaauw et al

Experimental determination of k0 nuclear data for the cesium radionuclides

The current (official) k (0) nuclear data for the cesium radionuclides was determined circa 1987 and was partially adopted from older literature. In this work we aimed at the redetermination of experimental k (0), Q (0) and (r) factors with metrological care. The results are in good agreement with more recent results from other authors. Our Q (0) and (r) values were 2-15 and 25 % different, respectively. The k (0) factors were determined according to the cadmium subtraction technique, resulting in values 3-7 % different than the official ones. Our precision and accuracy is discussed

Environmental monitoring for safeguards using k(0)-standardized neutron activation analysis

Multi-elemental analysis of samples containing uranium by Instrumental Neutron Activation Analysis (INAA) has been generally known as a problematic task due to the numerous radioisotopes produced from the uranium fission when the sample is irradiated with thermal neutrons. In k(0)-standardized NAA (k(0)-NAA), it is possible to correct for this effect through the introduction of k(0)-fission factors. In this work, the aim was to determine the feasibility of k(0)-NAA as a screening technique for accurate and isotopic ratio determination (we further refer to the technique as k(0)-UNAA) and where k(0)-UNAA could be positioned versus the other screening techniques. In k(0)-UNAA, we use an adapted formalism to determine the n(U-235)/n(U-238)-ratio. Several swipe samples spiked with different isotopic ratios were analyzed and demonstrate the feasibility of k(0)-UNAA as a accurate and reliable screening method for the quantification of environmental samples up to several tens of ng. At these levels the uranium concentration can be quantified with a reasonable uncertainty and reliable n(U-235)/n(U-238) isotopic ratio analysis could also be performed

Validation of the determination of tin by k0-instrumental neutron activation analysis in foodstuff

Several recent intercomparisons and certification exercises have shown that the determination of tin in e.g. food samples and plastics is not straightforward. k (0)-Instrumental Neutron Activation Analysis (k (0)-INAA) offers some intrinsic quality control opportunities for the determination of the tin content in samples since several monitoring radioisotopes are formed. In this work we will discuss the validation of the determination of tin by k (0)-INAA using different reference materials. The results show that only few of these radioisotopes are reliable in terms of accuracy and that from a survey of the tin content in a range of canned foods detection limits as low as 2 mg/kg can be achieved