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  • Author or Editor: I. Stamatelatos x
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Abstract  

A technique was developed for the identification of inhomogeneities in activity distribution and the correction of their effect on the interpretation of gamma spectrometry data in Large Sample Neutron Activation Analysis. The method was based on collimated gamma scanning using a germanium detector to obtain the activity pattern in the bulk sample and Monte Carlo simulations in order to correct the experimental data for the effect of the inhomogeneous activity distribution. The method was experimentally evaluated in the case of a large cylindrical reference sample of 2 L in volume containing quartz as matrix material and a known source of radioactivity and an excellent agreement was observed. The discussed technique improves the trueness of quantitative analysis of large samples with inhomogeneous activity distribution.

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Abstract  

We investigated the spatial dependence of the sensitivity of nitrogen measurement with a prompt gamma neutron activation analysis (PGNAA) system for small animals and developed an analysis procedure that permits the reduction of systematic errors due to that dependence. The analysis procedure is based on neutron and photon transport calculations performed using the MCNP code in order to evaluate the sensitivity of the PGNAA facility. The system can be calibrated experimentally using a small number of phantoms of known size and composition. The calculation approach can then be used to predict responses for animal body sizes and shapes relatively to those experimentally determined and to include the effect of tissue inhomogeneities. Our calculations were verified by experimental measurements performed for a set of cylindrical inhomogeneous phantoms. The calculated to experimental ratios observed were within 6%.

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Abstract  

A large sample neutron activation analysis (LSNAA) facility is under development at GRR-1 research reactor, NCSR ‘Demokritos’, to perform multi-element, non-destructive, contamination-free analysis of large volume samples. Correction algorithms have been derived to account for thermal neutron and gamma-ray self-attenuation in macroscopically homogeneous samples, as well as the photon detection efficiency to voluminous samples, based on no prior knowledge of the sample matrix composition. In the present study Monte Carlo simulations were performed to estimate the influence of inhomogeneities of major (matrix) and trace element on the accuracy of the technique. Types of inhomogeneities that can lead to severe errors in the analysis were depicted. The potential of including inhomogeneity tests in the measuring procedure to ensure the method’s applicability was examined.

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Abstract  

Large Sample Neutron Activation Analysis (LSNAA) was applied to perform non-destructive elemental analysis of a ceramic vase. Appropriate neutron self-shielding and gamma ray detection efficiency calibration factors were derived using Monte Carlo code MCNP5. The results of LSNAA were compared against Instrumental Neutron Activation Analysis (INAA) results and a satisfactory agreement between the two methods was observed. The ratio of derived concentrations between the two methods was within 0.7 and 1.3. Estimation of the activity level decay with time showed that the vase could be released from regulatory control at about 3 months post-irradiation. This study provided an analytical procedure for bulk sample analysis of precious and archaeological objects that need to be preserved intact and cannot be damaged for sampling purposes.

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Abstract  

A benchmark experiment was performed for Neutron Activation Analysis (NAA) of a large inhomogeneous sample. The reference sample was developed in-house and consisted of SiO2 matrix and an Al–Zn alloy “inhomogeneity” body. Monte Carlo simulations were employed to derive appropriate correction factors for neutron self-shielding during irradiation as well as self-attenuation of gamma rays and sample geometry during counting. The large sample neutron activation analysis (LSNAA) results were compared against reference values and the trueness of the technique was evaluated. An agreement within ±10% was observed between LSNAA and reference elemental mass values, for all matrix and inhomogeneity elements except Samarium, provided that the inhomogeneity body was fully simulated. However, in cases that the inhomogeneity was treated as not known, the results showed a reasonable agreement for most matrix elements, while large discrepancies were observed for the inhomogeneity elements. This study provided a quantification of the uncertainties associated with inhomogeneity in large sample analysis and contributed to the identification of the needs for future development of LSNAA facilities for analysis of inhomogeneous samples.

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Abstract  

Serum aluminum levels were determined by instrumental neutron activation analysis in 31 patients undergoing long-term haemodialysis. Aluminum-28 1.778 MeV (T 1/2=2.24 min) γ-rays produced by the thermal neutron reaction27Al(n,γ)28Al were detected. Successive irradiation of the samples at epithermal neutron fluence was performed to correct for the interference from the fast neutron reaction31P(n,α)28Al. Serum aluminum level in this group of subjects was adequately represented by a lognormal distribution with a mean and variance of 16.5 μg/l and 16.8 μg/l, respectively. The results obtained were found to be in agreement with serum aluminum determination performed by electrothermal atomic absorption spectrophotometry (r 2=0.97). Instrumental neutron activation can provide a rapid technique to routinely monitor long-term haemodialysis patients in order to identify individuals at greater risk to develop aluminum toxicity.

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