An approach is presented for the performance control of -ray spectrometers allowing for the identification of sources of malfunctioning. The traditional check consists of inspection of the FWHM of the peaks of the control source, their peak positions and areas followed by an energy calibration. The resolution and shape of a peak in the spectrum originating from an applied pulse generator are more responsive to the electronic noise than the -ray peaks. At the laboratory for INAA in Delft, the traditional energy calibration has been replaced by a spectrometer performance control which complements the traditional method by (1) an analysis of the shape of the pulser peak, (2) an inspection of the low energy and high energy tails of the peaks, (3) specification of quantified criteria for approval of the performance, (4) generation of control charts for visual inspection of the parameters determined and (5) extensive documentation of all operational characteristics for eventual correlations. This paper demonstrates how this approach to SPC enables to identify sources which cause malfunctioning of the spectrometer.
The laboratory for INAA in Delft processes several thousands of samples per year for multielement determinations in a variety
of matrices. Samples are measured on different spectrometers (using well-type and coaxial detectors), by different persons
and using many different analytical protocols. All these results should be consistent with each other with respect to the
degree of accuracy, i.e., the combination of trueness and precision. A rigorous internal quality control program has been
implemented with automatic on-line evaluation. Annually an additional evaluation of the internal quality control results is
carried out using statistical techniques. The Naji-plot approach has proven to be an important graphical tool since it provides
direct insight in both trueness and precision. It is demonstrated that the degree of accuracy of the results obtained under
the large variety of operational parameters is under constant improvement.
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.
Authors:P. Bode, M. Bueno, K. Goraieb, and M. Koster-Ammerlaan
The Compton continuum in large sample neutron activation analysis has a measurable contribution from scattering of gamma-rays
in the sample itself besides from scattering in the detector. The continuum, therefore, contains information on the sample’s
composition, which may be made available by chemometrics. This hypothesis was tested on four types of animal fodder with similar
amounts of mineral supplements. First results indicate indisputable discrimination of the sample types if using peakless parts
of the gamma-ray spectra of the natural radioactivity of the materials as well as of those obtained after neutron activation
of 1 kg samples. It indicates that the valuable information on differences in, e.g., organic constituents may be obtained
by analyzing the Compton continuum.
Authors:I. Stamatelatos, F. Tzika, T. Vasilopoulou, and M. Koster-Ammerlaan
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.
Authors:T. Vasilopoulou, F. Tzika, M. Koster-Ammerlaan, and I. Stamatelatos
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.
Authors:M. Vasconcellos, P. Bode, G. Paletti, M. Catharino, A. Ammerlaan, M. Saiki, D. Fávaro, A. Byrne, R. Baruzzi, and D. Rodrigues
Biomonitoring of mercury contamination of Brazilian Indian population groups living in the Xingu Park, a reservation situated in the Amazonic region, has revealed very high levels of mercury in hair samples as compared to controls. Total mercury was determined by INAA in most of the tribes living in the Park and methylmercury was determined by CVAAS in samples with total mercury above 10 mg/kg. Due to the fact that selenium seems to protect animals against the toxic effects of methylmercury, it was considered also of interest to determine its concentrations in the hair samples with very high mercury levels. Selenium was determined by INAA via the short-lived radionuclide 77mSe (T1/2 = 17.45 s). The correlations between selenium and mercury concentrations in Brazilian controls and in the Indian population groups are discussed.