The facilities for instrumental neutron activation analysis in Delft are described. Technical details of the fast rabbit systems, the normal pneumatic rabbit system, the large sample facility and the various coaxial and well-type Ge-detector spectrometers and sample changers are given. The capacity of the facilities is in the order of 15,000 samples per annum for full multi-element analysis. The capacity of the fast rabbit systems for INAA for use with short half-life radionuclides is even larger. The facilities are accessible for use by scientists from other establishments and countries as well.
The increased need of NAA laboratories to find beneficiaries and external funding to ensure the exploration costs requires a new view on the organization of the research and development. A higher degree of professionalism may be attained via introduction of quality management principles, e.g., via a project management of the research task. Guidance documents have been developed for this, and are being discussed. Examples are given of the use of quality- and project-management in the research activities of the NAA laboratory in Delft. Subjects discussed are internal training and qualification, planning, sources of error and documentation.
Increasingly govemmental bodies and industry require that supporting analytical laboratories have their quality assurance program implemented in a quality system by international standards such as derived from the ISO-25 guide. Neutron activation analysis (NAA) laboratories may have to deal with this trend too. In universities and research laboratories the need for it, and the implications of total quality management system are sometimes misconceived by unfamiliarity with the issue. The laboratory for INAA in Delft has been accredited for its quality system since 1993. Some of the tangible improvements since the introduction of quality management are presented. Four strategical considerations are given to consider the introduction of quality management at NAA laboratories, viz. with respect to the role of NAA for the validation of other methods, the role of NAA in the certification of reference materials, the preservation of knowledge and the acceptance of NAA as a respectable method.
Expressions have been derived which link detection limits in INAA with the Ge-detector specifications, provided by the vendors: relative efficiency, peak-to-Compton ratio and resolution. Situations have been distinguished in which a peak has to be detected on a Compton background, or on a natural background. The expressions allow for a direct indication of the improvement in detection limits, or the shortening of tumaround-time to attain equal detection limits, when a given detector is replaced by a detector with better specifications.
Detection limit improvement factors have been calculated to predict the improvement in detection limits when replacing a traditional 20% coaxial detector by either a 100% coaxial detector, a well-type detector or by equipping the detector with a Compton suppression shield. It is show that, when measuring sources on the end-cap of the detector, a system with a Compton suppression shield and a 100% detector have almost equal potentials. But when e.g. geometrical consideration larger source-to-end-cap distances are required with coaxial detectors, the well-type detector offers the best outlook for attaining better detection limits. The calculated predictions have been compared with experimental observations and the results agree well.
Many plastic products have relatively short life cycles. Upon final destruction, inorganic additives used for pigmentation, as polymer stabilizer or as flame retarding agent are being released to the environment. In a rapidly increasing way governmental authorities are setting limits to the use of inorganic additives, in particular cadmium, in plastics. INAA has attractive characteristics which may make authorities decide to select it for the control programs in relation to the enforcement of pollution standards. In this paper, an evaluation is made of the use of INAA for such analysis; analysis protocols, sensitivities, and observed levels of other trace elements are being discussed.
Quality control, as applied in normal activation analysis by the simultaneous analysis of well-characterized quality control
samples, blanks and sometimes duplicates, cannot fully be applied in large sample analysis. Well characterized control samples
are, e.g., not available at the size of large samples. Different approaches have to be developed to monitor and to control
sources of errors in this new type of chemical analysis. Some of the measured sample parameters dealing with gamma-ray and
neutron attenuation can only vary between well known values of elemental constants. These parameters can much easier be determined
in large sample analysis than with samples in the milligram range, thus offering an outlook for direct verification of the
quality of the related correction algorithms. Some examples are given here from experience with the kg-scale neutron activation
analysis at the facilities in Delft.
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.
An epithermal neutron activation analysis (ENAA) procedure has been evaluated in which samples packed in polyethylene capsules are irradiated during 15 minutes only, and induced activities are counted using well-type Ge-spectrometry. The evaluation was carried out in order to improve on ease of operation in ENAA. Biological and sediment reference materials have been analyzed. Compared to routine INAA, an improvement in detection limits was observed for As, Au, Cd, Mo, Ni, Sb, Sm, Sr, Ta, U, W and Zn. By Au–Zr neutron flux monitors, epithermal flux gradients have been determined. Concentrations found in the reference materials were generally in agreement with certified and consensus values.
The definitions used for the k0-constant, the coincidence correction factorc and the detector efficiency
f in the k0-method for NAA provide no means to interprete or correct for interference by artificial peaks. In this paper, extended descriptions for the detector efficiency are proposed to deal with escape peaks. For sum peaks caused by true coincidence, a k1-constant is defined as an alternative for the k0-constant, by separating the k0 into a part related to activation and a part related to spectrometry. The k1-constant is based on experimental data, just like the k0-constant.