A method is presented for the automatic identification of the elements present in a sample and the calculation of the corresponding
concentrations from the energies and peak areas determined by a spectrum analysis computer program. A preliminary interpretation
list is produced in which the possible isotopes are given for each peak in the spectrum. This list is based only on the gamma-ray
energies and half-lives of the isotopes. A careful analysis of this list yields groups of identified elements at four different
significance levels. The determination of the corresponding concentration is based on the single-comparator method. The procedure
is included in an automatic activation analysis system but can also be used separately.
The measuring system described in this paper, developed for non-destructive neutron activation analysis, consists of a semiconductor
detector gamma-ray spectrometer and a sample changer coupled to a PDP-9 computer via a CAMAC interface system. CAMAC modules
implemented in this system are an ADC interface, a sample changer control, display unit, a timer and a time-of-the-year clock.
The spectra are accumulated in a section of the computer memory. The computer is further used for experiment control and for
the analysis and interpretation of the measured gamma-ray spectra.
The procedure in use at our institute for the extraction of the desired element concentrations in a sample from the peak data
obtained by a spectrum analysis program, is described in detail. The method is based on the use of zinc as a single comparator
and takes into consideration primary activation products as well as their daughter isotopes. After assigning isotopes to spectrum
peaks on the basis of γ-ray energies, the list of possible isotopes is reduced to a list of present isotopes with their concentrations
using criteria based on half life, specificity and intensity of γ-rays. For elements not observed, detection limits are estimated.
The procedure has been used extensively during the last two years and has shown to produce reliable results.
At I.R.I. a new, fully automated facility for short half life INAA is being developed and installed at the Institutes 2 MW reactor. The fast rabbit transfer system is constructed only of plastic and carbonfiber parts, so that rabbit contamination is minimized. This system is automated in such a way that it can operate safely without direct supervisionn; the sequence of irradiations and measurements is optimized by a computer program for a given set of samples and analysis procedures. The rabbit system is controlled by an Apple IIe-computer connected to the central PDP 11/44 system of the Radiochemistry department. For a given set of samples and required analysis procedures (irradiation-, decay-, and measurement times) the central computer calculates an optimal sequence of individual actions (transfer from and to the reactor, sample storage or detector) to be carried out by the system. This sequence is loaded into the Apple-computer as a series of commands together with timing information. Actual control of the procedure occurs through the peripheral computer, which makes the system independent of delays or break-downs of the central multi-user computer system. Hardware, software and operating characteristics of the fast rabbit system will be discussed.
Practical application of oow energy gamma rays and X-rays in I.N.A.A. was restricted because of the complexity of the X-ray
spectrum and sample self-absorption. This paper describes a method for the calculation of sample self-absorption on the basis
of the actual sample spectra only, as measured with a high resolution semiconductor X-ray detector. In the 20–400 keV energy
range, the attenuation coefficient can be represented by a three parameter function of photon energy. This was verified by
measuring the transmission of photons of different energies through a range of materials. Experiments with neutron irradiated
U.S.G.S. standard reference materials with known major oxide composition showed that self-absorption thus calculated from
the observed spectra is in good agreement with the results of theoretical calculations based on known attenuation coefficients.
The system for routine instrumental neutron activation analysis, in use for several years at the IRI at Delft, has been evaluated.
Basis of this evaluation are: quality of the results, costs per analysis, capacity and ease of operation. A comprehensive
description of the analysis system and associated hardware and software is included.
In order to obtain reliable data about short-lived isotopes for use in thermal neutron activation analysis, experiments have
been carried out using a fast rabbit transfer system. Half-lives of 28 short-living isotopes have been measured by using a
counting system with a fixed dead-time. A Ge(Li) spectrometry system was used to determine the most important γ-ray energies
and intensities of these isotopes. For the half-lives an accuracy of better than 1% was attained, while for the γ-ray energies
the accuracy was 0.1 keV.
After 25 years of rapid development, neutron activation analysis is now in a period of maturing. In this period, the attention
has to be focussed on improvement of sensitivity, precision and accuracy without an appreciate increase of analysis costs,
and on decrease of the costs per analysis without affecting sensitivity, precision and accuracy. The present paper indicates
how already well known technical and physical possibilities can be used to reach this aim.
Authors:M. de Bruin, P. Korthoven, R. Duin, F. Groen, and C. Bakels
Simultaneous and non-destructive determination of many trace elements can play an important role in the identification of
the origin of samples in forensic investigations. However, when studying natural materials, great problems mostly arise in
the interpretation of the analysis data. This paper gives a description of a statistical analysis procedure applied to concentration
data obtained by non-destructive neutron activation analysis. The results obtained, when using this technique for the identification
of flint artifacts, are discussed.