This paper reports on the last year's two major activities of our nuclear instrumentation group it the field of high rate and high resolution gamma spectrometry which were mainly devoted to the needs of activation analysis of short-lived nuclides. The first of the projects was the completion of a state-of-the-art spectrometry system for very high counting rates which has been installed at the fast inrradiation and transport facility of the TRIGA reactor and now is the main instrument for the short-lived work of our radiochemistry group. Based on a laboratory-designed gated integrator pulse processing system and equipped with an Ortec Gamma-X detector of 20% relative efficiency with cooled FET and transistor reset preamplifier, it exhibits a basic resolution of 2.3 keV at 1332 keV which at a counting rate of 1.1 million cps of60Co degrades to 3.4 keV. An essential feature of the system is a novel quantitative pileup rejector of the pulse counting type which has been specially designed for high rejection efficiency and at the same time, for the reliable exemption of valid events, and thus is a necessary prerequisite for quantitative real-time correction of counting losses by means of the Virtual Pulse Generator method. The second project included the successful implementation of the novel Preloaded Filter Technique (applied for patent), a new method for high resolution and high throughput processing of nuclear detector signals which, in contrast to conventional techniques, does not rely on a fixed pulse processing time per event which up to now was the main reason for pulse pileup and limited throughput, but, at the latest, terminates the filtering process of an individual event at the instant of arrival of the next event which results in optimized throughput and, at the same time, in a self-adapting, counting rate dependent shaping time. To that aim, the delta-noise filter of the system must be preloaded with the best estimate of the final result of the filtering process which is simply the unfiltered signal amplitude, to make sure that at the instant of termination of the filtering process the output of the filter deviates from the final value not more than by the decaying noise amplitude. Complemented by counting rate dependent step-noise filtering, this technique made possible the creation of a spectrometry system for all purposes which at low to medium counting rates is comparable to the best of the semi-Gaussian amplifiers and at high counting rates to the gated integrator. An experimental implementation of the Preloaded Filter combined with an Ortec Gamma-X detector of 15% relative efficiency resulted in a basic resolution of 1.9 keV at 1332 keV at a counting rate of 5000 cps slowly degrading to 3.2 keV at a counting rate of 650 000 cps of60Co.
A new counting geometry with a simple sample changer was constructed to enable cyclic and pseudocyclic short-time activation
analysis. With the new system it is possible to cycle a sample, or successively an indefinite number of samples up to 20 times.
The sample changer acts at the same time as sample catcher for two n-type HPGe detectors and can release the sample into a
well-type HPGe detector. The new system enables the simultaneous counting of the irradiated samples by means of two endcap
HPGe detectors, and subsequent counting by means of the well HPGe detector or both detector types. A well detector ensures
a high counting efficiency which improves the sensitivity of a large number of short lived nuclides. Some standard reference
materials (i.e., BCR-176, NIST SRM 1633b, IAEA-336, 335b, 335c) were prepared and analysed in replicates. The results indicate
that up to 46 nuclides can be determined in BCR-176 if the samples are irradiated with and without the6LiD converter. An automatic evaluation programme was developed that determines the FWHM calibration parameters for each spectrum
for accurate peak-area estimation at high count rates.
A simple but highly effective rabbit system is made from an aluminum in-core part, inexpensive plastic tubing and an industrial
compressed air generator. Large sample containers have a volume of 25 ml. Smaller containers of 5 ml automatically separate
from a transport capsule. A transport time of below 0.5 second enables fast neutron activation analysis (FNAA). A software
implementation of a Loss-Free Counting multi-channel analyzer, storing immediately into the multi-megabyte memory of a low-cost
486 or Pentium type PC, enables the real-time control of a rabbit system as well as the collection of up to 1000 pairs of
simultaneously recorded loss-corrected and non-corrected spectra of 16 k channels each, in a true sequence without time gaps
in between, at throughput rates of up to 200 kc/s. By automatically adapting the noise filtering time to individual pulse
intervals, the Preloaded Digital Filter (PLDF) combines low- to medium-rate resolutions comparable to those of high-quality
Gaussian amplifiers with throughput rates of up to 100 kc/s, and high-rate resolutions superior to those of state-of-the-art
gated integrator systems. The combination of rabbit system, PLDF and software based multi-channel analyzer provides a low-cost
but powerful solution for NAA at Triga reactors in developing countries.
A system for Automatic NAA is based on a list of specific saturation activities determined for one irradiation position at
a given neutron flux and a single detector geometry. Originally compiled from measurements of standard reference materials,
the list may be extended also by the calculation of saturation activities from k0 and Q0 factors, and f and a values of the irradiation position. A systematic improvement of the SRM approach is currently being performed by pseudo-cyclic
activation analysis, to reduce counting errors. From these measurements, the list of saturation activities is recalculated
in an automatic procedure.
Authors:G. Westphal, G. Cadek, N. Kerö, Th. Sauter, and P. Thorwartl
Adapting it's processing time to the respective pulse intervals, the Preloaded Filter (PLF_ pulse processor offers optimum resolution together with highest possible throughput rates. The PLF algorithm could be formulated in a recursive manner which made possible it's implementation by means of a large field-programmable gate array, as a fast, pipe-lined digital processor with 10 MHz maximum throughput rate. While pre-filter digitization by an ADC with 12 bit resolution and 10 MHz sampling rate resulted in a poorer resolution than that of an analog filter, a digital PLF based on an ADC with 14 bit resolution and 10 MHz sampling rate, surpassed high-quality analog filters in resolution, throughput rate and long-term stability.
Authors:G. Westphal, K. Jöstl, B. Lipp, and P. Schröder
Combined with a new, low dead-time transistor reset preamplifier, the Preloaded Filter pulse processor offers up to 49% higher throughput rates than a conventional gated integrator system, at resolutions which are better by 0.3 to 0.4 keV. Low rate resolution of the Preloaded Filter is better than that of the gated integrator by more than 40%.
Authors:G. Westphal, F. Grass, Th. Kasa, and J. Schmidt
As a basic step to quality assurance of INAA via shortlived isomeric transitions replicate analyses of NaCl are performed on an INAA facility specifically designed for this purpose and from the series of net peak areas of Na-24m (20 ms) and Cl-38m (715 ms) it can be shown that the system is quantitative and that its precision is determined by counting statistics. Future improvements of the method may be expected from gamma spectrometry systems with higher throughput and better resolution than that of the now used gated integrator. A resolution comparison between a gated integrator and the recently developed Preloaded Filter is given on the basis of the detector applied in this investigation and substantial improvements are examplified.
Authors:F. Grass, G. Westphal, H. Lemmel, and J. Sterba
Exhaust systems of modern cars contain catalysts for the reduction of CO, NOx and hydrocarbons. These catalysts are made of ceramic materials with a large surface on which platinum metals catalyse the
oxidation. The catalysts contain approximately 2 g of platinum and 0.4 g of rhodium. Recently platinum is being replaced by
palladium. During driving the platinum-group elements (PGEs) are expelled from the tip in fine particles and are deposited
in the environment. For a projected study of emissions from cars driven on streets and highways it is important to know which
elements can be measured by short time activation analysis without any chemical procedure.
Authors:G. Westphal, F. Grass, H. Lemmel, and J. Sterba
A system for rapid automatic neutron activation analysis is governed by software performing irradiation control, neutron flux
monitoring and gamma-spectrometry with real-time correction of counting losses as well as spectra evaluation, nuclide identification
and calculation of concentrations in a fully automatic flow of operations.1,2 Elemental concentrations are derived from a list of experimentally determined specific saturation activities. To expand this
list, the “k0_IAEA” software3 is presently under evaluation and will be reported on in this paper. At a Triga reactor, reactor pulse activation may enhance
the sensitivity for very short half-lives,4 and will be presented in our paper.
Authors:J. Sterba, M. Blaauw, G. Steinhauser, M. Bichler, F. Grass, and G. Westphal
Combining the powers of a fast pneumatic transport system and the Automatic Activation Analyzer (AAA) of the Atominstitut
in Vienna with the newest version of the IAEA k0-Software, the application of the k0-method to the determination of short-lived radionuclides becomes easily possible. By calculating Asp-values with the IAEA
software, the often expensive and time-consuming measurement of Asp-values using certified reference materials is reduced
to quality control checks. Measurements clearly show that the two approaches are equivalent, especially since both take self-absorption
and neutron self-shielding into account. In this way it is possible to expand the library of the AAA with many hitherto unobtainable
Asp-values. At the same time, using highly accurate Asp-values already measured for many short-lived radionuclides, k0-values for those can be produced with a simple procedure.