Neutron Multiplicity Counting with the Analysis of Continuous Detector Signals
Doktorsavhandling, 2021

Neutron multiplicity counting is a non-destructive assay method for determining the mass of fissile materials (primarily plutonium) using the measured values of the singles, doubles and triples detection rates. Traditionally, the detection rates are obtained from the counting statistics of neutron detectors. The main problem with this approach is that it is sensitive to the overlapping of pulses which, especially at high count rates, lead to dead time losses in the counting electronics. This feature limits the applicability of the method to the measurement of samples with low emission intensities. To overcome this constraint, an alternative version of neutron multiplicity counting has been developed. The new approach is based on the direct analysis of the continuous voltage signals of the detectors (primarily fission chambers). Since the procedure does not rely on counting individual pulses, it is inherently free from dead time losses caused by their overlapping. As a result, the proposed method provides an alternative to traditional multiplicity counting, especially when measuring high intensity samples, like spent nuclear fuel. The thesis presents the complete process of establishing the new version of multiplicity counting. Based on a stochastic model of continuous detector signals, expressions are derived for some of their one- two and three-point (in time) moments (including their mean, covariance function and bicovariance function) and it is shown that the singles, doubles and triples detection rates can be recovered from them. In a computational study, detector signals are simulated and analysed in order to investigate the effect of certain parameters (the measurement time, the detection efficiency, the amplitude of the electronic noise and the intensity of non-neutron pulses) on the estimated values of the detection rates. To demonstrate the practical use of the proposed method, measurements have been performed using a Cf-252 source and the detection rates recovered from the moments of the recorded signals were compared with reference values obtained with pulse counting.

fission chamber

fissile material assay

neutron multiplicity counting

Kolmogorov equation

master equation

nuclear safeguards

Online via Zoom
Opponent: Honorary Professor Iván Lux, Hungarian Atomic Energy Authority, Hungary

Författare

Lajos Nagy

Chalmers, Fysik, Subatomär, högenergi- och plasmafysik

An extended theory of multiplicity counting from fission chamber signals in the current mode

International Conference on Mathematics and Computational Methods Applied to Nuclear Science and Engineering, M and C 2017,; (2017)

Paper i proceeding

Two- and three-point (in time) statistics of fission chamber signals for multiplicity counting with thermal neutrons

Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,; Vol. 929(2019)p. 148-155

Artikel i vetenskaplig tidskrift

Multiplicity counting using the two- and three point statistics of fission chamber signals – Theory and experimental demonstration

International Conference on Mathematics and Computational Methods Applied to Nuclear Science and Engineering, M and C 2019,; (2019)p. 2866-2875

Paper i proceeding

Multiplicity counting from fission chamber signals in the current mode

Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,; Vol. 839(2016)p. 92-101

Artikel i vetenskaplig tidskrift

Multiplicity counting from fission detector signals with time delay effects

Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,; Vol. 884(2018)p. 119-127

Artikel i vetenskaplig tidskrift

A new method to measure the mass of fissile materials

Fissile materials (e.g. uranium and plutonium) are used to generate electricity in nuclear power plants, however – with the wrong intentions - they can also be used to produce nuclear weapons. To ensure that they are used for peaceful purposes only, the International Atomic Energy Agency sends inspectors to nuclear facilities around the World regularly, who measure the mass of all the fissile materials present and compare the results with their own records.

Determining the mass of fissile materials requires special methods. One of them, called neutron multiplicity counting, utilizes a special property of these materials: they emit neutrons in groups of varying sizes at random times, which can be detected. When a neutron is detected, a voltage impulse is produced in the detector. The value of the mass can then be obtained from the number of pulses observed in a certain amount of time. In some cases, however, neutrons are detected so frequently, that pulses overlap and cannot be counted anymore.

To solve this problem, this thesis presents a new version of neutron multiplicity counting which, instead of counting individual pulses, relies on analyzing the statistical properties of the entire voltage signal of the detector. Besides describing the theoretical mathematical basis of this method, the thesis investigates its properties with computer simulations and demonstrates its use in an experiment.

Drivkrafter

Hållbar utveckling

Innovation och entreprenörskap

Ämneskategorier

Subatomär fysik

Styrkeområden

Energi

ISBN

978-91-7905-448-9

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4915

Utgivare

Chalmers tekniska högskola

Online via Zoom

Opponent: Honorary Professor Iván Lux, Hungarian Atomic Energy Authority, Hungary

Mer information

Senast uppdaterat

2021-06-14