| 641. |
- Elter, Zsolt, 1987, et al.
(författare)
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Performance of Higher Order Campbell methods, Part I: review and numerical convergence study
- 2016
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Ingår i: Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 821, s. 66-72
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Tidskriftsartikel (refereegranskat)abstract
- This paper investigates, through numerical simulations, the performance of a signal analysis method by which a high temperature fission chamber can be used over a wide range of count rates. Results reported in a previous paper (Elter et al., 2015 [I]) indicated that the traditional Campbell method and the pulse mode cannot provide a sufficient overlap at medium count rates. Hence the use of the so-called Higher Order Campbell (HOC) methods is proposed and their performance is investigated. It is shown that the HOC methods can guarantee the linearity (i.e. correctness) of the neutron flux estimation over a wide count rate, even during transient conditions. The capabilities of these methods for suppressing parasitic noise (originating from various sources) are verified.
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| 642. |
- Elter, Zsolt, 1987, et al.
(författare)
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Performance of Higher Order Campbell methods, Part II: calibration and experimental application
- 2016
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Ingår i: Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 835, s. 86-93
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Tidskriftsartikel (refereegranskat)abstract
- Applying Higher Order Campbelling methods in neutron flux monitoring with fission chambers is advantageous due to their capabilities to suppress the impact of unwanted noises and signal contributions (such as gamma radiation). This work aims to verify through experimental results that the basic assumptions behind the Higher Order Campelling methods are valid in critical reactors. The experiments, reported in this work, were performed at the MINERVE reactor in Cadarache. It is shown that the calibration of a fission chamber and the associated electronic system is possible in higher order mode. With the use of unbiased cumulant estimators and with digital processing, it is shown that over a wide count rate range, accurate count rate estimation can be achieved based on signal samples of a few ms, which is a significant progress compared to similar experimental results in the literature. The difference between the count rate estimated by pulse counting and by the Higher Order Campelling is less than 4%. The work also investigates the possibility of monitoring transient events. For this purpose, a control rod drop event was followed in Higher Order Campbelling mode.
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| 643. |
- Elter, Zsolt, 1987, et al.
(författare)
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Remark on the applicability of Campbelling techniques for transient signals
- 2016
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Ingår i: Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 813, s. 10-12
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Tidskriftsartikel (refereegranskat)abstract
- The signals of fission chambers are traditionally evaluated with Campbelling methods at medium count rates. Lately there has been a growing interest in the extension of Campbelling methods to cover a wider range of count rates. These methods are applied to measure the neutron flux in the stationary state of the reactor. However, there has not been any attempt to generalize these techniques for transient reactor states. This short note is devoted to a discussion of this question. It is shown through analytic and numerical calculations that for practical reasons the traditional, stationary Campbelling methods can be applied for transient scenarios as well. (C) 2016 Elsevier B.V. All rights reserved.
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| 644. |
- Enqvist, Andreas, 1981, et al.
(författare)
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Calculation of the light pulse distributions induced by fast neutrons in organic scintillation detectors
- 2010
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Ingår i: Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 618:1-3, s. 266-274
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Tidskriftsartikel (refereegranskat)abstract
- We develop a fully analytic and self-contained description of the amplitude distribution of light pulses in an organic scintillation detector due to a monoenergetic source of fast neutrons. To this end, two classes of problems have to be handled. One is a formula for the light pulse amplitude distribution for the complete life history of neutrons slowing down in a mixture of hydrogen and carbon as a statistical average over all collision sequences that can occur, accounting also for neutron leakage. A complete solution is given in terms of a non-recursive convolution integral expansion with respect to the various possible collision histories. These latter are dependent on the collision probabilities of neutrons of a given energy. The second is the calculation of this collision probability from analytical expressions for the geometry of the detector, in the present case a right cylinder. This quantity was taken from Monte Carlo simulations in previous work. Recursive formulae are derived for the probabilities of arbitrary collision sequences, and quantitative results are given for up to five consecutive collisions of all combinations. These probabilities can be used to determine how to truncate the non-recursive expansion of the full light amplitude distribution in quantitative work. The calculational method serves to lend insight and understanding into the structure of the pulse height spectra, as well as it provides a computationally cheap method of generating a large number of such spectra for various detector compositions, sizes and neutron energies, for the development and test of new spectrum unfolding techniques. (C) 2010 Elsevier B.V. All rights reserved.
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| 645. |
- Enqvist, Andreas, 1981, et al.
(författare)
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Initial Evaluation for a Combined Neutron and Gamma 1 Ray Multiplicity Counter
- 2010
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Ingår i: Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 621:1-3, s. 493-497
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Tidskriftsartikel (refereegranskat)abstract
- Multiplicity counters for neutron assay have been extensively used in materials control and accountability for nonproliferation, and nuclear safe- guards. Typically, neutron coincidence counters are utilized in these fields. In this paper we present a measurement system that makes use not only of neutron (n) multiplicity counting but also of gamma ray (γ) multiplicity counting and the combined higher-order multiples containing both neutrons and gamma rays. The benefit of this approach is in using both particle types available from the sample, leading to a reduction in measurement time when using more measurables. We present measurement results of n, γ, nn, nγ, γγ, nnn, nnγ, nγγ and γγγ multiples emitted by a 252Cf source and a 239Pu-Be19 source. A dual radiation measuring system proposed here could use extra measurables either to improve the statistics when compared to a neutron-only system, or alternatively be used for extended analysis and interpretation of sample parameters.The study presented here provides an answer to the questions regarding the ability to measure both neutron and gamma ray coincidences at once. The results show that the measurements system proposed in this study has a potential to be valuable in the area of nuclear nonproliferation and homeland security.
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| 646. |
- Enqvist, Andreas, 1981, et al.
(författare)
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Measurement and simulation of neutron/gamma-ray cross-correlation functions from spontaneous fission
- 2008
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Ingår i: Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 595:2, s. 426-430
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Tidskriftsartikel (refereegranskat)abstract
- For the first time, a technique is presented for the measurement of total and separate neutron and gamma-ray cross-correlation functions from a spontaneous fission source. The cross-correlation functions are unique for given material-geometry configuration, thus represent signatures that can be used for the identification of radioactive materials. The measurement technique allows for the collection of fast coincidences within a time window of the order of a few tens of nanoseconds. A digital pulse shape discrimination technique is used, which allows for the accurate acquisition of the coincidences in all particle combinations. Specifically, separate neutron–neutron, neutron–gamma-ray, gamma-ray–neutron, and gamma-ray–gamma-ray coincidences are acquired with two liquid scintillation detectors. The measurements are compared to results obtained with the MCNP-PoliMi code, which simulates neutron and gamma-ray coincidences from a source on an event-by-event basis. This comparison leads to relatively good qualitative agreement.The measurements and simulations of the separate neutron and gamma-ray contributions to the total cross-correlations provide new signatures that can be obtained using existing experimental systems built to accurately identify nuclear materials. This research has direct applications in the areas of nuclear nonproliferation and homeland security.
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| 647. |
- Enqvist, Andreas, 1981, et al.
(författare)
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Sample characterization using both neutron and gamma multiplicities
- 2010
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Ingår i: Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 615:1, s. 62-69
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Tidskriftsartikel (refereegranskat)abstract
- Formulas for multiplicity counting rates (singles, doubles, etc.), used for the unfolding of parameters of an unknown sample, can be derived from those for the corresponding factorial moments. So far such rates were derived only for neutrons. The novelty of this paper is related to the derivation of the individual gamma and mixed neutron-gamma detection rates as well as to investigation of the possibilities and actual algorithms for the sample parameter unfolding. Taking the individual gamma and mixed neutron-gamma moments up to third order, together with the neutron moments, there will be nine auto- and cross-factorial moments and corresponding multiplicity rates, as well as a larger number of unknowns than for pure neutron detection. The total number of measurable multiplicities exceeds the number of unknowns, but on the other hand the structure of the additional equations is substantially more complicated than that of the neutron moments. Since an analytical inversion of the highly non-linear system of over-determined equations is not possible, the use of artificial neural networks (ANNs) is suggested, which can handle both the non-linearity and the redundance in the measured quantities in an effective and accurate way. The use of ANNs is demonstrated with good results on the unfolding of various combination of multiplicities for certain combinations of the unknown parameters, including the sample fission rate, the leakage multiplication and the alpha and gamma ratios. (C) 2010 Elsevier B.V. All rights reserved.
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| 648. |
- Enqvist, Andreas, 1981, et al.
(författare)
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The Detection Statistics of Neutrons and Photons Emitted from a Fissile Sample.
- 2009
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Ingår i: Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 607:2, s. 451-457
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Tidskriftsartikel (refereegranskat)abstract
- The purpose of this paper is to present new analytical derivations to describe the emission and detection statistics of neutrons and photons generated in and emitted from fissile samples, with absorption included. The results of the analytical approach are compared with and validated by corresponding Monte Carlo simulations. The joint statistics of the generated and detected neutrons and photons are also described. The analytical model described in this paper accounts for absorption and detection, thus extending the model presented in previous studies. By using this new, improved model, one can investigate the relative feasibilities of measuring neutrons, gamma photons or combinations thereof, for the analysis of a specific fissile sample. It is seen that for larger mass samples photon absorption in the sample strongly decreases the multiplicity of emitted photons, whereas this is not the case for neutrons. The model correctly accounts for this strong self-shielding effect and confirm previous observations that although photons have a larger initial (source) multiplication, neutrons might be more favourable to measure in the case of large samples because of the increasing self-shielding effect for gamma photons.
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| 649. |
- Enqvist, Andreas, 1981, et al.
(författare)
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The number distribution of neutrons and gamma photons generated in a multiplying sample
- 2006
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Ingår i: Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 566:2, s. 598-608
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Tidskriftsartikel (refereegranskat)abstract
- The subject of this paper is an analytical derivation of the full probability distribution of the number of neutrons and photons generated in a sample with internal multiplication by one source emission event (spontaneous fission), and its comparison with Monte Carlo calculations. We derive recursive analytic expressions for the probability distributions P(n) up to values of n=N for which the cumulative probability Click to view the MathML source is equal to or larger than 0.99. The derivation is achieved using the symbolic computation language Mathematica. With the introduction of a modified factorial moment of the number of neutrons and gamma photons generated in fission, the resulting expressions could be brought to a formally equivalent form with those for the factorial moments of the total number of neutrons and photons generated in the sample. The results were compared to Monte Carlo calculations, and excellent agreement was found between the analytical results and the simulations. The results show that the probability distributions change with increasing sample mass in such a way that the ?bulk? of the distribution changes only slightly, but a tail develops for higher n values. This tail is the main reason for the increase of the factorial moments with increasing sample mass, an effect that was observed in earlier studies.
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| 650. |
- Eriksson, L, et al.
(författare)
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An investigation of sensitivity limits in PET scanners
- 2007
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Ingår i: NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT. - : Elsevier BV. - 0168-9002. ; 580:2, s. 836-842
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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