| 1. |
- Al-Adili, Ali, et al.
(författare)
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Neutron-multiplicity experiments for enhanced fission modelling
- 2017
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Ingår i: EPJ Web of Conferences. - : EDP Sciences. - 9782759890200
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Konferensbidrag (refereegranskat)abstract
- The nuclear de-excitation process of fission fragments (FF) provides fundamental information for the understanding of nuclear fission and nuclear structure in neutron-rich isotopes. The variation of the prompt-neutron multiplicity, ν(A), as a function of the incident neutron energy (En) is one of many open questions. It leads to significantly different treatments in various fission models and implies that experimental data are analyzed based on contradicting assumptions. One critical question is whether the additional excitation energy (Eexc) is manifested through an increase of ν(A) for all fragments or for the heavy ones only. A systematic investigation of ν(A) as a function of En has been initiated. Correlations between prompt-fission neutrons and fission fragments are obtained by using liquid scintillators in conjunction with a Frisch-grid ionization chamber. The proof-of-principle has been achieved on the reaction 235U(nth,f) at the Van De Graff (VdG) accelerator of the JRC-Geel using a fully digital data acquisition system. Neutrons from 252Cf(sf) were measured separately to quantify the neutron-scattering component due to surrounding shielding material and to determine the intrinsic detector efficiency. Prelimenary results on ν(A) and spectrum in correlation with FF properties are presented.
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| 2. |
- Jansson, Kaj, 1987-, et al.
(författare)
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Measurement of the 6Li(n,α)t neutron standard cross-section at the GELINA facility
- 2017
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Ingår i: ND 2016. - Les Ulis : EDP Sciences. - 9782759890200
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Konferensbidrag (refereegranskat)abstract
- The Li-6(n,alpha)t reaction cross-section is an established standard due to its relatively high crosssection as well as its high Q-value. However, it is only considered a neutron standard up to 1 MeV, because in the neutron energy region 1-3 MeV there exist discrepancies of several per cents between recent measurements [1,2] and evaluated data files [3]. It has been speculated [4] that neglecting of the particle leaking effect might be part of the explanation why there is a disagreement in this region. Based on R-matrix calculations, in the region around 2 MeV, one also expects three excitation levels of Li-7 to significantly influence the cross section [5]. In order to resolve these discrepancies, we perform measurements at the GELINA facility at JRC-Geel with two Frisch-gridded ionisation chambers. The Li-6(n,alpha)t cross section is measured relative to the U-235(n,f) standard. In order to solve previous encountered problems [6], the setup has been modified and moved to a new flight path station. In this proceeding we show that several problems have been eliminated and discuss possible solutions to newly arisen problems, due to the changed experimental conditions. Preliminary results from new data taken during 2016 with the updated setup are presented.
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| 3. |
- Jansson, Kaj, 1987-
(författare)
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Measurements of Neutron-induced Nuclear Reactions for More Precise Standard Cross Sections and Correlated Fission Properties
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is difficult to underestimate the importance of neutron cross section standards in the nuclear data field. Accurate and precise standards are prerequisites for measuring neutron cross sections. Two different projects are presented here with the aim of improving on neutron standards.A simulation study was performed for an experiment intended to measure the cross sections of H(n,n), 235U(n,f), and 238U(n,f) relative to each other. It gave the first estimates of the performance of the experimental setup. Its results have aided the development of the experimental setup by setting limits on the target and detector design.A second neutron-standard project resulted in three measurements of 6Li(n,α)t relative to 235U(n,f). Each subsequent measurement improved upon the previous one and changed the experimental setup accordingly. Although, preliminary cross sections were agreeing well with evaluated data files in some energy intervals, the main goal to measure the cross section up to 3 MeV was not reached.Mass yields and energy spectra are important outcomes of many fission experiments, but in low yield regions the uncertainties are still high even for recurrently studied nuclei. In order to understand the fission dynamics, one also needs correlated fission data. One particular important property is the distribution of excitation energy between the two nascent fission fragments. It is closely connected to the prompt emission of neutrons and γ’s and reveals information about how nucleons and energy are transferred within the fissioning nucleus.By measuring both the pre and post neutron-emission fragment masses, the cumbrance of detecting neutrons directly is overcome. This is done using the fission spectrometer VERDI and the 2E-2v method. In this work I describe how both the spectrometer, the analysis method, and the calibration procedures have been further developed. Preliminary experimental data show the great potential of VERDI, but also areas that call for more attention. A previously overlooked consequence of a central assumption was found and a correction method is proposed that can correct previously obtained data as well.The last part of this thesis concerns the efficiencies of the fission product extraction at the IGISOL facility. The methodology of the fission yield measurements at IGISOL are reliant on assumptions that have not been systematically investigated. The presented work is a first step of such an investigation that can also be used as a tool for optimising the setup for measurements of exotic nuclei. A simulation framework connecting three different simulation codes was developed to investigate the produced yield of fission products in a buffer gas. Several different variants of the setup were simulated and the findings were generally accordant with previous estimates. A reasonable agreement between experimental data and the simulation results is demonstrated.
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| 4. |
- Jansson, Kaj, 1987-, et al.
(författare)
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Simulated production rates of exotic nuclei from the ion guide for neutron-induced fission at IGISOL
- 2017
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Ingår i: European Physical Journal A. - : Springer. - 1434-6001 .- 1434-601X. ; 53:12
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Tidskriftsartikel (refereegranskat)abstract
- An investigation of the stopping efficiency of fission products, in the new ion guide designed for ion production through neutron-induced fission at IGISOL in Jyväskylä, Finland, has been conducted. Our simulations take into account the new neutron converter, enabling measurements of neutron-induced fission yields, and thereby provide estimates of the obtained yields as a function of primary proton beam current. Different geometries, targets, and pressures, as well as models for the effective charge of the stopped ions were tested, and optimisations to the setup for higher yields are suggested. The predicted number of ions stopped in the gas lets us estimate the survival probability of the ions reaching the downstream measurements stations.
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| 5. |
- Jansson, Kaj, 1987-, et al.
(författare)
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The new double energy-velocity spectrometer VERDI
- 2017
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Ingår i: ND 2016: INTERNATIONAL CONFERENCE ON NUCLEAR DATA FOR SCIENCE AND TECHNOLOGY. - Les Ulis : EDP Sciences. - 9782759890200
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Konferensbidrag (refereegranskat)abstract
- VERDI (VElocity foR Direct particle Identification) is a fission-fragment spectrometer recently put into operation at JRC-Geel. It allows measuring the kinetic energy and velocity of both fission fragments simultaneously. The velocity provides information about the pre-neutron mass of each fission fragment when isotropic prompt-neutron emission from the fragments is assumed. The kinetic energy, in combination with the velocity, provides the post-neutron mass. From the difference between pre- and post-neutron masses, the number of neutrons emitted by each fragment can be determined. Multiplicity as a function of fragment mass and total kinetic energy is one important ingredient, essential for understanding the sharing of excitation energy between fission fragments at scission, and may be used to benchmark nuclear de-excitation models. The VERDI spectrometer design is a compromise between geometrical efficiency and mass resolution. The spectrometer consists of an electron detector located close to the target and two arrays of silicon detectors, each located 50 cm away from the target. In the present configuration pre-neutron and post-neutron mass distributions are in good agreement with reference data were obtained. Our latest measurements performed with spontaneously fissioning 252Cf is presented along with the developed calibration procedure to obtain pulse height defect and plasma delay time corrections.
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| 6. |
- Ledoux, X., et al.
(författare)
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The neutrons for science facility at SPIRAL-2
- 2017
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Ingår i: ND 2016. - Les Ulis : EDP Sciences.
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Konferensbidrag (refereegranskat)abstract
- Numerous domains, in fundamental research as well as in applications, require the study of reactions induced by neutrons with energies from few MeV up to few tens of MeV. Reliable measurements also are necessary to improve the evaluated databases used by nuclear transport codes. This energy range covers a large number of topics like transmutation of nuclear waste, design of future fission and fusion reactors, nuclear medicine or test and development of new detectors. A new facility called Neutrons For Science (NFS) is being built for this purpose on the GANIL site at Caen (France). NFS is composed of a pulsed neutron beam for time-of-flight facility as well as irradiation stations for cross-section measurements. Neutrons will be produced by the interaction of deuteron and proton beams, delivered by the SPIRAL-2 linear accelerator, with thick or thin converters made of beryllium or lithium. Continuous and quasi-mono-energetic spectra will be available at NFS up to 40 MeV. In this fast energy region, the neutron flux is expected to be up to 2 orders of magnitude higher than at other existing time-of-flight facilities. In addition, irradiation stations for neutron-, proton- and deuteron-induced reactions will allow performing cross-section measurements by the activation technique. After a description of the facility and its characteristics, the experiments to be performed in the short and medium term will be presented.
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| 7. |
- Solders, Andreas, 1976-, et al.
(författare)
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Simulations of the stopping efficiencies of fission ion guides
- 2017
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Ingår i: Nd 2016. - Les Ulis : EDP Sciences. - 9782759890200
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Konferensbidrag (refereegranskat)abstract
- With the Ion Guide Isotope Separator On-Line (IGISOL) facility, located at the University of Jyväskylä, products of nuclear reactions are separated by mass. The high resolving power of the JYFLTRAP Penning trap, with full separation of individual nuclides, capacitates the study of nuclides far from the line of stability. For the production of neutron-rich medium-heavy nuclides, fissioning of actinides is a feasible reaction. This can be achieved with protons from an in-house accelerator or, alternatively, with neutrons through the addition of a newly developed Be(p,xn)-converter. The hereby-obtained fission products are used in nuclear data measurements, for example fission yields, nuclear masses, Q-values and decay spectroscopy. Prior to separation, the ionized reaction products are stopped in a helium-filled gas cell, referred to as the ion-guide. In this work we present simulations of the stopping of fission products in an ion guide developed for neutron-induced fission. The production and extraction rates are evaluated and compared against experimental values.
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| 8. |
- Tarrio, Diego, et al.
(författare)
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Characterization of the Medley setup for measurements of neutron-induced fission cross sections at the GANIL-NFS facility
- 2017
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Ingår i: ND 2016. - Les Ulis : EDP Sciences. - 9782759890200
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Konferensbidrag (refereegranskat)abstract
- Neutron-induced fission cross sections of 235U and 238U are widely used as standards for monitoring of neutron beams and fields. An absolute measurement of these cross sections at an absolute scale, i.e., versus the H(n,p) scattering cross section, is planned with the white neutron beam under construction at the Neutrons For Science (NFS) facility in GANIL. The experimental setup, based on PPACs and ΔE-ΔE-E telescopes containing Silicon and CsI(Tl) detectors, is described. The expected uncertainties are discussed.
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