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Rakopoulos, Vasileios
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Publications (10 of 27) Show all publications
Solders, A., Al-Adili, A., Canete, L., Eronen, T., Gao, Z., Jokinen, A., . . . Vilén, M. (2020). Fission yield studies using mass measurement techniques: Invited talk.
Open this publication in new window or tab >>Fission yield studies using mass measurement techniques: Invited talk
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2020 (English)Other (Other academic)
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-407032 (URN)
Available from: 2020-03-18 Created: 2020-03-18 Last updated: 2020-03-23Bibliographically approved
Al-Adili, A., Rakopoulos, V. & Solders, A. (2019). Extraction of angular momenta from isomeric yield ratios: Employing TALYS to de-excite primary fission fragments. European Physical Journal A, 55(4), Article ID 61.
Open this publication in new window or tab >>Extraction of angular momenta from isomeric yield ratios: Employing TALYS to de-excite primary fission fragments
2019 (English)In: European Physical Journal A, ISSN 1434-6001, E-ISSN 1434-601X, Vol. 55, no 4, article id 61Article in journal (Refereed) Published
Abstract [en]

The generation of angular momentum in fission is difficult to model, in particular at higher excitation energies where data are scarce. Isomeric yield ratios (IYR) play an important role in deducing angular momentum properties of fission fragments (FF), albeit this requires some assumptions and simplifications. To estimate FF angular momentum, fission codes can be used to calculate IYRs and compare them to experimental data. Such measurements have systematically been performed at the IGISOL facility using novel experimental techniques. In conjunction, a new method has been developed to infer the angular momentum of the primary FF using the nuclear reaction code TALYS. In this work, we evaluate this new method by comparing our TALYS calculations with values found in the literature and with results from the GEF fission code, for a few well-studied reactions. The overall results show a consistent performance of TALYS and GEF, as well as of many reported literature values. However, some deviations were found, possibly pinpointing the need to re-examine some of the reported literature values. A sensitivity analysis was also performed, in which the role of excitation energy, neutron emission, discrete level structure and level density models were studied. Finally, the role of multiple chance fission, of relevance for the reactions studied at IGISOL, is discussed. Some literature data for this reaction were also re-analyzed using TALYS, revealing significant differences.

National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-383514 (URN)10.1140/epja/i2019-12731-5 (DOI)000466093100001 ()
Available from: 2019-05-16 Created: 2019-05-16 Last updated: 2019-05-16Bibliographically approved
Rakopoulos, V., Lantz, M., Pomp, S., Solders, A., Al-Adili, A., Canete, L., . . . Penttilä, H. (2019). Isomeric fission yield ratios for odd-mass Cd and In isotopes using the phase-imaging ion-cyclotron-resonance technique. Physical Review C, 99(1), Article ID 014617.
Open this publication in new window or tab >>Isomeric fission yield ratios for odd-mass Cd and In isotopes using the phase-imaging ion-cyclotron-resonance technique
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2019 (English)In: Physical Review C, ISSN 2469-9985, Vol. 99, no 1, article id 014617Article in journal (Refereed) Published
Abstract [en]

Isomeric yield ratios for the odd-A isotopes of Cd119-127 and In119-127 from 25-MeV proton-induced fission on natural uranium have been measured at the JYFLTRAP double Penning trap, by employing the phase-imaging ion-cyclotron-resonance technique. With the significantly improved mass resolution of this novel method isomeric states separated by 140 keV from the ground state, and with half-lives of the order of 500 ms, could be resolved. This opens the door for obtaining new information on low-lying isomers, which are important for nuclear structure, fission, and astrophysics. In the present work the experimental isomeric yield ratios are used for the estimation of the root-mean-square angular momentum J(rms) of the primary fragments. The results show a dependency on the number of unpaired protons and neutrons, where the odd-Z In isotopes carry larger angular momenta. The deduced values of J(rms) display a linear relationship when compared with the electric quadrupole moments of the fission products.

National Category
Subatomic Physics
Research subject
Physics with specialization in Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-364949 (URN)10.1103/PhysRevC.99.014617 (DOI)000456288000005 ()
Funder
EU, FP7, Seventh Framework Programme, 605203Swedish Radiation Safety AuthoritySwedish Nuclear Fuel and Waste Management Company, SKB
Available from: 2018-11-06 Created: 2018-11-06 Last updated: 2019-02-11Bibliographically approved
Pomp, S. (2019). Isomeric yield ratio measurements at IGISOL/JYFLTRAP for assessing angular momenta of fission fragments: Invited talk, NUSTAR seminar series, GSI.
Open this publication in new window or tab >>Isomeric yield ratio measurements at IGISOL/JYFLTRAP for assessing angular momenta of fission fragments: Invited talk, NUSTAR seminar series, GSI
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2019 (English)Other (Other academic)
National Category
Subatomic Physics
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-396855 (URN)
Available from: 2019-11-11 Created: 2019-11-11 Last updated: 2019-11-11
Rakopoulos, V., Lantz, M., Solders, A., Al-Adili, A., Mattera, A., Canete, L., . . . Pomp, S. (2018). First isomeric yield ratio measurements by direct ion counting and implications for the angular momentum of the primary fission fragments. Physical Review C: Covering Nuclear Physics, 98(2), Article ID 024612.
Open this publication in new window or tab >>First isomeric yield ratio measurements by direct ion counting and implications for the angular momentum of the primary fission fragments
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2018 (English)In: Physical Review C: Covering Nuclear Physics, ISSN 2469-9985, E-ISSN 2469-9993, Vol. 98, no 2, article id 024612Article in journal (Refereed) Published
Abstract [en]

We report the first experimental determination of independent isomeric yield ratios using direct ion counting with a Penning trap, which offered such a high resolution in mass that isomeric states could be separated. The measurements were performed at the Ion Guide Isotope Separator On-Line (IGISOL) facility at the University of Jyvaskyla. The isomer production ratios of Ge-81, Y-96,Y-97 Sn-128(,1)30, and Sb-129 in the 25-MeV proton-induced fission of U-na(t) and Th-232 were studied. Three isomeric pairs (Ge-81, Y-96, and Sb-129) were measured for the first time for the U-na(t)(p, f) reaction, while all the reported yield ratios for the Th-232(p, f) reaction were determined for the first time. The comparison of the experimentally determined isomeric yield ratios with data available in the literature shows a reasonable agreement, except for the case of Sn-130 for unspecified reasons. The obtained results were also compared with the GEF model, where good agreement can be noticed in most cases for both reactions. Serious discrepancies can only be observed for the cases of Y-96(,)97 for both reactions. Moreover, based on the isomeric yield ratios, the root-mean-square angular momenta (J(r)(ms)) of the fission fragments after scission were estimated using the TALYS code. The experimentally determined isomeric yield ratios, and consequently the deduced J(rms), for Sn-130 are significantly lower compared to Sn-128 for both fissioning systems. This can be attributed to the more spherical shape of the fragments that contribute to the formation of Sn-130, due to their proximity to the N = 82 shell closure. The values of J(rms) for Sb-129 are higher than Sn-128 for both reactions, despite the same neutron number of both nuclides (N = 78), indicating the odd-Z effect where fission fragments with odd-Z number tend to bear larger angular momentum than even-Z fragments. The isomer production ratio for the isotopes of Sn is more enhanced in the U-na(t)(p, f) reaction than in Th-232(p, f). The opposite is observed for Y-96 and Y-97. These discrepancies might be associated to different scission shapes of the fragments for the two fission reactions, indicating the impact that the different fission modes can have on the isomeric yield ratios.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-362692 (URN)10.1103/PhysRevC.98.024612 (DOI)000441464600003 ()
Funder
EU, FP7, Seventh Framework Programme, 605203Swedish Radiation Safety AuthoritySwedish Nuclear Fuel and Waste Management Company, SKBAcademy of Finland, 275389 284516 312544 295207
Available from: 2018-10-18 Created: 2018-10-18 Last updated: 2018-11-06Bibliographically approved
Rakopoulos, V. (2018). Isomeric yield ratio measurements with JYFLTRAP: In quest of the angular momentum of the primary fragments. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
Open this publication in new window or tab >>Isomeric yield ratio measurements with JYFLTRAP: In quest of the angular momentum of the primary fragments
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, isomeric yield ratios of twenty nuclides produced in the fission of natU and 232Th by protons at 25-MeV and natU by high-energy neutrons were studied. The experiments were performed at the IGISOL-JYFLTRAP facility at the University of Jyväskylä. It is the first time that direct ion counting is used for the determination of the intensities of the states of interest, thus avoiding dependency on knowledge of nuclear decay schemes and properties. This was possible due to the superior resolution of a Penning trap which was utilized for this work. Two different techniques were employed, namely the sideband cooling technique and the phase-imaging ion-cyclotron-resonance technique. With the former, a mass resolving power of m/δm = 105 can be routinely achieved, while the latter, which was recently implemented at JYFLTRAP, offers an increase in the mass resolving power by a factor of ten. In addition, isomeric yield ratios were also determined by means of γ-ray spectroscopy.

From a comparison of the same isomeric pair from two different reactions, a dependency on the fissioning system can be observed. This indicates an effect of the fission mode to the yield ratio. Moreover, the evolution of the odd-A isotopes of Cd and In in the mass range A = 119 - 127 exhibit two distinguishably different trends. The ratios for the isotopes of In decrease with increasing mass, while the ratios for the isotopes of Cd are almost constant until mass number A = 125, where an increase can be noticed.

The origins of the angular momentum in the fission fragments is one of the long-standing questions regarding the fission process. Surprisingly, fission fragments have been observed to carry a considerable amount of angular momentum, even from fissioning systems with very low (or even zero) angular momentum. So far, the angular momentum can only be inferred from other fission observables, such as the isomeric yield ratios. In this work, a methodology was developed in order to deduce the root-mean-square angular momentum (Jrms) of the primary fragments by employing the nuclear reaction code TALYS.

Lower values of Jrms for the more spherical nuclei, near the closed-shell neutron configuration at N = 82, and higher ones for fragments with odd proton number have been deduced, in agreement with other studies. Moreover, a correlation between the angular momentum of the primary fragments with the electric quadrupole moments of the products was observed for the isotopes of In. The data can be used to gain insight into scission configuration and as guide for models that propose mechanisms for the generation of the angular momentum. Furthermore, the observed correlation is an indication of the role that the repulsive Coulomb force, together with the shape of the nascent fragment, play in the generation of the fragments’ angular momentum.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 120
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1743
Keywords
Nuclear physics, nuclear fission, angular momentum, fission fragments, isomeric yield ratios, fission products, scission, Penning trap.
National Category
Subatomic Physics
Research subject
Physics with specialization in Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-364951 (URN)978-91-513-0502-8 (ISBN)
Public defence
2018-12-13, 80127, Ångström, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2018-11-23 Created: 2018-11-06 Last updated: 2019-11-12
Pomp, S., Mattera, A., Rakopoulos, V., Al-Adili, A., Lantz, M., Solders, A., . . . Rinta-Antila, S. (2018). Measurement of fission yields and isomeric yield ratios at IGISOL. In: Hambsch, FJ Carjan, N Rusko, I (Ed.), Scientific Workshop on Nuclear Fission Dynamics And The Emission of Prompt Neutrons and Gamma Rays (Theory-4): . Paper presented at 4th Scientific Workshop on Nuclear Fission Dynamics and the Emission of Prompt Neutrons and Gamma Rays (Theory), JUN 20-22, 2017, Varna, BULGARIA. , Article ID UNSP 00017.
Open this publication in new window or tab >>Measurement of fission yields and isomeric yield ratios at IGISOL
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2018 (English)In: Scientific Workshop on Nuclear Fission Dynamics And The Emission of Prompt Neutrons and Gamma Rays (Theory-4) / [ed] Hambsch, FJ Carjan, N Rusko, I, 2018, article id UNSP 00017Conference paper, Published paper (Refereed)
Abstract [en]

Data on fission yields and isomeric yield ratios (IYR) are tools to study the fission process, in particular the generation of angular momentum. We use the IGISOL facility with the Penning trap JYFLTRAP in Jyvaskyla, Finland, for such measurements on Th-232 and U-nat targets. Previously published fission yield data from IGISOL concern the Th-232(p,f) and U-238(p,f) reactions at 25 and 50 MeV. Recently, a neutron source, using the Be(p,n) reaction, has been developed, installed and tested. We summarize the results for (p,f) focusing on the first measurement of IYR by direct ion counting. We also present first results for IYR and relative yields for Sn and Sb isotopes in the 128-133 mass range from U-nat(n,f) based on gamma-spectrometry. We find a staggering behaviour in the cumulative yields for Sn and a shift in the independent fission yields for Sb as compared to current evaluations. Plans for the future experimental program on fission yields and IYR measurements are discussed.

Series
EPJ Web of Conferences, ISSN 2100-014X ; 169
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-361441 (URN)10.1051/epjconf/201816900017 (DOI)000433153800016 ()978-2-7598-9031-6 (ISBN)
Conference
4th Scientific Workshop on Nuclear Fission Dynamics and the Emission of Prompt Neutrons and Gamma Rays (Theory), JUN 20-22, 2017, Varna, BULGARIA
Available from: 2018-12-10 Created: 2018-12-10 Last updated: 2018-12-10Bibliographically approved
Mattera, A., Pomp, S., Lantz, M., Solders, A., Rakopoulos, V., Al-Adili, A., . . . Gorelov, D. (2018). Production of Sn and Sb isotopes in high-energy neutron induced fission of natU. European Physical Journal A, 54, Article ID 33.
Open this publication in new window or tab >>Production of Sn and Sb isotopes in high-energy neutron induced fission of natU
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2018 (English)In: European Physical Journal A, ISSN 1434-6001, E-ISSN 1434-601X, Vol. 54, article id 33Article in journal (Refereed) Published
Abstract [en]

The first systematic measurement of neutron-induced fission yields has been performed at the upgraded IGISOL-4 facility at the University of Jyvaskyla, Finland. The fission products from high-energy neutron-induced fission of U-nat were stopped in a gas cell filled with helium buffer gas, and were online separated with a dipole magnet. The isobars, with masses in the range A = 128-133, were transported to a tape-implantation station and identified using gamma-spectroscopy. We report here the relative cumulative isotopic yields of tin (Z = 50) and the relative independent isotopic yields of antimony (Z = 51). Isomeric yield ratios were also obtained for five nuclides. The yields of tin show a staggered behaviour around A = 131, not observed in the ENDF/B-VII. 1 evaluation. The yields of antimony also contradict the trend from the evaluation, but are in agreement with a calculation performed using the GEF model that shows the yield increasing with mass in the range A = 128-133.

National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-328572 (URN)10.1140/epja/i2018-12462-1 (DOI)000428637900002 ()
Funder
EU, FP7, Seventh Framework Programme, 605203Swedish Radiation Safety AuthoritySwedish Nuclear Fuel and Waste Management Company, SKB
Available from: 2017-08-26 Created: 2017-08-26 Last updated: 2018-11-06Bibliographically approved
Al-Adili, A., Jansson, K., Tarrio, D., Hambsch, F.-J., Gook, A., Oberstedt, S., . . . Pomp, S. (2018). Studying fission neutrons with 2E-2v and 2E. In: Hambsch, FJ Carjan, N Rusko, I (Ed.), SCIENTIFIC WORKSHOP ON NUCLEAR FISSION DYNAMICS AND THE EMISSION OF PROMPT NEUTRONS AND GAMMA RAYS (THEORY-4): . Paper presented at 4th Scientific Workshop on Nuclear Fission Dynamics and the Emission of Prompt Neutrons and Gamma Rays (Theory), JUN 20-22, 2017, Varna, BULGARIA. , Article ID UNSP 00002.
Open this publication in new window or tab >>Studying fission neutrons with 2E-2v and 2E
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2018 (English)In: SCIENTIFIC WORKSHOP ON NUCLEAR FISSION DYNAMICS AND THE EMISSION OF PROMPT NEUTRONS AND GAMMA RAYS (THEORY-4) / [ed] Hambsch, FJ Carjan, N Rusko, I, 2018, article id UNSP 00002Conference paper, Published paper (Refereed)
Abstract [en]

This work aims at measuring prompt-fission neutrons at different excitation energies of the nucleus. Two independent techniques, the 2E-2v and the 2E techniques, are used to map the characteristics of the mass-dependent prompt fission neutron multiplicity, 7(A), when the excitation energy is increased. The VERDI 2E-2v spectrometer is being developed at JRC-GEEL. The Fission Fragment (FF) energies are measured using two arrays of 16 silicon (Si) detectors each. The FFs velocities are obtained by time-of-flight, measured between micro-channel plates (MCP) and Si detectors. With MCPs placed on both sides of the fission source, VERDI allows for independent timing measurements for both fragments. Cf-252(sf) was measured and the present results revealed particular features of the 2E-2v technique. Dedicated simulations were also performed using the GEF code to study important aspects of the 2E-2v technique. Our simulations show that prompt neutron emission has a non-negligible impact on the deduced fragment data and affects also the shape of 17(A). Geometrical constraints lead to a total-kinetic energy-dependent detection efficiency. The 2E technique utilizes an ionization chamber together with two liquid scintillator detectors. Two measurements have been performed, one of Cf-252(sf) and another one of thermal-neutron induced fission in U-235(n,f). Results from Cf-252(sf) are reported here.

Series
EPJ Web of Conferences, ISSN 2100-014X ; 169
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-361440 (URN)10.1051/epjconf/201816900002 (DOI)000433153800001 ()978-2-7598-9031-6 (ISBN)
Conference
4th Scientific Workshop on Nuclear Fission Dynamics and the Emission of Prompt Neutrons and Gamma Rays (Theory), JUN 20-22, 2017, Varna, BULGARIA
Available from: 2018-12-10 Created: 2018-12-10 Last updated: 2019-02-28Bibliographically approved
Mattera, A., Al-Adili, A., Lantz, M., Pomp, S., Rakopoulos, V. & Solders, A. (2017). A methodology for the intercomparison of nuclear fission codes using TALYS. In: Plompen, A.; Hambsch, FJ.; Schillebeeckx, P.; Mondelaers, W.; Heyse, J.; Kopecky, S.; Siegler, P.; Oberstedt, S. (Ed.), ND 2016: International Conference On Nuclear Data For Science And Technology. Paper presented at ND 2016: International Conference on Nuclear Data for Science and Technology, September 11-16, 2016, Bruges, Belgium.. Les Ulis: EDP Sciences, 146, Article ID 04047.
Open this publication in new window or tab >>A methodology for the intercomparison of nuclear fission codes using TALYS
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2017 (English)In: ND 2016: International Conference On Nuclear Data For Science And Technology / [ed] Plompen, A.; Hambsch, FJ.; Schillebeeckx, P.; Mondelaers, W.; Heyse, J.; Kopecky, S.; Siegler, P.; Oberstedt, S., Les Ulis: EDP Sciences, 2017, Vol. 146, article id 04047Conference paper, Published paper (Refereed)
Abstract [en]

Codes for the calculation of fission observables are frequently used to describe experimentally observed phenomena as well as provide predictions in cases where measurements are missing. Assumptions in the models, and tuning of parameters within the codes, often result in a good reproduction of experimental data. In this work we propose a methodology, coded in the newly developed program DELFIN (De-Excitation of FIssion fragmeNts), that can be used to compare some of the assumptions of the various models. Our code makes use of the fission fragments information after scission and processes them in an independent and consistent fashion to obtain measurable fission observables (such as ν(A) distributions and Isomeric Fission Yield ratios). All the available information from the models, such as fragments' excitation energies, spin distributions and yields are provided as input to DELFIN that uses the nuclear reaction code TALYS to handle the de-excitation of the fission fragments. In this way we decouple the fragments relaxation from the actual fission models. We report here the first results of a comparison carried out on the GEF, Point-by-Point and FREYA models for thermal fission of 235U and 239Pu and spontaneous fission of 252Cf.

Place, publisher, year, edition, pages
Les Ulis: EDP Sciences, 2017
Series
EPJ Web of Conferences, ISSN 2100-014X
National Category
Subatomic Physics
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-317442 (URN)10.1051/epjconf/201714604047 (DOI)000426429500146 ()978-2-7598-9020-0 (ISBN)
Conference
ND 2016: International Conference on Nuclear Data for Science and Technology, September 11-16, 2016, Bruges, Belgium.
Funder
Swedish Nuclear Fuel and Waste Management Company, SKBSwedish Radiation Safety Authority
Available from: 2017-03-14 Created: 2017-03-14 Last updated: 2018-07-03Bibliographically approved
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