Experimental and theoretical study of the isomeric cross section of the 197Au(n,2n) reaction

65 σ.Εθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Φυσική και Τεχνολογικές Εφαρμογές”Στην παρούσα εργασία, η ενεργός διατομή της αντίδρασης 197Au(n,2n) προσδιορίστηκε πειραματικά σχετικά με την ενεργό διατομή της αντίδρασης 27Al(n,α)24Na σε ενέργειες προσπιπτόντων νετρονίων μεταξύ 9.0 και 10.5 MeV μέσω της τεχνικής ενεργοποίησης. Η σχεδόν μονοενεργειακή δέσμη νετρονίων παρήχθη μέσω της αντίδρασης 2H(d,n)3He στον επιταχυντή Tandem van de Graaff 5.5 MV του ΕΚΕΦΕ `` Δημόκριτος '' και μελετήθηκε ως προς τη συνεισφορά ``παρασιτικών'' νετρονίων χρησιμοποιώντας την τεχνική πολλαπλής ενεργοποίησης και τον κώδικα αποσυνέλιξης SULSA. Οι ενεργές διατομές τροφοδότησης της δεύτερης ισομερούς κατάστασης (12-) του 196Au και το άθροισμα των ενεργών διατομών τροφοδότησης της θεμελιώδους (2-) και πρώτης ισομερούς στάθμης (5-) προσδιορίστηκαν ανεξάρτητα. Βοηθητικές προσομοιώσεις Monte Carlo πραγματοποιήθηκαν με τον κώδικα MCNP. Θεωρητικοί υπολογισμοί των ανωτέρω ενεργών διατομών στην ενεργειακή περιοχή 8 με 25 MeV πραγματοποιήθηκαν με τους κώδικες STAPRE-F, EMPIRE και TALYS, οι οποίοι συγκρίθηκαν και ως προς την υλοποίηση του Γενικευμένου Μοντέλου Υπερρευστού (Generalised Superfluid Model). Τα θεωρητικά αποτελέσματα συγκρίθηκαν με παλαιότερα αποτελέσματα στην ίδια περιοχή μαζών αποκαλύπτοντας την ισχυρή εξάρτηση από το ενεργειακό διάγραμμα των συμμετεχόντων πυρήνων.In the present work, the 197Au(n,2n) reaction cross section is experimentally determined relative to the 27Αl(n,a)24Na reaction at incident neutron energies of 9.0 to 10.5 MeV by means of the activation technique. The quasi-monoenergetic fast neutron beam was produced via the 2H(d,n)3He reaction at the 5.5 MV Tandem Van de Graaff accelerator at the NCSR ``Demokritos'' and was studied to determine the contribution of background ``parasitic'' neutrons using the multiple foil activation technique and the SULSA unfolding code. The cross sections for the population of the second isomeric state (12-) of 196Au and the sum of the ground (2-) and first isomeric state (5-) population cross sections were independently determined. Auxiliary Monte Carlo simulations were performed with the MCNP code. Theoretical calculations of the above cross sections in the 8 to 25 MeV region were carried out with the use of the STAPRE-F, EMPIRE and TALYS codes, which were also compared in their implementation of the Generalised Superfluid Model (GSM). The theoretical results are compared with previous work in the same mass region and the strong dependence on the level scheme of the nuclei involved was revealed.Ανδρέας Ι. Τσιγγάνη

Measurement of the 241Am and the 243Am neutron capture cross sections at the n_TOF Facility at CERN

The capture cross sections of 241Am and 243Am were measured at the n_TOF facility at CERN in the epithermal energy range with a BaF2 Total Absorption Calorimeter. A preliminary analysis of the 241Am and a complete analysis of the 243Am measurement, including the data reduction and the resonance analysis, have been performed.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Measurement and analysis of 241Am(n,gamma) cross sections with C6D6 detectors at the n_TOF facility at CERN

The 241Am(n,gamma) cross sections have been measured at the n TOF facility at CERN using C6D6 liquid scintillators and time of flight spectrometry. The results in the resolved resonance range bring new constraints to evaluations below 150 eV, and the energy upper limit can be extended from 150 eV to 320 eV. The analysis goes from thermal energy to 150 keV, and the unresolved resonance range cross section turns out to be larger than expected by evaluations or otherwise measured by previous works. The thermal cross section is found to be σth = 740 ± 74 barns, which is larger than expected by evaluations and most previous measurements.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Measurement of the neutron capture cross section of the fissile isotope 235U with the CERN n_TOF total absorption calorimeter and a fission tagging based on micromegas detectors

Current and future nuclear technologies require more accurate nuclear data on (n,γ) cross sections and the α-ratios of fissile isotopes. Their measurement presents several difficulties, mainly related to the strong fission γ-ray background competing with the weaker γ-ray cascades used as the experimental signature of the (n,γ) process. A specific setup was used at the CERN n TOF facility in 2012 for the measurement of the (n,γ) cross section and α-ratios of fissile isotopes and used for the case of the 235U isotope. The setup consists of a set of micromegas fission detectors surrounding the 235U samples all placed inside a segmented BaF2 Total Absorption Calorimeter.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Spin measurements of n+87Sr for level density studies

We have used the 4π BaF2 gamma-ray detector array at the n_TOF neutron time-of-flight facility at CERN for an experiment in order to determine the spins of resonances of n+87Sr by measuring the gamma-ray spectra and multiplicity distributions. The first results are presented here. We have assigned the orbital momentum to all evaluated resonances on the basis of their neutron widths. Further we have assigned the spin J to 16 s-wave resonances on based the population of low-lying levels.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Measurement of the 238U radiative capture cross section with C6D6 at the CERN n_ TOF facility

We present the preliminary results of the 238U(n,γ) reaction cross section measurement, performed on April 2012 at the CERN n_TOF facility using C6D6 scintillation detectors over an energy range from thermal to 1 MeV. The goal of this measurement, which is part of a larger proposal, is to reach an uncertainty of 2% in the cross section. The experimental set-up and the methods used to obtain this result are described.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Investigation of neutron-induced reactions at n_TOF: an overview of the 2009-2012 experimental program

The neutron time-of-flight facility n_TOF is operating at CERN (Switzerland) since 2001, having started in 2009 a new campaign with an upgraded spallation target. The ambitious program carried out includes a large number of experiments in nuclear technology, astrophysics, basic physics, detector development and medical applications. This paper is devoted to the physics program at n_TOF and the measurements performed between 2009 and 2012. Special attention is given to those experiments that have been most challenging, are more important for a particular field, have reached unprecedented levels of accuracy, or have been carried out for the first time ever.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

New measurement of the 25Mg(n,γ) reaction cross section

The 25Mg(n,γ) reaction cross-section has been measured at the neutron time-of-flight facility n TOF at CERN in 2003 and, with an improved experimental set up, in 2012. These measurements aim at addressing important astrophysical issues related to the s process. Highly-enriched 25Mg samples have been used, in the form of MgO powder and Mg metal sample. Preliminary results from the 2012 measurement and comparisons with previous data are shown.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

High-precision measurement of the 238U(n, gamma) cross section with the Total Absorption Calorimeter (TAC) at n_TOF, CERN

The neutron capture cross section of 238U is fundamental to the design and operation of current reactors and future fast nuclear reactors, and thus must be measured to a high level of accuracy. An experiment has been performed at the CERN n TOF facility using a 4π Total Absorption Calorimeter (TAC) to measure the capture cross section in the resolved resonance region between 1 eV and 25 keV. A preliminary analysis of the TAC data is presented with particular emphasis to the experimental background in this energy region of interest.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

The CERN n_TOF Facility: Neutron beams performances for cross section measurements

This paper presents the characteristics of the existing CERN n TOF neutron beam facility (n TOF-EAR1 with a flight path of 185 meters) and the future one (n_TOF EAR-2 with a flight path of 19 meters), which will operate in parallel from Summer 2014. The new neutron beam will provide a 25 times higher neutron flux delivered in 10 times shorter neutron pulses, thus offering more powerful capabilities for measuring small mass, low cross section and/or high activity samples.Peer reviewe