The 238U fission cross section is an international standard beyond 2 MeV where the fission plateau starts. However, due to its importance in fission reactors, this cross-section should be very accurately known also in the threshold region below 2 MeV. The 238U fission cross section has been measured relative to the 235U fission cross section at CERN – n_TOF with different detection systems. These datasets have been collected and suitably combined to increase the counting statistics in the threshold region from about 300 keV up to 3 MeV. The results are compared with other experimental data, evaluated libraries, and the IAEA standards.

The neutron-induced fission cross section of 237 Np was experimentally determined at the high-resolution and high-intensity facility n TOF , at CERN, in the energy range 100 keV to 9 MeV, using the 235 U( n,f ) and 238 U( n,f ) cross section standards below and above 2 MeV, respectively. A fast ionization chamber was used in order to detect the fission fragments from the reactions and the targets were characterized as far as their mass and homogeneity are concerned by means of α spectroscopy and Rutherford backscattering spectroscopy respectively. Theoretical calculations within the Hauser-Feshbach formalism have been performed, employing the EMPIRE code, and the model parameters were tuned in order to successfully reproduce the experimental fission cross-sectional data and simultaneously all the competing reaction channels.

The 235U neutron-induced cross section is widely used as reference cross section for measuring other fission cross sections, but in the resonance region it is not considered as an IAEA standard because of the scarce experimental data covering the full region. In this work, we deal with a new analysis of the experimental data obtained with a detection setup based on parallel plate ionization chambers (PPACs) at the CERN n_TOF facility in the range from 1 eV to 10 keV. The relative cross section has been normalised to the IAEA value in the region between 7.8 and 11 eV, which is claimed as well-known. Comparison with the ENDF/B-VII evaluation and the IAEA reference file from 100 eV to 10 keV are provided.

The U238 to U235 fission cross section ratio has been determined at n_TOF up to ˜1 GeV, with two different detection systems, in different geometrical configurations. A total of four datasets has been collected and compared. They are all consistent to each other within the relative systematic uncertainty of 3–4%. The data collected at n_TOF have been suitably combined to yield a unique fission cross section ratio as a function of neutron energy. The result confirms current evaluations up to 200 MeV. Good agreement is also observed with theoretical calculations based on the INCL++/Gemini++ combination up to the highest measured energy. The n_TOF results may help solve a long-standing discrepancy between the two most important experimental datasets available so far above 20 MeV, while extending the neutron energy range for the first time up to ˜1 GeV.

The neutron-induced fission cross section of 234U has been measured at the CERN n_TOF facility relative to the standard fission cross section of 235U from 20 keV to 1.4 MeV and of 238U from 1.4 to 200 MeV. A fast ionization chamber (FIC) was used as a fission fragment detector with a detection efficiency of no less than 97%. The high instantaneous flux and the low background characterizing the n_TOF facility resulted in wide-energy-range data (0.02 to 200 MeV), with high energy resolution, high statistics, and systematic uncertainties bellow 3%. Previous investigations around the energy of the fission threshold revealed structures attributed to s-vibrational levels, which have been confirmed by the present measurements. Theoretical calculations have been performed, employing the TALYS code with model parameters tuned to fairly reproduce the experimental data

We present the analysis of the resolved resonance region for the 234U(n,f) cross section data measured at the CERN n_TOF facility. The resonance parameters in the energy range from 1 eV to 1500 eV have been obtained with the SAMMY code by using as initial parameters for the fit the resonance parameters of the JENDL-3.3 evaluation. In addition, the statistical analysis has been accomplished, partly with the SAMDIST code, in order to study the level spacing and the Mehta-Dyson correlation.

C.Dom ingo-Pardo, U.Abbondanno, G .Aerts, H. Alvarez, F.Alvarez-Velarde, S.Andriam onje, J.Andrzejewski, P.Assim akopoulos, L.Audouin, G .Badurek, P.Baum ann, F.Be cv a r, E.Berthoum ieux, S.Bisterzo, F.Calvi~ no, M .Calviani, D.Cano-O tt, R.Capote, C.Carrapi co, P.Cennini, V.Chepel, E.Chiaveri, N.Colonna, G .Cortes, A.Couture, J.Cox, M .Dahlfors, S.David, I.Dillm an, R.Dol ni, W .Dridi, I.Duran, C.Eleftheriadis, M .Em bid-Segura, L.Ferrant, A.Ferrari, R.Ferreira-M arques, L.Fitzpatrick, H.Frais-K oelbl, K .Fujii, W .Furm an, R.G allino, I.G oncalves, E.G onzalez-Rom ero, A.G overdovski, F.G ram egna, E.G riesm ayer, C.G uerrero, F.G unsing, B.Haas, R.Haight, M .Heil, A.Herrera-M artinez, M .Igashira, M .Isaev, E.Jericha, F.K appeler, Y.K adi, D.K aradim os, D.K aram anis, M .K erveno, V.K etlerov, P.K oehler, V.K onovalov, E.K ossionides, M .K rti cka, C.Lam boudis, H.Leeb, A.Lindote, I.Lopes, M .Lozano, S.Lukic, J.M arganiec, S.M arrone, C.M assim i, P.M astinu, A.M engoni, P.M .M ilazzo, C.M oreau, M .M osconi, F.Neves, H.O berhum m er, M .O shim a, S.O ’Brien, J.Pancin, C.Papachristodoulou, C.Papadopoulos, C.Paradela, N.Patronis, A.Pavlik, P.Pavlopoulos, L.Perrot, R.Plag, A.Plom pen, A.Plukis, A.Poch, C.Pretel, J.Q uesada, T.Rauscher, R.Reifarth, M .Rosetti, C.Rubbia, G .Rudolf, P.Rullhusen, J.Salgado, L.Sarchiapone, I.Savvidis, C.Stephan, G .Tagliente, J.L.Tain, L.Tassan-G ot, L.Tavora, R.Terlizzi, G .Vannini, P.Vaz, A.Ventura, D.Villam arin, M .C.Vincente, V.Vlachoudis, R.Vlastou, F.Voss, S.W alter, H.W endler, M .W iescher, and K .W isshak

The Am-243 neutron capture cross section has been measured at the n_TOF facility(1) in the 0.7 eV-2 keV energy range. The n_TOF Total Absorption Calorimeter(2) (TAC) composed by 40 BaF2 crystals has been used in the measurement for detecting the electromagnetic cascades produced in the 243Am(n,gamma) reactions. All current evaluations in the resolved resonance region are based essentially in fission measurements and in only one transmission measurement.(3) The analysis of the measurement has been finished recently, and it is ready for its distribution to the EXFOR nuclear database. In addition, the data obtained with the TAC provide valuable information on the level density in the compound nucleus Am-244 and on its electromagnetic de-excitation scheme. In particular, the 243Am data, will be combined with data from previous measurements of Am-241, (240)pu, Np-237 and U-233,U-234,U-236 and with future measurements of U-235,U-238 for a systematic investigation of the photon strength functions in actinides.

Neutron-induced fission cross section measurements of 233U, 243Am and 241Am relative to 235U have been carried out at the neutron time-of-flight facility n_TOF at CERN. A fast ionization chamber has been employed. All samples were located in the same detector; therefore the studied elements and the reference 235U target are subject to the same neutron beam.

We have measured the neutron capture cross sections of the stable magnesium isotopes Mg-24,Mg-25,Mg-26 in the energy range of interest to the s process using the neutron time-of-flight facility n_TOF at CERN. Capture events from a natural metal sample and from samples enriched in Mg-25 and Mg-26 were recorded using the total energy method based on (C6H6)-H-2 detectors. Neutron resonance parameters were extracted by a simultaneous resonance shape analysis of the present capture data and existing transmission data on a natural isotopic sample. Maxwellian-averaged capture cross sections for the three isotopes were calculated up to thermal energies of 100 keV and their impact on s-process analyses was investigated. At 30 keV the new values of the stellar cross section for Mg-24, Mg-25, and Mg-26 are 3.8 +/- 0.2 mb, 4.1 +/- 0.6 mb, and 0.14 +/- 0.01 mb, respectively.

Neutron-induced fission cross section measurements of U-233, Am-243 and Am-241 relative to U-235 have been carried out at the neutron time-of-flight facility n_TOF at CERN. A fast ionization chamber has been employed. All samples were located in the same detector; therefore the studied elements and the reference U-235 target are subject to the same neutron beam.

The yield of the neutron capture reaction ${}^{232}$Th($n,\ensuremath{\gamma}$) has been measured at the neutron time-of-flight facility n_TOF at CERN in the energy range from 1 eV to 1 MeV. The reduction of the acquired data to the capture yield for resolved resonances from 1 eV to 4 keV is described and compared to a recent evaluated data set. The resonance parameters were used to assign an orbital momentum to each resonance. A missing level estimator was used to extract the $s$-wave level spacing of ${D}_{0}=17.2\ifmmode\pm\else\textpm\fi{}0.9$ eV.

The neutron capture cross section of Np-237 was measured between 0.7 and 500 eV at the CERN n_TOF facility using the 4 pi BaF2 Total Absorption Calorimeter. The experimental capture yield was extracted minimizing all the systematic uncertainties and was analyzed together with the most reliable transmission data available using the SAMMY code. The result is a complete set of individual as well as average resonance parameters [D-0 = 0.56(2) eV, = 40.9(18) meV, 10(4)S(0) = 0.98(6), R' = 9.8(6) fm]. The capture cross section obtained in this work is in overall agreement with the evaluations and the data of Weston and Todd [Nucl. Sci. Eng. 79, 184 (1981)], thus showing sizable differences with respect to previous data from Scherbakov et al. [J. Nucl. Sci. Technol. 42, 135 (2005)] and large discrepancies with data Kobayashi et al. [J. Nucl. Sci. Technol. 39, 111 (2002)]. The results indicate that a new evaluation combining the present capture data with reliable transmission data would allow reaching an accuracy better than 4%, in line with the uncertainty requirements of the nuclear data community for the design and operation of current and future nuclear devices.

The neutron-induced fission cross section of ${}^{245}$Cm was measured at n_TOF in a wide energy range and with high resolution. The energy dependence, measured in a single measurement from 30 meV to 1 MeV neutron energy, has been determined with 5$%$ accuracy relative to the ${}^{235}$U(n,f) cross section. In order to reduce the uncertainty on the absolute value, the data have been normalized at thermal energy to recent measurements performed at ILL and BR1. In the energy range of overlap, the results are in fair agreement with some previous measurements and confirm, on average, the evaluated cross section in the ENDF/B-VII.0 database, although sizable differences are observed for some important resonances below 20 eV. A similar behavior is observed relative to JENDL/AC-2008, a reactor-oriented database for actinides. The new results contribute to the overall improvement of the databases needed for the design of advanced reactor systems and may lead to refinements of fission models for the actinides.

The yield of the neutron capture reaction Th-232(n, gamma) has been measured at the neutron time-of-flight facility n_TOF at CERN in the energy range from 1 eV to 1 MeV. The reduction of the acquired data to the capture yield for resolved resonances from 1 eV to 4 keV is described and compared to a recent evaluated data set. The resonance parameters were used to assign an orbital momentum to each resonance. A missing level estimator was used to extract the s-wave level spacing of D-0 = 17.2 +/- 0.9 eV.

The decay from excited levels in medium and heavy nuclei can be described in a statistical approach by means of Photon Strength Functions and Level Density distributions. The study of electromagnetic cascades following neutron capture based on the use of high efficiency detectors has been shown to be well suited for probing the properties of the Photon Strength Function of heavy (high level density) and/or radioactive (high background) nuclei. In this work we have investigated for the first time the validity of the recommended PSF of actinides, in particular U-235, Np-238 and Pu-241. Our study includes the search for resonance structures in the PSF below S-n and draws conclusions regarding their existence and their characteristics in terms of energy, width and electromagnetic nature.

Neutron-induced fission cross sections for Pb-nat and Bi-209 were measured with a white-spectrum neutron source at the CERN Neutron Time-of-Flight (n_TOF) facility. The experiment, using neutrons from threshold up to 1 GeV, provides the first results for these nuclei above 200 MeV. The cross sections were measured relative to U-235 and U-238 in a dedicated fission chamber with parallel plate avalanche counter detectors. Results are compared with previous experimental data. Upgraded parametrizations of the cross sections are presented, from threshold energy up to 1 GeV. The proposed new sets of fitting parameters improve former results along the whole energy range.

Neutron-induced fission cross-sections of minor actinides have been measured using the n_TOF white neutron source at CERN. Geneva, as part of a large experimental program aiming at collecting new data relevant for nuclear astrophysics and for the design of advanced reactor systems. The measurements at n_TOF take advantage of the innovative features of the n_TOF facility, namely the wide energy range, high instantaneous neutron flux and good energy resolution. Final results on the fission cross-section of U-233, (245)cm and Am-243 from thermal to 20 MeV are here reported, together with preliminary results for Am-241. The measurement have been performed with a dedicated Fast Ionization Chamber (FIC), a fission fragment detector with a very high efficiency, relative to the very well known cross-section of U-235, measured simultaneously with the same detector.