Eur. Phys. J. A (2019) 55: 120
https://doi.org/10.1140/epja/i2019-12802-7

Regular Article - Experimental Physics

Measurement of the ²³⁵U(n, f) cross section relative to the ⁶Li(n, t) and ¹⁰B(n,) standards from thermal to 170 keV neutron energy range at n_TOF

¹ INFN Laboratori Nazionali del Sud, Catania, Italy
² Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Bari, Italy
³ Agenzia nazionale per le nuove tecnologie (ENEA), Bologna, Italy
⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Bologna, Italy
⁵ Dipartimento di Fisica e Astronomia, Università di Bologna, Bologna, Italy
⁶ European Organization for Nuclear Research (CERN), Genève, Switzerland
⁷ University of Lodz, Lodz, Poland
⁸ Institut de Physique Nucléaire, CNRS-IN2P3, Univ. Paris-Sud, Université Paris-Saclay, F-91406, Orsay Cedex, France
⁹ Technische Universität Wien, Wien, Austria
¹⁰ CEA Irfu, Université Paris-Saclay, F-91191, Gif-sur-Yvette, France
¹¹ Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain
¹² Charles University, Prague, Czech Republic
¹³ University of Manchester, Manchester, UK
¹⁴ Department of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia
¹⁵ University of York, York, UK
¹⁶ University of Santiago de Compostela, Santiago de Compostela, Spain
¹⁷ Universitat Politècnica de Catalunya, Barcelona, Spain
¹⁸ Universidad de Sevilla, Sevilla, Spain
¹⁹ Dipartimento di Fisica, Università degli Studi di Bari, Bari, Italy
²⁰ Instituto de Física Corpuscular, CSIC - Universidad de Valencia, Valencia, Spain
²¹ Paul Scherrer Institut (PSI), Villingen, Switzerland
²² Instituto Superior Técnico, Lisbon, Portugal
²³ Joint Institute for Nuclear Research (JINR), Dubna, Russia
²⁴ Goethe University Frankfurt, Frankfurt, Germany
²⁵ Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania
²⁶ Japan Atomic Energy Agency (JAEA), Tokai-mura, Japan
²⁷ European Commission, Joint Research Centre, Geel, Retieseweg 111, B-2440, Geel, Belgium
²⁸ Karlsruhe Institute of Technology, Campus North, IKP, 76021, Karlsruhe, Germany
²⁹ National Technical University of Athens, Athens, Greece
³⁰ School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK
³¹ Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116, Braunschweig, Germany
³² INFN Laboratori Nazionali di Legnaro, Legnaro (PD), Italy
³³ Consiglio Nazionale delle Ricerche, Bari, Italy
³⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Trieste, Italy
³⁵ Dipartimento di Fisica e Astronomia, Università di Catania, Catania, Italy
³⁶ University of Ioannina, Ioannina, Greece
³⁷ University of Vienna, Faculty of Physics, Vienna, Austria
³⁸ University of Granada, Granada, Spain
³⁹ Department of Physics, University of Basel, Basel, Switzerland
⁴⁰ Centre for Astrophysics Research, University of Hertfordshire, Hatfield, UK
⁴¹ Bhabha Atomic Research Centre (BARC), Mumbai, India
⁴² Australian National University, Canberra, Australia

^* e-mail: finocchiaro@lns.infn.it

Received: 21 March 2019
Accepted: 10 June 2019
Published online: 26 July 2019

Abstract

The ²³⁵U(n, f ) cross section was measured at n_TOF relative to ⁶Li(n, t) and ¹⁰B(n,) , with high resolution ( m) and in a wide energy range (25meV-170keV) with 1.5% systematic uncertainty, making use of a stack of six samples and six silicon detectors placed in the neutron beam. This allowed us to make a direct comparison of the yields of the ²³⁵U(n, f ) and of the two reference reactions under the same experimental conditions, and taking into account the forward/backward emission asymmetry. A hint of an anomaly in the 10-30keV neutron energy range had been previously observed in other experiments, indicating a cross section systematically lower by several percent relative to major evaluations. The present results indicate that the cross section in the 9-18keV neutron energy range is indeed overestimated by almost 5% in the recently released evaluated data files ENDF/B-VIII.0 and JEFF3.3, as a consequence of a 7% overestimate in a single GMA node in the IAEA reference file. Furthermore, these new high-resolution data confirm the existence of resonance-like structures in the keV neutron energy region. The results here reported may lead to a reduction of the uncertainty in the 1-100keV neutron energy region. Finally, from the present data, a value of b·eV has been extracted for the cross section integral between 7.8 and 11eV, confirming the value of b·eV recently established as a standard.