Accessing neutron-induced cross sections of actinides. Advantages and challenges of direct and indirect methods, seminar by P. Marini (CENBG)

par Dr Paola Marini

vendredi 27 févr. 2015, 10:30 → 11:30 Europe/Paris

seminar room, 2nd floor (bat 27)

Nuclear cross sections play an important role in nuclear physics and its applications. In particular, cross sections for reactions of neutrons and light charged particles on a wide range of isotopes and energies (from several keV to tens of MeV) are needed both for nuclear energy applications [1] and nuclear astrophysics [1], and give access to fundamental nuclear properties.In the panorama of nuclear energy applications, precision and accuracy of fundamental nuclear data are crucial because they determine the precision of key parameters (for instance the neutron multiplication factor) of Generation IV reactors and accelerator driven systems. There are two complementary methods to measure neutron-induced reaction cross sections, in which the Aval du Cycle et Énergie Nucléaire (ACEN) group of Bordeaux is involved: a direct method, based on the irradiation with neutrons of the sample of interest, and an indirect method, based on transfer (or surrogate) reactions, to produce the nucleus of interest and study its decay. Both methods face constraints on the target choice, and experimental and/or theoretical difficulties to infer the cross section of interest from the measured quantities. Direct measurements have four main requirements: the availability of a neutron source in the needed energy range and of the target of interest, and the full control over the experimental setup and the normalization of the cross section method. The latter relies on the measurement of the neutron flux via a known standard used as reference (typically 235U(n,f), 238U(n,f) and 237Np(n,f) cross sections). Recently, important discrepancies with respect to the evaluations were pointed out by the nTOF Collaboration in the 237Np(n,f) cross section [2]. To avoid systematic errors associated to the use of these standards, the ACEN group measures the neutron flux with respect to the 1H(n,p) elastic cross section, which is the best known standard in the energy range between 0.1 and 50 MeV. I will discuss the method, its validation at neutron energies above few MeV and the challenges associated to extending it at neutron energies below 1 MeV. However, data for many relevant isotopes are often not experimentally accessible via direct measurements due to their short half-life. The group develops therefore an alternative method, the so-called surrogate reaction method. Proposed for the first time in the 70's [3], it is an indirect method which aims at determining compound nucleus reaction cross sections involving short lived and/or difficult-to-produce targets. The method is based on the assumption of the independence of the compound nucleus decay probability in a given channel on the formation channel (Bohr hypothesis): the same compound nucleus B* formed in a neutron-induced reaction (n+A- >B*) is now formed in a transfer reaction on a slightly different (but more accessible) target nucleus (d+D- >B*+b). It is important to notice that the spin-parity population of the compound nuclei produced in neutron-induced and transfer reactions may be different, due to differences in the spin and parity in the entrance channel of both reactions, and the different relative angular momentum that can be transferred in the two reactions. This limits the applicability of the method to reactions weakly dependent on the details of the structure of the compound nucleus. I will review the open questions of this method, the first results of a recently performed experiment and a new strategy aiming at overcoming the present limitations of the method. [1] J. E. Escher et al. Rev. Mod. Phys., 84:353, 2012. and refs. therein. [2] C. Paradela, L. Tassan-Got, L. Audouin, B. Berthier, I. Duran, L. Ferrant, and S. Isaev. Phys. Rev. C, 82:034601, 2010. [3] H. C. Cramer, J. D. Britt. Nucl. Sci. En., 41:177, 1970.