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Neutron-induced cross sections via the surrogate method

Authors: Boutoux, Guillaume;

Neutron-induced cross sections via the surrogate method

Abstract

Neutron-induced cross sections of short-lived nuclei are needed for fundamental and applied physics as nuclear energy or astrophysics. However, very often the high radioactivity of the samples makes the direct measurement of these cross sections extremely difficult. The surrogate reaction method is an indirect way of determining neutron-induced cross sections through transfer or inelastic scattering reactions. This method presents the advantage that in some cases the target material is stable or less radioactive than the material required for a neutron-induced measurement. The method is based on the hypothesis that the excited nucleus is a compound nucleus whose decay depends essentially on its excitation energy and on the spin and parity state of the populated compound state. Nevertheless, the spin and parity population differences between the compound-nuclei produced in the neutron and transfer-induced reactions may be different. This work reviews the surrogate method and its validity. Neutron-induced fission cross sections obtained with the surrogate method are in general good agreement. However, it is not yet clear to what extent the surrogate method can be applied to infer radiative capture cross sections. We performed an experiment to determine the gamma-decay probabilities for 176Lu and 173Yb by using the surrogate reactions 174Yb(3He,p)176Lu* and 174Yb(3He,alpha)173Yb*, respectively, and compare them with the well-known corresponding probabilities obtained in the 175Lu(n,gamma) and 172Yb(n,gamma) reactions. This experiment provides answers to understand why, in the case of gamma-decay, the surrogate method gives significant deviations compared to the corresponding neutron-induced reaction. In this work, we have also assessed whether the surrogate method can be applied to extract capture probabilities in the actinide region. Previous experiments on fission have also been reinterpreted. Thus, this work provides new insights into the surrogate method.; Les sections efficaces neutroniques des noyaux de courte durée de vie sont des données cruciales pour la physique fondamentale et appliquée dans des domaines tels que la physique des réacteurs ou l'astrophysique nucléaire. En général, l'extrême radioactivité de ces noyaux ne nous permet pas de procéder à des mesures induites par neutrons. Cependant, il existe une méthode de substitution (" surrogate " dans la littérature) qui permet de déterminer ces sections efficaces neutroniques par l'intermédiaire de réactions de transfert ou de réactions de diffusion inélastique. Son intérêt principal est de pouvoir utiliser des cibles moins radioactives et ainsi d'accéder à des sections efficaces neutroniques qui ne pourraient pas être mesurées directement. La méthode est basée sur l'hypothèse de formation d'un noyau composé et sur le fait que la désexcitation ne dépend essentiellement que de l'énergie d'excitation et du spin et parité de l'état composé peuplé. Toutefois, les distributions de moments angulaires et parités peuplés dans des réactions de transfert et celles induites par neutrons sont susceptibles d'être différentes. Ce travail fait l'état de l'art sur la méthode substitution et sa validité. En général, la méthode de substitution fonctionne très bien pour extraire des sections efficaces de fission. Par contre, la méthode de substitution dédiée à la capture radiative est mise à mal par la comparaison aux réactions induites par neutrons. Nous avons réalisé une expérience afin de déterminer les probabilités de désexcitation gamma du 176Lu et du 173Yb à partir des réactions de substitution 174Yb(3He,p)176Lu* et 174Yb(3He,alpha)173Yb*, respectivement, et nous les avons comparées avec les probabilités de capture radiative correspondantes aux réactions 175Lu(n,gamma) et 172Yb(n,gamma) qui sont bien connues. Cette expérience a permis de comprendre pourquoi, dans le cas de la désexcitation gamma, la méthode de substitution donne des écarts importants par rapport à la réaction neutronique correspondante. Ce travail dans la région de terres rares a permis d'évaluer dans quelle mesure la méthode de substitution peut s'appliquer pour extraire des probabilités de capture dans la région des actinides. Des expériences précédentes sur la fission ont aussi pu être réinterprétées. Ce travail apporte donc un éclairage nouveau sur la méthode de substitution.

Country
France
Keywords

fonctions de poids, "C6D6", efficacités de détection d'une cascade-gamma, "fission", "radiative capture", "TALYS code, noyau composé, [PHYS.NEXP]Physics [physics]/Nuclear Experiment [nucl-ex], "neutron-induced cross sections", "angular momentum distribution", surrogate reaction method", "pulse height weighting functions", distributions de moments angulaires, fission, surrogate, Hauser-Feshbach, "gamma-cascade detection efficiency", TALYS, "gamma-decay probability", "3He-induced transfer reactions", "compound nucleus", "Hauser-Feshbach", [PHYS.NEXP] Physics/Nuclear Experiment, méthode de substitution, probabilité de desexcitation gamma, "Weisskopf-Ewing", capture radiative, germanium, "PHWT", "neutron emission", réactions de transfert induites par 3He, "spins", "germanium", Weisskopf-Ewing, sections efficaces neutroniques, émission de neutrons, spins, C6D6

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citations
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
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