Cross section for inelastic neutron ``acceleration'' by178Hfm2

Abstract

The scattering of thermal neutrons from isomeric nuclei may include events in which the outgoing neutrons have increased kinetic energy. This process has been called inelastic neutron acceleration, or INNA, and occurs when the final nucleus, after emission of the neutron, is left in a state with lower energy than that of the isomer. The result, therefore, is an induced depletion of the isomer to the ground state. A cascade of several γ 's must accompany the neutron emission to release the high angular momentum of the initial isomeric state. INNA was previously observed in a few cases, and the measured cross sections were only in modest agreement with theoretical estimates. The most recent measurement of an INNA cross section was σ INNA = 258 ± 58 b for neutron scattering by 177 Lu m . In the present work, an INNA cross section of σ INNA = 168 ± 33 b was deduced from measurements of the total burnup of the high-spin, four-quasiparticle isomer 178 Hf m2 during irradiation by thermal neutrons. Statistical estimates for the probability of different reaction channels past neutron absorption were used in the analysis, and the deduced σ INNA was compared to the theoretically predicted cross section.

Nuclear structure and depletion of nuclear isomers using electron linacs

“…Research on depletion of 180m Ta still represents the archetype for present and future efforts, in that a partial synthesis [21] of depletion and spectroscopic results underscored the importance of nuclear structure. With the exception of neutron capture [16,17], which proceeds through formation of a compound nucleus, isomer depletion occurs via transitions within a nuclide and requires the presence of one or more intermediate states (IS) lying higher in energy than the isomer. To serve as an intermediate state, a nuclear level must be capable of excitation from the isomer, and the level must have a branch of its decay that leads directly or by cascade to the ground state.…”

“…So far, induced depletion has been experimentally demonstrated for four isomers, using various stimulae: 180m Ta (by bremsstrahlung [13] and Coulomb excitation [14]); 68m Cu (by Coulomb excitation [15]); and 177m Lu and 178m2 Hf (by thermal neutrons [16,17]). Induced depletion of 178m2 Hf by low-energy (~ 10 keV) photons was first reported in 1999 [18], but remains to be duplicated by independent studies [19,20].…”

Metastable nuclear isomers as dark matter accelerators

“…1 where the form-factor for different ∆J are no longer suppressed for larger momentum exchange. Many standard model projectiles [9][10][11] have in fact been employed to induce down-scattering of isomers, and extract the excess energy in the process. We are going to show that there are classes of DM particles that indeed lift this momentum suppression, and their direct detection scenarios would greatly benefit from the scattering off metastable isomers.…”

Direct evidence for inelastic neutron “acceleration” byLu177m

“…The 258 ± 58 b high value of the thermal INNA reaction cross section for 177 Lu m has been indirectly deduced from burn-up [9] and capture cross sections [10]. Such an analysis has recently been achieved for INNA by the 178 Hf m2 with an estimated cross section of 168 ± 33 b [8]. In the case of 177 Lu m , the radiative capture cross section has been extracted under two reasonable hypothesis: (i) the 177 Lu m (n,γ ) 178 Lu m cross section corresponds to the total radiative capture cross section; (ii) the existence of an unknown long-lived isomer which could trap a part of the γ decay is very unlikely.…”

“…Thanks to the usually high cross section of thermal neutron interaction, such a process could be used to induce a fast de-excitation of an isomer. The 160-day J π = 23/2 − isomer in 177 Lu (Ex = 970 keV) and the 31-year J π = 16 + 178 Hf m2 [8] are good candidates to directly observe the inelastic neutron acceleration reaction. The 258 ± 58 b high value of the thermal INNA reaction cross section for 177 Lu m has been indirectly deduced from burn-up [9] and capture cross sections [10].…”