Isobaric analog impurities from neutron capture and transmission by magnesium

Abstract

Reso»rice neutron interactions with the stable isotopes of magnesium were measured at the Oak Ridge electron linear accelerator time-of-flight facility. The transmission of a natural metal sample (78.7% Mg} was measured at 200 m and capture by natural metal and by enriched isotope samples was measured at 40 m. Twenty-six reso»races in Mg+ n up to 1.8 MeV were fitted with Breit-Wigner multilevel parameters. The data were sufficient to assign spin and parity to 19 of these. The capture data were analyzed for resonances up to 850 keV for Mg+n, 265 keV for 2 Mg+n (17+ resonances), and 440 keV for Mg+n (4 resonances). Average capture at stellar interior temperatures was calculated. The Mg+ n data serve to assess the isospin impurities in three isobaric analog states. Three other states exhibit reduced neutron widths each several percent of the Wigner limit which may be understood in terms of simple shell model configurations.

Resonance neutron-capture cross sections of stable magnesium isotopes and their astrophysical implications

“…The differences between the present capture kernels and those of Ref. [9] do not appear to indicate this neutron sensitivity effect.…”

“…[9] and activation experiments by Mohr et al [20,21] are consistent with the 044615-2 direct radiative capture (DRC) component [22] that is not covered by the TOF technique. For this part, the thermal capture cross section reported in Ref.…”

“…[9]. Moreover, the spin and parity assignment of this level was confirmed since the χ 2 /DOF (χ 2 is chi-squared and DOF is the number of degrees of freedom) value of the fit (in the energy range 15-30 keV) was 1.6 assuming J π = 2 + and 5.6 assuming J π = 3 + .…”

“…The present capture data were analyzed together with transmission data from ORELA [9], available from the Experimental Nuclear Reaction Data (EXFOR) database [35], using the R-matrix code SAMMY [36]. A simultaneous resonance shape analysis of capture and transmission data results in much more reliable resonance parameters than can be obtained through independent analyses of the various data sets.…”

“…Because the energy range of interest for s-process temperatures is slightly larger than the one covered by the evaluation, Koehler [8] recently reanalyzed * Cristian.Massimi@bo.infn.it † Deceased. existing data [9] to derive an improved set of resonance parameters. His analysis included very high resolution data for the total cross section obtained with a metallic sample of natural Mg and high-resolution capture data measured with an enriched 25 Mg sample, both from experiments at the Oak Ridge Electron Linear Accelerator (ORELA) neutron time-of-flight facility [10].…”