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The U-236(n,f) cross section has been measured in the energy range from 0.5 eV to 25 keV at the Geel Electron Linear Accelerator neutron time-of-flight facility of the Institute for Reference Materials and Measurements in Geel, Belgium. A highly enriched U-236 sample was mounted back-to-back with a B-10 sample in the center of a Frisch-gridded ionization chamber, hence realizing a 2 pi detection geometry. A U-235(n,f) cross-section control measurement was performed in the same experimental conditions. Special attention has been given to the fission resonance integral I-f and to the strongest resonance at 5.45 eV, for which a resonance analysis has been performed yielding Gamma(f) = 1.7 mu eV Both values are highly overestimated in the literature.

The Cl-36(n,p)S-36 and Cl-36(n,alpha)P-33 reaction cross sections have been studied with resonance neutrons at the linear accelerator GELINA of the IRMM in Geel (Belgium) and have been determined up to approximately 250 keV using the time-of-flight technique. In this energy region, 17 resonances were observed, whereas eight were identified before. For some resonances the resonance strength, the spin, and the total width could be determined. From the obtained cross section data, the MACS has been calculated by numerical integration. These updated MACS values were used in stellar models to trace the origin of the rare isotope S-36.

Neutron induced reactions on 26Al, 36Cl and 41Ca and their astrophysical implications In this work (n,p) and (n,alpha) reactions on 26Al, 36Cl and 41Ca are studied as a function of the neutron energy. The measurements were performed at the high resolution GELINA time-of-flight facility of the IRMM in Geel, Belgium. Besides the nuclear physics information obtained from the resonance analysis of the reaction cross sections, these reactions are of importance in the understanding of the observed 36S and 26Al solar abundances. In the case of 26Al, the 26Al(n,alpha)23Na cross section up to 45 keV has been determined. Six resonances are observed. For three of them, the total level width and the spin could be calculated. For most of the resonances the obtained resonance parameters are in agreement with previous data. The calculated Maxwellian Averaged Cross Section values (MACS) used in stellar model calculations confirm that 26Al is indeed severely depleted by neutron captures in AGB stars. In the (n,p) and (n,alpha) measurements on 36Cl, eighteen resonances are observed in the energy region up to 250 keV, whereas eight were identified before. Only the lowest energy resonance shows a significant (n,alpha)-contribution of (76+-7)%, which is in perfect agreement with the value reported before. Furthermore, for four resonances, the resonance strength, spin, total and partial width Gamma_p could be determined. They are in good agreement with previous data, but the achieved accuracy is better. The calculated MACS values are used in stellar model calculations to trace the origin of 36S and reveal that the weak component of the s-process occurring in massive stars accounts for almost the entire production of solar 36S. The 41Ca(n,alpha)38Ar measurement is the first ever reported in the resonance region and affects the 36S abundance through 41Ca(n,alpha)38Ar(n,gamma)39Ar(n,alpha)36S. Twelve resonances are observed in the energy region up to 45 keV. For most of them the area, the total width, the spin and a value for Gamma_n/Gamma_p could be determined. After extension of the energy range, with a complementary measurement at the GELINA facility, the calculated MACS values will be used in stellar models.

This paper reports on the results of measurements of the (n,p) and (n,alpha) reactions on Al-26 and Cl-36, in the neutron energy region up to 140 keV and 350 keV, respectively. The experiments were carried out at the GELINA neutron facility of the IRMM (Geel, Belgium). For both reactions several resonances could be identified which were never observed before. Using the obtained cross section data, reaction rates were calculated and compared with other experimental and theoretical values.

New high resolution measurements of the Cl-36 (n,p)S-36 and Cl-36(n,alpha)P-33 reaction cross sections are reported. Some 20 resonances have been identified in the neutron energy region from 0.1 keV to 350 keV.