Experimental determination of the ${}^{26}\mathrm{Al}$($n,\alpha$)${}^{23}\mathrm{Na}$ reaction cross section and calculation of the Maxwellian averaged cross section at stellar temperatures

The ${}^{26}\mathrm{Al}$($n,\alpha$)${}^{23}\mathrm{Na}$ reaction cross section has been studied at the linear accelerator GELINA of the Institute for Reference Materials and Measurements in Geel, Belgium, and has been determined up to a neutron energy of about 100 keV using the time-of-flight technique. Six resonances could be observed in this energy region, whereas before only one had been identified experimentally. For four of them, resonance parameters such as resonance energy, total width, area, and spin of the state could be determined. From the obtained ${}^{26}\mathrm{Al}$($n,\alpha$)${}^{23}\mathrm{Na}$ cross section data, Maxwellian averaged cross section (MACS) values were calculated by numerical integration. Since neutron induced reactions are among the major destruction mechanisms of ${}^{26}\mathrm{Al}$ in our Galaxy, these new MACS values contribute to a better understanding of the observed ${}^{26}\mathrm{Al}$ abundance.

Figure 1

Frisch gridded ionization chamber used in this work. The sample is mounted in the middle of the cathode, which is the lowest plate in the picture. The top plate is the anode; the plate in the middle is the Frisch grid.

Figure 2

Level scheme for the ${}^{26}\mathrm{Al}$($n,{\alpha }_i$)${}^{23}\mathrm{Na}$ and ${}^{26}\mathrm{Al}$($n,p_i$)${}^{26}\mathrm{Mg}$ reactions. The energies are given in MeV.

Figure 3

Background fit through the black resonance regions, represented by the dots, for the ${}^{10}\mathrm{B}$($n,\alpha$)${}^7\mathrm{Li}$ reaction.

Figure 4

Anode vs TOF spectrum for the ${}^{26}\mathrm{Al}$($n,{\alpha }_i$) measurement.

Figure 5

${}^{26}\mathrm{Al}$($n,{\alpha }_0+{\alpha }_1$)${}^{23}\mathrm{Na}$ cross section determined in this work (black line) compared with the ${}^{26}\mathrm{Al}$($n,{\alpha }_0$)${}^{23}\mathrm{Na}$ cross section obtained by Koehler et al. [11] (gray line).

Figure 6

Voigt fits to the ${}^{26}\mathrm{Al}$($n,{\alpha }_0+{\alpha }_1$)${}^{23}\mathrm{Na}$ cross section data for the four resonances.

Figure 7

Total fitted width represented by triangles compared with the calculated experimental broadening (gray line) for the four observed resonances.

Figure 8

${}^{26}\mathrm{Al}$($n,{\alpha }_0+{\alpha }_1$)${}^{23}\mathrm{Na}$ MACS values calculated by numerical integration of the obtained cross section data (black line). A comparison is made with the experimental values from Koehler et al. [11] and Skelton et al. [13] and with theoretical values. The latter ones are represented by gray symbols.

Figure 9

Cross section data obtained for the first resonance at 5.9 keV (black line) compared with the data of Koehler et al. [11] (gray dashed line). The resonance area obtained by Koehler is much higher (but less precise) than the area obtained in this work, as indicated by the ${\omega }_{{\alpha }_0}$ value in Table .