Precise measurement of the $²⁷Al$(n, 2n)$^26gAl$ excitation function near threshold and its relevance for fusion-plasma technology

A new accurate measurement of the Al-27(n,2n)Al-26 excitation function leading to the ground state of Al-26 (t(1/2) = 7.1 x 10(5) years) in the near-threshold region (E-th = 13.55 MeV) was performed, with the goal to achieve relative cross-sections with the highest accuracy possible using proven methods. In addition, the measurements were also designed to provide good absolute cross-section values, since absolute cross-sections axe important for radioactive waste predictions in future fusion reactor materials. Samples of Al metal were irradiated with neutrons in the energy range near threshold (E-n = 13.5-14.8 MeV) in Vienna and St. Petersburg, and at 14.8 MeV in Tokai-mura. In addition, irradiations with neutrons of higher energies (17 and 19 MeV) were performed in Tubingen, to obtain also cross-section values well above threshold. The amount of Al-26 produced during the irradiations was measured via accelerator mass spectrometry (AMS). With this system, a background as low as 3 x 10(-15) for Al-26/Al-27 isotope ratios was obtained, corresponding to a (n,2n) cross-section of 0.04 mb. Utilizing AMS, cross-sections with much higher precision and considerably closer to the threshold than in previous investigations were measured. A substantial improvement in the knowledge, of this excitation function was obtained. Its expected strongly non-linear behaviour near threshold makes the production of Al-26 sensitive to temperature changes in a deuteriuin-tritium (D-T) fusion plasma. The prerequisite for such an application as a temperature monitor, namely a very well-known shape of the excitation function, was met. A quantitative prediction of the sensitivity of this method for monitoring the temperature in a D-T fusion plasma was therefore possible.