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Electrical Engineering, Measurement and Control Technology
A detailed measurement of the 27Al(n,2n)26Al reaction cross sections was performed in the near-threshold region (Eth=13.54 MeV), and its possible applicability for ion temperature measurements was investigated. The production of the long-lived radionuclide 26Al (t1/2=7.2×105 a) is of considerable interest to the fusion reactor program. Particularly long-lived radionuclides may lead to a significant long-term waste-disposal. Al-containing materials and Si carbide are candidate materials for fusion-reactor systems. The Al(n,2n) reaction and the two step process 28Si(n,np+d)27Al(n,2n) are the dominating processes for the formation of 26Al in a fusion reactor.1 The 27Al(n,2n)26Al reaction is expected to vary strongly with neutron energy above threshold. An accurate description of the excitation function is necessary to estimate the production of 26Al in a typical D–T fusion environment. From the existing data on cross sections it was not possible to produce an unambiguous excitation function. We started therefore a project to determine this excitation function more accurately. It has been pointed out by Smither and Greenwood2 that the 27Al(n,2n)26Al reaction can be used as a monitor to determine the ion temperature in a D–T fusion plasma. This method makes use of the neutron energy distribution as a sensitive function of the plasma ion temperature. The temperature sensitivity is most pronounced if the excitation function is strongly nonlinear and if the threshold falls within the energy region of the emitted neutrons: For the 27Al(n,2n)26Al reaction the threshold lies at 13.54 MeV and the (n,2n) reaction is expected to a strongly varying function of the neutron energy near threshold. Al samples were irradiated with 14 MeV neutrons generated via the T(d,n)4He reaction at three different laboratories under different conditions. The produced 26Al was measured using the extremely sensitive method of accelerator mass spectrometry (AMS). 26Al/27Al isotope ratios as low as 10−15 could be measured with the Vienna Environmental Research Accelerator (VERA) corresponding to cross-section values as low as 0.03 mbar. The results from the different neutron irradiations agree very well with each other. The absolute cross section values could be measured to 5% (mainly systematic errors). The slope of the excitation function is reproduced to better than 1% from our measurements. With this improved knowledge the sensitivity for monitoring the ion temperature in a D–T plasma was investigated. For monitoring the ion temperature of a thermal plasma a sensitivity of a few percent can be achieved by this method. In case of nonthermal plasmas with neutral beam injection (NBI) the mean neutron energy depends on the direction under which the plasma is viewed. A very high sensitivity for the case of backward viewing for NBI is obtained for the 27Al(n,2n)26Al reaction reflecting the closer distance of the mean neutron energy to the reaction threshold (Eth). Together with the very sensitive method of AMS this reaction may serve as a useful tool in plasma diagnostics. © 2001 American Institute of Physics.
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