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Measuring the 12C(n,2n)11C Cross Section
May 2014 to May 2016
Reaching a net positive energy output is one of the longterm goals of fusion research. This would allow for the construction of extraordinarily efficient fusion power plants. One approach to fusion is by inertial confinement, in which hydrogen isotopes are compressed by the ablation of a spherical shell using lasers, creating a hot and dense core.
The areal density of this core is the product of its density and radius. Once the areal density surpasses the Lawson Criterion, the nuclear fuel is ignited and becomes a viable source of energy. Carbon-12 can be used as a diagnostic to measure the areal density of an inertial confinement fusion reaction.
To make this measurement, the carbon is exposed to the fusion reaction. High-energy neutrons from fusion scatter from the carbon, removing neutrons from their nuclei. This causes the carbon to become unstable and decay into boron by releasing a positron.
The fraction of carbon that decayed was measured using NaI(Tl) scintillation detectors that counted coincidence 511 keV gamma from positron annihilation.
This left a gap in measuring the areal density of the fusion reaction. There was no measurement of how likely it was that fusion neutrons would cause the carbon to decay - the cross section of the 12C(n,2n)11C reaction.