Nanoparticles Slow Down Neutrons With Spin

Neutrons are of course one of the two primary constituents of the nucleus. If not found in a nucleus, the neutron must therefore have gained the energy to escape the strong nuclear force and will probably be travelling very, very fast. The term “cold neutron” is not really a comment of temperature but really about the speed of the neutron with the title being given to neutrons of energy less than 10−7 eV. Being able to slow neutrons down is very important especially in nuclear physics as the way to trigger nuclear fission is to convince a neutron to enter the fissile nucleus which will only do when travelling at slow speeds. Simply isolating slow neutrons from the the full Boltzmann range is a very inefficient task as you can imagine the amount of low enough energy is a considerably small percentage.

One of the methods that has been previously suggested is to use magnetic nanoparticles or shell based particles like C60, with low neutron absorptions, submerged in liquid helium as a medium for the neutrons to pass through. Because the nanoparticles have low friction and can flow easily they have no trouble absorbing the impact energy of neutrons that pass through. Being highly coherent in this cold state means that the impact chance is considerably greater and the impacts themselves can be viewed under simple momentum and energy conservation laws. During the experiments this process was able to increase the number of cold neutrons by over a hundred times.

However the “simple” view on the collisions ignores any rotational effects that may have been present. Since these logically could have had a considerable effect the current study worked on the same basis as the previous but wished to acknowledge the ability of the nanoparticles to spin. Through a series of theoretical predictions it was shown that these rotational effects are considerably great enough to be included. It was found that due to energy and momentum being stored in rotational energy of the nanoparticles an extra decease of neutron speed of 40% could be achieved if these effects were optimised. This means that the medium has the potential to be 40% more efficient per collision and considering the nanoparticles would be expected to scatter hundreds of neutrons this possible 40% gain in efficiency is very promising.

Paper links: Rotational Effects of Nanoparticles for Cooling down Ultracold Neutrons


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