# Experimental beta decay spectrum of free neutrons below 150 keV?

From a pedagogical point of view, beta decay is a really sweet example of relativistic kinematics, and seemingly the best and most natural example to use would be the decay of free neutrons. This is a nice example because the daughter has Z=1, so the Coulomb shift is pretty negligible, and that simplifies the theory a lot.

I found a high-precision measurement of the electron's energy spectrum in a thesis by Hickerson, with the UCNA collaboration, which does fit nicely with the simplest kinematic model at the energies where their experiment had data. However, their experiment basically has a cut-off below 150 keV. They modeled the cut-off carefully, but basically it's not that nice pedagogically not to have something at the low energies that is directly comparable to experiment.

Is there any measurement that extends to lower energies, even one with large error bars?

Kappa is the energy of the electron relative to the maximum available in the decay. D is the probability distribution of kappa, normalized so that $$\int D d\kappa=1$$.