Neutron Lifetime Experiments Using the Beam Method: Past, Present, and Future

Abstract

The oldest method for measuring the neutron lifetime combines the rate of neutron decay in a slow neutron beam, found by counting the final state charged particles, with absolute determinations of the neutron flux and the effective decay volume to obtain the specific activity of the beam and hence the radioactive decay constant. This is known as the "beam method." The most recent beam neutron lifetime experiment 1 achieved an experimental uncertainty of 3.4 s, dominated by systematic limitations of the neutron flux measurement. Recent ultracold neutron "bottle" experiments have obtained uncertainties at or just below the 1 s level. The effort to reduce the neutron lifetime uncertainty to 0.1 s will benefit from both approaches because, while they are both challenged by difficult systematic effects at that level, the nature of these effects is very different for the two methods. We present a brief review of the principles and history of the beam method and outline a path for future beam experiments to improve the neutron lifetime uncertainty to 0.1 s and below.