Monte Carlo simulations of a neutron lifetime experiment with a big gravitational trap

Abstract

A Monte Carlo model was developed for our current neutron lifetime experiment using storage of ultracold neutrons (UCN) in a big gravitational trap. The model allows us to calculate neutron trajectories in the given geometry, taking into account gravity, and has been used in simulations that reproduce all stages of the experiment. The simulation was proven able to reproduce the time-dependence of the detected neutron count rates. For investigation of systematic effects, the value of the neutron lifetime resulting from the simulation of the experimental procedure was compared to the value entered as an input parameter. In particular, the systematic uncertainty associated with the method of calculating the effective UCN collision frequency in the trap was found not to exceed 0.1 s. In addition, calculations showed that up to 1% uncoated area of the copper trap has a negligible influence on the neutron lifetime result.

Precise Measurements of the Decay of Free Neutrons

“…At present, this nuclear value is more precise than the neutron value by a factor 2.5, and an order of magnitude better than the value from the rare (1.0×10 -8 ) pion decay (132). Altogether, |𝑉 𝑢𝑑 | 0−0 = 0.97373(31), |𝑉 𝑢𝑑 | 𝑛 = 0.97377 (78), |𝑉 𝑢𝑑 | 𝜋 = 0.97390(290). 9.…”

“…The UCN energy intervals corresponding to the successive tilt angles are 0-40.1, 40.1-50.5, and 50.5-56.6 neV. The total process is simulated by Monte Carlo, which reliably reproduces the UCN rates over more than four orders of magnitude (77). The longest measured storage time is only 2.5 s below the extrapolated result, τ n = 881.5(0.7)(0.6) (the first error is statistical, the second systematic).…”