Neutron optics of the PSI ultracold-neutron source: characterization and simulation

Abstract

The ultracold neutron (UCN) source at the Paul Scherrer Institute serves mainly experiments in fundamental physics. High UCN intensities are the key for progress and success in such experiments.A detailed understanding of all source parameters is required for future improvements. Here we present the UCN source components, elements of the neutron optics, the characterization of important related parameters like emptying times, storage times or transmission probabilities of UCN which are ultimately defining the UCN intensity delivered at the beamports. We also introduce a detailed simulation model of the PSI UCN source, used to analyze the measurements and to extract surface parameters.

Measurement of the Permanent Electric Dipole Moment of the Neutron

“…Using the same trap geometry as in Ref. [15] and a softer initial UCN spectrum [33], we use the same value for ϵ and η. The error from heating was estimated to be 1 × 10 −30 e:cm, while the error from rotational motion dominates: 2 × 10 −28 e:cm.…”

The design of the n2EDM experiment

“…The prediction of the number of detected neutrons in the n2EDM apparatus is based on comprehensive Monte-Carlo simulations of the PSI UCN source, guides, and the experiment, treated as one system, performed with the MCUCN code [17,28].…”

“…The final sensitivity listed in the first column is that actually achieved in 2016; that of the second column represents the achievable sensitivity in n2EDM after an assumed 500 days of data taking, which could be achieved in four neutron optics and the UCN flux were calibrated using dedicated test measurements of the achievable density at the West-1 beamport in 2016 [29,30]. The simulation parameters and values are listed in Table 4 of [17]. These are: optical (Fermi) potential, loss per bounce parameter, fraction of diffuse (Lambertian) reflections, and the attenuation constant of the windows.…”

“…This was very close to the values reported in [33] for DLC (on aluminum foil at 300 K), and in [34] for dPS. We used optical potentials of 230 neV for DLC [17] and 165 neV for dPS, with the latter being the average of measured and theoretical values [34] (because of a large Fig. 5 Simulated energy spectra of the detected UCNs.…”

“…The most sensitive neutron EDM search delivered a result of d n = (0 ± 1.1 stat ± 0.2 sys ) × 10 −26 e cm, which sets an upper limit of |d n |< 1.8 × 10 −26 e cm (90% CL) [4]. This measurement was performed with the apparatus originally built by the RAL/Sussex/ILL collaboration [14], which went through continuous upgrades of almost all subsystems and was also moved to the source of ultracold neutrons (UCNs) [15][16][17] at the Paul Scherrer Institute (PSI). Arguably the most important of the upgrades were the addition of an array of atomic cesium magnetometers [18] and of a dual spin detection system [19].…”