Detection of molecular hydrogen in a neutron beam lifetime experiment

TL;DR

This work assesses whether residual molecular hydrogen in a cold-beam neutron lifetime experiment could bias proton counting via charge exchange that creates H$_{2}^+$ in the proton trap. Using the BL2 apparatus, the authors identify H$_{2}^+$ through timing and energy signatures and employ GEANT4, SRIM, and COMSOL-based simulations to model trapping, transport, and detection. They quantify detection efficiencies and estimate potential lifetime biases, finding that under typical configurations the effect is small (a few percent admixture of H$_{2}^+$ changes the proton efficiency by roughly 1% or less, corresponding to sub-s-second shifts in the lifetime), and thus unlikely to explain discrepancies with recent UCN measurements. For the earlier BL1 result, they establish an upper bound on H$_{2}^+$ admixture and conclude any potential bias would be $<0.5$ s, reinforcing that residual H$_2$ is not the source of the neutron lifetime tension but should be monitored in future higher-precision beam experiments.

Abstract

One method of determining the free neutron lifetime involves the absolute counting of neutrons and trapped decay protons. In such experiments, a cold neutron beam traverses a segmented proton trap inside a superconducting solenoid while the neutron flux is continuously monitored. Protons that are born within the fiducial volume of the trap are confined radially by the magnetic field and axially by the electrostatic potential supplied by trap electrodes. They are periodically released and counted, and the ratio of the absolute number of neutrons to protons is proportional to the neutron lifetime. Systematic error can be introduced if protons in the trap are lost, gained, or misidentified. The influence of molecular hydrogen interactions is of particular interest because of its ubiquitous presence in ultrahigh vacuum systems. To understand how it could affect the neutron lifetime, measurements were performed on the production and detection of molecular hydrogen in an apparatus used to measure the neutron lifetime. We demonstrate that charge exchange with molecular hydrogen can occur with trapped protons, and we determine the efficiency with which the molecular hydrogen ions in the trap are detected. Finally, we comment on the potential impact on a neutron lifetime experiment using this beam technique. We find that the result of the beam neutron lifetime performed at NIST is unlikely to have been significantly affected by charge exchange with molecular hydrogen.

Figures (21)

• Figure 1: Saturated vapor pressure of common gases as a function of temperature. (Reprinted with permission from Ref. BarCohen2016.)
• Figure 2: Layout of the apparatus for this work. The neutron beam travels left to right and is detected downstream from the trap. Decay protons are detected upstream of the trap near the bend. Although it is one contiguous vacuum system, there are two thin, silicon windows that can isolate the bore from the rest of the vacuum system.
• Figure 3: Photo of the segmented electrode proton trap.
• Figure 4: Schematic of the proton trap and detector (top) during a trapping mode. The diagrams below it illustrate the electrostatic potentials in the trap during the three modes of a trapping cycle.
• Figure 5: 2-dimensional histogram of the arrival time versus the energy after the door voltage of the trap is lowered.