Experimental study of ultracold neutron production in pressurized superfluid helium

We investigate experimentally the pressure dependence of the production of ultracold neutrons (UCNs) in superfluid helium in the range from saturated vapor pressure to 20 bar. A neutron velocity selector allows the separation of underlying single-phonon and multiphonon processes by varying the incident cold neutron (CN) wavelength in the range from $3.5$ to 10 Å. The predicted pressure dependence of UCN production derived from inelastic neutron scattering data is confirmed for the single-phonon excitation. For multiphonon-based UCN production we found no significant dependence on pressure, whereas calculations from inelastic neutron scattering data predict an increase of 43(6)% at 20 bar relative to saturated vapor pressure. From our data we conclude that applying pressure to superfluid helium does not increase the overall UCN production rate at a typical CN guide.

Figure 1

Schematic of the setup (not to scale): The CN beam passed through the velocity selector (a), a secondary CN guide (b), which could be closed off with a fast shutter (c), and a beam monitor with circular orifices (d) before it entered the cryostat through a series of thin aluminum windows (e). The He-II target was kept in the Ni-coated stainless-steel pressure container (g); CN entrance and exit sides from aluminum were covered with nickel half-spheres (q). Ultracold neutrons were guided by a polished stainless-steel tube (f), which was separated by an aluminum window from the production volume, to the ${}^3\mathrm{He}$ UCN detector (m). The CN beam exited the cryostat through aluminum windows (h) before entering a chopper (k) and TOF detector (l) for TOF spectral analysis. Pressure was applied via the filling line (p) connected to the bottom of the pressure container and measured in a static line with a gauge (o).

Figure 2

Time-of-flight spectra of the CN beam measured with an empty production volume, for various velocity selector rotation speeds at $-5^{\circ }$ [solid (green) line] and $+5^{\circ }$ (dashed black line) selector alignment. Each raw spectrum was normalized to area unity and then multiplied by the respective monitor count rate corrected for deadtime and $1/v$ efficiency. The difference in integral counts for $-5^{\circ }$ and $+5^{\circ }$ was due to the larger transmission of the velocity selector at $-5^{\circ }$.

Figure 3

Count rate observed in the UCN detector (a). Shown is the combined data of all measurements at SVP and $\lambda \sim 8.7$ Å (maximum of UCN production, ${\lambda }_{\text{cor}}=8.92$ Å). Vertical lines indicate the three different phases. The solid line in phase (iii) is the best fit (see text) to the data with a reduced ${\chi }^2=1.04$. Decay time constants $\tau$ for different pressures were constant within the error bars (b): The red line represents the weighted mean; the blue line, the error of the weighted mean.

Figure 4

Measured wavelength-resolved UCN production rate per incident CN for $p=\mathrm{SVP}$ [(red) circles], $p=6$ bar (black squares), $p=11$ bar [(blue) diamonds], $p=16$ bar [(magenta) stars], and $p=20$ bar [(brown) triangles]. Lines are best fits of logarithmic normal distributions to the data with a reduced ${\chi }^2=1.3$, 1.2, 2.0, 1.9, and 2.6, respectively.

Figure 5

Ultracold neutron production per incident CN from multiphonon processes for $p=\mathrm{SVP}$ [(red) circles], $p=6$ bar (black squares), $p=11$ bar [(blue) diamonds], $p=16$ bar [(magenta) stars], and $p=20$ bar [(brown) triangles]. (a) Data measured after subtracting the single-phonon part (see text); (b) calculations from inelastic scattering data for $p=\mathrm{SVP}$ [(red) circles], $p=5$ bar (black squares), $p=10$ bar [(blue) diamonds], $p=15$ bar [(magenta) stars], and $p=20$ bar [(brown) triangles].

Figure 6

Comparison of the pressure dependence of single-phonon UCN production from this experiment [(brown) diamonds] and from calculations from scattering data [(blue) $\mathsf{X}$'s] [32, 33]. Both dependencies have been normalized to the respective production rate at SVP.