TUCAN Sets New Detected Ultracold Neutron Rate

Ultracold neutrons are a scarce experimental resource: their low kinetic energy permits storage, transport through guides, and precision measurements of neutron properties, but their usable flux is constrained by source production and extraction. The TRIUMF UltraCold Advanced Neutron collaboration reports commissioning results for TUCAN, a new superthermal source coupled to TRIUMF's 520-MeV cyclotron. With the completed liquid-deuterium moderator installed, the source detected up to $1.47(2)\times10^7$ UCNs after a 60-second irradiation and accumulation cycle, and up to $6.75(3)\times10^5$ UCN/s in continuous operation.

Core Contribution

The paper's central contribution is an operating high-intensity UCN source rather than a new detector or a simulated design. TUCAN converts spallation neutrons through successive cooling stages: heavy water and liquid deuterium moderate the neutron spectrum, then inelastic scattering in superfluid $^4$He near 1.1 K produces the ultracold population. The authors' commissioning data connect that physical design to measured counts outside the radiation shielding.

The key design choice is the liquid-deuterium moderator. Figure 4 varies its fill volume and finds that the normalized UCN yield rises by a factor of 29.1(5) from the empty to filled-moderator condition. That is more informative than a single peak-rate measurement because it isolates the component responsible for much of the source gain. The paper also compares the volume trend with MCNP6 predictions, normalizing simulation and measurement over overlapping-uncertainty points; the largest simulation-to-data discrepancy occurs with an empty liquid-deuterium vessel.

Technical Approach

A 520-MeV proton beam strikes a tungsten target, producing neutrons that enter the moderation and conversion assembly. Figure 1 presents the source as a cut-open system: the tungsten target, heavy-water and liquid-deuterium regions, superfluid-helium UCN converter, and guide path toward the East experimental port. UCNs are accumulated in the source and delivered through guides after gate valves are opened, allowing the collaboration to measure both batch extraction and sustained operation.

The experiments vary beam current, moderator configuration, storage time, and operating mode. For batch measurements, the authors integrate UCN counts over a 180-second counting interval following a 60-second irradiation, then fit counts against the preceding average proton current with a zero-intercept linear model. A separate storage measurement fits the beam-off decay after 60 seconds of irradiation with two exponentials. The fitted storage constants show a slight increase at higher proton current, although the reported plot does not by itself establish the mechanism behind that trend.

The data-acquisition chain also matters at these rates. A time-histogram correction identifies dropped events from buffer overrun in a run at 26.6(3) proton-current units: the inferred detected count changes from $1.34(1)\times10^7$ to $1.47(2)\times10^7$. The paper therefore treats the quoted high-count result as a corrected measurement rather than assuming the raw detector total was complete.

Results and Analysis

The strongest empirical result is the continuous-operation measurement. With a 33-second target irradiation and a titanium foil, the count-rate trace remains largely constant after saturation during the 1,150-second beam-on period shown in Figure 5. The peak reported continuous rate is $6.75(3)\times10^5$ detected UCN/s. The authors state that this exceeds the detected rate at any other UCN source; the paper provides the measured TUCAN rate but does not present a side-by-side external-source benchmark in the supplied material.

The batch and moderator scans make the performance claim more convincing than the headline rate alone. Counts scale close to linearly with beam current across three configurations, while adding liquid deuterium produces a large increase in integrated counts. The 29.1(5)-fold moderator-volume gain is especially consequential because it is measured under controlled variation rather than inferred from a design model. Still, the evidence is commissioning-focused: it shows source behavior and detector-side counts, not downstream gains in a specific neutron-physics measurement. The source appears ready to expand the available UCN population for such experiments, but delivered performance will depend on guide transmission, storage, and each experiment's acceptance.