Which device did Oppenheimer credit Bohr with clarifying in early 1945?

Oppenheimer credited Niels Bohr with clarifying the design problem of modulated neutron initiators in early 1945.

A neutron initiator had to release neutrons at precisely the right moment to start the chain reaction in a fission weapon. The problem was especially important for the plutonium implosion design, where the core had to be compressed rapidly and symmetrically before the reaction began. A poorly timed neutron burst could reduce the weapon’s efficiency or cause a premature reaction.

Bohr visited Los Alamos under the code name Nicholas Baker and discussed the issue with Robert Oppenheimer and other scientists. Oppenheimer later recalled that the initiator had been a stubborn puzzle, but that Bohr clarified what needed to be done in February 1945. The eventual wartime device, known as Urchin, used polonium and beryllium and sat at the center of the plutonium core.

This answer is easy to confuse with implosion lenses, which shaped the explosive shock wave, or calutron magnets, which helped separate uranium isotopes. Cloud chambers were detection instruments, not the device Oppenheimer associated with Bohr’s clarification.