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Pharos Tzero Chopper

Pharos is equipped with a Tzero chopper that prevents fast neutrons from entering the instrument. Without a Tzero chopper, some of the fast neutrons would undergo a series of inelastic collisions inside the instrument and thereby produce an undesired, spectrally broad background. The Tzero chopper comprises a cylindrical rotor assembly that contains a beam stop assembly with a 341 kg Inconel steel beam stop and a counterweight. The rotor assembly rotates on precision ball bearings at typically 40 Hz. To ensure reliable operation with this very sizeable rotational energy, the Tzero rotor has to be balanced to high precision (better than 25 microns peak-to-peak vibration amplitude) on a dedicated test stand prior to installation.

The term "Tzero" refers to the precise synchronization of the Tzero phase to the master trigger (time zero when pulse is fired) of the LANSCE accelerator. The Pharos Tzero chopper is phase-locked to the master trigger with an accuracy of typically 10-20 ms. With a distance from the moderator of 14 m, a rotational frequency of 40 Hz, a beam size of 50 mm, and a rotor radius of 283 mm, the Tzero beam stop blocks neutrons with energies greater than ~2 eV.