200 mm  58 Ni 99% specular guide 200 mm  DLC 50% diffuse “T” 65.5 mm  DLC 99% specular guides 100% diffuse SS flange SD2 Cu gap Stopping UCN positions

SD2 Flapper

First bend 90% specular DLC Cu gaps Stopping UCN positions

Pump-out port and gate valve 1 90% specular DLC Entrance to gate valve 2

PPM Magnetic field regionAl window and fixture 6.4 mm  Monitor hole /-0.1x UCN/p 6.4 mm  Monitor hole /-0.3x UCN/p 65.5 mm  Entrance to GV /-0.04x10 -9 UCN/p 100% polarized Stopping UCN positions

Where do the UCN go? (flapper obstruction not simulated) Lost in the SD % Lost in the vertical guides Lost in the SS flange 5.15 Lost in the DLC guides before the Al window 2.71 Lost in the Cu gaps 0.86 Lost in the Al window 0.56 Lost in the DLC guides after the Al window 0.07 Leaks and numerical losses 0.04 Still in the system* (at 240 s) 0.83 Exited the monitoring holes 0.15 Arrived at gate valve * System defined out to gate valve 2