Dear Regan,
your problem is not the density (3.58 g/cc according to your input), rather the isotopic composition, which turns out to be the natural one, where 136Xe is one order of magnitude less abundant than in your intended composition. This is due to a classical and evergreen mistake in the MATERIAL card, where you input the isotope mass number in the wrong place (as a general rule, let WHAT(2) - atomic weight - and WHAT(4) - material index - be empty).
Additionally, you can remove all LOW-MAT cards (applying to group-wise neutron treatment), in view of the LOW-PWXS presence.
Also, I’m not sure I understand what you aim to achieve with the PHYSICS/DECAYS card (muons naturally decay in FLUKA by default).
Let us know if the anticipated yield remains unmatched.