Minimum pb alpha particle energy lower than expected

[PhysicsDan]

In this PR #5033
the initialisation on the pb fusion alpha particle momentum is modified to better match the experimental distribution.

There is a small issue identified and is detailed in the PR. The minimum energy of the alpha particles created in the decay of the $^$Be* nucleus is less than expected when a proton beam irradiates a stationary boron atom.

We should expect
min simulation energy > min analytical energy
and
max simulation energy > max analytical energy

Originally posted by @PhysicsDan in #5033 (comment)