Unknown bump in Bragg curve 5.49 MeV alphas in dry air

Dear FLUKA experts,

I’ve been simulating a simple 5.49 MeV alpha pencil beam source in a (10 x 10 x 10) cm box of air. I’m using a simple USRBIN to score the energy deposition in the box of air however I’m observing a bump in the right tail of the peak as pointed out in the Figure.

I’m not sure as to why this is occurring and would expect a smooth decline as seen in the second Figure:

test1.flair (2.1 KB)
test1.inp (1.6 KB)

That’s the effect of the (alpha) transport threshold. If, for the sake of accuracy, you lower the latter from the default value of 100 keV to the minimum value of 4 keV, by means of the PART-THR card, the curve smoothens as expected.