Particle fluence spectrum

Dear Experts,
I am running simulations where i have a beam of different particles of energy 0.1Gev transfering through a gold target 5cm.I when i use proton or alpha particles beam i see a discontinuity in the spectrum
between 60-100kev where there is no fluence ,as shown in the pictures above.why is this happening?

the first image is for protons and the second for alpha particles.
also i observe a big error at high energies for alpha particles(i mean the vertical lines ,if i have understood corectly they represent error).is there a reason for that.
thank you and im wating forward for your answer.

The spectrum is cut at the (default) transport threshold of 100 keV applying to protons and alphas. The fluence at a few ten keV may pertain to photons, for which a lower default cutoff of 33 keV applies.

These are not errors (the statistical errors appear to be small in most points), rather artificial fluctuations from one energy to another, due to the request of an excessive energy resolution (i.e. number of bins) with respect to the default step size, linked to the default fraction of kinetic energy lost in a charged particle step. You shall decrease the number of energy bins, or adjust the aforementioned step size by means of FLUKAFIX. Take a look at this lecture, slides 27 to 29, where (on slide 29) you will see the unnatural effect you got.