At short time constants the series noise component is dominant and at longer time constants the parallel noise is more significant. Therefore optimum shaping is achieved where series and parallel noise are equal.

If the amplifier shaping is much shorter than the time taken to collect the generated charge then ballistic deficit occurs. It is observed in the spectrum as a pronounced tailing at the low energy side of the peak.

Change the shaping time control in the simulation.  You should detect a slight increase in the noise level at lower levels.