To plot an acceleration from Explicit Dynamics, as was desired in the discussion you linked to, then you need to filter the data.
Creating an SRS is a filter operation. You decide on the upper and lower frequency limits to plot the data. That is a filter too.
I would not use results from this coarse mesh. The wait time in Explicit Dynamics is mostly driven by the smallest element in the model and less with the node count. With sufficient RAM, there is little incremental wait time by making the plate have elements that are closer in size to the elements in the impactor. You want at least 4 elements through the thickness of the plate and let them have an aspect ratio no more than 2 so if the thickness of the plate is t, then the element thickness is t/4 while the element side length is t/2. It is ideal if the elements are cubic with all sides equal.
The mesh on the impactor has small elements on the axis. Remesh the impactor to have one element on the axis that is similar in size to the elements around the circumference. This will reduce your wait time more than using fewer coarse elements on the plate.
Please make displacement, velocity and acceleration SRS plots and insert the images here once you have your data.