Your spectrum shows the expected background peak at just under 60keV - but the second peak at around 78 KeV is interesting.
I suggest that you don't sand the nickel, but pickle it in a 50/50 mix of citric and tartaric acid at around 60C for around 30 minutes. This will (I think) solubilise the oxides. The acid shouldn't be too strong, no more than 5 grams of each per litre of water. Rinse well afterwards.
ps - try it on a small piece first.