Dark matter detector: ionisation and background radiation challenge

A liquid-xenon detector records particle interactions partly by collecting electrons freed from xenon atoms. Describe what happens when a neutral xenon atom is ionised in this way, and state the sign of the charge left on the atom.

The detector is placed deep underground to reduce unwanted background signals. Explain why the overlying rock reduces some background radiation but does not remove every ordinary source of radiation near the detector.

In a simplified classroom test, a counter records 12, 16, 14 and 18 background counts in four separate $60\mathrm{s}$ intervals. The equipment and surroundings are unchanged. In the next $60\mathrm{s}$ interval it records 21 counts. Calculate the mean background count per $60\mathrm{s}$ and the excess in the final interval above that mean. A pupil says, 'The excess proves that a new kind of particle arrived.' Explain why the conclusion is not justified, and describe one useful next measurement to test it.