To enable differentiation between a particle and a ghost particle, a detector system resolves radiation from a ghost particle from radiation from an actual particle. The detector system outputs at least two detector signals corresponding to intensities of radiation being incident on different parts of the detector system or the detector system outputs at least two detector signals corresponding to intensities of radiation with different wavelengths being incident on the detector system. If radiation is received from a ghost particle, not each of the at least two detector signals has a level above a predetermined threshold level, whereas radiation received from a particle results in the signals having substantially a same level above a threshold level. The detector system may include a radiation detector device configured to generate the first detector signal in response to radiation incident on at least one predetermined part of the radiation detector device and a radiation blocking assembly configured to prevent radiation not originating from a particle from being incident on the predetermined part of the detector device.