A detector includes a sonde having a housing and comprising a scintillator disposed within the housing and a calibration source coupled to the scintillator to fluoresce the scintillator at a known wavelength of electromagnetic radiation. The detector further includes an electromagnetic radiation sensing device coupled to the scintillator and disposed within the housing and a first programmable/re-programmable processing module (PRPM) coupled to the electromagnetic radiation sensing device and disposed within the housing. The PRPM is programmed to process signals from the electromagnetic sensing device based on a user-defined analysis mode selected from the group of modes consisting of filtering, windowing, discriminating, and counting.