1. A method for measuring the concentrations of multiple radioxenon isotopes in a gaseous sample of xenon, the method comprising: providing a detector comprising N sub-detectors surrounding a sample cell, wherein the N sub-detectors are sensitive to both electrons and photons from radioxenon decays, specifically including beta decays, conversion electrons, x-rays, and gamma-rays; providing signal processing electronics for the N sub-detectors that can detect both single and coincident events within a single sub-detector, detect coincident events between multiple sub-detectors, and measure the detected events' respective energies; introducing the sample of xenon into the sample cell; during a measurement interval, whenever a coincidence between two events is detected, recording the two events' two measured energies as a point in a two-dimensional histogram, whenever a coincidence between three events is detected, recording the three events' three measured energies as a point in a three-dimensional histogram, and establishing regions of interest (ROIs) within both the two-dimensional histogram and the three-dimensional histogram that correspond to decay paths of the multiple radioxenon isotopes to be measured; and after the measurement interval, computing estimates of the concentrations of the multiple radioxenon isotopes, based on the number of counts collected in each of the ROIs.