” In 1948 he discovered that sodium iodide, activated by thallium, made an excellent scintillation counter. In 1950, with J. A. McIntyre, he found that well-formed crystals of this material provided remarkable energy-measuring devices for gamma rays and energetic particles and thus could be used as spectrometers in addition to gamma-ray and particle counters of high efficiency. “https://www.nobelprize.org/prizes/physics/1961/hofstadter/biographical/