Assertion: gamma ray | /r/Desires | concrete
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Source sentences:
text: Alpha particles (helium nuclei consisting of two protons and two neutrons) may be stopped completely by a sheet of paper, beta particles (high-speed electrons) can be stopped by perspex, while gamma rays (like X-rays, but with a shorter wavelength) may need lead or concrete to efficiently stop them - but can be stopped by any material providing there is enough of it.