| helium | <element> A gas. Certain types of radiation therapy used charged, radioactive particles of helium. (16 Dec 1997) |
|---|---|
| helium speech | The peculiar high-pitched, often unintelligible speech sounds produced when one breathes a mixture of up to 80 per cent helium and 20 per cent oxygen. (05 Mar 2000) |
| helium-3 | The rare stable isotope of helium (1.37 parts per million of ordinary helium); produced by the beta decay of tritium. (05 Mar 2000) |
| helium-4 | The common helium isotope, making up 99.999% of natural helium; it is emitted in the form of alpha rays (which are helium nuclei), from a variety of radionuclides. (05 Mar 2000) |