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Alpha decay

Alpha decay

Emission of alpha particles by a decaying radioactive atom

Alpha-decay (α-decay) - a type of radioactive decay of a nucleus, which results in the emission of twice the magic 4He helium nucleus - an alpha particle. This decreases the mass number of the nucleus by 4 and the atomic number by 2.

Theory

Alpha decay from the ground state is observed only in sufficiently heavy nuclei, such as radium-226 or uranium-238. Alpha-radioactive nuclei appear in the nuclides table beginning with atomic number 52 (tellurium) and mass number about 106-110, and with atomic number over 82 and mass number over 200 almost all nuclides are alpha-radioactive, though alpha-decay may not be the dominating mode of decay. Among natural isotopes alpha-radioactivity is observed in several nuclides of rare earth elements (neodymium-144, samarium-147, samarium-148, europium-151, gadolinium-152), and also in several nuclides of heavy metals (hafnium-174, wolfram-180, osmium-186, platinum-190, bismuth-209, thorium-232, uranium-235, uranium-238) and in short-lived decay products of uranium and thorium.

Alpha decay from highly excited states of the nucleus is also observed in a number of light nuclides, such as lithium-7. Among light nuclides alpha decay from ground state is experienced by helium-5 (decays into α + n), lithium-5 (α + p), beryllium-6 (α + 2p), beryllium-8 (2α) and boron-9 (2α + p).

The alpha particle experiences a tunneling transition through a potential barrier caused by nuclear forces, so alpha decay is an essentially quantum process. Since the tunneling effect probability depends exponentially on the barrier height[3], the half-life of alpha-active nuclei exponentially increases with decreasing alpha-particle energy (this fact constitutes the content of the Geiger-Nettol law). At alpha-particle energies of less than 2 MeV the lifetime of alpha-active nuclei significantly exceeds the time of existence of the universe. Therefore, although most natural isotopes heavier than cerium are in principle capable of decay through this channel, for only a few of them such decay has actually been documented.

The escape velocity of an alpha particle ranges from 9400 km/s (neodymium isotope 144Nd) up to 23 700 km/s for polonium isotope 212Po.

History

Alpha decay was first identified by the British physicist Ernest Rutherford in 1899. At the same time in Paris, French physicist Paul Villard carried out similar experiments, but did not have time to separate the radiation before Rutherford did. The first quantitative theory of alpha decay was developed by the Soviet and American physicist Georgy Gamov.

Danger to living organisms

Being quite heavy and positively charged, alpha particles from radioactive decay have a very short range in matter, and when travelling in the medium they quickly lose energy at a short distance from the source. This causes all the energy of the radiation to be released in a small volume of matter, increasing the chances of cellular damage if the radiation source enters the body. However, external radiation from radioactive sources is harmless since alpha particles can be effectively trapped by a few centimetres of air or tens of micrometres of dense matter - such as a sheet of paper or even the horny dead layer of the epidermis (the skin surface) - without reaching living cells. Even touching a pure alpha source is not dangerous, although it should be remembered that many alpha sources also emit much more penetrating types of radiation (beta particles, gamma rays, sometimes neutrons). However, ingestion of an alpha source results in significant radiation exposure. The quality factor of alpha radiation is 20 (greater than all other types of ionising radiation, except for heavy nuclei and fission fragments). This means that in living tissue an alpha particle causes an estimated 20 times as much damage as a gamma ray or beta particle of equal energy.

All of the above applies to radioactive sources of alpha particles with energies not exceeding 15 MeV. Accelerator-generated alpha particles can have much higher energies and create a significant dose, even when the body is exposed to external radiation.

Timeline

Further Resources

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Alpha Decay

Web

December 22, 2011

alpha decay | Definition, Example, & Facts

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References

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