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Alpha, beta, and gamma

Research begun by people such as New Zealander Ernest Rutherford soon after the discovery of nuclear radiation indicated that different types of rays are emitted. Eventually, three types were distinguished and named alpha α size 12{ left (α right )} {} , beta β size 12{ left (β right )} {} , and gamma γ size 12{ left (γ right )} {} , because, like x-rays, their identities were initially unknown. [link] shows what happens if the rays are passed through a magnetic field. The γ size 12{γ} {} s are unaffected, while the α size 12{γ} {} s and β size 12{β} {} s are deflected in opposite directions, indicating the α size 12{α} {} s are positive, the β size 12{β} {} s negative, and the γ size 12{γ} {} s uncharged. Rutherford used both magnetic and electric fields to show that α size 12{α} {} s have a positive charge twice the magnitude of an electron, or + 2 q e size 12{+2 lline q rSub { size 8{e} } rline } {} . In the process, he found the α size 12{γ} {} s charge to mass ratio to be several thousand times smaller than the electron’s. Later on, Rutherford collected α size 12{γ} {} s from a radioactive source and passed an electric discharge through them, obtaining the spectrum of recently discovered helium gas. Among many important discoveries made by Rutherford and his collaborators was the proof that α size 12{γ} {} radiation is the emission of a helium nucleus . Rutherford won the Nobel Prize in chemistry in 1908 for his early work. He continued to make important contributions until his death in 1934.

The figure shows north and south poles of a magnet through which three rays labeled as alpha beta and gamma are passed. After passing through a magnetic field the alpha ray is slightly deflected toward the right. The beta ray is deflected toward the left and the gamma ray is not deflected.
Alpha, beta, and gamma rays are passed through a magnetic field on the way to a phosphorescent screen. The α size 12{γ} {} s and β size 12{β} {} s bend in opposite directions, while the γ size 12{γ} {} s are unaffected, indicating a positive charge for α size 12{γ} {} s, negative for β size 12{β} {} s, and neutral for γ size 12{γ} {} s. Consistent results are obtained with electric fields. Collection of the radiation offers further confirmation from the direct measurement of excess charge.

Other researchers had already proved that β size 12{β} {} s are negative and have the same mass and same charge-to-mass ratio as the recently discovered electron. By 1902, it was recognized that β size 12{β} {} radiation is the emission of an electron . Although β size 12{β} {} s are electrons, they do not exist in the nucleus before it decays and are not ejected atomic electrons—the electron is created in the nucleus at the instant of decay.

Since γ size 12{γ} {} s remain unaffected by electric and magnetic fields, it is natural to think they might be photons. Evidence for this grew, but it was not until 1914 that this was proved by Rutherford and collaborators. By scattering γ size 12{γ} {} radiation from a crystal and observing interference, they demonstrated that γ size 12{γ} {} radiation is the emission of a high-energy photon by a nucleus . In fact, γ size 12{γ} {} radiation comes from the de-excitation of a nucleus, just as an x ray comes from the de-excitation of an atom. The names " γ size 12{γ} {} ray" and "x ray" identify the source of the radiation. At the same energy, γ size 12{γ} {} rays and x rays are otherwise identical.

Properties of nuclear radiation
Type of Radiation Range
α size 12{α} {} -Particles A sheet of paper, a few cm of air, fractions of a mm of tissue
β size 12{β} {} -Particles A thin aluminum plate, or tens of cm of tissue
γ size 12{γ} {} Rays Several cm of lead or meters of concrete

Ionization and range

Two of the most important characteristics of α size 12{α} {} , β size 12{β} {} , and γ size 12{γ} {} rays were recognized very early. All three types of nuclear radiation produce ionization in materials, but they penetrate different distances in materials—that is, they have different ranges . Let us examine why they have these characteristics and what are some of the consequences.

Questions & Answers

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Source:  OpenStax, Concepts of physics. OpenStax CNX. Aug 25, 2015 Download for free at https://legacy.cnx.org/content/col11738/1.5
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