10/14/2014 Beta Decay

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Beta Decay

Beta decays

Beta particles are electrons or positrons (electrons withpositive electric charge, or antielectrons). Beta decay occurswhen, in a nucleus with too many protons or too manyneutrons, one of the protons or neutrons is transformed intothe other. In beta minus decay, a neutron decays into aproton, an electron, and an antineutrino: n p + e - +. Inbeta plus decay, a proton decays into a neutron, a positron,and a neutrino: p n + e+ +n. Both reactions occur becausein different regions of the Chart of the Nuclides, one or theother will move the product closer to the region of stability.These particular reactions take place because conservationlaws are obeyed. Electric charge conservation requires that ifan electrically neutral neutron becomes a positively chargedproton, an electrically negative particle (in this case, anelectron) must also be produced. Similarly, conservation oflepton number requires that if a neutron (lepton number = 0)decays into a proton (lepton number = 0) and an electron(lepton number = 1), a particle with a lepton number of -1(in this case an antineutrino) must also be produced. Theleptons emitted in beta decay did not exist in the nucleusbefore the decaythey are created at the instant of thedecay.

10/14/2014 Beta Decay

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To the best of our knowledge, an isolated proton, a hydrogennucleus with or without an electron, does not decay. Howeverwithin a nucleus, the beta decay process can change a protonto a neutron. An isolated neutron is unstable and will decaywith a half-life of 10.5 minutes. A neutron in a nucleus willdecay if a more stable nucleus results; the half-life of thedecay depends on the isotope. If it leads to a more stablenucleus, a proton in a nucleus may capture an electron fromthe atom (electron capture), and change into a neutron and aneutrino.

Proton decay, neutron decay, and electron capture are threeways in which protons can be changed into neutrons or vice-versa; in each decay there is a change in the atomic number,so that the parent and daughter atoms are differentelements. In all three processes, the number A of nucleonsremains the same, while both proton number, Z, and neutronnumber, N, increase or decrease by 1.

In beta decay the change in binding energy appears as themass energy and kinetic energy of the beta particle, theenergy of the neutrino, and the kinetic energy of the recoilingdaughter nucleus. The energy of an emitted beta particlefrom a particular decay can take on a range of valuesbecause the energy can be shared in many ways among thethree particles while still obeying energy and momentumconservation.

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