Alpha Decay Because the binding energy of the

Alpha Decay Ø Because the binding energy of the alpha particle is so large (28. 3 Me. V), it is often energetically favorable for a heavy nucleus to emit an alpha particle n Nuclides with A>150 are unstable against alpha decay Ø Decay alpha particles are monoenergetic n Ea = Q (1 -4/A) 1

Alpha Decay ØTypical alpha energies are 4 < Ea < 8 Me. V n But half-lives vary from 10 -6 s to 1017 y! ØThe decay probability is described by the Geiger-Nuttall law n log 10λ = C – D/√E w λ is the transition probability w C, D weakly depend on Z w E is the alpha kinetic energy ØThe Geiger-Nuttall law can be derived using QM to calculate the tunneling probability 2

Alpha Decay Ø Geiger-Nuttall law 3

ØMonoenergetic alphas 4

ØCommon alpha sources ØSince d. E/dx is so large for alpha particles the sources are prepared in thin layers 5

Beta Decay Ø β- decay n n n β- decay β+ decay Electron capture (EC) Ø β- decay is the most common type of radioactive decay n All nuclides not lying in the valley of stability can βdecay Ø β- decay is a weak interaction n n The quark level Feynman diagram for β- decay is shown on a following slide We call this a semileptonic decay 6

Beta Decay 7

Beta Decay 8

Beta Decay ØBecause beta decay is a three body decay, the electron energy spectrum is a continuum 9

Beta Decay ØThe Q value in beta decay is effectively shared between the electron and antineutrino n The electron endpoint energy is Q Note these are atomic masses 10

Electron Capture ØProton rich nuclei can undergo electron capture in addition to β+ decay n n n e- + p -> n + n EC can occur for mass differences < 2 mec 2 Most often a K or L electron is captured EC will leave the atom in the excited state Thus EC can be accompanied by the emission of characteristic fluorescent x-rays or Auger electrons w e. g. 201 Tl ->201 Hg x-rays from EC was used in myocardial perfusion imaging 11

Characteristic X-rays ØNuclear de-excitation n n Gamma ray emission Internal conversion (IC) ØAtomic de-excitation n n x-ray emission Auger electron emission w Assume the K shell electron was ejected n n L to K transition == Ka M to K transition == Kb 12

Characteristic X-rays ØSimplified view 13

Auger Electrons ØEmission of Auger electrons is a competitive process to x-ray emission n For Auger electrons e. g. , EKLL = EK – EL 1 – EL 2 ØThe Auger effect is more important in low Z (Z < 15) elements because the electrons are more loosely bound ØThe fluorescent yield is defined as the fraction of characteristic x-rays emitted from a given shell after vacancy 14

Characteristic X-rays and Auger Electrons 15

Beta Sources Ø Most beta sources also emit gamma rays Ø Like alpha sources, beta sources must be thin because of d. E/dx losses 16

Gamma Decay ØGammas (photons) are emitted when a higher energy nuclear state decays to a lower energy one n n Alpha and beta decays, fission, and nuclear reactions often leave the nucleus in an excited state Nuclei in highly excited states most often de-excite by the emission of a neutron or proton If emission of a nucleon is not energetically possible, gamma emission or internal conversion occurs Typical gamma ray energies range from 0. 1 to 10 Me. V 17

Conversion Electrons Ø A competing process to gamma decay is internal conversion (IC) n In IC, the excitation energy of a nucleus is transferred to one of the electrons in the K, L, or M shells that are subsequently ejected n The electrons are called conversion electrons n IC is more important for heavy nuclei where the EM fields are large and the orbits of inner shell electrons are close to the nucleus n Internal conversion is a competing process to gamma emission 18

Conversion Electrons Ø Examples are seen in the electron spectra shown in the two figures n The first figure is particularly simple and shows three conversion lines arising from the transfer of 1. 4 Me. V to electrons in the K, L, and M shells Ø Note that the conversion electrons are monenergetic 19

Conversion Electrons 20

Conversion Electrons 21

Conversion Coefficients Ø Gamma emission and IC compete n λtotal = λgamma + λIC Ø Conversion coefficient α == λIC/λgamma n We can break this up according to the probabilities for ejection of K, L, and M shell electrons w α = αK + αL + αM + … 22

Conversion Coefficients Ø Increase as Z 3 Ø Decrease with increasing transition energy n Opposite to gamma emission Ø Increase with the multipole order n May compete with gamma emission at high L Ø Decrease with atomic shell number as 1/n 3 Ø Thus we expect K shell IC to be important for low energy, high multipolarity transitions in heavy nuclei 23

Conversion Coefficients 24

Conversion Electrons Ø Common conversion electron sources n These sources are the only practical way to produce monoenergetic electrons in the ke. V-Me. V range in the laboratory 25

Gamma Sources Ø Gamma sources usually begin with beta decay to put the nucleus in an excited state n n Encapsulation of the source absorbs the electron Typical gamma energies are ~1 Me. V 26

Gamma Sources Ø There also annihilation gammas Ø In β+ decay (e. g. 22 Na) the emitted positron will usually stop and annihilate producing two 0. 511 Me. V gammas 27

Neutron Sources ØNuclei that decay by neutron decay are rarely found in nature n n Exotic nuclei can be produced in high energy processes in stars or at heavy ion accelerators There are no direct neutron sources for the laboratory ØNeutron sources can be produced using spontaneous fission or in nuclear reactions 28

Neutron Sources Ø Spontaneous fission n n Many of the transuranic nuclides have an appreciable spontaneous fission decay probability e. g. 252 Cf (most widely used since t 1/2=2. 6 years) Dominant decay is alpha emission Spontaneous fission x 32 smaller Yield is 2. 5 x 106 n/s per μg of material 29

Neutron Sources Ø (a, n) sources n n Make a n source using an a beam Usually the source consists of an alloy of the alpha emitter plus target (e. g. Pu. Be) There is an accompanying large gamma decay component associated with these sources that make them troublesome Even though the emitted alpha is monoenergetic, the alpha beam is not due to d. E/dx losses w Hence the neutrons are not monoenergetic 30

Neutron Sources 31