NM Basic Sci.Intro.Nucl.Phys. 06/09/2011

Introduction to Nuclear and Nuclear Decay

Larry MacDonald [email protected] Basic Lectures

September 6, 2011

Atoms Nucleus: ~10-14 m diameter ~1017 kg/m3 clouds: ~10-10 m diameter (= size of ) water : ~10-10 m diameter ~103 kg/m3

Nucleons ( and ) are ~10,000 times smaller than the atom, and ~1800 times more massive than . (electron size < 10-22 m (only an upper limit can be estimated))

Nuclear and atomic units of length 10-15 = femtometer (fm) 10-10 = angstrom (Å)


mostly empty space ~ one trillionth of volume occupied by Water Hecht, Physics, 1994 (wikipedia)

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Mass and Units and Mass-Energy Equivalence

Mass atomic mass unit, u (or amu):

mass of 12C ≡ 12.0000 u = 19.9265 10-27 kg

Energy Electron volt, eV ≡ attained by an electron accelerated through 1.0 volt 1 eV ≡ (1.6 x10-19 Coulomb)*(1.0 volt) = 1.6 x10-19 J

2 E = mc c = 3 x 108 m/s speed of

-27 2 mass of , mp = 1.6724x10 kg = 1.007276 u = 938.3 MeV/c -27 2 mass of , mn = 1.6747x10 kg = 1.008655 u = 939.6 MeV/c -31 2 mass of electron, me = 9.108x10 kg = 0.000548 u = 0.511 MeV/c

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Elements Named for their number of protons X = element symbol Z () = number of protons in nucleus N = number of neutrons in nucleus A A A X A (atomic ) = Z + N Z X N Z X [A is different than, but approximately equal to the atomic weight of an atom in amu] Examples; ,

A Electrically neural atom, Z X N has Z electrons in its 16 208 atomic orbit. Otherwise it is ionized, and holds net electric 8 O8 82 Pb126 charge. ! ! ! ! Z ! !

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Nuclide Groups/Families

A is a nucleus with a specific Z and A ~1500 exist ( typically lists distinct Z)

Nuclides with the same Z (#protons) are N (#neutrons) are A (#) are Isobars

A nuclide with the same Z and A (& thus also N) can also exist in different (excited & ground) energy states; these are Isomers

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N vs. Z Chart of Nuclides

N > Z for the majority (N = Z for low Z elements)

The line of stability ( band) represents the stable nuclei.

Distribution of stable nuclei: Z N #stable nuclei even even 165 even odd 57 odd even 53 odd odd 4

279 stable nuclei exist isobars (all have Z < 84) isotones ~1200 unstable (radioactive) isotopes (65 natural, remaining are human- made)

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Nuclear Shell Structure Schematic energy diagrams • Similar to atomic structure, the nucleus can be E=0: is unbound (free) modeled as having quantized allowed energy states E<0: particle is bound (e.g. in (shells) that the nucleons occupy. nucleus, in an atom) • The lowest energy state is the . E>0: free & has excess energy (can be potential or kinetic) • Nuclei can exist in excited states with energy greater than the ground state. E • Excited nuclear states that exist for > 10-12 sec. are metastable states (isomeric). • Nucleons held together by the ‘strong ’; short range, but strong. • This overcomes the repulsive electrostatic force of similar charged protons • Also similar to : → Electrons swirl around in clouds about the nucleus; likewise, the nucleus is a dynamic swirl of nucleons. → Nucleons, like electrons, are paired in energy states - each with opposite . → Closed electron shells lead to chemically inert . Magic numbers of nucleons (analogous to closed shells) form particularly stable nuclei. [email protected] Hecht, Physics, 1994 7

Binding Energy

The mass of a nuclide is less than the mass of the sum of the constituents. The difference in energy is the .

The consequence is that energy is liberated when nucleons join to form a nuclide.

The binding energy per dictates results when nuclides break apart (fission) or fuse together (fusion)

(keep in mind that binding are thought of as negative, as in energy level diagrams on previous slide)


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Radioactive Decay Unstable nuclei change (decay) towards stable states The transformation involves emission of secondary ():

A A" [*] Z X ! Z"Y +W +Q

parent daughter nucleus radiation additional energy transforms nucleus [possibly excited *] + particle(s) + liberated in the decay

Q can be shared between the X, Y, and W particles. Y is frequently unstable itself.

Conservation principles: • Energy (equivalently, mass) • Linear • Angular momentum (including intrinsic spin) • Charge are all conserved in radioactive transitions

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Radioactive Decay Processes The decay processes are named for the (primary) radiation particle emitted in the transition:

• alpha A A#4 Z X" Z#2Y +$ + Q

• beta A X A Y ± Q isobaric Z "Z!1 + # + $ + ! alternative mechanism to β+ decay is

! A[ m] [*] A Z X " Z X + # isomeric alternative mechanism is

The (net charge) on particles can also be specified (upper-right)

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Decay Time

The rate at which decay is governed by a characteristic decay time constant, λ (units of λ are inverse-time, i.e. or rate) N t = N e"#t ( ) 0 N(t) = number of radionuclides at time t

N0 = number at time t = 0 λ = characteristic decay time constant ! The half-life, T1/2, is the time it takes for a sample to decay to one-half of its original number, or half of its original activity. ln(2) 0.693 T1/ 2 = = " " # t & "% ( N t = N 2 $ T1 / 2 ' ( ) 0 [email protected] ! 11

! An is the same as a nucleus; (two protons and two neutrons) 4 +2 " =2 He

General form of alpha decay process

! A A#4 4 +2 Z X" Z#2Y+2 He + Q

• Alpha particle always carries Q energy as kinetic energy (monoenergetic) • Alpha decay! occurs with heavy nuclides (A > 150) • Commonly followed by isomeric emission of , • which can also result in (see internal conversion slide)

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Beta Decay A beta(minus, β-) particle is indistinguishable from an electron. There are also beta(plus, β+) particles. These are indistinguishable from electrons, except with positive charge (of the same magnitude).

In β- decay, a nuclear neutron is converted into a proton (Z→Z+1) In β+ decay, a nuclear proton is converted into a neutron (Z→Z-1)

- A A $ β Z X"Z+1Y + # + % + Q The general form: + A A + β Z X"Z#1Y + $ + % + Q

! 18 18 + e.g. 9 F" 8 O + # + $ + 0.635MeV In each case the decay products include! a ( " ) or an anti-neutrino ( " ) have no charge, spin 1/2, and mass ~ 0.1 - 1 eV (?) the is polyenergetic the fixed Q is shared! by β and ν in continuous way

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Electron Capture An alternative (and competing mechanism) to β+ decay is electron capture. In electron capture, a proton is converted to a neutron, as in β+ decay, however, rather than emitting a β+, an orbital electron (usually from inner electron shells) is captured by the nucleus, conversion of a proton to a neutron occurs, and a neutrino (and additional energy, Q) are emitted from the decay process:

A " A Z X + e #Z"1Y + $ + Q

Capture of an electron creates a vacancy in an inner , which is filled by another electron from a higher shell. This results in characteristic x-rays, or Auger electrons. An example of e.c. relevant to nuclear medicine is the following decay: ! 201 " 201 81Tl + e # 80 Hg + $ + Q

None of the products of this decay are used in imaging, rather, characteristic x-rays filling the vacancy are detected by gamma cameras.

Characteristic x-rays also mono-energetic (transitions between electron orbits), but several nearby orbital energies can give rise to apparent! spread of energies.

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Gamma Emissions Gamma decay is an isomeric transition that follows the occurrence of alpha or .

A[ m] [*] A Z X " Z X + # The parent in this case (which is the daughter of the preceding α or β decay, or electron capture) can be in an , * ,that (essentially) immediately transitions to a lower state via emission of a gamma, or it can be in a metastable state m, which can have a life-time of between 10-12 sec. and ~600 years. Decay of metastable states also follow the law, and thus have characteristic decay times. !

Internal Conversion • Alternatively, the energy liberated from the isomeric transition can be delivered to an electron ejected from the atom (like Auger electrons vs. char. x-rays). • Again, electrons rearrange to fill the vacancy left by the i.c. electron, resulting in characteristic x- rays and/or Auger electrons. • Gamma emission and i.c. electron compete in the same nuclide decay.

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Decay Schemes

ENERGY increasing

Bushberg Z increasing

Example: 99mTc

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N vs. Z Chart of Nuclides

N > Z for the majority (N = Z for low Z elements)

The line of stability (gold band) represents the stable nuclei.


isotones isotopes

Hecht, Physics, 1994 [email protected] 17

Highlights Line of Stability: N = Z for low Z, N > Z for heavier elements (Z > 20) Isotopes (const. Z, number of protons) Isotones (const. N, number of neutrons) Isobars (const. A, number of protons and neutrons (atomic mass number)) Radioactive Decay Alpha (2 protons, 2 neutrons) mono-energetic followed by other decays

Beta +/-: Z changes by one, emits β, conserve charge poly-energetic Beta+ vs. electron capture; nucleus loses unit charge

Isomeric transitions: transitions between excited states, no change in Z, A, N mono-energetic gamma emission vs. internal conversion

Decay Time Dependence "#t N(t) = number of radionuclides at time t Exponential N t = N e N = number at time t = 0 ( ) 0 0 λ = characteristic decay time constant # t & "% ( alternatively (equivalent) $ T1 / 2 ' ln(2) 0.693 N t = N 2 T1/ 2 = = ( ) 0 " " ! [email protected] 18

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