NE Introduction to Nuclear Science Spring 2012 Classroom Session 3: Radioactive Decay Types Radioactive Decay and Growth Isotopes and Decay Diagrams Nuclear Reactions Energy of nuclear reactions Neutron Cross Sections Activation Calculations

Reminder Load TurningPoint Reset slides Load List 2

Let’s do some accounting… Mass of Oxygen Atom: Mp= amu Mn= amu Me=5.48e-4 amu 3 Mass Defect = Binding Energy (BE) Energy (BE) 1 amu = MeV

4 Chart of the Nuclides Z N Isobars Isotopes Isotones

Notice radioactive decay stabilizes atoms: Question: Do fission products normally have  - or  + decay? 5

Reaction Energetics Reaction reactants and products If  E is positive: reaction exothermic releases energy If  E is negative, reaction endothermic requires energy Endoergic and exoergic is sometimes used A + B  C + D +  E

The Energy Released (or consumed), Q Change in BE: Or since BE is related to mass defect Change in M: A + B  C + D +  E Preferred! because we have table B.1. Remember: The Equation Has to Be BALANCED!

Please remember… BALANCE! Before starting to work

Balancing Reactions nucleons  = 16+1 Charges (+)  = (-)  =  e - missing 0 1 So in reality the reaction is: Calculating Q…

Q-value for the reaction is: Using atomic mass tables: Endothermic reaction. Only a few fission neutrons can do it

A beryllium target is irradiated in a proton accelerator to produce 10 B. What is Q of the reaction? MeV MeV 3. 3 MeV MeV MeV

For clicker

13 Excited Nuclei Many reactions involve excited nuclei Sometimes long lived states (isomers) Excitation energy has to be added to the mass of the excited nuclei when calculating Q e.g. The mass of 22 Ne* at 1274 MeV is:

Decay Series The radioactive minerals contain many nuclides All of them decay by either  or  decay   A changes by 4, Z by 2   A does not change, A by 1 Th has one long lived isotope 232 Th U has two long lived 235 U, 238 U Series identified by relation Parent to Dauthers mass: A in multiples of 4 14 There are 3 natural series

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16 NoticeBranching

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Series are: A = 4n --- Thorium Series A = 4n+2 -- Uranium Series A = 4n+3 – Actinium Series Which one is missing? A = 4n+1 – Neptunium Series (Artificial) 18

It was there from the beginning… but notice: half life of 237 Np is relatively low. 19

20 Main Radioactive Decay Modes (Table 5.1 -page 89-Shultis) Decay TypeDescriptionEmission Gamma (  ) Decay of excited nucleus Gamma photon alpha (  ) Alpha particle is emitted Alpha particle negatron (  - )n  p + +e - +  Electron and anti- neutrino positron (β + ) p +  n+e + + Positron and neutrino Electron Capture (EC) Orbital e - absorbed: p + +e -  n +  Neutrino proton (p)Proton ejectedProton neutron (n)Neutron ejectedNeutron Internal Conversion (IC) Electron (K-Shell) ejected*Electron Spontaneous Fission (sf) Fission fragments

Comments: ,  +,  - are common modes of decay Long T 1/2 usually are  -emitters n, p emission are rare (excess p + atoms)  is predominant for Z>83 (above Bismuth) and atoms away from the line of  -stability. Some high Z atoms (Z>96) have dominant spontaneous fission  mostly dominates again at Z>105

Modes of Decay 22 ,  +,  - are common modes of decay Long T 1/2 usually are  - emitters n, p emission are rare (excess p + atoms)  is predominant for Z>83 (above Bismuth) and atoms away from the line of  -stability. Some high Z atoms (Z>96) have dominant spontaneous fission  mostly dominates again at Z>105

Solving momentum and KE equations Remember the conditions: 1. Parent nucleus at rest (usually the case) 2. Binary products only (not  -decay, but OK to E max ) 3. Calculate the correct Q (excited states are prevalent, and balance) 4. Finally, there usually reaction paths with many outcomes, therefore multiple Q-values

24 Kinetic Energy of Radioactive Decay Products Parent nucleus is at rest (E th ~ eV~17 o C) Conservation of Linear Momentum and Kinetic Energy requires products to travel in opposite directions (2 product). m 1 v 1 =m 2 v 2 Q=½ m 1 v ½ m 2 v 2 2 What is the energy of emitted particle? (it is what we measure) (it is what we measure) v1v1v1v1 m2m2m2m2 v2v2v2v2 m1m1m1m1 m1m1m1m1 m2m2m2m2 Original atom that will split in 2 pieces

25 Kinematics of radioactive decay… Notice 2:1

Warm up: What % of the energy should go to the  -particle? % 2. 2% 3. 50% 4. 10% 5. 1%

Example of  -spectroscopy? Pa U Np Pu Am Cm

Find Q for: MeV MeV MeV MeV MeV

For Clicker slide: Q=( )* =5.638MeV

What is the KE of the  particle in the radioactive decay of 241 Am? (3 min) MeV MeV MeV MeV

For Clicker slide: KE  =5.638*237/(237+4)=5.545 MeV

Notice: If alpha particle ALWAYS leaves with exactly the same energy. We would expect to detect a monoenergetic beam of  ’s. In reality…

The real alpha spectrum of 241 Am is: At least 5 different  energies… Why? Excited Nuclei!

The real decay path of 241 Am There are actually 6 alpha peaks Last two peaks are too close to be resolved Notice frequencies (%’s) Every decay path happens all the time but not with equal probability Look in your book: Page Am Taken from J. K. Beling, et al. Phys. Rev. 87 (1952)

35 Diagram means: Energy of the  -particle? Same old same old But Q is different each time

36 3.6

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By the way Notice also

There are a lot more hard to see peaks

So how is the “real” diagram? For that we need the TABLE OF ISOTOPES 40

Diagram 241 Am - 1 of 2 41

Diagram 241 Am - 2 of 2 42

The Table also includes a more complete list of particles emitted during decay 43

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45  ’s  ’s

46 Main Radioactive Decay Modes (Table 5.1 -page 89-Shultis) Decay TypeDescriptionEmission Gamma (  ) Decay of excited nucleus Gamma photon alpha (  ) Alpha particle is emitted Alpha particle negatron (  - )n  p + +e - +  Electron and anti- neutrino positron (β + ) p +  n+e + + Positron and neutrino Electron Capture (EC) Orbital e - absorbed: p + +e -  n +  Neutrino proton (p)Proton ejectedProton neutron (n)Neutron ejectedNeutron Internal Conversion (IC) Electron (K-Shell) ejected*Electron Spontaneous Fission (sf) Fission fragments