Lecture 35 – Chapter 22, Sections 4-6 Nuclear Reactions

• Fission Reactions

• Fusion Reactions

• Stellar Radiation

• Radiation Damage

Induced Nuclear Reactions

• Reactions in which a nuclear projectile collides and reacts with another nucleus

• Neutron-Capture Reactions– usually exothermic, the produced nuclide usually decays by proton or γ emission.

• Binuclear reactions – collision of two nuclei at very high energy.

pCNnN m 11

146

157

10

147 +→→+

HCNnN m 31

126

157

10

147 +→→+

pOFN m 11

178

189

42

147 +→→+ α

Making Synthetic Elements

• Elements Z=43, 61, 85 and Z > 92 are not naturally occurring on Earth.

• They can be made from nuclear reactions.

• Cyclotron– a positive particle accelerator used to fabricate some of the “unnatural” elements.– Doesn’t work well for heavy

particles• Linear accelerator– a heavy-ion

accelerator.• Accelerators mainly used to study

what happens during nuclear reactions.

Nuclear Fission• Fission splits a nucleus into two fragments, which are themselves usually

unstable• The reaction also releases neutrons and loads of energy.

nSrXenU 10

9538

13854

10

23592 3++→+

The total energy released could be determined by the same kind of mass difference calculation we used earlier.

Nuclear Fission

• Critical Mass– the amount of material that is just large enough to recapture one neutron for every fission reaction, thus causing another fission, which causes another, which…– Thus, fission becomes self-sustaining.

Reaction growing exponentially = bomb

Nuclear Reactors• If number of neutrons absorbed can be carefully controlled, then the

rate of reaction can be kept constant– Constant release of energy = useful

• The rate of fission is controlled by adjusting the number of recaptured neutrons with 112Cd

• Moderator (water or graphite) slows neutrons – smaller mass needed to become critical

Nuclear Fusion

• Fusion is easiest for lightest nuclides (smallest charge)• Generates a small amount of radioactive by-products• Fusion can be induced from a particle accelerator

– Small scale does not release useful amounts of energy• Fusion begins when temperature is high enough to overcome Coulombic

repulsion – Not dependent on mass of reactants (i.e. no critical mass)

• So, just get some H hot � fusion! Easy!– Critical temperature is 107 K

• The whole trick is to contain the fusion reaction

• Fusion not contained == bomb• Fusion bomb can be BIG because of lack of critical mass

HHeLinLi

nHeHeHHm

m

31

42

73

10

63

10

42

52

31

21

+→→+

+→→+

Tokamak

• One solution is a toroid of magnets – a tokamak reactor

• Heat provided by electrical resistance, neutron beams and RF

Inertial Fusion

• Second possibility is to confine the plasma with the same beams that supply heat (laser or particle)

• Momentum of incoming beams holds fuel in place

Stellar Nuclear Reactions

• First-generation star– Initial fuel almost entirely hydrogen

• Eventually creates elements up to iron (Z=26)

• Supernovae– the explosion of a star that ejects its nuclides into space.

• Second-generation stars create still-heavier elements

• Our sun is probably a third-generation star

HHeBeHe

e

HeHH

HHeHH

m

m

11

42

64

32

01

01

32

11

21

01

21

22

11

11

22

2

+→→

→+

+→+

+→→+

−+

+

γβγ

β

Effects of Radiation

• Radiation Damage – Radiation that passes through matter rips off electrons – ionization– One α particle can generate more than 105 cations– The cations are often chemically reactive.

• Immediate health effects– The formation of ions destroys living cells.– Cells that divide most rapidly tend to be the most easily damaged –

bone marrow, white blood cells, blood platelets, the lining of the GI tract, the cells in the gonads

• Long-term effects– Alteration of DNA– Genetic mutations of offspring

Tissue Penetration

Effects of Radiation on DNA• Oxidation

– Breakage of DNA strand

– Chemical modification of bases

• Methylation of bases

• Hydrolysis of bases

• Damage can be induced by gamma rays, X-rays, UV-light and reactive oxygen species generated by normal metabolic processes

• If one strand is damaged, this can usually be repaired

• Double strand damage is harder to repair

γγγγ

Radiation Shielding

• Because alpha particles are massive – easily stopped

– ~ 1 mm of solid material (paper, cloth, skin)

• Beta particles penetrate much farther

– 10-100 mm of dense solid (plastic, metal, concrete)

• Gamma rays are difficult to shield

– Several meters of concrete/metal

– Several cm of lead

Today• Finish CAPA #20

• A little review anyone?

• ‘Death by Chocolate’ seminar by Howard Peters

– Schaap 1000 7:30 pm

Thursday• ‘Death by Chocolate’ seminar by Howard Peters

– Wichers Auditorium 3:00 pm

Friday• Chem seminar Mary Rodgers (Wayne St.) Transition Metal Complexes

– Schaap 1000 4:00 pm

• Don’t forget CAPA #21 due Saturday!

• (Last seminar of semester coming up next Wednesday)