Nuclear Reactions. Thornton and Rex, Ch. 13. Otto Hahn and Lise Meitner

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1 Nuclear Reactions Thornton and Rex, Ch. 13 Otto Hahn and Lise Meitner

2 Reaction Kinematics Consider a general reaction, A (x, y) B or A + x Æ y + B with target A at rest. Ex. 9 Be a 2 Æ 1 n C 6 or equivalently, 9 Be 4 (a,n) 12 C 6 Conservation of energy gives: M A c 2 + m x c 2 + K x = m y c 2 + K y + M B c 2 + K B The difference between final and initial kinetic energies is called the Q-value. Q = K y + K B - K x = M A c 2 + m x c 2 - (m y c 2 + M B c 2 )

3 If Energy is released, Q > 0 fi Exothermic If Energy is converted to mass, Q < 0 fi Endothermic Two of the most important exothermic reactions are Fission and Fusion.

4 Q uick uiz How many α particles and β particles are produced in the decay chain 232 Th 90 --> 224 Ra 88? (A) (B) (C) (D) (E) 1 α and 1 β 2 α and 2 β 3 α and 3 β 2 α and 1 β 1 α and 2 β

5 Q uick uiz How many α particles and β particles are produced in the decay chain 232 Th 90 --> 224 Ra 88? (A) (B) (C) (D) (E) 1 α and 1 β 2 α and 2 β 3 α and 3 β 2 α and 1 β 1 α and 2 β α decays reduce A by 4 and Z by 2 β decays don t change A and increase Z by 1 ==> Nα = ( )/4 = 2 ==> 2Nα - Nβ = = 2 ==> Nβ = 4-2 = 2

6 Neutron Activation Neutrons - uncharged, can penetrate close to the nucleus, can induce reactions (Neutron Activation) s - Enrico Fermi bombarded elements from Hydrogen to ranium with neutrons. On ranium, Fermi was unable to identify the final products of the reactions. He expected elements heavier than ranium, such as in the expected process: n 0 Æ Æ 239 Np e -1 + n

9 Fission n Ba n n n Kr

10 Comments on Fission The observed reaction is one of many possible reactions: n 0 Æ 141 Ba Kr n undergoes fission does not. ranium ore contains 99.3% -238 and only 0.7% Fission occurs more easily if the neutron is slow (allowing more time for the reaction to occur.)

11 Q > 0, so energy released (~200 MeV). (Cf. Binding Energy vs. Atomic Weight) Since heavier nuclei are more neutron rich, the fission process results in the release of extra neutrons. (Cf. plot of N vs. Z)

14 nder the right conditions, the extra neutrons could cause more -235 to fission. This would release even more neutrons, etc., resulting in a chain reaction. n (The idea of chain reaction was patented by Leo Szilard in 1933, before he had any idea what nuclei might participate!)

15 After his and Meitner s realization of the process of nuclear fission, Frisch returned to Copenhagen and communicated the news to Niels Bohr, who was leaving for the.s. on the next day. In New York, in mid-january 1939, Bohr told Enrico Fermi who had arrived in exile on January 2. Soon groups all over the world were investigating the fission of ranium and its potential to cause a chain reaction. The worsening international situation in the months before the start of World War II resulted in major projects being set up in several countries (.S., Britain, France, Germany and Japan) in order to produce the first nuclear weapons. The largest effort was the Manhattan Project in the nited States. This was started at Columbia niversity by Fermi in nder the leadership of J. Robert Oppenheimer at a secret lab in Los Alamos, NM, the project produced the atomic bombs that exploded in the Alamogordo desert in New Mexico (July ) and over Hiroshima (August ) and Nagasaki (August ).

16 Two technical problems that had to be solved in order to achieve a chain reaction in ranium: 1) Fission occurs if -235 captures a slow neutron, but the neutrons emitted in fission are fast. The device must contain a substance which slows the neutrons down. A moderator is an element whose nuclei don t absorb neutrons and which are relatively light so that in collisions with neutrons they will absorb energy and thus slow the neutrons down. Typical moderators are water, heavy water (D 2 O), or Carbon (graphite). 2) The sample must be enriched with enough -235 relative to -238.

17 An additional technical problem must be overcome to achieve controlled nuclear fission, as for power generation. A variable amount of an additional material that is highly efficient in capturing neutrons must be inserted in the ranium. => Cadmium rods The first self-sustaining nuclear reactor, using graphite as moderator, was built under the stands of the football stadium at the niversity of Chicago by Enrico Fermi on December 2, 1942.

Nuclear Properties. Thornton and Rex, Ch. 12

Nuclear Properties. Thornton and Rex, Ch. 12 Nuclear Properties Thornton and Rex, Ch. 12 A pre-history 1896 Radioactivity discovered - Becquerel a rays + (Helium) b rays - (electrons) g rays 0 (EM waves) 1902 Transmutation observed - Rutherford and