Nuclear reaction

Rutherford in 1919 , thought that it ought to be possible to penetrate a nucleus with a massive high speed particle such as alpha particle and thereby either produce a nucleus with greater mass no or induce an artificial disintegration .Rutherford was successful in bombarding nitrogen with alpha particle & obtaining as a result an oxygen nucleus & a proton according to the equation:-

Such a process was termed as nuclear reactionnuclear reaction. In a nuclear reaction the sum of the initial atomic numbers is equal to the sum of final atomic numbers, a condition imposerd by conservation of charge . The sum of the initial mess nos is also equal to sum of final mass nos. but the initial rest mass is not equal to the sum of the final rest mass. The diff. Between the rest masses is equal to the nuclear reaction energy .The nuclear reactions at low energy are mostly of type

A + a ---------- B + p + q

.

Where A is the target nucleus, a is the impinging particle . B & p the products. B is usually a light nucleus or a gamma ray. If the reaction energy Q is positive , the reaction is termed as exothermic . On the other hand if it is negative the reaction is termed as endothermic

A + a ---------- B + p + q

 

ENERGY FROM NUCLEUS

 When we get energy from coal by burning the fuel in a furnace, we are doing so by tinkering the atoms of carbon and oxygen , rearranging their outer electrons into more stable combinations . When we get energy from uranium in a nuclear reactor , we are again burning a fuel , then we are tinkering with its nucleus , rearranging its nucleons into more stable configurations . The nucleons are held in nuclei by a strong force. In both atomic & nuclear burning , the release of energy is accompanied by a decrease in mass acc. To the equation Q= - ∆mc2 Thus for the same energy release a much larger mass of available mass is consumed in a chemical fuel than in a nuclear fuel.

Soon after the discovery of neutron by Chadwick, Enrico Fermi found that when neutrons bombard various elements , new radioactive elements are produced . He predicted that neutrons being uncharged , would be a useful nuclear projectile unlike the proton or alpha particle , it experience no repulsive coulomb force when it nears a nuclear surface .

NUCLEAR FISSION

In late 1930’s, the physicist Lise Meitner and chemists Otto Hann & Fritz Strassmann , working in Berlin 7 following up the work of Fermi and co-workers , bombarded solutions of uranium salts with such thermal neutrons . They found that after the bombardment of thermal radionuclides were produced. In 1939 after repeated tests, one of the radionuclides was identified as barium.

The splitting up of a uranium nucleus by splitting up of a thermal neutron into two equal parts was known as fission (By Frisch).

In a typical 23592U fission event, a nucleus 235

92U absorbs a thermal neutron, producing a compound nucleus 236

92UIn a highly excited state. It is this nucleus which actually undergoes fission , splitting into two fragments. The following figure shows the distribution by mass number of the fragments produced when 235

92U is bombarded with thermal neutrons. The most probable mass numbers occurring in about 7% of the events are centered around A=95 and A=140. The two fragments formed between them rapidly emit two neutrons, leaving 140Xe & 94Sr as fission fragments . Thus the overall fission process for this event can be represented by the reaction equation, 235

92U +n ----- 23692U----140

54Xe + 9438Sr + 2n

The fragments 140Xe & 94Sr are both highly unstable , undergoing beta decay until each reaches a stable end product . For Xe the decay chain is 140

54Xe 14055Cs -140

56Ba 14057La 140

58Ce

For Strontium the decay chain is 94

38Sr9439Y94

40ZrWe can estimate that the energy released by the fission of a high mass nuclide by examining the total bond energy per nucleon ∆Ebn before and after the fission. Fission can occur because the total mass energy will decrease i.e. ∆Ebn will increase so that the products of the fission are more tightly bound. Thus the total energy Q released by the fission is Q=(Total final binding energy)- (Initial binding energy)

If the fission takes place in a bulk solid most of this disintegration energy which first goes into the kinetic energy of the decay products, appears eventually as an increase in an internal energy of the system Five or six per cent of the disintegration energy however is associated with neutrinos that are emitted during the beta decay of the primary fission fragments. This energy is carried out of the system , as the neutrinos interact very weakly with matter & is lost.

The distribution by mass no. of the fragments that are found when many fission events of 235U are examined

A device for producing a continuous supply of heat energy from nuclear fission. Certain radioactive atomic nuclei, on being struck by neutrons, generate additional neutrons. This is self-sustaining if the speed of the neutrons is not too great. A nuclear reactor therefore has (i) a 'fuel', which may be uranium 235 or 238, or plutonium 239; (ii) a moderator, to control the speed and number of neutrons; and (iii) a heat exchange system, to utilize the heat generated (generally by operating the steam-driven turbines of a conventional electric power station).

A boiling water reactor uses the cooling water itself as the source of steam for the turbines. In a pressurized water reactor, the coolant is water under such pressure that it reaches a high temperature without evaporation, and is used to heat boiler water via a heat exchanger. A gas-cooled reactor uses carbon dioxide or some other gas as a coolant, heating turbine water via a heat exchanger.

 A fast reactor has no moderator, and generally uses liquid sodium as a coolant. A breeder reactor uses uranium 238 enriched with plutonium 239; it produces more Pu 239, and is the type of reactor used to generate material for atomic weapons. Some nuclear reactors are built and used solely for research purposes. Nuclear power sources are an important source of energy in several countries (1990 figures): Belgium (60%), France (75%), Hungary (48%), and Sweden (46%). The UK and USA both obtain c.20% of their power from this source.

Nuclear fusion – Energy generation in starsNuclear fusion – Energy generation in stars Some examples are :- 11H + 1

1H ---------- 21H + e + ν + 0.42 MeV

In this reaction 2 protons combine to form a deuteron and a positron with a release of 0.42 MeV of energy.Two deuterons combine to form a light isotope of helium. 21H + 2

1H --------- 31H + 1

1H + 4.03 MeV

In this reaction 2 deuterons combine to form a triton and a proton energy released when 2 light nulei combine to form a single larger nucleus the process is known as NUCLEAR FUSION.

All these reactions we find that two positively charged particles combine to form a larger nucleus. Such a process is hindered by the coulomb repulsion that acts to prevent the two positively charged particles from getting close enough to be within the range of their attractive nuclear forces and thus ‘fusing’. The height of this coulomb barrier depends on the charges and the radii of the two interacting nuclei. To generate useful amount of energy, nuclear fusion must occur in bulk matter. The process of raising the temperature of the material until the particles have enough energy – due to their thermal motions alone – to penetrate the coulomb barrier is known as THERMO NUCLEAR FUSION.

The temperature at which protons in a proton gas would have enough energy to overcome the coulomb barrier between them is then given by the equation

3/2 k T = Kavwhere Kav is the average kinetic energy of the proton , T is the temperature of the proton gas and k is the BOLTZMANN CONSTANT.The temperature of the core of the sun is only about 1.5 X 107 K. Therefore, even in the sun if the fusion is to take place, it must involve protons whose energies are far above the average energy.

The energy generation in stars takes place via thermo nuclear fusion.

Thus for thermo nuclear fusion to take place extreme conditions of temperature and pressure are required, which are available only in the interior of stars.

FUSION REACTIONS IN SUN

The fusion reaction in sun is a multi step process in which hydrogen is burnt into helium, hydrogen being the ‘fuel’ and helium the ‘ashes’. The proton – proton cycle by which this occurs is represented by following sets of reactions := 11H + 1

1H -------- 21H + e + ν + 0.42 MeV

e+ + e- -------- γ + γ + 1.02 MeV 21H + 1

1H -------- 32He + γ + 5.49 MeV

 32He + 3

2He -------- 42He + 1

1H + 11H + 12.86 MeV

For the fourth reactions to occur , the first 3 reactions must twice , in which caste 2 light he nuclei unite to form ordinary He or nucleus . CARBON CYCLE (For energy generation in sun) 12

6C + 11H --------- 13

7N + γ + 1.93MeV 13

7N -------------- 136C + e + ν + 1.20MeV

  13

6C + 11H -------- 13

7N + γ + 7.6MeV  13

7N + 11H ------ 15

8O + γ + 7.39MeV  15

8O ------------ 157N + e + ν + 1.71MeV

 15

7N + 11H ------- 12

6C + 4=He + 4.99MeV

.

The overall reaction can be written as 41

1H + 126C ----- 4

2H + 126C + 2e+ + 2ν + 3γ + 24.8 MeV

It may be noted that in this set of reactions carbon is not destroyed in the process but acts only as a CATALYST.

The burning of hydrogen in the Sun’s core is alchemy on a grand scale in the sense that one element is turned into another. It has been going for about 5 X 109 y, and calculations show that there is enough hydrogen to keep the Sun going for about the same time into the future. In about 5 billion years, however, the Sun’s core, which by that time will be largely helium, will begin to cool and the Sun will start to collapse under it’s own gravity. This will raise the core temperature and cause the outer envelope to expand, turning the Sun into what is called a RED GIANT.

If the core temperature increases to 108 K again, energy can be produced through fusion once more – this time by burning helium to make carbon. As a star evolves further and becomes still hotter, other elements can be formed by other fusion reactions.

The reactions in stars take place via thermonuclear fusion

CONTROLLED THERMONULEAR FUSIONThe first thermonuclear reaction on Earth occurred at Eniwetok Atoll on November 1, 1952, when USA exploded a fusion device, generating energy equivalent to 10 million tons of TNT.

A sustained and controllable source of fusion is considerably more difficult to achieve. It is being pursued vigorously in many countries around the world because fusion reactor is regarded as THE FUTURE POWER SOURCE.

The p-p cycle discussed earlier is not suitable for an Earth-bound fission reactor as it is extremely slow. The most attractive reaction for terrestrial use appears to be the two deuteron-deuteron reactions ,21H + 2

1H --------- 32H + 3.2 MeV

21H + 2

1H --------- 32H + 4.03 MeV

and the deuteron-triton reaction,21H + 3

1H --------- 42H + n + 17.59 MeV

Requirements for a successful thermonuclear

reactor There are requirements for a successful thermonuclear reactor:(a) A HIGH PARTICLE DENSITY : The density of the interacting particles must be large enough to ensure that d-d collision rate is high enough. At the high temperatures required, the deuterium would be completely ionized, forming neutral plasma.(b) A HIGH PLASMA TEMPERATURE : The plasma must be hot enough to penetrate the Coulomb barrier that tends to keep the interacting particles apart. A plasma ion temperature of 35keV, corresponding to 4 X 108 K has been achieved in the laboratory.

(c) A LONG CONFINEMENT TIME : A major problem is containing the hot plasma long enough to maintain it at a density and temperature sufficiently high to ensure the fusion of enough fuel. No solid container can withstand the high temperatures required. Therefore, clever confining techniques, such as MAGNETIC CONFINEMENT and INERTIAL CONFINEMENT, are being explored.

Efforts are being made world over to achieve thermonuclear fusion in laboratory. When that happens, humanity will be gifted with a source of unlimited and unpolluted energy.

• PHYSICS N.C.E.R.T CLASS – XII

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