Mech Sem7 Me2034nol

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RAJALAKSHMI ENGINEERING COLLEGEThandalam, Chennai 602 105

NOTES ON LESSON

Faculty Name : Mr.A.Khadeer Ahmed Designation : Lecturer

Subject Name : Nuclear Engineering Code : ME2034

Year : IV Semester : VII

Degree & Branch : B.E.-MECHANICAL ENGG. Section : A

OBJECTIVE

To gain some fundamental knowledge about nuclear physics, nuclear reactor, nuclear fuels,reactors and safe disposal of nuclear wastes.

TEXT BOOK

1. Thomas J.Cannoly, Fundamentals of nuclear Engineering John Wiley 1978.

REFERENCES

1. Collier J.G., and Hewitt G.F, Introduction to Nuclear power, Hemisphere publishing,

New York. 1987

2. Wakil M.M.El., Power Plant Technology McGraw-Hill International, 1984.


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UNIT I- NUCLEAR PHYSICS

Introduction of Nuclear Engineering

Nuclear engineering involves the design of systems and processes in whichnuclear physics and radiation plays an important role. Although the traditional focus ofnuclear engineering is the nuclear power industry, students with bachelor of science

degrees in nuclear engineering also pursue careers in health and medical physics,

plasma physics, plasma processing, and environmental mediation. Further, because of

the breadth of the nuclear engineering curriculum, graduates are prepared to work in a

number of technical areas outside the nuclear engineering field.

Nuclear energy, both from fission and fusion, offers a promising approach tomeeting the nation's energy needsan approach that may preserve jobs, raise the

standard of living, and alleviate the depletion of natural resources including natural gas,petroleum, and coal. Nuclear energy will also be required to provide electricity on the

moon or Mars and to propel space vehicles if we are to explore or colonize the solar

system. Since the discovery of fission 50 years ago, electricity is being produced

commercially in a several hundred billion-dollar industry. Applications of radioactive

tracers have been made in medicine, science, and industry. Radiation from particle

accelerators and materials made radioactive in nuclear reactors are used worldwide to

treat cancer and other diseases, to provide power for satellite instrumentation, to

preserve food, to sterilize medical supplies, to search for faults in welds and piping, and

to polymerize chemicals. Low energy plasmas are used in the manufacture of

microelectronics components and to improve the surface characteristics of materials.

High energy plasmas offer the possibility of a new energy source using thermonuclear

fusion. Because the breadth and rate of change in this field requires that the nuclear

engineer have a broad educational background, the curriculum consists of physics, math,

materials science, electronics, thermodynamics, heat transfer, computers, courses in the

humanities and social science areas, and numerous elective courses. Courses of a

specific nuclear engineering content come primarily in the third and fourth years.

The curriculum prepares students for careers in the nuclear industry and

governmentwith electric utility companies, in regulatory positions with the federal or

state governments, or for major contractors on the design and testing of improved

reactors for central station power generation or for propulsion of naval vessels.

The curriculum also prepares the graduate for work in many areas where a broad

technical background is more important than specialization in a specific field. Thus, the

graduate is also prepared to work in any area where a broad engineering background is

helpful, such as management, technical sales, or law. The curriculum gives students


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excellent preparation for graduate study in the fission and fusion areas, medical and

health physics, applied superconductivity, particle accelerator technology, and other

areas of engineering science in addition to study in areas such as materials science,

physics, mathematics, and medicine.

OBJECTIVES OF THE NUCLEAR ENGINEERING Educate students in the fundamental subjects necessary for a career in nuclear

engineering, and prepare students for advanced education in it and related fields;

Educate students in the basics of instrumentation, design of laboratory

techniques, measurement, and data acquisition, interpretation and analysis;

Educate students in the methodology of design;

Provide and facilitate teamwork and multidisciplinary experiences throughout the

curriculum

The nuclear engineer is concerned with the application of nuclear science and

technology for the benefit of humankind. The safe, economic development of nuclearenergy is a major area of activity for the nuclear engineer. The nuclear engineer is also

concerned with the uses of radiation in medical diagnostics and therapy, preservation of

food by irradiation, and the uses of radiation in industry for improving products and

making measurements. The nuclear engineer is prepared to design a nuclear power

reactor, determine how to operate a nuclear power plant most efficiently, and assist in

the evaluation of environmental factors in existing nuclear power plants. With the

rapidly expanding use of radiation in fields such as medical diagnostics and therapy and

food irradiation, there is continuous demand for specialists in radiation protection and

health physics. The safe, long-term storage of nuclear waste is also a challenging

technical problem requiring engineers with knowledge of basic nuclear engineering.

Nuclear engineering includes the use of radiation in medicine for treatment anddiagnostics; design, development and operation of nuclear power systems; numeric

simulation of nuclear systems; health physics and radiation protection; biomedical

engineering and radiation imaging; nondestructive examination of materials and

structures using radiation techniques; nuclear energy for space power and propulsion;

and using radiation in food processing, industrial processing and manufacturing control.

Nuclear model of an atom


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The nuclear model of the atom describes how the three basic sub atomic particles, the

proton, the neutron and the electron are arranged.

The nucleus is the centre of the atom and is positive in charge. It is made up of protons

and neutrons.

Negative electrons orbit the atom. The atom is made up mostly of empty space.

The nuclear model of the atom consists of a nucleus (meaning: 'nut' or 'kernel') which

is surrounded by orbiting electrons.

The atom is made up mostly ofempty space.

The nucleus is made up of protons and neutrons.

Protons are positive, neutrons are neutral and electrons are negative.

In a neutral atom the number of protons (positive charge) = the number of electrons

(negative charge)

Protons determine the identity of an element.

The number of protons is called the Atomic Number. Each element has a unique

Atomic Number. eg. All atoms of Carbon have an Atomic Number of 6. ie. they all

contain 6 protons. All atoms of oxygen contain 8 protons. ie. They have an Atomic

Number of 8. The Atomic Number for each element can be found in the Periodic Table.

Neutrons help stabilise atoms. If there are too many or too few neutrons the atombecomes unstable. Atoms of the same element that contain a different number of

neutrons are called isotopes.
http://www.google.co.in/imgres?imgurl=http://abyss.uoregon.edu/~js/images/rutherford_atom.gif&imgrefurl=http://abyss.uoregon.edu/~js/cosmo/lectures/lec07.html&h=428&w=608&sz=5&tbnid=tlZ7nyroJ-bBxM:&tbnh=96&tbnw=136&prev=/search%3Fq%3DNuclear%2Bmodel%2Bof%2Ban%2Batom%26tbm%3Disch%26tbo%3Du&zoom=1&q=Nuclear+model+of+an+atom&hl=en&usg=__EASH3Awfcxnnd_x2dMJ0-5yvJ6o=&sa=X&ei=Jqy3TdO_LofRrQfwo5nJDQ&ved=0CBgQ9QEwAg


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Electrons are involved in chemical reactions. During a reaction electrons are either

transferred or shared between chemical species. The noble gases are very unreactive

because they have a complete number of electrons in their outer shell.

The Rutherford model orplanetary model is a model of the atom devised by Ernest

Rutherford. Rutherford directed the famous Geiger-Marsden experiment in 1909, whichsuggested on Rutherford's 1911 analysis that the so-called "plum pudding model" ofJ. J.

Thomson of the atom was incorrect. Rutherford's new model for the atom, based on theexperimental results, had the new features of a relatively high central charge

concentrated into a very small volume in comparison to the rest of the atom and

containing the bulk of the atomic mass (the nucleus of the atom).

Rutherford's model did not make any new headway in explaining the electron-structure

of the atom; in this regard Rutherford merely mentioned earlier atomic models in which

a number of tiny electrons circled the nucleus like planets around the sun, or a ring

around a planet (such as Saturn). However, by implication, Rutherford's concentrationof most of the atom's mass into a very small core made a planetary model an even more

likely metaphor than before, as such a core would contain most of the atom's mass, in an

analogous way to the Sun containing most of the solar system's mass.

In 1911, Rutherford came forth with his own physical model for subatomic structure, as

an interpretation for the unexpected experimental results. In it, the atom is made up of a

central charge (this is the modern atomic nucleus, though Rutherford did not use the

term "nucleus" in his paper) surrounded by a cloud of (presumably) orbiting electrons.

In this May 1911 paper, Rutherford only commits himself to a small central region of

very high positive or negative charge in the atom.

"For concreteness, consider the passage of a high speed particle through an atom

having a positive central charge Ne, and surrounded by a compensating charge ofNelectrons."

From purely energetic considerations of how far alpha particles of known speed would

be able to penetrate toward a central charge of 100 e, Rutherford was able to calculate

that the radius of his gold central charge would need to be less (how much less could not

be told) than 3.4 x 1014 metres (the modern value is only about a fifth of this). This was

in a gold atom known to be 1010 metres or so in radiusa very surprising finding, as it

implied a strong central charge less than 1/3000th of the diameter of the atom.

The Rutherford model served to concentrate a great deal of the atom's charge and mass

to a very small core, but didn't attribute any structure to the remaining electrons and

remaining atomic mass. It did mention the atomic model of Hantaro Nagaoka, in which

the electrons are arranged in one or more rings, with the specific metaphorical structure
http://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Ernest_Rutherfordhttp://en.wikipedia.org/wiki/Ernest_Rutherfordhttp://en.wikipedia.org/wiki/Geiger-Marsden_experimenthttp://en.wikipedia.org/wiki/Plum_pudding_modelhttp://en.wikipedia.org/wiki/J._J._Thomsonhttp://en.wikipedia.org/wiki/J._J._Thomsonhttp://en.wikipedia.org/wiki/Atomic_masshttp://en.wikipedia.org/wiki/Atomic_nucleushttp://en.wikipedia.org/wiki/Electronshttp://en.wikipedia.org/wiki/Sunhttp://en.wikipedia.org/wiki/Solar_systemhttp://en.wikipedia.org/wiki/Atomic_nucleushttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Alpha_particlehttp://en.wikipedia.org/wiki/Elementary_chargehttp://en.wikipedia.org/wiki/Goldhttp://en.wikipedia.org/wiki/Hantaro_Nagaokahttp://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Ernest_Rutherfordhttp://en.wikipedia.org/wiki/Ernest_Rutherfordhttp://en.wikipedia.org/wiki/Geiger-Marsden_experimenthttp://en.wikipedia.org/wiki/Plum_pudding_modelhttp://en.wikipedia.org/wiki/J._J._Thomsonhttp://en.wikipedia.org/wiki/J._J._Thomsonhttp://en.wikipedia.org/wiki/Atomic_masshttp://en.wikipedia.org/wiki/Atomic_nucleushttp://en.wikipedia.org/wiki/Electronshttp://en.wikipedia.org/wiki/Sunhttp://en.wikipedia.org/wiki/Solar_systemhttp://en.wikipedia.org/wiki/Atomic_nucleushttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Alpha_particlehttp://en.wikipedia.org/wiki/Elementary_chargehttp://en.wikipedia.org/wiki/Goldhttp://en.wikipedia.org/wiki/Hantaro_Nagaoka


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of the stable rings of Saturn. The so-called plum pudding model of J.J. Thomson had

also had rings of orbiting electrons.

The Rutherford paper suggested that the central charge of an atom might be

"proportional" to its atomic mass in hydrogen mass units u (roughly 1/2 of it, in

Rutherford's model). For gold, this mass number is 197 (not then known to greataccuracy) and was therefore modeled by Rutherford to be possibly 196 u. However,

Rutherford did not attempt to make the direct connection of central charge to atomicnumber, since gold's place on the periodic table was known to be about 79 u, and

Rutherford's more tentative model for the structure of the gold nucleus was 49 helium

nuclei, which would have given it a mass of 196 u and charge of 98 e, which was much

more in keeping with his experimentally-determined central charge for gold in this

experiment of about 100 e. This differed enough from gold's "atomic number" (at thattime merely its place number in the periodic table) that Rutherford did not formallysuggest the two numbers (atomic number and nuclear charge) might be exactly the

same.

A month after Rutherford's paper appeared, the proposal regarding the exact identity of

atomic number and nuclear charge was made by Antonius van den Broek, and laterconfirmed experimentally within two years, by Henry Moseley.

concept of massenergy equivalence connects the concepts ofconservation of mass and

conservation of energy, which continue to hold separately. The theory of relativity

allows particles which have rest mass to be converted to other forms of mass which

require motion, such as kinetic energy, heat, or light. However, the mass remains.

Kinetic energy or light can also be converted to new kinds of particles which have restmass, but again the energy remains. Both the total mass and the total energy inside a

totally closed system remain constant over time, as seen by any single observer in a

given inertial frame. In other words, energy cannot be created or destroyed, and energy,

in all of its forms, has mass. Mass also cannot be created or destroyed, and in all of its

forms, has energy. According to the theory of relativity, mass and energy as commonly

understood, are two names for the same thing, and neither one is changed or

transformed into the other. Rather, neither one appears without the other. Rather thanmass being changed into energy, the view of relativity is that rest mass has beenchanged to a more mobile form of mass, but remains mass. In this process, neither the

amount of mass nor the amount of energy changes. Thus, if energy changes type and

leaves a system, it simply takes its mass with it. If either mass or energy disappears froma system, it will always be found that both have simply moved off to another place.
http://en.wikipedia.org/wiki/Plum_pudding_modelhttp://en.wikipedia.org/wiki/Atomic_mass_unithttp://en.wikipedia.org/wiki/Atomic_numberhttp://en.wikipedia.org/wiki/Atomic_numberhttp://en.wikipedia.org/wiki/Periodic_tablehttp://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Antonius_van_den_Broekhttp://en.wikipedia.org/wiki/Henry_Moseleyhttp://en.wikipedia.org/wiki/Conservation_of_masshttp://en.wikipedia.org/wiki/Conservation_of_energyhttp://en.wikipedia.org/wiki/Rest_masshttp://en.wikipedia.org/wiki/Kinetic_energyhttp://en.wikipedia.org/wiki/Masshttp://en.wikipedia.org/wiki/Plum_pudding_modelhttp://en.wikipedia.org/wiki/Atomic_mass_unithttp://en.wikipedia.org/wiki/Atomic_numberhttp://en.wikipedia.org/wiki/Atomic_numberhttp://en.wikipedia.org/wiki/Periodic_tablehttp://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Antonius_van_den_Broekhttp://en.wikipedia.org/wiki/Henry_Moseleyhttp://en.wikipedia.org/wiki/Conservation_of_masshttp://en.wikipedia.org/wiki/Conservation_of_energyhttp://en.wikipedia.org/wiki/Rest_masshttp://en.wikipedia.org/wiki/Kinetic_energyhttp://en.wikipedia.org/wiki/Mass


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Fast-moving objects and systems of objects

When an object is pushed in the direction of motion, it gains momentum and energy, but

when the object is already traveling near the speed of light, it cannot move much faster,

no matter how much energy it absorbs. Its momentum and energy continue to increase

without bounds, whereas its speed approaches a constant valuethe speed of light. Thisimplies that in relativity the momentum of an object cannot be a constant times the

velocity, nor can the kinetic energy be a constant times the square of the velocity.

The relativistic mass is defined as the ratio of the momentum of an object to its velocity.[4] Relativistic mass depends on the motion of the object. If the object is moving slowly,

the relativistic mass is nearly equal to the rest mass and both are nearly equal to the

usual Newtonian mass. If the object is moving quickly, the relativistic mass is greater

than the rest mass by an amount equal to the mass associated with the kinetic energy of

the object. As the object approaches the speed of light, the relativistic mass grows

infinitely, because the kinetic energy grows infinitely and this energy is associated withmass.

The relativistic mass is always equal to the total energy (rest energy plus kinetic energy)

divided by c2.[3] Because the relativistic mass is exactly proportional to the energy,relativistic mass and relativistic energy are nearly synonyms; the only difference

between them is the units. If length and time are measured in natural units, the speed of

light is equal to 1, and even this difference disappears. Then mass and energy have the

same units and are always equal, so it is redundant to speak about relativistic mass,

because it is just another name for the energy. This is why physicists usually reserve the

useful short word "mass" to mean rest-mass.

For things made up of many parts, like an atomic nucleus, planet, orstar, the relativistic

mass is the sum of the relativistic masses (or energies) of the parts, because energies are

additive in closed systems. This is not true in systems which are open, however, if

energy is subtracted. For example, if a system is bound by attractive forces and the workthey do in attraction is removed from the system, mass will be lost. Such work is a form

of energy which itself has mass, and thus mass is removed from the system, as it is

bound. For example, the mass of an atomic nucleus is less than the total mass of the

protons and neutrons that make it up, but this is only true after the energy (work) of

binding has been removed in the form of a gamma ray (which in this system, carries

away the mass of binding). This mass decrease is also equivalent to the energy requiredto break up the nucleus into individual protons and neutrons (in this case, work and

mass would need to be supplied). Similarly, the mass of the solar system is slightly less

than the masses of sun and planets individually.
http://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Momentumhttp://en.wikipedia.org/wiki/Velocityhttp://en.wikipedia.org/wiki/Kinetic_energy#Kinetic_energy_of_rigid_bodieshttp://en.wikipedia.org/wiki/Relativistic_masshttp://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence#cite_note-3%23cite_note-3http://en.wikipedia.org/wiki/Rest_masshttp://en.wikipedia.org/wiki/Kinetic_energyhttp://en.wikipedia.org/wiki/Kinetic_energyhttp://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence#cite_note-tipler-2%23cite_note-tipler-2http://en.wikipedia.org/wiki/Unit_of_measurementhttp://en.wikipedia.org/wiki/Natural_unitshttp://en.wikipedia.org/wiki/Atomic_nucleushttp://en.wikipedia.org/wiki/Planethttp://en.wikipedia.org/wiki/Starhttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Momentumhttp://en.wikipedia.org/wiki/Velocityhttp://en.wikipedia.org/wiki/Kinetic_energy#Kinetic_energy_of_rigid_bodieshttp://en.wikipedia.org/wiki/Relativistic_masshttp://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence#cite_note-3%23cite_note-3http://en.wikipedia.org/wiki/Rest_masshttp://en.wikipedia.org/wiki/Kinetic_energyhttp://en.wikipedia.org/wiki/Kinetic_energyhttp://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence#cite_note-tipler-2%23cite_note-tipler-2http://en.wikipedia.org/wiki/Unit_of_measurementhttp://en.wikipedia.org/wiki/Natural_unitshttp://en.wikipedia.org/wiki/Atomic_nucleushttp://en.wikipedia.org/wiki/Planethttp://en.wikipedia.org/wiki/Star


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The relativistic mass of a moving object is bigger than the relativistic mass of an object

that is not moving, because a moving object has extra kinetic energy. The rest mass ofan object is defined as the mass of an object when it is at rest, so that the rest mass is

always the same, independent of the motion of the observer: it is the same in all inertial

frames.

For a system of particles going off in different directions, the invariant mass of the

system is the analog of the rest mass, and is the same for all observers. It is defined asthe total energy (divided by c2) in the center of mass frame (where by definition, thesystem total momentum is zero). A simple example of an object with moving parts but

zero total momentum, is a container of gas. In this case, the mass of the container is

given by its total energy (including the kinetic energy of the gas molecules), since the

system total energy and invariant mass are the same in the reference frame where the

momentum is zero, and this reference frame is also the only frame in which the objectcan be weighed.

As is noted above, two different definitions of mass have been used in special relativity,

and also two different definitions of energy. The simple equation E= mc is notgenerally applicable to all these types of mass and energy, except in the special case thatthe momentum is zero for the system under consideration. In such a case, which is

always guaranteed when observing the system from the center of mass frame,E= mc istrue for any type of mass and energy that are chosen. Thus, for example, in the center of

mass frame the total energy of an object or system is equal to its rest mass times c, auseful equality. This is the relationship used for the container of gas in the previous

example. It is not true in other reference frames in which a system or object's total

energy will depend on both its rest (or invariant) mass, and also its total momentum.

In inertial reference frames other than the rest frame or center of mass frame, the

equationE= mc remains true if the energy is the relativistic energy and the mass therelativistic mass. It is also correct if the energy is the rest or invariant energy (also the

minimum energy), andthe mass is the rest or invariant mass.

However, connection of the total or relativistic energy (Er) with the rest or invariantmass (m0) requires consideration of the system total momentum, in systems and

reference frames where momentum has a non-zero value. The formula then required to

connect the different kinds of mass and energy, is the extended version of Einstein's

equation, called the relativistic energymomentum relationship:

or

Here the (pc)2 term represents the square of the Euclidean norm (total vector length) ofthe various momentum vectors in the system, which reduces to the square of the simple
http://en.wikipedia.org/wiki/Inertial_framehttp://en.wikipedia.org/wiki/Inertial_framehttp://en.wikipedia.org/wiki/Invariant_masshttp://en.wikipedia.org/wiki/Center_of_mass_framehttp://en.wikipedia.org/wiki/Mass_in_special_relativity#The_relativistic_energy-momentum_equationhttp://en.wikipedia.org/wiki/Euclidean_normhttp://en.wikipedia.org/wiki/Inertial_framehttp://en.wikipedia.org/wiki/Inertial_framehttp://en.wikipedia.org/wiki/Invariant_masshttp://en.wikipedia.org/wiki/Center_of_mass_framehttp://en.wikipedia.org/wiki/Mass_in_special_relativity#The_relativistic_energy-momentum_equationhttp://en.wikipedia.org/wiki/Euclidean_norm


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momentum magnitude, if only a single particle is considered. Obviously this equation

reduces to E= mc when the momentum term is zero. For photons where m0 = 0, theequation reduces to Er = pc.

Binding energy and the "mass defect"

Whenever any type of energy is removed from a system, the mass associated with the

energy is also removed, and the system therefore loses mass. This mass defect in the

system may be simply calculated as m = E/c2, but use of this formula in such

circumstances has led to the false idea that mass has been "converted" to energy. This

may be particularly the case when the energy (and mass) removed from the system is

associated with the binding energy of the system. In such cases, the binding energy isobserved as a "mass defect" or deficit in the new system and the fact that the released

energy is not easily weighed may cause its mass to be neglected.

The difference between the rest mass of a bound system and of the unbound parts is thebinding energy of the system, if this energy has been removed after binding. For

example, a water molecule weighs a little less than two free hydrogen atoms and an

oxygen atom; the minuscule mass difference is the energy that is needed to split the

molecule into three individual atoms (divided by c), and which was given off as heatwhen the molecule formed (this heat had mass). Likewise, a stick of dynamite in theory

weighs a little bit more than the fragments after the explosion, but this is true only so

long as the fragments are cooled and the heat removed. In this case the mass difference

is the energy/heat that is released when the dynamite explodes, and when this heat

escapes, the mass associated with it escapes, only to be deposited in the surroundings

which absorb the heat (so that total mass is conserved).

Such a change in mass may only happen when the system is open, and the energy and

mass escapes. Thus, if a stick of dynamite is blown up in a hermetically sealed chamber,

the mass of the chamber and fragments, the heat, sound, and light would still be equal to

the original mass of the chamber and dynamite. If sitting on a scale, the weight and masswould not change. This would in theory also happen even with a nuclear bomb, if it

could be kept in an ideal box of infinite strength, which did not rupture or pass radiation.

Thus, a 21.5 kiloton (9 x 1013joule) nuclear bomb produces about one gram of heat and

electromagnetic radiation, but the mass of this energy would not be detectable in an

exploded bomb in an ideal box sitting on a scale; instead, the contents of the box would

be heated to millions of degrees without changing total mass and weight. If then,however, a transparent window (passing only electromagnetic radiation) were opened in

such an ideal box after the explosion, and a beam of X-rays and other lower-energy light

allowed to escape the box, it would eventually be found to weigh one gram less than it

had before the explosion. This weight-loss and mass-loss would happen as the box was
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cooled by this process, to room temperature. However, any surrounding mass which had

absorbed the X-rays (and other "heat") would gain this gram of mass from the resulting

heating, so the mass "loss" would represent merely its relocation. Thus, no mass (or, in

the case of a nuclear bomb, no matter) would be "converted" to energy in such a

process. Mass and energy, as always, would both be separately conserved.

Massless particles

Massless particles have zero rest mass. Their relativistic mass is simply their relativistic

energy, divided by c2, or m(relativistic) = E/c2. The energy for photons is E = h where

h is Planck's constant and is the photon frequency. This frequency and thus the

relativistic energy are frame-dependent.

If an observer runs away from a photon in the direction it travels from a source, havingit catch up with the observer, then when the photon catches up it will be seen as having

less energy than it had at the source. The faster the observer is traveling with regard tothe source when the photon catches up, the less energy the photon will have. As an

observer approaches the speed of light with regard to the source, the photon looks

redder and redder, by relativistic Doppler effect (the Doppler shift is the relativistic

formula), and the energy of a very long-wavelength photon approaches zero. This iswhy a photon is massless; this means that the rest mass of a photon is zero.

Two photons moving in different directions cannot both be made to have arbitrarily

small total energy by changing frames, or by moving toward or away from them. The

reason is that in a two-photon system, the energy of one photon is decreased by chasingafter it, but the energy of the other will increase with the same shift in observer motion.

Two photons not moving in the same direction will exhibit an inertial frame where the

combined energy is smallest, but not zero. This is called the center of mass frame or the

center of momentum frame; these terms are almost synonyms (the center of mass frame

is the special case of a center of momentum frame where the center of mass is put at the

origin). The most that chasing a pair of photons can accomplish to decrease their energyis to put the observer in frame where the photons have equal energy and are moving

directly away from each other. In this frame, the observer is now moving in the same

direction and speed as the center of mass of the two photons. The total momentum of

the photons is now zero, since their momentums are equal and opposite. In this frame

the two photons, as a system, have a mass equal to their total energy divided by c2. Thismass is called the invariant mass of the pair of photons together. It is the smallest massand energy the system may be seen to have, by any observer. It is only the invariant

mass of a two-photon system that can be used to make a single particle with the same

rest mass.
http://en.wikipedia.org/wiki/Planck's_constanthttp://en.wikipedia.org/wiki/Relativistic_Doppler_effecthttp://en.wikipedia.org/wiki/Inertial_framehttp://en.wikipedia.org/wiki/Center_of_masshttp://en.wikipedia.org/wiki/Center_of_momentumhttp://en.wikipedia.org/wiki/Invariant_masshttp://en.wikipedia.org/wiki/Planck's_constanthttp://en.wikipedia.org/wiki/Relativistic_Doppler_effecthttp://en.wikipedia.org/wiki/Inertial_framehttp://en.wikipedia.org/wiki/Center_of_masshttp://en.wikipedia.org/wiki/Center_of_momentumhttp://en.wikipedia.org/wiki/Invariant_mass


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If the photons are formed by the collision of a particle and an antiparticle, the invariant

mass is the same as the total energy of the particle and antiparticle (their rest energy plus

the kinetic energy), in the center of mass frame, where they will automatically be

moving in equal and opposite directions (since they have equal momentum in this

frame). If the photons are formed by the disintegration of a single particle with a well-

defined rest mass, like the neutralpion, the invariant mass of the photons is equal to restmass of the pion. In this case, the center of mass frame for the pion is just the frame

where the pion is at rest, and the center of mass does not change after it disintegrates

into two photons. After the two photons are formed, their center of mass is still moving

the same way the pion did, and their total energy in this frame adds up to the mass

energy of the pion. Thus, by calculating the invariant mass of pairs of photons in a

particle detector, pairs can be identified that were probably produced by pion

disintegration.

Radioactive decay

Alpha decay is one example type of radioactive decay, in which an atomic nucleus

emits analpha particle, and thereby transforms (or 'decays') into an atom with a mass

number4 less and atomic number2 less. Many other types of decays are possible.

Radioactive decay is the process by which an atomic nucleus of an unstable atom loses

energy by emitting ionizing particles (ionizing radiation). The emission is spontaneous,

in that the atom decays without any interaction with another particle from outside the

atom (i.e., without a nuclear reaction). Usually, radioactive decay happens due to a

process confined to the nucleus of the unstable atom, but, on occasion (as with the

different processes ofelectron capture and internal conversion), an inner electron of theradioactive atom is also necessary to the process.

Radioactive decay is a stochastic (i.e., random) process at the level of single atoms, in

that, according to quantum theory, it is impossible to predict when a given atom will

decay.[1] However, given a large number of identical atoms (nuclides), the decay rate for

the collection is predictable, via the Law of Large Numbers.

The decay, or loss of energy, results when an atom with one type of nucleus, called the

parent radionuclide, transforms to an atom with a nucleus in a different state, or adifferent nucleus, either of which is named the daughter nuclide. Often the parent anddaughter are different chemical elements, and in such cases the decay process results innuclear transmutation. In an example of this, a carbon-14 atom (the "parent") emits

radiation (abeta particle, antineutrino, and a gamma ray) and transforms to a nitrogen-

14 atom (the "daughter"). By contrast, there exist two types of radioactive decay

processes (gamma decay and internal conversion decay) that do not result in
http://en.wikipedia.org/wiki/Pionhttp://en.wikipedia.org/wiki/Pionhttp://en.wikipedia.org/wiki/Invariant_masshttp://en.wikipedia.org/wiki/Radioactivityhttp://en.wikipedia.org/wiki/Atomic_nucleushttp://en.wikipedia.org/wiki/Alpha_particlehttp://en.wikipedia.org/wiki/Alpha_particlehttp://en.wikipedia.org/wiki/Mass_numberhttp://en.wikipedia.org/wiki/Mass_numberhttp://en.wikipedia.org/wiki/Atomic_numberhttp://en.wikipedia.org/wiki/Atomic_nucleushttp://en.wikipedia.org/wiki/Ionizing_radiationhttp://en.wikipedia.org/wiki/Nuclear_reactionhttp://en.wikipedia.org/wiki/Electron_capturehttp://en.wikipedia.org/wiki/Internal_conversionhttp://en.wikipedia.org/wiki/Stochastichttp://en.wikipedia.org/wiki/Quantum_mechanicshttp://en.wikipedia.org/wiki/Radioactive_decay#cite_note-not-predict-0%23cite_note-not-predict-0http://en.wikipedia.org/wiki/Law_of_Large_Numbershttp://en.wikipedia.org/wiki/Radionuclidehttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/Nuclear_transmutationhttp://en.wikipedia.org/wiki/Carbon-14http://en.wikipedia.org/wiki/Beta_particlehttp://en.wikipedia.org/wiki/Antineutrinohttp://en.wikipedia.org/wiki/Gamma_rayhttp://en.wikipedia.org/wiki/Nitrogen-14http://en.wikipedia.org/wiki/Nitrogen-14http://en.wikipedia.org/wiki/Gamma_decayhttp://en.wikipedia.org/wiki/Internal_conversionhttp://en.wikipedia.org/wiki/Pionhttp://en.wikipedia.org/wiki/Invariant_masshttp://en.wikipedia.org/wiki/Radioactivityhttp://en.wikipedia.org/wiki/Atomic_nucleushttp://en.wikipedia.org/wiki/Alpha_particlehttp://en.wikipedia.org/wiki/Mass_numberhttp://en.wikipedia.org/wiki/Mass_numberhttp://en.wikipedia.org/wiki/Atomic_numberhttp://en.wikipedia.org/wiki/Atomic_nucleushttp://en.wikipedia.org/wiki/Ionizing_radiationhttp://en.wikipedia.org/wiki/Nuclear_reactionhttp://en.wikipedia.org/wiki/Electron_capturehttp://en.wikipedia.org/wiki/Internal_conversionhttp://en.wikipedia.org/wiki/Stochastichttp://en.wikipedia.org/wiki/Quantum_mechanicshttp://en.wikipedia.org/wiki/Radioactive_decay#cite_note-not-predict-0%23cite_note-not-predict-0http://en.wikipedia.org/wiki/Law_of_Large_Numbershttp://en.wikipedia.org/wiki/Radionuclidehttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/Nuclear_transmutationhttp://en.wikipedia.org/wiki/Carbon-14http://en.wikipedia.org/wiki/Beta_particlehttp://en.wikipedia.org/wiki/Antineutrinohttp://en.wikipedia.org/wiki/Gamma_rayhttp://en.wikipedia.org/wiki/Nitrogen-14http://en.wikipedia.org/wiki/Nitrogen-14http://en.wikipedia.org/wiki/Gamma_decayhttp://en.wikipedia.org/wiki/Internal_conversion


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transmutation, but only decrease the energy of an excited nucleus. This results in an

atom of the same element as before but with a nucleus in a lower energy state. An

example is the nuclear isomer technetium-99m decaying, by the emission of a gamma

ray, to an atom oftechnetium-99.

Nuclides produced as daughters are called radiogenic nuclides, whether they themselvesare stable or not. A number of naturally occurring radionuclides are short-lived

radiogenic nuclides that are the daughters of radioactive primordial nuclides (types ofradioactive atoms that have been present since the beginning of the Earth and solar

system). Other naturally occurring radioactive nuclides are cosmogenic nuclides,

formed by cosmic ray bombardment of material in the Earth's atmosphere or crust. For a

summary table showing the number of stable nuclides and of radioactive nuclides in

each category, see Radionuclide.

The SIunit of activity is the becquerel (Bq). One Bq is defined as one transformation

(or decay) per second. Since any reasonably-sized sample of radioactive materialcontains many atoms, a Bq is a tiny measure of activity; amounts on the order of GBq

(gigabecquerel, 1 x 109 decays per second) or TBq (terabecquerel, 1 x 10 12 decays per

second) are commonly used. Another unit of radioactivity is the curie, Ci, which wasoriginally defined as the amount of radium emanation (radon-222) in equilibrium with

one gram of pure radium, isotope Ra-226. At present it is equal, by definition, to the

activity of any radionuclide decaying with a disintegration rate of 3.7 10 10 Bq. The use

of Ci is presently discouraged by the SI.

Types of decay

As for types of radioactive radiation, it was found that an electric or magnetic field

could split such emissions into three types of beams. For lack of better terms, the rays

were given the alphabetic names alpha,beta, and gamma, still in use today. While alpha

decay was seen only in heavier elements (atomic number 52, tellurium, and greater), the

other two types of decay were seen in all of the elements.

In analyzing the nature of the decay products, it was obvious from the direction of

electromagnetic forces produced upon the radiations by external magnetic and electric

fields that alpha rays carried a positive charge, beta rays carried a negative charge, and

gamma rays were neutral. From the magnitude of deflection, it was clear that alpha

particles were much more massive thanbeta particles. Passing alpha particles through avery thin glass window and trapping them in a discharge tube allowed researchers to

study the emission spectrum of the resulting gas, and ultimately prove that alpha

particles are helium nuclei. Other experiments showed the similarity between classical

beta radiation and cathode rays: They are both streams of electrons. Likewise gamma

radiation and X-rays were found to be similar high-energy electromagnetic radiation.
http://en.wikipedia.org/wiki/Nuclear_isomerhttp://en.wikipedia.org/wiki/Technetium-99mhttp://en.wikipedia.org/wiki/Technetium-99http://en.wikipedia.org/wiki/Radiogenic_nuclidehttp://en.wikipedia.org/wiki/Stable_isotopehttp://en.wikipedia.org/wiki/Radionuclidehttp://en.wikipedia.org/wiki/Primordial_nuclidehttp://en.wikipedia.org/wiki/Cosmogenic_nuclidehttp://en.wikipedia.org/wiki/Radionuclidehttp://en.wikipedia.org/wiki/International_System_of_Unitshttp://en.wikipedia.org/wiki/International_System_of_Unitshttp://en.wikipedia.org/wiki/Becquerelhttp://en.wikipedia.org/wiki/Curiehttp://en.wikipedia.org/wiki/Radiumhttp://en.wikipedia.org/wiki/Isotopehttp://en.wikipedia.org/wiki/Isotopehttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Greek_alphabethttp://en.wikipedia.org/wiki/Alpha_particlehttp://en.wikipedia.org/wiki/Beta_particlehttp://en.wikipedia.org/wiki/Gamma_rayhttp://en.wikipedia.org/wiki/Telluriumhttp://en.wikipedia.org/wiki/Electromagnetic_forcehttp://en.wikipedia.org/wiki/Alpha_decayhttp://en.wikipedia.org/wiki/Beta_decayhttp://en.wikipedia.org/wiki/Gamma_rayhttp://en.wikipedia.org/wiki/Alpha_particleshttp://en.wikipedia.org/wiki/Alpha_particleshttp://en.wikipedia.org/wiki/Beta_particleshttp://en.wikipedia.org/wiki/Neon_lamphttp://en.wikipedia.org/wiki/Emission_spectrumhttp://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Cathode_rayhttp://en.wikipedia.org/wiki/Electronshttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Nuclear_isomerhttp://en.wikipedia.org/wiki/Technetium-99mhttp://en.wikipedia.org/wiki/Technetium-99http://en.wikipedia.org/wiki/Radiogenic_nuclidehttp://en.wikipedia.org/wiki/Stable_isotopehttp://en.wikipedia.org/wiki/Radionuclidehttp://en.wikipedia.org/wiki/Primordial_nuclidehttp://en.wikipedia.org/wiki/Cosmogenic_nuclidehttp://en.wikipedia.org/wiki/Radionuclidehttp://en.wikipedia.org/wiki/International_System_of_Unitshttp://en.wikipedia.org/wiki/Becquerelhttp://en.wikipedia.org/wiki/Curiehttp://en.wikipedia.org/wiki/Radiumhttp://en.wikipedia.org/wiki/Isotopehttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Greek_alphabethttp://en.wikipedia.org/wiki/Alpha_particlehttp://en.wikipedia.org/wiki/Beta_particlehttp://en.wikipedia.org/wiki/Gamma_rayhttp://en.wikipedia.org/wiki/Telluriumhttp://en.wikipedia.org/wiki/Electromagnetic_forcehttp://en.wikipedia.org/wiki/Alpha_decayhttp://en.wikipedia.org/wiki/Beta_decayhttp://en.wikipedia.org/wiki/Gamma_rayhttp://en.wikipedia.org/wiki/Alpha_particleshttp://en.wikipedia.org/wiki/Alpha_particleshttp://en.wikipedia.org/wiki/Beta_particleshttp://en.wikipedia.org/wiki/Neon_lamphttp://en.wikipedia.org/wiki/Emission_spectrumhttp://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Cathode_rayhttp://en.wikipedia.org/wiki/Electronshttp://en.wikipedia.org/wiki/Electromagnetic_radiation


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The relationship between types of decays also began to be examined: For example,

gamma decay was almost always found associated with other types of decay, occurring

at about the same time, or afterward. Gamma decay as a separate phenomenon (with its

own half-life, now termed isomeric transition), was found in natural radioactivity to be a

result of the gamma decay of excited metastable nuclear isomers, in turn created from

other types of decay.

Although alpha, beta, and gamma were found most commonly, other types of decaywere eventually discovered. Shortly after the discovery of the positron in cosmic ray

products, it was realized that the same process that operates in classicalbeta decay can

also produce positrons (positron emission). In an analogous process, instead of emitting

positrons and neutrinos, some proton-rich nuclides were found to capture their own

atomic electrons (electron capture), and emit only a neutrino (and usually also a gamma

ray). Each of these types of decay involves the capture or emission of nuclear electronsor positrons, and acts to move a nucleus toward the ratio of neutrons to protons that has

the least energy for a given total number ofnucleons (neutrons plus protons).

Shortly after discovery of the neutron in 1932, it was discovered by Enrico Fermi that

certain rare decay reactions yield neutrons as a decay particle (neutron emission).Isolatedproton emission was eventually observed in some elements. It was also found

that some heavy elements may undergo spontaneous fission into products that vary in

composition. In a phenomenon called cluster decay, specific combinations of neutronsand protons (atomic nuclei) other than alpha particles (helium nuclei) were found to bespontaneously emitted from atoms, on occasion.

Other types of radioactive decay that emit previously seen particles were found, but bydifferent mechanisms. An example is internal conversion, which results in electron and

sometimes high-energy photon emission, even though it involves neither beta nor

gamma decay. This type of decay (like isomeric transition gamma decay) did not

transmute one element to another.

Rare events that involve a combination of two beta-decay type events happening

simultaneously (see below) are known. Any decay process that does not violate

conservation of energy or momentum laws (and perhaps other particle conservation

laws) is permitted to happen, although not all have been detected. An interesting

example (discussed in a final section) isbound state beta decay ofrhenium-187. In this

process, an inverse of electron capture, beta electron-decay of the parent nuclide is notaccompanied by beta electron emission, because the beta particle has been captured into

the K-shell of the emitting atom. An antineutrino, however, is emitted.
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Decay modes in table form

Radionuclides can undergo a number of different reactions. These are summarized in

the following table. A nucleus with mass numberA and atomic numberZis representedas (A, Z). The column "Daughter nucleus" indicates the difference between the new

nucleus and the original nucleus. Thus, (A 1, Z) means that the mass number is oneless than before, but the atomic number is the same as before.

Mode of decay Participating particlesDaughter

nucleus

Decays with emission of nucleons:

Alpha decay An alpha particle (A = 4,Z= 2) emitted from nucleus(A 4,

Z 2)

Proton emission Aproton ejected from nucleus(A 1,

Z 1)

Neutronemission

A neutron ejected from nucleus (A 1,Z)

Double proton

emissionTwo protons ejected from nucleus simultaneously

(A 2,Z 2)

Spontaneousfission

Nucleus disintegrates into two or more smaller nucleiand other particles

Cluster decayNucleus emits a specific type of smaller nucleus (A1,Z1) smaller than, or larger than, an alpha particle

(A A1,ZZ1) +(A1,Z1)

Different modes of beta decay:

decayA nucleus emits an electron and an electron

antineutrino(A,Z+ 1)

Positron

emission (+

decay)

A nucleus emits apositron and a electron neutrino (A,Z 1)

Electron capture

A nucleus captures an orbiting electron and emits a

neutrino the daughter nucleus is left in an excited

unstable state

(A,Z 1)

Bound statebeta decay

A nucleus beta decays to electron and antineutrino,

but the electron is not emitted, as it is captured intoan empty K-shell;the daughter nucleus is left in anexcited and unstable state. This process is suppressed

except in ionized atoms that have K-shell vacancies.

(A,Z+ 1)

Double beta

decayA nucleus emits two electrons and two antineutrinos (A,Z+ 2)
http://en.wikipedia.org/wiki/Mass_numberhttp://en.wikipedia.org/wiki/Atomic_numberhttp://en.wikipedia.org/wiki/Alpha_decayhttp://en.wikipedia.org/wiki/Alpha_particlehttp://en.wikipedia.org/wiki/Proton_emissionhttp://en.wikipedia.org/wiki/Protonhttp://en.wikipedia.org/wiki/Neutron_emissionhttp://en.wikipedia.org/wiki/Neutron_emissionhttp://en.wikipedia.org/wiki/Neutronhttp://en.wikipedia.org/wiki/Proton_emissionhttp://en.wikipedia.org/wiki/Proton_emissionhttp://en.wikipedia.org/wiki/Spontaneous_fissionhttp://en.wikipedia.org/wiki/Spontaneous_fissionhttp://en.wikipedia.org/wiki/Cluster_decayhttp://en.wikipedia.org/wiki/Beta_decayhttp://en.wikipedia.org/wiki/Beta_decayhttp://en.wikipedia.org/wiki/Beta_decayhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Electron_antineutrinohttp://en.wikipedia.org/wiki/Electron_antineutrinohttp://en.wikipedia.org/wiki/Positron_emissionhttp://en.wikipedia.org/wiki/Positron_emissionhttp://en.wikipedia.org/wiki/Beta_decayhttp://en.wikipedia.org/wiki/Beta_decayhttp://en.wikipedia.org/wiki/Beta_decayhttp://en.wikipedia.org/wiki/Positronhttp://en.wikipedia.org/wiki/Electron_neutrinohttp://en.wikipedia.org/wiki/Electron_capturehttp://en.wikipedia.org/wiki/Beta_decay#Bound-state_.CE.B2-_decayhttp://en.wikipedia.org/wiki/Beta_decay#Bound-state_.CE.B2-_decayhttp://en.wikipedia.org/wiki/Double_beta_decayhttp://en.wikipedia.org/wiki/Double_beta_decayhttp://en.wikipedia.org/wiki/Mass_numberhttp://en.wikipedia.org/wiki/Atomic_numberhttp://en.wikipedia.org/wiki/Alpha_decayhttp://en.wikipedia.org/wiki/Alpha_particlehttp://en.wikipedia.org/wiki/Proton_emissionhttp://en.wikipedia.org/wiki/Protonhttp://en.wikipedia.org/wiki/Neutron_emissionhttp://en.wikipedia.org/wiki/Neutron_emissionhttp://en.wikipedia.org/wiki/Neutronhttp://en.wikipedia.org/wiki/Proton_emissionhttp://en.wikipedia.org/wiki/Proton_emissionhttp://en.wikipedia.org/wiki/Spontaneous_fissionhttp://en.wikipedia.org/wiki/Spontaneous_fissionhttp://en.wikipedia.org/wiki/Cluster_decayhttp://en.wikipedia.org/wiki/Beta_decayhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Electron_antineutrinohttp://en.wikipedia.org/wiki/Electron_antineutrinohttp://en.wikipedia.org/wiki/Positron_emissionhttp://en.wikipedia.org/wiki/Positron_emissionhttp://en.wikipedia.org/wiki/Beta_decayhttp://en.wikipedia.org/wiki/Beta_decayhttp://en.wikipedia.org/wiki/Positronhttp://en.wikipedia.org/wiki/Electron_neutrinohttp://en.wikipedia.org/wiki/Electron_capturehttp://en.wikipedia.org/wiki/Beta_decay#Bound-state_.CE.B2-_decayhttp://en.wikipedia.org/wiki/Beta_decay#Bound-state_.CE.B2-_decayhttp://en.wikipedia.org/wiki/Double_beta_decayhttp://en.wikipedia.org/wiki/Double_beta_decay


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Double electron

capture

A nucleus absorbs two orbital electrons and emits

two neutrinos the daughter nucleus is left in an

excited and unstable state

(A,Z 2)

Electron capturewith positron

emission

A nucleus absorbs one orbital electron, emits one

positron and two neutrinos

(A,Z 2)

Double positron

emissionA nucleus emits two positrons and two neutrinos (A,Z 2)

Transitions between states of the same nucleus:

Isomeric

transition

Excited nucleus releases a high-energy photon

(gamma ray)(A,Z)

Internal

conversion

Excited nucleus transfers energy to an orbital electron

and it is ejected from the atom(A,Z)

Radioactive decay results in a reduction of summed rest mass, once the released energy

(the disintegration energy) has escaped in some way (for example, the products mightbe captured and cooled, and the heat allowed to escape). Although decay energy is

sometimes defined as associated with the difference between the mass of the parentnuclide products and the mass of the decay products, this is true only of rest mass

measurements, where some energy has been removed from the product system. This is

true because the decay energy must always carry mass with it, wherever it appears (see

mass in special relativity) according to the formula E= mc2. The decay energy isinitially released as the energy of emitted photons plus the kinetic energy of massive

emitted particles (that is, particles that have rest mass). If these particles come tothermal equilibrium with their surroundings and photons are absorbed, then the decay

energy is transformed to thermal energy, which retains its mass.

Decay energy therefore remains associated with a certain measure of mass of the decay

system invariant mass. The energy of photons, kinetic energy of emitted particles, and,later, the thermal energy of the surrounding matter, all contribute to calculations of

invariant mass of systems. Thus, while the sum of rest masses of particles is not

conserved in radioactive decay, thesystem mass and system invariant mass (and also thesystem total energy) is conserved throughout any decay process

Nuclear cross section

The nuclear cross section of a nucleus is used to characterize the probability that a

nuclear reaction will occur. The concept of a nuclear cross section can be quantified

physically in terms of "characteristic area" where a larger area means a larger

probability of interaction. The standard unit for measuring a nuclear cross section

(denoted as ) is thebarn, which is equal to 1028 m or 1024 cm. Cross sections can be
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measured for all possible interaction processes together, in which case they are called

total cross sections, or for specific processes, distinguishing elastic scattering and

inelastic scattering; of the latter, amongst neutron cross sections the absorption cross

sections are of particular interest.

In nuclear physics it is conventional to consider the impinging particles as pointparticles having negligible diameter. Cross sections can be computed for any sort of

process, such as capture scattering, production of neutrons, etc. In many cases, thenumber of particles emitted or scattered in nuclear processes is not measured directly;

one merely measures the attenuation produced in a parallel beam of incident particles by

the interposition of a known thickness of a particular material. The cross section

obtained in this way is called the total cross section and is usually denoted by a or T.

The typical nuclear radius is of the order of 1012 cm. We might therefore expect the

cross sections for nuclear reactions to be of the order of r or roughly 1024 cm andthis unit is given its own name, the barn, and is the unit in which cross sections areusually expressed. Actually the observed cross sections vary enormously. Thus forslow

neutrons absorbed by the (n, ) reaction the cross section in some cases is as much as1,000 barns, while the cross sections fortransmutations by gamma-ray absorption are in

the neighborhood of 0.001 barn.

Macroscopic cross section

Nuclear cross sections are used in determining the nuclear reaction rate, and are

governed by the reaction rate equation for a particular set of particles (usually viewed as

a "beam and target" thought experiment where one particle or nucleus is the "target"[typically at rest] and the other is treated as a "beam" [projectile with a given energy]).

For neutron interactions incident upon a thin sheet of material (ideally made of a single

type ofisotope), the nuclear reaction rate equation is written as:

where:

rx : number of reactions of type x, units: [1/time/volume] : neutron beam flux, units: [1/area/time]

x : microscopic cross section for reactionx, units: [area] (usuallybarns or cm2). A : density of atoms in the target in units of [1/volume]

: macroscopic cross-section [1/length]

Types of reactions frequently encountered are s: scattering, : radiative capture, a:absorption (radiative capture belongs to this type),f: fission, the corresponding notation
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for cross-sections being: s, , a, etc. A special case is the total cross-section t, which

gives the probability of a neutron to undergo any sort of reaction (t = s + + f + ...).

Formally, the equation above defines the macroscopic neutron cross-section (forreaction x) as the proportionality constant between a neutron flux incident on a (thin)

piece of material and the number of reactions that occur (per unit volume) in thatmaterial. The distinction between macroscopic and microscopic cross-section is that the

former is a property of a specific lump of material (with its density), while the latter isan intrinsic property of a type of nuclei.

UNIT-II- NUCLEAR REACTIONS AND REACTION MATERIALS

Nuclear fission

An induced fission reaction. A slow-moving neutron is absorbed by the nucleus

of a uranium-235 atom, which in turn splits into fast-moving lighter elements (fission

products) and releases three free neutrons.

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which

the nucleus of an atom splits into smaller parts (lighter nuclei), often producing freeneutrons andphotons (in the form ofgamma rays). The two nuclei produced are most

often of comparable size, typically with a mass ratio around 3:2 for common fissile

isotopes.[1][2] Most fissions are binary fissions, but occasionally (2 to 4 times per 1000

events), three positively-charged fragments are produced in a ternary fission. The

smallest of these ranges in size from a proton to an argon nucleus.

Fission is usually an energetic nuclear reaction induced by a neutron, although it isoccasionally seen as a form of spontaneous radioactive decay, especially in very high-

mass-number isotopes. The unpredictable composition of the products (which vary in a

broad probabilistic and somewhat chaotic manner) distinguishes fission from purely

quantum-tunnelling processes such as proton emission, alpha decay and cluster decay,

which give the same products every time.

Fission of heavy elements is an exothermic reaction which can release large amounts of

energy both as electromagnetic radiation and as kinetic energy of the fragments (heating

the bulk material where fission takes place). In order for fission to produce energy, the

total binding energy of the resulting elements must be less than that of the starting

element. Fission is a form of nuclear transmutation because the resulting fragments arenot the same element as the original atom.

Nuclear fission produces energy fornuclear powerand to drive the explosion ofnuclear

weapons. Both uses are possible because certain substances called nuclear fuels undergo
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fission when struck by fission neutrons, and in turn emit neutrons when they break

apart. This makes possible a self-sustaining chain reaction that releases energy at a

controlled rate in a nuclear reactor or at a very rapid uncontrolled rate in a nuclear

weapon.

The amount offree energy contained in nuclear fuel is millions of times the amount offree energy contained in a similar mass of chemical fuel such as gasoline, making

nuclear fission a very tempting source of energy. The products of nuclear fission,however, are on average far more radioactive than the heavy elements which are

normally fissioned as fuel, and remain so for significant amounts of time, giving rise to

a nuclear waste problem. Concerns over nuclear waste accumulation and over the

destructive potential ofnuclear weapons may counterbalance the desirable qualities of

fission as an energy source, and give rise to ongoing political debate over nuclear

power.

Fission reactors

Critical fission reactors are the most common type ofnuclear reactor. In a critical fission

reactor, neutrons produced by fission of fuel atoms are used to induce yet more fissions,

to sustain a controllable amount of energy release. Devices that produce engineered butnon-self-sustaining fission reactions are subcritical fission reactors. Such devices use

radioactive decay orparticle accelerators to trigger fissions.

Critical fission reactors are built for three primary purposes, which typically involve

different engineering trade-offs to take advantage of either the heat or the neutronsproduced by the fission chain reaction:

power reactors are intended to produce heat for nuclear power, either as part of agenerating station or a local power system such as a nuclear submarine.

research reactors are intended to produce neutrons and/or activate radioactivesources for scientific, medical, engineering, or other research purposes.

breeder reactors are intended to produce nuclear fuels in bulk from moreabundant isotopes. The better known fast breeder reactormakes 239Pu (a nuclear

fuel) from the naturally very abundant 238U (not a nuclear fuel). Thermal breeder

reactors previously tested using 232Th to breed the fissile isotope 233U continue tobe studied and developed.

While, in principle, all fission reactors can act in all three capacities, in practice the

tasks lead to conflicting engineering goals and most reactors have been built with only

one of the above tasks in mind. (There are several early counter-examples, such as the

HanfordN reactor, now decommissioned). Power reactors generally convert the kinetic

energy of fission products into heat, which is used to heat a working fluid and drive a
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heat engine that generates mechanical or electrical power. The working fluid is usually

water with a steam turbine, but some designs use other materials such as gaseous

helium. Research reactors produce neutrons that are used in various ways, with the heat

of fission being treated as an unavoidable waste product. Breeder reactors are a

specialized form of research reactor, with the caveat that the sample being irradiated is

usually the fuel itself, a mixture of 238U and 235U. For a more detailed description of thephysics and operating principles of critical fission reactors, see nuclear reactor physics.

For a description of their social, political, and environmental aspects, see nuclear reactor

Chain reactions

A uranium-235 atom absorbs a neutron and fissions into two new atoms (fission

fragments), releasing three new neutrons and some binding energy. 2. One of those

neutrons is absorbed by an atom of uranium-238 and does not continue the reaction.

Another neutron is simply lost and does not collide with anything, also not continuing

the reaction. However one neutron does collide with an atom of uranium-235, whichthen fissions and releases two neutrons and some binding energy. 3. Both of those

neutrons collide with uranium-235 atoms, each of which fissions and releases between

one and three neutrons, which can then continue the reaction.

Main article:Nuclear chain reaction

Several heavy elements, such as uranium, thorium, and plutonium, undergo bothspontaneous fission, a form ofradioactive decay and induced fission, a form ofnuclearreaction. Elemental isotopes that undergo induced fission when struck by a free neutron

are called fissionable; isotopes that undergo fission when struck by a thermal, slow

moving neutron are also called fissile. A few particularly fissile and readily obtainableisotopes (notably 235U and 239Pu) are called nuclear fuels because they can sustain a

chain reaction and can be obtained in large enough quantities to be useful.

All fissionable and fissile isotopes undergo a small amount of spontaneous fission which

releases a few free neutrons into any sample of nuclear fuel. Such neutrons would

escape rapidly from the fuel and become a free neutron, with a mean lifetime of about

15 minutes before decaying to protons and beta particles. However, neutrons almostinvariably impact and are absorbed by other nuclei in the vicinity long before this

happens (newly-created fission neutrons move at about 7% of the speed of light, and

even moderated neutrons move at about 8 times the speed of sound). Some neutrons will

impact fuel nuclei and induce further fissions, releasing yet more neutrons. If enough

nuclear fuel is assembled in one place, or if the escaping neutrons are sufficiently

contained, then these freshly emitted neutrons outnumber the neutrons that escape fromthe assembly, and asustained nuclear chain reaction will take place.
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An assembly that supports a sustained nuclear chain reaction is called a critical

assembly or, if the assembly is almost entirely made of a nuclear fuel, a critical mass.

The word "critical" refers to a cusp in the behavior of the differential equation that

governs the number of free neutrons present in the fuel: if less than a critical mass is

present, then the amount of neutrons is determined by radioactive decay, but if a critical

mass or more is present, then the amount of neutrons is controlled instead by the physicsof the chain reaction. The actual mass of a critical mass of nuclear fuel depends stronglyon the geometry and surrounding materials.

Not all fissionable isotopes can sustain a chain reaction. For example, 238U, the most

abundant form of uranium, is fissionable but not fissile: it undergoes induced fission

when impacted by an energetic neutron with over 1 MeV of kinetic energy. But too few

of the neutrons produced by 238U fission are energetic enough to induce further fissions

in 238U, so no chain reaction is possible with this isotope. Instead, bombarding 238U withslow neutrons causes it to absorb them (becoming 239U) and decay bybeta emission to239

Np which then decays again by the same process to239

Pu; that process is used tomanufacture 239Pu in breeder reactors. In-situ plutonium production also contributes to

the neutron chain reaction in other types of reactors after sufficient plutonium-239 has

been produced, since plutonium-239 is also a fissile element which serves as fuel. It is

estimated that up to half of the power produced by a standard "non-breeder" reactor isproduced by the fission of plutonium-239 produced in place, over the total life-cycle of

a fuel load.

Fissionable, non-fissile isotopes can be used as fission energy source even without a

chain reaction. Bombarding 238U with fast neutrons induces fissions, releasing energy as

long as the external neutron source is present. This is an important effect in all reactorswhere fast neutrons from the fissile isotope can cause the fission of nearby 238U nuclei,

which means that some small part of the 238U is "burned-up" in all nuclear fuels,

especially in fast breeder reactors that operate with higher-energy neutrons. That same

fast-fission effect is used to augment the energy released by modern thermonuclear

weapons, by jacketing the weapon with 238U to react with neutrons released by nuclearfusion at the center of the device

Fission bombs

The mushroom cloud of the atom bomb dropped on Nagasaki, Japan in 1945 rose

some 18 kilometers (11 miles) above the bomb's hypocenter.

One class ofnuclear weapon, afission bomb (not to be confused with the fusion bomb),otherwise known as an atomic bomb or atom bomb, is a fission reactor designed toliberate as much energy as possible as rapidly as possible, before the released energy
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causes the reactor to explode (and the chain reaction to stop). Development of nuclear

weapons was the motivation behind early research into nuclear fission: the Manhattan

Project of the U.S. military during World War II carried out most of the early scientific

work on fission chain reactions, culminating in the Trinity test bomb and the Little Boy

and Fat Man bombs that were exploded over the cities Hiroshima, andNagasaki, Japan

in August 1945.

Even the first fission bombs were thousands of times more explosive than a comparablemass ofchemical explosive. For example, Little Boy weighed a total of about four tons

(of which 60 kg was nuclear fuel) and was 11 feet (3.4 m) long; it also yielded an

explosion equivalent to about 15 kilotons ofTNT, destroying a large part of the city of

Hiroshima. Modern nuclear weapons (which include a thermonuclearfusion as well asone or more fission stages) are literally hundreds of times more energetic for their

weight than the first pure fission atomic bombs, so that a modern single missile warheadbomb weighing less than 1/8 as much as Little Boy (see for example W88) has a yield

of 475,000 tons of TNT, and could bring destruction to 10 times the city area.

While the fundamental physics of the fission chain reaction in a nuclear weapon is

similar to the physics of a controlled nuclear reactor, the two types of device must beengineered quite differently (see nuclear reactor physics). A nuclear bomb is designed to

release all its energy at once, while a reactor is designed to generate a steady supply of

useful power. While overheating of a reactor can lead to, and has led to, meltdown and

steam explosions, the much loweruranium enrichment makes it impossible for a nuclear

reactor to explode with the same destructive power as a nuclear weapon. It is also

difficult to extract useful power from a nuclear bomb, although at least one rocket

propulsion system, Project Orion, is intended to work by exploding fission bombsbehind a massively-padded and shielded vehicle.

The strategic importance of nuclear weapons is a major reason why the technology of

nuclear fission is politically sensitive. Viable fission bomb designs are, arguably, within

the capabilities of many being relatively simple from an engineering viewpoint.

However, the difficulty of obtaining fissile nuclear material to realize the designs, is the

key to the relative unavailability of nuclear weapons to all but modern industrialized

governments with special programs to produce fissile materials (see uranium enrichment

and nuclear fuel cycle).

Uranium production and purification

The discovery of fission led to two potential routes to the production of fissile

material for the first nuclear weapons by the United States in the 1940s. The first

involved separating uranium-235 from uranium-238 isotopes in natural uranium by

gaseous diffusion. The second path produced plutonium-239 by bombarding fertile
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uranium-238 in a nuclear reactor. But both approaches began with mining of uranium

ore. Today, the production of fissile fuel for nuclear power reactors uses many methods

originally developed for producing nuclear weapons. This unit addresses the metallurgy

of uranium, its conversion into gaseous uranium

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