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Atomic and Nuclear Physics



Atoms - Atoms are the smallest unit of an element that chemically behaves the same way the element does. When two chemicals react with each other, the reaction takes place between individual atoms at the atomic level.

Atomic Structure

  • In the early 20th century, a New Zealand scientist working in England, Ernest Rutherford, and a Danish scientist, Niels Bohr, developed a way of thinking about the structure of an atom that described an atom as looking very much like our solar system.
  • An atom is composed of three basic particles – electrons, protons and neutrons.
  • Nucleus of an atom consists of protons and neutrons.
  • Electrons revolve in atomic orbit.

Properties of atomic and sub atomic particles


Particle
Mass
Charge
Spin
Lifetime
Photon
0
 0
1
stable
Electron
0.511 MeV
-1
1/2
stable
Pi 0
135 MeV
 0
0
8.40 x 10 -17 sec
Pi +
140 MeV
+1
0
2.60 x 10 -8 sec
Pi -
140 MeV
-1
0
2.60 x 10 -8 sec
Proton
938 MeV
+1
1/2
stable
Neutron
940 MeV
 0
1/2
887 sec


Why some atoms are radioactive?

Atoms found in nature are either stable or unstable. An atom is stable if the forces among the particles that make up the nucleus are balanced. An atom is unstable (radioactive) if these forces are unbalanced if the nucleus has an excess of internal energy. Unstable atoms are called radionuclides. The instability of a radionuclide's nucleus may result from an excess of either neutrons or protons. An unstable nucleus will continually vibrate and contort and, sooner or later, attempt to reach stability by some combination of means:

  • ejecting neutrons, and protons
  • converting one to the other with the ejection of a beta particle or positron
  • the release of additional energy by photon (i.e., gamma ray) emission.

Cathode rays
  • Cathode rays (also called an electron beam or e-beam) are streams of electrons observed in vacuum tubes.
  • They were first observed in 1869 by German physicist Johann Hittorf, and were named in 1876 by Eugen Goldstein kathodenstrahlen, or cathode rays.

Properties of Cathode Rays

1. Cathode rays travel in straight lines. That is why, cathode rays cast shadow of any solid object placed in their path. The path cathode rays travel is not affected by the position of the anode.

2. Cathode rays consist of matter particles, and posses energy by the virtue of its mass and velocity. Cathode rays set a paddle wheel into motion when it is placed in the path of these rays one the bladder of the paddle wheel.

3. Cathode rays consist of negatively charged particles. When cathode rays are subjected to an electrical field, these get deflected towards the positively charge plate (Anode).

We know that a positively charged body would attract only a negatively charged body, therefore the particles of cathode rays carry negative charge.

Cathode rays also get deflected when these are subjected to a strong magnetic field.

4. Cathode rays heat the object only which they fall. The cathode ray particles possess kinetic energy. When these particles strike an object, a part of the kinetic energy is transferred to the object. The causes a rise in the temperature of the object.

5. Cathode rays cause green fluorescence on glass surface, i.e., the glass surface only which the cathode rays strike show a colored shine.

6. Cathode rays can penetrate through thin metallic sheets.

7. Cathode rays ionize the gases through which they travel.

8. Cathode rays when fall only certain metals such as copper, but rays produced. The X-rays are not deflected by electrical or magnetic fields. X-rays pass through opaque materials such as black paper, but stopped by solid objects such as bones.

9. Cathode rays travel with speed nearly equal to that of light.

Anode rays –
  • Canal rays are streams of positive ions in a rarefied gas. Due to their having a larger mass than electrons, canal rays are more penetrating and less easily bent than cathode rays (electron beams).since they are positive ions they move towards the cathode
  • They were first observed in Crookes tubes during experiments by the German scientist Eugen Goldstein, in 1886.
  • Later work on anode rays by Wilhelm Wien and J. J. Thomson led to the development of mass spectrometry.

Properties of Canal Rays

1. These rays travel in straight line.

2. These rays can penetrate through small thickness of matter.

3. These rays get deflected by electric and magnetic field in direction opposite to cathode rays.This shows that they are positively charged.

4. These rays produce fluorescence.

Radioactivity

  • Radioactivity - Unstable atomic nuclei will spontaneously decompose to form nuclei with a higher stability. The decomposition process is called radioactivity.
  • Radiation - The energy and particles which are released during the decomposition process are called radiation.
  • Natural Radioactivity - When unstable nuclei decompose in nature, the process is referred to as natural radioactivity.
  • Induced Radioactivity- When the unstable nuclei are prepared in the laboratory, the decomposition is called induced radioactivity.

There are three major types of natural radioactivity:

Alpha Radiation- Alpha radiation consists of a stream of positively charged particles, called alpha particles, which have an atomic mass of 4 and a charge of +2 (a helium nucleus). When an alpha particle is ejected from a nucleus, the mass number of the nucleus decreases by four units and the atomic number decreases by two units. For example:

23892U → 42He + 23490Th

The helium nucleus is the alpha particle.

Beta Radiation- Beta radiation is a stream of electrons, called beta particles. When a beta particle is ejected, a neutron in the nucleus is converted to a proton, so the mass number of the nucleus is unchanged, but the atomic number increases by one unit. For example:

234900-1e + 23491Pa

The electron is the beta particle.

Gamma Radiation- Gamma rays are high-energy photons with a very short wavelength (0.0005 to 0.1 nm). The emission of gamma radiation results from an energy change within the atomic nucleus. Gamma emission changes neither the atomic number nor the atomic mass. Alpha and beta emission are often accompanied by gamma emission, as an excited nucleus drops to a lower and more stable energy state.

Properties of Alpha, Beta and Gamma Particles

Type of radiation emitted & symbol
Nature of the radiation
(higher only)
Nuclear Symbol
(higher only)
Penetrating power, and what will block it (more dense material, more radiation is absorbed BUT smaller mass or charge of particle, more penetrating)
Ionising power - the ability to remove electrons from atoms to form positive ions
(c) doc b
Alpha
a helium nucleus of 2 protons and 2 neutrons, mass = 4, charge = +2
(c) doc b
Low penetration, biggest mass and charge, stopped by a few cm of air or thin sheet of paper
Very high ionising power, the biggest mass and charge of the three radiation's, the biggest 'punch'!
(c) doc b
Beta
high kinetic energy electrons, mass = 1/1850, charge = -1
(c) doc b
Moderate penetration, 'middle' values of charge and mass, most stopped by a few mm of metals like aluminium
Moderate ionising power, with a smaller mass and charge than the alpha particle
(c) doc b
Gamma
very high frequency electromagnetic radiation, mass = 0, charge = 0
(c) doc b
Very highly penetrating, smallest mass and charge, most stopped by a thick layer of steel or concrete, but even a few cm of dense lead doesn't stop all of it!
The lowest ionising power of the three, gamma radiation carries no electric charge and has virtually no mass, so not much of a 'punch' when colliding with an atom

Natural Radioactivity in soil

How much natural radioactivity is found in a volume of soil that is 1 square mile, by 1 foot deep? The following table is calculated for this volume (total volume is 7.894 x 105 m3) and the listed activities. It should be noted that activity levels vary greatly depending on soil type, mineral make-up and density (~1.58 g/cm3 used in this calculation). This table represents calculations using typical numbers.
Natural Radioactivity by the Square Mile, 1 Foot Deep
Nuclide
Activity used
in calculation
Mass of Nuclide
Activity found in the volume of soil
Uranium
0.7 pCi/g (25 Bq/kg)
2,200 kg
0.8 curies (31 GBq)
Thorium
1.1 pCi/g (40 Bq/kg)
12,000 kg
1.4 curies (52 GBq)
Potassium 40
11 pCi/g (400 Bq/kg)
2000 kg
13 curies (500 GBq)
Radium
1.3 pCi/g (48 Bq/kg)
1.7 g
1.7 curies (63 GBq)
Radon
0.17 pCi/g (10 kBq/m3) soil
11 µg
0.2 curies (7.4 GBq)

Total:
>17 curies (>653 GBq)


Radioactivity in Food

Every food has some small amount of radioactivity in it. The common radionuclides in food are potassium 40 (40K), radium 226 (226Ra) and uranium 238 (238U) and the associated progeny. Here is a table of some of the common foods and their levels of 40K and 226Ra.
Natural Radioactivity in Food
Food
40K
pCi/kg
226Ra
pCi/kg
Banana
3,520
1
Brazil Nuts
5,600
1,000-7,000
Carrot
3,400
0.6-2
White Potatoes
3,400
1-2.5
Beer
390
---
Red Meat
3,000
0.5
Lima Bean
raw
4,640
2-5
Drinking water
---
0-0.17

Radioactivity in Human body

You are made up of chemicals, and it should be of no surprise that some of them are radionuclides, many of which you ingest daily in your water and food. Here are the estimated concentrations of radionuclides calculated for a 70,000 gram adult based ICRP 30 data:
Natural Radioactivity in your body
Nuclide
Total Mass of Nuclide
Found in the Body
Total Activity of Nuclide
Found in the Body
Daily Intake of Nuclides
Uranium
90 µg
30 pCi (1.1 Bq)
1.9 µg
Thorium
30 µg
3 pCi (0.11 Bq)
3 µg
Potassium 40
17 mg
120 nCi (4.4 kBq)
0.39 mg
Radium
31 pg
30 pCi (1.1 Bq)
2.3 pg
Carbon 14
22 ng
0.1 µCi (3.7 kBq)
1.8 ng
Tritium
0.06 pg
0.6 nCi (23 Bq)
0.003 pg
Polonium
0.2 pg
1 nCi (37 Bq)
~0.6 fg

Nuclear Fission- The Nuclear Fission is a reaction when the nucleus of an atom, having captured a neutron, splits into two or more nuclei, and in so doing, releases a significant amount of energy as well as more neutrons. These neutrons then go on to split more nuclei and a chain reaction takes place.

Nuclear Fusion- Nuclear Fusion is a process where nuclei collide and join together to form a heavier atom, usually deuterium and tritium. When this happens a considerable amount of energy gets released at extremely high temperatures: nearly 150 million degrees Celsius. At extreme temperatures, electrons are separated from nuclei and a gas becomes a plasma—a hot, electrically charged gas.

Fissile Isotopes: Fissile isotopes are isotopes of an element that can be split through fission. Only certain isotopes of certain elements are fissile.

For example, one isotope of uranium, 235U, is fissile, while another isotope, 238U, is not. Other examples of fissile elements are 239Pu and 232Th.

An important factor affecting whether or not an atom will fission is the speed at which the bombarding neutron is moving.

If the neutron is highly energetic (and thus moving very quickly), it can cause fission in some elements that a slower neutron would not. For example, thorium 232 requires a very fast neutron to induce fission. However, uranium 235 needs slower neutrons.

If a neutron is too fast, it will pass right through a 235U atom without affecting it at all.

Splitting the Uranium Atom: Uranium is the principle element used in nuclear reactors and in certain types of atomic bombs. The specific isotope used is 235U. When a stray neutron strikes a 235U nucleus, it is at first absorbed into it. This creates 236U. 236U is unstable and this causes the atom to fission. The fissioning of 236U can produce over twenty different products. However, the products' masses always add up to 236. The following two equations are examples of the different products that can be produced when 235U fissions:
  • 235U + 1 neutron ->2 neutrons + 92Kr + 142Ba + ENERGY
  • 235U + 1 neutron ->2 neutrons + 92Sr + 140Xe + ENERGY

Chain Reaction-  A chain reaction refers to a process in which neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus in turn produces neutrons, and the process repeats.
The nuclear chain reaction is classified into two types – 1. Controlled Chain Reaction and 2. Uncontrolled Chain Reaction

Controlled Chain Reaction - In controlled chain reaction, the chain reaction is first accelerated so that the neutrons are built up to a certain level and thereafter; the number of fission producing neutrons is kept constant. This is called controlled chain reaction. Such a controlled chain reaction is used in nuclear reactors to obtain a steady energy.
Uncontrolled Chain Reaction - In uncontrolled chain reaction, the number of neutrons is allowed to multiply indefinitely and the entire energy is released all at once. This is uncontrolled chain reaction. This type of reaction takes place in atom bombs.
Nuclear Reactor- A nuclear reactor is a system that contains and controls sustained nuclear chain reactions. Reactors are used for generating electricity, moving aircraft carriers and submarines, producing medical isotopes for imaging and cancer treatment, and for conducting research.
Components of Nuclear Reactor
Core - The core of the reactor contains all of the nuclear fuel and generates all of the heat. It contains low-enriched uranium (<5% U-235), control systems, and structural materials. The core can contain hundreds of thousands of individual fuel pins.
Coolant - The coolant is the material that passes through the core, transferring the heat from the fuel to a turbine. It could be water, heavy-water, liquid sodium, helium, or something else. In the US fleet of power reactors, water is the standard.
Turbine - The turbine transfers the heat from the coolant to electricity, just like in a fossil-fuel plant.
Containment - The containment is the structure that separates the reactor from the environment. These are usually dome-shaped, made of high-density, steel-reinforced concrete. Chernobyl did not have a containment to speak of.
Cooling Towers - Cooling towers are needed by some plants to dump the excess heat that cannot be converted to energy due to the laws of thermodynamics. These are the hyperbolic icons of nuclear energy. They emit only clean water vapor.
Types of Reactors
   PWR: Pressurized Water Reactor, as of 2003, 212 were active worldwide.  Water is both the coolant and moderator and is kept at a high pressure 70 to 150 atm.
    BWR: Boiling Water Reactor, like PWR, water is both the coolant and moderator.  Although the water is kept at a lower pressure 70 atm and thus produces steam.  The steam directly powers the turbine, thus simplifying the design.  The only downside is that over time the turbine accumulates radioactivity (Nitrogen 17 with a half-life of seven seconds).
    Breeder Reactor: Uses both fissle and fertile elements to produce reaction.  Because of this, breeder reactors can use materials that are more widely available.  Operates by using fast neutrons to convert fertile elements into fissile elements.
Uses of Nuclear Reactor - The primary use of nuclear reactors is to produce electricity, which is the only way to use nuclear energy on a large scale. Small reactors are used for research, and are also used to produce radioisotopes for medical and industrial use. Reactors are also used to power some naval vessels and submarines.

Hydrogen Bomb - The hydrogen bomb, one of the mankind's most destructive weapon -also known as the H-Bomb or thermonuclear bomb works on the principle of nuclear fusion, where isotopes of hydrogen i.e. Deuterium and tritium combine or fuse under extremely high temperatures to form helium. Edward Teller and other American scientists developed the first hydrogen bomb, which was tested at Enewetak atoll on Nov. 1, 1952.
Mass Energy Relation
Einstein demonstrated that neither mass nor energy was conserved separately, but that they could be treated as one for the other and only the total "mass-energy" was conserved. The relationship between the mass and the energy is expressed as
E = m c 2
where m is the mass, c is the speed of light, and E is the energy equivalent of the mass.

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