Physics Notes: Nuclear

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1. In 1896, Henri Becquerel discovered that a photographic plate was darkened in the presence of Uranium.
Marie and Pierre Curie isolated two radioactive substances, Polonium and Radium.
In 1897, Thomson discovered that the atom was a ball of positive charge filled with moving negatively charged electrons. This was termed the Plum pudding model.
In 1899, Rutherford discovered that a Uranium compounds produced three different kinds of radiation.
In 1911, Rutherford forwarded the idea of the atom being a dense positive nucleus in the middle of the atom, with electrons moving around it. Most of the mass resided in the nucleus. Also proposed that the nucleus consisted of positively charged protons as well as negatively charged electrons.
In 1932, Chadwick proposed the presence of neutrons and that a nucleus is made up of neutral neutrons and positively charged protons. Neutrons are particles found in the nucleus that are electrically neutral and that have a mass almost identical to the proton.
Bohr proposed that the atom had negatively charged electrons that were orbiting the nucleus.
At a later time, the Quantum Mechanical Model was introduced. In this model, the structure of the whole atom is described entirely by mathematical equations.
In 1934, Enrico Fermi and Emilio Segré bombarded Uranium with neutrons, producing new radioactive isotopes.
In 1939, German scientists Otto Hahn and Fritz Strassmann found that Barium was produced by bombarding Uranium with neutrons. Lisa Meitner and Otto Frisch proposed that the neutrons caused the uranium to divide into two smaller nuclei, accompanied by a tremendous release of energy.

Physics Study Guide pg 72

Physics Text Book pg 149

2. Atomic Number: The number of protons in a nucleus (designated by the letter Z).
Atomic Mass Number: The total number of protons and neutrons (designated by the letter A).

3. Isotope: An isotope is an atom that contain the same number of protons but a different numbers of neutrons in the nucleus. The mass number changes but the atomic number stays constant.

4. An atomic mass unit is approximately 1.6606*10^-27 kg or approximately 1/12 the mass of a carbon-12 atom.

Stawa set 16

Physics Study Guide pg 73

5. Protons and Neutrons are held together by an attractive force termed the strong nuclear force. This force is greater then the repulsion force that is present between the positively charged protons. Since the strong nuclear force only acts over a small distance, the very large distant nucleons are unable to attract each other strongly enough thus fall apart as the protons repel.
The nuclei with Z=40 tend to be stable nuclei as they tend to have equal numbers of protons and neutrons. For nuclei with Z greater than 40, these are stable as they have more neutrons than protons. There are no stable nuclei with Z greater than 82. They are all radioactive. Due to their size, no number of neutrons can overcome the electric repulsion between protons.

Physics Study Guide pg 75-76

Physics Text Book pg 153

Physics Text Book pg 158-159

6. Alpha Particle: It is essentially a helium nucleus. It contains two protons and two neutrons. It has a charge of +2 and a mass of A=4. Alpha particles are positively charged thus move towards the cathode when a potential difference is present. When a nuclei decays by emitting an alpha particle, the number of protons is reduced by two and its mass is reduced by four. Alpha particles are emitted by very large nuclei where the strong nuclear force is insufficient to hold the nuclei together. It is abbreviated to (a) or 4/2 He. Alpha radiation is the least penetrating of the radiation types. It can be stopped (or absorbed) by a few centimeters of air or just a sheet of paper. It is a fairly large particle, which is one reason why alpha particles do not penetrate things very well but are very good ionisers. These particles have an emission velocity up to 10% the speed of light.

Beta Particle: Beta particles are a negatively charge electron emitted by the nucleus. It is not an orbital electron, but one created in the nucleus by the decay of a neutron into a proton and an electron. The electron then shoots out at a high speed of about 30-90% the speed of light. It has a charge of -1 and essentially has no mass. Since Beta particles are negatively charged thus move towards the anode when a potential difference is present. Weak nuclear forces are involved in the production of a beta particle in the nucleus as Beta particles are emitted by nuclei that have too many neutrons relative to the number of protons. It is abbreviated (B) or 0/-1 e. Beta radiation can penetrate air and paper but can be stopped by a thin sheet of metal.

Gamma Radiation: Gamma radiation is composed of high-energy photons. It is emitted by excited state nuclei. Gamma radiation has no charge and no mass. It is abbreviated by (Y). Gamma radiation can penetrate air, paper or thin metal. It can only be stopped by many centimeters of lead or by many meters of concrete. Gamma radiation is uncharged so is unaffected by electric fields. When a radioactive atom gives off gamma radiation it will be emitted with a velocity of the speed of light. This is like other types of electromagnetic radiation such as light and radio waves, which can travel long distances.

Property	Alpha Particle	Beta Particle	Gamma Particle
Description	Helium nucleus	High speed electron	High energy photon
Symbol	4 He 2	0 e -1	0 y 0
Charge	+2	-1	Neutral
Mass	4u	0.00055u	Nil
Emission Velocity	10% speed of light	30-90% speed of light	The speed of light
Penetration	Few cm air	12 mm foil	10 cm of lead or concrete
Ionizing Potential	Strong	Moderate	Little

Physics Study Guide pg 74-75

Physics Text Book pg 155

Physics Text Book pg 160-162

7. The strength of a radioactive source is determined by the number of disintegrations of its radioactivity per second. The unit for this is the becquerel (Bq). One disintegration per second is equal to one Becquerel. This is a very low rate and it is common for radioactive sources to be quoted in kilobecquerels (kBq), megabecquerels (MBq) or gigabecquerels (GBq). The becquerel replaces the curie. There are 37 thousand million becquerels in a curie.
Radioactivity dispersed in another material may be expressed as, megabecquerels per kilogram for natural radioactivity of some rocks, or as becquerels per cubic metre when describing the radioactive content of air. These units do not describe your radiation dose but represent the strength of radioactivity at a given place.

The measure of absorbed radiation dose is the gray (Gy) and it replaces the old unit, the Rad. The absorbed dosage is the energy absorbed per kilogram of tissue and calculated using the formula Absorbed Dose = Energy Absorbed/Mass of absorbing tissue.

However, while the energy delivered by different particles may be the same, the effect on living cells can be quite different. Alpha particles and neutrons are approximately ten times as damaging as beta particles and gamma rays for the same amount of energy deposited. The dose equivalent, the Sievert (Sv), is the important unit to assess the effects of ionizing radiation on living cells, especially human beings. It does not measure the same thing as the gray. The Sievert replaces the old unit, the Rem, (the roentgen equivalent for man). Therefore One Sievert will have the same effect no matter what type of radiation.
Dose Equivalent = Absorbed Dose * Quality Factor

8. In all radioactive decay, the classical conservation laws hold. Energy, linear momentum, angular momentum, and electric charge are conserved. Also, the nucleon number is conserved. When balancing a nuclear reaction, mass and charge are conserved.

When a nucleus decays, it becomes the nucleus of another atom.
Original Nucleus = Parent Nucleus
Resultant Nucleus = Daughter Nucleus
Daughter Nucleus + emitted particle = decay produced

9. The activity is proportional to the number of nuclei not yet decayed.

Physics Text Book pg 166

10. Half-life (T1/2): The time it takes for half the original amount of the substance to decay.
It is also the time it takes for the number of atoms you start with to halve. If you were measuring the radioactivity with a Geiger Muller detector, the count rate (e.g. the number of counts per second) would halve in this time. The greater the time it takes for the number of decays to halve, the greater its stability as its activity is low.

Number of Half-lives	0	1	2	3	4	5	n...
Number of Nuclei left	N	N/2	N/4	N/8	N/16	N/32	N/2^n

Physics Study Guide pg 78

Physics Text Book pg 165-166

11. Radioactive Decay is a random process as you are unable to predict which actual nuclei will decay next but rather you know only how many nuclei will decay in a period of time.

Physics Study Guide pg 78

Physics Text Book pg 165

12.

Calculate the half-life of this isotope from the graph
Model Answer. It is always better to give as much detail in your answer as possible. Show your working For example: The start reading is 80 counts
Half this reading is 40 counts
It takes 6 days to reach 40 counts.

Stawa Set 17

Physics Study Guide pg 79

Physics Text Book pg 165

13.
L=O/2^N where L = Left O = Original N = Number of Half-lives
N=T/H where N = Number of Half-lives T = Time H = Length of Half-life

Stawa Sets 17+18

Physics Study Guide pg 79

Physics Text Book pg 169

14. Uses of radioactivity
1. Radioactive materials are used to measure the thickness of films in, for example, a canning factory.
2. In hospitals they use radioactive materials to see what is happening inside the body - they call these tracers.
3. Radioactive materials are used in smoke detectors and to check welds in pipes.
4. Because they kill cells radioactive materials are used to kill germs. We use them to irradiate food and to irradiate cancer cells.
5. Radioactive dating Radioactivity can also be used to work out how old something is. When an unstable nucleus splits up (disintegrates) it emits radiation and turns into a different atom. As something radioactive gets older, it emits less radiation. Scientists measure radioactivity to check the age of fossils and rocks -they call this radioactive dating.
6. Nuclear Bombs: the splitting of nucleons thus releasing energy.

Radioactive materials are used in different ways for different purposes.
For example, if it has a short half-life the radioactivity decays quickly. This is good for when you use one as a body tracer, but bad when you use one in a smoke alarm where you would have to keep changing it.
Similarly, the radioactive material needs to give off useful radiation.
For example: Gamma radiation is less useful for testing aluminum film because it goes through the film with little change.
For a body tracer gamma radiation is safer and can be detected easily.

Physics Study Guide pg 81-84

Physics Text Book pg 167

Physics Text Book pg 174-176

Physics Text Book pg 184-189

15. Mass Defect: The difference in the mass of a nucleus and the sum of the masses of its constituent particles.
Nuclear Binding Energy: The amount of energy that must be put into a nucleus to break it into its constituent particles. We can also look at a nucleus in terms of the forces that hold it together. The electric force described by Coulomb predicts that the nucleus should fly apart (since positive charges repel other positive charges). Another short-range attractive force must be acting within the nucleus.

Stawa Set 16

Physics Study Guide pg 88-89

Physics Text Book pg 167

16. In the equation E = mc^2
E stands for energy and m is mass. The letter c is the speed of light (from the Latin word for speed, celeritas), which is 300,000,000 meters per second. No one had thought of mass as a form of energy before. This equation meant that it was possible for mass to be converted into energy. People soon noticed that this is what was happening in nuclear reactions. If you take the mass of a Uranium atom, and compare it with the masses of the atoms and other particles that come out of a nuclear fission reaction (when the Uranium nucleus breaks apart), you find that a little bit of mass is missing. Einstein's law explains that the mass has become energy - the kinetic energy (energy of movement) of the new particles speeding away from the explosion. And because the speed of light is so high, a little mass becomes a lot of energy. This energy becomes heat that can be used in nuclear reactors to make electricity. One of the conclusions which Einstein reached while working on his theory of relativity is that mass and energy are different forms of the same entity. Since they are different forms, you should be able to convert an amount of energy into an amount of mass and vice versa. The equation which gives the correct conversion is E = mc^2 which says that if you have a mass m, it is equivalent to c^2 time m amount of energy. Since c is the speed of light, you can see that the amount of energy available from a small amount of mass is huge.
c^2 = (3 x 10^8)^2 = 9 x 10^16.
In theory, one kg of mass can be converted into 9 x 10^16 joules of energy!

Einstein's equation is e = mc^2? Seriously, though, if you look at the units in the equation, you will see that energy = mass * (distance/time)^2
= mass * (distance/time^2) * distance
= force * distance
which is one of the definitions of 'energy'.

Stawa Set 15

Physics Study Guide pg 89

Physics Text Book pg 180-181

Physics Text Book pg 183

eg. If the mass of a Uranium atom is 50 gm, and the masses of the atoms and other particles that come out of a nuclear fission reaction (when the Uranium nucleus breaks apart) is 45 gm, you find that a little bit of mass is missing. Einstein's law explains that the mass has become energy.

Physics Study Guide pg 90

Physics Study Guide pg 93

Physics Text Book pg 178-183

18. Chain Reaction: Neutrons produced by the fission of one nucleus induces the fission of other nuclei. The critical mass refers to the mass of fissile material required to sustain un uncontrolled chain reaction such as the one that occurs in a bomb. When an atom bomb is detonated, two separated masses of fissile material, each less then critical mass, are forced together by a chemical explosive. The two combined masses are then greater than the critical mass and an explosion results.

Physics Study Guide pg 91

Physics Text Book pg 185-186

19. The reaction produces heat which is transferred to the water which is acting as a moderator and coolant. This hot liquid is transported to the condenser where it turns water into steam. The steam is ran through a turbine that is connected to a generator. This is connected to wired which is connected to houses.

20. Some radiation comes from rocks so expect to find it in the ground and the building materials that make our houses. As a result the soil, the plants and our food can be slightly radioactive too.
Some rocks actually give off radioactive Radon gas. All around us, radiation called cosmic rays (cosmic means 'universal') reaches us from space.
There's little we can do about natural radiation - but man has added to it. We meet Ray machines when we travel by air, or have a chest X-Ray in hospital. This exposes us to extra radiation. Man has also exploded radioactive nuclear weapons and made nuclear power stations which add radiation to the world.

21. When radiation collides with neutral atoms or molecules they may become charged ions (ionized). When this happens to molecules in living cells it can cause damage to the genes called a mutation. This can make cells turn into cancer cells. The cells go out of control, divide rapidly and cause serious problems. The larger the dose of radiation a cell gets, the more chance the cell will turn into a cancer cell. But, if the dose of radiation is very high it can kill the cell completely. We use this idea to kill cancer cells.

Dangers of radiation (higher)
Different types of radioactivity present us with different dangers.
Beta and gamma radiation are the most dangerous because they can penetrate our skin and damage the cells inside us.
Alpha radiation is less dangerous because it is unlikely to reach living cells inside us.
But if we get radiation inside us - either by accident or because of a hospital test - the story is very different. When it's inside us, alpha radiation is the most dangerous because it is easily absorbed by cells. Beta and gamma radiation are less dangerous because they are more likely to pass right through a cell than to be absorbed.

The Dangers of Radiation:

Type of radiation	Inside the body	Outside the body
Alpha	Most dangerous	Least dangerous
Beta	Less dangerous	More Dangerous
Gamma	Least dangerous	Most dangerous - although it's likely to pass straight through you.

Ionizing radiation does not 'build up' in your body, however, radiation effects may appear following exposure to large amounts of radiation, e.g., sunburn from too sunlight. All radiation carries energy which may damage living cells either by killing them or by changing their structure and function but it would take a very large dose to kill sufficient numbers of your cells to cause your death. In order to cause death, you need to receive a radiation dose several thousand times as large as the dose received normally each year from the environment. To cause death, you would need to be exposed more or less in one hit, not spread out over a year. (Compare with sunlight: spread out over a year it gives you a suntan, but in one day of sun baking it could cause your death by sunstroke). Over an extended period, the body can repair most small damage from almost any cause, including radiation, but if the dose is acute (i.e., all in one short period) more serious damage may occur. Two kinds of radiation damage can occur: damage to any of the cells of your body (somatic effects) which may put you at risk, and damage to your reproductive cells (genetic effects) which may put future generations at risk. There are many different somatic effects but the most important long-term effect is cancer induction. You have a 40:1000000 chance of getting a fatal cancer from a dose of radiation equal to one year of natural background. Very roughly, your lifetime dose of natural background might give you a 1:500 chance of dying of cancer. Note, you have a 1:4 chance of dying of cancer from all causes! Your total radiation dose from natural background up to the time your children are conceived (not after) might result in about a 1:3000 chance of generating genetic diseases that are passed on. Note that about 1:10 of all lovelorn children carry some type of genetically related defect.

Dosage	Effects
0.25 Sv	Reduction in white blood cells
1 Sv	Possible vomiting, Nausea
4 Sv	Vomiting, diarrhea, drop in blood cell count, about 50 of exposed victims die within weeks from failure of blood forming organs
5 Sv	Loss of hair, 50% die
6 Sv	Damage to stomach and intestine walls with loss of fluids, immediate radiation sickness, bloody diarrhea, extreme thirst, death within 3 weeks
10 Sv	Severe damage to central nervous system, death within days.

Lethal dose symbol
LD-50 means 50 of exposed will die
LD-50\25 means 50 of exposed will die within 25 days.

Created : 28 October 2000
Last modified : 26 November 2000
Author : Chad Silver email:Chaddysi@start.com.au
Site maintained by : Chad Silver

Component	Material	Function
Fuel	Uranium-235 or Uranium Dioxide	Pprovides energy for the fission reaction to occur.
Moderator	Graphite or Heavy water	It slows down the neutrons to facilitate the fission process.
Control rods	Cadmium Steel	Absorbs neutrons to prevent uncontrolled chain reactions.
Coolant	Water, Liquid Sodium	Removes heat to prevent meltage.
Shield	Reinforced concrete or lead	Protects the workers and others from radioactive leakage.