Ionising Radiation and Nuclear Reactions

Electromagnetic Radiation

Given off by atoms as electrons jump from one energy level to another, and includes ordinary visible light. Different types of EMR all travel at a speed of 3.00 x 10^8 ms^-1 (in a vacuum) and vary only in their frequency and wavelength.

EMR with high frequency has enough energy to remove electrons from atoms - hence it is called ionising radiation.

The electromagnetic spectrum

Nucear Radiation

Radiation produced as a result of the nucleus of an atom disintegrating is called nuclear radiation - alpha, beta, gamma particles.

• Gamma are both electromagnetic radiation and nuclear radiation

Nuclear Radiation and Ionising Radiation

All nuclear radiation is ionising radiation - when it interacts with an atom, it gives outer electrons of that atom enough energy to break way from the atom, thereby leaving as an ion.

Particles knocking off electrons

• Both alpha and beta particles knock electrons off their atoms

  • Alpha → pulls the electrons

  • Beta → pushes them

  • Gamma particles give energy direct to the electron by being completely absorbed by it

• Alpha and beta particles lose kinetic energy in the process of detaching electrons, slowing down and eventually stopping.

• Ionised atoms then have different chemical properties → results in changes to object.

Disintegrating Nuclei and Neutrons

Disintegrating nuclei can also eject neutrons → Zero electrical charge (do not directly cause ionisation in a single step/interaction.

Fast neutrons can be absorbed into a stable atom, making that nucleus unstable and more likely to emit ionising radiation of another type.

Neutrons are penetrating → they have no charge

Atoms

• Nucleus → consists of protons (+) and neutrons (0).

• Orbiting electrons → have negative charge equal in magnitude to the proton charge.Protons and neutrons collectively called nucleons (mp = mn = 1840 me)

• Protons and neutrons are themselves composed of smaller particles called quarks

• Number of protons determines the element (atomic number)

• Atomic mass number = no. of protons + number of neutrons

  • isotopes

Strong Nuclear Force

• Nucleus is held together by a force stronger than the repulsive force between the protons, and independent of charge (otherwise the neutrons would not be affected)

• acts over extremely small distances, so does not affect intermolecular attraction

Stability in nuclei

• the closer the ratio of neutrons to protons is to 1, the more stable te nucleus

• they tend to redress the balance by ejecting nuclear particles → undergo nuclear decay

• Such nuclei is said to be radioactive, ejected particles under go nuclear decay

  • said to be radioactive + particles ejected are nuclear radiation

Radioactive isotope

A radioisotope

• only 300 of the known 1700 known isotopes are stable - the rest are radioactive

• every isotope of every element with more than 83 protons is radioactive

Alpha Particles

Beta Particles

Gamma Radiation

Nuclear Penetrate Diagram

Effects of Radiation on Humans

• Ionising radiation causes atoms to lost electrons (become ions) and thus become charged

• These ions are chemically reactive and damage or even kill cells, or affect mechanism that regulates cell division - resulting in uncontrolled cell division - a tumour

Ionising radiation can have

1. somatic effects

2. genetic effects

3. beneficial effects

Organs in which many cell divisions occur (e.g. bone marrow, lungs) are more vulnerable to radiation damage

Somatic effects

short term harm caused by damage to or destruction of cells

Genetic effects

Long-term harm caused by damage to cells in reproductive organs (these mutations are then passed on to succeeding generations)

Beneficial effects

e.g. radiation used to kill malignant cancer cells

Time after Radiation

show up about a month after exposure

• e.g. damage to reproductive organs, eye lenses, bone marrow, gastrointestinal system, CNS

time between exposure and appearance of damage is called the latent period

Delayed effects = effects that appear months/years after radiations → leukaemia and cancerous tumours

Two main genetic effects of radiation are:

• chromosome aberrations (changes in the actual number or structure of the chromosomes)

• Genetic mutations that can lead to deformed births in future generations

Percent of natural/man made radiation

Beneficial effects of radiation

1. radioactive tracers e.g. Iodine-123

2. Smoke detectors

3. Radiation therapy

4. PET scans

5. X-ray pictures

6. Food irridation

People and radiation

A person does not become radioactive after being irradiated. The radiation will create ions and damage cell biology, but does not cause the production of further radiation.

The only way a person can become radioactive is to ingest a radioactive substance, which emits radiation from within the person until the activity drops to zero.

Nuclear equations

Spontaneous transmutation reactions → Alpha and beta particles

Artificial transmutation → a managed process that changes one nuclide into another (by neutron bombardment)

If nucleus has too many neutrons: reduce the number by changing a neutron into a proton and an electron, + ejecting the electron as a Beta particle.

In most cases, after A or B emission, nucleus is still unstable + loses more energy by immediate emission of Y ray.

Protons + Quarks

Each proton has two up quarks (+2/3) and one down quark (-1/3) →

• Charge = positive 1

• denoted uud

Neutron and quarks

one up quark and two down quarks

• total charge = 0

• denoted udd

A neutron can also be viewed as the union of a proton and an electron (or B- particle)

Nucleus + quarks

This explains how an electron can come from the nucleus:

Ve = antineutrino

Similarly a positron (B+) can be emitted from a nucleus

Ve =neutrino

*In nuclear reactions, energy, momentum and charge are conserved. This is achieved by the emission of the neutrino and antineutrino in above reactions.

Decay Chains

• resulting ‘daughter’ nucleus may be unstable and undergo further decay. This process can continue until stable atom is produced.

• Half life of daughter nucleus may be different from mother.

Detecting Radiation

1. Geiger Counter (Geiger Muller Tube)

2. Film Badges

3. Thermoluminescent dosimeter

4. Wilson Cloud Chamber

Geiger Counter

Film Badges

Principle → Radiation blackens photgraphic film

Filters of various thicknesses are attached to the film badge to distinguish between X-rays and Y-rays and B-rays

Film badges are not accurate

Thermoluminescent Dosimeter

Certain materials store a small amount of the energy absorbed from radioation and release it later when heated in the form of light.

Can detect X, Y and B rays

Wilson Cloud Chamber

Radiation Units and Becquerels

Atomic mass unit = 1/12 mass Carbon 12 = 1.6661 × 10^-27

• The becquerel (Bq) is a unit of radioactivity

• 1 Bq = 1 transformation per second

• Activity = N/t (number of transformations/time)

The gray

a measure of radioactive absorbed dose

1Gy = 1JKg^-1

Absorbed dose = energy absorbed (E) / Mass of affected body part (m)

Unit = Gy

Sieverts

a measure of dose equivalent

Dose equivalent = absorbed dose x quality factor

1Sv = 1Jkg^-1 x QF

Biological effect of radiation on animal + plant depends on radiation absorbed dose and type of radiation.

Each type has its own quality factor (relative biological effectiveness), which is multiplied by radiation absorbed dose to obtain the dose equivalent.

Lethal Dose

1Sv = massive dose, cause severe illness

>4 Sv = likely to kill

Recommended 1mSv (public) and 50 mSv (radiation ind. workers) / year

LD50 = radioactive dose that gives recipient a 50% chance of survival (around 4Sv)

LD50/25 = 50% will die within 25 days

RADIATION DOSE VARIES INVERSELY WITH THE SQUARE OF THE DISTANCE FROM THE SOURCE:

• e.g. if A is x3 as far from source as B, A will recieve 1/9 radiation of B

Half-Life

• The activity of a sample of radioactive material (i.e. no. of nuclei decaying/second) is proportional to the number of nuclei that are yet to decay

• a random process

• Half lives vary enormously for different radioactive substances, from microseconds to billions of years. The longer the half-life, the more stable is the isotope.

Half Life Graphs + Formulas

Carbon Dating

The amount of radioactive C14 in atmosphere is constant, the amount of decay being matched by the creation of new C14 by neutron bombardment from cosmic rays.

Formation: N14+neutron → C14 + Hydrogen

Decay: C14 → N14 + B-1

The half life for beta decay is 5760 years

Living things take in C14 and C12 from atmosphere (breathing + eating)

Ceases once they die → amount of C14 declines. By measuring the amount of C14 left in dead wood/skeletons, age can be calculated

**theory assumes proportion of C14 in atmosphere has been constant over the last several thousand years.

Mass Defect

Mass of the assembled particles in the atom is less than the sum of the masses of the individual components

Mass defect = the difference between the isotopic mass of the atom and the sum of the masses of its neutrons, protons and extra-nuclear electrons

Einstein’s equation

E=mc²

Binding Energy

If we regard energy + mass as equivalent: Protons and neutrons lose binding energy as they come together (stone).

Binding energy = the energy that would be required to separate the nucleus into isolated particles.

Particles lose energy when they form a nucleus

Energy must be given to the particles to break up the nucleus

Mass Defect ≡ Binding Energy

Key notes

• hydrogen has no binding energy (no mass defect) as its nucleus has only one particle; i.e. mass (H) = mass of one proton

• Binding energy per nucleon indicates stability of the nucleus - higher it is = more stable.

• electron-volt (eV) is a non-S.I unit of energy 1eV = 1.6022 × 1m10^-19

• 1u = 931 MeV

• Energy released or absorbed in nuclear reactions is often given in MeV

Nuclear Fusion + Fission Overview

B.E nucleon increases as we move along periodic table - up to 26 (iron{ most stable element} ), decreases again after atomic number 26

• This is because there are 2 competing forces at work:

  • protons pushing each other apart

  • strong nuclear force trying to hold things together

• The forces have a crossover point because the strong nuclear force has a very short range but electric force has a long range

• as nucleus gets bigger, electric forces starts to win out

Nuclear Fission

Nuclei with atomic number >26 tend to disintegrate to produce more stable nuclei and release energy in the process

The splitting of a nucleus into two lighter nuclei - the sum of the 2 masses of the lighter nuclei is less than that of the original nucleus

Lost mass is released as energy

Nuclear Fusion

Nuclei with atomic number <26 tend to combine to produce more stable nuclei, releasing energy in the process

The union of 2 nuclei into a single nucleus such that the mass of the single nucleus is less than the sum of the masses of the original nuclei

Lost mass = released as energy

Nuclear Fusion and Fission - Energy

• Nuclear fusion releases enormous amounts of energy but also requires a large amount of energy → electrostatic repulsion forces between the reacting nuclei must be overcome

• More energy is released per nucleon in nuclear fusion than in nuclear fission because a greater percentage of the mass is transformed into energy

Fusion reactions on the sun

The heat needed to bring the protons close enough to react is available on the sun but not on earth (except by a fission bomb)

Nuclear Fusion Equations

• To calculate how much energy is released: calculate the mass difference between products and reactants:

  • if difference = negative, energy is released in the reaction

  • if difference = positive, reaction absorbs/requires energy

Chain Reactions

• U235, 238, Thorium-232, Plutonium-239 undergo fission when they capture neutrons

• e.g: U-235 undergoes fission by capturing slow neutrons: 3 reactions possible:

  • 

Average number of neutrons produced in the fission of U235 = 2.5

** The fission products undergo further disintegration, usually B emission

Since each U-235 fission releases more than one neutron, a chain reaction may occur; reaction becomes self sustaining.

Controlled and uncontrolled reactions

Uncontrolled: If released neutrons are continually able to find other nuclei to split, then we have exponential growth; uncontrolled chain reaction, basis of an atomic weapon

Controlled: If chain reaction is strictly controlled so than only one extra neutron is allowed to initiate one further nuclear reaction each time, basis for power station.

Neutron Flux

the number of neutrons emitted per reaction that are suitable for further reaction

• Some emitted neutrons may “leak” through the surface, others may be captured by a nucleus but not cause fission.

Critical mass

• the mass of uranium beyond which an uncontrolled chain reaction will occur. (an explosion)

• If less than the critical mass, most neutrons can escape from the piece of uranium.

• If more, most neutrons remain inside the uranium to produce more splittings

• Critical mass for U235 = few kilograms, but depends on the size and shape

  • **presence of other uranium isotopes + impurities affect whether reaction = critical

Nuclear Reactors

• the neutron flux is maintained at 1 by varying

  • ratio of U-235 to U-238

  • the physical arrangement of the fuel rods

  • type of moderator used to slow down neutrons (enhancing their chances of being captured by U-235 nuclei)

• the first neutron comes from spontaneous fission

• a “fast breeder reactor” uses a coolant that isn’t an efficient moderator but uses U-238 as fuel (readily captures high energy neutrons)

Nuclear Reactor Diagram

ion

Neutron Radiation

• Neutrons do not cause ionisation themselves, but are likely to collide with hydrogen nuclei in our bodies, releasing high energy protons that becomes dangerous ionising force

• LD = 6.5 Sv

Nuclear Tech

• Pros

  • efficient, virtually unlimited supply, “clean”

• Cons

  • dangers of radiation, risk of accidents, problem of disposal of radioactive waste products - depends on radioactive waste and type of radiation it produces

Conventional Tech

• Pros

  • safer, employs more people

• Cons

  • less efficient, chemical pollution, limited fuel supply