Rutherford Scattering
The discovery of the nucleus:
- Before 1913 scientist believed an atom was like a plum pudding, a positive mass with electrons dotted around inside.
- It wasn’t until Ernest Rutherford did his experiment to prove otherwise.
Rutherford detected a narrow beam of alpha particles through a thin sheet of gold foil, and observed the movement of the alpha particles on a fluorescent screen.
From the results, he deduced :
- As most of the alpha particles passed through the golf foil without any deflection. Most of the space in atom is empty.
- As some of the alpha particles were deflected at large angles, they must approached some positively charged region responsible for the deflection. This positively charged region is called nucleus.
- As very few alpha particles are deflected so, it is assumed that volume occupied by the nucleus is very small.
- Since alpha particles , which are comparatively denser are deflected by nucleus, it shows that almost the complete mass of the atom is in nucleus.
Evaluation chamber- No air molecule will interfere with the alpha particles
foil is very thin – alpha radiation is low penetrating, thick foil will stop it.
Ensure all alpha particles are traveled at same speed.(same KE)
- speed effects, how much the particles are deflected to keep it their, all alpha particles are traveled with same velocity.
Narrow beam of alpha – if the beam was widely dispersed, it would be difficult to see which particles were deflected and which were just more widely dispersed.
Rutherford used the results to determine the modern model of atom , we use today, and by working out the number of alpha particles deflected, he could estimate the size of the gold nucleus.
The danger of the radioactivity
Ionizing radiation can damage living cells.
- It can destroy all the membrane that causes cell to die.
- Can damage the DNA , which can effect the cell division and can lead to the cancerous tumors.
Exposure to the ionizing radiation can effect the health of the affected person and possibly their future children.
therefore when using radio active material
- they must be stored carefully, in lead lined container, and kept under strict regulation for those who use them.
- they sources allowed in contact with skin, wear gloves and handle with tool so that the person is out of range.
- Liquid and gas sources should be contained in the sealed container so that they can’t be inhaled.
- Limit the time exposed to the source, no longer then necessary.
- Follows square root law
Background radiations
Natural radiations around us due to
- Cosmic rays
- radioactive material in the rocks, soil and air.
- Nuclear weapon/power
- air travel etc.
Radioactive Decay
Radioactive decay is –
- Random
- Spontaneous
- Have a constant half life
Half life (T1/2) – the radioactive half life of a substance is the time taken for half the radioactive nuclei in any sample to undergo radioactive decay.
Decay constant (λ) – the probability of an individual nucleus decaying per second.
T1/2 = In 2 / λ
T1/2 – half life time (s)
Activity (A) – the average number of decays per second.
A = λ N
A = Activity(Bacquerel Bq = s-1) λ = Decay Constant N= number of molecules
Radioactive isotopes in use-
Carbon dating- living plants and trees contain small amounts of radioactive isotope carbon 14which is formed in the atmosphere from cosmic rays and absorbed during photosynthesis.
C.A has a half life of 5510 years, so there is negligible decay during the lifetime of the plant.
Once the plant has died no further carbon is taken so the proportion of C14 decreases as it decays. As activity is proportional to the number of atoms still left to decay, measuring the activity of the dead sample allows us to calculate its age. Provided are have the activity of the same mass of living wood.
Argon dating – ancient rocks contain trapped argon gas as a result of the decay of the radioactive isotope of potassium.
The potassium isotope always decays by β– emission into calcium. This process is 3X more probable.
The effective half life of the decay of 19 40K is 1250 million years. The age of the rock can be calculated by measuring the portion of Argon 40 to potassium 40.
For every N potassium 40 atoms present , there must have been N+ q potassium atoms.The radioactive decay equation N = Noe– λ t can then be used to find the age of the sample.
Tracking- radioactive tracers can be injected into pipes, plants and even people so that they can be tracked and traced to give insight on leaks, plant uptake and uptake of iodine by the thyroid gland.
Thickness monitoring- radioactive isotopes can measure thickness, by putting e β– emittor above a metal sheet and a detector below and monitoring the e– that pass through.
Nuclear Instability
- For light stable isotopes, N= Z.
- As Z increases beyond to stable atoms have more neutrons than protons.
- Alpha emitter mainly Z ≥ 80 N ≥120.
- Beta emitters occur in neutron rich nucleus.
Energy levels– when isotopes decay, they can decay into an exated veoin of an element. This will then de exate, releasing a photon.
The technetium generator
- Technetium decayed from molybdenum is a constant gamma source with half life of 6 hours.
- It is often used in medicine, as it is reliable and its physical and biological half life as short, minimizing radiation and cleaning from the body quickly.
Nuclear radius
Nuclear Energy
Knowledge of the energy associated with nuclear binding and how we hamess it.
Binding Energy– of the nucleus is the work that must be done to separate a nucleus into its constituent neutrons and protons.
Mass Defect (or deficit) – ∆m of a nucleus is defined as the difference between the mass of the separated nucleons and the mass of the nucleus.
Mass can have a unit called atomic mass unit u.
lu = 1.661×10-27kg
lu = 931.5MeV
Fission and Fusion
- A nucleus with more binding energy per nucleon is more stable.
Thermal nuclear reactor
Fuel rods – consist of radioactive isotope uranium 235 ≈ a fissionable material.
Control rods – made of boron, they are lowered to absorb excess neutrons.
Moderator – water slowsdown (removes KE) from neutrons so more are at the correct range to be absorbed and cause fission.
Heat exchanger – turns water in steel pipes to steam to take the turbines.