The Atom, the Nucleus and Radioactivity

Rutherford’s Experiment Rutherford bombarded a very thin piece of gold foil with alpha (α) particles, which are actually the nuclei of helium atoms. He found that: ► Most α particles were undeflected and passed straight through the foil. ► Some were deflected through small angles. ► A very few number were turned back through an angle larger than 90 o. Click here… Click here…

Experimental Setup

► He explained this by using the nuclear model for atoms, i.e. each gold atom had a small positively charged nucleus at its centre containing most of the mass. This explanation is as follows: ► Nucleus is very small compared to the size of the atom. the atom is mostly empty space, hence most α particles were able to pass straight through.

► All the charge of an atom is at the centre so when a α-particle passed near the nucleus it was deflected as like charges repel. ► If an α particle was to collide head on or nearly head on it was deflected through an angle larger than 90 o. ► The relatively light, negatively charged electrons orbit the nucleus in various orbits.

► When light from a luminous source undergoes dispersion the resulting pattern is called an emission spectrum. ► A continuous spectrum is produced by an incandescent solid or liquid. This spectrum is not characteristic of the material producing it. ► A line spectrum is produced when the atoms of a gas have enough energy to give out coloured light. This spectrum is characteristic of the gas producing it.

Helium Spectrum Hydrogen Spectrum

How these are formed…

► An energy level is a fixed energy value that an electron can have in an atom. h f = E 2 - E 1 ► where h= Planck’s constant, E 2 = energy of higher orbit, E 1 = energy of lower orbit ► Example… Example…

► The atomic number (Z) of an element is the number of protons in the nucleus of an atom of that element. ► The total number of protons and neutrons in the nucleus of an atom is called the ► mass number (A) of that atom. Number of neutrons in the nucleus = Mass Number – Atomic Number i.e. Number of neutrons = A – Z

► Atoms of an element that have the same number of protons but different numbers of neutrons are called isotopes of the element.

Radioactivity ► Radioactivity is the disintegration or decay of the nuclei of certain atoms with the emission of one or more types of radiation.

Radiation

► The 3 types of radiation are: ► Alpha ► Beta ► Gamma

Alpha ► Alpha (α) radiation is fast moving helium nuclei ejected from the nuclei of radioactive atoms. It has the least penetrating power; a sheet of paper will stop this radiation.

Beta ► Beta (β) radiation is high-speed electrons ejected from the nuclei of radioactive atoms. These have medium penetrating power; a sheet of aluminium should stop this radiation.

Gamma ► Gamma (γ) radiation is high frequency electromagnetic radiation (with frequencies above those of normal X-rays) emitted from the nucleus of a radioactive atom. These have high penetrating power; 1-2 m of concrete would be needed to stop this radiation.

Problem : Calculate the number of  - particles and β-particles emitted in the decay of U 238 to Ra

Solution :   = He 4 β = e 0 Therefore a β emission has no effect on the mass number. Step 1 : Find the number of  emissions. By how many multiples of 4 the mass number decreases = 12 = ( 4 x 3) => 3  -emissions

If there had only been 3  -emissions the result would have been U 238  3He 4 + Rn 226 Step 2 : Find the number of β-emissions. Rn 226 ->2e 0 + Ra 226 ( always subtract 86 – ( -2 ) = = 88 ) ► ► Ans : 3  -emissions and 2 β-emissions 88 86

To demonstrate the ionizing affect of radioactivity Procedure: Bring a radioactive source close to the cap of a charged Gold Leaf Electroscope Observation: Leaves collapse Conclusion: The charge on the G.L.E. became neutralised by the ionised air.

Radon Gas (mainly from granite rock) is the main source of background radiation, which in turn is responsible for almost all the radiation we get exposed to over our lifetime. The Becquerel (Bq) is the unit of activity. One Bq = one disintegration per second.

Law of Radioactive Decay ► The number of nuclei decaying per second (i.e. the activity) is directly proportional to the number of nuclei undecayed. ► Rate of decay = λN ► The half-life of a radioactive isotope is the time taken for half of the undecayed atoms to undergo decay.

► The half-life of a radioactive element is also the time taken for its activity to decrease by half. where λ is the decay constant

► The Geiger-Muller tube when radiation passes through a window it ionises argon gas producing positive argon gas and negative electrons, these pick up high speeds in a strong electric field and cause an avalanche effect by producing more ions and electrons. On reaching the anode a pulse of current flows onto external circuit.

► Solid-state detector: consisting of a reverse biased p-n junction connected to a counting device. When radiation strikes depletion layer, some electron hole pairs formed. These charge carriers move by the influence of the voltage across it and so a pulse of current is formed.

► A mole of any substance is the amount of that substance that contains as many particles as there are atoms in exactly 12 grams of carbon. ► This number is 6.02x10 23, and is called Avogadro’s number.

Fission, Fusion and Nuclear Energy Hiroshima

FISSION ► the breaking up of a large nucleus into two smaller nuclei with the release of energy ► the breaking up of a large nucleus into two smaller nuclei with the release of energy and neutrons ► Chain Reaction Chain Reaction Chain Reaction

► Natural Uranium is made up of two isotopes: U-235 (0.7%), and U-238 (99.3%). ► Only U-235 undergoes fission. ► This occurs if it is bombarded with fast- moving or slow-moving neutrons, but is more likely to occur if the neutrons are relatively slow moving. ► This reaction is represented as follows: U n 1  Ba Kr n + K.E.

E = mc 2 ► ► The total mass on the left-hand side is greater than the total mass on the right-hand side. The mass which has disappeared has been converted (re-manifested) into the kinetic energy of the particles on the right

► The neutrons produced are fast moving and may trigger further fission. ► If the mass of the sample is above a certain critical mass, the process will become self- sustaining and a chain reaction will occur. ► If the mass is below the critical mass the reaction will simply fizzle out ► The Atom The Atom The Atom

Atomic Mass Unit (a.m.u.) ► ► ; we used to use the mass of a Hydrogen nucleus (which is a proton) as our basic unit, but in 1960 it was changed to 1/12 th the mass of a carbon 12 atom (because it was easier to measure). ► ► This is now known as the unified atomic mass unit (u). ► ► 1 a.m.u. = 1.67 x kg

NUCLEAR REACTOR The fuel is natural Uranium. The moderator is either Graphite or Heavy Water. This slows down the fast moving neutrons to enable further fission in U- 235 rather that being absorbed in U-238. The control rods absorb neutrons. They look like sleeves.

NUCLEAR REACTOR Lowering them over the fuel rods prevents the neutrons from one fuel rod reaching the next rod, and so they control the rate of the reaction. Lowering them completely causes the reaction to stop. 1 a.m.u. = 1.67 x kg,

Nuclear Fusion ► the joining together of two small nuclei to form one larger nucleus with the release of energy. The energy we get from the sun comes from nuclear fusion reactions in the sun ( 2 H = Deuterium (Hydrogen with one neutron), 3 H = Tritium)

Advantage of Fusion over Fission ► Less radioactive waste. ► Deuterium is readily available from the oceans. ► No dangerous chain reactions.

Why is a fission reactor a more viable source of energy than a fusion reactor? ► ► Easier to initiate reaction ► ► Fission can be more easily controlled ► ► There is, as of yet, no way to maintain and control the high temperatures required for fusion reactors. There are a couple of prototypes, e.g. ITER in France, but all are a long way from becoming commercial.

The effect of Ionising Radiation on humans depends on: ► ► The type of radiation (whether it’s alpha, beta or gamma) ► ► The activity of the source (in Bq) ► ► The time of exposure ► ► The type of tissue irradiated

Precautions when dealing with Ionising Radiation ► Make sure sources are properly shielded. ► Keep sources as distant as possible from human contact, eg use a pair of tongs ► Use protective clothing.