The Atom, the Nucleus The Atom, the Nucleus and Radioactivity and Radioactivity

RutherfordRutherford’’s Experiments ExperimentRutherford bombarded a very thin pieceRutherford bombarded a very thin piece

of gold foil with alpha (α) particles, of gold foil with alpha (α) particles,

which are actually the nuclei of helium which are actually the nuclei of helium

atoms. atoms.

He found that:He found that:► Most α particles were undeflected and Most α particles were undeflected and

passed straight through the foil.passed straight through the foil.► Some were deflected through small angles.Some were deflected through small angles.► A very few number were turned back through an angle larger A very few number were turned back through an angle larger

than 90than 90oo..

Click here…Click here…

Experimental SetupExperimental Setup

► He explained this by using the nuclear model for He explained this by using the nuclear model for atoms, i.e. each gold atom had a small positively atoms, i.e. each gold atom had a small positively charged nucleus at its centre containing most of charged nucleus at its centre containing most of the mass. This explanation is as follows:the mass. This explanation is as follows:

► Nucleus is very small compared to the size of the Nucleus is very small compared to the size of the atom. the atom is mostly empty space, hence atom. the atom is mostly empty space, hence most α particles were able to pass straight most α particles were able to pass straight through.through.

► All the charge of an All the charge of an atom is at the centre so atom is at the centre so when a α-particle passed when a α-particle passed near the nucleus it was near the nucleus it was deflected as like charges deflected as like charges repel.repel.

► If an α particle was to If an α particle was to collide head on or nearly collide head on or nearly head on it was deflected head on it was deflected through an angle larger through an angle larger than 90than 90oo..

► The relatively light, The relatively light, negatively charged negatively charged electrons orbit the electrons orbit the nucleus in various orbits.nucleus in various orbits.

► When light frWhen light fromom a luminous source undergoes a luminous source undergoes dispersion the resulting pattern is called an dispersion the resulting pattern is called an emission emission spectrumspectrum. .

► A A continuous spectrumcontinuous spectrum is produced by an incandescent is produced by an incandescent solid or liquid. This spectrum is not characteristic of the solid or liquid. This spectrum is not characteristic of the material producing it.material producing it.

► A A line spectrumline spectrum is produced when the atoms of a gas is produced when the atoms of a gas have enough energy to give out coloured light. This have enough energy to give out coloured light. This spectrum is characteristic of the gas producing it.spectrum is characteristic of the gas producing it.

Helium SpectrumHelium Spectrum

HHydrogenydrogen Spectrum Spectrum

How these are formed…How these are formed…

►An energy level is a fixed energy value An energy level is a fixed energy value that an electron can have in an atom.that an electron can have in an atom.

h f = Eh f = E2 2 - E- E11

►where h= where h= PlanckPlanck’’s constant, s constant, EE22 = energy = energy

of higher orbit, of higher orbit, EE11 = energy of lower orbit = energy of lower orbit

►Example…Example…

► The The atomic numberatomic number (Z) of an element is the (Z) of an element is the number of protons in the nucleus of an atom of number of protons in the nucleus of an atom of that element.that element.

► The total number of protons and neutrons in The total number of protons and neutrons in the nucleus of an atom is called the the nucleus of an atom is called the

►mass number mass number (A) of that atom.(A) of that atom.

Number of neutrons in the nucleus = Number of neutrons in the nucleus = Mass Number – Atomic NumberMass Number – Atomic Number

i.e. i.e. Number of neutrons = A – ZNumber of neutrons = A – Z

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

Radioactivity

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

Radiation

►The 3 types of radiation are:The 3 types of radiation are:

► AlphaAlpha► BetaBeta►GammaGamma

Alpha

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

Beta

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

Gamma

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

Problem : Calculate the number of -particles and β-particles emitted in the decay of U238 to Ra 226

92 88

Solution := He4 β = e0 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. 238 - 226 = 12 = ( 4 x 3) => 3 -emissions

-1

If there had only been 3-emissions the result would have been U238 3He4 + Rn226

Step 2 : Find the number of β-emissions.Rn226->2e0 + Ra 226

( always subtract 86 – ( -2 ) = 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 DecayLaw of Radioactive Decay►The number of nuclei decaying per The number of nuclei decaying per

second (i.e. the activity) is directly second (i.e. the activity) is directly proportional to the number of nuclei proportional to the number of nuclei undecayed.undecayed.

►Rate of decay = Rate of decay = λλNN

►The half-lifeThe half-life of a radioactive isotope of a radioactive isotope is the time taken for half of the is the time taken for half of the undecayed atoms to undergo decay. undecayed atoms to undergo decay.

21T

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

where where λ is the decay constantλ is the decay constant

2ln

21

T693.0

21

T

► The Geiger-Muller The Geiger-Muller tube tube

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

►Solid-state detector: Solid-state detector:

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

►A A molemole of any of any substance is the substance is the amount of that amount of that substance that substance that contains as many contains as many particles as there are particles as there are atoms in exactly 12 atoms in exactly 12 grams of carbon . grams of carbon .

►This number is This number is 6.02x106.02x102323, and is called , and is called AvogadroAvogadro’’s number. s number.

Fission, Fusion and Fission, Fusion and Nuclear EnergyNuclear Energy

HiroshimaHiroshima

FISSIONFISSION

►the breaking up of a large nucleus into the breaking up of a large nucleus into two smaller nuclei with the release of two smaller nuclei with the release of energyenergy and neutrons

► Chain ReactionChain Reaction

► Natural Uranium is made up of two Natural Uranium is made up of two isotopes: U-235 (0.7%), and U-238 isotopes: U-235 (0.7%), and U-238 (99.3%). (99.3%).

►Only U-235 undergoes fission. Only U-235 undergoes fission. ►This occurs if it is bombarded with fast-This occurs if it is bombarded with fast-

moving or slow-moving neutrons, but is moving or slow-moving neutrons, but is more likely to occur if the neutrons are more likely to occur if the neutrons are relatively slow moving.relatively slow moving.

►This reaction is represented as follows:This reaction is represented as follows:

U235 + n1 Ba141 + Kr92 + 3 n + K.E.

E = mc2

►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 The neutrons produced are fast moving and may trigger fast moving and may trigger further fission. further fission.

► If the mass of the sample is If the mass of the sample is above a certain above a certain critical critical massmass, the process will , the process will become self-sustaining and become self-sustaining and a a chain reaction chain reaction will occur. will occur.

► If the mass is below the If the mass is below the critical mass the reaction critical mass the reaction will simply fizzle outwill simply fizzle out

► The AtomThe Atom

Atomic Mass Unit (a.m.u.)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 1/12th 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 10-34 kg

NUCLEAR REACTORNUCLEAR 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 REACTORNUCLEAR REACTORLowering 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 10-34 kg,

Nuclear Fusion Nuclear Fusion

►the joining together of two small the joining together of two small nuclei to form one larger nucleus nuclei to form one larger nucleus with the release of energy.with the release of energy.

The energy we get from the sun comes from nuclear fusion reactions in the sun

(2H = Deuterium (Hydrogen with one neutron), 3H = Tritium)

Advantage of Fusion over Advantage of Fusion over FissionFission

► Less radioactive Less radioactive waste.waste.

► Deuterium is Deuterium is readily available readily available from the from the oceans.oceans.

► No dangerous No dangerous chain reactions.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 Precautions when dealing with Ionising RadiationRadiation

► Make sure sources Make sure sources are properly shielded.are properly shielded.

► Keep sources as Keep sources as distant as possible distant as possible from human contact, from human contact, eg use a pair of tongseg use a pair of tongs

► Use protective Use protective clothing.clothing.