physics spm:Radioactive 1

CHAPTER 5: RADIOACTIVITY

The structure of an Atom

An atom consists of a nucleus which made of protons and neutrons. It also has electrons orbiting the nucleus.

2. Protons and neutrons are called nucleons.

ParticleCharge Value of

charge / CMass/kg Relativ

e mass

Proton(p) Positive +1.6 x 10-19 1.67 x 10-27 1

Neutron(n)

neutral 0 1.67 x 10-27 1

Electron(e)

negative -1.6 x 10-19 9.11x 10-31 1/1840

Rutherford Atomic Model

1. Hans Geiger and Ernest Marsden carried out an experiment proposed by Ernest Rutherford.

Rutherford’s Explanation

The result of the experiment shows that: i) Most of the alpha particle passed through the gold foil in their original position. ii) A very small number of alpha particles were deflected by the gold foil.

iii) A very small number of alpha particles were bounced back by the gold foil.

3. The conclusion of the experiment: i) The atomic mass is placed at the center of the atom and is positively charged. ii) The atom has a lot of empty space.

iii) The electrons orbiting the nucleus make up most of the atomic volume

Proton number and nucleon number

1. The atoms of different elements are given different symbols.

2. 2. The nuclide notation of an atom gives the symbol of the element:

3. a) The proton number (atomic number), Z. For a neutral atom, the number of protons equals the number of electrons.

4. b) The nucleon number, A is a total number of protons and neutrons.

3. The number of neutrons, N = Nucleon number, A - atomi

number, Z N = A - Z

1114694

Symbol Number of fundamental particles

NucleonNumber

, A

Proton number

, Zproton neutron electron

H 1 0 1 1 1

C 6 8 6 14 6

Be 4 5 4 9 4

5.2 Analysing Radioactive Decay.

1. Radioactivity is the spontaneous disintegration (decay) of a unstable nucleus into a more stable nucleus with the emission of a radioactive rays or energetic particles photons. The decay occurs randomly.

2. The process is said to be spontaneous because it is not influenced by any physical factors such as temperature, time and pressure. It happens on its own, cannot be controlled and not affected by its chemical position.

3. A nucleus is unstable if it is too big. All nuclei with Z > 83 or A > 209 are unstable

.4. The emission of radioactive rays is random means that

a) Emission occurs at irregular intervals.

b) Emission does not occur at the same rate.

RADIOACTIVE DECAY VIDEO

http://www.youtube.com/watch?v=vuGvQjCOdr0

Unstable nucleus

emission of a radioactive rays or energetic particles photons

. There are three types of radioactive emissions : a) alpha particles- b) beta particles - c) gamma rays -

5. Example of stable and unstable isotopes

12614616815819820682208822108221482 O, Pb, Pb,

Element Stable isotopes Unstable isotopes

Carbon C C

Oxygen O O O

Lead Pb Pb Pb

126

14

6

16 15 19

8 88

20682

21082

21482

Characteristics of alpha particles, beta particles and

gamma rays

Absorption of Radiation

Radioactive decay.

A) Alpha Decay A A - 4 4

X Y + He Z Z - 2 2

Parent daughter particle nucleus nucleus (alpha)

In alpha decay, the proton number is reduced by 2 while the nucleon number is reduced by 4.

Beta Decay A A 0 X Y + e Z Z + 1 -1 Parent daughter particle nucleus nucleus (beta)

In beta decay, the proton number is increased by 1 because the new proton formed but the nucleon number is unchanged because of the loss of 1 neutron.

C) Gamma Decay A A X Y + Z Z Parent daughter particle nucleus nucleus (gamma

Example : 210 206 4 Po Pb + He + 84 82 2

There is no change in the proton number and nucleon number for a nuclide

that emits a gamma ray photon.

Write the types of radiation

Half-life.

1. Radioactive decay is a random and spontaneous process. This means that all the unstable nuclei do not decay at the same time, some decay earlier while others will decay at a much later time.

2. As the time progresses the number of atoms undecayed or remaining decreases while the number of atoms decayed or disintegrated increases. 3. The half-life, T1/2 of a radioactive sample is the time taken for the

number of radioactive atoms in the sample to be reduced to half of its original number.

4. After one half-life the activity, mass and the number of atoms remaining of any radioactive substance is halved.

Half-life.

181878 (½)3N= N–N =

Half-life T0 1T1/2 2T1/2 3T1/2

Number ofundecayed atoms.

N (½)1 N = ½N (½)2N = ¼ N N

% atoms undecayed

100 % 50 % 25 % 12.5 %

Mass 64 g 32 g 16 g 8 g

Activity(s-1) 120 s-1 60 s-1 30 s-1 15 s-1

Number of atoms decayed

N – N = 0 N–½N =½ N N–¼N = ¾ N N

Mass havedecayed

0 g 32 g 48 g 56 g

% atomsdecayed

0 % 50 % 75 % 87.5 %

Decrease in mass = initial mass – final mass = 80 g -2.5 g

= 77.5 g

Usage of Half-life

Usage of Half-life

• Half-Life in ArcheologyCarbon-14 has a half-life of 5600 years.1. Humans, animal and plants absorb Carbon-14 through carbon dioxide gas in the atmosphere. A small amount of carbon in CO2 exist as Carbon-14.

2. Living animals and vegetables have a constant amount of Carbon-14 because the C-14 decayed will always be replaced.However, for dead beings the amount of C-14 in it will decreasebecause new C-14 will not be absorbed causing its reactivity to decrease.

3. When an antique or human skull are found, their age can be determine by :Measuring the activity of C-14 in it.

Determine the ratio of decay C-14 against intact C-14

Example 1 :An old human skull is found buried in the sand. The skull producesradioactive ray with reactivity of 30 pulses per second. A new skullproduces reactivity of 100 pulses per second. If the background pulse is20 pulses per second and the half-life of C-14 is 5700 years, how oldis the skull?

Original reactivity = 100 – 20 = 80Reactivity of old skull = 30 – 20 = 10Decay process : T1/2 T1/2 T1/2 1. 40 20 10 Decay has passed through 3 half-lives ( 3T1/2 )The age of the old skull = 3 x 5700 = 17100 years.

Radioactive Detectors

1. Most method of detecting nuclear radiation are based on these two properties

(a) The ionizing effect of the radiation. When a radioactive ray is

emitted into the air, the air molecules around it will turn into ions. (charged molecules). The alpha particle has the highest ionization power, followed by beta and gamma ray.

(b) The ability of the radiation to blacken a photographic-plate or film.

1. Photographic plate or film.

1. A photographic plate works on the principle that radioactive radiation can cause a chemical change on the plate and produce a dark trace.

2. The photographic plate can detect, and.

3. It can be used as a special badge or tag to record the dosage of radiation a person (worker) is exposed to.

2. Scintillation detector

1. A scintillation detector is used to detect gamma radiation.

2. 2. The detector has sodium iodide which flashes when it is hit by gamma.

3. 3. The light flash (scintillation) causes photoelectron to be ejected from the photocathode.

4. These electrons are pulled towards anode, X ( photomultiplier) causing more electrons to be emitted. The electrons emoted are pulled to other anode Y, Z etc which multiply the number of electrons emitted successively.

5. A pulse is therefore produced and will be measured by an electronic counter.

3. Gold Leaf Electroscope

1. The electroscope is positively charged. The gold leaf is deflected because same charges repel.

2. When the charged plate of the electroscope is exposed to the source

of alpha particles, the gold leaf will collapse. 3. This due to the ionizing effect of alpha

particles, both positive and negative ions are produced in the air.4. negative ions are attracted towards the

positive ions on the surfaceof the disc and positive charges are neutralized.5. The deflection of the gold leaf decreases.

This indicates the presence of the alpha particles.

4. Spark Counter

1.A high voltage supply between the wire and the wire gauze is increase slowly until spark is observed.2.The voltage is then reduced slowly until no spark is observed (4 kV).3.A source of alpha particles is then brought closer to the wire gauze.4.The air molecules in the counter is become ionized.5.The positive ions and negative ions produced will be attracted to the wire gauze and thin wire respectively.6.As the ions moves towards their respective terminals more collisions with air molecules occur causing secondary ionization.7.The heat and light energies generated from these collisions producethe spark.1.The intensity of the radiation is indicated by the number of sparks produced per unit time.

It cannot detect beta particles and gamma rays

5. Geiger-Muller Tube (G-M Tube)

1.A Geiger-Muller (G-M) tube consist of an aluminium tube. It is filled with Argon gas at low pressure.2.The central tungsten wire acts as anode(+ve electrode) and the aluminium wall of the tube acts as cathode ( -ve electrode).1.G-M is connected to high voltage before being used.2.When a radioactive particle enters the tube through mica window and the argon gas atoms are ionized.3.The positively charged ions and electrons produced will acceleratetowards the cathode and anode respectively.1.As the ions accelerate, they collide with other atoms, hence producing more ions(secondary ions)1.The anode will collect all these electrons to produce an electric pulse in the circuit. The strength of the pulse is increased by an amplifier and then transfer to a recorder.1.The recorder are also record all readings when there are radioactivebrought closer to G-M. This called the background pulse•The background pulse are produced by

1.radioactive material from earth and the surroundings such asstones, soil, sand, etc.

1.cosmic rays and sun rays.

Actual reading = Reading recorded - Background reading.

10.

6. Cloud Chamber

• A cloud chamber is a detector which displays the tracks of charged particles.

• The felt strip round the top is soaked with alcohol. This causes the air in chamber to be saturated.

• The lower part is filled with solid Carbon dioxide, CO(dry ice). • Sponge is used to push CO towards the metal plate. As a result,the air

space becomes supersaturated.• When the radioactive rays enter the upper part, the ionization of air will occur. Saturated alcohol vapour will move above the ions and

condensed. Droplets of liquid alcohol on the ions will cause the formation of misty tracks.• Steps to ensure the clear tracks :

the transparent perspex cover is rubbed with a soft cloth to produce charges which will remove all ions in the chamber before

radioactive rays enter.The cloud chamber must be placed horizontally to ensure smooth flow particle in it.

The light must shine on the area supersaturated with vapour and not the black base of the chamber in order to avoid heating it.

Radiation Track Characteristics and explanation.

Alpha particle

- thick because have strongest ionizing power. A lot of alcohol droplets are formed along the ions produced along the track.- straight because not easily deflected by air molecules with its greater mass.- same length because each particle has equal amount of energy.

Beta particle

- thin because ionizing power is weak.- curvy because the particles are light and deflected by air molecules. - different length because each particle has different amount of energy.

Gamma ray

- thin, short and scattered because it has the lowest ionizing power.