CHAPTER 5: RADIOACTIVE5. 1 THE NUCLEUS OF AN ATOM5.1.1 The composition of the nucleus Nucleon number, A is defined as the total number of protons and neutrons in a nucleus.Number of neutrons, N = A - ZDefine nucleon number (A)Define proton number (Z)The electrons move in orbits around the nucleus.A proton has a unit positive charge.Proton number, Z, is defined as the number of protons in a nucleusThe number of electrons = the number of protonsAn element is identified by its proton numberA nucleus consists of a number of protons and neutrons.

Protons and neutrons also known as nucleons.A neutron is an uncharged particle of about the same mass as the proton.An atom is neutral because it contains an equal number of negatively charged electrons. So the net charge is zero.Each atom has a very small and very dense core called nucleus. Most of the mass of atom is contained in the nucleusMatter is made up of very small particles called atomsThere are a lot of empty spaces within atom

5.1.2 NuclideA nuclide is an atom of a particular structure. Each element has nucleus with a specific number of protons.

Nuclide notationNucleon number

AChemical symbol of the element

XProton number

Z

Example: Proton number of carbon: 6 Carbon nucleus has 6 protons Nucleon number of carbon is 12 Number of neutrons in carbon nucleus is 12 6 = 6

12 C 6

5.1.3 Isotopes Isotopes are atoms of the same element with the same number of protons but different number of neutron

Isotopes of an element contain the same number of protons and the same number of electrons. So isotopes have the same chemical properties chemical reactions involve the electrons in an atom. However they have different physical properties because their mass is different. Some isotopes exist naturally. Isotopes can also be made artificially.

Example:Carbon -12 : 12 C 6Carbon -14 : 14 C 6

Deuterium : 2 H 1Tritium : 3 H 1

Oxygen-16 :16 O 8Oxygen-17 : 17 O 8

Helium-3 : 3 He 2Helium-4 : 4 He 2

Exercise 5.11) A nucleus contains protons and neutrons. Which of these particles experiences an electrostatic force?

__________________________________________________________________2) Gold (Au) has 79 protons and 118 neutrons. Write the symbol for this nuclide.

__________________________________________________________________3) An isotope of nitrogen can be represented by 15 . How many of the following N particles does it have ? 7

a) Protons: _____________________b) Neutrons:____________________c) Electrons:____________________

4) An atom of element Y has a proton number 5 and a nucleon number 11. Identify the element

_______________________________________________________________________

5.2 RADIOACTIVE DECAY

5.2.1 RadioactivityRadioactivity is the spontaneous and random disintegration (decay) of an unstable nucleus accompanied by the emission of energetic particles or photons.

The nuclei of some atoms are unstable. The nucleus of an unstable atom will decay to become more stable by emitting radiation in the form of a particle or electromagnetic radiation. Emission occurs at irregular intervals Emission does not occur at the same rateThe emission of radioactive rays is randomSpontaneous processThe process is not triggered by any external factors such as temperature, pressure, time, etc.There is no way to tell which nucleus will decay, and cannot predict when it is going to decay.Random process

Exercise 5.2.11) Describe what happens to an atom when it undergoes radioactive decay.

___________________________________________________________________________

___________________________________________________________________________2) A scientist measures the count rate from a radioactive substance over a period of 20 minutes. The table shows the results of his measurements.

i. From the table, estimate the half-life of the substance.

Half-life=______________minutesii. How many half-lives elapsed during the 20 minute experiment?

Number of half-lives =___________________________iii. If the scientist had taken readings for 25 minutes, what might the count rate have been at the end of his experiment?

Count rate after 25 minutes =______________________counts/s

5.2.2 Characteristics of the three types of Radioactive Emissions

RadioactiveemissionsAlpha particles

Beta particles

Gamma rays

SymbolHelium nuclei or 4 He2 (2 p and 2 n)Electrons,

Electromagnetic radiation.

NaturePositive charged , helium nucleus (two protons and two neutrons)ElectronElectromagnetic waves with frequencies higher than 3 x 1019 Hz

Mass (atomic mass unit)4(high)(low)0

Charge+2 electric charges-1 electric charge0

SpeedUp to 3 x 107 m s-1(Slow)Up to 2.7 x 108 m s-1(Fast)3 x 108 m s-1 (speed of light)

EnergyFor a particular source,all particle are emittedwith the same KEFor a particular source- particle emitted havevarious KENil

In an electricfieldBends to negative plat(slight)Bends to positive plate(great)Does not bend , showing that it is neutral

In magneticfieldBends a little showingthat it has a big mass.Direction of the bendindicates that it is positively charges(slight)Bend a lot showing thatit has a small mass.Direction of the bendindicates that it ispositively charged(great)Does not bend showingthat it is neutral

IonisingpowerStrongestMediumWeakest

PenetratingpowerLowMediumHigh

Stopped byA thin sheet of paperA few millimeters ofaluminumA few centimeters oflead or concrete

Range in airA few centimetersA few metersA few hundred meters

Ionizing effect

Visible tracks in a cloud chamber for alpha particle, beta particle and gamma rays Radioactive emission has an ionizing effect The 3 types of radiation are highly energetic and use their energy to remove electrons from the air molecules when they pass through air. The ionization of an atom produces positive ion and negative ion (electron) Due to their different charges and masses, they have different ionizing abilities.not produce clear or continuous tracks due to their low ionizing power

Gamma Very fast beta particles - thin, straight tracks. The slower beta particles - short, thick tracks which curve in random direction.

Beta Dense strong ionization power straight tracks - the alpha particle has a large mass and momentum so it is not easily deflected

Alpha

Penetrating power The penetrating effect of alpha, beta and gamma radiation depends on their ionizing power. Radiation which has a stronger ionizing power will have a lower penetrating effect. The radiation emission loses some of its energy each time an ion pair is produced. Alpha particles lose energy very quick as they move through a medium. After a short distance in the medium, the alpha particles would have lost almost all energy. So alpha particles have the lowest penetrating power.

Alpha particles can be stopped by paper, beta particles go through paper but can be stopped by thin metal. Gamma rays can go through paper and metal sheet and can only be stopped by thick lead or concrete.

Interaction with electrical field Alpha and beta particles are deflected in an electric field because they are charged. The deflections are in opposite direction because they carry opposite charges. The deflection of beta is larger than alpha because mass of beta < mass of alpha Gamma rays are not deflected because they do not carry any charge.

Interaction with magnetic field

Alpha particles and beta particles are also deflected when they pass through a magnetic field while gamma rays are unaffected. The direction of the deflection of alpha particles in the magnetic field can be found using Flemings left-hand rule.

5.2.3 Radioactive Decay Radioactive decay is the process by which unstable atomic nuclei emit subatomic particles or radiation. When a radioactive nucleus decays, its nucleus breaks up, emits an alpha particle or beta particle and energy, and forms a new atom of a different element. A parent nuclide X changes into a daughter nuclide Y.

Alpha decayBeta decayGamma decay

A A-4 4 X Y + He + energyZ Z-2 2 A A 0X Y + e Z Z+1 -1 A AX X + Z Z

The daughter nuclide has 2 protons less and 2 neutrons less than the parent nuclide. The proton number Z decreases by 2 and its nucleon number A decreases by 4 A neutron changes to proton and electron during beta decay. Proton stays within the nucleus and electron is emitted as beta particle. The nucleus loses a neutron but gains a proton. Proton number increase by 1, nucleon number remains unchanged Gamma emission does not change the structure of the nucleus, it just makes the nucleus more stable. Gamma rays are emitted at the same time together with either an alpha or beta particle. When a nucleus ejects an alpha or beta particle, there is often some excess energy produced which will be released as gamma rays

230 226 4 Th Ra+ He+energy 90 88 2

238 234 4 U Th+ He+energy92 90 214 14 0 C N+ e + energy 6 7 -1

209 209 0 Pb Bi+ e + energy 82 83 -1210 206 4 Po Pb + He + 84 82 2

60 60 Co Co + 27 27

RAD I D O E SA C EC A RT Y II EV SE

Sometimes the daughter nuclide of a radioactive does is still unstable. It will eventually decay into another nuclide which is also unstable. This process continues as a radioactive decay series until a stable nuclide is reached. Each decay will emit either an alpha particle or a beta particle and may be gamma rays.

Exercise 10.2.31) Uranium -238. 238 U 92 Emits an alpha particle and disintegrates into a thorium atom. State the nucleon number and proton number of the thorium isotope formed.

2) Strontium 90.90 Sr 38 decays to yttrium-90

5.2.4 Changes in the proton number and nucleon number in Radioactive Decay A A-4 4Z Z-2 2 New element -particle

1)XR+He

Complete the equation to show the emission of alpha particlei. 212 208+PoPb

84 82

ii. 238 4 +HeU

922iii. 2224He+Rn

80 22) energy+e+RX A 0 Z+1 -1 New element -particle

Complete the equation to show the emission of beta particlei. 90 0e+Sr

38 -1

ii. 239 0e+Pa

91-1

iii. 14 0e+N

7-1

3) AA 0ZZ 0 Element element -rayhigh energy low energyenergy++XX

Complete the equation to show the combination emission of gamma ray, beta particle and alpha particle. Identify the element (emission) A, B, C, D and E in the symbolically formi. 229 225+90 88

ii. 233+ 91 92iii. 225 4+ 87 2

Exercise 5.2.41) Complete the following table about the particle in an atom. The first row has been filled in as an example.ParticleMassChargeLocation

Proton1 unit+1 unitIn the nucleus

Neutron

Electron

i. Which of the particles in the table make up of -particle?

_________________________________________________________

ii. On the same scale as indicated by the table, state

a) the mass of an -particle: ______________________

b) the charge of an -particle: ____________________

5.2.5 Detectors of radioactive emissions

NoRadioactive EmissionDetectorsReason

1Alpha, beta and gammaGeiger Muller Tube ( G.M Tube )

The radioactive emission enters the tube through the mica window and ionizes the neon gas. The electrons and positive ions are attracted towards the anode and cathode respectively. When electrons are collected by the anode, a pulse of current is produces. The pulses of current are counted by a scaler or ratemeter. The scaler gives the number of counts over a certain period of timeie counts per minute / counts per second. Initially the GM tube is switched on without the presence of any radioactive substance. The reading displayed by the ratemeter is known as the background count rates. When the GM tube is used to detect radioactive emission, the background count rate is subtracted from the count rate obtained. Alpha is positive charge, unlike charge are attracted in a strong electric field between the two electrodes. The exposure ofradiation over one week -AGM tube connected to the scaler. Minimum intensity of a radiation.-AGM tube connected to a ratemeter.Notes: Background radiation gives reading to the GM tube even though there is no radioactive source. Background radiation is always present due to natural radioactivity in the ground, bricks or buildings and cosmic radiation.

NoRadioactive EmissionsDetectorsReasons

2Beta and gamma.Photographic Films

Is worn by worker in nuclear power stations and in radiation laboratories. The badge contains a photographic film in a light-proof packet The parts of the film which had received radiation will be darkened. The degree of darkening indicates the amount of radiation the person had been exposed to.Cannot penetratethrough the metal

3Alpha and betaGold Leaf Electroscopes

When an electroscope is charged, the gold leaf sticks out, because the charges on the gold repel the charges on the metal stalk. When a radioactive source comes near, the air is ionized and starts to conduct electricity. This means the charge can leak away, the electroscope discharges and the gold leaf falls.Strong ionizing power

NoRadiationEmissionsDetectorsReason

4Alpha, beta and gammaCloud Chambers

It shows the path traveled by the ionizing radiation in air. The radioactive produces ions in the air that is saturated with alcohol vapour. The alcohol vapour condenses on the ions to make the tracks of the radiation visible. Alpha particles are best for this because it ionization power is high. Tracks of alpha thick and straight. Beta thin and twist Gamma thinner than beta.

5.2.6 Half Life The half-life T of a radioactive substance is the time for half of the radioactive nuclei to decay

Example:1) Determine the half-life from a decay curve

256 128 T T = Radioactive decay T = 3 hours

2) Pa takes 20.8 hours to shrinks from 80 g to 5 g.How many half lives are there?

80 g 40 g 20 g 10 g 5 g T T T T

(This decay process has taken the time of 4 half lives) 20.8 hours= 4 T , T = 20.8 / 4 = 5.2 hours.

Exercise 5.2.61) A radioactive of gamma rays has a half life of 4 days. A Geiger counter placed 3 m from the source initially has a count rate of 21600 per minute. After 8 days, the counter is moved back to a distance of 6 m from the source and its rate, in counts per minute is then.

2) The number of radioactive nuclides in two different samples P and Q are initially 4 N and N respectively. If the half life of P is t and that of Q is 3 t, the number of radioactive nuclides in P will be the same as the number of radioactive nuclides in Q after a time of

3) The activity of a sample of radioactive isotope decrease to of its initial value in 32 s. How much more time would be required for the activity to decrease to 1/64 0f its initial value?

4) A detector is used for monitoring and source and a reading of 240 units is observed. After a time equal to twice the half life of the source, the reading has fallen to 66 units. If a 5 mm thick lead sheet is inserted between the source and the detector, the reading would probably be

5) A counter is placed near a very weak radioactive source which has a half life of 2 hours. The counter registers 95 count/ min at noon and 55 count / min at 2 p.m. The expected count rate, in count / min , at 6 p. mm on the same day is

5.3 RADIOISOTOPES

5.3.1 The application of radioisotope

RadioisotopesUse In Mechanism

Americium -241Smoke detector Contain a weak radioactive source such as americium-241. Alpha particles emitted from the source and ionized air molecules. The ionized air molecules conduct electricity and small current flows in the smoke detector. When smoke enters the detector, the soot particles in the smoke absorb some of the alpha particles. This causes a decrease in the current that flows in the smoke detector and trigger the alarm. Americium-241 has a long half-life, 460 years so that the substance will last longer.

In Industries

RadioisotopesUse In Mechanism

Beta - rayThickness control

A radioactive sends radiation through the sheet material as it comes off the production line. Beta radiations are used for thin sheets. A radiation detector on the other side of the sheet measures the intensity of the radiation passing through the sheet. The amount of radiation received by the detector depends on the thickness of the rubber sheet. If the sheet is too thin, the reading of the detector increases. A signal is sent from the roller control to the rollers so that the pressure on the sheets can be reduced.

Cobalt-60Checking welds A radioactive substance which emits beta particles is added to a fluid in a pipeline to measure the flow rate in the pipeline and to find leaks. The radiation produced by the radioactive substance can be detected with a GM tube counter placed above ground. A larger increase in the count rate will indicate that there is leak in that area.

In Medical FieldUse InExplanation

Sterilising Medical instruments are sterilized after packing by a brief exposure to gamma rays. Gamma rays can be used to kill bacteria, mould and insects in food.

Radioactive tracers Nuclear medicine is a branch of medicine that uses radiation to provide information about the function of the specific organs of a patient or to treat disease. A radioisotope is taken in by a patient through the digestive system, by inhalation or through the blood vessels by injection. The radiation emitted enables organs such as thyroid, bones, heart, and liver to be easily imaged by imaging equipment. Disorders can then be detected. Characteristics of radioisotope:a) Half-life is medium, long enough to examine metabolic processes in the body and short enough to minimize the radiation dose to the patient.b) Emits low energy gamma rays which can pass through the body without much ionisation of the molecules.

Cancer treatment Gamma rays can kill living cells. They are used to kill cancer cells without having to resort to surgery. This is called radiotherapy. It is important to administer the correct dosage. Excessive radiation will damage too many healthy cells. Insufficient radiation will not stop the cancer from spreading.

In ArcheologyRadioisotopesExplanation

Carbon-14 Carbon-14 is a radioisotope with a half-life of 5730 years and decays by emitting beta particles. Living animals and plants have a known proportion of carbon-14 in their tissues which remains constant. When living things die, the amount of carbon-14 in their body decreases at a known rate. The amount of carbon-14 left in a decayed plant or animal can be used to tell its age.

In AgricultureExplanation

By measuring the radioactivity of the stem and leaves, scientists can find out how much fertilizer has been absorbed by the plant. Radioisotopes are used to kill pests and parasites and to control the ripening of fruits.

Exercise 5.3.11) A factory produces aluminum sheets of thickness 1 mm each. The thickness of the sheets is monitored by a gauge. A source is used in the gauge.Explain why and sources are not used in gauge.

_______________________________________________________________2) Iodine 131 is a radioisotope which decays by emitting particle and rays. It is used in hospitals to test the kidneys of patients. During the test, iodine 131 solutions is injected into the bloodstream of a patient. As the blood passes through the kidney, iodine -131 will be absorbed by the kidney and eventually excreted out of the body with the urine. If the kidney is not functioning properly, both the absorption and excretion rates of iodine-131 will decrease. A detector is placed near the kidneys of the patient to detect the activity of the radiation coming from the kidneya) Using X to denote the daughter nucleus, write down an equation for the decay of an iodine 131 nucleus

b) Explain why the particles emitted by iodine 131 fail to reach the detector

___________________________________________________________________c) The half life of iodine 131 is 8 daysi. State the meaning of half life

________________________________________________________________

________________________________________________________________

ii. For safety purpose, the activity of iodine 131 solution in the test should not exceed 1.5 x 108 disintegrations per second. .When an iodine -131 solution is prepared its activity is 6 x 10 8 disintegrations per second. How many days preparation would the solution be suitable for the test?

iii. Besides iodine -131, technetium -99 m is another radioisotopes which is also used in the kidney test. Technetium -99 m emits radiation only and its half- life is 6 hours. Which if these two sources do you think is more preferable for use in the kidney test? Explain your answer.

________________________________________________________________________________________________________________________________________________________________________________________________

5.4 NUCLEAR ENERGY5.4.1 Atomic Mass Unit (a. m. u) The atomic mass unit it the unit of mass for atoms and subatomic particles such as the proton, neutron an electron 1 atomic mass unit or 1 u is of the mass of the carbon-12 atom. The mass of one carbon-12 atom is 1.99265 x 10-26 kg1 u = kg 1 u = 1.66 x 10-27 kg 1 a.m.u = x mass of C 12 atom

Exercise 5.4.11) Determine the energy in unit eV for reaction shown below 236 88 144 1

92 36 56 0 n+energyy+4Ba+KrU

Where U = 236.04556 a.m.uKr = 87.91445 a.m.uBa = 143.92284 a.m.un = 1.00867 a.m.uc = 3.00 x 108 m s -11 MeV = 1.00 x 10-13 J

5.4.2 Mass and Energy

Relate the release of energy in a nuclear reaction with a change of mass according to the equation E = mc2

Both nuclear fission and fusion release a large amount of energy. The source of this energy is from the loss of mass in nuclear reactions. The sum of the masses of the particles before the reaction is more than the sum of the masses of the particles after the reaction. Mass defect is =sum of the masses before reaction sum of the masses after reaction Mass and energy can be exchanged one for the other. A loss of mass in a nuclear reaction means that the mass had changed to energy. The relationship between the mass and the energy:

E = mc2

Speed of light = 3.0 x 108 m/sEnergy released, in joules, J

Loss of mass or mass defect, in kg

5.4.3 Nuclear Fission Nuclear fission is a process in which a heavy nucleus splits into two or more lighter nuclei. Fission occurs when the nucleus of an atom is bombarded with a neutron. The energy of the neutron causes the target nucleus to split into two (or more) nuclei that are lighter than the parent nucleus, releasing a large amount of energy during the process.

Difference between nuclear fission and radioactive decay: The nucleus is divided into two large fragments of roughly equal mass. There is a significant mass defect. Other neutrons are emitted in the process.

Example: A slow neutron hitting a uranium-235 nucleus, causing it to split producing strontium-90, xenon-143 and three neutrons + energy. 235 1 143 90 1 U + n Xe + Sr + 3 n + energy 92 0 54 38 0 Another reaction:235 1 143 90 1 U + n Ba + Kr + 3 n + energy 92 0 54 38 0

Exercise 5.4.21) Hydrogen nuclei fuse together in the Sun. The nucleus of one isotope of hydrogen contains one proton and has the nuclide notation 1 . Other H 1 isotopes of hydrogen have the nuclide notations 2 and 3 . H H1 1

a. State the number of protons and the number of neutrons in a nucleus of each of the two other isotopes of hydrogen.

______________________________________________________________________________________________________________________________________________

b. Nuclei may fuse when they come togetheri. Explain why nuclei do not easily come together__________________________________________________________________________________________________________________________________

ii. Explain why nuclei are able to come together in the center of the Sun.

_________________________________________________________________

5.4.4 Chain Reactions

Example: Critical MassIn order for a chain reaction to occur, the sample of uranium must have a certain minimum mass known as critical mass

5.4.5 The generation of electricity from nuclear fissionElectricity can be generated from the energy released by fission reactions. A nuclear power station consists of a nuclear reactor and a generator.

Process:

A nuclear reactorIt produces tremendous amount of energy through nuclear fission.

Uranium fuel rodsThe nuclei are split by neutrons in a controlled chain reaction, releasing a large amount of energy. The energy released heats up the cold gas that passes through the reactor core.

Graphite moderatorActs as a moderator to slow down the fast neutrons produced by the fission. Slower neutrons are more readily captured by the uranium nuclei.

Boron or cadmium control rodThe boron control rods absorb neutrons. It can control the rate of fission reaction. When rods are lowered into the reactor core to absorb some of the neutrons, the rate of the fission reaction reduced.

Concrete shieldPrevents leakage of radiation from the reactor core.

CoolantTake away the heat from the nuclear reactor. Substances with high specific heat capacity such as water and carbon dioxide are used.

Heat exchangerHeat energy from the very hot gas is used to boil the water into steam

TurbinesThe turbines are rotated by the flow of steam under high pressure

Coil and magnetThe coils are rotated by the turbines. Electricity is generated by electromagnetic induction.

Justify the use of nuclear fission in the generation of electricity

5.4.6 Nuclear fusionNuclear fusion is the combining of two lighter nuclei to form a heavier nucleus, releasing a vast amount of energy during the process.

Example: Nuclear fusion is believed to be the process by which energy is released by the Sun. When two hydrogen-2 nuclei moving at high speeds collide, they can join together to produce heavier nucleus. A large amount of energy is released.

Another example: A hydrogen bomb uses the principle of nuclear fusion for its design.

5.5 THE IMPORTANCE OF PROPER MANAGEMENT OF RADIOACTIVE SUBSTANCESSources of background radiation Our bodies are always exposed to the natural background radiation. The level of background radiation is very low and is not harmful to humans.

5.5.1 The negative effects of radioactive substances When radioactive emissions strike living cells, it can cause ionization to the molecules of the cells. This may cause the cells to be killed, resulting in tissue damage. At low doses of radiation, the damaged tissues can repair itself rapidly. High doses of radiation can cause burn effects known as radiation burns. The ionization effect of radiation can also cause genetic damage to the molecules of the cells. This may lead to the formation of cancerous cells and tumor development. If the radioactive substances get inside the body, the most harmful effects come from the alpha particles because they have the highest ionization power. If the radioactive source is outside the body, the greatest danger is from gamma sources because gamma rays have the highest penetrating power. The alpha particles would not penetrate clothing and is highly unlikely to reach living cells in the body.Somatic effect Somatic effect appears in the person exposed to radiation. The seriousness of the damage depends on the dose of radiation received. Fatigue, nausea, hair loss, radiation burns, cataracts Blood disorder (leukemia), organ failure, death

Genetic effect Genetic effect affects the reproductive cells and can lead to defective offspring in the future generations of the exposed person. Birth defects, congenital defects, premature death, chromosome abnormalities, cancer in later life.

5.5.2 Handling of radioactive substances

5.5.3 The management of radioactive waste In a nuclear reactor, a very large amount of energy is produced from a very small amount of fuel. However, much of the waste is radioactive and therefore must be carefully managed as hazardous waste. Radioactive waste consists of variety materials requiring different methods of management to protect people and the environment.Low-level waste Comprises 90% of the volume but only 1% of the radioactivity of all radioactive waste. Generated from hospitals, laboratories and industry, as well as the nuclear fuel cycle. Comprises paper, rags, tools, clothing, filters Contains small amounts of mostly short-lived radioactivity Buried in shallow landfill sites. Often compacted or incinerated (in a closed container) before disposal to reduce its volume.

Intermediate-level waste Makes up 7% of the volume and has 4% of the radioactivity of all radioactive waste. Contains higher amounts of radioactivity and may require special shielding Comprises resins, chemical sludge, reactor components and contaminated material from reactor decommissioning Solidified in concrete or bitumen for disposal Short-lived waste (mainly from reactors) is buried. Long-lived waste (from reprocessing nuclear fuel) is disposed deep underground

High-level waste Consists of only 3% of the volume of all radioactive waste but it holds 95% of the radioactivity. Mainly used fueled rods or liquid waste from fuel processing Contains highly-radioactive fission products and some heavy elements with long-lived radioactivity. Generates large amount of heat and requires cooling, as well as special shielding during handling and transport. Vitrified by incorporating it into borosilicate glass which is sealed inside stainless steel containers for eventual disposal deep underground.

Exercise 5.51) In a research laboratory using radioactive materials, safety precautions have to be observed. Some of the safety precautions adopted by the laboratory are listed below. On the lines after each precaution, state reasons why it is a wise precautiona) Radioactive materials should only be picked up using long-handled tools.

________________________________________________________________________b) Food must not be taken where radioactive materials are being used.

________________________________________________________________________c) The researchers must wash their hands after the source has been put away safely.

________________________________________________________________________

Reinforcement Chapter 5Part A: Objective question.1) A positively charged ion (+1) has an atomic number of Z. Find its number of electrons and , number of neutrons Number of electrons Number of neutronsA. ZZB. Z-1ZC. Z+1ZD. Z-1 not enough information2) Which of the following is deduced from the Rutherfords scattering experiment?A. There are neutrons inside a nucleus.B. The sign of the charge of the nuclei is the same as the sign of alpha particlesC. Electrons are embedded in a nucleus.D. Electrons are light in mass compared with the nucleus.E. Electrons have the opposite charge compared with the nucleus.3) Isotopes have the same I. number of protonsII. number of neutronsIII. half lifeA. I onlyB. III onlyC. I and II onlyD. II and III only4) Which of the following best represents the magnitude of the radius of an atom?A. 10 -2 mB. 10 -6 mC. 10 -10 mD. 10 -14 m

5) Which of the following conclusions could be deduced from Rutherfords scattering experiment?I. The positive charge in an atom is confined to a very small region.II. The volume of an atom mainly consists of empty spaceIII. Electrons in an atoms are orbiting in different orbitsA.I onlyB.III onlyC.I and II onlyD.II and III only6) The proton number of an atom is the number of A. Neutrons in the nucleus.B. protons in the nucleusC. neutrons and electrons in the nucleusD. protons and neutrons in the nucleus7) A small amount of a radioactive isotope contains 72 billion unstable nuclei. The half-life of the isotope is 4 hours. How many unstable nuclei would remain after 12 hours?A. 6 billionB. 9 billionC. 18 billionD. 24 billion8) How many nucleons are in a nucleus of 30 K 19 A. 19B. 20C. 30D. 58

9) A radioactive source emits radiation that can pass through a sheet of paper but not through thick aluminum

What does this show about the radiation?A. It is alpha-particlesB. It is beta-particles.C. It is gamma-raysD. It is a mixture of alpha-particles and gamma-rays.10) A sample of a radioactive isotope is decaying. Which atoms will decay first?A. impossible to know, because radioactive decay is randomB. impossible to know, unless the age of the material is knownC. atoms near the center, because they are surrounded by more atomsD. atoms near the surface, because the radiation can escape more easily

Part B: Structure question1) Some pond water becomes contaminated by the release of radioactive waste. The radioactivity of a sample of the contaminated water is tested every week for 5 weeks. The results are shown in the table below.Time/weeks0112345

Activity count8004402401307040

a. Plot these values on graphb. Draw the best curve through your points.c. Use your graph to find the half-life of the radioactive material in the sample. Show clearly on the graph how you obtained your answer.Half-life of radioactive material = ________________ weeksd. Suggest two major sources of background radiation.i. ii.

2. a. When a nucleus decays by emitting an -particle, what happens toi. the number of neutrons in the nucleus____________________________________________________ii. the number of protons in the nucleus____________________________________________________iii. The charge on the nucleus?_____________________________________________________b. On 1st January 1900, a sample of a particular radioactive nuclide had an activity of 3200 count / min. The nuclide has a half-life of 22 years. Calculate the activity of the nuclide remaining in the sample on 1st January 1966

Activity = _____________________count/min4 | Page