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RADIOACTIVITY PHYSICS NOTES, REVISION QUESTIONS & ANSWERS

RADIOCTIVITY

INTRODUCTION

Radioactivity is a process where an unstable nuclide breaks up to yield another nuclide of different composition with emission of particles and energy

Radioactive decay is the spontaneous disintegration/decay of a radioactive  nuclide.

Radioisotopes are isotopes which are radioactive

.

Radioactivity is a nuclear reaction and not a chemical reaction

        similarities:between  Nuclear and chemical reaction

(i)-both involve the subatomic particles; electrons, protons and neutrons in an  atom

(ii)-both involve the subatomic particles trying to make the atom more stable.

(iii)-Some form of energy transfer to the environment take  place.

Differences between chemical reactions and nuclear reactions

Nuclear reaction Chemical reaction
Takes place within the nucleus and involves neutrons and protons Takes place on the outer energy elevel and only involves valency electrons
Release large amounts of heat energy Much less energy released
Not affected by environmental factors such as temperature Are affected by environmental factors such as temperature and pressure
New element  formed No new element formed

 

 

CHARACTERISTICS OF RADIOACTIVITY

All atoms with atomic number above 82 are radioactive

Radioactivity reactions are spontaneous and produce a lot of energy

Radioactivity is not affected by external factors like temperature and pressure

Types of radiation

There are three types of radiations emitted when radioactive nuclides disintegrate

 

(i)alpha(α) particle decay

 

(ii)Beta (β) particle decay

 

iii)Gamma (y) particle decay
       I.          Is positively charged and are attracted to the negative  plate of electric field

 

is negatively charged  hence attracted to the positive plate of electric field. No charge
     II.          Has mass number 4 and atomic number 2 therefore equal to a charged helium atom ( 42he2+)

 

no mass number  and atomic number  negative one(-1)  therefore equal to a fast moving electron (0 -1e)

 

 has  no mass number  and atomic number therefore equal to   electromagnetic waves.
they show a lesser deflection by electric filed ,due to their large mass Show are greater deflection due to the lesser mass Not deflected
have very low penetrating power and thus can be stopped  a thin sheet of Have medium penetrating power and thus can be stopped  thin sheet of aluminum foil.

 

very high  penetrating power and thus can  be stopped by a thick block of lead..
       I.          have high ionizing power thus cause a lot of damage to living cells.

 

Have medium ionizing power thus cause less damage to living cells than α particle. have very low ionizing power thus cause  less  damage to living cells  unless on prolonged exposure

 

 

Alpha decay;

a nuclide undergoing α-decay has its mass number reduced by 4 and its atomic number reduced by 2

 

Examples of alpha decay

 

210 84 Pb                        206 82 Pb            + 42He 2+

 

226 88 Ra                       222 88 Rn +          42He 2+

 

 

complete the equations below

 

266 106 Sg                      mn RF     +          42He 2+

 

251 98 Cf                       238 92U   +          …………………

 

285 112 Cn                     pq Hs      +          2 42He 2+

 

z a Es                           235 93 Np +          3 42He 2+

 

288 114 Uuq                   278 104 Rf   +       ………………

 

226 88 Ra                       222 88 Rn +          42He 2+

 

beta (β) decay

  1. v) a nuclide undergoing β -decay has its mass number remain the same and its atomic number increase by 1

 

Examples of beta (β) decay

22888Ra                                     22889Ac              +  0-1e

    

  22888Ra                         22892Th             +         

 

.    23290Th                      23291Pb                          +

                     

 lkTh                  21293Np            + 30-1e

 

Gamma y -decay

 

  1. v) a nuclide undergoing y -decay has its mass number and its atomic number remain the same.

 

 

The sketch diagram below shows the penetrating power of the radiations from a radioactive nuclide.

 

 

 

radioactive nuclide      sheet of paper       aluminium foil       thick block of lead

(radiation source)          (block α-rays)         (block β-rays)        block y-rays)

 

 

 

                                                                                                                                                                                                                                                                                                  α-rays              β-rays                          y-rays

           

The sketch diagram below illustrates the effect of electric /magnetic field on the three radiations from a radioactive nuclide

 

 

 

 

Radioactive disintegration/decay naturally produces the stable 20682Pb nuclide /isotope of lead.Below is the 238 92 U natural decay series. Identify the particle emitted in each case

 

 

B:NUCLEAR FISSION AND NUCLEAR FUSION

Radioactive disintegration/decay can be initiated in an industrial laboratory through two chemical methods:

  1. a) nuclear fission
  2. b) nuclear

a)Nuclear fission

Nuclear fission is the splitting process of a a heavy unstable nuclide releasing lighter nuclides, and a large quantity of energy   when bombarded /hit by a fast moving neutron

Nuclear fission is the basic chemistry behind nuclear bombs made in the nuclear reactors.

 

 

 

 

 

 

Examples of nuclear equations showing nuclear fission

 

 

Supply the missing information to te equations below

10 n  +  235 b U                        9038 Sr  + a54Xe + 310 n  +  energy

 

10 n   +        2713 Al                       2813 Al        +   y  +   energy

 

23592 U  + 10 n                          14757 La +    8735  Br + —- + energy

 

 

10 n  +  235 b U                   10 n  + ………….. energy

 

24796Cm  + 10 n                         ……….+    10 n       + energy

 

23595U  + 10 n                            ……….+     14256 Ba +310 n    +   energy

 

 

 

 

 

 

 

 

NUCLEAR FUSION.

Nuclear fusion is the process which smaller nuclides join together to form larger / heavier nuclides releasing a large quantity of energy..

Nuclear fusion is the basic chemistry behind solar/sun radiation.

Two daughter atoms/nuclides of Hydrogen fuse/join to form Helium nuclide on the surface of the sun releasing large quantity of energy in form of heat and light

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

21H      +    21H                        abHe       +      10 n + energy

        

 

   21H       +       21H                        ………..      +    11 H+ energy

 

11H                        42He       +         ………….+ energy

 

147N     +     ………….                      178O      +           11 H+ energy

 

5324N     +      42He       .                      10n      +      …………….+ energy

 

 

 

 

Similarities between nuclear fusion and nuclear fission

 

In both  a large quantity of energy

Both processes results in chain reactions

In both cases sub-atomic particles such as neutrons accompany the peocess

 

 

Differences between nuclear fusion and nuclear fission

Nuclear nuclear fission nuclear fusion
Heavy nucleus is split to smaller nuclei Smaller nuclei combine to form heavy nucleus
Have a lower activation energy Have a higher activation energy
Produces larger  amount of energy than nuclear fusion Produces relatively lower amount of energy

 

 

 

: HALF LIFE PERIOD (t1/2)

 

The half-life period is the time taken for a radioactive nuclide to spontaneously decay/ disintegrate to half its original mass/ amount.

It is usually denoted t 1/2.

The rate of radioactive nuclide disintegration/decay is constant for each nuclide.

 

 

The table below shows the half-life period of some elements.

 

Element/Nuclide Half-life period(t 1/2 )
238 92 U 4.5 x 109 years
14 6 C 5600 years
229 88 Ra 1620 years

 

The less the half life the more unstable the nuclide /element.

The half-life period is determined by using a Geiger-Muller counter (GM tube)

.A GM tube is connected to ratemeter that records the count-rates per unit time.

This is the rate of decay/ disintegration of the nuclide.

If the count-rates per unit time fall by half, then the time taken for this fall is the half-life period.

 

 

 

APPLICATIONS OF HALF LIFE

 

  1. Carbon dating
  2. Detecting leakages
  • Monitoring plant growth
  1. In medicine to monitor plant growth.

 

 

 

 

Examples

a)A radioactive substance gave a count of 240 counts per minute but after 6 hours the count rate were 30 counts per minute. Calculate the half-life period of the substance.

If  t 1/2  = x

 

then  240         120       60        30

From 240 to 30 =3x =6 hours

=>x  = t 1/2 = ( 6 / 3 )

= 2 hours

  1. b) The count rate of a nuclide fell from 200 counts per second to 12.5 counts per second in 120 minutes.

Calculate the half-life period of the nuclide.

 

 

 

 

 

 

 

 

 

  1. c) After 6 hours the count rate of a nuclide fell from 240 counts per second to 15 counts per second on the GM tube. Calculate the half-life period of the nuclide.

 

 

 

 

 

 

 

 

 

 

  1. d) Calculate the mass of nitrogen-13 that remain from 2 grams after 6 half-lifes if the half-life period of nitrogen-13 is 10 minutes.

 

 

 

  1. e) What fraction of a gas remains after 1hour if its half-life period is 20 minutes?

 

 

 

 

 

 

 

 

  1. f) 348 grams of a nuclide A was reduced to 43.5 grams after 270days.Determine the half-life period of the nuclide.

 

 

 

 

 

 

 

 

 

 

 

 

 

  1. g) How old is an Egyptian Pharaoh in a tomb with 2grams of 14C if the normal 14C in a present tomb is 16grams.The half-life period of 14C is 5600years.

           

 

 

 

 

 

 

 

 

 

  1. h) 100 grams of a radioactive isotope was reduced 12.5 grams after 81days.Determine the half-life period of the isotope.

                       

 

 

 

 

 

 

 

 

A graph of activity against time is called decay curve.

A decay curve can be used to determine the half-life period of an isotope since activity decrease at equal time interval to half the original

 

The graph below shows the rate of decay of carbon-14

 

 

 

 

(i)From the graph show and determine the half-life period of the isotope.

 

From the graph t 1/changes in activity from:

( 100 – 50 )      =>   ( 5700 – 0 )  =  5700  years

( 50 – 25 )      =>   ( 11400 – 5700 )  =  5700 years

Thus      t ½  5700 years

 

(ii)Why does the graph tend to ‘O’?

Smaller particle/s will disintegrate /decay to half its original.

             There can never be ‘O’/zero particles

 

 

The table below shows the change in mass of a radioactive isotope with time

 

Time (days) 0 2 4 6 8 10 12 14 16 18
Mass (g) 10.0 8.7 7.5 6.2 5.0 4.1 3.4 2.9 2.5 2.3

 

 

 

On the grid provided ,plot a graph of the percentage of bismuth remaining against time.  (3mks)

  1. From the graph determine
  2. half life of the radioisotope

 

  1. the mass after the 7th day

 

 

  • the mass after the 20th day

 

 

  1. The table below shows the measurements of radioactivity in counts per minute from a radioisotope iodine-128
Counts per minute 240 204 176 156 138 122 112
Time in days 0 5 10 15 20 25 30

 

  1. Plot a graph of counts per minute against time

 

 

 

 

 

 

 

  1. Use your gaph to determine the half life of iodine-128

 

 

 

 

  1. From youethe graph determinecount rate after;
  2. 12 minutes

 

 

 

  1. 22minutes

 

 

 

 

  1. After how many minutes was the count rate ;
  • 160 counts per minute

 

 

 

  • 197 counts per minute

 

 

 

A quantity  of  44Y was monitored with a GM tube and the folllowinf results were obtained over a period of 70 minutes.

Counts per minute 800 580 427 305 225 165 122 85
Time 0 10 20 30 40 50 60 70

 

 

  1. the grid provided plot a graph of counts per minute against time.

 

 

  1. Determne the half life of Y

 

 

  • On Starting with 32g of 44Y,how much of the isotope would remain after 110 minutes.

 

 

 

 

 

 

  1. Give two applications of half life

 

 

 

 

 

 

 

 

 

 

 

 

E: APPLICATION AND USES OF RADIOCTIVITY.

The following are some of the fields that apply and use radioisotopes;

a)Medicine: -\

  • Treatment of cancer to kill malignant tumors through radiotherapy e,g colbalt-60 and caesium-137
  • Sterilizing hospital /surgical instruments by exposing them to gamma radiation.
  • -to monitor growth in bones and healing of fractures
  • For providing power in heart pacesetters

 

  1. b) Agriculture:
  • monitor plant growth by tracing the route of the radioisotope.
  • Radioactive phosphorus is used to determine rate of absorption of phosphate fertilizers

 

  1. c) Food preservation:

 X-rays are used to kill bacteria in tinned food to last for a long time.

  1. d) Chemistry:

 To study mechanisms of a chemical reaction, one  reactant is replaced in its structure by a radioisotope e.g.

During esterification the ‘O’ joining the ester was discovered comes from the alkanol and not alkanoic acid.

During photosynthesis the ‘O’ released was discovered comes from   water.

 

 

 

  1. e) Dating rocks/fossils:

Comparing the mass of 14C in living and dead cells, to determine their age,

 

F: DANGERS OF RADIOCTIVITY.

  1. Exposure to theses radiations causes chromosomal and /or genetic mutation in living cells.
  2. Living things should therefore not be exposed for a long time to radioactive substances.
  3. One of the main uses of radioactive isotopes is in generation of large cheap electricity in nuclear reactors.
  4. Those who work in these reactors must wear protective devises made of thick glass or lead
  5. Accidental leakages of radiations usually occur
  6. In 1986 the Nuclear reactor at Chernobyl in Russia had a major explosion that emitted poisonous nuclear material that caused immediate environmental disaster
  7. In 2011, an earthquake in Japan caused a nuclear reactor to leak and release poisonous radioactive waste into the Indian Ocean.
  8. The immediate and long term effects of exposure to these poisonous radioactive waste on human being is of major concern to all environmentalists.

 

Control

Proper use,storage and disposal of radioactive materials

Regular checks of equipment which emit radiations

 

            Revision quiz   RADIOACTIVITY

 

 

 

  1. 1993 Q P1A 7

The Table below gives the rate of decay for radioactive element Y.

 

Number of days Mass (g)
0 384
270 48

 

Calculate the half-life of the radioactive element Y.

 

 

 

  1. 1995 P1A Q30

(a)      100g of radioactive   23391 Pa    was reduced to 12.5g after 81 days.

Determine the half-life of Pa.                                                                   (2 marks).

 

 

 

  1. b) 23391 Pa decays by Beta emission. What is the mass number and the atomic

number of the element formed?                                                  (1 mark)

 

 

  1. 1996 P1A Q 20

Complete the diagram below to show how α and β particles from radioactive can be

distinguished from each other. Label your diagram clearly.                         (3 marks)

 

 

 

 

 

 

 

 

Source of radiation                                           Paper                            Metal foil

 

  1. 1997 P1A Q 7

M grammes of a radioactive isotope decayed to 5 grammes in 100 days.

The half –life of the isotope is 25 days.

(a) What is meant by half-life?                                                                            (1 mark)

 

…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

(b) Calculate the initial mass M of the radioactive isotope.                     (2 marks)

 

………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

  1. 1998 P1A Q1

An isotope of Uranium 234 94U  decays by emission of an alpha particle to thorium. Th.

 

 

(a). Write the equation for the nuclear reaction undergone by the isotope.        (1 mark)

 

……………………………………………………………………………………………………………………

(b). Explain why it is not safe to store radioactive substances in containers made from

Aluminum sheets.                                                                                           (1 mark)

………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

  1. 2000 Q 13

A radioactive isotope X2 decays by emitting two alpha (a) particles and one

beta (β) to from  214

Bi

83

(a) What is the atomic number of X2?

…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

 

(b) After 112 days, 1/16 of the mass of X2 remained. Determine the half life of X2

 

…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

  1. 2002 Q 10

                The graph below represents a radioactive decay series for isotope H.

Study it and answer the questions that follow

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(a) Name the type of radiation emitted when isotope H changes to isotope J.

 

……………………………………………………………………………………………………………….

(b) Write an equation for the nuclear reaction that occur when isotope J changes to isotope K

 

 

 

……………………………………………………………………………………………………………………

  1. c) Identify a pair of isotope of an element in the decay series

 

…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

 

100 g of a radioactive substance was reduced to 12.5 g in 15.6 years.

Calculate the half – life of the substance.                                                        (2 marks)

 

………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

           9.

(a)  Complete the nuclear equation below.                                                               (1 mark)

 

37 18A…..                   3719B  +………..

 

 

…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

(b)     State one:

(i)         Use of radioisotopes in agriculture                                                                    (1mark)

 

……………………………………………………………………………………………………………….

(ii)        Danger associated with exposure of human beings to radioisotopes           (1 mark)

 

……………………………………………………………………………………………………………….

  1. 2007 Q 14
  2. a)    Distinguish between nuclear fission and nuclear fusion.                                 (2 marks)

 

……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

Describe how solid wastes containing radioactive substances should be disposed of.                                                                                                                                                         (1 mark)

 

……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

11,.          2008 Q 24

  1. a) A radioactive substance emits three different particles. Give the symbol of the particle

with the highest mass.                                                                                              (1 mark)

…………………………………………………………………………………………………………………..

  1. b) (i)  Find the values of Z1 and Z2 in the nuclear equation below

                                        Z1                1                  94                140              1

U   +  n           Sr   +     Xe +2 n

92                0                  38                Z0                0                 

 

 

 

 

 

  1. ii) What type of nuclear reaction is represented in represented in b (i) above?

(1mark)

 

 

 

 

 

 

 

 

 

 

 

Time (minutes)

Give the name of the:

  1. a) Process taking place between t0 and t                                                   (1mark)

 

……………………………………………………………………………………………………………………

  1. b) Energy change that occurs between t3 and t4

 

                    ……………………………………………………………………………………………………………….

  1. 2009 Q 6d P2

   (d) Naturally occurring uranium consist of three isotopes which are radioactive.

Isotopes            234 u                235u                 238u

Abundance       0.01%               0.72%               99.27%

(i) Which of these isotopes has the longest half-life? Give reasons.                      (1 mark)

 

…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

(ii) Calculate the relative atomic mass of uranium.                                                  (2 marks)

 

……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

(iii) 235 92U is an alpha emitter .If the product  of the decay of this nuclide

is thorium (Th) .Write a nuclear equation for the process.                    (1 mark)

 

 

……………………………………………………………………………………………………………….

  1. iv) State one use of radioactive isotopes in the paper industry (2 marks)

 

………………………………………………………………………………………………………………………………………………………………………………………………………………………………..

 

 

 

 

 

 

 

  1. 2011 Q 2

            Complete the nuclear equation below:

131          131

I       Xe +

53             54

The half life of   13153 I   is 8 days.

Determine the mass of    13153I remaining if 50 grammes decayed for 40 days.

………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

Give one harmful effect of radioisotopes.                                                 (1 mark)

……………………………………………………………………………………………………………….

  1. 2012 Q9 P1

120g of iodine – 131 has a half life of 8 days decays for 32 days. On the grid provided,

plot a graph of the mass of iodine – 131 against time.                                            (3 marks)

 

 

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