RADIOACTIVITY PHYSICS NOTES, REVISION QUESTIONS & ANSWERS

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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

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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 ^m_n RF + ⁴₂He ²⁺

²⁵¹ ₉₈ Cf ²³⁸ ₉₂U + ^{…………………}

²⁸⁵ ₁₁₂ Cn ^p_q 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

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 +

^l_kTh ²¹²₉₃Np + 3+ ⁰_-1e

Gamma y -decay

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:

a) nuclear fission
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 + ⁸⁷₃₅Br + —- + 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 ^a_bHe + ¹₀ n + energy

²₁H + ²₁H ……….. + ¹₁ H+ energy

4 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

Carbon dating
Detecting leakages

Monitoring plant growth

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

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.

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.

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.

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

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

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.

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 ¹/₂changes 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)

From the graph determine
half life of the radioisotope

the mass after the 7^th day

the mass after the 20^th day

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

Plot a graph of counts per minute against time

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

From youethe graph determinecount rate after;
12 minutes

22minutes

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

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

Determne the half life of Y

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

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

b) Agriculture:

monitor plant growth by tracing the route of the radioisotope.
Radioactive phosphorus is used to determine rate of absorption of phosphate fertilizers

c) Food preservation:

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

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.

e) Dating rocks/fossils:

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

F: DANGERS OF RADIOCTIVITY.

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

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.

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).

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

number of the element formed? (1 mark)

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

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)

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

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)

2000 Q 13

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

beta (β) to from 214

(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

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

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

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)

(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)

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

2007 Q 14
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

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

with the highest mass. (1 mark)

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

b) (i) Find the values of Z₁ and Z₂in the nuclear equation below

_{Z1 1 94 140 1}

U + n Sr + X_e +2 n

^{92 0 38 Z0 0}

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

(1mark)

Time (minutes)

Give the name of the:

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

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

b) Energy change that occurs between t₃and t₄

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

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)

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

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

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

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)

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

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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