year 11 gcse physics unit 1 - wordpress.com · year 11 plum pudding model gcse physics unit 1 in...

Year 11 GCSE Physics Unit 1

Radioactivity By the end of this section you should be able to: 1.4.1 research the historical development of the model of atomic structure

from the 'plum pudding' model to the present Rutherford-Bohr model (w 0)b, (iv)c);

1.4.2 describe, in outline, the Rutherford alpha-particle scattering experiment and its principal results (tv - (i)c);

1.4.3 explain how the evidence provided by the Rutherford alpha-particle scattering experiment led to the 'plum pudding' model of the atom being replaced by the Rutherford-Bohr model (w - (iv)c):

1.4.4 describe the structure of atoms in terms of protons, neutrons and electrons;

1.4.5 recall the relative charge and relative mass of protons, neutrons and electrons;

1.4.6 describe a nucleus in terms of atomic number Z and mass number A, using

the notation X (iv - (iii)c); and

1.4.7 explain what an isotope is.

The Atom

Atoms are made up of 3 types of particle. The proton, the neutron and the

electron. The protons and the neutrons are found in the nucleus (a very small

dense area in the centre of the atom). The electrons orbit around the nucleus.

Particle Relative Charge Relative Mass

Proton \

Neutron o 1

Electron -A

The idea of atoms is an old one -it began n Ancient Greece, about 2500 years

ago. (The word 'atom' means 'indivisible' in Greek.) I t was only in 1919 that

Rutherford carried out an experiment which gives us our modern day picture of

the atom. Before that, many different scientists had ideas about what as atom

might be like.

2 *

Year 11

Plum Pudding Model

GCSE Physics Unit 1

I n 1803, John Dalton argued that the idea of atoms could explain the

differences between elements. He thought of atoms as like tiny solid billiard

balls that could not be broken up. This was his 'model' of an atom.

From about 1870 onwards many scientists studied cathode-ray tubes. I n 1895 in

Paris, Jean Perrin showed that the cathode rays were negatively-charged

particles. I n 1897 in Cambridge, J . J . Thomson managed to make measurements

of these particles, which he called electrons. From his results it seemed that

the electrons were smaller that atoms (about 1/1840 of the mass of the lightest

atom, hydrogen).

I n 1904 Thomson suggested a new 'model'

the atom. This was his 'plum-pudding' model

or 'currant-bun' model. He thought the

negatives electrons were embedded in a

positive blob of matter.

Nuclear Model

of electrons (the plums)

Positive matter (the pudding)

I n Manchester, Ernest Rutherford suggested an experiment to f ire alpha

particles at tin gold foil. From the results, Rutherford suggested in 1911 that

atoms have positive nucleus (with most of the mass), surrounded by the negative

electrons.

I n 1919, Rutherford found a way of changing atoms

and showed that protons existed. I n 1932 James

Chadwick discovered neutrons, which helped to

explain isotopes. Since then many other

have been discovered.

particles

19

Year 11

Structure of the Atom

GCSE Physics Unit 1

Mass number - sometimes called the nucleon number (number of protons and neutrons in nucleus)

Atomic number - sometimes called the proton number (number of protons = number of electrons)

Why are atoms electrically neutral?

H o , praYryr * n o . A<*rVrc-r\<^

3 Li

Definition of isotope:

Definition of ion: oYnr^n 4Wi f V->ce>

Questions:

1. Complete the table:

Symbol No. of

protons No. of

neutrons No. of

electrons Atomic No. Mass No.

2 2 2 1 L-f

12r (o a m 6 8 11 i

8 §- IV=> l> to

8 8 17

ItFe 2 u O f 5 u

2b 30 2 U 26

2. Which of the atoms in question 1 are isotopes of the same element? 1 2 r 14 r


Research:

Find out how the I r i sh Physicist S . J . Stortey contributed to research on the

structure of the atom.

Alpha Scattering Experiment

Rutherford conducted an experiment where he fired alpha particles at thin gold

foil. Detectors were used to find how the alpha particles were scattered..

2 £ r-f-WcA f\-rYrr>r\<=K

'cZarry^X^: -P 33 found ^ pcxAc^

source of M-particles

Most of the particles went straight through the foil but some of them were scattered

back towards the source. This is described as the equivalent of a machine gun being fired

at tissue paper! You wouldn't expect any of the bullets to come back - but with the gold

foil that what happened.

Results

• Most particles passed straight through - /oV-ryv^ >-W->OIP rvnn=Hi i nf.

Some particles had deflected slightly ° 3 r

• Very few particles came straight back - m ^ V ' V- V -^vv«?»ti^i^r\ x

<2Pl


Atomic Structure and Isotopes

Q l The diagram opposite shows the particles that constitute an atom. * *

a) Name the particles labelled A, B and C.

b) What stops the electrons from flying away from t ^nucleus?

c) How many neutrons are there in the nucleus if there are 16 nucleons in this atom? <g

Q2 The following paragraph describes the structure

neuWo

of an atom. Copy and complete.

All atoms consist of a

positive charge and

r\t \c~\<&, and a number of t^]^*r~v-vr>^\ » The

is made up of p r ^ W - n - J and neutrons. pr ,—VT-TY^ have a are electrically neutral. Most of the :

of the atom is concentrated here but rt takes up a relatively small ',/rJome The (^\f~rY/ orbit the <-M iri&x t3 • They carry a negative charge (and are

really really ^ T W D A X a proton or neutron is about '/^.u r,

the proton are almost -tV\g ?y . /v\g .

) . The ratio of the mass of an electron to the mass of . The masses of the and

Q3 Complete the table opposite which summarises the relative mass and electrical charges of the sub-atomic particles.

Particle Relative Mass

Eler.frio Charge

ProfOn 1 \ + V; Neutron 1 . o

Electron 3

Source

Thin Gold Leaf

Q4 The diagram below shows the apparatus used by Lord Rutherford to probe the structure of the atom.

a) Name the particles that are directed at the gold f j i l . ptyoftAzs

b) Why does this apparatus need to operate in a vacuum? QS> t*. pa/he ld Cr\u peneyvcxX'A

c) Which of the detectors measures the highest count rate? yC dt/sortaed

d) Some particles are detected at Y. Explain this re< roôa observation using your knowledge of atomic -^ 'dp ' . structure. a x ' e o S i s -KJe ce repei\«c>

+-oe oc pcx /V \c ie3 e) Just a very small fraction of the incident particle*

are scattered more than 90° by the foil (some of these are detected by detector Z). What does this tell you about the nuclei of the gold atoms? s«"wa\\

f) Gold was chosen as the target for this experiment Give a reason for this choice. <gîd vje/»-j "^voêable .

M > f ^ M > f ^ Detector X m > Detector X m >

g) Explain why a gaseous target would be unsuitable

•ko/cszY CXrA TYXCLS^ r-oV- r e a c h Physics for CCEA Questions 1 - 3 . Page 63

^ded^cVcK ex ten t -VWdc

3>l


Radioactive Decay By the end of this section you should b£ able to: 1.4.8 recall that some nuclei are unstable and disintegrate emitting alpha, beta

or gamma radiation randomly and spontaneously, and that such nuclei are described as radioactive;

1.4.9 recall that alpha particles are helium nuclei consisting of two protons and two neutrons, beta particles are fast electrons, and gamma radiation is an electromagnetic wave of high energy;

1.4.10 describe nuclear disintegrations in terms of equations involving mass numbers and atomic numbers (w - (Hi)c):

1.4.11 recall, through demonstrations of computer simulations, the range of alpha, beta and gamma radiations, that alpha radiation is stopped by a few centimetres of air or a thin sheet of paper, that beta radiation is stopped by several metres of air or a thin sheet of aluminium, and that gamma radiation easily passes through all of these but can be blocked by lead;

1.4.12 know what background activity is its source and how it is taken into account when measuring activity;

1.4.13 know what ionisation is and recall that radioactive emissions cause dangerous ionisations and the steps taken to minimise the risk to those who use ionising radiations;

Alpha Decay

An alpha-particle is a helium nucleus, written \He or \

I t has 4 nucleons: 2 p^dfcy-a ^ 2_

Alpha particles are n/7 iVivJe\vj charged and have a relative charge of -t-2.

Beta Decay

A beta-particle is a fast moving electron , written "<? or XP

Beta particles are D g y h u M i j charged and have a relative charge of - ;

Gamma Decay

A gamma ray is a very high energy electromagnetic wave, written or

Gamma rays have no r.Vno./'y o r v

Year 11 SCSE Physics Unit 1

Nuclear Stability

Some nuclei are stable and some are radioactive.

Definition of radioactive nuclei: / ir^^Yr^e n i , y j p i n -H qV PmiV-rs

_ [o cf ^ CO,c\\rb\r^

The graph shows the line you get if you plot the number of neutrons against the

number of protons for stable nuclei.

Stable Nuclei

• At f i rst the number of neutrons is equal to the number of protons. I f all

nuclei were like this, the graph would follow the dashed line.

• Bigger atoms tend to be less stable. To be more stable they need more

neutrons, this is why the graph rises above the dashed line.

Unstable Nuclei

• The shaded areas show the nuclides that are unstable.

• As they decay radioactively, they form new nuclides that are always closer

to the main line of stable nuclei.

#-* «~< o

I \> Z <+i o —H

N u m b e r of F ro tons ( Z )


Radioactive Decay

When an unstable nucleus decays it emits an alpha particle, a beta particle or a

gamma ray. When this happens, the process is random and spontaneous.

The decay being random means that it is impossible to know which individual

nucleus will decay next.

The decay being spontaneous means that the nucleus will decay when it decides

to - it is not possible to make it decay (e.g. by heating or applying pressure to it)

or stop it from decaying.

Remember it is the nucleus that decays, not the atom itself. When the nucleus

decays it will change into the nucleus of another element because the number of

protons and neutrons inside it will have changed.

Radiations and Electric fields

Radioactive source

pi. Mark on the positive and negative terminals to the electric plates and label which line represents a, $ and y radiation.

^rvOSS ma'ams

Being positively charged, alpha particles are deflected by both magnetic and

electric fields. Having a negative charge, beta particles are also deflected by

both types of field but in the opposite direction to that of alpha particles.

Gamma rays have no charge so are unaffected by magnetic and electric fields

314


Radioactive Decay Equations

When a nucleus of a radioisotope (radionuclide) emits an alpha-particle it loses 4

nucleons: 2 protons and 2 neutrons.

222 86 Ra 218

84 Po + 'He

When a nucleus emits a beta-particle, it emits an electron. This happens

because a neutron has changed into a proton and an electron. The proton stays

in the nucleus and the electron is emitted at high speed as it is too energetic

and electrons are not permitted in the nucleus.

"Be +

When a nucleus emits a gamma ray the nucleus does not change as no particles

are emitted, only excess energy.

Questions:

For each of the following, write a balanced nuclear equation

1) 2Hu decays by a-emission

2) ™Th decays by a-emission ^ T t ^ - J 5 ^ £ c + 1*

112

3) 284 decays by a-emission

4) 2

8

27Fr decays by fb-emission

5) 289.4c decays by (S-emission

6) ™Th decays by ^-emission ^ T u + - ^

^ A e -U> ^ TV> f : p

35

The Periodic Table of the Elements

3 4 5 6 7 0

(V (2) Key

1.0 H

hydrogen 1 (13) (14) (15) (16) (17)

4.0 He

helium 2

6.9

Li lithium

3

9.0

Be beryllium

4

relative atomic mass atomic number

name atomic (proton) number

10.8 B

boron 5

12.0 C

carbon 6

14.0 N

nitrogen 7

16.0 O

oxygen 8

19.0

F lluorine

9

20.2 Ne neon

10 23.0

Na sodium

11

21.3

Mg magnesium

12 (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)

27.0 Al

aluminium 13

28.1 Si

silicon 14

31.0 P

phosphorus 15

32. t s

sulphur 16

35.5 CI

chlorine 17

39.9 Ar

argon 18

39.1 K

potassium 19

40.1

Ca calcium

2 0

45.0 Sc

scaiKtium 21

47.9

Ti titanium

22

50.9 V

vanadium 23

52.0 Cr

chromium 24

54.9 Mil

manganese 2b

55.8

Fe iron 2b

58.9

Co cobalt

27

5B.7 Ni

nickel 28

63.5

Cu copper

29

65.4

Zn zinc 30

69.7 Ga

gallium 31

72.6 Ge

germanium 32

74.9

As arsenic

33

79.0 Se

selenium 34

79.9 Br

bromine 36

83.8 Kr

krypton 36

85.5 87.6 B8.9 91.2 m q 95 9 98.9 104 1 103 9 106 4 107 9 119/1 ; ; • ] ; : 1 ID 7 HO? C Rb

rubidium 37

Sr strontium

38

Y yttrium

39

Zr zirconium

40

Nb niobium

41

Mo molybdenum

42

Tc technetium

43

Ru ruthenium

44

Rh rhodium

45

Pd palladium

46

Ag silver

47

1 IgiH

Cd cadmium

48

t I '(.<.>

In indium

49

Mo./

Sn tin 50

121 8

Sb antimony

51

\r t .O

Te tellurium

52

126.9 «

iodine 53

131.3

Xe xonon

54 132.9

Cs caesium

55

137.3 Ba

barium SB

138.9 La *

lanthanum 57

178.5 Hf

hafnium 72

180.9

Ta tantalum

73

183.9

w tungsten

74

186.2 Re

rhenium yfi

190.2 Os

osmium 7fi

192.2

lr iridium

77

195.1 Pt

platinum 78

197.0

Au gold 79

200.6

Hg mercury

on

204.4 TI

thallium

207.2 Pb lead

209.0 Bi

bismuth

210.0 Po

polonium

210.0 At

astatine

222.0 Rn

radon

1284} Bh

bohnium 107

192.2

lr iridium

77

195.1 Pt

platinum 78

197.0

Au gold 79 00 8 I 82 83 84 85 86

[223.0]

Fr tranciuin

87

|22G.0| Ra

radium 88

1227] Ac |

actinium 89

[2011 Rf

rutherfordium 104

[262]

Db dubnium

105

[266]

Sg seaborglurn

106

1284} Bh

bohnium 107

[27/] Hs

hasslum toe

[268] Mt

meitnerium 109

[271] Ds

darmstadium 110

[272|

Rg roentgertium

111

Elements with atomic numbers 112-116 have be not fully authenllcad

en reporter. I but

* 5 8 - 7 l Lanthanktes

t 9 0 - 1 0 3 Actinides


t 9 0 - 1 0 3 Actinides

140.1 Ce

cerium 58

140.9 Pr

praseodymium 59

144.2 Nd

neodymlum 60

144.9

Pin promethium

61

150.4 Sm

samarium 62

152.0

Eu europium

63

157.3 Gd

gadolinium 64

158.9 Tb

terbium 65

162.5 Dy

dysprosium 66

164.9 Ho

holiuium 67

167.3 Er

erbium 08

168.9 Tm

thulium 69

173.0 Yb

ytterbium 70

175.0 Lu

lutetium 71


t 9 0 - 1 0 3 Actinides 232.0 Th

thorium 90

231.0

Pa protactinium

91

238-0

u uranium

92

237.0 Np

neptunium 93

239.1

Pu Plutonium

94

243.1

Am amerioiurn

95

247.1

Cm curium

96

217.1 Bk

foOlMilllTl 97

252.1 Cf

californium 98

[252]

Es einsteinium

99

[257] Fm

fermium 100

[258] Md

mendeleviuin 101

[259] No

nobelium 102

[260] Lr

iawrenciuni 103

Year 11 GCSE Physics Un i t l

Decay Equations

Fill in the missing information for each of the disintegrations.

1 226 „ R a 2 l l R n +

O 210 D

2. MPo -> Pb + \a

3. 14C->

—> Ac "f" 2 oc

2U3>

95 ^ -> 2 3 9

6. 711

8 >

223

7. u692Sm -> + 4a

8. ^Pm -> 14.5

62

loi -> 245 99

10. l£L& -» ™Cf + 152, > K

11. Write a nuclear equation to represent the following decays.

a) Caesium-120 decays to form Barium ss ofcs

b) Samarium-146 decays to form Neodymium lot

8S c) Astatine-198 decays to form Radon

d) Plutonium-239 decays to form Uranium

e) Gadolinium-150 decays to form Samarium

f ) Lead-211 decays to form Bismuth

g) Astatine-218 decays to form Radon

h) Polonium-214 decays to Lead PQ

A t —o

s Zi> 63

2-l« 8S

1^ A J d 2_ 6V.

3*?

A t —> 2IC,

2 ' 8 „ 8U -•P

M 7.'


The Decay Series of Uranium Uranium-238 is one of the radioactive elements in the earth's interior that contributes to the immense amount of heat inside the earth. Below is the decay series for a U-238 nucleus, snowing all of the steps of decay that occur before it finally reaches a stable state. Fill in all blanks of either the type of decay that occurs to get from one step to another, or with the name of the isotope produced at a certain step.

U 238 Th-234 Pa 234 U 234 U 238

alpha

Th-234

beta .

Pa 234

. t >eta .

U 234

Po 218

\ 1

Po 210

_ alpha j

Po 210

_

Bi 210

Physics for CCEA Questions 4 - 6 , Page 15

Year 11 GCSE Physics Un i t l

Ionisation Ability

All types of radioactivity form ion. They are called 'ionising radiation'.

When alpha particles, beta particles or gamma rays collide with a material they

can knock an electron off an atom in the material forming an ion.

The ability of radioactivity to form an ion depends on its mass, the larger the

mass the greater the ionising ability. This means c*. - /M/hcA^s are

the most ionising and Y - <"^J^ a r c the least ionising.

One alpha particle can ionise 10 000 atoms. However, because it puts all of its

energy into atoms, it very quickly runs out of energy. This is why alpha particles

cannot penetrate through much.

Radioactivity can be detected because it forms ions.

Questions

When radiation travels through matter it can cause ionisation.

a) Explain what is meant by the term "ionisation'1? - rOcKaKcn -sW^ps co-N oY<=r*y

The diagram below shows a simplified drawing of an experiment to demonstrate that radiation can ionise matter.

o(_ cx^ e\ecV-/cn

The space between the plates is filled with argon gas at low pressure. A current is measured.

b) Name the two different particles formed when ion &• radiation from the source ionises an argon atom. e^cWcr\

c) Describe how this leads to a <z~ p i c x ^ r o current in the circuit. 4 u e ,io>* Source

Mefal Plates

d) The argon gas is removed from between the plates, leaving a vacuum behind. Explain why there is now no current flow. ^ ^y-^y^

teniae.

3^


Detecting Radiation

Photographic Film - <<, $ "o"

All three types of radiation will blacken

photographic film

Gold-leaf Electroscope - cx

A charged leaf will fall when a radioactive source

is brought near to it because the air near the

source will be ionised. A negative leaf will attract

the positive ions and so will be discharged.

Spark Counter -

A spark counter is a thin wire near a piece of

metal gauze. A high voltage between the wire

and gauze is adjusted until it is almost, but not

quite, sparking. A radioactive source brought

near to it will ionise the air making the air a

better conductor allowing sparks to be

produced.

Geiger-Muller Tube (G-M Tube) <] j2>, #

This is a metal tube with a thin wire down

the centre. I t contain gas at low pressure.

I t works on the same principle as the spark

counter, but the voltage is lower so that no

spark is produced. Instead a pulse of current

is produced. This is amplified and passed to a

scaler which counts the pulses.

radioactive source in tweezers

?\ ions

leaf falls

radium

G-M tube

thin " window

Up

scaler ratemeter

loudspeaker

Year 11

Penetration Power

GCSE Physics Unit 1

The ability of radioactivity to pass through materials is called its penetrating

ability. I t depends on the size of the radioactive particle.

The bigger the particle, the more likely it is to collide with the atoms of the

material. The collision will stop the particle going through the material. With

every collision the particle loses energy until eventually it has no KE left.

Alpha particles are the biggest and are least able to penetrate a material.

Paper will stop them and even in air a-particles only travel for a few centimetres

before they are stopped.

Beta particles are stopped by a few millimetres of aluminium.

Gamma rays are the most able to penetrate and will even find their way through

metres of concrete. Gamma rays are only reduced by lead; 2.5cm of lead will

only reduce their intensity by about 50%.

a-source

^-source

y-touroe

paper aluminium lead ( - m m ) (2.5 cm)

ct>/4bc-vOj

31

J


Finding the range of beta particles in aluminium

Beta particles are particles that can ionise materials through which they pass

and they will continue to move through these materials until they have

completely used up all the energy they had when they left the nucleus. In this

experiment we will look at the thickness of material needed to absorb the

electron, in other words to take away all of its energy.

Method

Safety Precautions Needed

k l g f f> •=*r-\ / C g ^ r^oVv-kM^€».J IQOOH<? go l ^ n r ! t x \ 4 U ^-A^j Q g W i A H

Count Rates:

cow/7i rate = total counts

time taken

Example:

A radioactive rock registers 2094 counts in 3 hours, the count rate is:

2094

count rate = = 689 counts per hour

3

4 1


Corrected Count Rate:

This is when we remove background coun

radioactive source.

Example:

A student finds that her antique watch ii

paint?) A Geiger Counter records a tota

the count rate, assuming a background o1

count rate = —^— = \Q5cpm

corrected count rate = measure

corrected count rate = 1 0 5 - 2 0

(You can use counts per second or counts

Background Radiation

Everyday of our lives we are exposed to i

the time this is completely harmless as i1

Sources of Background radiation are:

•

r from that \

> radioactive

of 210 coun

20cpm?

d rate - bad

= &5cpm

per minute <

-adiation tha

arises from

which came from our

(must be the luminous

ts in 2 minutes. What is

iground rate

is well)

t is all around us. Most of

natural sources.

-r-A*=S

0 J J f Y ¥ r l i m \ - > c - rrvi « , , c T -=zcrr-

—>

4 \


Dangers of Radiation

Radiation is harmful because of its penetrating nature and its ability to ionise.

Your body is a finely tuned machine, designed to carry out complex chemical

reactions between neutral atoms. I f you start turning those neutral atoms into

ions, suddenly the reactions don't work. Radiation will also damage living cells

and the DNA inside them; if this happens the cells may become cancerous.

I n order to quantify the effects of ionising radiation on tissue we define a

quantity called the absorbed dose. The absorbed dose is the energy absorbed

per kilogram of tissue. I t is measured in units called grays (1 gray = 1 J/Kg).

The greater the dose of radiation a cell gets, the greater the chance that the

cell will become cancerous. However, very high doses of radiation can kill the

cell completely. We use this property of radiation to kill cancer cells.

The sievert is another unit for dose of radiation. A dose of 1 Sv all at once wil

make you sick but if you receive this dose over a long period of time it will cause

less damage to your body.

How radioactive are you?

0.05//Sv - Sleeping next to someone

0.1^/Sv - Eating one banana

5/JSV - Dental X-ray

10/ySv - Background dose received by an average person on an average day

70/y5v - Living in a stone, brick or concrete building for 1 year

lOOj/Sv - Chest X-ray

lOmSv - Average CT-scan

36mSv - Smoking 1.5 packs a day for 1 year

lOOmSv - Lowest 1 year dose clearly linked to increased risk of cancer

2Sv - Severe radiation poisoning, sometime fatal

8Sv - Fatal dose

50Sv - 10 mins next to Chernobyl reactor core after explosion and meltdown

4 4 r


Radiation and the Body

How dangerous each type of radiation is to the body depends on

whether it is inside or outside the body.

Inside the body: Alpha radiation is most dangerous because it is easily

absorbed by the cells. Beta and gamma radiation are not as dangerous because

they are less likely be absorbed by a cell and will usually just pass Straight

through it.

Outside the body: Alpha radiation is least dangerous because it is unlikely to

reach living cells inside the body. Beta and gamma radiation are more dangerous

because they can penetrate the skin and damage the cells inside.

Radiation Badge

Whilst radioactivity can be used to treat cancer cells, it is also a major cause of

cancer. Consequently people who work with radioactivity have legal limits imposed on

them on how much daily exposure is acceptable. This is checked by the use of a small

badge which contains photographic film that blackens when radiation

is incident on it. The more radiation the badge receives the darker

the film becomes when it is developed. To get an accurate measure

of the dose received, the badge contains different materials that

the radiation must penetrate to reach the film. These

may include aluminium, copper, lead-tin alloy and plastic

There is also an open area at the centre of the badge.

photographic film sealed in thin plastic

How do people working with radioactivity protect themselves?

poVrf^iv yrv^ ind^xk, - p^^rVr? nrpiof^ -rh^ritH QA PQ./

4 5

Questions: Total [20]

1. Name a detector that can detect all 3 types of radiation.

U - M £ u h £ , p W V ^ r y ^ h U r f\\r^ ; r\r\\r\ cW\/v\)oe>/ [1]

2. What is meant by the mass number of an element? [1]

3. Uranium is radioactive, what is meant by radioactive?

4. What speed do gamma-rays travel at?

5. What is an ion?

6. What is an isotope?

[1]

[1]

ir +UQV Wn^ ^ r v ^ n o prdronQ [1]

7. What is a beta particle? d d p no . or:

8. What is an alpha particle?

[1]

[1]

9. What type(s) of radiation is/are the most dangerous when outside the body? Explain your answer : K

[2]

10. What type(s) of radiation is/are the most dangerous when inside the body? Explain your answer :

[2]

11. Radioactive particles can be harmful to living cells. a) Which type of radiation can be harmful to living cells?

rX , fi , X b) What process usually has to happen for damage to occur?

[1]

[1] c) Why are cells that have been slightly altered so dangerous?

d) Wnat do we call the condition commonly caused by tliese cells? [1]

12. Give the symbols for alpha, beta and gamma radiation (include the mass and atomic numbers). g


Working with Radioactivity By the end of this section you should be able to: 1.4.14 through mathematical modelling, based on demonstrations or computer

simulations, explain the meaning of the term half-life, carry out simple calculations involving half-life and be able to determine half-life from appropriate graphs; and

1.4.15 describe some uses of radioactivity in industry, medicine and agriculture f> - (iv)a).

Half Life

Radioactivity does not last forever. Once an atomic nucleus has decayed, it is

not the same. A radioactive rock will contain many billions and trillions of atoms,

so the number of possible decays is vast.

As a radioactive sample decays, the number of non-decayed nuclei is less than

before. So the number of decoys in a given time (count rate) will fall.

Scientists find it useful to talk about the half life of a radioactive sample. This

is a measure of time.

The half life of a radioactive sample is \\rr,<=> -\p\r_g^ rn Vvl^ 4W>

or -, -. • ; , . • • • • • :

i c k u n f c j to (H\ to W n \ r iV.T

The half of some radioactive sources is very large indeed. I t all depends on the

type of element.

Sample Half life Th-90 14 060 000 000 years U-238 4 471 000 000 years C-14 5 730 years Am-241 423 years Na-24 15 hours Mt-109 30 min Pa-234 70 sec

4t


I t is possible to find out the half life of a radioactive substance from a graph of

the count rate against time. The graph shows the decay curve for a radioactive

substance. What is the half life of the substance?

Half l ife= Ida

Time (Days)

I f we know the half life of an isotope we can work out how much of that isotope

will be left after a certain amount of time.

Example:

The half life of H-3 is 4500 days. I f we had 12 grams to begin with, how much

would there be after 13 500 days?

G

e S

3 @ * p © L S

I f we know how much time an isotope has been left for, we can work out what

the half life is for it.

Example:

I f we started with 120g of Pa-234, and had 1.88g after 7 minutes, what is the

half life? \2o^ -H? k<3cj —s>

W>\p life.

Year 11

Questions:

GCSE Physics Unit 1

1. Strontium-90 has a half life of 28 years. I t is a fb emitter and may be

absorbed into human bone. How much time must pass before its activity falls

to 1/32 of its original value? Why would it be dangerous in our food chain?

2. While animals are alive the proportion of Carbon-14 in them remains

constant. But once they die the C-14 decays. Suppose a modern bone

contains 80 units of C-14, and an old bone contains just 10 units. How old is

the bone if C-14 has a half life of 5700 years?

3. A sample of Bismuth-214 has an activity of 64 becquerel (that is 64 atoms

decay in 1 second). I t has a half life of 20 minutes. What is the activity after

a) one half life?

b) one hour?

c) two hours?

I4. Plot a graph using the data of question 3.

5 . A patient suffering from cancer of the thyroid gland is given a dose of

radioactive iodine-131, with a half life of 8 days, to combat the disease. He

is temporarily radioactive and his nurses must be changed regularly to

protect them. I f his radioactivity is initially 4 times the acceptable level,

how long is it before the special nursing rota can be dropped?

5) 4 * accep^y^- ^e)

apbb 1 ba\p I'pe - 9 1 x a c e

QpYsu 2 h a \ ( C ^ ( X - - 5 Ixj a c c

Lauei

48

0 !4 2>2_

S Vx>\p h u e s . • 2 3 v^r^ , x S

r b e n e r ^ n c ^ v ^ X j o >s LA_>e/"e r e > ^ c g V \ s i ^ v - ^ c i e .

5 o )

i p / t x i o c x c K u < a u s c r - c p s s i m r v W / t u J - H Q J O C e l l s o e d d

2) %o —=? ^ 4 0 —^> 2 o (3)

t o

b u - ^ 5 1 \U= 15 = 2 U 1 ^ I f c ^

2o qo "So \CO 12-0


T o f i n d t h e ha l f - l i fe o f P r o t a c t i n i u m - 2 3 4 . Protactinium-234 2( 49-|Pa) is a radioisotope which decays quite quickly, by emitting beta-particles.

• First measure the count-rate with no radioactive source. This is the background count. What causes this?

• Then put the source under the G-M tube and watch the ratemeter. Record the count-rate every 20 seconds for about 5 minutes.

• Put your results in a table: • Subtract the background count to get the

corrected count-rate in the third column.

• Plot a graph of the corrected count-rate (y-axis) against time (x-axis) .

J s e your graph to answer the following questions: i) How long did it take for the count-rate to fall

to half? >) How long did it take to halve again? ;) Choose any point on your graph. How long

did it take to halve from this point? I) From your answers to (a ) , (b), (c) what is the

half-life of Protactinium-234?

In an experiment like the one above, the following results were obtained:

Time (s) 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300

Count rate (per s ) 97 SO 67 55 45 38 32 27 23 19 16 13 11 9 8 7

Corrected count rate (per s)

The background count was 2 per second. a) Complete the table. b) Plot a graph for the corrected count-rate against time. c) What is the half-life?

4^

Time Count-rate Corrected count-rate seconds counts per second counts per second

0

10

HO

60

SO

100

120

m

160 1 180

ZOO

110

mo

160

ISO

^00

^ n e \ s O 2 0 6 4 0 ( 0 0 So ICO r2o m o Ifao i"8o 2 o o 22o ZUo 2jbo ZSo 3 o o

c p s q > ? o W SS i i S SB 31 2^ 2 3 R l(o 13 H ^ 8 ^ U 5 5 5 ^ 3 ^ ^ z s 2 ^ ^ 14 1| ^ 3 fc, S

4o 80 n o I b o 2 0 0 2JUO 2 ^ 0


Assessed Homework

1. Complete the following decay equations

iBa + l ) 55

b) Co 56 25 Mg +

P

a

[4]

[4]

c) What other radiation could have been emitted with these decays? [1]

2. Radon has an atomic number of 86 and a mass number of 220. I t emits an

alpha particle to become thorium A (polonium), which emits another alpha

particle to become thorium B (radioactive lead). Thorium B then emits a beta

particle to become thorium C (bismuth). What is the atomic and the mass

number of thorium C? Show your equation. [7]

3. The half-life of Bi is 20 minutes. What fraction of a sample will remain

after 2 hours? [3]

4. A ratemeter records a background count rate of 2 cps. When a radioactive

source is held near the count rate is 162 cps. I f the half-life of the source

is 5 minutes, what will the recorded count rate be 20 minutes later? [4]

5. A radioactive source of half-life 2 min gives a count of 1600 cpm (at time 0) .

Draw a table to show the counts per min at times 0, 2, 4, 6,10 minutes. [4]

6. An ionization chamber was connected to a pulse electroscope and an alpha

source held near it. Beyond a certain distance no pulses were produced. For

some small distances the pulse rate varied as follows:

Distance from source/cm 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Number of pulses per min 100 90 68 44 26 14 8 4 1

Plot a graph to estimate the range of the alpha particles. [8]

Total [35]

50

2.2Q _ l_v CD •S2. D . +

AVTJYMC . n o - 8 ^ W Q S n o = X i

3) X H c o - 3 s l c H a l f H u e s ^

> - 'x :? ^ - £ ) / ( ~ H ® -1 * tr

U) 2C - U V-cAp LuifcS CQjnr- rata. - itcSL-X - lUaoCD

I b o CD CD

S ) " T T n n e i H ^ t / ^

Clprv-v

c

R

O 2. l O

2o b o -go l a o

(33 po-oV~2> CD O J ^ O S ,

© 3 C C \ € 3

risoV'r^SS

»

Year 11

Uses of Radioactivity

GCSE Physics Unit 1

i m e s h

I Am-241

1 NOJ>0

Smoke Detectors

Smoke alarms contain a weak source of

Americium-241. This emits alpha particles

which ionise the air, so that it conducts

electricity and a small current flows. I f

smoke particles enter the alarm they absorb

the alpha particles reducing the current so the alarm sounds. Americium-241

has a half life of 460 years. Why is this helpful?

alarm

Thickness control

The thickness of paper, plastic or aluminium foil

can be controlled by measuring how much beta

radiation passes through the material to a G-M

tube. I n a sheet-steel factory gamma radiation machbwry t to cotitroi

iDllara

is used. What happens when the G-M tube detects too much radiation?

Checking Welds

I f a gamma source is placed or

photographic film on the other

one sic

weak

ie of a meta

points or air

weld and a piece of

bubbles will show up on the

film as darken spots.

Si


Sterilising

Gamma rays can be used to kill bacteria, mould and

insects in food, even after the food has been

packaged. This prolongs the shelf-life of the food

but it sometimes can change the taste of the food.

tSamma rays are also used to sterilise hospital

equipment, especially plastic syringes that would be damaged by heating.

Carbon Dating

There's a small amount of radioactive carbon-14 in all living Carbon dioxide

organisms. The ratio of C-14 to C-12 inside a living organism taksscarbon-u T into the food

remains constant. When they die no new C-14 is taken in by

the dead organism. The C-14 it conta ined at the time of

death decays over time. By comparing the amount of C-14

left in the dead organic material to the amount of C-14 in a

living organism the approximate time since it died can be calculated,

Dating Rocks

Some rock contain traces of uranium-238. U-238 has a half life of 4500 million

years. By measuring how much uranium is left in the rock its approximate age

can be calculated.

Radiotherapy

Cobalt-60 emits gamma rays and

can be used to treat tumours.

The tumour is irradiated from

many angles and planes. Why?

Gamma rays

_ _

5 2


Medical Tracer

Doctors use radioactive tracers for medical imaging. Technetium-99 is a

common tracer and is relatively safe for use inside the body because:

• it only emits gamma-rays. The y-rays can be detected outside the body by a

'gamma camera' and are the least ionising type of radiation.

• it has a short half-life of 6 hours so it decays before it can do much damage

to the cells of the body but long enough for medical tests to be completed,

i.e. for the tracer to have made its way to the part of the body to be imaged.

Why are a-sources not used as medical tracers?

— : i 3~

Industrial Tracer

Leaks from pipelines carrying oil or gas can be

traced by injecting a beta emitting radioisotope

into it. This saves digging it all up. Isotopes are

chosen so that it has a half life of only a few hours or days. This is so that it

remains long enough to be detected but not so long that it remains a safety

problem. Why are beta emitters used and not alpha or gamma emitters?

=3 Research:

Find out how radioactivity is used in agriculture.

" ( ~y=v<-$ kr» r=%=» - -~~ <~-" <

55>

Year 11

Questions;

GCSE Physics Unit 1

1. Look at the diagram below showing how the thickness o metal sheet is kept constant by the use of a radiation source

S c o n c e

a) Name A, B and C. What type of radiation is X? ^ |

b) Suppose the thickness of the metal passing C increases. How does the system detect this change, and how does it return the thickness to its preset value? I I r a s r ^ c h a h c A SeYecYed

c) The radioactive isotope used here must have a long half-life. Explain what wou ld go wrong if the half-life was only two hours. - coVWcxWcrv cfp - c^r- T o o H K c V c . a s

d) What type of radiation would you choose if you wanted to monitor the thickness of cardboard? g

e) Explain why gamma radiation would be the wrong choice of radiation in d)

Y n c K affected b y | c o r d b c c v / c |

2- Gamma-emitting isotopes can be used to find out whether containers or pipes are leaking or not. An engineer wants to test an underground water pipe for leaks without digging up the road. It is buried one metre below the pavement.

a) Describe what the engineer would do to carry out his test.

.voy^cr" iSc=A-^pG. imV-o p i p e . , j2> cmY

L o W 2 j e <T d e h a r v - € » d is> leexte. b) The isotope needs to have a half-life of about a week. What problems could occur if it was much

longer or shorter than this?

L—& remcx^ I n a ^ o J S p=r - t o o \<=r^ , C c L i S £ d O A A a g t + o S u ^ O J A c i ^ n .

3. Copy and complete the table summarising the uses of radioactive isotopes.

Use bf Radioactive.-isotope

Alpha, beta or gamma emitter?

Short, medium' or long half-life?

ason for choosing short, medium or long >]' half-life

Tracers in medicine ^C\dY

»ed -to recdoos l e o e l <=p

Tracers in industry P ry\0cJiOrv\ n<

ft?

:ecJ"ViA^«. -to c^rrdr o_>l- o e e d ^raJ. v-f=> *aOiHc /qr+o/ -bZRr"

Sterilisation of food

Cr C\\

•\ p^cV"c/<_j l i n e , CJ^\i\ov^=Ve s i o

Thickness control (paper) % Cc

Thickness control (metal sheets) a

5*4

Year 11 GCSE Physics

irradiation dose surgical temperatures radioactive sterilise gamma instruments damage

exposed microbes fresh safe emitter

Xnnr\rv^a radiation can be used to _ food, keeping it p/>?^ for

A high dose of _

longer. The process kills harmful rrwrrdr^f^ . but does less r V l r v n t y to food, as it doesn't involve exposure to high •\pjrr\Qfi>\r\Y\ )<TP^, like boiling. The food is not roA\c\arVw x? afterwards, so it is perfectly

? o p g to eat. The isotope needs to be a very strong <Pnr\\Yret of gamma rays. This method can also be used to sterilise

Unit 1

The diagram shows a design for a smoke detector that could be fitted in a house. A weak radioactive source causes ionisation between the electrodes. The ions are attracted to one of the electrodes, and there is a small current.

Battery

1 "T" Radioactive

Resistor

Alarm

J source

Ammeter linked fo alarm

a) What type of source would be suitable for this application? c< ^^p^/rfc^

b) What happens when smoke enters the detector? How does this set off the alarm? ' n o \cr^s>, m o J-fSc*-*.

c) Some consumers might be worried about the presence of a radioactive source in the detector. Hov would you reassure them? — ^ trapped k»-j <p\cx^n C O S S

6. In the Health Serv ices , radiation is used in the treatment of many cancers .

a) What type of radiation is generally used? &

b) What does the radiation do? \Cx\\ co^c^rc^s o - U s

c) W h y does the radiation need to be very well-targeted?

The medica l physicists w h o are responsible for calculat ing the doses need to ensure that the dose of radiation is not too low or too high.

d) What cou ld happen if the dose is too low? ndr fc-vH cp\ csV

e) What cou ld happen if the dose is too high? c ^ ^ s e

56

Subdrance J&adioacftuitL A v\

| Half *' llf<2_. AtiHovt^\cpvv\ Subdrance J&adioacftuitL A v\

| Half *' llf<2_. AtiHovt^\cpvv\

J( I' 7naer2f~> 16ODO

r D *

1 u

to Kcor3 "Soco 2 4 0 c J ^ Q / S 1 5 0 0 0

a 2 to J

U Y S3 b o o c o LOhcK S o u r c e te> rrcek

S3 b o o c o

a) o V a c e / £ a

o ) ^ c \ d i o i - K e x^po j C Guar>^mc\ G x

<d) r r \ eo2o r ' .oo j tlmvc\cr\eS2> o p ^

e') finding cmcte\ ieats c/\










=<S. o n e »—>

WoVvcesV- o o r n

(xiWicH S^Az-ees u J C o l d ,

' OC ^ hLcK(23V C o m

j


Radioactivity for Energy

By the end of this section you should be able to: 1.4.16 describe nuclear fission in simple terms and be aware that it is a form of

energy used in the generation of electricity (fission equations are not required);

1.4.17 know that for fission to occur the uranium 235 or plutonium 239 nucleus must f i rst absorb a neutron and then split into two smaller nuclei and release two or three fission neutrons;

1.4.18 know that the fission neutrons may go on to start a chain reaction; 1.4.19 discuss and debate some of the political, social, environmental and ethical

issues relating to the use of nuclear energy to generate electricity (w -

1.4.20 describe nuclear fusion in simple terms and be aware that it is the source of a star's energy;

1.4.21 describe nuclear fusion in terms of an equation involving mass numbers and atomic numbers {w - (iii)c): and

1.4.22 appreciate the potential of nuclear fusion to solve the world's energy needs provided the technological difficulties of fusion reactors can be overcome (w - (iv)a, (iv)b).

Nuclear Fission

Occurs when a large nucleus splits into two or more smaller nuclei

e.g. When a neutron is absorbed by a uranium nucleus it splits in 2 and releases 3

neutrons that can fission other uranium nuclei. Some energy is also released

as heat. (Note: either 2 or 3 neutrons are released in each fission

dependant on what daughter nuclei are released)

O — neutron

uranium nucleus splits Into smaller nuclei and some

neutron hits uranium nucleus

more neutrons } 1 fission

reaction of cha

daughter nuclei

these neutrons hit more

uranium nuclei O

O

J

5k


• I n a nuclear reactor these fission reactions are controlled. I f we let the

reaction go uncontrolled the temperature inside the reactor core would get

to hot and a meltdown would result. We control the reactor temperature by

removing excess fission neutrons and hence control the number of fissions.

• I n an atomic bomb we let the reaction go uncontrolled.

• I n electricity generation if 1kg of uranium-235 undergoes nuclear fission it

can release about the same amount of energy as 2 million kg of coal.

• Nuclear fission results in the production of radioactive waste.

Radioactive waste

High level waste

The fuel rods from nuclear reactors are extremely radioactive. They have long

half lives and so they remain hazardous for thousands of years. This waste is

sealed into glass bocks (in a process called 'vitrification'). The blocks are put in

sealed containers and buried deep underground, sometimes in old mines.

Intermediate level waste

Other components from nuclear power stations are much less radioactive than

the fuel rods. These components are usually sealed in cement inside steel drums

and buried underground.

Low level waste

This includes the clothing and instruments used by workers in nuclear power

stations and research labs. I t can be stored in concrete vaults.

Research:

What caused the disaster at Chernobyl? 2b April qg=


Nuclear Fusion

Occurs when two small nuclei join to make one larger nucleus

e.g. When a hydrogen nuclei and a deuterium nuclei join to make a helium nuclei.

Equation: [H + \H j - * \He + energy O — hydrogen-1

nuclei collide and fuse together

< D — hydrogen-2

\ 7

Q)— helium-3

Another equation: \H + \H\-> \He + In + energy • Fusion reaction is the basis of the hydrogen bomb (thermonuclear bomb).

• The sun converts mass into energy in this way.

• Nuclear fusion only occurs when the nuclei collide at very high speed.

Nuclear fusion and electricity Production

For a nuclear fusion reaction like this to happen, extremely high temperatures

and pressures are required along with a very high density of particles. These

perfectly describe the conditions inside a star. These conditions are extremely

difficult to replicate in a reactor.

The problem with recreating nuclear fusion is that two positive nuclei will

mutually repel one another. So we need to get the nuclei moving at high speeds

to overcome this repulsion.

I f we could get it to work, it would solve the world's energy problems. We could

get the fuel (hydrogen) from water molecules and the helium produced is just an

inert gas.

Physics for CCEA Questions 1 -6 , Pages 72 + 73

Physics for You Questions 1 - 14, Pages 363 - 36$

53-

year 11 gcse physics unit 1 - wordpress.com · year 11 plum pudding model gcse physics unit 1 in...

Documents