Alpha decay

Alpha particles consist of two protons plus two neutrons.

They are emitted by some of the isotopes of the heaviest elements.

Example: The decay of Uranium 238

U238

92Th

234

90α

4

2+

Uranium 238 decays to Thorium 234 plus an alpha particle.

Notes:

1. The mass and atomic numbers must balance on each side of the equation: (238 = 234 + 4 AND 92 = 90 +2)

2. The alpha particle can also be notated as:He

4

2

QuestionShow the equation for Plutonium 239 (Pu) decaying by alpha emission to Uranium (atomic number 92).

Pu239

94U

235

92α

4

2+

Beta decay

Beta particles consist of high speed electrons.

They are emitted by isotopes that have too many neutrons.

One of these neutrons decays into a proton and an electron. The proton remains in the nucleus but the electron is emitted as the beta particle.

Example: The decay of Carbon 14

C14

6N

14

7 β-

0

-1+

Carbon 14 decays to Nitrogen 14 plus a beta particle.

Notes:

1. The beta particle, being negatively charged, has an effective atomic number of minus one.

2. The beta particle can also be notated as:e

0

-1

QuestionShow the equation for Sodium 25 (Na), atomic number 11, decaying by beta emission to Magnesium (Mg).

Na25

11Mg

25

12 β-

0

-1+

Gamma decayGamma decay is the emission of electromagnetic radiation from an unstable nucleus

Gamma radiation often occurs after a nucleus has emitted an alpha or beta particle.

Example: Cobalt 60

Co60

27γ

0

0+Co

60

27

Cobalt 60 with excess ENERGY decays to

Cobalt 60 with less ENERGY plus gamma radiation.

Do Now copy and completeChanging elements

Both alpha and beta decay cause the an isotope to change atomic number and therefore element. Alpha decay also causes a change in mass number.

Decay type Atomic number Mass number

alpha DOWN by 2 DOWN by 4

beta UP by 1 NO CHANGE

gamma NO CHANGE NO CHANGE

Complete the decay equations below:

Fe59

26Co

59

27 β-

0

-1+

Ra224

88Rn

220

86α

4

2+

N16

7O

16

8 β-

0

-1+

(a)

(c)

(b)

Write equations showing how Lead 202 could decay into Gold. (This cannot happen in reality!)

Pb202

82Hg

198

80α

4

2+

Pt194

78Au

194

79β

-0

-1+

Element Sym Z

Platinum Pt 78

Gold Au 79

Mercury Hg 80

Thallium Tl 81

Lead Pb 82

Bismuth Bi 83

Hg198

80Pt

194

78α

4

2+

There are other correct solutions

Choose appropriate words to fill in the gaps below:

When an unstable nucleus emits an alpha particle its atomic number falls by _______ and its mass number by ______.

Beta particles are emitted by nuclei with too many ________. In this case the atomic number increases by ______ while the ________ number remains unchanged.

Gamma rays consist of ______________ radiation that is emitted from a nucleus when it loses ________, often after undergoing alpha or beta decay.

electromagneticenergy masstwofour one

WORD SELECTION:

neutrons

electromagnetic

energy

mass

two four

one

neutrons

Today’s lesson

• Use the term half-life in simple calculations, including the use of information in tables or decay curves.

• Give and explain examples of practical applications of isotopes.

• Title Half-life

½ - life – copy please

• This is the time it takes for half the nuclei present in any given sample to decay

half-life (t½)

Number of nuclei undecayed

timeA graph of the count rate against time will be the same shape

Different ½ - lives

• Different isotopes have different half-lives

• The ½-life could be a few milliseconds or 5000 million years!half life applet

half-life (t½)

Number of nuclei undecayed

time

Examples

• A sample of a radioactive isotope of half life 2 hours has a count rate of 30 000 counts per second. What will the count rate be after 8 hours?

Examples

ActivityThe activity of a radioactive source is equal to the number of decays per second.

Activity is measured

in bequerels (Bq)

1 becquerel

= 1 decay per second

Half life

Henri Becquerel discovered

radioactivity in 1896

Question 1At 10am in the morning a radioactive sample contains 80g of a radioactive isotope. If the isotope has a half-life of 20 minutes calculate the mass of the isotope remaining at 11am.

10am to 11am = 60 minutes

= 3 x 20 minutes

= 3 half-lives

mass of isotope = ½ x ½ x ½ x 80g

mass at 11 am = 10g

Question 2Calculate the half-life of the radioactive isotope in a source if its mass decreases from 24g to 6g over a period of 60 days.

24g x ½ = 12g

12g x ½ = 6g

therefore TWO half-lives occur in 60 days

half-life = 30 days

Example 2 – The decay of source ZSource Z decays with a half-life of three hours.

At 9 am the source has an activity of 16000 Bq

The activity halves every three hours.

Time Activity (Bq)

9 am

12 noon

3 pm

6 pm

9 pm

midnight 500

1000

2000

4000

8000

16000

When will the activity have fallen to 125 Bq? 6 am

Example 3 – The decay of isotope X

Isotope X decays to Isotope Y with a half-life of 2 hours.

At 2 pm there are 6400 nuclei of isotope X.

Time Nuclei of X

Nuclei of Y

2 pm

4 pm

6 pm

8 pm

10 pm

midnight 200

400

800

1600

3200

6400

6200

6000

5600

4800

3200

0

When will the nuclei of isotope X fallen to 25? 6 am

Question 3A radioactive source has a half-life of 3 hours.

At 8 am it has an activity of 600 Bq.

What will be its activity at 2 pm?

at 8 am activity = 600 Bq

2 pm is 6 hours later

this is 2 half-lives later

therefore the activity will halve twice

that is: 600 300 150

activity at 2 pm = 150 Bq

Question 4 – The decay of substance P

Substance P decays to substance Q with a half-life of 15 minutes. At 9 am there are 1280 nuclei of substance P.

Complete the table.

Time Nuclei of X

Nuclei of Y

9 am

9:15

9:30

9:45

10 am

10:15 40

80

160

320

640

1280

1240

1200

1120

960

640

0

How many nuclei of substance X will be left at 11 am? 5

Question 5A sample contains 8 billion nuclei of hydrogen 3 atoms. Hydrogen 3 has a half-life of 12 years. How many nuclei should remain after a period 48 years?

48 years = 4 x 12 years

= FOUR half-lives

nuclei left = ½ x ½ x ½ x ½ x 8 billion

nuclei left = 500 million

Experiment Dicium 25

Finding half-life from a graph

0

100

200

300

400

500

600

0 20 40 60 80 100 120

time (seconds)

num

ber

of n

ucle

i

half-life

The half-life in this example is about 30 seconds.

A more accurate value can be obtained be repeating this method for a other initial nuclei numbers and then taking an average.

Question 6

0

100

200

300

400

500

600

700

800

900

0 10 20 30 40 50 60 70 80 90 100

time (seconds)

acti

vity

(B

q)

Estimate the half-life of the substance whose decay graph is shown opposite.

The half-life is approximately 20 seconds half-life

Question 7The mass of a radioactive substance over a 8 hour period is shown in the table below.

Draw a graph of mass against time and use it to determine the half-life of the substance.

Time (hours)

0 1 2 3 4 5 6 7 8

Mass (g) 650 493 373 283 214 163 123 93 71

The half-life should be about 2 hours:

Choose appropriate words or numbers to fill in the gaps below:

The ________ of a radioactive substance is the average time taken for half of the _______of the substance to decay. It is also equal to the average time taken for the ________ of the substance to halve.

The half-life of carbon 14 is about _______ years. If today a sample of carbon 14 has an activity of 3400 Bq then in 5600 years time this should have fallen to ______ Bq. 11200 years later the activity should have fallen to ____ Bq.

The number of carbon 14 nuclei would have also decreased by ______ times.

eight half-life5600 425 activity1700

WORD & NUMBER SELECTION:

nuclei

eight

half-life

5600

425

activity

1700

nuclei

Revision Simulations

Half-Life - S-Cool section on half-life and uses of radioactivity including an on-screen half-life calculation and an animation showing thickness control.

BBC AQA GCSE Bitesize Revision: Detecting radiation Natural sources of background radiation Artificial radiation Half life

Alpha Decay - PhET - Watch alpha particles escape from a Polonium nucleus, causing radioactive alpha decay. See how random decay times relate to the half life.

Uses of radioactive isotopes

Smoke detection

• Uses

Thickness control

Thickness control

Used as Tracers

Used as Tracers

Killing microbes

Killing microbes

Checking welds

Used as Tracers

Carbon dating – write notes using the book page 265

Summary sheet

“Can you………?”

Test!

Thursday

27th September 2012

Can you answer the questions on pages 261 and

265?