ks4 radioactive decay

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KS4 Physics

Radioactive Decay


Radioactive Decay

Rutherford's experiments

Notation

Summary activities

Radioactive decay & half-life

Nuclear power

Contents


Radioactive waste from nuclear power stations is an environmental concern. The problem is the waste stays radioactive for thousands of years.

Radioactive waste

The current solutions are:

1. Store it at the nuclear power station until is full.

2. Dump it far out at sea.

3. Store it deep underground in non-permeable rock.


1. What are the three types of radiation?

2. Which type of radiation is the most penetrating?

3. Why is radioactive waste not stored in permeable rock?

4. Why should nuclear power stations not be situated in geologically active regions?

Alpha, beta and gamma

Gamma

It could contaminate water that seeps through the rock.

Earthquakes could cause radioactive spills.

Radiation questions


Scientists once believed that atoms were spheres of positive charge with negative charges spread throughout.

This resembled a plum-pudding, so it was called the ‘plum-pudding’ model.

This was wrong!

What we used to think

How did we discover current ideas about the structure of the atom?


Ernest Rutherford and his team of scientists performed a famous experiment in Manchester.

They fired alpha particles at a piece of very thin gold foil (only a few atoms thick).

If the ‘plum-pudding’ model of the atom was correct, the alpha particles should pass straight through and only be slightly deflected.

This did not happen.

Rutherford's team


1. Most of the alpha particles went straight through the foil.

2. Some alpha particles were deflected through large angles.

3. A very few alpha particles were reflected straight back.

What Rutherford’s team observed


Observation Conclusion

Most alpha particles went straight through the foil.

A few were deflected through large angles.

A very few were reflected straight back.

Atoms are mostly space.

The nucleus is very small compared to the size of the atom and it contains most of the mass and all the positive charge.

Rutherford’s conclusions


Pretend you are Ernest Rutherford and you have just completed your investigation.

Write a letter to a fellow scientist describing your observations and findings. Include the impact you think it will have on current thinking.

Dear Dr Banner,

I am writing to…

Task


Radioactive Decay


Notation

Summary activities


Nuclear power

Contents


A

X Z

Element symbol

Mass number (A)

The number of protons plus the number of neutrons in a neutral atom.

Atomic number (Z)

The number of protons (which is the same as number of electrons) in a neutral atom.

Notation


12

C 6

75

As 33

127

I 53

Name the elements below and calculate how many protons (P), neutrons (N) and electrons (E) they have.

Notation exercise

P = 6N = 6E = 6

carbon P = 33N = 42E = 33

arsenic P = 53N = 74E = 53

iodine


12

C 6

13

C 6

14

C 6

Notation exercise

Calculate how many protons (P), neutrons (N) and electrons (E) these atoms have.

P = 6N = 6E = 6

P = 6N = 7E = 6

P = 6N = 8E = 6


Isotopes and radioisotopes

These atoms are all carbon – what is the difference between them?

They have different numbers of neutrons.

What are atoms of the same element with different numbers of neutrons called?

Isotopes

What are isotopes that are unstable and emit radiation to become more stable called?

Radioisotopes

12

C 6

13

C 6

14

C 6


Radioactive Decay


Notation

Summary activities


Nuclear power

Contents


Why is it that there are different types of radiation?

What is going on inside the nucleus?

The three types of decay are…

alpha

beta

gamma

Radioactive decay


A decreases by 4 (A – 4)Z decreases by 2 (Z – 2)

What is emitted?

Description of decay:

Example of decay:

Effect on A and Z:

alpha particle (helium nuclei)

238 234 4

U Th + + energy 92 90 2

2 neutrons and 2 protons are emitted from the nucleus.

Alpha decay


A stays the same (A)Z increases by 1 (Z + 1)

What is emitted?


Example of decay:

Effect on A and Z:

High-energy electron

1 neutron in the nucleus decays into a proton and a high-energy electron, which is emitted.

14 14 0

C N + + energy 6 7 -1

Beta decay


A stays the same (A)Z stays the same (Z)

What is emitted?


Effect on A and Z:

High-energy electromagnetic radiation

nucleus changes shape into a more stable shape, resulting in gamma radiation being emitted

Gamma decay


The time it takes the number of radioactive nuclei in a sample to decrease by 50%.

The time it takes the count rate from a radioisotope to decrease by 50%.

There are two definitions of half-life:

You must learn both of these definitions!

Half-life


Decay rate (counts/min

)

Time (min)

80

60

40

20

2 4 6 8

How can you calculate the half-life of the radioisotope represented by this graph?

Calculate the time it takes the count rate to decrease from 80 per min to 40 per min.

2 mins

Double-check that the time it takes the count rate to decrease from 40 per min to 20 per min is the same

2 mins

The half-life of the radioisotope is 2 mins.

Graphing half-life


Radioactive Decay


Notation

Summary activities


Nuclear power

Contents


Carbon dating

All living things absorb a little radioactive carbon-14 when they feed and breathe, as well as the normal carbon-12.

When living things die, they stop taking in carbon-14 and so the carbon-14 present at death slowly decays to carbon-12 (half-life is 5,600 years).

The amount of radioactivity from the decaying carbon-14 can be used to calculate the age of bones, wood, paper and cloth.


A fresh bone gives a radioactive count of 170 counts per minute. Another ancient bone of the same mass gives a count rate of 50 counts per minute. The background count is 10 counts per minute.

How old is the bone?

Counts due to bones are 170 – 10 = 160 (fresh) and

50 – 10 = 40 (ancient)

The count rate of the carbon-14 has fallen to one quarter of its original value, i.e. 160/2 = 80, 80/2=40.This is two half lives.

So the bone is 5,600 x 2 = 11,200 years old

Counts due to bones are 170 – 10 = 160 (fresh) and

50 – 10 = 40 (ancient)

The count rate of the carbon-14 has fallen to one quarter of its original value, i.e. 160/2 = 80, 80/2=40.This is two half lives.

So the bone is 5,600 x 2 = 11,200 years old

Carbon dating: example


When a nucleus decays it gives out heat energy.In a nuclear power station, the uranium-235 atoms decay and give out energy and neutrons.

Nuclear power

Each time a uranium atom splits it produces 2 or 3 neutrons (depending on the reaction). These go on to hit other uranium atoms, which causes them to decay. A chain reaction is set up where more and more energy is released. In a nuclear reactor the process is carefully controlled so that neutrons are absorbed harmlessly and the energy released is controlled.

In a nuclear bomb the reaction is not controlled, and this causes the explosion!


Fast neutron from previous decay cause the uranium nucleus to split.

Nuclear power – fission


Kr

Ba

n

n

n

n

fissi

onuranium

In this reaction, a neutron from a previous decay can lead to more

and more decays.

This is called a chain reaction.

moredecays

Nuclear power


Radioactive Decay


Notation

Summary activities


Nuclear power

Contents


atomic number – The number of protons in the nucleus of an atom.

half-life – The time it takes the number of radioactive nuclei in a sample to decrease by 50%. This is also the time it takes the count rate from a radioisotope to decrease by 50%.

isotopes – Different versions of the same element, which have the same number of protons but different numbers of neutrons.

mass number – The total number of protons and neutrons in the nucleus of an atom.

radioisotope – An natural or artificially-created isotope of an element that is radioactive.

Glossary


Anagrams


Multiple-choice quiz

ks4 radioactive decay

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