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Physics > 7 Radiation and half-life > Flashcards

7 Radiation and half-life Flashcards

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1
Q

How did Henri Becquerel test his idea that uranium emitted x-rays after being exposed to sunlight?

A

He placed some wrapped, unused photographic plates in a drawer with some samples of uranium ore on top of them.

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2
Q

What had he found out about these images after developing them?

A

He found a strong image of the ore on the plates when he developed them.

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3
Q

Why was this strong image found?

A

He realised that this was due to a new type of ionising radiation. He had discovered radioactivity.

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4
Q

What is the unit of radioacitivity?

A

Becquerel.

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5
Q

What is a Becquerel?

A

It is a measure of how many unstable nuclei are disintegrating (breaking up) pre second.

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6
Q

What does one Becquerel mean in terms of disintegration?

A

One becquerel means a rate of one disintegration per second.

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7
Q

What is the size of a Becquerel?

A

The Becquerel is a very small unit.

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8
Q

What would a KBq represent?

A

An average of 1000 disintegrations per second.

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9
Q

What can we use to detect radiation?

A
  • Photographic film.
  • Geiger-Müller detector.
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10
Q

How do scientists use photographic film to see exposure to radiation?

A

Some scientists who work with radioactive materials wear a piece of photographic film in a badge.

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11
Q

What do scientists then check in relation to this badge?

A

If the film becomes fogged (unclear) it shows that the scientist has been exposed to a certain amount of radiation. These badges are checked regularly to ensure that scientists are not exposed to too much ionising radiation.

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12
Q

What is an image that shows the basic construction of a Geiger-Müller tube?

A
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13
Q

What components does the Geiger-Müller tube consist of?

A

It is a glass tube with an electrically conducting surface on the inside. The tube has a thin window made of mica (a naturally occurring mineral that can be split into thin sheets). The tube contains a special mixture of gases at very low pressure.
In the middle of the tube, electrically insulated from the conducting surface, there is an electrode. This electrode is connected, via a high-value resistor, to a high-voltage supply, typically 300-500 V.

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14
Q

What happens when ionising radiation enters the tube?

A

When ionising radiation enters the tube it causes the low pressure gas inside to form ions, The ions allow a small amount of current to glow from the electrode to the conducting layer. This is detected by an electronic circuit.

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15
Q

What is the GM tube usually linked up to?

A
  • A counting circuit.
  • Rate meters.
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16
Q

Why are counting circuits used?

A

This keeps a count of how many ionising particles (or how much γ radiation) have entered the GM tube.

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17
Q

Why are rate meters used?

A

These measure he number of ionising events per second, and so give a measure of the radioactivity in Becquerels. Rate meters usually have a loudspeaker output so the level of radioactivity is indicated by the rate of clicks produced.

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18
Q

What is background radiation?

A

It is a low-level ionising radiation that is produced all the time.

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19
Q

How is background radiation produced?

A

It has a number of sources. Some of these are natural and some are artificial.

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20
Q

What are some of the sources and their percentages?

A
  • Ground and buildings. 14%
  • Radon gas. 50%
  • Medical. 14%
  • Nuclear power. 0.3%
  • Cosmic rays. 10%
  • Other. 0.2%
  • Food and drink. 11.5%
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21
Q

What are the two types of natural background radiation?

A
  • From earth.
  • From space.
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22
Q

What is natural background radiation from earth?

A

Some of the radiation we receive comes from rocks in the Earth’s crust.

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23
Q

How did the Earth’s crust become radioactive?

A

When the Earth was formed, around 4.5 billion years ago, it contained many radioactive isotopes. Some decayed very quickly but others are still producing radiation. Some of the decay products of these long-lived radioactive materials are also radioactive, so there are radioactive isotopes with much shorter half-lives still present in the Earth’s crust.

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24
Q

What is an example of an element that decays very slowly?

A

One form of uranium.

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25
Q

What are uraniums products?

A

Two of its decay products are gases.

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26
Q

What are the names of these gases?

A
  • Radon.
  • Boron.
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27
Q

What are some characteristics of radon?

A

Radon-222 is a highly radioactive gas produced by the decay of radium-226

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28
Q

What are some characteristics of thoron?

A

Thoron, or radium-220, is an isotope of radium formed by the decay of a radioactive isotope of thorium (thorium-232).

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29
Q

What happens because these decay products are gases?

A

They come out of radioactive rocks.

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30
Q

What happens because these gases are dense?

A

They are dense gases so they build up in the basements of buildings

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31
Q

Why does the amount of background radiation produced in this way vary from place?

A

Some parts of the Earth’s crust have higher amounts of radioactive material.

32
Q

What is an example of how radiation differs from place to place?

A

In Cornwall in the UK, for example, where the granite rock contains traces of uranium, the risk of exposure to radiation from radon gas is greater than in some other parts of the UK.

33
Q

Why is there natural background radiation from space?

A

Violent nuclear reactions in stars and exploding stars called supernovae produce cosmic rays (very energetic particles) that continuously hit the Earth. Lower energy cosmic rays are given out by the Sun.

34
Q

Why do we not feel the effect of these cosmic rays?

A

Our atmosphere gives us fairly good protection from cosmic rays but some still reach the Earth’s surface.

34
Q

What is radiation in living things?

A

The atoms that make up our bodies were formed in the violent reactions that take place in stars that exploded (supernovas) billions of years ago.

35
Q

How do these atoms make us radioactive?

A

Some of these atoms are radioactive so we carry our own personal source of radiation around with us.

36
Q

What is another way that we (as living things) contribute to radioactivity?

A

Also, as we breathe we take in tiny amounts of the radioactive isotope of carbon, carbon-14.

37
Q

What are the two things that Carbon-14 does in relation to radioactivity?

A
  • Carbon-14 behaves chemically just like the stable isotope, carbon-12, we continuously renew the amount of the radioactive carbon in our bodies.
  • Carbon-14 and other radioactive isotopes are eaten by humans (and animals which are in turn eaten by humans) because they are present in all living things.
38
Q

What is another source of radioactivity?

A

Artificial radiation.

39
Q

Why is artificial radiation a thing?

A

This is because we use radioactive materials for may purposes.

40
Q

What are the most prevalent forms of artificial sources of background radiation?

A
  • Generating electricity in nuclear power stations.
  • Testing nuclear weapons in the atmosphere has also increased the amounts of radioactive isotopes on the Earth.
41
Q

What is a tracer?

A

The use of substances that can be detected to show the movement of liquids through a person’s body or pipes in machinery.

42
Q

Where are radioactive tracers used?

A

In industry and medicine, and radioactive materials are also used to treat certain forms of cancer.

43
Q

What is important to note when talking about the main sources of background radiation?

A

The majority of background radiation is natural - the amount produced from medicinal use in industry is very small indeed.

44
Q

What is a characteristic of radioactive decay?

A

Radioactive decay is a random (unpredictable) process, just like throwing a coin.

45
Q

What is a characteristic stemmed from the fact that decay is a random process?

A

We do not know which nuclei will disintegrate (break down) at a particular time.

46
Q

What happens if we have a sample of a radioactive material in terms of atoms?

A

It will contain millions of atoms.

47
Q

What can we deduce about the amount of atoms that will decay bearing in mind that decay is a random process?

A

The process of decay is random, so we don’t know when an atom will decay but there will be a probability that a certain fraction of them will disintegrate in a particular time. This is the same as in the coin toss - there was a 50% probability that the coins would land heads each time we conducted a trial.

48
Q

What happens once an unstable nucleus has decayed?

A

It is out of the game - it won’t be around to disintegrate during the next period of time

49
Q

What happens if we plot a graph of number of disintegrations per second against time for a radioactive isotope?

A

We would therefore expect the rate of decay to fall as time passess because there are fewer nuclei to decay.

50
Q

Why is the number of radioactive decay likely to vary from trial to trial?

A

The model becomes less and less reliable as the number of atoms becomes smaller. With real radioactive decay the number of coins becomes smaller. With real radioactive decay we use a measure of activity called the half life.

51
Q

What is half life?

A

It is the length of time it takes for the activity of an amount of radioactive substance to halve.

52
Q

What happens if the amount of radioactive matter has halved?

A

Then the activity of the decay halves.

53
Q

Is the half-life the same for all radioactive substances?

A

No, it is different for different isotopes.

54
Q

What is a picture of the graph showing the half-life period for a radioactive isotope?

A
55
Q

How do we measure the half-life of a radioactive material?

A

We must measure the radioactivity of a sample at regular times.

56
Q

What peice of apparatus is used to measure the radioactivity of a sample at regular times?

A

A Gieger-Muller tube linked to a rate meter.

57
Q

What must we do before taking measurements with a Geiger-Muller meter from a sample?

A

We must measure the local background radiation.

58
Q

What do we do with this value of background radiation?

A

We must subtract the background radiation measurement from measurements taken from the sample so we know the radiation produced by the sample itself.

59
Q

What do we do after the subtraction?

A

We then measure the rate of decay of the sample at regular time intervals.

60
Q

Where is the rate of decay shown?

A

By the count rate on the rate meter.

61
Q

In what kind of table should the results be recorded in?

A
62
Q

What is the rate of decay proportional to?

A

To the amount of radioactive isotope present.

63
Q

What does the graph of the rate of decay against time look like?

A
64
Q

What is the half-life of uranium-238?

A

4.5 billion years.

65
Q

What is the decay process of uranium-238?

A

Alpha particle emission.

66
Q

What is the half-life of radium-226?

A

1590 years.

67
Q

What is the decay process of radium-226?

A

Alpha particle emission / Gamma ray emission

68
Q

What is the half-life of radon-222?

A

3.825 days.

69
Q

What is the decay process of radon-222?

A

Alpha particle emission.

70
Q

Which isotopes are best for medical use?

A

Isotopes with short half-lives are suited to medical use.

71
Q

Why are isotopes with short half-lives best for medical use?

A

This is because the activity of a source will rapidly become very small as the isotope decays very quickly.

72
Q

Which isotopes are best for dating samples of organic material?

A

They will need to have very long half-lives.

73
Q

Why are isotopes with long half-lives better for dating samples of organic material?

A

This is because the activity will become difficult to measure accurately if it drops below a certain level.

74
Q

What kind of graphs can be used to find the half life?

A

Graphs of activity, in Becquerels, against time can be used to find the half-life of an isotope.

75
Q

How can knowing the half-life of a substance help us?

A

It can be used to make predictions of the activity of the radioisotope at a later time.

76
Q

PRACTICE HALF-LIFE CALCULATIONS.

A

Physics (24 decks)

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