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Physics Year 2 > Radioactivity > Flashcards

Radioactivity Flashcards

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1
Q

What is a dosimeter badge? Who wears it?

A

1) A badge that monitors the cumulative amount of radiation that the body of the person wearing it has absorbed. It takes into account the relative biological effects of ionising radiation
2) People who work with ionising radiation

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2
Q

During Rutherford’s scattering experiment, why was it important that the alpha particles were all monoenergetic? What made this easy to do?

A

1) Slower moving α-particles would be deflected by a larger angle than faster ones. Therefore all the α-particles had to have the same KE
2) All α-particles emitted from a particular radioactive source all have the same KE

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3
Q

What is the equation for the intensity of gamma radiation as it varies with distance?

A

I = k/x²

Where I is intensity, x is distance from source and k is Nhf/4π

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4
Q

How does a Geiger-Muller tube work?

A

It registers a pulse of electricity each time an ionising particle, e.g. α or β, enters the tube. The GM tube is connected to a digital counter which keeps count of the number of ionising particles entering the tube

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5
Q

Why do some radioisotopes require cooling as part of their storage?

A

Because of the ‘decay heat’ generated by radioactive decay. The energy of the ionising radiation emitted from these isotopes is converted to the thermal energy of the surrounding atoms

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6
Q

What did Rutherford conclude about the nucleus due to back scattering in his experiment?

A

The nucleus was a dense object with mass much larger than that of the α-particles. Applying the conservation of momentum to an elastic collision between a moving object and a stationary one shows that the moving object can only bounce back if it is much smaller in mass than the stationary one

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7
Q

The half life of a radioactive isotope is the time for each of the following to half:

A
  • the number of radioactive atoms, N
  • the activity, A
  • the corrected count rate, C
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8
Q

Why does the intensity of beta radiation not follow the inverse square law? Why does gamma radiation follow the inverse square law?

A

1) Because a significant proportion of it’s energy is absorbed as it ionises the air molecules
2) The absorption of gamma radiation by air is negligible

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9
Q

Describe radioactive decay

A

A process where an unstable nucleus loses energy by emitting radiation

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10
Q

What is radioactive dating?

A

A technique used to determine the age of an object based on the amount of a naturally occurring radioactive isotope present in the material from which the object is made

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11
Q

The activity of a sample of a specific isotope decreases…

A

… with the decreasing number of radioactive atoms present

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12
Q

The gamma emitter used in an experiment to demonstrate the inverse square law must have…

A

… a half-life long enough so that it’s activity doesn’t change during the experiment

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13
Q

What are essential safety precautions when handling alpha and beta sources?

A

Using long tongs and minimising the time for which the sources are removed from their boxes which must be lead lined

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14
Q

How can the corrected count rate be found from results collected from a GM tube?

A

Dividing the number of counts by the measured time interval, then subtracting the background count

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15
Q

Why is gamma the most harmful to humans outside the body?

A

It is the most penetrating

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16
Q

How is background radiation accounted for when using a GM tube?

A

The number of counts during a fixed period with no radioactive source are measured

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17
Q

Give 3 examples of artificial sources of background radiation

A

> Sources from medical applications
Waste from the nuclear power industry
Fallout from nuclear weapons testing

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18
Q

Why is carbon dating less accurate for samples less than 200 years old or older than 60000 years?

A

1) The change in the ratio of C-14 to C-12 is too small to be measured accurately
2) There are too few remaining C-14 atoms to be measured accurately

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19
Q

What does a shorter half life and larger decay constant mean for the activity of a particular radioisotope,

A

On a per mole basis they have a higher initial activity and are therefore ver dangerous and require remote-controlled handling and cooling

20
Q

If a radioactive sample contained N atoms, the number of decays per second is equal to…

A

… the probability of decay per second (λ s^-1) x N

21
Q

Explain carbon dating

A
  • It is a method used to date artefacts that are made of organic materials, based on the amount of C-14 present
  • There are 3 naturally occurring isotopes of carbon: C-12 & C-13 which are both stable, and C-14 which decays into N-14 by beta radiation
  • C-14 is created in the earth’s atmosphere by cosmic rays. Plants take up atmospheric CO2 which contains traces of C-14, about 1 in 10^12 carbon atoms is C-14
  • Plants are eaten by animals, so there is a constant exchange of C-14. When the organism dies, the exchange stops and amount of C-14 decreases. The time since the death the organism can be determined by comparing the amount of C-14 still present with the amount likely to have been present when it was living
22
Q

In Rutherford’s scattering experiment why did the beam of α-particles need to be narrow and parallel?

A

So that the scattering angle could be measured accurately

23
Q

In Rutherford’s scattering, experiment why did the gold need to be thin? What was the container evacuated?

A

1) To make it most likely that a particular alpha particle would only experience one scattering
2) Alpha particles can only travel a few cm in air

24
Q

What is the half life of a radioactive isotope? How does the decay constant λ affect activity and half life of a particular radioisotope?

A

1) The time for the activity of that sample to halve

2) The activity of an isotope with a large decay constant falls more rapidly and has a shorter half life

25
Q

The nucleon number expressed as a mass in grams approximates to…

A

… the mass of one mile of that isotope (it’s molar mass)

26
Q

How is the intensity of electromagnetic radiation defined?

A

The radiation energy per second passing through an area of 1m² normal to the radiation

27
Q

Explain choice of half-life in applications of radioisotopes, give some examples

A

Choice of half life depends on what the radioisotope will be used for. A beta emitter used to monitor an industrial process producing sheets of paper needs a long half life so it’s activity remains constant for a long period of time. Radioisotopes used as tracers in medical diagnosis need a short half life so that the time they are active inside then body is kept to a minimum.

28
Q

What factors limited the accuracy of Rutherford’s value for the upper limit for the radius of the nucleus?

A
  • The apparatus used did not allow detection of a head on collision with a scattering angle of 180°
  • α-particles are hadrons and are therefore interact via the SNF, not just the coulomb (electrostatic) repulsion. Only affected by SNF if they are less that 3fm apart
  • The finite size of the α-particles introduces uncertainty in the calculation of the distance of closest approach. The recoil of the gold nucleus would cause a slight change in the distance of closest approach
29
Q

Why are alpha particles more dangerous than beta particles when inside the body?

A

All their energy is deposited in a small volume causing more damage to cells. They are highly ionising. The energy from β-particles is distributed over a greater volume

30
Q

What is the nature of alpha, beta and gamma?

A

Alpha - two protons and two neutrons
Beta - an electron
Gamma - high frequency electromagnetic radiation

31
Q

What is the number of decays per second defined as? What is the equation for it?

A

1) The activity of a sample, measured in becquerel (Bq), where 1 Bq is equal to one decay per second
2) A = λN

32
Q

Why are beta particles more dangerous than alpha particles outside the body?

A

They are smaller and more penetrating so can penetrated sufficiently to cause damage to surface tissues and the eyes. α-particles are very big and and very ionising, therefore they have a very short range. All their energy is dissipated in the dead outer layers of the skin.

33
Q

Probability…

A

… gives the fraction of a large sample of random events that would show a particular outcome

34
Q

Why is beta radiation less ionising and more penetrating than alpha particles?

A

They are much smaller and have half the charge, therefore they are less likely to interact with matter

35
Q

What is a modern technique used for carbon dating?

A

Accelerator mass spectrometry (AMS)

36
Q

Explain potassium-argon dating

A
  • This allows you to determine how long ago s rock was formed based on the ratio of argon-40 to potassium-40 contained within the rock.
  • Potassium-40 is radioactive and has a half life of 1250 million years, with two possible modes of decay, either forming argon-40 or calcium-40, with the decay to calcium-40 being eight times more likely.
  • Originally the material from which the rock was made was molten and the argon escaped as a gas, however when it solidified to rock the argon got trapped in the rock. Using mass spec. the ratio of potassium-40 to argon-40 can be determined, which can then be used to calculate how old the rock is
37
Q

What is the equation for half-life? What does this show?

A

T1/2 = ln2/λ

38
Q

Give 5 medical uses of radioisotopes?

A

> Small quantities of alpha emitting isotope radium-233 are injected into tumorous tissue to directly kill cancer cells
β emitting isotope iodine-131 is used to treat thyroid cancer
β+ emitters (C-12, N-13, F-18) are chemically added to compounds used by body processes to act as tracers in conjunction with a PET scan
Low-energy gamma sources are used as tracers to produce images of internal organs. They are combined with a compound that is taken up by a certain part of the body. Gamma radiation emitted is detected by a gamma camera which tracks the compound
Gamma radiation used to sterilise medical equipment

39
Q

How was Rutherford able to predict the size of the nucleus? What enabled him to do this?

A

1) By calculating the upper limit for the radius of the nucleus and calculating a value for distance of closest approach
3) Back scattering in his experiment

40
Q

What is varied when measuring the absorption of beta radiation? Alpha radiation?

A

1) The thickness of aluminium

2) The distance between the alpha source and the GM tube

41
Q

What does one mole of a substance contain?

A

The same number of entities as there are atoms in 12g of pure C-12, that number being the Avogadro constant

42
Q

Give 3 examples of natural ways we are exposed to background radiation

A

> Some rocks contain unstable isotopes (potassium, uranium, thorium) which decay to other radioactive products, some of which are gases that can seep out of the ground. An example is granite
Radioactive isotopes in the ground that dissolve in water are taken up by plants and animals which then enter our food chain
Cosmic rays, which include particles and gamma radiation, from the sun and other sources in the solar system

43
Q

Give 3 industrial uses of radioisotopes

A

> An alpha emitter, polonium-210, is used as a static eliminator to neutralise static electricity in production of paper, plastics, textiles etc
β source strontium-90 used to manufacture materials requiring specific thicknesses (paper, aluminium foil etc). Thickness controlled by measuring the amount of radiation that gets through. The detector is linked to a computer which controls the pressure of the rollers
Gamma emitters used as industrial tracers. E.g. a small amount of the radioactive gas can be put into a pipeline, the gamma intensity is measured above ground to determine position of a leak

44
Q

Give an example of an occupation that would require the wearing of a radiation dosimeter badge

A

A radiographer or worker in the nuclear power industry

45
Q

In Rutherford’s scattering experiment how were the scattered α-particles detected?

A

By scintillations on a zinc sulphide screen on the end of a microscope

46
Q

Why are alpha particles deflected less than beta particles of the same KE? Why are they deflected in the opposite direction?

A

1) Alpha particles are more massive

2) They have opposite charges

Physics Year 2 (11 decks)

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