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AQA A-level Physics > Chapter 26 - Radioactivity > Flashcards

Chapter 26 - Radioactivity Flashcards

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1
Q

What are the 3 types of radiation?

A
  • alpha
  • beta
  • gamma
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2
Q

Draw and briefly describe rutherford’s alpha-scaterring experiement?

A
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3
Q

What were the results of rutherford’s experiement and what did this tell us about the atom?

A
  • most alpha particles passed straight through the foil (most of atom is empty space)
  • 1 in 2000 alpha particles were deflected (centre of atom (nucleus) must be positive
  • 1 in 10000 alpha-particles were reflected back (most atom’s mass is concentrated in a small refion at the centre called the nucleus)
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4
Q

How can the ionising effect of each type of radiation be investigated?

A

Ionisation chamber and a picoammeter.

Ions created in the chamber are attracted to the oppositely charged electrode where they are discharged. Electrons pass through picoammter as a result of ionisation in the chamber. The current is proportional to the number of ions per second created in the chamber.

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5
Q

Rank the 3 types of radiation in terms of ionising power from strongest to weakest?

A

alpha radiation - strongly ionising

beta radiation - weakly ionising

gamma radiation - very weakly ionising

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6
Q

Describe the particle tracks for alpha and beta in a cloud chamber

A

alpha: straight tracks
beta: wispy tracks

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7
Q

What is alpha radiation absorbed by?

A

layer of paper/dead skin cells

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8
Q

What is beta radiation absorbed by?

A

5mm of metal

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9
Q

What is gamma radiation absorbed by?

A

several cm of lead

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10
Q

*Draw and breifly describe experiment for investigating absorption of materials

A

Measure count rate with no absorber and minus background count rate. Then measure count rate with an absorber in a fixed position between the source and tube, then calculate corrected count rate and so this for varying thickness of absorber.

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11
Q

What is the count rate?

A

Number of counts per second

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12
Q

What is contained in a geiger-muller tube?

A

Argon at low-pressure

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13
Q

What is the purpose of the the thin mica window in a geiger-muller tube?

A

Allows alpha and beta particles to enter the tube (gamma photons can enter the tube through the walls too)

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14
Q

What runs down the middle of the geiger-muller tube?

A

A metal rod at a positive potential

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15
Q

What happens when a particle of ionising radiation enters the GM tube?

A

Particle ionises the gas atoms along its track. Negative ions attracted to the rod, positive ions attracted to the wall. Ions produces further ions

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16
Q

How is a count recorded in a geiger-muller tube?

A

Ions cause a pulse of charge to pass round the circuit through a resistor, causing Pd pulse across resistor - which is recorded as a count.

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17
Q

What is alpha radiation’s range in air?

A

Few centimetres in air

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18
Q

What is beta radiation’s range in air?

A

Metre in air

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19
Q

What is gamma radiation’s range in air?

A

Unlimited range in air (follows inverse square law)

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20
Q

How do the radiation types deflect in a magnetic field?

A

Alpha and beta deflect in opposite directions.

Beta deflects more than alpha and gamma is not deflected

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21
Q

Define intensity of radiation?

A

Radiation energy per second passing normally through a unit area

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22
Q

Write the equation for the inverse square law

A

I = k/r2

k = nhf/4π

I (intensity) of the radiation varies with the inverse square of distance, r.

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23
Q

Describe how the inverse-square law can be verified experiementally. (for a gamma source)

A

Use a gieger-muller tube to measure the count rate (C) at different measured distances,d, from the tube and the background count rate (CO) without the source present. The corrected count rate, C-CO, is proportional to the intensity of the radiation

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24
Q

Why is ionising radiation harmful?

A

It damages living cells by damaging DNA - causing uncontrolled cell division. Possibly causing cancer

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25
Q

What is dose of radiation measured in?

A

Sieverts (Sv)

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26
Q

What does the biological effect of ionising radiation depend on?

A

Dose recieved and the type of radiation

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27
Q

How can exposure to ionising radiation be reduced?

A
  • Wear film badge to monitor exposure to ionising radiation. If overexposed, the wearer shouldn’t be allowed to continue working with the equipment
  • Storage of radioactive materials should be in lead-lined containers
  • reduce radiation dose to minimum
  • No source should be allowed to touch skin (solid sources transferred using handling tools, liquids + gases sealed in containers)
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28
Q

What is background radiation?

A

Naturally occuring radiation due to cosmic radiation, radioactive materials in rocks + soils etc

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29
Q

What does background radiation vary with?

A

Local geological features

e.g. radon gas can be accumulate in poorly ventilated areas of buildings in certain locations

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30
Q

What are the main sources of background raiation in the UK?

A
  • Air (e.g. Radon gas)
  • Medical uses
  • ground and building
  • Nuclear weapons
  • Nuclear power
  • Air travel
  • Food + drink
  • Cosmic rays
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31
Q

How should radioactive materials be stored?

A
  • Should be in lead-lined containers (thick enough to reduce the gamma radiation from the sources in the container to about background level)
  • Under lock and key
  • record of sources
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32
Q

What does random nature of radioactive decay mean?

A

Every nucleus of a radioactive isotope has an equal probability of undergoing radioactive decay in any given time interval. Which atom that decays is chance - can’t be known which particular nuclei will decay next

33
Q

What is number of nuclei that disintegrate (delta N) in a time interval proportional to?

A
  1. ) N, the number of nuclei of x remaining at time t
  2. ) Duration of the time interval (delta t)
34
Q

What is the rate of disintegration (activity) equation?

A
35
Q

What equation represents number of nuclei (N) against time (t) graph?

A
36
Q

Given mass of a radioacive isotope is proportional to the number of nuclei, N, of the isotope; what is the equation of mass throughout the reaction?

A
37
Q

What is activity of a radioactive isotope?

A

Number of nuclei of the isotope that disintegrates per second (rate of change of the number of nuclei of the isotope)

units: Becquerel (Bq)

38
Q

Give the equation for activity of a sampe nuclei of an isotope

A
39
Q

What is half life of a radioactive isotope (T1/2​)?

A

Time taken for mass of the isotope to decrease to half the initial mass

or

Time taken for number of nuclei of the isotope to decrease to half the initial number

40
Q

How do you find the half life from a mass of radioactive sample - time graph?

A

Draw line from half of original mass of isotopeand draw line down to time at which this happens

41
Q

What is the equation for corrected count rate for a given time in decay?

A
42
Q

What is the decay constant? (lamba)

A

Probability of an individual nucleus decaying per second

43
Q

What’s the equation for half life?

A

T1/2 = ln2/decay constant

44
Q

How does carbon dating work?

A

Living plants + trees contain a small percentage of carbon-14 formed in the atmosphere as a result of cosmic rays knocking out neutrons from nuclei.

Measuring the decay of C-14 (T1/2 = 5570 years) of a dead sample - always an estimate for the age of the sample.

45
Q

What is a radioactive tracer?

A

Used to follow the path of a substance through a system

46
Q

What are some key features a radioactive tracer should have?

A
  • Half life which is stable enough for the necessary measurements to be made and short enough to decay quickly after use.
  • Emit beta radiation or gamma radiation so it can be detected outside the flow path.
47
Q

What are some applications of radioactive tracers?

A
  • Detecting underground pipe leaks (Beta or gamma emitter/tracer injected into the flow. Detector on surface used to detect leakage)
  • Modelling oil reservoirs mathematically to improve oil recovery (beta emitter titrated water HO31 containing water injected into oil reservoir)
  • Investigating the uptake of fertilizers by plants (plant watered with solution containing fertilizer (containing phosphorus). Radioactivity of leaves proportional to amount of fertilizer.
  • Monitoring uptake of iodine by thyroid gland (given sodium iodide solution containing radioactive 53I131. Activity measured)
48
Q

What is the choice of a radioactive isotope for a particular application determind by?

A
  • Half life + type of radiation needed
  • Toxicity and biochemical suitability of the pharmaceuticals to which it is attracted.
  • Whether or not a stable product is needed
49
Q

How is radiation used in the manufacture of metal foil?

A
  • Thickness monitoring

rollers squeeze plate metal on a continous production line. Detector measures amount of radiation passing through the foil. If detector reading drops, foil is too thick, so signal is fed back to control system to make rollers move together to make the foil thinner.

  • Source is a beta emitter with a long half life
50
Q

Whatcan you tell from an N-Z graph?

A

Nuclear stability

51
Q

Where is the stability belt found on N-Z graph?

A

Curve upwards with increasing neutron-proton ratio from the origin to N = 120 , Z = 80 approximately

52
Q

For light isotopes (proton number (Z) from 0 to 20) what pattern do the stable nuclei follow on the N-Z graph?

A

Follow a straight line of N=Z

53
Q

What happens to the stable nuclei as Z increases beyond 20?

A
  • Have more neutrons than proton (neutron/proton ratio increases)
  • Extra neutrons bind the nucleus together without introducing repulsive electrostatic forces as more protons would do
54
Q

Where are alpha emitters found on the N-Z graph, and why?

A
  • Occur beyond Z=60 (have more neutrons than protons but too large to be stable)
  • Because the strong nuclear force between the nucleons is unable to overcome the electrostic force of repulsion between protons
55
Q

Where are Beta- emitters found on the N-Z graph, and why?

A
  • Left of the stability belt

Isoptopes are neutron rich here, that become more stable by converting a neutron into a proton and emitting a beta parrticle

56
Q

Where are Beta+ emitters found on the N-Z graph, and why?

A
  • Right of the stability belt

Isotopes are proton rich and become stable by converting proton nto a neutron and emitting a B+ particle (and neutrino)

57
Q

*How can emission of an alpha particle, B- particle and B+ particle be represented on a N-Z graph?

A
58
Q

On a N-Z graph, what does electron capture lie in the same region as?

A

B+ emitters

59
Q

Why would a gamma photon be emitted after an unsafe nucleus emits an alpha or beta particle?

A

Daughter nucleus formed in an excited state

60
Q

What is a technetium generator?

A

Used in hospitals to produce a source which emits gamma radiation only

61
Q

What is a metastable state?

A

When a radioactive isotope forms in an excited state (emits gamma radiation) after alpha emission or beta emission and stays excited long enough to be seperated from the parent isotope

62
Q

In the technetium generator, how is Tcm-99 formed?

A

After beta emission of the molybdenum isotope - 99Mo42 whch has a half life of 6 hours and decays to ground state by gamma emission. A technetium generator has an ion exchange coloumn containing emmonium molybdenum exposed to neutron radiation several days earlier to make a significant number of the molybdenum nuclei unstable. A solution of sodium chloride is then passed through the column, some of the chloride ions exchange with pertechnate ions but not with molybdenate ions so the solution that emerges conatins 99Tc43m

63
Q

What are examples of diagnostic (medical) uses of 99Tc43m?

A
  • Monitoring blood flow through the brain -use external detectors after a small quantity of sodium pertechnate solution is administered intravenously.
  • gamma camera- designed to image internal organs and bones by detecting gamma radiation from sites in the body where gamma - emitting isotope such as 99Tcm is located. Bone deposits can be located using phosphate tracer labelled with 99Tcm. Consists of photomultiplier tubes in a lead shield behind a lead collimator grid which ensures each tube only detects gamma photons emitted from nuclei located at a well defined spot.
64
Q

What are the 2 ways of measuring nuclear radius?

A
  1. ) Distance of closest approach of scattered particle
  2. ) Electron diffraction
65
Q

Describe distance of closest approach method in finding nuclear radius?

A

Rutherford’s scattering experiment used to caonclude alpha particles that bounce back and are deflecting through 180 degrees(reflected) will have stopped a short distance from the nucleus (point where electrical potential energy equals it’s kinetic energy - conservation of energy)

closest approach = nuclear radius (max value)

66
Q

What is the equation for initial kinetic energy of the alpha particles? (using coulombs’s law)

A

Initial Ek = Eelec = Q(nucleus) x q(alpha)/ 4πEor

Q/q = charge

r = distance of closest approach

67
Q

Describe the electron diffraction method for calculating nuclear radius

A

Beam of high energy electrons is directed at a thin solid sample of an element, the incident electrons is diffracted (due to de broglie wavelength same as diameter of the nucleus - 10-15m) by the nuclei of the atoms in the foil. Beam produced by accelerating electrons through a potential difference of the order of million volts. Detector used to measure the number of electrons per second diffracted through different angles

68
Q

What is the typical value for a nucleus diameter?

A

10-15m

69
Q

What happens to the number of electrons detected as an angle theta of the detctor to the zero order beam is increased?

A

Number of electrons diffracted into the detector decreases, then increases slightly, then decreases again.

70
Q

*Draw a graph of angle of diffraction against detector reading

A
71
Q

What is used to calculate the diameter of the nucleus in electron diffraction?

A

Angle of first minimum

angle (min) = 1.22 x wavelength/ 2R

where R is radius

72
Q

What is the equation showing nuclear radius’ (R) dependence on mass number (A)?

A

R = r0A1/3 where ro = 1.05 fm

73
Q

*What does a graph of R against A1/3 show?

A
  • Straight line through the origin
  • Gradient = ro
74
Q

What is special about density of the nucleus?

A

Density is constant, independent of radius

nucelons are seperated by the same distance regardless of the size of the nucleus (evenly spaced).

75
Q

Calculate the denisty of a nucleus

A

V = 4/3 π R3 = 4/3π (roA1/3)3 = 4/3 π ro3A

Since mass, m = Au where u = 1.67 x10-27Kg

density = Au/4/3ro3 A= 1u/4/3ro3 = 1.67x10-27/4/3π(1.05x10-15)3 = 3.4 x 1017 Kgm-3

76
Q

What is the advantage of doing elelctron diffraction over distance of closest approach to find nuclear radius?

A

Electrons are leptons, therefore don’t interact with nucleons via strong force (like alpha particles do) so they can get closer to nucleus - increasing accuracy/resolution

Easier to produce high energy electrons - as they are easier to accelerate

Electrons give less recoil

77
Q

Why does an alpha particle cause more ionisation than a beta particle, when they have the same kinetic energy?

A

Alpha particle has more mass and has twice as much charge as a beta particle.

78
Q

What type of radiation is likely to be the most appropriate for sterilisation of surgical equipment

A

gamma radiation because it is very / the most penetrating so will irradiate all sides of the instruments.

Also, instruments can be sterilised without removing the packaging.

79
Q

Why won’t irradiated surgical instruments become radioactive once sterilised with gamma radiation?

A

To become radioactive the nucleus has to be affected which (ionising) radiation does not do.

Energy of photons not high enough to induce nuclear decay

AQA A-level Physics (31 decks)

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