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Physics Year 13 > Chapter 25 - Radioactivity > Flashcards

Chapter 25 - Radioactivity Flashcards

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1
Q

key features of alpha radiation

A
  • consists of positively charged alpha particles (He Nucleus)
  • charge = +2e
  • made of two protons and two neutrons
  • very ionising
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2
Q

key features of beta radiation (both types)

A
  • Beta-minus = fast moving electrons
  • charge = -e
  • Beta-plus = fast moving positrons
  • charge = +e
  • both are weakly ionising
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3
Q

key features of gamma radiation

A
  • high energy photons
  • wavelength = 10^-13m
  • no charge
  • travel at speed of light
  • barely ionising
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4
Q

state/draw what occurs to these types of radiation in an electric/magnetic field where the magnetic field goes into the paper and the electric field has +ve at top, -ve at bottom

how can we explain this

A
  • alpha curves down towards the -ve plate
  • beta-minus curves up towards the positive plate
  • beta-plus curves down to the -ve plate (more than alpha)
  • gamma is undeflected
  • think about opposite charges attracting or FLHR
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5
Q

describe the procedure for the absorption practical

A

1) set up a radiation source pointing at a GM tube, connected to a counter, measure the distance from the source to the tube and keep constant
2) measure the count of background radiation for 30 seconds, repeat 3 times, calculate an average activity
3) place your source in the radiation holder and record count for 10 seconds, calculate an activity, and correct for background rate
4) place an absorbing material between the source and the GM tube, record for another 10 seconds and calculate corrected activity
5) repeat for other absorbing materials and radiation types and compare

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6
Q

what will absorb the different types of radiation

A 
alpha = few cm of air, paper
beta = few metres air, few mm of aluminium
gamma = lots of lead
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7
Q

what is meant by ionising and rank the types of radiation in order of their increasing ionising ability

A
  • their ability to remove an electron from atoms they interact with
  • gamma, beta, alpha
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8
Q

what are some safety precautions that should be taken when dealing with radiation

A
  • store radioactive sources in a lead lined box
  • use long handled tongs
  • wear gloves
  • point away from people
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9
Q

what are the typical speeds of the types of radiation

A 
alpha = 10^6ms^-1
beta = 10^8ms^-1
gamma = 3x10^8ms^-1
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10
Q

what occurs in alpha decay, write a generalised equation

A

(A,Z)X —-> (A-4,Z-2)Y + (4,2)(alpha)

  • two protons and two neutrons are removed/emitted from the nucleus
  • nucleon number decreases by 4
  • proton number decreases by 2
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11
Q

what occurs in beta-minus decay, write a generalised equation

A

(A,Z)X —-> (A,Z+1)Y + (0,-1)(e-) + antineutrino
n —> p + e- + antineutrino

  • caused by the weak nuclear force, when a nucleus has too many neutrons for stability
  • a neutron breaks down into a proton, electron and antineutrino
  • nucleon number remains constant
  • atomic number increases by 1
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12
Q

what occurs in beta-plus decay, write a generalised equation

A

(A,Z)X —-> (A,Z-1)Y + (0,1)(e+) + neutrino
p —> n + (e+) + neutrino

  • caused by weak nuclear force
  • occurs when a nucleus has too many protons for stability
  • proton breaks down into neutron, positron and neutrino
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13
Q

what occurs in gamma radiation, write a generalised equation

A

(A,Z)X —-> (A,Z)Y + gamma

  • caused when there’s an excess of energy in the nucleus following a decay
  • nothing changes
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14
Q

what is conserved in a nuclear decay

A
  • nucleon number
  • atomic number
  • charge number
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15
Q

what is the pattern of stability in nuclei

A

up to about Z = 20, the line for stability follows N = Z

beyond this, the line curves up and there more neutrons than protons in the stable isotopes

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16
Q

what are the two key features of radioactive decay

A
  • it is random and spontaneous
17
Q

what is meant by radioactive decay being random

A
  • we cannot predict when any given nucleus will decay

- there is a constant probability of decay in any given nucleus

18
Q

what is meant by radioactive decay being spontaneous

A
  • nuclei are not affected by nearby decays

- not affected by external factors

19
Q

define the half life of an isotope

A

“the half life of an isotope, t1/2, is the average time taken for half the number of active nuclei in the sample to decay”

20
Q

what simple equation can we make using half life and what is a useful rearrangement

A

N = No x (1/2)^n
where n is the number of half lives ‘completed’

this can be rearranged to
n = log(1/2)(N/No)
or
n = log2(No/N)

21
Q

define the activity of a sample

A

“the activity, A, of a source is the rate at which nuclei decay or disintegrate, the number of decays per unit time”

22
Q

what is the equation for activity, and the units

A 
A = deltaN/deltaT
activity = Bq (becquerels)
23
Q

what can we say about proportionalities to do with the activity of a source, what is the constant of proportionality

A
  • activity must be proportional to the number of nuclei in the source

A = kN

k in this case is the decay constant

24
Q

define the decay constant

A

“the decay constant, lambda, is the probability of an individual nucleus decaying per unit time (second)”

25
Q

what is our equation linking activity and number of nuclei

A

A = Lambda (N)

26
Q

which equation is obtained by finding the solution to the activity equation, what are it’s derivatives

A

N = No e^-(lambda)(t)

as N is directly prop to activity and mass
A = Ao e^-(lambda)(t)
M = Mo e^-(lambda)(t)

27
Q

how can we derive the identity linking half life and decay constant

A 
using N = No e^-(lambda)(t)
at t(1/2), N = No/2
so
No/2 = No e^-(lambda)(t)
1/2 = e^-(lambda)(t)
ln(1/2)  = -(lambda)(t)
Lambda(t) = ln(2)
28
Q

how can you determine the half life of a radioactive source

A

1) set up a GM tube and counter
2) record background count for 30 seconds, repeat 3 times, calculate an average activity
3) place radioactive source pointing towards GM tube, measure distance between and keep constant
4) start a stopwatch, record count for 10 seconds each half minute and determine the corrected count rate each time
5) plot a graph of corrected count rate against time
6) find half life at 3 different points and take an average

OR Plot ln(A/(corrected)) against time, determine lambda from the graph and t1/2 accordingly

29
Q

how can we model radioactive decay using an iterative method

A

1) start with No undecayed nuclei
2) Choose a very small (relative to half life) time period (deltaT)
3) using DeltaN = (lambda)(deltaT)(N) calculate the proportion/number of nuclei decaying each time
4) calculate number of undecayed nuclei after each time period using
N = No - deltaN

5) repeat for subsequent multiples of DeltaT

30
Q

how can we make our iterative radioactive modelling more accurate

A

use smaller time intervals

31
Q

why do all living things on earth contain carbon-14

A
  • carbon is stored in fats, lipids, glucose etc.

- a certain proportion of all carbon is carbon 14

32
Q

what is the half life of carbon 14

A

5700 years

33
Q

how is carbon 14 produced

A
  • cosmic rays interact with atoms in the upper atmosphere producing neutrons
  • these react with nitrogen 14, to produce carbon 14

(1,0)n + (14,7)N —> (14,6)C + (1,1)p

34
Q

what are some limitations to carbon dating

A
  • it assumes the ratio of carbon-12 to carbon-14 remains constant, this may not be the case if CO2 emissions, volcanic eruptions and exposure to radiation have been present, as this can change it
  • the activity of small samples of carbon is very small and can be difficult to detect due to background radiation
35
Q

how can we date rocks

A

use the radioactive decay (through beta-minus) of rubidium-87

half life = 49 billion years

36
Q

describe how carbon dating works (4)

A
  • living things stop taking in carbon after they die
  • the initial ratio of carbon-14 to carbon-12 can be calculated by taking data from a living sample
  • the current ratio of carbon-14 to carbon-12 can be calculated
  • the age of the sample can be determined using a half life of 5700 years and
    X = Xoe^-(lambda)t
37
Q

why is a ln(A) against t graph more reliable than an A against t graph for determining decay constant or half life

A
  • exponential plot gives decay curve, harder to draw
  • easier to see anomalies with a straight line than a curve
  • the exponential curve is more affected by the random/spontaneous nature of radioactive decay than the straight line, more susceptible to random error
  • easier to average out the random error using a straight line
    etc.

Physics Year 13 (15 decks)

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