chapter 25 - Radioactivity

Question 1

What are the three types of radiation?

Answer 1

alpha, beta, gamma

Question 2

What is ionising radiation?

Answer 2

Radiation that removes electrons from atoms, creating positive ions

Question 3

What is alpha decay?

Answer 3

When an alpha particle (2 protons and 2 neutrons) is emitted from the nucleus

Question 4

What are the properties of alpha radiation?

Answer 4

• Very ionising
• Very short range
• Very low penetrating
• Charged (+2e)

Question 5

What’s beta- decay?

Answer 5

Decomposition of down quark into up quark (neutron into proton) to make an electron and antineutrino

Question 6

What’s beta+ decay?

Answer 6

Decomposition of up quark into a down quark (proton into neutron) to make a positron and neutrino

Question 7

What are the properties of beta radiation?

Answer 7

• Ionising
• Short range
• Low penetrating
• Charged (-1e or +1e)

Question 8

Whats gamma decay?

Answer 8

Emission of high energy gamma photons from an unstable atoms nuclei

Question 9

What is the wavelength of gamma rays?

Answer 9

Less than 10^-13

Question 10

What are the properties of gamma radiation?

Answer 10

• Low ionising
• Long range
• Highly penetrating
• No charge

Question 11

How can you use electric fields to distinguish between radiations?

Answer 11

• Beta- radiation (electrons) will be deflected towards +ve plate because they’re -ve
• Beta+ radiation (positrons) and alpha radiation (helium nuclei) will be deflected towards -ve plate as they’re both +ve ( alpha deflected less due to its heavier mass)
• Gamma radiation (photons) won’t be deflected at all as they have no charge

Question 12

How can you use magnetic fields to distinguish between radiations?

Answer 12

• Beta- radiation (electrons) will be deflected downwards as they’re -ve
• Beta+ radiation (positrons) and alpha radiation (helium nuclei) will be deflected upwards as they’re both +ve
• Gamma radiation (photons) won’t be deflected at all as they have no charge

Question 13

How do you stop alpha?

Answer 13

A few centimetres of air or a thin sheet of paper

Question 14

How do you stop beta?

Answer 14

1 meter of air or 1-3 mm of aluminium

Question 15

How do you stop gamma?

Answer 15

A few centimetres of lead

Question 16

Why is radiation dangerous?

Answer 16

Ionising which causes damage to living cells

Question 17

What is the alpha decay equation

Answer 17

A: X :Z → A-4: X :Z-2 +4He2

226Ra88→222Rn86+4He2

Question 18

What kind of nuclei undergoes beta minus decay?

Answer 18

One with too many neutrons to be stable

Question 19

What kind of nuclei undergoes beta positive decay?

Answer 19

One with too many protons to be stable

Question 20

What kind of nuclei undergoes beta positive decay?

Answer 20

One with too many protons to be stable

Question 21

What’s the beta- decay equation?

Answer 21

A: X :Z → A: X :Z+1 + e− + anti-neutrino

14C6→ 14N7 + e− + anti-neutrino

Question 22

What’s the beta+ decay equation?

Answer 22

A: X :Z → A: X :Z-1 + e+ + νe

23Mg12 → 23Na11 + e+ + νe

Question 23

What kind of nucleus emits gamma radiation?

Answer 23

One with a surplus of energy (especially just after alpha or beta emission)

Question 24

What’s the gamma decay equation?

Answer 24

A: X :Z → A: X :Z + γ (photon)

137 56Ba→137 56Ba+γ

Question 25

What is a decay chain?

Answer 25

A parent nuclei decays into a radioactive daughter nuclei, which then becomes the parent for a subsequent decay process until it has a stable nucleus

Question 26

How do you describe radioactive decay?

Answer 26

As a random and spontaneous event

Question 27

Why is radioactive decay random?

Answer 27

- Cannot predict when a particular nucleus in a sample will decay - Cannot predict which nucleus will decay next - Each nucleus in a sample has the same chance of decaying per unit time

Question 28

Why is radioactive decay spontaneous?

Answer 28

- Not affected by presence of other nuclei in sample | - Not affected by external factors (like temperature or pressure)

Question 29

What's half life?

Answer 29

The time taken for the radioactivity of a specified isotope to fall to half its original value

Question 30

How do you calculate half life?

Answer 30

Half life = ln(2)/λ

Question 31

How can you simulate half life with dice?

Answer 31

- Throw a large number of dice at the same time (e.g. 100 dice) - Remove all the dice showing a particular number from the group and record the number removed - Throw the lower number of dice again and repeat - Once you've gotten down to one or so dice you stop - Plot a graph of number dice threw against the number of throws - You can use the graph to see that you should have a relatively stable half life

Question 32

What is activity?

Answer 32

The rate at which a source of unstable nuclei decays

Question 33

What is the unit of activity?

Answer 33

Becquerel (Bq) = s^-1

Question 34

What is activity reliant on?

Answer 34

Number of undecayed nuclei and half life

Question 35

How do you calculate activity?

Answer 35

A = λN

Question 36

What's the decay constant?

Answer 36

The probability of decay of an individual nucleus per unit time

Question 37

What's the decay constant?

Answer 37

The probability of decay of an individual nucleus per unit time

Question 38

What are the units for decay constant?

Answer 38

s^-1 or h^-1 or y^-1

Question 39

What is the exponential decay equation for activity?

Answer 39

A = Ao x e^(-λt) ``` A = activity Ao = initial activity λ = decay constant t = time ```

Question 40

What is the exponential decay equation for number of undecayed nuclei?

Answer 40

N = No x e^(-λt) ``` N = number of undecayed nuclei No = initial number of undecayed nuclei λ = decay constant t = time ```

Question 41

Why does activity decrease exponentially?

Answer 41

Because activity is directly proportional to the number of undecayed nuclei. Since the number of undecayed nuclei is decreasing exponentially, the activity also has to decrease exponentially

Question 42

How can you determine the half life of protactinium?

Answer 42

- Find the background count - Shake protactinium bottle for 15 seconds to dissolve the protactinium - Place a GM tube directly in front of the bottle without touching it - Take a ten second count of decaying protactinium every 30 seconds - Make sure to zero the count right before every 10 second count - Take the background count away from each of the 10 second counts - Plot graph of corrected count rate against time - You can then find half life by seeing time it takes for count rate to half

Question 43

Why is protactinium difficult to use and transport?

Answer 43

It has a very short half life so has decayed before you get to use it

Question 44

How can you make protactinium?

Answer 44

Get a bottle filled with Uranyl(VI) nitrate and organic solvent - Uranium-238 in uranyl nitrate decays into Thorium-234 - Thorium-234 decays into Protactinium-234 - Protactinium-234 is soluble in organic solvent so dissolves and rises to top of the bottle

Question 45

What is carbon dating?

Answer 45

The process of using the radioactive isotopes in carbon 14 to calculate the age of an artifact

Question 46

How does carbon dating work?

Answer 46

- Living organisms take in carbon-12 and carbon-14 in known ratios while alive - After it dies the carbon-14 isotopes continue to decay while carbon-12 stays same - Therefore the ratio of C-12 to C-14 in fossils can be measured and their age calculated

Question 47

What are the limits of carbon dating?

Answer 47

- Assumes ratios of carbon-12 and carbon-14 have stayed same over time - Carbon-14 in small amounts so activity is comparable to the background rate