Chapter 14 - Radioactivity

Question 1

When is gamma emitted?

Answer 1

After alpha or beta decay to release excess energy from the nuclei.

Question 2

Define positron?

Answer 2

A positively charged particle with mass equal to that of an electron (form of antimatter).

Question 3

Learn on the table in notes?

Answer 3

Now

Question 4

Define neutrino?

Answer 4

An uncharged particle with almost no mass emitted during beta decay.

Question 5

Word equation for beta minus decay?

Answer 5

Neutron -> proton + electron + antineutrino

Question 6

When are alpha or beta particles emitted?

Answer 6

When a nucleus with a large imbalance between its protons and neutrons becomes stable it emits alpha or beta.

Question 7

Word equation for beta plus decay?

Answer 7

Proton -> neutron + positron + neutrino

Question 8

Define weak interaction/nuclear force?

Answer 8

The nuclear force responsible for beta decay.

Acts on both quarks and leptons

Question 9

What is ionisation?

Answer 9

When fast moving particles (alpha or beta) drag electrons away from atoms when they collide.

Question 10

What happens to Gamma Alpha and beta particles when they pass through an electric field?

Answer 10

• alpha particles are pulled slightly towards the negative plate.
• beta particles are pulled heavily towards the positive place (less mass than alpha therefore deflect more)
• gamma rays move in a straight line and are unaffected.

Question 11

What happens to Gamma Alpha and beta particles when they pass past a magnet?

Answer 11

• beta particles are attracted towards the magnet.
• alpha particles are deflected away from the magnet.
• gamma rays are unaffected by the magnetic field so continue in a straight line.

Question 12

What materials and thicknesses will block Alpha beta and gamma radiation?

Answer 12

Alpha - paper
Beta - lead (few mm)
Gamma - lead (few cm) of concrete (few m)

Question 13

What piece of equipment can be used to detect Alpha beta and gamma radiation?

Answer 13

Geiger-müller tube

Question 14

Name two other piece of equipment that can detect alpha radiation?

Answer 14

Cloud chamber or solid-state detector.

Question 15

Two words to describe decay?

Answer 15

Spontaneous and random

Question 16

What makes decay spontaneous?

Answer 16

• that decay of a nucleus isn’t affected by the presence of other nuclei
• that decay of nuclei can’t be affected by external factors such as temp. or pressure

Question 17

What makes decay random?

Answer 17

• that it’s impossible to predict when a nucleus will decay

- that each nucleus in the sample has same chance of decaying per unit time

Question 18

Define decay constant (λ)?

Answer 18

The probability that an individual nucleus will decay per unit time interval (s^-1 or h^-1 etc.).

Question 19

Define activity?

Answer 19

The rate of decay or disintegration of nuclei in a radioactive sample (in decays/second = Bq).

Question 20

Activity equation? (2)

Answer 20

A=λN

N = number of undecayed nuclei in sample

A=ΔΝ/Δt

Question 21

Define count rate?

Answer 21

The number of particles or gamma-ray photons detected per unit time by a Geiger-Müller tube.

Question 22

Two reasons why the count rate is not the true value?

Answer 22

• some radiation absorbed by sample

- some radiation not from sample due to background radiation

Question 23

Define half life?

Answer 23

The mean time taken for half the number of active nuclei in a radioactive sample to decay.

Question 24

Exponential decay equations for count rate (R), number of undecayed nuclei (N) and activity rate (A)?

Answer 24

N = N0e^(-λt)

R = R0e^(-λt)

Α = Α0e^(-λt)

0 means original/initial value

Question 25

Define carbon dating?

Answer 25

A technique used to date relics using the carbon-14 isotope.

Question 26

Explain how carbon dating works?

Answer 26

All living organisms absorb small amounts of carbon-14. It has a half life roughly 5800 years. Once an organism dies it does not absorb any more carbon-14, and so the remaining radioactive isotope decays gradually. Therefore the time since death can be determined using a mass spectrometer to measure the activity.

Question 27

Explain four problems with carbon dating?

Answer 27

• recently dead organisms mean not enough change in carbon-14 content therefore it won’t work (min 100years).
• modern organisms may have less carbon-14 sent so many fossil fuse have been burnt.
• such a small amount of carbon-14 means there is a low activity rate so it can take a while.
• +-40 years uncertainty on carbon-14 half life.

Question 28

What are the two half life equations?

Answer 28

N/N0 = e^(-λt(1/2)) = 0.5

Therefore

λ = ln(2)/t(1/2)

t(1/2) is half life

Question 29

Why is the activity for very old samples measured over a period of time and then averaged?

Answer 29

Activity is very small/decay is random

Question 30

Why can carbon dating not be used for extremely old samples?

Answer 30

The activity is so low it cannot be differentiated from the background radiation