Topic P6- Radioactivity

Question 1

What is an isotope?

Answer 1

Atoms of the same element- they have the same number of protons but a different number of neutrons.
They have a different nuclear mass but the same nuclear charge.

Question 2

Describe the ways Nuclei can Decay:

Answer 2

When nucleus decays, it will spit out one or more of four types of radiation: alpha, beta, gamma and neutron.
In this process, the nucleus will often change into a new element as the nucleus changes its charge, it mass or both.

An alpha particle is two neutrons and two protons, the same as a helium nucleus. They have a relative mass of 4 and a charge of +2. They are relatively big and heavy and slow moving.

A beta particle is simply an electron with virtually no mass and a charge of -1. Beta particles move quite fast and are quite small. For ever beta particle emitted, a neutron turns to a proton in the nucleus.

After spitting out an alpha or beta particle, the nucleus might need to get rid of some extra energy. It does this by emitting gamma rays- a type of EM wave. Gamma rays have no mass and no charge, they are just energy. So, they don’t change the element of the nucleus that emits them.

If a nucleus contains a lot of neutrons, it may just throw out a neutron. The number of protons stays the same, but it now has a different nuclear mass, so it becomes an isotope of the same element.

Question 3

Compare the penetration properties of alpha, beta and gamma radiation:

Answer 3

Alpha particles don’t get very far before they start colliding with atoms- they have the shortest range in a material.

Beta particles can travel quite far before hitting an atom.

Gamma radiation travels a long way before hitting an atom- so gamma radiation has the longest range in a material.

Question 4

What is count rate?

Answer 4

Count rate is the number of radioactive particles that reach a detector in a given time.
The further the radiation has to travel, the higher the chance it will be absorbed by the material it is travelling through.
This means the count rate decreases the further the detector is from a radioactive source.

Question 5

What materials can block: alpha particle radiation, beta particle radiation and gamma ray radiation?

Answer 5

Alpha particles can be blocked by paper.
Beta particles can be blocked by thin aluminium.
Gamma rays can be blocked by thick lead.

Question 6

What is the nuclear equation for alpha radiation?

Answer 6

When a nucleus emits an alpha particle:

• the mass number decreases by 4- as it loses two protons and two neutrons.
• the atomic number decreases by 2 because it has two less protons.

226 222 4
Ra → Rn + α
88 86 2

Question 7

What is the nuclear equation for beta radiation?

Answer 7

When a nucleus emits a beta particle a neutron changes into a proton, so:

• the mass number does not change- it has lost a neutron but gained a proton.
• the atomic number increases by 1, because it has one more neutron.

14 14 0
C → N + β
6 7 -1

Question 8

What is the nuclear equation for gamma radiation?

Answer 8

When a nucleus emits a gamma ray:
-the mass number and the atomic number don’t change.

234 234
Pa → Pa + γ
91 91

Question 9

Describe what happens when an electrons becomes ‘excited’ to higher energy levels:

Answer 9

• Electrons in an atom sit in different energy levels or shells.
• each energy level is a different distance from the nucleus.
• An inner electron can move up one or more energy levels in one go if it absorbs EM radiation with the right amount of energy.
• When it does move up, it moves to a partially filled (or empty) shell and is said to be ‘excited’.
• The electron will then fall back to its original energy level, and in doing so will lose the same amount of energy it absorbed.
• the energy is carried away by EM radiation.

Question 10

What is ionisation (in radiation):

Answer 10

• if an outer electron absorbs radiation with enough energy, it can move so far that it leaves the atom.
• It is now a free electron and the atom is said to have been ionised.
• The atom is now a positive ion,
• It’s positive because there are now more protons than electrons.
• An atom can lose more than one electron.
• The more electrons it loses, the greater its positive charge.

Question 11

How do florescent tubes work?

Answer 11

• They use excited electrons to produce light.
• Florescent tubes contain mercury vapour.
• Electrons are accelerated through the mercury vapour, which ionises some of the mercury atoms, producing more free electrons.
• When this flow of free electrons collides with electrons in other mercury atoms, the electrons in the mercury atoms are excited to higher energy levels.
• When these excited electrons return to their original energy levels, they emit radiation in the UV range of the EM spectrum.
• A compound called a phosphor coats the inside of the tube and absorbs this radiation, exciting its electrons to higher energy levels.
• These electrons then cascade down the energy levels emitting many different frequencies of radiation, all in the visible part of the EM spectrum.