Nuclear Decay

Question 1

What is the name given to the low levels of radiation from both natural and man made sources that the environment exposes us to?

Answer 1

Background radiation

Question 2

What is the instrument used to measure radiation?

Answer 2

The Geiger-Muller tube

Question 3

In what unit are radiation levels measured in?

Answer 3

counts per second, this unit is called the becquerel(Bq)

Question 4

What accounts for half of the background radiation on Earth?

Answer 4

Naturally radioactive gases in the atmosphere, particularly Radon, radon is produced in the decay of uranium ore present in certain rocks (especially granite)

Question 5

How should you determine the ‘average’ background radiation in your area using a GM tube?

Answer 5

As radioactive decay is a random and spontaneous process, the activity must be measured over a long period of time (30 minutes or more) and then an average calculated

Question 6

How does background count affect investigations into nuclear radiation?

Answer 6

The background count will skew the results of the investigation, therefore the background radiation must be measured separately and deducted from the count measured in the main part of the investigation.

Question 7

What does decay mean?

Answer 7

The change of a nucleon from one type to another or the change in composition or energy state of the nucleus as a whole.

Many nuclei are slightly unstable and there is a small probability that in any given time, they may decay.

Question 8

What are the three main types of nuclear radiation?

Answer 8

Alpha, Beta and Gamma radiation.

Question 9

How are the three types of nuke radiation different?

Answer 9

Each one comes about through a different process within the nucleus, each one is composed of different particles and each one has different properties.

Question 10

Explain in terms of Kinetic energy how a radiation particle is absorbed by a substance when a nuclear decay occurs.

Answer 10

1. The particle emitted will leave the nucleus with a certain amount of kinetic energy.
2. As the particle travels it will ionize particles in it’s path, losing a small amount of that kinetic energy at each ionization.
3. When all the kinetic energy is transferred the particle stops and is absorbed by the substance it is in at that moment.

Question 11

What are alpha particles?

Answer 11

They consist of 2 protons and neutrons, the same as a Helium nucleus.

Question 12

Why are alpha particles highly ionizing?

Answer 12

Because it’s a relatively large particle with a significant positive charge (+2e)

Due to it ionizing so much, it quickly loses it’s kinetic energy and is easily absorbed.

A few centimeters traveled through air is enough to absorb them. They are also completely blocked by paper and skin

Question 13

What is a Beta particle and how is it made?

Answer 13

An electron emitted at a high speed from the nucleus when a neutron decays into a proton.

Question 14

Why is the Beta particle much less ionizing than the Alpha particle? How is it stopped?

Answer 14

Due to it’s single negative charge and much smaller size.

Several meters of air or a thin sheet of aluminium are needed to absorb Beta particles.

Question 15

What is the least ionizing nuclear radiation?

Answer 15

Gamma rays, they are high energy, high frequency, electromagnetic radiation. These photons have no charge and no mass and will rarely interact with particles in their path.

Question 16

How are gamma rays said to have been “absorbed?”

Answer 16

Although they can never be completely absorbed, their energy can be reduced by several centimeters of lead or several meters of concrete.

If their energy is reduced to a ‘safe’ level, they are said to have been ‘absorbed.’

Question 17

Can Ionizing radiations interact with particles which make up human cells?

Answer 17

Yes, there may be so much radiation that cells die as a result.

Where there is less ionisation, the molecules of DNA in the cells may change slightly. DNA mutations can cause cells to have an increased tendency to become cancerous.

Question 18

Describe how alpha radiation affects the body.

Answer 18

Inside the body, it’s highly ionizing - very dangerous, radiation poisoning and cancer is possible.

Outside, it is absorbed by the surface layer of dead skin cells. So no danger.

Question 19

Describe how beta radiation affects the body.

Answer 19

Inside, moderate ionisation and danger, exposure should be minimised.

Outside, moderate ionisation and danger, ‘close’ exposure should be minimized.

Question 20

Describe how gamma radiation affects the body.

Answer 20

Inside, minimal ionisation - cancer danger from long term exposure.

Outside, same as inside

Question 21

What is the decay constant?

Answer 21

Probability that a nucleus will decay, for each second that it exists.

Since radioactive decay is a spontaneous and random process, any radioactive nucleus may decay at any moment.

Question 22

Is the likelihood that a particular nucleus will decay, affected by factors outside the nucleus?

Answer 22

No, each nucleus acts entirely independently.

Question 23

Explain what is meant by ‘activity,’ A?

Answer 23

The number of nuclei decaying per second.

A= (dN/dt)

Naturally, when the sample is larger, then the number that decay in a second will be greater. So the activity is proportional to the number of nuclei in that sample.

Mathematically expressed as,

A= -λN, the minus sign means N decreases with time.

Question 24

What is the formula for the rate of decay of nuclei?

Answer 24

check bottom section of page 132

Question 25

What is the half life of a radioactive sample?

Answer 25

The time in which the strength of the source decays to half it’s original value?

ln2/λ