Detection of Radioactive Emission Archives

How do you detect radioactivity?

Detection of Radioactive Emission:

Radioactive emission is invisible to the naked eye. It cannot be detected by any of our senses because it has no smell, no taste and makes no sound.
The detection of radioactive emissions is by the ionising effect of radiation on the atoms of matter.

An energetic particle or photon that passes through a medium can knock an electron out of an atom in that medium. This process is known as ionisation and it produces charged particles called ions.

Figure illustrates the ionising process when an energetic particle interacts with an atom of a medium.
Types of Radioactive Emissions 5

The ionisation of an atom produces an ion pair which consists of an electron as the negative ion and a positively charged atom as the positive ion.

Therefore radioactive emission is also known as ionising radiation.

Detectors of radioactive emission detect the ion pairs produced by the radioactive emission.

Table gives a list of detectors and the type of radioactive emission they can detect.

Name of detector	Type of radiation that can be detected	Observation when radiation is detected
Photographic badge	β-particles γ-rays	Darkening of the photographic plate in the badge
Geiger- Muller tube with ratemeter	α-particles β-particles γ-rays	The ratemeter shows a count rate higher than the background count.
Cloud chamber	α-particles β-particles γ-rays	Tracks with specific characteristics are formed in the cloud chamber.
Spark counter	α-particles	Sparks are seen and heard between the wire gauze and the wire below it.

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Geiger-Muller Tube Types of Radioactive Emissions 6

The Geiger-Muller tube (or the GM tube) is a very sensitive and useful detector of radiation. It can detect a-particles, /3-particles and y-rays.

A radioactive emission enters the tube through the mica window and ionises the argon gas inside the GM tube.

The ions produce a pulse of current that is counted by a scaler or ratemeter.

The scaler gives the number of counts over a certain period of time

The ratemeter gives the count rate in counts per second or counts per minute.
Types of Radioactive Emissions 7

Initially the GM tube is switched on without the presence of any radioactive substance. The reading displayed by the ratemeter is known as the background count rate.

When the GM tube is used to detect radiation from a radioactive substance, the background count rate is subtracted from the count rate obtained.

Cloud Chamber

A cloud chamber shows the path travelled by ionising radiation in air.

In a cloud chamber, radiation produces ions in the air that is saturated with alcohol vapour.

The alcohol vapour condenses on the ions to make the tracks of the radiation visible.

Figure shows the tracks-of alpha particles, beta particles and gamma rays.
Types of Radioactive Emissions 17

Spark Counter Types of Radioactive Emissions 9

A spark counter consists of a wire gauze and a thin wire below it.

A high voltage is applied between the gauze and the wire. The voltage is adjusted until it is just below the value required to produce sparks.

When a radioactive source is brought near the wire gauze, the radiation ionises the air between the wire gauze and the thin wire. The motion of the ions to the gauze and the wire causes sparks to be produced. The sparks can be seen and heard.

Spark counters are suitable for detecting alpha particles. Beta-particles and gamma rays produce too few ions to produce sparks.

Photographic Badge Types of Radioactive Emissions 10

The photographic badge is worn by workers in nuclear power stations and in radiation laboratories.

The badge contains a photographic film in a light¬proof packet.

The film is developed at the end of each month.

The parts of the film which had received radiation will be darkened. The degree of darkening indicates the amount of radiation the person had been exposed to.

Radioactivity: Types of Radioactive Emissions

What is radioactivity in physics?

Radioactivity:

You may have heard of this statement
“Danger : Radiation Kills”. Do you know that radiation can kill for a good purpose?
The syringe and needle used by a doctor is cleaned by gamma radiation. What is radiation and how does it kill bacteria and other organisms?
In 1896, the French scientist Henri Becquerel discovered that some uranium salts give out invisible rays that can go through thick paper and darken a photographic plate.
These rays or radiation are given out by the nuclei of the uranium atoms.
The nuclei of some atoms are unstable. An unstable nucleus will decay to become more a stable nucleus by emitting radiation, in the form of a particle or electromagnetic radiation.
Radioactivity is the spontaneous disintegration of an unstable nucleus accompanied by the emission of an energetic particle or a photon.
Figure gives a diagrammatic representation of a radioactive disintegration.
The energetic particle or photon is also known as radioactive emission.
The atoms of an element can exist as isotopes. Some isotopes have nuclei that are stable while some isotopes have nuclei that are unstable.
Stable nuclei remain as they are and do not undergo any changes.
Unstable nuclei will undergo radioactive disintegration or radioactive decay to become more stable.
Table gives some examples of stable and unstable isotopes.

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List three types of radiation that are produced during radioactivity

Types of Radioactive Emissions:

There are three kinds of radioactive emissions:
(a) Alpha particles (α-particles)
(b) Beta particles (β-particles)
(c) Gamma rays (γ-rays)
Alpha particles, beta particles and gamma rays can be represented diagramatically as in Figure 5.9:
An alpha particle:
(a) is a helium nucleus
(b) consists of two protons and two neutrons
(c) is positively charged
(d) is very heavy compared to a beta particle
(e) moves slower than a beta particle; up to about 10% of the speed of light
A beta particle:
(a) is a high energy electron
(b) is negatively charged
(c) is very much lighter than an alpha particle
(d) moves at very high speed; up to 99% of the speed of light
Gamma rays:
(a) are electromagnetic waves
(b) have very high frequencies and short wavelengths (10^-13m < λ < 10^-11 m)
(c) do not carry any charge
(d) move at the speed of light in a vacuum

Characteristics of Radioactive Emissions

Alpha particles, beta particles and gamma rays ionise the atoms of the medium through which they pass to produce ion pairs.
Alpha particles have a relatively larger mass and are positively-charged. They have the highest ionising power and produce the most ion pairs in a medium.
Beta particles have a much smaller mass and are negatively-charged. They have a lower ionising power than a-particles.
Gamma rays are electromagnetic waves and have the lowest ionising power.
The ionisation power of α, β and γ is indicated by the number of ion pairs produced per cm travelled in air as shown in Table.
The radioactive emission loses some of its energy each time an ion pair is produced.
Alpha particles which have the highest ionising power lose energy relatively quickly as they move through a medium. After a short distance in the medium, the alpha particles would have lost almost all of its energy. Therefore, alpha particles have the lowest penetrating power.
Gamma rays which have the lowest ionising power, have the highest penetrating power.
Figure shows the approximate range of the three radioactive emissions in air.
Figure compares the penetrating powers of different radioactive emissions.
Alpha and beta particles are deflected in an electric field because they are charged. The deflections are in opposite directions because they carry opposite charges.
However, the alpha particles go through a smaller deflection than the beta particles as their mass is much larger.
Figure compares the paths of radioactive emissions in an electric field. Gamma rays are not deflected because they do not carry any charge. Note that the paths of the alpha and beta particle are curved only in the region between the plates.
Figure shows the effect of a magnetic field on alpha particles.
Alpha particles and beta particles are deflected when they pass through a magnetic field while γ-rays are unaffected. Alpha particles and beta particles follow circular paths in a magnetic field.
The direction of the deflection of alpha particles in a magnetic field can be found using Fleming’s Left Hand Rule where the
- First finger: Direction of the magnetic field
- Second finger: Direction of motion of the alpha particles
- Thumb: Direction of deflection
Beta particles are deflected in the opposite direction along a circular path with a smaller radius, as shown on Figure.
Figure shows the paths taken when a beam consisting of alpha particles, beta particles and gamma rays move in a magnetic field.

Table summarises the nature and characteristics of the three types of radioactive emissions.

Properties	α-particles	β-particles	γ-rays
Nature	Helium nucleus Consists of 2 protons and 2 neutrons	Fast moving electrons	Electromagnetic waves
Symbol	₂He⁴	_-1e⁰	–
Charge	+2e	-e	No charge
Speed	Up to 10% the speed of light	Up to 99% the speed of light	Speed of light
Ionising power	Strong	Medium	Very weak
Penetrating power	Low penetrating power Stopped by a sheet of paper	Medium penetrating power Stopped by a few millimetres of aluminium	High penetrating power A few centimetres of lead can absorb a significant amount of it
Range in air	Several centimetres	Several metres	Several hundred metres
Effect of magnetic field	Small deflection	Larger deflection in the opposite direction of the a-particles	No deflection
Effect of electric field	Small deflection towards a negatively-charged plate	Larger deflection towards a positively-charged plate	No deflection
Tracks in cloud chamber	Straight and thick lines	Thin and wavy lines	Very fine short lines