Nuclear Radiation

Rutherford's Scattering Experiment

At the start of the 20th century, Ernest Rutherford devised an experiment to investigate the structure of atoms.

Positively-charged alpha particles were fired at a very thin piece of gold foil in the apparatus shown below. Because of the vacuum, the alpha particles were able to travel freely.

Every time an Alpha particle hit the fluorescent screen, the screen glowed for a short time. The microscope was moved all around the outside of the circular fluorescent screen, so that the number of alpha particles hitting the screen at every position could be observed.

The video below explains the observations Rutherford's team recorded.

Rutherford deduced 3 main points about atoms from these results:

1. Because most of the positively-charged alpha particles passed straight through the gold atoms in the foil, most of the atom must be empty space.

2. Because only very, very few positively-charged alpha particles were deflected backwards in almost the direction they had come from, most of the mass of the atom must be concentrated in a very small central area.

3. Because some of the positively-charged alpha particles were deflected backwards by the nucleus, the nucleus must be positively-charged.

Because of these findings we can deduce the atom's structure:-

Where:-

Proton - Positive charge (+1), mass of 1 Atomic Mass Units (A.M.U.).

Neutron - No charge (0), mass of 1 Atomic Mass Units (A.M.U.).

Electron - Negative charge (-1), mass negligible.

The diagram below shows a summary of the history of the structure of the atom:-

Radioactivity and Decay

The symbol for an atom is often written in the form:-

Nuclear Decay

There are three types of radioactivity that can be emitted from atomic nuclei during radioactive decay:-

1. Alpha particles.

2. Beta particles.

3. Gamma rays.


Alpha Decay

Alpha decay takes place when an alpha particle (consisting of 2 protons plus 2 neutrons) is ejected from an atom's nucleus.

Beta Decay

Beta decay takes place when a neutron in the nucleus decays into a proton and an electron. The proton stays in the nucleus (so the atomic number increases by 1) while the electron is ejected from the atom's nucleus as a beta particle.

Gamma Decay

Gamma rays are photons of electromagnetic energy - They are not particles. When gamma rays are ejected from an atom's nucleus, this does not change the mass number or atomic number of the atom. It does however change the energy state of the nucleus.

Ionisation

When one of the three types of radiation is incident (hits) another atom, the radiation can cause ionisation, which removes an outer electron from the nucleus, creating an ion.

Each type of radiation ionises matter by different amounts, and this affects how far the radiation can travel through a material, the more ionising the radiation, the shorter its range:-

The video below gives a brief summary of Nuclear Radiation and decay:-

The diagram below shows a summary of the Three types of Radioactive Decay:-

Detecting Radioactivity

When a Radioactive source gives out any of the three radiation types above, it is possible to detect this radiation through a variety of methods :-

1. A Geiger-Muller Tube and Counter.

3. Photographic Film/Plates and Film Badges.

3. Bubble and Cloud Chambers.


The diagram below shows a summary of the three types of Radioactive Decay:-


The Geiger-Muller Tube and Counter

The Geiger-Muller tube is used to detect radiation in real time. Whenever radiation enters through the thin Mica Window (so thin that even Alpha particles can pass through), the radiation ionises the gas within the tube. These ions allow a current to flow between the casing and the central electrode, which is registered by the counting circuit. This can be used to measure the activity of a source (see below).


Photographic Plates and Film Badges

In 1896, Henri Becquerel created the first radiation detector by accident when investigating how certain compounds show Phosphorescence (glow) when exposed to bright light. During his research, he left some photographic plates in a closed drawer, under a box of Uranium salts and when these were developed several days later, showed a darkening that could only be explained by some form of rays leaving the uranium salts.

Radioactivity darkens Photographic film in a similar way to visible light, the more intense the radiation, the darker the plate will become. This allows scientists to detect exposure over long time periods using a film Badge:-

The above diagram shows the contents of a standard film badge. A small piece of photographic film is sealed into the back of the badge, and has different material shields placed in from of it (numbers in above diagram refer to different blocking shields), which allows the type of radiation being detected to be determined.


Cloud Chambers

Cloud chambers are used to visually detect radiation. They both work using trails to show the path of radiation as it passes through the detector:-

The above image shows the trails left within a cloud chamber by the passage of radiation through it.

The Cloud chamber consists of a sealed vessel containing a supersaturated vapour. This vapour is held below the point at which it would condense, but as it lacks condensation nuclei, it remains in a vapour form.

When radiation passes through the vapour, atoms in the vapour become ionised. The ionised vapour acts as condensation nuclei and droplets form in the air along the path the radiation took.

Note - This is similar to the process which creates vapour trails from the engines of high altitude aircraft.

The patterns that the clouds take can be used identify the type of radiation.