RADIOACTIVITY

listened to with a Geiger counter!

Discoveries
 

One day in 1896, Henri Becquerel (by chance according to legend?) arranged in his cupboard, a packet of uranium salt beside an unexposed photographic plate. Several days later, he took out the plate and  developed it. To his surprise, he noticed that the photographic plate had been exposed without having been exposed to the light. Having repeated this experiment, he concluded that Uranium spontaneously emits what he called "uranic rays".
 

In 1898, Marie Curie discovered that pitchblend, a uranium ore, emits more radiation than uranium itself. She deduced that this ore contains, in very small quantities, one or more elements much more active that uranium. With the assistance of her husband Pierre Curie and after two years of effort, she arrived at isolating two new elements: Polonium (named thus in tribute to her homeland) and Radium. It was then that Marie Curie invented the word "radioactivity".
 

The 3 radiations
 

There exist three varieties of radioactivity characterised by the emission of different rays emitted by the nucleus of the atom:

• a (alpha) rays  are stopped by 6 cm of air. They are composed of alpha particles made up of two protons and two neutrons (in fact a helium nucleus). The particle is therefore positively charged. These alpha particles are nothing more than fragments of unstable heavy nuclei which reorganise themselves to become lighter and more stable nuclei (thus non radioactive!).

• b (beta) rays are stopped by an aluminium screen. They are notably composed of electrons and are therefore negatively charged. Beta radiation is identical to the cathode radiation in your TV!

In reality, there exist two sorts of beta ray: the b- which are electrons with their negative charge (the most frequent form of radioactivity!) the b+ which would be discovered later, which are antielectrons with a positive charge. (see weak interaction)

• g (gamma) rays are extremely penetrating and can pass straight through a safe. They are composed of high energy photons (particles of light). They are nothing but pure energy without any mass. These g photons emitted by the nucleus are due to the internal rearrangement of nuceons within the nucleus. These nucleons are organised in concentric layers (like an onion), a little like electrons are around the nucleus. Whenever a nucleon slides from one of the outer nuclear layers into a more internal layer and therefore one which is less energetic, it releases energy in the form of a g photon.

These three varieties of radioactivity are not emitted simultaneously. Each nuclear reaction of an atom emits only one single type of radiation at a time!

For example, radioactive Uranium-238 emits an alpha ray and thus loses 4 nucleons (2 protons + 2 neutrons): U 238 thereby transforms itself into Thorium-234 (because 2 protons less - that changes an atom!).
 

Transmutation and half-life
 

Radioactivity is a spontaneous transmutation of one atom into another atom with the emission of radiation.

These transmutations, ancient dreams of the alchemists of the middle ages, take place more or less rapidly, depending upon a characteristic interval of a radio-element which is called its radioactive half-life.
Let's imagine an extremely large population of radioactive atoms, all identical: the half-life of this population is equal, by definition, to the length of time after which half of the atoms which composed the population at the start have transmuted into other elements; after a second period of time, the remaining population is once again divided by two and therefore makes a quarter of the initial number, and so on.
This half-life can vary, depending upon the radio-element, from a few fractions of a second to several milliard years.  Tellurium 128 has a half-life of 1.5 x 10²⁴ years, that is to say one hundred thousand milliard, milliard times the age of the Universe... (one hundred thousand billion, billion in american english)
 
 

Natural or artificial?
 

Radioactivity has not been invented by man. It has existed since the beginning of the universe: We speak of natural radioactivity when it is due to the durable radio-elements formed in the stars which have not yet found their stable state: they will end up transforming themselves into stable atoms. This radioactivity is very important and releases a very large amount of energy which without doubt maintains the magma in a molten state beneath the earths crust.
For example uranium, thorium, radium and the isotopes carbon 14, radon 222 and  potassium 40 are natural radioactive elements present in the minerals of the earth, in thermal waters and in the air.

We speak of articial radioactivity when referring to elements fabricated by man. In this case, these atoms are very heavy  (with a high atomic number Z), very unstable and therefore have a very short half-life. Physicists create these artificial radio-elements by bombarding natural atoms whith protons or alpha particle: the nuclei of these atoms acquire additional protons which transform them into new heavier atoms. Radioactivity is produced frequently by large unstable nuclei....
This transformation of atomic nuclei is called a nuclear reaction of which much talk is heard about two types: nuclear fission and nuclear fusion.
 

Nuclear fission

Whenever the nucleus of a heavy atom (like uranium 235) fissions (fragments) into two smaller nuclei, it produces a remarkable event: the sum of the masses of these two remaining nuclei is less than the mass of the original large nucleus. Where has the missing mass gone? It has transformed itself into pure energy (Einsteins mass-energy equivalence), an enormous quantity of energy. Furthermore, in the case of uranium 235, the fission of the nucleus can be provoked by  a single neutron and, a very important detail, this fission in turn produces other neutrons which can themselves break apart other uranium nuclei ...We are about to witness a chain reaction.

To see a short explanetary film ( .avi format of 991 KBytes) on the chain reaction, click on the atomic explosion below:
 

If this reaction is controlled within a nuclear power station, it produces energy which drives an electric turbine.  If this reaction is not controlled and gets carried away, it produces an explosion of the A bomb type seen at Hiroshima.

Nuclear fusion

In broad terms it's the inverse of fission.

Two light atomic nuclei (like hydrogen) crash into each other and fuse together into a single bigger nucleus. Now the final mass of this big nucleus is smaller than the sum of the masses of the two initial nuclei, which is where we get an enormous release of energy produced by the annihilation of this difference of mass.
In order to be able to provoke such a fusion reaction, it is necessary to force the nuclei, all positively charged, to move together and to overcome their mutual repulsion (like two lovers who repulse each other): This is not possible except at very high temperatures (the temperature corresponding to the intensity necessary to get the particles to crash into each other). This is why the nuclear fusion reaction is also called a thermonuclear reaction (thermo = heat).
 
 

This uncontrolled reaction is used in the hydrogen bomb or H bomb.
This reaction is also seen in the heart of our Sun where temperatures reach hundreds of millions of degrees. Nuclear fusion there is auto-regulated by an equilibrium between the pressure of the explosion produced and the force of gravity which crushes the enormous mass of the Sun in on itself. This reaction is the source of the energy which gives rise to life on Earth.
 

crazyflash: Atomic Radio

Radio Active with DJ Atomic emits 3 types of program:  The ALPHA Romeo style, very chic but very relaxed. Little penetrating at the 'audi-matter' level. The BETA Bondieu style a touch green, but good electronic music. The GAMMA Toutissu style very commercial, very energetic and bright.