PHYSICS PROJECT ON

RADIOACTIVITY

INDEX INTRODUCTION THREE TYPES OF RADIOACTIVE DECAY

alpha decay beta decay gamma decay

LAW OF RADIOACTIVE DECAY

Radioactivity refers to the particles which are emitted from nuclei as a result of nuclear instability. Because the nucleus experiences the intense conflict between the two strongest forces in nature, it should not be surprising that there are many nuclear isotopes which are unstable and emit some kind of radiation.

INTRODUCTION

There are three main types of radiation: Alpha radiation Beta radiation

Gamma radiation

Three Types of Radioactive Decay

ALPHA DECAY

The reason alpha decay occurs is because the nucleus has too many protons which cause excessive repulsion. In an attempt to reduce the repulsion, a Helium nucleus is emitted. The way it works is that the Helium nuclei are in constant collision with the walls of the nucleus and because of its energy and mass, there exists a nonzero probability of transmission. That is, an alpha particle (Helium nucleus) will tunnel out of the nucleus.

Here is an example of alpha emissionwith AMERICIUM-241:-

Alpha Decay of Americium-241 to Neptunium-237.

BETA DECAY

BETA DECAY is one process that unstable atoms can use to become more stable. There are two types of beta decay, beta-minus and beta-plus.During beta-minus decay, a neutron in an atom's nucleus turns into a proton, an electron and an antineutrino. The electron and antineutrino fly away from the nucleus, which now has one more proton than it started with. Since an atom gains a proton during beta-minus decay, it changes from one element to another. For example, after undergoing beta-minus decay, an atom of carbon(with 6 protons) becomes an atom of nitrogen (with 7 protons).During beta-plus decay, a proton in an atom's nucleus turns into a neutron, a positron and a neutrino. The positron and neutrino fly away from the nucleus, which now has one less proton than it started with. Since an atom loses a proton during beta-plus decay, it changes from one element to another. For example, after undergoing beta-plus decay, an atom of carbon (with 6 protons) becomes an atom of boron (with 5 protons).Although the numbers of protons and neutrons in an atom's nucleus change during beta decay, the same.

Beta decay occurs when the neutron to proton ratio is too great in the nucleus and causes instability. In basic beta decay, a neutron is turned into a proton and an electron. The electron is then emitted. Here's a diagram of beta decay with hydrogen-3:

There is also positron emission when the neutron to proton ratio is too small. A proton turns into a neutron and a positron and the positron is emitted. A positron is basically a positively charged electron. Here's a diagram of positron emission with carbon-11:

Positron Decay of Carbon-11 to Boron-11.

The final type of beta decay is known as electron capture and also occurs when the neutron to proton ratio in the nucleus is too small. The nucleus captures an electron which basically turns a proton into a neutron. Here's a diagram of electron capture with beryllium-7:

Electron Capture of Beryllium-7. It decays to Lithium-7.

GAMMA DECAY

In gamma decay, depicted in Fig. 3-6, a nucleus changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation (photons). The number of protons (and neutrons) in the nucleus does not change in this process, so the parent and daughter atoms are the same chemical element. In the gamma decay of a nucleus, the emitted photon and recoiling nucleus each have a well-defined energy after the decay. The characteristic energy is divided between only two particles.

Here's a diagram of gamma decay with helium-3:

Gamma Decay of Helium-3

Energy level diagram showing the emission of gamma rays by a 60Co27

nucleus subsequent to beta decay.

60Co27

Et = 1.17 MeV

Et = 1.33 MeV

The HALF - LIFE describes the amount of time needed for half of a sample of unstable atoms or particles to undergo decay. Thallium-208, for example, decays into lead-208 with a half-life of 3.05 minutes.

MEAN LIFE

In radioactivity, average lifetime or mean life of all the nuclei may be thought of as the sum of the lifetimes of all the individual unstable nuclei in a sample, divided by the total number of unstable nuclei present. The mean life of a particular species of unstable nucleus is always 1.443 times longer than its half-life .

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