1. Introduction

Names: Bryan Chang Ding Lun, Walter Koh Li Zhi, Kopparthi Sandeep, Sean Michael Koh
Class: S2-07
Group Reference: F

We, Group F of S207, wish to have a greater understanding of the factors that affects particle decay and radiation. For our experiment, we will specifically be focusing on how the distance of the Geiger counter from the radiation source affects the radioactivity of Americium-241. We will also be trying to find out the half life of americium by using formulae.
Firstly, radioactive particles are particles that are unstable. This instability is caused by the excess energy the particle has due to the ratio between it’s protons and it’s neutrons. Since all particles want to be more stable or be in their lowest energy state.

Hence, these particles will decay into more stable forms by losing protons, neutrons, electrons or just energy.

This loss of particles is known as radiation. There are 3 types of radiation. Alpha, Beta and Gamma radiation.

Alpha radiation is an ionizing radiation that consists of Alpha particles
Beta radiation is another type of radiation which consists of Beta particles, they are either electrons and positrons.

An alpha particle   consists of 2 Protons and 2 Neutrons. It is a form of helium and has a charge of +2 and a mass of 4. They are relatively slow and heavy and they have a low penetrating power as shown in Figure 1.  Since alpha particles have a large charge,  they ionise other atoms strongly.
A beta particle has a charge of -1 and it’s mass is about 1/2000th of a proton. Thus, beta particles are the same as an electron. Compared to alpha particles, beta particles are faster and lighter.  They have a medium penetrating power, as shown in Figure 1 and they ionise atoms as they move along. However, their process of ionisation is not as strong as that of the alpha particles.
A gamma ray , as the name suggests is a ray, which can be thought of as a burst of energy. They are waves, unlike the above mentioned forms of radiation, which are particles. They have a high penetration power, as seen in Figure 1. It needs a thick material, such as lead to reduce them significantly. They do not ionise other atoms directly. However, they do cause other atoms to emit other particles, which will result in ionisation.  Gamma rays are emitted along with alpha and beta particles 
    This radiation spreads in all directions about the source, and
therefore when it is a distance(for example, x) from the source it is spread over the surface of a sphere of radius x and area 4πx2. If E is the energy radiated per unit time by the source, the intensity (energy per unit time per unit area) is given by Intensity = E/4πx2 (S-cool, n.d.). Thus the energy varies as the inverse square from the source as seen in figure 2 and 3.

Figure 2. Figure 3.

In the decay chain(term used to describe the chain of decay of  particle until it becomes stable) of Americium-241, it undergoes mostly Alpha decay (United States Environmental Protection Agency, 2014). In order to connect this information to the Geiger Counter, one can understand the effects of the change in various factors on the amount of radiation detected by the Geiger Counter. The different factors can alter the radioactivity in a variety of ways.

It is important to take note is that the Geiger Counter can detect cosmic radiation which is created when subatomic particles that originate from different galaxies and the Sun interact with the Earth’s atmosphere (Science Clarified, n.d.). The count of cosmic radiation is ranges from 2 to 14 counts.

There are a variety of cosmic rays such as:

  • Galactic Cosmic Rays
    - Come from outside the Solar System

  • Anomalous Cosmic Rays
    - Come from the interstellar space at the edge of the heliospace

  • Solar Energetic particles
    - Associated with solar flares and other energetic solar flares

There are several radioactive materials in the world. However, for this experiment, we are specifically using Americium-241 for several reasons :

  • Americium has a long half life (432.7 years)
  • Easily accessible - Can be found in a variety of industrial equipment including smoke detectors which we are using for our experiment
  • It mostly emits Alpha radiation. This is why it is used in Ionizing smoke detectors.

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