Unit 1: Nuclear Physics (Lab: PHYDSC353P)
Table of Contents
1. Laboratory Objectives
The objective of this practical course is to provide students with hands-on experience in Nuclear Instrumentation. You will learn to operate the Geiger-Muller (G.M.) Counter, handle radioactive sources safely, and analyze the interaction of radiation with matter.
2. Characteristics of G.M. Counter (Plateau Curve)
Before performing any experiment, the Operating Voltage of the G.M. counter must be determined. This is done by plotting the Counting Rate vs. Applied Voltage.
- Threshold Voltage: The minimum voltage at which counts start appearing.
- Plateau Region: The range of voltage where the counting rate is relatively independent of the applied voltage.
- Operating Voltage: Usually chosen as the midpoint of the plateau.
3. Verification of Inverse Square Law
Radioactive radiation from a point source spreads out equally in all directions. This experiment verifies that the intensity (I) of radiation is inversely proportional to the square of the distance (d) from the source.
Procedure: Record the counts for a fixed duration at varying distances. Plotting 1/√(Counts) vs. d should yield a straight line.
4. Absorption Coefficient of Gamma Rays
Gamma rays are attenuated as they pass through materials like lead or iron. The reduction in intensity follows an exponential law.
Where I0 is the initial intensity, x is the thickness of the absorber, and μ is the Linear Absorption Coefficient.
5. Absorption of Beta Particles in Aluminum
Unlike gamma rays, beta particles have a finite range. This experiment determines the Mass Absorption Coefficient and the range of beta particles in Aluminum (Al) foils. It helps in understanding the energy of the beta source.
6. Dead Time of G.M. Counter
The Dead Time is the short interval of time after a count is recorded during which the counter is "incapable" of responding to another radiation particle.
- Two-Source Method: A common method to measure dead time by comparing counts of two sources separately and then together.
7. Statistical Fluctuation in Radioactive Decay
Radioactive decay is a random process. This experiment involves taking a large number of readings for a source with a long half-life to verify the Poisson Distribution or Gaussian Distribution of the counts.
Lab Exam Focus Corner
Frequently Asked Questions
- Why do we subtract Background Radiation? Because cosmic rays and natural isotopes in the environment contribute to the count. True count = Total count - Background count.
- What is 'Quenching' in a G.M. tube? The process of preventing multiple discharges after a single ionizing event, usually achieved by adding a halogen gas.
Common Mistakes
- Source Handling: Never touch a radioactive source with bare hands; use tweezers. Always return the source to its lead container immediately after use.
- Parallax in Distance: Ensure the distance between the source and the G.M. window is measured precisely along the central axis of the tube.
Practical Tips
Tip: Always check the Slope of the Plateau. A good G.M. tube should have a plateau slope of less than 5% per 100V. If the slope is too steep, the tube may be near the end of its life.