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CIE A-Level Physics Notes

23.2.2 Decay Constant and Activity

Introduction

Radioactive decay, a spontaneous and random process, involves the transformation of an unstable atomic nucleus into a more stable configuration. Key to this process are the decay constant and activity, which quantitatively describe the decay behaviour of radioactive isotopes.

Understanding Radioactive Decay

The Concept of Radioactive Decay

Radioactive decay is the spontaneous transformation of an unstable atomic nucleus into a more stable one. This process occurs through the emission of radiation, which can include alpha particles (helium nuclei), beta particles (electrons or positrons), and gamma rays (high-energy electromagnetic radiation). Each type of radiation has different properties and levels of penetrative power.

Diagram showing β− decay in which the parent nucleus emits an electron and an antineutrino. The daughter nucleus has one more proton and one less neutron than its parent.

β− decay

Image Courtesy Openstax College

The Randomness of Radioactive Decay

A fundamental aspect of radioactive decay is its randomness. It's impossible to predict exactly when a specific atom will decay. This unpredictability is a core feature of quantum mechanics and is key in differentiating radioactive decay from deterministic processes.

Decay Constant (λ)

Definition of Decay Constant

The decay constant, symbolised as λ (lambda), represents the likelihood of a single atom decaying per unit time. It quantifies the intrinsic decay rate of a particular radioactive isotope.

Characteristics of Decay Constant

  • Intrinsic Property: Unique to each radioactive isotope, the decay constant is independent of external factors like temperature or chemical state.
  • Units of Measurement: Measured in reciprocal seconds (s-1), the decay constant has dimensions of time-1.
  • Implication of High/Low Values: A high decay constant indicates a high probability of decay in a short time, signalling a rapidly decaying isotope. Conversely, a low decay constant indicates a more stable isotope with a longer lifespan.

Activity (A)

Definition of Activity

Activity, denoted as A, measures the number of decay events in a radioactive sample per unit time. It quantifies the overall rate of decay in a given sample.

Characteristics of Activity

  • Units of Measurement: The unit of activity is the Becquerel (Bq), where 1 Bq equates to one decay per second. Another common unit is the Curie (Ci), although the Becquerel is the SI unit.
  • Dependence on Decay Constant and Nuclei Number: Activity is directly proportional to the decay constant and the number of undecayed nuclei present in the sample.
  • Implication in Practical Applications: High activity materials are more hazardous due to their high rate of radiation emission. Activity measurements are crucial in areas like radiation therapy, where precise dosages are essential.

Calculation of Activity

Deriving the Activity Formula

The formula for calculating activity is derived from the relationship between the decay constant and the number of undecayed nuclei. It is given by:

A = λN

where A represents the activity in becquerels (Bq), λ is the decay constant in reciprocal seconds (s-1), and N denotes the number of undecayed nuclei.

Step-by-Step Example

Consider a sample with a decay constant of 0.001 s-1 and 5000 undecayed nuclei. The activity is calculated as:

A = 0.001 x 5000 = 5 Bq

This example demonstrates how activity can be determined using the decay constant and the number of undecayed nuclei, providing a practical understanding of these concepts.

Application in Real-World Scenarios

Radiometric Dating

Radiometric dating techniques rely on the decay constant to estimate the age of artifacts and geological samples. By measuring the remaining activity and knowing the original number of nuclei and the decay constant, scientists can calculate the age of a sample.

Nuclear Medicine

In nuclear medicine, understanding the decay constant and activity of isotopes is crucial for safe and effective treatment. Isotopes with specific decay constants are chosen to ensure they remain active for the required duration in diagnostic or therapeutic procedures.

Environmental Safety

Activity measurements are essential in assessing the safety and environmental impact of radioactive materials. Managing waste from nuclear reactors and other radioactive sources requires a thorough understanding of the activity of various isotopes.

Conclusion

Grasping the decay constant and activity is essential for students studying nuclear physics and its applications. These concepts not only underpin our understanding of radioactive decay but also have practical implications in diverse fields like archaeology, medicine, and environmental science.

FAQ

Determining the decay constant of a radioactive isotope experimentally involves measuring the rate of decay of the isotope over a given time period. This is typically done by observing the decrease in the number of undecayed nuclei or the decrease in activity over time. One common method is to use a detector to measure the radiation emitted by the isotope, which directly corresponds to the number of decay events. By recording the activity at different times, scientists can plot a decay curve. The slope of this curve in a semi-logarithmic plot gives the decay constant. This method relies on the exponential nature of radioactive decay, described by the equation N = N0e(-λt), where N is the number of undecayed nuclei at time t, N0 is the initial number of nuclei, and λ is the decay constant. By fitting experimental data to this equation, the decay constant can be extracted. This process requires precise measurement techniques and careful data analysis to ensure accuracy, as the decay constant is crucial for applications in fields like nuclear medicine, radiometric dating, and nuclear energy.

The decay constant for a given isotope does not change over time. It remains constant throughout the life of the isotope. This stability of the decay constant is due to its dependence on the internal structure of the atomic nucleus, which does not change over time. The forces governing the stability and decay of the nucleus, such as the strong nuclear force and electrostatic repulsion between protons, remain constant for a particular isotope. Therefore, the probability of decay of the nucleus, as quantified by the decay constant, remains unchanged. This consistency is a fundamental aspect in nuclear physics, particularly in the study of radioactive decay and its applications. It ensures predictability and accuracy in calculations involving radioactive decay, such as in radiometric dating methods or in nuclear medicine, where the decay constant is used to predict the behaviour of radioactive isotopes over time.

The decay constant plays a crucial role in determining the age of archaeological finds through radiometric dating methods, such as carbon-14 dating. In these methods, the decay constant provides the necessary rate information to calculate the time elapsed since the death of a biological organism. The basic principle involves measuring the remaining amount of a radioactive isotope in the sample and comparing it to the original amount. The decay constant, an intrinsic property of the isotope, allows for the calculation of the time period over which decay has occurred. By applying the decay equation, which integrates the decay constant, scientists can back-calculate the time elapsed since the isotope started decaying, which corresponds to the death of the organism. This technique, reliant on the stability and known value of the decay constant, has revolutionised fields like archaeology and palaeontology, enabling accurate dating of ancient organic materials. The reliability and precision of the decay constant are paramount in these applications, as even slight variations can lead to significant errors in age determination.

The decay constant is considered an intrinsic property of a radioactive isotope because it is inherent to the isotope and independent of external factors. This means that the decay constant remains constant regardless of changes in environmental conditions such as temperature, pressure, or chemical state. The reason for this lies in the nature of radioactive decay, which is governed by the forces within the nucleus, specifically the balance between the strong nuclear force and the electrostatic force. These forces are not influenced by external physical conditions but are determined by the internal structure and composition of the nucleus. Therefore, the decay constant, which quantifies the rate at which a particular isotope undergoes radioactive decay, is solely a characteristic of the isotope itself. This property is crucial in nuclear physics and radiometric dating, as it allows for consistent and reliable measurements and calculations regarding the decay of isotopes, irrespective of external conditions.

The decay constant is intrinsically linked to the stability of a nucleus. A higher decay constant signifies that a nucleus is more prone to radioactive decay, indicating lesser stability. This is because the decay constant represents the probability of decay of a nucleus per unit time. For isotopes with a high decay constant, the likelihood of decay events occurring within a given timeframe is greater, implying that these isotopes are less stable and tend to disintegrate more quickly. Conversely, a lower decay constant suggests greater stability, as the chances of the nucleus undergoing decay are reduced. This stability is a result of the balance of forces within the nucleus, where strong nuclear forces effectively overcome the repulsive forces between protons. Therefore, understanding the decay constant provides insights into the nuclear stability of isotopes, which is fundamental in nuclear physics, especially when exploring the nature of different elements and their isotopes in the periodic table.

Practice Questions

A sample of a radioactive isotope has a decay constant of 0.0023 s^-1. Initially, there are 8000 undecayed nuclei in the sample. Calculate the activity of the sample.

To calculate the activity of the sample, we use the formula A = λN, where A is the activity, λ is the decay constant, and N is the number of undecayed nuclei. Substituting the given values, we get A = 0.0023 s-1 x 8000 = 18.4 Bq. This calculation shows that the sample has an activity of 18.4 becquerels, meaning it experiences 18.4 decay events per second. Understanding this concept is crucial in nuclear physics, especially in applications like radiometric dating and nuclear medicine.

Explain why a high decay constant indicates a more hazardous radioactive material in terms of radiation safety.

A high decay constant implies a high probability of an atom decaying in a given time, leading to a greater rate of radioactive decay. This results in a higher activity, meaning more decay events per second, and consequently, more radiation emission in a shorter period. In radiation safety, this is a significant concern as increased radiation can pose greater health risks due to higher exposure levels. Therefore, materials with higher decay constants are generally considered more hazardous, necessitating stricter safety measures and handling protocols to protect against radiation exposure.

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