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Questions and Answers
What does the count rate from a pure beta emitter do when the absorber thickness increases?
What does the count rate from a pure beta emitter do when the absorber thickness increases?
The count rate decreases rapidly at first and then slowly.
What is the end point of the absorption curve in the context of beta particles?
What is the end point of the absorption curve in the context of beta particles?
The range of beta particles.
The range of beta particles depends entirely on the material of the absorber.
The range of beta particles depends entirely on the material of the absorber.
False
What is the definition of areal density in the context of beta particles?
What is the definition of areal density in the context of beta particles?
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What is the relationship between density thickness and linear thickness?
What is the relationship between density thickness and linear thickness?
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Using density thickness makes it impossible to compare the absorption properties of different materials for beta particles.
Using density thickness makes it impossible to compare the absorption properties of different materials for beta particles.
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What is the purpose of using a universal curve of beta ray range versus energy?
What is the purpose of using a universal curve of beta ray range versus energy?
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In the context of beta particles, what are delta rays?
In the context of beta particles, what are delta rays?
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Beta particles have a significantly different mass than orbital electrons.
Beta particles have a significantly different mass than orbital electrons.
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What is the main cause of energy loss for beta particles?
What is the main cause of energy loss for beta particles?
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The amount of energy lost by a beta particle depends on its kinetic energy but not its distance of approach to an electron.
The amount of energy lost by a beta particle depends on its kinetic energy but not its distance of approach to an electron.
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What is the formula to calculate the kinetic energy of an ejected electron during ionization by a beta particle?
What is the formula to calculate the kinetic energy of an ejected electron during ionization by a beta particle?
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In many ionizing collisions, a single ion pair is always produced.
In many ionizing collisions, a single ion pair is always produced.
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What is the purpose of using photographic emulsion in the study of beta particle interactions?
What is the purpose of using photographic emulsion in the study of beta particle interactions?
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What is the definition of specific ionization in the context of beta particles?
What is the definition of specific ionization in the context of beta particles?
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Specific ionization is highest for high-energy beta particles.
Specific ionization is highest for high-energy beta particles.
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What is the primary mechanism behind bremsstrahlung?
What is the primary mechanism behind bremsstrahlung?
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What is the approximate formula used for calculating the fraction of incident beta energy converted to X-ray photons during bremsstrahlung?
What is the approximate formula used for calculating the fraction of incident beta energy converted to X-ray photons during bremsstrahlung?
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Higher atomic numbers lead to a lower likelihood of bremsstrahlung.
Higher atomic numbers lead to a lower likelihood of bremsstrahlung.
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How is the extrapolated range of alpha particles determined?
How is the extrapolated range of alpha particles determined?
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The mean range of alpha particles is always significantly higher than the extrapolated range.
The mean range of alpha particles is always significantly higher than the extrapolated range.
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What is the typical shape of the alpha particle absorption curve, and why?
What is the typical shape of the alpha particle absorption curve, and why?
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Increasing the thickness of an alpha absorber only reduces the number of alpha particles that pass through.
Increasing the thickness of an alpha absorber only reduces the number of alpha particles that pass through.
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What is the formula for the approximate range of alpha particles in air, for energies below 4 MeV?
What is the formula for the approximate range of alpha particles in air, for energies below 4 MeV?
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What is the formula for approximating the range of alpha particles in any medium, given their range in air?
What is the formula for approximating the range of alpha particles in any medium, given their range in air?
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What is the primary energy loss mechanism for alpha particles?
What is the primary energy loss mechanism for alpha particles?
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Alpha particles produce a large number of ion pairs per unit distance traveled.
Alpha particles produce a large number of ion pairs per unit distance traveled.
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Why are gamma rays different from beta or alpha particles in terms of absorption?
Why are gamma rays different from beta or alpha particles in terms of absorption?
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The attenuation of gamma rays in a good geometry setup can be represented by a linear equation in a semi-log graph.
The attenuation of gamma rays in a good geometry setup can be represented by a linear equation in a semi-log graph.
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What is the meaning of the attenuation coefficient (μ) in the context of gamma ray attenuation?
What is the meaning of the attenuation coefficient (μ) in the context of gamma ray attenuation?
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The mass attenuation coefficient (μm) has the same unit as the linear attenuation coefficient (μ).
The mass attenuation coefficient (μm) has the same unit as the linear attenuation coefficient (μ).
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What is the relationship between the linear attenuation coefficient (μ), mass attenuation coefficient (μm), and density (ρ)?
What is the relationship between the linear attenuation coefficient (μ), mass attenuation coefficient (μm), and density (ρ)?
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What does the atomic attenuation coefficient (μa) represent?
What does the atomic attenuation coefficient (μa) represent?
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The microscopic cross section (σ) is a measure of area and thus represents the same physical quantity as the linear attenuation coefficient (μ).
The microscopic cross section (σ) is a measure of area and thus represents the same physical quantity as the linear attenuation coefficient (μ).
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What is the relationship between the microscopic cross section (σ), the atomic density (N), and the macroscopic cross-section (∑)?
What is the relationship between the microscopic cross section (σ), the atomic density (N), and the macroscopic cross-section (∑)?
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The attenuating properties of matter are independent of the energy of the incident gamma rays.
The attenuating properties of matter are independent of the energy of the incident gamma rays.
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For energies between 0.7 and 5 MeV, most materials have the same gamma ray attenuating properties when considering the mass attenuation coefficient (μm).
For energies between 0.7 and 5 MeV, most materials have the same gamma ray attenuating properties when considering the mass attenuation coefficient (μm).
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For shielding gamma rays, the density of the shielding material is less important than the atomic number.
For shielding gamma rays, the density of the shielding material is less important than the atomic number.
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For very low and very high gamma ray energies, materials with high atomic numbers are less effective shields than those with low atomic numbers.
For very low and very high gamma ray energies, materials with high atomic numbers are less effective shields than those with low atomic numbers.
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Study Notes
Radiation Biophysics Lecture 7
- This lecture covers the interaction of radiation with matter, focusing on beta and alpha particles, and gamma rays.
Beta Particles
- Attenuation: The count rate from a pure beta emitter decreases rapidly with increasing absorber thickness, then slowly reaching a point where the detector only records background radiation.
- Range: The range of beta particles is the maximum distance they travel, determined by their maximum energy. It depends on the absorber material and beta particle energy.
- Range-Energy Relationship: The required absorber thickness for a given beta energy decreases as the density of the absorber increases.
- Areal Density: This is the number of absorber electrons in the path of beta particles (electrons/cm²).
Mechanism of Energy Loss
- Ionization and Excitation: Beta particles lose energy through interactions with orbital electrons, causing ionization and excitation. The energy loss depends on the distance of approach to the electron and the kinetic energy.
- Delta Rays: Ejected electrons with enough energy to travel a significant distance, creating a trail of ionizations.
- Specific Ionization: This refers to the number of ion pairs produced per unit path length. It's high for low-energy beta particles, decreasing with increasing energy.
- Bremsstrahlung: X-rays emitted when high-speed beta particles are deflected by strong nuclear forces, losing energy.
Alpha Particles
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Range-Energy Relationship: The mean range of alpha particles is the range most accurately determined. Alpha particle absorption curves are flat due to monoenergetic nature. Increasing thickness reduces alpha particle energy, not the number of particles.
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Extrapolated Range: Alpha particle absorption stops abruptly (sharp decrease) when the absorber thickness reaches the range of alpha particles.
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Range in air and other mediums: The range of alpha particles is inversely related to the square root of the atomic weight of the medium.
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Energy Transfer: Alpha particles cause significant energy loss through ionization. The specific ionization is high due to their high electric charge and relatively slow velocity. The energy loss is mainly via ionization and varies comparatively little over a wide range of energies.
Gamma Rays
- Attenuation: Gamma rays can only be reduced in intensity by increasing absorber thickness. Unlike beta and alpha particles, they aren't completely absorbed.
- Good Geometry: Collimated beams and positioning the source, absorber, and detector for precise measurements. The absorber should be thin enough to minimize the scattering of the photons. No scattering material should be present near the detector.
- Attenuation Coefficient: This gives the fraction of the gamma beam that is attenuated per unit thickness of the absorber.
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Linear and Mass Attenuation Coefficients:
-Linear attenuation coefficient (cm⁻¹): When absorber thickness is measured in centimeters.
- Mass attenuation coefficient (cm²/g): When absorber thickness is measured in grams per square centimeter.
- Relationship between the coefficients: μ₁ (cm⁻¹) = μm (cm²/g).p(g/cm³)
- The atomic attenuation coefficient (μ₁): The fraction of the gamma ray beam that is attenuated by a single atom in absorber.
- Microscopic and Macroscopic cross sections: The Microscopic cross section (σ) is used to calculate macroscopic cross section (Σ). ∑=σN where N is the number of absorber atoms per cubic centimeter.
Energy Dependence of Attenuation
- The attenuating properties of matter (e.g., shielding materials) systematically vary with atomic number and incident gamma ray energy.
- For specific energies (0.7 to 5 MeV), the mass attenuation coefficient for most materials is relatively constant.
- Absorbers with higher atomic numbers are generally more effective at attenuating gamma rays at energies outside the range stated above.
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Description
Explore the interaction of radiation with matter in this lecture focused on beta and alpha particles, as well as gamma rays. Understand key concepts like attenuation, range, and the mechanism of energy loss through ionization and excitation. Ideal for students studying radiation biophysics.
