Lecture 6 Flashcards
The beta count from a source drops with _________ thickness of an absorbing material.
Increasing
Why do beta sources have a maximum range?
All particles are absorbed at a certain material thickness so none can get through after that range.
What does the maximum range of beta sources depend on?
- Beta ray energy (in some cases)
- Density of electrons in the material
Define density thickness
The product of density and thickness for a material, equal to the range of a beta particle. The units of density thickness are the same as the units for the range of a beta particle.
How can the range of a beta particle be found through several different materials?
The density thicknesses can be added together.
Describe the range-energy curve for a beta particle
What are the two mechanisms for beta energy loss?
- Ionisation and excitation
- Bremsstrahlung
What is the equation for the range of a beta particle in air at STP when its energy is below 0.8 MeV?
R = range in air
E = particle energy (MeV)
What is the equation for the range of a beta particle in air at STP when its energy is above 0.8 MeV?
R = range in air
E = particle energy (MeV)
How can the tracks of beta particles be observed?
Observations can be made for beta particles in a cloud chamber.
What is the average energy lost by a beta particle for every collision in air?
34eV
What is the result of a beta particle collision in air?
Energy is lost due to the generation of an ion pair.
How can the linear rate of energy loss be calculated for different energies of beta in air?
Using the number of ion pairs generated per cm (and multiplying the value by 34eV).
What does the maximum range of alpha sources depend on?
The number of collisions of the particle with electrons as these cause the energy of the alpha particle to deplete.
When does the total count of an alpha source decrease?
When all KE is depleted.
Describe the count rate v. absorber thickness curve for an alpha particle
What is the equation for the range of an alpha particle in air at STP?
R = range in air
E = particle energy (MeV)
The range of an alpha particle in a material other than air is __________ proportional to the density and proportional to the _________ ____ of the atomic mass.
Inversely
Square root
What is the equation for the range of an alpha particle in a material at STP?
Rₐᵢᵣ = range in air
ρₐᵢᵣ = density of air
Aₘ = atomic mass of material
Rₘ = range in material
ρₘ = density of material
Aₐᵢᵣ = atomic mass of air
Describe the graph of the energy deposited v. absorber thickness for an alpha particle
What is the Bragg peak
A pronounced peak on a plot of energy loss of ionising radiation during its travel through matter. It occurs immediately before an alpha particle comes to rest.
What are the mechanisms for neutron energy loss?
- Elastic scattering
- Capture (usually followed by the release of a gamma photon)
What are the two types of neutron?
- Fast neutrons
- Thermal neutrons
What is a fast neutron?
A neutron with a high initial kinetic energy (so high velocity).
What is a thermal neutron?
A neutron whose energy distribution matches that of the Maxwell-Boltzmann distribution in the surrounding material.
What is the equation for the Maxwell-Boltzmann energy distribution?
E = energy (in J)
k_B = Boltzmann constant
T = temperature
What is the shape of a neutron attenuation curve?
Exponential
For a neutron, the _________ ___________ is often quoted instead of the linear or mass attenuation coefficient.
Nuclear cross-section
What is the equation for the intensity of a beam of neutrons in terms of the nuclear cross-section?
I(x) = intensity
I₀ = initial intensity
µ(x) = linear attenuation coefficient
x = distance
σ = nuclear cross section
N = atomic density
What does the nuclear cross section depend on?
- Energy
- Nucleus
- Nuclear reaction
Define fast diffusion length
The average distance travelled in a (non-fissile) material before the neutrons are fully thermalised.
Define thermal diffusion length
The attenuation of the thermal neutrons in the medium (before capture). It is exponential with distance.
What is the fast diffusion length in water?
5.75 cm
What is the thermal diffusion length in water?
2.88 cm
What distance of water is an effective shield for neutron sources
~25 cm
