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PHYS3008: Health Physics > Lecture 6 > Flashcards

Lecture 6 Flashcards

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1
Q

The beta count from a source drops with _________ thickness of an absorbing material.

A

Increasing

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2
Q

Why do beta sources have a maximum range?

A

All particles are absorbed at a certain material thickness so none can get through after that range.

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3
Q

What does the maximum range of beta sources depend on?

A
  • Beta ray energy (in some cases)
  • Density of electrons in the material
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4
Q

Define density thickness

A

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.

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5
Q

How can the range of a beta particle be found through several different materials?

A

The density thicknesses can be added together.

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6
Q

Describe the range-energy curve for a beta particle

A
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7
Q

What are the two mechanisms for beta energy loss?

A
  • Ionisation and excitation
  • Bremsstrahlung
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8
Q

What is the equation for the range of a beta particle in air at STP when its energy is below 0.8 MeV?

A

R = range in air
E = particle energy (MeV)

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9
Q

What is the equation for the range of a beta particle in air at STP when its energy is above 0.8 MeV?

A

R = range in air
E = particle energy (MeV)

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10
Q

How can the tracks of beta particles be observed?

A

Observations can be made for beta particles in a cloud chamber.

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11
Q

What is the average energy lost by a beta particle for every collision in air?

A

34eV

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12
Q

What is the result of a beta particle collision in air?

A

Energy is lost due to the generation of an ion pair.

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13
Q

How can the linear rate of energy loss be calculated for different energies of beta in air?

A

Using the number of ion pairs generated per cm (and multiplying the value by 34eV).

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14
Q

What does the maximum range of alpha sources depend on?

A

The number of collisions of the particle with electrons as these cause the energy of the alpha particle to deplete.

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15
Q

When does the total count of an alpha source decrease?

A

When all KE is depleted.

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16
Q

Describe the count rate v. absorber thickness curve for an alpha particle

A
17
Q

What is the equation for the range of an alpha particle in air at STP?

A

R = range in air
E = particle energy (MeV)

18
Q

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.

A

Inversely
Square root

19
Q

What is the equation for the range of an alpha particle in a material at STP?

A

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

20
Q

Describe the graph of the energy deposited v. absorber thickness for an alpha particle

A
21
Q

What is the Bragg peak

A

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.

22
Q

What are the mechanisms for neutron energy loss?

A
  • Elastic scattering
  • Capture (usually followed by the release of a gamma photon)
23
Q

What are the two types of neutron?

A
  • Fast neutrons
  • Thermal neutrons
24
Q

What is a fast neutron?

A

A neutron with a high initial kinetic energy (so high velocity).

25
Q

What is a thermal neutron?

A

A neutron whose energy distribution matches that of the Maxwell-Boltzmann distribution in the surrounding material.

26
Q

What is the equation for the Maxwell-Boltzmann energy distribution?

A

E = energy (in J)
k_B = Boltzmann constant
T = temperature

27
Q

What is the shape of a neutron attenuation curve?

A

Exponential

28
Q

For a neutron, the _________ ___________ is often quoted instead of the linear or mass attenuation coefficient.

A

Nuclear cross-section

29
Q

What is the equation for the intensity of a beam of neutrons in terms of the nuclear cross-section?

A

I(x) = intensity
I₀ = initial intensity
µ(x) = linear attenuation coefficient
x = distance
σ = nuclear cross section
N = atomic density

30
Q

What does the nuclear cross section depend on?

A
  • Energy
  • Nucleus
  • Nuclear reaction
31
Q

Define fast diffusion length

A

The average distance travelled in a (non-fissile) material before the neutrons are fully thermalised.

32
Q

Define thermal diffusion length

A

The attenuation of the thermal neutrons in the medium (before capture). It is exponential with distance.

33
Q

What is the fast diffusion length in water?

A

5.75 cm

34
Q

What is the thermal diffusion length in water?

A

2.88 cm

35
Q

What distance of water is an effective shield for neutron sources

A

~25 cm

PHYS3008: Health Physics (19 decks)

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