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Dosimetry > Indirectly ionising radiation > Flashcards

Indirectly ionising radiation Flashcards

1
Q

What is the effective atomic number?

A

The atomic number of an element which would interact in the same way as the composite material

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2
Q

What is the equation of effective atomic number?

A

Zbar = (sum(alphan.Zn^2.94))^(1/2.94)

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3
Q

Describe Compton scatter

A

The incident photon has energy»binding energy so it interacts with a sea of free electrons. It creates scattered photons, bremsstrahlung, and delta rays through elastic collisions.
The incoming photon interacts with an electron, depositing some of its energy with the electron and both are scattered.

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4
Q

What is the energy of the photon after Compton scatter?

A

hv’ = hv0(1/(1+alpha(1-cos(phi)))

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5
Q

What is the energy transferred in Compton scatter?

A

Etr = hv0((alpha(1+cos(phi)))/(1+alpha(1-cos(phi))))

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6
Q

What is alpha in Compton scatter?

A

alpha = hv0/m0c^2

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7
Q

What is cos(theta), the angle of electron scatter, given by in Compton scatter?

A

cos(theta) = (1+alpha)tan(phi/2)

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8
Q

In Compton scatter, at low and high energiess which particle gets the majority of the energy?

A 
low = photon
high = electron
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9
Q

What does Compton scatter depend on?

A

The number of free electrons = Na Z/A

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10
Q

At high energies in Compton scatter, what direction are the photon and electron scattered in?

A

Forward direction

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11
Q

What is the mass energy transfer coefficient?

A

u/density, linear attenuation coefficient with only a dependence on the composition of the medium and not the density.

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12
Q

What is KERMA?

A

KERMA is the initial kinetic energy of all charged particles released as a result of interactions in mass dm. It includes bremsstrahlung, energy deposited outside the region of interest by particles created in it. Only defined for neutral particles

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13
Q

What is the equation for KERMA?

A

KERMA = psi.(utr/density).e^(-up.x)

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14
Q

What is the absorbed dose?

A

The total energy imparted by ionising radiation to mass dm. Considers energy deposited in dm by radiation produced outside the region dm, but not energy deposited outside dm by charged particles created inside dm.

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15
Q

What is the equation for absorbed dose?

A

D = psi.(utr/density).e^(-up.x) dx

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16
Q

At depth dx what is the fluence decreased by a factor of?

A

e^(-ue.dx), ue = electron linear attenuation coefficient

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17
Q

What gives the fraction of energy re-emitted as bremsstrahlung?

A

g

18
Q

How does g affect the mass attenuation coefficient equation?

A

uen/density = utr/density (1-g)

19
Q

What are the requirements for measuring absorbed dose?

A

In an infinite medium

Beyond Dmax and build-up region

20
Q

What is the equation for the Bragg Cavity Theory?

A 
Dmed = JW/e . Dgas
Dmed = dQ/dm . W/e . Dgas
21
Q

What is the Fano theorem?

A

In a medium of given composition, exposed to a given flux of primary radiation, the flux of secondary radiation is also uniform and independent of density of the medium as well as density variations from point to point.

22
Q

What are the requirements of the Fano theorem?

A

Interaction coefficients are independent of density
Cavity in finite medium and is homogeneous
Uniform irradiation
Thickness of chamber wall»range of secondary electrons
Chamber wall needs the same electron density as medium.

23
Q

What is the equation for dose from the Fano theorem?

A

Dmed = J (Wair/e) Smed,gas (uen/density)med,gas

24
Q

What is the linear attenuation coefficient?

A

A measure of non-attenauted primary beam after it passes through an absorber, u
It is a measure of the energy removed from a beam

25
Q

How is the linear attenuation coefficient derived?

A

Expose an absorber of thickness dx to primary beam. Place a detector at point P such that it only measures unattenuated primary beam. At P, dN is proportional to N0.dx. So dN=-uN0dx.

26
Q

What is the cross section of interaction?

A

sigma = probability of intersection/unit particle fluence

27
Q

How can linear attenuation coefficient be derived from the cross section?

A

dN=B.sigma.N0.dx, so u = B.sigma where B is the number of interaction centres per unit volume and B=Na.density/Molar mass.

28
Q

What is the energy transfer coefficient?

A

The energy removed from the photon beam which becomes kinetic energy for electrons in the absorber.
utr = Etr/hv0 .u, where Etr is the average energy transferred into KE per interaction and Etr/hv0 is the average fraction of energy transferred to KE.

29
Q

How is the energy absorption coefficient defined?

A

The energy deposited in medium, uen = utr(1-g)

30
Q

How is the mass attenuation coefficient defined?

A

u/density, so it is u which only depends on the composition of the medium and not the density.

31
Q

What is the process of the Photo-Electric Effect?

A

An incoming photon has enough energy to ionise an inner shell electron. A higher shell electron then drops to the lower energy state, emitting a characteristic x-ray.

32
Q

How are Auger electrons produced?

A

The characteristic x-ray which is produced in the photo-electric effect ionises another electron in the atom.

33
Q

What is the mass attenuation coefficient for the photo-electric effect proportional to?

A

tau/density prop 1/hv0^3 prop Z^3

34
Q

What is the mass energy transfer coefficient for the photo-electric effect?

A

tauk/density = tau/density . (hv0-hvc)/hv0 = tau/density . (1-hvc/hv0) where hvc is the average energy transferred to KE.

35
Q

What is the equation for exposure in a gas?

A

X = dQ/dm

36
Q

Describe how a free air ionisation chamber works

A

A beam from a source is aimed through a diaphragm of known area into a region of free air. A high polarising voltage is applied. Ions are produced and are attracted to the electrodes, losing all of their energy before they reach there.
Guard electrodes ensure that only ions created in the measuring volume are collected.
The distance between the plates must be greater than double the range of the secondary electrons.

37
Q

Derive KERMA from exposure in a free air ionisation chamber.

A

X = Q/A.L.density = psi(uen/density)(e/W)
uen/density = utr/density . (1-g)
So, KERMA = X/1-g (W/e)

38
Q

What requirement allows KERMA = X(W/e) for superficial energies?

A

g«1

39
Q

How is dose to medium found from dose to air?

A

Dmed = KERMA. (uen/density)med/(uen/density)air

40
Q

What are the requirements of making an ionisation chamber at superficial energies?

A

Need electronic equilibrium betwen air on the inside and graphite
All the electrons produced in air are stopped by the graphite
Photo-electric effect dominates so air equivalence between the layers is needed.
A measuring electrode and insulator need to be introduced.

Dosimetry (11 decks)

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