Dosimetry equipment I

Question 1

What is meant by exposure and electronic equilibrium? (4 marks)

Answer 1

• exposure = X = delta Q / delta m = charge librated per unit mass
• the conditions of electronic equilibrium are that the number of electrons entering and leaving a mass of dm is equal
• from a practical perspective this occurs when the minimum dimension of the detector is&raquo_space; than the range of the electrons

Question 2

How does a FREE air ionisation chamber occur? (6 marks)

Answer 2

• photon beam passes between two parallel plates
• voltage applied between the plates is above the saturation voltage to prevent recombination
• photons ionise the air, plates collect the ions
• mass of air is defined by beam and plate geometry
• circuit measures charge collected
• used at the NPL for kV beams

Question 3

What the purpose of a primary standard? (4 marks)

Answer 3

• provides an absolute dose measurement from first principles
• used to provide a chain of traceability in measurements
• used or calibration of secondary standards
• located at the NPL in the UK (free air (kV beams) and graphite calorimetry (electrons and MV beams))

Question 4

How can dose to air measurements be converted to dose to water? (4 marks)

Answer 4

D_w = M x N_k x B_w x [mew / rho]

• M = reading
• N_k = calibration factor
• B_w = backscatter factor
• mew / rho = ratio pf mass absorption coefficients

Question 5

How are thimble chambers constructed and how do they work? (6 marks)

Answer 5

• graphite shell with an air cavity and an Al electrode in the centre
• graphite shell thickness is greater than the range of the electrons, and similar Z to air but denser
• 1 mm wall = 1 m of air equivalence
• large chambers are more sensitive but lack spatial resolution used for environmental monitoring
• smaller chambers used for beam profiles / PDDs
• dose is proportional to exposure

Question 6

How does a parallel plate chamber work and when it is used? (4 marks)

Answer 6

• two long parallel electrode plates with a small separation but a volume
• front window to allow photons in
• thin in direction of meaurement
• used in high dose gradients such as the build-up region

Question 7

How does a calorimeter work? (5 marks)

Answer 7

• measures small temperature changes to determine the energy deposited
• E = mc delta T
• core is thermally isolated
• temp measured using thermistors
• graphite used as specific heat capacity of water is too high and difficulties with impurities

Question 8

What is the purpose of a secondary standard? (4 marks)

Answer 8

• calibrated against a national standard
• NPL provides correction factors
• high quality instrument
• used to cross-calibrate field instruments

Question 9

For a field chamber what corrections are applied to determine the dose to water? (5 marks)

Answer 9

D_w = R x N_d x k_ion x k_ftp x k_e

R = reading
N_d = calibration factor
k_ion = ion recombination factor
k_ftp = pressure and temp correction
k_e = electrometer factor

Question 10

Why do we calibrate instruments? (3 marks)

Answer 10

• ensures doses are accurate and consistent between centres
• provides chain of traceability
  
  • > primary -> secondary -> field instruments