HMPH208: Assignment
Question 1
i.
a)
b)
c)
d)
e)
f)
g)
h)
ii.
iii.
iv.
Define the following terms:
Radiation Protection
Exposure
Particle Fluence
Energy Fluence
Kerma
Absorbed Dose
Effective Dose
Dosimetry
[10]
Describe the ICRU sphere and explain the evaluation of equivalent dose with the aid
of a diagram
[10]
A patient has a chest x-ray. The area of the chest exposed to the x-ray beam is
approximately 500 cm2 and the intensity of the x-ray beam is 0.3 W/m2. The patient is
exposed for 0.2 seconds. Hospital regulations state that the absorbed dose must be kept
below 0.0020 Gy.
a) What is the power of the beam to which the patient is exposed?
[2]
b) What is the maximum human-equivalent dose for the patient?
[3]
3
A pocket air- wall chamber has a volume of 2.5cm and a capacitance of 7pF. Initially
charged at 200V, the reader showed a potential difference of 170V after the chamber
was worn. Given that density of Air= 1.293x10-6kg/cm3.What exposure can be
inferred?
[5]
Question 2
i. Draw a labelled diagram of a gas filled detector.
ii. With the aid of a diagram describe and explain the regions of a gas filled detector.
iii. Describe detector quenching and how it is achieved?
[5]
[10]
[5]
iv. A pocket air- wall chamber has a volume of 2.5cm3 and a capacitance of 7pF. Initially
charged at 200V, the reader showed a potential difference of 170V after the chamber was
worn. Given that density of Air= 1.293x10-6kg/cm3.What exposure can be inferred? [5]
v.
[5]
Question 3
i.
ii.
iii.
Calculate the thickness of Perspex of density 1.2g/cm3 that is required to stop all β
particles from P-32. Assume density thickness of 0.7g/cm2
[2]
A γ-ray detector having a 100% counting efficiency is positioned in a constant field,
making 10 measurements of equal duration of 100 sec. The average number of rays
detected (counts) per measurement is 1.00x 105. What is the mean value of the count
rate?
[2]
In a unique exposure situation, the mean absorbed dose to the following organ doses
have been calculated and have been delivered from the following radiation types
incident on the body of an irradiated individual:
Organ
Red Bone Marrow
Stomach
Thyroid
Calculate:
Mean Absorbed
Dose (Gy)
Radiation Type
0.25
0.35
0.45
364keV Photons
9keV Neutrons
90keV Neutrons
Tissue
Weighting
Factor
0.12
0.12
0.04
a) The organ equivalent doses to the red bone marrow, stomach, and thyroid.
b) Effective dose to the exposed individuals.
[7]
iv.
An 8 MeV photon penetrates into a 200g mass of tissue and undergoes a single, pair
production interaction. Both the e+ and e-particles created in this interaction dissipate
all their kinetic energy within the mass through secondary ionization and a
bremsstrahlung photon of energy 1.1 and 2MeV respectively are produced and escape
from the mass without further interaction. The positron, after expending all its kinetic
energy interacts with an ambient electron within the mass and they mutually
annihilate. Both photons created in the annihilation radiation escape the mass before
further interaction. Calculate:
a) The Kerma
[2]
b) Absorbed dose the tissue receives from these interactions
[3]
v. Describe the Bragg-Cavity principle and its use in cavity ionization chambers to estimate
tissue doses.
[2]
vi. What assumptions must be made regarding the primary electron energy distributions within
the cavity.
[2]
vii. How can the number of electron -ion pairs created by the radiation field in the cavity be used
to calculate the dose imparted in the wall material?
[2]
viii. A chamber satisfying the Bragg- Gray conditions contains 0.15g of gas with a W value of 33
Ev. Ip-1. The ratio of the mass stopping power of the wall and the gas is 1.03. What is the
current when the absorbed dose rate in the wall is 10mGy/h?
[3]
ix. Calculate the absorbed dose measured in a tissue equivalent chamber with a 1cm3 air filled
cavity within the chamber which is exposed to a 60Co source for 10 minutes. A total of
2.25x109 electrons are collected from the chamber as a result of ionization events in this
period and the average stopping power ratio of tissue to air (Sm/Sg) =1.137 for 60Co gamma
rays
[5]
Question 4
i.
ii.
iii.
iv.
v.
vi.
Distinguish between strongly penetrating radiation and weakly penetrating radiation? [4]
Distinguish between directly ionizing radiation and indirectly ionizing radiation
[4]
Why are operational quantities necessary in the field of radiation protection?
[2]
If dividual monitoring is not feasible, how should doses to individuals be assessed?
[2]
Why are filters used in passive dosimeters?
[2]
With the aid of a diagram describe charged particle equilibrium and the relationship
[6]
between dose and Kerma.
vii. Determine the linear attenuation coefficient µ in terms of photon fluence I entering a small
absorber of thickness dx in which the fluence is reduced by an amount dl
[3]
viii. How is the linear attenuation coefficient related to the maas attenuation coefficient?
[1]
ix. The mass attenuation coefficient for Pb for photon energies of 59.5keV and 662keV are
5.13cm2/g and 0.11 cm2/g respectively. The density of lead is 11.34g/cm3. Estimate the
thickness of Pb absorber placed in front of a well focussed photon beam which would
reduce the dose imparted by the photon flux to 1% of the original dose which would be
imparted in the absence of Pb shielding for photon energies of:
a) 59.5keV
[3]
b) 662keV
[3]
END OF ASSIGNMENT