radiation units: activity & exposure x-rays radiation ... · activity & exposure x-rays...
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24
Lecture 24 Radioactivity
Radiation units:
Activity & Exposure
X-rays
Radiation protection
Applications
Effect of radiation on human body
Depends on
•Energy
•Type of radiation
•Region of body
Radioactivity
Nuclear radiation and x-rays
Penetrate body
No immediate pain or other sensation
Large or repeated small doses
Reddened skin
Lesions
cancers
Main hazard
Caused by ionisation
Reactive ions produced
(hydroxyl Ion OH-)
Interfere with chemical operation of cell
Cells damaged or destroyed
Genetic damage or mutation may occur
Radiation units: Activity & Exposure
Activity of radiation source
•Number of disintegrations per second
Si unit of activity
•Becquerel (Bq)
1 (Bq) = 1 disintegration per second
gram of Radium has an activity of 3.7x1010Bq
106 times more active than
many medical radiation sources
Exposure Absorbed dose
Energy per unit mass absorbed by material
in the path of the radiation beam
SI unit (joules per kg) called gray (Gy)
1Gy = absorbed energy of 1Joule /kg
Radioactivity
NN
t
Effect of radiation on human body
“Biologically equivalent dose”
Concerns
Effects of different types of radiation
Absorbed dose multiplied by weighting factor
compares the effect of the radiation with the
effect of X-rays on tissue.
Unit is the Sievert (Sv)
Weighting factor for X-rays =1
Weighting factor a particles =20
Different types of radiation → different effects
Radioactivity
Equivalent dose (Sv)
= weighting factor x absorbed dose (Gy)
“Biologically equivalent dose”
Effect of radiation on human body
Radiation Weighting (Sv/Gr)
X-rays, g rays
(Energy 200keV)
1
Electrons (b
particles)
1-1.5
Slow neutrons 3-5
protons 10
a particles 20
Weighting factor is known as
the relative biological effect (RBE)
Equivalent dose (Sv)
= RBE x absorbed dose (Gy)
Radioactivity
“Biologically Effective dose”
Depends on what part of the body are
exposed to radiation
Some organs are more sensitive to
radiation than others. A tissue weighting
factor is used to take this into account.
Different parts of body→ different effects
Radioactivity
Sv has the same units as Gy (Joule/kg)
Example: a particles 20 times more damaging on
tissue than X-rays. For 1 Gy dose of a particles
the biologically equivalent dose of X-rays
would be 20 Sv.
Gy multiplied by weighting factor→Sv
Biological effect proportional to
amount of ionization produced in
tissue Proportional to energy deposited
Effect of radiation on human body
Radioactivity
Summary
Name Meaning Unit
Activity or
Decay rate
Number of
disintegrations
per /second
Bq
Absorbed
dose
Energy absorbed
per unit mass
Joule/Kg
Gray (Gy)
Biologically
Equivalent
dose
Effects of different
types of radiation
Absorbed
dose x
weighting
factor.
Sievert
(Sv)
Weighting
factor
Relative biological
effect Sv
Gy
Half life Question
What is the activity of 1g of Strontium 90 if its
half-life is 28 years?
decay rate = -N = (0.693/T1/2) N
N
t
Activity =
N = number of atoms in 1g of Sr90
Definition of Avogadro’s number NA
Number of Carbon atoms in 12g of Carbon
NA = 6.02 x1023
90 g of Sr contain NA atoms of Sr
Number of atoms in 1g N = 23 2116.02 10 6.7 10
90
g
g
Activity = (0.693/T1/2) 6.7 x1021
Activity = 5.3x1012Bq
A 1x1010Bq radioactive source has a half-life of
12 yr and is considered safe if its activity is less
than 3.7 x104bq How much time must pass before
the source is safe?
N = N0(0.5)n
Decay rate = -N = (0.693/T1/2) N
Problem
Decay rate = 1 x1010 disintegrations/second
12 years =12x52x7x24 x3600 =3.78 x108 s
Decay rate = 1 x1010 = (0.693/T1/2) N
N = 5.45 x1018 nuclei
Half life
Radioactivity
Domestic application
Smoke Detector
Americium 241
Half life 432 years 241Am
Mass ~ 0.3mg
Activity ~ 37x103Bq
Current
Detector
alarm
Ionisation Chamber
Effective Dose
• Background is between 1 and 2 mSv yr-1
Average Annual Radiation Dose to the Irish
Population
Medical
Diagnostics
12.1%
Terrestrial
14.1%
Within the Body
12.1%
Thoron Decay
3.0%
Cosmic Rays
9.1%
Global Fallout
0.3%
Radon
49.3%
Source Dose
Dental x-ray 0.01mSv
Chest x-ray 0.02mSv
Seven hour flight 0.05mSv
Annual Dose Limit 1mSv (+Background)
Head CT Up to 1.5mSv
Background Radiation Approx 2 mSv yr-1
Cosmic Radiation for
domestic airline pilot 4mSv yr-1
Dose limit for Radiation
Workers 100mSv over 5 years
Radiation Protection in Perspective
Maximum energy of x-rays is
determined by the accelerating
voltage (energy of the incident electron)
X-rays
High dc voltage ≈50kV
+ -
high energy electrons
x rays
Production
1. Bremsstrahlung
•Braking radiation.
•90% of x-rays produced.
•Produces continuum of x-rays
•Continuous range of x-ray wavelengths
X-Rays
X-ray production: 2 mechanisms
X-ray
Energetic
electron
Electron with
less energy
Incident electrons undergo strong deceleration
and hence high energy EM waves (x-rays) are
emitted
Any accelerating or decelerating charge
will emit electromagnetic radiation
Atom 1
Characteristic
X-ray emitted
Electron ejected
from inner shell
Energetic
electron
Electron drops to
lower energy level
inner shell
X-Rays
2. Ionisation of the absorber atom:
● By ejection of an electron from the inner orbit
followed by the filling of the vacancy by an
electron falling in from an outer orbit.
●10% of x-rays produced in this manner
X-ray production: 2 mechanisms
x-rays characteristic of the target
material produced
X-Rays
Bremsstrahlung
radiation (x-rays)
X-rays characteristic
of target material
X-ray emission spectrum
Wavelength (nm)
Inte
nsit
y
0.1 0.2
X rays
Properties
•Like visible light but
shorter wavelength (higher frequency)
•Produce fluorescence in some materials
•Uncharged
•Reflected and refracted like light
•Affect photographic film
•Heavier elements like Ca absorb x-rays
better than C, O, N. so bone absorbs x-rays
better than muscle and air.
•Produce ionisation in materials
Radiation protection
Time of exposure
• constant activity source, dose is directly proportional to exposure time
• Sensitive x-ray film helps keep note of exposure time
Shielding
• Shielding placed between person and source
to absorb radiation
• Lead aprons
Lead has high electron density
Radiation uses up energy interacting with lead
Minimise Exposure
Distance from source
• Radiation levels around source (non-directional)
decrease in proportion to distance squared
X-ray
tube
Collimator
X-ray
detector
Sheets of lead or
aluminium, etc.
Radiation protection
Shielding
Attenuation is its reduction due to
the absorption and scattering of some
of the photons out of the beam
I = intensity of beam
I0 = intensity of beam with no attenuator
x = thickness of attenuator
μ = linear attenuation coefficient.
(Constant dependent on the substance
& energy of x-rays)
0
xI I e m
Shielding Half Value Layers (HVL)
Radiation protection
-µx
0I = I e
-µx00
I = I e
2
0II =
2let
ln2= xm
ln 2HVL x
m
%
Inci
den
t ra
dia
tion
HVL’s
Ra
dia
tion
Tra
nsm
itte
d
50
25
12.5
6.25
3.12
1.60
1 2 3 4 5 6
The Half Value Layer (HVL) is the thickness
of a material that will reduce the beam
intensity by half
Depends on material.
e.g. 2.5mm for Al, equivalent for Pb is 0.1mm
Radiation protection
Example
The HVL for Pb for a particular energy x-ray
is 0.1mm. By how much will an x-ray beam
be reduced, if a lead sheet 1.5mm thick is
placed in its path?
Beam reduction factor will be 32768.
1.5 mm is 15 HVLs.
Each HVL reduces the beam by a factor of 2.
Beam reduction will be 2x2x2x2….x2. 15 times.
= 215 = 32,768.
Inverse Square Law
Radiation protection Distance
powerIntensity
area
1 2
14
PI
r 2 2
24
PI
r
2
1 2
2
2 1
I r
I r
Consider imaginary
spheres
r2
r1
Isotropic
source
Radiation dose is reduced by moving away
from source By how much?
Person or object
As the person gets further away, the sphere that
intersects with them gets larger and larger
Fraction = Area of person 4 π r12
Fraction = Area of person 4 π r22
Radiation protection
Example
1) A person is working near a radioactive source
and wants to decrease their dose rate by a
factor of 10. How far away do they have to
move?
2
1 2
2
2 1
I r
I r
2
1 2
21 1
10
I r
I r
2
2
2
1
10r
r 2
1
10 3.16r
r
They have to move 3.16 times further away
The intensity (dose / area) falls off as 1/r2 so
moving 4 times as far away will decrease the
dose rate by a factor of 16.
Radiation protection
Example
A person’s hand receives a radiation dose at a
rate of 50mSvh-1 at a distance of 1cm from a
source. What would the dose rate be if the
person’s hand is 18cm from source?
2
1 2
2
2 1
I r
I r
21
2
2
1850 ( )
1
Sv hr
I
m
11
2 2
50 ( )0.15 ( )
18
Sv hrI Sv hr
mm
Dental X rays
Early detection of diseases
It is used to help view general tooth condition
such as cysts, tumors,
gum disease or abscesses that exist in the
bone surrounding the teeth.
In the past, the exposure lasted several
seconds, whereas now, the exposure times
are set at tenths of seconds.
Faster film speed has dramatically reduced the
amount of radiation exposure to the patient by
reducing exposure time.
find cavities between the teeth
· see tartar on the roots
· find worn-out fillings
Bite wing X ray
Dental X rays
Background radiation dose ≈3 mSv per year
Radiation dose from a dental x-ray ranges from
0.04 to 0.15 mSv
effects of radiation exposures are cumulative
Source Exposure
(mSv)
Dental (Bite wings) 0.038
Dental (Full-mouth) 0.15
Chest 0.08
Outer space (per year) 0.5
Natural sources (per year) 3
X-Rays
Medical Applications
• Tomography
Technique for obtaining a cross-sectional image
Very high quality image
Often uses x-rays to image
• CT – Computerized tomography
Rotates x-ray tube around patient
Uses large array of detectors
Collects x-rays penetrating patient and constructs image
Allan MacLeod Cormack and Godfrey Hounsfield
• Nobel in medicine in 1979
Radiotherapy
Radiation used for cancer treatment
Cancer cells are rapidly dividing
• Therefore are sensitive to ionizing radiation
• Improves survival rates for some types
Often used with chemotherapy
• Chemicals that inhibit cell division
• Side effects similar to those of radiotherapy
• Radiotherapy easier to localize
• Side effects of radiotherapy more localized
X-rays used in a dental surgery typically have a
wavelength of 0.03 nm. What is the frequency of
these rays.
cf
8
9
3 10 1
0.03 10
msf
m
1910f Hz
A mobile phone transmits at a frequency of
1.75 x 108Hz. At what wavelength does it operate?
8 1
8
3 101.7
1.75 10
c msm
f Hz
Exercise