24
Lecture 24
Radioactivity
Radiation units:
Activity & Exposure
X-rays
Radiation protection
Applications
Radioactivity
Effect of radiation on human body
Depends on
•Energy
•Type of radiation
•Region of body
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
Radioactivity
Radiation units: Activity & Exposure
Activity of radiation source
•Number of disintegrations per second
N
  N
Si unit of activity
t
•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
Effect of radiation on human body
Different types of radiation → different effects
“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.
“Biologically equivalent dose”
Unit is the Sievert (Sv)
Equivalent dose (Sv)
= weighting factor x absorbed dose (Gy)
Weighting factor for X-rays =1
Weighting factor a particles =20
Radioactivity
Effect of radiation on human body
Radiation
X-rays, g rays
(Energy 200keV)
Electrons (b
particles)
Weighting (Sv/Gr)
1
Slow neutrons
3-5
protons
a particles
10
20
1-1.5
Weighting factor is known as
the relative biological effect (RBE)
Equivalent dose (Sv)
= RBE x absorbed dose (Gy)
Radioactivity
Sv has the same units as Gy (Joule/kg)
Gy multiplied by weighting factor→Sv
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.
“Biologically Effective dose”
Depends on what part of the body are
exposed to radiation
Different parts of body→ different effects
Some organs are more sensitive to
radiation than others. A tissue weighting
factor is used to take this into account.
Biological effect proportional to
amount of ionization produced in
tissue
Proportional to energy deposited
Radioactivity
Effect of radiation on human body
Summary
Name
Meaning
Unit
Activity or
Decay rate
Number of
disintegrations
per /second
Energy absorbed
per unit mass
Bq
Absorbed
dose
Biologically Effects of different
Equivalent types of radiation
dose
Weighting
factor
Relative biological
effect
Joule/Kg
Gray (Gy)
Absorbed
dose x
weighting
factor.
Sievert
(Sv)
Sv
Gy
Half life
Question
What is the activity of 1g of Strontium 90 if its
half-life is 28 years?
Activity =
decay rate
N
t
= -N = (0.693/T1/2) N
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 =
1g
6.02 1023   6.7 1021

90 g
Activity = (0.693/T1/2) 6.7 x1021
Activity = 5.3x1012Bq
Half life
Problem
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
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
Radioactivity
Domestic application
Smoke Detector
Americium 241
Half life 432 years
241
Am
Mass ~ 0.3mg
Activity ~ 37x103Bq
Ionisation Chamber
alarm
Current
Detector
Effective Dose
• Background is between 1 and 2 mSv yr-1
Average Annual Radiation Dose to the Irish
Population
Terrestrial
14.1%
Radon
49.3%
Medical
Diagnostics
12.1%
Within the Body
12.1%
Global Fallout
0.3%
Cosmic Rays
9.1%
Thoron Decay
3.0%
Radiation Protection in Perspective
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
X-rays
Production
high energy electrons
x rays
+
High dc voltage ≈50kV
Maximum energy of x-rays is
determined by the accelerating
voltage (energy of the incident electron)
X-Rays
X-ray production: 2 mechanisms
1
Energetic
electron
Atom
Electron with
less energy
X-ray
Any accelerating or decelerating charge
will emit electromagnetic radiation
Incident electrons undergo strong deceleration
and hence high energy EM waves (x-rays) are
emitted
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
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-rays characteristic of the target
material produced
Characteristic
X-ray emitted
Energetic
electron
Electron ejected
from inner shell
Electron drops to
lower energy level
inner shell
X-Rays
X-ray emission spectrum
X-rays characteristic
of target material
Intensity
Bremsstrahlung
radiation (x-rays)
0.1
Wavelength (nm)
0.2
X rays
Properties
•Like visible light but
shorter wavelength (higher frequency)
•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
•Produce fluorescence in some materials
Radiation protection
Minimise Exposure
Distance from source
• Radiation levels around source (non-directional)
decrease in proportion to distance squared
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
Radiation protection
Shielding
Attenuation is its reduction due to
the absorption and scattering of some
of the photons out of the beam
X-ray
detector
X-ray
tube
Collimator
Sheets of lead or
aluminium, etc.
I  I 0e
mx
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)
Radiation protection
Shielding
Half Value Layers (HVL)
The Half Value Layer (HVL) is the thickness
of a material that will reduce the beam
intensity by half
I = I0 e
-µx
let I =
I0
2
ln2=m x
I0
= I0e-µx
2
%
Incident radiation
50
25
12.5
6.25
3.12
1.60
123456
m
Radiation Transmitted
HVL  x 
ln 2
HVL’s
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?
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.
Beam reduction factor will be 32768.
Radiation protection
Distance
Radiation dose is reduced by moving away
from source
By how much?
Inverse Square Law Consider imaginary
spheres
Isotropic
source
power
Intensity 
area
P
I1 
4 r12
r1
r2
P
I2 
4 r22
I1 r22
 2
I 2 r1
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?
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.
2
2
2
1
I1 r

I2 r
r22
10  2
r1
I1
r22
 2
 I1  r1
 
 10 
r2
 10  3.16
r1
They have to move 3.16 times further away
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?
I1 r22
 2
I 2 r1
50m Sv(hr ) 1 182
 2
I2
1
50m Sv(hr ) 1
1
I2 

0.15
m
Sv
(
hr
)
182
Dental X rays
It is used to help view general tooth condition
Early detection of diseases
such as cysts, tumors,
gum disease or abscesses that exist in the
bone surrounding the teeth.
Bite wing X ray
find cavities between the teeth
· see tartar on the roots
· find worn-out fillings
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.
Dental X rays
Source
Dental (Bite wings)
Dental (Full-mouth)
Chest
Outer space (per year)
Natural sources (per year)
Exposure
(mSv)
0.038
0.15
0.08
0.5
3
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
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
Exercise
X-rays used in a dental surgery typically have a
wavelength of 0.03 nm. What is the frequency of
these rays.
f 
c

3 108 ms  1
f 
9
0.03 10 m
f  10 Hz
19
A mobile phone transmits at a frequency of
1.75 x 108Hz. At what wavelength does it operate?
1
c
3 10 ms
 
 1.7m
8
f 1.75 10 Hz
8