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