radioactivity and radioactive decay reynaldo s. jimenez nuclear training center philippine nuclear...

RADIOACTIVITY and

RADIOACTIVE DECAY

REYNALDO S. JIMENEZNuclear Training Center

Philippine Nuclear Research Institute

What is RADIATION ?

Has been around since the earth was formed 4500 million years ago.

Can be detected, measured and controlled.

87% of radiation dose comes from natural sources, e.g... cosmic, food we eat, our homes

13% result of man’s activities..

- Medical Applications (diagnosis and treatment of disease)- Industrial application (inspection of welds, detection of cracks in or cast metal)- Research application (dating of antiquities, food preservation)

Radiation – any form energy that

travels and dissipates.

Ionizing Radiation – Its interaction with matter may result to creation of charged particles.

Non-Ionizing Radiation – its interaction with matter will not result to creation of charged particles.

Everything in nature, every creature

and every material contains, and

always has contained, radioactive

substances.

You are radioactive yourself,

and so is your dog, your coffee,

your seatmate,and your mother-in-

law.

Radiation is all around us.

Sunshine is one of the most familiar forms of radiation.

IONIZING RADIATION

Potentially harmful or beneficial to Humans…depending on how it is used.

Short wavelength = high energy

Long wavelength =

low energy

What is IONIZING RADIATION?

- the kind of radiation which is a result of the radioactive process

- that which changes the physical state of atoms which it strikes causing them to be electrically charged or “ionized”

e-

Neutral atom

e-

e-

Ionized atom

SOURCES OF RADIOACTIVITY

Natural Sources Artificial (Man-made)

accounts for 15% of the total

radiation burden 97% of man-made

radiation is due to diagnostic medical exposures

Natural Sources• Terrestrial• Floors and

walls of our homes, schools or offices

• Food, water and air

• Muscles, bones and tissues

• Cosmic radiation or rays

Background Radiation• Value of background radiation is not stable, may vary widely from place to place and from time to time, depending, for instance, on the structure and wetness of the soil, the seasons, changes in weather, wind direction and the level above sea• Radiation emitted from natural radioactive substances in our environment and from the cosmos

Artificial Sources– Dental and other

medical x-rays– Radiation used to

diagnose diseases and for cancer therapy

– Industrial uses of nuclear techniques

– Consumer products such as luminous wrist watches, ionization smoke detectors

– Fallout from nuclear weapons testing

– Small quantities of radioactive materials released to the environment from coal and nuclear power plants

What is RADIOACTIVITY ?

RADIOACTIVTY

is the process by

which certain

atoms

spontaneously

emit high energy

particles or ra

ys

from th

eir

nucleus.

Where does it come from? How does it happen??

Radiation comes from the nucleus of the atom.

. . . RADIATION . . . radiation . . .

•Everything in the world is composed of different types of matter (chemical elements).

•Each element consists of very small parts called the "Atoms".

ATOM

electron

NUCLEUS

ATOM

Typical diameter: ~ 10-10 m

Typical diameter: ~ 10-15 m

NUCLEUS

neutron

proton

particle mass charge location

proton 1 amu + in nucleus

neutron 1 amu no charge

in nucleus

electron 1/1850 amu

- around nucleus in various energy

levels

Subatomic Particles

1 amu = 1.675 x 10-27 kg

Radioactivity depends on the structure of the nucleus.

A nucleus should contain “appropriate” # of neutrons to become stable non-radioactive

C-12 C-13C-10 C-11 C-14 C-15

Stable configuration (For Z ≤ 20) : # of neutrons = or a bit higher than # of protons

An unstable nucleus has too much energy in it. An atom cannot hold this energy forever.

Unstable nuclei make substances radioactive.

Sooner or later, the atom must get rid of the excess energy

. . . and return to its normal (stable) state.

radiation

WHY, again, do certain ATOMS DECAY?

Atoms with too much energy in their nuclei are called "radioactive".

Some nuclear arrangements are less stable than others.

A radioactive isotope decays to form a more stable

nucleus.

… get rid of their excess energy

(DECAY) by emitting radiation.

Radioactive isotopes…

They decay by spitting out:

- mass (alpha particles)- charge (beta particles)- energy (gamma rays)

PARENT and DAUGHTER

PARENT NUCLIDE – the original nuclide which undergoes radioactive decay

DAUGHTER NUCLIDE (or progeny) - the more stable nuclide which

results from radioactive decay

NUCLIDE - any atomic species characterized by the number of protons and number of neutrons

notations: AzXN X-A AX

where X: symbol of element Z: atomic number = no. of protons N: number of neutrons

A: atomic mass = Z + N Examples:

6027Co33 Co-60 60Co

32

15P17 P-32 32P

ALPHA, •A helium nucleus, 4

2He- Consists of 2 protons and 2 neutrons

•heavy ( Mass : 4 units or 7340 times beta particle )•Charge : +2•High energy ( Energy range : 4 to 8 MeV )•Slow moving ( Speed : 2 X 107 m/s)•Emitted when the nucleus is too big•Least penetrating but much damage where it penetrates

- Limited range : < 10 cm in air ; 60 microns in tissue•Easily Shielded (e.g., paper, skin)

(1) alpha decay - emission of an alpha particle (a He nucleus), resulting in a decrease in both mass and atomic number.

Example: Alpha Decay of Americium-241 to Neptunium-237

αNp Am 4

2

237

93

241

95

- decay•Heavy nuclei more massive than Pb decay by this method•Atomic Number, Z, decreases by 2•Atomic Mass Number, A, decreases by 4•Products are a new element and an particle

αYX 42

4A2Z

AZ

Beta,

• A fast moving electron originating from the nucleus• Emitted when the nucleus has too many neutrons• Comes from a neutron which has changed into a p+

and an e-• Very light ( Mass : 0.00055 amu )• Charge : -1• Energy dependent on radionuclide - ( Energy range : several KeV to 5 MeV ) Range : ~ 12' / MeV in air ; few mm in tissue• Shielding (aluminum and other light (Z<14) materials,

plastics)

(2) Beta decay - emission of a beta particle (an electron from the nucleus), resulting in an increase in atomic number.

- DECAY•A radioactive nucleus that undergoes - decay has a neutron in its nucleus convert into a p+ and an e-

•Atomic Number, Z, increases by 1

•Atomic Mass Number, A, remains the same

Example: Beta Decay of Hydrogen-3 to Helium-3. He H 3

2

3

1

YX A

1ZAZ

Gamma, •Not a particle but a burst of very high energy as electromagnetic radiations of very high frequency•Results from the transition of nuclei from excited state to their ground state•No mass, 0 charge•Have highest energy of all EM radiations•Very dangerous (can do a lot of biological damage)•Energies well defined and characteristics of the emitting radionuclide (up to several MeV)•Speed : speed of light• Long range : km in air ; m in body•Shielding : large amounts of lead or concrete

(3) Gamma decay - This is the photon that carries the energy that is emitted.  The wavelength is in the order of 10-11 to 10-14 m (higher energy than x-rays).

DECAY•Only energy is released•Parent and daughter atoms are the same chem'l element•Atomic Number, Z, remains the same•Atomic Mass Number, A, remains the same

XX A

Z

A

Z

Example: Gamma Decay of Helium-3.

HeHe 3

2

3

2

POSITRON, +

• Similar to e- but opposite charge

• Comes from a proton which has changed into a neutron and a positron

p+ n + +

• The neutron stays in the nucleus and the positron ejected at high speed

• Charge : +1

POSITRON DECAY•Occurs in nuclei which have an excess of protons•Atomic Number, Z, reduces by 1•Atomic Mass Number, A, remains the same

1A

1-ZAZ YX

•Example: Positron Decay of Carbon-11 to Boron-11. B C 11

5

11

6

Comes from a proton which has changed into a neutron and a

positron

p+ n + +

ELECTRON CAPTURE•Occurs in atoms of excess protons•e- in the innermost shell (K-shell) is captured by a p+ in the nucleus to form a neutron• always accompanied by emission of x-rays• x-rays emitted are characteristics of the progeny nuclide•Atomic Number, Z, reduces by 1•Atomic Mass Number, A, remains the same

ray- x YX A1-Z

AZ

Example: Electron Capture of Beryllium-7. It decays to Lithium-7.

LiBe 7

3

7

4

(4) positron emission - emission of a positively charged electron (positron) from the nucleus, resulting in a decrease in the atomic number.  A positron has the same mass as an electron, but opposite in charge.  In other words, inside the nucleus, a proton is being converted into a neutron.

(5) electron capture - This happens in heavy atoms in which an inner shell (1s) electron is captured by the nucleus, resulting in a decrease in atomic number. This process has the same effect as positron emission.

Summary of Radioactive Decay Modes

Decay Mode Symbol Common Source

Change in Z

Change in N

Change in A

Alpha Heavy Nuclei - 2 - 2 - 4

Beta - Excess Neutrons

+ 1 - 1 0

Gamma Excited Nuclei

0 0 0

Positron + Excess Protons

- 1 + 1 0

Electron Capture

Excess Protons

- 1 + 1 0

Decay Chains / Decay Series

A radionuclide may decay to a nuclide which is also radioactive.

This radionuclide may in turn give rise to another radionuclide…

… and this will be repeated until the atom finally reaches a stable form.

This path to stability is called a DECAY CHAIN or a DECAY SERIES.

CHART OF NUCLIDES• a plot of Z vs. N of all known nuclides

• provides several important data concerning the isotopes

• may be used to determine how a particular nuclide will decay• may be used to determine the progeny of a parent nuclide• decay modes are given in order of abundance

• From the chart it is possible to find nuclides which are stable.

• Stable nuclides form a rough band running diagonally up and to the right on the chart.

• General Rule : The closer a nuclide is to the line of stability, the more stable it is.

238 234 230 226 222 218 214 210 206

U-92Pa-91Th-90Ac-89Ra-88Fr-87Rn-86At-85Po-84Bi-83Pb-82Tl-81

U

Th Th

Ra

Rn

Po

Pb

Bi

Po

Tl

Pb

Bi

Po

Pb

U

Pa

Atomic Mass NumberA

tom

ic e

lem

en

t an

d n

um

ber

Uranium-238 Series

Alpha in

Beta-

out

n

out

Original nucleus

n

in

Beta+

out

Alpha

out

Ato

mic

Mass N

um

ber

Atomic element and number

NUCLEAR EQUATIONS

•Nuclear equations show how atoms decay.

•Similar to chemical equations

- must still balance mass and charge

NUCLEAR EQUATIONS

Example: A patient is injected with radioactive phosphorus. What happens to the phosphorus?

Is this equation balanced?

You must see if the mass and charge are the same on both sides.

Charge +15 (protons) + 16 (protons) - 1 ()-------------------------------------- +15 total charge +15 total charge

Yes, it is balanced.

Mass 15 protons 16 protons 17 neutrons 16 neutrons-------------------------------------32 total mass 32 total mass

3215P17 32

16S16 +

DECAY PARAMETERS: characteristics of specific radionuclide

•ACTIVITY, A - number of disintegrations of a nucleus occurring per second

•DECAY CONSTANT, - the fractions of atoms which undergo decay per unit time

•HALF-LIFE, T1/2 - time taken for half the atoms of a radionuclide to undergo radioactive decay

RADIOACTIVE DECAY LAW

N = No e - T

where :

No : original number of nuclei present

T : time which passed

: radioactive decay constant

N : remaining nuclei after time T

UNITSACTIVITY

- described by the number of nuclear disintegrations per unit time.

1 Becquerel (Bq) = 1 disintegration per second - named after Henri Becquerel, discoverer of radioactivity

1 Curie (Ci) = 37 billion disintegrations per second

- named after Marie Curie who discovered and named radium and polonium

1 Ci = 3.7 x 1010 Bq

Activity: how much is present?Activity – tells how many unstable nuclei decay in a second and emit radiation

Activity of an object need not be in any proportion to its size.

High activity Low activity

Radiography isotope

Low level waste

ACTIVITY, AProportional to the number of unstable nuclei

A = N

Can be written in the form:

A = Ao e - T

where: Ao: original activity of radionuclide

T : time which passed : radioactive decay constant A : remaining activity after decay time

Half-Life

Each radioisotope has its own half-life.

Half-life values can range from milliseconds to billions of years.

Half-life examples:

•Molybdenum-99 67 hours• Iodine-131 8 days• Phosphorus-32 14.3 days• Iron-59 45 days• Cobalt-60 5.3 years• Carbon-14 5760 years• Uranium-235 710 million years

HALF-LIFE, T1/2 - time taken for half the atoms of a

radionuclide to undergo radioactive decay

To determine relationship between and T1/2 :

at T=T1/2 , N = No / 2

substituting to N = No e -T

taking ln of both sides:

21T

e21

21T

693.0

21T

oo eN

2N

ln (1/2) = ln e(-T1/2)

- 0.693 = - T1/2

CALCULATING ACTIVITY

Activity is also defined as:

Where A : activity at time T

Ao : initial activity

n : number of half-lives which

have elapsed, i.e.

no

2A

A

21T

Tn

EXAMPLE OF ACTIVITY CALCULATION

PROBLEM:

P-32 has a half-life of 14.3 days. On Jan. 10, 2006, the activity of the P-32 sample was 10 mCi. What will the activity be on February 6, 2006?

no

2A

A METHOD 1: USE Eqn. 1

SOLUTIONS:

Given: T = 27 days : Time interval bet Jan. 10, Feb. 6, 2006

T1/2 = 14.3 days : Half-life of P-32

Ao = 10 Ci : Activity of source on Jan. 10, 2006

Activity of P-32 on Feb. 6, 2006 is 2.698 mCi.

substituting to Eqn. 1 :noAA

2

91.117.30

5.57

21

y

y

T

Tn

27 days

14.3 days1.89

mCimCimCi

A 698.2706.3

10

2

1089.1

METHOD 2 : USET

oeAA Eqn. 2

Substituting to Eqn. 2 :

A = 10 e – (0.048 / day) x (27 days)

= 10 e - 1.308

= 10 x 0.27 = 2.7023 Ci

Activity of P-32 on Feb. 6, 2006 is 2.7023 Ci.Note: Difference is due to rounding adjustments.

12

21

103.217.30

693.0693.0 yxyT

Where

0.048 / day 14.3 days

EXAMPLE OF ACTIVITY CALCULATION

PROBLEM:

A Cs-137 source had an activity of 800 MBq on Jan. 1, 1973. What will its activity be on July 1, 2030?

no

2A

A METHOD 1: USE Eqn. 1

SOLUTIONS:

Given: T = 57.5 y : Time interval bet Jan. 1, 1973 and July 1, 2030

T1/2 = 30.17 y : Half-life of Cs-137 (from Table of Nuclide)

Ao = 800 MBq : Activity of source on Jan. 1, 1973

Computation for T

year , mo. , day

Tf = July 1, 2030 2030 , 7 , 1

Ti = Jan 1, 1973 1973 , 1 , 1---------------------------------- 57 yrs , 6 mo. , 0 days

57 yrs + 6 mo / 12 mo/yr + 0 days

57 yrs + 0.5 yrs -------------------------------------

T = Tf - Ti = 57.5 yrs

91.117.30

5.57

21

y

y

T

Tn

MBqMBqMBq

A 21376.3

800

2

80091.1

Activity of Cs-137 on July 1, 2030 is 213 MBq.

subsituting to Eqn. 1 :noAA

2

METHOD 2 : USET

oeAA Eqn. 2

Substituting to Eqn. 2 :

A = 800 e – (0.023 / y) x (57.5 y)

= 800 e - 1.32

= 800 x 0.267 = 214 MBq

Activity of Cs-137 on July 1, 2030 is 214 MBq.Note: Difference of 1 MBq is due to rounding adjustments.

12

21

103.217.30

693.0693.0 yxyT

Where

0.023 / y

Sample computation for T

Given: Tf = 3:20 pm

Ti = 10:00 am

---------------------------------

5 hrs + 20 mins

Convert hrs to mins:

(5 hrs) X (60 mins/hr) = 300 mins

Add the 20 mins:

300 mins + 20 mins = 320 mins = T

3 + 12 = 15 hrs , 20 mins = 10 hrs, 0 mins

Seatwork: Sample computation for TGiven: Tf = Sept. 12, 1997 Ti = Jan. 1, 1966

--------------------------------- T = Tf – Ti 31 yrs + 8mos. + 11 days

Convert 8 mos. to yrs: (8 mos.) X (1 yr/12 mos.) = 0.6666 yrConvert 11 days to yrs: (11 days) X (1 yr / 365 days) = 0.0301 yrAdd all the numbers converted to yrs: (31 + 0.6666 + 0.0301) yrs = 31.6967 yrs or 31.70 yrs = T

1997 , 9 , 12 1966 , 1 , 1

Seatwork: 32P has a half life of 14.3 days. At 0 day it

has an activity of 1mCi. Compute for the activity of 32P after the:

1) 1st half life : 2) 2nd half life: 3) 3rd half life: 4) 4th half life:

EXAMPLE OF HALF-LIFE CALCULATION

PROBLEM : A sample is counted and found to have 952 counts per minute. Seven minutes later it is measured again and has a count of 148 counts per minute. A background measurement gave 6 counts per minute. What is the half-life of the sample?

SOLUTIONS :

GIVEN:

Initial Activity : Ao = 952 - 6 = 946 cpm

Activity 7 minutes after : A = 148 - 6 = 142 cpm

This means that the number of half-lives (n) in 7 minutes is 2.74.

METHOD 1 : UsenoAA

2

66.6142

9462

A

Aon

n log 2 = log 6.66

74.23010.0

8235.0

2log

66.6logn

74.2

72

1 T

Using2

1T

Tn

n

TT

21

Therefore, the half-life of the sample is 2.55 mins.

T1/2 = 2.55 minutes

UseT

o

eA

A

TA

Aln

o

TAA

ln o

TA

Aln o

METHOD 2 :

A = Ao e -T

7142946

ln

7

66.6ln

7

89.1

271.0

Then use:

21

693.0

T

693.0

21 T

271.0

693.02

1 T

T1/2 = 2.55 minutes

THANK YOU for your attention.