Radioactivity and Radioactive Decay.suneD 08

8/14/2019 Radioactivity and Radioactive Decay.suneD 08

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Radioactivity and

Radioactive Decay


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Objectives

To be able to:

define the term radioactive decay

use the terms parent nuclide and progeny correctly

list the types of ionizing radiation & their properties

state the quantities & units used in the measurement ofradiation

describe decay mechanisms in terms of changes to the

parent nuclide & the types of radiation emitteduse the chart of the nuclides to predict progeny resultingfrom radioactive decay

define the terms half-life & decay constant


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Objectives (cont)

define the terms activity & becquerel

relate the becquerel to curie and convert from one to the

other

calculate the amount of activity of a nuclide remainingafter a specific period

calculate the half-life of a nuclide given sample data

use the chart of nuclides to describe decay chains and

seriesdescribe the sources of both natural & artificial

radionuclides


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Radioactive Decay

Spontaneous changes in the nucleus ofan unstable atom

Results in formation of new elements(progeny)

Accompanied by a release of energy,either particulate or electromagnetic orboth


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Radioactive Decay

Why some nuclides decay

the structure of the nucleus determines

whether or not it will be radioactive

if a nucleus does not have a stable

arrangement, it will decay and form a more

stable nuclide

Nuclear instability is related to whether the

neutronto proton ratio is too high or too low


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Ionizing Radiation

Any particle or ray which has sufficient

energy to remove e s from atoms

Unit of energy: eV (electron volt)

1 eV = 1.6 x 10 -19joules

keV (1000 eV), MeV (1,000,000 eV)


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consists of 2 p and 2 n tightly bound

together

He

emitted w/ a well-defined energy which is

characteristic of the particular

radionuclide from w/c it was emitted

usually emitted by heavy nuclei

elements, e.g. U, Ra

4

2

Alpha Particle ()


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an e w/c is ejected from the nucleus of the

radionuclide at high speed

the proton remains in the nucleus & the e

is emittedhas a small mass (1/1840 u)

has a single negative charge

are emitted w/ a distribution of energies up toa max. energy w/c is dependent on the

particular radionuclide

Beta Particle ()


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em radiation w/c comefrom the nucleusof an atomas a result of radioactivedecay

em radiationconsists of packets of energy

(photons) w/c are transmitted in the form ofwaves at the speed of light

- includes non-ionizing radiation, e.g.radiowaves, microwaves, heat, visible light, & uv

have the highest energy of all em radiationno mass & no charge

Energy released are well defined &characteristic of the emitting radionuclide

Gamma Rays ()


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IONIZING

RADIATION

Potentially harmful or beneficial to

humansdepending on how it is used.

Short wavelength

= high energyLong wavelength

= low energy

Energy Spectrum


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Positrons (+)

is similarto an electron w/ the same mass

but an opposite (+) charge

it comes from a proton w/c has changed intoa neutron & positron

the neutron stays in the nucleus & the

positron is ejected at high speed


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X - rays

electromagnetic radiation with no mass & nocharge

produced when atomic e -s undergo a change inorbit

Neutrons

particles found in the nucleus of the atom

has a mass of 1 u and no charge


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Summary of the properties of

Ionizing Radiation

Type of radiation Symbol Mass (u) Charge

alpha 4 + 2

beta - 1/1840 -1

gamma 0 0

positron + 1/1840 +1

X-ray x 0 0

neutron n 1 0


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Using the chart of the nuclides

to find decay modes

the way in w/c decay occurs is different for

each particular radionuclide & their decay

modes differ in both the particles produced& the energy of the emitted particle or ray

provides information on the radioactive

decay modes and energies

use to find both stable & unstable nuclides


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Isotopes

There are many

isotopes. Mosthave moreneutrons thanprotons. Someare stable butmost are unstable(radioactive).

equal number of protons and neutrons


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The Line of Stability

N>Z

- The closer the

nuclide to the line of

stability, the morestable it is


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Modes of Radioactive Decay


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Emission of a highly energetic heliumnucleusfrom the nucleus of a radioactive

atom

Occurs when neutron to proton ratio is toolow

Results in a decay product whose atomic

number is 2 less than the parent and whose

atomic mass is 4 less than the parent

Alpha particles are monoenergetic

Alpha Emission


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Alpha particle

charge +2

Alpha Particle Decay


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Alpha Particle Decay


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Alpha Decay Example

Ra-226 decays by alpha emission

When Ra-226 decays, the atomic massdecreases by 4 and the atomic numberdecreases by 2

The atomic number defines the element, sothe element changes from radium to radon

226

Ra 222Rn + 4He88 86 2


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Nuclear ReactionsAlpha Decay


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Emission of an electron from the nucleus of

a radioactive atom ( n p+ + e-1)

Occurs when neutron to proton ratio is toohigh(i.e., a surplus of neutrons)

Beta particles are emitted with a wholespectrum of energies (unlike alpha particles)

Beta Emission


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Beta Spectrum


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Beta particle

charge -1

Beta Particle Decay


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Beta Particle Decay


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Nuclear Reactions

Beta Decay


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Beta Decay of 99Mo


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Gamma Ray Emission

Monoenergetic radiations emitted fromnucleus of an excited atom following

radioactive decay

Rid nucleus of excess energy

Have characteristic energies which can beused to identify the radionuclide


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Gamma Ray Emission

Gamma radiation


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Gamma Ray Emission


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Isomeric Transition

process whereby a metastable radionuclide

emits a gamma ray thus removing excess

energy from the nucleus

metastable radionuclide is one which hasexcess energy in the nucleus, e.g., 99mTc.

also called isomer

IT Mo42

99Tc

43

99m + - Tc + 43

99


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Positron (Beta+) Emission

Occurs when neutron to proton ratio is

too low ( p+ n + e+ ) Emits a positron (beta particle whose

charge is positive)

Results in emission of 2 gamma rays(more on this later)


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Positron (Beta+) Emission


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Positron Decay

O8

15

N

7

15+ +


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Nuclear Reactions

Positron Decay or Electron Capture

Positron Emission

Electron Capture


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X-Ray Production

electron fillsvacancy Electronejected

Characteristic

x-rays


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Internal Conversion

another process w/c removes excess energy

from the nucleus; an alternative to gamma

ray emission

excess energy is being transferred to anorbital e-from the inner shells of the atom

These ejected electrons are called Auger

electronsand have very little kinetic energy

Electron& x-rayare emitted instead of a

gamma ray


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Internal Conversion


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Orbital Electron Capture

Also called K Capture

Occurs when neutron to proton ratio is toolow

Form of decay competing with positronemission

One of the orbital electrons is captured bythe nucleus: -1e + +1p n Results in emission of characteristic x-rays


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Photon Emission

DifferenceBetween

X-Rays and

Gamma

Rays

S f R di ti D M h i


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Summary of Radioactive Decay Mechanisms

Decay

Mode

Characteristic of

Parent Radionuclide

Change in

Z

Change in

N Comments

Alpha Heavy nuclei -2 -2 Alphas Monoenergetic

Beta Excess neutrons +1 -1 Beta Energy Spectrum

Positron Excess protons -1 +1Positron Energy

Spectrum

Electron

CaptureExcess protons -1 +1

K-Capture;

Characteristic X-rays

Emitted

Gamma Excess energy 0 0 GammasMonoenergetic

Internal

ConversionExcess energy 0 0

Ejects Orbital

Electrons;

characteristic x-rays

and Auger electrons

emitted


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Types of radiation

Ionizing radiation (IR) Particles

Alpha Beta Neutron

Electromagnetic Gamma & X-ray

Non-ionizing radiation(NIR)

Electromagnetic Visible light Microwave Radiofrequency Extremely low

frequency

Lasers in a classical concert
http://en.wikipedia.org/wiki/Image:Laser_effects.jpg


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Decay Parameters

Physical quantities w/c describe the way in

w/c radionuclide decays:

decay constant

activity

half-life


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Radioactive Decay Law & Decay Constant

N = N0e -Twhere:

N0 - the number of radioactive nuclei

present at T = 0,

- the radioactive decay constantT - the elapsed time

N - the number of radioactive nuclei

remaining after the elapsed time


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Activity

Indicates the number of radionuclides

disintegrating per second (dps or s-1)

A = N or A = A0e-T

The SI unit is the becquerel (Bq)

1 Bq = 1 disintegration per second


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Radioactive decay constant ()= 0.693/T1/2

where T1/2

is half-life


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Half-life (T1/2)

The time it will take for the activity of the

radioactive source to decrease to one-half

of its original value

The activity of a source is dependent onthe half-life of the particular radionuclide

Each radionuclide has each characteristic

half-life

- T= T1/2 N = N0/2 A = A0/2

-


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Half-Life and Decay Constant

The relationship between half-life and

decay constant is:

T=0.693


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Half-life

Radionuclide Half-life Application

Cesium-137 30.17 y Industrial gauging,

medical therapy

Americium-241 433 y Industrial gauging

Cobalt-60 5.25 y Industrial gauging,

radiography,

medical therapy

Iodine-131 8.1 d Medical

diagnosis/therapy

Iodine-125 60 d Medical

diagnosis/therapy


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Half-Life


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Activity

Indicates the number of radionuclides

disintegrating per second (dps or s-1)

The SI unit is the becquerel (Bq)

1 Bq = 1 disintegration per second

M lti l & P fi f B


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Multiples & Prefixes of Bq

Multiple Prefix Abbreviation

1 ------- Bq

1,000,000 Mega (M) MBq

1,000,000,000 Giga (G) GBq

1,000,000,000,000 Tera (T) TBq

1 x 1015 Peta (P) PBq


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Relationship of Units of Activity

Becquerel (Bq) = 1 dps

Curie (Ci) = 3.7 x 1010dps

1 Ci = 3.7 x 1010 Bq

1 mCi = 3.7 x 10 7Bq1 Ci = 3.7 x 10 4Bq


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Useful Conversions for Units of Activity

Curies to Becquerels Becquerels to Curies

1 Ci = 37 kBq 1 Bq = 2.7 x 10 -11Ci

1 mCi = 37 MBq 1 kBq = 2.7 x 10 -3Ci

1 Ci = 37 GBq 1 MBq = 2.7 x 10 -5Ci = 27 Ci

102

= 37 TBq

1 GBq = 2.7 x 10-2

Ci = 27mCi

1 TBq = 2.7 x 10 Ci = 27 Ci


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Examples

1. Cesium -137 source has an activity of 20

mCi. This is equivalent to:

a) Bq,

b) MBq,

c) GBq

2. A radioactive source has an activity of 800MBq. Convert this to: a) mCi, c) Ci


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Radioactive Decay

Activity (A)

disintegration

time

time (t)

A = A0e -T


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Radioactive Decay

The amount of activity A remaining

after n half-lives is given by

A

Ao

1

2n=


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Calculating Activity

A=

A 0

2 n

where:

A is the activity at time, T

A0is the initial activity

n is the number of half-lives w/c has

elapsed

n = T/T1/2


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Examples

1. Co-60 has an activity of 5.55 GBq as ofFeb. 19, 2000. What was its activity last

May 19, 2008? (T1/2= 5.27 yrs.)

2. Cs- 137 source used in mining industry

has an activity of 740 MBq as of March

30, 1990. What will be its activity onDec. 30, 2008? ( T1/2= 30 yrs.)


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Sources of ionizing radiation:

Natural and

Artificial or man-made


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Sources of IR

Natural sources Cosmic radiation

Terrestrial radiation

Naturally occurring radioactive material (NORM)Artificial or man-made sources

Electrically generated radiation

Accelerator produced radioisotopes Reactor produced radioisotopes


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Background Radiation

Galacticradiation is a

component of

the background

radiation on

earth.

Natural sources


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Cosmic radiation

contributes to thebackground radiation

on earth. The earths

atmosphere provides

shielding from most ofthe cosmic radiation.

Natural sources


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cosmicradiationcomes fromoutside theearth Cosmogenic

formed as aresult of

cosmic rayinteractions


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terrestrial radiationcomes from the

rocks of the earth

U-238 Rn-222

Th-232 Rn-220


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Terrestrial Nuclides

Nuclide Half-life Natural Activity

226Ra 1,600 yr 16 Bq/kg in limestone and 48Bq/kg in igneous rock

222Rn 3.82 days Noble gas; average annualair concentrations in US

range from 0.6 to 28 Bq/m3

40K 1.28 x 109yr 0.037 to 1.1 Bq/g in soil



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There are three decay chains that occur in nature:

the uranium series, beginning with U-238

the thorium series, beginning with Th-232

the actinium series, beginning with U-235

Once upon a time there was also a neptunium series,which originated with 241Pu, that has a half-life ofonly 14 years. The only remaining member of this

series is 209Bi with a half-life of 2 x 1018years.


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Primordialexisting since the creation of the earth(have half-lives in the order of a billion (10 9) years

Nuclide Half-life Natural Activity

235U 7.04 x 108yr 0.711% of all natural

uranium

238U 4.47 x 109yr 99.275% of all natural

U; 0.5 to

4.7 ppm total U in

common rocks

232Th 1.41 x 1010yr 1.6 to 20 ppm in

common rocks


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Th-232

Th-228

Ra-228

Ac-228 Ra-224

Rn-220

Po-216

Pb-212

Po-212

Tl-208

Pb-208

1.4 x 1010 y

5.8 y

6.1 h

1.9 y

3.7 d

56 s

0.15 s

Bi-212

61 m

(stable)

Thorium-232

Decay Series

11 h

300 ns

3.1 m

61 m

Th-232

Th-228

Ra-228

Ac-228 Ra-224

Rn-220

Po-216

Pb-212

Po-212

Tl-208

Pb-208

1.4 x 1010 y

5.8 y

6.1 h

1.9 y

3.7 d

56 s

0.15 s

Bi-212

61 m

(stable)

Thorium-232

Decay Series

11 h

300 ns

3.1 m

61 m

Th-232

Th-228

Ra-228

Ac-228 Ra-224

Rn-220

Po-216

Pb-212

Po-212

Tl-208

Pb-208

1.4 x 1010 y

5.8 y

6.1 h

1.9 y

3.7 d

56 s

0.15 s

Bi-212

61 m

(stable)

Thorium-232

Decay Series

11 h

300 ns

3.1 m

61 m


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238U series


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Nuclide Half-life

Energy (MeV)

alpha beta Gamma (Photons/trans)

U-238 4.51 x 109

y 4.18Th-234 24.10 days 0.193, 0.103 0.092(0.04), 0.063(0.03)

Pa-234 1.175 min 2.31 1.0 (0.015), 0.076(0.0063), I.T.

U-234 2.48 x 105y 4.763

Th-230 8.0x 104y 4.685 0.068 (0.0059)

Ra-226 1,622 y 4.777

Rn-222 3.825 d 5.486

Po-218 3.05 m 5.998 0.0186(0.030

Pb-214 26.8 m 0.65 0.352(0.036), 0.295(0.020),

0.242(0.07)

Bi-214 19.7 m 5.505 1.65, 3.37 0.609(0.295), 1.12(0.1310)

Po-214 1.64 x 10-4

s 7.680Tl-210 1.32 m 1.96 2.36(1), 0.783 (1), 0.297(1)

Pb-210 19.4 y 0.017 0.0467(0.045)

Bi-210 5.00 d 1.17

Po-210 138.40 d 5.298 0.802(0.000012

Pb-206 Stable

U series


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Internal radioactivity

Radioactivityin diet

lead-210

polonium-210

potassium-40

Radionuclides Found


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Radionuclides Found

in Your Body

Nuclide*

Total Mass of

Nuclide Found

in the Body

Total Activity of

Nuclide Found in

the Body

Daily

Intake of

Nuclides

Uranium 90 g 30 pCi (1.1 Bq) 1.9 g

Thorium 30 g 3 pCi (0.11 Bq) 3 g

40K 17 mg 120 nCi (4.4 kBq) 0.39 mg

Radium 31 pg 30 pCi (1.1 Bq) 2.3 pg

14C 95 g 0.4 Ci (15 kBq) 1.8 g

*Uranium, Thorium and Radium are elements

Artificial (man made) sources of IR


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Artificial (man-made) sources of IR

Medical

Industrial (nuclear and non-nuclear industry)

Accidental releases from nuclear industries

Consumer products

R l ti R di ti


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Example Special Units SI Units (37x)

environmental samples picocurie 10-12 millibequerel 10-3

laboratory standard nanocurie 10-9 becquerel 100in-vitro tracer microcurie 10-6 kilobequerel 103

nuclear medicine millicurie 10-3 megabequerel 106

industrial source curie 100 gigabequerel 109

teletherapy source kilocurie 103 terabequerel 1012

irradiator megacurie 106

petabequerel 1015

Relative Radioactive

Source Activity

Global average individual background


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Global average individual background

ionizing radiation doses (mSv/y)

TOTAL = 2.69 Source:UNSCEAR

Cosmic

0.39

Terrestrial

0.46Internal K-40, C-14

0.23

Radon1.3

Medical

0.3Fallout

0.007Occupational

0.002

Discharges

0.001Products

0.0005

TOTAL FROM

NATURAL 2.38

TOTAL FROM ARTIFICIAL 0.31


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References

1. Module 2.3, IAEA/ANSTO DistanceLearning Project

2. IAEA Lecture Materials for the Post

Graduate Educational Course in RadiationProtection and Safe Use of Radiation

Sources

Th k


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Thank you

Documents

Radioactivity and Radioactive Decay.suneD 08