chemical and physical properties: radioactivity & radioisotopes

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Radioactivity & Radioisotopes

University of Lincoln presentation

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http://chemistryfm.blogs.lincoln.ac.uk/


Isotopes

In 1913 Soddy proposed the existence of ISOTOPES

Definition: Atoms of the same elements with different atomic masses

Frederick Soddy

Nobel Prize (Chemistry) 1921



Henri Becquerel

Marie & Pierre Curie

Radioactivity discovered in 1896



Stable v. Radioactive Isotopes

0

200

400

600

800

1000

1200

1400

1600

StableRadioactive

There are approximately 1,700 isotopes known to exist



Chart of the Nuclides



Black squares denote STABLE isotopes

Z

N



Nuclear Stability

• The stability of the nucleus depends on both N and Z– Z≤20 N=Z N/Z = 1– 20<Z≤92 N>Z N/Z = 1–1.6– Z>92 Spontaneous fission

• If N/Z < or > stable ratio, the nucleus is radioactive



Chart of the Nuclides & Radioactivity

Z

N

Neutron R

ICH

Neutron D

EFICIENTN/Z = 1–1.6

N/Z > 1.6

N/Z < 1



Chart of the Nuclides & Radioactivity

Neutr

on R

ICH

Neutro

n D

EFIC

IEN

T

E

STABLE

N/Z <1

Need to gain n

+

N/Z>1.6

Need to lose n

-



– Decay (Negatron emission)

X X + –A A

Z Z+1

n p

Parent Daughter Negatron

It is easier to convert a neutron to a proton, than expel a neutron from the nucleus



Decay

E A

ZXAm

Z+1X–

A

Z+1X

– decay (nearly) always results in a daughter in an excited state – if this excited state is fairly long-lived it is called a meta-stable state (m)

XS energy is lost by

expelling a -ray



+ Decay (Positron emission)

X X + +A A

Z Z-1

p n

Parent Daughter Positron

It is easier to convert a proton to a neutron, than expel a proton from the nucleus



Decay

• Nuclei that are simply too big (too many n and too many p) need to lose both n and p as quickly as possible

= Helium nucleus He 2 protons + 2 neutrons

4

2



Chart of the Nuclides

-emitters



Common Radioactive Emissions

Emission Symbol Nature Mass ChargeAlpha He

nucleus4.0026 2+

Beta electron 0.00055 1–

X-ray X-ray EMR None 0

Gamma EMR None 0

Positron + positively charged electron

0.00055 1+

Proton p Proton 1.0073 1+

Neutron n neutron 1.0087 0



Half-life (t½)

The time taken for the activity of a radioisotope to reach half it’s original value



Half-Life (t½)For example, suppose we had 20,000 atoms of a radioactive substance. If the half-life is 1 hour, how many atoms of that substance would be left after:

Time Number of atoms remaining

% of atoms remaining

1 Hour (one lifetime)

10,000 50%

2 hours (Two lifetimes)

5,000 25%

3 hours (Three lifetimes)

2,500 12.5%



Radioactivity

0

50

100

150

200

250

300

350

400

450

500

0 5 10 15 20 25 30

Time (minutes)

Num

ber of

rad

ioac

tive

ato

ms pr

esen

t

One half life

Two half lifes



Radioactivity

• Decay Equation:

At = A0e-t

At = activity at time t

A0 = activity at time 0 (initial activity)

= decay constant (rate constant)t = time

First Order reaction



Radioactivity

• Decay Equation:

Ln(At) = Ln(A0) - t

Intercept Gradient

Straight line graph



Biological Effects of Radiation

• Radiation passing through cells of living tissue ions and free radicals

• These react with compounds in the cell, disrupting or altering the normal metabolic processes




• These changes can result in:– Death of the organism or animal– Reduced ability of cells to divide– Abnormal cell division– Changes in genetic material– Increase in the rate of aging




Mainly due to the radiolysis of water:

H2O + radiation H+ + OH + e–

OH immediately reacts with neighbouring molecules, such as proteins and DNA

foreign substances (also H2O2 is formed)

disrupt/change normal metabolic processes

The hydroxyl free radical is very reactive



Cascade effect

Radiation

Initial disruption

1st generation of foreign substances that cause further

disruption

Initial disruption has now been magnified 8 times

Continuation in cascade leads to a level of disruption with which the body cannot cope



Penetrating Power of Radiation

n

Skin & paper

5mm brass 6mm Al

Pb & concrete

Very thick concrete (2m)



Absorbed Dose

• The amount of energy absorbed by the tissue

• Units – the Gray (Gy)– 1 Gy = 1 Jkg-1

– An absorbed dose of 10 Gy is lethal for most mammals• Although the absorbed energy is very low

(10 Jkg-1), the disruption it causes to biological processes in the tissue will result in death



Dose Equivalent

• Different radiation types cause different amounts of damage– In order for ‘dose’ to meaningful, need

to be able to define it in terms of ‘damage done’• Dose equivalent defines the damage done in

man

• Units – Sievert (Sv)



Dose Equivalent

Dose Equivalent = Absorbed Dose (Gy) x Q

Where Q is the empirical quality factor

, X Q = 1

Fast n, p Q =10

Q =20



Dose Equivalent

In theory, 100 Sv -radiation will cause the same biological effect in man as a

dose of 100 Sv radiation

BUT the absorbed doses are 100 Gy and 5 Gy, respectively



Illicit Radioactive Sources

Dirty Bombs –Radiation Dispersal Devices

(RDD)



Dirty Bombs

• Conventional explosives wrapped in radioactive material – NOT atomic bombs



Dirty Bombs

A SMART PHONE that can detect radiation may soon be helping the police to find the raw materials for radioactive “dirty bombs” before they are deployed.

The phones will glean data as the officers carrying them go about their daily business, and the information will be used to draw up maps of radiation that will expose illicit stores of nuclear material.

New Scientist (December 2004)



Depleted Uranium

• t½ U-238 = 4.5 x 109 y

– Not exactly ‘radioactive’

– 1 atom will decay every 4.5 x 109 y


Acknowledgements

• JISC• HEA• Centre for Educational Research and

Development• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org

chemical and physical properties: radioactivity & radioisotopes

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neutron n neutron

stable isotopes z n

time t

radioactivity decay

z z20n

nuclides radioactivity