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Radioactivity & Radioisotopes
University of Lincoln presentation
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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
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Henri Becquerel
Marie & Pierre Curie
Radioactivity discovered in 1896
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Stable v. Radioactive Isotopes
0
200
400
600
800
1000
1200
1400
1600
StableRadioactive
There are approximately 1,700 isotopes known to exist
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Chart of the Nuclides
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Black squares denote STABLE isotopes
Z
N
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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
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Chart of the Nuclides & Radioactivity
Z
N
Neutron R
ICH
Neutron D
EFICIENTN/Z = 1–1.6
N/Z > 1.6
N/Z < 1
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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
-
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– 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
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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
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+ 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
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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
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Chart of the Nuclides
-emitters
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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
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Half-life (t½)
The time taken for the activity of a radioisotope to reach half it’s original value
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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%
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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
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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
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Radioactivity
• Decay Equation:
Ln(At) = Ln(A0) - t
Intercept Gradient
Straight line graph
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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
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Biological Effects of Radiation
• 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
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Biological Effects of Radiation
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
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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
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Penetrating Power of Radiation
n
Skin & paper
5mm brass 6mm Al
Pb & concrete
Very thick concrete (2m)
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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
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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)
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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
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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
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Illicit Radioactive Sources
Dirty Bombs –Radiation Dispersal Devices
(RDD)
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Dirty Bombs
• Conventional explosives wrapped in radioactive material – NOT atomic bombs
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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)
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Depleted Uranium
• t½ U-238 = 4.5 x 109 y
– Not exactly ‘radioactive’
– 1 atom will decay every 4.5 x 109 y
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Acknowledgements
• JISC• HEA• Centre for Educational Research and
Development• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org