Introduction to Marine Radiochemistry
1. Isotopes and radioactive decay
2. Mathematical description of radioactive decay
3. Decay of a parent isotope to a stable daughter
Example: radiocarbon dating
4. Decay series
Math
U, Th series in oceanography
Examples : Th isotopes
Atoms and Chemical Elements
Atom: a nucleus surrounded by electrons:
Atomic radius ~ 10-8 cm
Nuclear radius ~ 10-12 cm
Nuclear density ~ 1014 g/cm3 !
The nucleus consists primarily of
positively charged protons
electrically neutral neutrons
A chemical element is characterized by a specific number of
protons in its nucleus; different isotopes of an element contain
different numbers of neutrons
Notation
Z = atomic number (= number of protons in nucleus)
N = neutron number
A = Z + N = mass number
A
238
92U
(the “92” is redundant with “U” and
is usually omitted)
Z
Or, an element with several isotopes:
12 13 14
6 C, 6 C, 6 C
Holden, N. E., and F. W. Walker. Chart of the Nuclides. 11th ed. Schenectady, NY: General Electric Co., 1972.
Image removed due to copyright restrictions.
Chart of the nuclides: expanded view
Holden, N. E., and F. W. Walker. Chart of the Nuclides. 11th ed. Schenectady, NY: General Electric Co., 1972.
Image removed due to copyright restrictions.
The unstable nuclides -- “radionuclides”
If so many nuclides are unstable, why are they around?
1. Formed during initial nucleosynthesis, but decay very slowly: e.g.,
238U, 235U, 232Th
2. Formed by decay of slowly-decaying parent isotope
3. Formed by a naturally occurring process, e.g., comsogenic isotopes: 14C
4. Anthropogenic: e.g. nuclear bomb testing and nuclear energy production,
e.g., Pu isotopes, 3H, 137Cs,…
Example : E- Decay
Neutron 
o proton electron
For example,
40
19 K
40

o20
Ca E
Q Energy
Ndaughter Nparent1
Zdaughter Zparent1
Adaughter Aparent
•We can measure
E particles
The energy emitted is always the same for a given transition; the energy of the
β particle varies up to a maximum.When Eβ < Emax , a neutrino is emitted (υ)
−
Number of β Particles
Distribution of β energies:
3.0
_ 0.05 MeV
Emax= 1.350 +
0
− ,1
β
.6
− ,1
β
%
Μ
eV
.77
eV
Μ
Energy spectrum of beta particles emitted by 40
19 K. Note that most of the beta particles
are emitted with energies that are about one-third of the maximum or end point energy.
During each beta decay, a neutrino is emitted with a kinetic energy equal to the difference
between the maximum energy and the kinetic energy of the associated beta particle.
80
1.5
%
0.5
1.0
−
Energy of β Particles in million
Electron Volts (MeV)
2.0
20
0
Energy in Million Electron Volts (MeV)
27 Mg
12
27 Al
13
1.0
Excited
states
γ, 0.1707 MeV
γ, 0.8438 MeV
γ, 1.0144 MeV
Ground
state
0
27 AI
13
13
12
Atomic Number (Z)
Figure by MIT OCW.
Figure by MIT OCW.
Example : D decay
Decay by emission of a 4He nucleus…
228
o224
90Th 
88 Ra
Energy
D Energy
§ M ·
1
2 ¨
M D vD ¨1 D ¸
¸
2
© M p ¹
Kinetic energy of
D particle
“recoil” energy
Ndaughter Nparent2
Zdaughter Zparent2
Adaughter Aparent4
228Th
90
3
28%
8,
20
γ
0.133
α, 5
.421
γ
0.217
, 71%
0.2
0.0
Excited States of 224Ra
α,
5.3
38,
α,
5.
0.3
0.
.17
5
α,
4%
5.4
0.1
Parent
%
0.2
γ 0.169
Energy in million electron volts (MeV)
5.519 MeV
γ
0.084
224Ra
88
88
Ground State
Daughter
90
Atomic Number (Z)
Atomic #
Decay scheme diagram for the alpha decay diagram
Ra . In this case,
Th to 224
for the alpha decay of 228
88
90
four different alpha particles are emitted with four
complementary gamma rays.
Figure by MIT OCW.
(3)
' 14 C
§ date 1950 ·
1000 ¨e f m 1¸
¹
©
Incoming
Cosmic-Ray
Proton
Shatters
Nucleus of
Atmospheric
Atom
Neutron so
Produced
Net Result:
+
Enters
Nucleus of
Atmospheric
14
N Atom
N-14 + n
(7p+7n) + n
14
N Atom
Becomes
C Atom
14
+
Among the Fragments
is a Neutron
Knocks Out
a Proton
14
C+p
(6p+8n) + p
Net Result:
14
C Atom is Radioactive
Mean Lifetime:
Tm= 8200 years
Eventually
14
the C Atom
Spontaneously
Ejects an Electron
Half Lifetime:
T1/2 = 5700 years
p
n-e­
Hence:
14
C-e-
14
N
(6p+8n) - e­
(7p+7n)
14
14
C Atom Becomes N Atom
The "life cycle" of a carbon-14 atom. Created in the atmosphere by the collision of a
neutron (produced by primary cosmic-ray protons) with a nitrogen atom, the average
14
C atom "lives" for 8200 years. Its life is terminated by the ejection of an electron
with returns the atom to its original form, 14N.
14
C is formed in the atmosphere.
Rate of formation depends on cosmic ray flux
1/2 life = 5730 years
Figure by MIT OCW.
Tree-ring Calibration Curve
Radiocarbon in Atmosphere
8000
cal BP
7000
Radiocarbon 'age' BP
600
500
Δ14C %
400
300
200
6000
5000
4000
3000
ATMOSPHERE
100
2000
0
1000
-100
30000 25000 20000 15000 10000 5000 [0]
cal BC
cal AD
cal BP
140
11000
10000
9000
0
7000 6000 5000 4000 3000 2000 1000
8000
120
Δ14C %
0
BC
Calender date
1000
2000
AD
Detail of Calib Curve
100
600
80
500
40
9000
8000
7000
6000
The atmosphere Δ14C record as derived from: dendrochronologically
calibrated bidecadal tree-ring 14C measurements (
from 9440 BC
to AD 1950), a cubic spline through the 234U/230Th-calibrated coral
14C averages corrected by 400 14C yr (
from 20,000 to 9440 BC),
and a smooth transition from an assigned pre-25,000 BC steady-state
). The coral measurements (Bard et al. 1993) are
value of 500% (
shown as circles with 2 σ error bars. For the detailed coral/tree-ring
comparison in the inset, the coral ages (1 σ error bars) were converted to cal ages using the marine calibration curve (Fig. 17) with ΔR = 0.
Figure by MIT OCW.
Radiocarbon years BP
60
400
300
200
100
0
1400
1500
1600
1700
1800
1900
2000
cal AD
Figure by MIT OCW.
Reservoir Effects
Cal BP
10000
8000
6000
4000
2000
0
200
Atmosphere
Δ14C %0
100
Surface Ocean
0
Thermocline
-100
Deep Ocean
-200
10000
6000
8000
Cal BC
4000
2000
[0]
Cal AD
Atmospheric Δ14C (bidecadal values) as used for the model calculations and calculated
surface ocean (0-75 m), thermocline (75-1000 m), and deep ocean (1000-3800 m) Δ14C
values. These estimates are from a model of the ocean / atmosphere carbon cycle.
Figure by MIT OCW.
Anthropogenic Perturbations of Atmospheric 14C content
1. The Suess Effect
de Vries
effect
14
C %0
+20
+10
O
?
?
Suess Effect
-10
-20
1900 1800 1700 1600 1500 1400 1300 1200 1100 1000
Historical Date, A.D.
Deviation of the initial radiocarbon activity in per mil of wood samples of known
age relative to 95 percent of the activity of the oxalic acid standard of the National
Bureau of Standards. The observed activities were corrected for carbon isotope
14
fractionation and recalculated using a value of 5730 years for the half-life of C.
The decline in the radiocarbon content starting at about 1900 results from the
introduction of fossil CO2 into the atmosphere by the combustion of fossil fuels
(Suess effect). The anomalously high radiocarbon activity around 1710 and 1500
A.D. is known as the "de Vries effect." Its causes are not understood.
Figure by MIT OCW.
Northern
Hemisphere
900
800
14
C %0
700
600
500
Geosecs Indian
1100
1000
Geosecs Atlantic
Geosecs Pacific
2. Bomb Testing
Radiocarbon in
Atmospheric CO2
Southern
Hemisphere
400
300
200
100
0
-100
1954 1956 1958 1960 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992
Atmospheric Nuclear Weapon Tests
1954
1992
14
Observed time history for the C/C ratio in ground level air. Before 1970, a clear difference can be seen between the northern and southern
hemispheres, and also a clear seasonality appears in both records. These features are related to injections of bomb 14C from stratosphere to
troposhpere (mainly in the northern hemisphere). Times and magnitudes of the atmospheric weapons tests are shown. Although the test ban
treaty was implemented at the begining of 1963, several small Chinese and French atmospheric tests were conducted after this time.
Figure by MIT OCW.
Abundances, Half-Lives, and Decay Constants of the Principle
Naturally Occurring Isotopes of Uranium and Thorium
Isotope
Abundance %
Half-Life y
Decay Constant y-1
Reference
1.537 x 10-10
1.55125 x 10-10
9.722 x 10-10-10
9.8485 x 10
1
2
3
2
2.806 x 10-6
4.990 x 10-11
4.948 x 10-11
4
5
6
238
U
99.2739
4.510 x 109
4.468 x 109
235
U
0.7204
234
U
0.0057
232
Th
100
0.7129 x 109
0.7038 x 109
2.48 x 109
13.890 x 109
14.008 x 109
1. Kovarik and Adams (1955)
2. Jaffey et al. (1971)
3. Fleming et al. (1952)
4. Strominger et al. (1958)
5. Picciotto and Wilgain (1956)
6. LeRoux and Glendenin (1963)
U
234
92
β
Pa
234
Th
230
Atomic Number
90
U
238
α
Th
234
Ra
226
88
Rn
218
86
β
At
218
Po
210
84
Po
214
Bi
210
82
β
Pb
206
β
Rn
222
α
Po
218
Bi
214
Pb
210
Pb
214
The 238U Decay Series
Ti
210
Ti
206
Hg
206
80
124
126
128
130
132
134
136
138
140
142
144
146
Neutron Number
The Decay of 238U to Stable 238Pb
Figure by MIT OCW.
Parent, N1
Daughter, N2
200
N1(T1/2 = 10 h)
S = -λ1N1
100
80
60
Log N
40
Equal Decay Rates
20
S = -λ2N2
10
8
6
N2(T1/2 = 1 h)
4
2
1
λ(N2) = 1 hr
0
2
4
6
8
Time in Hours
10
12
Decay of a long-lived parent (N1) to a short-lived daughter (N2).
The half-life of the parent is 10 hours, while that of the daughter
is 1 hour. The number of daughter atoms increases as a function
of time from an initial value of zero. Eventually in this system
the ratio of parent to daughter assumes a constant value equal to
λ2 - λ1/λ1, and a radiochemical equilibrium is thereby established.
Note that the ordinate is in units of log N which is related to 1n N
by 1n N = 2.303 log N.
Figure by MIT OCW.
Decay Series Chemical Properties and the Potential for Radioactive Disequilibrium in the Oceans
U-238 SERIES
Th-232 SERIES
U-235 SERIES
Np
Soluble
U
Particle
Reactive
Pa
U-234
U-235
4.51x103y
2.48x105y
7.13x108y
α
Th-234
Th
Ac
Soluble
U-238
Ra
24.1d
Po-234
Po-231
1.18m
3.2x104y
β−
Th-230
Th-232
Th.228
Th-231
Th-227
7.52x106y
1.39x1010y
1.90y
25.6h
18.6d
α
Ac-228
Ac-227
6.13h
22.0y
Ra-226
Ra-228
Ra-224
Ra-223
1622y
5.75y
3.64d
11.4d
Fr
Gas
Rn
At
Po
Particle
Reactive
Rn-222
Rn-220
Rn-219
3825d
54.5s
3.92s
α
Po-218
Po-214
Po-210
Po-216
3.05m
1.6x10-4s
138.4d
0.158s
Bi
Pb
Tl
65%
Po-212
Po-215
3.0x10-7s
1.83x10-3s
Bi-214
Bi-210
Bi-212
Bi-211
19.7m
5.0d
60.5m
2.16m
Pb-214
Pb-210
26.8m
22.3y
Pb-206
Pb-212
35%
Pb-211
Pb-208
Pb-207
36.1m
10.6h
Tl-208
Tl-207
3.1m
4.79m
Figure by MIT OCW.
The U238 Series in Seawater
U238
Th234
Green
U234
ca, 10-a total
Th230
Ra226
Pb210
Secular
equilibrium
Po210
0
1
2
3
Concentration in seawater/(d.p.m.1-1)
Concentrations of the longer-lived members of the U238 decay series in seawater arranged in
descending order within the series. The values chosen are representative of deep ocean water.
Somewhat different relationships are found in the surface waters. Shaded areas represent the
suspended particulate fraction. Deficiencies of Th230 and Pb210 relative to their parent nuclides
result from scavenging. A Th234 deficiency is not found because of the short half-life, but a
significant uptake by the suspended particles is evident.
Figure by MIT OCW.
25
Depth (m)
75
234
Th
238
U
125
175
225
0
1
2
3
Total Activity (d.p.m. l-1)
234
Typical profile of Th in the upper open ocean. Shaded area represents
disequilibrium between total 234Th and 238U. Data taken from VERTEX
3 (ref. 11.)
Figure by MIT OCW.
234
Th / 238U Disequilibrium - Surface Ocean
1000
Slower
3
τscav (days)
800
600
Faster
400
200
1
1
λ234
2
0
= 34.8 days 0.0
0.2
0.4
0.6
0.8
1.0
A234 / A238
Figure by MIT OCW.
230Th
in the Deep Ocean
Thorium-230 Profiles
200
0
1000
NH Pacific
Slope
SAR Sea
NH Atlantic
DEPTH, K
2000
3000
4000
5000
~ 1dpm / 1000 L
6000
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
TH-230, DPM/1000 L
Thorium-230 distributions measured in ocean-water profiles by alpha-counting.
Figure by MIT OCW.
Figure by MIT OCW.
Excess Pb = 210 (dpm/g)
-1
1
10
100
0
0
2
4
6
8 10 12
0
20
Depth (cm)
Depth (cm)
10
30
100
PC-7
200
40
50
s = 0.28 cm/y
BC-5
s = 0.34 cm/y
300
210
14
Depth profiles of excess Pb and C age (organic carbon fraction) for a box core and
a piston core collected from the eastern Bransfield strait. The agreement in accumulation rates
indicates that bioturbation has a negligible effect on the distribution of these naturally occurring
radionuclides in the seabed.
Figure by MIT OCW.