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

MathU, Th series in oceanographyExamples : 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

Z

238U (the “92” is redundant with “U” and

92 is usually omitted)

Or, an element with several isotopes:

12C ,136 C ,14

6 C6

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,

40Ca � E �

Zdaughter Zparent�1 •We can measure

Adaughter Aparent

40K ����o�20 � Q� � Energy19

Ndaughter Nparent�1

E particles

Num

ber o

f β − P

artic

les

Energy of β − Particles in million

Emax= 1.350 +_ 0.05 MeV

1.51.00.50

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.

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 )

Distribution of β energies:

Electron Volts (MeV)

Atomic Number (Z)

Ground state

Excited states

Ener

gy in

Mill

ion

Elec

tron

Volts

(MeV

)

0

1.0

2.0

3.0

12 13

γ, 0.8438 MeV γ, 1.0144 MeV

γ, 0.1707 MeV

27 13 AI

27 13 Al

27 12 Mg

20%β −, 1.60 ΜeV

80%β −, 1.77 Μ

eV

Figure by MIT OCW.

Figure by MIT OCW.

Example : D decay

Decay by emission of a 4He nucleus…

228Th����o�88 224Ra �D �Energy90

Energy 1 MD vD

2 ¨�¨�§� 1 �

MD ¸�·�

2 ©� ¸�M p ¹�

Kinetic energy of “recoil” energy D particle

Ndaughter Nparent�2Zdaughter Zparent�2Adaughter Aparent�4

En

erg

y i

n m

illi

on

ele

ctro

n v

olt

s (M

eV) 5.4

0.3

0.2

0.1

0.0 Ground State Daughter

Parent 5.519 MeV

228Th90

Excited States of 224Ra

α, 5.173

α,5.

338,

28%

α,5.4

21, 71%

α, 5.2

08, 0

.4%

0.2%

γ 0.217

γ 0.084

γ 0.133

γ 0

.16

9

224Ra88

88 90

Atomic Number (Z)

Atomic #

Decay scheme diagram for the alpha decay diagram

for the alpha decay of 228Th to 224Ra . In this case,90 88

four different alpha particles are emitted with four

complementary gamma rays.

Figure by MIT OCW.

(3) '14 C 1000 §̈�©�� e ��date �1950 � f m � 1 ·̧�¹

��

Incoming Cosmic-Ray

Proton Shatters Nucleus of Atmospheric

Atom

Among the Fragments is a Neutron

14C Atom is Radioactive

Mean Lifetime: Tm= 8200 years

Half Lifetime: T1/2 = 5700 years

Neutron so Produced

Enters Nucleus of Atmospheric14N Atom

Knocks Out a Proton

Eventually the14C Atom Spontaneously Ejects an Electron

Net Result:

+

N-14 + n (7p+7n) + n 14N Atom

Becomes 14C Atom

+

14C + p(6p+8n) + p

Net Result:

n-e­ pHence:

-

14 14C-e- N (6p+8n) - e­ (7p+7n)

14C Atom Becomes 14N 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 average14C atom "lives" for 8200 years. Its life is terminated by the ejection of an electron with returns the atom to its original form, 14N.

14C is formed in the atmosphere.

Rate of formation depends on cosmic ray flux

1/2 life = 5730 years

Figure by MIT OCW.

Radiocarbon in Atmosphere

cal BP

cal BC cal AD cal BP

600

500

400

Δ 14 C

% 300

200 ATMOSPHERE

100

0

-100 30000 25000 20000 15000 10000 5000 [0]

11000 10000 9000 8000 140

120

100

9000 8000 7000 6000

80

60

40

Δ 14 C

%

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 value of 500% ( ). The coral measurements (Bard et al. 1993) are 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.

Tree-ring Calibration Curve

8000

7000

Rad

ioca

rbon

yea

rs B

P R

adio

carb

on 'a

ge' B

P

6000

5000

4000

3000

2000

1000

0 7000 6000 5000 4000 3000 2000 1000 0 1000 2000

BC AD Calender date

600

500

400

1400 1500 1600 1700 19001800 2000 cal AD

Detail of Calib Curve

300

200

100

0

Figure by MIT OCW.

Figure by MIT OCW.

200

-200

100

-100

0

8000 4000 [0]10000 6000 2000

10000 8000 6000 4000 2000 0

Atmosphere

Surface Ocean

Thermocline

Deep Ocean

Cal BP

Cal BC Cal AD

Δ 14 C

%0

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.

Reservoir Effects

Figure by MIT OCW.

Figure by MIT OCW.

2. Bomb Testing

110011001000 900 800 700 600 500 400 300 200 100100

0 -100

1954 1956 1958 1960 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992

Northern Hemisphere

Southern Hemisphere

Geo

secs

Atla

ntic

Geo

secs

Indi

an

Geo

secs

Pac

ific Radiocarbon in

Atmospheric CO2

Atmospheric Nuclear Weapon Tests 1954 1992 Observed time history for the 14C/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.

� 14C

%0

Figure by MIT OCW.

Anthropogenic Perturbations of Atmospheric 14C content

1. The Suess Effect

? ?

Suess Effect

de Vries effect+20

-20

+10

-10

O

� 14 C

%0

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 fractionation and recalculated using a value of 5730 years for the half-life of 14C. 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.

92

90

88

86

84

82

80

124 126 128 130 132 134 136 138 140 142 144 146

206

206

210

210

214

214

218 222

226

230

234

234

234

218

218

214210

210

206

Pb Pb Pb

Po Po Po

Bi Bi

Ti Ti

Hg

Rn Rn

Ra

β

β

β

β α

α

At

Th Th

U 238 U

Pa

Neutron Number

Ato

mic

Num

ber

The Decay of 238U to Stable 238Pb

Abundances, Half-Lives, and Decay Constants of the Principle Naturally Occurring Isotopes of Uranium and Thorium

Isotope ReferenceAbundance % Half-Life y

99.2739

Decay Constant y-1

238U

235U

234U

232Th

0.7204

0.0057

100

4.510 x 109 1.537 x 10-10

1.55125 x 10-104.468 x 109

0.7129 x 109

0.7038 x 109

2.48 x 109

13.890 x 109

14.008 x 109

9.722 x 10-10

9.8485 x 10-10

2.806 x 10-6

4.990 x 10-11

4.948 x 10-11

1

1. Kovarik and Adams (1955)

2 3 2 4 5 6

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)

The 238U Decay Series

Figure by MIT OCW.

Parent, N1

Daughter, N2

200

100 80 60

40

20

10 8 6

4

2

1 0 2 4 6 8 10 12

Time in Hours

Equal Decay Rates Lo

g N

N1(T1/2 = 10 h)

N2(T1/2 = 1 h)

S = -λ1N1

S = -λ2N2

λ(N2) = 1 hr

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

Np

Soluble U

Pa

Th

Ac

Ra

Fr

Rn

At

Po

Bi

Pb

Tl

U-238

4.51x103y

Th-234

24.1d

Th-230

7.52x106y

Po-234

1.18m

U-234

2.48x105y

Ra-226

1622y

Rn-222

3825d

Po-218

3.05m

Po-214

1.6x10-4s

Pb-214

26.8m

Bi-214

19.7m

Pb-210

22.3y

Bi-210

5.0d

Pb-206

Po-210

138.4d

Th-232

1.39x1010y

Ra-228

5.75y

Ac-228

6.13h

Th.228

1.90y

Ra-224

3.64d

Rn-220

54.5s

Po-216

0.158s

Pb-212

10.6h

65%

35%

Po-212

3.0x10-7s

Bi-212

60.5m

Tl-208

3.1m

Pb-208

U-235

7.13x108y

Th-231

25.6h

Po-231

3.2x104y

Ac-227

22.0y

Th-227

18.6d

Ra-223

11.4d

Rn-219

3.92s

Po-215

1.83x10-3s

Pb-211

36.1m

Bi-211

2.16m

Tl-207

4.79m

Pb-207

α

α

β−

α

Soluble

Particle Reactive

Particle Reactive

Gas

U-238 SERIES U-235 SERIESTh-232 SERIES

Figure by MIT OCW.

The U238 Series in Seawater

U238

Th234

Green

U234

ca, 10-a total Th230

Ra226

Pb210

Secular equilibrium

0 1 2 3

Po210

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.

0 1 2 3

25

75

125

175

225

Dep

th (m

)

Total Activity (d.p.m. l-1)

234Th

238U

Typical profile of 234Th 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.

1000

800

600

400

200

0 0.0 0.2 0.4 0.6 0.8 1.0

3

21

Fast

er

Slow

er

τ sca

v (da

ys)

A234 / A238

= 34.8 daysλ234

1

234Th / 238U Disequilibrium - Surface Ocean

Figure by MIT OCW.

NH Pacific

NH Atlantic

Slope SAR Sea

0

1000

2000

3000

4000

5000

6000

200

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

~ 1dpm / 1000 L

TH-230, DPM/1000 L

230Th in the Deep Ocean

Thorium-230 Profiles

Thorium-230 distributions measured in ocean-water profiles by alpha-counting.

DEP

TH, K

Figure by MIT OCW.

Figure by MIT OCW.

Excess Pb = 210 (dpm/g)

Dep

th (c

m)

-1 1 10 100 0 2 4 6 8 10 12 0 0

10

Dep

th (c

m) 20

30

100

PC-7 200

s = 0.28 cm/y40 BC-5

50 s = 0.34 cm/y

300

Depth profiles of excess 210Pb and 14C 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.