Radionuclides in the Environment

6 Radioactive Fallout in Astronomical Settings Plutonium-244 in the Early Environment of the Solar System

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P. K. KURODA University of Arkansas, Fayetteville, Ark. 72701 The half-life of Pu (8.2 X 10 years) is short compared with the age of the earth (4.5 Χ 10 years), and hence this nuclide is now extinct. However, the time interval (Ξ) be tween the element synthesis in stars and formation of the solar system may have been comparable with the half-life of Pu. It has been found recently in this laboratory that various meteorites contain excess amounts of heavy xenon isotopes, which appear to be the spontaneous fission decay products of Pu. The value of Ξ calculated from the experimental data range between 1 to 3 Χ 10 years. The process of formation of the solar system from the debris of supernova is somewhat analogous to the formation of fallout particles from a nuclear explosion. 244

7

9

244

244

8

In 1952 Harold C. Urey (19) made the following statement: D r . E d w a r d Teller remarked recently that the origin of the earth was somewhat like the explosion of the atomic bomb: the physical effects are often temporary, but the chemical effects, such as radioactive and non radioactive elements, remain. It is possible by a study of these substances to learn much about the bomb, and also about the origin of the earth. W e have attempted to follow Urey's suggestion and maintained two research projects side by side in our laboratories during the past decade: (a) studies on the debris from the nuclear weapons tests, and (b) studies on the origin of the earth and the solar system. In pursuing a research problem as complex as this, it was felt desir able to have encouragement from a leading astronomer. The statements by Opik (13) i n "The Oscillating Universe" seem to fulfill this require ment. H e compares the formation of new stars to meteorological phe nomena—a cycle like that of precipitation and evaporation of water on 83 Freiling; Radionuclides in the Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1970.

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earth. According to Opik (13), the "clouds" (the nebula) produce brilliant "raindrops" (new born stars), which emerge from a cosmic "rainstorm" (supernova explosion). Expanding the ideas expressed b y these investigators, we attempt in this report to find an analogy between the processes of formation and transport of "hot" single particles from nuclear weapons and the forma tion of the solar system from the debris of supernova explosions.


Radioactive

Fallout

in Geophysical

Settings

The energy release from an atom bomb equivalent to 20,000 tons of T N T is roughly 8 Χ 10 ergs ( 7 ) . The fireball cools rapidly, and accord ing to Freiling et al ( 6 ) , the cooling rate is about 10°K./sec. at 1000°K. Thus, the time interval between the nuclear explosion and condensation of the single fallout particles is somewhat less than one minute. Some of the fission product decay chains have gaseous and volatile members, whose half-lives lie i n the range of seconds to minutes. F o r example: 20

3.2-min. »Kr ->

15.4-min. »Rb - » 50.4-day S r ->

10-sec. K r - »

72-sec. R b ->

9.7-hr. S r ->

16-sec.

66-sec.

12.8-day

8

8

9 1

140

91

X e ->

140

Cs - »

89

58-day Y - *

91

1 4 0

9 1

Ba - »

In a decay chain such as β· β- β~ A —» Β —» C ->

where A is a gaseous precursor, Β is a volatile nuclide such as an alkali metal, and C is a fairly long lived nonvolatile nuclide, the nuclide C w i l l become depleted i n single particles if the half-lives of A and Β are comparable with the time interval ( B ) between nuclear detonation and formation of the single particles. Thus, the following relationship should hold: + β. e-λΒΞ + A

C = C*+A.

j \

(ΕΓ

Χ α Ξ

-

β- ΒΞ) λ

(1)

Β Α where C is the number of atoms of C that should have been present i n the particle, C * is the number of atoms of C found i n the particle, A and Β are the number of atoms of A and Β produced directly from the fissioning nuclide (independent yields expressed i n terms of number of atoms ), and λ and λ are the decay constants of A and B, respectively (4). Figure 1 shows some of the single fallout particles collected after the Chinese nuclear explosion on M a y 14, 1965 (14). Figure 2 shows the mass-yield distribution of the fission products i n some of the single fallout particles ( 5 ) . The values of 3 calculated i n this manner range from 30-50 sec, as shown in Table 1 ( 4 ) . Λ

Α

Λ

Β

Freiling; Radionuclides in the Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1970.


6.

K U B O D A

85

Fallout in Astronomical Settings

Science

Figure 1. Autoradiograph of single fallout particles collected after the Chinese nuclear explosion, May 14, 1965 (14) (Copyright 1966 by the American Association for the Advancement of Science)

The eastward movement of the nuclear debris injected into the atmosphere at L o p Nor (90°Ε and 40°Ν) can be computed as shown in Figure 3 (10). Whenever the fresh nuclear debris completes one cycle around the earth, there is usually a sudden increase i n the con centrations of short lived isotopes such as 33-day C e , 50.4-day S r , 65-day Z r , as well as the number of single fallout particles in the unit volume of air, as shown in Figures 4 and 5 (1, 16, 17). 141

89

95

Radioactive

Fallout

in Astronomical

Settings

In studying astrochemical phnomena, we must depend on the mete orite samples until the samples of moon and other planetary objects become available to us. The time and distance factors involved are enormously large. According to Opik (13), we consider the following sequence of events. Roughly 4.5 billion years ago a supernova explodes at a place within a radius of some 100 light years from the relative loca tion where the solar system is situated now in our galaxy. The fresh


86

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T H E

E N V I R O N M E N T


RADIONUCLIDES

SW01V

JO HBGWTÏN

Journal of Geophysical Research

Figure 2. Mass-yield distribution of the fission products in single fallout particles collected at Osaka, Japan and Fayetteville, Ark. after the May 9, 1966 Chinese nuclear explosion (4) The curves show the general shape of the U fission mass-yield curve. The yields at masses 89, 91, and 140 are down because of their gaseous and volatile precursors (see text). The yields at masses 103 and 129 are also low owing to the volatUe propties of the elements Ru and Te 235


6.

87


K U R O D A

debris from the supernova expands at an initial velocity of 1000 miles/ sec, collecting interstellar dust and increasing its mass. The explosion of supernova w i l l be visible from nearby stars situated a few thousand light years away, just as the explosion of a supernova i n the constellation Taurus was visible on July 4,1054 A . D . to the Chinese astronomers. Since the distance scale is so large, the 1054 supernova explosion is still visible today as the famous Crab Nebula, 914 years after the explosion. The diameter of the Crab Nebula is now 7 light years, and the "fresh" debris are still expanding at a velocity of some 800 miles/sec. Downloaded by UNIV OF CINCINNATI on May 30, 2016 | http://pubs.acs.org Publication Date: January 1, 1970 | doi: 10.1021/ba-1970-0093.ch006

Table I.

Time Interval (H) Between Nuclear Detonation and Formation of Single Fallout Particles Ξ, Seconds

Nuclides Used for Calculation

Osaka Particle 1, 16-μ Diameter

Osaka Particle 2, 13-μ Diameter

Fayetteville Particle 1, 6-μ Diameter

i40Ba/93Zr Y/ Zr Ce/ Zr

36 34 20

38 42 43

51 N.D. N.D.

9 1

1 4 1

9 5

9 5

The "shock wave" of fresh debris from a supernova explosion travels a great distance. For example, it has been calculated that the Crab Nebula w i l l attain a diameter of 70 light years 23,000 years from now; 140 light years i n 260,000 years; 210 light years in 1.3 million years; and 280 light years i n 4 million years. After that the expanding shell w i l l begin to dissipate because the velocity of expansion drops to about 1-2 miles/sec. (3,600-7,200 miles/hr.), which is the velocity of the molecules of interstellar gas. A n object which is about 300 light years i n diameter and which closely resembles the terminal stage of a supernova explosion is visible today in the constellation Orion (the Orion H a l o ) . A number of very bright young stars are located within the Orion Halo, and their ages are estimated to be less than 5-10 million years; obviously their formation was triggered by the "shock wave" of the supernova explosion. If the sun and the solar system originated under the astronomical conditions described above, the materials used for the solar system must have contained some fresh "debris" from the supernova. Burbidge et al (2) reported i n 1956 that the chemical elements heavier than uranium can be synthesized under conditions such as those existing i n supernova. Type I supernovae, during their explosion, release 1 0 - 1 0 ergs of energy. A most remarkable property of the Type I supernova is that their light curve falls off exponentially, decreasing to half its value i n 55 ± 1 days; this agrees surprisingly w e l l with the spontaneous fission half-life of C f . 49

50

254


88

R A D I O N U C L I D E S IN

THE

ENVIRONMENT

The half-life of C f is unfortunately too short for "tracer" studies of the condensation process of the solar system if we recall that the materials thrown out of the supernova must travel a great astronomical distance and the time period involved is of the order of magnitude of 5 to 10 million years between the supernova explosion and formation of the sun and the solar system.


2 5 4

Science

Figure 3. Air trajectory in the Northern Hemisphere during May and June 1965 (10). This figure indicates the air movement at the height of 5650 meters in the troposphere, which is known to give a representative over-all average value of the tropospheric air movement. Circled numbers show dates; numbers between circles show average wind velocities in meters/sec. The fresh debris from the Chinese nuclear explosion of May 14, 1965 travelled eastward and circled the earth (Copyright 1965 by the American Association for the Advancement of Science)


6.


K U R O D A

89

According to W o o d (20) and Goldstein and Short (8) most of the meteorites seem to have cooled at a rate of l°-10°C./million years i n the temperature range between 300° and 700°C. This suggests that the 10.0

NUCLEAR I ARRIVAL OF EXPLOSION 1/FlkST WAVE

5.0 4.0 S r

89

/ S r

90

3.0-

THIRD CYCLE


2.0-

, i-o-

0.50.4 0.3 0.2

0.05-

~r~ I

14 20

MAY 1965

10

JUNE

- τ ιο

-τ— 20

20

JULY

-ι— 20

I

—r

SEPT.

AUG.

10


Figure 4. Variation of the Ce/ Ce and Sr/ Sr ratios in rain at Fayetteville, Ark., after the nuclear explosion of May 14, 1965 (17). 141

144

89

90

0: Ce/ Ce ratio in rain. X: Sr/*°Sr ratio in rain. 141

144

89

Curve I (which is approximately linear) is given by the equa tion i4i

C e /

i44

4.6 e~°-

01S9t

C e :

exp (0.0176Î) +0.52

where t = 0 on May 14,1965 (the date of nuclear explosion). The above empirical equation can be derived theoretically, and the deviation of the experimentally observed Ce isotope ratio data from Curve I illustrates the extent to which the tropospheric atmosphere is not instantly and uniformly mixed; this enables us to follow the eastward movement of the nuclear debris around the world planetary condensation and cooling process was slow and lasted perhaps several hundred million years. Thus, in studying the condensation process of the solar system using meteorites we must search for a suitable


90

RADIONUCLIDES

I N

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"tracer," whose half-life is about 100 million years. Such an isotope is found in the decay chain of one of the isotopes of C f : si.

66y a 2.55f/


2 4 8

Cm a

4.7 X 10 t/ 8

si.

si.

6.55 X

10™y

.244PU

240PU"

1.2 Χ ΙΟ !/

β a 8.2 X Wy

2

4

0

N

?

3

m

11

;

6

0

α 6760!/

m

si. 2 4 0

U"l4.lA

236Û

2 X 10 t/ 16

α 2.39 Χ 10 !/ 7

si. 2 3 2

Th"

> 10 !/ 20

α 1.41 Χ 10 !/ 10

P u was discovered i n the debris of the Bikini test i n 1952, and its decay constants have been redetermined by Fields et al. to be as follows: α-decay (8.18 ± 0.26) X 10 t/, spontaneous fission decay (6.55 ± 0.32) X 10 y (5). P u may be considered as an ideal "tracer" nuclide for these studies because of its decay characteristics. Its existence or absence i n the early solar system can be considered as a crucial test for or against the theories of the synthesis of chemical elements i n stars. The α-decay products of ^ P u are difficult to identify because they all decay to the long lived T h . Fortunately, however, the spontaneous fission decay rate of P u is not too slow. Thus, P u produces a reason ably large quantity of stable xenon isotopes such as X e , X e , X e , and X e . Since the abundance of xenon i n the earth or i n meteorites is extremely low, the production of fissiogenic xenon by P u may produce 2 4 4

7

10

2 4 4

2 3 2

2 4 4

2 4 4

131

132

134

136

2 4 4


Fallout

K U R O D A


6.

in Astronomical

• 10 20 MAY 1966

ι 30

91

Settings

10 20 JUNE

30

• ' 10 20 JULY

1

1——I

30

10 20 AUGUST

Science

Figure

5. Variation of the number of single fallout particles, Ce/ Ce ratio in the ground-level air at FayetteviUe, Ark.

141

144

Ce (1)

141

and

Radioactivities have been extrapolated back to the time of the bomb explosion (May 9, 1966). Top: Calculated mean diameters of particles. An apparent over-all average diameter ( D ) of the particles in the daily air filter samples was calculated by the method first proposed by Baugh et al. (1). Ο: from Ce data, X: from Zr data. Values of D calculated from the Ce and Zr data do not agree well, owing to the effect of atmospheric fractionation of nuclear debris, but they range between ca. 2 and 4μ. The Ce/ Ce production ratio in the induced fission of 1 7 by fission spectrum neutrons shown here was calculated from the fission yields given by Zysin (21) (Copyright 1967 by the American Association for the Advancement of Science) 141

141

141

144

95

95

ω5


92

RADIONUCLIDES

I N

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the so-called xenon isotope "anomalies" either i n terrestrial or meteoritic xenon (9). These anomalies are interpreted as being caused by the decay of I and P u i n the early history of the solar system (9, 15, 18). 129

2 4 4

β'

129!

• i29 1.7 X 10 y

Xe

( ble ) sta

7

244p

u

to

2 3 2

Th

8.2 X 10 t/ Downloaded by UNIV OF CINCINNATI on May 30, 2016 | http://pubs.acs.org Publication Date: January 1, 1970 | doi: 10.1021/ba-1970-0093.ch006

7

spontaneous fission

131-136χ

β

6.55 X 10 t/ 10

Just as earlier we were able to observe mass-yield distributions of the fission products from the fissionable nuclide used i n the Chinese nuclear device, it is possible to "see" part of the mass-yield curve from the fission of P u , which was synthesized originally i n a supernova. Figure 6 shows the mass-yield distribution of the excess fissiogenic xenon observed in the meteorite Pasamonte (15). The quantities of excess fissiogenic xenon actually found i n the meterorites agree with the calculated value, based on astronomical models described by Opik (13). According to Opik (13), the supernova explosion sweeps up and compresses into an expanding shell a mass of interstellar gas of the order of 15,000 solar masses, 1000 times greater than the original mass of the supernova. This means that the fresh debris from a supernova is diluted 1000 times by old debris. If the P u / U ratio in the fresh supernova debris is Ρ and that in the interstellar matter (old debris) is Q, the initial ^ P u / ! ! ratio (a) in the mixture of matter from which the solar system was formed is 2 4 4

2 4 4

2 3 8

2 3 8

Q has been calculated to be 0.0235 (12) according to the continuous nucleosynthesis model. Then we have « = 0.025

(3)

Similarly, the initial I / I ratio (β) i n the mixture of matter from which the solar system was formed is 1 2 9

1 2 7

β = 0.004

(4)

The isotopic compositions of xenon extracted from about a dozen achondrites and several chondrites recently have been determined care-


6.

93


K U R O D A

/ Λ

/

(

S. /

•

/

/

/

/

/

/

/

• Downloaded by UNIV OF CINCINNATI on May 30, 2016 | http://pubs.acs.org Publication Date: January 1, 1970 | doi: 10.1021/ba-1970-0093.ch006

I 1

; / /

/

>

•

/

κa

I" Κ

1'1 . C

MASS NUMBER i


Figure 6.

Mass-yield distribution of the fissiogenic xenon isotopes in the meteorite Pasamonte (15)


94

RADIONUCLIDES

I N

T H E


fully i n this laboratory (11). A l l of these meteorite samples contain excess X e and X e . The abundances of I and U i n these meteorites have also been determined ( 3 ) . If the excess X e ( X e ) and the excess X e ( X e ) found in the meteorites are the decay products of the extinct nuclides I and P u , the ratios / I and ' X e / U should be a function of the time interval ( H ) between the supernova explosion and "formation" of the meteorites. The values of a and β can be expressed i n terms of X e and Xe and the amounts of U and I found i n the meteorites today. Thus 1 2 9

136

1 2 9

1 3 6

1 2 9 r

1 3 6 /

1 2 9

2 4 4

1 2 9 r

1 3 6

1 2 9 r

(ΐ «/Χθ/υ)

ο1)8

(i29r e/l)

o l ) s


3

and

X

= α'· " β

=

^ .

e

λ

1 3 6 /

(5)

Ξ

- x ^

(6)

where a' = 3.4 Χ 10"

ce. S T P « / X e / g r a m Tj

4

(7)

13

and β' = 7.1Χ

Ι Ο " cc. S T P 1

1 2 9

0ie/gram I

(8)

The spontaneous fission of U contributes to the production of X e i n the meteorites. This contribution is small, however, and amounts to 2 3 8

1 3 6 /

(

Radionuclides in the Environment

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