』OURNAL OF SCIENCE &. ENGINEERING, MAY, 1966 VOL ][ PP 153~ 156-

Nuclear Chemistry and its Applications

N. Matsuura, University of T oky。

There are so many scientific and industrial applications in the field of nucleat

chemistry that I could only state here about a limited subject. My present item is in

the chemistry of elements in the inoganic and geological fields. I guess you know

the name of the last element of Periodical Table, 103 Lawrencium (Lw). A question

arises what would be the last element that human kind could find in future. Why

are certain elements found abundant in nature while other elements missed or so

rarely found? The periodic table well accounts the chemical and physical properties

of the chemical elements and can be applied successfully to classify the elements

into several groups. But it failed to predict what is the abundance of the elements

on the earth. The nuclear chemistry throw light on this problem as well as on many

other difficulties.

A scientist tried to count the age of the earth in the history of universe en the

rate of sodium chloride flowing into oceans from rivers before the discovery of

radioactive elements. The approaches to the dating of the earth’s history have been made possible by means of radioactive isotopes. Take an example the uranium has

two isotopes, 238U with a half life cf 4.5 × 109 years and 235U with 7. 0 x 108 years. If

two isotopes were present in just the same amount at the time of the birth of the

earth, the longer lived isotope would have much larger abundance than shorter one.

The abundance ratio actually found at present date is

aasu；描U = 137 .8= (e-t /T2a8) / (e•/ T2a5)

under the disintegration law of radioactive element which follows exponential func­

tion law.: The calculation yields t = 6xl09 years for the age of the earth.

However, there is no reliable evidence on the assumption made for the abund­

ance of equal weight of two isotopes at the time of ereation. The nuclear chemistry

has provided in the last decades a lot of information about the nuclear reactions,

out of which a possible way of synthesizing heavy elements from the lightest elem­

ent of hydrogen was suggested, The selected pathway of such a nuclear syntheses

should explain the natural abundance of the isotopes in our solar system including

our earth, as well as many astronomical and geological observations. The isotopic

abundance investigated statistically on the minerals of earth’s crust and on the

meteorite has been improved in accuracy by means of the activation analysis which

presents the data of the ingredients in the mineral as low as parts per billion,

10-9g/g. The activation analysis is carried out conviniently in the nuclear institute,

for instance in the nuclear reacto~ located in Tsing Hua University in T aiwan. This

method of analysis is based on the element made radioactive by a nuclear reaction

一 153一

and has many advantages other than its extremely, rapid and simple, dcne without

chemical treatment in many cases. Besides, the commonly cccurring elements 丘， O,C,

1旬， Si etc. have very little reaction with neutron, er very small cress section for

neutron, then a successful application will be found in the isotope abundance deter~ mination. Even though a lot of problems to be solved are there before the average

abundance of the elements is decided from the various sources (terrestial rocks,

meteorites, etc), we can recognize the important part played by the activation analysis

method on this problem.

The schematic view of the logarithumic abundance ratio of the elements is Fig.

1. There are so many fluctiations in the figures from Urey’s compilation plotted

against atomic weight or isotopic mass that the law of periodic table could not be

applied. However, from nuclear chemistry it will be noted that those elements whose

atomic weights are multiples of four (Oddo,s rule in nuclear chemistry) and more

especially those which are multiples of two (Hark妞，s rule) are more abundant than

their immediate neighbors. There are also maxima corresponding to the magic

numbers 8，間， 82 and 126. The detailed discussion was made from nuclear stability

and nuclear reaction cress section on the drawn cures. Now it is possible to outline

the main process by which our elments were made from the simplest one, hydrogen.

(1) Bethe’s theory of Hydrogen Burns in Solar

System 411日＝九He十2e＋十27.8 MeV (exothermic)

(2) Alpher, Bethe and Gamow theory.

All the matter started at time zero as a dense

neutron fluid, whose expansion and beta decay of

neutron to proton led to the creation of all elem‘

ents by neutron capture nuclear reaction while the

universe and galaxy were being made.

(3) B2HF theory based on the chemical evidence

ns學脫第三期

::r: 因

于4

。十aB

、‘

、’ ‘w 、、

H: Haiifigkeit in german. Isotopic Mass A

Fig. 1 Log. abundance ratio (Si=l06)

log H against Atomic Mass

126

1t from detailed abundance.

A large fraction of synthesis of elements

above the iron region was by fast neutron capture

on a very fast time scale with sudden cut off

neutron gamma process. Supernova explosions

give an attractive mechanism for gamma proposed

by the theory.

~：Fe-= 13 ﹔ He十4﹔n一124.5 MeV endothermic

~M＋ ~n→拉~M→A+1•+1M 十e-

Bind ing energy of n higher than 2 MeV

The B2HF theory presumes 238U ;2asu = 1/1. 65 at the begining time of solar system

from calculation based on nuclear rapid synthetic process stated as above. Then we

can expect the age of the ~arth may be modified to the value 4.5× 109 years from

the relation described earlier, but slight modification should be necessary for the

equation. From this figure of the age of our globe the following t ransuranium

-154 一

Nuclear Chemistry and its Applications

elements, even though they are the isotopes of longest half lives hither to known. are all missing.

log a, V-..332

20~ 、 Th\

、.t6l- 大~2扭Uranium

12 • 、代~2~Jn叫m

。 E

4

。

『 4年

36 .'8 40 42 Z'/A

Hig 2

2a1Np 2.2 × 105 years

244Pu 7.6 × 107 years

3-17cm 1.6× 107 years

2的Am 7.6× 103 years 249Bk

2s1cf

2s4Es

7 × 103 years

6.6 × 102 years

280 days 257Fm 4.5

255Md 1.5 years

hours (e一〉257Lw 8 sec

104 0.3 sec (fission)

The nuclear synthesis by rapid neutron cap­

ture is retarded at a break point of magic

number as the binding energy falls to 2. 0

Me V or less for the successive neutron cap­

ture. And it has its end when the mass of

neucleus attains a certain value of threshold

for the nuclear fission. Spontaneous fission caused by natural neutron comming from

cosimic ray degradation and radioactive minerals by alpha·neutron reaction is the

common way of disintegration for the heaviest elements in stead of alpha or beta

disintegration. The natural fission cross section (rate of fission) for the trans·

uranium elements and thorium of even mass is represented in Fig. 2. The coordinate

scale is taken in logarithumic unit of fission half life, year and the abscissa is a

function of Z2 /A, atomic number Z and atomic mass A. The critical Z2/ A value

indicated by the dotted line predicts that the value extrapolated from the straight

line in Fig 2 gives a half-life of a few second for spontaneous fission for Z2/A being

approximately 43. ”The Z2 /A rule is interpretsd from the energy threshold for

fission given by

E=4π.2(A/2)i • r20 · a-4圳的音• r20 • a+ (3/5) _J_至1?22__一一王星空泛Surface energy (s) term (A/2)1/3日 Al/3月

Coulomb repulsion cf proton

E=C-f.(Z2/A), C and f are constant values. (see Fig 2)

(22/ A)crit =(5/3)8π﹒ ro·s/e2=45

From this simplyfied theoretical consideration we can not obtain a full understanding

of nuclear chemistry on the heaviest elements. Nevetheless, we could say that no

greater amount of the heaviest elements in periodic table has not been prepared than

the following figures.

Pu tons, Cm grams, Bk and Cf micro grams, Es and Fm, micro micro grams and

102~ 104 atoms.

Appendix: Illustration of Astronomical Evolution of the Stars.

一 155 一

n'J, 學u. 第三期

Gravitational Contraction

一→

戶一令

女 H stars (H Burning)

*H伽S(II. geneH:. ti on)

－－－卦，

-tr H stars O giant Red Star

(He Burning)

－一→’

Supernova neutron capture (Rapid proceas).

Sun

還多 gi州 (]f. g…

一 156 一