Malley M.C. Radioactivity: A History of a Mysterious Science

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A NEW SCIENCE

128

exhausted. Ironically, these  ndings, combined with other cir-

cumstances, led radioactivity down a path which ended its exis-

tence as a separate research  eld.

FROM RADIOACTIVITY TO NUCLEAR

AND PARTICLE PHYSICS

Right before the war Rutherford’s student Ernest Marsden

reported a tantalizing result. While experimenting with alpha

particles, he noticed high speed hydrogen atoms in the apparatus.

Perhaps hydrogen, like helium, was a by-product of radioactive

disintegration.

 is possibility intrigued Rutherford. Marsden le Cambridge

and could not continue experiments while he served in the army.

War research claimed most of Rutherford’s time, but he managed

to continue Marsden’s investigations sporadically. To his surprise,

Rutherford found that radioactive decay was not responsible for

hydrogen’s appearance.  e hydrogen in Marsden’s apparatus

came from nitrogen atoms in the air.

Apparently an energetic alpha particle could chip o a piece

of a nitrogen atom, producing hydrogen atoms. A er years of vain

a empts to in uence radioactive disintegration, Rutherford real-

ized that he and Marsden had accidentally caused an ordinary ele-

ment to disintegrate!

If alpha particles could split o hydrogen atoms from other

atoms, hydrogen might be an atomic building block.  ese results

bolstered long-standing suspicions about the atom’s composition.

 ey also presented an exciting possibility. Perhaps powerful

alpha particles could be used to disrupt other atoms and uncover

information about nuclear forces and structure.

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Rutherford’s results inspired new research with alpha and beta

particles and e orts to get more powerful projectiles for bom-

barding atoms. To increase the energies of these subatomic bul-

lets, scientists invented machines that could accelerate particles

to high speeds.  e electrostatic generator, the linear accelerator,

and the cyclotron developed in the 1920s and 1930s were the  rst

in a series of increasingly bigger and more ingenious devices for

smashing atoms.

Several important textbooks on radioactivity appeared in the

1920s. Laboratories in Paris and Vienna published scores of papers

on radioactivity researches, while sites like Berlin and Cambridge

were not far behind. Meyer and von Schweidler cited 1,561 authors

for the period 1916–26 in their textbook Radioaktivität.  e  eld

was healthy, but interests were changing.

Rutherford assessed the discipline in his 1930 textbook,

Radiations  om Radioactive Substances.  e title embodies the

change he recognized, a shi from studying radioactive transfor-

mations and decay relationships to understanding the rays and

their interactions with ma er.  ese studies, he believed, would

reveal information about two persistent puzzles in radioactivity:

the structure of the nucleus and the energy changes involved in

its transformations. Bombarding ma er with high speed particles

was the new frontier of research.

During the 1920s physicists had increasingly directed their

a ention towards the atomic nucleus and the subatomic particles

resident in the universe.  eoreticians became interested in the

previously experimental science of the atom.  ey applied quan-

tum theory and wave mechanics to the nucleus. By delineating

probability’s reign in the physics of the atom, the new theories

placed chance at the heart of nature.  ough some physicists still

desired causal explanations for radioactive decay and the quantum,

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130

they were willing to move beyond those seemingly impenetrable

mysteries to questions they could answer.

In 1931 the  rst International Conference on Nuclear Physics

convened in Rome.  e next year Chadwick identi ed an uncha-

rged subatomic particle, the neutron. Of signi cant theoretical

import, the neutron became another tool for probing the atom’s

interior. A positively charged counterpart to the electron, the

positron, was discovered in cosmic rays by Carl Anderson.

In 1934 Irène Curie and her husband Frédéric Joliot produced

radioactive phosphorus, the  rst of a sequence of arti cially cre-

ated radioactive isotopes. Many of these isotopes became useful

for medicine and industry, for instance as radioactive tracers, for

treating cancer, and as ionization sources in smoke detectors.

Experiments with arti cial isotopes eventually led to the dis-

covery of nuclear  ssion in 1939 by German chemists O o Hahn

and Fritz Strassmann and Austrian physicist Lise Meitner. In  s-

sion, an unstable nucleus breaks apart, producing two atoms of

lesser atomic weight. Commonly known as “spli ing the atom,”

this process releases huge amounts of energy.

Without announcement or fanfare, radioactivity had been

superseded by nuclear physics and particle physics.  at once fas-

cinating phenomenon remained interesting mainly for its applica-

tions. Another new  eld, nuclear chemistry, took the place vacated

by radiochemistry, whose major problems had been solved by the

early 1920s. Cosmic ray studies, born from radioactivity research,

were swallowed up by particle physics.

And so the mystery which had enticed scientists for several

decades was never solved on its original terms. New ways of con-

ceptualizing physics replaced the older ideas of physical systems

operating through chains of cause and e ect. Successes of the new

approaches convinced most scientists to view probability as a basic

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131

principle of the physical world requiring no further explanation.

 e mystery of radioactivity was replaced by a new set of questions

about nuclear structure, particles and  elds, quantum transitions,

symmetry and invariance, and increasingly abstruse mathemati-

cal schemes.  e solidly experimental science of radioactivity

transmuted into new theoretical species, nurtured by their experi-

mental o shoots and trailed by a series of practical consequences.

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PART TWO

MEASURING AND USING

RADIOACTIVITY

And now you are in possession of salts of pure radium! . . . What a pity it is

that this work has only theoretical interest, as it seems.

—Władisław Skłodowski to Marie Curie, 1902

Circumstances soon superseded these words, written by

Marie Curie’s beloved father only a few days before his death.

Radioactivity’s potential for practical applications fueled a bur-

geoning industry for radioactive materials. This new industry had

far-reaching effects.

Radiation’s visible e ects on living tissues stimulated searches for

medical uses, with unforeseen consequences. Demand for radioac-

tive materials spurred widespread exploration and business ventures

to process and manufacture radioactive materials. New instruments

opened up  esh avenues for exploring the unknown, as well as more

options for practical applications of radioactivity.

International standards for radioactivity facilitated research,

medicine, and industry. Measuring methods and instruments grew

MEASURING AND USING RADIOACTIVITY

134

more sophisticated and eventually transformed the scale of research.

Research, medicine, and industry became entangled with social, politi-

cal, and ethical questions raised by the gradual recognition of radioac-

tivity’s powers.

135

Methods and Instruments

 ings have their due measure.

—Horace, Satires

CRUCIAL CHOICES

When scientists select a particular method to investigate a phe-

nomenon, they implicitly accept underlying assumptions about

the phenomenon and narrow the range of options to consider.  e

ways that experimenters gather and record data determine what

they can observe and limit the possibilities for interpreting their

results. Methods chosen to investigate radioactivity in uenced

the growth and direction of the new science.

Early experimenters could use photography, electrical measur-

ing devices, and  uorescent minerals to study radioactivity. Each

method had particular strengths and limitations, and each sug-

gested a di e r en t i nte r p ret a t ion of r a d ioa c t i v i ty. To em pl oy ph oto g-

raphy implied the new rays were similar to light. Henri Becquerel

used photography for his  rst experiments because he was search-

ing for a new kind of light. Since electrical methods record ion-

ization, they could suggest a resemblance to x rays, cathode rays,

MEASURING AND USING RADIOACTIVITY

136

or ultraviolet light. Testing with  uorescent materials implicated

ultraviolet light or cathode rays.

Photography was familiar and simple in concept. Photographs

captured direct visual images, useful for qualitative studies and

for recording the paths of rays de ected by electric and mag-

netic  elds. Unfortunately, this method had serious drawbacks.

Photographic images were di cult to quantify and interpret. For

instance, it was hard to determine intensity of radiation from pho-

tographic impressions. Photography could not be used for short-

lived radioactive products because longer exposure times were

needed to produce useful images. Photography is especially prone

to error, because many agents besides radioactivity can a ect pho-

tographic plates and  lm.

Scientists used electrical devices to study ionization produced

by radioactive bodies. Fi ed with scales to measure the movement

of electroscope leaves or electrometer needles and strings, electro-

scopes and electrometers could be extremely sensitive and yield

precise quantitative values. According to Pierre Curie, his electro-

scope was 10,000 times more sensitive than the spectroscope, an

instrument which could detect vanishingly small quantities of an

element.

Since electrical instruments responded readily and could be

easily modi ed, they could be adapted for all strengths and types

of radiations. Unlike photographic plates, electrical devices could

detect short-lived radioactive bodies. Because of their advantages,

electrical methods soon displaced photography in radioactivity

research.

Becquerel rays create striking visual e ects in  uorescent

minerals, making them candidates for detecting radioactivity.

Nineteenth-century researchers had used screens made of  uo-

rescent barium platinum cyanide to detect rays.  ese screens

METHODS AND INSTRUMENTS

137

became popular for studying cathode rays and x rays and were

later used to investigate Becquerel rays.  is simple and colorful

method was not suited for most quantitative studies.

A er Ernest Rutherford and Hans Geiger showed in 1908 that

alpha particles create individual  ashes of light when they strike

 uorescent zinc sul de (Sidot’s blende),  uorescent screens could

be used for quantitative work. By counting scintillations, research-

ers were actually counting alpha particles. Zinc sul de screens

became widely used for this purpose.

Since alpha, beta, and gamma rays a ect photographic plates,

 uorescent materials, and electrical devices di erently, the instru-

ment chosen determined what was recorded and could in uence

the conclusions drawn from data. Other details of experimental

arrangements might also interfere with interpretations.

For instance, paper or glass coverings on photographic plates

allowed beta and gamma rays to pass through, but blocked weak

alpha rays. When researchers separated uranium and thorium from

their beta-emi ing decay products in 1901, they found no activ-

ity in the uranium and thorium. Sir William Crookes, Becquerel,

Rutherford, and Frederick Soddy mistakenly concluded that these

elements were not radioactive. Only later did they learn that ura-

nium and thorium sent out alpha rays. Since the alpha rays could

not pass through the paper or glass coverings in the earlier experi-

ments, they had not been detected.

STANDARDIZING THE MEASURES

At  rst, laboratories developed their own methods for measuring

radioactivity and determining the amount of radioactive material

in preparations. As research proliferated, it became increasingly