Lecture 14 - Experimental Achievements in Physics
- We have considered the input of science into technological
development, new processes and products
- Technology is involved in science as well, for example,
through experimentation
- Before the advent of large particle accelerators and billion
dollar scientific projects, experimental science relied on
much simpler methods and technologies
- We will consider two examples of experiments from the late
19th century, experiments that contributed directly to the later
development of nuclear power
Particle Science in the 19th Century
- Most scientists accepted some form of atomic theory by the
end of the 19th century, with an analysis of phenomena in
terms of forces and particles
- William Crookes (1832-1919) observed negatively charged
cathode rays in 1876
- William Conrad von Rontgen (1845-1923) discovered X-rays
accidentally, noting that they could fog photographic plates
- X-rays were produced when cathode rays (streams of
electrons) hit a glass tube, the point on the tube would glow,
emitting X-rays
Becquerel’s Experiments with Uranium
- Henri Becquerel (1852-1908) knew of Rontgen’s
experiments, and decided to investigate phosphorescent
materials (materials that would glow)
- He took a photographic plate and wrapped it in heavy black
paper, blocking exposure to sunlight
- He then took a thin crust of potassium uranyl sulphate
crystals and placed it on top of the wrapped photographic
plate, and exposed the crystals to sunlight
- This process produced exposures on the covered plate much
like those from X-rays, confirming his expectation that there
was a link between rays, sunlight and phosphorescence
- Becquerel thought that sunlight caused rays of some sort to
be produced from the phosphorescent sample, and these rays
caused the exposure on the plate, as the heavy black paper
blocked the plate from exposure to sunlight
- He had planned on exposing potassium uranyl sulphate to
varying amounts of sunlight and then testing the intensity of
the photographic exposure from the exposed sample
- However, due to intermittent sunlight conditions, Becquerel
decided to store the plates and samples in darkness for
several days
- When he removed the samples to expose them to sunlight, he
noted that the plates had strong exposures despite having
been stored in the dark.
- Becquerel then set up an “isolation box” experiment to
determine if the plate exposures were accidents
- Leaving a fresh plate and a potassium uranyl sulphate sample
that had not been exposed to sunlight in a sealed box for five
hours, he noted a strong exposure on the photographic plate
- He concluded that it may require light to produce
phosphorescence, but that “penetrating rays” could be
produced without such stimulation
- Becquerel then obtained the purest possible sample of
uranium metal he could find, produced in an electric arc
furnace
- He reasserted his claim that uranium emits rays without any
photo-stimulation
- He determined that pure metallic uranium emits more
powerful rays than uranium salts
- He discussed how uranium salts that had been isolated from
light for three months were still emitting rays without any
perceptible loss in strength
- Becquerel had discovered a constant, powerful source of
radiation from a particular element, one that could not be
traced back to sunlight and was different than X-rays
- From the point of view of technology, only very simple
technologies were needed to make this fundamental
discovery
- The 20th century saw a rapid increase in the scale and cost of
scientific experimentation, Becquerel and Rutherford’s
experiments were some of the last to make fundamental
discoveries
- After Becquerel, the Curies, Pierre (1859-1906) and Marie
(1867-1934) investigated the properties of radium, uranium
and polonium
- They discovered that these elements contained vast amounts
of energy (the notebooks Marie kept while extracting radium
from pitchblende are still radioactive today)
Rutherford’s Experiments with Transmutation
- The idea of transmutation, conversion of one element into
another, was very old
- Before the transformation of chemistry into a modern
science, alchemists attempted to transform elements
- Alchemy originated in Egypt, adopted by Plato’s school,
revived in the Renaissance, used by Newton
- Though there were other important transformations, ancient
alchemists sought primarily to transform base metals into
gold
- Alchemical transformation processes involved repeated
distillations or heating materials and cooling them repeatedly
- What Rutherford did was much more basic, he altered the
atomic structure of matter through bombardment with
particles
- Rutherford noted that radiation came in different varieties,
alpha and beta radiation
- Importantly, Rutherford argued that the emission of radiation
was a natural process that occurred in certain unstable
elements
- This was important, as elements were previously thought to
be indivisible or ultimate, the “building blocks” of matter
- Becquerel’s discovery was important to Rutherford’s work,
as Rutherford used alpha particles and radiation to bombard
substances to discover their structure
- Radiation was thus used as a tool to investigate nature
- Rutherford was a gifted experimentalist, consider his “gold
foil experiment”
- Rutherford took a thin gold foil and bombarded it with
positively charged alpha particles
- Most of the particles passed through the foil, proving that
atoms were made up mostly of space
- However, periodically an alpha particle would bounce back,
indicating that it had hit a heavy, positively charged object
(the nucleus)
- Rutherford described it as similar to firing a rifle shell at a
piece of tissue paper and having it reflect back
- Simple physical experiments using basic technologies
produced knowledge of the physical structure of matter
- Also, the experiments were interpreted in terms of simple
physical models, particles striking particles and the like
- The details of his experiment are not important, what matters
is the straightforward way in which Rutherford came to his
conclusions
- He bombarded nitrogen atoms with alpha particles, and then
used scintillation screens (screens that lit up when hit by
particles) to detect the presence of hydrogen
- This hydrogen was produced by the disintegration of nitrogen
atoms upon bombardment
- Rutherford had achieved the transformation of elements
using simple, mechanical methods
- Importantly, he did not need expensive machinery or vast
amounts of energy to achieve these results, all that was
required was a radioactive source and a controlled
experimental set-up
Conclusions
- The 20th century was marked by the advent of expensive “big
science” projects, but much simpler technology was used to
make progress into the ultimate structure of matter
- Scientific discoveries are frequently cumulative, Becquerel’s
discovery of radiation from uranium led to the use of
radiation in the investigation of matter
- Simple experiments into matter were possible in the late 19th
century as there were methods for producing pure samples
(e.g. the use of the electric arc furnace to produce pure
samples of uranium metal)
- These simple experiments shattered an understanding of
matter that had been dominant for hundreds of years
- They challenged the idea that elements were stable ultimate
constituents of matter, and they realized an ancient dream,
that of transmutation of elements from one into another
- The instability of matter, and the transformation of the
elements are two important components of nuclear power