SPU-22: The Unity of Science from the Big Bang to the Brontosaurus and Beyond

Lecture 11

5 March 2014

Science Center Lecture Hall A

Outline of Lecture 11(Single Demo Day)

Demo introduction Statement on light Discovery of X-rays and Radioactivity Applications of both: - Structure of atom: periodic table, protons, isotopes, and neutrons - Resolution of age of earth conflict Demo conclusion

Nature Of Light

Light, as noted before, is very difficult to model properly. Model that seems to “work” for all observed phenomena involving light is called “quantum electrodynamics,” fancy name for complicated theory which we do not here explore. It explains/predicts seemingly inconsistent behavior of light: It sometimes acts like wave (as we first modeled light) and sometimes as particle, as in interactions with atoms. How can Nature be like that? It is

Discovery of X-RaysWilhelm Roentgen, in late 1895, experimented with Crookes tube to study beta rays (electrons)

Shielded tube from light with black cardboard

Discovered scintillation on screen ≈ meter distant

Followed up: Found that these “X-rays” penetrated paper, books,…; had no charge (How done?) Pasteur again re serendipity

Took photo of wife’s hand (see next slide)

Was popular in shoe stores (see next slide plus two)

First X-ray Picture of Human(Bertha Roentgen’s Hand)

Wilhelm Roentgen (1845-1923)

Late 20th Century Cartoon

Fluoroscope: Shoe Fitting (1930s - 50s)

X-Ray SpectroscopyScene setting: Unfinished business on atomic structure. Nucleus has positive charge. But what is its composition?

Enter Henry Moseley who in c. 1913 bombarded different elements with high energy electrons from Crookes-like tube and obtained X-rays. Discovered empirical, linear relation (see next slide) between elements’ atomic numbers and square root of x-rays’ frequencies, as predicted by Bohr’s model of atom. Energy difference between electron orbits is proportional to square of atomic number in Bohr’s model

Moseley’s (Empirical) Law

Meaning of Moseley’s Law

So what? Previously elements/atoms were given numbers based solely on atomic weight or based on consistency with chemical properties, gathered in periodic table

Moseley had found connection between atomic number and physical property of atom

Discovery Of RadioactivityIn 1896, Henri Becquerel, following discovery of of X-rays, studied materials that gave off radiation after being exposed to sources of energy, such as sunlight. He had been studying uranium salt this way. One day rather cloudy; he placed uranium salt in drawer where also had stored unexposed photographic plate. For some reason, he later looked at plate and found had been darkened. Figured out cause; studied emissions of uranium (see next slide plus two). Student, Marie Curie and husband, Pierre (next slide plus one), joined study. Marie discovered radium and coined term, “radioactivity.” It stuck

Henri Becquerel (1852-1908)

Marie (1867-1934) [and Pierre (1859-1906)] Curie

Study Of Radioactivity: Schematic

Atom Puzzles Remain

Why atomic mass usually greater than twice mass of positive charges in atom? Why different mass atoms often have same chemical properties?

Discovery Of Proton

Not easy to attribute to single individual

After discovery of electron, scientists realized that positively charged entities must exist. Why?

Rutherford fired alpha particles into nitrogen gas and detected hydrogen nuclei coming out. This result led to theory hydrogen nucleus was proton and was charged constituent of all atomic nuclei

Discovery of Isotopes

Frederick Soddy (1911) and isotopes: Some elements, which are radioactive decay products, showed differences, although elements could not be distinguished chemically. Answer: isotopes (= nuclei had same number of protons, but different amounts of neutral component)

“Put colloquially, their atoms have identical outsides, but different insides”

What was this constituent of nuclei that was electrically neutral??

Electron-proton combination? Ruled out by various (arcane for this course) arguments

Frederick Soddy (1877-1956)

Discovery Of Neutron

James Chadwick (1932) demonstrated, via scattering experiments, existence of neutral particle that came from nucleus. Estimated its mass; result within one part in c. 2,000 of modern value; close to mass of

proton (~0.1% heavier)

Solves problem of mass-charge discrepancy of atomic nuclei. (Atomic number for most nuclei accounts for less than half mass of nucleus)

Outside of nucleus, neutron unstable: decays in c. 10 minutes. Proton stable for at least ~1034 years

James Chadwick (1891-1974)

Structure Of Atom Concluded(At Last!)

What is an atom? - Nucleus, composed of positively charged protons and uncharged neutrons; former largely control chemical properties of atom

- Electrons, equal in number to protons, surround nucleus - Different elements have different numbers of protons/electrons

What Is Radioactivity?

Realization of alchemists’ dream: conversion of one element (specific type of atom; now we can say it) into another element, although not exactly iron into gold on demand

Changeover - for element that has radioactive property (by no means all do) - occurs randomly, but on overall reliably predictable schedule, because of truly huge number of atoms involved

For each radioactive element available, one can determine in laboratory its “half-life” very accurately, virtually no matter how long or how short (see next four slides)

R

Radioactive Decay And Decay Products (Thorium X= Radon)

Some Element Pairs Used For Radioactive Dating And The Parent Half-Lives

Potassium 40 (19) - Argon 40 (18): 1.25 Gyr Rubidium 87 (37) - Strontium 87 (38): 48.8 Thorium 232 (90) - Lead 208 (82): 14.0 Uranium 238 (92) - Lead 206 (82): 4.5

Note 1: Number following name of element is number of neutrons + protons in nucleus; second number (in parentheses) is atomic number of nucleus, i.e., number of protons in nucleus.

Note 2: Decay chains are of different lengths and complexities

Radioactivity As Clock

Rutherford (1904) suggestion: Could use radioactivity as clock. How? Isolate known (measured) amount of radioactive element; monitor amount to determine elapsed time

Talked of technique with Kelvin in audience (next slide)

How apply to Earth? Not trivial! First, need to know half lives of relevant element(s). Rest of procedure tricky, as we will see in part

Rutherford On Kelvin (1904) “I came into the room, which was half dark, and

presently spotted Lord Kelvin in the audience and realized that I was in for trouble at the last part of the speech doing with the age of the earth, where my views conflicted with his. To my relief, Kelvin fell fast asleep, but as I came to the important point, I saw the old bird sit up, open an eye and cock a baleful glance at me! Then a sudden inspiration came, and I said Lord Kelvin had limited the age of the earth provided no new source of heat was discovered. That prophetic utterance refers to what we are now considering tonight, radium! Behold! The old boy beamed upon me.”

How Measure Age Of Earth Via Radioactivity? (Introduction)

Look at “isolated” portion of rock with radioactive parent and daughter elements present. Separately, measure half-life of parent

Assume only parent present at formation of rock. Assume no parent atoms left, and no daughter atoms entered system since rock formation. If true, age determination “straightforward,” but only of rock! (See next slide.)

Reading Radioactivity Clocks: What Are Some Specific Potential Pitfalls?

First, given sample rock, what are we measuring age of? In effect, time elapsed since rock first formed. How formed? Presumably solidified from liquid form (not true for all rocks)

Possible problems: When originally formed, “pocket” in rock may have already had, for whatever reason, some unknown amount of daughter element in addition to parent; rock may have undergone one of more partial melting episodes; some of parent and/or daughter element may have escaped, and different amounts of either may have entered

Redundancy To (Partial) Rescue

Use two (or more) independent methods of dating, as in our astronomy lab

Choose two different parent/daughter pairs that have different susceptibilities to various possible systematic errors. Caution: One must use pairs that have half lives suitable for problem at hand (what does this statement mean?)

Early Results For Earth’s Age From Use Of Radioactivity Clock

After Rutherford’s initial idea for using radioactivity as a clock (shades of Galileo and Io), the field of dating expanded greatly

By early 1920s fairly reliable results were being obtained. Age of earth was estimated at between 1 and 8 billion years, well more than previous estimates made by other means. There were many skeptics among geologists

Typical Geologist Reaction: F. W. Clarke, U.S. Geological Survey (1924)

“From chemical denudation, from paleontological evidence, and from astronomical data, the age of the earth has been fixed with a noteworthy degree of concordance at something between 50 and 150 millions of years. The high values found by radioactive measurements are therefore to be suspected until the discrepancies shall have been explained.” [Note irony: Age range Clarke here endorses encompasses most of the values Kelvin gave half century earlier.]

So How Can One Best Use Radioactivity To Estimate Age Of

Earth? Look for oldest rock and determine its age. How

does one know, in advance, oldest rock? One doesn’t!

Determine age of every rock and see which is oldest. Not very practical!

How do we even know that oldest rock now around has been here since birth of Earth? We don’t. We can at best obtain reliable lower bound on earth’s age

How Then Do We Estimate Age Of Earth?

Search for “pristine” bodies, those that were around at formation of planetary system.

Meteorites are good prospects: Modern radioactive dating results are remarkably concordant, yielding values of about 4.567 billion years. Easy number to remember and fairly reliable for earth’s age, based on models of planet formation

Triumph Of Physics

Overwhelming evidence from radioactive dating and redundant decay chains convinces virtually all scientists. Estimates of geologists of earth’s age totally eclipsed. “Rationality” triumphed, based on science that withstood extensive criticism and emerged with clean hands

Where, Specifically, Did Kelvin Go Wrong?

Kelvin made predictions about earth’s age and sun’s shining. Both were wrong due to radioactivity and nuclear fusion, respectively, neither known or even dreamed about in the mid 19th century.

For earth: Radioactivity provided a continuous source of substantial heat and thus earth didn’t just cool from original high temperature as Kelvin assumed.

For sun: Fusion, which we didn’t discuss, primarily of hydrogen into helium at high temperature and pressure near sun’s center, provided huge and nearly inexhaustible supplies of energy (recall: E=mc2), which will keep earth warm for about another 5 billion years

Ernest Rutherford (1904)

“The discovery of the radioactive elements, which in their disintegration liberate enormous amounts of energy, thus increases the possible limits of the duration of life on this planet [via the sun’s radiation], and allows the time claimed by the geologists and biologists for the process of evolution.”

Demonstration Of RadioactivityDust contains radioactive elements, radon being most

relevant because of its continuous seepage from Earth (via uranium decay)

We measure, with suitable instrument, constant, albeit small, fraction of those atoms on balloon that decay radioactively. We display on chart count rate of decay as parent atoms are converted into chain of daughter atoms. To show this behavior more clearly, we started experiment this morning before class and are looking now at results at end of class

Results From Dust Experiment

See screen with counts of radioactive decays vs. time

Why is curve so “jagged”? Concept of smoothing via averaging