Radioactivity- alchemy of an atom

by

Dr. Adriana Banu

JMU Department of Physics and Astronomy

January 28, Meet the Scientist: Saturday Morning Physics at JMU’12

A few fundamentals…

1896 Mendeleyev

92 building blocks(chemical elements)

Periodic Table of Elements

What chemists like…

o all matter (solids, liquids and gases) is composed of atomso chemical element = matter that is composed of only one type of atomo atom = the smallest particle of any element that still retains the characteristics of the element

not to scale

Atom = nucleus + electrons

(10-10 m)

+Ze

-e

Nucleus = protons + neutrons

(10-14 m)

Electron (discovered in 1897 by J. J. Thomson)Neutron (discovered in 1932 by J. Chadwick)

19061935

Stable and Unstable AtomsHow do atomic particles interact inside the atom?o there are forces within the atom that account for protons (positive charges) and electrons (negative charges): like charges repel and unlike charges attract each other

r

q1 q2F21 F12

r

q1 q2F21 F12

Attractive force between q1 and q2.

Repulsive force between q1 and q2.

What makes the protons stay together in an atom?o the strong nuclear force which opposes and overcomes the electrical repulsion force between the protons

The energy associated with the strong force is called the binding energy.

Stable and Unstable Atoms

Does the nucleus of an atom ever lose particles?o stable(unstable) nucleus: the binding energy is(not) great enough to hold the nucleus together.

Unstable atoms will lose neutrons or protons as they attemptto become stable. They are called radioactive atoms.

3 types observed: , and

What is a radioactive decay?o spontaneous breakdown of an atomic nucleus resulting in the release of nuclear radiation What is nuclear radiation?o energy and matter released during radioactive decay

What nuclear physicists like…

Z

~ 3000 currently known nuclides~ 270 stables only !~ 7000 expected to exist

Color Key:Stable+ emission- emission particle emissionSpontaneous fission

N

A chemical element (X) is uniquely identified by the atomic number Z !XA

Z N

Nuclides that have the same Z but different N are called isotopes !Mass number: A = N + Z

What is radioactivity?o emission of nuclear radiation due to a change in the nucleus

Transmutation occurs when a radioactive element attempts to become stabilized and transforms into a new element.

Radium-226(88 protons + 138 neutrons)

Radon-222(86 protons + 136 neutrons)

He-4(2 protons + 2 neutrons)

The Beginnings…

It was 1895 in Europe…

the discovery of X-rays

1869 – first observation of “cathode rays” Wilhelm Conrad Röntgen

1901 – Nobel Prize for Physics (first ever)

Modern “Alchemy”: radioactivity1896 Becquerel discovered radioactivity

A. H. Becquerel Pierre Curie Marie CurieThe Nobel Prize in Physics 1903

1898 – Marie Curie discovered two new radioactive elements: Polonium & Radium

The Nobel Prize in Chemistry 1911

Uranium mineral

Curies’ work on radioactivity… Marie Curie studied the “uranium rays” through their electrical effects using the quartz piezoelectroscope, an unusually sensitive electroscope invented by Pierre Curie himself (see picture below)

Classical electroscope used to detect electrical effectsin the air

(briefly)Marie Curie’s legacy

o coined the word “radioactivity” to describe each substance’s power to give off invisible ionizing rays/radiationso radioactivity is an atomic propertyo discovery of new radioactive elements- Polonium and Radiumo isolated Radium and determined its atomic weighto suggested that Becquerel rays might be pieces of matter

o the first scientist honored with 2 Nobel prizes (physics & chemistry)o first female professor at the prestigious University of Paris (Sorbonne)o founded a private Radium Institute (1914) to conduct research in chemistry, physics and medicine (the beginning of radiotherapy)

Rutherford joins the questMeet the “uranium rays”:

Ernest RutherfordThe Nobel Prize in Chemistry 1908

“for his investigations into the disintegration of the elements and the chemistry of radioactive substances”

Rutherford – father of nuclear physics

fflufluorescentscreen

non-deflectedparticles

deflected particles

gold foil

radioactive source

Discovery of the nucleus!

“Father of nuclear physics” in his own words:“It was as though you had fired a fifteen inch shell at a piece of tissue paper and it had bounced back and hit you.”

1 kg of radium would be converted into 0.999977 kg of radon and alpha particles.

The loss in mass is only 0.000023 kg = 23 mg!

Energy = mc2 = mass x (speed of light)2

= 0.000023 x (3 x 108)2 = 2.07 x 1012 joules.Equivalent to the energy from over 400 tonnes of TNT!!!

Radium-226(88 protons + 138 neutrons)

Radon-222(86 protons + 136 neutrons)

He-4(2 protons + 2 neutrons)

1 kg Ra (nuclear) 4*105 kg TNT (chemical)

238Pu

Where does the energy come from?

Vanishing radioactivityo no matter how much radioactive material is present initially, half of it will disappear after a time interval known as the “half-life”, T1/2

o after two half-lives, only a quarter of the original material remains…

o radioactivity is a random process that follows the laws of probability

Exponential decay law:teNN 0

2/1

693.0T

(decay probability)

Radioactive half-lifeHalf-lives for various radioisotopes can range from a fewmicroseconds (10-6 s) to billons of years:

Radioisotope Half-life

Polonium-215 0.0018 seconds

Radon-222 38 seconds

Sodium-24 15 hours

Iodine-131 8.07 days

Cobalt-60 5.26 years

Caesium-137 30.17 years

Radium-226 1600 years

Carbon-14 5730 years

Uranium-234 0.25 million years

Potassium-40 1.3 billion years

Uranium-238 4.5 billion years

~ 99% ~ 0.7% ~ 0.006%

How to measure the decay of radioactive isotopes?Basic unit of measure the radioactivity: curie (C)1 C = 37 x 109 decays/second

ExposureAmount of radiation striking an object: Röntgen (R)

DoseAmount of energy absorbed by an object exposed to radiation: gray (Gy)(in USA: 1 rad = 0.01 Gy)

Dose equivalentAmount of biological damage causedby radiation: Sievert (Sv)(in USA: 1 rem = 0.01 Sv = 10 mSv)

Human body is naturally radioactive

This is the story of a mysterious science whichentered history at the onset of major transitionsin science, and itself triggered these changes…

From its modest beginnings as a minor phenomenon,radioactivity quickly developed into a major

research field…

The radioactive decays of naturally occurring mineralscontaining uranium and thorium are in large part

responsible for the birth of the study of nuclear physics…

A remarkable episode in modern science,

radioactivity has the dual attractions of a fascinatinghistory and dramatic consequences for humanity…

What did we learn?

Got questions?