TRANSMUTATION AND DECAY SERIES

HOW URANIUM CHANGES TO LEAD

or

WHAT HAPPENS IN BETWEEN

Kevin Lavarnway Schroon Lake Central School

START WITH A RADIOISOTOPE

• An isotope is an atom of the same element with the same number of protons and a different number of neutrons

• Some isotopes are naturally unstable and spontaneously change to another isotope of a different element

• This change from one element to another is called TRANSMUTATION.

THIS IS NOT A CHEMICAL CHANGE!

• Chemical reactions involve atoms rearranging by breaking and forming bonds involving electrons

• TRANSMUTATION involves changes in the nucleus that change the actual identity of the element

• These reactions are called NUCLEAR because they involve the atom’s nucleus

NOTATION

Uranium – 238

238 mass number ( p + n )

U

92 atomic number ( # p )

TRANSMUTATION INVOLVES THE RELEASE OF RADIATION

• A Radioisotope emits, or gives off, radiation from its nucleus

• Each isotope emits a certain type of radiation; it has a specific decay mode

• The radiation can be in the form of particles or rays

ELECTROMAGNETIC SPECTRUM

LOWER ENERGY HIGHER ENERGY

TYPES OF RADIATION

• Pure energy given off as rays is called gamma radiation

• Radiation particles could be alpha or beta particles

ALPHA PARTICLE

• The same as a helium nucleus• 2 protons and 2 neutrons

4 mass of 4 amu He 2 Has a 2+ charge

because of 2 protons

LOWEST PENETRATING POWER

BETA PARTICLE

• The same characteristics as an electron(not from an energy level—emitted from the

nucleus)

0 essentially no mass

e -1 charge of -1

HIGHER PENETRATING POWER

GAMMA RADIATION

• Pure electromagnetic energy with high frequency and short wavelength

• No mass or charge associated with it

MOST PENETRATING POWER

PENETRATING POWER OF NUCLEAR RADIATION

NUCLEAR TRANSMUTATION(natural)

Original New

Radioisotope Isotope + Radiation

238 234 4

U Th + He

92 90 2

A CLOSER LOOK…

U-238 has alpha decay Total mass before and after must be “equal”

238 234 4 U Th + He 92 90 2

Total of atomic numbers must also balance

How can you use decay mode information?

Can you determine the new isotope if you know the decay mode?

For example, Th-232 is an alpha emitter.

232 4

Th ? + He

90 2

STRATEGY…

*mass numbers must balance

232 = 228 + 4

Th ? + He

90 = 88 + 2

*atomic numbers must balance

How do we know what the new isotope is?

CHECK THE PERIODIC TABLE!

The new isotope has a mass number of 228 and an atomic number of 88.

228

?

88 each element has a unique

atomic number…

The new isotope is Radium-228 which is a beta emitter!

What happens to Ra-228?

228 0

Ra ? + e

88 -1

(beta particle)

Balance the mass numbers and the atomic numbers…careful of that -1)

And the result of the decay is…

mass number stays same 228 Ac atomic number increases by 1 89

Actinium a new element with a greater atomic #

This is a beta emitter as well! When will it stop???

…not until a stable isotope is formed!

The change from one radioisotope to another in a specific sequence is called a decay series

We will explore some transmutations that are part of the uranium-238 decay series...

You will be given two radioisotopes and their decay modes. Determine what the product of each transmutation will be…

Start off examples are:

U-238 is an alpha emitter:

238 234 4

U Th + He

92 90 2

Th-234 is a beta emitter:

234 234 0

Th Pa + e

90 91 -1

Take your time and think it out…

1. Write the symbol of the radioisotope in the proper notation on the reactant side

2. Write the notation for the radiation type emitted on the product side

3. Total up the mass numbers on top so the total mass is balanced on each side

4. Total up the atomic numbers on the bottom so they are balanced as well

5. Use Periodic Table to identify the element by its atomic number

Some practice for you…

• You and a partner will receive a sheet of paper with two sample transmutations---tear in half so you each have two reactions

• Complete each transmutation individually and then compare the results with your partner…

• Decide on the correct answers and we will share them with the entire class by completing the decay series…

U-238 decay series

U-238 Th-234 Pa-234 U-234

Th-230 Ra-226 Rn-222 Po-218

Pb-214 Bi-214 Po-214 Pb-210

Bi-210 Po-210 Pb-206 (stable)

One very special isotope…

U-238 Th-234 Pa-234 U-234

Th-230 Ra-226 Rn-222 Po-218

Pb-214 Bi-214 Po-214 Pb-210

Bi-210 Po-210 Pb-206 (stable)

Rn-222 is a gas!

Are all of the steps just alike?

It looks like all transmutations are identical to every other when it is written out in a straight line or sequence…

but some are alpha and some are beta decays… they are not all the same.

Is there a better way to show what is really happening?

Let’s try an x-y axis to show it…

Atomic number

Ma

ss

nu

mb

er

Follow the directions on the hand-out to change the linear picture into a 2-dimensional display.

Use the results to get the range of atomic numbers you need and the range of mass numbers you need to fit on your graph.

Once you figure it out, place your Post-It note for your radioisotope on the wall chart in the right location.

Now for some practice on your own

You have just worked with the decay series for U-238 and discovered a sequence of radioisotopes (including radon gas!)

U-235 is a different isotope of uranium. It has its own decay series as it too eventually changes to lead.

Give the U-235 decay series a try and practice your skills… good luck!