Save these instructions in your binder. Do not turn in.

The Shape of Things
Atomic Target Practice


This lab is incomplete. The lab handout used in class was borrowed from another teacher and is not yet available in electronic form.
Introduction

It is one of the most famous experiments of all time. More than 25 years after conducting the experiment, Ernest Rutherford described the results this way:

It was about as credible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you.
The experiment itself was actually the culmination of a series of experiments, carried out over a five-year period, dealing with the scattering of high-energy alpha particles by various substances. What is “Rutherford Scattering” and why was it so important?

 

Background

Ernest Rutherford received the Nobel Prize in Chemistry in 1908 for his investigations into the disintegrations of the elements as a result of radioactive decay. Among the products of the radioactive decay of elements are alpha particles—small, positively charged, high-energy particles. Alpha particles are in fact completely ionized 4  2He nuclei. In trying to learn more about the nature of alpha particles, Rutherford and his co-workers, Hans Geiger and Ernest Marsden, began styding what happened when a narrow beam of alpha particles was directed at a thin peice of metal foil. Alpha particles are a type of nuclear radiation, traveling at about 1/10 the speed of light. As expected for such high-energy particles, most of the particles penetrated the thin metal foil and were detected on the other side. What was unexpected was that a few—a very few, to be sure—of the alpha particles were actually reflected back toward the source, having been “scattered” or had their paths bent due to their encounters with the metal atoms in the foil target. The number of alpha particles that were reflected back depended on the atomic mass of the metal. Gold atoms, having the highest atomic mass of the metals studied, gave the largest amount of so-called “back scattering”.

Rutherford’s scattering experiments

No instrument allows the atom’s nucleus to be observed directly, so scientists have to find other ways to learn about it. Scientists at Jefferson Lab in Virginia use a machine called an electron accelerator to probe atomic nuclei. This accelerator takes electrons, forms them into a beam about the width of a human hair and speeds them up to nearly the speed of light. This fast moving beam of electrons is then directed at a target. Some of the electrons in the beam interact with some of the atoms in the target. When this occurs, the electron changes direction and some particles could fly out of the nucleus. Detectors placed around the target record the paths of the electron and nuclear fragments. These collisions provide scientists with clues about the structure of the nucleus. By bouncing a marble off of a hidden target, you simulate the experiments done at particle accelerator laboratories like Jefferson Lab.


Materials

Several wooden shapes hidden underneath carboard
A marble
A ramp (use a book or a piece of paper)
A pen or marker
Paper to mark marble trajectories



Procedure

Prepare your lab notebook by putting the title of the lab and the date at the top of the first empty page in your lab notebook. Remember to make an entry in the table of contents and to number the pages as you go along. When writing in your lab notebook you should always say what you did, what you observed and what it means. See the Data section for a hint about how to organize the material in your lab notebook for this lab.
  1. It is your group’s mission to discover the shape under the cardboard but you cannot:
  2. Cover the cardboard with two pieces of scrap paper that you have taped together on the long edge.
  3. Use the ramp to roll the marble under the cardboard.
  4. Watch what the marble does. Does it hit something and bounce back out? Does it not hit anything? You can use these clues to discover what is hidden under the cardbaord.
Marble.Scatter.Ex (6K)
  1. Keep track of what the marble does by drawing its path on top of the cardboard. Remember, you can’t actually see what the marble is doing, so you will have to estimate what path it took. Try repeating the same path several times.
  2. If the marble looks like it hit something under the cardboard, draw its shape on the paper you placed over the cardboard. See the example.
  3. Roll the marble under the cardboard many different times from many different directions.
  4. Once all of your team members think they know what shape is hidden under the cardboard, call your teacher over to check your results. Don’t turn the cardboard over at all so that the other groups do not receive an unfair advantage!


page break


Data

Make a data table recording the letter associated with each shape, a description of the shape you found, the dimensions of the shape in cm, and a drawing of the shape.


Questions

  1. What was the most difficult part of this experiment?
  2. What can you do in your experiment that the scientists at Jefferson Lab (who are investigating atomic structure) can’t?
  3. Which was the easiest shape to find? Why?
  4. Which was the hardest shape to find? Why?


To Turn In

For this lab turn in a sheet of paper with your data tables neatly transcribed onto them.

Last updated: Dec 03, 2006      Home