MadSci Network: Physics

Re: if a neutron is separated from an atom could it pass through other atoms?

Date: Wed Dec 10 03:19:54 2003
Posted By: Michael Kay, Haz. Mat Mgmt, Health Physics, Nuclear Science, President and Consultant AMBRY, Inc
Area of science: Physics
ID: 1068623906.Ph

That is a good question to introduce the interaction of radiation with 
matter. I will try to illustrate what is happening on the atomic level 
with some examples that might be easier to understand, but are on a scale 
you are familiar with. You mention the three particles-the proton, the 
neutron, and the electron-that are the most commonly encountered when 
considering nuclear reactors and radioactive materials. These particles 
form two groups: the proton and electron are charged particles, and the 
neutron is an uncharged, or neutral, particle. The property of charge is 
the most important in understanding the interaction of these particles 
with matter, or, as you put it, other atoms (like those that make up a 
table). The electron has a charge of -1 (the negative sign comes from the 
original definition of the charge on an electron being negative). The 
proton has a charge of +1 where the plus sign indicates the opposite 
charge from the electron. All atoms (except the hydrogen atom consisting 
of one proton and one electron) have a nucleus containing protons and 
neutrons at the center. Almost all the weight of the atom is in the 
nucleus because the proton and the neutron each weigh about 1837 times as 
much as an electron. An atom is mostly empty space--a heavy center with 
light electron "balls" whirling around it to form a spherical cloud. 

Atoms of any element in a pure state are neutral, therefore there are as 
many electrons in orbits around the atom as there are protons in the 
nucleus. When an electron is knocked out of an atom, we say the atom is 
ionized, and has a positive charge. To knock an electron out of an 
orbital takes energy, so if another electron comes speeding through a 
group of atoms, the negative charges of the electrons will repel the 
speeding electron, and the positive nuclear charge will attract it. Each 
time the speeding electron knocks another electron out of an atom, it 
loses some of its energy. If the electron is attracted to a positively 
charged nucleus, and its path is changed so it appears the electron is 
going around a curve, the electron loses energy again. In each encounter, 
the energy loss is small, therefore it takes a lot of collisions and 
curves to slow the speeding electron to a speed where it can be captured 
by a nucleus that has an open orbital. The speeding electron is 
constantly being deflected, and its path is not a straight line. The 
number of atomic electrons and charges on the nucleus depends on the 
material the speeding electron is traveling through. If it is made of 
light elements such as hydrogen, carbon, or oxygen (like wood of a 
table), there will be fewer electrons in a cube of the material than if 
it were made of a metal such as iron, gold, or lead. There will be many 
more collisions and curves in the path due to the high positive charges 
on the nuclei of the heavier atome. Therefore, the speeding electrons 
will not get as far in those materials. While a thin piece of metal 
easily stops speeding electrons, a 1-inch thick table top will also stop 
almost all of the electrons. So, the electrons will probably not get 
through the table.

A fast moving proton has a charge of +1, and will attract every electron 
it comes near, causing a large amount of ionization along its path. Since 
the proton weighs so much more than the electron, it is not deflected by 
a collision--just like a bowling ball would not be deflected by a 
collision with a ping-pong ball. The only time a fast moving proton would 
be deflected is if it comes very close to a nucleus. So generally, the 
path of a proton is a straight line. The proton causes so much 
ionization, with each ionization costing some energy, that its path is 
short. Imagine a bowling ball passing through a mass of pingpong balls. 
The bowling ball goes straight until it is stopped by the large number of 
collisions, each taking a part of its energy, both from the collision and 
from the interaction of the negatively charged electrons with the 
positively charged proton. When the proton comes to rest, it captures one 
of the free electrons an becomes an ordinary hydrogen atom. 

So we see that no matter if the charge on the speeding particle is 
positive or negative, the interaction with the charges of the orbital 
electron or the nucleus will slow it down quickly; just how quickly 
depends on the number of electrons and positive charges, so heavier 
elements slow them down faster. 

Now we come to neutrons. Neutrons are not charged, so they are unaffected 
by the charges of the electrons or of the nuclei. They do not attract or 
ionize the electrons, so they do not lose much energy if they hit an 
electron. The only way they can lose energy is by a direct collision with 
a nucleus. What is the best material to stop a neutron? Since we are 
dealing with collisions, let's use the billiard ball example. If the cue 
ball hits a bowling ball, it will bounce off, changing direction, but it 
will not lose much energy. So the first generality about stopping 
neutrons is that heavy elements may not be effective. Now let's look at 
the cue ball hitting another billiard ball. Any amount of energy can be 
transferred to the ball that is hit. The cue ball may come to a dead stop 
in one direct, head-on collision where it transferred all its energy to 
the ball it hit. The cue ball may also glance off at an angle with only 
some of its energy lost. Since the neutron-as-a-cue-ball has a weight of 
one, the greatest energy transfers will occur with nuclei-as-balls having 
a very low atomic weight. The lightest is normal hydrogen, atomic weight 
1, and it is the most effective material to stop neutrons. The light 
elements are all effective. So water, plastic, wood, parafin, and light 
elements such as lithium and boron are very good neutron absorbers. 
Still, it may take many inches, or even feet, of these materials to 
absorb all the neutrons from a nuclear reactor or from a radioactive 
source that emits neutrons. If you placed a neutron detector on top of 
the table, and had a neutron source under the table, there would probably 
be a measurable amount of neutrons coming through the table.

If you would like to read some more about this, some references are

William Ehmann and Diane Vance, "Radiochemistry and Nuclear Methods of 
Analysis", pages 173 to 175, Wiley Interscience, New York, 1991

On the Internet, there is a question and answer format website from the 
Health Physics Society (their information is reviewed for accuracy):
Radiation Basics-Neutrons: This covers History, Properties, and the 
Billiard Ball Model.
The paragraph on shielding and interactions of neutrons is very good.
This explains what happens after the neutron is slowed down; how may be 
captured by a nucleus, making that nucleus radioactive.

To see how neutrons interact differently from electrons or x-rays, go to:
This site has some very good pictures taken using neutrons, and also has 
a good neutron basics section.

Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2003. All rights reserved.