Download Radiation and Radioactive Decay

13.1 Radiation and Radioactive Decay
Physics Tool box
 Alpha decay occurs when an unstable nucleus emits a particle, often denoted
as
4
2
He , which consists of two protons and two neutrons. The resulting
daughter nucleus is of a different element and has two protons and two
neutrons fewer than the parent.
 decay, a neutron is replaced with a


proton and a  particle (a electron). In  decay, a proton is replaced with

a neutron and a high speed  (a high speed positron).
 Beta decay assumes two forms. In
 Gamma decay is the result of an excited nucleus that has emitted a photon
and dropped to a lower state.
Henri Becquerel, in 1896, discovered that an ore containing uranium had the ability to
darken a photographic plate even when the plate was completely covered and protected
from light. This process was unaffected by physical treatments such as heating and
cooling, and is now known as a fundamental property of atoms, called radioactivity. It
was the nucleus of a radioactive element that emits radiation as it decomposes
(decays).
It was Rutherford (at McGill university in Montreal), revealed that radioactive emissions
to be of three types:
 Alpha particles ( )
 Beta particles ( )
 Gamma rays ( )
Alpha Decay
The alpha particles were found to consist of two protons and two neutrons. Since the
nucleus of the most abundant isotope of Helium consists of these particles, the 
particle is often denoted as
4
2
He (the Helium nucleus). When a radioactive material
emits an  particle, the nucleus of one of its atoms loses two protons and two
neutrons. The loss of the two protons changes the atom from one type of element to
another (transmutation). Smoke detectors use this process to help save lives.
If an atom of element X transmutes to element y by emitting an
a nuclear reaction has taken place.
A
Z
 particle, we say that
X  AZ42Y 24 He
Where A is the atomic mass number (the number of particles, or nucleons, in the
nucleus) and Z is the atomic number (the number of protons).
Example
Given the following reaction,
218
84
Po  X 24 He , determine X
Solution:
218
84
Po  X 24 He
218
84
Po 
2184
84 2
218
84
Po 
214
82
X 24 He
X 24 He
Looking at a periodic table, with atomic number 82, is lead (Pb).
 particles emitted in a nuclear decay have the ability to penetrate about 5cm of air (a
few sheets of paper), thus must possess some form of kinetic energy. Now since kinetic
energy is a conserved quantity, you must ask,” where dose this kinetic energy come
from?”. It takes the theory of binding energy and the strong nuclear force to answer
that question.
Protons (positive charges) within a nucleus repel each other through electric force.
Therefore for a nucleus to contain protons, there must exist a force stronger than the
nuclear force to hold the protons together. This force is called the strong nuclear
force. This force is only effective over short distances (or order of 1.5 
10 m , the
radius of a small nucleus). Since the strong force is attractive, work must be done to
break the nucleus apart.
15
Binding Energy
Experiments have shown that the mass of an atomic nucleus is always less than the
sum of the masses of its constituent protons and neutrons. The mass difference in
2
terms of energy (E=mc ) is called the binding energy and it illustrates that positive
work would have to be done to break the nucleus apart.
The above graph shows the approximate binding energies per nucleon for all the
elements. You notice that the binding energy is at a maximum at a mass around 56
(which is the case for iron) and decreases steadily afterward. By looking at the curve
we notice that the nuclei near the middle of the period table are held together more
strongly than other nuclei.
Why is it that the middle elements have the highest binding energies? The nucleus is
the battleground between the strong nuclear (only active at a distance of a small
nucleus) and the electromagnetic forces. As you move up the period table toward iron,
you find that the nuclei have more nucleons, all pulling toward each other by the strong
nuclear force, thus the binding energy increases. As we pass iron, we again have even
more nucleons, however the distance between them is increasing and the mutual
attraction due to the (short-range) strong nuclear force is weakened. But the proton
pairs are still feeling the electromagnet repulsion force. As we approach elements such
as radium, polonium, and uranium, the binding energy is so low that the nucleus is not
stable at all. When strained to the breaking point, two protons and two neutrons form
inside the nucleus and it gets released as an  particle with kinetic energy.
Beta Decay
The beta particles,
, emitted in some forms of radioactive decay may be negatively
charged electrons or positively charged electrons, called positrons (anit-matter). When
 decay releases electrons, it is called  decay; when it releases positrons, it is

called  decay.
the
The
 particle is symbolized as
decay is more common than
0
1
e if it is an electron and
0
1
e if it is a positron. 
 decay.
It is important to realize that the electron released in Beta decay, does not come from
the electrons orbiting the nucleus, but rather from inside the nucleus via one of the
following reactions.
1
0
n  11 p 01 e e
energy 11 p  01 n 01 e e
Where
e is an anti-neutrino, and e is a neutrino.
The both the neutron decay into a proton and the proton decay into a neutron is
governed by the weak nuclear force.
Note: please think that a neutron is just the combination of a proton and a electron. It
is more complicated and a complete explanation requires the understanding of
quarks (see a section 13.4)
Example
An atom of potassium-40 can transmute into an atom of some other element by
emitting a
 particle. Represent this reaction in symbol, and identify the daughter
element.
Solution:
40
19
K  2040Ca 10 e , the element is Calcium.
Gamma Decay
Unlike alpha and beta decay,
decay results in the production of photons that have
zero mass and no electric charge. This decay is believed to occur when a highly excited
nucleus drops to a lower energy state. If the reaction only produce a gamma decay,
then no transmutation takes place (element remains the same).
Gamma decays typically occur simultaneously with alpha and beta decays.
Pb 
211
82
The
Bi 10e 
211
83
photons (gamma rays) are similar to x-rays except they have a higher frequency
(in reality the frequency ranges for the x-rays and gamma ray overlap) and thus higher
energy.
Anti-matter
We have seen that in some reaction we have created a particle called an anti-neutrino.
You may ask how does the neutrino and the anti-neutrino compare. The answer come
from our current model which holds that every particle has its own antiparticle (in a few
cases, the particle is its own antiparticle). The antiparticle is like the mirror image to the
particle.
The concept of antiparticle comes from the Diracs solution of Energy and mass with
E 2 m2 c4
E mc 2
It was the mc which indicates that a particle can have negative energy. The presence
of these particle was confirmed in the early 1930’s.
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