Transcript RADIOACTIVE DECAY

RADIOACTIVE DECAY
Berçin Kutluk
Alpha Decay
Why do elements undergo radioactive decay?
Some nuclei are stable, while others undergo radioactive
decay. How do we distinguish one from the other?
The answer lies in conservation of energy. A nucleus will decay
if there is a set of particles with lower total mass (a lower
mass will mean less energy and a nucleus having less energy is
more stable) that can be reached by decay process or simply
by fission, a process in which a massive nucleus splits into two
less massive ones.
The mass of a nucleus is determined by the sum of the
energies of all its constituents. The energies of the
constituents depend on their masses, their motion, and their
interactions (a process in which a particle decays or it
responds to a force due to the presence of another particle, as
in a collision).
and energy conservation
In alpha-decay an atom ejects an alpha particle, which is simply a helium atom without any
electrons. In doing so the parent atom decays into a lighter particle. An example of this is a
uranium-238 atom decaying into into a thorium-234 atom and an alpha particle (helium-4
nucleus, i.e. 2 protons and 2 neutrons). A schematic diagram illustrates this:
This type of decay occurs naturally in uranium and is an example of "spontaneous decay".
The uranium atom doesn't just break apart. As it decays, each of the two resulting
elements (the thorium and α-particle) fly apart at high speed. In other words they both
have kinetic energy.
It is possible to measure the mass of the original uranium atom and the masses of the two
resultant particles. This is done by measuring the momentum of each particle as it strikes
a sensor. When these measurements are taken it is found that the total mass of the two
smaller particles is less than the mass of the original uranium particle. Some mass must
have been turned into (mostly kinetic) energy.
Energy Changes in Alpha Decay
In alpha decay, the atomic number changes, so the original (or parent) atoms
and the decay-product (or daughter) atoms are different elements and
therefore have different chemical properties.
In the alpha decay of a nucleus, the change in binding energy - amount of
energy that must be supplied to break an atomic nucleus
into its component fundamental particles appears as the kinetic energy of the
alpha particle and the daughter nucleus.
Because this energy must be shared between these two particles, and because
the alpha particle and daughter nucleus must have equal and opposite momenta
because of the law of conservation of momentum, the emitted alpha particle
and recoiling nucleus will each have a well-defined energy after the decay.
Because of its smaller mass, most of the kinetic energy goes to the alpha
particle. To express it more concretely, we know that momentum is the product of
mass and velocity of an object. Since alpha particle has a smaller mass, it
will take the larger energy – and therefore have higher speed while the
decayed element is heavier and will move slower. The momentum at the
beginning is zero – assuming that the vibrational energy of atoms is minimum
and because of this balance, it will be the same again.
ZONE of NUCLEAR STABILITY
As you move higher in atomic number on the periodic table you
find that the number of neutrons increases much faster than the
number of protons in stable elements. If these elements have too
many neutrons they are said to be "heavy". Neutrons do stabilize
the nucleus (and thus the atom) blocking the interaction between
protons (protons repel each other at a certain distance because of
Coulomb repulsion force). Too many (or too few) protons though,
makes the atom unstable and that is when, in some cases, an alpha
particle is emitted. By emitting an alpha particle the atom is able
to increase the stability by reducing the ratio of protons to
neutrons. This can be shown simply by saying that a nucleus has 60
protons and 80 neutrons (purely hypothetical). The ratio initially is
0.75 (60/80) but when an alpha particle is emitted the ratio
becomes 0.74 (58/78).
The aim of the elements is to enter the stability zone,
as in the decay of polonium, in the next example.
Too much/few protons make the atom unstable and the
atom emits an alpha particle to become stable by
reducing the proton/neutron ratio. 210
Po84 84/210 = 0.667
The polonium nucleus has 84 protons and 126
206Pb
82/124, or 0.661
neutrons. The ratio of protons to neutrons is
82
Z/N = 84/126, or 0.667. A 206Pb nucleus has 82
protons and 124 neutrons, which gives a ratio
of 82/124, or 0.661. This small change in the
Z/N ratio is enough to put the nucleus into a
more stable state, and as shown in Figure,
brings the "daughter" nucleus (decay product)
into the region of stable nuclei in the Chart of
the Nuclides.
EXPRESSING IT AS AN EQUATION
Alpha decay can be expressed as an equation in which an
element X decays to Y, emitting an alpha particle and a
discrete energy, in millielectronvolts:
AX
Z
 A-4YZ-2 + α
210Po
84

206Pb
82
+ 4He2 + 3,5MeVα
A small note: As we can see from the decay of
polonium, an element becomes stable when it
decays into lead. Pb is in fact the heaviest
element that is stable. However, elements lighter
than Pb can also decay due to an unbalance in
proton/neutron ratios.
Alpha Decay
&the Strong Force
What are strong forces?
The nucleus of helium
contains two protons. They
are both positively charged
and will repel each other. So
why don’t protons go flying
out the atom and stay bound
in a helium nucleus? There
must be another force that
holds them together.
This force
is the
Strong
Nuclear
Force.
What kind of a force is it?
The strong nuclear force binds
protons and neutrons together to
make the nucleus.
The strong nuclear force is actually a
force between quarks and is carried
by particles called gluons – which are
force carriers. Protons and neutrons
are made of quarks and they feel the
strong nuclear force as well.
Protons
would fly
out of the
nucleus due
to repulsion
if there was
no strong
force
Think of a force carrier like this. I have a stick in my hand
and push my friend with that stick. He/she will fall down –
because the force I exerted does work. This force is
transferred – or “carried” – by the stick. The stick is a force
carrier.
“CARRY”ing strong nuclear force
They are called gluons because they “glue” the quarks together.
Gluons are massless, travel at the speed of light, and possess a property
called color. Analogous to electric charge in charged particles, color is of
three varieties, designated as red, blue, and yellow, and analogous to
positive and negative charges - three anticolor varieties. Quarks change their
color as they emit and absorb gluons, and the exchange of gluons maintains
proper quark color balance.
Unlike other forces, the force between quarks increases as the distance
between the quarks increases. Up to distances about the diameter of a
proton, quarks behave as if they were free of one another, a condition called
asymptotic freedom. As the quarks move farther apart, the gluons that move
between them utilize the energy that they draw from the quark's motion to
create more gluons...the larger the number of gluons exchanged among
quarks, the stronger the binding force. The gluons thus appear to lock the
quarks inside the elementary particles, a condition called confinement.
Gluons can also bind with one another to form composite particles called
glueballs.
But if the strong force is so “strong”,
how does an alpha particle get free of
the nucleus. The force carriers – the
gluons – get even stronger at long
distances. Therefore, it seems
impossible for an alpha particle to
detach itself from the strongly packed
nucleus....
The RANGE of strong forces...
SF has a very short range (about 10-15 metres - the size of the
nucleus. And the attraction of quarks is at proton and neutron level
(it is predictable that when neutrons are too much the balance is
disturbed)
- Because of this, only very close to the nucleus can the proton feel
its attraction. The range of the electric forces (which are pushing
out) are much bigger.
- If a sudden position change in nucleus occurs, the balance
between these forces
İs upset and the elctric force dominates; activating the protonproton repulsions and causing a part of the nucleus to break
free.
- In fission, for example, the nucleus can break more or less in half.
But WHY helium?
The "binding energy" of a particular isotope is the amount of
energy released at its creation; you can calculate it by
finding the amount of mass that "disappears" and using
Einstein's famous equation. The binding energy is also the
amount of energy you'd need to add to a nucleus to break it
up into protons and neutrons again; the larger the binding
energy, the more difficult that would be.
It happens that the 4He nucleus is held
together exceptionally tightly--it has a much
larger binding energy than other light nuclei.
This makes alpha particles the easiest type of
clump to spit out.
Breaking out of the nucleus...
Particles can appear in places where energy can't move them, for
example, water in a cup does not have the energy to push itself up
over the lip of the cup. However, in the microscopic world, particles
can appear in places where energy can't take them. For instance,
the alpha particle of the element radium can move itself away from
the atomic nucleus and through the outside of the nucleus – we
know that this is alpha decay itself.
For particles, the surface can appear to be like a wall. Particles in
an atomic nucleus do not have enough energy to break through
surface tension. However, particles do have wave energy and they
use this energy to break through surfaces and exit the core. As the
process appears like the particle has traveled through a tunnel in a
mountain, it is given the name "tunnel effect."
...the tunnel effect.
Alpha-Decay Theory confirmed the mysteries of
quantum mechanics. When two objects
approach each other their atoms touch at the
point closest to the other object. At that
moment, the nature of the electrons
surrounding the objects' atoms are slightly
changed, each taking on some characteristics of
the other – such as energy. By electromagnetic
repulsions, the alpha particle can break free of
the nucleus and its strong force. This is known
as the tunnel effect and was first proposed by
George Gamow, the originator of the Big Bang
theory, in 1928.
How deep can they go?
Once the alpha particle is free from the tight grip of the nucleus you
may wonder how long does it last on its own? How deep can it
penetrate into say human skin?
These are important questions when dealing with radiation.
Alpha particles usually have energies ranging from 4MeV to
10MeV. This amount of energy is not sufficient enough to
even pass through paper. To compare to some of the other
types of radiation, beta decay can emit a particle
(electron) that has an energy of 200MeV which could pass
through 17cm of aluminum and gamma decay emits a a
photon of energy that could probably pass a meter or more
into the aluminum. Thus it can be said that alpha particles
are of relatively weak energies compared to other types of
radiation...
SPONTANEITY OF ALPHA-DECAY
-Alpha decay is an exothermic process. As the
nucleus becomes more stable, it liberates a net
amount of energy due to this. Therefore, the
tendency for minimum enthalpy is fulfilled.
-Also, the decay’s entropy can be said to be
positive; the places the particles can exist
increase (i.e. the micro states increase). A clump
from the nucleus is released, making the number
of particles two. Particles accelerate suddenly to
conserve their momentum – randomness per unit
time increases. Therefore, alpha decay also
satisfies the tendency or maximum randomness.
Under these conditions, it can be said that
Alpha decay is always spontaneous.
SMOKE DETECTORS
...an interesting use of alpha radiation
One interesting use of alpha decay can be found in
smoke detectors, which seems like an unlikely place.
Smoke detectors contain the radioactive isotope
americium-241 which was obtained from the decay of
plutonium. Alpha particles from the americium collide
with oxygen and nitrogen particles in the air creating
charged ions. An electrical current is applied across the
chamber in order to collect these ions. When there is
smoke in this chamber the alpha particles are absorbed
by the smoke. This lowered the number of ions in the
air and thus the electrical current is reduced setting of
the alarm. Americium-241 is quite safe because the
alpha particles usually travel only a few centimeters in
air.
THE SMOKE-DETECTOR
Figure shows a simplified set up of a
smoke detector (not including the
alarm mechanism and I'm sure some
other important
pieces).
Rutherford – the first successful
alchemist
• Transformed N to O
– 14N + a  17O + 1H
• Why wouldn’t it be practical to convert Pb
into Au?
• The conversion of platinum into gold has
been achieved by bombarding platinum-198
with neutrons to produce platinum-199.
This isotope, in turn, decays to gold-199
with the loss of a subatomic particle. What
subatomic particle is lost by the platinum as
it becomes an atom of gold?