Transcript Document

Before the Gamma Radiation, there was...
The beginning:
The strong nuclear force (= strong force) is one
of the four basic forces in nature (the others
being gravity, the electromagnetic force, and the
weak nuclear force). As its name shows us, it is
the strongest of the four. But, it also has the
shortest range, meaning that particles must be
extremely close before it performs its effects.
Its main job is to hold together the the
subatomic particles of the nucleus = the protons
+ neutrons = the nucleons.
We have learned, previously, that like charges
repel, and unlike charges attract.
R
R
A
If you consider that the nucleus of all atoms except H
contain more than one proton in them, and each proton
carries a postive charge, why do the nuclei of these
atoms stay together? The protons must feel a repulsive
force from the other neighboring protons.
This is where the strong nuclear force comes in.
The strong nuclear force is created between
nucleons by the exchange of particles called
mesons (chargeless hadrons made up of 1 quark
and 1 antiquark). This exchange is like constantly
hitting a ping-pong ball back and forth between
two people. As long as this meson exchange can
happen, the strong force is able to hold the
participating nucleons together.
The nucleons, though, must be extemely close together
in order for this exchange to happen. The distance
required is about the diameter of a proton or a neutron.
If a proton or neutron can get closer than this distance
to another nucleon, the exchange of mesons can occur,
and the particles will stick to each other. If they can't
get that close, the strong force is too weak to make
them stick together, and other competing forces (usually
the electromagnetic force) can make the particles move
apart.
Beyond
barrier: SNF
present
In the case of
approaching protons, the
closer they get, the
more they feel the
repulsion from the other
proton As a result, in
order to get two protons
close enough to begin
exchanging mesons, they
must be moving
extremely fast
(which means the temperature must be really
high), and/or they must be under very high
pressure so that they are forced to get close
enough to allow the exchange of meson to create
the strong force.
The nuclear force is independent from charge,
which means two protons attract each other the
same rate as 2 neutrons or a proton and a
neutron. Once the electrostatic barrier is passed,
the repulsion force is far too little compared to
the strong nuclear force to show its effect
anyway.
One thing that helps reduce the repulsion
between protons within a nucleus is the presence
of any neutrons. Since they have no charge they
don't add to the repulsion already present, and
they help separate the protons from each other
so they don't feel as strong a repulsive force
from any other nearby protons. Also, the
nucleus is tightly packed so that nucleons can
exchange mesons easily. This way, a nucleus is
not destroyed.
When the mass of a nucleus, for example
4 He is measured, and when the mass of
2
the nucleons of that nucleus, for this case
2 neutrons and 2 protons, are measured
seperately outside of the nucleus, which one
do you think was heavier?
The mass of a nucleus is always less than the sum
of the individual masses of the protons and
neutrons which make it up. When forming a
nucleus, the nucleons transform some of their
masses into the form of energy. The nuclear
binding energy can be measured by Einstein’s
favourite formula;
Nuclear binding energy = Dmc2
Where Dm is the difference between the masses
of individual nucleons and the nucleus.
The stability of a nucleus depends mainly on A, the
mass number and Z, the atomic number. Up to the
mass number 30 or 40, a nucleus has approximately
the same nb. of neutrons and protons to be stable.
Bigger nuclei must have more neutrons than protons
since as Z gets bigger, repulsive forces get bigger.
When nucleus gets big enough, no neutron is enough
to keep it stable. After, Z= 82, no nuclei is stable.
Such unstable nuclei are radioactive, which means
they undergo radiations in order to become stable.
A nucleus having very much protons compared to
neutrons will never be stable, yes, but this does
not mean that a nucleus with many neutrons and
little protons will be stable. To understand this
we may look at this graph, also present in our
holy book Zumdahl:
The changing of one element to another to
become more stable through radioactivity is
transmutation. It can occur by alpha or beta
radiation. (or else some other nuclear reactions
such as nuclear bombardment but I will not deal
with it now)
A gamma ray is simply a high energy photon – a pack
of energy-. It is chargeless, pure energy. It has no
mass as well.
After alpha or beta decay, a nucleus is often
left in an excited state -that is, with some
extra energy. It then "calms down" by
releasing this energy in the form of a very
high-frequency photon, or electromagnetic
wave, known as a gamma ray.
After a decay reaction, the nucleus is often in an
“excited” state. This means that the decay has resulted
in producing a nucleus which still has excess energy to
get rid of. So, the emission of gamma rays is a way for
a high energy nucleus to reduce its energy and become
more stable. (This is due to one of the 2 universal driving
forces, the tendency of minimum energy)
As, or if, you have noticed, i have mentioned
that the nucleus is left in an excited state, and
it returns to the ground state by emitting pure
energy, gamma rays. But how, how can a nucleus
be in an excited state?
•It may occur because of a violent collison with
another particle- changing the arrangement of
nucleons
•Or more commonly and more related to our
business, the nucleus, after a previous
radioactive decay may remain in an excited
state.
You may have seen that what I have written
seems to suggest that there are energy levels in a
nucleus, just like the shells of electrons. Just like
an atom, a nucleus itself can be in an excited
state, and, when jumping down to a lower state it
emits a photon. This can be explained by:
Previously on this slide show:
(***Talking about energy levels in nucleus***)
(...)this can be explained by:
THE NUCLEAR SHELL MODEL
Although not yet clearly explained, it is suggested
that the nucleons exist in an interacting, many-body
system, and that each nucleon moves in an average
field created by all other nucleons. The motion of
each nucleon is governed by the average attractive
force of all the other nucleons. The resulting orbits
form "shells," just as the orbits of electrons in atoms
do.
Yet going on with the explanation...
For nuclei to be stable, there are some “magic
numbers”. These are the numbers of neutrons and
protons in a nucleus. If a nucleus has that much p or
n, it is found to be more stable than the others.
This numbers are usually even, for symmetry.
(symmetry provides strength in bounds and thus
stability.)
These magic numbers are:
For protons: 2, 8, 20, 28, 50, 82.
For neutrons: 2, 8, 20, 28, 50, 82, 126.
Do you remember Noble Gases?
They contained the number of electrons that were
completely filling an electron shell. Since the shell
was completely filled, they were not active for
reacting chemically, thus were called STABLE.
The magic numbers for the nucleus is just like that!
Nucleons at that numbers are thought to fill a
nuclear shell completely, thus, the nucleus with filled
shells are more stable.
When a nucleus of an atom undergoes a nuclear
reaction (or a collision), at the end of that reaction,
its nucleons can be disorganized. They can be
arranged at shells so that they have excess energy.
A nucleon can stay at a higher shell, although, say,
there is a space at the lower shell. The nucleus
rearranges these particles to, as much as possible,
completely fill its shells (The lower ones first then
the higher ones). By this filling, and jumping down
process, the nucleus EMMITS THE EXCESS
ENERGY, just like an atom with an excited electron
emmiting energy when the electron jumps to a lower
level of energy. This energy given off is ultraviolet
radiation when an atom goes to a lower energy state,
and the energy given off when a NUCLEUS is going
to a lower energy state is called, guess what,
GAMMA RADIATON!!!!! (phew, hardly made the
connection)
If you have questions regarding the previous
slides, please ask now since although I felt I
should explain the nuclear shell model and tried, I
couldn’t and now I will probably be unable to
answer any of your questions properly, but to my
favor I would like to point out that the nuclear
shell model is not yet truly accepted or clearly
explained even by scientists. It is a strong
theory, though, in my opinion, because with the
even numbers it also suggests SYMMETRY.
Previous knowledge:After a nuclear interaction,
when the nucleus of the reactant had undergone a
beta or an alpha radiation, the nucleus still has
excess energy. Instead of having another alpha or
beta radiation, the nucleus gives out the excess
energy in the form of gamma rays. So gamma rays
frequently accompany natural decay reactions and
particle reactions.
The presence of gamma decay is favoured by the theory that
energy is quantized in atomic level; that is: Energy is given
off in discrete amounts called quanta. Instead of giving off a
high amount of energy at once, it is more probable and easy
for an excess energy to be given in steps. So, a nucleus,
instead of giving off its whole excess energy at once by beta
or alpha, gives some by beta or alpha and the rest by gamma.
This way, to rearrange its particles after giving off energy is
much easier.If the nucleus had given off all its energy at
once, rearranging the nucleons would have been harder,
nuclear orbits would be shuffled a lot as to cause hardships
reorganizing. Plus, the kinetic energy of a beta particle, an
antineutrino or an alpha particle may not be as high as for the
nucleus to give off all its excess energy, an additional particle
with high kinetic energy may be needed.

0
0
How can
5
12B
decay to the ground state of
6
12C?
Right path- most probable
Left path
–least
probable
1 megaelectron volt = 1.60217646 × 10-13 Joules
Gamma Decay of He-3
Dysprosium
In gamma decay a nucleus changes from a
higher energy state to a lower energy state
through the emission of electromagnetic
radiation (photons).
•Gamma photons have no mass and no electrical
charge-they are pure electromagnetic energy.
•Because of their high energy, gamma photons
travel at the speed of light
•Their wavelength is short and frequency high
showing they are really fast and of high energy.
•The number of protons (and neutrons) in the
nucleus does not change in this process, so the
parent and daughter atoms are the same
chemical element.
•Highly concentrated gamma-rays can kill living cells
High-energy radiation kills
cells by damaging their
DNA, thus blocking their
ability to grow and
increase in number.
•In cancer treatments, focused gamma rays can be used
to eliminate malignant cells, known as radiotherapy
•Needs a lead block or a thick concrete block to be
stopped. (Lead has a high density and it is not
radioactive.)
•Has weak ionizing property (no charge no
mass)
Radiation
that falls
within the
“ionizing
radiation"
range has
enough
energy to
remove
tightly
bound
electrons
from
atoms, thus
creating
ions.
An example of a nuclear interaction that
results with gamma emission:
A gamma ray is released to lower the energy state of
Thorium. As seen, the atomic and mass numbers of
Thorium stays the same, only on the right side of the
equation it is more stable.
Protactinium
1) Total Nucleon Number (TOP VALUES) =Total
number of protons and neutrons
2) Total electric charge (BOTTOM VALUES)
Are kept the same.
Type of
radiation
emitted &
symbol
Nature of the
radiation
(higher only)
Nuclear
Symbol
(higher only)
Penetrating power, and what will
block it (more dense material, more
radiation is absorbed BUT smaller
mass or charge of particle, more
penetrating)
Ionising power - the ability to remove
electrons from atoms to form positive ions
a helium nucleus
of 2 protons and 2
neutrons, mass =
4, charge = +2
Low penetration, biggest mass
and charge, stopped by a few cm
of air or thin sheet of paper
Very high ionising power, the biggest
mass and charge of the three
radiation's, the biggest 'punch'!
high kinetic
energy electrons,
mass = 1/1850,
charge = -1
Moderate penetration, 'middle'
values of charge and mass, most
stopped by a few mm of metals
like aluminium
Moderate ionising power, with a
smaller mass and charge than the alpha
particle
very high
frequency
electromagnetic
radiation, mass =
0, charge = 0
Very highly penetrating, smallest
mass and charge, most stopped
by a thick layer of steel or
concrete, but even a few cm of
dense lead doesn't stop all of it!
The lowest ionising power of the three,
gamma radiation carries no electric
charge and has virtually no mass, so
not much of a 'punch' when colliding
with an atom
Alpha
Beta
Gamma
Question 1:
What is the nature of a gamma ray?
Answer
Question 2:
What is the mass of a gamma ray? Compared to
alpha and beta particles, therefore, is it more or
less energetic?
Answer
Question 3:
What is needed to stop the penetration of a
gamma ray?
Answer
Question 4:
Is gamma ray a short or a long range force?
Answer
Question 5:
Does the parent nucleus change into a different
element nucleus during gamma radiation?
Answer
Question 6:
Give the magnetic (electrical) difference
between alpha, gamma and beta
Answer
Question 7:
The sum of the values at the top and the bottom are
the same in a radioactive decay reaction. What are
these? The Atomic Number at the bottom and the Mass
Number at the top?
Answer
Question 8:
What happens to the parent nucleus when it undergoes
gamma decay?
Answer
Question 9:
Tell a use of the gamma rays that we have learned
Answer
Question 10:
What is transmutation? Does it occur in an individual
gamma decay?
Answer
Question 11:
The free nucleons or the nucleus consisting of the
nucleons, which has greater mass? How does the force
between nucleons in a nucleus arise?
Answer
Question 12:
Is the strong nuclear force a long or a short range
force? What about the electrical force? Tell how
these two fight in a nucleus and say which overcomes.
Answer
Question 13:
Explain the theory of how the attraction force
between two nucleons is attained in the nucleus.
Answer
Answer 1:
Gamma rays are high energy photons, they are
chargeless pure energy
Back
Answer 2:
Massless gamma rays have higher energy than alpha
and beta. Alpha and Beta are particles with high
kinetic energy, but gamma itself IS energy.
Back
Answer 3:
A lead block or a thick concrete wall
Back
Answer 4:
It is a very long range force, it is effective in
great distances and has high penetrating ability
Back
Answer 5:
No, the parent nucleus stays the same, it only
gives off excess energy and becomes more
stable.
Back
Answer 6:
Alpha: positively charged (+)
Beta: negatively charged (-)
Gamma: Chargeless
Back
Answer 7:
Nooo! Neither mass nor number of
protons is necessarily conserved in a
nuclear reaction. What is conserved is
the nucleon number (top) and the
electrical charge (down)
Back
Answer 8:
A parent nucleus that undergoes gamma decay
gives off the remaining excess energy after a
nuclear reaction and jumps to a lower energy
state,becoming more stable,hopefully entering the
zone of stability. (nuclear shell model)
Back
Answer 9:
Since gamma rays can kill living cells, we focus
them to kill malignant tumour cells! (The enemy
of my enemy is my friend)
Back
Answer 10:
Transmutation is the changing of one element to
another to become more stable through
radioactivity. It can occur by alpha or beta
radiation but NOT gamma radiation, since we have
learned, hopefully, that gamma radiation is only
related to energy and does not change the number
of nucleons.
Back
Answer 11:
The free nucleons have greater mass. The
difference is the measure of the nuclear binding
energy, nucleons forming a nucleus lose some of
their mass into energy for binding.
Back
Answer 12:
Back
Strong nuclear force: Short ranged
Electrical force: Long ranged
Although Electrical force is effective in greater
distances and the protons in the nucleus would tend
to repel each other, the strong nuclear force is
greater than electrical force. Once a nucleon passes
into the region where SNF is effective, the 2
nucleons stick together, overcoming the repulsive
force between (+)ly charged protons. Before passing
into the effective SNF region, the closer the 2
nucleons get, the greater the repulsive force,
however this fact can be eliminated since the
particles move very fast and quaickly enter the SNF
region.
Answer 13:
The nucleons are bound to each other by
constant particle exchange, these very small
particles are called mesons, and as long as
meson exchange goes on, the attractive nuclear
force goes on. As if you are playing volleyball
with a friend and you are bound until one of you
quits or the ball falls.
Unanswered Questions :(
•How do neutrons help keep the nucleus together?
What is the gluing effect of neutrons?
•If the neutrons help keep nucleus together, why,
in beta decay, a neutron is transformed into a
proton?
•Giancoli Physics Text Book
•Zumdahl Chemistry Text Book
•Student Presentations: /inspired by Negehan Demirci’s Nuclear
Stability presentation.
•http://www.arpansa.gov.au/basics/gamma.htm
•http://www.revision-notes.co.uk/revision/917.html
•http://www.lbl.gov/abc/wallchart/chapters/06/1.html
•http://www.orau.gov/reacts/gamma.htm
•http://www.ndted.org/EducationResources/HighSchool/Radiography/nuclearreact
ions.htm
•http://www.chm.bris.ac.uk/webprojects2002/sidell/
DECAY.htm
•http://www.phy.uct.ac.za/courses/phy300w/np/ch1/
node50.html#SECTION00045100000000000000
•http://physics.uoregon.edu/~courses/dlivelyb/ph161
/L25.html#Radioactive_decay
•http://hyperphysics.phyastr.gsu.edu/hbase/forces/funfor.html
•http://64.233.187.104/search?q=cache:2Csg7odpZr
UJ:www.westrain.org/documents/84/NP3_12_2003.doc+radioactive+decay+reactions&hl=tr
•http://aether.lbl.gov/www/tour/elements/stellar/str
ong/strong.html
•http://physics.bu.edu/py106/notes/RadioactiveDecay.
html
•http://www.halexandria.org/dward472.htm
•http://www.epa.gov/radiation/understand/ionize_nonio
nize.htm
•http://www.epa.gov/radiation/understand/gamma.htm
•http://acept.la.asu.edu/PiN/mod/light/colorspectrum/
gamma.html
•http://imagers.gsfc.nasa.gov/ems/gamma.html
•http://library.tedankara.k12.tr/chemistry/vol2/nucle
ar%20stability/z216.htm