1. This Week Atoms and nuclei What are we made of? The periodic table
Why does it stop?
How were the elements made?
Radioactive decay
Useful but can be toxic
Discovery of X Rays: Cathode Rays and TV sets
Medical imaging
Limit your dose
9/17/2018
Physics 214 Spring 2015

2. The basic building blocks
The basic building blocks that make up atoms are
charge mass
Proton x x 10-27kg
Neutron x 10-27kg
Electron -1.6 x x 10-31kg
Photon E = hf p = h/λ
h = Planck’s constant = x 10-34J.s
9/17/2018
Physics 214 Spring 2015

3. The nucleus
The nucleus was found by Rutherford in scattering charged particles from a gold foil.
He observed that occasionally the charged particle was deflected through a large angle indicating it had “hit something hard”
The nucleus of any atom is composed of protons and neutrons. The protons have positive electric charge so repel each other. If they are close enough the strong force binds them together and the nucleus is stable
9/17/2018
Physics 214 Spring 2015

4. Atoms
Atoms are made up with a central nucleus of protons and neutrons surrounded by a number of electrons equal to the number of protons.
The notation we use is 2He4
2 is the atomic number = number of protons (and electrons)
4 is the mass number = number of protons + neutrons
Note atomic mass is the actual mass of the nucleus in atomic mass units with Carbon set to be 12 units and atomic mass in general is not an integer
H2O gives the number of atoms required to make a molecule
water = 2 Hydrogen plus 1 Oxygen
In a nuclear reaction energy, charge, and atomic mass are conserved.
Atoms can decay
Atoms can join together to form new atoms or molecules
Chemistry is determined by the electrons
9/17/2018
Physics 214 Spring 2015

5. Isotopes
For a given number of protons there is a nucleus that is most stable for a particular number of neutrons.
Isotopes are when for the same number of protons the number of neutrons is different from the most stable configuration.
Since the number of electrons is the same the chemical properties are “identical” but the nucleus can be unstable.
The larger the difference between ideal and the isotope the more unstable the nucleus becomes.
Nuclei have been made artificially that do not exist in nature.
9/17/2018
Physics 214 Spring 2015

6. Radioactive decay
The general rule is that if a lower energy configuration exists for the same set of particles then they will try to change into that configuration.
Each radioactive element has a characteristic half life, that is the time for half the sample to decay. Radioactive elements are clocks which tick at a specific rate.
The amount remaining after N half lives is ½ x ½ x ½ ………..
Half lives vary from billions of years to a fraction of a second.
Those in the range of greater than thousands of years are used to date objects on earth back to it’s formation 4.6 billion years ago
C years I million
U billion
9/17/2018
Physics 214 Spring 2015

7. Types of nuclear decays
A proton can change into a neutron and emit a positron (anti particle of electron)
1e0
Atomic number decreases by mass number is the same
A neutron can change into a proton and emit an electron
-1e0
Atomic number increases by mass number is the same
A nucleus can emit an alpha particle which is 2 neutrons and 2 protons (actually a helium nucleus)
2He4
Atomic number decreases by 2 mass number decreases by 4
Photons can be emitted
No change in Atomic number or mass number.
9/17/2018
Physics 214 Spring 2015

8. What is the daughter?
When an element decays it usually changes to a new element.
The notation we use is 3Li7 where 3 is the number of protons, called the atomic number, and 7 is the number of protons plus neutrons, called the mass number.
Any decay or nuclear reaction conserves charge and mass number.
2He4 + 7N14 = 8O17 + 1H nuclear reaction
88Ra226 = 86Rn He nuclear decay
9/17/2018
Physics 214 Spring 2015

9. Energy and atoms
When we make atoms from the constituents energy is released. The sun’s energy comes from fusion E = Mc2
When we make molecules by joining atoms energy is released. Burning is Carbon plus oxygen making CO2
Electrons in atoms have specific energy levels
When an electron is removed from an atom the atom will capture an electron and emit photons of specific energies
When many atoms are emitting photons we get spectral lines, that is electromagnetic radiation of specific wavelengths and colors. Each atom has it’s own “fingerprint”
9/17/2018
Physics 214 Spring 2015

10. Waves and particles
Particles have both discrete properties such as charge and energy but also wave properties.
λ = h/p and f = E/h
The theory which explains behavior at small distances is called quantum mechanics. Electron orbits in atoms are given by a three dimensional probability distribution.
Particle beams can interfere just like the light beams in the two slit experiment. The answer to the question “which slit did the particle go through?” is “ it went through both!!”
9/17/2018
Physics 214 Spring 2015

11. Periodic table n # electrons (2L+1) x 2 = 2 L = 0
If we start with hydrogen we make new atoms by adding protons and neutrons and adding electrons to be equal to the protons. In principle we can make an atom with any mixture of protons and neutrons but if the balance between the forces is not correct the nucleus will be unstable and decay.
There are three numbers that specify an electron and no two electrons in an atom can have the same set of numbers.
The three numbers are n which defines an energy shell, s which defines the spin and has only two values and L which defines the angular momentum which is a positive integer from 0 to n-1 in each shell.
Most chemistry involves the behavior of the electrons. When a shell is full the atom is chemically inert e.g. noble gases. When a shell has just one or a few electrons it is chemically very active
n # electrons
(2L+1) x 2 = 2 L = 0
(2L+1) x 2 = 8 L = 0 or 1
(2L+1) x 2 = 18 L = 0 or 1 or 2
9/17/2018
Physics 214 Spring 2015

12. Cathode rays and TV sets
Using a high electric field one can strip electrons from atoms and accelerate them. Magnetic and electric fields can then be used to shape and steer the beam.
Heating a filament increases the flow of electrons.
The original observation was called cathode rays because the beam came from the negative terminal.
Today a CRT TV set produces an electron beam which is raster scanned over a surface containing strips of phosphors which when hit with the electrons produce the colors that form the image.
The accelerating voltage is typically 30,000 volts and
X rays are produced as the beam hits the screen
9/17/2018
Physics 214 Spring 2015

13. Medical imaging
Medical imaging uses electromagnetic radiation, sound or ingestion of radioactive substances
9/17/2018
Physics 214 Spring 2015

14. X rays
On 8 November 1895, Wilhelm Conrad Roentgen ( ) was experimenting on cathode rays when an object across the room began to glow.
Soon there were X ray portrait studios and X ray machines were used by magicians and you could by X ray machines from catalogs for home use
First x ray picture
9/17/2018
Physics 214 Spring 2015

15. X ray imaging
An electron beam is accelerated through 50,000 volts and collides with a tungsten target. The deceleration produces X rays and X rays also come from inner shell electrons being ejected.
A CAT scan uses X rays to image a thin slice of the human body. Images are taken of successive slices and computers are used
to produce 3 D rotatable images
X rays are absorbed the denser the material so they are very good for imaging bones
9/17/2018
Physics 214 Spring 2015

16. CAT images
9/17/2018
Physics 214 Spring 2015

17. Magnetic resonance imaging
MRI is an interplay of magnetism and resonance.
The body is made up mostly of water and fat .
Both of these contain a large number of Hydrogen atoms.
These atoms have a spin and act like a magnet
If we put the human body inside a large magnet,
which has a strength much larger than the magnetic
field of earth then the tiny spins of the hydrogen atoms
align themselves with the magnetic field.
9/17/2018
Physics 214 Spring 2015

18. Precession (like a top)
In a magnetic field the nuclei do not actually line up with the external field but the direction of the nuclear magnet precesses around the direction of the external field like a top. The frequency of rotation depends on the external field. At 1.0 T the protons spin at approx. 42 million times a second At 1.5T, they spin at approx. 63 million times a second
An essential feature of MRI is that there is a magnetic field gradient across the patient so that the precession frequency is position dependent B0
9/17/2018
Medical imaging

19. MRI in practice
An Radio Frequency pulse is sent in with a specific frequency and the protons with that frequency absorb energy. When the pulse stops ( ~ few milliseconds) the protons relax and re emit the energy. This energy is detected as a current in a coil and the characteristics of that current determine the density of protons at that scan point.
9/17/2018
Physics 214 Spring 2015

20. MRI images of the brain sagittal Axial coronal 9/17/2018
Physics 214 Spring 2015

21. What is Nuclear Imaging ?
Nuclear Medicine
What is Nuclear Imaging ?
The process involves injecting into the body a small amount of chemical substance tagged with a short lived radioactive tracer. Depending on the chemical substance used, the radiopharmaceutical concentrates in the part of the body being investigated and gives off gamma rays. A gamma camera then detects the source of the radiation to build a picture. These are called scans.
9/17/2018
Physics 214 Spring 2015

22. Nuclear Imaging Scans
Brain Scans These investigate blood circulation and diseases of the brain such as infection, stroke or tumor. Technetium is injected into the blood so the image is that of blood patterns.
Thyroid Uptakes and Scans These are used to diagnose disorders of the thyroid gland. Iodine 131 is given orally , usually as sodium iodide solution. It is absorbed into the blood through the digestive system and collected in the thyroid.
Lung Scans These are used to detect blood clots in the lungs. Albumen, which is part of human plasma, can be coagulated, suspended in saline and tagged with technetium.
Cardiac Scans These are used to study blood flow to the heart and can indicate conditions that could lead to a heart attack. Imaging of the heart can be synchronised with the patient's ECG allowing assessment of wall motion and cardiac function.
Bone Scans These are used to detect areas of bone growth, fractures, tumors, infection of the bone etc. A complex phosphate molecule is labeled with technetium. If cancer has produced secondary deposits in the bone, these show up as increased uptake or hot spots.
9/17/2018
Physics 214 Spring 2015

23. Radioisotopes Used in Nuclear Medicine
For imaging Technetium is used extensively, as it has a short physical half life of 6 hours. However, as the body is continually eliminating products the biological half life may be shorter. Thus the amount of radioactive exposure is limited.
Technetium is a gamma emitter. This is important as the rays need to penetrate the body so the camera can detect them.
It has such a short half life, it cannot be stored for very long because it will have decayed. It is generated by a molybdenum source (parent host) which has a much greater half life and the Tc extracted on the day it is required. The molybdenum is obtained from a nuclear reactor and imported. For treatment of therapy, beta emitters are often used because they are absorbed locally.
9/17/2018
Physics 214 Spring 2015

24. Radioactive elements The half life needs to be short so that an image
can be obtained in a reasonable time and also that the radioactive dose be non hazardous. Isotopes have to be made close to point of use.
Labeling agent Half-life
carbon minutes
oxygen minutes
fluorine minutes
bromine minutes
The different radioactive elements provide different images.For example oxygen-15 provides an image of how oxygen is being absorbed. Flourine is used to study brain metabolism and carbon for blood flow.
9/17/2018
Physics 214 Spring 2015

25. PET scan Positron – electron tomography
requires ingesting a radioactive
element that decays with positron
emission. The positron annihilates
with an electron and produces
a pair of back to back photons.
Since there are many decays in a small volume one can intersect many photon pairs to find the number of photons/unit volume.
PET scans are used, for example, to image blood flow in the Brain.
9/17/2018
Physics 214 Spring 2015

26. PET Scan for Cancer
PET can help doctors locate the presence of cancer/infection anywhere in the body. Because cancers are multiplying and require energy for growth, the PET scan is designed to detect any mass that is growing fast. The PET scan involves the use of radioactive glucose which is injected into the body. The glucose is taken up by the cancer cells and this activity can be monitored by the PET scan. PET scan has the ability to identify tumors in their very early phase. The PET scan can also detect the spread of cancer in other parts of the body.
PET scanning is the most sensitive test for detecting cancer and its location.  When used in conjunction with other radiological tests like Ultrasound, CT Scans and/or MRI, it is very effective in the detection of cancer or its spread.
9/17/2018
Physics 214 Spring 2015

27. PET images
Resting
Music
Thinking
Visual
PET can be used to study changes in the brain due to different stimuli
9/17/2018
Physics 214 Spring 2015

28. Gamma Knife
A Gamma Knife contains 201 cobalt-60 sources of approximately 30 curies (1.1 TBq), in a circular array in a heavily shielded assembly. The device aims gamma radiation through a target point . The patient wears a helmet that is fixed to the skull. Gamma Knife therapy, radiation to kill cancer cells and shrink tumors. Gamma Knife radiosurgery is able to accurately focus many beams of high-intensity gamma radiation to converge on one or more tumors. Each individual beam is of relatively low intensity, so the radiation has little effect on intervening brain tissue and is concentrated only at the tumor itself.
9/17/2018
Medical imaging