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 7/20/2016 Physics 214 Spring 2016 1
The basic building blocks The basic building blocks that make up atoms are charge mass Proton 1.6 x 10-19 1.672 x 10-27 kg Neutron 0 1.675 x 10-27 kg Electron -1.6 x 10-19 9.1 x 10-31 kg Photon 0 E = hf p = h/λ h = Planck s constant = 6.626 x 10-34 J.s 7/20/2016 Physics 214 Spring 2016 2
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 7/20/2016 Physics 214 Spring 2016 3
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 2 He 4 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 H 2 O 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 7/20/2016 Physics 214 Spring 2016 4
3E-05 Cloud Chamber As charged particles pass through a gas electrons are knocked out of the atom and the ions produced can act as seeds for a super saturated liquid to condense. This leaves a visible track of bubbles of liquid. Cosmic rays are bombarding the earth continuously and radioactive decays also produce charged particles which can be observed in a cloud chamber. If the chamber is in a magnetic field the tracks are curved The first observation of the positron (anti particle of the electron) was made in a cloud chamber 7/20/2016 Physics 214 Spring 2016 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. 7/20/2016 Physics 214 Spring 2016 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 14 5730 years I 129 1.6 million U 238 4.47 billion http://www.physics.purdue.edu/class/applets/phe/lawdecay.htm 7/20/2016 Physics 214 Spring 2016 7
Types of nuclear decays A proton can change into a neutron and emit a positron (anti particle of electron) 1e 0 Atomic number decreases by 1 mass number is the same A neutron can change into a proton and emit an electron -1e 0 Atomic number increases by 1 mass number is the same A nucleus can emit an alpha particle which is 2 neutrons and 2 protons (actually a helium nucleus) 2He 4 Atomic number decreases by 2 mass number decreases by 4 Photons can be emitted No change in Atomic number or mass number. 7/20/2016 Physics 214 Spring 2016 8
What is the daughter? When an element decays it usually changes to a new element. The notation we use is 3 Li 7 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. 2He 4 + 7 N 14 = 8 O 17 + 1 H 1 nuclear reaction 88Ra 226 = 86 Rn 222 + 2 He 4 nuclear decay 7/20/2016 Physics 214 Spring 2016 9
Energy and atoms When we make atoms from the constituents energy is released. The sun s energy comes from fusion E = Mc 2 When we make molecules by joining atoms energy is released. Burning is Carbon plus oxygen making CO 2 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 7/20/2016 Physics 214 Spring 2016 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!! 7/20/2016 Physics 214 Spring 2016 11
Periodic table 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 1 (2L+1) x 2 = 2 L = 0 2 (2L+1) x 2 = 8 L = 0 or 1 3 (2L+1) x 2 = 18 L = 0 or 1 or 2 7/20/2016 Physics 214 Spring 2016 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 7/20/2016 Physics 214 Spring 2016 13
Medical imaging Medical imaging uses electromagnetic radiation, sound or ingestion of radioactive substances 7/20/2016 Physics 214 Spring 2016 14
X rays On 8 November 1895, Wilhelm Conrad Roentgen (1845-1923) 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 7/20/2016 Physics 214 Spring 2016 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 7/20/2016 Physics 214 Spring 2016 16
CAT images 7/20/2016 Physics 214 Spring 2016 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. 7/20/2016 Physics 214 Spring 2016 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 B 0 7/20/2016 Physics 214 Spring 2016 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. 7/20/2016 Physics 214 Spring 2016 20
MRI images of the brain sagittal Axial coronal 7/20/2016 Physics 214 Spring 2016 21
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. 7/20/2016 Physics 214 Spring 2016 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. 7/20/2016 Physics 214 Spring 2016 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. 7/20/2016 Physics 214 Spring 2016 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 carbon-11 oxygen-15 fluorine-18 bromine-75 Half-life 20.3 minutes 2.03 minutes 109.8 minutes 98.0 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. 7/20/2016 Physics 214 Spring 2016 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. 7/20/2016 Physics 214 Spring 2016 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. 7/20/2016 Physics 214 Spring 2016 27
PET images Resting Music Visual Thinking PET can be used to study changes in the brain due to different stimuli 7/20/2016 Physics 214 Spring 2016 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. 7/20/2016 Physics 214 Spring 2016 29