Nuclear Transmutations ( Nucleosynthesis )

Transcription

1 Nuclear Transmutations ( Nucleosynthesis ) Nucleosynthesis reactions in the interior of stars have produced nearly all of the naturally occurring elements on earth. In a nuclear transmutation a nucleus is struck by a another particle (proton, neutron or another nucleus), inducing a nuclear reaction. This is how heavier elements are made from lighter elements. Rutherford was the first to observe transmutation of an element by alpha bombardment. N was transmuted into O. Why large? 14 7 N α 17 8 O+ 1 1 p To do nucleosynthesis on earth, we must overcome LARGE energy barriers by simulating the very high energy conditions found inside a star. Two ways to do this: (1) Heat to VERY high temperatures (stars) (2) Use particle accelerators (cyclotron or linear accelerator). Today, the use of particle accelerators and neutrons as the bombarding particle have greatly extended nuclear reaction chemistry. All the elements past U are man-made through this process of acceleration or neutron bombardment. A linear accelerator uses a series of tubes with alternating voltage. A particle is accelerated from one tube to the next by repulsion. Schematic diagram of a cyclotron accelerator 1 Accelerator Facilities Foothill CERN - Geneva, Switzerland Nobel Prizes in Physics The Nobel Prize in Physics 1976 The Nobel Prize in Physics 1984 The Nobel Prize in Physics 1988 The Nobel Prize in Physics 1992 SLAC - Stanford Linear Accelerator Nobel Prizes in Physics Charm: The 4th Quark Quarks Revealed: Structure Inside Protons and Neutrons Tau: The Third Electron-Like Particle Nobel Prize in Chemistry Molecular basis of Eukaryotic Transcription 2

2 Transuranium Element Synthesis All of the elements past uranium are man-made in the laboratory. The synthesis of these transuranium elements is one of the most important achievements of particle physicists during the 1940 s and 1950 s. Research continues today with over 25 elements synthesized. Beta emission after neutron bombardment is the key to many synthesis. Remember, beta emission increases the atomic number by one. Below are a few of the reactions used to create these man-made elements Neptunium, Np: U n U Np β 1940-Plutonium, Pu: Np Pu β 1944-Americium, Am: Pu n Pu Am β Heavier nuclei like He and C can also be accelerated and used in bombardment: 1949-Berkelium, Bk: Am α Bk n 1969-Rutherfordium, Rf: Cf C Rf n 3 Using Neutrons for Element Transmutations 1. Nuclear transmutation reactions using neutrons as the bombarding particle do not require the neutrons to be accelerated to high speeds. Why not? 2. In fact neutrons cannot be accelerated. Why not? 3. The neutrons needed for this are produced by the reactions that occur in nuclear reactors. 4. Neutrons are used in the medical industry to create unstable nuclei that are gamma and/or beta emitters. These isotopes are used for targeted cancer radiation therapy or as tracers within the body to diagnose illness. Example of nuclear chemistry in medicine is the formation of cobalt Co is used for γ-radiation gamma knife treatment of cancer. The steps in the synthesis are shown below. Write the appropriate equations. 1. Neutron bombardment of iron-58 to produce iron-59: 2. Beta emission from iron-59: 3. Neutron bombardment of cobalt-59 to produce cobalt-60: Write the net synthesis: 4

3 Biological Effects of Radiation As matter absorbs radiation (alpha, beta or gamma) several process can occur. Radiation is usually classified into non-ionizing or ionizing radiation. 1. Non-ionizing Radiation Generally of lower energy, such as visible light electromagnetic radiation or slow moving neutrons. Causes heating of matter when absorbed. No bonding or nonbonding electrons are ionized in the interaction. Can cause thermal burns. 2. Ionizing Radiation Higher energy radiation that ionizes an electron from an atom or a molecule. Includes alpha, beta, gamma, X-rays and higherenergy UV radiation. Far more harmful to biological systems than non-ionizing radiation. Ionizing radiation usually results in the formation of free radicals, substances with one or more unpaired electrons. Since the body is 70% water, most free radicals are formed from ionizing water. Formation of water ion-radical: energy + H2O > H2O + + e Further free radical formation: H2O + + H2O > H3O + + OH e + H2O > H + OH Free radical OH Damage inflicted depends on radiation type, half life, length of exposure, biological behavior of the radioactive isotope and tissue exposed. Alpha Particles: Penetrate very little, stopped by skin because of high mass and charge. But, can cause severe damage in their path due to ability to cause ionization and to knock atoms out of molecules. Less harm is cause if only skin is exposed to alpha radiation, but damage is severe if inhaled or ingested. Beta Particles: Causes less ionization, but is a more destructive external source due to deeper penetration. Free radicals are very reactive and do damage by breaking Gamma Rays: Can penetrate and pass bonds of other molecules. Severe damage, even death, of tissue through living tissue, causing severe can result. ionization along their pathway. 5 The most dangerous external source. Units for Measuring Radiation Exposure 1. Energy absorbed by living tissue rad: Radiation Absorbed Dose. 1 rad = 0.01 J/kg tissue. (Most often used in medicine.) 1.2. SI unit - Gray (Gy): 1 Gy = 100 rad = 1 J/kg tissue. 2. Since each type of radiation deposits energy in your body differently, the rad is multiplied by a Relative Biological Effectiveness factor, RBE to obtain the rem, rotegen equivalent man 2.1. rem = (rad)x(rbe) SI unit - Sievert (Sv): Sv = (1 Gy)x(RBE) = 100 rem RBE values: Alpha: RBE = 20 Beta: RBE = 1 Gamma: RBE = 1 Note: Exposure versus Contamination Exposure to radiation DOES NOT make something radioactive! Distance from source limits exposure. Contamination is when radioactive material is ingested, inhaled or on the surface. The material is still present and continues to emit radiation. 6

4 Radiation and Health Safety Sources of radiation exposure in the U.S. Radon Distribution in the U.S Rn Po + 2 He; t1/2 = 3.82 days Po Pb + 24 He; t1/2 = 3.11 min X-ray This is the hand of a physician who was exposed to repeated small doses of x-ray radiation for 15 years. The skin cancer appeared several years after his work with x-rays had ceased. Cancer incidence depends on radiation dose. From Meissner, William A. and Warren, Shields: Neoplasms, In Anderson W.A.D. editor; Pathology, edition 6, St. Louis, 1971, The C.V. Nuclear Mosby Co Chemistry 7 Radioactive Nuclides and Biological Behavior The biological behavior of radioactive substances that are ingested, inhaled, or absorbed through the skin plays an important role in the amount of physiological damage done. Examples: Uranium-235 and plutonium-239 both emit alpha particles and have long half-lives. Uranium is rapidly excreted. However, plutonium behaves like calcium and will be incorporated into bones and teeth, resulting in long term exposure. Naturally occurring strontium is nonradioactive and nontoxic at levels normally found in the environment, but strontium-90 is a radiation hazard. It is present in significant amount in spent nuclear fuel, in radioactive waste from nuclear reactors and in nuclear fallout from nuclear tests. Strontium-90 is a beta emitter that behaves very similarly to calcium. After entering the organism, most often by ingestion of contaminated food or water, about 70 80% of the dose is excreted. Most of the remaining strontium-90 is deposited in teeth, bones and bone marrow. Its presence in bones can cause bone cancer, cancer of nearby tissues, and leukemia. Radioactive iodine-131 is one of the elements likely to be released into the upper atmosphere after a nuclear event. Carried great distances on high speed winds, it can then drop down into the lower atmosphere, where it may be breathed into the lungs. It can also contaminate crops on the ground and be introduced into the body through food and drink. (Fruits and wines are particularly susceptible.) The problem is that the human thyroid gland has a tremendous affinity for iodine, radioactive or otherwise. In other words, the thyroid gland quickly absorbs radioactive iodine, where it can injure or even kill the gland. In fact, radioactive iodine is often administered by doctors specifically to kill the thyroid as a treatment in some thyroid diseases such as Grave's disease. The thyroid gland, is one of the largest endocrine glands in the body. Thyroid hormones act throughout the body, influencing metabolism, growth and development, and body temperature. During infancy and childhood, adequate thyroid hormone is crucial for brain 8 development.

5 Infamous Events-Chernobyl (April 26, 1986) On 26 April 1986, reactor number four at the Chernobyl plant, near Pripyat in the Ukrainian SSR, exploded. Nearly thirty to forty times more fallout was released than had been by the atomic bombings of Hiroshima and Nagasaki. The disaster resulted in the evacuation and resettlement of over 336,000 people. Cesium-137 is a radioactive isotope of cesium which is formed as one of the more common fission products in the nuclear fission of uranium-235 in nuclear reactors and nuclear weapons. It is among the most problematic of the short-to-medium-lifetime fission products because it easily moves and spreads in nature due to the high water solubility of cesium's most common chemical compounds, which are salts. 9 Infamous Events-Fukushima March 12, 2001 One of the substances released was radioactive iodine-131. On March 16, 2001 Japan s Nuclear Safety Commission recommended local authorities to instruct evacuees from the area under 40 years of age to ingest stable iodine as a precaution against ingestion of radioactive iodine

6 Medical Applications of : Diagnosis (Radiotracers) Nuclear Medicine Radiotracers: Requirements 1. A radioactive isotope is used that concentrates in the tissue to be studied. 2. A method of detecting the radiation. Usually a whole body machine so the emission can be mapped. 3. A computer algorithm to reconstruct an image from the detected radiation pattern Some Radionuclides Used as Radiotracers Positron Emission Tomography (PET) scans: PET scans are used to locate areas of increased chemical activity within the body. One technique involves the incorporation of the positron emitter 18F bonded to a glucose analog to measure glucose uptake, which is an indicator of energy metabolism. As positrons are given off, they immediately interact with an electron, mutual annihilation takes place. Two gamma rays are given off in the process and these are imaged. Areas of increased gamma emission are associated with higher glucose use. Cancer cells multiply rapidly, therefore have higher glucose use than normal cells. These PET scans show brain activity in a normal person (left) and in a patient with Alzheimer s disease (right). Red and yellow indicate relatively high activity within a region. 11 Medical Applications of : Cancer Therapy Cancer Treatment 1. Older treatments of cancer have used x-ray and gamma ray exposure to kill cancer cells. These treatments were not discriminatory and often did more damage to healthy tissue than the cancer tissue. 2. More modern treatments, such as Gamma-knife surgery spares healthy tissue. This treatment uses several hundred concurrent low-dose gamma beams that have a convergence point at the tumor. Most healthy tissue is spared while the tumor receives a high exposure. 3. Boron neutron capture therapy (BNCT) use 10B to capture neutrons. The 10B is injected into the tumor and a neutron beam is directed at the tumor. The 10B captures a neutron and then emits an alpha particle. The alpha particle damage is then confined to the tumor tissue. Emission stops when the neutron beam is removed. Some Radioisotopes Used in Radiation Therapy Each of these radioactive isotopes usually targets a specific organ or area of the body: P-32: blood diseases Co-60: gamma source Sr-90: bone cancer I-125: prostrate cancer I-131: thyroid cancer Cs-137: gamma source (external) Ir-192: gamma source (internal) 12

7 Other Applications of Smoke Detectors Ionization smoke detectors use Americium-241 as a source of low level alpha particle emission to ionize air. A difference in ionization due to smoke particles is detected and generates an alarm. An Americium container from a smoke detector Food Science: γ irradiation Gamma irradiation of food has been used for years to increases shelf life and prevent spoiling of food. The low dose gamma rays retard the growth of bacteria, molds and yeasts. Shelf lives can be doubled or tripled without refrigeration. This is still a controversial process. 13 Other Questionable Applications of Alexander Litvinenko was a former officer of the Russian Federal Security Service (FSB) and KGB, who fled from court prosecution in Russia and received political asylum in the United Kingdom. According to his wife and father, he was working for MI6 and MI5 after receiving the asylum. On 1 November 2006, Litvinenko suddenly fell ill and was hospitalized. He died three weeks later, becoming the first confirmed victim of lethal polonium-210-induced acute radiation syndrome. Litvinenko's allegations about the misdeeds of the FSB and his public deathbed accusations that Russian president Vladimir Putin was behind his unusual malady resulted in worldwide media coverage. Subsequent investigations by British authorities into the circumstances of Litvinenko's death led to Polonium-210 is a alpha emitter. This made it difficult for medical professionals to determine the source of Litvinenko's radiation sickness because they were performing tests to detect gamma rays. Spectroscopic tests on a large urine sample finally detected the alpha radiation. 14