Chapter 9: Nuclear Radiation Most isotopes with less than 20 protons are stable. proton to proton repulsions are minimalized by the neutrons. Isotopes of elements with 20 to 83 protons may be unstable. most even, even nuclei are stable and most odd, odd are unstable. Nuclear Radiation All isotopes with more than 83 protons are unstable. the nucleus is just too large. When an unstable nuclei decays, it releases _____________ in the form of small particles and / or energy. Isotopic Symbols In chapter three, we wrote isotopic symbols. Mass # = p + n Atomic # = p 131 53 I Learning Check How many protons, neutrons, and electrons does each have? 131 55 C s and 212 85 At Identify an isotope with 31 protons and 41 neutrons. Types of Particles There are several common particles that are produced when a nuclei decays. Alpha (a) – consists of two protons and two neutrons like a helium nuclei. Beta (b-) – is a high energy electron that comes from the nucleus. Gamma (g) – is a high energy photon that comes from the nucleus. Positron (b+) – is a positively charged electron from the nucleus. Types of Particles Radiation Protection To protect yourself from radiation in the workplace requires diligence. Proper shielding depends on the type of radiation. Alpha particles are the heaviest and are stopped by our skin. These require minimal shielding. Beta and positrons can penetrate as far as 0.5cm of tissue. These require thick clothing for shielding. Radiation Protection Gamma particles can pass through your entire body. These require several feet of concrete and lead for shielding. Radiation Protection Radiation decreases dramatically with distance. Like sound waves, if you double your distance, then the intensity decreases by ¼. E = 1 d 2 Nuclear Equations A nuclear equation shows a nuclear decay or other process. Mass and atomic numbers are always written. Sum of all mass numbers must be equal. Sum of all atomic numbers must be equal. Nuclear Equations 238 92 U Parent Nuclei 234 90 Th + Daughter Nuclei 4 2 He Radioactive particle Notice that the atomic numbers add to 92 and the mass numbers add to 238. Nuclear Equations Writing a Decay Smoke detectors contain Am-241 which is an alpha emitter. Write the balanced nuclear equation. Iodine-131, a beta emitter, is used to check thyroid function. Write the balanced nuclear equation. Writing a Decay Carbon-11, a positron emitter, is used to detect brain damage. Write the balanced nuclear equation. Cobalt-60, a gamma emitter, is used to destroy harmful pathogens in food. Write the balanced nuclear equation. Producing Radioactive Isotopes Many useful isotopes are produced from nuclear bombardment. collision of one nuclei by another Also known as _______________ All isotopes with atomic number greater than 92 are man-made. 249 98 Cf + 15 7 N 260 105 Db + 4 n 1 0 Medically Important Isotopes Many of these man-made isotopes are produced for medical purposes in hospitals. One example is Tc-99, which is widely used to detect tumors. 98 42 1 0 Mo + 99 42 Mo n 99m 43 99 42 Mo Tc + 0 -1 β Learning Check Identify the missing particle in the transmutation reaction below. 58 28 Ni + 1 1 H 4 2 He + ? Measurement and Detection A Geiger tube is often used to detect radiation. Consists of a metal tube filled with a gas like Argon. When radiation enters the tube, it ionizes the gas. The ions are attracted to a negative charged wire and the electrons are then counted. Measurement and Detection Activity The Curie (named for Marie Curie) was the amount of radiation for one gram of Radium. 1 Ci = 3.7 x 1010 disintegrations / second. A “disintegration” could be an alpha, beta, gamma, etc. produced. 3.7 10 10 disintigrations 1 C i sec Learning Check An I-131 source has an activity of 25mCi. How many beta particles are produced in 1.0 minutes? RAD, RBE, and REM RAD = radiation absorbed dosage 1 RAD = 1.0 x 10-2 J / kg of body tissue RBE = a multiplier for each type of radiation. alpha = 20; protons, neutrons = 10; betas, positrons, and gammas = 1. REM = radiation equivalent in humans REM = RAD x RBE Background Radiation We are all exposed to radiation EVERY single day. Ground contains radioactive isotopes like U-238. Air contains Rn-222 and C-14. Food contains K-40 and C-14. Water contains Rn-222. Background Radiation People who fly frequently or who live at high altitudes receive higher levels of cosmic rays. Fact: people living in Denver receive twice as much background radiation as we do. Fact: Cigarette smokers receive alpha radiation from Po-210 – which comes from phosphate fertilizers. Background Radiation FACT: CRT style TV’s emit radiation. Exposure to x-rays (essentially a beta particle) may have a cumulative effect. The average person receives about 170mrem per year. Radiation Levels LD50 = lethal dose for 50% of people receiving that dosage. A whole body exposure of _____ rem A dosage of ___ rem reduces the white blood cell count to zero. A dosage of 100 rem reduces these same cells significantly. Patients often die from secondary infections. Radiation Levels Exposure to 25 rem or less usually cannot be detected. Alexander Litvinenko, a former Russian spy, was given Po-210 in a drink on November 1st, 2006 and died a few weeks later. ...this may be the time to say one or two things to the person responsible for my present condition. You may succeed in silencing me but that silence comes at a price. You have shown yourself to be as barbaric and ruthless as your most hostile critics have claimed. You have shown yourself to have no respect for life, liberty or any civilized value. You have shown yourself to be unworthy of your office, to be unworthy of the trust of civilized men and women. You may succeed in silencing one man but the howl of protest from around the world will reverberate, Mr. Putin, in your ears for the rest of your life. May God forgive you for what you have done, not only to me but to beloved Russia and its people. OSHA Any person working with or around radiation wears a device to detect radiation exposure. The maximum dosage permitted is ___ rem per year. Most workers receive much less than 1 rem per year. Biological Effects Alpha particles cannot penetrate our skin, yet internally they can cause massive damage on soft tissues like the lungs and intestinal linings. Beta particles can cause a burn on the outer portion of the skin. Gamma particles penetrate completely and if exposed to large quantities is deadly. Biological Effects Radiation affects those cells in our body that undergo rapid cell division like bone marrow, intestinal lining, and the skin. Radiation tends not to affect cells that remain unchanged like our brain, liver, muscles, etc. How It Works… When radiation hits a molecule like water, it ionizes. H 2 O + radiation H 2O +1 + 1e - This water molecule reacts with another water molecule. H 2O +1 + H 2 O H 3O +1 + OH How It Works… The “dot” on the OH is an odd electron. Molecules with an odd electron are called _______________. ___________ are electron scavengers and interfere with electron transfer reactions – many of which are vital in the function of the body. Medical Applications Medical uses of radioisotopes fall into two categories. Diagnostic Therapeutic Diagnostic A standard x-ray cannot produce an image of an organ like the heart, liver, pancreas, blood vessels, etc. To illuminate the targeted region, a radioisotope is injected into the body. Radioisotopes used are low dosages with a short halflife. localized 0.1 to 50 rem doses are common CAT & PET scans A computerized tomography scan is an enhanced x-ray machine using multiple beams capable of producing a 3D image. A positron emission tomography scan uses a positron emitter to generate a 3D image. CAT & PET scans MRI In magnetic resonance imaging, a powerful magnetic field is used along with low energy radio frequency to generate an image. Based on the premise that hydrogen atoms have “spin.” In a powerful magnetic field, these hydrogen atoms can be made to flip between the two spin states. MRI Therapeutic In radiation therapy, radiation is used to target cancer cells. Radiation levels are in very high dosages. localized 4000 – 6000rem doses are common Cyberknife treatment uses computer technology to aim 100’s of x-ray beams precisely at a tumor. Learning Check The dosage of Tc-99 for a lung scan is 20.mCi/kg of body tissue. What dosage should a 121 pound patient receive? Half-Life A half-life is the amount of time that it takes for one- half of a radioisotope sample to decay. Half-life's can be very short to very long. Tc-99 has a half-life of 6 hours K-40 has a half-life of 1.25 x 109 years Half-Life Learning Check The isotope Cr-51 has a half-life of 28 days. How much of a 160.mg sample would remain after 112 days? A patient is injected with N-13, which has a half-life of 10 minutes. If the original activity of the sample is 40 mCi, what activity would be remain after 40 minutes? The amount of F-18 decreases from 40.mg to 10.mg in 220 minutes. What is the half-life of this radioisotope? Fission Nuclear fission is the process by which a larger nuclei is _____ into two smaller ones. During this process, a small percentage of the mass is converted to energy as predicted by Einstein. E = mc2 ; where c = speed of light. 1 x 10-3 g “lost” can generate 9.0 x 1010 kJ of energy. Only two fissionable isotopes are known. U-235 and Pu-239 Fission Begins when a neutron strikes a U-235 atom. The products are numerous – below is just one example. 235 92 U + 1 0 n 139 56 Ba + 94 36 Kr + 3 n + energy 1 0 On average, three new neutrons are produced. Each new neutron can split another U-235. Fission In a nuclear weapon, a ____________ of U235 is imploded producing an uncontrolled chain reaction. Fission Fission In a nuclear power plant, the quantity of U-235 cannot sustain a chain reaction. Control rods absorb excess neutrons. Waste products, with long half-life’s, from spent fuel rods are stored in large pools at the power plant. Fission Fusion Fusion involves taking two or more smaller nuclei and fusing them together. Once again, mass is converted to energy. This is the process by which all stars work. 1 1 H+ H+ H+ H 1 1 1 1 1 1 4 2 He + 2 β 0 +1 Fusion On our planet, fusion has been achieved. 2 1 H + 3 1 H 4 2 He + 1 0 n The temperatures required are extremely high. Currently, more energy is required to achieve fusion than we get back from the reaction. Research continues as this represents the “holy grail” of energy. Learning Check In one possible fission reaction for U-235, the U-235 is bombarded with a neutron producing Kr-91, three neutrons, and another nuclei. What is the unidentified nuclei?