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Transcript
13.1 Radiation and Radioactive Decay Physics Tool box Alpha decay occurs when an unstable nucleus emits a particle, often denoted as 4 2 He , which consists of two protons and two neutrons. The resulting daughter nucleus is of a different element and has two protons and two neutrons fewer than the parent. decay, a neutron is replaced with a proton and a particle (a electron). In decay, a proton is replaced with a neutron and a high speed (a high speed positron). Beta decay assumes two forms. In Gamma decay is the result of an excited nucleus that has emitted a photon and dropped to a lower state. Henri Becquerel, in 1896, discovered that an ore containing uranium had the ability to darken a photographic plate even when the plate was completely covered and protected from light. This process was unaffected by physical treatments such as heating and cooling, and is now known as a fundamental property of atoms, called radioactivity. It was the nucleus of a radioactive element that emits radiation as it decomposes (decays). It was Rutherford (at McGill university in Montreal), revealed that radioactive emissions to be of three types: Alpha particles ( ) Beta particles ( ) Gamma rays ( ) Alpha Decay The alpha particles were found to consist of two protons and two neutrons. Since the nucleus of the most abundant isotope of Helium consists of these particles, the particle is often denoted as 4 2 He (the Helium nucleus). When a radioactive material emits an particle, the nucleus of one of its atoms loses two protons and two neutrons. The loss of the two protons changes the atom from one type of element to another (transmutation). Smoke detectors use this process to help save lives. If an atom of element X transmutes to element y by emitting an a nuclear reaction has taken place. A Z particle, we say that X AZ42Y 24 He Where A is the atomic mass number (the number of particles, or nucleons, in the nucleus) and Z is the atomic number (the number of protons). Example Given the following reaction, 218 84 Po X 24 He , determine X Solution: 218 84 Po X 24 He 218 84 Po 2184 84 2 218 84 Po 214 82 X 24 He X 24 He Looking at a periodic table, with atomic number 82, is lead (Pb). particles emitted in a nuclear decay have the ability to penetrate about 5cm of air (a few sheets of paper), thus must possess some form of kinetic energy. Now since kinetic energy is a conserved quantity, you must ask,” where dose this kinetic energy come from?”. It takes the theory of binding energy and the strong nuclear force to answer that question. Protons (positive charges) within a nucleus repel each other through electric force. Therefore for a nucleus to contain protons, there must exist a force stronger than the nuclear force to hold the protons together. This force is called the strong nuclear force. This force is only effective over short distances (or order of 1.5 10 m , the radius of a small nucleus). Since the strong force is attractive, work must be done to break the nucleus apart. 15 Binding Energy Experiments have shown that the mass of an atomic nucleus is always less than the sum of the masses of its constituent protons and neutrons. The mass difference in 2 terms of energy (E=mc ) is called the binding energy and it illustrates that positive work would have to be done to break the nucleus apart. The above graph shows the approximate binding energies per nucleon for all the elements. You notice that the binding energy is at a maximum at a mass around 56 (which is the case for iron) and decreases steadily afterward. By looking at the curve we notice that the nuclei near the middle of the period table are held together more strongly than other nuclei. Why is it that the middle elements have the highest binding energies? The nucleus is the battleground between the strong nuclear (only active at a distance of a small nucleus) and the electromagnetic forces. As you move up the period table toward iron, you find that the nuclei have more nucleons, all pulling toward each other by the strong nuclear force, thus the binding energy increases. As we pass iron, we again have even more nucleons, however the distance between them is increasing and the mutual attraction due to the (short-range) strong nuclear force is weakened. But the proton pairs are still feeling the electromagnet repulsion force. As we approach elements such as radium, polonium, and uranium, the binding energy is so low that the nucleus is not stable at all. When strained to the breaking point, two protons and two neutrons form inside the nucleus and it gets released as an particle with kinetic energy. Beta Decay The beta particles, , emitted in some forms of radioactive decay may be negatively charged electrons or positively charged electrons, called positrons (anit-matter). When decay releases electrons, it is called decay; when it releases positrons, it is called decay. the The particle is symbolized as decay is more common than 0 1 e if it is an electron and 0 1 e if it is a positron. decay. It is important to realize that the electron released in Beta decay, does not come from the electrons orbiting the nucleus, but rather from inside the nucleus via one of the following reactions. 1 0 n 11 p 01 e e energy 11 p 01 n 01 e e Where e is an anti-neutrino, and e is a neutrino. The both the neutron decay into a proton and the proton decay into a neutron is governed by the weak nuclear force. Note: please think that a neutron is just the combination of a proton and a electron. It is more complicated and a complete explanation requires the understanding of quarks (see a section 13.4) Example An atom of potassium-40 can transmute into an atom of some other element by emitting a particle. Represent this reaction in symbol, and identify the daughter element. Solution: 40 19 K 2040Ca 10 e , the element is Calcium. Gamma Decay Unlike alpha and beta decay, decay results in the production of photons that have zero mass and no electric charge. This decay is believed to occur when a highly excited nucleus drops to a lower energy state. If the reaction only produce a gamma decay, then no transmutation takes place (element remains the same). Gamma decays typically occur simultaneously with alpha and beta decays. Pb 211 82 The Bi 10e 211 83 photons (gamma rays) are similar to x-rays except they have a higher frequency (in reality the frequency ranges for the x-rays and gamma ray overlap) and thus higher energy. Anti-matter We have seen that in some reaction we have created a particle called an anti-neutrino. You may ask how does the neutrino and the anti-neutrino compare. The answer come from our current model which holds that every particle has its own antiparticle (in a few cases, the particle is its own antiparticle). The antiparticle is like the mirror image to the particle. The concept of antiparticle comes from the Diracs solution of Energy and mass with E 2 m2 c4 E mc 2 It was the mc which indicates that a particle can have negative energy. The presence of these particle was confirmed in the early 1930’s. 2