Atomic Decay Radioactivity

Penetration depth of alpha (a), beta (P), and gamma (7) radioactivity. 

As you can see, y radiation is the most penetrating, and often the most destructive (y radiation is to blame for the increased rates of leukemia following the bombing of Hiroshima and Nagasaki, for example) however, this doesn t mean that a radiation is off the hook. If you recall the ex-KGB agent that was poisoned via radioactive sushi, he was killed by a very powerful a emitter. Sure, you can protect yourself from the a particles with a piece of 

Alpha decay is denoted by the symbol a (Greek alpha), and the radionuclide essentially just emits an atom of helium with no electrons 2 He see Chapter 2 for isotope symbolism. 

Remember that a proton or neutron is about 1,800 times larger than an electron, so an a particle with two neutrons and two protons is absolutely huge. Additionally, as this helium nuclei has two protons and no electrons, it s positively charged. Radionuclides that are rich in protons desire to undergo a decay to decrease their positive charge. For example, uranium-238 with 92 protons undergo a decay to become thorium-234 with 90 protons. Notice how the mass number went down by four (four nucleons are in an a particle), yet only two protons were lost (so the atomic number only decreaises by two). 

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Radon daughters The series of unstable isotopes that are formed as radon atoms undergo radioactive decay. 

Activity, nuclear Rate of radioactive decay number of atoms decaying per unit time, 518... 

Radioactive decay is a stochastic process that occurs at random in a large number of atoms of an isotope (see Textbox 13). The exact time when any particular atom decayed or will decay can be neither established nor predicted. The average rate of decay of any radioactive isotope is, however, constant and predictable. It is usually expressed in terms of a half-life, the amount of time it takes for half of the atoms in a sample of a radioactive isotope to decay to a stable form. 

We use expression (26.12), substituting the disintegration rate for the number of atoms, since we recognize that in this first-order reaction the rate is directly proportional to the amount of reactant, that is, the number of atoms. (All radioactive decay processes follow... 

The rate at which radioactive atoms decay is unaffected by the chemical or physical form of the nucleide and depends only on the number N of atoms present and the decay constant Vs-1 for that particular nuclide. In a single... 

Because of its importance in natural phenomena, e.g., radioactive decay and population dynamics, let us introduce the exponential distribution through an illustration, n atoms are assumed to decay over the time interval [0 — 0], each atom having the same probability of decaying at any time in this interval. In other words, the time at which an atom decays is uniformly distributed over [0 — 0]. Let us call N(t) the number of decay events between 0 and t. The probability that a single atom has not decayed at time t, is 1—f/0 (Figure 4.3). The probability that none of the n atoms has decayed at time t is... 

Marie (NLP 1903, NLC 1911 ) and Pierre (NLP 1903 ) Curie took up further study of Becquerel s discovery. In their studies, they made use of instrumental apparatus, designed by Pierre Curie and his brother, to measure the uranium emanations based on the fact that these emanations turn air into a conductor of electricity. In 1898, they tested an ore named pitchblende from which the element uranium was extracted and found that the electric current produced by the pitchblende in their measuring instrument was much stronger than that produced by pure uranium. They then undertook the herculean task of isolating demonstrable amounts of two new radioactive elements, polonium and radium, from the pitchblende. In their publications, they first introduced the term radio-activity to describe the phenomenon originally discovered by Becquerel. After P. Curie s early death, M. Curie did recognize that radioactive decay (radioactivity) is an atomic property. Further understanding of radioactivity awaited the contributions of E. Rutherford. 

Despite their instability, some unstable atoms may last a long time the half-life of uranium 238, for example, is about 4.5 billion years. Other unstable atoms decay in a few seconds. Radioactive decay is one of the topics of nuclear chemistry, and it involves nuclear forces, as governed by advanced concepts in chemistry and physics, such as quantum mechanics. Researchers do not fully understand why some atoms are stable and others are not, but most radioactive nuclei have an unusually large (or small) number of neutrons, which makes the nucleus unstable. And all heavy nuclei found so far are radioactive—nuclides with an atomic number of 83 or greater decay. 

Number of atoms of radioactive 87Rb and daughter 87Sr as a function of time in units of half-life. For each half-life the number of radioactive 87Rb atoms drops by a factor of two as the 87Rb atoms decay to 87Sr. After 10 half-lives, the parent isotope is effectively gone. 

The age of an art object can provide a valuable clue to whether it is real or a forgery. Because the half-life for a specific isotope is constant, half-life can be used to find the age of an object. The isotope put to use for radioactive dating is carbon-14. The half-life of carbon-14 is 5,730 years. The amount of carbon-14 in our atmosphere remains fairly constant. When an object such as a plant is alive, it absorbs C02. The carbon atoms in the C02 are made of a specific ratio of carbon-14 atoms to carbon-12 atoms. The carbon-14 atoms decay by emission of beta particles ... 

Uranium in nature may be measured either radiometrically or chemically because the main isotope - 238U - has a very long half life (i.e., relatively few of its radioactive atoms decay in a year). Its isotopes in water and urine samples usually are at low concentrations, for which popular analytical methods are (1) radiochemical purification plus alpha-particle spectral analysis, (2) neutron activation analysis, (3) fluorimetry, and (4) mass spectrometry. The radiochemical analysis method is similar in principle to that of the measurement of plutonium isotopes in water samples (Experiments 15 and 16). Mass spectrometric measurement involves ionization of the individual atoms of the uranium analyte, separation of the ions by isotopic mass, and measurement of the number of separated isotopic ions (see Chapter 17 of Radioanalytical Chemistry text). 

Tritium is the heavy isotope of hydrogen (section 5.2). Its symbol is 3H, or T. Tritium atoms are unstable and disintegrate radioactively, forming stable 3He atoms. The radioactive decay is accompanied by the emission of [1 particles, measurable in specific laboratories ... 

The rate of radioactive decay is by convention expressed as the half-life, T1/2, defined as the time span during which a given concentration of the radioactive element atoms decays to half their initial value. T1/2 of tritium is 12.3 years. Thus, after 12.3 years one-half the initial concentration of tritium atoms is left, after 24.6 years only one-quarter is left, and so on. A radioactive decay curve of tritium is given in Fig. 10.1. Using the decay curve it is possible to determine, for example, how many years it takes for a given amount of tritium to decay to 20% of the initial amount. The answer, obtained from Fig. 10.1, is 29 years. Similarly, one can determine what percentage of an initial amount of tritium will be left after 20 years. The answer is 32% (read from Fig. 10.1). 

The basic unit of measure for radioactivity is the number of atomic decays per unit time. In the SI system, this unit is the becquerel (Bq), dehned as one decay per second. An older, widely used measure of activity is the curie (Ci). Originally... 

Radioactive materials, such as californium, are hazardous to living cells. As the element s atoms decay, they emit energy and particles that damage or kill the cell. The damaged cells rapidly divide, producing masses called tumors. Cancerous cells can crowd out healthy cells, reduce or stop organ function, and break free to spread through the body. 

Radioactive Decay Radioactive elements are made up of atoms whose nuclei are unstable and give off atomic radiation as part of a process of attaining stability. The emission of radiation transforms radioactive atoms into another chemical element, which may be stable or may be radioactive such that it undergoes further decay. 

The introduction of time as a necessary consideration for understanding matter was a radical change to the conception of the elements, which since ancient times were thought to be eternal. To describe the life of a decaying radioactive element, in 1900 Rutherford introduced the concept of the half-life. The half-life of a substance is the time it takes for half of the atoms in a sample to decay (emit nuclear radiation) and, in effect, for the sample to become a different substance. Half-life can also be used to express the time it takes for the chemical or physical activity of the sample to decrease by half. 

The rate of decay of a radioactive source is called its activity and is simply the rate at which radioactive parent atoms decay to more stable daughter atoms. The relationship between the initial activity (Ao) at time zero and the activity (A) after an elapsed time t, is given by ... 

Here A represents the amount of decaying radionuclide of interest remaining after some time t, and Aq is the amount present at the beginning of the observation. The k is the rate constant, which is different for each radionuclide. Each atom decays independently of the others, so the stoichiometric coefScient a is always 1 for radioactive decay. We can therefore drop it from the calculations in this chapter and write the integrated rate equation as... 

The rate (frequency) of disintegration is an inherent property of a given radioisotope and varies widely among isotopes. The half-life (time of survival of half the initial set of radioactive atoms) may be millions of years (e.g., Np) to a fraction of a second (e.g., P). Since disintegrations in a small interval of time are rare and independent of other time intervals, disintegration in equal finite intervals follows the Poisson distribution. The probability of atoms still being radioactive, P(0) (in which 0 indicates no nuclear transformations), after t intervals with a decay constant 6 (= fraction of radioactive atoms decayed per interval, if seconds, 5 if days) thus equals... 

In PET, radioactive substances that emit positrons are introduced into a patient s bloodstream. As the radioactive atoms decay, the positrons they emit collide with electrons, producing gamma rays that escape from the body and are detected by an array of instruments surrounding the patient. Computer analysis of the amount and direction of gamma ray production, and comparison of the data collected for people with and without certain brain disorders provides doctors with valuable information. For example, PET scans of the brain have been used to study the movement of the medication L-dopa in the brains of people suffering from Parkinson s disease. In these procedures, fluorine-18 atoms are attached to L-dopa molecules, which are then injected into a patient. Each flourine-18 decays and emits a positron that generates gamma rays when it meets an electron. 

The Curies focused their early research on studying radioactivity and the energy produced as radioactive atoms decayed. They discovered polonium and radium, and identified thorium as a radioactive element. Marie and Pierre s lives were characterized by poverty, even though they were celebrated in the world of science. Marie... 

Some elements do not occur naturally, but can be synthesized. They can be produced in nuclear reactors, from collisions in particle accelerators, or can be part of the fallout from nuclear explosions. One of the elements most commonly made in nuclear reactors is technetium. Relatively large quantities are made every day for applications in nuclear medicine. Sometimes, the initial product made in an accelerator is a heavy element whose atoms have very short half-lives and undergo radioactive decay. When the atoms decay, atoms of elements lighter than the parent atoms are produced. By identifying the daughter atoms, scientists can work backward and correctly identify the parent atoms from which they came. 

An electron emitted from the nucleus of an atom by radioactive decay. 

The specific activity of a radionuclide relates activity (A) (the number of radioactive atoms decaying) to the total number (N) or total mass of atoms present ... 

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