Radioactivity Karolina H. Czarnecka, PhD Department of Molecular Bases of Medicine

Transcription

1 Radioactivity Karolina H. Czarnecka, PhD Department of Molecular Bases of Medicine

2 The periodic table is a tabular arrangement of the chemical elements, ordered by their atomic number (number of protons in the nucleus), electron configurations, and recurring chemical properties.

3 NUCLEAR FORCE and Radioactivity For a given number of protons, it is efficient only when a certain number of neutrons are present: if they are too scarce or too numerous, the glue no longer functions and the nucleus disintegrates -> radioactive nucleus. Instable nucleus is changing into more stable radioactive decay

4 Stable nuclei and radioactive nuclei The phenomenon of radioactivity is associated with the transformation of a nucleus, which "wants" to change into something more stable. A radioactive nucleus will choose among the various possible modes of radioactivity, that or those which provide it with the fastest return to stability. Thus, the lifetime of a radioactive nucleus depends only on the ease with which it can mutate into a more stable form.

5 Stable nuclide nuclides that are not radioactive and do not spontaneously undergo radioactive decay. When such nuclides are referred to in relation to specific elements, they are usually termed stable isotopes. 80 elements have at least one stable isotope which is never observed to decay, amounting to a total of about 254 stable isotopes. Hydrogen-1 (protium) Hydrogen-2 (deuterium) Helium-3 Helium-4no mass number 5 Lithium-6 Lithium-7no mass number 8

6 Unstable nuclide nuclides that are radioactive and can spontaneously undergo radioactive decay. thousands of isotopes have been characterized as unstable. These "radioisotopes" decay over time scales ranging from fractions of a second to trillions of years. too few or too many neutrons (in relation to the number of protons) will cause it to decay beta decay a nitrogen-16 atom (7 protons, 9 neutrons) is converted to an oxygen-16 atom (8 protons, 8 neutrons)[19] within a few seconds of being created.

7 unstable nuclei emit alpha particles (He nuclei) and beta particles (electrons) heavy nuclei are unstable and `decay', meaning that they spontaneously split into smaller nuclei and emit stray particles. This is called radioactivity.

8 Nuclear transmutation coming back to alchemy and the search of Philosopher's stone is the proces of conversion of one chemical element or isotope into another through nuclear reactions (in which an outside particle reacts with a nucleus) f.ex. fusion through radioactive decay (where no outside particle is needed).

9 ISOTOPES For nearly all chemical elements, there are several combinations of protons and neutrons which provide stability to the atom's nucleus. The different isotopes of a given element have the same atomic number but different mass numbers since they have different numbers of neutrons.

10 ISOTOPES The chemical properties of the different isotopes of an element are identical, but they will often have great differences in nuclear stability. For stable isotopes of light elements, the number of neutrons will be almost equal to the number of protons, but a growing neutron excess is characteristic of stable heavy elements.

11 more than 99% of the nuclei of oxygen atoms have 8 protons and 8 neutrons in less than one percent of cases, the oxygen nuclei have 8 protons and either 9 or 10 neutrons These three possible combinations constitute the three stable isotopes of oxygen. All other combinations lead to an unstable isotope of oxygen, that is a radioactive oxygen nucleus.

12 The element tin (Sn) has the most stable isotopes with 10, the average being about 2.6 stable isotopes per element. tiny difference in the spectral frequencies of hydrogen and deuterium comes from an essentially mechanical source, the slight change in the "reduced mass" associated with the orbiting electron. But for practical purposes the chemical behavior of the isotopes of any element are identical.

13 In Nuclear physics the proces of transmutation of one elemnts into other is posssible in the process of bombing (targetitng) the Mercurium with neutrinos (n), deuter or

14 NUCLEAR POWER The neutron is essential to the production of nuclear power. neutrons were used to effect many different types of nuclear transmutations. In 1938 the discovery of nuclear fission led to the construction of atom bombs due to the nucelar chain reaction The fission event produce neutrons, each of these neutrons might cause further fission events

15 NUCLEAR FISSION is either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei). The fission process often produces free neutrons and gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay.

16 Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original atom. Most fissions are binary fissions (producing two charged fragments) The two nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of products of about 3 to 2, for common fissile isotopes. In the liquid drop model, the two fission fragments are predicted to be the same size. The nuclear shell model allows for them to differ in size, as usually experimentally observed

17 NUCLEAR FISSION Simple method for producing radioactive nuclei consists in fragmenting stable nuclei into several pieces

18 EXCITED ATOMS Excitation is an elevation in energy level above an arbitrary baseline energy state. In quantum mechanics an excited state of a an atom / molecule / nucleus is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). The temperature of a group of particles is indicative of the level of excitation.

19 Atomic excitation hydrogen The ground state of the hydrogen atom corresponds to having the atom's single electron in the lowest possible orbit - "1s" ( which has the lowest possible quantum numbers). By giving the atom additional energy (for example, by the absorption of a photon of an appropriate energy), the electron is able to move into an excited state (one with one or more quantum numbers greater than the minimum possible). If the photon has too much energy, the electron will cease to be bound to the atom, and the atom will become ionised.

20 ATOMIC SPECTRA excited atoms emit light of certain wavelengths which correspond to different colors. The emitted light can be observed as a series of colored lines with dark spaces in between -> this series of colored lines is called a line or atomic spectra. Each element produces a unique set of spectral lines. no two elements emit the same spectral lines, elements can be identified by their line spectrum.

21 ATOMIC SPECTRA - examples The visible light spectrum is displayed at the top and line spectra for three elements - hydrogen, neon, and iron - are below.

22 Ionizing radiation is emitted when radioactive substances decay. Radioactive decay occurs when the nucleus of an atom spontaneously decays by emitting a particle. The four forms of ionizing radiation are: alpha particles beta particles gamma rays and indirectly, neutrons. All have enough energy to ionize atoms, in other words, remove one or more of the atom s electrons.

23 A nuclei which emits energy in the form of radiation is known parent radionuclide, which further transform to an atom with a nucleus in a different nucleus containing different numbers of nucleons. These new elements formed during the radioactive decay are known as daughter nuclei.

24 alpha particle 2 protons and 2 neutrons the equivalent of the nucleus of a helium atom. α particles readily ionize material they contact and transfer energy to that material s electrons. An α particle can travel several millimeters in air, but in general its range decreases with increasing density of the medium. α particles do not penetrate the outer layer of human skin, but if inhaled, alpha particles can damage lung tissue.

25 95 Am241 Np He U238 Th He Th Ra He Pu U He U Th He 4 88 Ra He 4

26 beta particle an electron -> effect of Negative beta decay an positron -> effect of Positive beta decay much lighter than an alpha particle. Thus, it takes beta particles a longer distance than alpha particles to lose energy. A medium-energy beta particle travels about one meter in air and one millimeter in body tissue.

27 beta minus decay beta decay does not change the mass number of the nucleus -> result in an increase of +1 in the atomic number because of the addition of a proton in the daughter nucleus beta decay decreases the neutron-to-proton ratio, moving the nucleus toward the band of stable nuclei.

28 beta plus decay beta decay does not change the mass number of the nucleus positron has the same mass as an electron but opposite charge, positron emission is the opposite of beta minus decay. Thus positron emission is characteristic of neutron-poor nuclei, which decay by transforming a proton to a neutron and emitting a high-energy positron

29 in positron decay a proton decays into a neutron, a positron with a neutrino particle.

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31 Gamma rays electromagnetic radiation A radioactive element may emit gamma rays (quanta, called photons) if the nucleus remaining after alpha or beta decay is in an excited state undergoes a transition to a lower-energy state by emitting a γ ray Gamma rays can penetrate much more deeply than alpha or beta particles; a high-energy gamma ray photon may pass through a person without interacting with tissue at all. When gamma rays interact with tissue, they ionize atoms.

32 X-Ray Emission Part of the electromagnetic spectrum with the wavelength range 10-8 m to m In electron capture (EC), an electron in an inner shell reacts with a proton to produce a neutron, with emission of an x-ray. The mass number does not change, but the atomic number of the daughter is lower by 1 than the parent W.C Roentgen, a German physicist discovered X-Rays in the year A photographic film wrapped in black color paper was found to be exposed when placed near a cathode-ray tube. Roentgen concluded that some invisible rays must have been emitted by the cathode ray tube and that it must have penetrated the black paper and hence could expose the photographic film.

33 Neutrons Unlike alpha and beta particles, they do not interact with electrons or cause ionization directly. however can ionize indirectly in a variety of ways: elastic collisions, inelastic scattering, nonelastic scattering, capture reactions, or spallation processes. These processes variously result in the emission of gamma rays, beta radiation, and, in the case of spallation, more neutrons.

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35 Ionizing radiation can be measured using units of electron volts, ergs, and joules The electron-volt (ev) - unit of energy associated with moving electrons around. An electron is tightly bound in a hydrogen atom (1 proton & 1 electron). It takes 13.6 electron-volts of energy to move this electron completely away from the proton. The atom is ionized The ionization energy of the tightly bound electron in hydrogen is 13.6 electron volts.

36 Mesuring Radioactivity The radioactivity of a substance is measured in the number of nuclei that decay per unit time. The standard international unit or radioactivity is called a becquerel (abbreviated Bq), which is equal to one disintegration per second (dps). Radioactivity is also measured in curies, a historical unit based on the number of disintegration per second in one gram of radium-226 (37 billion). 1 curie = 37 billion Bq. One picocurie (a trillionth of a curie) = Bq, and 1 Bq = 27 picocuries. Radioactivity is also measured in disintegration per minute (dpm). One dpm = 1/60 Bq.

37 Specific activity measures the radioactivity of a unit weight of substance. The units are curies per gram or becquerels per gram. This allows us to compare whether a substance is more or less radioactive than another. The specific activity of a radionuclide is inversely proportional to its atomic weight and its half-life.

38 Environmental and biological measurements of radioactivity are generally expressed as concentrations of radioactivity in soil, water, air, or tissue. Examples of units: picocuries per liter, becquerels per cubic meter picocuries per gram disintegrations per minute per 100 square centimeters. One picocurie (abbreviated pci) is (or ) curie.

39 The Becquerel (Bq) measures the activity of a radioactive source, giving the number of atoms which, within a particular time frame, transform and emit radiation. 1 Bq = 1 emission of radiation per second. 1 MBq = 1 mega Becquerel = 1,000,000 Bq 1 GBq = 1 giga Becquerel = 1,000,000,000 Bq 1 TBq = 1 tera Becquerel = 1,000,000,000,000 Bq

40 ABSORBING the exposure Placing your body near a radioactive source results in exposure. To evaluate the hazard from this exposure one must compute the absorbed dose. This is defined as the energy imparted to a defined mass of tissue. Dose is generally not uniform over the body. A radioactive substance can be selectively taken up by different organs or tissue.

41 Measuring the absorbed dose: GRAY & RAD The Gray (Gy) measures the absorbed dose, giving the energy transferred by ionizing radiations to the material upon encountering it. 1 Gy = 1 joule per kilogram = 100 rads 1 mgy = 1 milligray = Gy 1 μgy = 1 microgray = Gy 1 ngy = 1 nanogray = Gy Radiation doses are often calculated in the units of rad (radiation absorbeddose). One rad is 100 ergs/gram, 100 ergs of energy absorbed by one gram of a given body tissue.

42 MEASURING THE BIOLOGICAL EFFECT: THE SIEVERT The Sievert (Sv) evaluates the effects of ionizing radiation on living material. At equal doses, the effects of radioactivity on living tissue depends on the type of radiation (alpha, beta, gamma, etc.), on the organ concerned and also on the length of exposure. 1 msv = 1 millisievert = Sv 1 µsv = 1 microsievert = Sv

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46 Some units used in measuring ionizing radiation and radiation dose Unit Description Equivalent Rem (roentgen equivalent man) A unit of equivalent absorbed dose of radiation which takes into account the relative biological effectiveness of different forms of ionizing radiation, or the varying ways in which they transfer their energy to human tissue. The dose in rem equals the dose in rad multiplied by the quality factor (Q). Sievert (Sv) A unit of equivalent absorbed dose equal to 100 rem. Rad (radiation absorbed dose) Gray (Gy) Curie (Ci) Becquerel s (Bq) Disintegration s per second (dps) A unit of absorbed dose of radiation. Rad is a measure of the amount of energy deposited in tissue. A unit of absorbed radiation dose equal to 100 rad. Gray is a measure of deposition of energy in tissue. The traditional unit of radioactivity, equal to the radioactivity of one gram of pure radium-226. The standard international unit of radioactivity equal to one disintegration per second. The number of subatomic particles (e.g. alpha particles) or photons (gamma rays) released from the nucleus of a given atom over one second. One dps = 60 dpm (disintegrations per minute). rem = rad x Q Q=20 for beta and gamma radiation Q= 10 for neutrons 1 Sv = 100 rem Sv = Gy x Q 1 rad = 100 erg/gram 1 Gy = 100 rad 1 Ci = 37 billion dps = 37 billion Bq 1 Bq = 27 pci 1 dps = 1 Bq

47 Links to additional pages Animation of Bohr's Model c-theory-ii/51 Nuclear Reactions

48 Questions What is an alfa and beta decay? What is isotope? What are the units to mesure radioactivity? What are the units to mesure dose of absorption? What is a nuclide / Stable / unstable nuclide What is nuclear force? Absorption Spectrum and Emission Spectrum of an atom Rutheford / Bohr and Schrodinger models of atom structure What are neutrons for? Write a balanced nuclear equation to describe each reaction. the beta decay of S1635 the decay of P1530 by positron emission

49 More questions What makes something radioactive? an unstable nucleus YES/NO elements with an atomic number higher than 83 YES/NO More protons than neutrons YES/NO

50 1.A 1 g sample of 207 Po is allowed to decay. It has a half life of 6 hours. How much 207 Po will remain after 12 hours? 125 mg 250 mg 333 mg 500 mg 667 mg 2.What nucleus is formed as a result of the α-decay of Po? At Po Rn Pb Hg 3.What nucleus is formed as a result of the β-minus decay of 55 24Cr? Mn Mn Cr V V

51 More questions The true test of whether an element is radioactive is whether any of its isotopes -- or configurations -- are stable. If not, the substance is radioactive. It's true that all elements with an atomic number higher than 83 are radioactive, but there are also several radioactive elements with lower atomic numbers. What makes something radioactive? an unstable nucleus YES/NO elements with an atomic number higher than 83 YES/NO More protons than neutrons YES/NO