The Nucleus Chapter 24 5/9/2019

Facts about the nucleus - a tinny part of the atom (thousand times smaller than the atom) - positively charged (the amount of charge = charge of the electron cloud) - it carries almost all the mass of the atom - determines the nature of the atom (the element) - does not change during chemical reactions - studying the nucleus more difficult than studying the atom - quantum mechanical effects are predominant - unknown forces that hold the alike charges together - strange radiation emerging from nucleus: alpha, beta and gamma rays - must have an internal structure - must contain other elementary particles - a challenging problem for the XX century physics 1 5/9/2019

Radioactivity Henri Becquerel: strange rays coming from all uranium compounds --> should have a nuclear origin (not changed during chemical reactions, or different physical conditions) the strength of the radiation depends only on the amount of uranium present Pierre Curie and Marie Sklodowska Curie: thorium also radiates! NEW radiating elements: polonium and radium three different type of radiation identified: - alpha () radiation: small penetration in objects, positively charged particles, 2X charge of an electron, 4X the mass of H, (He atom nucleus) - beta () radiation: bigger, but still small penetration, negatively charged particles, mass and charge of the electrons, --> electrons! - gamma () radiation: high penetration in all materials, no charge, shown to be very high energy photons 2

New image of the Nucleus Radioactivity --> nuclei have internal structure What is inside? First theory: protons (p) (H nucleus) + electrons - to account for the  and  radiation - protons: existence proved by Rutherford and Chadwick (1919), a new accepted elementary particle - theoretical work shows that electrons cannot be in the nucleus! - something else must exist, which have a mass comparable with the mass of protons (to account for the elements known relative mass) discovery of neutrons (n) (Chadwick, 1924) - neutrons: mass of the proton and no charge, can disintegrate in proton and electron (but not = proton+electron) , explain the mass of the elements in the periodic table + ISOTOPES (same atoms with different mass = same number of protons, but different number of neutrons) existence of isotopes --> non integer relative masses of the elements protons and electrons are called nucleons isotopes of H: deuterium tricium 3

Nuclear reactions and the Alchemists’ Dream After radioactive decay the resulting nucleus (daughter) can have different charge from the original (parent) nucleus number of protons --> charge --> the nature of the atom atoms can thus change in other elements through nuclear reactions but not through chemical reactions! What kind of reactions are possible? Conservation laws that must must be obeyed: mass-energy, linear momentum, angular momentum, charge, nucleon number ……. Notations of nucleons --> Superscript: nr.of nucleons Subscript: nr. of elementary charge Example for nuclear reactions: note conservation of nucleon number and charge Total energy (including the rest-mass energy) and total momentum should also conserve! 4

Reactions that occur naturally nucleus emitting an  particle (radioactive  decay) nucleus emitting an electron (radioactive  decay) inverse  decay (electron capture) beta minus decay (positron emission) positron: the antiparticle of electron gamma ray emission: do not change the identity of the nucleus, it is the result of the relaxation of an excited nucleus 5

Law of the radioactive decay Equal amounts of different radioactive materials do not give off radiation at the same rate measure of radiation: activity (number of decays /unit time) unit for activity: 1 curie (Ci) = 3.7 x 1010 decays/second activity of a sample determined by two factors - number of radioactive atoms in the sample - type of nucleus basic law: the probability P that a radioactive nucleus will decay is the same at each time moment (until the nucleus decays) result: The amount of radioactive nucleus which are in the sample, decays exponentially in time (activity decays exponentially in time) analogy: throwing a given number of dice, and removing the 1 results (activity proportional with the number of remained dice) notion of half-life (the time it takes the activity to drop to one-half its original value) - constant during the evolution! half-lifes inversely proportional with the probability P different type of nucleus have different half-lifes, or different P values! - like throwing dice with other number of faces, and taking out again those showing: 1 6

Radioactive clocks The radioactive decay rate is unaffected by physical and chemical conditions on Earth Knowing (1) the half-life of a particular isotope (2) the products into which it decays (3) relative amount of parent and daughter atoms - we can calculate how long it has been decaying !!! --> radioactive clock Uranium-238 -- > Lead-206 (half-life of 4.5 billion years) appropriate to date the lifetime of Earth and other planets (Uranium is formed only during supernova, explosions!!) dating organic materials - ratio of the stable 12C and radioactive 14C isotopes - half-life for 14C: 5700 years - during the life of an organism the 14C is replaced through the metabolism 12C/14C is constant - after the organism dies 14C is not anymore replaced 12C/14C increases! --> possibility to date the death (not useful beyond 40,000 years) --> need other radioactive clocks 7

Interaction of the radiation with matter Radiation is invisible We detect radiation through their interaction with matter ,  and  rays interact strongly with matter, being stopped or de-accelerated by passing through materials  and  rays interact with matter through their charges -  and  produce the ionization of the matter, and loose their speed continuously; - distance they travel depends on their initial energy -  rays has much smaller penetration ranges than  rays interaction of  rays with matter is different, they do not lose their energy in bits and pieces a photon can loose or its entire energy in interactions, or non of it (energy of the photon can be used to (1) ionize an atom, (2) given to a free electron with creation of a new photon, (3) to create a particle-antiparticle pair) number of gamma photons surviving at various distances through a material --> exponential decay 8

Biological Effects of radiation The ionization caused by radiation passing through a living tissue can destroy organic molecules the effect on health depends on: - amount of radiation absorbed by the living tissue - biological effects associated with this absorption measures: rad : (radiation absorbed dose)--> amount of energy deposited in the material; 1 rad = 0.01 J/kg rem: (radiation equivalent in mammals) --> reflects the biological effects of radiation; different type of radiation have different effects 1 rad  radiation  1.7 rem exposure; 1 rad of photons  1 rem 1 rad  radiation  20 rem exposure; 1 rad of n or p  10 rem maximum recommended occupational dose: 5 rem/year maximum recommended rate for general public: 0.5 rem/year 9

10 Radiation around us It is impossible to get away from ALL radiation Cosmic rays: 0.03 rem/year Radon in buildings: 0.2 rem/year Surroundings (from natural radioactivity): 0.03 rem/year Consumer products: 0.005 rem/year Medical analyses (X-rays, isotope diagnoses): 0.05 rem/year From Nuclear power: 0.005 rem/year Total (normally) approx: 0.35 rem/year 10

11 Radiation Detectors Scintillation detectors - earliest: fluorescent screen --> for alpha particles (Rutherford) - television screen --> for electron detection Photomultiplier tubes: can detect single particles Geiger counter (ionization of gas to detect the radiation) Bubble chambers (reveals the track of the particles) 11

13 Summary Home work assignment : 637/1-9,13-21; 638/22-33,35-38 the nucleus of the atom has an internal structure radioactivity is a nuclear effect rather than an atomic one three type of radiation: (helium nuclei) ,  (electrons) and (photons) nuclei are composed of protons and neutrons isotopes of an element differ by the number of the neutrons in the nucleus radioactive decays are spontaneous, obey the known conservation laws and most often change the chemical nature of their atoms two factors determine the activity of a radioactive sample: the number of radioactive atoms, and the characteristic half-life all radiation interacts with matter depositing energy in the material leading to temperature increase, atomic excitation and ionization. The ionization caused by radiation passing through living tissue can kill or damage cells. Home work assignment : 637/1-9,13-21; 638/22-33,35-38 638/41-43,45-48; 639/51,54-57,9-16; 640/17,18 13