HW #11: 11.32, 11.34, 11.36, 11.44, 11.46, 11.48, 11.50, 11.52, 11.56, 11.58, 11.68, 11.76, 11.82, 11.84, 11.86

Transcription

1 Chemistry 11 Lectures & 3: Radiation; Common Decay Pathways; Radioactive Decay Rates; Detection and Quantification of Radiation; Chemical & Biological Effects of Radiation Chapter 11 in McMurry, Ballantine, et. al. 7 th edition HW #11: 11.3, 11.34, 11.36, 11.44, 11.46, 11.48, 11.50, 11.5, 11.56, 11.58, 11.68, 11.76, 11.8, 11.84, Learning bjectives: 1. Review and expand the fundamental terms associated with the nuclear atom. Define radioactivity and describe the energy associated with it 3. Define electromagnetic radiation (EMR) in terms of frequency, wavelength, and energy 4. Describe the interactions between EMR and chemical compounds 5. Describe what causes elements to be radioactive 6. Define the commonly encountered radioactive emissions 7. Write nuclear equations for commonly encountered radioactive emissions 8. Rationalize the type of radiation given off by a particular radioactive isotope 9. Describe radioactive decay as a first order process 10. Define intensity of radiation and associated units (Bq, Ci) 11. Describe the devices used to measure radiation 1. Quantify the relationship between distance from a source of radiation and intensity of radiation 13. Differentiate between energy and intensity of radiation 14. Distinguish between intensity of radiation (measured in Ci or Bq) and delivered dose of radiation (measured in R) 15. Distinguish between delivered dose of radiation (measured in R) and radiation dose absorbed (measured in rad or Gy) 16. Distinguish between radiation dose absorbed (measured in rad or Gy) and effect of dose in humans (measured in rem or Sv) 17. Describe what happens to ionizing radiation in the body 18. Rationalize why antioxidant vitamins may protect against radiation damage in humans 1

2 Nuclear Chemistry The preceding reactions we have encountered have dealt with changes in the (valence) electronic configuration between atoms. Nuclear reactions involve changes in the nucleus, with a tremendous difference in energies involved Electrons emitted from the nucleus are referred to as beta particles ( ) and have energies in the range of 1-3 MeV e or Question: What is radioactivity? Answer: The release of nuclear energy as either electromagnetic radiation (EMR) or high energy subatomic particles.k. but EMR covers a wide range of energies, including light or EMR in the visible range so we need a clearer definition Better Answer: The highly energetic emissions associated with stabilizing the nucleus in nuclear transformations is great enough to ionize perfectly stable atoms and compounds (see detecting radiation below) Before Continuing, Let s Revisit Some ld Definitions: Atomic Number Mass Number Isotope o o Stable Isotope Radioactive isotope Atomic Weight And Add a Few New Definitions: Nucleon Nuclide Radionuclide 1 An ev = 1.60 x J; 13.6 ev is the energy required to remove an orbiting electron from an individual hydrogen atom

3 Electromagnetic Radiation (EMR) Electromagnetic radiation quantized packets of energy that travel through space at the speed of light (c = 3.00 x 10 8 m/s) o The term photon was coined by G.N. Lewis from the Greek word for light but Albert Einstein developed the idea of the photon (Das Lichtquant,3 ) Wavelength () the distance between successive points in the period (or cycle) of the EMR in meters Frequency () the number of cycles that pass a fixed point per unit time o The standard unit is the hertz, where Hz = cycle/s The product of wavelength and frequency is a constant, the speed of light o c (m/cycle x cycles/s) o The speed of light, c = 3.00 x 10 8 m/s Consequently, if we know the frequency we can determine the wavelength and vice versa. Question: What is the wavelength associated with EMR of frequency Hz? This is the term used by Albert Einstein when he developed the idea of the photon. In particular, Einstein showed that shining a light on a metal surface could cause the ejection of an electron, which was dependent on the frequency of the light and not the intensity (it was for this photoelectric effect that he won the Nobel prize) 3 Say what you will about Wikipedia, but the entry on the photon is well written and highly recommended to the interested student 3

4 Photon Energies Light (EMR of specific wavelength or frequency) is quantized in terms of allowed energy. We consider light to be a stream of energy packets which we refer to as photons. This is given by E = h= hc/ h = Planck s constant = 6.63 x joule sec o A joule is the SI unit of energy and is related to the calorie by 1 calorie = J (recall 1 calorie is the amount of energy needed raise 1 gram of water 1 o C) 4 You should note from the equations relating photon energy to frequency and wavelength that energy is proportional to frequency and inversely proportional to wavelength In order to get a better feel for the energies associated with different frequencies of EMR, let s consider my latest invention The Gamma Ray Food Cooker: E = h = hc/ h = 6.63 x J s c = 3.0 x 10 8 m/s Microwave radiation has a wavelength of 10 - m. Let s compare it to EMR = m in the gamma range 4 For additional perspective, note a watt is defined as a J/s, so 1 joule would run a 10 W bulb for 0.1 s or 1 kj would run a 100 W bulb 10 s. Typical covalent bond energies are on the order of 40 kj/mol (100 kcal/mol) 4

5 For perspective, the wavelength of EMR in the visible spectrum runs from approximately 400 nm (4 x 10-7 m) to 700 nm (7 x 10-7 m) The mnemonic RY G. BIV follows the visible spectrum from lowest energy to highest Electrons are excited to higher energy levels in the UV and visible EMR range o Excitation energies (and resultant absorption of light) in the visible range gives rise to color At lower infrared energies, absorption of light causes changes in molecular motion, thus heat lamps focus on delivering infrared EMR Returning to our focus on nuclear chemistry, and x-ray EMR is of such high energy that it can only result from nuclear transformations Radioactive Emissions Why nuclear transformations occur in the first place: the belt of stability 5

6 Stability of 11 C vs. 1 C vs. 13 C vs. 14 C Notice this 1:1 n:p ratio deflects upwards as more charge is packed into the tiny space of the nucleus; e.g. Lead, atomic number 8, with the most common stable isotope being 06 Pb Question: What is the purpose of the neutron? Why is it unnecessary for the hydrogen atom ( 1 H) to have a neutron? Particles Commonly Encountered Typical species emitted from the nucleus in natural radioactive decay processes 5 Beta particles Alpha particles EMR in the x-ray & range 5 Neutrons may also be emitted in natural processes, typically if the nucleus is in an excited state after emission of another form of radiation. More often, neutrons are produced as a result of induced radioactive decay, such as the fission of 35 U for power 6

7 What Happens When a Nucleus Emits Radioactivity? While there is a conversion of mass to (considerable) energy in nuclear reactions (the famous E = mc equation), as a first approximation mass and charge are still conserved n Writing Nuclear Equations: While redundant, this is the one occasion where a left subscript for atomic number is necessary to help account for conservation of mass and charge U Th He Betaemission Typically for (lighter) elements that need to decrease n:p 1 n H e Examples Sr ? 1 e I ? 1 e Alpha emission Typically for (heavier) elements that need to raise the n:p ratio U Th He Questions: 6 Ra decays by alpha emission (1): What is the product of the nuclear reaction? Find Ra on the periodic table (): What are likely sites of toxicity of this radioactive nucleotide? Answer (1): Answer (): 7

8 Positron emission Common when n:p is too low; i.e., when the conversion of a proton to a neutron would come in handy Questions: 13 N and 11 C are positron emitters. What are the results of these decay processes? Answer: 13 N 11 C Question: Why are positron emitters much rarer than other radioactive isotopes? For instance, Carbon-11 or 11 C has a half-life of 0.38 min. Answer: Question: Why do PET scanners detect radiation? What happened to the mass? Answer: Conservation of linear momentum and total energy means must be produced (the above is a Feynman diagam) 8

9 Gamma Emission Really this is just a means of converting a high energy nucleus from an excited state to its ground state. 11 B B 11 5 * 5 Radioactive Decay Radioactive Decay is a First-rder Reaction Disintegrations are dependent on concentration; that is the number of nucei which undergo transmutation and emit radioactivity is proportional to the number that still remain The key result is half-life (t1/) is a constant for radioactive decay Question: A 1 mci 6 dose of radioactive 131 I intended for a patient with Grave s disease was delayed from its site of production for 16 days due to inclement weather and a routing problem. What is the radioactivity of the dose upon arrival? 131 I has a half life of 8 days. Generally, the amount of a radioactivity remaining is given by (N/No) = (1/) n, where N is the remaining activity, No is the initial activity, and n is the number of half-lives that have elapsed Question: What would the remaining radioactivity be if the 131 I had been delayed 6 days? 6 A Curie (Ci) is a unit of measurement for quantifying intensity of radiation (as opposed to energy of each particle). 1 Ci = 3.7 x disintegrations/second 9

10 Detecting & Measuring Nuclear Radiation ne characteristic of radiation is its ability to knock electrons out of their orbits when it encounters other matter This is particularly true of, and X-rays. Radiation with this ability is referred to as ionizing radiation If electrons are knocked out of an atom it becomes a positively charged cation The Geiger-Müller Counter 10

11 Radioactivity is measured in terms of energy and intensity Intensity: number of particles or photons/time; also referred to as disintegrations/time 1 curie (Ci) = 3.7 x dps (equivalent to decay rate of 1 g 6 Ra) SI intensity unit is the becquerel; 1 becquerel (Bq) = 1 dps An important point to bear in mind is that the intensity of radiation from a point source decreases as the square of the distance. That is, I I 1 d d 1 d1 better: I I1 d Question: A radioactive 15 I seed was placed in a prostrate and produced 30 kev X- rays and intensity = 10 mci at the border of the prostate, 1 cm away. What is the radiation intensity at the border of the testes, 3 cm away? Energy: the ability to do work (damaging or productive) for each particle We may quantify the energy of EMR by its frequency or wavelength: E = h= hc/, where h = Planck s constant = 6.63 x joule sec Thus rays are approximately 1000 times more energetic than X-rays Typical energy ranges for radiation are 1 to 10 MeV or 3.83 x to 3.83 x cal - for a single particle or photon 11

12 Radiation Dosimetry Radiation Dosimetry: The calculation of the absorbed radiation dose after exposure to ionizing radiation When considering radiation dosimetry and its effects it is important to consider the intensity of ionizing radiation, the overall energy of ionizing radiation, the ability of that radiation to be absorbed, and the ability of the absorbed radiation to elicit tissue damage Curies are a measure of intensity of radiation; how often a transmutation event fires off high energy radiation Roentgens (R) are a measure of the energy delivered by a source of radiation; recall that total energy = (energy/transmutation) x number of transmutations o o A roentgen is equivalent to the dissipation of 87.6 ergs/g dry air; this is a small quantity of energy as 1 erg = 10-7 joule As ionizing radiation produces ions, we may look at the energy delivered in terms of the charge developed; 1 R =.58 x 10-4 coulomb/kg Roentgens do not take into account the ability of a particular tissue to absorb radiation; this is quantified by the RAD or radiation absorbed dose. Before damage can occur, the tissue must absorb radiation o Example: For radiation 1 R yields.97 RAD in H,.96 RAD in muscle, and.93 RAD in bone; for x-ray photons, 1 R yields 3 RAD in bone 1

13 The difference between RAD and REM is a correction factor for the ability of a given type of radiation to cause tissue damage o REM = n(rad) n = 1 gamma and beta n = 5 for low energy neutrons n = 10-0 for high energy neutrons and alpha particles Finally, we must bear in mind penetrating power Question: Why does alpha radiation have the least penetrating power and gamma radiation have the greatest penetrating power? 7,8 Answer: As a consequence, radiation is far more damaging owing to its tremendous penetrating power unless you drink a big glass of 10 Po. 7 Interestingly, neutrons have high penetrating power (no charge) which makes them particularly hazardous given their high REM values 8 As would be expected from the concept of RAD, penetrating power is substance dependent. Water is commonly used to contain neutrons, though moderation of neutron numbers in nuclear reactors is achieved by the use of [commonly] boron containing control rods which can be raised or lowered into the aqueous reaction medium. See 13

14 So how does radiation exert a biologic effect? Ionization and the generation of free radicals: Energy + H H + + e - H + H + + H e - + H H + H - So radiation exerts a biologic effect by ionization and the generation of free radicals, and we are exposed to about 360 mrem annually 9. Is there anything we can do, outside of not living in a homemade of those glass logs Hanford would like to start making? Recall free radicals are also generated in the continual generation of energy by the mitochondria in cells during the 4 electron reduction of to H We are afforded some protection by our anti-oxidant vitamins, which are efficient scavengers of free radicals. That is, antioxidant vitamins are easily attacked by these reactive single electron species, sparing us! The main antioxidant vitamins are vitamin E, vitamin A, and vitamin C. The mechanism of hydroxyl radical scavenging by vitamin E is shown below H R. R + H H.. H. R H - H H + R H - H +. R H - H +. R. R + H 9 There is no acute biological effect between 5 and 5 rem, but some concern exists for the possibility in a slight increase in mutations leading to birth defects, cancer, etc. 14