Radioactive nuclei. From Last Time. Biological effects of radiation. Radioactive decay. A random process. Radioactive tracers. e r t.

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From Last Time Nuclear structure and isotopes Binding energy of nuclei Radioactive nuclei Final Exam is Mon Dec 21, 5:05 pm - 7:05 pm 2103 Chamberlin 3 equation sheets allowed

Dec 14, 2009 Phy208 Lecture 29 1 ~ equal # neutrons and protons Tues.

Emitted particle carries away energy Can strip electrons from atoms (ionizing radiation) break apart chemical bonds in living cells (radiation damage) Geiger counter

effectiveness) Radiation type RBE X-rays 1 Gamma rays 1 Beta particles 1-2 Alpha particles 10-20 Dose equivalent (Energy deposited / tissue mass) x RBE Units of Sv (sieverts)

Dec 14, 2009 Phy208 Lecture 29 4 Radioactive tracers Worked on radioactivity as student with Ernest Rutherford. Lodged in nearby boarding home.

dehevesy described his first foray into nuclear medicine: The coming Sunday in an unguarded moment I added some radioactive deposit [lead-212] to the freshly prepared pie and

1 From Last Time Nuclear structure and isotopes Binding energy of nuclei Radioactive nuclei Final Exam is Mon Dec 21, 5:05 pm - 7:05 pm 2103 Chamberlin 3 equation sheets allowed About 30% on new material Rest on topics of exam1, exam2, exam3. Week15HW covers material for final, but does not count toward HW grade. Tues. Dec 14, 2009 Phy208 Lecture 29 1 ~ equal # neutrons and protons Tues. Dec 14, 2009 Phy208 Lecture 29 2 Radioactive decay Unstable nuclei decay by emitting particle Can be photon (light particle), or matter particle. Emitted particle carries away energy Can strip electrons from atoms (ionizing radiation) break apart chemical bonds in living cells (radiation damage) Geiger counter Biological effects of radiation Radiation damage depends on Energy deposited / tissue mass (1 Gy (gray) 1J/kg) Damaging effect of particle (RBE, relative biological effectiveness) Radiation type RBE X-rays 1 Gamma rays 1 Beta particles 1-2 Alpha particles Dose equivalent (Energy deposited / tissue mass) x RBE Units of Sv (sieverts) [older unit rem, 1 rem0.01 Sv] Common units msv (10-3 Sv), mrem (10-3 rem) Common safe limit 500 mrem/yr (5 msv/yr) Tues. Dec 14, 2009 Phy208 Lecture 29 3 Tues. Dec 14, 2009 Phy208 Lecture 29 4 Radioactive tracers Worked on radioactivity as student with Ernest Rutherford. Lodged in nearby boarding home. Suspected his landlady was serving meals later in week recycled from the Sunday meat pie. His landlady denied this! dehevesy described his first foray into nuclear medicine: The coming Sunday in an unguarded moment I added some radioactive deposit [lead-212] to the freshly prepared pie and on the following Wednesday, with the aid of an electroscope, I demonstrated to the landlady the presence of the active deposit in the soufflé. George de Hevesy A random process Radioactive decay is a random process It has some probability of occurring. For one nucleus, Prob( decay in Δt) rδt r decay rate For N nuclei, # decays ΔN N x Prob(decay) rnδt # decays / s ΔN/Δt rn N N o e r t Tues. Dec 14, 2009 Phy208 Lecture 29 5 Tues. Dec 14, 2009 Phy208 Lecture

2 Radioactive half-life Example of random decay. Start with 8,000 identical radioactive nuclei After one half-life, half the nuclei have decayed. Undecayed nuclei t0 t1 yr t2 yr t3 yr Every half-life, half the atoms decay Tues. Dec 14, 2009 Phy208 Lecture 29 7 Radioactive decay question A piece of radioactive material is initially observed to have 10,000 radioactive nuclei. 3 hours later, you measure 1,250 radiaoctive nuclei. The half-life is A. 1/2 hour B. 1 hour C. 3 hours D. 8 hours In each half-life, the number of radioactive nuclei, and hence the number of decays / second, drops by a factor of two. After 1 half life, 5000 are left undecayed. After 2 half lives, 1/2 of these are left: 2,500 After 3 half lives there are 1,250 left. Tues. Dec 14, 2009 Phy208 Lecture 29 8 Radioactive decay question A piece of radioactive material is initially observed to have 1,000 decays/sec. It s half life is 2 days. Four days later, you measure A. 1,000 decays / sec B. 500 decays / sec C. 250 decays / sec D. 125 decays / sec Tues. Dec 14, 2009 Phy208 Lecture 29 9 Decay rate r (Units of s -1 ) Quantifying radioactivity Prob( nucleus decays in time Δt ) r Δt Activity R (Units of becquerel (1 Bq1 s -1 ) or curie (1 Ci3.7x10 10 s -1 ) Mean # decays / s rn, N# nuclei in sample Half-life t 1/2 (Units of s) time for half of nuclei to decay t 1/2 N N o e r t ln2 r r Tues. Dec 14, 2009 Phy208 Lecture Different types of radioactivity Three different types of decay observed: Alpha decay Gamma decay (First three letters of Greek alphabet). Ernest Rutherford (1899): "These experiments show that the uranium radiation is complex and that there are present at least two distinct types of radiation - one that is very readily absorbed, which will be termed for convenience the alpha-radiation, and the other of more penetrative character which will be termed the beta-radiation." Tues. Dec 14, 2009 Phy208 Lecture Example of α decay Heavy nucleus spontaneously emits alpha particle nucleus loses 2 neutrons and 2 protons. It becomes a different element (Z is changed) Example: 92 protons 146 neutrons U 4 2 He protons 2 neutrons 234 Alpha particle Th 90 protons 144 neutrons Tues. Dec 14, 2009 Phy208 Lecture

Decay sequence of 238 U Radon Zone 1 Highest Potential (greater than 4 pci/l) Zone 2 Moderate Potential (from 2 to 4 pci/l) α decay Radon is in the 238 U decay series Radon is an α emitter that

Dec 14, 2009 Phy208 Lecture 29 14 Activity of Radon 222 Rn has a half-life of 3.83 days. Suppose your basement has 4.0 x 10 8 such nuclei in the air. What is the activity?

3 Decay sequence of 238 U Radon Zone 1 Highest Potential (greater than 4 pci/l) Zone 2 Moderate Potential (from 2 to 4 pci/l) α decay Radon is in the 238 U decay series Radon is an α emitter that presents an environmental hazard Inhalation of radon and its daughters can ionize lung cells increasing risk of lung cancer Tues. Dec 14, 2009 Phy208 Lecture Tues. Dec 14, 2009 Phy208 Lecture Activity of Radon 222 Rn has a half-life of 3.83 days. Suppose your basement has 4.0 x 10 8 such nuclei in the air. What is the activity? We are trying to find number of decays/sec. So we have to know decay constant to get RrN r t 1/ days 86,400s/day s R dn rn s nuclei 836decays/s dt 1Ci R 836 decays/s decays/s 0.023µCi Tues. Dec 14, 2009 Phy208 Lecture Tues. Dec 14, 2009 Phy208 Lecture Decay sequence of 238 U But what are these? α decay decreases by one increases by one Electron (beta particle) emitted But nucleus has only neutrons & protons. Nucleus emits an electron (negative charge) Must be balanced by a positive charge appearing in the nucleus. This occurs as a neutron changing into a proton Tues. Dec 14, 2009 Phy208 Lecture Tues. Dec 14, 2009 Phy208 Lecture

Changing particles Neutron made up of quarks. One of the down quarks changed to an up quark. New combination of quarks is a proton.

radioactive carbon dating. Half-life 5,730 years. Tues. Dec 14, 2009 Phy208 Lecture 29 19 Tues.

Steady-state achieved in atmosphere, with 14 C: 12 C ratio ~ 1:1 trillion (1 part in 10 12 ) Carbon-dating question The 14 C: 12 C ratio in a fossil bone is found to be 1/8 that of the ratio

As long as biological material alive, atmospheric carbon mix ingested (as CO 2 ), ratio stays fixed. After death, no exchange with atmosphere. Ratio starts to change as 14 C decays A.

4 Changing particles Neutron made up of quarks. One of the down quarks changed to an up quark. New combination of quarks is a proton. beta decay example 14 6 C 14 7 N + e 8 neutrons 6 protons 7 neutrons 7 protons 14 nucleons 14 nucleons 6 positive charges 7 positive charges + 1 electron + 1 negative charge Used in radioactive carbon dating. Half-life 5,730 years. Tues. Dec 14, 2009 Phy208 Lecture Tues. Dec 14, 2009 Phy208 Lecture Radiocarbon dating 14 C has a half-life of ~6,000 years, continually decaying back into 14 N. Steady-state achieved in atmosphere, with 14 C: 12 C ratio ~ 1:1 trillion (1 part in ) Carbon-dating question The 14 C: 12 C ratio in a fossil bone is found to be 1/8 that of the ratio in the bone of a living animal. The half-life of 14 C is 5,730 years. What is the approximate age of the fossil? As long as biological material alive, atmospheric carbon mix ingested (as CO 2 ), ratio stays fixed. After death, no exchange with atmosphere. Ratio starts to change as 14 C decays A. 7,640 years B. 17,200 years C. 22,900 years D. 45,800 years Since the ratio has been reduced by a factor of 8, three half-lives have passed. 3 x 5,730 years 17,190 years Tues. Dec 14, 2009 Phy208 Lecture Tues. Dec 14, 2009 Phy208 Lecture Other carbon decays Too few neutrons Lightest isotopes of carbon emit positron antiparticle of electron, has positive charge! Too many neutrons Gamma decay Alpha decay (alpha particle emitted), (electron or positron emitted), can leave nucleus in excited state Nucleus has excited states just like hydrogen atom Emits photon as it drops to lower state. 9 C 6 9 B 5 This is antimatter Nucleus also emits photon as it drops to ground state This is gamma radiation 3 neutrons 6 protons 4 neutrons 5 protons + e + Extremely high energy photons. Ni Ni Tues. Dec 14, 2009 Phy208 Lecture Tues. Dec 14, 2009 Phy208 Lecture

Decay summary Alpha decay Nucleus emits He nucleus (2 protons, 2 neutrons) Nucleus loses 2 protons, 2 neutrons Beta - decay Nucleus emits electron Neutron changes to proton in nucleus Beta + decay

What particle is it? Na atomic number 11 Ne atomic number 10 A. Alpha B. Electron beta C. Positron beta D.

5 Decay summary Alpha decay Nucleus emits He nucleus (2 protons, 2 neutrons) Nucleus loses 2 protons, 2 neutrons Beta - decay Nucleus emits electron Neutron changes to proton in nucleus Beta + decay Nucleus emits positron Proton changes to neutron in nucleus Gamma decay Nucleus emits photon as it drops from excited state Decay question 20 Na decays in to 20 Ne, a particle is emitted? What particle is it? Na atomic number 11 Ne atomic number 10 A. Alpha B. Electron beta C. Positron beta D. Gamma 20 Na has 11 protons, 9 neutrons 20 Ne has 10 protons, 10 neutrons So one a proton (+ charge ) changed to a neutron (0 charge) in this decay. A positive particle had to be emitted. Tues. Dec 14, 2009 Phy208 Lecture Tues. Dec 14, 2009 Phy208 Lecture Radiation Therapy 50-% of cancer patients treated with radiation Goal: disable cancerous cells without hurting healthy cells X-rays or γ-rays ( Co) from 20 KV to 25 MV Exposure from laboratory source ( decays/s) Co source has an activity of 1 µcurie Each decay: 1.3 MeV photon emitted Assume all absorbed by a 1 kg section of your body for 1 hour Energy absorbed in 1 kg ( ev )( J /ev )( decays/s) ( 1hr) ( 30s/hr) J What dose do you receive? ( J /kg) 1rad /( 0.01J /kg) rad rem ( ) A. 0.5 rem B. 0.3 rem C. 0.1 rem D rem E rem Tues. Dec 14, 2009 Phy208 Lecture Tues. Dec 14, 2009 Phy208 Lecture

Final Exam. Physics 208 Exit survey. Radioactive nuclei. Radioactive decay. Biological effects of radiation. Radioactive tracers

Final Exam. Physics 208 Exit survey. Radioactive nuclei. Radioactive decay. Biological effects of radiation. Radioactive tracers Final Exam Mon, Dec 15, at 10:05am-12:05 pm, 2103 Chamberlin 3 equation sheets allowed About 30% on new material Rest on topics of exam1, exam2, exam3. Study Tips: Download blank exams and take them. Download