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Radioactivity and Nuclear Energy

I. Introduction

In 1896 Henri Becquerel accidently discovered in his Paris laboratory that theelement uranium is able to expose covered photographic plates, to ionize gases, and to

cause certain substances such as zinc sulfide to glow in the dark. Becquerel concludedthat uranium gives off some kind of invisible but penetrating radiation, a property he calledradioactivity.

Soon afterward, Pierre and Marie Curie, in the course of extracting uranium fromthe ore pitchblende at the same laboratory, found two other elements that are alsoradioactive They named one polonium, after Marie Curie's native Poland. The other,which turned out to be thousands of times more radioactive than uranium, was calledradium.

II. Why are Substances Radioactive?

A. Radioactivity = the spontaneous emission of radiation or particles from the nucleiof certain elements or compounds

B. Some isotopes of atoms (nuclides) have unstable nuclei. Nuclei are often unstable ifthey are exceptionally large (large numbers of protons and neutrons or anincompatible proton to neutron ratio) or if the nucleus exists in an exited state. Thesenuclei will often split (or decay) forming new more stable nuclei or lower energynuclei. When this stabilization occurs, radioactivity is often given off.

Radioactivity Radioactivity

Unstable nucleus Unstable nucleus Stable nucleus

C. Of the approximately 2000 known nuclides, 279 are not radioactive.

D. Except for hydrogen, every isotope of every element has a nucleus containing at leastone neutron for every proton. Apparently, the tremendous repulsive forces betweenprotons in the nucleus are moderated by the presence of neutrons. The more protons,the more additional neutrons beyond the exact number of protons are needed to serveas a buffer zone.

E. Above bismuth, with 83 protons and 126 neutrons, all isotopes are radioactive.Beyond this point, there is apparently no nuclear super glue sufficient to hold all thepositively charged protons in close proximity.

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III. Five Main Types of Radioactivity

A. Alpha particles

1. Alpha particles consist of 2 protons and 2 neutrons. They are helium nuclei.Alpha particles have a +2 charge.

2. Symbols 42He

3. Alpha particle production is a very common mode of decay for heavy radioactivenuclides.

4. Alpha particles are relatively large particles. Radioactivity consisting of alphaparticles is not very penetrating. A sheet of paper will stop alpha particles. Theycannot penetrate the dead layer of skin on the surface of your body. An intenseexternal dose of alpha particles, however, will produce a burn on the skin. Alphaparticles can do a great deal of damage if they are emitted inside the body, whichmight result from inhaling or swallowing an alpha emitter.

B. Beta particles

1. Beta particles are electrons (-1 charge). These electrons in the nucleus arise in thetransformation of a neutron into a proton and an electron by a complex series ofsteps. Once the electron is produced, it is ejected with a high velocity from the

atom. The net effect of-particle production is to change a neutron to a proton.

2. Symbols -01

e

3. Beta particles are smaller particles than alpha particles and are more penetratingthan alpha particles. A piece of wood will stop beta particles. Beta particles canpenetrate the dead outer layer of skin but will be stopped within the skin layer,causing damage to the skin tissue and making it appear burned. Beta particleswill not penetrate to the inner organs of the body, but serious damage can occur ifa beta emitter is taken internally.

C. Gamma rays

1. Gamma rays are a form of electromagnetic radiation (high-energy photons oflight). Gamma rays have no mass or charge. A nuclide in an excited nuclearenergy state can release excess energy by producing a gamma ray. They areusually emitted in conjunction with alpha or beta particles.

2. Symbol

D. Positron Particles

1. A positron is a particle with the same mass as the electron but opposite charge.

Production of a positron appears to change a proton to a neutron.

2. Symbol01 e

E. Electron capture

1. Electron capture is a process where one of the inner-orbital electrons is capturedby the nucleus. The net effect of electron capture is change of a proton into aneutron. Gamma rays are usually produced along with electron capture.

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IV. Radioactivity Decay Equations

A. Both the atomic number and mass number must be conserved in nuclearreactions.

B. Example Problems :

*Complete the following radioactive decay equations and circle the radioactivity emitted.

1.8035 Br __________ + -

01

e

2. As __________ + -01

e

3. __________ 136 C +

01 e

4.21085 At

20683Bi + __________

5. __________ 23893Np +

42 He +

**Write balanced nuclear equations for each of the following processes.

1. 22688

Ra produces an particle:

2. 214

82

Pbproduces a particle:

3. 116 C

produces a positron

***Supply the missing particle in each of the following nuclear equations.

1. 20180

Hg + ____ 20179

Au + 00

2. 3819

K 3818

Ar + _____

3. 22286

Rn 21884

Po + _____

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V. Half-lives of Radioisotopes

A. Half-life- = the time required for the number of nuclides in a radioactive sample toreach half the original number of nuclides.

B. The half-life is a characteristic constantfor a particular radioactive material.

C. Equations:

1.ln N

N0= k t

where N is the number of atoms present at time t, N0 is the number of atomspresent at t=0, and k is the rate constant for the disentegration of the atoms.

2. t1/2 =0.693

kwhere t1/2 is the half-life of the element.

D. Examples:

Radioisotope Half-life Radioactivity Emitted

Hydrogen-3 12 years Beta

Carbon-14 5730 years Beta

Phosphorus-32 14 days Beta

Potassium-40 1.28 X 109 years Beta and gamma

Cobalt-60 5 years Beta and gamma

Iodine-131 8 days Beta and gamma

Cesium-137 30 years Beta

Polonium-214 1.6 X 10-4 seconds Alpha and gamma

Radium-226 1600 years Alpha and gamma

Uranium-235 7.1 X 108 years Alpha and gamma

Uranium-238 4.5 X 109 years Alpha

Plutonium-239 24,400 years Alpha and gamma

E. Example Problems :

1. Give the radioactivity decay equation for cesium-137. Starting with10.0 g of cesium-137, how long would it take before you only had 2.50 g?

2. Give the radioactivity decay equation for radium-226. How much of a 100. gsample of radium-226 would remain after 8000 years?

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VI. Carbon Dating

A. Radiocarbon dating is based on the radioactivity of146

C, which decays via -particles:

146

C 01

e + 147

N

B. Carbon-14 is not only continually breaking down into nitrogen-14, however; it also iscontinuouslyproducedin the atmosphere when highe-energy neutrons from spacecollide with the nitrogen:

147

N + 10n 14

6C + 1

1H

C. Therefore, the amount of carbon-14 in the atmosphere remains constant. As plants usecarbon dioxide in photosynthesis (and we consume plants), living tissue maintains thesame % of carbon-14. When a plant dies however, there is no longer a source ofcarbon-14 to replace that lost to radioactive decay, so the plants C-14 levels begindiminishing. If you know the half-life of carbon-14, you can then calculate when anorganism died by how much below normal atmospheric concentration its C-14 levels are.

Example:Assume the half-life of C-14 is 5730 years. How old is a childs wooden toy, if theC-14 levels in it are found to be 1/8 that of trees which are alive today.

VII. Nuclear Transmutation Reactions

A. Nuclear transmutation = reaction in which a high-speed particle collides with anucleus to produce a different nucleus. Radioactivity is often emitted in this process.The high speeds of the bombarding particles are achieved with particle accelerators.

B. Elements with atomic numbers 93 through 109 are synthesized using transmutationreactions of uranium.

C. Complete the following transmutation equations:

1.23994 Pu +

42 He __________ +

10n

2. __________ +126 C 4

10n +

25198 Cf

3. 24998

Cf + _____ 260105

Unp + 4 10n

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VIII. Nuclear Energy

A. Basic Concepts

1. Medium-sized nuclei are the most stable. Therefore, combining two small nucleito get a medium-sized one (fusion) or breaking apart large nuclei to yield medium-sized ones (fission) both yield much energy.

2. In a nuclear reaction yielding energy, MASS IS LOST. The mass that is lost hasbeen converted to energy. The amount of energy generated by a given loss inmass is provided by Einsteins equation:

E = m c2

where E is the energy produced, m is the loss in mass, and c is the speed of light.

Example:Consider the formation of deuterium from hydrogen and a neutron:

11H + 1

0n 2

1H

The actual masses are:

11

H = 1.6736 x10 24 g ; 10

n = 1.6750 x 1024 g ; sum= 3.3486 x 1024 g

However, the mass of deuterium is 3.3446 x 10-24 g, representing a total loss of4.03 x 10 -27 g. Therefore, how much energy is released by a single atom of deuterium?

B. Nuclear fission = process in which a heavy element nucleus splits into two ormore lighter nuclei (with release of enormous amounts of energy) as a result ofnuclear bombardment.

Example : An atom of fissionable (splittable) fuel such as23592 U is struck by a neutron

and broken into 2 high speed fragments, 2 or 3 neutrons, and large amounts ofenergy.

10 n +

23592 U

14156 Ba +

9236 Kr + 3

10 n + energy

In this process the 235U absorbs a neutron to become 236U. 236U is so unstable that itexplodes to give the products above. These new nuclei are radioactive themselvesand extremely dangerous. This process can be used in both atomic bombs andnuclear power plants. The primary difference in the two processes is that in anatomic bomb the majority of neutrons produced react farther with additional fuel in anuncontrolled chain reaction, and in a nuclear power plant only a few neutrons react

further and the fission reaction self-propagates in a controlled manner.

C. Nuclear fusion = the process of combining two light nuclei to form a heavy, morestable nucleus with a release of enormous amounts of energy

1. Occurs on the sun (2H e)

2. According to accepted theory (recently disputed) nuclear fusion requires hightemperatures (about 40 million K) in order to produce nuclear speeds fast enoughto overcome nuclear repulsions.

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IX. Effects of Radiation and Radioactivity on Cells

A. Radiation damage to organisms can be classified as somatic or genetic damage.

1. Somatic damage = damage to the organism itself, resulting in sickness or death. The

effects may appear almost immediately if a massive dose of radiation is received; forsmaller doses, damage may appear years later, usually in the form of cancer.

2. Genetic damage = damage to the genetic machinery of reproductive cells, which can"surface" as problems afflicting the offspring of the organism

B. The biological effects of a particular radiation depend on several factors:

1. Penetrating ability of radiation--The particles and electromagnetic rays produced inradioactive processes vary in their ability to penetrate human tissue.a. Gamma rays are highly penetrating.b. Beta particles can penetrate approximately 1 cm into the skin layer.

c. Alpha particles are stopped by the skin.

2. Ionizing ability of the radiation--Because ions behave quite differently from neutralmolecules, radiation that removes electrons from molecules in tissues seriously disturbstheir functions. Alpha particles, beta particles, fast-moving neutrons, gamma rays, x-rays, and cosmic rays possess a certain amount of energy as they hit tissue moleculeswhich is transferred to the tissue. These forms of particles and radiation are all capable ofproducing ions within the tissue. An ion pair is the electron and positive ion that areproduced during an ionization collision of an atom and radiation. Many ion pairs areproduced by a single "particle" of radiation because such a particle must undergo manycollisions before its energy is reduced to the level of the surrounding material. Theelectrons ejected from and atom frequently have enough energy to bombard neighboringmolecules and cause further ionization. The ionizing effect is what makes radiation

harmful to living materials. In living matter, the formation of ions and free radicalsdisrupts cellular function.

The ionizing ability of radiation varies dramatically:a. Gamma rays penetrate very deeply but cause only occasional ionization.b. Alpha particles, although not very penetrating, are very effective at causing ionization

and produce serious damage. Therefore, the ingestion of a producer of alphaparticles, such as plutonium, is particularly damaging.

c. Beta particles are intermediate between gamma rays and alpha particle in ionizingability.

3. Chemical properties of radiation source--When a radioisotope is ingested, its capacity tocause damage depends on how long it remains in the body .Example: Krypton-85 and strontium-90 are both beta particle producers. Becausekrypton is a noble gas and is almost chemically inert, it passes through the body quicklyand does not have much time to do damage. Strontium, on the other hand, is chemicallysimilar to calcium. It can collect in the bones, where it may cause leukemia and bonecancer.

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X. Some Units to Measure Radiation Doses

A. Becquerel (Bq)

1. 1 becquerel = 1 disintegration (nuclear transformation) per second

2. Does not describe the type of radiation or the effect of radiation on tissue or other

matter

B. Curie (Ci)

1. 1 curie = 3.7 X 1010 disintegrations (nuclear transformations) per second

2. Does not describe the type of radiation or the effect of radiation on tissue or othermatter

C. Rad (rad)-- r adiation a bsorbed d ose--measures amount of energy absorbed bythe tissue

1. R adiation a bsorbed d ose is influenced by the radioactivity source, energy of theradiation, distance of radiation source form the tissue, the nature of the tissue, andlength of exposure to radiation.

2. 1 rad = absorbed dose of radiation which results in the transfer of 100 ergs ofenergy to 1 g of irradiated tissue

3. Lethal dose--about 1000 rads (0.0024 calorie) to entire human body

D. Rem--roentgen equivalent man--absorbed dose of radiation which will produce thesame biological effect as 1 rad of therapeutic x-rays

1. The rem takes into account the fact that different radiation when absorbed bytissue in equal quantities produces different biological effects.

2. For example, to produce the effect of 1 rem takes about 1 rad of beta particles butonly 0.1 rad of alpha particles. Alpha particles are much more damagingbiologically even though they are less penetrating. Over the path that theypenetrate they crash though tissues producing dense clusters of ionization alongtheir short pathways. Beta particles are more penetrating but zip through tissuescreating more scattered ionization.

E. LET--linear energy transfer--average energy released per unit of path length ofradiation

1. High LET--neutrons and alpha particles--do not travel far but where they travelthey produce closely packed groups of ion pairs

2. Low LET--beta particles, x-rays, gamma rays, and cosmic rays--travel longdistances. Leave behind sets of ion pairs which are spread out.

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XI. Detection of Radiation

A. Geiger-Mller counter = detects radiation by amount of ionization of a gas(usually argon) in a tube

-- -- - -

- --

gas

(detects charges)

Incoming Radiation

Will Click or Turn a Counter(detects beta particles best)

B. Scintillation counter = detects radiation because radiation strikes a surface coatedwith a chemical which will produce flashes of light when struck by radiation (NaI).

C. Film badges = degree of darkening of a negative indicates the radiation exposure

XII. Protection against Radiation

A. Shielding

1. Alpha particles--paper stops--not very penetrating

2. Beta particles--wood stops

3. Gamma rays and x-rays--thick concrete stops--very penetrating. Lead apronsare often used.

B. Distance--farther away from radiation source the lower the amount of exposure

XIII. Typical Annual Radiation Exposure of an Average Person in the United States

A. Natural Sources

1. Cosmic radiation About 26%

2. Rocks and soil About 24%

3. Minerals in the body About 11%

B. Human Activities

1. Medical x-rays About 26%

2. Nuclear medicine About 7%

3. Radioactive fallout About 2%

4. Other About 4%

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XIV. Radioisotopes and Medicine

A. Radiotracers = a radioactive nuclide, introduced into an organism for diagnosticpurposes, whose pathway can be traced by monitoring its radioactivity. Radioactiveand their nonradioactive counterparts differ in their nuclear properties, but not in theirchemical properties. Thus, body chemistry is not upset by the presence of a smallamount of radioactive substance whose nonradioactive form is already present in the

body. Radiotracers provide sensitive and nonsurgical methods for learning aboutbiological systems, for detecting disease, and for monitoring the action andeffectiveness of drugs.

B. Desirable properties of potential radioactive tracers:

1. At low concentrations, the radioisotope must be detectable by instrumentationplaced outside the body. Almost all radiotracers are gamma emitters, because thepenetrating power of alpha and beta particles is too low.

2. The radioactive tracer must have a short half-life (usually 3 hours to less than 10days).

3. The radioactive tracer must be eliminated by the body not remain in the bodyindefinitely.

4. The chemical properties of the radioactive tracer must be compatible with normalbody chemistry.

C. Radioisotopes used in diagnosis

1. Technetium-99m

(a) Source is not natural

99

42 Mo 99m

43 Tc + -0

1e + m = metastable (in high energy state)

(b) Decays as follows

99m43 Tc

9943Tc +

(c) Advantages:

1. Gamma rays are easy to detect.

2. Short half-life (6.02 hours) but long enough for it to localize in

the body

3. No worse than an x-ray biologically

4. Used to detect cancer and heart disease and for tumor detection in theliver, spleen, and bone.

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2. Iodine-131

(a) Not natural, produces beta and gamma radiation, t1/2 = 8.1 days

(b) Used to detect thyroid disease. Patients drink a solution containing NaI thatincludes iodine-131, and the uptake of the iodine by the thyroid gland is

monitored with a scanner.

3. Others and their applicationDecay

Nuclide Half-life Area of the Body Studied Mode

Iron-59 45.1 days red blood cells beta

Molybeum-99 67 hours metabolism beta

Phosphorus-32 14.3 days eyes, liver, tumors beta

Chromium-51 27.8 days red blood cells electron capture

gamma

Strontium-87 2.8 hours bones electron capturegamma

Xenon-133 5.3 days lungs beta

Sodium-24 14.8 hours circulatory system beta

D. Radiation therapy

1. Cancer cells divide more rapidly than normal cells. Cells that reproduce at a rapidrate are more sensitive to radiation damage. Therefore, cancer cells are moresensitive to radiation than normal cells, and low doses ofx-rays and gamma rays are often used to treat cancer.

2. Teletherapy = use of radiation to destroy cancerous tissue not removable bysurgery. Often use x-rays or gamma rays (cobalt-60).

3. Brachytherapy = insert radioisotopes by needle or seed into the area to betreated. Often use gold-198 or iridium-192. Used often for skin cancers.