lec 4 - radioactivity

7/30/2019 Lec 4 - Radioactivity

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RADIATION AND RADIOACTIVITY

Radiation is the process taking place in the atoms nucleus which allows unstable elements tobecome stable. Unstable elements give off radiation.

TYPES OF RADIATION

Type of Nuclear Particle Particle Particle

Decay Change Mass (amu) Charge Description

Alpha Emission of 4 +2 Helium nucleus

4

2

BetaElectron Emission of 0 -1 Electron

0

-1

Capture of 0 -1 Electron

0

-1

Positron Emission of 0 +1 Positron

0

+1

Gamma Emission of rays no no High-energy

particle particle radiation

HALF-LIFE

Def: Time that it takes for one-half the radioactive atoms to go to a stable form. It

ranges from nanoseconds to millions of years. Uranium 238 has a half-life of 4.5 billionyears.

Biological Half-Life Time it takes an organism to metabolize and get rid of thecompound. If you had radioactive carbon, it would take 5723 years for itschemical half-life. If the carbon was in ethanol (booze), most of the carbon wouldbe gone as carbon dioxide within 24 hours.

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Nuclear Reactions

Alpha Decay

226 222 4 0 Ra = radium Rn = radon

Ra --------- Rn + + 88 86 2 0

211 4 Bi = bismuth

Bi ----------- + ______________83 2

Beta Emission0

Electron -1

70 0 70 Ga = gallium Ge = germanium

Ga -------- + Ge31 -1 32

113 0 Ag = silver

Ag ------ + _____________47 -1

Positron Emission0

+1

30 0 30 P = phosphorus Si = silicon

P --------

+ Si15 +1 14

23 0 Mg = magnesium

Mg -------- + _________12 +1

What would happen if a proton (p) gave off a positron?1 0

p ------ + ________+1 +1

Electron CaptureAn electron is captured on the left side of the equation. In the previous cases, the electrons

were given off or emitted on the right side of the equation..

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0

-1

87 0 87 Rb = rubidium Kr = krpton

Rb + ---------> Kr37 -1 36

73 0 As = arsenic Ge = Germanium

As + ---------> ______33 -1

Emission means a particle is given off. The radioactive particle is on the right side of the equation

Capture means a particle is added. The radioactive particle is on the left side of the equation.

Nuclear Fission1

A heavy nucleus is bombarded with neutrons. n0

U = uranium I = iodine Y = ytterbium

235 1 135 97 1U + n ------> I + Y + 4 n + energy

92 0 53 39 0

Ba = barium

235 1 139 1U + n -------> Ba + ______ + 3 n + energy

92 0 56 0

Sn = tin Mo = molybdenum

235 1 131 103 1U + n --------> Sn + Mo + _____ n + energy

92 0 50 42

We use neutrons to bombard the nucleus and we get more neutrons as products along with a newelement. These neutrons can then bombard new nuclei and get more neutrons and radioactiveelements. This is a chain reaction.

Nuclear Fusion

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Two nuclei combine to form a new nucleus.

2 1 3H + H -------> He

1 1 2

Problems:

Tell whether the new element has a higher or lower atomic number than the starting element forthe following cases.

a) alpha decay

b) electron capture

c) electron emission

d) positron emission

e) nuclear fission

f) nuclear fusion

USES OF RADIOISOTOPES

A) Tracers - study where the compound or element goes. The original amount is in too small aconcentration to measure. By using a radioactive element in the compound, thecompound can be 'seen' by the radioactivity coming off, by using a counter ora radiograph.

B) Irradiation of foods It kills microorganisms responsible for spoilage.

C) Radiation therapy Used in the treatment of cancer. Radiation can kill cells. The rapidlydividing cells, like cancer cells, are more susceptible to being killed. Gettingthe dosage correct is important, since normal cells will also be killed in theprocess.

D) Diagnostic uses1. Iodine-131 in KI. Used for hyperthyroid and thyroid cancer. The thyroid produces

a hormone, thyroxin, which has iodine as part of the molecule. The radioactiveiodine will concentrate in the thyroid. The radiation given off will kill the rapidlydividing cells. The half-life of Iodine-131 is 8.1 days. Beta and gamma rays are

given off.

2. Gadolinium-153 Used to measure bone demineralization. It gives off gamma and x-rays.Scanning the bone and measurement of the bone densities are determined.

3. Technetium-99m The 'm' means metastable. It will give off radiation and go to beinga more stable element.

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99m 99

Tc ----------> Tc + 43 43

It has a half life of 6 hours. Technetium-99 is formed from Molybdenum-99.

4. Positron Emission Tomography - PET scansUse positron emitting isotopes. The positron reacts with an electron and twogamma rays are formed. The gamma rays are given off in opposite directions.The gamma rays are measured. The instrument looks like a big donut. The patient'shead or body is passed into this hole and the radiation recorded. The computerpart of the instrument can 'see' where the positron and electron collided.

11 11 0

C -----------> B + e (aka )6 5 +1

0 0

e + e -------> 2 +1 -1

E) Carbon datingCarbon-14 is formed by the bombardment of Nitrogen-14 with neutrons

14 1 14 1N + n ---------> C + H

7 0 6 1

Carbon-14 has a half life of 5730 years. If an old dead plant had one half the C-14which is found in the earth's CO2, that means the plant was alive one half-life ago or5730 years.The shroud of Turin was found to be from the time period of 1260 to 1390 A.D. So itis not the shroud used to wrap Christ.

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lec 4 - radioactivity

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