lecture 11 - nassau community college 152/152 lecture 1… · · 2012-05-23homer trapped in the...

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Copyright © 2009 Charles Hicks

Lecture 11Professor Hicks

Inorganic Chemistry (CHE152)


Radioactive decays are all

first order processes


Half-Lives of Various Nuclides

Nuclide Half-LifeType of

Decay

Th-232 1.4 x 1010 yr alpha

U-238 4.5 x 109 yr alpha

C-14 5730 yr beta

Rn-220 55.6 sec alpha

Th-219 1.05 x 10–6 sec alpha

2



Carbon-14 dating

Earth

Earth’s atmosphere

space

cosmic radiation promotes

Transmutation

of nitrogen-14 to carbon-14

N +147 n1

0 C +146 p1

1

CO2146

photosynthesisC6H12O6

(sugar)

donuts

study the slides on

C-14 for their content


Carbon-14 dating

Earth

Earth’s atmosphere

space

eating the donuts makes

Homer slightly radioactive

Rest in

Peace

when Homer dies he stops replenishing the

carbon-14 in his body and the amount that

is there starts to decay

3


Carbon-14 dating

• almost all life is part of the same food chain

• plants make sugars

• animals eat plants

• animals eat animals

• %carbon-14 is constant for an organism constantly taking in food

• when they die they stop replenishing carbon-14 and the clock on its decay starts

• t ½ = 5730 years

• [14C] = [14C]initiale-kt

• useful for dating 1000-50,000 years


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Carbon-14 dating% C-14 (compared to living

organism)Object’s Age (in years)

100% 0

90% 870

80% 1850

60% 4220

50% 5730

40% 7580

25% 11,500

10% 19,000

5% 24,800

1% 38,100


Kinetics of radioactive decay

• first order process

• [Uranium-238] = [Uranium-238]initiale-kt

• [ ]’s can be replaced with any quantity that

proportional to the amount of element:

mass, moles, volume under constant T, P,

or rate of radiation emission

• Shorter half-lifemore decay per second

- we say the sample is hotter and in many

cases it is at higher temperature

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Example: An ancient skull gives 4.50 particles/min∙gC. If a

living organism gives 15.3 particles/min∙gC, how old is the

skull?

= e-kt

1

2

[C-14]

[C-14]initial

= e-kt1/2

(4.5/15) = e -0.0001209*t

1

2= e-k*5730

k = 1.209 x 10-4 yr-1

= 0.0001209 yr-1

ln(1/2) = -k*5730

t = 9958 yr

1) use half-life t½ to find k 2) use the k value to solve for t

use ratio of rates of particle

emission instead of ratio of [ ]’s



Mass Defect and

Binding Energy

• when a nucleus forms, some of the mass of the

separate nucleons is converted into energy

• the difference in mass between the separate

nucleons and the combined nucleus is called the

mass defect

• the energy that is released when the nucleus

forms is called the binding energy

E = mc2

Albert Einstein Lise MeitnerLise Meitner

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homer on the

earth’s surface

separated protons, neutrons

homer trapped in the well b/c he

does not have energy to escape

heat

released

heat

released

binding energy = heat released when nucleus forms

it comes from mass energy

binding

energy=

carbon-12

nucleus

+

+

+ N

N

N

+

+

+ N

N

N

+ +

+

+

+

N

N

N

N

N

N

+N

mass

energy

total mass =

12.000000 amu

12.0984 amu

total mass

6 1.00866

+ 6 1.00783

E = mc2

c = speed light

m = mass defect

0.0984 amu here

E = binding energy


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• compare elements binding energy per nucleon

• Iron-56 has the most stable nucleus

• energy will be released when:


Calculate the amount of energy released

when a helium-4 nucleus is formed from

two protons and two neutrons. This is also

referred to as the binding energy of a

helium-4 nucleus.

Mass He-4 = 4.00150

Mass Neutron = 1.00866

Mass proton = 1.00728

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Fission/Fusion• a few nuclei are so unstable that

if their nucleus is hit by a neutron, the large nucleus splits into two smaller nuclei - this is called fission

• small nuclei can be smashed together to make a larger nucleus - this is called fusion

• both fission and fusion release enormous amounts of energy– fusion releases more energy per

gram than fission

Lise Meitner


Fission


where does the energy from

fission/fusion come from?

• during nuclear fission, mass is converted into energy as a more stable nucleus forms

• the energy can be calculated from E=mc2

• U-235 that undergoes fission produces about 1.7 x 1013 J per mole

– a very exothermic chemical reaction produces around 106 J per mole

http://upload.wikimedia.org/wikipedia/en/b/b3/Lise_Meitner_1900.jpg

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Fission Chain Reaction• a chain reaction occurs when a reactant in

the process is also a product of the process

– in fission it is neutrons

– small amount of neutrons start the chain

• many of the neutrons produced in fission are

either ejected from the uranium before they

hit another U-235 or are absorbed by the

surrounding U-238

• minimum amount of fissionable isotope

needed to sustain the chain reaction is called

the critical mass


if sample is very dilute most neutrons will escape

- they will not initiate another reaction

if sample is absorbing more than 1 neutron on the

average then the process will very rapidly accelerate

- this is called a Criticality Excursion


Criticality excursions

• Samples that are undergoing fission, releasing

neutrons, can experience sudden accelerations

due to nuclear chain reactions

• Caused by

• Precipitations

• Neutrons reflected back

• Change in shape of container

• Powerful bursts of radiation emitted

All increase the number of

emitted neutrons that are

absorbed initiating the next

step of the chain reaction

http://wps.prenhall.com/wps/media/objects/1056/1081591/media/9738.html

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Nuclear Power Plants - Core• fissionable material is stored in long tubes,

called fuel rods, arranged in a matrix

• between the fuel rods are control rods

made of neutron absorbing material

– B or Cd

– neutrons needed to sustain the chain reaction

• when the rods are lowered slow down

ejected neutrons, called a moderator

– allows rate of chain reaction to be controlled


Nuclear Fusion


Fusion

• potentially safer fuels than fission

H2

1H

3

1He

4

2n

1

0+ +

products not radioactive isotopes!tritium relatively safe

t ½ = 12 years

beta decay

problems

1) activation energy is high T ~ 108 K

2) hard to create/control high enough

temperatures

http://wps.prenhall.com/wps/media/objects/169/173571/NuclearPowerPlant.html




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Fusion


heat

released

overall

reaction

(heat absorbed)

Cold Fusion????


Describe the effect of raising or lowering

the control rods in a nuclear fission reactor

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Estimate the energy released when 1.0 mole of Am-241

undergoes alpha decay.

Mass Am-241 = 241.00471 amu

Mass Np-237 = 236.99715 amu

Mass He-4 = 4.00150 amu

Compare the value you obtain to the amount of energy obtained

from combustion of 1 mole of octane 5.0 x106 J/mol


Example: If you have a 1.35 mg sample of Pu-236,

calculate the mass that will remain after 5.00 years

Pu-236 has a half life of 2.86 yr

1) use half-life t½ to find k 2) use the k value and the t of interest

to solve for mass at that time

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Blocking radiation

• An elements ability to block X-rays

increases with nuclear size (Z)

• Pb is often used as shielding because it is

almost the heaviest element that is not

radioactive itself

• Soft body tissue is mostly H2O, which is

mostly O-16 by mass

• Bone and cartilige contain substantial

amounts of Ca and P which are larger

nuclei absorb more radiation


X-ray imaging• Medical x-rays are of the “shadows” of

denser body tissue – bones, cartilage, etc

X-ray

source


CT Scans

• Series of x-rays used to construct a 3-

dimensional image

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PET Scan

• imaging technique based on positron emission

positron + electron gamma rays

• isotope incorporated into a molecule

• molecule ingested

• positrons emitted from places molecule is concentrated

• gives a three dimensional “x-ray” of the region the molecule binds

• example F-18 labeled drugs that bind in the brain’s addiction centers

- F-18 labeled cocaine, heroine


Barium

Enema

• radiation absorption increases with size of nucleus

• bones Ca, P causes them to absorb more than

soft tissue mostly H, O, C, N

• colon can be imaged with x-rays if it is filled with

BaSO4 because Ba absorbs X-rays well


Radiation therapy

• Cancerous tumors dosed with radiation

• Can be performed with focused beams, or

• “seeding” radioactive material near the

tumor, or

• Molecules labeled with radioactive

isotopes that target specific organs

• Radiotherapy works by damaging the

DNA of the cancer cells

http://en.wikipedia.org/wiki/Image:PET-MIPS-anim.gif

http://en.wikipedia.org/wiki/Image:Colonic_Herniation_08787.jpg

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Radiation therapy

External beam radiation therapy

• Cobalt-60 often used as a source of

radiation

• It is a beta emitter

• beam is focused on

the site of the tumor


Radiation therapy

Internal radiotherapy• I naturally accumulates in the thyroid gland

• I-131 is radioactive and can be used to treat

thyroid cancer

• I-131 after being ingested a NaI-131

accumulates in thyroid where it undergoes -

decay dosing the thyroid cancer

Non-radioactive iodine is eaten to prevent damage to the

thyroid caused by I-131 released from nuclear bombs or

accidents

It works by flooding the body with I- so that most of the I-

at the thyroid gland is not radioactive


Radiation therapy

Internal radiotherapy• Prostate cancer treated with radioactive seeds

• Palladium-103 is an example of a seed material

• It has t½ = 17 days

• decays by electron capture emitting gamma rays

Pd-103 seeds

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Dangers of Radioactivity• high energy radioactive decay products called

Ionizing Radiation

• Ionizing radiation can directly cleave DNA

- called primary damage, or

- can break molecules up forming species that

cause damage by chemical reactions

- called secondary damage

• most common secondary pathway is water

absorbing radiation forming hydroxyl radicals

• radical = odd number of electrons

• Very reactive and an oxidizing agent

OH


Relative Biological Effectiveness

(at causing damage)

Type and Energy Range Relative Biological Effectiveness

X and Gamma rays 1

Electrons 1

Neutrons (energy dependent) 10

Protons 5

Alpha Particles 20

you will be provided with this table on the exam


Dose

• amount of energy absorbed in an exposure has a unit

called the gray (Gr)

1 gray =1 Joule

kg tissue

gray is a large unit for typical exposures

A more common unit is the Radiation Absorbed Dose (RAD)

0.01 gray = 1 RAD

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REM’s

dose in REM = dose in RAD x RBE

• unit used to measure total risk associated with a dose

• accounts for differences in extent of damage caused

by different particle types

dose (REM) effect

5 none felt, yearly limit radiation workers

50 none felt, red blood cell count lowered

1000 near 100% fatal

• typical x-ray deliver 1-100 mREM

• background level is 30-60 mREM per year

• barium enema recently? you got 400 mREM


Factors besides size of dose• Getting a dose in regions that are mostly

muscle (legs, arms) is much less dangerous

than the same dose in your organs

• Distributing a dose over time is much less

damaging

• Cells have repair mechanisms

• If repair can keep up much less damage can

accumulate

- DNA single strand breaks are repairable

- Double strand breaks – not very repairable


Dangers of radiation and

penetrating power

• gamma radiation strongly penetrating requires thick layers of most protecting materials

• without protection dangerous doses quickly delivered

• and emitters easy to block

• and emitters very dangerous if ingested since they can deliver large doses accumulated over time

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Why do beta emitters not have to be kept

in a thick-walled lead containers, but

precautions must be taken to prevent

ingesting them?


Calculate how many REMs were delivered

in 10 barium enemas if the dose for each

barium enema is 0.40 RAD


How many RAD of alpha particles could a

radiation worker be dosed with before they

reached their yearly safety limit of 5.0

REM?

lecture 11 - nassau community college 152/152 lecture 1… · · 2012-05-23homer trapped in the...

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