Nuclear Chemistry

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APPLICATIONS OF NUCLEAR CHEMISTRY

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USES OF RADIOISOTOPES

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TRACER APPLICATION TRACERS

–A radioisotope used to follow the path of a chemical process

– The fact that it can be detected or “traced” as it decays makes it possible to determine its location at all times.

• Example: – C-14 is the most useful tracer isotope & has been

used to study photosynthesis & the pathway followed by C in many organic processes.

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Tracer Applications in Medicine Isotope Half-life Body part Medical application

CE-141 32.5 days Intestines, blood Gastrointestinal diagnosis, measuring myocardial blood flow

Fe-59 44.5 days Blood (RBC) Studies of red-blood cells & bone marrow

P-32 14.3 days Eyes, liver, tumors Treatment of leukemia

Au-198 2.7 days Liver Liver imaging

I-131 8.04 days Thyroid Treatment of hyperthyroidism

I-125 60 days Thyroid, liver Imaging thyroid, liver function

Tc-99m 6 hours Brain Lung Thyroid/Spleen

Detection of tumors or blood clots Location of blood clots Measurement of size & shape

Na-24 14.8 hours Circulatory system Measure extracellular fluids

Tl-201 73 hours Heart, arteries Diagnostic aid (thallium stress test) 5

Tracer Applications in Medicine

6 Thallium “stress test”

Tracer Applications in Medicine

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Tracer Applications in Medicine

HIDA (hepatobiliary iminodiacetic acid scan) 8

Tracers in Photosynthesis

• 6 CO2 + 6 H2O C6H12O6 + 6 O2 • Radioactive 14C has helped to determine the path

of C in photosynthesis.

• Starting with 14CO2, it was possible to isolate the intermediate products during photosynthesis & measure the amount of radioactivity of each C-containing compound.

• Provides experimental evidence that CO2 in the environment is chemically converted to glucose in plants.

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STRUCTURAL DETERMINATION

• Formula of thiosulfate (S2O32-)

• Chemists were uncertain whether the 2 S atoms occupied equivalent positions in the ion.

• Thiosulfate using S enriched with radioactive S-35 isotope which act as a “label” for S atoms were prepared.

• Reaction was reversed & found that all the “labels” are found S precipitate only.

• the 2 S atoms are not structurally equivalent. 10

RADIATION THERAPY

• It has been known that exposure to high-energy radiation can induce cancer in humans & other animals.

• Cancer is characterized by uncontrolled cellular growth.

• On the other hand, it is also well established that cancer cells can be destroyed by proper radiation treatment.

• In radiotherapy, a compromise is sought.

• The radiation to which the patient is exposed must be sufficient to destroy cancer cells without killing too many normal cells & it is hoped, without inducing another form of cancer.

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Radiation therapy

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Radiation therapy

• Some radioisotopes are used in the treatment of cancer because they have the ability to kill living tissues

Isotopes Half-life Type of radiation emitted

Use in therapy

I-131 8 days Beta, gamma Thyroid therapy

Co-60 5.3 days Beta, gamma Cancer therapy

P-32 14.3 days Beta, gamma Leukemia therapy

Y-90 64 hours Beta, gamma Implantation tumor & cancer therapy

Rn-222 3. 8 days Alpha, gamma Cancer therapy of bladder, uterus & kidney

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Radiation therapy

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PET SCAN • Positron emission

tomography (PET) scans use

radioisotopes that emit

positrons when the nucleus

decays. Once formed, a

positron combines with an

electron to form 2 rays which

creates a scan of an organ.

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Medical Imaging w/o Radioactivity • Not based on nuclear reactions & do not utilize

radioactivity: X-rays; CT scan; MRI

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Medical Imaging w/o Radioactivity

• X-rays – Form of High-energy type of radiation but lower than rays

– Map of bone & internal organs is created on an x-ray film.

– Dense bones is clearly visible making it able to find fracture

• CT (computed tomography) scan – Also uses x-rays

– Provide high resolution of “slices” of the body

– CT scans in head are used to diagnose bleeding & tumors in the brain

• MRI (magnetic resonance imaging) – Uses low-energy radio waves to visualize internal organs

– Do not damage cells; good for visualizing soft tissues.

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DATING APPLICATION Radiocarbon Dating

– Produced when atmospheric N is bombarded by cosmic rays

• C-14 decays

• C-14 enters the biosphere as CO2, which is taken up in plant photosynthesis. Plant-eating animals in turn exhale C-14 in CO2. Eventually C-14 participates in many aspects of the C cycle.

• Ratio of 14C to 12C remains constant in living matter.

But when an individual plant or an animal dies,

the C-14 isotope in it is no longer replenished.

• Same change occurs when C atoms are trapped in

coal, petroleum or wood preserved underground &

in mummified bodies. 18

HCnN 1

1

14

6

1

0

14

7

βNC 0

1-

14

7

14

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Radiocarbon dating

• The decreasing ratio of 14C to 12C can be used to estimate the age of a specimen.

• By measuring the decay rates of the fresh sample & the old sample, age of old sample can be calculated.

• Valuable tool for estimating objects (containing C atoms) dating back 1000 to 50,000 yrs.

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Radiocarbon dating

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Dating Using U-238 Isotopes

• Suitable for estimating age of rocks in the earth & extraterrestrial objects.

• Half-life for U-234 to Pb-206 4.51 x 109 yr

• Mass ratio Pb-206/U-238 is compared

– If ratio = 0.866 4.51 x 109 yrs

– If lower means < 4.51 x 109 yrs

– If greater suggests > 4.51 x 109 yrs

• Age of oldest rock ( probably age of Earth) at 4.5 x 109 or 4.5 billion years.

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Dating Using K-40 Isotopes

• Most important techniques employed in geochemistry.

• Relevant mode of decay in dating:

• Accumulation of gaseous Ar-40 is used to gauge the age of specimen.

• When K-40 atom in mineral decays, Ar-40 is trapped in the lattice of the mineral & can escape when material is melted.

• Ratio of Ar-40 to K-40 & its t ½ (1.2 x 109 yr) makes it possible to establish the age of rocks.

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AreK 40

18

0

1-

40

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RADIOACTIVITY IN THE ENVIRONMENT

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RADIACTION SOURCES • NATURAL

– IR, UV & visible radiation from the sun; Background radiation

• ARTIFICIAL – Radio waves from radio & tv sets; Microwaves; xrays

K-40 Rb-87 Th-232 U-239

Land elemental abundance (ppm) 28000 112 10.7 2.8

Land activity (Bq/kg) 870 102 43 35

Ocean elemental conc (mg/L) 339 0.12 1 x 10-7 0.0032

Ocean activity (Bq/L) 12 0.11 4 x 10-7 0.0040

Ocean sediments elemental abundance (ppm) 17000 - 5.0 1.0

Ocean sediments activity (Bq/kg) 500 - 20 12

Human body activity (Bq) 4000 600 0.08 0.4 24

Background Radiation

• Is the radiation in the environment

How much Radiation is Safe?

• The average exposure to background radiation is 300 millirems per year at sea level. Radiation exposure is slightly higher at higher elevation.

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What are sources of background radiation?

• Largest contribution comes from natural sources:

– Radon gas (major source) t½ is 3.8 days; inert gas that is tasteless & odorless; escapes from rocks & soil & enters the ground through air & water. In homes, it can seep through basements; when inhaled, it is deposited in the respiratory track & increases the risk of cancer.

– Cosmic rays a form of short wavelength electromagnetic radiation comes from outer space. People living in high altitude receive more cosmic radiation than people living at sea level

– Terrestrial radiation (other than Rn) comes from rocks & soil particularly C-14, K-40, Th-223 & U-238. Being present in food, air & water, it enters the body through ingestion & inhalation.

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Other sources of radiation

• Human activities:

–X-rays (diagnostic or therapeutic)

–Nuclear medicine

–Consumer products

• Rn in water supplies

• Building materials: stone, bricks, concrete

• Tobacco (naturally occurring Po-210 found in leaves)

–Nuclear power industry & Nuclear fallout 27

Units of radiation

Aspect of Radiation Common Unit Definition

Radioactivity (emission or disintegration)

Curie (Ci) Becquerel (Bq)

1 Ci = 3. 7 x 1010 disintegration /second

1 Mci = 3.7 x 104 Bq

Absorbed Dose Rad (radiation absorbed dose)

Gray (Gy)

1 rad = 100 ergs/g absorbing body

1 gray = 100 rad

Dose equivalent Rem (Roentgen equivalent man)

Sieverts (Sv)

1 rem = 1 roentgen of high voltage of xray damage to

humans 1 Sv = gray × quality factor *

* Depends on type of radiation

Exposure Roentgen (R) 1 R = 1 Coulomb/kg of air 28

Permissible levels of Radiation Exposure

Description Permissible levels

Accumulated/yr (mrems) Single dose (mrems)

Adult 5,000 3,000

Minor 500 300

Fetus 500 before birth or no more than 50 per month

Humans get the ff millirems such as:

• + 2 annual exposure due to watching 4 hours of TV per day

• + 2 one chest x-ray

• +20 000 therapeutic radioactive I treatment for thyroid gland 29

Effects of Radiation on Human Body

Dose (x 103 mrems) Effects

< 50 No detectable short term effect

50 – 250 Fatigue, nausea, reduction in WBC & platelets

250 – 500 Same effects as 50 – 250 x 103 rems, but more severe; vomiting; diarrhea; susceptible to infection due to low WBC; hair loss; hemorrhaging due to impaired clotting mechanism; 50% die within months

500 – 1000 Damage to cardiovascular system, intestinal tract & brain; death within weeks

10 000 – 10 000 Same effect as 500 – 1000 x 103 rems but more severe; coma & death within hours

100 000 Instant death 30

Average Annual Exposure

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Biological Effects of Radiation on the Body

• Radiation may cause either direct or indirect damage to biological system.

• Generally classified as somatic or genetic.

– Somatic injuries are those that affect the organism its own lifetime.

• Sunburn, skin rash, cancer & cataracts

– Genetic damage means inheritable changes or gene mutations.

• A person whose chromosomes have been damaged or altered by radiation may have deformed offspring.

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Factors that Influence Radiosensitivity of Cells

1) Activity of cell. Actively reproducing cells are more affected by radiation than inactive cells. This explains why a person exposed to radiation will likely become sterile because reproductive cells are active cells.

2) Age of cells. Young cells are more radiosensitive than mature cells. Children are more affected by radiation than older people.

3) Type of cells. Some cells are more radiosensitive than others. Example, blood-forming organs are radiosensitive. This explains why people exposed to radiation have high risk of acquiring leukemia.

4) Chemicals. Certain chemicals increase or decrease radiosensitivity. Benzypyrene (present in automobile exhaust, cigarette smoke & charcoal-grilled meat) activate cells rendering them more radiosensitive.

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Chemical Basis of Radiation Damage

• Ionizing radiation

– radioactive particles or gamma rays can remove electrons form atoms & molecules in its path, leading to formation of ions & radicals (short-lived & highly reactive)

• Example: water is irradiated with rays

• The electron (in the hydrated form) can subsequently react with H2O or OH- to form atomic H & with O to produce the superoxide ion, O2

-(a radical)

• In tissues, O2- & other FR attack cell membranes & a host of

organic compounds such as enzymes & DNA molecules. 34

Extent of Radiation Damage

• Depends on the activity & energy of the radiation, length of exposure & whether the source is inside or outside the body

• particularly harmful outside the body

• stopped by skins but within body, particularly dangerous

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RADIATION DETECTION

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Instruments Used for Detecting Radiation

• Photographic plates & film

–Method used by Henri Becquerel

– The greater the extent of exposure, the darker the area of the developed negative

– Example: film badges

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Instruments Used for Detecting Radiation

• Geiger Muller counter (Geiger counter) –Detect based on the ionization of matter

caused by radiation

– Ions & electrons produced by the ionizing radiation permit conduction of an electric current.

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Instruments Used for Detecting Radiation

• Scintillation counter

– Detect & measure radiation based on the tiny plashes of lights produced when radiation strikes a suitable phosphor (substances when excited gives off light as electrons return to lower-energy states)

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CHEMICAL IMPACT

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END OF NUCLEAR CHEM PRESENTATION

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