based on the number of protons and neutrons, an atom can be stable or unstable. generally, small...
TRANSCRIPT
Based on the number of protons and neutrons, an atom can be stable or unstable. Generally, small atoms require an equal number of p & n for stability, larger atoms require more neutrons.
4.1 Natural Radioactivity
To obtain a more stable state, unstable nuclei disproportionate and emit high energy particles and/or electromagnetic radiation (photons) spontaneously.
The name for this process is radioactivity and the substances undergoing such, are said to be radioactive.
radioactive Stable
Alpha
Beta
Gamma
Radiation strikes molecules in its path
e-s may be knocked away, forming unstable particles (ions, radicals)
:. Called ionizing radiation
These can cause undesirable chem. reactions which may lead to malignant tumors, leukemia, anemia, or genetic mutations.
Biological effect of radiation
Nuclear Reactions
All nuclear equations show a balance on mass number & atomic number between the reactants and products:
C14
6 e
0
-1 N14
7 +
14 14
6 6
Po209
84 H
e
4 2
? ?
? +
Top: 209=4+XX = 205
bottom: 84=2+XX = 82
symbol: 82 = Pb
Po209
84 H
e
4 2
Pb
205
82 +
Mass number
Atomic number
5
Rn He Ra 21886
42
22288
► 4He, helium nucleus, contains 2 protons and 2 neutrons ► most ionizing, but least penetrating
Alpha Emission ()
The daughter nuclides produced by -decay are often obtained in an excited state. The excess energy associated with this excited state is released when the nucleus emits a photon in the -ray portion of the electromagnetic spectrum.
The metastable 60mCo nuclide has a half-life of 10.5 minutes. Since electromagnetic radiation carries neither charge nor mass, the product of -ray emission by 60mCo is 60Co.
Most of the time, the -ray is emitted within 10-12 seconds after the -particle. In some cases, gamma decay is delayed, and a short-lived, or metastable, nuclide is formed, which is identified by a small letter m written after the mass number.
60mCo, for example, is produced by the electron emission of 60Fe.
Gamma Emission ()
Technetium (Tc-99m) is a gamma emitter.
It can be injected intravenously as a solution of sodium pertechnatate, NaTcO4.
Gamma rays emitted by the technetium are detected to produce a scintigram.
whole-body bone scintigrams obtained in an adult demonstrate normal anatomy.
Extensive osseous metastases from lung carcinoma.
The most widely used radioisotope for medical diagnostics.
99
Pa e Th 23491
01
23490
Turns a neutron into a proton: n p + e- β particle is a high energy electron
Beta Emission
Radioactive Iodine ( I-131) therapy is a treatment for an overactive thyroid, a condition called hyperthyroidism.
I131
53 Xe131
54
0
-1+ g+ e
The radioactive iodine that is not taken up by your thyroid is rapidly eliminated through body fluids such as urine, feces, saliva, and perspiration.
How Long Does the Radioiodine Stay in the Body?
Radioiodine is collected by the thyroid gland. The radiation given off by this form of iodine decreases the function of the thyroid cells and inhibits their ability to grow. This is the desired medical effect of this treatment. Some of the other tissues in your body will receive some incidental radiation. This small amount of radiation has NOT been shown to produce any adverse effect.
b emissions
g emissions
89.9%7.3%
131 53 I
131 54 Xe
131 54 Xe*
90 % 10 %
1111
Ne e Na 2210
01
2211
► Turns a proton into a neutron: p n + e+ positron has a charge of +1 and negligible mass► Antimatter, anti-electron
Positron Emission
Positron emission tomography (PET)The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule.
Fludeoxyglucose is a radiopharmaceutical used in the medical imaging a glucose analog, with the positron-emitting radioactive isotope fluorine-18 substituted for the normal hydroxyl group at the 2' position in the glucose molecule.
14
• occurs when an inner orbital electron is pulled into the nucleus• no particle emission, but atom changes• proton combines with the electron to make a neutron
Tc e Ru 9243
01
9244
Electron Capture
p + e- → n
Practice
1. An alpha particle is the same as: a. helium-5 nucleus b. helium-4 nucleus
c. helium-3 nucleus d. proton
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2. Which of the following is the most penetrating? a. alpha particles b. beta particles
c. Positrons d. gamma rays
3. What type of shielding is required for gamma radiation? a. Gloves b. paper and clothing
c. lead or concrete d. Lab coat and shoes
4. The damaging effects of radiation are due to? a. Production of poison b. Generation of isotopes
c. Generation of radicals d. Production of new isotopes
Practice
5. The nuclear reaction, 8B 8Be + 0e is an example of:
5 4 +1
6. Write the Nuclear Equation for the Positron Emission From K-40
7. Write the nuclear equation for the beta decay of cobalt-60.
a. gamma radiation
b. positron emission
c. beta decay
d. alpha decay
8. Write a nuclear equation for the electron capture by Be-7
The half-life of a radioisotope is the time for the radiation level to decrease to one-half of the original value.
4.4 Half-life of a radioisotope
Radioisotopes that arenaturally occurring tend to have long half-livesused in nuclear medicine have short half-lives
To solve a problem using t1/2
1. Use the half-life as a conversion factor:
1 half-life8.0 days
Example: Use the half-life given above to calculate how many half-lives in 40 days?40 days x 1 half-life = 5 half-lives
8.0 days
2. Reduce the amount of sample by ½ for every half-life.Example: How many grams left over, after 5 half-lives, if started with 100 g?
100 x ½ x ½ x ½ x ½ x ½ = 3.125 g100 x (½)5
100 x 1/32
If it helps any, consider:Amount remaining = starting amount x (½)n n = number of half-lives
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9. ) F-18, which has a half-life of 110 min, is used in PET scans. If 100 mg of F-18 is shipped at 8:00 am, how many mgs of the radioisotope are still active if the sample arrives at the radiology laboratory at 1:30 pm?
a) 50 mg b) 25 mg c) 12.5 mg
Practice
10. Phosphorus-32, a radioisotope used in the treatment of leukemia, has a half-life of 14 days. If a sample contains 8.0 g of phosphorus-32, how many grams of phosphorus-32 remain after 42 days?