Nuclear Chemistry Study Guide: Structures of Life Study Guide

Role of Nuclear Medicine in Healthcare

• Nuclear Medicine Technologist: A professional who works in a hospital setting using nuclear medicine to diagnose and treat various medical conditions. Their responsibilities include:

  • Preparing radioisotopes for patient administration.

  • Operating scanners that convert radiation from the patient's body into diagnostic images.

• Nuclear Pharmacy: A specialized environment where radioisotopes are handled. Protective measures include:

  • Wearing protective clothing and gloves.

  • Using lead glass shields on syringes for handling radioactive materials.

Characteristics of Natural Radioactivity

• Unstable Nuclei: Radioactivity originates from unstable nuclei, which are commonly found in:

  • Elements with atomic numbers $20$ and higher.

  • Nuclei where nuclear forces are insufficient to offset the electrostatic repulsions between protons.

• Radiation Emission: To become more stable, these nuclei emit small particles of energy known as radiation. Forms of radiation include:

  • Alpha ($\alpha$) particles: Identical to a helium nucleus ($_{2}^{4}\text{He}$).

  • Beta ($\beta$) particles: High-energy electrons ($_{-1}^{0}e$).

  • Positrons ($\beta^{+}$): Anti-electrons ($_{+1}^{0}e$).

  • Gamma ($\gamma$) rays: Pure electromagnetic energy ($_{0}^{0}\gamma$).

Isotopes and Radioisotopes

• Radioisotope Definition: An isotope of an element that is capable of emitting radiation. An element may have one or more radioactive isotopes.

• Naming Conventions: The name of a radioisotope specifically includes its mass number.

  • Example: Iodine-131: A radioisotope used for thyroid disorders. It has a mass number of $131$ and an atomic number of $53$. Its atomic symbol is $_{53}^{131}\text{I}$.

• Stability Comparison:

  • Magnesium: Stable isotope is $_{12}^{24}\text{Mg}$; radioactive isotope is $_{12}^{27}\text{Mg}$.

  • Iodine: Stable isotope is $_{53}^{127}\text{I}$; radioactive isotope is $_{53}^{131}\text{I}$.

  • Uranium: Has no stable isotopes; radioactive isotopes include $_{92}^{235}\text{U}$ and $_{92}^{238}\text{U}$.

Biological Effects and Radiation Protection

• Ionizing Radiation: Strikes molecules in its path, knocking away electrons and forming unstable ions like $\text{H}_{2}\text{O}^{+}$. This triggers undesirable chemical reactions and damages cells.

• Cell Sensitivity: Cells that divide rapidly are most sensitive to damage, including cells in bone marrow, skin, and reproductive organs. This damage can lead to cancer.

• Shielding Requirements:

  • Alpha Partices: Require paper and heavy clothing for protection.

  • Beta Particles: Require lab coats or gloves.

  • Gamma Rays: Require dense shielding such as lead or thick concrete.

• Exposure Limitation:

  • Minimizing the amount of time spent near the radioactive source.

  • Increasing the distance from the radioactive source.

Nuclear Equations and Radioactive Decay

• Radioactive Decay: A process where a nucleus breaks down by emitting radiation. In balanced nuclear equations:

  • The sum of mass numbers for reactants and products must be equal.

  • The sum of atomic numbers for reactants and products must be equal.

• Alpha Decay: A nucleus emits an alpha particle ($_{2}^{4}\text{He}$). The new nucleus has a mass number decreased by $4$ and an atomic number decreased by $2$.

  • Example: $_{251}^{98}\text{Cf} \rightarrow _{96}^{247}\text{Cm} + _{2}^{4}\text{He}$

  • Example (Americium-241): $_{95}^{241}\text{Am} \rightarrow _{93}^{237}\text{Np} + _{2}^{4}\text{He}$

• Beta Decay: An electron ($_{-1}^{0}e$) is emitted when a neutron in the nucleus breaks down into a proton and a beta particle. The atomic number increases by $1$, while the mass number remains the same.

  • Example (Potassium-42): $_{19}^{42}\text{K} \rightarrow _{20}^{42}\text{Ca} + _{-1}^{0}e$

  • Example (Cobalt-60): $_{27}^{60}\text{Co} \rightarrow _{28}^{60}\text{Ni} + _{-1}^{0}e$

• Positron Emission: A proton is converted into a neutron and a positron ($_{+1}^{0}e$). The mass number remains unchanged, but the atomic number decreases by $1$.

• Gamma Radiation: Energy ($_{0}^{0}\gamma$) is emitted from a metastable (unstable) nucleus, often indicated by the letter "m" after the mass number. Mass and atomic numbers do not change.

  • Example: $_{43}^{99m}\text{Tc} \rightarrow _{43}^{99}\text{Tc} + _{0}^{0}\gamma$

Transmutation: Producing Radioactive Isotopes

• Definition: The conversion of a stable nucleus into a radioactive one by bombarding it with small particles (protons, neutrons, or alpha particles).

• Examples of Bombardment:

  • Boron-10 bombarded by an alpha particle: $_{5}^{10}\text{B} + _{2}^{4}\text{He} \rightarrow _{7}^{13}\text{N} + _{0}^{1}n$

  • Nickel-58 bombarded by a proton: $_{28}^{58}\text{Ni} + _{1}^{1}H \rightarrow _{27}^{55}\text{Co} + _{2}^{4}\text{He}$

  • Technetium-98 bombarded by a neutron: $_{43}^{98}\text{Tc} + _{0}^{1}n \rightarrow _{41}^{95}\text{Nb} + _{2}^{4}\text{He}$

Radiation Measurement and Units

• Instruments: A Geiger counter is used to detect beta and gamma radiation by creating electrical currents from ions produced by radiation. Radiation counters are critical for monitoring sites like the Fukushima Daiichi nuclear power plant.

• Units of Activity:

  • Curie (Ci): $3.7 \times 10^{10}\text{ disintegrations/s}$, based on the activity of $1\text{ g}$ of radium.

  • Becquerel (Bq): The SI unit, defined as $1\text{ disintegration/s}$.

• Units of Exposure and Damage:

  • Rad (radiation absorbed dose): Measures the amount of radiation absorbed by $1\text{ g}$ of material (e.g., body tissue).

  • Rem (radiation equivalent in humans): Measures the biological effects. Calculated as: $\text{Biological damage (rem) = Absorbed dose (rad) } \times \text{ Factor}$.

• Relative Biological Factors:

  • Beta and Gamma: Factor = $1$.

  • High-energy protons and Neutrons: Factor = $\approx 10$.

  • Alpha Particles: Factor = $20$ (most internal damage).

• Conversions:

  • $1\text{ rem} = 1000\text{ mrem}$.

  • $1\text{ Sievert (Sv)} = 100\text{ rem}$.

Radiation and Health

• Annual Exposure: The average person in the U.S. receives $360\text{ mrem}$ annually. Sources include buildings, food/water (Potassium-40), air, and cosmic radiation from the sun.

• Radiation Sickness: The severity of effects depends on the dose received at one time.

• LD50 (Lethal Dose): The whole-body radiation dose expected to cause death in $50\%$ of the population. For humans, this is approximately $500\text{ rem}$.

• Food Irradiation: The FDA approves doses of $0.3\text{ kGy}$ to $1\text{ kGy}$ produced by Cobalt-60 or Cesium-137. This kills bacteria like Salmonella and E. coli and extends shelf life for produce (e.g., strawberries, tomatoes).

Half-Life of Radioisotopes

• Definition: The time required for the radiation level (activity) of a sample to decay to one-half of its original value.

• Decay Curve: For Iodine-131 (half-life = $8\text{ days}$), the amount of radioactive sample halves every $8\text{ days}$.

• Common Half-lives:

  • Carbon-14: $5730\text{ years}$.

  • Potassium-40: $1.3 \times 10^{9}\text{ years}$.

  • Strontium-90: $38.1\text{ years}$.

  • Iodine-123: $13\text{ hours}$.

• Calculations Example: For a $36\text{ mg}$ sample of Strontium-90 over $152.4\text{ years}$:

  • Number of half-lives = $152.4 / 38.1 = 4$.

  • Decay: $36 \text{ mg} \rightarrow 18 \text{ mg} \rightarrow 9 \text{ mg} \rightarrow 4.5 \text{ mg} \rightarrow 2.25 \text{ mg}$. (Result: $2.2\text{ mg}$ remaining).

Medical Imaging and Therapy

• Diagnostic Radioisotopes: Short half-lives are preferred. Body cells do not distinguish between stable and radioactive atoms; once incorporated, their emissions provide organ images.

  • Iodine-131: Used for thyroid scans.

  • Positron Emission Tomography (PET): Uses short-lived positron emitters (C-11, O-15, N-13, F-18) to study brain function and blood flow. Positrons combine with electrons to produce gamma rays for 3-D imaging.

• Non-Radioactive Imaging:

  • Computed Tomography (CT): Uses a computer to monitor the absorption of $30,000$ X-ray beams through tissue layers.

  • Magnetic Resonance Imaging (MRI): Non-invasive technique exciting protons in hydrogen atoms using a strong magnetic field; energy absorbed is converted to color images.

• Brachytherapy: Internal radiation therapy for prostate cancer.

  • Permanent: Implantation of $40+$ seeds containing Iodine-125, Palladium-103, or Cesium-131.

  • Temporary: Iridium-192 delivered via long needles for $5$ to $10\text{ minutes}$.

Nuclear Fission and Fusion

• Nuclear Fission: The splitting of a large nucleus into smaller nuclei when bombarded with neutrons, releasing vast "atomic energy."

  • Reaction: $_{0}^{1}n + _{92}^{235}\text{U} \rightarrow _{92}^{236}\text{U} \rightarrow _{56}^{139}\text{Ba} + _{36}^{94}\text{Kr} + 3_{0}^{1}n + \text{energy}$.

  • Chain Reaction: Each U-235 fission produces neutrons that trigger further fissions.

• Nuclear Power Plants: Use fission to produce steam for electricity generators. Control rods absorb neutrons to prevent an uncontrolled chain reaction; uranium is kept below critical mass.

• Nuclear Fusion: Small nuclei combine to form larger nuclei at extremely high temperatures ($100,000,000\,^{\circ}\text{C}$), releasing massive energy. Occurs in the sun and stars.

  • Produces less waste than fission.

  • Hydrogen isotopes combine to form helium.