Concise Notes on Radiation Biology

Sources, Discovery, and Types of Radiation

• Radiation: Energetic particles/waves through a vacuum or matter.
• Uses: Medicine, academics, industry, electricity generation, agriculture, archaeology, space exploration, law enforcement, geology.

Sources of Radiation

• Natural:
  • Cosmic rays.
  • Gamma rays from the earth.
  • Radon decay products in air.
  • Radionuclides in food.
• Artificial:
  • Medical radiographic diagnosis (largest source).
  • X-rays and radiation therapy.
  • Nuclear weapons testing fallout.
  • Radioactive waste from nuclear industries.
  • Industrial gamma rays.
  • Lesser sources: glow-in-the-dark watches, microwave ovens, cell phones, antique glass.

Discovery of Radiation

• Infrared radiation: William Herschel (1800), detected beyond the red spectrum using a prism and thermometer.
• Ultraviolet radiation: Johann Wilhelm Ritter (1801), rays darkened silver chloride preparations faster than violet light.
• Radio waves: Heinrich Hertz (1887), produced artificially using electrical circuits.
• X-rays: Wilhelm Röntgen (1895), noticed fluorescence on a coated glass plate.
• Radioactivity: Henri Becquerel, uranium salts fogged photographic plates; Marie Curie named the behavior.
• Radium: Marie and Pierre Curie (1899), discovered in pitchblende.
• Alpha and beta rays: Ernest Rutherford (1899), differentiated by penetration and charge.
• Gamma rays: Paul Villard (1900), discovered a penetrating type of radiation from radium; Rutherford named it gamma rays in 1903.
• Cosmic rays: Victor Hess (1912), carried electrometer in balloon flights.
• Neutron radiation: Chadwick (1932), discovered with the neutron.

Types of Radiation

• Ionizing Radiation:
  • Directly ionizing: Disrupts atomic structure, causing chemical and biological damage through kinetic energy.
  • Indirectly ionizing: Produces secondary electrons (charged particles) after energy absorption (e.g., X and gamma rays).
• Non-ionizing Radiation:
  • Electromagnetic radiation without enough energy to ionize atoms/molecules.
  • Causes excitation (electron movement to higher energy state).
  • Threshold: less than 10 eV, or 33 eV (energy to ionize water).

Uses of Radiation

• Medicine: Diagnosis, monitoring, and treatment (e.g., X-rays, radiation therapy, CAT/CT scans).
• Academic/Scientific: Research, carbon dating, neutron activation analysis.
• Industrial: Irradiation for sterilization, improving food production, gauges for measuring thickness and density.
• Nuclear Power Plants: Electricity generation through nuclear fission.
• Communication: All modern communication systems use electromagnetic radiation.

Atoms, Nuclides, Radionuclides, Alpha Emission

Atoms

• Basic unit of matter with a nucleus (protons and neutrons) and electrons.
• Classified by number of protons (element) and neutrons (isotope).
• Over 99.94% of mass concentrated in the nucleus.
• Electrons have stable energy levels and determine chemical properties.

Protons

• Subatomic particle with positive charge ($p$ or $p^+$).
• Number of protons determines atomic number.
• Diameter: 1.6–1.7 fm.
• Free protons make up 90% of cosmic rays.

Neutrons

• Subatomic particle with no charge ($n$ or $n^0$).
• Nucleons: protons and neutrons.
• Neutron number determines isotope.
• Free neutrons undergo beta decay.

Nuclear Force

• Force between nucleons, binding protons and neutrons.
• Attractive at 1 fm, insignificant beyond 2.5 fm, repulsive below 0.7 fm.
• Residual effect of strong force mediated by gluons.

Electron

• Subatomic particle with a negative elementary electric charge $e^-$.
• No known components or substructure, generally thought to be an elementary particle.
• Electrons contribute less than 0.06% to an atom's total mass.
• Electrons are bound to atoms with attractive Coulomb force.
• Beta decay is an example of how electrons may be created.

Nuclides

• Atomic species characterized by proton number (Z), neutron number (N), and nuclear energy state.
• Isotopes: same Z, different N.
• Isobars: equal mass number A, different Z.
• Isotones: equal N, different Z.

Radionuclides

• Unstable nuclides achieve stability by nuclear transformations (radioactive decay).
• Emit alpha or beta particles.
• May emit gamma rays to transition to ground level.

Alpha Particles

• Two protons and two neutrons ($\alpha$ or $\alpha^{2+}$ ); helium nucleus.
• Kinetic energy of about 5 MeV, 5% the speed of light.
• Low penetration depth; stopped by a few cm of air or skin.

Beta Particles

• High-energy electrons or positrons emitted by radioactive nuclei.
• Beta decay: $β^−$ (electron) and $β^+$ (positron).

Gamma Radiation

• High-energy photons emitted from unstable nucleus.
• Ionizing radiation.

X-Radiation

• Electromagnetic radiation produced by slowing electron beams.
• Ionizing radiation; harmful to living tissue.

Neutron Radiation

• Free neutrons; indirectly ionizing.
• More penetrating than alpha or beta radiation.

Cosmic Radiation

• High-energy particles originating outside Solar System.
• Primary cosmic rays: protons and alpha particles.
• Secondary cosmic rays: neutrons, pions, positrons, and muons.

Radioactive Decay

• Unstable isotope transforms to stable isotope, emitting particles.

Alpha Decay

• Emission of alpha particle, reducing mass number by 4 and atomic number by 2.
• Example: $\begin{aligned} {}<em>{92}^{238}\text{U} \rightarrow {}</em>{90}^{234}\text{Th} + {}_{2}^{4}\text{He} \end{aligned}$

Beta Decay

• In the case of beta decay that produces an electron emission, it is referred to as beta minus ($\beta^−$), while in the case of a positron emission as beta plus ($\beta^+$).
• For electron omission, an electron antineutrino is also admitted while positron emission is accompanied by an elector neutrino.
• Example: n Æ p + e - + $v^-$.
• Example: p Æ n + e+ + $v$.
• Beta + decay cannot occur in an isolated proton because it requires energy due to mass of neutron.
  +

Electron Capture

• Electron comes in contact with a nucleus and neutrino is emmited.
• Decay is also called K-capture because the innermost electron of an atom belongs to the K-shell of the electronic configuration of the atom, and this has the highest probability to interact with the nucleus.

Gamma Decay

• Emission of gamma rays from excited nuclear state.
• Isomeric transition: gamma decay from metastable excited states.
• Example: $\begin{aligned} {}^{60}<em>{27}\text{Co} \rightarrow {}^{60}</em>{28}\text{Ni*} + e^- + \nu<em>e + \gamma + 1.17 \text{ MeV} \rightarrow {}^{60}</em>{28}\text{Ni} + \gamma + 1.33 \text{ MeV} \end{aligned}$

Nuclear Fission

• Splitting of atomic nucleus into smaller parts, releasing energy.
• Binary fissions (two charged fragments) and ternary fissions (three fragments).

Fission Fragments

• Atomic fragments left after fission.
• Ternary fission also produces light nucleus such as helium-4 (90%) or tritium (7%).

Radioactive Decay Series

• Transformation of one nuclide into another until a stable nuclide results.
• Thorium series, radium (uranium) series, and actinium series.

Loss of Energy

• Energetic charged particles interact with matter, losing energy via excitation, ionization, and radioactive losses.

Specific Ionization

• Number of primary and secondary ion pairs produced per unit length.
• Increases with charge, decreases with velocity.

Bremstrahlung

• Electron interactions with atomic nuclei, emitting X-rays.
• Probability is proportional to $Z^2$ of the absorber.
• $\frac{\text{Energy loss by bremsstrahlung}}{\text{Energy loss by ionization}} = \frac{ZE_k}{800}$

Electromagnetic Radiation

• Includes radiowaves, microwaves, visible light, ultra violet light, X rays and γrays.
• Electromagnetic radiation waves are essentially characterized by their energy which varies inversely with the wavelength, the quantum of energy associated with the waves progressively increases from radiowaves with least energy to X and with highest energy, and X and γ ray photons have the ability to eject an electron from its orbit in an atom (are ionizing radiations).

Ionization

*The process of removing one or more electrons from atoms by the incident radiation leaving behind electrically charged particles (an electron and a positively charged ion) which may subsequently produce significant biological effects in the irradiated material (Figure 3.2).

*Other radiations of the electromagnetic spectrum fall short of the energy required to remove an electron from an atom and they are called non-ionizing radiations. Non-ionizing radiations are generally considered harmless to biological tissues at levels below those that cause heating effects, although there remain controversies in this area and research is ongoing. Cellular phones, radar, infrared, radiowaves, microwaves, visible light, ultrasound fall into this category.

Interactions of electromagnetic radiation

*When electromagnetic radiation travels through matter, it can be transmitted without transferring any energy or its intensity may be reduced by interaction with the traversed material.

*Biological effects arise when electromagnetic radiations, mainly X rays or γ rays, are either scattered or absorbed by the atoms of tissues/organs. Quantum theory considers electromagnetic radiation as streams of packets/bundles of energy called photons.

$E = h\nu= hc/ \lambda$
*Where

• E is the energy of the photon,
• h is Planck’s constant, and νis the frequency of the photon.

Energy

*The energy of a photon is directly related to its frequency and inversely to wavelength, λ

• $c= \lambda\nu$ Where c is the velocity of light.

Photoelectric absorption

In photoelectric absorption, the photon interacts with a bound inner shell electron in the atom of the absorbing medium and transfers its entire energy to the electron ejecting it from the occupied atomic shell

• The incident photon disappears and the energy transferred is used to overcome the binding energy of the electron and the remainder appears as kinetic energy of the resulting photoelectron.

$Kinetic Energy (electron) = h\nu– E b$
*
Where hνis the energy of incident photon, and Eb is the binding energy of the electron.

• The photoelectric effect is the dominant energy transfer mechanism for X and γ ray photons having energies below 50 keV in biological tissues, but it is much less important at higher energies

Compton Effect

• This process occurs when an incident photon interacts with an outer orbital electron whose binding energy is very low compared with that of the incident photon
• In this interaction, the incident photon transfers energy to an atomic electron causing its ejection from the atom.
• The scattered electron (a secondary charged particle) travels some distance in matter and eventually loses energy by further ionization and excitation events to become part of the material

Pair production

When a photon of high energy ( >1.02 MeV) interacts with atoms of the medium, the incident photon can be spontaneously converted into the mass of an electron and positron pair by interaction of the Coulomb force in the vicinity of the nucleus

• A positron is the anti-matter equivalent of an electron and it has the same mass as an electron, but it has a positive charge equal in strength to the negative charge of an electron.

Half value layer

• The thickness of absorber that reduces the photon intensity to one half is called the half value layer (HVL).
  $I (x) = I0 e - µx$
  Where I (x) = the intensity at thickness x, I0 = is the initial intensity on the surface of the absorber, µ= n×σis the absorption coefficient measured in cm −1 , , n = the number of atoms per cm3 in the material, σ= the absorption cross section in cm 2 , and x = the thickness of material in cm

Linear energy transfer

As the LET increases, so does the biologic damage. This is known as the relative biologic effectiveness (RBE).

• $\text{LET}=(−dE/dX)$

Dosimetry

*The measurement of any type of ionizing radiation received by a mass of any type of matter.

Stochastic effect:

*A radiation effect whose probability of occurrence increases with increasing dose, but whose severity isindependent of total dose

Deterministic (Non stochastic) effect

*A radiation effect characterized by a threshold dose. The effect is not observed unless the threshold dose is exceeded. Once the threshold dose is exceeded in an individual, the severity of injury increases with increasing dose.

Threshold dose:

*The minimum radiation dose at which a specified deterministic effect can occur.

Exposure:

• A measure of the strength of a radiation field at some point in air. This isthe measure made by a survey meter.

Absorbed dose:

• Radiation dose is the energy (Joules) absorbed per unit mass of tissue and has the (S.I.) units ofgray (1 Gy = 1 J/ kg).

• A rad is defined as a dose of 100 ergs of energy per gram of the given material.

• The gray (Gy) is defined as a dose of one joule per kilogram.

• To convert from traditional to SI units, multiply the rad by 0.01 or multiply the gray by 100 to equal therad.

Equivalent dose

*The equivalent dose (HT) is a measurement used to compare the biologic effects or damage an exposed individual might expect to occur from different types of ionizing radiation.

*rem (roentgen equivalent man) is the traditional unit and the sievert is the SI unit for the equivalent dose. To convert from rem to sievert, multiply the rem by 0.01 or from sievert to rem, multiply the sievert by 100 to equal the rem.

Quality Factor:

• The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness.

Effective dose

*Effective Dose is used to estimate the risk of radiation or its biological consequence in animals and man

Collective dose  

*Collective dose is defined as the received per person in Sv multiplied number of persons exposed per year

Relative Biological Effectiveness

$\text{RBE}=\frac{DR}{D x}$

Kerma

*kinetic energy released in the medium

Air kerma:

*The energy released per unit mass of a small volume of air when it is irradiated by an xo ray beam

Chemical dosimeters

*The Fricke chemical dosimeter is based on chemical change by absorption of radiation and used to measure, X, γ and electron doses

Thermoluminescence dosimeters (TLD):

*Consist of types of radioactive isotopes that cause electrons in the crystal's atoms to jump to higher energy states, where they stay trapped due to intentionally introduced impurities in the crystal, until heated.

Geiger-Muller Counter

*It detects the emission of nuclear radiation — alpha particles, beta particles, or gamma rays — by the ionization produced in a low-pressure gas in a Geiger–Müller tube , which gives its name to the instrument.

*GM counter has been used in particulate detection, X and gamma ray detection, neutron detection, and gamma-measurement personnel protection and process control.

Ionization chambers

• measure the current flow which occurs due to the ionization of the air molecules exposed to radiation

Proportional Counter

*A type of gaseous ionization detector device used to count particles of ionizing radiation. A key feature is its ability to measure the energy of incident radiation, and it is widely used where discrimination between radiation types isrequired, such as between alpha and beta particles.

Film dosimetry

*Special radiographic films have been developed for verification of dose in radiotherapy practice. This has proved useful for measuring dose profiles but the method has limited accuracy and dose range for determination of absolute radiation doses

Calorimetry Dosimetry System

*recognized as the best approach for establishing absorbed dose standards