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The Physics Behind Radiation

Author: Ruoxi Lin

Editors: Flynn Ma, Viola Chen

Artist: Jade Li

When we hear of radiation in the news, it’s often portrayed as an unyielding, dangerous force that obliterates cities and destroys people’s lives: think of the Chernobyl disaster or the Three Mile Island incident. Considering the multiple cancer fatalities and irreversible damage these infamous incidents have left, it’s reasonable why people fear radiation. However, as technology has developed over time, radiation has been a more trusted, harnessable energy source in many fields–especially the medical field–where it remains the driving force behind radiotherapy. How are we able to control this useful yet unstable force and implement it into modern-day medical equipment, such as X-rays? The answer lies in physics.

In the context of X-ray radiation (not referring solely to X-rays used in a medical setting, but the type of electromagnetic radiation itself), energy is measured with Planck’s equation E=hf=hc/λ, where E is energy, h is Planck’s constant, f is frequency (the number of waves that pass a given point in a second), c is the speed of light, and λ (lambda) is wavelength. Planck’s constant is the universal constant that defines quantum energy and has a value of 6.62607015×10⁻³⁴ J·s. When utilizing Planck’s equation, it’s also important to acknowledge that E refers to the energy of a photon, or packet of electromagnetic energy. Frequency can be calculated by dividing wave speed by wavelength; this is seen when substituting c/λ for f, as all electromagnetic radiation travels at the speed of light. 

X-rays are produced by X-ray tubes, which are vacuum tubes that turn electrical power into X-rays. These tubes have a cathode end, a usually negative end where electricity flows out from, and an anode end, a usually positive end where electricity goes. Currents applied to the cathode heat it and force it to emit electrons, which collide with the anode end. The energy of the electrons is converted to X-ray photons, either becoming Bremsstrahlung X-rays or Characteristic X-rays. Bremsstrahlung X-rays occur when electrons, upon coming close to the atom of the anode, are slowed down and deflected by a force field around the nucleus. Some of the electron’s energy, usually of varying amounts (in kilovolts), is given off in the form of an X-ray photon, either once or multiple times. Characteristic X-rays are produced when electrons from the cathode orbit the inner shell of the anode target atom, before being ejected by another electron, and then replaced by an outer shell– the process of the latter leading to another release in energy. X-ray photons produce prominent doses of radiation targeted at patients during X-ray imaging procedures via photoelectric or Compton scattering, which describes how radiation spreads out in different directions due to interactions with the target matter’s electron shells and the X-ray.

With the evolution of physics alongside medical technology, radiation dosage can be calculated using standard convolution, superposition principles, and Monte Carlo simulations. The superposition and convolution principles of Monte Carlo simulations use computational algorithms that take multiple random samples to predict possible outcomes to explain risk. The Superposition principle, used in all linear systems and functions, states that when waves overlap, the resulting disturbance or response is the sum of the displacements in the individual waves. The Convolution principle is much more difficult but can be described as the integral, or area under a curve defined by a function, that shows the overlap of one function after it is shifted over another function. Using these two principles, physicists can create model equations to determine where radiation dosage would be concentrated and where the radiation is headed. Both result in fast and accurate results but are not as accurate as Monte Carlo simulations, which track the route of individual X-ray photons, including where they enter and possible scattering. These methods produce precise dose tables through computer simulation and model molecular interactions well, but are usually more time-consuming and expensive. Measuring this dosage has been a fundamental achievement that still guides radiology today.

While it is not acknowledged in the medical field as much as chemistry or biology, physics has allowed us to make essential discoveries with X-ray technology and other forms of modern neuroimaging techniques. From widely known equations to in-depth algorithms, physics has changed the course of radiology in unbelievable ways, and its evolution alongside radiation technology will allow us to provide better diagnoses for patients and improve analyses of industrial materials, food irradiation methods in agriculture, radiation detection technology in space exploration, and so much more. 



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