Arthur Holly Compton
Compton scattering is a significant concept in the subject of physics. It is also known as the Compton effect. It showcases the outcome when a particle collides with a target at an angle. When x-rays are used, the result is that they scatter upon colliding with the target leading to an increase in the wavelength of the subsequent x-ray. The increase in wavelength signifies a reduction in the amount of energy held with the particle. At the time, the discovery was valuable because it helped in the development of theory and knowledge, especially as it relates to the behavior of light. In this paper, an assessment of the phenomenon, alongside its historical background, key scientific concepts, and applications, will occur.
Arthur Holly Compton
Arthur Holly Compton made the discovery that was subsequently named after him. He discovered this notion while researching at Washington University. It was the culmination of multiple events that had taken place before. The scholar graduated from the University of Wooster, and later, Princeton University (American Physical Society n.p). While in the latter, he developed several theories. Nonetheless, the main one contented that it is possible to examine the nature of the arrangement of atoms in matter by evaluating the intensity of the x-ray reflections beamed onto crystals.
Given his pioneering research while at Princeton University, the scholar received a National Research Council (NRC) Fellowship. He opted to undertake the scholarship at Cambridge University in Britain (American Physical Society n.p). Nevertheless, while there, he faced major challenges, especially concerning the lack of adequate equipment. Consequently, he shifted to Washington University in St. Louis. While there, he improved his laboratory equipment consistently, a factor that led to his discovery of the Compton effect.
Historical Relevance of the Experiment
The discovery of the Compton effect has valuable historical relevance. Therefore, it is crucial to evaluate the historical background of the series of experiments that led to the discovery, if one is to understand its significance. Notably, Arthur Compton was not the pioneer scientist to study the scattering of electromagnetic radiation— which had until then focused on gamma rather than x-rays. Two conceptual problems were evident in the previous attempts. Firstly, it concerned the comparison of qualities of two different beams (the primary and secondary beams). A portion of scientists believed that a difference between the incident and secondary rays existed; however, it was difficult to prove such a convergence in a practical setting. Secondly, a poor understanding of the nature of the absorption techniques was another challenge.
Additionally, even if the researchers could explain the differences in energy between the primary and secondary beams, Bartlett holds that it was difficult to measure the difference using an objective methodology (124). This situation prevented the development of a comprehensive theory on the matter. Further, it also prohibited the practical application of scattering, like measuring the density of matter, which has become more prevalent today.
Bartlett highlights some of the attempts to research the scattering effects before Compton. This clamor arose following the discovery of x-rays in 1895, and subsequently, radioactivity in 1896 (Bartlett 122). Nonetheless, despite the invention, a limited understanding of electromagnetic radiation forms existed. A scholar, Eve, undertook experiments on scattering targeted at radium ad using gamma rays. The researcher concluded that secondary rays are highly homogenous when compared to the incident ones. Therefore, the scholar theorized that the magnitude of absorption between the two beams differed.
Other scholars that paved the ground for the eventual discovery of the Compton effect include Kleeman. The scientist concluded that scattered rays have a higher absorption capacity when compared to the incident beam (Bartlett 122). Another scholar that pioneered the study is Florance. His theoretical development of the concept was significant for the eventual Compton scattering. Florance was the first to incorporate geometry. From his experiments, he deduced that the angle of the target varied the quality and absorbency of the secondary rays. For instance, when the primary radiation is beamed toward a target whose is angle is greater than 90°, then the emerging ray is weaker than the incident ray (Bartlett 122). However, when the angle is less than 90°, the energy of the emerging ray does not vary significantly from that of the primary beam. Thus, Florance concluded that the positioning of the electroscope (the source of the gamma rays) is a critical consideration when setting up the experiments. Yet, such ideas only remained qualitative since no objective means of measuring the effects existed.
By the time Compton was undertaking the experiment, the underlying belief among most scholars on the subject was as summed up by George William. The latter held that “All substances, when exposed to a beam of x-rays themselves give out x-rays, which are identical with the primary rays in quality, and can, in fact, be conveniently regarded as so many unchanged primary rays which have been merely scattered or deviated by the substance” (Bartlett 124). Compton did not perceive the understanding of absorbency properties to be a priority. Instead, he argued that the comprehension of the spectral attributes of a radioactive ray was the more immediate concern. Mostly, he believed that it is impossible to evaluate the scattering effects of an electromagnetic beam without, first, understanding its underlying characteristics. He held this to be the major failure of the existing school of thought.
Bartlett contends that the overarching belief at the time was based on Thomson’s theory of scattering. However, despite being the most dominant, it was inherently flawed; yet, many researchers did not object to it because of the respect that Thomson commanded in the scientific community (124). This aspect makes Compton’s discovery even more revolutionary because it went against the popular dogma at the time.
The Experiment
Compton used an x-ray tube to experiment. The tube is constructed in a unique manner such that it can convert electricity into x-rays — the technique is more conventional today. The scholar then used the machine to emit electromagnetic radiation, which was then targeted at a graphite surface. The rationale for using the graphite surface is that as a form of a carbon compound, a relatively advanced understanding of its structure already existed by that time; hence, it would provide a suitable platform on where the researcher could prove his hypotheses.
Based on the experiment, Compton concluded that such targeting increased the wavelength of the ensuing x-rays. However, one of the main challenges at the time was the measurement of the increase in the wavelength of the scattered rays. The researcher solved this problem by once more, focusing the scattered ray onto a crystal. The crystal them radiated the energy into an ionization chamber. The tool measures the power of electromagnetic radiation based on the number of photons that emerge (Bartlett 120). The ability if this chamber to provide an accurate depiction of the subsequent energy arises from its relatively low electric field; hence, preventing the multiplication of ions.
The Main Concepts of the Experiment
The Compton effect refers to the decline in energy that an electromagnetic ray (gamma or x-ray) experiences when it encounters matter. The resultant reduction in energy implies an increase in the wavelength — a wave with a higher wavelength contains less energy when compared to one with a shorter one. This circumstance arises because, in the latter, the particles in the matter are more compacted; thus, they hold more incident energy.
Compton found that as the scattering angle increased (based on the positioning of the target matter), so did the wavelength of the secondary ray. Notably, the researcher used x-rays for his experiments. However, he held that the assumptions are also applicable to other forms of electromagnetic energy, including light. Therefore, when the constituent elements of light (photon) hit a target (graphite), the particle loses some of its energy because of the concepts of conservation in energy and momentum. Consequently, the ensuing photon particle holds less energy, a factor that causes an increase in its wavelength.
The experiment illustrates a concept that had, hitherto, been considered impossible — inelastic scattering. It refers to the variation in wavelength when a charged photon hits a target. Until then, the underlying belief was that only elastic scattering was possible. In the latter approach, a particle retains all its energy upon hitting matter. This scenario was prevalent because, by that time, the understanding of waves was advanced. For the most part, waves illustrate elastic scattering when radiated against a target. Accordingly, the outcome of Compton’s experiment had significant implications for physics at the time. It showed that wave theory did not provide enough explanations for the properties of light. This result led to the classification of light as a quantum phenomenon.
Compton also gave a theoretical explanation for the occurrence of the effect. He assumes that the x-ray particles are like the photons in light. He then proceeds to argue that they scatter upon hitting a target. In the process, the photons lose some of their energy to the electrons of the target matter. He noted the reduction in the photon’s energy and subsequent increase in the secondary its wavelength. This situation occurs because of Einstein’s theory on special relativity — energy must always be conserved (Weinert 115). Therefore, the transfer of energy from the photon to an electron must be reflected in the secondary wave. This outcome is indicated in the form of an increase in wavelength and low energy. It implies that the initial energy is shared among a growing number of particles; hence, the concept of scattering. Essentially, the total energy of the primary photon must total that of the secondary photon plus the displaced proton. The equation below provides a simplistic overview of the concept of energy preservation inherent in the Compton effect.
Energy of the primary = Energy of the secondary photon + Energy of the displaced electron.
This relationship gives rise to the Compton equation as stated below,
λ − λ0 = h/mec (1 − cos θ)
Where,
λ – wavelength of the secondary photon.
λ0 – wavelength of the initial incident photon.
me – mass of the electron.
θ – angle of the target at the time of scattering.
A detailed explanation of the outcome indicates an inclination to quantum theoretical concepts. According to Weinert, when an x-ray hits a target, it does not act on all the electrons uniformly (116). Instead, it focuses on a single electron. Subsequently, the affected electron refracts the x-ray in a direction that is consistent with the initial scatter angle. The consequence of this scenario is that it leads to a change in the momentum of the x-ray; thus, necessitating a shift from the electron worth the same amount of momentum as the refracted x-ray. The incident energy of the x-ray will be equal to that of the secondary one. Nonetheless, the energy does not reflect the total initial energy present in a photon since some energy is lost. This loss occurs because of the kinetic effort of the electron when shifting. Therefore, the frequency of the successive ray will reduce to accommodate the ‘lost’ energy; hence, leading to an increase in the secondary wavelength (λ).
The experiment became notable because it solved a major issue in the physics discourse at the time. Back then, the understanding of the properties of light was inadequate. Consequently, most of its properties were still a subject for debate. Nevertheless, Compton’s experiment, and the resulting theory, illustrated that some aspects of the behavior of light are equivalent to those of normal particles. This understanding acted as an impetus for research into the various probable uses of the knowledge. Accordingly, he was awarded the Nobel peace prize for physics in 1927. Further, the effect was christened with a name the same as his. Principally, the notion of Compton scattering has numerous practical uses. Below is an overview of its application.
Applications of Compton’s Effect
It is crucial to ensure that any theoretical proposition also has practical implications. This aspect is necessary because individuals should not conduct research merely for the sake of it. Compton’s effect has numerous practical applications. Its most common use is in radiobiology. Given the differential wavelengths of the primary and secondary waves upon hitting a target, one can learn of the attributes of matter. Therefore, in radiotherapy, it may, for instance, be used to evaluate whether a body tissue is normal or dead; thus, providing objective grounds on which to institute treatment.
Compton’s scattering is also vital in the field of electron densitometry. According to Islami rad, it refers to the ability to measure the number of free electrons in an element (245). Therefore, it serves a vital function in the identification of elements in the periodic table. Further, it also provides suitable theoretical and practical bases with which to study the wave properties of different forms of energy. This aspect has had positive effects on some sectors, especially imaging.
Conclusion
The Compton effect is a foundational discourse in physics. It provided a platform for understanding various issues regarding electromagnetic forms of energy like light, gamma-, and x-rays. The discovery of this phenomenon by Arthur H. Compton occurred at a time when numerous other experiments on the subject occurred. However, by the time of the discovery, none of the theoretical propositions had been proved. Therefore, Compton’s ability to prove the differences in wavelength and frequency of the primary and secondary rays was revolutionary.
Works Cited
American Physical Society. “Arthur Holly Compton Laboratory of Physics, Washington University, St. Louis.” Www.Aps.Org, 2020, www.aps.org/programs/outreach/history/historicsites/compton.cfm. Accessed 3 May 2020
Bartlett, Albert Allen. “Compton Effect: Historical Background.” American Journal of Physics, vol. 32, no. 2, 1964, pp. 120–127, 10.1119/1.1970139. Accessed 3 May 2020.
Islami rad, S. Z. “Optimization of Energy Window for Gamma Densitometer Based Backscatter Method in Oil Industry.” Russian Journal of Nondestructive Testing, vol. 52, no. 4, 2016, pp. 245–249, 10.1134/s1061830916040045. Accessed 3 May 2020.
Weinert, Friedel. “Compton Experiment (or Compton Effect).” Compendium of Quantum Physics, vol. 7, 2009, pp. 115–117, 10.1007/978-3-540-70626-7_35. Accessed 3 May 2020.