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| Fig. 1:Experimental Setup to determine existence of β-rays. (Source: N. Martelaro) |
Scientific experiments are the foundation for building our understanding of the physical world. While experiments may seem complex to the non-scientist, in reality, many influential scientific discoveries have been made with beautifully simple experiments. One such example is Earnest Rutherford's experiment on the nature of uranium radiation and its ability to pass through various materials. [1] Using the fact that the radiation from Uranium ionizes a gas, thus creating charges particles, Rutherford was able to measure the current produced by a sample of uranium placed between two charged metal plates. By placing metal foils on top of the uranium, Rutherford showed that part of the ionizing radiation was stopped while another part appeared to pass through the foils. This helped to confirm the work of Becquerel, showing that there were two components of the ionizing radiation. [2] These two rays were given the names α-rays and β-rays. While Becquerel's works suggested the existence of the two types of rays, there was still little understanding of their nature. Specifically, through how much and what types of material did these rays pass through? Rutherford aimed to explore this using his simple and beautiful experiment and was able to show the differences between α and β radiation absorption. This experiment would later lead to him advising Geiger and Mardsen's famous gold foil experiment, whereupon α particles moving through gold foil were shown to sometimes deflect, suggesting the positive nuclear core model of the atom that we know today. [3] This report gives an overview of the design and results of Rutherford's experiment, which is described in detail in his 1899 paper. [1] It should be noted that Rutherford details a number of experiments in his paper, only the experiment that confirmed the existence of α and β rays will be discussed here.
At the time of Rutherford's experiment, uranium was know to emit an ionizing radiation similar to x-rays. When subjected to a gas, this radiation would create positively and negatively charged particles. This allows the gas to be a temporary conductor of electricity and would allow an electric potential and current to be measured. From Becquerel's work, it was known that the radiation would penetrate solid material, but it was not known through how much material. It was also known that the rays emitted from uranium had varying powers.
From this theory, Rutherford hypothesized that the rays emitting from uranium would be complex, composed of different types of rays. He proposed that testing how well the rays penetrated metal foils may help to show what the characteristics of the rays were.
Rutherford's experimental setup was quite simple. A diagram of it is shown in Fig. 1. A sample of uranium is placed between two plates A and B. Plate A is charged by a battery to 50 V, while plate B is connected to the sensing element of an electrometer. This creates an electrical potential between the plates. Due to the ionizing radiation coming from the uranium, the gas in between the plates will become electrically charged. Positive ions will move away from plate A while negative ion will move toward it. This will induce a small current between the plates. As this current flows in the gas, it will create a charge on the sensing element of the electrometer.
The electrometer works by having four separated quadrants. The quadrants are hollow inside, much like a bicycle tire without a tube. The diagonally opposing quadrants are connected together electrically. One set of quadrants is connected to earth ground while the other acts as the sensing quadrant and is connected to Plate B. A metal vane (or needle) is placed inside this hollow area of the quadrants. The vane hangs from a thread, allowing it to spin inside of the quadrants. The vane is then charged. When a charge is induced on Plate B and the sensing quadrant of the electrometer, it begins to spin the vane due to the opposing electrical forces, similar to how magnets with the north poles facing each other will repel each other. The degree that the vane turns is associated with the voltage, while the rate that the vane turns is associated with the current.
To explore the nature of the radiation, Rutherford covered the uranium with a thin sheet of metal foil. He then measured the "rate of leak" given by the electrometer vane when in constant motion (indicating a specific amount of current, and thus an amount of charge induced by the ionizing radiation). By placing successive sheets of foil, Rutherford was able to see how much the rate of leak diminished, indicating how much of the radiation was blocked.
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| Fig. 2: Leak rate per minute from uranium radiation with increasing numbers of Dutch metal (brass) foils 8 × 10-5 cm thick) [1] (Source: N. Martelaro) | ||||||||||||||||||||||||||||||||||
Rutherford first began by adding sheets of Dutch metal (brass foil), on top of the uranium and measured the leak rate per minute in the electrometer scale divisions. The second column of the table in Fig. w shows the leak rate as each layer of foil was added. The third column shows the ratio that the leak rate had decreased from the previous layer, helping to show the effect of each layer on blocking the radiation. The exponential nature of the attenuation with increating thickness is evident in Fig. 2.
Rutherford then used thicker aluminum foil (0.0005 cm thick) to block the radiation from the uranium. Adding four layers of the aluminum foil blocked much of the radiation, as shown in Fig. 3. However, after the fourth layer, it took another eight layers of aluminum to decrease the leak rate from 9.4 to 7. This simple test shows that there appear to be two components of ionizing radiation from the uranium, one that is blocked very easily (corresponding to the radiation that is blocked with the first four layers of aluminum) and one that is barely blocked by the aluminum (corresponding to the leak rate even after 12 layers are added).
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| Fig. 3: Leak rate per minute from uranium radiation with increasing numbers of aluminum foils 5 × 10-4 cm thick) [1] (Source: N. Martelaro) | ||||||||||||||||||||||
Rutherford described these two components of the radiation as α-rays and β-rays.
After understanding that the radiation from uranium was composed of α- and β-rays, Rutherford then extended his experiment to explore the penetration of β-rays. From the earlier results, he knew that he could block all of the α-rays with a few sheets of material. He found that this could be done with aluminum, tin, and even paper. With the α- rays blocked by 0.005 cm of aluminum, Rutherford added aluminum sheets to explore the penetration of β-rays.
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| Fig. 4: Uranium leak rate/min for increasing layers of aluminum after α-rays are blocked. The leak rate is set to 1 at 0.005 cm of aluminum denoting the base rate for β-rays. [1] (Source: N. Martelaro) | |||||||||||
The results of this experiment, reproduced in Fig. 4, show that β-rays appear to only have one component and a fairly constant penetrating power. This shows why the decrease in detected leak rate was fairly linear only when the β-rays were tested without α-rays. Rutherford does note though that there may be another component to the radiation from uranium, but that it must be so small or so penetrating that it was undetectable with his experimental setup. In 1903, Rutherford would later go on to discover γ-rays (gamma-rays), a third ionizing radiation with very high penetration.
Rutherford's beautiful experiment is one example of how we can understand nature without needing complex and expensive equipment. Though some of the measurement equipment used in Rutherford's day, such as the electroscope, was an intricate mechanical measurement tool, it still relies upon simple principles of nature. By understanding that ionizing radiation could create a current in a gas between charged plates and by testing how that current was changed when trying to block the radiation, Rutherford was able to understand and characterize two fundamental components of nuclear radiation.
What is even more impressive is that Rutherford's brilliant experiment can easily be recreated today with more modern radiation detectors, such as the Geiger counter and naturally occurring uranium ore. [4,5] Both Geiger counters and small samples of uranium ore can readily be purchased online for less than $200. One could then recreate Rutherford's analysis using grocery store aluminum foil. Overall, Rutherford's experiment shows how with just enough understanding of the world as we know it, simple tools, and a bit of creativity, we can create simple ways to explore nature and better our understanding.
© Nikolas Martelaro. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] E. Rutherford, "Uranium Radiation and the Electrical Conduction Produced by It," Philos. Mag. 47, 109 (1899).
[2] H. Alaeian, "An Introduction to β-Ray Spectroscopy," Physics 241, Stanford University, Winter 2014.
[3] E. Rutherford, "The Scattering of α and β Particles by Matter and the Structure of the Atom," Philos. Mag. Ser. 6 21, 669 (1911).
[4] T. English, "Radiation Detectors," Physics 241, Stanford University, Winter 2015.
[5] A. Lange, "Nature's Radioactive Material," Physics 241, Stanford University, Winter 2011.