Alpha, Beta and Gamma Rays: It’s History And Discovery

In this video I am going to talk about the three major types of nuclear radiation: alpha, beta, and gamma, distinguished by how powerful they are, alpha being easier to block than beta which is easier to block than gamma. There are quite a few videos about what these radiations are but almost nothing on how we know what they are. Why was alpha and beta radiation discovered before gamma, how did these discoveries relate to the discovery that a beam in a vacuum tube called cathode rays were really a beam of electrons, and why did it take so long to discover that alpha radiation was made of helium nuclei! Ready for the real if slightly convoluted story?  Let’s go!

Let’s start in the summer of 1898 when a 27-year-old grad student at Cambridge named Ernest Rutherford read a very interesting article in the scientific papers from Paris. A woman named Marie Sklodowska Curie had used a very sensitive electricity meter to measure how radioactive uranium and thorium are.  As Rutherford was finishing his Ph.D. on the electrical effects of x-rays it seemed like a good idea to use Curie’s method to study what was called “Becquerel rays” in the same way.  Therefore, Rutherford took a scale and placed a layer of uranium on a plate with a gap, and measured the current from the electrified air.  He then put a very thin layer of gold in the gap to see if, and how, the radiation changed.  To his delight, the gold seemed to block some of the rays.  Two leaves of gold lowered the current even more.  Three pieces lowered the current, too, but not by as much.  When he switched to a thicker aluminum after a few pieces there was basically a steady current, no matter how much metal he added.  Rutherford quickly realized that there must be at least two types of radiation, one that was easily blocked and one that wasn’t, and decided to name them after the Greek letters for a and b, alpha and beta.  On September 1, 1898, Rutherford published his conclusion: “uranium radiation is complex, and there are present at least two distinct types of radiation – one that is readily absorbed, which will be termed for convenience the alpha radiation, and the other of a more penetrative character, which will be termed the beta radiation.”[1]  Rutherford missed realizing that there is a third type of radiation because gamma radiation is very difficult to measure with electricity and his uranium did not produce enough to be measured with an electricity meter.  In fact, he even admitted in this paper that, “it is possible that other types of radiation of either small intensity or very great penetrating power may be present.[2]”  Yep!

Ernest Rutherford
Ernest Rutherford

Meanwhile, Marie Curie discovered a new highly radioactive substance in uranium ore, that she called polonium.  Polonium was so much more radioactive than uranium that she made up the term “radioactive” to describe it and her husband Pierre was so enthralled that he dropped his research to work with her in this new field.  Then, in December 1898, Marie and Pierre Curie published that there was a second substance hidden in the ore that was even more radioactive, so radioactive that they called it radium.  Radium was special: for example, it seemed to violate the laws of thermodynamics, it was always hot and it glowed bright enough to read by without changing shape or having any input energy source!  Now everyone wanted a piece but only the Curies in France and a German Chemist named Friedrich Giesel (who started working on it when he read about Polonium) had any, and while they were working on isolating it: they weren’t sharing much except to their closest associates.  Speaking of Giesel, in August 1899, some friends of Giesel’s in Germany (named Elster and Geitel) decided that since cathode rays are easily moved by a magnet it would be, “interesting to determine whether the conductivity caused by the Becquerel rays (radio-activity) can also be altered by a magnetic force.[3]”  They didn’t notice anything from the weak Uranium, but a piece of radium on loan from Giesel did seem to be lowered by the magnetic field.  Giesel and some other friends named Mayer and Schweidler then wondered if the magnet actually slowed down the radiation of the radium or deflected some of the rays.  They, therefore, decided to see if they could see the effect of magnets on radiation in a photograph and by November 1899, found that the radiation from radium was distinct into two types, one that was undisturbed by magnetic fields and one that curved like cathode rays (or a beam of electrons)[4].  Giesel and his friends missed seeing the gamma rays because they did not have very active material and the film is much more reactive to alpha and beta radiation than to gamma radiation (gamma radiation is hard to see electrically AND with photographs, that is why it took so long to find).  Also, since alpha particles are very heavy, like 8000 times as massive as beta, the magnetic fields used weren’t strong enough to move the alpha particles much and that was what they thought was non-deviatable.  Inspired by the Germans, by January of 1900, Pierre Currie set up an electrical system to quantitatively study the magnetic effect on radiation and validated that some of the radiation from radium was divertable with a magnet and some were not and was able to determine that, “The deflected rays are the most penetrating.[5]” Therefore, in France and Germany, it seemed more descriptive to use the term non-deviatable radiation instead of alpha radiation and deviatable radiation instead of beta radiation (Rutherford stubbornly stuck with his nomenclature though). 

Two months later in March 1900, Marie and Pierre Curie reported that the deviatable rays from radium contain negative charges, and concluded that “the deviable rays of radium, as with the beam of the cathode ray, carry [negative] electricity[6]”.  This conclusion about the deviable ray being similar to cathode rays is what attracted the interest of a shy chemist named Paul Villard and led him to discover gamma rays.  See 3 years earlier, in 1897, Rutherford’s advisor, JJ Thomson had determined that if he increased the vacuum for cathode rays tubes he could get the cathode rays to move with electrically charged plates and not just by a magnet (the trace amount of air was acting like a Faraday cage and keeping the rays from feeling the electrical force).  Thomson then balanced the electric force and the magnetic force and used this electromagnetic gauntlet to determine the speed of the beam.  He then used that information to determine that the beam was made of teeny tiny negatively charged particles that were eventually called electrons.  However, there was an experiment that seemed to invalidate JJ Thomson’s electron theory.  When cathode rays travel through thin metal materials, they make it through the same electromagnetic gauntlet which implied, if Thomson was correct, that cathode rays tunnel through metal but keep the same speed, which seemed impossible.  Villard, who was the pro-electron theory, wondered if it could work out if the cathode ray didn’t tunnel through the metal as much as cause the point of contact to be a new source of cathode rays. To validate his theory, Villard conducted an experiment where he had part of a cathode ray hit an insulated sheet at an angle and found that the beam was split in two where the part of the beam that hit the metal emerged perpendicular to the surface and ended up at a distance from the center. 

Radiation

It was in the middle of these experiments on cathode rays that Villard read about Marie Curie’s results with the charge of deviatable rays from radiation.  Villard decided to study if the penetrating deviable radiation acted like cathode rays when it hit metal.  Luckily, he was working in the same laboratory building as Henri Becquerel who had discovered radiation in the first place with uranium and who had been supplied with radium from the Curies.  Villard then borrowed some radium from Becquerel and put it in a lead container with a hole in it so that the radiation came out in a beam and then had half the beam of radiation hit a metal sheet at 45 degrees and recorded the response on a photographic plate that was wrapped in black paper (thus blocking out the alpha particles).  He got a very strange result.  He did get an image from some of the beams moving perpendicularly from the metal, however, a trace amount of the beam seemed to go straight through the thin metal sheet.  It immediately appeared clear to him that there were two types of beams that can travel through paper, one that acts like a cathode ray (a beam of electrons) and one that can travel straight through thin metal like an x-ray.  Villard then wondered if both beams were movable with magnets so he removed the metal in the middle and added a magnet.  He then found two distinct beams on the photograph, one that was unmoved by the magnet and one that was moved by a few centimeters, however, the unmoved beam could not be diminished even when going through several sheets of paper and an aluminum strip or even a thin piece of lead.  After Villard pointed it out, other people validated Villard’s discovery.  However, most people were way more interested in alpha or beta radiation and there wasn’t much interest in Villard’s work and even Villard turned to other subjects.

Now we go back to Ernest Rutherford.  Back in 1898, while he was publishing about the alpha and beta radiation, Rutherford moved from England to Canada to be a professor at McGill University which had, “a swell lab[7]” so that he could “knock the shine out of the Yankees![8]” In Canada, Rutherford started doing amazing work (like discovering a new radioactive gas and the half-life of radiation and then using the half-life of radioactive materials to determine the age of the Earth). And with his good friend Friedrick Soddy, Rutherford discovered that radioactivity often involves one chemical element decaying into another element called transmutation.  Soddy recalled, “I remember quite well standing there transfixed as though stunned by the colossal import of the thing and blurting out…’Rutherford, the Thorium is disintegrating and transmuting itself into an argon gas.’  Rutherford replied, “For Mike’s sake, Soddy, don’t call it transmutation.  They’ll have our heads off as alchemists… Make it a transformation.[9]”).  By the way, Rutherford and his friends accomplished all of this despite the fact that he only had feeble radium that was too weak to light up an x-ray screen!  For example, Rutherford was convinced for years that alpha radiation was made of charged particles which means they should be moved by a magnet but it was incredibly difficult to measure as they are so heavy, especially with such weak radium.  In fact, it wasn’t until February 1903 when Rutherford was able to experimentally demonstrate that alpha particles from his weak radium were, “deviable by a strong magnetic and electric field. The deviation is in the opposite sense of the cathode rays, so the radiations must consist of positively charged bodies projected with great velocity.[10]”  This is also the paper that Rutherford decided to name the third type of radiation “gamma rays” as gamma is the third letter of the Greek alphabet and he wanted to start his paper with a short description of the three types of radiation[11].

Marie Curie obviously read Rutherford’s paper about radiation as in her thesis presented 4 months later in June 1903, she not only included that Rutherford had found that alpha particles are deviatable by magnets she also, for the first time, named the three types of radiation, “according to the notation adopted by Mr. Rutherford, by the letters alpha, beta, [and] gamma.[12]”  Mere months after publishing this thesis, the Nobel committee announced that Marie Curie was being awarded 1/4th of the 1903 Nobel Prize for physics!  The public and the newspapers fell over themselves in excitement.  Suddenly, everyone wanted to know about radium, and Curie’s thesis became the bible of radioactive research the three types of radiation were then referred to as alpha, beta, and gamma as they have been known ever since. 

Let’s take a moment to go back to Curie’s phenomenal thesis and her definition of alpha, beta, and gamma radiation.  She said that “1) the alpha rays are very slightly penetrating… and the magnetic field acts very slightly upon them… in the same manner as with cathode rays but the direction of deflection is reversed…” “2) The beta rays are less absorbable as a whole then the preceding ones.  They are deflected by a magnetic field in the same manner as cathode rays” and “3) the gamma rays are penetrating rays, unaffected by a magnetic field, and comparable to Rontgen rays (x-rays).[13]”  These definitions are still considered correct today, and the only big thing she missed was that gamma rays are composed of helium nuclei, and for that discovery, we need to go back to Ernest Rutherford and his friend Friedrick Soddy.

See about a month before Marie Curie defended her thesis, Rutherford’s friend Frederick Soddy moved from Canada to England to work with a famous Chemist named William Ramsay.  Years later Soddy recalled, “Our trouble here was the same as in Montreal. We had quite an insufficient amount of radiation for our investigations.  Then, by the most extraordinary chance, the whole future prospect was changed.[14]” What happened was, while going for a walk, Soddy “casually” happened to see a shop with “pure radium” from “Professor Giesel in Germany” on sale!  A few weeks later Rutherford visited his friend in London and Soddy wasted no time in telling Rutherford of his amazing find.  Soddy recalled that Rutherford, “was absolutely bowled over and became as excited as a schoolboy over the coming holidays.  With thirty precious milligrams of pure radium bromide, we bounded back to [the laboratory] and we both immediately repaired to the darkroom with some metal foils and a bit of x-ray screen.  The effect was terrific; it was like a person born blind suddenly being given sight, for though Rutherford had made a special study of the Becquerel rays, this was the first time he had ever seen them… Now he had a visual demonstration of what he had found out in the dark – so to speak.[15]”  Rutherford kindly loaned his 30 mg of radium to Soddy and his new advisor William Ramsay while he went on a European vacation and while Rutherford was celebrating Marie Curie’s Ph.D. defense in Paris, Soddy and Ramsay demonstrated with two different radium samples that radium emanates helium gas[16].  Rutherford, Soddy, and Ramsay were then all fully convinced that alpha particles were really charged helium atoms but it was still very difficult to prove it.  In fact, it took until November of 1908 for Rutherford conclusively proved that alpha particles are charged by helium atoms and that, “the charge is twice the unit charge carried by the hydrogen atom set free in the electrolysis of water.[17]

Just a few months after that, in mid-1909, Rutherford was asked by his coworker named Heinz Geiger for a suggestion on a good easy project for a new student and Rutherford suggested that he could use some of his radium and, “see if any a-particles can be scattered through a large angle[18]” off of a super-thin piece of metal.  Now Rutherford just wanted to get the student used to the equipment as he was convinced alpha particles were too heavy and energetic to be moved by a thin metal.  Instead, a few days later Rutherford was shocked to learn that some of the a-particles did bounce off the thin piece of metal.  Years later Rutherford recalled that “It was quite the most incredible event that has ever happened to me in my life.  It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.[19]” Rutherford thought about it for over a year and decided that “this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus.[20]”  And thus the idea of the nucleus was born.  With the concept of the Nucleus, it became clear that alpha particles were really helium nuclei. 

One comment about beta radiation, it turns out that the beta radiation that people were experimenting with was only one kind of beta radiation.  See, in 1934, Marie and Pierre Curie’s daughter, Irene, and her husband Frederic Joliot conducted an experiment where they bombarded aluminum with alpha particles and found that the resulting material emitted radiation that was positive but had the weight of an electron.  Now the existence of a “positive electron” or a positron had been found two years earlier in cosmic rays but this was the first example of the emission of positrons from radioactive decay.  For this reason, beta radiation is usually split into two types: beta negative decay (where an electron is emitted), and beta positive decay (where a positron is emitted).  And that is how alpha, beta, and gamma radiation were discovered and named, and how we discovered that alpha radiation is made of helium nuclei, beta is made of electrons or positrons, and gamma is composed of high energy massless electromagnetic radiation. 

To understand how radiation works in the nucleus, you need to discuss the neutron which was first thought of (and named) by… you guessed it, my man Ernest Rutherford in 1920.  And I am going to tell that story very soon (I promise), but first I want to talk a bit about helium.  Now you might be surprised to learn that the discovery of helium didn’t predate the discovery of radiation by very much.  In fact, helium was only discovered on Earth just three years before Rutherford discovered alpha particles by Friedrick Soddy’s boss William Ramsay!  Before that, it was just found in the sun which is why it is named after the Greek God of the Son: Helios.  But wait, how did they discover an element in the sun?  And how did Ramsay get it on Earth and what does that have to do with Kansas and the tragedy of the Hindenburg and how did it end up in party balloons?  The fascinating history of helium is next time on the lightning tamers.       

Thanks for watching and thanks to my patrons for supporting me (link below).  If you want to know more about Marie Curie I have 2 videos about her, I also have two videos about Rutherford and 1 about JJ Thomson, and.. well, I have a lot of videos to check them all out!  If you haven’t yet, hit the subscribe button, and the bell button, and then make a comment about how confusing this all is or how much you love me or how much you hate me (the algorithm doesn’t care).   


[1] Rutherford, Ernest, (January 1899) “Uranium Radiation and the Electrical Conduction Produced by It” Philosophical Magazine Vol. XLVII p. 116

[2] Rutherford, Ernest, (January 1899) “Uranium Radiation and the Electrical Conduction Produced by It” Philosophical Magazine Vol. XLVII p. 116

[3] J Elster und H. Geitel “Weitere Versuche an Becquerel-strahlen” Annalen der Physik Band 69 (Sept 14, 1899) p. 88

[4] Giesel, M “Ueber die Ablenkbarkeit der Becquerelstrahlen im magnetischen Felde” Annalen der Physik Vol 305, Issue 12 January 1, 1899  

[5] Curie, P “Action of the Magnetic Field on the Becquerel Rays” (January 8, 1900) Comptes Rendus No 2, Tome CXXX p. 74

[6] Curie, P and Curie, M “On the Electric Charge of Deviable Rays of Radium” (March 5, 1900) Comptes Rendus No 10, Tome CXXX p. 648

[7] Rutherford, Ernest to Newton, Mary Aug 11, 1895, quoted in Eve, A. S. Rutherford p. 56

[8] Rutherford, Ernest to Newton, Mary Aug 3, 1898, quoted in Eve, A. S. Rutherford p. 55

[9] Soddy, Fredrick, quoted in Schwoerer, H., Magil, J. Beleites, B. Lasers and Nuclei p. 131

[10] Rutherford, E (Feb 1903) “The Magnetic and Electric Deviation of the easily absorbed Rays from Radiation” Philosophical Magazine Vol V – Sixth Series p. 177

[11] Rutherford, E (Feb 1903) “The Magnetic and Electric Deviation of the easily absorbed Rays from Radiation” Philosophical Magazine Vol V – Sixth Series p. 177

[12] Curie, M Radio-Active Substances (1904) p. 33

[13] Curie, M Radio-Active Substances (1904) p. 33

[14] Frederick Soddy Quoted in Howorth, M Pioneer Research on the Atom (1958) p. 98

[15] Frederick Soddy Quoted in Howorth, M Pioneer Research on the Atom (1958) p. 112

[16] Ramsay, W, and Soddy, F “Experiments in Radioactivity, and the Production of Helium from Radium” (July 28, 1903) Proceedings of the Royal Society (Vol 72) p. 206

[17] Rutherford, E, and Royds, T “The nature of alpha particle from Radioactive Substances” Phil. Mag. 17 p. 286 (Jan 1909)

[18] Rutherford, Ernest “Forty Years of Physics” in Background to Modern Science, 1938 p 48 

[19] Rutherford, Ernest “Forty Years of Physics” in Background to Modern Science, 1938 p 48 

[20] Rutherford, Ernest “Forty Years of Physics” in Background to Modern Science, 1938 p 48 

Fascinating article about the history of Rutherford’s radium: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4123660/#FN6

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