What are alpha rays? How are they produced?
Alpha "rays" are actually high speed particles. Early researchers tended to refer to any form of energetic radiation as rays, and the term is still used. An alpha particle is made up of two protons and two neutrons, all held together by the same strong nuclear force that binds the nucleus of any atom. In fact, an alpha particle really is a nucleus - it's the same as the nucleus of a common atom of helium - but it doesn't have any electrons around it, and it's traveling very fast. Alpha particles are a type of ionizing radiation.
To describe the production of alpha particles, we have to define radioactive decay. This process can be thought of as follows. Certain combinations of neutrons and protons in a nucleus are stable. For example, in a stable bismuth atom there are 83 protons and 126 neutrons. This is called bismuth-209 (126 + 83 = 209). It will always be bismuth-209*. But if we were to add one more neutron to this atom, and make it bismuth-210, it would now be unstable, or radioactive. The atom will eventually spontaneously change or "decay", to become more stable. There are only certain ways it can do this. One way is to emit an alpha particle. In this transition, it spits out a piece of itself (the alpha particle), and becomes more stable. The alpha particle is the radiation given off during the process of "alpha decay". Since it lost two protons and two neutrons, the old bismuth atom is now an atom of thallium-206. Now, this thallium is more stable, but is also radioactive. It will decay again (but not by alpha decay), this time becoming a completely stable atom of lead. Only relatively "heavy" atoms - like bismuth - can go through alpha decay. Lighter radioactive elements go through other types of transitions to become stable. There are plenty of these radioactive materials naturally present on the Earth, which is how these radiations were discovered.
Another way to produce alpha particles is to "force" an atom to emit one. This is done by taking advantage of certain properties of various atoms. Here's an example. If we take some regular atoms of boron-10 (five protons, five neutrons), and expose this boron to a field of slow-moving neutrons, some of the boron atoms will absorb a neutron. When this happens, the outcome is not what you'd expect. The boron-10 does not just become stable boron-11. A likely possibility is that the "excited" boron atom will emit an alpha particle, becoming stable lithium in the process. There are other atoms that behave in this fashion.
Although alpha radiation travels very fast, it can easily be blocked or shielded. Alpha particles have an electric charge because of the protons. As they move through matter, they are constantly interacting with other charged particles, such as electrons. This process transfers the motion (energy) of the alpha particle to the electrons, actually knocking the electrons free in the process. This is known as ionization. These interactions cause the alpha particle to lose its energy and come to rest. Imagine a cue ball as it is traveling along on a pool table, running into other billiard balls and eventually stopping. With alpha particles, this happens in a very short distance, even in air. Alpha particles will loose all their energy in just a couple inches of travel in air. Once an alpha particle is stopped, it grabs the first two free electrons it can find, and becomes a plain old atom of helium.
Alpha radiation is not hazardous if the source is external to the body. Alpha particles don't penetrate deeply enough into the body to reach living tissue. If the source of the alpha radiation is internal to the body, then the ionization we mentioned earlier can damage living tissue. So, safety practices for handling alpha-emitting materials are centered on preventing inhalation or ingestion of the material.
For a huge listing of information about radiation, see The Radiation Information Network.
*[Editor's Note - On April 23, 2003, it was reported that bismuth-209 is not a stable isotope, but decays into thallium-205 through alpha decay. Bismuth-209 has an extremely long half-life, roughly 1.9×1019 years. More information can be found here.]
Author:
Keith Welch, Radialogical Controls Group (Other answers by Keith Welch)
April 3rd, 2015 | By Mirion Technologies
Ionizing radiation takes a few forms: Alpha, beta, and neutron particles, and gamma and X-rays. All types are caused by unstable atoms, which have either an excess of energy or mass (or both). In order to reach a stable state, they must release that extra energy or mass in the form of radiation.
Alpha Radiation
Alpha radiation occurs when an atom undergoes radioactive decay, giving off a particle (called an alpha particle) consisting of two protons and two neutrons (essentially the nucleus of a helium-4 atom), changing the originating atom to one of an element with an atomic number 2 less and atomic weight 4 less than it started with. Due to their charge and mass, alpha particles interact strongly with matter, and only travel a few centimeters in air. Alpha particles are unable to penetrate the outer layer of dead skin cells, but are capable, if an alpha emitting substance is ingested in food or air, of causing serious cell damage. Alexander Litvinenko is a famous example. He was poisoned by polonium-210, an alpha emitter, in his tea.
Beta Radiation
Beta radiation takes the form of either an electron or a positron (a particle with the size and mass of an electron, but with a positive charge) being emitted from an atom. Due to the smaller mass, it is able to travel further in air, up to a few meters, and can be stopped by a thick piece of plastic, or even a stack of paper. It can penetrate skin a few centimeters, posing somewhat of an external health risk. However, the main threat is still primarily from internal emission from ingested material.
Gamma Radiation
Gamma radiation, unlike alpha or beta, does not consist of any particles, instead consisting of a photon of energy being emitted from an unstable nucleus. Having no mass or charge, gamma radiation can travel much farther through air than alpha or beta, losing (on average) half its energy for every 500 feet. Gamma waves can be stopped by a thick or dense enough layer material, with high atomic number materials such as lead or depleted uranium being the most effective form of shielding.
X-Rays
X-rays are similar to gamma radiation, with the primary difference being that they originate from the electron cloud. This is generally caused by energy changes in an electron, such as moving from a higher energy level to a lower one, causing the excess energy to be released. X-Rays are longer-wavelength and (usually) lower energy than gamma radiation, as well.
Neutron Radiation
Lastly, Neutron radiation consists of a free neutron, usually emitted as a result of spontaneous or induced nuclear fission. Able to travel hundreds or even thousands of meters in air, they are however able to be effectively stopped if blocked by a hydrogen-rich material, such as concrete or water. Not typically able to ionize an atom directly due to their lack of a charge, neutrons most commonly are indirectly ionizing, in that they are absorbed into a stable atom, thereby making it unstable and more likely to emit off ionizing radiation of another type. Neutrons are, in fact, the only type of radiation that is able to turn other materials radioactive.
The reason why alpha particles heavily dominate as the proton-neutron mix most likely to be emitted from most (not all!) radioactive components is the extreme stability of this particular combination. That same stability is also why helium dominates after hydrogen as the most common element in the universe, and why other higher elements had to be forged in the hearts and shells of supernovas in order to come into existence at all.
Here's one way to think of it: You could in principle pop off something like helium-3 from an unstable nucleus - that's two protons and one neutron - and very likely give a net reduction in nuclear stress. But what would happen is this: The moment the trio started to depart, a neutron would come screaming in saying look how much better it would be if I joined you!! And the neutron would be correct: The total reduction in energy obtained by forming a helium-4 nucleus instead of helium-3 would in almost any instance be so superior that any self-respecting (and energy-respecting) nucleus would just have to go along with the idea.
Now all of what I just said can (and in the right circumstances should) be said far more precisely in terms of issues such as tunneling probabilities, but it would not really change the message much: Helium-4 nuclei pop off preferentially because they are so hugely stable that it just makes sense from a stability viewpoint for them to do so.
The next most likely candidates are isolated neutrons and protons, incidentally. Other mixed versions are rare until you get up into the fission range, in which case the whole nucleus is so unstable that it can rip apart in very creative ways (as aptly noted by the earlier comment).