Note-A-Rific: Radioactivity
History
In 1896 Henri Becquerel found that a certain rock had special properties.
- Caused a photographic plate (old fashioned type of film) to go dark, as if it had been exposed to light.
- The rock must be emitting some new kind of radiation, even though we can’t see it.
· Weird part is you don’t need to do anything to make the rock emit the radiation.
- The rock that Becquerel was using contained traces of uranium.
Shortly after this, Marie and Pierre Curie isolated two other radioactive elements, polonium and radium.
- No matter what physical or chemical stresses they placed on these elements, they continued to emit radiation.
- Since nothing they did could stop the radioactivity, they believed that the radioactivity must come from deep within the atom.
- Radioactivity actually results from the decay (disintegration) of an unstable nucleus.
- They were talking about a process that fundamentally changes the atoms of the element itself.
Where chemical reactions involve the rearrangement of electrons, radioactivity involves the rearrangement of nucleons!
- Note: in 1934, after having received two Nobel Prizes (Physics & Chemistry), Marie Curie died of leukemia from years of exposure to radioactive elements.
Ernest Rutherford and others started studying the radiation that was emitted by these elements.
· Found three distinct forms of radiation, based on their ability to pass through objects and their deflection in magnetic fields…
1. Alpha (a): could barely pass through a single sheet of paper. Deflected as a positive particle in a magnetic field.
2. Beta (b): can pass through about 3mm of aluminum. Deflected as a negative particle in a magnetic field.
3. Gamma (g): can pass through several centimetres of LEAD! Not deflected in a magnetic field.
Alpha Decay
An alpha particle is actually the nuclei of a helium atom *****************. A bunch of these streaming out of a radioactive sample is an alpha ray.
· Each alpha particle has a charge of +2e
· The alpha particles themselves are very stable.
· Since the atoms of the radioactive sample are emitting these nuclei, it must change the element somehow.
· Each atom is losing 2 protons and 2 neutrons when it goes through an alpha decay.
· If an element emits a rays, check the periodic table for an element with 2 less protons.
Example:
Radium 226 (
) will emit a rays. It will form an alpha particle and
radon.
à 
+ 
Ra is a “Parent Nucleus”
Rn is a “Daughter Nucleus”
He is an alpha particle
Changing one element into another is called transmutation.
Notice how the total number of nucleons and protons on each side is the same.
The basic formula for an alpha decay is…
à
+ 
Alpha decay happens in elements further down the periodic table because the strong nuclear forces in the atom are not able to hold a very large nucleus together.
· In the process of alpha decay energy is released because the total mass of the daughter nucleus plus the alpha particle is less than the mass of the original parent nucleus.
· The “missing” mass isn’t really missing. It’s been turned into energy following Einstein’s famous formula E = mc2.
Example: How much energy is released when Uranium-238 decays to Thorium-234
This is an alpha decay. The reaction for it would be…
à
+ 
It is possible to look up the total masses of these atoms in text books. They would be…
238.0508u à 234.0436u + 4.0026u
Add up the stuff on the right side…
238.0508u à 238.0462u
There’s 0.0046u unaccounted for after the reaction has occurred. Since 1u = 931.5MeV, the energy released in this reaction is 4.3MeV. This energy is found (mostly) in the kinetic energy of the alpha particle and daughter nucleus moving away from each other.
· Where does the rest of the energy go? You’ll find out soon!
Beta Decay
In beta decay, a beta particle is emitted.
· This beta particle is in fact an electron! But where did the electron come from…
· Remember that decay involves the nucleus, and there are no electrons in the nucleus
· Also, we know from experiments that after a beta decay, the number of electrons in the atom stays the same
In fact, one neutron turns into a proton and an electron.
- Neutrons are just a little more massive than protons, so the neutron is big enough to split into a proton and an electron.
n à p+ + e-
· The electron is thrown out of the atom.
· The proton stays in the nucleus.
· The effect of a beta decay is that you transmutate the parent atom into the element with one more proton, one more step on the periodic table.
Example: What are the products of a beta decay of carbon 14
à 
+
e- + neutrino
Several things to notice…
· The number of nucleons (A) stays the same, since in the nucleus one neutron “disappears” and is replaced by one proton.
· The number of protons (Z) has increase by one (so a transmutation occurs).
· The electron emitted is not an orbital electron; it is “created” in the nucleus.
· The neutrino (rest mass = 0, charge = 0, symbol = v) is emitted when a neutron changes into an electron and proton.
o Neutrinos were discovered by an Italian physicist who couldn’t get the right numbers when he calculated conservation of momentum and energy for beta decays.
o By assuming that neutrinos must be there, conservation of energy and momentum held true for this decay.
o Since neutrinos have no charge (like neutrons) and no mass, he thought of them as “baby” neutrons…
bambino à “baby” in Italian
neutrino à baby neutrons
The basic formula for beta decay is…
à
+ e- + v
Gamma Decay
Gamma rays are photons with very high energy (high frequency on EM spectrum).
Sometimes the nucleus can be in an “excited” state, a high energy state.
- Possibly due to a collision with another particle, or the nucleus is excited after an a or b decay.
- When the nucleus jumps down to a lower energy state, it releases energy as a photon, a gamma ray.
- The values of A and Z stay the same! No transmutation happens. All we are doing is releasing some energy from an excited nucleus
Example: An atom of Boron-12 goes through a beta decay to form an excited atom of Carbon-12. The excited Carbon-12 atom then goes through a gamma decay. Write the reactions that represent these decays.
à
+ e- + v ç
normal beta decay
à
+ g ç the gamma decay releases 4.4.MeV
You have no way of knowing how much energy is released in a gamma decay, unless you have a reference book to look up that reaction.
· If you know the wavelength or frequency of the gamma ray emitted, you can calculate its energy, or vice-versa.
Example: What is the wavelength of the gamma ray emitted in the example above?
E = h f = h c/l
l = h c/E
= (4.14 x 10-15Js) (3.00 x 108m/s) / (4.4 x 106eV)
l = 2.8 x 10-13 m
If you look at the frequencies in the textbook, you’ll see that X-rays and Gamma Rays overlap…they are in fact quite similar except for their origins.
· X-rays are created when an electron strikes an atom.
· Gamma rays are emitted in a nuclear process