What are we doing today Decay Types of Radiation Properties of nuclear radiation Decay and Probability Protactinium

Radiation Radiation: The process of emitting energy in the form of waves or particles. Where does radiation come from? Radiation is generally produced when particles interact or decay. A large contribution of the radiation on earth is from the sun (solar) or from radioactive isotopes of the elements (terrestrial). Radiation is going through you at this very moment!

Isotopes What’s an isotope? Two or more varieties of an element having the same number of protons but different number of neutrons. Certain isotopes are “unstable” and decay to lighter isotopes or elements. Deuterium and tritium are isotopes of hydrogen. In addition to the 1 proton, they have 1 and 2 additional neutrons in the nucleus respectively. Another prime example is Uranium 238, or just 238 U.

Radioactivity By the end of the 1800s, it was known that certain isotopes emit penetrating rays. Three types of radiation were known: 1)Alpha particles (  ) 2)Beta particles (  ) 3)Gamma-rays (  ) By the end of the 1800s, it was known that certain isotopes emit penetrating rays. Three types of radiation were known: 1)Alpha particles (  ) 2)Beta particles (  ) 3)Gamma-rays (  )

Where do these particles come from ?  These particles generally come from the nuclei of atomic isotopes which are not stable.  The decay chain of Uranium 238 produces all three of these forms of radiation.  Let’s look at them in more detail…

Alpha Particles (  ) Radium R protons 138 neutrons Radon Rn 222 Note: This is the atomic mass number, which is the number of protons plus neutrons 86 protons 136 neutrons + n n p p   He) 2 protons 2 neutrons The alpha-particle  is a Helium nucleus. It’s the same as the element Helium, with the electrons stripped off !

Beta Particles (  ) Carbon C 14 6 protons 8 neutrons Nitrogen N 14 7 protons 7 neutrons + e-e- electron (beta-particle) We see that one of the neutrons from the C 14 nucleus “converted” into a proton, and an electron was ejected. The remaining nucleus contains 7p and 7n, which is a nitrogen nucleus. In symbolic notation, the following process occurred: _ n  p + e ( +  Yes, the same anti-neutrino we saw previously

Gamma particles (  ) In much the same way that electrons in atoms can be in an excited state, so can a nucleus. Neon Ne protons 10 neutrons (in excited state) 10 protons 10 neutrons (lowest energy state) + gamma Neon Ne 20 A gamma is a high energy light particle. It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum. A gamma is a high energy light particle. It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum.

Gamma Rays Neon Ne 20 + The gamma from nuclear decay is in the X-ray/ Gamma ray part of the EM spectrum (very energetic!) Neon Ne 20

How do these particles differ ? Particle Mass* (MeV/c 2 ) Charge Relative to the electronic charge Gamma (  ) 00 Beta (  ) ~0.5 Alpha (  ) ~ * m = E / c 2

Rate of Decay  Beyond knowing the types of particles which are emitted when an isotope decays, we also are interested in how frequently one of the atoms emits this radiation.  A very important point here is that we cannot predict when a particular entity will decay.  We do know though, that if we had a large sample of a radioactive substance, some number will decay after a given amount of time.  Some radioactive substances have a very high “rate of decay”, while others have a very low decay rate.  To differentiate different radioactive substances, we look to quantify this idea of “decay rate”

Half-Life The “half-life” (t ½ ) is the time it takes for half the atoms of a radioactive substance to decay. For example, suppose we had 20,000 atoms of a radioactive substance. If the half-life is 1 hour, how many atoms of that substance would be left after: 10,000 (50%) 5,000 (25%) 2,500 (12.5%) 1 hour (one lifetime) ? 2 hours (two lifetimes) ? 3 hours (three lifetimes) ? Time No. atoms remaining % of atoms remaining

Decay constant,   The decay constant is the probability a given nucleus will decay in one second  The rate of decay is directly proportional to the number of un-decayed atoms dN/dt  NordN/dt = - N  This is a differential equation – when it is solved it leads to  N = No e – t N 0 = starting number of particles  = number of particles at time t

Lifetime Note by slight rearrangement of this formula: Fraction of particles which did not decay : N / N 0 = e - t Number of half lives Time (min) Fraction of remaining neutrons N = No e – t

 Not all particles have the same half-life.  Uranium-238 has a half life of about 4.5 billion (4.5x10 9 ) years !  Some subatomic particles have half-lives that are less than 1x sec !  Given a batch of unstable particles, we cannot say which one will decay.  The process of decay is statistical. That is, we can only talk about either, 1) the half-life of a radioactive substance, or 2) the “probability” that a given particle will decay.  Not all particles have the same half-life.  Uranium-238 has a half life of about 4.5 billion (4.5x10 9 ) years !  Some subatomic particles have half-lives that are less than 1x sec !  Given a batch of unstable particles, we cannot say which one will decay.  The process of decay is statistical. That is, we can only talk about either, 1) the half-life of a radioactive substance, or 2) the “probability” that a given particle will decay.

Summary  Certain particles are radioactive and undergo decay.  Radiation in nuclear decay consists of , , and  particles  The rate of decay is give by the radioactive decay law:  Some elements have half-lives ~billions of years.  Subatomic particles usually have half-lives which are fractions of a second…  Certain particles are radioactive and undergo decay.  Radiation in nuclear decay consists of , , and  particles  The rate of decay is give by the radioactive decay law:  Some elements have half-lives ~billions of years.  Subatomic particles usually have half-lives which are fractions of a second… N = No e – t