Radioactivity Section 20.2

Radioactivity Wilhelm Rontgen discovered glowing fluorescent screen on far side of a room where he was experimenting with a CRT (cathode ray tube)-called them X-rays Henri Becquerel experimented to see if luminescent materials would give off X rays when exposed to light. Wrapped photographic film to shield it from natural light & placed under phosphorescent uranium salts. When he exposed salts to sunlight, silhouettes of the crystals appeared when he developed the film. He thought salts absorbed sunlight and reemitted the energy as X-rays. One day, it was cloudy, he stored the salts and film in a drawer. When he developed film, he found it had been exposed while in the drawer. This was the first recorded observation of radioactivity

Radioactive Isotopes Radioactive materials were those which emitted high energy particles and rays. Rutherford discovered alpha particles (Helium 2+ nuclei). The particles displayed the spectrum of helium. He also discovered beta particles and showed they were identical to electrons Paul Villard discovered another form of radioactivity he called gamma rays- a more penetrating type of radiation. Gamma rays are very high frequency EM waves

Radioactive Isotopes

Radioactivity Alpha particles are attracted to negative plate, beta particles to positive plate and gamma rays are unaffected

Alpha Decay When a radioactice isotope emits an alpha particle, it loses 2 protons and 2 neutrons. The original nucleus is called the parent nucleus and the resulting nucleus is called the daughter. Only very large nuclei such as U, Ra, Po, Bi, Th emit alpha particles

Alpha decay note conservation of mass and electric charge are required in a nuclear reaction

Alpha decay Since Radium nucleus has been changed into Radon, reaction is called a transmutation. Reaction takes place because the products are more stable than the reactant. Total BE of products increases and mass defect goes into Kinetic energy

Alpha decay Alpha particles have a fairly large mass and usually high speed therefore they are energetic enough to knock electrons out of some atoms creating ions. Alpha radiation is aka ionizing radiation

Beta decay When a radioactive isotope emits a beta particle, it appears that an electron is lot from inside the nucleus. How can this be?

Beta decay A neutron transforms into a proton and a high energy electron (beta particle).This means that the nucleon number remains the same, but the atomic number must increase by 1

Beta decay

Experimentation showed that linear momentum, angular momentum and mass was not conserved in beta decay reactions. This was a violation of fundamental laws of physics. Wolfgang Pauli proposed that there must be a particle which accounted for the missing energy and momentum. Over 25 years later, the particle was discovered- the antineutrino- a form of antimatter. It has a very small mass and can travel at or close to speed of light. Shortly after, another form of beta decay was discovered where a positive electron or positron was emitted. Here, a proton changes into a neutron and a neutrino is also emitted. Again, these type of reactions proceed to greater stability of the products. The two types of beta decay are also known as β + and β – decay.

Gamma Decay

When a nucleus decays by alpha or beta decay, the daughter nucleus is often left in an excited state. The nucleus wants to be in its ground state so a gamma ray is emitted. The gamma ray photon has a much greater energy than a photon emitted by an atom, so its penetrating power is the greatest of the forms of radiation. Gamma radiation has no electric charge and it is too energetic to be absorbed through electron jumps between energy levels in the atom. Whenever gamma radiation is absorbed, energetic electrons are freed, cations are left behind. This explains why gamma radiation is biologically damaging. It is ionizing radiation and damages the DNA in our cells, kills cells, causes radiation sickness, increases incidence of cancer

Gamma radiation

Gamma Decay