P.7 - Radioactivity

Atoms and Radiation

• Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable. This is a random process called radioactive decay

• Activity is the rate at which a source of unstable nuclei decays. Activity is measured in becquerel (Bq)

• Radiation can be detected using an instrument called a Geiger Counter

• Count-rate is the number of decays recorded each second by a detector

  Could be unstable by:

• extra neutrons

• extra protons

• excess energy

Types of Nuclear radiation

• Alpha particle (α) - this consist of 2 neutrons and 2 protons, same as helium

• Beta particle (β) - a high speed electron ejected from the nucleus as a neutron turns into a proton

• Gamma ray (γ) - electromagnetic radiation emitted from the nucleus

• Neutron

Atomic and Mass Number

• Atomic number — the number of protons inside the nucleus of an atom

• Mass number — the number of protons and neutrons in the nucleus of an atom

Changes in the Nucleus

• Atoms are very small, having a radius of about 1 × 10^-10 meters

• Most of the of an atom is concentrated in the nucleus

• The radius of a nucleus is less than 1/10,000 of the radius of an atom

• All atoms of a particular element have the same number of protons

• Atoms have no overall electrical charge

  I an atom, the number of electrons is equal to the number of protons in the nucleus

• Atoms turn into ions if they lose or gain outer electron/s

• Atoms of the same element can have a different number of neutrons, these atoms are called isotopes of that element

• In a nuclear equation, an alpha particle may be represented by the symbol: ₄²He

• In a nuclear equation, an beta particle may be represented by the symbol: _-1⁰e

Discovery of the Nucleus

Plum Pudding Model

• Overall charge is neutral

• Number of positive and negative charges are equal

• Electrons were in a sea of positive charge

Alpha - scattering experiment

• Most particles passed straight through

• Some particles passed through at different angles

• Some particles deflected back

• Electrons can move up an orbit by absorbing electromagnetic radiation

• Electrons move down an orbit by emitting electromagnetic radiation

• The electromagnetic radiation is a photon

Alpha, Beta, Gamma

• Radioactive contamination is the unwanted presence of material containing radioactive atoms on other materials

• Irradiation is the process of exposing an object to nuclear radiation. The irradiation object does not become radioactive

• It is important for the findings of studies into the effect of radiation on humans to be published and shared with other scientists so that the findings can be checked by peer review

Activity and Half-Life

• Activity- number of nuclei that decay per second — activity = number of nuclei decayed / time

• Count rate - number of counts per second from a Geiger Counter — count rate = number of counts from Geiger Counter / time

• Half life (1) - time taken for the count rate to reduce to half of it’s initial value

• Half life (2) - time taken for the number of un-stable radioactive isotope nuclei to half

• Half life (3) - time taken for mass of unstable radioactive isotope nuclei to half

Nuclear radiation in medicine

• Nuclear radiations are used in medicine for the exploration of internal organs and control or destruction of unwanted tissue

• Background radiation is around all the time

• Radiation does is measured is sieverts (Sv) 1000mSv = 1Sv

• Level of background radiation does may be affected by occupation and/or location

Nuclear Fission

• Nuclear fission is the splitting of a large, unstable atomic nucleus

• Uranium-235 and Plutonium-239 are fissionable materials commonly used in fission reactors

• The nucleus must absorb a neutron for fission to occur

• Cor at the centre contains rods of enriched Uranium fuel surrounded by a moderator

• Moderator - slows down fast neutrons so the fission chain reaction doesn’t break down

• Control rods control the speed of the chain reactions

• As control rods are pushed into the core, they slow down the fission reaction, as they are puller out, the chain reaction increases

• Control rods absorb neutrons

Nuclear Fusion

• Nuclear fusion is the joining of 2 light nuclei to form a heavier nucleus

• In this process, some of the mass may be converted into the energy of radiation