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PHS PHYSICS 2 2018 - ATOMIC PHYSICS (RADIOACTIVITY (CHARACTERISTICS OF THE…
PHS PHYSICS 2 2018 - ATOMIC PHYSICS
RADIOACTIVITY
DETECTION OF RADIOACTIVITY
There is a small amount of radiation around us all the time because of radioactive materials in the environment. This is called background radiation. It mainly comes from natural sources such as soil, rocks (radon-222 gas), air, building materials, food and drink, even in space.
The
Geiger-Müller tube
detects alpha, beta and gamma radiation. The 'window' at the end is thin enough for alpha particles to pass through. If an alpha particles enters the tube, it ionizes the gas inside. This sets off a high-voltage spark across the gas and a pulse of current in the circuit. A beta particle or burst of gamma radiation has the same effect.
The
Cloud chamber
is useful for studying alpha particles because it makes their tracks visible. The chamber has cold alcohol vapour in the air inside it. The alpha particles make the vapour condense, so you see a trail of tiny droplets where each particle passes through. At one time, cloud chambers were widely used in nuclear research, but they have since been replaced by other devices.
CHARACTERISTICS OF THE THREE KINDS OF EMISSION
Radioactive emissions occur
randomly
over space and time
Alpha Particles
each particle has 2 protons and 2 neutrons
electrical charge is +2
high mass compared to Beta particle (4)
not very penetrating - can be stopped by a thick sheet of paper, skin or a few centimetres of hair
ionizing ability is strong
deflected by electric and magnetic fields
Beta Particles
each particle is an electron (nucleus decay). Neutron decays into a proton and electron. An electron is shot out from the nucleus and new element is formed.
electrical charge is -1
low mass (1/1800)
penetrating - stopped by a few millimetres of aluminium or other metal
ionizing ability is weak
deflected by electric and magnetic fields - much lighter and negative therefore deflected more and in opposite direction to alpha
Gamma Rays
electromagnetic waves (similar to x-rays)
electrical charge is 0
has no mass
very penetrating - never completely stopped, though lead and thick concrete will reduce intensity
ionizing ability is very weak
not deflected by electric or magnetic fields because there is no charge
RADIOACTIVE DECAY
The
spontaneous transformation
of an
unstable atomic nucleus
into a lighter one, in which radiation is released in the form of alpha particles, beta particles or gamma rays,
Alpha
Uranium 238 decays to Thorium 234 plus He atom
Loss of alpha particle leaves nucleus with 2 less protons and neutrons therefore a new element is formed.
Beta
Radium 228 decays to Actinium 228 plus beta particles
if an isotope is unstable because it has too many neutrons, one can turn into a proton and electron. The electron shoots out of the nucleus with high energy and the extra proton remains in the nucleus
Gamma
with some isotopes, the emission of an alpha or beta particle from a nucleus leaves the protons and neutrons in an 'excited' arrangement. As the protons and neutrons rearrange to become more stable, they lose energy. This is emitted as a burst of gamma radiation.
gamma emission by itself causes no change in mass number or atomic number.
HALF-LIFE
The
time taken
for the
radioactivity
of a specific isotope to be
halved
is the
half life
every isotope has its own specific half life
a sample of a radioactive material contains billions of atoms
it is a completely random and unpredictable event for a specific atom of that isotope to decay
SAFETY PRECAUTIONS
As radioactive materials can force electrons out of atoms and turn them into ions, they need to be stored in special lead containers. They are handled with much care and special clothing as they can damage the DNA of cells.
THE NUCLEAR ATOM
NUCLEUS
The nucleus contains protons which have a positive charge, and neutrons which are the same mass as protons but have no charge.
The
atomic (proton) number
Z, describes the number of protons in an atom
The
mass (nucleon) number
A, symbolises the number of protons and neutrons in an atom
A
nuclide
is a distinct type of atom or nucleus characterised by a specific number of protons and neutrons
Alpha Decay
thorium- 230
Atomic mass decreases by 4 and atomic number decreases by 2, due to He 4, 2
Beta Decay
Thallium-206
Atomic mass stays the same but atomic number increases by 1 as a neutron turns into a proton plus 1 electron.
ISOTOPES
An
isotope
is a form of a chemical element whose atomic nucleus contains a specific number of
neutrons
, in addition to the number of protons that uniquely defines the element.
For elements with atomic numbers up to around 20, isotopes are stable when the neutron/proton (N/P) ratio is equal to 1.
As the nucleus increases in size, more neutrons are required to stabilise the large repulsive forces between protons. Therefore the band of stability increased to N/P = 1.4 to 1.5.
There are no stable isotopes for elements greater than element 83.
Isotope Research Document
