Radioactive decay GapFill

At the centre of each atom is a nucleus, which takes up very little volume but contains most of the mass of the atom. When a positive particle is fired at this nucleus, it is deflected in a process known as  Rutherford scatteringThomson avoidanceCoulomb diffractionMarsden deflection. This nucleus can change in various ways, known as nuclear decay, where a nucleus changes its composition by ejecting another particle.
Common particles ejected during nuclear decay include:

•  Alpha particlesNeutrinosProtonsBeta particles, which are identical to a helium nucleus. These can't pass through many materials, and are stopped by paper or a few centimetres of air, but can easily ionise materials they interact with. Nuclei which decay in this way are used in  starting a chain reactionmedical tracingexternal radiotherapysmoke detectors where they are safe to use because they can't reach any other objects (or people) through the air.
•  Beta particlesAlpha particlesNeutrinosProtons, which are electrons. These have a greater range, and can travel a few metres in air, but can be stopped by  paperthick sheets of leadthick walls of concretethin sheets of metal, which makes them great for applications in detecting the thickness of this material.
•  ProtonsGamma raysBeta particlesNeutrinos, which are high-energy photons. These travel the furthest distance and are the most difficult to stop. The intensity of radiation decreases away from their source, following the law  I = k²/xI = k/x²I = √(k/x)I = k/x.

When performing any experiment using radioactive materials, it is important to take safety precautions, such as  wearing goggleshandling them with tongstying your hair backletting them cool down first. It's also important to take into account  thermalatmosphericbackgroundcosmic microwave radiation, by taking measurements of the radiation levels without any sources, and subtracting this from any radiation measurements you take.
The probability of any particular atom decaying at a given point is impossible to predict, but it is possible to predict how an entire radioactive sample should behave over time. A sample's half-life is the time taken for half of the particles in a sample to decay, or the time taken for the activity of a sample to halve. More generally, this can be written as  N = N₀^(-λ)N = N₀e^(-λt)N = e^(-λt)N = N₀e^(-λ), where λ is the decay constant of the sample, and is equal to  number of particles ÷ activitynumber of particles - activityactivity ÷ number of particlesactivity - number of particles.