Radioactivity

Atoms and Radiation

There are three types of nuclear radiation emitted are alpha radiation, beta radiation and gamma radiation.

it is not possible to predict when an unstable nucleus will decay, it is random.

it is not affected by external conditions.

it cannot be sped up by temperature or pressure change.

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The discovery of the nucleus

rutherford used a particle to probe inside atoms.

he found that some of the a particles were scattered through large angles.

Alpha particles have a positive charge

Alpha particle scattering experiment:

Rutherford, Geiger and Marsden fired alpha particles at a very thin gold foil.

most a particles passed through the foil, most of the atom is empty space

some a particles were deflected through small angles, suggests the nucleus is charged.

small amount of a particles were deflected at large angles, suggests the nucleus has a large mass.

The 'nuclear model'

every atom has a positively charged nucleus where most of the mass of the atom is concentrated. electrons orbit the nucleus, most of atom is empty space.

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Changes in the nucleus

isotopes of an element are atoms with the same number of protons but different numbers of neutrons. - same atomic number, different mass numbers

Example of an isotope  - uranium 238 which has 92 protons and 146 neutrons

Alpha particles consist of two protons and two neautrons. Relative atomic mass of 4, relative charge +2.

When an unstable nucleus emits alpha particles -  atomic number goes down by two, mass number goes down by 4

Beta particles is a high speed electron from the nucleus, emiited when a neautron in the nucleus changes, relative atomic mass of 0, relative charge -1

Atomic number goes up by one, mass number is unchanged

Gamma radiation - no change in the atomic number/mass number. no charge/mass. it is a electromagnetic wave released by the nucleus

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Alpha, beta and gamma radiation

Ionisation - radiation knocking electrons out, atoms become positively charged bc they lose electrons

Ionisation in a living cell can damage/kill cell

An object is irradiated if it is exposted to ionising radiation 

Radioactive contamination is unwanted pesence of materials containing radioactive atoms 

Alpha radiation is stopped by paper, consists of particles, each composed of two protons and two neutrons. Has the greatest ionising powers

Beta radiation is stopped by a thin sheet of metal, consists of fast moving electrons emitted from nucleus. less ionising than alpha radiation

Gamma radiation is stopped by thick lead, consists of electromagnetic radiation

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Activity and half-life

The activity of a radioactive source is the number of unstable atoms in the source that decay per second

The half-life of a radiactive isotope is the average time it takes for the number of nuclei of the isotope in a sample to halve

the count rate of a Geiger counter caused by a radioactive source decreases as the activity of the source decreases

count rate after n half-lives = initial count rate/ 2^n

Number of unstable nuclei after n half-lives = initial number of unstable nuclei / 2^n

The number of atoms of a radioactive isotope and the count rate both decrease by half every half-life

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Nuclear radiation in medicine

Radioactive isotopes are used in medicine for medical imaging and treatment of cancer and as tracers to monitor organs

How useful a radioactive isotope depends on its half-life and the type of radiation it gives out

Radioactive tracers - patient ios injected with a radioactive isotope that emits gamma radiation, the movemnmt of the radiation through the organ is monitored using a gamma detector placed outside the body

Medical imaging - patient is injected with a radioactive isotope of gamma radiation, the isotope is absorbed by the organ and the gamma camera outside the body, connected to the computer dectects the radiation

gamma radiation is used for cancer treatment bc it penetrates deep into the body, the isotope must have a half-love long enough to complete the imaging/treatment but short enough to avoid exposing the patient to radiation

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Nuclear fission

Nucleus fission is the splitting of an atomic nucleus into two smaller nuclei

uranium 238 is non-fussionable

For fission to occur, the uranium 235 or plutonium 239 nucleus must absoprb a neutron - this is called induced fission

Spontaneous fission is where fission occurs without a neutron being absorbed

A chain reaction occurs when the neutrons released from each fission event cause further events to occur

in a nuclear reactor the fission process is controlled, one fission neutron per fission. on average this goes on to produce further fission.

Control rods in the reactor core absorb surplus neutrons 

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Nuclear Fusion

Nucleur fusion is the process of forcing two nuclei close enough together that they from a single larger nucleus

Some of the mass of the smaller nuclei is converted to energy 

two light nuclei collide at very high speeds

nuceur fusion is the prossess by which energy is released in stars including the sun.

A fusion must be at very high temperatures before nuclear fusion can take place

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Nuclear issues

High levels of background radiation can be harmful as it ionises substances it passes through, nuclear power contributes top very littlen to background radiation

Radon gas is an alpha emitting isotope that seeps into houses 

nuclear waste contains many different radioactive isotopes that emit nuclear  for many years  - dangerous bc it can cause cancer

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