Edexcel Physics Unit 7 notes

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Unit 7 : Radioactivity.(part 1)

7.1 Atomic structure

The atom consists of nucleus located in the centre which is made up of the protons and neutrons.

Protons: Positively charged particle. Have same mass of neutron.

Neutrons: Neutral charge and mass is same as proton.

Electrons: Circulating around the nucleus of an atoms, have a negative charge.

An atom is electrically neutral as it has equal number of protons and electrons and positive charges cancels out the negative charges.

Ions: There can be a positive ion or negative ion; if electron is taken away from an atom it becomes positively charged as the number of protons in that particular atom is more than the electrons

A negatively charged ion will have a greater number of electrons than protons.

Ions are found in pairs as a charged particle will attract another ion with opposite charges and will become a pair.

The process of making ion is called as Ionisation.

Mass number/ Nucleon number:

The mass of atom is number of protons and neutrons.

Atomic number/ Proton number: (symbol Z)

The number of protons in a nucleus. Atom is identified based on proton number.

In an atom (electrically neutral), there are equal number of protons and electrons. These electrons help in determining the chemical properties of an atom.

 

For example: (pictures available in download) 

Isotopes:

Atoms of same element with same proton number but different masses (nucleon number) are called isotopes.

Radioactivity:

In 1896, Henry Becquerel revealed radioactivity.

In chemical reactions, the number of electrons change. The number of particles emitted per second are equal to the activity of source, which is measured in becquerel (Bq). However, the number of protons and neutrons remain same, signifying that the nucleus of an atom is stable and unreactive.

In some cases, the nucleus will throw out some particles through a random process depending upon nature of nucleus. The rate at which particles are emitted is not dependent upon other factors such as temperature and chemical. Example most commonly used is Radium.

4 types of radioactive emission:

Alpha particles:

Nuclei of helium atoms, made up of 2 protons and 2 neutrons (mass number-4). Atomic number is 2. When an alpha particle is emitted the nucleon number of original atom changes and is reduced by 4 and atomic number is reduced by 2. This is called alpha decay.

Beta Particles (β): electrons emitted when a neutron particle from the nucleus is ejected and turned into a proton and electron. Since electron is very small in mass, it makes no difference to the mass number while leaving the nucleus of an atom. This process increases the atomic number by one. This whole process is called Beta decay and results in change of proton number so element changes.

Always make sure that the equation for alpha and beta particle is balanced.

Gamma Ray:

Along with alpha and beta decay, there is always a gamma ray given out.

·        These are electromagnetic waves similar to light and radio.

·        Carry a lot of energy away from the nucleus, so the nucleus becomes stable after giving out alpha and beta rays.

·        They have no mass or charge, so no change in mass or atomic number of nuclei.

Neutron emission: In case the nucleus is unstable, the neutron is emitted and they are dangerous emission

Causes the mass number to reduce by 1 but the atomic number does not change. 

Ionisation

Production of ions due to alpha, beta or gamma radiation is called ionisation. These ions cause damage to tissues of the body.

Experiment:

Effect of radium by holding a charged gold leaf electroscope.

The electroscope is initially charged positive so the metal gold repels away from the metal log. When radium is brought closer to the metal log, gold leaf falls. This happens because the electroscope has been discharged. Discharged means alpha particles emitted from the radioactive radium causes ionisation and release ions in air above electroscope. Ions pulls electrons out of air molecules.

Positive and negative ions are made, positive ones repelled and negative ones are attracted from the electroscope and electroscope is neutralised (remember the electroscope is discharged because of ions and not because of the alpha particles).

Radiation properties:

Particle detection:

Geiger-Muller (GM) tube is an electronic counter that helps in detecting radiation based on their properties.

When any of the alpha, beta or gamma radiation enters the tube. The gas atoms get ionised and produce ions. These ions produce a pulse of current that activates the counter attached to the GM tube. The tube then counts the number of ionising particles entering the tube.

Practical: range and penetration of radiation:

Alpha particles can travel up to 5 cm and can be stopped by a sheet of paper. These particles also ionise strongly with air.

Beta particles travel up-to few meters and is stopped by a sheet of aluminium few meters thick. Beta particles ionise less strongly than alpha particles.

Gamma rays can travel through air and cover greater distances and can be stopped by a very thick piece of lead. Ionises very weakly in air

Dangers of radiation:

Ions produce by radiation cause production of chemicals which are harmful for the body. Alpha particles are most damaging as they ionise strongly when they enter into the body through respiration of air containing radioactive gas. These radiations only travel a short distance so it can be preventable by maintaining a safe distance.

Gamma rays travel long distance and can enter easily through the body so they also have to be kept away from the human body.

Radiation from background:

Radioactive rocks: uranium, thorium, radon and potassium.

Radon gas emits alpha particles, if it is inhaled it can cause cancer of lungs.

Ionising radiation emitted from the sun and outer space is called cosmic rays. These are the sources of background radiation.

In some occupation, there is excessive exposure to radiation such as radiographers. They are exposed to X-rays on routine basis and so they have to make sure that their exposure is as minimal as possible.

Nuclear power stations have nuclear reactors which have neutrons being produced. These neutrons are source of danger for workers

 Radioactive decay:

A random process which causes the nuclei of a radioactive substance to decay by emission of alpha or beta particles from the nuclei.

Half-life:

Time taken for a radio-active substance to decay half number of atoms in a radio-active substance. (Number of nuclei are halved in number is the half-life of a substance)

Symbol t½.

Radioisotope’s half-life: is Radio-active material isotopes half-life.

(Isotopes: atom of same element with same neutron number but different proton number).

Experiment:

Half-life of protactinium measured with instrument shown in the picture below:  

Uranium decays to produce Protactinium as a product this is dissolved in aqueous layer in plastic bottle. When plastic bottle is shaken, protactinium is dissolved in upper dense layer. Then this protactinium decays which is measured by GM tube placed near the upper layer. The GM tube is connected to ratemeter which measures every 10 seconds and the graph produced at the ratemeter helps in understanding the half-life of elements. In this case the half-life of protactinium is 70 secs.

Practical:

Background count rate correction:

A radioactive source is left at some distance from the GM tube will still give readings at the ratemeter. These are because of background radiation and is called as background count.

Hence the background rate can be first calculated by keeping at some distance from the radioactive source and then the GM tube can be brought closer to the radioactive source.

The radioactive source can be calculated by subtracting the final reading from the background count rate.

Uses:

Archaeological dating:

All living things contain carbon of different isotopes. Carbon-14 is a radioactive isotope having a half-life of 5700 years. So, measuring the amount of carbon-14 in ancient objects help calculate their age.

Stability and half-life:

Isotopes: elements having same proton number but different nucleon number are known as isotopes.

Unstable isotopes undergo radioactive decay and they decay more quickly than the stable radioactive isotopes. These unstable isotopes have less half-life than the stable ones.

Radioactive material uses:

Used in medicine and industry. People working with radio-active material wear radiation badges to record the amount of exposure to radiation.

Medicine:

Radioactive tracers: Helps in examining the inside structures of the body. Iodine-123 helps to see thyroid gland, which is important to control the rate of functioning of our body. The tracer needs to be isotope with short half-life.

The surgical medical equipment is all sterilised with cobalt-60 with as this radio-active isotope emits energetic gamma rays and these helps kill bacteria.

Cobalt-60 is also used in treatment of cancers by directing a strong beam of radiation to kill cancerous tissues to kill the cancer. This is called radiotherapy treatment. These treatments have serious side-effects.

Industry:

Radioactive tracers help to detect leaks in underground pipes, by placing the radioactive substance into pipe and a GM tube helps in detecting increase in radiation level and so leaks can be detected.

This detecting time and money as the whole length does not need to be dug up.