Radioactivity

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Radioactivity : 

Radioactivity By Anay Kakade Std. IX Roll No. 07

What is the structure of the Atom? : 

What is the structure of the Atom? Consists of a small positively charged nucleus with negatively charged electrons orbiting around it at relatively large distances Radius of the nucleus = 10-15m Radius of atom = 10-10m Thus the atom is mostly empty space 2

The nucleus of the atom consists of two types particles: : 

The nucleus of the atom consists of two types particles: They are 1) neutrons which have no charge and 2) protons which are positive Atomic No. of an element, Z = the No. of protons in the nucleus of an atom Mass No. of an element, A = total No. of protons+ the total No. of neutrons in the nucleus of the atom No. of neutrons in an atom of an element = A - Z Isotopes =Atoms of an element having the same No. of protons (Atomic No.) but different No. of neutrons (hence different mass numbers) All elements occur in more than one isotope 3

What is Radioactivity? : 

What is Radioactivity? Nuclei of certain isotopes are unstable (i.e. they contain excess amount of energy) . In order to become more stable, they need to emit the excess amount of energy. This process is known as Radioactive Decay. Radioactivity = The spontaneous disintegration or decay of the nuclei of certain atoms with the emission of one or more types of radiation Artificial Radioactivity- Many isotopes can be made radioactive by bombarding them with neutrons 4

There are three different types of radiation that can be emitted by a nucleus: : 

There are three different types of radiation that can be emitted by a nucleus: : 1. α (Alpha)-Radiation 2. β (Beta)- Radiation 3. γ (Gamma)-Radiation 5

What is α (Alpha)-Radiation? : 

What is α (Alpha)-Radiation? Consists of two protons and two neutrons stuck together (i.e. the nucleus of a helium atom) Are deflected by electric and magnetic fields (i.e. are charged) Can be stopped by a thin sheet of paper or about 5cm of air -low penetrating power α-particles are relatively large particles, thus they have lots of collisions with atoms of the materials through which they pass. During these collisions the α-particles’ energy can cause ionisation of the materials. α- particles cause lots of ionisation Emission of an α-particle (He24) decreases the Atomic No. by 2 and the Mass No. by 4 6

What is β (Beta)-Radiation? : 

What is β (Beta)-Radiation? β – particles are high speed electrons ejected from the nuclei of radioactive atoms It occurs when a neutron in the nucleus splits to become a proton and an electron. The proton remains in the nucleus and the electron (β- particle) is emitted at high speeds β – particles are more penetrating than α – particles (since they are smaller particles they have less collisions and so penetrate further). The fact that they have less collisions means that they cause less ionisation. They are deflected by electric and magnetic fields (i.e. they are charged particles) When a β particle is emitted, the Mass No. stays the same (since the mass of an electron is very small) and the Atomic No. increases by one (as an extra proton is created with the β particle. 7

What is γ (Gamma)-Radiation? : 

What is γ (Gamma)-Radiation? γ -Radiation is high frequency electromagnetic radiation. When they are emitted from the nucleus, the nuclear structure stays the same, it simply represents a loss of energy 8

What is the activity of a radioactive sample? : 

What is the activity of a radioactive sample? The activity (A) of a radioactive sample is the No. of nuclei of the substance decaying per second It is measured in Becquerel (Bq) 9

The Law Of Radioactive Decay states that: : 

The Law Of Radioactive Decay states that: The No. of nuclei decaying per second (the activity) is directly proportional to the No. of nuclei present dN/dt  N dN/dt = constant N dN/dt = λN where λ is the decay constant 10

The half-life (T1/2) of a radioactive isotope is: : 

The half-life (T1/2) of a radioactive isotope is: The time taken for half of the nuclei to decay (in other words the time taken for the activity to fall to half its starting level) T1/2 = ln 2 / λ 11

How is radiation detected? : 

How is radiation detected? Radiation is mostly detected through the ionisation it causes when it interacts with matter. Two main types of detectors 1. Geiger-Müller tube 2. Solid State detector 12

The Geiger-Müller Tube is used to detect radiation: : 

The Geiger-Müller Tube is used to detect radiation: Consists of a cylindrical tube with a thin mica window at one end to allow the radiation to pass. The container contains argon gas at low pressure There is a wire through the centre of the cylinder and a large potential difference is supplied between the case and wire, the wire being + w.r.t. the case. When radiation enters through the window, it causes ionisation of some of the argon atoms into + ions and – electrons. 13

Geiger-Müller Tube : 

Geiger-Müller Tube The + ions move towards the – case and the electrons towards the wire. As the electrons get nearer to the wire they feel a force of attraction due to the wires electric field, and so they speed up These fast moving electrons collide with other gas atoms and cause further ionisation. This effect is called a Townsend avalanche and results in a large No. of electrons hitting the central wire. Thus a pulse of current is sent along the wire to the counter . The counter registers this pulse of current as one radiation event. The counter circuit must then reset itself before it can count another event. The time taken to reset is called the dead time. If the pulses are arriving faster than the dead time the system will not be able to count all the pulses. 14

There are two types of counters for detecting radiation: : 

There are two types of counters for detecting radiation: These are either 1. Scalar – which counts all radiation events from the time it is switched on 2. Rate meter – which counts the No. of pulses per second 15

Solid State Detectors are also used to detect radiation: : 

Solid State Detectors are also used to detect radiation: This works on the fact that when radiation strikes certain semiconducting materials it creates electron-hole pairs. It consists of a reverse biased pn junction with a pd across it, connected to a counter. When radiation strikes the depletion layer, it causes electron-hole pairs to be formed. This causes a current across the depletion layer which is then amplified, and then registers on the counter as one radiation event 16

Radioisotopes are used in various fields: : 

Radioisotopes are used in various fields: Medicine Medical imaging – trace amounts of short half life isotopes can be ingested and the path of the isotope traced by the radiation given off Cancer treatment – radiation kills cancerous cells more easily than healthy cells Sterilisation – γ – rays can be used to kill germs and hence sterilise food and plastic equipment Industry – used to trace blockages in pipes, or to test the thickness of materials (by putting a source on one side of the material and detector on the other) Carbon dating Once a living organism dies, it is no longer taking in any Carbon. C14 is radioactive, and decays over time. By measuring the activity of C14 in an object and comparing it with the amount of C14 which was present initially you can estimate when the organism died 17

Radioisotopes are also used for: : 

Radioisotopes are also used for: Carbon dating: Once a living organism dies, it is no longer taking in any Carbon. C14 is radioactive, and decays over time. By measuring the activity of C14 in an object and comparing it with the amount of C14 which was present initially you can estimate when the organism died Smoke detectors: A radioactive source ionises the air between two electrodes. Thus current flows between them If smoke particles enter this space they stick to the ions and the current is reduced. This reduced current triggers the alarm 18

Is Radioactivity It A Health Problem? : 

Is Radioactivity It A Health Problem? The Alpha, Beta and Gamma particles all add energy to the body’s tissues. The effect is called the Ionizing Energy. It can alter DNA. Even though Alpha particles are not very penetrative if the decaying atom is already in the body (inhalation, ingestion) they can cause trouble. 19

Slide 20: 

20 Thank you!