Physics Topics can be challenging to grasp, but the rewards for understanding them are immense.
Properties of Alpha, Beta and Gamma Rays
Rutherford’s experiment: The radioactive emissions from radioisotopes when subjected to a strong magnetic field at right angles to the plane of the radiations show different deflections. Fig. shows the experimental arrangement.
Experimental arrangement:
G | A container, nearly evacuated and placed in a dark room. |
L | A small, deep and thick walled lead container. |
R | A mixture of different radioisotopes |
S | Slit on the lid of the lead container that allows radiations to come out upwards. |
P | A photographic plate. |
B0 | A strong magnetic field perpendicular to the plane of paper and directed downwards. |
Observation: When the photographic plate is examined after a considerable Length of time, three distinct lines are seen on the plate.
- Line A: It shows a small deviation of some emissions to the left.
- Line B: This shows a significant deviation of some emissions to the right.
- Line C: It shows a part of the radiation not affected by the magnetic field.
Inference: The inferences from the observations are that a mixture of radioactive samples can emit three types of radiations.
- α-rays (alpha rays): Applying Fleming’s left hand rule it is seen that line A is produced by the comparatively heavy, positively charged stream of high speed particles called α – rays or α – particles.
- β-rays (beta rays): By similar analysis, the line B is made by light, negatively charged stream of high speed particles called β-rays or β-particles.
- γ-rays (gamma rays): The emission, that is not deflected by the magnetic field and produces line C, consists of γ -rays or γ-radiations. Obviously, γ-radiation is not a stream of charged particles.
Discussions:
i) Identical results will be obtained when instead of a magnetic field an electrical field is applied from the right to the left along the plane of the paper.
ii) No radioactive isotope can emit all three radiations α, β and γ simultaneously. Hence, a mixture of different types of radioisotopes needs to be kept in R to obtain the results described. Generally any radioactive sample contains parent isotopes as well as daughter isotope. If this daughter isotope is also radioactive then we can get three types of rays. For example, this event may happen when parent isotope emits α-rays and daughter isotope emits β-rays.
Alpha rays
Nature of α-rays: Measurements of charge q and specific charge \(\frac{q}{m}\) establish that α-rays are high speed, stream of particles.
i) α – particles are positively charged and its charge q = + 2e, that is it contains two units of elementary charge [e = +1.6 × 10-19C] as that of a proton or electron.
ii) Mass of an α – particle is four times the mass of a proton. Experimentally it is found that an α-particle is compara-ble lo a helium-4 nucleus. As helium-4 nucleus consists of two protons and two neutrons, so alpha particle is denoted by the symbol 2He4.
Properties of α -rays:
- Actually α -rays are not rays, they are a stream of high speed particles. Each of the particle is known as α-particle.
- Each α -particle is positively charged and It contains two units of elementary charge as that of electron.
- The mass of each α-particle is equal to mass of 4 protons.
- From its mass and charge it is concluded that α -particles are structurally identical with a helium nucleus.
- As α -particles are positively charged they can be deflected by electric or magnetic field.
- Initial velocity and kinetic energy of α-particles depend on radioisotopes from which α-particles are emitted. In most cases the initial velocity is nearly 10 cm ᐧ s-1 and initial kinetic energy is within the limit of 5 MeV to 10 MeV.
- From its high initial kinetic energy it can be concluded that α -particles are emitted from the nucleus of atom.
- α -particles have low penetrating power in comparison to β and γ -rays and can be completely absorbed in mm thick aluminium plate.
- As penetrating power is less, α -particles have high ionisation power in comparison to β and γ -rays. In gaseous medium α -particles dislodge orbiting electron and ionise the gas.
- It affects photographic plate. When it strikes fluorescent material (like zinc sulphide) it produces scintillation (flashes of light).
- In a gaseous medium α -rays cannot travel beyond a certain range. This range is determined by the nature of the α – emission source.
- α -rays are used in nuclear reactions and in artificial transmutation of one element into another.
Beta rays
Nature of β-rays: From experiment we know that like α -rays, β -rays are also a stream of fast moving particles.
From the measurement of charge q and specific charge \(\frac{q}{m}\) of β-rays it is proved that each β -particle is an electron, i.e.,
- charge of β -particle -e = -1.6 × 10-19 C and
- mass of β-particle = 9.1 × 10-31 kg
Since mass of electron is negligible compared to mass of proton, therefore the mass number of β-particle is taken as zero. Due to its unit negative charge β -particle is sometimes expressed as -1β0 or -1e0.
Properties of β-rays:
i) Actually β -rays are not rays rather they are a stream of high speed particles known as β -particles.
ii) Each β -particle is an electron.
iii) As β -particles are light and negatively charged they are significantly deflected by the electric or magnetic field.
iv) Initial velocity as well as kinetic energy of each particle depends on radioisotopes from which the particle is emitted. Initial velocity may take any value from zero to velocity of light. Similarly kinetic energy ranges from zero to certain upper limit. Generally this value ranges from 5 MeV to 10MeV.
v) From its high initial kinetic energy it can be concluded that they are emitted from the nucleus of atom. Inside a nucleus when a neutron transforms into a proton, an electron is generated. As nuclear force has no influence on electron, it cannot confine the electron in the nucleus and so it comes out. Actually this electron is β -particle and not orbital electron.
vi) The penetration power of β -rays is 100 times greater than that of α -rays but \(\frac{1}{100}\) part that of γ -rays. It is completely absorbed by 1 cm thick aluminium plate.
vii) β -rays can ionise gas but its ionising power is \(\frac{1}{100}\) of that of α -rays.
viii) It affects photographic plate and produces weak scintillation on falling on a fluorescent screen.
ix) β -rays are used in nuclear reaction and artificial transmutation.
x) Whenever a β -particle is emitted from the nucleus of a radioactive element a massless, chargeless particle called neutrino is formed, the existence of which was originally suggested by Wolfgang Pauli in the year 1930. The name was given by Fermi (in 1934) while giving his neutrino theory of β -decay. k was detected in 1956 by Reines and Cowan.
Comparison between cathode rays and β -rays:
Similarities:
- Both are streams of moving electrons.
- Both possess penetrating and ionising properties.
- Both affect photographic plates and exhibit fluorescence when falling on compounds like zinc sulphide etc.
- Both are deflected by an electric or a magnetic field.
Dissimilarities:
Cathode rays | β-rays |
1. The flow of electrons from cathode to anode, due to an electrical discharge of any gas at 0.01 mm Hg pressure forms cathode rays. | 1. The stream of electrons emitted from a radioactive nucleus due to spontaneous disintegration is called β-rays. |
2. The electrons orbiting around the nucleus of an atom are emitted in the form of cathode rays. It is an extra-nuclear phenomenon. | 2. β-rays originate due to the disintegration of the nucleus i.e., when a neutron transforms into a proton an electron is generated which is emitted out as a β-ray. It is a nuclear phenomenon. |
3. Its emission depends on an external source of energy. | 3. Radioactivity substances emit β-rays spontaneously without any external influence. |
4. Less energetic. | 4. Highly energetic particles. |
5. The penetrating and ionizing power of cathode rays are comparatively small. | 5. The penetrating and ionising power of β-rays are greater than those of cathode rays. |
Gamma rays
Nature of γ -rap:
i) γ -rays are electromagnetic rays like light rays and with the same speed as that of light in vacuum.
ii) According to Planck’s quantum theory, γ -ray is constituted of a stream of photons. As frequency is high, so energy of each photon is also high. Its wavelength is shorter than that of X -rays, ranging from 0.005 Å to 0.5 Å.
Example: The energy of γ -ray photon of wavelength 0.01 Å is 1.24 MeV.
Properties of γ -rays:
i) Like light, γ -rays are electromagnetic waves and travels with the same speed as that of light in any medium.
ii) The wavelength of γ -rays is in the range from 0.005Å to 0.5Å.
iii) γ -rays are neutral and therefore they are not deflected by an electric or a magnetic field.
iv) According to quantum theory, γ -rays comprise of high energy photons. The energy of each photon is considerably high and can measure up to a few MeV.
v) The high energy of γ -ray photons implies that γ -rays are emitted from the nucleus. When α and β -particles are emitted from a nucleus the nucleus acquires an excited energy state. In order to return to the ground state γ -rays are emitted.
vi) The penetrating power of γ -rays is high in comparison with α or β -rays. It can penetrate a few centimetres of lead plate. The ratio of the penetrating power of α, β and γ -rays is 1 : 102 : 104.
vii) Ionising power of γ -rays is comparatively less than that of α and β -rays.
viii) γ -ray, like X -rays, undergoes diffraction.
ix) γ -rays, can affect photographic plates and adversely affect the cells of the human body. Therefore, for the treatment of cancer and tumour γ -rays are used. Powerful γ -ray burst are used to probe star formation.
x) γ -rays are used in nuclear reaction and artificial transmutation operations.
xi) γ -ray photon of energy of a few MeV or more, when passing close to a heavy nucleus changes into an electron and a positron (particle identical to electron but with positive charge). This is called “pair production” which is an example of energy changing to mass hence the energy associated with a γ -ray photon can be taken as E = 2me × c2J.
Comparison between X-rays and γ -rays:
Similarities:
- Both X -rays and γ -rays are electromagnetic waves.
- Both can create fluorescence and affect photographic plates.
- Both have ionising and penetrating power.
- Crystals can diffract both X -rays and γ -rays
- Both X – rays and γ -rays remain unaffected by an electric or a magnetic field.
- X -rays and γ -rays travel with the speed of light in vacuum.
Dissimilarities:
X-rays | γ-rays |
1. X-rays are emitted when the electrons of an orbit undergo transition from one energy level to another. | 1. γ-rays are emitted when the daughter nucleus makes a transition from excited state to ground state. |
2. Its emission depends on an external source or influence. | 2. γ-rays are emitted spontaneously from radioactive substances. |
3. Wavelength ranges from 0.1Å to 50 Å | 3. Wavelength ranges from 0.005 Å to 0.5 Å and hence more energetic than X – rays. |
4. Ionising and penetrating power are comparatively low. | 4. Ionising and penetrating power are comparatively high. |
5. The energy of the photons is comparatively less. | 5. The energy of the photons is comparatively more. |