1) Gamma rays are electromagnetic radiation emitted during nuclear transitions between excited and lower energy states. They were discovered in 1900 and have shorter wavelengths than X-rays. 2) Gamma ray properties include being unaffected by electric and magnetic fields and having penetrating abilities dependent on their energy. Their energies can range from thousands to millions of electron-volts. 3) Gamma emission and absorption follow selection rules regarding angular momentum and parity conservation. Transitions are characterized by their electric or magnetic multipole type, such as electric quadrupole or magnetic dipole.

1. GAMMA RAYS
Nature of 𝛾 rays:
Nucleus --- left in an excited state following 𝛼 𝑜𝑟 𝛽 disintegration.
𝛾 rays - Excited nucleus makes a transition to the lower state by emitting electro – magnetic
radiation.
First observed by - French scientist – P. V. Villard [1900] – different from 𝛼 𝑜𝑟 𝛽 rays by
observing their greater penetrating power.

2. Comparison of properties of 𝛾 rays:
Figure: Penetrating power Figure: Undeflected beam of 𝛾 rays in a magnetic field

3. Figure: Path of 𝛾 rays in
electric field.

4. Figure: Range of 𝛾 rays in the
electromagnetic spectrum

5. Emission of 𝛾 rays analogous to the emission of electromagnetic radiation from the excited
states of atom.

6. The energy of the photon emitted is only few electrons – volts.
Measurement of the energies of 𝛼 𝑜𝑟 𝛽 show that these have energies of about million 106
electron – volt region.
Transitions between nuclear energy states – results in the emission of photons with energies
ranging from – few thousand electron volt to several million electron volt.
Characteristic X – rays emitted due to the transitions of the electrons in the inner – shells of
atom have energies up to several thousand electron – volts.

7. Wave – length of characteristic X – rays = 10−10m.
Wave – length of the 𝛾 rays are even shorter than X – rays.
Using the energy - wave length relation:
𝐸 = ℎ𝑣 =
ℎ𝑐
𝜆
=
12412.5
𝜆
eV
Wave – lengths of the 𝛾 rays of energies 104
, 105
, 106
eV are 1.24 𝐴 , 0.124 𝐴 , 0.0124 𝐴

8. Representation of 𝛾 decay:
Example:

9. Example:

10. Some Decay Scheme:
Cesium Decay scheme (Ce)

11. Decay Scheme: Cobalt (Co)

12. Sodium Na Decay scheme:

13. DIFFERENCE BETWEEN 𝛾 – PHOTON AND X –RAYS:
 𝛾 – photon is emitted due to transition of nuclei between different energy states of the nuclei
[ different energy levels of the nuclei]
X – rays are emitted due to transition of electrons from outer shell to some inner shell [ or
orbitals]

14. ELECTROMAGNETIC RADIATION:

15. Electric and Magnetic field distribution: [associated with electro – magnetic radiation]
Sr No Electric Magnetic
1 Monopole ------
2 Dipole Dipole
3 Quadrupole Quadrupole
4 Octupole Octupole

16. Figure: Electric monopole and associated
electric field

17. Figure: Radiating Electric Field

18. Figure: Radiation from an
oscillating dipole.

19. Figure: Oscillating Electric Dipole

20. Figure: Magnetic Dipole

21. Figure: Magnetic Quadrupole Distribution

22. 𝛾 – photons that come out during the decay are
electro – magnetic radiation.
𝛾 – photons can be classified like above
distribution.
𝛾 – photons carry angular momentum.

23. Electric
Distribution
Notation
Dipole E1
Quadrupole E2
Octupole E3
Magnetic
Distribution
Notation
Dipole M1
Quadrupole M2
Octupole M3

24. where the number denotes the angular momentum.
General notation: EL, ML
𝛾 – photons that come out during decay is characterised by angular momentum ‘L’, that is
related to distribution.

25. 𝜸 – Ray Emission: Selection Rules:
The angular momentum carried by the photon w.r.t the origin:
𝐿 = 𝐼𝑖 - 𝐼𝑓
where 𝐼𝑖 and 𝐼𝑓 are the angular momenta of the initial and final states respectively.
For a given 𝐼𝑖 and 𝐼𝑓, 𝐿 can have the following values, in accordance with the vector model:
| 𝐼𝑖 - 𝐼𝑓| ≤ 𝐿 ≤ | 𝐼𝑖 + 𝐼𝑓| -----------------------------[1]
Equation [1] is the selection rules which sets the limits on the possible multipolarities of the
transition.

26. Parity:
The transitions are also governed by the parity selection rule.
Parity is conserved in an electromagnetic process.
Parity selection rules for electric multipole transition [EL] and magnetic multipole transition
[ML]:
For electric multipole transition [EL]:
Parity is even when L is even.
Parity is odd when L is odd. PARITY is given by: (−1)𝐿

27. For magnetic multipole transition [ML]:
Parity is odd for even value of L.
Parity is even for odd value of L.
Parity is given by: (−1)𝐿+1

28. 1) Example: Considering the transition: 0+ 0+
2+ 884 KeV 𝐼𝑖 = 0, 𝐼𝑓 = 0
From angular momentum conservation:
0+ 691 KeV L = 0
However, L=0 does not exist.
E.C. 𝛾 – photon Transition does not occur.
0+
Excess energy is given to electron in atom.

29. Transition: 2+ 0+
𝐼𝑖 = 2+, 𝐼𝑓 = 0+
Condition for allowed values of angular momentum: | 𝐼𝑖 - 𝐼𝑓| ≤ 𝐿 ≤ | 𝐼𝑖 + 𝐼𝑓|
Allowed value of the angular momentum: L = 2.
 Quadrupole transition
Conservation of parity must be obeyed.
For conservation of parity, the transition is purely electric.
Allowed transition: Electric Quadrupole [E2]

30. MOSSBAUER EFFECT
The Mössbauer effect, or recoilless nuclear resonance fluorescence, is a physical phenomenon
discovered by Rudolf Mössbauer in 1958.
 It involves the resonant and recoil-free emission and absorption of gamma radiation by atomic
nuclei bound in a solid.
In the Mössbauer effect, a narrow resonance for nuclear gamma emission and absorption results
from the momentum of recoil being delivered to a surrounding crystal lattice rather than to the
emitting or absorbing nucleus alone.

31. When this occurs, no gamma energy is lost to the kinetic energy of recoiling nuclei at either the
emitting or absorbing end of a gamma transition: emission and absorption occur at the same
energy, resulting in strong, resonant absorption.

32. Experimental set – up of
Mossbauer Effect:

33. Figure:

34. Figure:

35. Figure: