ALPHA PARTICLE SPECTRUM? - DETAILED EXPLANATION

We have discussed that every alpha(𝛼) emitter has only one associated 𝛼 energy. This is experimentally true for many 𝛼 emitter where one finds that velocity spectrum of alpha particle from these isotopes is always a shar line spectrum. This is to be expected that since the emission of an alpha particle is a result of energy transition between two different nuclear states.

in 1930, S. Rosenblum, in France proved by means of spectograph that the 𝛼 particles from Thc(Bi²¹²), all of which had been thought to have same energy, actually were consisted of number of groups of particles with slightly different energies. The 𝛼 particles form a given radioactive substance were collimated with slits and after deflection thru 180⁰ with strong magnetic field, formed lines on photographic plate.

For example, ₈₄Po²¹⁴ decays to ₈₂Pb²¹⁰ byemitting 4 groups of  𝛼 particles having ranges in air 6.91cm, 7.79 cm, 9.04cm and 11.51cm. These ranges correspond to energies 7.68MeV, 8.28MeV, 9.07MeV & 10.51 MeV respectively.

Another example is decay of ₉₀Th²²⁸. 

₉₀Th²²⁸     →    ₈₈Ra²²⁴ + 𝛼

It comprises of five groups of 𝛼-particles with different energies.

𝛼-particle which is emitted by transformation of excited state of parent nucleus to ground state of daughter nucleus will have maximum energy.

In the above example, out of 5, four groups of  𝛼-particles leave the daughter nuclei in excited state. The fifth group of 5.42MeV 𝛼-rays take one to ground state of daughter nuclei. We can note from figure that excited states of nuclei reach ground state by emitting 𝜸-rays shown by vertical wavy lines.

Thus the fine structure of 𝛼-spectrum tells us about energy levels in daughter nuclei. We emphasize that existence of these different 𝛼-energy groups and 𝜸-rays proves the existence of nuclear energy levels.

Long Range 𝛼-particles:

If the parent 𝛼-emitter emits 𝛼-particles when it is in an excited state, then we get long range 𝛼-particles.  This is because the energy of excitation becomes available to 𝛼-particles as they reach the ground state of daughter nuclei.

The following figure illustrates the emission of long range 𝛼-particles from ₈₄Po²¹⁴.

𝛼ₒ - normal 𝛼-group corresponding to transition between ground states of  ²¹²Po and ²⁰⁸Pb.

𝛼₁, 𝛼₂ - these groups originate from transitions from excited states of parent ²¹²Po to daughter ²⁰⁸Pb directly.

Thus, nuclear spectroscopic studies of long range 𝛼-emitters provide information about nuclear energy levels of parent.