Table of Nuclides

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1.3: Decay
Origin of gamma quanta after Decay

Most radionuclides that emit beta-minus particles also emit gamma radiation. The explanation I was given in college was that during the emission of a beta (or alpha for that matter), the nucleus has "extra" energy, as the nucleons  reassemble. The nucleus also recoils.  These events cause the nucleus to give off this energy... and it is done so in the form of electro-magnetic waves (gamma rays).  So, how can one know that the gamma rays are emitted AFTER the beta-minus decay transforming Cs-137 to Ba-137m?  If they occurred during the beta-minus emission, they would be attributed to Cs-137.  Right?

I guess what I am asking essentially is how we know that Cs-137 does not transform directly to Ba-137 (without first transforming to Ba-137m -isomeric transition).

 I am not expert in this field. However I will try to explain what I know.

 Usually the parent nuclide (Cs-137) is in ground state. When it decays
with beta emission. The nuclide immediately turn into higher element (Ba-137) in
excited state. Then the excited nucleus become stable by emitting gamma quanta.
 If parent nuclide is in excited state (meta), then it can emit gamma first then decay with beta and residual emit gamma. In this case, one can distiguish the origin by  different half life of strength of gamma quanta.
 So, one may assumes the gamma quanta is attributed to the excited state of Ba-137.

  94.4 0f Cs-137 transforms to Ba-137m and the remaining is assumed to be decay into ground state of Ba-137 directly. This fact may be derived by precise
measurement of total gamma rays from the decay of Cs-137. ( And also observing beta spectrum )

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