"METASTABLE" -

"m", small case m, when appended to an isotope name, means metastable. An isotope that is in a metastable state has pent up energy inside that WILL be released at some time, but all normal quantum transitions are forbidden. This energy comes about from normal nuclear decay of the metastable isotope's parent atom. Usually when an atom decays, there is an immediate rearrangement of the energy levels quite quickly in the daughter. So much so, that we often think of the Gamma ray that is released as part of the decay. It is not. Radioactive decay involves the transformation of the nucleus in terms of physical particles, into a new nuclide. Gamma radiation usually comes from the nucleus of the daughter nuclide as it rebalances its own energy to a ground state or more stable state.
In some instances, the new atom holds the energy for a finite period of time, sometimes microseconds, sometime hours. This final release of energy is called Isometric Transition (abbrev. IT), and allows the nucleus to fall to the stable state or nearer to a stable state. Thus, Ba-137m has the same weight, number of nucleons etc. as stable Ba-137. Ditto Tc-99m > Tc-99m, although they have vastly different properties as far as radiation and half-life.
IsoMERs are two or more nuclides that have same atomic and mass numbers, but differing in other properties. IsoTOPES have the same atomic numbers, but different atomic weights and energy ( different number of Neutrons), while IsoTONES are nuclides with the same number of Neutrons, but different numbers of Protons.

Sometimes the Gamma process involves the released photon smacking into an inner shell electron, disappearing but imparting its energy to the electron, which takes off from the atom- these are the IC or INTERNAL CONVERSION electrons. This void is filled from another shell, and that process alone creates a characteristic X-ray photon, whose energy is equal to the difference of the two electron's binding energy. If the excitation energy to the escaping electron comes from the atomic region rather than the nucleus, it is called an Auger Electron.

If a photon comes from the nucleus, it is called Gamma, if it comes from outside the nucleus, it is called an X-ray. The origin is the ONLY difference. Likewise, electrons, be they regular ones or antiparticle positrons, that come from the nucleus are called Betas. If they come from outside the nucleus ( shell area, or atomic area) they are called electrons. These is some disagreement among scientists about calling EC a Beta decay, since the electron goes into the nucleus, not out of it. Even the folks at Oak Ridge who make my Fe-55 disagree about this.

Pa-234m Generator

The Spectrum Techniques Ba-137m generator is nice, and affordable.

Those short half life isotopes are fun to chart. It is quite true that Cs-137 is actually a pure beta emitter, but is always found in the presence of the progeny Ba-137m, which gives the typical gamma signature we look for in spectrum analysis.

http://www.qsl.net/k0ff/Ba137M.jpg/Cs-137%20with%20NO%20filter-%20See%2032%20keV-.jpg

Only a few isotopes are truly gamma free beta minus emitters, H3, C-14, Ni-63 Tc-99, Tl-204, Pm-147 Sr-90/Y-90 and P-32 being the most common. Any time betas are flying about, X-rays are also being created by them. These can be bremsstrahlung or of the "characteristic" type, so there is always a lot of "noise" on the scan. Setting up a NaI(Tl) probe for spectrum analysis includes adding a beta shield to the source holder for this reason.
We have a small group of spectrum analysis enthusiasts at:
http://groups.yahoo.com/group/GammaSpectrometry/





Scan details:
Setup:
3M3 3" x 3" Nai(Tl), Canberra 2007P base, Canberra 727 lead shield-graded to copper. USC-20 set as MCS= Multi Channel Scaler mode, HP laptop