Total absorption spectroscopy

Total absorption spectroscopy is a measurement technique that allows the measurement of the gamma radiation emitted in the different nuclear gamma transitions that may take place in the daughter nucleus after its unstable parent has decayed by means of the beta decay process.^[1] This technique can be used for beta decay studies related to beta feeding measurements within the full decay energy window for nuclei far from stability.

It is implemented with a special type of detector, the "total absorption spectrometer" (TAS), made of a scintillator crystal that almost completely surrounds the activity to be measured, covering a solid angle of approximately 4π. Also, in an ideal case, it should be thick enough to have a peak efficiency close to 100%, in this way its total efficiency is also very close to 100% (this is one of the reasons why it is called "total" absorption spectroscopy). Finally, it should be blind to any other type of radiation. The gamma rays produced in the decay under study are collected by photomultipliers attached to the scintillator material. This technique may solve the problem of the Pandemonium effect.

There is a change in philosophy when measuring with a TAS. Instead of detecting the individual gamma rays (as high-resolution detectors do), it will detect the gamma cascades emitted in the decay. Then, the final energy spectrum will not be a collection of different energy peaks coming from the different transitions (as can be expected in the case of a germanium detector), but a collection of peaks situated at an energy that is the sum of the different energies of all the gammas of the cascade emitted from each level. This means that the energy spectrum measured with a TAS will be in reality a spectrum of the levels of the nuclei, where each peak is a level populated in the decay. Since the efficiency of these detectors is close to 100%, it is possible to see the feeding to the high excitation levels that usually can not be seen by high-resolution detectors. This makes total absorption spectroscopy the best method to measure beta feedings and provide accurate beta intensity (I_β) distributions for complex decay schemes.

In an ideal case, the measured spectrum would be proportional to the beta feeding (I_β). But a real TAS has limited efficiency and resolution, and also the I_β has to be extracted from the measured spectrum, which depends on the spectrometer response. The analysis of TAS data is not simple: to obtain the strength from the measured data, a deconvolution process should be applied.