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The 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. This technique can be perfectly suited 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 (''Iβ'') distributions for complex decay schemes. In an ideal case, the measured spectrum would be proportional to the beta feeding (''Iβ''). But a real TAS has limited efficiency and resolution, and also, the ''Iβ'' 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. == Analysis method for TAS data == The complex analysis of the data measured with the TAS can be reduced to the solution of a linear problem: d = Ri given that it relates the measured data (''d'') with the feedings (''i'') from which the beta intensity distribution ''Iβ'' can be obtained. ''R'' is the response matrix of the detector (meaning the probability that a decay that feeds a certain level gives a count in certain bin of the spectrum). The function ''R'' depends of the detector but also of the particular level scheme that is being measured. To be able to extract the value of ''i'' from the data ''d'' the equation has to be inverted (this equation is also called the "''inverse problem''"). Unfortunately this can not be done easily because there is similar response to the feeding of adjacent levels when they are at high excitation energies where the level density is high. In other words, this is one of the so-called "ill-posed" problems, for which several sets of parameters can reproduce closely the same data set. Then, to find ''i'', the response has to be obtained for which the branching ratios and a precise simulation of the geometry of the detector are needed. The higher the efficiency of the TAS used, the lower the dependence of the response on the branching ratios will be. Then it is possible to introduce the unknown branching ratios by hand from a plausible guess. A good guess can be calculated by means of the Statistical Model. Then the procedure to find the feedings is iterative: using the expectation-maximization algorithm to solve the inverse problem,〔 〕 the feedings are extracted; if they don't reproduce the experimental data, it means that the initial guess of the branching ratios is wrong and has to be changed (of course, it is possible to play with other parameters of the analysis). Repeating this procedure iteratively in a reduced number of steps, the data is finally reproduced. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Total absorption spectroscopy」の詳細全文を読む スポンサード リンク
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