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.

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.

The gamma rays produced in the decay under study are collected by photomultipliers attached to the scintillator material.

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.

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).

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.

Then the procedure to find the feedings is iterative: using the expectation-maximization algorithm to solve the inverse problem,[2] Then the procedure to find the feedings is iterative: using the expectation-maximization algorithm to solve the inverse problem,[3] 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).

This is important in electron capture decay, as it can affect the results of any x-ray gated spectra if the internal conversion is strong.

It is not possible to produce gamma sources that emit all the energies needed to calculate accurately the response of a TAS detector.

During the measurement there are additional statistical processes that affect the energy collection and are not included in the Montecarlo.

Since the peaks reproduced with the Montecarlo do not have the correct width, a convolution with an empirical instrumental resolution distribution has to be applied to the simulated response.

[10] In this station it is also possible to implant the beam directly in the center of the TAS, by changing the position of the rollers.

It is made of one piece of NaI(Tl) material cylindrically shaped with φ = h = 38 cm (the largest ever built to our knowledge).

It also allows the placement of ancillary detectors in the opposite side to measure other types of radiation emitted by the activity implanted in the tape (x rays, e−/e+, etc.).

[14] Surrounding the TAS there is a shielding box 19.2 cm thick made of four layers: polyethylene, lead, copper and aluminium.

The purpose of it is to absorb most of the external radiation (neutrons, cosmic rays, and the room background).

Hypothetical beta decay seen by high-resolution (germanium mainly) and TAS detectors. There is a change in philosophy when measuring with a TAS. With a germanium detector (Ge), the energy peaks corresponding to individual gammas are seen, but the TAS detector gives a spectrum of the levels populated in the decay (ideal TAS). The TAS detector has less resolution but higher efficiency.
Lucrecia measuring station where the shielding can be seen in white as well as the beam line that delivers the radioactive species.