Beta Decay Total Absorption Spectrometer
The distribution of -decay probabilities I to the different levels of the final nucleus is a quantity of fundamental interest because it reflects the overlap of parent and daughter nuclear wave functions. Measurements of this distribution provide a stringent test of nuclear structure theoretical models. This type of verification is particularly important when these models are used to make predictions of decay properties far from stability, as is the case of the half-lives used in the astrophysical r-process calculations. On the other hand with well calibrated theoretical models closer to the stability one can deduce nuclear structure properties from I-distributions, as for example the shape of the nucleus. -decay intensity distributions can be used to calculate the energy distribution of -particles, neutrinos and -ray cascades emitted in the decay. This finds application for example in the calculation of the decay heat of the irradiated nuclear fuel, due to the disintegration of accumulated fission products, of importance in the context of reactor operation and safety assessment. Other such application will be the calculation of reactor neutrino spectra of importance for -oscillation experiments or homeland security.
The available experimental information on I comes largely from high resolution Ge spectroscopy. However, except for simple decays proceeding to a limited number of excited levels, it suffers from the systematic error known as the pandemonium effect. For large level densities, the fragmentation of the -strength into many excited levels and the large number of de-excitation paths makes impossible the isolation of individual primary transitions and shifts the apparent -intensity to low excitation energies. Total absorption spectroscopy solves this problem because it relies on the detection of the full decay -ray cascade rather than the individual transitions. It is the only method capable of providing accurate I distributions for complex decays below the particle emission threshold.
Figure 1. Nuclear landscape. In red, existing TAS data. In blue, the r-process path.
Figure 2. Upper panel: TAS spectrum for the decay of 104Tc. Lower panel: The obtained beta intensity compared with the high resolution data
Ideally a Total Absorption Spectrometer (TAS) will cover a 4 solid angle, will be thick enough to have 100% peak efficiency, and will be insensitive to -particles. In this case the measured spectrum corresponds to I widened by the spectrometer energy resolution. In practical spectrometers none of this conditions are met and the -intensity must be obtained by deconvolution of the measured spectrum with the calculated spectrometer response. A large solid angle coverage and peak efficiency of the spectrometer guarantee the insensitivity of the calculated response to the decay path and therefore the accuracy of the result.
The largest source of systematic error in total absorption spectroscopy comes from contaminations in the spectrum. Ancillary detectors (-detector for +/--decay, X-ray detector for EC-decay) can be used to eliminate the background contamination. In the case of EC-decay they can also provide isotopic separation, but in general the isobaric contaminations should be eliminated either a posteriori from the data (from separate measurements) or a priori from the radioactive beam by selective ionization (laser or chemical) or extreme mass resolution (Penning trap). For neutron rich nuclei, -delayed neutrons produce capture or inelastic -rays in the detector which must be distinguished from the source -rays.
The presently proposed TAS at DESIR is an existing compact design (diameter 25cm, length 25cm) which consists of 12 BaF2 crystals arranged in cylindrical geometry and provides detection efficiencies larger than 98% for -ray cascades. An inner hole of diameter 5cm allows the placement close to the source of either a thin Si detector or a thin plastic scintillation detector for tagging the -particles. Additionally a small Ge detector can be used to tag the X-rays coming from the EC process. The good timing of the BaF2 should allow us to discriminate the -delayed neutron induced signals from the source -rays due to their slightly longer time of flight. The segmentation of the detector will allow us to constrain the -ray cascade multiplicity as a function of excitation energy and will help to further reduce the uncertainties in the spectrometer response. A dedicated data acquisition system has been developed including an active gain matching system of the different PMT using the Ra contaminant -emissions as reference signals.
Figure 3. Twelve fold segmented BaF2 TAS
Currently we are working on the development of a new spectrometer based on the new scintillation material LaBr3:Ce. This material will offer a much improved energy resolution which on one hand will allow resolving narrower features in the intensity distribution and on the other hand will improve the accuracy of the result through a better localization of the contaminations and a fine adjustment of the spectrometer response function.
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