STUDY OF THE β DECAY OF FISSION PRODUCTS WITH THE DTAS DETECTOR ∗

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Introduction
The β decay of fission products is responsible for most of the energy released in a nuclear reactor after shut-down.The so-called decay heat varies as a function of cooling time, and can be calculated from the nuclear data corresponding to all nuclei produced during the fission process by means of the summation method.This method consists of summing the activities of the nuclide involved weighted with the mean γ, β and α energies released per decay, as it is represented by the following equation: where f (t) is the power function, Ēi is the mean decay energy of the i th nuclide (β, γ and α components), λ i is the decay constant, and N i (t) is the number of nuclide i at cooling time t.Ēβ and Ēγ depend on the β-intensity distribution, I β , for those nuclei that undergo β decay.
The main limitation of this method is related to the lack of information for some nuclei.In particular, I β data available in the international databases that come from high resolution experiments with Ge detectors, may suffer systematic uncertainties due to the Pandemonium effect [1], which occurs when feeding to high-energy levels is underestimated due to the modest efficiency of such detectors to high-energy γ rays.Total Absorption Gamma-Ray Spectroscopy (TAGS) has been shown to avoid this systematic uncertainty, and some of the most important contributors to the decay heat have been already measured with this technique, giving rise to important improvements in decay heat calculations [2].In TAGS measurements, the detection efficiency is maximized by using large scintillator crystals covering a solid angle of ∼ 4π to absorb the entire γ cascades de-exciting the daughter nucleus after the β decay, rather than detecting individual γ rays.The "inverse problem" d = R(B)f has to be solved in order to determine I β with this technique [3], where d represents the experimental data, f is the feeding distribution we wish to determine, and R is the response function of the detector, that depends on the branching ratio matrix of the decay (B) and is calculated with Monte Carlo (MC) simulations [4].

Experiment
Measurements of several fission products of interest in reactor technology were carried out in 2014 at the upgraded IGISOL IV (Jyväskylä, Finland) [5] using the new segmented Decay Total Absorption Gamma-Ray Spectrometer (DTAS) for the first time [6].This detector has been developed for the Decay SPECtroscopy (DESPEC) experiment at FAIR, and it is made of 18 rectangular NaI(Tl) crystals of 150 mm × 150 mm × 250 mm [7].
The experiment was carried out with protons from the K130 cyclotron impinging on a natural uranium target.The resulting fission fragments were separated and driven to the JYFLTRAP double Penning-trap system [8] where the beam was purified.The nuclei were extracted from the trap and implanted on a tape placed in vacuum in front of a plastic β detector at the centre of DTAS.The software sum of the 18-crystal was reconstructed offline and β-γ coincidences were required in order to obtain a spectrum free from environmental background.The DTAS spectrometer was characterized with Geant 4 [9] MC simulations of calibration sources.
Among all the cases measured in this campaign, here we will focus on the decay of 103 Mo and 103 Tc, that were pointed as priority one decays for the decay heat by the Nuclear Energy Agency (NEA) [10].

Analysis
103 Mo: The β decay 103 Mo → 103 Tc, with Q β = 3.635 MeV and T 1/2 = 67.5 s, was measured with an implantation rate of 800 nuclei/s.The first step in the analysis consists of identifying all the contaminants.Since the decay of the daughter was also measured, this contamination was subtracted and normalized according to Bateman equation.The summing-pileup contamination was also taken into account.It was calculated with a MC procedure based on the random superposition of two stored events within the ADC gate, and it was normalized with a theoretical expression based on [11].The detector response to the decay was calculated with the information of the decay scheme at low excitation energies [12] (a very incomplete information for this decay, in fact no evaluation for I β was done before), and the nuclear statistical model at high excitation energies.Figure 1 (left) shows the quality of the reproduction of the measured spectrum with this response.From our preliminary analysis, 9% of the total I β is obtained above 1621.1 keV, the last populated level known so far [12]. 103Tc: The decay 103 Tc → 103 Ru with Q β = 2.662 MeV and T 1/2 = 54.2 s was measured with an implantation rate of 200 nuclei/s.The summingpileup is the only contaminant, and the detector response was calculated as described above.The nice reproduction of the experimental spectrum is shown in figure 1 (right).From this preliminary analysis, 1.4% of the I β is obtained above 1065.6keV, the last populated level known [13].Moreover, we can compare the absolute γ intensities measured with Ge detectors in [13], with the γ intensities determined from our I β distribution.For example, at 346 keV, the main level, we obtain a 27% against the 28% from ENSDF.
In conclusion, we have obtained for the first time decay intensities for 103 Mo and our data for 103 Tc show a small amount of new intensity detected.Finally, in Table I, we present a preliminary evaluation of the mean energies obtained with the I β distributions determined from our analysis.For 103 Tc, the new intensity does not increase Ēγ and decrease Ēβ as expected, because we obtained a larger value for the ground state feeding.