Beta-strength and anti-neutrino spectra from total absorption spectroscopy of a decay chain 142 Cs → 142 Ba → 142 La

. Beta decays of mass A = 142 isobaric chain starting from 142 Cs have been investigated by means of Modular Total Absorption Spectrometer (MTAS) and on-line mass separation at Oak Ridge National Laboratory. The beta strength distribution derived for 142 Cs decay from MTAS spectra is showing signiﬁcant differences in β -feeding pattern when compared to the values listed at nuclear databases. MTAS results are shifting the associated anti-neutrino energy spectrum towards lower energies. A decay pattern deduced for 142 Ba is similar to earlier reported results.


Introduction
The Modular Total Absorption Spectrometer (MTAS) array has been designed, constructed, characterized and applied to the decay studies of 238 U fission products at the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory [1][2][3]. MTAS measurements yield true β-strength distribution verifying and helping to develop the microscopic description of β-decay. In particular, the decay heat release in fission products during a nuclear fuel cycle as well as respective anti-neutrino energy spectra can be determined from the complete β-decay scheme. Over seventy decay products of 238 U fission products measured at ORNL include 22 decays of high priority for decay heat analysis [4]. These measurements include the 142 Cs activity assessed recently as the third most important contributor to the high energyv spectra in nuclear reactors [5][6][7]. The significant differences between β-feeding values derived from MTAS measurement and Evaluated Nuclear Structure Data Files (ENSDF) database are substantially impacting the reference reactor antineutrino flux and its spectral properties, see, e.g., [8]. January 2016 at the On-Line Test Facility, namely the mass separator on-line to the ORNL Tandem accelerator. MTAS detector array consists of 19 NaI(Tl) hexagonal shape detectors. Each crystal is ∼53 cm long and about 20 cm maximum diameter. The weight of NaI(Tl) detector material in MTAS array is ∼1000 kg, covering over 99% of the solid angle around the measured activities. The entire array is surrounded by over 5000 kg of lead and SWX-227A neutron shielding foam. This shielding reduces the laboratory gamma background by a factor of 1000. MTAS auxiliary detectors include two 1-mm-thick Si β-trigger detectors (each divided into 7 strips, each ∼8.5 mm wide), placed at the center of the NaI(Tl) array around the tape transporting the activities [1]. The signals detected in these silicon counters are used as β-triggers. Applying coincidence condition between silicon detectors and MTAS suppresses typical laboratory background by at least three orders of magnitude. The γ -ray efficiency for full-energy deposition of a single γ -ray in on-line conditions is about 81% at 300 keV and about 71% at 6 MeV [3]. The MTAS thresholds for individual detector, each from 18 regular modules, is about 25 keV. For the central module it is around 30 keV.

MTAS measurement and ENSDF database comparison
We evaluate the energy spectra measured with MTAS to determine β-feeding intensities to known and unknown energy levels in the daughter nuclei, including the ground state. The evaluation method is based primarily on the techniques applied in the analysis of other total absorption spectrometers spectra, see e.g., [9][10][11]. However, our analysis is profiting from the increased efficiency and high segmentation of MTAS [2,3,8]. The MTAS detector response function was simulated with the GEANT4 toolkit. This generated MTAS response function is verified with through measurements of single γ -ray and γ -cascades emitted in the decay of calibration sources like 137 Cs, 54 Mn, 65 Zn, 60 Co and 24 Na. MTAS energy spectra are divided into two response regimes. These correspond to energy levels below threshold energy with de-excitation γ -paths known from high energy resolution gamma spectroscopy data and to unknown β-fed levels above the threshold energy grouped in 25 keV wide bins. More details of MTAS spectra analysis method are given in [1][2][3]8]. In this paper we present MTAS results on measured 142 Cs and 142 Ba activities compared to GEANT4 simulations using currently available ENSDF entries [12]. Beta-gated MTAS spectra are analyzed to reduce background. Also the pile-up analysis is included into our evaluation procedure [2]. The decay chain of the A = 142 includes the very short-lived 142 Cs (1.7 s) isotope which decays to 142 Ba activity having a half-life of 10.6 min, followed by the decay of 142 La (91.1 min) to stable 142 Ce.

Decay of 142 Ba
There were several measurements of 142 Ba decay, see, e.g., [13], and summarized recently in Nuclear Data Sheets [12]. These measurements were done using highresolution Ge detectors as well as total absorption gamma spectroscopy (TAGS) [14]. Interestingly, the 142 Ba decay is a rare case where the high-resolution measurements point to practically the same β-decay pattern as deduced from the TAGS measurement. It could be explained by a relatively low beta decay energy value of Q β = 2212(5) keV. Indeed, 142 Ba MTAS spectrum is in general agreement with earlier data (Fig. 1). Figure 2 presents the comparison of an overall fit of the simulated decays following the ENSDF entry for the 142 La level scheme populated in 142 Ba decay (colored components) and their sum (orange), to the measured MTAS spectrum (black). The sum of the individual decay paths follows well MTAS measured data. The γ -rays with intensity much smaller then 10% of the strongest γ -ray de-exciting each level were not taken into account in this particular simulation. The intensities of β-feeding determined from MTAS measurement were compared to the values from ENSDF entries and Greenwood's measurements [14]. Examples of the results for main levels are listed in Table 1.

Decay of 142 Cs
The MTAS-measured energy spectrum for 142 Cs decay is compared to the simulated MTAS response using decay data listed in the current ENSDF entry in Fig. 3. The reduction of the ground-state β-feeding and the β-feeding to the first-excited 2 + state at 360 keV is clearly visible, as well as the presence of new β-fed levels at higher excitation energies. The derived β-feeding intensities for 142 Cs decay with Q β = 7325(9) keV are presented in Fig. 4. The MTAS-determined 142 Cs β-feeding to the 142 Ba ground-state is 44(2)%, lower than 56% listed in ENSDF. The upper limit for beta-feeding to the first-excited 2 + state in 142 Ba is 0.5% [8], at least an order of magnitude lower than reported earlier 7% [12].

High energy part of reactor anti-neutrino spectrum for 142 Cs
The 142 Cs beta decay was found to be among top 3 contributors to the high energy reactorv spectrum in recent evaluations [5][6][7]. We have re-evaluated the spectrum of anti-neutrinos emitted in this decay basing on new MTAS data. The change in thev energy spectrum of 142 Cs is shown in Fig. 5. The fraction ofv with energy above 5 MeV changes from 20% to 14(1)% the totalv flux from 142 Cs decay, while the fraction of 142 Csv with energy below 1.8 MeV increases from 11% to 23(3)%   . 142 Csv energy spectrum derived from MTAS data (black) compared to thev energies obtained from the latest ENSDF entry (cyan). [8]. Such change can be expected for complex decays of fission products, with high Q-value and influenced by the Pandemonium Effect [15] after the measurement using the total absorption technique. It shows the reduction of previously reported feeding to low-lying states and a corresponding increase of β-strength at higher excitations in the daughter nucleus. It shifts the correspondingv spectrum to lower energies.

Summary
The Modular Total Absorption Spectrometer (MTAS) was used to measure the beta-gamma activities in the mass A = 142 decay chain. Two isobars from this chain, 142 Ba and 142 Cs, are presented here. For both of them, the new β-feeding pattern was obtained. For 142 Ba decay, with low Q β of 2.2 MeV, MTAS spectra analysis reproduce known decay pattern obtained previously using high resolution gamma spectroscopy as well as total absorption measurements [14]. MTAS measurements of 142 Cs, with much higher Q β of 7.3 MeV, resulted in a major modification of the decay scheme. The groundstate β-feeding is reduced from 56% (ENSDF) to 44(2)% (MTAS). For the β-feeding to the first-excited 2 + state at 360 keV only a limit of β-feeding ∼0.5% can be given, while the ENSDF value is listed as 7%. Our measurements are showing also the presence of new β-fed levels at higher excitation energies in 142 Ba. MTAS measurements confirmed the importance of 142 Cs decay as one of the top three contributors to the high energyv spectral shape in nuclear reactors, and as so far the most important activity changing the expected high-energy anti-neutrino flux. MTAS results for 142 Cs shift the totalv-spectrum toward lower energies. It results in a reduction of reactor v anomaly, roughly from 94.6(22)% to 95.7(22)% and is enhancing high-energy "bump" from ∼ 10% to ∼ 12% excess, for the reference anti-neutrino flux [8] calculated using a summation method.