Measurement of neutron capture cross section and capture gamma-ray spectrum of 128 Te in keV-neutron energy region

. The neutron capture cross section of 128 Te was measured by the TOF method in the energy regions of 15 – 91 keV and 550 keV. The cross section was determined with accuracies of 4-7%. The results are compared with past experimental data and evaluations of JENDL-4. A large disagreement with the recent activation measurement at around 550 keV was observed.


Introduction
Tellurium isotopes are produced in nuclear reactors as a fission product. Recent evaluation of neutron nuclear data of Tellurium isotopes has been made in a series of nuclear data evaluation of fission products [1]. On the other hand, 128 Te is one of the candidate nuclides for neutrino-less double beta decay searches. Neutroninduced reactions are major source of background in the rare-event measurement. Accurate (n,) cross section data of 128 Te are necessary to evaluate background [2]. However, most of experimental data of the neutron capture cross section of 128 Te at neutron energies below 1 MeV are old and have large uncertainties. Thus, in this work, new measurements were carried out to determine the capture cross section of 128 Te using the time-of-flight (TOF) method in the keV energy region.

Experiments and data analysis
A detailed description of the experimental procedure has been given elsewhere [3]. Experiments were performed at the Laboratory for Zero-Carbon Energy at the Tokyo Institute of Technology. The incident neutrons were generated through the 7 Li(p,n) 7 Be reaction by a pulsed proton beam (1.5 ns width × 4 MHz) from a 3 MV-Pelletron accelerator bombarding a lithium target. Experiments were carried out in the two different neutron energy regions: 15 -100 keV and around 550 keV. For the low energy experiments, the proton energy was set at 1.901 MeV, 20 keV above the reaction threshold. The near-threshold energy allows neutrons to be produced in a kinematically limited forward cone with an angle of 50°. This kinematically collimated neutron beam achieved low background measurements. For the high energy experiments, the proton energy was set around a resonance energy of the 7 Li(p,n) 7 Be reaction, producing high-intensity neutrons in all the directions. The incident neutron energy spectrum was * Corresponding author: buchi@zc.iir.titech.ac.jp measured by the TOF method. Neutrons were detected with two different-sized 6 Li glass scintillation detectors for the low and high energy experiments. The scintillator sizes were 5 mm diam. × 5 mm thick for the low energy experiments and 102 mm diam. × 6.4 mm thick for the high energy experiments. The flight lengths were 300 mm and 4.59 m for the low and high energy experiments, respectively. The trigger signal for TOF measurement was generated with a capacitive beam pick-off monitor in a beam duct upstream from the neutron source.
Neutron capture -rays from the sample were detected with an anti-Compton NaI(Tl) spectrometer [4]. The NaI(Tl) spectrometer consists of a main NaI(Tl) detector (152 mm diam. × 305 mm length) and an annular anti-Compton NaI(Tl) detector (330 mm outer diam. × 172 mm inner diam. × 356 mm length) surrounding the main NaI(Tl) crystal. The spectrometer was shielded with borated polyethylene, borated paraffin, potassium-free lead and cadmium. In addition, a front shield of 6 LiH cut down scattered neutrons from the sample to the NaI(Tl) spectrometer while -rays from the sample were allowed. The detection angle of rays with respect to the neutron beam axis was 125°, at which the angular dependent term of dominant E1 transition vanishes.
An isotopically enriched 128 Te sample was used. The sample was a metal pellet with a diameter of 16 mm. The isotopic enrichment of 128 Te was 99.95%. The net weight of 128 Te was 2.977 g. A gold disk with a diameter of 15 mm was used as a standard sample for neutron capture cross section. The flight length from the neutron source to the sample was 120 mm in the low energy experiments and 200 mm in the high energy experiments.
Both the TOF and pulse height (PH) of detected events by the NaI(Tl) spectrometer were recorded eventby-event in list-format data files. Measurements with neutron spectrum or proton beam intensity. The total measurement times are listed in Table 1.  Fig. 1. A prominent peak around 150 ns is rays from the 7 Li(p,) 7 Be reaction in the neutron source. The neutron capture events with the sample appear around 130 ch, which corresponds to a neutron energy of 550 keV. The foreground and background TOF gates were set in TOF spectra for analysis as shown in Fig. 1. The foreground and background PH spectra were obtained by sorting data into PH spectra by the foreground and background gates. The constant background pulse-height spectrum was built from the background gate. The time-dependent background was obtained from the blank measurement. The obtained PH spectra are shown in Fig. 2. The net PH spectrum was made by subtracting the background PH spectrum from the foreground PH spectrum. The net PH spectrum for the high energy experiments is shown in Fig. 3.
For data analysis of the low energy experiments, four TOF gates, 15 -25, 25 -35, 35 -55, 55 -91 keV, were set. The time resolution of the TOF measurement is about 3 ns, corresponding to 4 keV in energy resolution at a neutron energy of 30 keV. The energy widths of the TOF gates were chosen as to be much wider than the energy resolution.   The capture cross sections were obtained from the pulse height spectra by the pulse-height weighting technique [5]. The neutron capture yield was calculated as: where is the pulse-height channel, ( ) is the pulseheight spectrum, ( ) is the weighting function, is the neutron binding energy of 129 Te, and . is the incident neutron energy in the centre-of-mass system. The neutron capture cross section of 197 Au of JENDL-4 was used as the standard cross section.

Results and discussion
Preliminary results of the present measurements are shown in Fig. 4. Taking into account statistical and experimental systematic errors, the cross section was determined with uncertainties of 4-7%. Past experimental data [2,[5][6][7] and the evaluated values from JENDL-4.0 [9] are also shown. The evaluated cross sections are averaged in the same energy bins as the present experimental data for comparison.
In the neutron energy region below 100 keV, the measurements of Macklin et al. [5] are lower than the present experimental results. In addition, the Macklin's data have large errors especially near the 90 keV region. The experimental data of Bergman et al [7] is in a good agreement with the present data but their data fluctuate larger than the present results by 20 -40% on average.
In the high energy region above 100 keV, the results of Dovbenko et al. [6] is considerably smaller than the present results. The most recent data by Tornow et al. are also smaller than the present results at an energy of 550 keV. Their data is 28% smaller than the present value. They measured the cross section by the activation technique, measuring the decay -rays from radioactive 129 Te after neutron irradiation. Tellurium-129 has an isomer state at an excitation energy of 105 keV. The half-life of the isomer state is 33.6 days. In the activation measurement, only the ground state transition was measured due to the long half-life of the isomer state. On the other hand, prompt -rays from the neutron capture reaction were detected in the present work. The excitation energy of the isomer state is small. Thus, the isomer-state transition from the capture state does not affect the present experimental results. This suggests that the difference between the present and Tornow's results may come from the isomer production.

Conclusions
The neutron capture cross section of 128 Te was measured by the TOF method in the energy regions of 15 -91 keV and 550 keV. The cross section was determined with accuracies of 4-7%. Disagreement with the recent activation measurement at around 550 keV was observed. The isomer state contribution may explain the disagreement. Further investigation of the contribution of the isomer state production is needed.