Neutron transmission and capture measurements and analysis of Dy from 0.01 to 550 eV (original) (raw)

2017, Progress in Nuclear Energy

High-resolution transmission and capture measurements of M Ni-enricned targets have been made at the Oak Ridge Electron Linear Accelerator (ORELA) from a few eV to 1800 keV in trarsmissior. and from 2.5 keV to 5 MeV in capture. The transmission data from I to 450 keV were analyzed with a muiti-level R-matrix code which uses the Bayes' theorem for the fitting process. This code provides the energies and neutron widths of toe resonances inside the 1-to 450-keV region as well as a possible parameterization for outside resonances to describe the smooth cross section in this region. The capture data were analyzed with a least-squares fitting code using the Breit-Wigner formula. From 2.S to 450 LeV, 166 resonances were seen in both sets of data. Correspondence between the energy scales shows a discontinuity around 300 keV which makes the matching of resonances at higher energies difficult. Eighty-nine resonances were seen in the capture data only. Average parameters for the '0 observed »wave resonances were deduced. The average level spacing D" was found to be equal to I5.2_± 1.5 keV, the strength function, S 0 , equal to (2.2 ± 0.6) X 10-4 and the average radiation width, H,, equal to 1.30 ± 0.07 eV. The staircase plot of the reduced level widths and the plot of the Lorentz-weighted strength function averaged over various energy intervals show possible evidence for doorway states. The level densities calculated with the Fermi-gas model for £ = 0 and for £ > 0 resonances were compared with the cumulative number of observed resonances, but the analysis is not conclusive. The average capture cross section as a function of the neutron incident energy is compared to the tail of the giant electric dipole resonance prediction. ? The experimental conditions of the capture measurements are given in Sect. 4 wiih a discussion of the various corrections applied to the raw data. The capture data analysis made with a least-squares fitting code using the Breit-Wigner formalism is reported in Sect. 5, Section 6 deals with problems encountered in the energy scales of the two sets of data. In Sect. 7, the resonance parameters obtained from the analysis of both data sets are reported and their uncertain ties discrised. Comparisons ?<iin previously reported resonance parameters for *°Ni are made in Sect. 8. The behavior of the average resonance parameters is analyzed in Sect. 9. Possible evidence for door way states is discussed. 2. TRANSMISSION MEASUREMENTS AND DATA PROCESSING The transmission measurements were made by the time-of-flight technique using neutron pulses from the ORELA v/ater-moderated tantalum target and a 78.203-m flight path. Measurements were made on two samples of *°Ni enriched to 99.7% with thicknesses of 0.00736 and 0.0744 atoms/barn. At the location o. the samples 9 m from the neutron target, the neutron beam was collimated to a diameter of 2.4 cm. A neutron house monitor was used to normalize the sample-in sample-out data to compensate for variations in the neutron production rate during the runs which were of _ few days duration. Two different neutron detectors and techniques were used to cover the energy region investi gated. For the energy region from 200 eV to 240 keV, transmission data were acquired using a 1.3-cmthick, I 1-cm-diam 'Li glass scintillation detector. The electron beam burst was 40 nsec wide, producing a beam power on the target of 40 kW at 800 Hz and the moderator distance (for resolution) was 2.0 cm. Measurements were made on both samples, the thicker one mainly for the higher-energy region and the thinner one for the low-energy resonances. Two filters were inserted in the beam at 5 m: a 1-g/cm 2 l0 B filter (1/e transmission at ~l keV) to eliminate low-energy neutrons from preceding bursts, and a 0.6-cm-thick lead filter to reduce the gamma flash. The 'Li glass scintillator wzs mounted OR an RCA-4522 photomultiplier tube which was gated off during the gamma flash and the succeeding-8 psec. During the measurements three sources of backgrounds were monitored: (I) a background arising from 2.2-MeV gamma rays produced by neutron capture in the water moderator of the target which has a 17.6-jisec half-life time dependence and is the main background at early times and amounts to-3%; (2) a time-independent room background which is the dominant background at long times; and (3) a background produced by neutrons scattered by the detector, air around the detec tor, and the concrete walls and floor of i u-detector statin which later return to the detector. This background is estimated from blacking-out resonances in "Ni and, at lower energies, from an auxiliary measurement using a 2M U sample. Additional details on these backgrounds are reviewed in ref. 2. By use of a 2-cm-thick, 7.5-cm-diam K'E-110 proton recoil scintillation detector, 1 transmission data were obtained from 4 to 240 keV, overlapping the 6 Li d?ta, and also extended upward to 1770 keV. The phototube was gated off for-5 //sec during and after the gamma flash to reduce interference from afterpulsing. The electron beam bum was 10-nsec wide producing a beam power of 16 kW at 1000 Hz. The energy resolution, A£/£, obtained with the NE-110 scintillator was 0.1% below 100 !.«V and 0.35-JE(Mc\)% at higher energies. Both *°Ni samples were measured under these conditions. Two Alters were inserted in the neutron beam at 5 m: the 1-g/cm 1 l0 B filter and a 0.6-cm-thick 2> *U filter. Three sources of background were present: (I) gamma rays in the beam produced by nei tron capture in the water moderator, (2) a conitant beam-independent background, and (3) a 47S keV gamma ray from the l0 B<n.o-y) reaction produced from the absorption of neutrons by the boron in the Pyrex face of the photo-multiplier and moderated by the NE-110 plastic scihtiliator. To aid in the determination of these m ORNL-DWG 81-17832 1 1 *Vli TRANSIiSSION DA1A AND FfT U-6 GLASS DETECTOR (0.00736 atoms/bom) Q6i rl»-' mtmryf^^ JjTyfifTrif^t ^<«ll»W»«B»*»>>« 1 Ni CAPTURE CROSS SECTION DATA (b) (0.00368 otoms/barn) __ 1 10 '.5 NEUTRON ENERGY (keV) 20 Fig. I. Ni-60 transmission data jnd Tit shown with the *°Ni capture data from 5 to 20 keV.