Title: 207Pb (n,2ngamma)206Pb Cross-Section Measurements by In-Beam Gamma-Ray Spectroscopy
Citation: AIP Conference Procceding - Capture Gamma-ray Spectrometry and Related Topics vol. 819 p. 561-562
Publisher: American Institute of Physics
Publication Year: 2006
JRC Publication N°: JRC35515
URI: http://publications.jrc.ec.europa.eu/repository/handle/JRC35515
Type: Contributions to Conferences
Abstract: Innovative systems for energy production and/or waste transmutation require new nuclear data in a wide neutron energy range. For (n,xn) reactions with x>1 at En> 6-8 MeV, experimental data are very rare and evaluated data show large discrepancies. Thus, in the frame of the n-TOF project a program to measure (n,xn) cross-sections was initiated. This paper presents 207Pb(n,2ngamma)206Pb cross-section mesurements with a lead target enriched in 207Pb at the GELINA white neutron beam at IRMM, Geel, Belgium. this experiement was performed in parallel with (n,n'gamma) measurements by the detection of prompt gamma rays in HPGe detectors. The main difficulty was the presence of a strong gamma flash which only arrives a few µsec before the fastest neutrons (2.5µsec for 20 MeV neutrons). For the reactions corresponding to high energy neutrons, pile up of pulses fromthe gamma flash together with events from neutron induced gamma rays may occur. This condition deteriorates the energy resolution in the gamma spectra and increases significantly the dead time. To solve these problems we have developed a data acquisition system based on digital pulse processing techniques. Signals from the preamplifier of the HPGe crystals were sampled at a rate of 65 MS/s using 14bit Flash ADC modules developed at IReS, Strasbourg. These modules use built-in-Field-Programmable Gata Array (FPGA) to analyse the samples online. For the energy determination, the trapezoidal shaping technique was used. The algorithm for time determination consisted in a digital timing filter followed by a digital constant fraction discriminator. The lead sample enriched to 92% in 207Pb was placed at 200m from the neutron source. For the 207Pb(n,n'gamma) measurements, conventional electronics was used, while for the (n,2ngamma) measurements two of the HPGe detectors (70% and 100%) relative efficiency were also read out with digital electronics. The neutron flux was measured with a 235U fission-chamber. The data were collcdted from three months. We will present the cross section data for incident neutron energies between 6 and 20 MeV for the main gamma transitions in 206Pb. Our data will be compared to the results obtained by Vonach et al. and to the TALYS and EMPIRE-II code predictions. With the help of the model calculations the (n,2n) cross section values can be deduced and will presented.
JRC Institute:Institute for Reference Materials and Measurements

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