Nuclear Instruments and Methods in Physics Research A 572 (2007) 866–873 Comparison of the measured and the calculated total thermal neutron cross-section of Pb G. Muhrer à , T. Hill, F. Tovesson, E. Pitcher Los Alamos National Laboratory, Los Alamos, 87545, NM, USA Received 27 October 2006; received in revised form 29 November 2006; accepted 11 December 2006 Available online 31 December 2006 Abstract Lead is one of the materials commonly used in the design of neutron sources. However, with the increased accuracy expectations of the predictions for these sources it is no longer prudent to represent the neutron cross-section of lead in the thermal regime by the free gas model. We have therefore generated a scattering kernel for lead for which we have replaced the ideal gas approximation by a generalized crystallographic model, which includes the crystal structure of lead and the inelastic scattering in terms of the phonon spectrum of the material. These theoretical results then were compared to measurements. r 2007 Elsevier B.V. All rights reserved. PACS: 28.20.Cz; 25.40.Dn; 25.40.Fq Keywords: Scattering kernel 1. Introduction There has been a significant effort in recent years to benchmark the performance of spallation neutron sources [1,2] and to compare them to calculations [3,4]. While these comparisons in general have been satisfying there have been cases [5,6] where the results have not lived up to the expectations. One of the possible reasons for this is that the thermal cross-section for most of the materials used in the calculations is approximated by the free gas model [7]. While this is a good first approximation for amorphous solids, it is not for crystalline solids such as metals. There is a high demand for new scattering kernels for moderator and reflector materials, and of particular interest is lead, which is very commonly used as a reflector material and is therefore of great importance to the spallation physics community. However, the method which will be presented for the development of a lead scattering can also be used for any other face centered cubic material and at any temperature. 2. Experimental results Since the data currently available from the National Nuclear Data Center [8] do not have the necessary resolution to make an accurate comparison with the calculated cross-section we decided to measure the total neutron cross-section for Pb at flight path 5 at the Manuel Lujan Neutron Scattering Center at the Los Alamos National Laboratory. Fig. 1 shows the experimental setup. In order to obtain the total cross-section for lead a 3.8 cm and a 7.6 cm thick block of lead, respectively, were placed in the beam between the end of the shutter and the beginning of the collimator as indicated in Fig. 1. In both cases the transmission spectrum was measured with a fission chamber downstream of the collimator. This detector measures the time-dependent neutron beam flux after passing through the transmission samples by using the 235 U(n,f) standard reaction. The parallel-plate ionization chamber with a 200 mg=cm 2 deposit of 235 U (99.9% isotopic purity) is placed 2.96 m downstream of the transmission sample. The data acquisition system records the detector pulse–height response and the neutron time- of-flight for each event, so that fission events can be ARTICLE IN PRESS www.elsevier.com/locate/nima 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.12.021 à Corresponding author. Tel.: +1 505 665 8806; fax: +1 505 667 2676. E-mail address: muhrer@lanl.gov (G. Muhrer).