-Neutron capture and transmission measurements were performed by the time-of-flight tech

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Abstract. Recently a method for high accuracy total cross section measurement in the energy range of 24 keV to 940 keV using an iron filtered beam was developed at RPI. Measurements the total cross section of carbon and beryllium are discussed. A new neutron detection system was developed at RPI and the first measurement with this system is reported here. Filtered beam measurementsThe idea of a filtered beam measurement was previously explored at RPI for transmission measurements of deuterium [1]. Recently a 238 U filtered beam was also used [2] and further explored in ref. [3]. The iron filtered beam utilized a 30 cm thick iron piece placed in a collimated neutron beam between the neutron source and the sample being measured. The experiment was done using the conventional time-offlight (TOF) method with the detector placed at a flight path distance of 25.55 m.Most of the neutrons that interact with the thick iron filter are removed from the beam. At neutron energies corresponding to resonance potential interference where the total cross section exhibits a minimum, the filter is nearly transparent to neutrons. The resulting filtered beam has about 19 peaks of intense neutron flux; in some the transmission through the filter is 60%. The main advantage of the filtered beam method is the removal the time dependent neutron and gamma background which imposes limitations on the accuracy of TOF measurements. In a filtered beam experiment the typical signal to noise ratio varies between 500-5000 depending on the selected peak. The accuracy obtained for graphite and beryllium total cross sections measured using this method is about 1%. Better accuracy can be obtained with more careful attention to details such as dead time correction and beam stability normalization.We tested the filtered beam method with a sample of graphite (carbon) for which the total cross section is well known. The method was then used on beryllium. Open beam data is shown in figure 1. It is interesting to note that the counts between the resonances are mostly dominated by resolution broadening and not off energy background neutrons or gammas. The background is mainly room background and neutrons that pass around the filter and can be measured at long time of flight. Transmission is calculated by integrating the peak in the open and sample data, correcting each for the background, and taking the ratio of the background corrected sample to the open count rates. The transmission is converted to cross section by the simple relation σ = −ln(T )/N where a Presenting author, e-mail: danony@rpi.edu The total cross section measured using a 7 cm thick graphite sample compared with the ENDF/B-VII.0 evaluation for carbon. The energy error bar represents the width of the peak (in fig. 1) that was summed.T is the transmission and N [atom/barn]is the atom density of the sample. The total cross section of a 7 cm thick graphite sample is shown in figure 2 together with the ENDF/B-VII.0 [4] evaluation for 12 C. All the points with statistical error of 1% or less ar...

Abstract-The purpose of the present work is to accurately measure the neutron cross sections of samarium The most signijicant isotope is