Measurements with the high flux lead slowing-down spectrometer at LANL

Abstract

A Lead Slowing-Down Spectrometer (LSDS) was recently installed at LANL [D. Rochman, R.C. Haight, J.M. O’Donnell, A. Michaudon, S.A. Wender, D.J. Vieira, E.M. Bond, T.A. Bredeweg, A. Kronenberg, J.B. Wilhelmy, T. Ethvignot, T. Granier, M. Petit, Y. Danon, Characteristics of a lead slowing-down spectrometer coupled to the LANSCE accelerator, Nucl. Instr. and Meth. A 550 (2005) 397]. The LSDS is comprised of a cube of pure lead 1.2m on the side, with a spallation pulsed neutron source in its center. The LSDS is driven by 800MeV protons with a time-averaged current of up to 1μA, pulse widths of 0.05–0.25μs and a repetition rate of 20–40Hz. Spallation neutrons are created by directing the proton beam into an air-cooled tungsten target in the center of the lead cube. The neutrons slow down by scattering interactions with the lead and thus enable measurements of neutron-induced reaction rates as a function of the slowing-down time, which correlates to neutron energy. The advantage of an LSDS as a neutron spectrometer is that the neutron flux is 3–4 orders of magnitude higher than a standard time-of-flight experiment at the equivalent flight path, 5.6m. The effective energy range is 0.1eV to 100keV with a typical energy resolution of 30% from 1eV to 10keV. The average neutron flux between 1 and 10keV is about 1.7×109n/cm2/s/μA. This high flux makes the LSDS an important tool for neutron-induced cross section measurements of ultra-small samples (nanograms) or of samples with very low cross sections. The LSDS at LANL was initially built in order to measure the fission cross section of the short-lived metastable isotope of U-235, however it can also be used to measure (n,α) and (n,p) reactions. Fission cross section measurements were made with samples of 235U, 236U, 238U and 239Pu. The smallest sample measured was 10ng of 239Pu. Measurement of (n,α) cross section with 760ng of Li-6 was also demonstrated. Possible future cross section measurements include fission and (n,p) and (n,α) reaction in radioactive samples.