Final Task Report on NRF Measurements of Photon Scattering Resonances in Plutonium at the High Voltage Research Laboratory of MIT Page: 3 of 8
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Final Task Report on
NRF Measurements of Photon Scattering Resonances in
Plutonium at the High Voltage Research Laboratory of MIT
Micah S. Johnson, Dennis P. McNabb, Eric B. Norman
January 27, 2007
Measurements of nuclear resonance fluorescence (NRF) states in 239Pu
were conducted using the (y,y') reaction with Bremsstrahlung photons. The
cross-sections of NRF states in 239Pu were determined by taking the ratio of
transition intensities in 239Pu to the measured intensity of the 2212-keV
transition of 27A.
The bremsstrahlung experiments were conducted at the 3-MV Van de
Graaff electron accelerator at the High Voltage Research Laboratory (HVRL)
at the Massachusetts Institute of Technology (MIT). Bremsstrahlung photons
were produced by impinging an electron beam onto a " radiator" with 102-[m
Au backed by 1-cm Cu (used for cooling and electron cleanup). The radiator
was electrically isolated and the average current was recorded for analysis. The
bremsstrahlung photons then pass through a 20-cm long collimator with a 2.50
half-angle, conic opening. The photon beam was aligned by steering the
electron beam on the radiator and observing the tune of the beam with an x-ray
imager downstream of the target.
There were 2 targets for the Pu runs. Each target was a 1.5-cm diameter
disk of ~ 4 g of Pu enriched to 930 239Pu that was encapsulated in ~ 25-g
Nitronic-40 holder with a 1-mil stainless steel window. The manganese of the
Nitronic-40 (and other elements) did produce some contamination but also
served as an internal energy calibration that will be discussed later. The outer
diameter of the Nitronic-40 holder was approximately 2.5 cm. For
normalization we placed a thin disk of Al (approximately 1.2 mm thick and 1.6-
cm in diameter (similar to Pu diameter of 1.5 cm)) on the window of the Pu
holder. We used two different target configurations during the Pu
measurements. One, we used a single target at the central maximum of the
bremsstrahlung beam. Second, we used two Pu targets side-by-side separated
by approximately 1 cm and centered on the flux. Runs with the additional
target increased our statistics by a factor of 2. The flux over this range was
estimated with MCNP to be flat and was consistent with the online x-ray
Scattered y-rays were measured with two HPGe detectors at 1200 with
respect to the beam direction in the lab frame. Both detectors were encased in
thick Pb shielding to reduce background from room scatter. The openings to
the detectors were covered with Pb absorbers of various thicknesses (e.g. 1/4",
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Johnson, M S; McNabb, D P & Norman, E B. Final Task Report on NRF Measurements of Photon Scattering Resonances in Plutonium at the High Voltage Research Laboratory of MIT, report, February 23, 2007; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc891132/m1/3/: accessed November 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.