Neutron emission asymmetries from linearly polarized γ rays on nat Cd, nat Sn, and 181 Ta

Transcription

1 Neutron emission asymmetries from linearly polarized γ rays on nat Cd, nat Sn, and 8 Ta Clarke Smith, Gerald Feldman, and the HIγS Collaboration George Triangle C. Smith, G. Feldman (GWU) Washington University Universities Nuclear Laboratory (TUNL) Asymmetric neutron emissions /

2 High Intensity γ-ray Source at TUNL HIγS produces γ rays by Compton backscattering free-electron-laser photons from stored electrons intense and nearly monochromatic circularly or linearly polarized neutron emission induced by incident 0-0 MeV photons via (γ, n), (γ, n), and (γ, f) in both fissile and nonfissile targets C. Smith, G. Feldman (GWU) Asymmetric neutron emissions /

3 Photofission studies at HIγS using linearly polarized incident γ rays, neutron emission is azimuthally asymmetric asymmetry depends on: emitted neutron energies nuclear species of the target can possibly be exploited for applications involving isotopic identification C. Smith, G. Feldman (GWU) Asymmetric neutron emissions /

4 Neutron detection outgoing neutrons detected by 8 liquid-scintillator neutron detectors mounted at: φ lab = 0, 90, 80, 70 θ lab = 55, 7, 90, 07, 5, targets positioned in center of array, tilted in θ lab and φ lab (each at 5 ) to double effective thickness time of flight measured over 58 cm flight path C. Smith, G. Feldman (GWU) Asymmetric neutron emissions /

5 Neutron emission asymmetries asymmetrical production of neutrons allows construction of I /I, the ratio of yields parallel to yields perpendicular to the plane of γ-ray polarization I I = N left + N right N top + N bottom N is the number of neutron counts per detector in an energy bin of width E = MeV generate plots of I /I as a function of outgoing neutron energy E n potentially useful for isotopic identification C. Smith, G. Feldman (GWU) Asymmetric neutron emissions 5 /

6 Targets fissile targets nonfissile targets Mar. 009 Feb. 00 Jul. 00 Jul. 0 8 U 5 U, 8 U 9 Pu, Th - nat Pb, 09 Bi, nat Fe, nat Cu, nat Cr nat Pb 9 Be, nat Cd, nat Sn, 8 Ta nat La, nat Hg, nat Dy C. Smith, G. Feldman (GWU) Asymmetric neutron emissions 6 /

7 July 00 nat Cd, nat Sn, and 8 Ta runs γ rays produced by HIγS were incident on nat Cd, nat Sn, 8 Ta data were taken at incident photon energies E γ = MeV for nonfissile targets with high (γ, n) thresholds, measured asymmetries result unambiguously from (γ, n) e.g. nat Cd, nat Sn, 8 Ta targets chosen for low (γ, n) thresholds and high expected I /I values E γ (MeV) nat Cd - - X X X X nat Sn - - X - X X 8 Ta X X X X - X C. Smith, G. Feldman (GWU) Asymmetric neutron emissions 7 /

8 Data analysis with ROOT data collected includes: pulse height time-of-flight (ToF) pulse-shape discrimination (PSD) neutrons distinguished from Compton-scattered photons by D cuts on ToF vs. PSD plots E n calculated using ToF techniques: E n = m n ( ) x, where x = 8 m t additional data taken using 5.5 MeV circularly polarized γ rays used to check for systematic differences in detector efficiency corrections applied to data taken using linearly polarized γ rays C. Smith, G. Feldman (GWU) Asymmetric neutron emissions 8 /

9 cut placed on PSD data.. Neutrons were subsequently isolated from photons with two-dimensional cuts on the plots of ToF vs. PSD, as shown below. ToF projection Neutrons Neutrons shown in red shown in red! Neutrons! Photons! TOF vs. PSD plot, showing two-dimensional cut to isolate neutrons inside the red box. One-dimensional histograms showing TOF and PSD projections. The results of the -D cut are highlighted in red (isolated neutrons). PSD projection. These nonrelativistic neutrons had their energies (En) determined by ToF techniques using the following: C. Smith, G. Feldman (GWU) Asymmetric neutron emissions 9 /

10 circularly polarized linearly polarized C. Smith, G. Feldman (GWU) Asymmetric neutron emissions 0 /

11 Neutron asymmetry plots for various E γ (θ lab = 90 ).5.5 Cd Sn Ta.0 MeV MeV MeV MeV MeV MeV C. Smith, G. Feldman (GWU) Asymmetric neutron emissions /

12 Neutron asymmetry plots for various θ lab (E γ = 5.5 MeV).5 Cd Sn Ta C. Smith, G. Feldman (GWU) Asymmetric neutron emissions /

13 Summary linearly polarized γ rays were incident on nat Cd, nat Sn, and 8 Ta we observed an azimuthal asymmetry in neutron emission that was relatively distinct for nat Cd, nat Sn, and 8 Ta asymmetries also distinct from previously studied targets at HIγS future work determine characteristic I /I vs. E n curves for additional fissile and nonfissile targets collect data for targets with multiple isotopic components to gauge ability to determine isotopic composition C. Smith, G. Feldman (GWU) Asymmetric neutron emissions /