Publications

Publications by members of the Experimental Subatomic Physics group.

Near threshold (γ,π<sup>0</sup>) reactions for He<sup>4</sup> and C<sup>12</sup>

The total cross section and angular distributions for the reaction 4He(γ,π0) from a cryogenic liquid helium target have been measured within 25 MeV of threshold using tagged photons and a large acceptance π0 spectrometer at the Saskatchewan Accelerator Laboratory. The reduced isovector amplitude p(+)3 has been determined from the total cross-section measurement using a distorted-wave impulse approximation analysis. Refinements from earlier analytical methods, specifically an improved background event rejection scheme and a corrective tagged photon energy calibration analysis, have also yielded an improved estimate on the p(+)3 amplitude for the 12C(γ,π0) reaction explored previously.

Author(s): 
M. G. Barnett, R. Igarashi, R. E. Pywell, and J. C. Bergstrom
Journal Reference: 
Phys. Rev. C 77, 064601 (2008)

Light output response of BC-505 liquid scintillator

Data on the light output response of BC-505 liquid scintillator to neutrons ranging in energy from 0.5 to 12 MeV have been fitted with the parameterization of Chou using the GEANT4 toolkit to simulate the response of the scintillators. Excellent agreement is obtained with the parameters image and image

Author(s): 
R.E. Pywell, B.D. Sawatzky, J. Ives, N.R. Kolb, R. Igarashi, W.A. Wurtz
Journal Reference: 
Nucl. Inst. Meth. in Phys. Res. A 565 (2006) 725–730

High resolution measurement of the <sup>16</sup>O(γ,pn)<sup>14</sup>N<sub>0,1,2,…</sub> reaction

The exclusive 16O(γ,pn)14N0,1,2,… reaction was measured for the photon energy range Eγ=98.5 to 141.0 MeV with an excitation energy resolution of 2.8 MeV. Protons were detected at 76° and 82° and coincident neutrons were detected at the opening angle for quasifree deuteron photodisintegration. Only the T=0, 1+, 3.95 MeV state in the residual 14N nucleus was significantly populated. This corresponds to a preferred L=0 angular momentum transfer to the recoil nucleus as is expected for quasifree kinematics. The absence of significant population to other discrete states is evidence that absorption on correlated proton-neutron pairs with an angular momentum transfer of L>~2 is largely suppressed for the low recoil momentum range of the current measurement. As a consequence of this fact, and the absence of unnatural parity states, only absorption on proton-neutron pairs in relative motion l=0,2 is expected for all possible shell model couplings. No significant population of the T=1 state at 2.31 MeV is consistent with previous measurements, demonstrating that absorption on proton-neutron pairs in an isospin triplet is suppressed. The results of the measurement to discrete states in the residual 14N nucleus are compared to microscopic calculations which include contributions above that of the mean field due to the short range and tensor parts of the nuclear potential. Clear separation in the contribution of (1p)-2 and (1p)-1(1s)-1 proton-neutron pairs is seen to occur at an excitation energy of 20±2 MeV. Significant population of the (1p)-1(1s)-1 continuum above 20 MeV indicates that absorption on L=1 nucleon pairs is also important near quasifree kinematics.

Author(s): 
K. R. Garrow, F. Adimi, N. R. Kolb, J. Lu, R. E. Pywell, D. M. Skopik, J. M. Vogt, J. W. Jury, A. Kuzin, M. N. Thompson, and J. Ryckebusch
Journal Reference: 
Phys. Rev. C 64, 064602 (2001)

Nuclear Compton scattering from <sup>12</sup>C and bound-nucleon polarizabilities

Compton scattering cross sections from 12C have been measured at scattering angles θγ = 35° - 150° using tagged photons of Eγ = 84 - 105 MeV. Attempts to extract nucleon polarizabilities from the data were hampered by model ambiguities. These included uncertainties in the strength of the electric quadrupole and quasideuteron total photon absorption channels, and in the parametrizations of meson-exchange effects and nuclear form factors. These ambiguities led to large variations in the extracted values of the effective polarizabilities of the bound nucleon. Inelastic Compton scattering cross sections from the 4.44 MeV first-excited state were also obtained.

Author(s): 
B. J. Warkentin, D. L. Hornidge, R. Igarashi, J. C. Bergstrom, E. L. Hallin, N. R. Kolb, R. E. Pywell, D. M. Skopik, J. M. Vogt, and G. Feldman
Journal Reference: 
Phys. Rev. C 64, 014603 (2001)

Thickness measurement for thick actinide targets.

We report the measurement of the thickness and composition of several films of actinide material used for measuring photofission cross sections in two experiments performed at the Saskatchewan Accelerator Laboratory (SAL) and at the Thomas Jefferson National Accelerator Facility (JLab). We characterized films of  237Np, 232Th, 238U, 235U, and  233U. The films had thicknesses of the order of 1 mg/cm2, and were deposited on aluminum foils 100 μm thick. By the activities and the energy losses of the α-particles emitted by the films we were able to determine the composition of each film and to measure its thickness with a precision of up to 2%

Author(s): 
J.C. Sanabria, B.L. Berman, P.L. Cole, R.E. Pywell, V.G. Nedorezov, A.S. Sudov, G.Ya. Kezerashvili
Journal Reference: 
Nucl. Inst. Meth. in Phys. Res. A 457 (2001) 402-410

Quasifree Compton Scattering from the Deuteron and Nucleon Polarizabilities

Cross sections for quasifree Compton scattering from the deuteron were measured for incident energies of Eγ = 236 - 260 MeV at the laboratory angle θγ = -135°. The recoil nucleons were detected in a liquid-scintillator array situated at θN = 20°. The measured differential cross sections were used, with the calculations of Levchuk et al., to determine the polarizabilities of the bound nucleons. For the bound proton, the extracted values were consistent with the accepted value for the free proton. Combining our results for the bound neutron with those from Rose et al., we obtain 1-sigma constraints of αn = 7.6-14.0 and βn = 1.2-7.6.

Author(s): 
N. R. Kolb, A. W. Rauf, R. Igarashi, D. L. Hornidge, R. E. Pywell, B. J. Warkentin, E. Korkmaz, G. Feldman, and G. V. O'Rielly
Journal Reference: 
Phys. Rev. Lett. 85, 1388 (2000)

Two-deuteron photodisintegration of <sup>4</sup>He at E<sub>γ</sub> = 150–250 MeV

We have performed a measurement of the cross section for the 4He(γ ,dd) reaction with photon energies between 150 and 250 MeV. Both reaction particles were detected in coincidence. We obtained absolute cross sections at six angles and two energy bins (150–190 MeV and 190–250 MeV). A Legendre polynomial fit to the angular distribution shows a sin2θ distribution which suggests the isospin-forbidden E1 transition. However, a transition matrix element (TME) analysis using only strictly allowed transitions shows that the angular distribution can be described by just two TMEs: E2 s- and d-wave absorption on the D state of 4He. We performed an inverse direct-capture calculation in order to determine if the D state could contribute to such a high degree to the cross section. This calculation indicates that 90% of the cross section arises from absorption on the D state but fails to reproduce the observed sin2θ angular distribution.

Author(s): 
B. J. Rice, R. S. Canon, E. A. Wulf, H. R. Weller, K. Keeter, N. R. Kolb, C. Mueller, R. E. Pywell, G. A. Retzlaff, D. M. Skopik, and G. Feldman
Journal Reference: 
Phys. Rev. C 61, 064612 (2000)

Elastic Compton Scattering from the Deuteron and Nucleon Polarizabilities

Cross sections for elastic Compton scattering from the deuteron were measured over the laboratory angles θγ=35°–150°. Tagged photons in the laboratory energy range Eγ=84–105MeV were scattered from liquid deuterium and detected in the large-volume Boston University NaI spectrometer. Using the calculations of Levchuk and L'vov, along with the measured differential cross sections, the isospin-averaged nucleon polarizabilities in the deuteron were estimated. A best-fit value of (α¯−β¯)=2.6±1.8 was determined, constrained by dispersion sum rules. This is markedly different from the accepted value for the proton of (α¯−β¯)p=10.0±1.5±0.9.

Author(s): 
D. L. Hornidge, B. J. Warkentin, R. Igarashi, J. C. Bergstrom, E. L. Hallin, N. R. Kolb, R. E. Pywell, D. M. Skopik, J. M. Vogt, and G. Feldman
Journal Reference: 
Phys. Rev. Lett. 84, 2334 (2000)

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