The surrogate nuclear reaction method can be used to determine neutron-induced reaction cross sections from measured decay properties of a compound nucleus created using a different reaction and calculated formation cross sections. The reliability of $(n,\ensuremath{\gamma})$ cross sections determined using the Weisskopf-Ewing and ratio approximations are explored for the $^{155,157}\mathrm{Gd}$$(n,\ensuremath{\gamma})$ reactions. Enriched gadolinium targets were bombarded with 22-MeV protons and $\ensuremath{\gamma}$ rays were detected in coincidence with scattered protons using the Silicon Telescope Array for Reaction Studies/Livermore-Berkeley Array for Collaborative Experiments (STARS/LiBerACE) silicon and germanium detector arrays. The $\ensuremath{\gamma}$-emission probabilities for the $^{154,156,158}\mathrm{Gd}$ compound nuclei were measured at excitation energies up to 12 MeV. It is found that the approximations yield results that deviate from directly measured $^{155,157}\mathrm{Gd}$$(n,\ensuremath{\gamma})$ cross sections at low energies. To extract reliable cross sections, a more sophisticated analysis should be developed that takes into account angular-momentum differences between the neutron-induced and surrogate reactions.

/pdf/measurement-of-g-emission-branching-ratios-for-154-156-158-3hoj3u694h.pdf

Measurement of γ -emission Branching Ratios for 154,156,158 Gd Compound Nuclei: Tests of Surrogate Nuclear Reaction Approximations for ( n,γ ) Cross Sections

R. O. Hughes,1,* C. W. Beausang,1 T. J. Ross,1,4 J. T. Burke,2 N. D. Scielzo,2 M. S. Basunia,3 C. M. Campbell,3 R. J. Casperson,2 H. L. Crawford,3 J. E. Escher,2 J. Munson,3 L. W. Phair,3 and J. J. Ressler2 1Department of Physics, University of Richmond, 28 Westhampton Way, Richmond, Virginia 23173, USA 2Lawrence Livermore National Laboratory, Livermore, California 94551, USA 3Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 4Department of Physics, University of Surrey, Guildford GU2 7XH, United Kingdom (Received 20 December 2011; published 21 February 2012)

/pdf/utilizing-p-d-and-p-t-reactions-to-obtain-n-f-cross-sections-2xrwq50pnl.pdf

Utilizing ( p , d ) and ( p , t ) reactions to obtain ( n , f ) cross sections in uranium nuclei via the surrogate-ratio method

The $^{240}\mathrm{Am}(n,f$) cross section has been measured for the first time above 4 MeV, using the surrogate-ratio method over the neutron energy range of 200 keV to 14 MeV. The reactions $^{243}\mathrm{Am}(p,tf$) and $^{238}\mathrm{U}(p,tf$), which proceed through the fissioning excited nuclei $^{241}\mathrm{Am}$* and $^{236}\mathrm{U}$*, were used as surrogates for the desired $^{240}\mathrm{Am}(n,f$) and $^{235}\mathrm{U}(n,f$) reactions. The experiment was fielded using the STARLiTeR detector system with a recently commissioned VME-based data acquisition system. The 38.4-MeV proton beam used in these measurements was provided by the K150 cyclotron at the Texas A Cyclotron Institute. The measured $^{240}\mathrm{Am}(n,f$) cross section disagrees with many of the most recent evaluations, and a reevaluation is recommended.

/pdf/measurement-of-the-am-240-n-f-cross-section-using-the-3mcpoy2e3m.pdf

Measurement of the Am 240 ( n , f ) cross section using the surrogate-ratio method

Nuclear Data Sheets for A=155

Neutron Cross Section Measurements

A segmented Si telescope and HPGe array is used to study the $^{156}\mathrm{Gd}$($p,d\ensuremath{-}\ensuremath{\gamma}$)$^{155}\mathrm{Gd}$ direct reaction by $d$-$\ensuremath{\gamma}$ and $d$-$\ensuremath{\gamma}$-$\ensuremath{\gamma}$ coincidence measurements using 25-MeV protons. The present investigation is the first time that this $N=91$ nucleus and the $N=90$ region---which is known for a rapid change from vibrational to rotational character, several low-lying ${0}^{+}$ states in the even-even nuclei, and large Coriolis ($\ensuremath{\Delta}\ensuremath{\Omega}=1$) plus $\ensuremath{\Delta}N=2$ mixing in the even-odd nuclei---have been studied by particle-$\ensuremath{\gamma}$ coincidence following a direct reaction with light ions. Gamma-ray energies and branches, excitation energies, angular distributions, and cross sections are measured for states directly populated in the ($p,d$) reaction. A new low-energy doublet state at 592.46 keV (previously associated with the $K=0\ensuremath{\bigotimes}{\frac{3}{2}}^{\ensuremath{-}}[521]$ bandhead) and several new $\ensuremath{\gamma}$-ray transitions (particularly for states with excitation energies $g1$ MeV) are presented. Most notably, the previous $\ensuremath{\nu} {\frac{7}{2}}^{+}[404]$ systematics at and around the $N=90$ transition region are brought into question and reassigned as $\ensuremath{\nu} {\frac{5}{2}}^{+}[402]$. This reassignment makes the $\ensuremath{\nu} {\frac{1}{2}}^{+}[400]$, $\ensuremath{\nu} {\frac{3}{2}}^{+}[402]$, and $\ensuremath{\nu} {\frac{5}{2}}^{+}[402]$ orbitals, which originate from the $3{s}_{1/2}$, $2{d}_{3/2}$, and $2{d}_{5/2}$ spherical states, respectively, responsible for the three largest cross sections to positive-parity states in the ($p,d$)$^{155}\mathrm{Gd}$ direct reaction. These three steeply upsloping orbitals undergo $\ensuremath{\Delta}N=2$ mixing with their $N=6$ orbital partners, which are oppositely sloped with respect to deformation. The presence of these steeply sloped and crossing orbitals near the Fermi surface could weaken the monopole pairing strength and increase the quadrupole pairing strength of neighboring even-even nuclei, which would bring $\ensuremath{\nu} $2p-2h ${0}^{+}$ states below $2\ensuremath{\Delta}$. Indeed, this could account for a large number of the low-lying ${0}^{+}$ states populated in the ($p,t$)$^{154}\mathrm{Gd}$ direct reaction.

/pdf/particle-g-spectroscopy-of-the-p-d-g-155-gd-reaction-neutron-57jwt0nqvw.pdf

Particle-γ Spectroscopy of the (p,d-γ) 155 Gd Reaction: Neutron Single-quasiparticle States at N=91

The internal surrogate ratio method allows for the determination of an unknown cross section, such as $(n,\ensuremath{\gamma})$, relative to a better-known cross section, such as $(n,f)$, by measuring the relative exit-channel probabilities of a surrogate reaction that proceeds through the same compound nucleus. The validity of the internal surrogate ratio method is tested by comparing the relative $\ensuremath{\gamma}$ and fission exit-channel probabilities of a $^{236}\mathrm{U}{}^{*}$ compound nucleus, formed in the $^{235}\mathrm{U}(d,p)$ reaction, to the known $^{235}\mathrm{U}(n,\ensuremath{\gamma})$ and $(n,f)$ cross sections. A model-independent method for measuring the $\ensuremath{\gamma}$-channel yield is presented and used.

/pdf/relative-sup-235-u-n-gamma-and-n-f-cross-sections-from-sup-4uir0eo0qc.pdf

Relative {sup 235}U(n,{gamma}) and (n,f) cross sections from {sup 235}U(d,p{gamma}) and (d,pf)

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Particle-γ Spectroscopy of the (p,d-γ) 155 Gd Reaction: Neutron Single-quasiparticle States at N=91

Relative {sup 235}U(n,{gamma}) and (n,f) cross sections from {sup 235}U(d,p{gamma}) and (d,pf)