Total Photoneutron Cross Section Research Articles

Measurement of the Se80(γ,n) reaction with linearly polarized γ rays

This paper reports the results of the first measurements of the differential cross section of the $^{80}\mathrm{Se}(\ensuremath{\gamma},n)^{79}\mathrm{Se}$ reaction with a linearly polarized gamma-ray ($\ensuremath{\gamma}$-ray) beam. The cross section was measured at three incident $\ensuremath{\gamma}$-ray beam energies: 15.6, 15.8, and 16.0 MeV, with a beam energy spread of 3.0% full width at half-maximum (FWHM). The differential cross section for the excitation spectrum in $^{79}\mathrm{Se}$ was measured at two scattering angles in the plane of the beam polarization: $\ensuremath{\theta}={90}^{\ensuremath{\circ}}$ and ${135}^{\ensuremath{\circ}}$, and at one angle in the plane perpendicular to the plane of polarization: $\ensuremath{\theta}={90}^{\ensuremath{\circ}}$. The total photoneutron cross sections determined from these data are between 0.8 and 1.3 standard deviations smaller than previously published results. The excitation spectra measured in this work were fit with a Hauser-Feshbach model. Better fits to the data were obtained with a constant-temperature formulation of the nuclear level density (NLD) than with a Fermi-gas NLD model. The parameters for the constant-temperature NLD model obtained in this work are consistent with those obtained for medium-mass nuclei in previous studies.

Photoneutron cross-section measurements in the Bi209 ( γ,xn ) reaction with a new method of direct neutron-multiplicity sorting

Photoneutron cross sections were measured in the $^{209}\mathrm{Bi}$($\ensuremath{\gamma}, xn$) reaction with $x=1--4$ at energies from the neutron threshold to 40 MeV using quasimonochromatic laser Compton-scattering $\ensuremath{\gamma}$-ray beams. A novel technique of direct neutron-multiplicity sorting with a flat-efficiency detector was used. Results are compared with the positron in-flight annihilation data of Livermore and Saclay as well as with bremsstrahlung data. The present neutron yields are consistent with the latest photoactivation measurement with bremsstrahlung. The giant dipole resonance component of the total photoneutron cross section is reasonably reproduced by model calculations of the Hartree-Fock Bogoliubov plus quasiparticle random phase approximation based on the Gogny D1M interaction for both E1 and M1 components. In contrast, the quasideuteron component of the total photoneutron cross section requires a factor of $\ensuremath{\sim}3$ enhancement of the Levinger parameter. Furthermore, a survival of large $(\ensuremath{\gamma},n)$ cross sections above 20 MeV may indicate the surface effect on the two-component particle-hole state density. The threshold behavior of photoneutron emission is discussed in terms of the average neutron kinetic energy.

Partial Photoneutron Cross Sections for 207,208Pb

Using linearly-polarized laser-Compton scattering γ-rays, partial E1 and M1 photoneutron cross sections along with total cross sections were determined for 207,208Pb at four energies near neutron threshold by measuring anisotropies in photoneutron emission. Separately, total photoneutron cross sections were measured for 207,208Pb with a high-efficiency 4π neutron detector. The partial cross section measurement provides direct evidence for the presence of pygmy dipole resonance (PDR) in 207,208Pb in the vicinity of neutron threshold. The strength of PDR amounts to 0.32%−0.42% of the Thomas-Reiche-Kuhn sum rule. Several μN2 units of B(M1)↑ strength were observed in 207,208Pb just above neutron threshold, which correspond to M1 cross sections less than 10% of the total photoneutron cross sections.

Total and partial photoneutron cross sections for Pb isotopes

Using quasimonochromatic laser-Compton scattering $\ensuremath{\gamma}$ rays, total photoneutron cross sections were measured for ${}^{206,\phantom{\rule{0.16em}{0ex}}207,\phantom{\rule{0.16em}{0ex}}208}$Pb near neutron threshold with a high-efficiency 4$\ensuremath{\pi}$ neutron detector. Partial $E1$ and $M1$ photoneutron cross sections along with total cross sections were determined for ${}^{207,\phantom{\rule{0.16em}{0ex}}208}$Pb at four energies near threshold by measuring anisotropies in photoneutron emission with linearly polarized $\ensuremath{\gamma}$ rays. The $E1$ strength dominates over the $M1$ strength in the neutron channel where $E1$ photoneutron cross sections show extra strength of the pygmy dipole resonance in ${}^{207,\phantom{\rule{0.16em}{0ex}}208}$Pb near the neutron threshold corresponding to 0.32$%$--0.42$%$ of the Thomas-Reiche-Kuhn sum rule. Several ${\ensuremath{\mu}}_{N}^{2}$ units of $B(M1)\ensuremath{\uparrow}$ strength were observed in ${}^{207,\phantom{\rule{0.16em}{0ex}}208}$Pb just above neutron threshold, which correspond to an $M1$ cross section less than 10$%$ of the total photoneutron cross section.

Evaluated cross sections of the σ(γ, nX) and σ(γ, 2nX) reactions on 112, 114, 116, 117, 118, 119, 120, 122, 124Sn isotopes

A combined analysis of experimental data on total and partial photoneutron reaction cross sections, obtained using bremsstrahlung γ-radiation and quasi-monoenergetic annihilation photon beams, was performed for nine Sn isotopes. The partial reactions σeval(γ, nX) and σeval(γ, 2nX) cross sections were evaluated using an approach free of the shortcomings of experimental neutron multiplicity sorting methods. This approach involves calculations within the photonuclear reaction model, based on Fermi gas densities and considering the effects of nucleus deformation, the isospin splitting of its giant dipole resonance (GDR), and experimental data on the total photoneutron cross sections σexp(γ, xn) = σexp(γ, nX) + 2σexp(γ, 2nX) = σexp(γ, n) + σexp(γ, np) + … + 2σexp(γ, 2n) + 2σexp(γ, 2np) + …. The evaluated σeval(γ, nX) and σeval(γ, 2nX) reactions cross sections were obtained using the introduced transition multiplicity functions Ftheor = σtheor(γ, 2nX)/σtheor(γ, xn) = σtheor(γ, 2nX)/[σtheor(γ, nX) + 2σtheor(γ, 2nX) + …]; and σeval.(γ, 2nX) = Ftheor.σexp.(γ, xn) = σeval(γ, nX) = (1 − 2Ftheor)σexp(γ, xn). The evaluated partial reaction cross sections were used to assess the total photoneutron reaction cross sections σeval(γ, sn) = σeval(γ, nX) + σeval(γ, 2nX) + … as functions of the mass number A. The GDR features of 112, 114, 116, 117, 118, 119, 120, 122, 124Sn isotopes were studied and are discussed here.

Photoneutron angular distributions for 32S

Angular distributions of the energy spectra of photoneutrons emitted in the reaction 32S(γ, n0)31S were measured over the range of excitation energies from 18 to 29 MeV. Coefficients of Legendre polynomials fitted to the data gave no indication of photon absorption other than through electric dipole transitions, except at energies above 27 MeV. The measured value of the normalized Legendre coefficient a2 is − 1.0 ± 0.1 across most of the giant resonance region supports a simple shell model interpretation that the 2s12 → 2p32 single-particle transition (of channel spin 0) is the dominant mechanism for photoabsorption followed by neutron decay to the ground state of 31S. Comparison of the present results for the ground state channel with the total photoneutron cross section shows that up to an excitation energy of 18.5 MeV nearly all neutron-emitting transitions proceed to the ground state of 31S even though many channels for excited states are open. The ground state photoneutron channel exhausts about 6% of the TRK sum rule.

Ground-state photoneutron reactions in sup18O.

Differential cross sections have been measured for the reaction $^{18}\mathrm{O}$(\ensuremath{\gamma},${\mathrm{n}}_{0}$${)}^{17}$O over the region of excitation energy from 14 to 26 MeV. The angle-integrated cross section for the ground-state transition reveals that this channel accounts for less than 20% of the total photoneutron cross section in the structured pygmy resonance region (near 14 MeV) and is a small fraction (10--15 %) of the cross section in the region of the giant resonance (near 25 MeV). The values of angular distribution coefficients fitted to the data are consistent with a description of this reaction in which electric dipole excitations dominate the cross section in the pygmy resonance. Narrow regions exist near 15.0, 16.0, and 20.0 MeV where nonzero ${a}_{1}$ coefficients are observed indicating the absorption of non-E1 radiation. The measured cross section and ${a}_{2}$ coefficients are compared to a direct-semidirect calculation which gives reasonable agreement and suggests that f-wave neutron emission dominates the ground-state channel and that there is little justification for the introduction of E2 amplitudes other than a pure direct E2 term.

Absolute photoneutron cross sections for Zr, I, Pr, Au, and Pb.

The photoneutron cross sections for /sup nat/Zr, /sup 127/I, /sup 141/Pr, /sup 197/Au, and /sup nat/Pb have been measured near the peak of the giant dipole resonance, with monoenergetic photons, in such a way that the absolute cross sections for these nuclei were intercalibrated, using /sup 141/Pr as a benchmark. The measured peak total photoneutron cross sections are, within an uncertainty of 2--3 %, 174, 252, 340, 502, and 602 mb for /sup nat/Zr, /sup 127/I, /sup 141/Pr, /sup 197/Au, and /sup nat/Pb, respectively. These results resolve outstanding discrepancies among previous measurements.

Photoneutron and photofission cross sections for 238U and 232Th using neutron capture gamma rays

The photofission and total photoneutron cross section of 238U and 232Th have been measured as a function of energy between 5 and 11 MeV. The photons used were those produced in the neutron capture reaction in the Tangential Beam Port Facility of the University of Virgina Reactor. The capture γ ray sources used were the following; Al, Cr, Co, Cu, Fe, Ni, N, S and Ti. A computer code was used to calculate the spectrum of each capture γ ray beam used in the irradiations. This calculation accounted for the attenuation in the beam and the contribution from neutron capture in A1 and H. A second code iteratively solved for the best fit cross section curve for the experimentally obtained yield data.

Ground state photoneutron reactions in 17O.

Photoneutron angular distributions and differential cross sections have been measured for the reaction $^{17}\mathrm{O}$(\ensuremath{\gamma},${\mathrm{n}}_{0}$${)}^{16}$O over the excitation energy region from 10 to 24 MeV. The angle-integrated ground state cross section was obtained and compared to the total photoneutron cross section. The comparison indicates that the ground state channel dominates the pygmy resonance region below ${E}_{\ensuremath{\gamma}}$=16 MeV but contributes very little (<10%) to the giant resonance region (near 22 MeV). Legendre coefficients extracted from the angular distribution data suggest that electric dipole transitions make up nearly all of the absorption strength in the region studied except in narrow regions near 10.8, 15.1, 17.3, and 22.3 MeV, where small but significantly nonzero values of the ${a}_{1}$ coefficient are observed. Values of the ${a}_{2}$ coefficient, interpreted in an E1 approximation, suggest a relatively pure single-particle composition of many of the states observed in the pygmy resonance region from 10 to 16 MeV.

Photoneutron cross sections for 14C.

The photoneutron cross sections for $^{14}\mathrm{C}$ [\ensuremath{\sigma}(\ensuremath{\gamma},1n) and \ensuremath{\sigma}(\ensuremath{\gamma},2n)] have been measured up to 36 MeV using monochromatic photons. The cross section for the emission of a single neutron displays a very prominent resonance near 15 MeV which appears to decay primarily to the ground state of $^{13}\mathrm{C}$. The (\ensuremath{\gamma},2n) cross section is sharply peaked at 26 MeV and is large compared with those for $^{12}\mathrm{C}$ and $^{13}\mathrm{C}$. The integrated total photoneutron cross section up to 36 MeV is 126\ifmmode\pm\else\textpm\fi{}12 MeV mb (61% of the Thomas-Reiche-Kuhn sum-rule value). The $^{14}\mathrm{C}$ photoneutron cross sections are interpreted in terms of the reaction kinematics and the competition among the particle channels in order to provide information on the isospin properties of the major E1 states in $^{14}\mathrm{C}$ and on the validity of the weak core-coupling model.

Experimental photoneutron cross sections of natural zirconium from 8 MeV to 134 MeV

The inclusive partial photoneutron cross sections and the total photoneutron cross section\(\sigma ^{(1)} (E_\gamma ) = \sum\limits_{i = 1}^{i_{\max } } {\sigma (\gamma ,in...)} \) for natural Zr have been measured from 8 MeV to 134 MeV, with monochromatic photons obtained by in flight annihilation of monoenergetic positrons. The integrated total photoneutron cross section up tillEγ=140 MeV is (1.59±0.11)σ0, whereσ0 is the Thomas-Reiche-Kuhn sum-rule. The integrated total photonuclear absorption cross section is evaluated to be (1.73±0.15)σ0. The behaviour of the total photoneutron cross section as a function of Eγ, in the 40 MeV <Eγ<140 MeV energy range, is well described by a modified version of the quasideuteron model.

Photoneutron cross sections for oxygen from 24–133 MeV

The inclusive partial photoneutron cross section σ(γ, ln…) and σ(γ, 2n…) and the total photoneutron cross section σ (1)( E γ ) = ∑ i = 1 σ( γ, in…) for 16O have been measured with mono-energetic photons in the energy region above the giant resonance and below the meson thresholds. Comparison with other data shows the dominance of the (γ, pn) process in this energy region and thus underscores the importance of the mechanism of photon absorption by a correlated proton-neutron pair in the nucleus; thus, satisfactory agreement of modern versions of the quasideuteron theory with the data has been obtained. The measured integrated total photoneutron cross section up to 140 MeV is 0.67 ± 0.07 TRK sum-rule units.

Fine structure in thePb208photoneutron cross section between 9.9 and 11.2 MeV

The partial differential photoneutron cross sections for $^{208}\mathrm{Pb}$ were measured in the energy region from 9.9 to 11.2 MeV with an energy resolution of about 100 keV. Neutrons leaving $^{207}\mathrm{Pb}$ in its ground state and first two excited states were resolved, and angular distributions were determined from measurements at seven angles from 45\ifmmode^\circ\else\textdegree\fi{} to 135\ifmmode^\circ\else\textdegree\fi{}. These data provide more information about the fine structure observed in earlier, poorer resolution measurements of the total photoneutron cross section and strongly support the interpretation of this structure as being due to electric dipole excitations. The measured partial cross sections provide a sensitive test for models attempting to describe the photon interaction cross section and the coupling of the excited states to the continuum. Calculations using a statistical model with transmission coefficients obtained from a conventional optical model are unable to explain even the average behavior of the branching ratios with energy. The measured angular distributions show clear evidence for interference of either $E2$ or $M1$ processes with the predominant $E1$ interaction. The interference terms have a magnitude approximately equal to that predicted by a simple direct-semidirect calculation including the electric dipole and isoscalar electric quadrupole giant resonance. However, this agreement is puzzling because the average of interferences between the many compound nuclear levels in this region of excitation would be expected to reduce anisotropies about 90\ifmmode^\circ\else\textdegree\fi{} toward zero.NUCLEAR REACTIONS $^{208}\mathrm{Pb}$($\ensuremath{\gamma}$,$n$), $E=9.9\ensuremath{-}11.2$ MeV; tagged photon resolution 120 keV; measured $\ensuremath{\sigma}({E}_{n},\ensuremath{\theta})$, $\ensuremath{\theta}=45\ifmmode^\circ\else\textdegree\fi{}\ensuremath{-}135\ifmmode^\circ\else\textdegree\fi{}$; observed interference of either $E2$ or $M1$ with $E1$.

Photoneutron cross sections forO17

Photoneutron cross sections involving the emission of one and two neutrons from $^{17}\mathrm{O}$ have been measured over the energy interval 8.5 to 39.7 MeV using monoenergetic photons from positron in-flight annihilation. The 6-MeV wide giant dipole resonance is observed to be centered at 23 MeV and a pygmy resonance is seen at about 13 MeV. Such structure as is apparent in the cross sections is not as pronounced as for the cases of $^{16}\mathrm{O}$ and $^{18}\mathrm{O}$. Comparison of the total photoneutron cross section with recent ground-state data indicates that much of the pygmy resonance decays to the ground or first excited state of $^{16}\mathrm{O}$, but that the giant dipole resonance decays mainly to highly excited states in the daughter. Excellent agreement is observed between the present results and a recent two-particle, one-hole shell-model calculation of the isospin-split giant dipole resonance states for this nucleus. New photoneutron cross-section results for $^{16}\mathrm{O}$ up to 39.7 MeV are reported as well.NUCLEAR REACTIONS $^{17}\mathrm{O}(\ensuremath{\gamma},n)$, ${E}_{\ensuremath{\gamma}}=8.5\ensuremath{-}39.7$ MeV and $^{16}\mathrm{O}(\ensuremath{\gamma},n)$, ${E}_{\ensuremath{\gamma}}=15.9\ensuremath{-}39.7$ MeV; measured $4\ensuremath{\pi}$ neutron yield for monenergetic photons; $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},1n)$, $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},2n)$, integrated cross sections, isospin splitting of the giant resonance for $^{17}\mathrm{O}$.

Fine structure in the208Pb(?,n) cross section

The total photoneutron cross section of208Pb was measured between 8 and 13 MeV using the bremsstrahlung photon facility from a 35 MeV linac. Considerable resonance structure was observed in the cross section, of which the peak around 9 MeV, as well as the structure around 10.8 MeV may be due toE2 excitations.

Alpha particles from the photodisintegration of 9Be in the photon energy region 18 to 26 MeV

Alpha particles from the 9Be(γ,n)8Be(16.6) and 9Be(γ,α0)5He reactions were studied in the photon energy region 18 to 26 MeV; the results yielded a combined integrated cross section of 13.1 ± 2 MeV mb and an upper limit on the integrated (γ,α0) cross section of 4.0 MeV mb. This agrees within error with the integrated cross section of Becchi, Meneghetti, Sanzone, and Vitale, 10 ± 2 MeV mb, which would contain about 50% of any contribution from the (γ,α0) reaction. These reactions together with the 9Be(γ,n1)8Be reaction (which has an integrated cross section of 2.4 ± 0.4 MeV mb) are the major reaction channels contributing to the total photoneutron cross section in this energy region. Their sum, 15.5 MeV mb, agrees well with the results of Nathans and Halpern and Hughes, Sambell, Muirhead, and Spicer but disagrees with that of Costa, Pasqualini, Piragino, and Roasio.

Angular distributions and cross sections of the photoneutron reaction in 9Be in the photon energy region 18–26 MeV

The 9Be(γ,n0)8Be and 9Be(γ,n1)8Be reactions were studied in the photon energy region 18 to 26 MeV. The integrated cross section of the (γ,n0) reaction was found to be less than 0.1 MeV mb in this energy region. The integrated cross section for the (γ,n1) reaction in the energy region is 2.4 ± 0.4 MeV mb, which is 10 to 15% of the total photoneutron cross section. The measured angular distributions for the (γ,n1) reaction are consistent with the population of 3/2+ and 5/2+ states of 9Be. These results are consistent with slightly re-interpreted predictions of Majling, Kukulin, and Smirnov.

The Chromium Photoneutron Cross Section

A high resolution measurement of the total photoneutron cross section of chromium from threshold to 27 MeV has been obtained. The result is compared with an earlier measurement and an interpretation in terms of isospin effects is made.

Photoneutron cross sections for 18O

Absolute ( γ, xn) cross sections for 18O were measured up to 33 MeV, using a quasimonoenergetic photon beam from positron annihilation in flight. The cross sections show pronounced structure in particular at energies below the main giant dipole resonance. The (γ, 2n) cross section is large in the whole energy region. The integrated total photoneutron cross section amounts to 0.53 in units of the classcial dipole sum. No pronounced isospin splitting could be observed in the photoneutron decay channel.