Photoneutron Yield Research Articles

Photoneutron cross section forO16

The photoneutron cross section for $^{16}\mathrm{O}$ has been measured with monoenergetic photons from 17 to 33 MeV. This measurement was carried out by subtracting the photoneutron yield for Si from that for Si${\mathrm{O}}_{2}$, thus reducing the uncertainty in the neutron detection efficiency associated with the neutron moderation by the hydrogen in a water sample. The present cross-section results agree with the mean of several previous measurements with monoenergetic photons, and resolve a prior discrepancy at the 22.1-MeV peak. The measured average photoneutron energy is quite large in the photon energy region from 26 to 28 MeV, signifying that ground-state transitions dominate the ($\ensuremath{\gamma}, n$) cross section there.NUCLEAR REACTIONS $^{16}\mathrm{O}(\ensuremath{\gamma}, n)$, measured cross section and average photoneutron energy up to 33 MeV, monoenergetic photons.

γ(d,n)preaction with polarized and unpolarized gamma rays

The asymmetry function for the $\ensuremath{\gamma}(d,n)p$ reaction is calculated for the Paris and supersoft core potentials for gamma-ray energies, ${E}_{\ensuremath{\gamma}}$, extending from 20 to 150 MeV, at center of mass angles 45\ifmmode^\circ\else\textdegree\fi{}, 90\ifmmode^\circ\else\textdegree\fi{}, and 135\ifmmode^\circ\else\textdegree\fi{}. The relative photoneutron yield at ${E}_{\ensuremath{\gamma}}=20$ MeV is also calculated. In addition, the differential cross section, total cross section, and polarization of the outgoing protons are reported. The Paris potential gives good agreement with the experimental data for the differential and total cross sections. However, the discrepancy at 0\ifmmode^\circ\else\textdegree\fi{} still remains. It is found that the asymmetry function cannot be used to distinguish between the various nucleon-nucleon potentials.NUCLEAR REACTIONS $\ensuremath{\gamma}(d,n)p$. Calculated asymmetry function; total and differential cross sections.

Improved calculation of photoneutron yields released by incident electrons.

Improved calculation of photoneutron yields released by incident electrons.

Absolute measurements of 235U and 239Pu fission cross-sections with photoneutron sources

the fission cross-sections of 235U and 239Pu for NaBe, LaBe, NaD and GaD photoneutrons have been measured absolutely. The neutron source strength was measured using a manganese bath to compare the photoneutron yield from the sources with the standard source NBS-II. Fission counts were accumulated with the source positioned symmetrically between two identical fission foils in an experiment package suspended in a low-albedo laboratory. Fission fragments passing through limited solid angle apertures were recorded on polyester track-etch films. The masses of the foil deposits were determined by microbalance weighings and confirmed by thermal neutron fission and alpha counting. After making a correction for the calculated energy distribution of the source neutrons, values of 1.471 ± 0.029, 1.274 ± 0.026, 1.162 ± 0.025 and 1.195 ± 0.026 barns were obtained for the 235U fission cross-section at the source median energies of 140, 265, 770 and 964 keV, respectively. Corresponding values of 1.469 ± 0.045, 1.515 ± 0.038, 1.670 ± 0.039 and 1.643 ± 0.038 barns were determined for 239Pu.

An Estimate of the Yield of Photoneutrons from Beryllium Using a LINAC as the X-Ray Source

A procedure is described for calculating approximate photoneutron yields from thick beryllium targets exposed to the x radiation produced in thick tungsten targets bombarded with 2- ..-->.. 8-MeV electrons. It is based on empirical relations developed for predicting the average thick tungsten target radiation yield and the average beryllium photoneutron cross section for the selected incident electron energy.

Analog Monte Carlo studies of electron−photon cascades and the resultant production and transport of photoneutrons in finite three−dimensional systems

A major computer program has been written to perform Monte Carlo studies of electron−photon cascades and the resultant production and transport of photoneutrons in finite three−dimensional systems. It is programmed in FORTRAN and is explicitly designed to be easily used and extended to yield a fast, adaptable, and versatile computer program that has wide application to problems in solid−state, nuclear, and reactor physics. It combines a simple but accurate procedure for sampling from an arbitrary differential probability distribution with efficient cross−section representations to perform complicated single−particle of many−particle calculations. The calculations of Alsmiller and Moran at ORNL and the experiments of Barber and George at the Stanford High Energy Physics Laboratory have been compared with results of the new computer code. The comparisons indicate that the results of the new Monte Carlo studies are in good agreement with the ORNL calculations of the photoneutron yield in cylindrical targets of Cu, Ta, Pb, and U for a wide variety and 100 MeV. The comparisons also yield good agreement with the Stanford experiments of the photoneutron yield in rectangular targets of Cu, Ta, experiments of the photoneutron yield in rectangular targets of Cu, Ta, experiments of the photoneutron yield in rectangular targets of Cu, Ta, Pb, and U for a wide variety of target thicknesses for incident electron energies ranging from 10 to 34 MeV. A series of Rensselaer Polytechnic Institute LINAC ’’bare’’ target experiments have been analyzed with the Monte Carlo program. The computational studies and the experimental results are in good agreement. In particular, there is excellent agreement on the angular distributions of photoneutrons from the tree spherical Pb targets as measured using foil detectors. the calculated results predicted quite faithfully the observed improvement in the angular distributions as the design of the spherical Pb target was modified to obtain a more isotropic distribution.

Photoneutrons from 13C at photon energies up to 14 MeV

The photoneutron yield of 13C was measured as a function of the maximum bremsstrahlung energy up to 14.5 MeV. The cross section for the 13C(γ, n) 12C reaction was obtained by using the inverted matrix method of Penfold and Leiss. The integrated cross sections from threshold to 10 MeV and to 14 MeV are 2 and 16 MeV ·mb, respectively. These values are in agreement with earlier experiments. The cross-section curve shows peaks at 6.2, 7.5, 8.2, 9.1, 11 and 13 MeV, the last two being in the pygmy resonance energy region. The angular distributions of photoneutrons were measured at different values of E γmax . The data were analyzed for the distribution of neutrons from each peak. The significant features of the cross-section curve and the angular distributions of neutrons from the peaks are discussed and compared with the theoretical ones reported by several authors. The integrated cross section from the threshold to 10 MeV is much smaller than those calculated on the basis of a pure single-particle model. The neutrons in the pygmy resonance energy region follow a 1+0.46 sin 2θ angular distribution, while a 1+1.5 sin 2θ distribution is expected from the single-particle picture of the pygmy resonance. These discrepancies are interpreted in terms of the mixing of core excitations and single-particle excitations.

The photoneutron cross section of 141Pr

The photoneutron yield from 141Pr has been measured in 50 keV steps from threshold to 17 MeV by counting the annihilation γ-rays following position decay of 140Pr. Leiss-Penfold analysis of the yield curves was used to derive the cross section. The results are compared with other recent measurements of the 141Pr( γ, n) 140Pr cross section, and the results of a collective-model calculation which shows good agreement with the cross section obtained in this experiment are presented.

Measurement of the photoneutron yield from thick copper and water targets and determination of the excitation function of the (?, n) reaction for O16 and Cu63 by means of the Belen'kii-Tamm equilibrium photon spectrum

Measurement of the photoneutron yield from thick copper and water targets and determination of the excitation function of the (?, n) reaction for O16 and Cu63 by means of the Belen'kii-Tamm equilibrium photon spectrum

Photodisintegration of Ca 40

The photoneutron yield from calcium has been determined at 50 keV intervals from threshold to 20 MeV, and in 200 keV intervals up to 30 MeV. Positon activities were counted between betatron beam bursts. Analysis by the Penfold-Leiss method gives a cross section curve with some quantity of resolved structure. This structure is not readily describable in detail using current particle-hole interaction theories of the photodisintegration process in calcium.

The emission of photoneutrons from natural lithium

The photoneutron yield curve of natural lithium has been determined for incident bremsstrahlung energies up to 18.5 MeV. This yield curve shows no gross structure in the region of 17 MeV as indicated in some previous measurements. The resulting cross section indicates the presence of three high energy levels in 7Li at 13.6 MeV, 15.3 MeV and 16.5 MeV. Additional measurements using a neutron detection system with a response deliberately designed to be sensitive to slow neutrons were made simultaneously with the above yield curve determination. This “slow neutron” yield curve suggests that the discrepancies amongst the earlier results may be due to the different responses of the various neutron detection systems used. An outline of a possible test to verify this suggestion is given.

Photoneutron yield and shielding problems of high energy cyclic electron accelerators

The equilibrium photon spectrum for various materials is used to calculate the photoneutron yield from the “giant resonance” region in the absorption of >30 Mev electrons. A scheme is proposed for neutron shielding design calculations applicable to cyclic electron accelerators.

Photoneutron Cross-Section Measurements on Gold Using nearly Monochromatic Photons

Continuously variable monochromatic gamma rays with an energy spread of 3% have been produced using the annihilation in flight of positrons accelerated in a linear electron accelerator. These gamma rays have been employed to measure the ($\ensuremath{\gamma}, n$) and ($\ensuremath{\gamma}, 2n$) cross sections for gold. A neutron counting method was used to determine the neutron multiplicity. The photoneutron yield for Au has a maximum of 535\ifmmode\pm\else\textpm\fi{}37 mb at 13.7\ifmmode\pm\else\textpm\fi{}0.3 MeV. The ($\ensuremath{\gamma}, 2n$) cross section was observed to have a threshold at 14.5\ifmmode\pm\else\textpm\fi{}0.5 MeV and a maximum of approximately 100 mb at 18 MeV. The integrated ($\ensuremath{\gamma}, n$) and ($\ensuremath{\gamma}, 2n$) cross sections, up to 25 MeV, were found to be 2.14\ifmmode\pm\else\textpm\fi{}0.15 and 0.83\ifmmode\pm\else\textpm\fi{}0.16 MeV-b, respectively. The value of $\ensuremath{\int}{0}^{\ensuremath{\infty}}\ensuremath{\sigma}{E}^{\ensuremath{-}2}\mathrm{dE}$ was determined to be 15.3\ifmmode\pm\else\textpm\fi{}1.5 mb/MeV. The level density parameter $a$ was found to be 17.1\ifmmode\pm\else\textpm\fi{}0.5 Me${\mathrm{V}}^{\ensuremath{-}1}$. The total cross section $\ensuremath{\sigma}(\ensuremath{\gamma}, n)+\ensuremath{\sigma}(\ensuremath{\gamma}, 2n)+\ensuremath{\sigma}(\ensuremath{\gamma}, pn)$ can be fitted with a single Lorentz curve having a peak cross section of 535 mb at 13.90 MeV and a width of 4.2 MeV.

The nuclear photoeffect in holmium and erbium

The photoneutron yield and photon scattering cross sections have been measured in the giant resonance region for holmium and erbium. Both cross sections were found to be the same within the experimental uncertainties for the two target materials. The absorption cross section integrated to 23 MeV is 3.20±0.48 MeV·b. The intrinsic nuclear quadrupole moment determined from the giant resonance splitting is 7.6±1.1 b. The observed scattering cross sections were large compared to that calculated from the observed absorption cross section by means of the optical theorem and the dispersion relation. The additional contribution to the scattering cross sections results from the optical anisotropy of these nuclei and can be predicted quantitatively from the expression for the scattering by a three-dimensional classical oscillator. A justification for the classical formulation is given. The data also suggest that in the energy region between 10 and 23 MeV at least 80% of the integrated absorption cross section is associated with charge oscillations along the three spatially correlated nuclear axes.

Some remarks concerning the determination of the photoneutron yield of thick specimens

Some remarks concerning the determination of the photoneutron yield of thick specimens

Photoneutron Cross Sections of Cobalt and Manganese

The total photoneutron yields of ${\mathrm{Mn}}^{55}$ and ${\mathrm{Co}}^{59}$ have been measured from threshold to approximately 30 Mev. Analysis of these data using the Leiss-Penfold matrix indicates that the cross sections for both elements show a splitting in the giant resonance region in accord with the predictions of the classical hydrodynamic model. The ${\mathrm{Mn}}^{55}$ peaks occur at energies of 16.8\ifmmode\pm\else\textpm\fi{}0.25 Mev and 19.75\ifmmode\pm\else\textpm\fi{}0.25 Mev corresponding to cross sections of 90 mb and 77 mb, respectively. ${\mathrm{Co}}^{59}$ maxima occur at 16.75\ifmmode\pm\else\textpm\fi{}0.25 Mev and 18.75\ifmmode\pm\else\textpm\fi{}0.25 Mev with cross sections of 109 mb and 92 mb. The cross sections $\ensuremath{\sigma}(\ensuremath{\gamma}, n)+\ensuremath{\sigma}(\ensuremath{\gamma}, 2n)+\ensuremath{\sigma}(\ensuremath{\gamma}, np)+\ensuremath{\cdots}$ integrated to 25 Mev are 627 Mev-mb for ${\mathrm{Mn}}^{55}$ and 709 Mev-mb for ${\mathrm{Co}}^{59}$. Breit-Wigner resonance lines were fitted to both cross sections and the intrinsic quadrupole moments determined from these fits are +0.78\ifmmode\pm\else\textpm\fi{}0.10 barn for manganese and +0.76\ifmmode\pm\else\textpm\fi{}0.11 barn for cobalt.

Photodisintegration ofLi6

The photodisintegration of ${\mathrm{Li}}^{6}$ by bremsstrahlung radiation of 17.3-Mev peak energy was investigated by use of coincidence techniques to detect neutron-proton coincidences and by measurement of the photo-proton energy spectra. The ${\mathrm{Li}}^{6}(\ensuremath{\gamma}, n){\mathrm{Li}}^{5}$ reaction is found to be responsible for 0.58\ifmmode\pm\else\textpm\fi{}0.13 of the photoneutron yield measured by Romanowski while the ${\mathrm{Li}}^{6}(\ensuremath{\gamma}, p){\mathrm{He}}^{5}$ reaction contributes 0.31\ifmmode\pm\else\textpm\fi{}0.04 of the yield. No angularly correlated neutron-proton coincidences were detected which would support a deuteron-alpha-particle model for the ${\mathrm{Li}}^{6}(\ensuremath{\gamma}, np){\mathrm{He}}^{4}$ reaction; however, the possible existence of this reaction is not eliminated.

Photoneutrons from Al.

The energy spectra of photoneutrons emitted at 90° from Al under bremsstrahlung of 24 and 30 MeV maximum energy were investigated by means of the recoil protons in photoemulsions. The difference between the two spectra shows that above 24 MeV photon energy the neutrons are emitted mainly by a direct process. This process gives a relevant contribution (>25%) to the photoneutron yield, at 30 MeV.

An accurate photo-neutron standard

Abstract An easily reproducible, RdTh-D2O, photo-neutron standard has been built. Its calibration is based on the associated charged particle method. We have determined its photoneutron yield to an accuracy of 1.1%. After calibration, the ionisation current of the RdTh source was compared to the ionisation current of a standard radium source, on a Curie chamber. The yield of any substandard can be determined, apart from small geometrical corrections, by a simple comparison of the RdTh source to the radium standard on the Curie chamber. Background neutrons from (α-n) reactions in the RdTh source contribute to 3% of the total yield. The number of neutrons emitted by the source used has been determined by the recoil proton method, using an ionisation chamber and by chemical integration in a manganese nitrate solution. The total yield of the standard is known to an accuracy of 1.25%.

Photoneutron Yields in the Rare-Earth Region

The total photoneutron yield curves for Sn, I, La, Ce, Sm, Tb, Ho, Er, Yb, Ta, Au, and Pb have been measured for x-ray energies from 7 to 40 Mev with about one percent statistical uncertainty. The Penfold-Leiss matrix was used to convert these yield curves to integrated neutron yield cross sections directly without smoothing the original activation curves. The cross sections derived from the integral curves were corrected for multiple neutron emission above the ($\ensuremath{\gamma}, 2n$) threshold using the statistical model. The widths found for the giant resonances for the closed-shell nuclei decreased from 5 Mev to 3.8 Mev in going from Sn to Pb. The widths for the elements having large nuclear deformations for most of their isotopes were considerably broader. These widths decreased slowly from 8.6 Mev for Sm to 6 Mev for Ta. These widths are consistent with the broadening of the giant resonance to be expected on the Danos model if values of the intrinsic quadrupole moment are taken from Coulomb excitation data. The neutron yield cross sections corrected for multiple neutron emission were integrated to 22 Mev. Using these integrals and defining $f$ by $\ensuremath{\int}\ensuremath{\sigma}\mathrm{dE}=0.06(\frac{\mathrm{NZ}}{A})f$, these data gave an average value of $f=1.34\ifmmode\pm\else\textpm\fi{}0.21$.