Photoneutron Spectra Research Articles

Photoneutron Angular Distributions for 4He

Differential cross sections for the reaction 4He(γ, n)3He have been measured at laboratory angles of 53°, 68°, 83°, 98°, 128°, and 143° for excitation energies between 22 and 32 MeV by the photoneutron time of flight technique. The photoneutron spectra were used to obtain the angular distribution coefficients in the Legendre series expansion for the differential photoneutron cross sections. Analysis of these coefficients indicates that the photodisintegration process in this nucleus, while dominated by E1 photon absorption followed by channel spin zero (no spin flip) p wave neutron emission, is affected by E1 channel spin one and E2 contributions. In the higher energy region (E > 27 MeV), the amount of E2 absorption observed was ~6% of the total and is dominated by the channel spin one E2 matrix element. The total cross section obtained from the measured differential cross sections yields a value of σ(γ, p)/σ(γ, n) near unity, in agreement with a calculation of this ratio which requires no charge symmetry breaking component of the nuclear force.

Background (γ,n) cross section inPb208

A series of experimental estimates of the nonresonant radiative neutron reaction cross section in $^{208}\mathrm{Pb}$ have been reported recently. To date, the results from ($\ensuremath{\gamma},n$) studies have been inconsistent with measurements of neutron-induced reactions. In an effort to resolve this discrepancy, the shape of the 41-keV resonance in the reaction $^{208}\mathrm{Pb}(\ensuremath{\gamma},n)^{207}\mathrm{Pb}$ has been studied in detail by simultaneous measurements of the photoneutron spectra at 90 and 135\ifmmode^\circ\else\textdegree\fi{}. A small asymmetry in the resonance shape implies the presence of a background cross section of ${1.3}_{\ensuremath{-}0.7}^{+2.7}$ mb. This value can be explained in terms of contributions from neighboring resonances plus a small direct-reaction component; there is no evidence for anomalous processes. The result is consistent with the most recent data from study of the reaction $^{207}\mathrm{Pb}(n,{\ensuremath{\gamma}}_{0})$.[NUCLEAR REACTIONS $^{208}\mathrm{Pb}(\ensuremath{\gamma},n)$, bremsstrahlung end points 7.9, 8.4, and 9.0 MeV; studied shape 41-keV resonance, deduced background $\ensuremath{\sigma}(\ensuremath{\gamma},n)={1.3}_{\ensuremath{-}0.7}^{+2.7}$ mb.]

Valence component in the threshold photoneutron spectrum ofZr91

Ground-state radiation widths (${\ensuremath{\Gamma}}_{\ensuremath{\gamma}0}$) for 36 ${p}_{\frac{3}{2}}$ photoneutron resonances in $^{91}\mathrm{Zr}$ have been measured from 5-225 keV above threshold at the Argonne National Laboratory threshold photoneutron facility. Reduced neutron widths (${{\ensuremath{\gamma}}_{n}}^{2}$) for the same resonances were obtained at the Oak Ridge Electron Linear Accelerator facility from total-cross-section measurements on $^{90}\mathrm{Zr}$. A strong correlation ($\ensuremath{\rho}=+0.59$) is observed between the two sets of widths. Because of a lack of knowledge of the relative phase of the compound-nucleus and valence amplitudes in the individual resonances, the average valence component cannot be deduced from the correlation analysis. However, one can obtain a mean compound-nucleus contribution by studying those resonances where the valence amplitude is expected to be negligible. By using such a procedure, an average valence component is obtained which is in excellent agreement with the average valence strength calculated from the valence model. The $s$-wave neutron strength function is ${S}^{0}=(6.2\ifmmode\pm\else\textpm\fi{}1.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ and the $p$-wave strength function is ${S}^{1}$ = (3.2 \ifmmode\pm\else\textpm\fi{} 0.8) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$. The reduced photon width for $E1$ transitions is $\overline{k}(E1)=(3.2\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, in agreement with the single-particle reduced width derived from the Weisskopf model. The neutron widths are observed to follow a Porter-Thomas distribution, while the photon widths appear to follow a ${\ensuremath{\chi}}^{2}$ distribution with two degrees of freedom.[NUCLEAR REACTIONS $^{91}\mathrm{Zr}(\ensuremath{\gamma},n)$, ${E}_{\ensuremath{\gamma}}=9.0$ MeV; deduced ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}0}$ of resonances. $^{90}\mathrm{Zr}$($n$, total), ${E}_{n}=5\ensuremath{-}225$ keV; measured $\ensuremath{\sigma}(E)$, deduced ${{\ensuremath{\gamma}}_{n}}^{2}$ of resonances. Determined correlation, calculated valence contribution to ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}0}$; calculated strength functions.]

A method for reducing bremsstrahlung-produced photoneutron spectra to partial (γ, n) cross sections

A new procedure is presented for reducing a series of bremsstrahlung-produced photoneutron spectra to partial (γ, n) cross sections to various possible states of the residual nucleus. The essentially new features of the method consist of the minor one of using linear interpolation to bridge gaps across regions where the cross sections cannot be calculated uniquely from the data, and the major one of requiring that the final computed partial cross section curves exhaust the entire yield spectra, not just the parts near the “tip” region which are usually used. The initially computed (and interpolated) “exact” cross sections are used to establish branching ratios to the various final states, which are held fixed in a fitting or normalization procedure. The method has the advantage that it permits reasonably good estimates of cross sections without requiring the large amount of data-taking the more conventional analysis would require, and it makes full use of the whole extent of the spectrum curves measured. The method is illustrated, and tested to a limited degree, by assuming cross sections for a fictitious nucleus Hy A . Suggestions for improvements of the method are also presented. The general procedure may be useful in analysis of data from experiments other than photoneutron reactions, especially those where the effective resolution is as unfavorable as it is where the bremsstrahlung photon spectrum is the controlling factor.

Photoproduction of high-energy neutrons in thick targets by electrons in the energy range 150 to 270 MeV

Photoneutron spectra with energies greater than 12 MeV produced by electrons incident on a thick lead target have been measured for primary electron energies between 150 and 266 MeV and at a fixed angle of 90 ° to the beam axis. Measurements of the neutron yield have furthermore been performed at a primary energy of 234 MeV as a function of target depth for the same lead target and as a function of the mass number for C, Al, Cu, Cd and Pb targets. The results were obtained with three independent neutron detectors: two proton recoil counters and one time-of-flight set-up. The high-energy regions of the spectra are compared with the predictions of the phenomenological quasi-deuteron model and a satisfactory agreement in shape and scale of the spectra is found. Additional high-energy neutrons from pion reabsorption processes were observed at electron energies of 234 and 266 MeV.

Excited States ofAr40Observed in the ReactionAr40(γ, n0)Ar39

We identify 27 excited states of $^{40}\mathrm{Ar}$ in the energy range 10.4-12.5 MeV from the photoneutron spectrum of $^{40}\mathrm{Ar}$ irradiated with 12.5-MeV bremsstrahlung. These levels are probably associated with the promotion of one of the $1{f}_{\frac{7}{2}}$ valence neutrons into the continuum. The distribution of radiation strength in this region suggests the presence of a single-particle resonance in $^{40}\mathrm{Ar}$ at \ensuremath{\sim} 10 MeV.

High-energy photoneutrons from 12C

Photoneutron spectra from the reaction 12C(γ, n) 11C were measured at bremsstrahlung end-tip energies from 64 to 128 MeV in 2 MeV steps. The spectra were obtained simultaneously at five angles (30°, 60°, 90°, 112° and 142°) using time-of-flight techniques. The measured cross sections for a fixed neutron energy exhibit kinematic resonances corresponding to the emission of a single neutron from the 1p shell in 12C. The observed angular distributions of these 1p shell neutrons are forward asymmetric, the asymmetry increasing with neutron energy. The results are compared with single-particle and cluster theories. It was found that the angular distributions and the differential cross section at 90° can best be explained by a quasi-alpha model. It is therefore concluded that dynamic correlations between nucleons in the same shell are more important than those between nucleons in different shells.

Evidence for isospin splitting of the giant dipole state in 26Mg

Direct evidence for T-splitting of the dipole state in 26Mg has been obtained by studying the energy spectra of photoneutrons from the two components. The states centered around 17 MeV decay strongly to the ground or low-lying states of 25Mg ( T = 1 2 ) whereas those around 22 MeV decay only to the known T = 3 2 states in 25Mg.

Ground state photoneutron angular distributions in the giant resonance of 16O

Photoneutron spectra for transitions to the ground state in the giant resonance of 16O were measured with high neutron energy resolution (ΔE ~ 60 keV) at angles of 50, 66, 82, 98, 114, 130, and 146 degrees relative to the direction of the incident bremsstrahlung beam. The spectra at the various angles were measured during successive experimental runs using a single 50 m time-of-flight spectrometer. The different angles were obtained by bending the electron beam relative to the direction of the flight path. Angular normalization was achieved by using a fast neutron monitor located at the end of a short flight path which was always normal to the reaction plane. Legendre coefficients for the angular distributions were extracted as functions of the excitation energy. From the energy dependence of the coefficients, interference effects are evident in the ground state (γ,n) cross section in the region from 17 to 30 MeV.

The energy distribution of antimonyberyllium photoneutrons

The proton-recoil proportional counting method with gamma-neutron discrimination has been used to measure the neutron spectrum of an antimony-beryllium source located in a lead gamma-shielding. Since the variations in the neutron spectrum caused by the lead and the source material are small, the photoneutron spectrum could be determined with good accuracy by a Monte Carlo neutron transport code. It turned out that the distribution of photoneutrons from the primary group has a width of 3·5 ± 1·5 keV and an average energy of 26·0 ± 1·3 keV. In addition to this primary neutron group a second neutron group at 363 ± 15 keV and with an intensity of 5·1 ± 1 per cent as compared with the primary group could be established. The neutron spectra for a series of representative source geometries are calculated and compared with previously published results.

Fine structure in the photoneutron spectra from praseodymium-141 and lead

Careful measurements of the energy spectrum of photoneutrons ejected from praseodymium and from natural lead show that there are peaks on top of the expected smooth spectrum. Comparison with the recent cross-section studies, which indicate structure on the giant dipole resonance for these nuclei, shows agreement between the energies of the peaks found in the present work and the energies En = E−Qi where E corresponds to a peak in the cross-section curve and Qi is the threshold for producing a given state in the daughter nucleus. Only ground state transitions (for Pr) or ground state and the first two excited state transitions (for Pb) need be invoked.

Investigation of high-energy photoneutrons from carbon

The spectra of photoneutrons produced by 103 MeV bremsstrahlung bombardment of 12C have been measured at lab angles of 30°, 60°, 90°, 112° and 142° using time-of-flight techniques. The angular distributions in the giant resinance region are compared with the predictions of 1p-1h theory. The angular distributions above the giant resonance region exhibit a forward anisotropy which increases with increasing neutron energy. The results are compared with single-particle and cluster theories. It is found that the angular distribution may be explained best using a quasi-alpha-particle theory.

On the photodisintegration of 51V, Ni, 55Mn and 59Co

The energy distribution and polarization of photoneutrons from 51V, natural nickel, 55Mn and 59Co have been measured. The targets were irradiated with the γ-ray beam ( E γ max = 85 MeV) of the Turin synchrotron, and the photoneutrons were detected with a diffusion cloud chamber filled with helium. The usual interpretation of the photoneutron spectra in terms of evaporation with a small contribution of direct reaction is not in agreement with the experimental results for neutron energies lower than 1.5 MeV. Qualitative discussion of other models is given to explain the experimental indication of photoneutron polarization found for 55Mn and 59Co.

Fast-Photoneutron Spectra due to 55-85-MeV Photons

Energy spectra of fast photoneutrons from eighteen elements (Li, Be, B, C, N, O, Al, S, Fe, Cu, Zn, In, Sn, Ta, Tl, Pb, Bi, and U) were measured at 67\textonehalf{}\ifmmode^\circ\else\textdegree\fi{}, using thin-target bremsstrahlung from the Rensselaer linac. The neutron detection system utilized a 100-meter flight path and had an over-all time resolution of 0.4 nsec/m. Measurements of the spectra from each target at bremsstrahlung energies of both 55 and 85 MeV permitted extraction of neutron difference spectra effectively due to photons between 55 and 85 MeV. Absolute normalization of the spectra was effected by comparison with the experimentally measured photoneutron spectrum of deuterium. Difference spectra were obtained for neutron energies greater than 10 MeV. Cross sections for production of these neutrons ranged from 59 \ensuremath{\mu}b/sr for lithium to 1.28 mb/sr for uranium. The experimental data were compared to the predictions of the quasideuteron model as recently modified by Levinger, with secondary-neutron effects estimated by Monte Carlo calculations. Good agreement was obtained for the absolute-neutron-production cross sections and the shapes of the neutron energy spectra over the range of targets studied.

Time-of-Flight Measurement of Photoneutrons from O16, Ca40, and P31

Energy spectra of photoneutrons from O 16 , Ca 40 and P 31 were measured by the time-of-flight method, using the JAERI (Japan Atomic Energy Research Institute) 20-Mev Electron Linac and its annexed 50-meter neutron flight path. The flight time of the emitted neutrons was measured with a fast time-of-flight system. The energy range of measurement covered almost the whole region of the giant resonance for each sample. Many fine structure peaks in the energy spectra were observed. The results were compared with the photoreaction and inverse photoreaction cross sections measured by several other authors. The agreement was satisfactory. Theoretical calculations of the dipole states in O 16 and Ca 40 by Elliott and Flowers, and Brown et al. , explain the gross features of the observed spectra but not the details.

Neutron Spectra from Monoenergetic Photons on Bismuth

Spectra of photoneutrons from a ${\mathrm{Bi}}^{209}$ target were obtained in the electric dipole giant-resonance region (12-16 MeV). A $\ensuremath{\gamma}$-ray energy resolution of 1.5% was attained using a bremsstrahlung monochromator; the neutron energies were measured with a time-of-flight technique with a resolution which varied from 10% at 2 MeV to 15% at 6 MeV. About 88% of the neutrons ejected by 14-MeV $\ensuremath{\gamma}$ rays are statistical. The inferred level density of ${\mathrm{Bi}}^{208}$ near its ground state is not a simple function of excitation energy, but from 4 to 7 MeV it increases exponentially with energy; in that energy range the temperature is about 1 MeV. The nonstatistical high-energy neutrons leave the ${\mathrm{Bi}}^{208}$ nucleus at the excitation energies at which the states are well represented as single-neutron holes in ${\mathrm{Bi}}^{209}$. These high-energy neutrons are intimately connected with the giant resonance. If the giant resonance is dominated by a single dipole state, the absolute width which characterizes the decay of the dipole state into the compound nucleus must increase by a factor of about 3 between 12 and 16 MeV.

Photoneutrons from 9Be

A measurement of the photoneutron spectrum from 9Be was made using a knock-on proton spectrometer consisting of a stilbene crystal, and incorporating pulse shape discrimination. The resulting spectrum is interpreted in terms of states in 9Be and confirms the existence of several doubtful levels and postulates the presence of at least one new level.

Photoneutron spectra and cross sections for 12C and 16O

Partial (γ, n) cross sections have been measured for two self-conjugate nuclei 12C and 16O by measuring the spectral intensity of emitted neutrons in conjunction with a determination of the time-integrated absolute photon flux. The maximum-energy photoneutrons which leave the residual nuclei in their ground state were isolated from other transitions, making it possible to study individual excited levels in 12C and 16O. By measuring the neutron spectra at several angles and taking advantage of the use of self-conjugate target nuclei, these isolated levels in 12C and 16O were found to be due to electric dipole photo-absorption with subsequent emission of d-wave neutrons.

Nature of the Dipole States of Oxygen-16

RECENTLY, two different analyses of the spectrum of photoneutrons from oxygen-16 on bombardment by 34-MeV bremsstrahlung have been published. The first of these, by Yergin et al.1, concluded that in the giant resonance region, the decay of the 22.3 and 24.3 MeV states (1−; T = 1) occurred with quite large probability via the 5.2 MeV doublet of oxygen-15 as well as via the oxygen-15 ground-state and the 6.2 MeV state of oxygen-15. These latter two decay channels are those which are expected to participate on the basis of the 1 particle-1 hole description of the 1−; T = 1 states of oxygen-16 (ref. 2). In the other analysis of the photoneutron spectrum, Tanner and Earle3 have concluded that the neutron spectrum of Yergin is quite consistent with transitions to the ground- and 62 MeV-states of oxygen-15 only, and that the 1 particle-1 hole description of the giant dipole resonance is thereby vindicated.

A 100 MHz digital timing system

A timing system with a channel width of 10 nsec is completely described. The equipment is required to be compatible with a digital magnetic tape recording system 4–7) but may be used independently. Measurements of the complex energy spectra of photoneutrons from the reactions S 32(γ,n) S 31 and Si 28 (γ,n) Si 27 are presented which clearly demonstrate the resolution and linearity of the system.