Function Of Incident Neutron Energy Research Articles

Calibration of LiBaF 3:Ce scintillator for fission spectrum neutrons

The scintillator LiBaF 3 doped with small amounts of Ce +3 has the ability to distinguish heavy charged particles (p, d, t, or α) from beta and/or gamma radiation based on the presence or absence of nanosecond components in the scintillation light output. Since the neutron capture reaction on 6Li produces recoil alphas and tritons, this scintillator also discriminates between neutron induced events and beta or gamma interactions. An experimental technique using a time-tagged 252Cf source has been used to measure the efficiency of this scintillator for neutron capture, the calibration of neutron capture pulse height, and the pulse height resolution—all as a function of incident neutron energy.

Probing reaction dynamics with the 196Pt(n,xnγ) reactions for x~15

Discrete \ensuremath{\gamma}-ray spectra have been measured as a function of incident neutron energy for nuclei produced in the ${}^{196}\mathrm{Pt}(n,xn\ensuremath{\gamma})$ reactions. Spectroscopy was done using the large-scale Compton suppressed Ge \ensuremath{\gamma}-ray spectrometer GEANIE. The ``white'' source neutron beam was produced at the Los Alamos Neutron Science WNR facility. Reaction neutron energy was determined using the time-of-flight technique. Reaction-channel yields were inferred from the measured intensity sum of the ${2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+}$ and the ${2}_{2}^{+}\ensuremath{\rightarrow}{0}_{1}^{+}$ transitions for the ${}^{196}\mathrm{Pt}(n,xn)$ reactions for $x<~15.$ Weisskopf-Ewing calculations (including precompound) done with the HMS-ALICE code correctly predict the bulk of the $(n,xn)$ reaction products for low multiplicity. However, they do not accurately predict yield ratios of the different $(n,xn)$ reactions for $x>~9.$ In addition, there is no consistent experimental indication of charged-particle reaction channels $(n,pxn)$ for incident neutron energies above 60 MeV where they are predicted to account for approximately $1/3$ of the total reaction cross section. Several possible causes are discussed for these discrepancies. Finally, the region of $E\ensuremath{-}J$ phase space populated in this reaction is probed for several of the strongest reaction channels through the observation of relative yields for different yrast and off-yrast states.

53Cr(n,n' gamma ) reactions and the level structure of 53Cr.

Gamma-ray decay of levels in the stable isotope $^{53}\mathrm{Cr}$ has been studied using $^{53}\mathrm{Cr}$(n,n'\ensuremath{\gamma}) reactions for incident neutron energies between threshold and 10 MeV. Measured gamma-ray production cross sections have been compared with earlier measurements and with cross sections calculated using precompound-compound-nucleus theory. Some of the present results are at variance with earlier experimental or evaluated results. For example, for the decay of the ${E}_{x}$=1537-keV level we are unable to explain variations in the measured branching ratios of the transition gamma rays as a function of incident neutron energy. The experimental data were analyzed within the framework of several theoretical model calculations of the level structure of $^{53}\mathrm{Cr}$. Quantitative discrepancies are discussed.

Neutron Damage Equivalence for Silicon, Silicon Dioxide, and Gallium Arsenide

Displacement- and ionization-energy transfers to Si, SiO2, and GaAs as functions of incident neutron energy were calculated using new cross section data and fine group structure in the NJOY code system. Neutron spectra determinations for several reactor neutron environments were made using new activation cross sections and a new technique with the SAND II code. Measurements of carrier removal rates in GaAs and of Si transistor gain degradation were made in representative neutron environments. Experimental results are compared to damage ratios predicted with the new spectra and NJOY displacement functions. For fission-like spectra, calculated Si damage ratios are in good agreement with those determined with ASTM E722-85 and with measured transistor damage ratios. Significant differences are found between Si NJOY and ASTM E722-85 for 14-MeV-to-reactor neutron damage ratios where NJOY gives better agreement with experimental data reported in the literature. In GaAs 14-MeV-to-reactor experimental damage ratios are smaller than predicted by calculated displacement ratios. This suggests that a more complex model of damage for majority carrier removal in GaAs is required. The use of incorrect damage functions is shown to adversely affect simulation fidelity in some representative neutron environments.

Efficiency calibration of large volume liquid scintillation detectors

An array of seven large (10 cm thick by 20 cm diameter) NE213 scintillation detectors for use in neutron time-of-flight experiments has been calibrated to determine a matrix of relative detection efficiency as a function of incident neutron energy and detector bias. This matrix is then used to determine the relative efficiency for neutron groups in time-of-flight spectra to which an efficiency-optimizing dynamic bias has been applied.

Construction and First Application of a TEPC Dose-Equivalent Meter for Area Monitoring

The performance of the new working area monitor, called the Jülich Counter, was studied. The results of new calculations on the statistical precision of the integral reading of this instrument are presented as a function of incident neutron energy and simulated diameter. Measurements with the Jülich counter, in the form of a prototype of a portable version, in different neutron-gamma radiation fields are presented. Measurements and calculations are performed on d(y) for the scattered neutron-gamma radiation from a water target, irradiated by a collimated fast neutron beam produced by 14 MeV deuterons incident on a beryllium target.

Effects of deformation on neutron total cross sections of even-ANd and Sm isotopes

Neutron total cross sections of $^{144}\mathrm{Nd}$ and $^{148}\mathrm{Sm}$ and total cross-section differences for the pairs of isotopes $^{144,142}\mathrm{Nd}$, $^{146,144}\mathrm{Nd}$, $^{148,144}\mathrm{Nd}$, $^{150,144}\mathrm{Nd}$, $^{150,148}\mathrm{Sm}$, $^{152,148}\mathrm{Sm}$, and $^{154,148}\mathrm{Sm}$ have been measured at 25 energies from 0.75 to 14 MeV. Each sample consisted of about 40 g of powdered oxide with an isotopic enrichment >94%. For each isotopic pair the total cross-section difference was determined directly by measuring the ratio of transmitted flux for matched samples. This difference was found to have an oscillatory behavior when plotted as a function of incident neutron energy. The oscillations have been explained as a deformation effect qualitatively in terms of the nuclear Ramsauer effect and quantitatively using a deformed optical model potential. Adding two neutrons to a nucleus in this mass region was found to have a different effect on the neutron total cross section than adding two protons. A coupled channel analysis was performed using as input data the present total cross sections, low-energy neutron scattering parameters, and elastic and inelastic (${2}^{+}$) differential cross sections for neutron scattering at 4 and 7 MeV. Excellent total cross-section fits were obtained for the Sm isotopes; however, the total cross-section fits for the Nd isotopes were only fair. Quadrupole deformation parameters have been deduced for all the isotopes studied. The sensitivity of neutron total cross-section differences to this parameter and to the real isospin term of the nuclear potential is discussed.NUCLEAR REACTIONS $^{142,144,146,148,150}\mathrm{Nd}$, ${E}_{n}=0.75\ensuremath{-}13.89$ MeV, $^{148,150,152,154}\mathrm{Sm}$, ${E}_{n}=0.75\ensuremath{-}14.49$ MeV; measured ${\ensuremath{\sigma}}_{n,\mathrm{tot}}(E)$. Enriched targets. Deduced coupled-channel optical-potential parameters, quadrupole deformation parameters.

Fission properties of 233Th

The fission fragment kinetic energy and mass distributions as a function of incident neutron energy were investigated for the 232Th(n, f) reaction. The neutron energy range studied, from 1.35 to 5.30 MeV, includes the fission threshold region where some large structures in the fission cross section are observed, which are attributed to vibrational states in the second (or third) well of the fission barrier. The variation of the mean total kinetic energy of the primary fragments with incident neutron energy is consistent with the results of recent measurements of the variation of the mean number of prompt neutron emitted per fission. Two distinct behaviors are observed, depending upon whether or not the fission proceeds below the barrier (through vibrational states). Information concerning the fission fragment mass dependance of the fragment kinetic energy variation as a function of neutron energy, is also presented. A possible explanation of the results is given in terms of a near adiabatic behavior of the 232Th(n, f) process. It is the first time that such a behavior is observed in the actinide region.

Neutrons and fission

The advent of the double-hump fission barrier that occurred about a decade ago was one of the great steps forward not only in fission but also in nuclear physics in general. This new development brought the two disciplines into much closer contact after years of independence. The double-hump shape of the fission barrier (figure 1) provided the clue to a unified explanation of a number of experimental results. Among these are fission isomerism, the phenomenon whereby a heavy nucleus can exist in a relatively long-lived excited state decaying by fission, and the appearance of several types of structure in the fission cross sections of many nuclei as a function of incident neutron energy.

Critical Magnetic Scattering in Manganese Fluoride

The angular dependence of the quasielastic magnetic scattering has been measured in Mn${\mathrm{F}}_{2}$ in the critical region $T>{T}_{N}$. The temperature dependence of the inverse correlation range and the staggered mode susceptibility for both transverse and longitudinal spin fluctuations were observed as a function of incident neutron energy over the range $56<{E}_{0}<134$ meV. The validity of the quasielastic approximation for this energy region was verified. The data for the longitudinal fluctuations were found to follow simple power laws with the critical exponents $\ensuremath{\nu}=0.634\ifmmode\pm\else\textpm\fi{}0.02$, $\ensuremath{\gamma}=1.24\ifmmode\pm\else\textpm\fi{}0.02$. An $\ensuremath{\eta}$ fitted to the data closest to ${T}_{N}$ gave $\ensuremath{\eta}=0.05\ifmmode\pm\else\textpm\fi{}0.02$. The transverse fluctuations are not divergent at ${T}_{N}$, although for $T>{T}_{N}$ they can be described by power-law fits taken with respect to a temperature ${T}_{\ensuremath{\perp}}\ensuremath{\approx}66\ifmmode^\circ\else\textdegree\fi{}$K by indices ${\ensuremath{\nu}}_{\ensuremath{\perp}}=0.63\ifmmode\pm\else\textpm\fi{}0.08$, ${\ensuremath{\gamma}}_{\ensuremath{\perp}}=1.47\ifmmode\pm\else\textpm\fi{}0.1$. The actual critical temperature is ${T}_{N}=67.458\ifmmode\pm\else\textpm\fi{}0.008$.

Neutron capture in 238U

The γ-ray spectrum following slow neutron capture in 238U has been measured as a function of incident neutron energy at the Brookhaven HFBR fast neutron chopper. The decay scheme proposed by previous (d, p) and high resolution thermal (n, γ) measurements has been substantially verified. Partial radiation widths for s-wave resonances up to 117 eV have been deduced. By using a simple multi-level analysis, the spectra between resonances from 21 eV down to thermal have been fitted. The behavior of the strong transitions to low-lying l = 1 states is explained using a direct capture model, while the transitions to low-lying collective states can be explained by level-level interference. The distribution of El transition strengths is not consistent with a χ 2 distribution of one degree of freedom.

Partition of the Average Energy Deposited in Germanium as a Function of Incident Neutron Energy

Partition of the Average Energy Deposited in Germanium as a Function of Incident Neutron Energy

Partition of the Average Energy Deposited in Silicon as a Function of Incident Neutron Energy

The average energy deposited in crystalline germanium per incident neutron has been calculated as a function of neutron energy. The calculations are for neutron energies below 4 MeV, where scattering events predominate and charged-particle production is minimal. A calculation at 14 MeV by Nichols is also included. As the incident neutron energy increases from 50 keV to 14 MeV, the average ionization energy deposited increases from \ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}12}$ to $\ensuremath{\approx}1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ (erg/g) per (neutron/${\mathrm{cm}}^{2}$), and the energy available for displacements (nonionization) increases from $\ensuremath{\approx}1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ to $\ensuremath{\approx}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ (erg/g) per (neutron/${\mathrm{cm}}^{2}$). The calculated neutron energy dependence of the energy available for displacements agrees with the energy dependence of carrier-removal experiments in germanium performed with monoenergetic neutrons. Comparisons of these calculations are made with analogous ones for silicon. The energy deposited into atomic processes per carrier removed for \ensuremath{\sim}40-\ensuremath{\Omega} cm $n$-type germanium is $\ensuremath{\approx}0.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ erg/carrier, and the corresponding number is about a factor of 2 larger in the case of $n$-type silicon.

Thermal and resonance neutron capture in Cl, Sc, V, Co and Ir

The γ-ray spectra emitted following capture of low-energy neutrons ( E n < 10 keV) were observed as a function of incident neutron energy. The neutron energies were selected by the time-of-light technique while the γ-rays were detected in a NaI(Tl) anticoincidence spectrometer. Significant variations of the γ-ray intensities as a function of neutron energy occur. Two previously unreported resonances at 470 ± 20 and 1080 ± 60 eV neutron energy are observed in scandium.

Elastic and Inelastic Scattering of Fast Neutrons fromTh232

The elastic and inelastic scattering of neutrons from thorium is studied throughout the incident neutron energy interval 0.3 to 1.6 Mev. Fast time-of-flight techniques are utilized in conjunction with an electro-magnetic pulsing and bunching system for the production of short and very intense neutron bursts. The differential elastic cross section is determined as a function of incident neutron energy. The cross sections for inelastic scattering resulting in the excitation of residual nuclear levels at 50\ifmmode\pm\else\textpm\fi{}2, 170\ifmmode\pm\else\textpm\fi{}10, 720\ifmmode\pm\else\textpm\fi{}20, 790\ifmmode\pm\else\textpm\fi{}20, 820\ifmmode\pm\else\textpm\fi{}20, 1050\ifmmode\pm\else\textpm\fi{}50, and 1150\ifmmode\pm\else\textpm\fi{}50 kev are measured. The angular distribution of inelastically scattered neutrons resulting in the excitation of the lower two nuclear levels is determined. The measured individual inelastic excitation functions are combined to obtain the total inelastic scattering cross section as a function of incident neutron energy. The experimental results are compared with the results of other measurements.

The response of a NaI(T1) crystal to monoergic neutrons

The response of a 1 3 4 ″ × 2″ NaI(T1) cylinder to neutrons in the pulse height region between 2.5 and 7 MeV gamma energy equivalent was studied as a function of incident neutron energy for a range of neutron energies between room scattered neutrons and 1.162 MeV. The Li 7(p, n) Be 7 reaction was used as a source of neutrons since it is relatively free from high energy gamma ray background. Monoergic neutron response curves were obtained for neutron energies of 166 keV, 341 keV, and 469 keV. All other distributions were for proton energies giving rise to two neutron groups. The pulse height distributions were measured for a fixed number of counts of a shielded long counter at each neutron energy. The long counter was calibrated against Ilford C2 emulsions for the case of 1.162 and 0.698 MeV neutrons in order to obtain the neutron flux from the long counter counts. The pulse height distributions are nearly linear between 4 and 6.6 MeV. The extrapolations of these distributions intersect the gamma equivalent axis at approximately the binding energy of the last neutron in I 128.