Prompt Neutron Research Articles

Prompt neutron emission spectra and multiplicities in the thermal neutron induced fission of235U

The emission spectra of prompt fission neutrons from mass and kinetic energy selected fission fragments have been measured in235U(n th,f). Neutron energies were determined from the measurement of the neutron time of flight using a NE213 scintillation detector. The fragment energies were measured by a pair of surface barrier detectors in one set of measurements and by a back-to-back gridded ionization chamber in the second set of measurements. The data were analysed event by event to deduce neutron energy in the rest frame of the emitting fragment for the determination of neutron emission spectra and multiplicities as a function of the fragment mass and total kinetic energy. The results are compared with statistical model calculations using shell and excitation energy dependent level density formulations to deduce the level density parameters of the neutron rich fragment nuclei over a large range of fragment masses.

Calibration of a prompt neutron activation analysis facility for the measurement of total body protein

Prompt neutron activation analysis with 252Cf has been used to measure total body protein. Since simultaneous irradiation and detection generated a high count rate at the detectors the nucleonic system was optimised to reduce distortions in the gamma -ray energy spectrum. Tissue-equivalent phantoms were used to calibrate the apparatus. A total of 53 studies was performed on 39 normal subjects. Ratios of total body protein to fat-free mass agreed well with those from other centres and with cadaver studies. Duplicate measurements of 14 subjects gave a coefficient of variation of +or-2.9% for a measurement of total body protein, which is close to that calculated from the known errors of the technique of +or-2.7%. The whole body dose equivalent was 0.17 mSv.

Prompt neutron emission from the spontaneous fission of 260Md.

We have made the first measurement of the number of neutrons emitted from the spontaneous fission of a nuclide in which very high fragment energies dominate the fission process. In bombardments of $^{254}\mathrm{Es}$, we produced a large sample of 28-d $^{260}\mathrm{Md}$, which was neutron counted in a 1-m-diameter spherical tank containing a Gd-doped scintillator solution. The average number of neutrons emitted per fission is only 2.58\ifmmode\pm\else\textpm\fi{}0.11, substantially less than for other actinides. A linear dependence of neutron multiplicity on fragment-excitation energy is observed to the highest values of total kinetic energy.

Measurement of Prompt Neutron Decay Constant and Large Subcriticality by the Feynman-α Method

The Feynman-α experiments were carried out using light-water-moderated and -reflected cores loaded with highly enriched uranium fuel at the Kyoto University Critical Assembly. An experimental technique using a multichannel scaler was developed to improve the accuracy of measurement and to shorten measuring time. Then, the βeff/l values of single and coupled cores with different neutron spectra were measured to demonstrate the capability of the present technique for measuring the prompt neutron decay constant α. Moreover, the Feynman-α method was applied to measuring large subcriticalities. Through these experiments, it is found that the present technique greatly improves the accuracy of a measurement, and the one-point reactor approximation is applicable to a tightly coupled core. It is also found that the subcriticality down to approximately -35 $ can be measured by this method if the position of the neutron detector is chosen carefully, and the present Feynman-α method can be applied to a subcriticality monitoring system.

Measurement of Prompt Neutrons Emitted from Thermal Neutron-Induced Fission of Uranium-233 by Double-Energy Double-Velocity Method

Measurement of Prompt Neutrons Emitted from Thermal Neutron-Induced Fission of Uranium-233 by Double-Energy Double-Velocity Method

Measurements of the Energy Dependence of Prompt Neutron Emission from233U,235U, and239Pu forEn= 0.0005 to 10 MeV Relative to Emission from Spontaneous Fission of252Cf

Measurements of the Energy Dependence of Prompt Neutron Emission from²³³U,²³⁵U, and²³⁹Pu for<i>E_n</i>= 0.0005 to 10 MeV Relative to Emission from Spontaneous Fission of²⁵²Cf

Prompt neutrons from the low-energy fission of atomic nuclei

The characteristics of prompt fission neutrons (PFN)--spectra and average number of neutrons--are important for nuclear reactor calculations. This paper takes into account the special features associated with the characteristics of the fission fragments. The Hauser-Feshbach statistical theory is used to calculate the characteristics of PFN with the following differences: a more accurate account of the cascade nature of neutron emission is presented, a different method of determining the average excitation energy of the fragments is used, and an expression for the level density which takes account of the shell structure of nuclei is given. Results are presented of a calculation of the spectra and multiplicity distribution of PFN for the most important cases, namely the spontaneous fission of Cf 252 and the thermal-neutron fission of U 235.

Prompt Neutron Emission Probabilities Following Spontaneous and Thermal Neutron Fission

Accurate neutron emission probability distributions (Pv) associated with spontaneous and induced fission are necessary in correcting for neutron multiplication in neutron coincidence counting. A reanalysis of previously published Pv distributions is presented as well as recent measurements. All data were obtained using a large liquid scintillator tank.

New formula for the spectrum of prompt neutrons from fission

New formula for the spectrum of prompt neutrons from fission

The Average Number of Prompt Neutrons and the Distributions of Prompt Neutron Emission Number for Spontaneous Fission of Plutonium-240, Curium-242, and Curium-244

(The average number of prompt neutron v /SUB p/ and the distributions of prompt neutron number probability P(v) for spontaneous fission of /sup 240/Pu, /sup 242/Cm, and /sup 244/Cm relative to v /SUB p/ (/sup 252/Cf) have been measured using a large gadolinium-loaded liquid scintillation counter with a coincidence method.)The results were v /SUB p/ (/sup 240/Pu)=2.141+ or 0.016, v /SUB p/ (/sup 242/Cm)=2.562 + or - 0.020, and v /SUB p/ (/sup 244/Cm)= 2.721 + or - 0.021. (The measured distributions of prompt neutron number were fitted with Gaussian curves by a weighted least-squares method.) The widths of Gaussian distribution are 1.149 + or - 0.047, 1.159 + or - 0.074, and 1.175 + or 0.098 for /sup 240/Pu, /sup 242/Cm, and /sup 244/Cm, respectively. (The results as well as a previous measurement of spontaneous fission of /sup 252/Cf show the linear variation of sigma with v /SUB p/ at the first order of approximation.) The data were fitted by a least-squares method, and the result is given by a sigma= 0.980+0.076v /SUB p/ . This fact demonstrates the trend that the width of the excitation energy distribution of fission fragments increases with the average excitation energy of themore » fission fragments in the range of nuclides mentioned above.« less

Neutron Capture Cross Sections of the Silver Isotopes 107Ag and 109Ag from 2.6 to 2000 keV

Prompt neutron capture from highly enriched samples of the stable silver isotopes was measured at the Oak Ridge Electron Linear Accelerator neutron time-of-flight facility. Resonance peaks were par...

A Measurement of the Average Number of Prompt Neutrons from Spontaneous Fission of Californium-252

The Oak Ridge National Laboratory large liquid-scintillator detector was used in a precise determination of v̄p, the number of neutrons emitted promptly, from spontaneous fission of 252Cf. Measurements of the detector efficiency over a broad energy region were made by means of a proton-recoil technique employing the Oak Ridge Electron Linear Accelerator “white” neutron source. Monte Carlo calculation of the detector efficiency for a spectrum representative of 252Cf fission neutrons was calibrated with these elaborate measurements. The unusually flat response of the neutron detector resulted in elimination of several known sources of error. Experimental measurement was coupled with calculational methods to correct for other known errors. These measurements lead to an unusually small estimated uncertainty of 0.2% in the value obtained, v̄p = 3.773 ± 0.007.

High rate spectroscopy for on-line nuclear coal analyzer (nucoalyzerTM conac)

A high count rate, time-variant Ge(Li) spectrometer has been developed for on-line coal analysis. The analyzer is being fabricated for use in a power generating station. Prompt neutron activation of coal samples is the basis of analysis, with 252 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cf</sub> as the source for irradiation. The spectroscopy system allows counting rates up to 150 k counts per second without significant loss in energy resolution or peak shape. The high data throughput allows the coal analyzer to be used for on-line process control. The coal analyzer will be discussed, with emphasis on the high-rate signal processing system. Results of analysis of coal samples will be presented.

Analysis of prompt neutron anisotropies in thermal neutron fission of235U

Prompt neutron anisotropies as a function of total fragment kinetic energy Ek have been calculated in the thermal-neutron-induced fission of 235U on the basis of neutron evaporation from fully accelerated fragments. There is a large discrepancy between the calculated and available experimental results at higher Ek. This discrepancy can be attributed to the emission of an isotropic component of prompt neutrons during the descent of the compound nucleus from saddle to scission. Although the fraction of these isotropic neutrons increases with Ek, the absolute number is almost constant as a function of Ek. The importance of the present results on the pre-scission neutrons with respect to the dynamics of the nucleus from saddle to scission is discussed.

A Simple Formula for Calculation of Prompt Neutron Yield from Spontaneous Fission of Transuranics

The simple statistical model statement relating the yield Y /SUB P/ of fission fragments to the effective neutron binding energy B /SUB n/ , namely Y /SUB P/ ..cap alpha.. exp(-B/sup -/ /SUB n/ /T), can be used as a basis for parallel developments, one leading to the well-known empirical delayed-neutron statement Y = Y/sub 0/exp(-K(3Z - A)), where Y is the number of delayed neutrons per fission. Repeating the development for prompt neutron emission leads to the analogous result v/sup -/ = v/sup -//sub 0/exp(-b(3Z - A)) where v/sup -/ is the prompt neutrons per fission. This semi-empirical result implies that a semi-logarithmic experimental plot of v/sup -/ against (3Z - A) should be a family of straight lines. Currently available experimental results justify this prediction. The theoretical precision of this semi-empirical formula is estimated to be + or -10% or better, depending mainly on the part and shape of the fission yield-mass curve taken into account. The existence of the above empirical and semi-empirical formulas strongly suggests that the yield of fission neutrons, as well as their spectrum, can be calculated by using the standard statistical model with non-adjustable parameters.

Prompt Neutron Multiplicity Distribution in the Spontaneous Fission of Curium-242

The prompt neutron multiplicity distribution for the spontaneous fission of 242Cm has been measured. The measurement has been carried out with a neutron multiplicity counter using hydrogen moderati...

Neutron multiplicity measurements of Cf and Fm isotopes

Prompt neutrons in coincidence with the fission fragments from the spontaneous fission of $^{250,252,254}\mathrm{Cf}$ and $^{257}\mathrm{Fm}$ were measured inside a 75-cm-diameter, Gd-loaded liquid scintillation counter having a neutron-detection efficiency of about 78%. Measurements for $^{256}\mathrm{Fm}$ were done just outside the counter with an efficiency of 31%. The kinetic energies of both fission fragments and the number of neutrons for each fission event were recorded. From these data, the fragment kinetic energies and masses and the neutron multiplicity distributions were determined for $^{250,252,254}\mathrm{Cf}$ and $^{257}\mathrm{Fm}$. Variances of neutron multiplicity distributions as a function of total fragment kinetic energy and the ratio of fragment masses have been calculated and are presented for all the nuclides studied.RADIOACTIVITY, FISSION $^{250,252,254}\mathrm{Cf}$, $^{257}\mathrm{Fm}$; measured neutron multiplicity distributions as a function of total kinetic energy and mass ratio. $^{256}\mathrm{Fm}$; measured average and variance of neutron multiplicity distributions as a function of total kinetic energy and mass ratio.

Resonance Neutron Capture in 138Ba and 140Ce and the Prompt Neutron Correction to ?-ray Detectors

The neutron capture cross sections of 138Ba and 140Ce have been measured with high energy resolution between 3 and 100 keY using the capture cross section facility at the 40 m station of the Oak Ridge Electron Linear Accelerator. The average s-wave level spacings are &lt;D)s = 6�3 � 1 �7 and 3�2 � 0�8 keY for 138Ba and 140Ce respectively. The deduced s-wave neutron strength functions are 104So = 1�0�0�4 and 1�54�0�53 respectively, and the average s-wave radiative widths are &lt;r,)s = 55�20 and 35�9meY. The p-wave neutron strength functions are 104S1 ~ 0�03 and 0�32�0�12 respectively. The average 30 keY Maxwellian capture cross sections are 3�9�0�8mb for 138Ba and 7�7 � 0�9 mb for 140Ce. A more rigorous treatment of the prompt neutron correction for l'-ray detectors is described, and it is shown that previously published results for 138Ba have underestimated this correction. The 140Ce data provide excellent corroboration for the calculated magnitude and time dependence of the prompt neutron correction.

Direct measurement of prompt neutrons emitted in fission ofRa226by 12 MeV protons

Prompt neutron energy distributions were measured at 0\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{} with respect to the fission axis, in correlation with the mass-energy distribution of the fragments, for the fission of $^{226}\mathrm{Ra}$ induced by 12 MeV protons. The average number and average kinetic energy of prefission and postfission neutrons were obtained as a function of fragment mass and total kinetic energy. The average number of prefission neutrons emitted is 0.33 \ifmmode\pm\else\textpm\fi{} 0.15 $\frac{n}{\mathrm{fission}}$ (this includes also contributions from scission neutrons). By comparing this result with calculations of the average number of "true" prefission neutrons, it was found that the number of true prefission neutrons is zero. The average number of postfission neutrons emitted from both fragments is 3.20 \ifmmode\pm\else\textpm\fi{} 0.20 with an average kinetic energy of 1.33 \ifmmode\pm\else\textpm\fi{} 0.07 MeV. The dependence of the number of postfission neutrons on the fragment mass can be interpreted as a combination of a sawtooth structure for the asymmetric mass division and a linearly increasing function for the symmetric mass division.NUCLEAR REACTIONS, FISSION $^{226}\mathrm{Ra}(p,xnf)$, $E=12$ MeV; measured coin. fragment energy, neutron velocity at $\ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{}, 90\ifmmode^\circ\else\textdegree\fi{}$ with respect to fragment; deduced number and kinetic energy of prefission, postfission neutrons.

Prompt neutron behaviour and the point reactor model in small critical and sub-critical fast assemblies

According to a point reactor model of nuclear kinetics the prompt neutron density in a sub-critical fissile assembly decays exponentially with a time constant, usually termed α, which is linearly related to the reactivity by a constant prompt neutron generation time. This linear relationship between α and reactivity extrapolates to prompt critical at α = 0. Departures from this model have been reported by several observers. In assemblies driven by a pulsed neutron source, even when initial transients have been allowed to die away the prompt decay is not always found to be exponential and extrapolation of the relationship between assigned decay constants and reactivity does not always yield prompt critical at α = 0. Precise measurements of prompt decay for some VERA assemblies demonstrate the limited range of validity of the point reactor model. The measurements are compared with SWANTIME calculations. This time-dependent code is a multi-group multi-region diffusion theory version of the prompt kinetic equations. Though this treatment is not designed to analyse the decay in simple terms such as a sum of exponential decays, it does show that a softening of the spectrum occurs as the population decays. For this softer spectrum the decay exponent is smaller than for the corresponding steady state calculation and is no longer proportional to reactivity and so cannot be used directly as a measure of the reactivity of a subcritical reactor. Reactivities based on the ratio of prompt to delayed contributions to the measured decay agree with those based on conventional period measurements used to calibrate control rods; if a linear relationship is assumed between alpha and reactivity, errors in estimating subcritical reactivities can be as large as 20 per cent. To some extent this also provides a check that the delayed neutron data, in terms of relative abundances and decay constants are adequate for both 235U and 239Pu.