Delayed Neutron Yields Research Articles

Experimental Determination of Photofission Neutron Multiplicities for 235U, 236U, 238U, and 232Th Using Monoenergetic Photons

The prompt neutron multiplicities for photofission of the four isotopes 235U, 236U, 238U, and 232Th have been measured with monoenergetic photons over the energy range from 5.5 to 18 MeV using the annihilation in flight of fast positrons. The delayed neutron yield has been measured for all four isotopes at 10.9- and 16.8-MeV photon energies. The ratio of first- to second-chance fission has been measured as a function of energy up to 17-MeV excitation energy for 236U and 238U photofission.

Calculated Time-Dependent Delayed Neutron Group Yields for Thermal-Neutron Fission of Uranium-235

The time-dependent unseparated six delayed neutron group yields are calculated for thermal-neutron fission of 235U. These calculations are based on a recent reference set of delayed neutron precursors, available fission data, and fission systematics. The unseparated calculated time-dependent group yields are compared with separated group yields (groups based on half-life values) and the experimental unseparated group yields. The comparisons indicate the magnitude of the error involved when separated group yields are used for evaluating the accuracy of delayed neutron data. A total delayed neutron yield of 160.62 ± 11.52 per 104 fissions was obtained for thermal-neutron fission of 235U.

Average neutron energies from separated delayed-neutron precursors

Average delayed-neutron energies for 16 isotopes of Br, Rb, I, and Cs have been measured by means of detector ring counting rate ratios in a polyethylene-moderated neutron counter system. An on-line mass spectrometer facility was used to give chemically and mass separated sources of delayed-neutron precursors. A simple expression predicts general trends of the average delayed-neutron energies. Average delayed-neutron energies of individual precursors were weighted by the corresponding delayed-neutron yields and combined to give average energies for all six delayed-neutron groups commonly used in reactor kinetics calculations.

Delayed Neutron Yield Calculations for the Neutron-Induced Fission of Uranium-235 as a Function of the Incident Neutron Energy

The total delayed neutron yield from 235U was calculated as a function of the energy of the neutron inducing fission. The calculations (based on fission data and fission systematics) extend from thermal to 15-MeV neutron energies. The energy-dependent delayed neutron calculations are generally consistent with experimental results from thermal to 14-MeV neutron energies. Delayed neutron yields per 104 fissions of 168.7 ± 16.7, 178.2 ± 17.2, and 88.9 ± 10.0 were obtained at thermal, fission-spectrum, and 15-MeV neutron energies, respectively. The energy dependence of the odd-even effect in the fission charge distribution was found to partially account for the near constant yield observed below 5-MeV neutron energy, as well as the rapid decrease in yield observed at the second-chance fission threshold.

Assay of fissile materials by a cyclic method of neutron activation and delayed-neutron counting

A cyclic method of thermal neutron activation and delayed-neutron counting provides a quantitative assay that is specific, rapid, and sensitive for fissile material in solid or liquid samples. In each activation/counting cycle the sample is transferred into a thermal neutron flux for activation and then to a detector for delayed-neutron counting. Computer calculations based on specific induced fission rates of the fissile material and on the half-lives and delayed-neutron yields of six beta-emitting precursor groups are used to optimize analyses of various fissile isotopes and of natural uranium. Corrections for interferences caused by the reaction 17O(n, p) → 17N → β 4.4s 17O ∗→ 16O+n(delayed) are made by using a cross section that was measured with water enriched in 17O. For an analysis time of 10 min/sample and a thermal neutron flux of 10 9 n/(cm 2·s), detection limits in 15 g soil samples are 0.5 ppm for natural uranium and 8 ppb for 239Pu.

On the (3 Z − A) dependence of delayed neutron yield from fission fragments

The letter is to show that the observed yield Y = Y o exp [3 Z − A] of delayed neutrons from fission is equivalent to Y = Y o exp [ −B n T ] where B n is the neutron binding energy of the emitter and T is the temperature of the excited nucleus.

Reevaluation of the Emission Probabilities of Delayed, Neutrons from Fission Products

The emission probabilities of delayed neutrons are reevaluated on the basis of recently determined fission yields. The accuracy of the probabilities for thermal and fast fission of 233U and 235U is examined by summation of the individual delayed-neutron yields of the precursors and by comparison with the unseparated group and directly measured total delayed-neutron yields.The good agreement of the results obtained by the two independent methods indicates that the derived probabilities are accurate enough for the calculation of delayed-neutr on yields for other fissioning systems and for reactor calculations.

Delayed-Neutron Data for Reactor-Physics Analysis

ecommended values for delayed-neutron data of principal interest in reactor-physics analysis are presented. These data are the absolute delayed-neutron yield, the delayed-neutron energy spectrum, and the group fractions and decay constants used to describe the time behavior of delayed-neutron production. The recommended values are the result of a review and evaluation of experimental data and empirical correlations. Use of these values is discussed, and several remaining problems are described.

Cross Section for the Delayed-Neutron Yield from the 9Be(n, p) 9Li Reaction at Neutron Energies of 14.5 and 14.9 MeV

The delayed-neutron production cross section for the 9Be(n,p)9Li reaction has been measured at incident neutron energies of 14.5 and 14.9 MeV. Cross-section values of 0.053 ± 0.005 and 0.210 ± 0.013 mb, respectively, have been obtained.

Delayed-neutrons from arsenic isotopes 84As, 85As and 86As

Short-lived arsenic isotopes were produced by thermal-neutron fission of 235U and isolated within 2·5 sec by volatilization of arsenic hydride. The existence of a strong delayed-neutron precursor of 2·05±0·05 sec half-life among the arsenic isotopes is confirmed and its assignment to 85As ascertained by milking of the daughter product 33 sec 85Se and 3·1 min 84Se, which is the final nucleus in the neutron decay branch of 85As. A delayed-neutron yield of 7·8 ± 1·2 neutrons/10 4 fissions was obtained for 85As. A new isotope, 0·9 ± 0·2 sec 86As, was detected by delayed-neutron counting and by following the decay of its most prominent γ-ray. The mass assignment was verified by milking of 54 sec 86Br. The delayed-neutron yield of 86As amounts to 0·57 (+0·25/−0·15) neutrons/10 4 fissions. Neutron emission was also detected in the decay of the 5·5 sec 84As, with a yield of 0·02 ± 0·01 neutrons/10 4 fissions. Using the measured delayed-neutron yields and measured or estimated fission yields, the following neutron emission probabilities were obtained: P n 84 = 0·13 ± 0·06%, P n 85 = 23 ± 3% and P n 85 = 3·8 (+1·7/−1·0)%. From the growth of 87Br in arsenic samples a half-life of about 0·3 sec and a fractional cumulative yield of 1·8 ± 0·9% were deduced for 87As.

Strength functions in the β-decay of short-lived fission products

Approximate β-strength functions have been derived from measurements of the β − feed to high-lying excited states in 40 neutron-rich nuclei. These nuclei, all produced in thermal fission, cover a broad range of nuclides from bromine to yttrium (84 ≦ A ≦ 95) and from silver to lanthanum (116 ≦ A ≦ 143). The β − feed was determined by non-ideal total γ-ray absorption. Above the cut-off energy expected from pairing, the strength functions in general increase smoothly with energy when no strong allowed single-particle strength is concentrated in the observed energy range. The increase is in these cases largely proportional to the nuclear level density. When allowed single-particle strength is likely to occur the strength functions show significant structure even at higher energies (≈5 MeV) and the average strength then corresponds to a higher value of log ƒt. These data are consistent with the theory of high-energy β-decay in terms of a giant resonance, but draw attention to the persistence of significant shell structure in the tail part of the resonance although the definite but smeared-out shell-model transitions are hindered as would be expected. Direct information on the β-strength is needed for the evaluation of delayed neutron yields. The data provide such information for the wellknown delayed neutron precursors 87Br and 137I.

Revised Delayed-Neutron Yield Data

Values of the yield of delayed neutrons from fission previously reported by Masters, Thorpe, and Smith and by Krick and Evans have been reevaluated following refinement of earlier measurements and calibrations. Except for 238U, the values for fast fission are in agreement with earlier results of Keepin.

Measurement of the Uranium-235 Content in a Spent Mtr Type Fuel Element Using the Delayed-Neutron Yield Technique

Measurement of the Uranium-235 Content in a Spent Mtr Type Fuel Element Using the Delayed-Neutron Yield Technique

On the Delayed Neutron Yields of the Longer-lived Halogen Precursors in Thermal Neutron Fission of 235U

On the Delayed Neutron Yields of the Longer-lived Halogen Precursors in Thermal Neutron Fission of ²³⁵U

Delayed Neutron Yields from 14.8 MeV Neutron Induced Fission of 238U, 232Th and 231Pa

Delayed Neutron Yields from 14.8 MeV Neutron Induced Fission of ²³⁸U, ²³²Th and ²³¹Pa

Cross Section for the Delayed-Neutron Yield from the 17O(n,p)17N Reaction at 14.1 MeV

(1970). Cross Section for the Delayed-Neutron Yield from the 17O(n,p)17N Reaction at 14.1 MeV. Nuclear Science and Engineering: Vol. 40, No. 1, pp. 136-138.

The Measurement of Absolute Delayed-Neutron Yields from 3.1- and 14.9-MeV Fission

A new method, utilizing a modulated neutron source operated in antisynchronism with a long counter, has been used to measure the absolute and relative delayed-neutron yields of five fissionable iso...

Measurement of mass- and time-dependence of delayed fission neutrons by an on-line mass separator

Delayed neutron emission for thermal neutron fission of235U is investigated by means of a helium-filled mass separator. At the focus of the separator, the neutron activity is detected in coincidence with theΒ decay preceding the neutron emission. Measurements of the activity build-up give the total delayed neutron yield and the yields for different times after fission. Measurements of the activity decay give the half-life distribution.

Delayed neutron precursors—II: Antimony and arsenic precursors separated chemically

A high-speed chemical technique is described for separating antimony, arsenic and germanium from fission products. The hydrides of these elements are formed inside a nuclear reactor by the Zn + H 2SO 4 reaction and are swept out by a fast gas flow. Further studies have been made of delayed neutron emission from fission product isotopes of Sb and As formed in the thermal neutron fission of 235U; the half-lives and delayed neutron yields are in good agreement with those measured in an earlier study using an electrolytic technique. The weighted average values from the two studies are as follows: half-lives (sec), 85,86As 2·028 ± 0·012, 134Sb 11·3 ± 0·3, 135Sb 1·696 ± 0·021; absolute delayed neutron yields (neutrons/10 4 fissions), 85,86As 9·7 ± 0·8, 134Sb 0·028 ± 0·004, 135Sb 3·5 ± 0·3. A search for other delayed neutron precursors of Sb, As and Ge with half-lives down to 0·5 sec gave negative results.

Delayed neutron precursors—I: Antimony and arsenic precursors separated by electrolysis

Using an improved electrolytic separation technique, further studies are reported of delayed neutron emission from Sb and As fission product isotopes. Previous results have been confirmed and a difference in half-life between the two short-lived delayed neutron precursors has been established. Least-squares fits of the data gave the following half-lives: 135Sb 1·59 ± 0·09 sec; 85,86As 2·03 ± 0·03 sec. In the thermal neutron fission of 235U, the absolute delayed neutron yields of these nuclides (neutrons/10 4 fissions) are: 135Sb 3·6 ± 0·5, 85,86As 9·4 ± 1·3. The longlived delayed neutron precursor detected previously ( t 1 2 14 ± 4 sec) has been shown to be due to one of the elements Sb, As, Sn, Ge, Se or Te, and not to halogen contamination. It appears to consist of two components, one with a half-life of 9–12 sec and the other much longer lived.