Thermal Neutron Fission Research Articles

Pulse height defect in an ionization chamber investigated by cold fission measurements

The cold fragmentation of 235U thermal neutron fission has been studied in a twin ionization chamber working with pure methane gas. Due to the very high fragment mass resolution, a careful investigation of the pulse height defect in this detector has been achieved. No dependence of the pulse height defect on the fragment mass has been observed. Therefore, it is easy to calibrate the energy response of such a detector. An accurate calibration method is proposed.

Fitting and optimal grouping on gamma reliability data : Duan Wei and C. K. Shau. IEEE Trans. Reliab.R-36, 595 (1987)

Exsting experimental thermal, fast, and 14-MeV neutron-induced fission-product cumulative and independent yieds have been compiled, corrected to common reference values, and listed in tabular form for the following fissile nuclides:Thermal-neutron fission: cumulative yields for 227Th, 229Th, 233U, 235U, 239Pu, 241Pu, 241Am, 242Am, 245Cm, 249Cf, 251Cf, 254Es, and 255Fm; independent yieds for 233U, 235U, 237Np, 238U, and 239Pu.Fast-neutron fission: cumulativ yields for 227Ac, 231Pa, 232Th, 233U, 235U, 237Np, 238U, and 239Pu; independent yields for 235U and 238U.14-MeV-neutron fission: cumulative yields for 231Pa, 232Th, 233U, 235U, 237Np, 238U, and 239Pu; independent yields for 232Th, 233U, 235U, 238U, and 239Pu.11-MeV-neutron fission: cumulative yields for 232Th.3-MeV-neutron fission: cumulative yields for 231Pa, 232Th, and 238U.1.1-MeV-neutron fission: cumulative yields for 237Np.From these experimental values the unknown independent yields are deduced empirically for thermal-neutron fission of 233U, 235U, 239Pu, and 241Pu; the fast fission of 232Th, 233U, 235U, 238U, 239Pu, 240Pu, and 241Pu (the chain yields for 240Pu and 241Pu used at this energy being predictions); and the 14-MeV-neutron fission of 232Th, 233U, 235U, and 238U.Finally, by the fitting of the preceding information to condition equations derived from the conservation laws, adjusted sets of chain and independent yields are calculated for thermal fission of 233U, 235U, 239Pu, and 241Pu; fast fission of 232Th, 233U, 235U, 238U, 239Pu, and 241Pu; and 14-MeV fission of 232Th, 233U, 235U, and 238U. The literature search is probably complete to the end of 1975; some 1976 results are included.This paper replaces and makes obsolete the following UKAEA reports: AERE-R7209, AERE-R7394, AERE-R7680, and AERE-R8152.

System reliability using binomial failure rate : Kyung C. Chae. Proc. a. Reliab. Maintainab. Symp., 136 (1988)

Exsting experimental thermal, fast, and 14-MeV neutron-induced fission-product cumulative and independent yieds have been compiled, corrected to common reference values, and listed in tabular form for the following fissile nuclides:Thermal-neutron fission: cumulative yields for 227Th, 229Th, 233U, 235U, 239Pu, 241Pu, 241Am, 242Am, 245Cm, 249Cf, 251Cf, 254Es, and 255Fm; independent yieds for 233U, 235U, 237Np, 238U, and 239Pu.Fast-neutron fission: cumulativ yields for 227Ac, 231Pa, 232Th, 233U, 235U, 237Np, 238U, and 239Pu; independent yields for 235U and 238U.14-MeV-neutron fission: cumulative yields for 231Pa, 232Th, 233U, 235U, 237Np, 238U, and 239Pu; independent yields for 232Th, 233U, 235U, 238U, and 239Pu.11-MeV-neutron fission: cumulative yields for 232Th.3-MeV-neutron fission: cumulative yields for 231Pa, 232Th, and 238U.1.1-MeV-neutron fission: cumulative yields for 237Np.From these experimental values the unknown independent yields are deduced empirically for thermal-neutron fission of 233U, 235U, 239Pu, and 241Pu; the fast fission of 232Th, 233U, 235U, 238U, 239Pu, 240Pu, and 241Pu (the chain yields for 240Pu and 241Pu used at this energy being predictions); and the 14-MeV-neutron fission of 232Th, 233U, 235U, and 238U.Finally, by the fitting of the preceding information to condition equations derived from the conservation laws, adjusted sets of chain and independent yields are calculated for thermal fission of 233U, 235U, 239Pu, and 241Pu; fast fission of 232Th, 233U, 235U, 238U, 239Pu, and 241Pu; and 14-MeV fission of 232Th, 233U, 235U, and 238U. The literature search is probably complete to the end of 1975; some 1976 results are included.This paper replaces and makes obsolete the following UKAEA reports: AERE-R7209, AERE-R7394, AERE-R7680, and AERE-R8152.

Computerized on-line measurement the yield of short-lived bromine and iodine isotopes from thermal neutron fission of235U

The short-lived bromine and iodine isotopes from thermal neutron fission of235U have been separated from fission products by one step solvent extraction combined with an ion exchange fast chemistry system. The measured gamma spectra have been acquisited by an automatic computer on-line system. The results include experimental independent fission yields of86Br,134mI,134gI,136mI, and136gI, the cumulative yields of87Br,88Br,137I,138I. The isomeric yield ratio of134I and136I has been determined and the iodine isotopic distribution curve has been constructed and analyzed.

Detection of fallout155Eu and207Bi in a210Pb-dated lake sediment core

Low-energy γ-ray spectrometry is used to detect fallout155Eu and207Bi in a210Pb-dated sediment core from McKay Lake, Ottawa, Canada. Unlike207Bi which is detected only in two core sections deposited in the mid 1970's,155Eu is consistently detectable to mid 1960's. A comparison of corresponding137Cs and155Eu inventories in the sediment core indicates that fallout155Eu derives primarily from the thermal neutron fission of235U. The derived flux of unsupported210Pb at the sediment/water interface is in agreement with previously estimated flux of atmospheric210Pb in the Great Lakes region.

Isomeric yield ratio of146La and84Br in the thermal neutron induced fission of235U

The independent isomeric yield ratio of146La and84Br in the thermal neutron fission of235U is reported for the first time with the values of 0.058±0.017 and 0.62±0.20, respectively. the yields have been determined using a fast radiochemical separation technique followed by γ-spectroscopy. The deduced rms angular momentum of84Br is 5.9 ℏ from the statistical model analysis and the rms angular momentum of146La is found too low to be determined.

Thermal-neutron fission cross section of 26.1-min 235Um.

The thermal-neutron fission cross section of $^{235}\mathrm{U}^{\mathrm{m}}$ has been measured relative to the ground-state cross section. A rapid radiochemical separation procedure was developed to provide sizeable (${10}^{10}$ to ${10}^{11}$ atom) samples that were reasonably free of the parent $^{239}\mathrm{Pu}$. From a series of eight measurements, the value of 1.42\ifmmode\pm\else\textpm\fi{}0.04 was obtained for the ratio ${\ensuremath{\sigma}}_{m}$/${\ensuremath{\sigma}}_{g}$.

Decay of 100Sr and a "pairing-free" K pi =1(+) rotational band in odd-odd 100Y.

The decay of $^{100}\mathrm{Sr}$ (193 ms) to the low-spin $^{100}\mathrm{Y}$ isomer (735 ms) was studied from mass-separated activity produced in thermal neutron fission of $^{235}\mathrm{U}$. \ensuremath{\gamma} singles and \ensuremath{\gamma}-\ensuremath{\gamma} coincidence measurements resulted in the placement of 67 \ensuremath{\gamma} transitions in a decay scheme with 26 levels below 2 Mev. The multipolarities of low-energy transitions were determined from internal conversion electron measurements. Logft values were deduced using an absolute \ensuremath{\gamma}-ray intensity determination for the A=100 decay chain. The ${1}^{+}$ levels in $^{100}\mathrm{Y}$ at 10.70 and 974.61 keV each receive \ensuremath{\sim}40% of the \ensuremath{\beta} feeding from $^{100}\mathrm{Sr}$. A ${K}^{\ensuremath{\pi}}$${=1}^{+}$ rotational band (with ${1}^{+}$, ${2}^{+}$, and ${3}^{+}$ levels at 10.70, 76.15, and 172.03 keV) is proposed which can be characterized as a nearly ``pairing-free'' band. Other levels in $^{100}\mathrm{Y}$ are discussed in terms of two-quasiparticle Nilsson orbitals.

Decay of 80Zn: Implications for shell structure and r-process nucleosynthesis.

The decay of $^{80}\mathrm{Zn}$ to levels in $^{80}\mathrm{Ga}$ has been studied from mass-separated activity produced in the thermal neutron fission of $^{235}\mathrm{U}$. Based on \ensuremath{\gamma} singles and coincidence measurements, 26 \ensuremath{\gamma} rays were placed in a level scheme for $^{80}\mathrm{Ga}$ with 13 excited states up to 2655 keV. The emission of a \ensuremath{\gamma} ray resulting from delayed neutron emission of $^{80}\mathrm{Ga}$ is also reported. The structure of the $^{80}\mathrm{Ga}$ level scheme is discussed. The properties of $^{80}\mathrm{Zn}$ and $^{80}\mathrm{Ga}$ reported here are of astrophysical interest for modeling the r process.

Mass separation of rare-earth elements by a high-temperature thermal ion source coupled with a He-jet system

By using a high-temperature thermal ion source coupled to a He-jet system, neutron-rich isotopes of rare-earth elements such as cerium, praseodymium, neodymium and promethium produced by the thermal-neutron fission of 235U were ionized and successfully separated. The temperature dependence of the ionization efficiency has been measured and found to be explained qualitatively by the vapour pressure of the relevant element. The characteristic temperature dependence of the ionization efficiency can be utilized for Z-identification of several isobars of rare-earth elements. The heaviest isotopes of neodymium and promethium, 155Nd and 156Pm, have recently been identified.

Composite Delayed Neutron Energy Spectra for Thermal Fission of235U

Composite delayed neutron energy spectra from the thermal neutron fission of /sup 235/U have been measured for eight delay-time intervals between 0.17 and 85.5 s. This experimental technique combines a helium-jet and tape transfer system with a beta-neutron time-of-flight spectrometer. The neutron energy range of 0.01 to 2.0 MeV is spanned with /sup 6/Li-glass, plastic, and liquid scintillators. Spectra are compared to ENDF/B-V as well as to individual precursors' data and average energies are tabulated for the present and previous compilations. An equilibrium spectrum is also calculated and compared to ENDF/B-V and individual precursor measurements.

Charge distribution study in the alpha-particle induced fission of232Th+ effect of excitation energy

The fractional cumulative yields of135I,138Xe and140Ba in the α particle (30 MeV) induced fission of232Th have been determined following the growth and decay of135gXe,138gCs and140La, respectively, employing high resolution gamma ray spectroscopy. The fractional cumulative yield values are 0.766±0.02, 0.813±0.03 and 0.991±0.004, respectively. The analysis of the data indicates a broader width of charge distribution (σ) compared to the normally observed σ=0.56±0.06 for thermal neutron fission of235U.

G factor of the 21+ state in 142Ba.

The g factor of the ${2}_{1}^{+}$ state in $^{142}\mathrm{Ba}$ was measured using the time integral perturbed angular correlation technique. The state was populated following the \ensuremath{\beta} decay of mass-separated $^{142}\mathrm{Cs}$ produced by thermal neutron fission of $^{235}\mathrm{U}$. A g factor of 0.48\ifmmode\pm\else\textpm\fi{}0.14 was measured. This result is analyzed together with previous results for $^{144}$,146Ba, using two different models: the hydrodynamic model including Greiner's correction for different proton and neutron pairing forces, and the interacting boson approximation IBA-2.

Simplified Method for Calculation of Neutron Capture Transformation Effects of Fission Products on Decay Power

A simplified method is proposed for the calculation of the effects of neutron capture transformations of fission products (FPs) on the decay power of FPs. The decay power of FPs after shutdown changes by the neutron capture transformations of FP nuclides during reactor operation. It is proposed to calculate the neutron capture transformation effects considering the production of the following 7 nuclides 103Ru, 134Cs, 136Cs, 148mPm, 148Pm, 154Eu and 156Eu by the neutron capture reaction of the direct mother nuclide alone giving a cumulative fission yield for the mother nuclide. The present method was assessed by com-paring the calculation results with the rigorous calculation results for the thermal-neutron fission of 235U irradiated between 1 and 5 yr in a light water reactor with thermal-nentron flux between 3 x 1013 and 6 x 1013 n/cm2·s and for the fast-neutron fission of 239Pu irradiated between 1 and 5 yr in a fast breeder reactor with total neutron flux between 3 x 1015 and 6 x 1015 n/cm2·s. It has been clarified that the present method can calculate the neutron capture transformation effects within the accuracy of ±1% of the decay power for the irradiation of 1yr and cooling time less than 109s irrespective of fission type and neutron flux. The accuracy varies little with neutron flux but considerably with irradiation time. For a irradiation of 5 yr the present method can calculate the capture effect within the accuracy of +1% and -5% of the decay power. The accuracy can be improved to ±1% of the decay power with the simple correction factors.

Yields of fission products produced by thermal-neutron fission ofCm243

On the basis of measured yields for 72 gamma rays and known nuclear data, cumulative fission-product yields were deduced for 69 fission products having half-lives between 36 seconds and 65 days representing 41 mass chains created during thermal-neutron fission of $^{243}\mathrm{Cm}$.

Reevaluation of Independent Fission Yields in Thermal-Neutron Fission of 235U

Reevaluation of Independent Fission Yields in Thermal-Neutron Fission of ²³⁵U

Identification and decay of neutron-rich 102Sr and level structure of A~100 Y nuclei.

The previously unreported decay of $^{102}\mathrm{Sr}$ to levels in $^{102}\mathrm{Y}$ has been studied from mass-separated activity produced in the thermal neutron fission of $^{235}\mathrm{U}$. A half-life for $^{102}\mathrm{Sr}$ was measured to be 68\ifmmode\pm\else\textpm\fi{}8 ms. Based on \ensuremath{\gamma} singles and coincidence measurements, 24 \ensuremath{\gamma} rays were placed in a level scheme for $^{102}\mathrm{Y}$ with nine excited states up to 1689 keV. Four excited states were assigned a ${J}^{\ensuremath{\pi}}$ of ${1}^{+}$ from logft values. Evidence for delayed-neutron emission from $^{102}\mathrm{Sr}$ is reported. These results are compared with systematics for other neighboring, highly deformed, odd-odd Y and Nb nuclei. The level structure of A\ensuremath{\sim}100 Y nuclei, which exhibit a very abrupt transition from spherical to highly deformed shapes, is reviewed. The possibility of the existence of bands with moments of inertia close to that of a rigid rotor is discussed.

Magnetic moments of the 21+ states in 146,148Ce.

The g factors of the 2/sub 1//sup +/ states in /sup 146,148/Ce have been measured using the time integral perturbed angular correlation technique. The levels were populated by the radioactive decay of mass-separated /sup 146,148/La which was produced by thermal neutron fission of /sup 235/U. The measured g(2/sub 1//sup +/) values are 0.24 +- 0.05 and 0.37 +- 0.06 for /sup 146/Ce and /sup 148/Ce, respectively. The results are interpreted as a probe of the number of valence protons participating in the collective motion. Data for Ba, Ce, Nd, and Sm nuclei in the region of N = 90 show a distinct change in the value of g(2/sub 1//sup +/) as the number of neutrons increases from 86 to 90. This change is attributed to the effects of the strong interaction between the ..pi..h/sub 11/2/ and ..nu..h/sub 9/2/ orbits which causes a gradual dissipation of the Z = 64 subshell gap.

Independent isomer yield ratio of90Rb

Abstract The independent isomer yield ratio for 90Rb from thermal neutron fission of 235U has been measured by use of a new technique involving a pulsed reactor and an on-line mass spectrometer facility. The observed isomer yield ratio of 8.7 ± 1.0 is one of the largest ratios measured for a low energy fission process. However, a statistical model analysis shows that the average angular momentum (J = 4.5) deduced from this isomer yield ratio is consistent with average angular momentum for other products from low energy fission.

Simplified method for calculation of neutron capture transformation effects of fission products on decay power.

A simplified method is proposed for the calculation of the effects of neutron capture transformations of fission products (FPs) on the decay power of FPs. The decay power of FPs after shutdown changes by the neutron capture transformations of FP nuclides during reactor operation. It is proposed to calculate the neutron capture transformation effects considering the production of the following 7 nuclides 103Ru, 134Cs, 136Cs, 148mPm, 148Pm, 154Eu and 156Eu by the neutron capture reaction of the direct mother nuclide alone giving a cumulative fission yield for the mother nuclide. The present method was assessed by com-paring the calculation results with the rigorous calculation results for the thermal-neutron fission of 235U irradiated between 1 and 5 yr in a light water reactor with thermal-nentron flux between 3 x 1013 and 6 x 1013 n/cm2·s and for the fast-neutron fission of 239Pu irradiated between 1 and 5 yr in a fast breeder reactor with total neutron flux between 3 x 1015 and 6 x 1015 n/cm2·s. It has been clarified that the present method can calculate the neutron capture transformation effects within the accuracy of ±1% of the decay power for the irradiation of 1yr and cooling time less than 109s irrespective of fission type and neutron flux. The accuracy varies little with neutron flux but considerably with irradiation time. For a irradiation of 5 yr the present method can calculate the capture effect within the accuracy of +1% and -5% of the decay power. The accuracy can be improved to ±1% of the decay power with the simple correction factors.