Thermal Neutron Fission Of 235U Research Articles

Determination of the absolute intensity of the 1205 keV [formula omitted]-ray emission from 91Y

A radiochemically pure solution of 91Y was produced by the thermal neutron fission of 235U followed by successive chemical separations to remove fission product impurities. The gamma emission rate of the 91Y 1205 keV gamma was measured using multiple high purity germanium gamma spectrometers previously calibrated for counting efficiency using a certificated mixed nuclide gamma standard. The activity concentration of the 91Y was subsequently standardised by liquid scintillation counting. From the combination the activity concentration and gamma emission intensity, the absolute intensity of the 1205 keV gamma emission was derived as 0.2297(39)%. This data agrees within the quoted uncertainties with the absolute intensity of 0.26(4)% published in nuclear data sheets A=91 (Baglin, 2013), but reduces the uncertainty by an order of magnitude.

Overview of research by the fission group in Trombay

Nuclear fission studies in Trombay began nearly six decades ago, with the commissioning of the APSARA research reactor. Early experimental work was based on mass, kinetic energy distributions, neutron and X-ray emission in thermal neutron fission of 235U, which were carried out with indigenously developed detectors and electronics instrumentation. With the commissioning of CIRUS reactor and the availability of higher neutron flux, advanced experiments were carried out on ternary fission, pre-scission neutron emission, fragment charge distributions, quarternary fission, etc. In the late eighties, heavy-ion beams from the pelletron-based medium energy heavy-ion accelerator were available, which provided a rich variety of possibilities in nuclear fission studies. Pioneering work on fragment angular distributions, fission time-scales, transfer-induced fission, γ-ray multiplicities and mass–energy correlations were carried out, providing important information on the dynamics of the fission process. More recently, work on fission fragment γ-ray spectroscopy has been initiated, to understand the nuclear structure aspects of the neutron-rich fission fragment nuclei. There have also been parallel efforts to carry out theoretical studies in the areas of shell effects, superheavy nuclei, fusion–fission dynamics, fragment angular distributions, etc. to complement the experimental studies. This paper will provide a glimpse of the work carried out by the fission group at Trombay in the above-mentioned topics.

A method for nondestructive assay of nuclear materials in facilities with a pulse neutron generator and a digital technology for discrimination between neutrons and photons

A method for determining the U-235 concentration in fuel assemblies of a high-power channel-type PBMK reactor is described. The measure of U-235 content of an analyzed sample is the number of neutrons from thermal-neutron fission of U-235, normalized to the number of gamma quanta produced in thermal neutron capture by hydrogen nuclei in the scintillator or by B-10 in the glass of a photomultiplier tube. A pulse neutron generator based on DT reaction is the neutron source, and an organic scintillator with the pulse shape discrimination between neutrons and gamma rays with the aid of the digital technology is a detector. The scintillator is also used as a neutron moderator. Simulation of the method shows that the U-235 content of the analyzed sample can be determined for 1 min with an accuracy of 1% or better. The efficiency of the method has been confirmed by experimental investigations on a model of the setup.

Fission yields of molybdenum in the Oklo natural reactor

The isotopic compositions of molybdenum in six uranium-rich samples from the Oklo Zone 9 natural reactor were accurately measured by thermal ionization mass spectrometry. The samples were subjected to an ion exchange separation process that removed the isobaric elements zirconium and ruthenium, with high efficiency and a low blank. Molybdenum possesses seven isotopes of which 92,94,96Mo are unaffected by the fission process, enabling the raw data to be corrected for isotope fractionation by normalising to 92Mo/96Mo, and to use 94Mo to correct for the primordial component in each of the fission-produced isotopes. This enables the relative fission yields of Mo to be calculated from the isotopic composition measurements, to give cumulative fission yields of 1:0.941:0.936:1.025 for 95,97,98,100Mo, respectively. These data demonstrate that the most important nuclear process involved in reactor Zone 9 was the thermal neutron fission of 235U. The consistency of the relative cumulative fission yields of all six samples from different locations in the reactor, implies that Mo is a mobile element in the uraninite comprising Zone 9, and that a significant fraction of molybdenum was mobilized within the reactor zone and probably escaped from Zone 9, a conclusion in agreement with earlier published work.

Separation of radionuclides in a multitracer prepared by thermal neutron fission of 235U

In response to the needs of research groups in Japan, a chemical separation procedure was developed for isolating specific radioactive isotopes from a multitracer prepared by the reaction of 235U fission induced by thermal neutrons. This efficient technique is based on solvent extraction and separates the multitracer isotopes into six groups. It is intended to be used in an automated rapid extraction system.

Analysis of 235U Fission by Selective Channel Scission Model

Until now, it has not been clarified why the fission product of 235U induced by thermal neutrons shows two peaks. It is possible to explain this problem by the present idea of channel-dependent fission barrier introduced by the selective channel scission (SCS) model. Fission of 235U induced by thermal neutrons was analyzed theoretically by the SCS model. Using the SCS model the fission barrier of each scission channel is calculated. If the excited energy of the nucleus is more than some of the channel-dependent fission barriers, those channels can be opened to achieve fission. It is understood that the distribution of fission barriers results in two peaks for 236U. Hence, the distribution of fission products from thermal neutron fission of 235U also gives two peaks. It is known that fission products are almost stable isotopes at the peaks. If the nucleus can be excited at a lower energy level than that by thermal neutrons, cleaner fission of 235U is possible.

Preparation of Multitracer by Thermal Neutron Fission of 235U

Preparation of Multitracer by Thermal Neutron Fission of 235U

Studies on the level structure of 152Nd

The structure of 152Nd, produced via β− decay of 152Pr, has been investigated using γ-γ coincidence, γ-γ angular correlation, and β-γ-γ triple coincidence measurements. Praseodymium activities were obtained by the thermal neutron fission of 235U and collected by the on-line isotope separator KUR-ISOL. Nineteen γ-rays and four excited states were newly found. The spins of six exited states were determined and the parities of three of them were also deduced. The spin and parity of the 152Pr ground state, which were previously considered as 4−, are proposed to be 4+ by the present study.

Conversion electron measurements of 152Nd

Multipolarities of γ-rays in 152Nd have been studied by conversion electron measurements in the β− decay of 152Pr. The praseodymium activities were obtained by the thermal neutron fission of 235U and followed with the on-line isotope separator KUR–ISOL. Conversion electrons and γ-rays were measured with a Si(Li) detector and a HPGe detector, respectively. The two γ-rays, 226 keV and 285 keV, were decided to have the multipolarities of the E1 transition. The spins and parities of the 1600 keV and 1542 keV levels were determined to be 3+ and 2+, respectively.

Range of products withA=131–136 in the reactor neutron fission of237Np,243Am by and thermal neutron fission of233U,235U, and239Pu

Range of products withA=131–136 in the reactor neutron fission of237Np,243Am by and thermal neutron fission of233U,235U, and239Pu

Precision determination of charge dispersion and distribution in the proton-induced fission of238U at 13.9MeV excitation

By means of a gas-jet transport system and the ordinary radiochemical stacked-target technique incorporated with the Geγ-ray spectrometry, 126 product nuclides with half-lives ranging from seconds through a year were identified and their yields were determined in the proton-induced fission of238U at 13.9 MeV excitation. The charge dispersion width was determined for mass chains and concluded to be constant with respect to the mass. The average of the deduced widths was 0.69±0.04 charge unit which was just equal to that of thermal-neutron fission of235U and also consistent with values reported for proton-induced fission of238U. From the width thus obtained, we determined the most probable chargesZ p for observed isobaric multiplets. Considering the pre-scission neutron systematics and an appropriate charge polarization between the complementary fragments we deduced from the obtainedZ p the post-scission neutron numberν post that turned out fairly consistent with Terrell’s neutron systematics for thermal-neutron-induced and spontaneous fission. Thus obtainedν post enabled us in turn to derive equations describingZ p as functions ofA. The obtained charge dispersion and charge distribution resulted in the mass-yield distribution resembling that of thermal-neutron fission.

Isomeric yields of130Sb,132Sb,134I, and136I in the thermal neutron fission of235U

Isomer yield ratios of130Sb,132Sb,134I and136I isomers formed in the thermal neutron fission of235U have been calculated from our previous experimental studies that led to the identification of these species. In those studies the iodine and antimony fractions formed in fission were rapidly separated and the decay of γ-rays belonging to each isomer pair were followed using Ge(Li) detectors and a multichannel analyzer. The isomer ratios were calculated from growth and decay considerations of these γ-rays. The results are compared with the recently published values obtained with an on-line isotope separator, those from LOHENGRIN, and those from model calculations. Angular momenta of fission fragments corresponding to the measured isomer yields have also been calculated.

Fine structure of mass and charge distribution in low energy fission

Studies of finer details in mass and charge distribution fission leads to a better understanding of the fission process. Experimental determination of independent and cumulative yields using radiochemical techniques as well as mass spectrometers and fission product recoil separators form the basis of such studies. It has been established that closed shells as well as an even number of nucleons influence both mass and charge distributions. The magnitudes of these effects may be estimated from existing experimental yield data and various fission models. Using our measurements of several fission yields and those existing in the literature we have calculated even-odd proton and neutron effects for various low energy fissioning systems. Where enough data existed, direct calculations were made, whereas for other cases the Zp-model of WAHL has been used. It is found that the even-odd proton effect is well established and pronounced in thermal neutron fission of235U and233U. Lesser effects were found for reactor neutron induced fission of232Th, thermal neutron fission of239Pu and spontaneous fission of245Cm and249Cf. No effect seems to exist in the thermal neutron fission of241Pu and the spontaneous fission of252Cf. The even-odd neutron effect is found to be much lower than the corresponding proton effect in235U and233U fissions and is nonexistent in the rest of the fissioning systems.

Nuclear decay studies of fission-product nuclides using an on-line mass separation technique

An isotope-separator-on-line (ISOL) system has been developed at the Idaho National Engineering Laboratory to enable a wide variety of nuclear decay studies to be made for fission-product radionuclides. The system is unique in that it utilizes the spontaneous fission source,252Cf, as the source of fission-product radioactivity. Fission products are transported to the ion source of the mass separator by the helium gas-jet technique. Mass-separated beams of previously unattainable rare-earth nuclides are produced with this system because of the higher yield of fission products with A>150, relative to that for thermal-neutron fission of235U, and the use of a relatively efficient ion source. Recent decay studies reported here include systematic measurements of rare-earth nuclide half-lives and comparison of them to theoretical prediction, a decay scheme investigation for154Nd, and β-strength function measurements for140Cs.

Comparison of chemical etching with the alkali-alcohol mixture and the ultrasonic etchings of fission fragment and alpha particle tracks in lexan polycarbonates

Abstract Tracks of the fission fragments from the thermal neutron fission of 235U and of the alpha particles from the (n, alpha) reaction of 6Li registered in Lexan samples respectively and etched by the mixed solution of alcohol and KOH, a short name PEW,4 the ultrasonic and the normal chemical etching processes at 40°C for 40 min and 50 min respectively were compared. The differences of the tracks revealed by these methods have been shown in photomicrographs. To compare the bulk etching rates, the rates of decrease in thickness of the Lexan samples were obtained for the PEW, ultrasonic, and the chemical etchings to be 12, 1.2, and 0.73μm/h respectively for the samples without the neutron irradiations, and 18.8,1.6, and 0.85 μm/h respectively for the samples irradiated by the neutrons fluence of 3 × 1014 neutrons/cm2.

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.

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.

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.