Cumulative yields of short-lived ruthenium isotopes in the thermal neutron induced fission of233U,235U and239Pu

Cumulative yields of short-lived ruthenium isotopes in the thermal neutron induced fission of²³⁵U,²³⁵U and²³⁹Pu have been determined using a fast radiochemical separation technique followed by gamma spectrometry. The cumulative yields of¹⁰⁷Ru and¹⁰³Ru in²³³U (n_th, f) and¹⁰⁷Ru and¹⁰⁹Ru in²³⁹Pu (n_th, f) are determined for the first time. The measured cumulative yields are converted to chain yields assuming normal charge distribution systematics for comparison with the literature data on chain yields.     

Measurement and evaluation of some low-energy fission yield

Methods of independent fission yield measurement are briefly reviewed and compared one to another, experimental yield data are compiled for some low-energy fissioning systems. Example of predicting fission yields in thermal-neutron induced fission of²³⁹Pu is evaluated with even-odd effect presented. Future directions of experiments and evaluations are discussed.     

Uncertainty of decay heat calculations originating from errors in the nuclear data and the yields of individual fission products

The calculation of the abundance pattern of the fission products with due account taken of feeding from the fission of²³⁵U,²³⁸U, and²³⁹Pu, from the decay of parent nuclei, from neutron capture, and from delayed-neutron emission is described. By means of the abundances and the average beta and gamma energies the decay heat in nuclear fuel is evaluated along with its error derived from the uncertainties of fission yields and nuclear properties of the individual fission products.     

The diminution of the neutron odd-even effect due to prompt neutron emission in low energy nuclear fission

Two fissioning systems [²³⁵U(n_th, f) and²⁵²Cf(sf)] have been studied to see the consequence of prompt neutron emission on odd-even effects. Monte Carlo calculations have been carried out for prompt neutron emission. Fragment and product yields have been analyzed by the WahlZ _p Model, the Method of Differences and Model Independent Calculations. The result of the wash-out effect of prompt neutron emission on the neutron odd-even effect has been shown; however, this was not seen with the protons.     

Fission product yields in the fast-neutron fission of238U

The fission yields of 38 fission products in the fast-neutron induced fission of²³⁸U have been determined using a rapid, multiscaling gamma-ray spectroscopic method. To obtain absolute yields for fission products having half-lives ranging from 32 s to 40 d, a total of 56 multi-scaling gamma-ray spectra were collected using various irradiation and cooling periods. Gamma-rays and photopeak areas of interest were assigned to the fission products by their energies and half-lives. Fission product activities were evaluated from spectral data using growth and decay calculations and fission yields were determined by normalizing the¹⁴⁰Ba yield to the average value from reported data. The depleted uranium target, covered with a boron-cadmium thermal neutron shield, was used to keep interference from the fission of²³⁵U minimal. Results for the cumulative fission yields, including 17 mostly short-lived fission products measured for the first time, are compared with previous measurements and with the recommended yields in recent evaluations. The agreement, and some discrepancies, in the comparisons are discussed. No explicit even-odd pairing effects are observed in the fission yield data for fast-neutron induced fission of²³⁸U.     

Influence of even nucleon numbers and closed shells on mass and charge distribution in low energy fission

Extensive experimental and evaluated yield data are now available for the fragments in low energy nuclear fission. Using these data we re-examined the influence of even nuclear numbers and closed shells on the mass and charge distribution in low energy fission. We used WAHL's Zp model and Ap model. We also examined the effects ofZ=50,N=82 and possiblyN=88 shells. A new method was developed based on two Gaussian curves for the even and odd products. TheEOZ andEON values calculated are based on these different methods and are seen to be consistent with each other in spite of the somewhat different definitions of the even-odd effect. The even-odd effect decreases with an increasing fissility parameter. TheEON values are substantially lower than the correspondingEOZ values, probably due to the effect of washing out the neutron pairing effect by prompt neutron emission. The magnitude of the even-odd effect varies with different mass regions. TheEOZ andEON values decrease as they go from asymmetric to symmetric regions in mass distributions.     

A fast radiochemical procedure for separating iodine from fission products

The iodine isotopes produced by thermal neutron fission of²³⁵U were separated by a fast chemical procedure, with a separation yield of 95%. The gamma-ray energies and relative intensities in the decay of¹³¹I,¹³²I,¹³³I,¹³⁴I and¹³⁵I were determined using a Ge(Li) detector.     

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, and^136gI, 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.     

Possible exit channel effect on isomer yield ratios

Isomer yield ratio measurements in fission are important in understanding the fission process. With the development of new instrumental techniques, a large number of yield data are now available. The experimental data on isomer yield ratios in the thermal neutron induced fission of²³⁵U are compared with those calculated from the simple statistical model byMadland andEngland. The method of calculation has been extended to the isotopes having more than one isomeric state. The results may be explained according to the multi-exit-channel model of fission.     

Dosage du237Np en trances dans l'uranium et le plutonium

The²³⁷Np content of²³⁸Pu or²³⁹Pu samples were determined by the gammaspectrometry of²³⁸Np formed by thermal neutron activation. The measurements were carried out on irradiated²³⁸Pu samples directly, and after the chemical separation of²³⁹Pu samples. The²³⁷Np content of natural uranium was determined from the ratio of the alpha-activities of²³⁸Pu and²³⁹Pu isotopes formed from the decay of neptunium isotopes produced by the activation of²³⁷Np and²³⁸U isotopes, respectively.      

Non-destructive determination of spontaneously fissioning nuclides by neutron coincidence counting using multichannel time spectroscopy

A non-destructive method for determining the amount of actinoids has been developed. The method is based on thermal neutron coincidence counting and employs a selective detection of neutrons resulting from the spontaneous fission of actinoids. The detection system is described in detail and the measurement results of²⁴⁴Cm as an example are presented. The results show that the measured fission rate of²⁴⁴Cm is consistent with the fission rate calculated from ENDF/B-V data and that the amount of²⁴⁴Cm can be determined within about 5% accuracy even in the presence of a large amount of actinoids, for example, up to 2.6·10⁶, 3.6·10⁴, or 1.6·10³ times in the mass ratio of²³⁹Pu,²⁴¹Am, or²⁴⁰Pu to²⁴⁴Cm, respectively.     

Radiochemical determination of the yields of transuranium nuclides produced from natural uranium irradiated in a reactor

Transuranium nuclides were produced by irradiating a pellet of natural uranium sulfide in the Japan Material Testing Reactor (JMTR). After irradiation, a successive separation of uranium, plutonium, americium and curium was carried out. The fractional concentrations of the nuclides²³⁸Pu,²³⁹Pu,²⁴⁰Pu,²⁴¹Am,²⁴³Am,²⁴²Cm and²⁴⁴Cm were determined by α-ray spectrometry, and those of²⁴¹Pu and^242mAm were estimated from the build-up of α-emitting daughters,²⁴¹Am and²⁴²Cm, respectively. As the yield of²⁴²Pu was too slight to be detected by α-counting, the neutron activation analysis of the plutonium fraction based on the²⁴²Pu(n, γ)²⁴³Pu reaction was carried out by γ-ray spectrometry, and it was shown that a few pg of²⁴²Pu could be determined. A burn-up of²³⁵U was also estimated by neutron activation analysis. The experimental results are compared with the calculated ones.     

Experimental evaluation of239Pu,238Pu and233U spikes for determining plutonium concentration by thermal ionization mass spectrometry and alpha-spectrometry

Experimental evaluation on the use of²³⁹Pu spike in Isotope Dilution-Thermal Ionization Mass Spectrometry (ID-TIMS),²³⁸Pu spike in Isotope Dilution Alpha Spectrometry (IDAS) and²³³U as a Non-Isotopic Diluent in Alpha Spectrometry (N-IDAS), for determing plutonium concentration in samples with burn-up values in the range of 1,000–10,000 MWD/TU is done. Precision is determined by analyzing replicate aliquots from different samples using each of the three spikes. Accuracy is established by comparing the results with those obtained by using well recognized spike²⁴²Pu in ID-TIMS. It is shown that the use of²³⁹Pu spike with the latest generation thermal ionization mass spectrometers gives the best precision (0.2%), whereas the precision values of 0.5 and 1% can be obtained by using²³⁸Pu and²³³U spikes, respectively, on a routine basis. Reasons for the difference in the precision values are discussed, along with the merits and drawbacks on the use of different spike isotopes.     

Limit of the INAA procedure for copper determination based on the64Cu 511 keV line

The²³⁵U/²³⁸U ratio is determined by neutron activation analysis counting the ϕ-rays of short half-lives fission products and²³⁹U. The effect of the neutron spectrum hardening using a⁶LiD converter is also demonstrated. The²³⁵U/²³⁸U ratio is determined using short irradiation, waiting and counting times.     

Simultaneous determination of trace uranium and thorium by radiochemical neutron activation analysis

A method for the simultaneous, radiochemical neutron activation analysis of uranium and thorium at trace levels in biological materials is described, based on a technique known as LICSIR, in which a double neutron irradiation is employed. In the first, long irradiation²³³Pa (27.0 d) is induced by neutron capture on²³²Th and then the sample is cooled for several weeks. A second short irradiation to induce²³⁹U (23.5 m) is followed by a rapid sequential radiochemical separation by solvent extraction of²³⁹U with TBP and²³³Pa with TOPO. Chemical yields of²³⁹U and²³³Pa were measured for each sample aliquot using added²³⁵U and²³¹Pa tracers from the -spectra of the separated fractions. The technique was validated by quality control analyses.     

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

The fractional cumulative yields of¹³⁵I,¹³⁸Xe and¹⁴⁰Ba in the particle (30 MeV) induced fission of²³²Th have been determined following the growth and decay of^135gXe,^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 () compared to the normally observed =0.56±0.06 for thermal neutron fission of²³⁵U.     

Determination of the 238U spontaneous fission decay constant without neutron irradiation

We have developed a methodology for measuring the decay constant of the spontaneous fission of ²³⁸U, l_f, using nuclear particle track detectors where thermal neutron irradiation is unnecessary. This methodology is based on the fact that the radiation damage caused by spontaneous fission of trans-uranium elements bearing a mass number close to 238 are similar to ²³⁸U spontaneous-fission ones. Loading a thick source of uranium (thickness greater than the fission fragment range) with a small amount of a suitable trans-uranium element (for instance, ²⁴²Pu, which presents a spontaneous fission half-life of 6.75^.10¹⁰ y), it is possible to determine the observation efficiency of a particle-track detector for fission fragments. Procedures concerning our thick source manufacture and uniformity tests of the trans-uranium distribution are also presented. These results make it possible for the exposure of thick uranium sources (without trans-uranium element) to lead to a l_f value.     

Using low resolution gamma detectors to detect and differentiate 239Pu and 235U fissions

When ²³⁹Pu and ²³⁵U undergo thermal neutron-induced fission, both produce significant numbers of beta-delayed gamma rays with energies in the several megaelectron volt range. Experiments using high energy-resolution germanium detectors have shown that it is possible to distinguish the fission of ²³⁹Pu from that of ²³⁵U. It is desirable to detect the presence of ²³⁵U or ²³⁹Pu using detectors that are less expensive and more rugged than high purity germanium detectors. To this end we demonstrate how differences in the energy spectrum and decay rates of the beta-delayed gamma rays can be used to identify ²³⁹Pu and ²³⁵U using low resolution plastic and liquid scintillator detectors. Experimental data are used to identify differences in the spectra and also to test the identification algorithms. Results to date are very promising.     

Spontaneous-fission decay constant of 238U measured by nuclear track techniques without neutron irradiation

A value of the decay constant for spontaneous fission of ²³⁸U, l_f, determined by particle-track detector techniques, is presented. The main source of systematic error in fission-track measurements of l_f, the so called neutron dosimetry, has been avoided. Instead of calibrating the neutron source and the usual mica detector through tracks of ²³⁵U fission induced by thermal neutrons, spontaneous-fission tracks and alpha-particle tracks of ²³⁸U and ²⁴²Pu were used. A value of l_f = (8.66±0.38)·10^-17 y^-1 has been obtained.     

Precision and accuracy in the determination of plutionium-239 /uranium-233, americium-241/ uranium-233 and curium-244 /uranium-233 alpha activity ratios by alpha spectrometry

Determination of²³⁹Pu/²³³U,²⁴¹Am/²³³U and²⁴⁴Cm/²³³U alpha activity ratios is required when using²³³U as a tracer for the determination of plutonium, americium and curium by alpha spectrometry. Precision and accuracy in the determination of these alpha activity ratios was evaluated by preparing synthetic mixtures from solutions of enriched isotopes of²³⁹Pu,²⁴¹Am,²⁴⁴Cm and²³³U. Separate synthetic mixtures were prepared for each of the three alpha activity ratios. The sources from the synthetic mixtures were prepared by direct evaporation method using tetra ethylene glycol /TEG/ as a spreading agent, alpha spectra were recorded by employing solid state silicon surface barrier detectors coupled to a 4 K analyzer and the alpha spectra were evaluated by a method based on the geometric progression decrease for the far tail of the spectrum. Large area detector /i.e. 450 mm²/ was observed to reduce the effect of nonhomogeneous distribution, if any, of the two elements present in the source. Precision and accuracy of about 1% is demonstrated for the determination of²³⁹Pu/²³³U,²⁴¹Am/²³³U and²⁴⁴Cm/²³³U alpha activity ratios using large area silicon surface barrier detector.