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 Z_p-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.     

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.     

Determination of pre-scission kinetic energy in low energy fission using isotonic charge distribution model

The spectral lines of the 478 keV prompt -ray from^7*Li produced in the¹⁰B(n, )^7*Li reaction were measured for a silicon wafer coated with a thin boron layer using neutron beams. The shapes of the Doppler broadened lines were found to depend greatly on the angle between the measurement axis and the boron layer surface. The angular dependence was interpreted according to whether the energetic^7*Li ions are ejected into air or into the substance in the forward or backward recoil of^7*Li to the -ray detector.     

Mass and charge distribution in the fission of 238U

The mass and charge distribution in the fission of ²³⁸U by protons of 15·6 MeV have been determined using Ge(Li) spectroscopy. The total fission cross section was found to be 1005 mb. For 10 shielded or quasi-shielded nuclides, correlation with a Gaussian charge dispersion was investigated using various mass formulae and charge distribution hypotheses. The postulate of minimum potential energy using a shell-effected mass equation was found to best correlate the data.     

Systematics of nuclear charge distribution in fission the ZP model

An extension of the conventional Z_P model has been used with data for ten fission reactions and the method of least-squares for derivation of parameters that describe the nuclear charge-distributions. Data for light and heavy fission products for each reaction were treated together by consideration of the complementarity of average mass numbers of precursors (A′_L+A′_H=A_F). This treatment gave calculated yields that are essentially equal for complementary elements (Z_L+Z_H=Z_F). The parameters determined, , the Gaussian dispersion width parameter, a linear function of Δ_Z=|Z_P-A′(Z_F/A_F)|, Z_P being the Z at maximum of the curve, and and , the even-odd proton and neutron yield enhancement factors, represented most data (A_H⋟130–150 and the complementary light A's) moderately well, better than did oscillating functions of and Δ_Z. The values were found to be approximately constant (0.48 to 0.62) and, generally, to increase with the average number of neutrons emitted, . The Δ_Z values at A′=140 were in the range of 0.42 to 0.50, and the slopes, ∂Δ_Z/∂A′_H, ranged from 0.0 to 0.03. The factor was found to be several times larger than the factor and to decrease with increasing excitation energy and with A and/or Z of the fissioning nucleus, as has been reported previously. The adequacy of the Z_P model for representation of data and for prediction of yields in the high-yield mass-number regions mentioned above are discussed, and the large uncertainties that are involved in extrapolating the model to near symmetric or to very asymmetric mass and charge divisions are pointed out.     

Characteristics of the asymmetric mass distribution in proton-induced fission of actinides

To study the correlation between fragment mass distributions and shell structures of fission fragments, fragment mass and energy distributions in proton-induced fission of ²³²Th and uranium isotopes, ^233,235,238U, were precisely measured using a double-TOF method. It was found that the lighter side of the heavier wing of asymmetric mass distributions shifts to larger fragment mass number corresponding to the N/Z value of the fissioning nucleus. The results are explained qualitatively by the change of the most probable mass number of fission fragments of Z = 50 proton-shell.     

Radiochemical studies on generator eluted113mIn chloride

^113mIn is milked out of a¹¹³Sn-^113mIn generator with dilute hydrochloric acid for use in nuclear medicine. The concentrations of the various impurities like Sn, Zr and their colloidal forms which may trap^113mIn activity have to be initially evaluated before releasing the generator for medicinal use. The authors have evaluated the purity of the^113mIn-chloride obtained from the generators in detail. The possibility of using a mixture of HCl and NaCl as an alternative eluent for^113mIn has also been investigated. It has been observed that this new eluent gives greater yields of^113mIn and renders the final manipulation of isotonicity of indium labelled compounds easy.     

Effect of protein stabilization on charge state distribution in positive- and negative-ion electrospray ionization mass spectra

Changes in protein conformation are thought to alter charge state distributions observed in electrospray ionization mass spectra (ESI-MS) of proteins. In most cases, this has been demonstrated by unfolding proteins through acidification of the solution. This methodology changes the properties of the solvent so that changes in the ESI-MS charge envelopes from conformational changes are difficult to separate from the effects of changing solvent on the ionization process. A novel strategy is presented enabling comparison of ESI mass spectra of a folded and partially unfolded protein of the same amino acid sequence subjected to the same experimental protocols and conditions. The N-terminal domain of the Escherichia coli DnaB protein was cyclized by in vivo formation of an amide bond between its N- and C-termini. The properties of this stabilized protein were compared with its linear counterpart. When the linear form was unfolded by decreasing pH, a charge envelope at lower m/z appeared consistent with the presence of a population of unfolded protein. This was observed in both positive-ion and negative-ion ESI mass spectra. Under the same conditions, this low m/z envelope was not present in the ESI mass spectrum of the stable cyclized form. The effects of changing the desolvation temperature in the ionization source of the Q-TOF mass spectrometer were also investigated. Increasing the desolvation temperature had little effect on positive-ion ESI mass spectra, but in negative-ion spectra, a charge envelope at lower m/z appeared, consistent with an increase in the abundance of unfolded protein molecules.     

Radiochemical studies on the separation of cobalt with sodium alginate

⁶⁰Co was used as indicator for studies on the separation of cobalt with sodium alginate. Effect of eight experimental parameters (digestion time, digestion temperature, mode of separation, concentration of reagents, pH, order of heating, ammonia, mixing of reagents) on the precipitation and separation of cobalt alginate have been investigated.     

Adsorption of fission products on a metal surface in nitric acid solutions: Radiochemical study using a multitracer

Adsorption behavior of fission products in nitric acid solution on various alloys and metals was studied by using a multitracer produced by neutron irradiation of UO₂. The adsorption behavior of the fission products ⁹⁹Mo, ¹³¹I, ¹³²Te, ¹⁴⁰La, and ¹⁴³Ce, and ²³⁹Np was simultaneously studied. Some chemical decontamination tests were also examined. Clear adsorption of ⁹⁹Mo, ¹³¹I, and ¹³²Te was observed, whereas adsorption of ¹⁴⁰La, ¹⁴³Ce, and ²³⁹Np was not. The adsorption characteristics were discussed by considering anion-exchange reaction and surface complexation.     

Radiochemical studies of the sorption behavior of strontium and barium

The sorption behavior of strontium and barium on kaolinite, bentonite and chlorite-illite mixed clay was studied by radioanalytical techniques using the batch method.⁹⁰Sr (29.1 y) and¹³³Ba (10.5 y) were used as radiotracers. Characterization of the solid matrices was done by FTIR and XRD spectrometries and specific surface area measurements. Synthetic groundwater was used as the aqueous phase. The variation of the distribution ratioR _d, as a function of metal ion loading was examined. The sorption isotherms were fitted to various isotherm models. The sorption energies were calculated to be in the range of 8–10 kJ/mol suggesting an ion exchange type of sorption mechanism. In detailed experiments, chlorite-illite mixed clay was first presaturated with K⁺, Sr²⁺, Ca²⁺ and Al³⁺ ions, respectively, prior to sorption studies with Ba²⁺ ions. The results of Ca²⁺ pretreated chlorite-illite were very similar to those of natural chlorite-illite, suggesting that the Ba²⁺ ion exchanges primarily with the Ca²⁺ ion on the clay minerals.     

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.     

Radiochemical neutron activation analysis of Mo and W in geochemical and cosmochemical samples with an emphasis on the correction of fission products

Radiochemical neutron activation analysis (RNAA) was applied to geochemical and cosmochemical samples to determine trace amounts of Mo and W. To determine the Mo concentration by NAA accurately, the contribution of the fission products of U should be corrected. For that reason, we developed a simple and effective method, where a contribution of fissiogenic ⁹⁹Mo was estimated by monitoring the ratio of uranium fission-product ⁹⁹Mo to ¹³³I. Mo concentrations corrected for fission with the W concentrations were consistent with the literature values, showing that ¹³³I was found to be an effective monitor for fission correction. Detection limits are estimated to be 10 ppb for Mo and W and 30 ppb for U under the present experimental conditions.     

Modifying the charge state distribution of proteins in electrospray ionization mass spectrometry by chemical derivatization

Electrospray ionization (ESI) of denatured proteins produces a broad distribution of multiply-charged ions leading to multiple peaks in the mass spectrum. We investigated changes in the positive-mode ESI charge state distribution produced by several chemical modifications of denatured proteins. Capping carboxylic acid groups with neutral functional groups yields little change in charge state distribution compared with unmodified proteins. The results indicate that carboxyl groups do not play a significant role in the positive charging of denatured proteins in ESI. The modification of proteins with additional basic sites or fixed positive charges generates substantially higher charge states, providing evidence that the number of ionizable sites, rather than molecular size and shape, determines ESI charging for denatured proteins. Fixed charge modification also significantly reduces the number of protons acquired by a protein, in that the charge state envelope is not increased by the full number of fixed charges appended. This result demonstrates that Coulombic repulsion between positive charges plays a significant role in determining charge state distribution by affecting the gas-phase basicity of ionizable sites. Addition of fixed-charge moieties to a protein is a useful approach for shifting protein charge state distributions to higher charge states, and with further work, it may help limit the distribution of protein ions to fewer charge states.     

Kinetics of small ion evaporation from the charge and mass distribution of multiply charged clusters in electrosprays

The distribution of charge z and radii R in clusters electrosprayed from formamide solutions of tetraheptylammonium bromide was investigated by selecting those within a narrow range of electrical mobilities Z(1) in a first differential mobility analyzer (DMA), reducing their charge to unity by passage through a neutralizing chamber containing a radioactive (alpha) source, and measuring the mobilities Z(z) of the resulting discrete set of singly charged clusters in a second DMA. After correcting for the polarization contribution to cluster drag, the tandem DMA data yield the range of radii present at detectable levels for each charge state up to z = 9. Because small ion evaporation from electrospray drops leads to charge loss when a drop reaches a certain critical radius R(crit)(z), the measured maximum and minimum cluster radii associated with a given z can be used to infer the activation energy Delta for ion evaporation as a function of drop charge and curvature. These results confirm the Iribarne-Thomson ion-evaporation mechanism, and support earlier theoretical expressions for the functional form of Delta(z,R). The different phenomenon of ion evaporation from metastable multiply charged dry clusters is also observed at characteristic times of 1 s. Its activation energy is estimated as approximately 0.3 eV larger than for ion evaporation from the drops. This new process complicates the interpretation of the present measurements in terms of ion evaporation from liquid surfaces, but introduces no radical change in the picture. It helps understand why salt clusters with more than two or three charges are harder to see in mass spectrometers than in mobility studies under ambient conditions. Copyright 2000 John Wiley & Sons, Ltd.     

Nuclear charge distribution in the spontaneous fission of252Cf: Determination of fractional cumulative yields of133mTe and133gTe

The fractional cumulative yields (FCY) of^133mTe and^133gTe in the spontaneous fission of²⁵²Cf were measured for the first time by a radiochemical method. The values ofFCY are 0.533±0.014 and 0.291±0.042 for^133mTe and^133gTe, respectively. The isomeric state to ground state fractional independent yield (FIY) ratio of¹³³Te,R, was found to be 3.5. The root-mean-square angular momentum of the primary fragment corresponding to the fission product¹³³Te, Jr.m.s.=8.8h, was estimated according to a simple one-parameter statistical model. The fractional cumulative yields from this work together with other literature data in the mass region A=131–141 are compared with the normal yields given by the empiricalZ _p model by Whhl. It suggests that both theN=82 neutrons shell and nucleus pairing effects are not apparent for the spontaneous fission of²⁵²Cf.     

Radiochemical studies of the retention of volatile iodine in aqueous solutions

Volatile molecular iodine and organic iodine species would be generated under radiation in the gas space and sump of nuclear power plant containments during a severe accident involving core melt. Engineered and currently installed safety systems might not effectively retain highly volatile iodine species. Fast and efficient conversion to non-volatile species within the containment would lower the potential risk of a significant release into the environment and therefore, methods must be sought to reduce iodine volatility to a minimum or, ideally, to remove it completely. This paper presents the results of an experimental programme to obtain a fast and efficient reduction of highly volatile organic iodide species in aqueous solution into iodide ions.