Independent Fission Research Articles

Distributions isotopiques des gaz rares et de leurs précurseurs dans la fission thermique de 239, 241Pu: Etude de l'effet “pair-impair”

The independent and cumulative fission yields of rare gas and halogen isotopes produced from the fission of 239Pu and 241Pu by thermal neutrons have been measured by means of on-line isotopic separation. Mass and charge distributions have been studied. Elemental yields can be deduced for the two fissioning systems. A comparison with 233U and 235U fission is made for the proton odd-even effect.

Independent yields of 148 mPm, 148gPm and 146Pm for thermal-neutron-induced fission of 233U and 235U

The independent fission yields of 148 m Pm, 148 g Pm and 146Pm have been measured for thermal-neutron-induced fission of 233U and 235U. The yields of 148 m Pm are (11.3 ± 1.3) × 10 −7% for 233U, and (6.7 ± 0.6) × 10 −9% for 235U. The yields of 148 g Pm are (3.7 ± 0.9) × 10 −7% for 233U and (2.8 ± 0.7) × 10 −9% for 235U. Upper limits for the yields of 146Pm have been set: 1.1 × 10 −8% for 233U and 4.5 × 10 −10% for 235U. The deviation of the very low fractional chain yields from the “normal” Gaussian charge distribution curve with a width parameter σ = 0.56 ± 0.06 is discussed.

Analyzing features of recoil spectrometers

Since 1973 two new recoil spectrometers for the study of the products of the thermal neutron induced fission are in operation: the parabola spectrometer LOHENGRIN and the gas-filled separator JOSEF. Their analyzing features have been considerably improved compared to older facilities of similar type. Thus the high mass and energy resolution and the good intensity of LOHENGRIN enable a systematic determination of the fractional independent fission yields. Because of the increased mass and nuclear charge resolution JOSEF offers special possibilities for the investigation of β-decay chains and of β-decaying isomeric states. The applications which recoil spectrometers find at heavy ion accelerators are the selection of the rare reaction products out of the intense beam of projectiles and the analysis of the basic properties such as mass, nuclear and ionic charge, kinetic energy and emission angle of the reaction products.

A recent source modification for noble gasses at the Los Alamos on-line mass analysis facility

The Los Alamos on-line mass analysis experiment at the Godiva-IV burst reactor facility has been modified to determine independent fission yields of noble gasses. The gasses are released from a stearate target and ionized by electron bombardment. The distance traveled by the gasses from the target to the ionization chamber is 20 cm. The efficiency of the electron bombardment source is lower than that of the surface ionization source that was employed to measure the yields of Rb and Cs. But this effect is compensated by the larger quantity of target metal that is possible when using a stearate target.

Independent fission yields of Rb and Cs from thermal-neutron-induced fission ofPu239

The relative independent fission yields of Rb and Cs from thermal-neutron-induced fission of $^{239}\mathrm{Pu}$ have been measured on line using a mass spectrograph and thermalized neutrons from a burst reactor. Independent yields were derived by normalizing the measurements to products of chain yields and fractional independent yields, estimating the latter from measured cumulative yields of Kr and Xe. Comparing the independent yields with those from $^{238}\mathrm{U}$ fission, the $^{239}\mathrm{Pu}$ results show shifts in isotopic yield distribution toward lower mass for both Rb and Cs and also toward the production of more Cs and less Rb when $^{239}\mathrm{Pu}$ is fissioned.NUCLEAR REACTIONS $^{209}\mathrm{Pu}(n, f)$, $E=\mathrm{thermal}$; measured relative abundances; deduced independent fission yields. Mass spectrograph, burst reactor.

Independent yields of 84Rb, 86Rb and 136Cs for thermal-neutron-induced fission of 233U, 235U and 239Pu

The independent fission yields of 84Rb (a positron emitter), 86Rb and 136Cs have been measured for thermal-neutron-induced fission of 233U, 235U and 239Pu. The yields of 84Rb are (6·6 ± 0·7) × 10 −8% for 233U, (2·4 ± 0·3) × 10 −9% for 235U, and (3·9 ± 0·4) × 10 −8% for 239Pu. The yields of 86Rb are (9·2 ± 0·9) × 10 −4% for 233U, (2·1 ± 0·2) × 10 −5% for 235U, and (1·9 ± 0·2) × 10 −4% for 239Pu. The yields of 136Cs are 0·105 ± 0·011% for 233U, 0·0074 ± 0·0007% for 235U, and 0·100 ± 0·010% for 239Pu. The half-life of 136Cs by least squares analysis of the decay of eleven independent samples is 13·16 ± 0·03 days.

Independent fission yields of Rb and Cs fromU238induced by fission-spectrum neutrons

The relative independent yields of Rb and Cs from $^{238}\mathrm{U}$ fission induced by fission-spectrum neutrons have been determined by means of a mass spectrograph on line with a nuclear burst reactor. The spectrograph ion source is a tantalum oven containing a target of high purity $^{238}\mathrm{U}$ in a graphite matrix. After irradiation, the radioactive fission products are deposited on the focal plane of the spectrograph and each mass position is analyzed quantitatively from its $\ensuremath{\beta}$ activities. Independent yields were calculated by normalizing the measurements using chain yield values and fractional independent yields that were calculated from recently measured cumulative yields for Kr and Xe. The results indicate a charge deviation from an unchanged charge distribution of 0.41 \ifmmode\pm\else\textpm\fi{} 0.07 nuclear charge units, a scaling factor of 1.27 for the number of neutrons associated with $^{238}\mathrm{U}$ fission over the prompt number from thermal-neutron fission of $^{235}\mathrm{U}$ for these fission products, and values of $\ensuremath{\sigma}$ for the isobaric charge distribution of 0.49 \ifmmode\pm\else\textpm\fi{} 0.04 and 0.60 \ifmmode\pm\else\textpm\fi{} 0.02 for Rb and Cs, respectively. An odd-even $Z$ effect is noticeable in that the fractional independent yields are generally lower by (20 \ifmmode\pm\else\textpm\fi{} 10)% for both Rb and Cs than the mean values estimated from a smooth theoretical model. The sum of the estimated independent yields for the Rb isotopes agrees within error with that for the isotopes of the complementary element Cs.NUCLEAR REACTIONS $^{238}\mathrm{U}(n,f)$, $E=\mathrm{fission}\mathrm{spectrum}$; measured relative abundances; deduced independent fission yields, charge deviation, odd-even $Z$ effect. Mass spectrograph, burst reactor.

Studies of the mass distribution in fission of uranium induced by 170 MeV protons

Cumulative and independent fission yields have been determined radiochemically for some long-lived tin and antimony isotopes formed in the reaction U(170 MeV p, f). By use of charge dispersion curves established earlier, chain yields in the mass range 117 ⩽ A ⩽ 126 were calculated. The mass distribution curve has been drawn in the same mass range and proves that a valley occurs in this region. Thus it is confirmed that the mass distribution curve is neither symmetric, nor has it a broad flat maximum.

Independent yields of 133,135Xe in the fission of 235U, 238U and 239Pu by 14 MeV neutrons

The direct fission populations of states in 133,135Xe have been measured for fission induced by 14 MeV neutrons. Targets consisting of metal foils of 235U, 238U and 239Pu were separately encapsulated and irradiated for periods of 1 to 8 min in a flux of 1.5 × 10 11 n cm 2 · s . The xenon isotopes of interest were extracted from the bulk fission products within 3 to 12 min after irradiation. The xenon γ-rays were then examined with a Ge(Li) detector and the populations of the 133,135Xe isomer and ground states were deduced. Computer analysis techniques were employed to extract the independent fission yield values from this data.

Independent Yields of Krypton and Xenon Isotopes in Thermal-Neutron Fission ofU235. Observation of an Odd-Even Effect in the Element Yield Distribution

The fractional and independent fission yields of krypton isotopes (masses 87 to 94) and xenon isotopes (masses 137-143) produced in thermal-neutron fission of $^{235}\mathrm{U}$ were measured using a mass separator operating in an on-line mode with a fission source exposed to time-controlled neutron fluxes. The noble-gas isotopes were collected simultaneously at given times and each mass was measured separately. The distribution of the isotopic yields of each element was found to be Gaussian, in general agreement with Wahl's estimates. The total independent yields of krypton and xenon, as well as their complementary elements, barium and strontium, were found to indicate an odd-even effect in the element yield distribution. Even-$Z$ elements are formed in higher yields than their odd-$Z$ neighbors (20-50% higher); the even-$Z$ yields are also higher than Wahl's normal element yields (\ensuremath{\sim}12% higher for Kr and Ba and \ensuremath{\sim}40% for Xe and Sr). Half-life values of 20 nuclides produced in the decay of krypton and xenon isotopes were measured as well.

On-line measurements of rare-gas fission yields in 14 MeV neutron fission

Abstract The independent and cumulative fission yields of rare-gas isotopes produced from the fission of 233 U, 235 U, 238 U and 232 Th by 14 MeV neutrons have been measured by means of on-line isotopic separation. Chain yields can be deduced from the cumulative yields and Gaussian curves fitted to the independent yield distributions. Values of Z p − Z UCD and σ Z have been compared with those obtained from other fissioning systems. The total number of neutrons, ν tot , emitted in 232 Th fission has been estimated.

Determination of the nuclear charge distribution in the chain 132 from thermal neutron fission of 235U and 233U

Fractional independent and fractional cumulative fission yields of the three principal constituents in the mass chain 132 produced by thermal neutron induced fission of 235U and 233U have been investigated using rapid chemical separations of tin, antimony and tellurium as hydrides. Values measured are: 132Sn (13·7 ± 0·8)% cumulative, 132Sb (48·7 ± 1·0)% independent and 132Te (37·6 ± 1·5)% independent for the fission of 235U, and 132Sn (2·5 ± 0·5)% cumulative, 132Sb (27·1 ± 3·0)% independent and 132Te (64·4 ± 4·0)% independent for the fission of 233U. The half-life of 132Sn has been determined as 39·0 ± 1·0 sec. A Gaussian charge dispersion curve with a width parameter σ = 0·59 fitted to the measured yields gives Z p -values of 51·28 −0·30 +0·10 and 51·75 −0·15 +0·10 or a deviation from unchanged charge density of ΔZ = −0·37 −0·30 +0·10 and ΔZ = −0·34 −0·15 +0·10 charge units for the fission of 235U and 233U respectively. No pronounced local effects attributable to the closed 50-proton and 82-neutron shells have been observed.

Computer analysis of fission data

A two part computer program has been written to aid in the analysis and interpretation of experimental fission data. The first part of the program calculates the population of all members of any radioactive decay chain at any point in time using as input the initial population distribution and the structure of the decay chain. The program may also be used to perform a time integration of the population for any member to give the number of decays during a specified time interval. The second part of the program uses as input the experimentally measured yield values derived from extractions at various times after fission of one or more members of a particular fission product decay chain. The independent fission yield to any given mass chain is assumed to be distributed according to a Gaussian function. The parameters Z p and w characterizing this distribution are varied so as to minimize the quantity X 2 which is based on the difference between the calculated and experimentally determined fission yield values. The application of the first part of the program is discussed in connection with several fission chain population problems. The use of a novel technique for deducting the values of the independent fission yield distribution parameters is described for neutron induced fission of 235U and 239Pu. This technique utilizes the second part of the computer program and was employed to extract the independent xenon yields for the fission chains with mass number 133 and 135.

Nuclear charge dispersion of light-mass fission products in the fission of 235U and 238U by medium-energy protons

Independent fission yields for 90Ym, 90Ym+g, 91Ym, 91Ym+g, and 92Y and cumulative yields for 91Sr, 92Sr, and 93Y in the fission of 235U and 238U by protons of energy from 20–85 MeV have been determined. Excitation functions and charge dispersion curves have been obtained. The most probable charge, Zp, moves closer to stability, ZA, with increasing energy. The values of ZA − Zp for the two fissioning systems are identical, unlike the situation in the heavy-mass region. The values of ZA − Zp for the light- and heavy-mass regions differ considerably, showing that the equal charge displacement hypothesis does not hold even at these moderate energies.

Fission Yields of Several Iodine Isotopes and Half-Life and Fission Yield ofTe135

Measurements have been made of the independent fission yields of several iodine isotopes in thermal-neutron-induced fission of $^{235}\mathrm{U}$. These yields were obtained by isolation of iodine-fission products at early times after fission by a new, fast radiochemical procedure. The fractional cumulative fission yield and half-life of $^{135}\mathrm{Te}$ were also determined from the experimental results. Fission yields measured are: $^{131}\mathrm{I}$, (4.2\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$% independent; $^{132}\mathrm{I}$, (1.74\ifmmode\pm\else\textpm\fi{}0.11)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}2}$% independent; $^{133}\mathrm{I}$, 0.176\ifmmode\pm\else\textpm\fi{}0.009% independent; $^{134}\mathrm{I}$, 0.90\ifmmode\pm\else\textpm\fi{}0.08% independent; $^{135}\mathrm{I}$, 0.44\ifmmode\pm\else\textpm\fi{}0.07 fractional independent; $^{135}\mathrm{Te}$, 0.52\ifmmode\pm\else\textpm\fi{}0.07 fractional cumulative. The half-life of $^{135}\mathrm{Te}$ was determined to be 11.2\ifmmode\pm\else\textpm\fi{}2.6 sec.

Independent yields of 82Br, 86Rb, 136Cs and 150Pm in spontaneous fission of 252Cf

Independent fission yields in the spontaneous fission of 252Cf were determined by radiochemical techniques for the shielded nuclides 82Br, 86Rb, 136Cs and 150Pm. They were found to be < 3·7 × 10 −6 per cent, 5·4 × 10 −6 per cent, 2·2 × 10 −2 per cent and 2·0 × 10 −2 per cent, respectively. The corresponding fractional chain yields lead to empirical Zp (most probable charge) values which are in good agreement with Zp values determined by physical methods (K X-rays).

Nuclear-Charge Distribution of Fission-Product Chains of Mass Numbers 131-133

New information on the nature of the nuclear charge distribution in thermal-neutron fission of $^{235}\mathrm{U}$ has been obtained from measurements of fission yields of the principal constituents in each of three adjacent mass chains. A regular variation in the width of the charge-distribution curves with mass number has been shown, and values of the most probable nuclear charge (${Z}_{p}$) have been determined. No effects attributable to the 50-proton or 82-neutron closed shells have been observed in the charge-distribution curves examined. Independent and cumulative fission yield values were determined for the principal constituents in each of the three mass chains 131, 132, and 133. Values measured are: $^{131}\mathrm{Sn}$, (1.28\ifmmode\pm\else\textpm\fi{}0.21)% cumulative; $^{131}\mathrm{Sb}$, (1.66\ifmmode\pm\else\textpm\fi{}0.40)% independent; $^{132}\mathrm{Sn}$, (0.59\ifmmode\pm\else\textpm\fi{}0.17)% cumulative; $^{132}\mathrm{Sb}$, (2.76\ifmmode\pm\else\textpm\fi{}0.35)% independent; $^{132}\mathrm{Te}$, (0.86\ifmmode\pm\else\textpm\fi{}0.10)% independent; $^{133}\mathrm{Sn}$, 0.015% cumulative; $^{133}\mathrm{Sb}$, (3.05\ifmmode\pm\else\textpm\fi{}0.39)% independent; $^{133m}\mathrm{Te}$, (2.19\ifmmode\pm\else\textpm\fi{}0.33)% independent; $^{133g}\mathrm{Te}$, (2.07\ifmmode\pm\else\textpm\fi{}0.31)% independent; and $^{133}\mathrm{I}$, (0.17\ifmmode\pm\else\textpm\fi{}0.06)% independent. The charge distribution curve for each mass chain was established from the fission yield data. The data were fitted to the Gaussian relationship, $P(Z)=A\mathrm{exp}\ensuremath{-}\frac{{(Z\ensuremath{-}{Z}_{p})}^{2}}{c}$. The widths of the curves as given by values of $c$ for mass chains 131, 132, and 133 are, respectively: 1.10\ifmmode\pm\else\textpm\fi{}0.05, 0.74\ifmmode\pm\else\textpm\fi{}0.08, and 0.57\ifmmode\pm\else\textpm\fi{}0.05. The data show a monotonic decrease in the width of the charge distribution curves with increasing fission-product mass number. Implications of this observation are discussed in respect to previous assumptions of a single charge distribution curve for all fission-product masses in low-energy fission. Values of ${Z}_{p}$ as given by the Gaussian curves for mass chains 131, 132, and 133 are, respectively: 50.72\ifmmode\pm\else\textpm\fi{}0.07, 51.07\ifmmode\pm\else\textpm\fi{}0.06, and 51.59\ifmmode\pm\else\textpm\fi{}0.04. These values are compared with published values derived from assumptions of a single charge distribution curve for all mass chains. Predicted effects of nuclear shells on charge distribution have been examined in the light of the present data. No effect was detected in mass chains 131, 132, and 133. New half-life values determined are: $^{131}\mathrm{Sn}$, 1.32\ifmmode\pm\else\textpm\fi{}0.23 min; $^{132}\mathrm{Sn}$, 1.00\ifmmode\pm\else\textpm\fi{}0.17 min; $^{132}\mathrm{Sb}$, 3.13\ifmmode\pm\else\textpm\fi{}0.33 min; and $^{133}\mathrm{Sb}$, 2.67\ifmmode\pm\else\textpm\fi{}0.33 min. The half-life of $^{133}\mathrm{Sn}$ was estimated to be 55 sec.

INDEPENDENT YIELDS OF 124Sb AND 126sb IN THERMAL NEUTRON INDUCED FISSION AND THEIR IMPLICATION ON THE EMPIRICAL Zp FUNCTION

The independent fission yields in thermal neutron induced fission of 235U have been measured to be (9.5 ± 1.4)•10−6% and (1.04 ± 0.17)•10−3% for 124gSb and 126gSb, respectively. The total independent yields for all isomers of these nuclides are estimated to be (1.2 ± 0.2)•10−5% for 124Sb and (3.7 ± 0.6)•10−3% for 126Sb using a Huizenga–Vandenbosch approach for estimating population of isomeric states.These data do not seem to support a 50 proton shell effect on the Zp function, using the "unique" charge distribution curve of Wahl. A revised Wahl plot based on recent information on neutron emission from individual fission fragments is given and discussed in view of shell effects in fission. It is shown that the effect of the Zp function of the 50 proton shell is very small.

Independent Fission Yield ofSb127

The fractional independent yield of ${\mathrm{Sb}}^{127}$ has been found to be 0.057\ifmmode\pm\else\textpm\fi{}0.010 from the thermal-neutron fission of ${\mathrm{U}}^{235}$. This value leads to a calculated value of 49.5\ifmmode\pm\else\textpm\fi{}0.1 for the "most probable charge" ${Z}_{P}$ for fission products with $A=127$ and implies that there is no pronounced effect of the 50-proton shell on the ${Z}_{P}$ function. It was also found that 40.4\ifmmode\pm\else\textpm\fi{}2.4% of fission-product ${\mathrm{Sb}}^{127}$ is formed by beta decay of 4.4-min ${\mathrm{Sn}}^{127}$ and that 53.9\ifmmode\pm\else\textpm\fi{}2.1% is formed by beta decay of 2.15-h ${\mathrm{Sn}}^{127}$.

Charge distribution in photofission of U238 and Th232

The independent fission yields of four fission products of U238 and three of Th232 photofission by thick-target bremsstrahlung of 23 MeV maximum energy, have been measured. The results are in general agreement with the proposal of equal charge displacement, and in strong disagreement with unchanged charge distribution. Both points for Nb96, however, are low by more than two orders of magnitude, an effect previously noticed by other workers for this nucleide. A number of cumulative yields, mainly on the wings of the mass-yield curve, have also been measured.