Neutron Odd-even Effect Research Articles

Mass and charge distributions in the very asymmetric thermal neutron induced fission of the odd-Z nucleus 242mAm

Yields of light fission products ( A = 68, 70–84, 87, 88, 94, 96, 98, 102 and 106–108), their kinetic energies and nuclear charge distributions ( A = 71–84, 87 and 88) in the thermal neutron induced fission of the odd-Z nucleus 242mAm(Z = 95) were measured using the mass-separator Lohengrin at the Institute Laue-Langevin in Grenoble (France). The mass yield curve shows a fine structure at A = 70, probably due to shell and/or odd-even effects affecting also the nuclear charge distribution. The analysis of isotopic chain yields gives evidence for a very low excitation energy of the lightest fission fragments observed. A preferential formation of fragments with even Z is found for this odd-Z compound nucleus. Calculated values for the local odd-even effect are comparable with those for the neighbouring even-Z fissile nuclides and increase from 13% to 30% with increasing asymmetry of the mass split ( A L = 84 to 68 and Z L = 35 to 28). The neutron odd-even effect shows a similar but less pronounced behaviour.

Excitation energy dependence of fragment characteristics for the photofission of 232Th

Independent and cumulative product yields were measured for the photofission of 232Th with bremsstrahlung with endpoint energies 6.5, 7.0, 8.0, 11.0, 12.0, and 14.0 MeV, applying γ spectrometric techniques on catcherfoils and pneumatically transported 232Th-samples. The independent heavy fragment yields for the fission of the 232Th compound nucleus at excitation energies in the vicinity of the fission barrier were deduced. Postneutron mass, isobaric charge, isotopic mass distributions, isotonic and elemental yield distributions and proton odd-even effects were obtained from these independent yields. In the mass distributions a maximum yield is observed for mass splits with heavy fragments in the region of A = 142, corresponding with a high production of Ba( Z = 56) - isotopes. A slightly increased yield is also observed for mass splits with heavy mass in the vicinity of A = 134. The latter effect increases with increasing compound nucleus excitation energy. The similarity between the mass distributions of the N = 142 fissioning systems 232Th, 234U and 236Pu is striking. For low excitation energy the proton odd-even effect in the element distributions amounts to 30%, while on the other hand no sizeable neutron odd-even effect could be deduced from the isotonic distributions. The proton odd-even effects remain constant up to compound nucleus excitation energies of about 7.85 MeV. For higher compound nucleus excitation energies the proton odd-even effect drops rapidly. A possible explanation of these observations in terms of pair breaking at the outer barrier is proposed.

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

Two fissioning systems [235U(nth, 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 WahlZp 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.

Mass, charge and energy distributions in the very asymmetric fission of 249Cf induced by thermal neutrons

Fission yields of fragments with mass numbers A = 69–87 and fragment kinetic energies for A = 74–87 in the very asymmetric neutron induced fission of 249Cf have been measured using the mass separator Lohengrin at the Institut Laue-Langevin in Grenoble. The chain yields observed in this very light mass region are considerably higher than predicted. An evaluation of the isotopic yields shows a strong increase of the proton odd-even effect from 7% to 17% with increasingly asymmetric fission. The neutron odd-even effect is increasing from 4% to 10%. These observations and a narrowing of the average nuclear charge distribution can be explained by a decreasing excitation energy with increasing asymmetry of the fission process.

Excitation energy dependence of charge odd-even effects in the fission of 238U close to the fission barrier

Independent yields of the heavy fragments have been determined for the photofission of 238U at excitation energies in the barrier region. Bremsstrahlung with six different endpoint energies between 6.1 and 11 MeV was used. The independent and cumulative yields of the fragments were measured using γ-spectrometry techniques. Fragment charge, isotopic mass, elemental yield distributions and proton and neutron odd-even effects are deduced from these independent yields. The experiments revealed a sizeable odd-even effect [ δ p = (29±2)%] in the element distributions, but on the other hand a small effect [ δ n = (3±2)%] in the isotonic distributions. The proton odd-even effect in the photofission of 238U remains fairly constant for excitation energies up to about 7.85 MeV. When the excitation energy of the fissioning nucleus is raised slightly higher, the proton odd-even effect drops off quickly.

Nuclear mass and charge distributions in the cold region of the spontaneous fission of 252Cf

The spontaneous fission of 252Cf was investigated experimentally in the cold-fission region. The fission-fragment mass- and nuclear-charge distributions were determined in total kinetic energy bins of 2 MeV width parallel to the average Qmax value as a function of mass up to (Qmax-TKE) equal to 15 MeV. In the evaluation bin with an average total kinetic energy of 11 MeV below the Qmax value, 220 nuclides were identified, 59 of them are extremely neutron-rich and lie outside the Karlsruhe Chart of Nuclides. The relative individual nuclide yields can be easily obtained from table 2 as a function of (Qmax-TKE). The fragment mass, nuclear charge and neutron number odd-even effects integrated over all mass splits of each (Qmax-TKE) bin were evaluated. There is no odd-even effect in the mass yield δΛ, except when (Qmax-TKE) becomes less than or equal to 3 MeV. The proton and the neutron odd-even effects δZ and δN integrated over all masses of one evaluation bin decrease linearly with (Qmax-TKE, however with different slopes. At (Qmax- TKE) = 9 MeV the neutron odd-even effect is about 8 times smaller than δZ. The proton odd-even effect as a function of fragment mass δZ(A) at constant (Qmax−TKE) shows an undulatory behaviour with about five fragment masses. This structure is still strong at the highest measured (Qmax − TKE) of 15 MeV, where δZ, the average, is largely reduced. The variance of the isobaric nuclear-charge distributions σz2(A) was found to be anti-correlated with δz(A). The structures in σz2(A) decrease with increasing (Qmax − TKE).The neutron odd-even effect δN(A) shows vehement changes with fragment mass from positive to negative values and from small to large amplitudes. It is concluded from the present results that δZ and δN cannot be interpreted as indicators of the intrinsic excitation energy at scission, that the upper limit for the intrinsic excitation energy in fission of 252Cf for all TKE values is 20 MeV, and that the structure of 5–6 mass units observed in many fission parameters of even-even fissioning systems stems from the shape of the surface of Q-values as a function of mass and nuclear charge.

A high-resolution multi-parametric study of 239Pu(n th, f) with the Cosi-Fan-Tutte spectrometer

The Cosi-Fan-Tutte spectrometer has been used to measure for the first time in one go the mass-nuclear charge-energy correlations of well-resolved light group masses for 239Pu(n th, f). About 2 × 10 6 events were collected and analysed. The global mass distribution is consistent with the Lohengrin data. The mean proton odd-even effects in yields and energy are δ p=(11.8±0.3)% and δE K o−e=0.49±0.15 MeV respectively. The δ p( E L) varies from ≈7% to ≈25% for E L=90 to 116 MeV. The present analysis consists of 21 values of E L compared to only 5 values for Lohengrin. The apparent mean neutron odd-even effect δ n=(3.5±0.1)% is much lower than the δ n= (6.5±0.7)% for Lohengrin. The mean value of conditional variance 〈 σ z 2( Z| A)〉=(0.42±0.02); the 〈 σ z 2( Z| A)〉 increases from 230Th to 250Cf and this behaviour turns out to be important for understanding the dynamics of the fission process.

Mass and nuclear charge yields for 237Np(2n th,f) at different fission fragment kinetic energies

The recoil mass separator LOHENGRIN of the Laue-Langevin Institute Grenoble has been used to measure for the first time, the yields of light fission fragments from the fissioning system: 239 93Np; this odd-Z nucleus is formed after double thermal neutron capture in a 239 93Np target. The mass distributions were measured for different kinetic energies between 92 and 115.5 MeV, but the nuclear charge distributions were determined only up to 112 MeV. These distributions are compared to the distributions obtained for the even-even system 240 94Pu. At high kinetic energy, the mass distribution shows a prominent peak around mass number A L = 106. These cold fragmentations are discussed in terms of a calculation based on a scission point model extrapolated to the cold fission case. As expected for an odd- Z fissioning nucleus, the nuclear charge distributions do not reveal any odd-even effect. The global neutron odd-even effect is found to be (8.1 ± 1.5)%. A simple model has been used to show that most of the neutron odd-even effect results from prompt neutron evaporation from the fragments.

High resolution measurements of mass, energy and nuclear charge correlations for 229Th(n th, f) with the cosi fan tutte spectrometer

The COSI FAN TUTTE spectrometer has been used to measure for the first time in one go the mass-charge-energy correlations for well-resolved light group masses for 229Th(n th , f). About 4.8 × 10 5 events were collected in a ≈ 3 months run. The global mass distribution shows structures and the distribution for E L = (112±0.5) MeV is dominated by mass numbers A L = 86 and 96. The mean kinetic energy 〈E L 〉 shows strong fluctuations as a function of the light fragment mass A L . The mean proton odd-even effects in yields and energy are д p = (40.0±1.0)% and д E o-e TKE = (1.35±0.17) MeV , respectively. The δp(E L ) varies by a factor of ≈ 3 for E L = 02 to 107 MeV with a plateau for E tL ≈ 96 to 103 MeV. The mean neutron odd-even effect is δn = (1.95±0.5)%). The mean value of the conditional variance is 〈σ 2 ( Z¦ A)〉 = (0.33±0.02) and it varies little with energy. The conditional variance seems to decrease as one goes from 252Cf to 230Th. These data are discussed in the context of existing results on other nuclei.

Mass and nuclear-charge yields for 249Cf(n th, f) at different fission-product kinetic energies

The recoil mass spectrometer Lohengrin of the Laue-Langevin Institute, Grenoble, has been used to measure the yields of light fission products from 249Cf(n th, f) as a function of A, Z, the kinetic energies and the ionic-charge states. The mass and nuclear-charge distributions integrated over all the ionic-charge states were determined for different kinetic energies E Loh between 87 and 112 MeV. The behaviour of ΔZ = Z− Z UCD as a function of A' differs strongly from that for 236U and 240Pu; it passes the zero value at ~7.5 mass units away from the center of the Z = 50 shell and it reaches ΔZ ∼ 1 at A' ∼ 123 without ever passing the shell. The mean proton and neutron odd-even effects on yields are δ p = (4.6 ±0.7)% and δ n = (9.5 ±0.7)%. The odd-even effects on mean proton and neutron kinetic energies are δE K o−e ( Z) = (0.20 ± 0.07) MeV and δE K o−e ( N) = (0.48 ± 0.16) MeV, respectively. The mean value of isobaric variance 〈 σ Z 2〉 = (0.43 ± 0.03) and, unlike the results for 236U and 240Pu it shows little dependence on kinetic energy; the present value along with the results for the other fissioning nuclei confirm that σ Z 2 results from zero-point isovector giant dipole vibration at the exit point.

Fission yields at different fission-product kinetic energies for thermal-neutron-induced fission of 239Pu

At the recoil spectrometer “Lohengrin” of the Institut Laue-Langevin in Grenoble, the yields of the light fission products from the thermal-neutron-induced fission of 239Pu were measured as a function of A, Z, the kinetic energy E and the ionic charge states q. The nuclear charge and mass distributions summed over all ionic charge states were determined for different light fissionproduct kinetic energies between 93 and 112 MeV. The proton odd-even effect which was measured to be (11.6 ± 0.6)% causes considerable fine structure in the yields. The average kinetic energy of even- Z elements in the light fission-product group is 0.3 ± 0.1 MeV larger than for odd- Z elements. The neutron odd-even effect is (6.5 ± 0.7)%. The comparison with previously published data 1) for thermal-neutron-induced fission of 235U reveals a correlation between the proton odd-even effect in the yield and in the kinetic energy of the elements. The dependence of the proton odd-even effect on the fragmentation is very similar for 235U and 239Pu when it is considered as a function of the nuclear charge of the heavy fission products. The isobaric variances σ z 2. for thermal-neutron fission of 235U and 239Pu coincide at all kinetic energies if the influence of the proton odd-even effect is averaged out. This supports the hypothesis that the magnitude of σ z 2 is determined only by quantum-mechanical zero-point fluctuations. The influence of the spherical shells Z = 50 and N = 82 on the fragmentation is discussed.

Distributions of fission products from various low-energy fission reactions and the systematics of the odd-even fluctuations

The dependence of the proton odd-even effect on the excitation energy of the fissioning nucleus and the mean kinetic energy of the fission fragments is examined in thermal neutron fission of $^{239}\mathrm{Pu}$ and in "fast" neutron fission of $^{235}\mathrm{U}$, $^{238}\mathrm{U}$, and $^{232}\mathrm{Th}$. The proton odd-even effect observed in thermal neutron fission is found to decrease with increasing excitation energy and/or with increasing atomic number of the fissioning even $Z$ nucleus. Possible factors responsible for the magnitude of the odd-even effect are discussed. An evaluation is made of a neutron odd-even effect, based on the observed mean proton odd-even effect, and consequently the mass distribution of fission fragments prior to neutron emission is calculated. A good fit is found between the calculated and the measured fragment mass yield curves and between calculated and measured neutron emission as a function of mass.NUCLEAR REACTIONS, FISSION $^{235}\mathrm{U}$($n,f$), $^{239}\mathrm{Pu}$($n,f$), $^{238}\mathrm{U}$($n,f$), $^{232}\mathrm{Th}$($n,f$), calculated odd-even effect; deduced systematics of the odd-even effect.

Nuclear charge distribution of fission products fromU235(nth),fof the masses 79 to 100

Passing mass separated recoil fission particles through a $\ensuremath{\Delta}E \mathrm{Si}$-surface barrier detector the nuclear charge distributions of fission products were measured in the mass chains 79 to 100. The average nuclear charge as well as the second, third, and fourth moment of the nuclear charge distribution reveal odd-even and shell effects.NUCLEAR REACTION, FISSION $^{235}\mathrm{U}({n}_{\mathrm{th}},f)$, measured nuclear charge distribution of $A=79$ to $A=100$, deduced element and isotone yields, proton and neutron odd-even effect.

Distributions isotopiques des gaz rares dans la fission par neutrons thermiques de 235U et 233U

Abstract The independent and cumulative fussion yields of rare gas isotopes produced from the fission of 233U and 235U by thermal neutrons have been measured by means of on-line isotopic separation. Gaussian curves can be fitted to the independent yield distributions. Values of Zp−ZUCD and σz have been obtained. Elemental yields can be deduced for the two fissioning systems. An estimation is made of the proton and neutron odd-even effects.