LOHENGRIN Spectrometer Research Articles

Identification of μs isomers in the fission products of241Pu(nth,f)

Several $\ensuremath{\mu}\mathrm{s}$ isomers were observed in neutron-rich nuclei in the mass range $A=88--109.$ These nuclei are produced by thermal neutron-induced fission of ${}^{241}\mathrm{Pu}.$ The detection is based on time correlation between fission fragments selected by the LOHENGRIN spectrometer, at ILL (Grenoble) and the $\ensuremath{\gamma}$-ray or conversion electron emission of the isomers. Among a dozen isomers studied, three new ones have been observed. The decay schemes of ${}^{88m}\mathrm{Br},$ ${}^{94m}\mathrm{Y},$ ${}^{96m}\mathrm{Rb},$ and ${}^{100m}\mathrm{Nb}$ are discussed in the framework of the spherical shell model. The yields and isomeric ratios of all these isomers have been measured.

Cold binary and ternary fission

Cold binary fission has been studied in the spontaneous decay of252Cf (sf). Cold fission for the mass split 132/120 is especially interesting because here it is the doubly magic132 Sn which is dominating the yield for a range of total excitation energies 0 ≤ TXE ≤10MeV For the reaction242Am(n,f) induced by thermal neutrons the yields of the heaviest clusters being emitted as ternary particles have been investigated at the mass spectrometer Lohengrin of the Institut Laue-Langevin. The emission of the heaviest clusters (33, 34, 35 Si) is a cold process with all three reaction partners being born in or very close to their ground states.

Cold fission of 233U(n th, f)

The recoil spectrometer Lohengrin of the Institute Laue-Langevin in Grenoble was used to measure the yields of light fission fragments separated according to their mass and nuclear charge for the mass-number range 76 ⩽ A L ⩽ 93 in the cold fission regime. Neutron emission being forbidden energetically, the correlated heavy fission fragments including their kinetic energies, as well as the total excitation energy TXE of both fragments were determined by conservation laws. Close to TXE = 0, the yields seem to be strongly influenced by the level density of the fragments. In order to obtain yields for the fission into the ground-states of the two fragments, the measured yields per excitation energy interval were divided by calculated level densities for the two fragment system and extrapolated to TXE = 0. These deduced ground-state yields are influenced by the Q values and by the potential energy of the fragments at the scission configuration. The observed trends in the ground-state yields as a function of nuclear charge and mass may qualitatively be reproduced by calculated penetrabilities through the scission barrier. In these calculations, no correspondence between the calculated penetrabilities and the ground-state yields is found if calculated fragment ground-state deformations are used. There is no difference in the influence of neutron and proton pairing on the ground-state yields.

Reduction of energy dispersion on a parabola mass spectrometer

The particle flux density at the mass spectrometer Lohengrin can be significantly increased if a portion of 40 cm along its parabola, corresponding to ±2.7% of the central energy value, is focused by using a dipole magnet with a shaped exit face. This device compensates the momentum dispersion of the first magnet and the energy dispersion of the electric field, increasing up to a factor of 7 the particle flux at the new focal position and diminishing the background from ions which underwent charge exchange collision and scattering in the spectrometer. Thus more accurate studies of rare fission events and the determination of exotic nuclei characteristics will become possible.

Beta-decay half-lives of neutron rich Cu and Ni isotopes produced by thermal fission of235U and239Pu

The half-lives of very neutron rich isotopes of Ni and Cu have been measured. The isotopes are produced in very asymmetric thermal fission of235U and239Pu at the I.L.L. high flux reactor. They are separated by means of the Lohengrin spectrometer. They are identified with aΔE-E ionization chamber and implanted in one of the 8 Si planar detectors where theβ-particles are also detected. The time correlations between the implantation and the detection ofβ-particles provide the half-life. The values obtained are compared to current theoretical predictions.

Characteristics of mass and nuclear charge distributions of229th(nth, f). implications for fission dynamics

The mass and nuclear charge distributions of fission fragments from229Th(nth, f) have been measured at several kinetic energies with the mass spectrometer Lohengrin (ILL-Grenoble). The average proton e-o effect, which reaches 41%, induces large oscillations in the parameters of the isotopic charge distribution. A comparison of the data from different fissile nuclei shows the importance of the last stage of the process for intrinsic excitations.

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.

Nuclide yields of light fission products from thermal-neutron induced fission of 233U at different kinetic energies

The yields of light fission products from thermal-neutron induced fission of 233U are measured as a function of their mass A, their nuclear charge Z, their kinetic energy E and their ionic charge state q at the recoil spectrometer Lohengrin of the Institut Laue-Langevin in Grenoble. The mass yields are determined by intercepting the fragments with an ionization chamber of high energy resolution positioned at the focal plane of the spectrometer. The nuclear charges and their yields are determined with the same ionization chamber by measuring the residual energy of fission products, selected monoenergetically by Lohengrin, behind a passive absorber made of parylene-C. The nuclear charge resolution enabled by this detector device is considerably improved to Z/d Z = 58. The nuclear charge and mass distributions summed over all ionic charge states are listed within the mass range 79 ⩽ A ⩽ 106 at 6 energies: E = 85.34, 90.41, 95.46, 100.50, 105.55 and 110.55 MeV. The energy-integrated nuclear charge and mass yields are also given. The isotonic and isotopic yields are shown. An odd-even effect in the yields is found for the protons as well as for the neutrons at all kinetic energies. The yield weighted total odd-even effect for the protons is found to be (22.1 ± 2.1)%, for the neutrons (5.4 ± 1.7)%. An odd-even effect for the protons in the mean kinetic energy is also observed. The displacement of the mean isobaric nuclear charges from the unchanged charge-density values and the variances of the isobaric nuclear-charge distributions reveal fine structures in their mass dependences.

Energy calibration of surface barrier detectors for fission fragments

The pulseheight, X, delivered in surface barrier detectors by incoming fission fragments of given energy, E, and mass, M, has been studied. Fragments separated as to energy and mass from the 235U(n, f) reaction induced by thermal neutrons were available from the Lohengrin spectrometer of the ILL. The energies on Lohengrin were calibrated by a time-of-flight technique. It could be shown that for the mass and energy ranges of unslowed fragments encountered in nuclear fission there is a linear relationship between E and X for any fixed M, and similarly a linear relationship holds between E and M for any fixed X. The calibration equation, therefore, reads E = ( a + a′ M) X + b + b′ M with a through b′ being constants characterizing the detector. In parallel to the above measurements, the pulseheight spectra from a 252Cf(sf) source were taken. The pulseheights X = P L and X = P H for the light and heavy fragment peaks of this spectrum were determined by a fitting procedure with Gaussians. Several detectors having been tested, it appears that at least for detectors of the same brand, a universal calibration scheme can be set up which correlates the detector constants a through b′ to the Cf spectrum parameters P L and P H. The accuracy of this calibration is sufficient for most practical purposes. The above calibration method has been put forward and in general use since many years [1]. However, the revised constants of the present investigation yield kinetic energies of fission fragments which are lower by about 1–2%. These energies are in excellent agreement with recent independent measurements of the kinetic energy release in fission. Along with the calibration the energy resolution of surface barrier detectors has been studied as a function of bias voltage on the detector and mass or energy of incoming ions.

Cold fragmentation in thermal-neutron-induced fission of 233U and 235U

The mass spectrometer Lohengrin of the Institut Laue-Langevin in Grenoble was used to measure fission-fragment mass yields in the mass range 80 ≤ A ≤ 107 for light-fission-fragment kinetic energies up to about 115 MeV for the reactions 233,235U(n th, f). The kinetic energies corresponding to a common fixed yield level for each isobar reflect the influence of the proton pairing energy, but not of the neutron pairing energy. By using calculated Q-values for the different mass splits, mass distributions at fixed total excitation energy are deduced from the data. At a fixed total excitation energy of about 7 MeV, the yield increases from very asymmetric mass splits ( A L ≈ 80) to more symmetric mass splits ( A L ≈ 105) by more than two orders of magnitude. This strong dependence on the mass split seems to be correlated with the decreasing surface-to-surface distance of the unaccelerated fission fragments in this range of mass splits, as calculated under the assumption that the total Q-value is represented by the mutual Coulomb repulsion of the two fragments. The influence of the fission-fragment ground-state deformations on the yield in cold fragmentation could not be detected unambiguously.

The thermal-neutron-induced alpha-accompanied fission of235U investigation of the low energy part of the alpha spectrum

The energy spectrum of the α-particles emitted in the thermal-neutron induced fission of235U was measured from 11.5 MeV down to 2 MeV using the parabola mass spectrometer Lohengrin at the I.L.L. high flux reactor. This low energy part of the energy spectrum presents a smooth connection with the energy spectra which have been recently reported above 7 MeV. The overall energy spectrum, which is known to be quasi-gaussian above 12 MeV, is slightly asymmetric at low energy, where the observed particles are 6% more than expected from a gaussian shape. As a consequence, all the values reported for235U for the rate of α-accompanied fission compared to binary fission have to be multiplied by 1.06. This asymmetry is about 2 times less important than the one reported for252Cf. No evidence was seen for any intense low energy component as reported before. The possible reasons for the existence of this asymmetry are discussed.

The ionic charge distribution of fission products and the influence of internal conversion on highly preionized heavy ions

At the mass spectrometer LOHENGRIN of the Institut Laue-Langevin the ionic charge state distributions of235U(n th,f)-fission products separated according to their mass, nuclear charge and kinetic energy were determined. The mean values and the widths of the ionic charge state distributions are compared with semiempirical predictions. Deviations between the experimental data and the estimation of Nicolaev and Dmitriev are found. Furthermore, the influence of the internal conversion process on the ionic charge state distribution of highly ionized fission products was established. Internal conversion is observed mainly for odd-odd nuclei and nuclei with 59 neutrons. The Auger cascade following the internal conversion process shifts the ionic charge state distribution by about 3 charge units. The yield of conversion electrons per fragment was determined in the mass range 85≦A≦103.

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.