Digitale Fenster für γ-γ-Koinzidenzmessungen mit Halbleiterdetektoren

Zusammenfassung Digital windows have been designed and built to avoid the disadvantages of normal single channel analysers setting the gate in γ-γ-coincidence experiments with two Ge(Li)-detectors.

The new neutron-rich isotope183Lu

The new neutron-rich isotope183Lu was produced in multinucleon transfer reactions by irradiatingnatW/Ta targets with 11.7 MeV/u136Xe ions, and identified by mass separation and decay spectroscopy. The measured half-life of183Lu is 58±4 s. The properties of the excited states of183Hf are discussed.

HIGH SPIN STATES OF 232Th, 234U AND 236U+

On the nuclear structure of 229Ra

14 pages, 3 tables, 6 figures.

Structure of the Triplet of low-lying states in101Mo

The properties of the triplet of low-lying states in101Mo have been studied through spectroscopy of theγ radiation following thermal neutron capture in100Mo and β− decay of101Nb and through a measurement of the proton angular distributions in the100Mo(d,p) reaction with 14 MeV deuteron energy. The half-lives of the 13.5 keV state and the 57.0 keV 5/2+ state have been measured as 226(7) and 133(7)ns, respectively. These values and the quadrupole/dipole mixing ratios of the 13.5 keV and 43.5 keV transitions yield spin and parity 3/2+ for the 13.5 keV level. The E2 components in the 13.5 (3/2+ →1/2+) and 43.5 keV (5/2+→3/2+) transitions are ≦ 8·10−4 and 54(9)%, respectively. The possibility of…

The nuclear structure of 229Th

Abstract The γ -rays following the β − decay of 229 Ac have been investigated by means of γ -ray singles and γγ -coincidence measurements using Ge detectors. Multipolarities of 40 transitions in 229 Th have been established by measuring conversion electrons with a mini-orange electron spectrometer. The half-lives of the 146.35, 164.53 and 261.96 keV levels have been measured using the advanced time delayed βγγ (t) method. The low-lying states in 229 Th and observed transition rates have been interpreted within the quasiparticle–phonon model with inclusion of Coriolis coupling. Two octupole correlated parity partner bands, with K π =5/2 ± and K π =3/2 ± , were identified in 229 Th.

Verfl�chtigung von Spaltprodukten aus festem Uran(IV)-fluorid und anderen Fluoriden

In der Kernehemie sueht man nach temperaturund strahlenresistenten Targetsystemen, aus denen naeh Kernreaktionen kurzlebige Nuklide kontinu: ierlich abgetrennt werden kSnne n. Dies ist insbesondere im Hinblick auf die yon der Gesellschaft fiir Sehwerionenforsehung in Darmstadt geplanten Experimente interessan~, bei denen die Synthese neuer, iibersehwerer Elemente versucht wird. Als Modell fiir Verdampfungsstudien aus festen Targets wurde die tr~gerfreie Verfliichtigung yon Spaltprodukten aus festem Uran(IV)-fluorid gew~hlt [1--3]. Im fliiehtigen Anteil konnten folgende Elemente identifiziert werden: Se, Kr, Zr, Iqb, Me, Te, Ru, Sn, Sb, Te, J und Xe. In einem Rohr mit Temperaturgradienten l~…

Evidence for Shape Coexistence in Neutron-Rich Rh and Ag Nuclei

Neutron-rich nuclei of Rh and Ag have been investigated by γ-ray spectroscopic measurements as well as single-particle transfer reactions like (d,3He) and (3He,d). The obtained level schemes of 105,107,109,111Rh and 113,115Ag show two different types of excitation: (I) proton-hole states related to a spherical shape of the nucleus and (II) proton-particle states with a rotational band-like structure (intruder band) pointing towards deformation of the nucleus. The “fingerprints” for such a shape coexistence and resulting systematics in these nuclei and the possible interpretation of the intruder band as a rotational band built upon the 1/2+[431] Nilsson configuration are discussed.

Decay properties of neutron deficient Kr isotopes

The decay properties of the neutron deficient isotopes73–77Kr and73–76Br have been studied at the ISOLDE facility at CERN. The total decay energiesQ, as determined fromβ + singles orβ + -γ coincidence measurements, are compared with mass formulae.

Precise energies of gamma rays from the 230Th and 228Th decay

Abstract Energies up to 300 keV of prominent gamma rays from the decay of 230 Th, 228 Th and some of the 228 Th descendants have been precisely measured by use of Ge(Li) detectors.

Evidence for intruder states in111Rh

Levels in111Rh have been investigated via the γ -rays following the β−-decay of 2.1 s111Ru. The Ru activity was produced in the fission of249Cf and separated chemically from the fission product mixture. The emitted γ-rays were studied by γ singles and γ(t) coincidence measurements. Evidence for intruder states in111Rh has been obtained. Their properties are discussed and compared with those in the lighter Rh isotopes.

The new neutron-rich isotope228Rn

2 pages, 1 table, 2 figures.-- PACS nrs.: 23.90.+w; 27.90.+b.-- Section: Short Notes.

An improved system for fast, continuous chemical separations (“SISAK 2”) in nuclear spectroscopic studies

Abstract An improved rapid, continuous solvent extraction system (“SISAK 2”) is described. The system is connected to a gas-jet installed at the Mainz reactor. It allows single or multistage chemical separations of liquid phases by means of specially designed centrifuges within ∼1 s per stage. The application of this system to study short-lived nuclides is exemplified for neutron-rich lanthanum and cerium isotopes produced by fission.

Odd neutron nuclei near A=100: Rotational bands in103Mo and105Mo populated in the? ? decays of103Nb and105Nb

Theβ − decays of103Nb and105Nb have been studied at the fission product separators JOSEF and LOHENGRIN. Half-lives of (1.5±0.2) s and (2.95±0.06) s, respectively, have been determined for these decays. Fromγ singles andγ-γ coincidence measurements extended level schemes for103Mo and105Mo have been established for the first time. The lowest energy levels of these nuclei are consistent with the interpretation as members of rotational bands built on a 3/2+ [411] Nilsson state. Evidence is presented for the location of the 9/2+ [404] configuration.

The decay of 8.7 min237Pa

The beta and gamma radiations of237Pa have been investigated employing semiconductor and scintillation spectrometers and coincidence techniques. Sources of237Pa were obtained after bombardments of238U with bremsstrahlung and 14-MeV neutrons and subsequent chemical separation. From the total of 18γ-rays following the decay of 8.7±0.2 min237Pa 17 transitions, representing 99.9% of theγ-ray intensity, could be placed in a level scheme of237U. AQ β- value of 2.25±0.1 MeV has been determined.

New nuclides: Neptunium-243 and neptunium-244

We have observedγ-rays following the β−-decay of243Np and244Np after chemical isolation of neptunium isotopes from the products of the reaction of 835 MeV136Xe with244Pu. The ground-state of 1.85-min243Np hasJ=5/2. The decay of 2.29-min244Np (probableJπ=7−) populates high-spin members of the ground state rotational band in244Pu.

Beta-decay of148,150,152Pr into levels of148,150,152Nd

Theβ−-decay of148,150,152Pr into levels of148,150,152Nd has been studied byγ-singles,γγ-coincidence and for148Pr also byγγ-angular correlation measurements. The praseodymium activities have been obtained by thermal neutron-induced fission of235U followed by on-line mass separation with the helium-jet coupled mass separator HELIOS. Decay schemes for these heaviest praseodymium nuclides are proposed. From the angular correlation measurements spin values for the seven lowest levels of148Nd are deduced. Spin and parity assignments for the lower levels in150,152Nd have been extrapolated from level systematics.

Lifetime measurements in99Sr and100Zr

The lifetime of the first excited state in99Sr has been determined by the delayed γ-γ coincidence method. The value of t/2=0.58(9) ns corresponds to a deformation parameter s=0.38(4). The deduced value confirms the previously proposed ν[411]3/2 Nilsson assignment for the99Sr ground state. In addition, lifetime values for the 2+ (t1/2=0.40(8) ns) and 0+ (t1/2=5.36(23) ns) states in100Zr are obtained, which deviate from older measurements and indicate a larger deformation of s=0.39(4).

Identification of Thorium-236

Abstract The new nuclide 236Th has been produced via the (γ, 2 p) reaction by irradiation of 238U with 140 MeV bremsstrahlung. After chemical separation of thorium, the half-life was determined to be 36 ± 3 min -from the growth-decay curve of the strongest γ-ray transition of the daughter nuclide, 9 min 236Pa.

Studies of short-lived fission products by means of the multistage solvent extraction system SISAK

The multistage solvent extraction system SISAK is described. The system is used for online studies of nuclides with half-lives down to ∼1 s. Some of the chemical separation procedures are presented, and a survey is given on the results obtained so far. The possiblity to gain access to nuclides with even shorter half-lives is also discussed.

Continuous separation and identification of neutronrich neptunium isotopes from heavy-ion reactions by means of the centrifuge system “sisak”

Abstract Neutron-rich neptunium isotopes formed in direct transfer reactions between 136 Xe projectiles and targets of 244 Pu have been separated from other reaction products with the on-line liquid-liquid extraction system SISAK. A four-detector delay method was used to determine the half-lives of the previously unknown isotopes 243 Np and 244 Np. From the decay of the 287.4 keV γ-line a half-life of 1.8 ± 0.3 min was determined for 243 Np. From the decay curves of the γ-lines at 162.7 and 216.7 keV, which fit into the known level scheme of 244 Pu, a half-life of 2.5 ± 0.3 min results for 244 Np.

The nuclear structure of $^{223}$Fr

Abstract The γ-rays following the β− decay of 223Rn have been investigated by means of γ-ray singles including multispectrum analysis, and γγ-coincidence measurements using Ge detectors. Multipolarities of 38 transitions in 223Fr have been established by measuring conversion electrons with a mini-orange electron spectrometer. Most of the observed transitions could be placed in a level scheme comprising 53 excited states of 223Fr. The level structure is interpreted in terms of K π = 3 2 ± and 1 2 ± parity doublet bands, and a second K π = 3 2 ± higher-lying parity doublet. The experimental data are compared with the theoretical predictions of the reflection-asymmetric rotor model.

The decay of105Tc to levels in105Ru

Theβ−-decay of 7.6-min105Tc has been investigated byβ- andγ-ray singles and coincidence measurements. The activities have been produced by thermal-neutron-induced fission of235U or239Pu and subsequent chemical separation of the technetium fraction from the fission product mixture. AQβ-value of 3.2±0.2 MeV has been determined. In a delayed coincidence experiment the lifetime of the first excited state in105Ru at 20.55 keV has been measured to be 340±15 nsec. A level scheme of105Ru is proposed and compared with the results of recent nuclear reaction studies like104Ru(d, p) and104Ru(n, γ). From beta branching ratios to levels in105Ru, ground state spin and parity of 5/2+ can be suggested for10…

Levels in104Ru populated by the ??-decay of104Tc

Theβ− — andγ-activities of 18.4 min104Tc have been studied with singles and coincidence measurements. Sources were prepared by chemical separation of technetium from the fission products of thermal-neutron-induced fission of235U. A revised level scheme of104Ru up to an excitation energy of 4.26 MeV is proposed. Fromβ-γ coincidence measurements, aQβ-value of 5.62±0.07 MeV has been obtained.

Decay of the 4.2-min152Pm into levels of152Sm

After bombardments of enriched152Sm with 14-MeV neutrons, three isomers of125Pm reaction, have been observed with half-lives of 4.2±0.2 min, 7.5±0.2 min and 15.0±1.0 min. Sources of pure 4.2-min152Pm were obtained by separating the promethium from its parent 11-min152Nd produced by thermal-neutron induced fission of235U. The beta and gamma radiations of the 4.2-min152Pm have been investigated using semiconductor and scintillation spectrometers. Most of theγ-ray transitions could be placed in a level scheme for 152Sm. Especially interesting is a level at 1083 keV for which the assignmentI π=0+ is proposed. In a 4πβγ experiment, the ground state feeding of152Sm in theβ −-decay of the 4.2-min1…

Search for stable octupole deformation in $^{225}$Fr

Levels in $^{225}$Fr have been studied by $\gamma$-ray and conversion-electron spectroscopy following the $\beta$-decay of $^{225}$Rn, and by the $^{226}$Ra(t,$\alpha$)$^{225}$Fr single-proton-pickup reaction. Multipolarities were obtained for $\sim$ 50 $\gamma$-transitions, and a level scheme was established in which definite spin-parity values were determined for over 20 of the 48 levels. Although low-lying rotational bands of both parities exist for $K=1/2$ and $K=3/2$, there is no experimental evidence to claim these are parity doublets. E1 strengths connecting the $K^\pi=3/2^\pm$ bands are intermediate between those for nuclides which are reflection symmetric and those claimed to be oc…

Direct identification of 103–107Mo by a rapid chemical separation procedure

Abstract The nuclides 103–107Mo have been identified by means of γ-ray spectroscopy after the separation of molybdenum from fission products of 235U and 239Pu by a rapid chemical procedure. Half-lives of 67.5 ± 1.5 s for 103Mo, 59.4 ± 0.8 s for 104Mo, 36.7 ± 1.0 s for 105Mo, 8.4 ± 0.5 s for 106Mo and 3.5 ± 0.5 s for 107Mo have been determined and the main γ-lines were assigned to the decay of these isotopes. Some evidence for 108Mo with a half-life ≤2 s has also been obtained.

Search for long-lived superheavy elements in the reaction of136Xe with238U

A search with radiochemical methods for long-lived superheavy elements in 238U targets bombarded with intense beams of136Xe ions produced negative results. A formation cross section of ≤1×10−35 cm2 is deduced at 95% confidence level for nuclides with half-lives between 1 and 200 d.

The deformedN=60 nucleus 41 101 Nb

The β− decay of101Zr has been investigated at the fission-product separators JOSEF and LOHENGRIN. The half-life of101Zr has been determined to 2.5(1) s and a level scheme for101Nb has been established fromγ ray singles as well as X/3-γ and γ—γ coincidence measurements. Conversion coefficients for transitions in101Nb and level half-lives between 10 ps and 2 ns have been determined. Three rotational bands are identified among the low-lying levels with band heads at 0 keV, 206 keV and 208 keV. The bands are probably based on the Nilsson configurations [422 5/2+], [301 3/2−] and [303 5/2−], respectively. The deformation has been determined to βq=0.40(4) and 0.41(8) for the ground state band and…

VERFLÜCHTIGUNG VON SPALTPRODUKTEN AUS FESTEM URAN(IV)-FLUORID UND ANDEREN FLUORIDEN

In der Kernehemie sueht man nach temperaturund strahlenresistenten Targetsystemen, aus denen naeh Kernreaktionen kurzlebige Nuklide kontinu: ierlich abgetrennt werden kSnne n. Dies ist insbesondere im Hinblick auf die yon der Gesellschaft fiir Sehwerionenforsehung in Darmstadt geplanten Experimente interessan~, bei denen die Synthese neuer, iibersehwerer Elemente versucht wird. Als Modell fiir Verdampfungsstudien aus festen Targets wurde die tr~gerfreie Verfliichtigung yon Spaltprodukten aus festem Uran(IV)-fluorid gew~hlt [1--3]. Im fliiehtigen Anteil konnten folgende Elemente identifiziert werden: Se, Kr, Zr, Iqb, Me, Te, Ru, Sn, Sb, Te, J und Xe. In einem Rohr mit Temperaturgradienten l~…

Decay of the heaviest isotope of neodymium:154Nd

The neutron-rich isotope154Nd has been separated from235U fission products with the HELIum-jet On-line mass Separator HELIOS and investigated by γ-singles, γγ- and γX-coincidence measurements. The Z- and A-assignment have been confirmed by the observation of promethium KX-rays and by the genetic relationship to the daughter product 1.7-min154Pm. A half-life of 26±2 s has been obtained for154Nd, and a partial decay scheme is proposed. A 40 s activity previously assigned to154Nd could not be confirmed.

Hafnium and Lutetium Isomers Produced in Heavy-lon Collisions of 7.6 MeV/u 40Ar, 8.5 MeV/u 84 Kr and 8.5 MeV/u 136Xe on natW Targets

Evidence for rotational bands in103Nb

The nucleus103Nb62 has been studied through the s− decay of the fission product103Zr at LOHENGRIN and JOSEF. The energies of the lowlying levels and the γ transitions indicate rotational bands based on the Nilsson proton configurations [422 5/2+] [303 5/2−] and [301 3/2−] at 0, 164 and 248 keV, respectively. The measured half-lives of (4.7±0.5) ns and (2.0±0.6) ns of the levels at 164 and 248 keV are consistent with the Nilsson-model estimates including pairing.

Saturation of deformation atN=60 in the Sr isotopes

From the halflife oft 1/2=3.91 (16) ns for the 21 + state in100Sr a deformation parameter ofβ=0.40(1) has been derived. Within the given uncertainties, this value is equal to the recently determinedβ-parameters for98Sr and99Sr, indicating that in the Sr isotopes saturation of deformation is reached immediately at its onset well before neutron midshell. This behaviour is reflected by the “twin” character of98Sr and100Sr with identical features of their unperturbed ground-state rotational bands. These observations are discussed in terms of strong two-nucleon interactions.

New neutron-rich isotopes of astatine and bismuth

Neutron-rich isotopes of astatine have been produced through spallation reactions with 600 MeV protons on a232Th target and identified by spectroscopic techniques combined with one-line mass separation at the ISOLDE facility. The half-lives of218At and219At have been remeasured to be 1.5(3) s and 57(4) s, respectively. Four new isotopes of astatine,220−223At, have been observed for the first time, and their half-lives were found to be 3.73(13) min, 2.3(2) min, 54(10) s, and 50(7) s, respectively. Another nuclide,216Bi, has been observed for the first time as the daughter product of the220At alpha decay, and its half-life has been measured to be 6.6(21) min.

Decay properties of neutron-rich niobium isotopes

The decay of neutron-rich niobium isotopes in the mass region $A\ensuremath{\simeq}100$ has been investigated by $\ensuremath{\gamma}$-ray singles and $\ensuremath{\gamma}$-$\ensuremath{\gamma}$ coincidence measurements. The isotopes were produced by thermal-neutron induced fission of $^{235}\mathrm{U}$, $^{239}\mathrm{Pu}$, and $^{249}\mathrm{Cf}$, and niobium was separated from fission-product mixtures with an automated chemical procedure. Isomerism was found in the even-mass niobium isotopes with the following half-lives: 1.5 and 3.1 sec for $^{100}\mathrm{Nb}$, 1.3 and 4.3 sec for $^{102}\mathrm{Nb}$, and 0.8 and 4.8 sec for $^{104}\mathrm{Nb}$. Half-lives and $\ensuremath{\gamma}$ rays…

Quadrupole deformed and octupole collective bands in 228Ra

Spins and parities for collective states in $^{228}$Ra have been determined from conversion electron measurements with a mini-orange $\beta$-spectrometer. The fast-timing $\beta\gamma\gamma(t)$ method has been used to measure lifetimes of T$_{1/2}$=550(20) ps and 181(3) ps for the $2^{+}_{1}$ and $4^{+}_{1}$ members of the K=0$^{+}$ band, and T$_{1/2} \leqslant$ 7 ps and $\leqslant$ 6 ps for the $1^{-}_{1}$ and $3^{-}_1$ members of the K = $0^{-}$ band, respectively The quadrupole moments, $Q_{0}$ deduced from the B (E2; 2$_{1}^{+} \rightarrow 0_{1}^{+}$) and B (E2; 4$_{1}^{+} \rightarrow 2_1^{+}$) rates are in good agreement with the previously measured value and the systematics of the reg…

Subshell closure effects on the collectivity of 36 88 Kr52 and the beta decay of88Br to levels in88Kr

The level structure of88Kr has been investigated by measurements on chemically separated bromine samples. A total of 146γ-rays have been assigned to the decay of88Br, and a level scheme with 58 levels is proposed. The collectivity of88Kr has been investigated within the framework of the Interacting Boson Approximation (IBA-2 model). Comparison of our detailedγ-ray data and calculations have led to the establishment of a protonhole boson desciption rather than protonparticle nature. This supports recent works establishingZ=40 as a major subshell closure.

The collective structure of106,108Ru

Levels in106,108Ru have been studied by measuringγγ coincidences andγγ directional correlations of theγ-rays following theβ−-decay of 36 s106Tc and 5 s108Tc. Tc activities were separated chemically from the fission products of thermal neutron induced fission of239Pu by the continuously running system “SISAK 2” and theγ-radiation was measured on-line. An extended level scheme for108Ru, spin assignments,E2/M1 multipole mixing ratios andB(E2) ratios for106Ru and108Ru have been obtained. The collective structure of the two nuclei is discussed in the framework of different nuclear models.

The new neutron-rich nuclei231Fr and231Ra

The new neutron-rich isotope231Fr has been produced in a spallation reaction of238U induced by 600 MeV protons and identified by mass-separation and decay spectroscopy. In addition, the so far unknown231Ra and the known231Ac have been observed as daughter products. The Z-assignments were performed via the decay of KX-rays, and genetic relationships. Half-lives of 17.5(8) s and 103(3) s were obtained for231Fr and231Ra, respectively.