Measurement of mass- and time-dependence of delayed fission neutrons by an on-line mass separator

Delayed neutron emission for thermal neutron fission of²³⁵U is investigated by means of a helium-filled mass separator. At the focus of the separator, the neutron activity is detected in coincidence with theΒ decay preceding the neutron emission. Measurements of the activity build-up give the total delayed neutron yield and the yields for different times after fission. Measurements of the activity decay give the half-life distribution. The mass-dependence of yield and half-life was investigated for the whole fission product mass region. The splitting of the total yield of delayed neutrons between light and heavy fission products has been determined. Based on Keepin's value of the total number of delayed neutrons per fission, a yield of 1.05 and 0.53 neutrons per 100 fissions has been obtained for light and heavy fission products, respectively. The experimentally determined mass- and time-dependences of delayed neutron emission are compared with those calculated from the known precursors. Unidentified neutron activities in the mass regions 85–88 (half-life 1 sec) and 96–100 (half-life 1.5 sec) are discussed. By means of theoretical considerations the yield difference for delayed neutrons between light and heavy fission products is interpreted and the new neutron activity is assigned to the precursors⁹⁸Y or⁹⁹Y.     

Heavy-ion induced fusion-fission systematics and the effect of the compound-nucleus spin distribution on fission-barrier determination

Fission and evaporation residue excitation functions have been measured for reactions using ¹⁸O and ¹⁹F projectiles to produce ¹⁵⁸Dy, ¹⁶⁸Yb, ¹⁷⁸W, ¹⁸⁸Pt and ²¹⁰Po compound nuclei. The results of the measurements were compared with statistical model calculations performed using the Hauser-Feshbach code ALERT1. The initial calculations used fixed parameters and showed excellent agreement with the data for all but the two lightest systems studied, ¹⁵⁸Dy and ¹⁶⁸Yb. Further calculations with different parameter sets gave better agreement for the light systems. It is concluded that the calculations are best able to describe the data in those situations for which the angular momentum distribution following fusion produces significant population in or above the region where the fission barrier is equal to the neutron binding energy. This is not the case for the light systems. The fission barriers for the heavy systems are consistent with those predicted by the rotating finite range model.     

The effect of the entrance channel on the fission of 216Ra

We have studied mass-energy distributions (MED) of fission fragments using two projectile-target combinations, ¹²C + ²⁰⁴Pb and ⁴⁸Ca + ¹⁶⁸Er, leading to the same compound nucleus ²¹⁶Ra at the excitation energy E*～40 MeV. It has been found that the contribution of the asymmetric mode in the case of the former reaction is 1.5%, and it is 30% in the case of the latter one. We connect such a sharp increase in the yield of asymmetric products in the ⁴⁸Ca + ¹⁶⁸Er reaction with the quasifission process, the MED of which have a clearly expressed shell structure. The characteristics of the fission fragment MED are of such a kind that they can be interpreted by analogy with the low-energy fission of heavy nuclei as a manifestation of an independent mode of nuclear decay which competes with the classical fusion-fission process.     

Investigation of the <Superscript>232</Superscript>Th and <Superscript>238</Superscript>U photofission

The average angular momenta of the ⁸⁴Br, ¹³²Sb, ¹³³Te, and ¹³⁵Xe fission fragments, produced as a result of ²³²Th and ²³⁸U photofission by bremmsstrahlung γ photons with an endpoint energy of 8.5 MeV, have been determined by measuring the isomeric yield ratios. The experimental isomeric yield ratios have been calculated taking into account the contribution from β decay of isobaric nuclei to the yields of isotopes studied.     

(EC + β<Superscript>+</Superscript>) decays of the 11/2<Superscript>-</Superscript> isomer and 1/2<Superscript>+</Superscript> ground state of <Superscript>143</Superscript>Dy

The 1/2⁺ ground state and a 11/2^- isomer of very neutron-deficient isotope ¹⁴³Dy were produced by irradiation of an enriched target of ¹⁰⁶Cd with ⁴⁰Ca and studied by using a helium-jet fast tape-transport system in combination with proton-γ, X-γ and γ-γ coincidence measurements. A simple ( EC + β⁺) decay scheme of ^143mDy with a half-life of 3.0(3) s and a tentative ( EC + β⁺) decay scheme of ^143gDy with a half-life of 5.6(10) s are proposed. As a by-product, the 347- and 545-keV γ transitions in ¹³⁸Sm following the β-delayed proton emission of ¹³⁹Gd decay and the 323-keV γ transition in ¹³⁹Eu following the β-delayed proton emission of ¹⁴⁰Tb decay could be observed for the first time. Received: 20 August 2002 / Accepted: 28 October 2002 / Published online: 11 February 2003 RID="a" ID="a"e-mail: xsw@ns.lzb.ac.cn Communicated by D. Schwalm     

Messung von HalbwertszeitenK-verbotener Übergänge ingg-Kernen der Seltenen Erden

The half-life of the 1,490 keV level in¹⁶²Dy has been measured in the decay of¹⁶²Ho^m (68 min) by delayed coincidences ofKX rays and (900-1,300) keV gamma radiation to be (1.94 ± 0.19) nsec. A half-life in¹⁷⁶Hf has been remeasured in the decay of¹⁷⁶Ta (8.1h) by delayed coincidences of KX rays and (1,100–1,200) keV gamma radiation to be (4.75 ± 0.25) nsec. In order to fix the level with this half-life in the level scheme of¹⁷⁶Hf, the gamma spectrum delayed to KX rays was measured. It is suggested that probably the 1,227.4 keV-level of¹⁷⁶Hf is delayed and has the spin 2^? or 3⁺.     

Spin-flipβ − decay of even-even deformed nuclei110Ru and112Ru

Neutron-rich nuclides¹¹⁰Ru and¹¹²Ru produced in symmetric fission of²³⁸U by 20 MeV protons have been studied at the IGISOL facility by means ofβ-ray,γ-ray and conversion electron spectroscopy. A total of 12 and 6γ-transitions were observed in the decays of¹¹⁰Ru and¹¹²Ru, respectively. Multipolarities were determined for a few transitions. The beta decay half-life was determined to be 11.6±0.6 s for¹¹⁰Ru and 1.75±0.07 s for¹¹²Ru. As a side product, a new value of 2.1±0.3 s for theβ half-life of the¹¹²Rh 1⁺ state was obtained. The decay energy measured with the plastic scintillator was 2.81 ±0.05 MeV for¹¹⁰Ru and 4.52 ±0.08 MeV for¹¹²Ru. The beta decay schemes of¹¹⁰Ru and¹¹²Ru isotopes indicate that the main fraction of their beta strength resides in two 0⁺→1⁺ spin-flip transitions with their logf t values between 4.4 and 4.7. The decay energies and the energies of the 1⁺ GT states are compared with the macroscopic-microscopic model calculations. The observed GT-strengths are discussed in the framework of the deformed single-particle model.     

Determination of the fission barrier of232Pu from β-delayed fission and the problem of the first barrier

A delayed fission activity with a half-life of (55±7) s and a production cross section of (5±1)nb has been observed in the irradiation of²³⁷Np with 104 MeVα particles. It is tentatively assigned to the electron capture decay of²³²Am followed by fission of²³²Pu With a probability of (1.3 _?0.8 ⁺⁴ )· 10^?2 A barrier height of (5.3±0.4) MeV is extracted for²³²Pu exceeding theoretical barrier heights by about 2MeV. Systematic deviations between experimental and theoretical barrier heights previously observed in particular for U and Th isotopes are now also established for neutron deficient Pu isotopes indicating severe defects in the procedure of calculating static fission barriers.     

Der Zerfall des95Sr

The nuclide⁹⁵Sr was isolated by chemical separation methods after thermal neutron induced fission of²³⁵U. It's decay properties were investigated employing semiconductor spectrometers and coincidence techniques. A half-life of 24.4±0.2 sec was found. Out of 29 gamma rays, 25 transitions representing 97% of the observed gamma-ray intensity were placed in a decay scheme comprising 19 excited states of⁹⁵Y.     

Limits on equal energy back-to-backγ-rays in252Cf decay

A search has been made for coincident, equal-energy, 180° correlatedγ-rays from²⁵²Cf fission decay. Theseγ-rays may arise if new neutral particles are produced in fission, and if they decay byγ-emission. The fraction of fissions which give rise to particles at rest with masses between 1.4 and 2.0 MeV, which then decay into twoγ-rays in a time ? 10^?19s, is less than 8 × 10^?7 at the 95% confidence level.     

The fissioning isomer 237Npm

A spontaneously fissioning isomer of ²³⁷Np with a half-life of 40±12 ns has been discovered. Threshold measurements indicate that the isomer excitation energy is 2.7 ± 0.3 MeV. It is concluded that spontaneous fission is not the principal decay mode for this shape isomer.     

Zur Gamma- und Röntgenstrahlung von Holmium-164

The photon spectrum in the decay of¹⁶⁴Ho was investigated using a high resolution solid state detector separating theK-X-rays of Dy, Ho and Er. The two γ-rays depopulating the 3⁺ and 2⁺ levels in¹⁶⁴Ho were found to have energies of 56.6 and 37.3 keV. The 91.4 keV radiation of Er from our sample seems to have a non resolved high energy component with about 2.5% intensity. It may be due to the cross-over transition 3⁺→1⁺ or a sum line. The half-life of the isomeric state of¹⁶⁴Ho was found to be 36.7 ± 1 min. The total transition rates between different levels were calculated from the measured photon intensities. The fraction 0.27 found in the branching of thee ^?-capture feeding the 2⁺-state of Dy has nearly the same value as the analog branching in the β^?-decay. Combining the measured complex ground state decay curve and the transition intensities of the isomeric and of the ground state the ground state half-live can be deduced and was found to be (28.1 ± 1.0) min. The corresponding ratio of the ground state and the isomeric state production rates is 2.8₄:1. A ratio of 0.04 was found for the cross section of¹⁶⁵Ho (γ, 3n),¹⁶²Ho and¹⁶⁵Ho (γ,n)¹⁶⁴Ho for a 90 MeV bremsspectrum.     

Beta decay of neutron-rich 118Rh and the lowest excited states in 118Pd

Beta decay of a refractory isotope ¹¹⁸Rh produced in symmetric fission and mass separated by the ion guide technique has been applied for the study of low-lying excited states of ¹¹⁸Pd. The yrast band in ¹¹⁸Pd has been observed up to a 6⁺ state and the lowest states of the asymmetric γ-band have been identified. The measured half-life of ¹¹⁸Rh is (300±60)ms. The systematics of the excited states in neutron-rich Pd-isotopes implies the saturation towards an O(6) symmetry at N = 70. Received: 19 June 2000 / Accepted: 1 August 2000     

Search for an 80 ms spontaneous fission activity in bombardments of 249Bk with 15N

A rotating drum system was used to search for an 80 ms spontaneous fission (sf) activity in the reaction of ¹⁵N with ²⁴⁹Bk. No such activity was found beyond a cross section limit of 0.5 nb. A sf activity with a half-life of 23 ± 2 ms and a maximum formation cross section of 19 ± 4 nb at 82 MeV was observed. The identity of this activity has not been determined.     

Der Zerfall des94Sr

The nuclide⁹⁴Sr was isolated by chemical separation methods after thermal neutron induced fission of²³⁵U. Its decay properties were investigated employing semi-conductor spectrometers and coincidence techniques. A half-life of 74.1±0.3 sec was found. Besides the 1428.3 keV γ-ray we found 4 new γ-rays which could be placed in a decay scheme comprising three excited states of⁹⁴Y. The 1⁺ states in theuu-nuclides^90,92,94Y and^88,90,92Rb are compared.     

Der Zerfall des163Tb

The beta and gamma radiations of¹⁶³Tb have been investigated employing scintillation and semiconductor spectrometers and coincidence techniques. The sources were produced by the¹⁶³Dy(n,p)¹⁶³Tb reaction. A half-life of 19.5 min has been measured without evidence for an isomeric state in the half-life range of 2 sec to 7 d; especially a 6.5 h activity previously assigned to¹⁶³Tb could not be confirmed. 76 of the total of 82 gamma-ray transitions, representing 99.6% of the gamma-ray intensity, could be assigned to 23 levels in¹⁶³Dy. From beta and beta-gamma coincidence measurements aQβ-value of 1.7 ± 0.1 MeV has been determined. Spin and parity of 3/2⁺ are deduced for the ground state of¹⁶³Tb with the Nilsson configuration [411↑]. The level at 884 keV is interpreted as aJ ^π=1/2⁺ state with a high contribution of the three-quasiparticle configurationp[411↑]-p[523↑]-n[523↓] because of an allowed unhindered beta-ray transition to this level.     

The study of the (α, α'f) reaction at 120 MeV on 232Th and 238U: (I). Fission probabilities and angular distributions in the region of the giant quadrupole resonances

The fission decay channel of ²³²Th and ²³⁸U has been investigated, using the (α, α'f) reaction at 120 MeV bombarding energy. The angular distributions of the fission fragments and the fission probabilities up to around 15 MeV excitation have been measured. No evidence for the fission decay of the giant quadrupole resonance has been found, although for ²³⁸U, a weakly excited structure is seen in the (α, α'f) spectrum at about 9.5 MeV excitation at backward angles with respect to the recoil axis. This effect is similar to what has been found in a (⁶Li, ⁶Li'f) experiment reported recently. The over-all feature of the fission probability for excitation energies above the fission barrier are well reproduced by statistical calculations.     

Study of the fission decay of the isoscalar monopole resonance via the 238U(α, α′) reaction at 0°

The fission decay of ²³⁸U has been investigated using inelastic scattering of 120 MeV ga-particles to excite the ²³⁸U nucleus. Angular correlations of the fission fragments have been measured for excitation energies between 5.7 and 15.7 MeV in coincidence with inelastically scattered α-particles between 0 and 3°. The difference in yield for fission in coincidence with inelastically scattered α-particles between 0–1.35° and 1.35–3° was used to deduce the fission decay of the giant monopole resonance. It was found that in the fission decay channel (22 ± 5)% of the E0 EWSR strength is located between 8 and 15 MeV excitation energy. The distribution of the deduced monopole strength is in agreement with recent theoretical calculations predicting splitting of the giant monopole resonance in deformed nuclei.     

Spontaneous fission of <Superscript>256</Superscript>Rf,new data

Spontaneous fission properties of the short-lived neutron-deficient ²⁵⁶Rf nucleus produced in the complete fusion reaction with a beam of multiply charged heavy ⁵⁰Ti ions from the U-400 cyclotron (FLNR, JINR) are experimentally investigated. Its half-life and decay branching ratio are measured. The average number of neutrons per spontaneous fission of ²⁵⁶Rf (\(\bar v = \;4.47 \pm \;0.09\)) is determined for the first time.     

A setup for the search for E0 decay of superdeformed 236U

Transitions between nuclear states with a large deformation difference may have large transition probabilities for E0 decay. Such an E0 decay has been found in the decay of the superdeformed ²³⁸U fission isomer back into the normal-deformed states [1]. However, a search for the E0 back-decay in ²³⁶U showed no result [2]. A setup has been developed to search for conversion electrons in this decay.