CP violation and the search for new physics

Beta-delayed proton decay of ⁵⁷Zn has been investigated at the GSI on-line isotope separator. The studied ⁵⁷Zn nuclei were produced in fusion evaporation reactions by using a 150 MeV ³²S beam on a ²⁸Si target. Beta-delayed protons were measured by a charged-particle telescope detector. The observed decay pattern was used to construct the level scheme of ⁵⁷Cu and to extract the beta feeding distribution. The experimental results are compared with shell-model calculations. An erratum to this article is available at .     

The upgrade of the kinematic separator VASSILISSA—experimental results and plans

Within the past twelve years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in heavy-ion-induced complete fusion reactions. In the course of the experimental work, fourteen new isotopes have been identified by the parent-daughter correlations in the region of elements with 92≤Z≤94. The study of the decay properties and formation cross sections of the isotopes of elements 110, 112, and 114 was performed using high-intensity ⁴⁸Ca beams and ²³²Th, ²³⁸U, and ²⁴²Pu targets. At the beam energies corresponding to the calculated cross-section maxima of the 3n evaporation channels, the isotopes ²⁷⁷110, ²⁸³112, and ²⁸⁷114 were produced and identified. For further experiments aimed at the synthesis of the superheavy element isotopes (Z≥110) with the use of intensive ⁴⁸Ca extracted beams, improvements in the ion optical system of the separator and the focal plane detector system have been made.     

The β-delayed one- and two-proton emission of 27S

In an experiment performed at the GANIL LISE3 facility, radioactive ²⁷S isotopes have been produced by projectile fragmentation of a 95 AMeV ³⁶Ar primary beam. After selection by means of the LISE3 separator, the isotope of interest was implanted in a silicon-detector telescope where its half-life ( T _1/2 = 15.5(15) ms) and its main decay branches were measured. Received: 19 October 2001 / Accepted: 5 December 2001     

The decay of the Tz = −12 nuclide 59Zn

The decay of ⁵⁹Zn was studied with an isotope separator on-line. From the β- and γ-rays the half-life and E^max(β⁺) of ⁵⁹Zn were found to be 183.7 ± 2.3 ms and 8100 ± 100 keV, respectively, and the ft-value and the GT matrix element were deduced. Delayed protons in the decay of ⁵⁹Zn were also studied.     

Isotope shift in the Ca I resonance line and changes in mean-square nuclear charge radii of the stable Ca isotopes

The isotope shift of all stable Ca isotopes was studied in the Ca I resonance line. Enriched isotopes were used in an atomic beam, passing through the center of a spherical Fabry-Perot interferometer. The measured isotope shifts were separated into mass shift and field shift by comparing the optical isotope shift with δ〈r ²〉 values derived from recent muonic x-ray measurements. The results are discussed together with known data on the isotope shift in the Ca I intercombination line and data from Hartree-Fock calculations. The following mean values are obtained for the change in nuclear charge distribution δ〈r ²〉 [fm²]∶ [40, 42] 0.23(3); [40, 43] 0.13(3); [40, 44] 0.28(4); [40, 46] 0.14(7); [40, 48] 0.00(2).     

Production and decay of269110

In an experiment carried out to identify element 110, we have observed anα-decay chain, that can be unambiguously assigned to²⁶⁹110. In a scries of preexperiments the excitation functions of the fusion reactions⁵⁰Ti +²⁰⁸Pb→²⁵⁸104^* and⁵⁸Fe +²⁰⁸Pb→²⁶⁶108^* were measured with high precision in order to get the optimum projectile energies for the production of these heavy elements. The cross-section maxima of the 1n evaporation channels were observed at excitation energies of 15.6 MeV and 13.4 MeV, respectively. These data result in an optimum excitation energy of 12.3 MeV of the compound nucleus for the production of²⁶⁹110 in the reaction⁶²Ni +²⁰⁸Pb→²⁶⁹110 + 1n. In irradiations at the corresponding beam energy of 311 MeV we have observed a decay chain of 4 subsequent a decays. This can be assigned to the isotope with the mass number 269 of the element 110 on the basis of delayed α-α coincidences. The accurately measured decay data of the daughter isotopes of the elements 108 to 102, obtained in the previous experiments, were used. The isotope²⁶⁹110 decays with a hair-life of (270 _-120 ⁺¹³⁰⁰ ) μs by emission of (11.132±0.020) MeV alpha particles. The production cross-section is (3.3 _-2.7 ^+6.2 ) pb.     

Observation of the isotopes264108 and265108

The experiment having led to the discovery of first isotopes of element 108, as published previously in Short Notes to this Journal [1, 2] are described and discussed in a final paper. Two isotopes of element 108 were produced by complete fusion of²⁰⁷Pb and²⁰⁸Pb, respectively, with⁵⁸Fe. Both isotopes areα-emitters. For the isotope with mass 265 threeα-decay sequences were observed. Theα-decay energy is (10.36±0.03) MeV, the half-life (1.8 _?0.7 ^+2.2 ) ms. For the isotope with mass 264, which is the heaviest doubly even isotope known at present, the decay sequence of one atom was found. The measured half-life is (76 _?36 ⁺³⁶⁴ ) μs. Our experimental results point towards an enhanced stability of the heaviest elements against spontaneous fission, which was already observed for element 106.     

Superheavy research in Japan

At RIKEN (The Institute of Physical and Chemical Research) in Japan, we have performed experiments to study the productions and decays of the heaviest elements produced by one-neutron emission channels of ²⁰⁸Pb and ²⁰⁹Bi based heavy-ion-induced fusion reactions. A gas-filled-type recoil separator has been used for collecting evaporation residues of the reactions separating them from high intensity beam particles. The reactions studied were ²⁰⁸Pb(⁵⁸Fe,n)²⁶⁵Hs, ²⁰⁸Pb(⁶⁴Ni,n)²⁷¹Ds, ²⁰⁹Bi(⁶⁴Ni,n)²⁷²Rg, ²⁰⁸Pb(⁷⁰Zn, n) ²⁷⁷112, and ²⁰⁹Bi(⁷⁰Zn,n)²⁷⁸113. In studies of the first four reactions we have provided the independent confirmations of the productions and their decays of the isotopes, ²⁶⁵Hs, ²⁷¹Ds, ²⁷²Rg, and ²⁷⁷112, as well as the decay properties of their decay daughters, previously studied by Hofmann et al., a group of Gesellschaft für Schwerionenforschung (GSI), Germany. In the last reaction, we observed two decay chains originated from the isotope ²⁷⁸113, assigned firstly by generic correlation of the alpha decay chains connected into the previously known decay of ²⁶⁶Bh and ²⁶²Db via previously unknown alpha decays of ²⁷⁸113, ²⁷⁴Rg and ²⁷⁰Mt.     

Production and decay of the isotope 271 Ds (Z = 110)

Production and decay of the isotope ²⁷¹Ds were studied. The isotope was produced by ²⁰⁸Pb + ⁶⁴Ni ²⁷¹Ds + n reaction. Fourteen -decay chains have been assigned to decays originating from the isotope ²⁷¹Ds. An excitation function of the production of this isotope was measured. The results have provided a good confirmation of production and decay of ²⁷¹Ds reported by Hofmann et al. The presence of an isomeric state in ²⁷¹Ds has been confirmed. The existence of a possible isomeric state in ²⁶⁷Hs is presented.Received: 22 September 2003, Revised: 26 December 2002, Published online: 10 August 2004PACS: 23.60. + e decay - 25.70.-z Low and intermediate energy heavy-ion reactions - 27.90. + b D. Kaji: Present address: Center for Nuclear Science, University of Tokyo, Wako-shi, Saitama 351-0198, Japan.A. Ozawa: Present address: University of Tsukuba, Tsukuba, Ibaragi 3058 577, Japan.     

Identification of the neutron-rich isotope174Er

The heaviest known erbium isotope, 3.3(2) min.¹⁷⁴Er, has been identified using on- line isotope separation and by ß and γ- ray spectroscopy. Twelve γ rays were placed in a preliminary decay scheme. The mass of the new isotope was established by the separator and the Z was deduced from Tm K x ray coincident ß and γ rays. In addition, the heaviest known isotope of holmium¹⁷¹Ho was observed with a half-life of 49(5) s.     

Penning trap assisted decay spectroscopy of neutron-rich 115Ru

Exotic, neutron-rich ¹¹¹Mo and ¹¹⁵Ru nuclei, produced in proton-induced fission of ²³⁸U target, were separated with the IGISOL mass separator. The separator was coupled to the JYFLTRAP Penning trap to select the ions of a single, desired element out of the isobaric IGISOL beam. Monoisotopic samples of ¹¹⁵Ru and ¹¹¹Mo ions were observed with a microchannel plate detector after the trap or were implanted on a catcher foil for gamma- and beta-ray coincidence spectroscopy. In spite of short data taking time new gamma transitions were identified in the beta decay of very neutron-rich ¹¹⁵Ru.     

The influence of ion species on ion bombardment induced preferential orientation

Abstract

Vacuum-deposited Cu specimens were implanted with 40 keV Cu⁺, Ni⁺, Ne⁺, Kr⁺ and Xe⁺ ions in an electromagnetic isotope separator. The specimens were investigated for texture by X-ray diffraction techniques on implanted as well as non-implanted areas of the same specimen. It was found that a preponderance of ?110? crystallographic axes parallel to the incident beam develops during irradiation but that the intensity of the effect is dependent on the ion species in an apparently unpredictable way. A previously suggested channelling mechanism for the origin of the effect is discussed in the light of the present findings and it is concluded that modifications to the proposed mechanism is needed to accommodate these results. It is further shown that certain implications of the results presented here could have an important bearing on ion bombardment related phenomena such as ion ranges, sputtering and materials modification by ion implantation.     

Possible new approaches to superheavy elements — the (HI,αχn) reaction and the neutron-rich beam

We first report the recent experiment on the²³²Th(⁴⁰Ar,α3n)²⁶⁵[106] reaction carried out at RIKEN to test the usefulness of the (HI,αχn)-type reaction for the production of very heavy elements. Reaction residues recoiling out of a thin ThO₂ target bombarded with 5.2 MeV/u⁴⁰Ar beam were separated from the beam by using a gas-filled recoil separator and implanted onto a two-dimensionally position-sensitive solid-state detector placed at the focal plane of the separator. The preliminary results of the analysis show that the cross section of the above reaction is around 1 nb, being about four times larger than that of the²⁴⁸Cm(²²Ne,5n)²⁶⁵[106] reaction reported in the literature. Secondly, we discuss briefly the merits and demerits of the use of a neutron-rich beam for the production of superheavy elements, in which we stress the possibility that the demerit of weak secondary-beam intensity is nearly cancelled out by the increase of the survival probability with the increase of the neutron number of the compound nucleus and also by the possible enhancement of near-barrier fusion.     

Design study of an in-flight projectile fragment separator for rare isotope beams

Radioactive isotope(RI) beams are used to investigate the characteristics of unstable nuclei. Fragment separators, which have large angular and energy acceptances, were required to obtain high RI beam intensity. Careful design is required due to the large high order aberrations induced by the large aperture magnets, which are used to collect rare isotopes obtained from a high energy primary heavy-ion beam hitting a target [1]. In our design study, a high energy ¹²C primary beam was used to produce neutron rich medium mass heavy ions such as ⁹Li. Mirror symmetric optics provides smaller high order aberration and thus a design study of a mirror symmetric in-flight projectile fragment separator was performed to obtain large angular and energy acceptances. We investigated the optimal material and thickness of the target for the production of a ⁹Li beam. Based on the simulation, a beryllium target was selected to give a large yield with a smaller energy spread of the secondary beam. We also investigated the optimal thickness of the aluminum energy degrader. The selections of the target material and thickness were investigated by using the code LISE++.After optimization of the material and the thickness of the target, we performed a design study of the optics of the in-flight separator for a high resolution with high acceptance. The designed optics of the in-flight separator consists of the four normal conducting quadrupole triplets, three sextupoles and two normal conducting dipoles. The horizontal and vertical angular acceptances of the designed separator are 40 mrad and 70 mrad, respectively. The separator has a mass resolution of 640 when the object size is taken to be 1 mm. The correction of the second order aberration in the designed optics was performed by three sextupole magnets. The path length of the designed separator is 20.183 m. The optics design and the high order aberrations were investigated by using the code ORBIT.     

Beta-delayed proton precursors with 59≦Z≦62

The new, beta-delayed proton emitters¹²⁸Pr(3.2 _?0.4 ^+0.5 s),¹³⁰Pm(2.2 _?0.4 ^+0.6 s), and¹³²Pm(5.0 _?0.6 ^+0.8 s) have been observed at the on-line isotope separator OASIS. Also studied were¹²⁹Nd and¹³³Sm. Our on-going study of the decay properties of very neutron-deficient lanthanides has led to detailed information about beta-delayed proton emitters with 59≤Z≤62. The isotopes were produced at the SuperHILAC's on-line isotope separator OASIS [1] in reactions of⁴⁰Ca projectiles on⁹²Mo and⁹⁶Ru targets enriched to >95%. Decay studies were carried out in a low background spectroscopy laboratory. The isotope of interest was passed through a slit in the focal plane of the mass separator, transported ionoptically to a fast cycling tape, and periodically positioned between a silicon charged particle telescope, a HPGe x-ray detector, and a GAMMA-X Ge detector. Decays involving beta-delayed gamma rays and protons along with any coincident positrons, x-rays and gamma rays were measured event-by-event and written to computer tape for subsequent replay and analysis.     

Reactions of40Ar with233U,235U,and238U at the barrier

Production cross sections for target-like transfer products in reactions of⁴⁰Ar with^233,235,238U at the barrier were determined using radiochemical techniques. The heaviest products detected are isotopes of californium (Z=98). In addition to the quasi-elastic component of the isotopic distributions observed in the vicinity of uranium, there are also relaxed contributions throughout the entire region. The peak positions of the isotope distributions of this component for fixed atomic number, after the transfer of > 3 charges, approach closely the minimum of the potential energy surfaces (PES). The experimental results thus indicate the cold formation of the reaction products. A search for long-lived heavy actinides produced, by fusion-evaporation, via isotopes of element 110 and their subsequent decay through α-decay chains, remained unsuccessful at a cross section limit of 21 pb.     

Search for new isotopes of element 112 by irradiation of 238U with 48Ca

The reaction ⁴⁸Ca+²³⁸U was investigated at the recoil separator VASSILISSA at the Flerov Laboratory of Nuclear Reactions, JINR Dubna, in attempts to synthesize new isotopes of element 112. The experiments were performed at two beam energies resulting in excitation energies of the compound nucleus of 33 and 39 MeV. The collected beam dose was 3.5 × 10¹⁸ and 2.2 × 10¹⁸, respectively. Two spontaneous fission events were observed at the lower beam energy, which tentatively were assigned to the new neutron rich isotope ²⁸³112 produced in the reaction ²³⁸U(⁴⁸Ca,3n)²⁸³112. The measured cross-section is (5.0^+6.3 _−3.2) pb and the half–life is (81⁺¹⁴⁷ ₋₃₂) s. No event was observed at the higher beam energy resulting in the upper cross–section limit of 7.3 pb. Received: 12 October 1998 / Revised version: 12 January 1999     

Beta-delayed proton spectroscopy of 24Si

In an experiment at the LISE3 facility of GANIL, we used the projectile fragmentation of a ³⁶Ar primary beam at 95 MeV/nucleon to produce the isotope ²⁴Si. The beta decay half-life of ²⁴Si has been determined to be T_1/2 = (140 ± 8) ms, in agreement with earlier measurements. In addition to the decay of the isobaric analog state, several proton peaks are observed for the first time allowing to identify most of the Gamow-Teller transitions to unbound states and to present a partial decay scheme for ²⁴Si. These results are compared with shell-model predictions.     

Excited states of 154Nd studied through the decay of 154Pr

The neutron-rich isotope ¹⁵⁴Pr, the heaviest isotope of praseodymium, has been investigated by γ-ray multispectrum scaling and γ-γ-(t), X-γ-(t) coincidence experiments. The isotope ¹⁵⁴Pr was separated from ²³⁵U fission products with the on-line isotope separator KUR-ISOL. The decay scheme of ¹⁵⁴Pr has been constructed consisting of 9 excited states and 12 transitions in ¹⁵⁴Nd, including 7 excited states newly found in the present experiment.     

Second experiment at VASSILISSA separator on the synthesis of the element 112

The upgraded separator VASSILISSA was used to confirm results of previous experiments on the synthesis of the heavy isotopes of the element 112 obtained in complete-fusion reactions of ⁴⁸Ca and ²³⁸U. An additional reason was the non-observation of any events in the same reaction studied with the BGS separator at Berkeley. Limits of for the atomic mass number of the observed spontaneously fissioning isotope were measured. The obtained results on the decay mode, half-life and production cross-section are in agreement with those obtained in our first experiment.Received: 10 June 2003, Revised: 29 July 2003, Published online: 2 December 2003PACS: 23.60. + e decay - 25.70.-z Low and intermediate energy heavy-ion reactions - 25.85.Ca Spontaneous fission - 27.90. + b