Neutron-rich Radioactive Ion Beams Research Articles

New program for measuring masses of silver isotopes near the N = 82 shell closure with MLLTRAP at ALTO

The ISOL facility ALTO, located at Orsay in France, provides stable ion beams from a 15 MV Tandem accelerator and neutron-rich radioactive ion beams (RIB) from the interaction of a γ-flux induced by a 50 MeV 10 μA electron beam in a uranium carbide target. A magnetic dipole mass separator and a resonance ionization laser ion source allow to select the ions of interest. New setups are under preparation to extend the fundamental properties measured at ALTO of ground and excited states of exotic nuclei. For example, high-precision mass measurements for an accurate determination of the nuclear binding energy. To perform these measurements two devices will be hosted at ALTO: a radio frequency quadrupole to cool and bunch (RFQCB) the continuous radioactive beam and the double Penning trap mass spectrometer MLLTRAP, commissioned off-line at the Maier-Leibnitz Laboratory (MLL) in Garching, Germany. The unique production mechanism using photo-fission at the ALTO facility allows for mass measurements in a neutron rich area of nuclides of major interest around 132Sn with less isobaric contamination than using proton drivers. In this context, we plan to measure neutron-rich silver isotopes (Z = 47, A > 121) to explore the possible weakening of the shell gap for Z < 50 and its impact on the A = 130 r- process nucleosynthesis. The ground-state light silver masses (A < 121), known with a precision below 20 keV, will be used for the on-line commissioning of MLLTRAP and to characterize the performance of the detection system. The status and timeline of the novel setup is presented.

Experimental testing and numerical simulations for life prediction of gate valve O-rings exposed to mixed neutron and gamma fields

EPDM O-rings of gate valves employed for the construction of a second-generation accelerator for the production of neutron-rich Radioactive Ion Beams were studied in order to assess their sealing performance both during the facility service time and the post-service storage phase. Several O-ring specimens were at first exposed to different dose levels of mixed neutron and gamma radiations. Correspondent modifications of physical and mechanical properties of the material were investigated by means of uniaxial tensile tests, dynamic mechanical analyses, aging, compression set and vacuum leak tests. A hyperelastic strain energy function was adopted to fit the mechanical response of the material as a function of the absorbed dose. The minimum squeeze degree that guarantees O-ring sealing efficiency at different irradiation levels was determined by varying the interference between O-rings and grooves. A finite element model of the vacuum leak test was then set up to assess the contact pressure level required to ensure sealing. Numerical simulations of the gate valve main O-ring were subsequently carried out. By comparison of the predicted contact pressure and strain levels with experimental results, a life prediction map, as function of the service time, the storage time and the O-ring squeeze degree, was proposed.

The SPES High Power ISOL production target

SPES (Selective Production of Exotic Species) is a facility under construction at INFN-LNL (Istituto Nazionale di Fisica Nucleare - Laboratori Nazionali di Legnaro), aimed to produce intense neutron-rich radioactive ion beams (RIBs). These will be obtained using the ISOL (Isotope Separation On-Line) method, bombarding a uranium carbide target with a proton beam of 40MeV energy and currents up to 200 μA. The target configuration was designed to obtain a high number of fissions, up to 1013 per second, low power deposition and fast release of the produced isotopes. The exotic isotopes generated in the target are ionized, mass separated and re-accelerated by the ALPI superconducting LINAC at energies of 10AMeV and higher, for masses in the region of A = 130amu, with an expected rate on the secondary target up to 109 particles per second. In this work, recent results on the R&D activities regarding the SPES RIB production target-ion source system are reported.

A combined experimental and numerical approach for the control and monitoring of the SPES target during operation at high temperature

The SPES project at INFN-LNL aims at the production of neutron-rich Radioactive Ion Beams (RIBs) using the ISOL (Isotope Separation On Line) technique. A 40MeV 200μA proton beam will directly impinge a uranium carbide target, generating approximately 1013 fissions per second. The target system is installed under vacuum inside a water-cooled chamber, and have to maintain high working temperatures, close to 2000°C. During operation the proton beam provides the heating power required to keep the target at the desired temperature level. As a consequence, its characteristics have to be strictly controlled in order to avoid undesired overheating. According to the original design of the control system, the proton beam can be suddenly interrupted in case of out of range vacuum or cooling water flow levels. With the aim to improve the reliability of the control system a set of temperature sensors has been installed close to the target. Their types and installation positions were defined taking into consideration the detailed information coming from a dedicated thermal–electric model that allowed to investigate the most critical and inaccessible target hot-spots.This work is focused on the definition and experimental validation of the aforementioned numerical model. Its results were used to appropriately install two type C thermocouples, a PT100 thermo-resistance and a residual primary beam current detector. In addition the numerical model will be used for the definition of appropriate thresholds for each installed temperature sensor, since it allows to define a relationship between the locally measured values with the overall calculated temperature field. In case of over temperatures the monitoring system will send warning signals or in case interrupt the proton beam.

Photo-ionization of aluminum in a hot cavity for the selective production of exotic species project

SPES (Selective Production of Exotic Species) is an Isotope Separation On-Line (ISOL) based accelerator facility that will be built in the Legnaro-Istituto Nazionale di Fisica Nucleare (INFN) Laboratory (Italy), intended to provide intense neutron-rich radioactive ion beams obtained by proton-induced fission of a uranium carbide (UCx) target. Besides this main target material, silicon carbide (SiC) will be the first to be used to deliver p-rich beams. This target will also validate the functionality of the SPES facility with aluminum beam as result of impinging SiC target with proton beam. In the past, off line studies on laser photoionization of aluminum have been performed in Pavia Spectroscopy Laboratory and in Laboratori Nazionali di Legnaro; a XeCl excimer laser was installed in order to test the laser ionization in the SPES hot cavity. With the new Wien filter installed a better characterization of the ionization process in terms of efficiency was performed and results are discussed.

Microscopic DC-TDHF study of heavy-ion potentials and fusion cross sections

We study heavy-ion fusion reactions at energies near the Coulomb barrier, in particular with neutron-rich radioactive ion beams. Dynamic microscopic calculations are carried out on a three-dimensional lattice using the Density-Constrained Time-Dependent Hartree-Fock (DC-TDHF) method. New results are presented for the 132Sn+40Ca system which are compared to 132Sn+48Ca studied earlier. Our theoretical fusion cross-sections agree surprisingly well with recent data measured at HRIBF. We also study the near- and sub-barrier fusion of 24,16O on 12C which is important to determine the composition and heating of the crust of accreting neutron stars.

Studies for aluminum photoionization in hot cavity for the selective production of exotic species project

Selective production of exotic species (SPES) is an ISOL-based accelerator facility that will be built in the Legnaro INFN Laboratory (Italy), intended to provide an intense neutron-rich radioactive ion beams obtained by proton induced fission of an uranium carbide target. Beside this main target, a silicon carbide (SiC) target will the first to be used to deliver some p-rich beams. This target will validate also the functionality of the SPES facility with aluminum beam as result of hitting SiC target with protons. In the past off-line studies on laser photoionization of aluminum have performed in Pavia Spectroscopy Laboratory and in Laboratori Nazionali di Legnaro where, recently, a XeCl excimer laser was installed in order to test the laser ionization in the SPES hot cavity. Results are promising to justify further studies with this technique, aiming a better characterization of the SPES ion extraction capability under laser photoionization.

Simulations for the future converter of the e-linac for the TRIUMF ARIEL facility

In the next years, TRIUMF activity will be focused on building a new facility to produce very intense neutron rich radioactive ion beams. Unlike others ISOL facilities, the e-linac primary beam, that will induce the fission, is an intense electron beam (50 MeV energy and 10 mA intensity). This challenging choice, which make this installation unique, despite the ALTO facility, makes an average fission rate of 1013–14fissions/s in the target.This beam is sent on an uranium carbide target (UCx), but due to its power, it is essential to insert a "converter" on the beam path to avoid a target overheating. The purpose of this converter is to convert electrons into Bremsstralhung radiation. The γ rays produce excite the dipole resonance of 23892U (15 MeV) inducing fission. Energy deposition, fission rate and thermal behavior were simulated using Monte Carlo techniques are presented in this paper

Discrimination of gamma rays due to inelastic neutron scattering in AGATA

Possibilities of discriminating neutrons and γ rays in the AGATA γ -ray tracking spectrometer have been investigated with the aim of reducing the background due to inelastic scattering of neutrons in the high-purity germanium crystals. This background may become a serious problem especially in experiments with neutron-rich radioactive ion beams. Simulations using the G eant4 toolkit and a tracking program based on the forward tracking algorithm were carried out by emitting neutrons and γ rays from the center of AGATA. Three different methods were developed and tested in order to find “fingerprints” of the neutron interaction points in the detectors. In a simulation with simultaneous emission of six neutrons with energies in the range 1–5 MeV and 10 γ rays with energies between 150 and 1450 keV, the peak-to-background ratio at a γ -ray energy of 1.0 MeV was improved by a factor of 2.4 after neutron rejection with a reduction of the photopeak efficiency at 1.0 MeV of only a factor of 1.25.

Localization of volatile isotopes on a cryotrap

Neutron-induced fission of uranium allows for the production of high-intensity neutron-rich radioactive ion beams. However, also large quantities of unwanted volatile radioactive species are produced that have to be hindered from contaminating the beamline and vacuum system of the facility. In the framework of radioprotection studies within the MAFF project at the FRM II in Garching with 1014fission events/s [D. Habs et al., The Munich accelerator for fission fragments MAFF, Nucl. Instr. and Meth. B 204 (2003) 739], the performance of a cryotrap system has been studied, designed to localize gaseous radioactivity close to its origin. These studies provide important radioprotection information for the planned EURISOL facility with 1015fission events/s. Design considerations of a compact cryotrap operated with cold helium gas at a saturation temperature around 18K will be presented. Activity distribution calculations of the fission source, the cryotrap and the subsequent vacuum system result in a prediction of the retention capability of the cryotrap system of 99.98%. These design calculations have been experimentally verified with three cryotrap prototypes differing in cold surface area as well as in their internal helium gas flow characteristics. Retention capabilities have been measured with and without passive shielding of the external thermal load (300K) using different tracer gases and an inclusive pressure-related diagnostics as well as mass-spectroscopic measurements.

Ion source development for the on-line isotope separator at JAEA

A forced electron beam induced arc discharge (FEBIAD-B2) type ion source and a surface ionization type one with a uranium target have been developed for the JAEA-ISOL. The ion sources were utilized to produce medium-heavy neutron-rich radioactive ion beams for acceleration with Tokai radioactive ion accelerator complex (TRIAC). In the FEBIAD-B2 type ion source, a target container containing about 1g/cm2 of 238UCx target is directly attached to a plasma chamber. While the surface ionization type one, 238UCx target is located inside the ion source as an internal target. With both ion sources, about 100 isotopes produced in the proton-induced fission of uranium with separation efficiencies of 0.1–30% have been mass-separated.

Multifoil UCx target for the SPES project --An update

The target system is one of the key issues for the facilities aimed at the production of neutron-rich radioactive ion beams. In the framework of the SPES project (Study for the Production of Exotic Species), the possibility of using a target configuration with a proton beam (40MeV, 0.2mA) directly impinging on multiple uranium carbide disks is investigated. The 238U fission fragments constitute the source for the exotic beams and for this purpose the disks are placed inside a graphite box at 2000 °C. The target is split into several thin disks in order to allow the cooling of the system by thermal radiation. In this way about ∼ 1013 fissions s -1 are obtained with a relative simple system and with relative low costs. Further steps have been performed compared to previous publications and now all the main parameters of the system have been analysed by means of calculation codes: the fission rates and the fission fragment distribution; the power deposition and the thermal analysis; the thermo-mechanical behaviours of the disks; the effusive and diffusive extraction release properties of the target.

Recent results at HRIBF using neutron-rich radioactive ion beams

Several recent experiments at the Holifield Radioactive Ion Beam Facility using neutron-rich radioactive ion beams (RIBs) are briefly described. These experiments contribute significantly to the growing list of techniques currently used with RIBs. Recent fusion-evaporation cross-section measurements below the Coulomb barrier for the 132Sn + 64Ni reaction suggest enhancement as compared to 124Sn + 64Ni reaction. These studies are being expanded to include reactions with 134Sn. The g factor of the first 2+ state in 132Te has been measured to be 0.35(5) through the recoil-in-vacuum technique. Angular distributions have been measured in the reaction 13C(134Te,135Te)12C at 550 MeV and used to suggest the spin and parity of a new 2109 keV state in 135Te as , suggesting a single-particle configuration of . Decay spectroscopy may be further enhanced using a ranging-out technique to reduce the isobaric impurity of neutron-rich beams.

Neutron transfer reactions with neutron-rich radioactive ion beams

Initial measurements are presented of the (d,p) reactions on neutron-rich N=50 isotones along the r-process path of nucleosynthesis with radioactive ion beams of 82Ge and 84Se. Prospects for measurements with unstable 130,132Sn beams are discussed.

Coulomb excitation and transfer reactions with neutron-rich radioactive beams

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of measurements around the 132Sn region, including Coulomb excitation and single-nucleon transfer. The B(E2;0+ → 2+) values for first 2+ excited states of even-even neutron-rich 132-136Te and 126-130Sn have been measured by Coulomb excitation in inverse kinematics. Neutron transfer onto a 134Te beam from 9Be and 13C targets, to populate single-particle states in 135Te, has also been studied. Gamma rays from the 13C(134Te, 12C) reaction were used to identify the νi13/2 state in 135Te, at an energy of 2109 keV. These and other results, and plans for future experiments with these neutron-rich beams, are presented.

Coulomb excitation and transfer reactions with rare neutron-rich isotopes

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of measurements around the 132Sn region, including Coulomb excitation, fusion-evaporation, and neutron transfer. The B ( E 2 ; 0 + → 2 + ) value for first 2+ excited states of even-even neutron-rich 132–136Te and 126–134Sn have been measured by Coulomb excitation in inverse kinematics. The results are discussed in terms of the shell model and the quasiparticle random phase approximation. Neutron transfer onto a 134Te beam, from 9Be and 13C targets to populate single-particle states in 135Te, has also been studied. Gamma rays from the 13C(134Te, 12C) reaction were used to identify the ν i 13 / 2 state in 135Te, at an energy of 2109 keV. These and other results, and plans for future experiments with these neutron-rich beams, are presented.

Nuclear structure studies with heavy neutron-rich RIBS at the HRIBF

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of exciting measurements around the 132 Sn region, including Coulomb excitation, fusion-evaporation, and neutron transfer. The B ( E 2 ; 0 + → 2 + ) value for first 2 + excited states of neutron-rich 132,134,136 Te and 126,128,130,132,134 Sn have been measured by Coulomb excitation in inverse kinematics. The results are discussed in terms of the shell model and the quasiparticle random phase approximation. Neutron transfer onto a 134 Te beam, from 9 Be and 13 C targets to populate single-particle states in 135 Te, has also been studied. Results, and plans for future experiments with these neutron-rich beams, are presented.

Synthesis and decay properties of superheavy atoms in nuclear reactions induced by stable and radioactive ion beams

This talk consists of two parts. The first one presents the results of investigations performed in 1998-2000 in Dubna on the synthesis of superheavy nuclei in reactions induced by 48Ca ions. The radioactive decay properties of the nuclei, indicating a considerable increase in the α-decay and spontaneous fission half-lives of the isotopes of elements 110-116 when approaching the closed neutron shell at N = 184, are given. In the second part the possible ways of advancing into the region of more neutron-rich nuclei, using stable and radioactive ion beams, are discussed. Since so far no intense radioactive ion beams are available, some experiments with stable beams are considered as a test for the suggested ideas.

Project of an advanced ISOL facility for exotic beams at LNL

In the framework of the European program to define a second generation Radioactive Ion Beam facility, LNL are proposing the construction in the next five–seven years of a specialized national facility for RIB originated by fission fragments produced by secondary neutrons. It consists on a two-accelerator ISOL-type facility to provide intense neutron-rich radioactive ion beams of highest quality, in the range of masses between 80 and 160. The conceptual design is based on a high intensity 50 MeV (100 kW) proton linac as driver and on the availability of the heavy-ion accelerator ALPI as post accelerator. The estimated neutron yield is 2×10 14 n/s at 0°, high enough to satisfy the demand for an advanced RIB facility. An intense R&D program on different items is actually in progress in collaboration with other Laboratories and University groups and is moving in a European context.