Maier-Leibnitz Laboratory Research Articles

Investigations of the structures of the Ru isotopes: 98Ru

As part of a systematic study of the nuclear structure of the Ru isotopes, 98Ru was investigated via the β-decay of 98Rh at iThemba LABS, and the 100Ru(p, t) reaction at the Maier-Leibnitz Laboratory. The combined data results in significant revision of the previous spin assignments and clarification of the nature of levels in 98Ru, as well as providing insights into the evolution of the structures across the Ru isotopic chain.

Fission resonances observed in the ^{237}Np(d,pf) reaction and the fission barrier topology of ^{238}Np

The fission probability of ^{238}Np was measured as a function of the excitation energy in the energy range of E^* = 5.4 - 6.2 MeV in order to search for transmission fission resonances. A radioactive ^{237}Np target was bombarded with deuterons of E_d = 12 MeV, whereas the energy of the protons was analyzed with a superior resolution of varDelta E = 8 keV. The experiment was performed at the Tandem accelerator of the Maier-Leibnitz Laboratory in Garching employing the ^{237}Np(d,pf) reaction. A group of fission resonances has been observed at excitation energies between E^* = 5.5 - 5.8 MeV, which could be ordered into three (superdeformed) rotational bands with a rotational parameter of hbar ^2/2varTheta = 3.507 keV, and identified as the first direct observation of transition states composing rotational bands in an odd-odd nucleus as they appear above the top of the outer fission barrier. Nuclear reaction code (talys1.95) calculations were also performed to extract the multi-humped fission barrier parameters of ^{238}Np by fitting them to the experimental data of the present (d,pf) and previous (n,f) experiments. The extracted barrier parameters also support the above interpretation of the observed resonances.

Spectroscopic Study of 39Ca for Endpoint Nucleosynthesis in Classical Novae

Classical novae are environments which can produce heavier elements up to mass A ∼ 40. These nuclides at the endpoint of nova nucleosynthesis consist of elements such as Ar, K, and Ca. There is an order of magnitude discrepancy with the predicted and theoretical abundances of these endpoint nuclides produced in a classical nova. The uncertainty in the theoretical 38K(p,γ)39Ca reaction rate has been shown to affect the abundances by an order of magnitude or more. The only direct measurement of this reaction rate was performed with the DRAGON facility at TRIUMF; however additional spectroscopic data could aid the interpretation of this data as well as motivate further study of this reaction rate. In this study, we present the preliminary results of a spectroscopic study of 39Ca using the 40Ca(d,t)39Ca reaction carried out at the Maier-Leibnitz Laboratory in Garching, Germany.

Accessing the Single-Particle Structure of the Pygmy Dipole Resonance in ^{208}Pb.

New experimental data on the neutron single-particle character of the Pygmy Dipole Resonance (PDR) in ^{208}Pb are presented. They were obtained from (d,p) and resonant proton scattering experiments performed at the Q3D spectrograph of the Maier-Leibnitz Laboratory in Garching, Germany. The new data are compared to the large suite of complementary, experimental data available for ^{208}Pb and establish (d,p) as an additional, valuable, experimental probe to study the PDR and its collectivity. Besides the single-particle character of the states, different features of the strength distributions are discussed and compared to large-scale shell model (LSSM) and energy-density functional plus quasiparticle-phonon model theoretical approaches to elucidate the microscopic structure of the PDR in ^{208}Pb.

Simulations of the novel double-Penning trap for MLLTRAP: Trapping, cooling and mass measurements

MLLTRAP is a double Penning trap mass spectrometer that was initially designed to be located at the Maier-Leibnitz-Laboratory (MLL) in Garching (Germany) for high precision mass measurements of exotic nuclei. A second double-trap assembly, dedicated this time to in-trap α decay spectroscopy, has been developed and is the object of this paper. This assembly can optionally replace the current mass-measurement trap electrode assembly during future spectroscopy campaigns at DESIR/SPIRAL2 in France. Though the previous assembly will be the instrument of choice for high-precision mass measurements, the new double-trap has been designed to be compatible with the ToF-ICR and PI-ICR mass measurement techniques in addition to its initial decay spectroscopy purpose, as it would be a significant operational advantage not to have to switch between assemblies on a regular basis. We have undergone a number of simulations to design this double trap system and estimate its future capabilities. These simulations concern the cooling of ions of interest in the first trap, mass measurements in the second trap and the application of the new Decay and Recoil Imaging (DARING) technique to measure lifetimes of first excited nuclear states populated by α decay. The latter will be described in details in a forthcoming publication and thus the present contribution should be considered as ”part one” of a two-part article on the design and simulation of the upcoming double-trap system for MLLTRAP. While the cooling and measurement techniques presented in this contribution have been extensively described and simulated in the past, this specific double-trap has a unique geometry, as the central electrode of the second trap has been replaced by a cubic arrangement of four Si-strip detectors. We study here the expected impact of this geometry on the mass measurement capabilities of the future trap.

Mass measurement of Re-190

In this paper, a measurement of the atomic mass and mass excess of are presented. This isotope and were produced at the Maier-Leibnitz Laboratory (MLL) in Munich in the 192Os(d, α)190Re and 194Pt(d, α)192Ir reactions. The Q3D magnetic spectrograph was used to measure the momenta of the α-particle ejectiles in order to reconstruct states in both 190Re and 192Ir. A mass calibration was performed using known energy levels in 192Ir. These measurements were used to obtain a new value of the mass excess of 190Re, −35583 ± 5 keV. The previously known literature value is −35640 ± 70 keV.

Application of in-trap [formula omitted] spectroscopy to lifetime measurements with MLLTRAP

MLLTRAP is a double Penning-trap designed for high precision mass measurements of exotic nuclei. It was built and commissioned off-line at the Maier-Leibnitz Laboratory (MLL) in Garching and is currently installed at the ALTO facility at IPNO, awaiting online commissioning. A complementary double-trap assembly dedicated to in-trap decay spectroscopy is being studied, in which the central electrode of the second trap has been replaced by an arrangement of four silicon-strip detectors (SSD) set at the trapping potential. By using the well-known buffer-gas cooling technique in the first trap we will be able to deliver a spot-like ion cloud of high purity to the detector trap. The decay products being emitted in-flight and not from an implantation material, we expect to achieve high-resolution decay spectroscopy of the confined ion bunch. In the case of heavy α emitters, α particles can be detected by the in-trap SSD, while conversion electrons are guided very efficiently by the magnetic field to another detector, thus allowing for unperturbed detection of both. In addition, once coupled with a position-sensitive electron detector, this spectroscopic trap will allow for indirect measurements of excited-state lifetimes in the region of heavy and super-heavy nuclei, via a new recoil distance measurement method.

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.

Shape coexistence in 94Zr studied via Coulomb excitation

In recent years, a number of both theoretical and experimental investigations have been performed focusing on the zirconium isotopic chain. In particular, state-of-the-art Monte Carlo shell-model calculations predict shape coexistence in these isotopes. In this context, the 94Zr nucleus, which is believed to possess a nearly spherical ground state, is particularly interesting since the purported deformed structure is basedon the low-lying 02+ state, making it amenable for detailed study. In order to provide definitive conclusionson the shapes of the low-lying states, two complementary experiments to study 94Zr by means of low-energy Coulomb excitation were performed. This data will allow the quadrupole moments of the 21,2+ levels to be extracted as well as for the deformation parameters of the 01,2+ states to be determined and, thus, definitive conclusions to be drawn on the role of shape coexistence in this nucleus for the first time. The first experiment was performed at the INFN Legnaro National Laboratory with the GALILEO-SPIDER setup, which, for the first time, was coupled with 6 lanthanum bromide scintillators (LaBr3:Ce) in order to maximize the γ-ray detection effciency. The second experiment was performed at the Maier-Leibnitz Laboratory (MLL) in Munich and used a Q3D magnetic spectrograph to detect the scattered 12C ions following Coulomb excitation of 94Zr targets.

Recent developments for AMS at the Munich tandem accelerator

The AMS setup at the Maier-Leibnitz-Laboratory (MLL) in Garching features a 14 MV tandem accelerator and two dedicated beam lines for high-sensitivity measurements of isotopes in all mass ranges. Especially in the medium mass range (70 < A < 120), AMS sensitivity is often limited by isobaric and isotopic background with only small relative difference in Z and/or A. Here, we present recent developments and upgrades at the MLL with special focus on improved sensitivity and new techniques for challenging AMS isotopes such as 93Zr and 99Tc. Upgrades of the measurement setup as well as radiopurity studies for AMS samples, which could be altered by atmospheric neutrons during transportation by aircraft, are discussed.

High-resolution (p,t) study of low-spin states in Pu240 : Octupole excitations, α clustering, and other structure features

A high-resolution $(p,t)$ experiment at $E_{p}$= 24 MeV was performed to populate low-spin states in the actinide nucleus ${}^{240}$Pu. The Q3D magnetic spectrograph of the Maier-Leibnitz Laboratory (MLL) in Munich (Germany) was used to identify the ejected tritons via $dE/E$ particle identification with its focal-plane detection system. Angular distributions were measured at nine different Q3D angles to assign spin and parity to the excited states based on a comparison with coupled-channels DWBA calculations. In total, 209 states have been excited in $^{240}$Pu up to an excitation energy of 3 MeV. Many previously known states have also been observed and their spin-parity assignments were confirmed. However, many of the populated states have been seen for the first time. The low-spin one-octupole phonon excitations, i.e. $K^{\pi} = 0^-,1^-,2^-,3^-$, could be observed and a new candidate for the $K = 3$ projection is proposed. Furthermore, the double-octupole or $\alpha$-cluster structure of the $0^+_2$ state in $^{240}$Pu has been studied in more detail. It is shown that the $0^+_2$ state in $^{230}$Th has a distinctly different structure. In addition, strongly excited $1^-$ states have been observed at 1.5\,MeV and 1.8\,MeV in $^{240}$Pu. The present study suggests that similar states might be observed in $^{230}$Th. At least two different and distinct structures for $J^{\pi} = 0^+$ states are present in the actinides. These are pairing states and states with enhanced octupole correlations. We have shown that it is crucial to consider negative-parity single-particle states being admixed to some $K^{\pi} = 0^+_2$ rotational bands to understand the $\alpha$-decay hindrance factors and enhanced $E1$-decay rates. Based on our analysis, we have identified the double-octupole or $\alpha$-cluster $K^{\pi} = 0^+$ candidates from $^{224}$Ra to $^{240}$Pu.

AMS of 93Zr: Passive absorber versus gas-filled magnet

Abstract Two different isobar separation techniques were tested for the detection of the long-lived fission product 93Zr ( T 1 / 2 = 1.64 · 106 a) using Accelerator Mass Spectrometry (AMS), i.e. a passive absorber and a gas-filled magnet, respectively. Both techniques were used in combination with a Time-of-Flight path for the identification of the stable neighboring isotopes 92Zr and 94Zr. The passive absorber was represented by a stack of silicon nitride foils for high flexibility regarding the thickness for optimal isobar separation. Ion beams with a large variety of energies, between 80 and 180 MeV, were provided for this experiment by the tandem accelerator at the Maier-Leibnitz Laboratory in Garching, Germany. With these beams, the stopping powers of 93Zr and 93Nb as a function of energy were determined experimentally and compared to the results obtained with the simulation program SRIM. Considerable discrepancies regarding the energy dependence of the two stopping power curves relative to each other were found. The lowest detection limit for 93Zr achieved with the passive absorber setup was 93Zr/Zr = 1 · 10−10. In comparison, by optimizing the gas-filled magnet set-up, 93Nb was suppressed by around six orders of magnitude and a detection limit of 93Zr/Zr = 5 · 10−11 was obtained. To our knowledge, these results represent the lowest detection limit achieved for 93Zr until now.

Analytical method for the determination of Np and Pu in sea water by AMS with respect to the Fukushima accident

A chemical separation procedure for plutonium (Pu) and neptunium (Np) was developed using extraction chromatography, mass spectrometry and radiometric analysis to determine their concentrations and isotopic ratios in sea water. 241Am, which causes isobaric background to 241Pu in mass spectrometric measurements, was successfully separated from the Pu fraction by this method. Water samples which were spiked with 242Pu and 237Np or 239Np, respectively, were used for chemical yield determination. The chemical yields of Pu and Np, which were determined by alpha and gamma spectrometry at the Radiochemie München (RCM), of more than 85% were obtained. The developed method was applied to analyze the concentration of Pu and Np in the certified reference material, IAEA-443, by Accelerator Mass Spectrometry (AMS) at the Maier–Leibnitz-Laboratory (MLL) to check the applicability of the method to sea water samples. The concentrations of 240Pu, 241Pu and 237Np obtained in this study are in agreement with the certified and literature values within the uncertainties. Due to strong isotopic interference of 239Pu with 238U, it was not possible to analyze the concentration of 239Pu. Some modifications of the chemical separation method to suppress the uranium (U) fraction are under consideration. This method can be used for the analysis of Pu and Np in Pacific Ocean water samples collected after the Fukushima accident.

Investigation of 0+states in mercury isotopes after two-neutron transfer

Using the high-resolution Q3D magnetic spectrograph at the Maier-Leibnitz Laboratory (MLL) Tandem accelerator in Munich, we studied 0+ excitations in the mercury isotopes 198 Hg, 200 Hg, and 202 Hg after two-neutron transfer. We only observed 4-6 excited 0+ states per nucleus up to about 3-MeV excitation energy, far fewer than in other experiments of this (p, t ) campaign. The results reveal a sharp drop in the number of low-lying 0+ states towards the 208 Pb shell closure. We discuss the low-energy 0+ state density as a function of the valence nucleon number N val . The 0+ excitation energies and the measured (p, t ) transfer cross sections indicate a structural change throughout the Hg isotopes, with the most notable result being the peaking in the cross section of the low-lying excited 02 + state in 200 Hg.

Recent developments at MLLTRAP: Improving the system for mass measurements and in-trap observation of nuclear decays

The MLLTRAP double Penning-trap setup, installed at the Maier-Leibnitz Laboratory in Garching, presently serves as an environment for development work for mass spectrometry and decay-spectroscopy experiments at future radioactive ion beam (RIB) facilities. Here, an m/q separator is presently being set up as an element for beam transport of charge-bred ions, which are required for mass measurements of highly-charged ions. Moreover, a novel type of experimental setup, dedicated to in-trap decay spectroscopy, is under development to be implemented in the future MATS facility at FAIR. It combines the high-resolution mass-purification capabilities of a Penning trap with a customized “detector-trap” setup, consisting of an arrangement of position-sensitive Si-strip detectors that replaces the conventional ring electrode of the trap, providing both, ion storage and detection of decay products. In this way, decay-spectroscopy experiments of mass-selected nuclides, trapped as a carrier-free source, thus avoiding any background or scattering effects, will become feasible via direct in situ spectroscopy of stored ions.

CALIFA Barrel prototype detector characterisation

Well established in the field of scintillator detection, Caesium Iodide remains at the forefront of scintillators for use in modern calorimeters. Recent developments in photosensor technology have lead to the production of Large Area Avalanche Photo Diodes (LAAPDs), a huge advancement on traditional photosensors in terms of high internal gain, dynamic range, magnetic field insensitivity, high quantum efficiency and fast recovery time. The R3B physics programme has a number of requirements for its calorimeter, one of the most challenging being the dual functionality as both a calorimeter and a spectrometer. This involves the simultaneous detection of ∼300MeV protons and gamma rays ranging from 0.1 to 20MeV. This scintillator – photosensor coupling provides an excellent solution in this capacity, in part due to the near perfect match of the LAAPD quantum efficiency peak to the light output wavelength of CsI(Tl). Modern detector development is guided by use of Monte Carlo simulations to predict detector performance, nonetheless it is essential to benchmark these simulations against real data taken with prototype detector arrays. Here follows an account of the performance of two such prototypes representing different polar regions of the Barrel section of the forthcoming CALIFA calorimeter. Measurements were taken for gamma–ray energies up to 15.1MeV (Maier-Leibnitz Laboratory, Garching, Germany) and for direct irradiation with a 180MeV proton beam (The Svedberg Laboratoriet, Uppsala, Sweden). Results are discussed in light of complementary GEANT4 simulations.

Developments in Penning trap (mass) spectrometry at MLLTRAP: Towards in-trap decay spectroscopy

A novel type of experimental setup, dedicated to in-trap decay spectroscopy in a Penning trap, is presently being developed at the double Penning-trap facility MLLTRAP, installed at the Maier-Leibnitz Laboratory in Garching. This new tool for α and electron decay spectroscopy is envisaged to be implemented in the future MATS facility at FAIR. It combines the high-resolution mass-purification capabilities of a first Penning trap with a customized ‘detector-trap’ setup, consisting of an arrangement of position-sensitive silicon (Si)-strip detectors that replaces the conventional ring electrode of the trap, providing both, ion storage and detection of decay products. In this way, decay-spectroscopy experiments of mass-selected nuclides, trapped as a carrier-free source, thus avoiding any background or scattering effects, will become feasible via direct in situ spectroscopy of stored ions.

Investigation of0+states in198Hg after two-neutron pickup

We studied ${0}^{+}$ states in ${}^{198}$Hg after the ${}^{200}$Hg${(p,t)}^{198}$Hg transfer reaction up to 3 MeV excitation energy. The experiment was performed using the high-resolution Q3D magnetic spectrograph at the Maier-Leibnitz Laboratory Tandem accelerator in Munich. In total, only four ${0}^{+}$ states were observed in ${}^{198}$Hg, significantly fewer than in other experiments of the $(p,t)$ transfer campaign. We discuss the low-energy ${0}^{+}$ state density as a function of the valence nucleon number ${N}_{\mathrm{val}}$ and test if the ${0}^{+}$ density can be used as a signature for the prolate-oblate shape-phase transition in the Hf-Hg region.

Transmission resonance spectroscopy in the third minimum of232Pa

The fission probability of ${}^{232}$Pa was measured as a function of the excitation energy in order to search for hyperdeformed (HD) transmission resonances using the ($d,pf$) transfer reaction on a radioactive ${}^{231}$Pa target. The experiment was performed at the Tandem accelerator of the Maier-Leibnitz Laboratory (MLL) at Garching using the ${}^{231}$Pa($d,pf$) reaction at a bombarding energy of ${E}_{d}=12$ MeV and with an energy resolution of $\ensuremath{\Delta}E=5.5$ keV. Two groups of transmission resonances have been observed at excitation energies of ${E}^{*}=5.7$ and 5.9 MeV. The fine structure of the resonance group at ${E}^{*}=5.7$ MeV could be interpreted as overlapping rotational bands with a rotational parameter characteristic to a HD nuclear shape (${\ensuremath{\hbar}}^{2}/2\ensuremath{\Theta}=2.10\ifmmode\pm\else\textpm\fi{}0.15$ keV). The fission barrier parameters of ${}^{232}$Pa have been determined by fitting talys 1.2 nuclear reaction code calculations to the overall structure of the fission probability. From the average level spacing of the $J=4$ states, the excitation energy of the ground state of the third minimum has been deduced to be ${E}_{\text{III}}={5.05}_{\ensuremath{-}0.10}^{+0.40}$ MeV.

Search for superheavy elements with292⩽A⩽310in nature with accelerator mass spectrometry

There is a possibility that small traces of long-lived superheavy elements ($Z\ensuremath{\geqslant}104$) still exist in nature. An ultrasensitive search for such superheavy elements has been conducted at the Maier-Leibnitz Laboratory in Garching (Germany) by means of accelerator mass spectrometry. A sample of raw platinum has been scanned for 13 different masses in the range $292\ensuremath{\leqslant}A\ensuremath{\leqslant}310$. The masses $A=292$ and 298 were scanned in pure osmium and pure lead fluoride, respectively. For each mass, several hours of background-free data were recorded. Since no events could be attributed to superheavy elements, upper limits on their abundances in the sample materials on the order of ${10}^{\ensuremath{-}14}$--${10}^{\ensuremath{-}16}$ were established.