[en] In a recent publication Marinov et al. (Phys. Rev. C 76, 021303(R), 2007) reported that isomeric states should exist in the neutron-deficient thorium-isotopes ²¹¹Th, ²¹³Th, ²¹⁷Th and ²¹⁸Th. These isotopes were found by use of a conventional mass spectrometer in abundances of 10^-11 relative to ²³²Th in a natural sample of thorium. Therefore they should be of primordial origin and have minimum half-lives of several 100 million years. This is surprising, as the ground-states of these α-emitters have lifetimes shorter than seconds and are surrounded by other short-lived α-emitting nuclei. So their production and half-life can not be explained within current models of nuclear and astrophysics. With the method of Accelerator Mass Spectrometry (AMS) at the Maier-Leibnitz-Laboratory in Munich this search could be repeated with higher sensitivity and complete reduction of background due to molecules. Hereby none of the four neutron-deficient thorium isotopes could be detected, the new upper limit for their abundance is an order of magnitude below the postulated value.

[en] First analyses of real ³⁶Cl-AMS samples were performed with the newly developed low memory-effect ion source at the DREsden Accelerator Mass Spectrometry (DREAMS) facility. Considerable improvements have been reached with respect to the overall ion source performance. Especially, parameters like current output, ion source fractionation effects, normalization factors, blank values and sulphur suppression factors have been investigated to enhance accuracy of ³⁶Cl-data. Applications cover a wide spectrum, which implies also highly variable ³⁶Cl/³⁵⁺³⁷Cl-ratios ranging from nearly background level of ∝10^-15 up to 10^-10. Samples from aquifers in arid regions for groundwater dating and modelling were analysed. Meteorite samples were measured to investigate the constancy of the galactic cosmic radiation, production rates from sulphur, and reconstruction of exposure histories of individual meteorites.

No abstract available

[en] As a result of a joint collaboration between the University of Utah, LLNL, the Technical University Munich and the Ludwig Maximilians University Munich, it became possible to determine A-bomb induced ⁶³Ni in pure copper samples from Hiroshima beyond a ground range of 1,000 m (see Chapter 9, Part B). The low ⁶³Ni activities induced in copper samples due to neutrons from the A-bomb explosion at large distance require, however, a careful discussion of the fraction of ⁶³Ni produced in these samples due to cosmic radiation. In this section, an analysis of the production of ⁶³Ni in copper samples due to cosmic radiation is performed. Production due to neutrons, protons, stopped muons, and photonuclear reactions is discussed. It is obvious from Figure 1 (Pfennig et al. 1995) that a variety of reactions induced by neutrons, protons, muons and photons can contribute to the production of ⁶³Ni in copper. The most important of these processes will be discussed here. Since the cross-sections for the production of ⁶³Ni in copper samples due to fast and stopped muons were not known, they were determined experimentally. (J.P.N.)

[en] Full text: ¹⁰Be measurements at DREAMS [DREsden Accelerator Mass Spectrometry facility] take up a large fraction of the AMS beam times at the 6MV accelerator at the ion beam center of HZDR. Currently, they are undertaken at a terminal voltage of 4.5 MV [Rugel et al., NIMB 2016]. Here, we investigated potential benefits from a change in accelerator terminal voltage in order to increase the efficiency of ¹⁰Be counting. Presently, after the stripping in Ar gas in the accelerator, Be2+ ions are directed towards a 1 μm thin SiN foil placed after the analysing magnet on the high-energy side that helps to suppress the ¹⁰B interference by differential energy loss and separation in an electrostatic analyser. After passage through the absorber foil the mean charge state of Be ions is increased and the 4+ charge state is selected and transported to the detector. In this mode of operation, losses of ¹⁰Be ion beam intensity on the way from the low-energy side of the system to the detector are dominated by these two charge exchange processes [Arnold et al., NIMB 2010]. However, there is only limited data for the recharge behaviour of Be in a stripper gas at energies relevant for the measurements at DREAMS [Hofmann et al., NIMB 1987; Niklaus et al., NIMB 1994]. For an argon gas stripper, Niklaus et al. [NIMB 1994], suggest lower terminal voltages for optimal transmission of ¹⁰Be2+. On the other hand, an increase of the overall energy of the Be2+ beam after the accelerator will certainly allow for a higher Be4+ yield after the passage through the absorber foil. In contrast to the original data by Niklaus et al. [NIMB 1994], we found that increasing the terminal voltage to ≥ 5MV does not reduce the yield of the Be2+ charge state after the accelerator. As a further recharge to the 4+ charge state is conducted in a foil after the analysing magnet it is desirable to hit the foil with the highest available energy/velocity to have optimal stripping of Be2+ to the naked ion. Thus, the efficiency of ¹⁰Be measurements can indeed be improved by increasing the terminal voltage, both at DREAMS and at other AMS facilities of a similar size that are using the absorber method with a charge exchange from 2+ to 4+ for isobar suppression. We present data on the performance of the system at higher beam energies documenting an increase in overall detection efficiency by 25%. Under these conditions the interfering isobar ¹⁰B is still well separated, and no additional interferences (e.g. from nuclear reactions) appear in our spectra.

[en] Radionuclides such as "2"3"6U and "2"3"9Pu were introduced into the environment by atmospheric nuclear weapon tests, reactor accidents (Chernobyl, Fukushima), releases from nuclear reprocessing facilities (Sellafield, La Hague), radioactive waste disposal, and accidents with nuclear devices (Palomares, Thule) [1]. Accelerator Mass Spectrometry (AMS) is the most sensitive method to measure these actinides. The DREsden AMS (DREAMS) facility is located at a 6 MV accelerator, which is shared with ion beam analytics and implantation users, preventing major modifications of the accelerator and magnetic analyzers. DREAMS was originally designed for "1"0Be, "2"6Al, "3"6Cl, "4"1Ca, and "1"2"9I. To modify the system for actinide AMS, a Time-of-Flight (TOF) beamline at the high-energy side has been installed and performance tests are on-going. Ion beam and detector simulations are carried out to design a moveable ionization chamber. Especially, the detector window and anode dimensions have to be optimized. This ionization chamber will act as an energy detector of the system and its installation is planned as closely as possible to the stop detector of the TOF beamline for highest detection efficiency.

[en] After successful measurements of ¹⁰Be, ²⁶Al and ⁴¹Ca at DREAMS (DResden Accelerator Mass Spectrometry), extensive test measurements of ³⁶Cl started. Besides the challenge of separating the stable isobar ³⁶S, which at DREAMS is accomplished by post-stripping and a split-anode-ionization-chamber, the problem of ion source memory must be solved. To characterize this effect we use ³⁵Cl/³⁷Cl samples of natural composition and ³⁵Cl-enriched samples with a ³⁵Cl/³⁷Cl-ratio >100. Similar measurements at the French AMS facility ASTER showed differences of 2-4% in the ³⁵Cl/³⁷Cl ratios of the highly enriched samples after 24 h of sputtering samples with natural isotopic ratios. To minimize the long-term-memory effect, two modified designs of the original source (HVEE) were constructed at DREAMS. A more open geometry was used to improve the vacuum level, and parts of the target loading system were modified to allow the exchange of the individual cathode aperture with each target.

No abstract available

[en] The AMS-measurements of ⁶³Ni in copper samples obtained from different distances in Hiroshima represent the first detection of fast neutrons after more than 50 years, and the first reliable bomb-induced fast neutron measurements beyond 700 m from the hypocenter. A major significance of these ⁶³Ni results is that they provide, for the first time, fast neutron measurements at the distances most relevant to atomic-bomb survivor locations (900-1,500 m). The measurement of ⁶³Ni in copper samples using AMS provides a substantial improvement in fast neutron detection compared with the ³²P measurements made in 1945. For example, the measurement-to-background ratio at about 400 m from the hypocenter is about 55 for ⁶³Ni and only about 2 for the ³²P measurements. Also, the ⁶³Ni measurements reach background levels at about 1,800 m from the hypocenter compared with only about 700 m for ³²P. At distances of ∼1,800 m from the hypocenter, out to at least 5,000 meters, the measurements appear to level off at a value on the order of 7x10⁴ atoms of ⁶³Ni/g of Cu, suggesting a background of approximately that magnitude. When this background is subtracted and the resulting data corrected to 1945, the measured ⁶³Ni in copper samples from Hiroshima are in good agreement with DS02 sample-specific calculations. Comparisons with DS86 calculations also show good agreement except at the Bank of Japan. The difference at that distance is significant when compared with DS86 calculations. (J.P.N.)

[en] Highlights: ► New 6 MV tandem accelerator in operation in Germany for AMS, IBA and HE-implantation. ► DREsden AMS (DREAMS) primarily used for radionuclides ¹⁰Be, ²⁶Al, ³⁶Cl, ⁴¹Ca and ¹²⁹I. ► Quality assurance by traceable calibration materials and interlaboratory comparisons. High accuracy data for future DREAMS users. ► Energy calibration of accelerator by ¹H(¹⁵N,γα)¹²C yield correction factor of 1.019. - Abstract: A new 6 MV electrostatic tandem accelerator has been put into operation at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). The system is equipped for accelerator mass spectrometry and opens a new research field at HZDR and the Helmholtz Association. It will be also used for ion beam analysis as well as for material modification via high-energy ion implantation. The research activity at the DREsden Accelerator Mass Spectrometry facility (DREAMS) based on a 6 MV Tandetron is primarily dedicated to the long-lived radioisotopes of ¹⁰Be, ²⁶Al, ³⁶Cl, ⁴¹Ca, and ¹²⁹I. DREAMS background levels have been found to be at 4.5 × 10⁻¹⁶ for ¹⁰Be/⁹Be, 8 × 10⁻¹⁶ for ²⁶Al/²⁷Al, 3 × 10⁻¹⁵ for ³⁶Cl/³⁵Cl and 8 × 10⁻¹⁵ for ⁴¹Ca/⁴⁰Ca, respectively. The observed background of 2 × 10⁻¹³ for ¹²⁹I/¹²⁷I originates from intrinsic ¹²⁹I from AgI produced from commercial KI. The introduction of quality assurance approaches for AMS, such as the use of traceable calibration materials and taking part in interlaboratory comparisons, guarantees high accuracy data for future DREAMS users. During first experiments an energy calibration of the accelerator has been carried out using the nuclear reaction ¹H(¹⁵N,γα)¹²C yielding an energy correction factor of 1.019.