G. Hollos

Measurement of 129I concentrations in the environment after the Chernobyl reactor accident

Abstract The Chernobyl nuclear reactor accident which occurred on April 26, 1986 is known to have injected into the atmosphere a pulse of a large number of radionuclides. The activities of several radionuclides present in the subsequent fallout have been measured in different locations throughout Europe by gamma-ray and beta counting. We present here measurements of concentrations of the long-lived radionuclide 129I ( T 1 2 = 1.6 × 10 7 yr ) in environmental samples collected in Israel and Europe following the nuclear reactor accident. The measurements were performed by accelerator mass spectrometry, using the 14UD Rehovot Pelletron Accelerator. Concentrations of 129I in rainwater samples collected in the Munich (West-Germany) area and in Israel during the fallout period were measured to be 2.6 × 1010 and 1.2 × 109 atoms I respectively, while a 1982 rainwater sample from Israel shows a 129I concentration of 8.2 × 107 atoms I . Three measurements of the ratio 129I/131I gave a mean value of 21, from which an effective operating time of the reactor of 1.5 to 2 yr prior to the accident can be estimated. The possible use of anthropogenic 129I as a tracing tool for global environmental processes is discussed.

Selective suppression of negative ions by lasers

Abstract A method of selective suppression of negative ions by electron photodetachment with a laser beam is investigated. The interaction of 532 nm photons from a Nd:YAG laser with negative ions of 32 S, 37 Cl and 40 Ca 16 O was studied. A strong depletion of 32 S − and 40 Ca 16 O − beam intensities, but no effect on 37 Cl − ions, was observed. Photodetachment cross sections of (1.0±0.2)×10 −17 cm 2 and (7±3)×10 −17 cm 2 were measured for 32 S − and 40 Ca 16 O − , respectively. Implications for accelerator mass spectrometry measurements are discussed.

Study of laser interaction with negative ions

Abstract Negative ions can be neutralized by detaching their additional electron through interaction with a laser beam. By properly choosing the laser wavelength, the process is highly selective; it can in principle enhance the discrimination power of an accelerator mass spectrometry system for ions of different elements (e.g. isobaric background) by allowing their separation prior to their injection into a tandem accelerator. We demonstrate this process in the case of the 59 Ni- 59 Co pair with the AMS system based on the Rehovot 14UD Pelletron accelerator and a pulsed Nd-YAG laser at the fundamental wavelength (1064 nm). A photodetachment cross section of (0.6 ± 0.3) × 10 −17 cm 2 was measured for 59 Co − and a suppression factor of 125 for the 59 Co isobaric background was achieved in a 59 Ni AMS measurement. The duty factor due to the pulsed laser was about 10 −4 . The laser-AMS system was also applied to the study of rare negative ions in the actinide region. Preliminary results on the laser interaction with uranium negative ions are reported.

Measurements of natural concentrations of 129I in uranium ores by accelerator mass spectrometry

Abstract An accelerator mass spectrometry system is described and utilized for measurements of 129I concentrations in natural and environmental samples. We report here on measurements of 129I isotopic abundances in iodine reagents and in iodine of mineral origin and of 129I concentrations in uranium ores of different origins. The 129I isotopic abundances for two measured contemporary iodine reagents and for iodine from a deep underground brine are 1.3 × 10−13 and about 4 × 10−14, respectively. 129 I U ratios in the range 10−13–10−12 are measured and compared to a simple model of 129I production by spontaneous and induced fission of uranium. No clear correlation with the uranium concentrations or residence times is observed.

Measurements of 41Ca spallation cross sections and 41Ca concentrations in the grant meteorite by accelerator mass spectrometry

Abstract Systematic measurements of spallogenic 41Ca in the iron meteorite Grant and in proton irradiated elemental Ni, Fe and Ti targets have been made by accelerator mass spectrometry using a 14UD Pelletron accelerator. Spallation (p, 41Ca) cross sections, were measured for proton energies between 40 and 600 MeV. For the Grant meteorite 5 samples at various depths, ranging from the post-atmospheric surface to centre, were measured in order to investigate the depth production profile of 41Ca. Using the measured excitation function for 41Ca on Fe, the production rate due solely to primary galactic cosmic rays is estimated and the importance of secondary flux production for 41Ca in a large meteorite is discussed.

The 36C1 ages of the brines in the Magadi-Natron basin, East Africa

The depression in the East African Rift which includes both Lake Magadi and Lake Natron forms a closed basin within which almost all the dissolved chloride originates in precipitation, since there is no important source of very ancient sedimentary chloride. This provides an ideal setting for the evaluation of the 36Cl methodology as a geochemical and hydrological tracer. The main source of recent water, as represented by the most dilute samples measured, is characterized by a 36C1/C1 ratio of 2.5 × 10−14, in agreement with the calculated value expected in precipitation. Surface evaporation increases the chlorinity of the local freshwater inflow by about a factor of 110 without changing the isotopic ratio, indicating that little chloride enters the system in the form of sediment leachate. A second type of brine found in the basin occurs in a hot deep groundwater reservoir and is characterized by lower 36C1/C1 ratios (<1.2 × 10-14). By comparing this value with the2.5× 10−14 in recent recharge, one obtains an approximate salt accumulation age of 760 Ka which is consistent with the time of the first appearance of the lake. These older brines also have lower 18O and 2H values which indicate that they were recharged during a climatically different era. The 36C1/C1 ratios in the inflowing waters and in the accumulated brine, together with the known age of the Lake Magadi basin, may be used to estimate the importance of the hypogene and epigene, as opposed to the meteoric, mode of 36C1 production. Such a calculation shows that the hypogene and epigene processes together contribute less than 6% of the total 36C1 present in the lake.

Measurement of the 129I/131I ratio in Chernobyl fallout

Rainwater collected in the Munich area approximately one week after the Chernobyl reactor accident was investigated for its content of the radio-isotopes 129I (T1/2 = 1.6 × 107 yr) and 131I (T1/2 = 8.04d). For the time of release, an isotopic ratio of 129I/131I = 19 ± 5 was found. This value was obtained from a gamma-ray activity measurement of 131I with a Ge detector and a concentration measurement of 129I with accelerator mass spectrometry. From the measured ratio an operating time of the reactor prior to the accident in the vicinity of two years can be estimated, which is in fair agreement with estimates from other long-lived to short-lived radioisotope ratios in the Chernobyl fallout. Some measurements of 131I activity in thyroids of persons living in the Munich area are also reported.

Electromagnetic isotope enrichment for accelerator mass spectrometry of 41Ca

Abstract A procedure to enrich the natural 41Ca abundance in calcium by variable amounts was developed at the GSI medium-current mass separator. The method was applied to increase the natural 41 Ca Ca ratios in three samples of modern deer bone, which were subsequently measured by accelerator mass spectrometry at the Rehovot 14UD Pelletron tandem accelerator. The following enrichment factors and original 41 Ca Ca ratios were measured for three bone samples, which all originate from approximately the same region in Germany: (i) 24.9 ± 1.3, (1.8 ± 0.7) × 10−14; (ii) 48.4 ± 2.4, (2.4 ± 0.6) × 10−14; (iii) 49.3 ± 2.5, (5.7 ±1.1)×10−14 41 Ca Ca ratios lie at the upper end of terrestrial values measured in various bone samples at other laboratories. The measured ratios altogether spread over two orders of magnitude.

Studies of negative ion formation and interaction with a laser beam

An experimental system composed of a standard negative ion source and an Nd:YAG laser is used to study the formation of negative ions by laser impact and the interaction of the laser with the negative ion beam. Using a Q‐switched Nd:YAG laser (about 107 W/cm2) impinging on the solid FeS sample of a Cs‐sputter source, pulses of negative ions of sulphur with a peak intensity of 3 mA and a width of 150 ns were observed. The time structure of the pulses is measured and shows a complex behavior, not yet fully understood. The same experimental system is used to analyze the production of rare negative ions in the lanthanide and actinide regions and to study their interaction with a laser beam. Cross sections for photodetachment in La− and Th− are measured.

Observation of high intensity negative ion pulses by laser impact

Abstract We report on the observation of intense pulses of sulphur negative ions extracted from a Cs-beam sputter source and produced by the impact of 532 nm photon pulses from a Nd-YAG laser on a FeS cathode. Peak currents of about 3 mA are obtained after extraction and preacceleration of the negative ions, which are subsequently analyzed by accelerator mass spectrometry. The time structure of the 32S− negative ion current is measured and shows a complex behavior not yet fully understood.