D. Berkovits

in uranium minerals and standards

Abstract We have recently extended our accelerator mass spectrometry (AMS) system to the detection of actinides for geophysical applications, taking 236 U as an interesting test case for this class of nuclides. We report on the development of the experimental setup and on preliminary measurements of 236 U in minerals and uranium standards. The isotopic abundance sensitivity is 1×10−11. The natural 236 U abundance of five U-rich minerals were measured to be between 1 and 33×10−11. 236 U is expected to be produced in situ by neutron capture on 235 U . Three uranium standards material were measured and showed a large 236 U contamination.

Experimental Limit to Interstellar 244Pu Abundance

Short-lived nuclides, now extinct in the solar system, are expected to be present in the interstellar medium (ISM). Grains of ISM origin were recently discovered in the inner solar system and at Earth orbit and may accrete onto Earth after ablation in the atmosphere. A favorable matrix for detection of such extraterrestrial material is presented by deep-sea sediments with very low sedimentation rates (0.8-3 mm kyr-1). We report here on the measurement of Pu isotopic abundances in a 1 kg deep-sea dry sediment collected in 1992 in the North Pacific. Our estimate of (3 ± 3) × 105 244Pu atoms in the Pu-separated fraction of the sample shows no excess over the expected stratospheric nuclear fallout content and, under reasonable assumptions, sets a limit of 0.2 244Pu atoms cm-2 yr-1 for extraterrestrial deposition. Using the available data on the ISM steady-state flux on Earth, we derive a limit of 2 × 10-11 g 244Pu (g ISM)-1 for the abundance of 244Pu in the ISM.

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.

High-power liquid-lithium target prototype for accelerator-based boron neutron capture therapy

A prototype of a compact Liquid-Lithium Target (LiLiT), which will possibly constitute an accelerator-based intense neutron source for Boron Neutron Capture Therapy (BNCT) in hospitals, was built. The LiLiT setup is presently being commissioned at Soreq Nuclear Research Center (SNRC). The liquid-lithium target will produce neutrons through the (7)Li(p,n)(7)Be reaction and it will overcome the major problem of removing the thermal power generated using a high-intensity proton beam (>10 kW), necessary for sufficient neutron flux. In off-line circulation tests, the liquid-lithium loop generated a stable lithium jet at high velocity, on a concave supporting wall; the concept will first be tested using a high-power electron beam impinging on the lithium jet. High intensity proton beam irradiation (1.91-2.5 MeV, 2-4 mA) will take place at Soreq Applied Research Accelerator Facility (SARAF) superconducting linear accelerator currently in construction at SNRC. Radiological risks due to the (7)Be produced in the reaction were studied and will be handled through a proper design, including a cold trap and appropriate shielding. A moderator/reflector assembly is planned according to a Monte Carlo simulation, to create a neutron spectrum and intensity maximally effective to the treatment and to reduce prompt gamma radiation dose risks.

Calcium resorption from bone in a human studied by 41Ca tracing

Abstract We are investigating long- and short-term processes that involve the resorption of the bone calcium in the human body, using accelerator mass spectrometry of 41Ca ( T 1 2 = 104000 years). The evolution of an injected dose of 41Ca has been followed over 900 days by measurements in urine and serum. The 41 Ca/Ca isotopic ratio rapidly decreased after injection and after about 100 days, reached a quasi-steady state of 1.5 × 10−11. Variations in this level would signal a change in rates of calcium resorption from the bone.

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.

Environmental 90Sr measurements

90Sr (T12 = 28.5 years) is a long-lived radionuclide produced in nuclear fission. Fast radiochemical detection of 90Sr in environmental samples is not feasible using current analytical methods. Accelerator Mass Spectrometry (AMS) measurements of 90Sr were made with the Rehovot 14UD Pelletron accelerator at a terminal voltage of 11 or 12 MV using our standard detection system. Injection of hydride ions (SrH3−) was chosen owing to high beam intensity and low Coulomb explosion effects. 90Sr ions were identified and discriminated from isobaric 90Zr by measuring time of flight, total energy and three independent energy-loss signals in an ionization chamber. A reference sample and a ground-water sample were successfully measured. The detection limit determined for a laboratory blank by the residual counts in the 90Sr region is 90SrSr = 3 × 10−13, corresponding in practice to (2−4) × 10790Sr atoms or about 0.5–1 pCi/L in environmental water samples.

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.

Note: Proton irradiation at kilowatt-power and neutron production from a free-surface liquid-lithium target

The free-surface Liquid-Lithium Target, recently developed at Soreq Applied Research Accelerator Facility (SARAF), was successfully used with a 1.9 MeV, 1.2 mA (2.3 kW) continuous-wave proton beam. Neutrons (~2 × 10(10) n/s having a peak energy of ~27 keV) from the (7)Li(p,n)(7)Be reaction were detected with a fission-chamber detector and by gold activation targets positioned in the forward direction. The setup is being used for nuclear astrophysics experiments to study neutron-induced reactions at stellar energies and to demonstrate the feasibility of accelerator-based boron neutron capture therapy.

Measurements of anthropogenic radionuclides in environmental samples

Abstract Measurements of anthropogenic 129 I and 41 Ca in two Greenland firn cores spanning the years 1935 to 1989 have been performed by accelerator mass spectrometry. The two sets of data points measured for 129 I show a discrepancy and prevent definitive conclusions. One of the sets indicates no large increase of 129 I concentrations in contrast to precipitation at lower latitudes. Data on the 41 Ca profile are preliminary and show a few points markedly above background. The feasibility of detection of 90 Sr in the environment by accelerator mass spectrometry is studied and first results are reported.