Soreq Applied Research Accelerator Facility Research Articles

Hydrogen blister formation in single crystal and polycrystalline tungsten irradiated by MeV protons

Single and polycrystalline tungsten samples were irradiated with 2.2 MeV protons at Soreq Applied Research Accelerator Facility (SARAF). Hydrogen blisters were obtained for both single crystal and polycrystalline samples, elucidating the role of grain boundaries in blister formation. The effect of temperature and flux on the critical formation dose for blisters and on their dimensions was studied. It was found that for single crystals, the critical formation dose is one order of magnitude higher than for polycrystalline tungsten at high temperature irradiation conditions. Upon reducing the irradiation temperature to ambient, the critical dose for formation of blisters in single crystals was reduced by a factor of three while in polycrystalline tungsten there was no significant change with temperature, thus indicating the role of grain boundaries in blister formation. Larger blisters were obtained in single crystals than in polycrystalline tungsten at ambient temperature conditions, identifying the grain boundaries as a preferential additional hydrogen trap. The height to area ratio of the blisters is found to be strongly temperature dependent and only weakly dependent on irradiation flux for both single and polycrystalline samples.

93Zr developments at the Heavy Ion Accelerator Facility at ANU

The long-lived radionuclide 93Zr t1/2 = (1.61 ± 0.05) Ma plays an important role in nuclear astrophysics and nuclear technology. In stellar environments, it is mainly produced by neutron capture on the stable nuclide 92Zr. On Earth high amounts of radioactive 93Zr are produced in nuclear power plants directly from 235U fission, but also by neutron capture on 92Zr, as Zr-alloys are commonly used as cladding for nuclear fuel rods.Despite its importance, the neutron capture cross section of 92Zr at thermal and stellar energies (keV) is not well known. Neutron irradiation of 92Zr and subsequent determination of produced 93Zr via AMS is a promising approach to resolve this issue. The main challenge in AMS measurements of 93Zr is the interference from the stable isobar 93Nb. The high particle energies available with the 14UD tandem accelerator at the Australian National University are ideal to tackle this challenge. Different sample materials, molecular ion species and sample holder materials were tested for their 93Nb background. Commercial ZrO2 powder irradiated with thermal neutrons from the reactor at the Atominstitut in Vienna (ATI) was used as reference material for AMS measurements. In contrast to literature reports and γ-activity measurements of 95Nb, which suggest that chemical Nb reduction works, elevated 93Nb contents were measured in chemically pre-treated samples. The reasons are under investigation. At the ANU we developed AMS for ∼210 MeV 93Zr ions using an 8 anode ionisation chamber. We achieved background levels of 93A/Zr ∼ 10−12 with acceptance of 2–8% of the 93Zr ions at the high-energy side. This is more than an order of magnitude better than previously reported. The 93Nb isobar was suppressed by a factor between 13,000 and 90,000 in the detector.This performance allows measurements of the thermal and stellar neutron-capture cross section of 92Zr for samples irradiated at the ATI and the Soreq Applied Research Accelerator Facility, respectively, using AMS.

Neutron spectrometry of a liquid Lithium based (p, n) beam at SARAF facility using the broad-energy range directional spectrometer CYSP

An intense neutron beam produced via 7Li(p, n)7Be reaction on a newly developed high-power liquid Lithium target (LiLiT) is available at SARAF (Soreq Applied Research Accelerator Facility) facility, Israel. Its spectrum was determined using the CYSP (CYlindrical SPectrometer), a new directional neutron spectrometer which incorporates the functionality of the Bonner spheres in a single cylindrical moderator. The advantages of this spectrometer are that the neutron spectrum is derived in only one exposure, and that its directional response is not affected by room- and air-scattered neutrons. The data from the CYSP spectrometer were analysed using the FRUIT unfolding code.

The Soreq Applied Research Accelerator Facility (SARAF): Overview, research programs and future plans

The Soreq Applied Research Accelerator Facility (SARAF) is under construction in the Soreq Nuclear Research Center at Yavne, Israel. When completed at the beginning of the next decade, SARAF will be a user facility for basic and applied nuclear physics, based on a 40 MeV, 5 mA CW proton/deuteron superconducting linear accelerator. Phase I of SARAF (SARAF-I, 4 MeV, 2 mA CW protons, 5 MeV 1 mA CW deuterons) is already in operation, generating scientific results in several fields of interest. The main ongoing program at SARAF-I is the production of 30 keV neutrons and measurement of Maxwellian Averaged Cross Sections (MACS), important for the astrophysical s-process. The world leading Maxwellian epithermal neutron yield at SARAF-I ($5\times 10^{10}$ epithermal neutrons/sec), generated by a novel Liquid-Lithium Target (LiLiT), enables improved precision of known MACSs, and new measurements of low-abundance and radioactive isotopes. Research plans for SARAF-II span several disciplines: Precision studies of beyond-Standard-Model effects by trapping light exotic radioisotopes, such as $^6$He, $^8$Li and $^{18,19,23}$Ne, in unprecedented amounts (including meaningful studies already at SARAF-I); extended nuclear astrophysics research with higher energy neutrons, including generation and studies of exotic neutron-rich isotopes relevant to the rapid (r-) process; nuclear structure of exotic isotopes; high energy neutron cross sections for basic nuclear physics and material science research, including neutron induced radiation damage; neutron based imaging and therapy; and novel radiopharmaceuticals development and production. In this paper we present a technical overview of SARAF-I and II, including a description of the accelerator and its irradiation targets; a survey of existing research programs at SARAF-I; and the research potential at the completed facility (SARAF-II).

Blister formation at subcritical doses in tungsten irradiated by MeV protons

The material response of tungsten to irradiation by MeV protons has been studied experimentally, in particular with respect to bubble and blister formation. Tungsten samples were irradiated by 2.2 MeV protons at the Soreq Applied Research Accelerator Facility (SARAF) to doses of the order of 1017 protons/cm2 which are below the reported critical threshold for blister formation derived from keV range irradiation studies. Large, well-developed blisters are observed indicating that for MeV range protons the critical threshold is at least an order of magnitude lower than the lowest value reported previously. The effects of fluence, flux, and corresponding temperature on the distribution and characteristics of the obtained blisters were studied. FIB cross sections of several blisters exposed their depth and structure.

Measurement of Pb208(n,γ)Pb209 Maxwellian averaged neutron capture cross section

The doubly magic $^{208}\mathrm{Pb}$ nucleus is a bottleneck at the termination of the $s$-process path due to its very low neutron capture cross section. This cross section is also important for the decomposition of $s$, $r$ processes and U/Th radiogenic decay contributions to the Pb-Bi solar abundances. The $^{208}\mathrm{Pb}(n,\ensuremath{\gamma})^{209}\mathrm{Pb}$ cross section was measured at the Soreq Applied Research Accelerator Facility Phase I using an intense quasi-Maxwellian neutron source produced by irradiation of the liquid-lithium target with a 1.5-mA continuous-wave proton beam at 1.94 MeV. The cross section was measured by counting the $\ensuremath{\beta}$ activity from the irradiated lead target. The measurement allowed us to evaluate the Maxwellian averaged cross section (MACS) at 30 keV obtaining a value of 0.33(2) mb. This has been compared with the earlier activation and time-of-flight measurements found in the literature. The MACS cross-sectional value of the $^{63}\mathrm{Cu}(n,\ensuremath{\gamma})^{64}\mathrm{Cu}$ reaction was determined in the same experiment and is compared to a recent published value.

Stellar 30-keV neutron capture in 94, 96Zr and the [formula omitted] photonuclear reaction with a high-power liquid-lithium target

A high-power Liquid-Lithium Target (LiLiT) was used for the first time for neutron production via the thick-target Li7(p,n)Be7 reaction and quantitative determination of neutron capture cross sections. Bombarded with a 1–2 mA proton beam at 1.92 MeV from the Soreq Applied Research Accelerator Facility (SARAF), the setup yields a 30-keV quasi-Maxwellian neutron spectrum with an intensity of 3–5×1010 n/s, more than one order of magnitude larger than present near-threshold Li7(p,n) neutron sources. The setup was used here to determine the 30-keV Maxwellian averaged cross section (MACS) of 94Zr and 96Zr as 28.0±0.6 mb and 12.4±0.5 mb respectively, based on activation measurements. The precision of the cross section determinations results both from the high neutron yield and from detailed simulations of the entire experimental setup. We plan to extend our experimental studies to low-abundance and radioactive targets. In addition, we show here that the setup yields intense high-energy (17.6 and 14.6 MeV) prompt capture γ rays from the Li7(p,γ)Be8 reaction with yields of ∼3×108γs−1mA−1 and ∼4×108γs−1mA−1, respectively, evidenced by the Zr90(γ,n)Zr89 photonuclear reaction.

Demonstration of a high-intensity neutron source based on a liquid-lithium target for Accelerator based Boron Neutron Capture Therapy

A free surface liquid-lithium jet target is operating routinely at Soreq Applied Research Accelerator Facility (SARAF), bombarded with a ~1.91MeV, ~1.2mA continuous-wave narrow proton beam. The experiments demonstrate the liquid lithium target (LiLiT) capability to constitute an intense source of epithermal neutrons, for Accelerator based Boron Neutron Capture Therapy (BNCT). The target dissipates extremely high ion beam power densities (>3kW/cm2, >0.5MW/cm3) for long periods of time, while maintaining stable conditions and localized residual activity. LiLiT generates ~3×1010n/s, which is more than one order of magnitude larger than conventional 7Li(p,n)-based near threshold neutron sources. A shield and moderator assembly for BNCT, with LiLiT irradiated with protons at 1.91MeV, was designed based on Monte Carlo (MCNP) simulations of BNCT-doses produced in a phantom. According to these simulations it was found that a ~15mA near threshold proton current will apply the therapeutic doses in ~1h treatment duration. According to our present results, such high current beams can be dissipated in a liquid-lithium target, hence the target design is readily applicable for accelerator-based BNCT.

A high-power liquid-lithium target (LiLiT) for neutron production

A high-power liquid-lithium target (LiLiT) was built and commissioned with a 1.2 mA, 1.91 MeV (2.3 kW) proton beam from the SARAF (Soreq Applied Research Accelerator Facility, Israel). The lithium film acts both as a neutron-producing target via the 7Li(p,n)7Be reaction with a yield of ~ 2 × 1010 n/s/mA and as a power beam dump. The neutron source is suited for the study of neutron capture reactions on nuclides (stable or radioactive) of low abundance for nuclear astrophysics and to the needs of boron neutron capture therapy (BNCT).

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.

High-power liquid-lithium jet target for neutron production

A compact liquid-lithium target (LiLiT) was built and tested with a high-power electron gun at the Soreq Nuclear Research Center. The lithium target, to be bombarded by the high-intensity proton beam of the Soreq Applied Research Accelerator Facility (SARAF), will constitute an intense source of neutrons produced by the (7)Li(p,n)(7)Be reaction for nuclear astrophysics research and as a pilot setup for accelerator-based Boron Neutron Capture Therapy. The liquid-lithium jet target acts both as neutron-producing target and beam dump by removing the beam thermal power (>5 kW, >1 MW/cm(3)) with fast transport. The target was designed based on a thermal model, accompanied by a detailed calculation of the (7)Li(p,n) neutron yield, energy distribution, and angular distribution. Liquid lithium is circulated through the target loop at ~200 °C and generates a stable 1.5 mm-thick film flowing at a velocity up to 7 m/s onto a concave supporting wall. Electron beam irradiation demonstrated that the liquid-lithium target can dissipate electron power areal densities of >4 kW/cm(2) and volume power density of ~2 MW/cm(3) at a lithium flow of ~4 m/s while maintaining stable temperature and vacuum conditions. The LiLiT setup is presently in online commissioning stage for high-intensity proton beam irradiation (1.91-2.5 MeV, 1-2 mA) at SARAF.

High-power electron beam tests of a liquid-lithium target and characterization study of 7Li(p,n) near-threshold neutrons for accelerator-based boron neutron capture therapy

A compact Liquid-Lithium Target (LiLiT) was built and tested with a high-power electron gun at Soreq Nuclear Research Center (SNRC). The target is intended to demonstrate liquid-lithium target capabilities to constitute an accelerator-based intense neutron source for Boron Neutron Capture Therapy (BNCT) in hospitals. The lithium target will produce neutrons through the 7Li(p,n)7Be reaction and it will overcome the major problem of removing the thermal power >5kW generated by high-intensity proton beams, necessary for sufficient therapeutic neutron flux.In preliminary experiments liquid lithium was flown through the target loop and generated a stable jet on the concave supporting wall. Electron beam irradiation demonstrated that the liquid-lithium target can dissipate electron power densities of more than 4kW/cm2 and volumetric power density around 2MW/cm3 at a lithium flow of ~4m/s, while maintaining stable temperature and vacuum conditions. These power densities correspond to a narrow (σ=~2mm) 1.91MeV, 3mA proton beam. A high-intensity proton beam irradiation (1.91–2.5MeV, 2mA) is being commissioned at the SARAF (Soreq Applied Research Accelerator Facility) superconducting linear accelerator.In order to determine the conditions of LiLiT proton irradiation for BNCT and to tailor the neutron energy spectrum, a characterization of near threshold (~1.91MeV) 7Li(p,n) neutrons is in progress based on Monte-Carlo (MCNP and Geant4) simulation and on low-intensity experiments with solid LiF targets. In-phantom dosimetry measurements are performed using special designed dosimeters based on CR-39 track detectors.

Effect of secondary electrons suppression on emittance measurement of an intense, low-energy beam

Transversal emittance measurements of intense low-energy beams involve use of collimators. As most of the beam is stopped by a collimator, a question arises whether the special conditions in the vicinity of the collimator influence emittance measurement. In particular, the secondary electrons emitted from the slit surface may affect the measurement of the beam phase distribution. We have observed significant modification in the measured phase space distribution of a 5-6 mA DC proton beam at the Soreq Applied Research Accelerator Facility low-energy transport after application of a weak magnetic field in the plane of the slit collimator. The periphery region of the phase distribution was mostly affected. The overall effect on the emittance value was as large as 20%.

Simulation of the neutron spectrum from the 7Li(p,n) reaction with a liquid-lithium target at Soreq Applied Research Accelerator Facility

The 7Li(p,n)7Be reaction has been used for the last 25 years to produce quasi-Maxwellian neutrons in order to measure Maxwellian-Averaged Cross-Sections in the relevant temperatures for stellar nucleosynthesis. A liquid-lithium target at the Soreq Applied Research Accelerator Facility is expected to allow us to perform such measurements at higher neutron intensities. Here we describe a Monte Carlo tool, SimLiT, developed to evaluate neutron spectra, intensities and angular distributions resulting from this reaction. We also demonstrate the feasibility to couple SimLiT with an advanced transport code, resulting in a powerful tool for planning and analysis of experiments using the 7Li(p,n) reaction as a neutron source.

Towards an intense radioactive 8Li beam at SARAF Phase I

The 8Li(α,n)11B reaction plays an important role in the r-process nucleosynthesis occurring in type II supernovae and in binary neutron stars. At high rates, this reaction can lead, despite the short 8Li half-life, to production of seed nuclei through a chain of reactions, which can then cause the formation of heavier nuclei via (n,γ) reactions. Several measurements of this reaction were carried out in the last few years using fast 8Li beams that are available at several RIB facilities at limited currents. A new production method for light Radioactive Ion Beams (RIB) via fast neutron reactions is conceptually able to provide orders of magnitude higher RIB currents, up to 1012 pps. The high efficiency of the two-target production method has been experimentally proven for 6He production, in a similar manner to the 8Li production. We present a unique apparatus for 8Li production and extraction at the Soreq Applied Research Accelerator Facility - SARAF. In 2011, it is expected that SARAF Phase-I will produce high intensity, 5 MeV deuteron beams for limited experiments. By using a LiF neutron converter, neutrons with energies of up to 20 MeV and a total yield of 1012 fast n/sec will be available per 1 mA of deuterons. As a secondary production target we are using discs of 65% porous B4C, mounted inside a high temperature furnace. 8Li, produced via the 11B(n,α)8Li reaction channel, diffuses out of the B4C target and is ionized in a thin rhenium surface ionizer. The total efficiency of 8Li RIB creation will be measured by means of alpha and beta measurements. Following the experiment at SARAF Phase I, better production and extraction schemes of light RIB, of great interest in various nuclear and astrophysics-related measurements, may become possible.

Use of a wire scanner for monitoring residual gas ionization in Soreq Applied Research Accelerator Facility 20 keV/u proton/deuteron low energy beam transport beam line

The ion source end of the Soreq Applied Research Accelerator Facility accelerator consists of a proton∕deuteron ECR ion source and a low energy beam transport (LEBT) beam line. An observed reduction of the radio frequency quadrupole transmission with increase of the LEBT current prompted additional study of the LEBT beam properties. Numerous measurements have been made with the LEBT bream profiler wire biased by a variable voltage. Current-voltage characteristics in presence of the proton beam were measured even when the wire was far out of the beam. The current-voltage characteristic in this case strongly resembles an asymmetric diodelike characteristic, which is typical of Langmuir probes monitoring plasma. The measurement of biased wire currents, outside the beam, enables us to estimate the effective charge density in vacuum.

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.

LiLiT - a Liquid-Lithium Target as an Intense Neutron Source for Nuclear Astrophysics at the Soreq Applied Research Accelerator Facility

An intense neutron source based on a high-intensity (2-4 mA) proton beam from the SARAF superconducting linear accelerator (Soreq NRC, Israel) and on a windowless forced-flow Liquid-Lithium Target (LiLiT) is being developed. The neutron source will be used primarily for studies of neutron-induced reactions of interest to nuclear astrophysics. The principle and design of the target, presently in construction, are discussed.

High power accelerator-based boron neutron capture with a liquid lithium target and new applications to treatment of infectious diseases

A new conceptual design for an accelerator-based boron neutron capture therapy (ABNCT) facility based on the high-current low-energy proton beam driven by the linear accelerator at SARAF (Soreq Applied Research Accelerator Facility) incident on a windowless forced-flow liquid-lithium target, is described. The liquid-lithium target, currently in construction at Soreq NRC, will produce a neutron field suitable for the BNCT treatment of deep-seated tumor tissues, through the reaction 7Li(p,n) 7Be. The liquid-lithium target is designed to overcome the major problem of solid lithium targets, namely to sustain and dissipate the power deposited by the high-intensity proton beam. Together with diseases conventionally targeted by BNCT, we propose to study the application of our setup to a novel approach in treatment of diseases associated with bacterial infections and biofilms, e.g. inflammations on implants and prosthetic devices, cystic fibrosis, infectious kidney stones. Feasibility experiments evaluating the boron neutron capture effectiveness on bacteria annihilation are taking place at the Soreq nuclear reactor.