Soreq Nuclear Research Center Research Articles

Opportunities for high-energy neutron- and deuteron-induced measurements for fusion technology at the Soreq applied research accelerator facility (SARAF)

The Soreq Applied Research Accelerator Facility (SARAF) will be based on a 40 MeV, 5 mA CW (continuous wave) proton/deuteron superconducting linear accelerator, currently under construction at Soreq Nuclear Research Center in Yavne, Israel. It is planned to commence operation during 2025. Experiments at SARAF could provide data on high-energy deuteron- and neutron-induced cross-sections, yields and radiation damage, which are invaluable for the design and operation of the International Fusion Materials Irradiation Facility-DEMO-Oriented NEutron Source (IFMIF-DONES), and fusion technology in general. Pulsed beams (∼1 nsec) of variable energy deuterons will irradiate a lithium target and generate pulsed neutron beams with energy up to ∼55 MeV, which will be used to measure energy-dependent neutron-induced differential cross-sections, utilizing time of flight techniques. Impinging continuous wave (CW) 40 MeV deuteron beams on a unique gallium-indium (GaIn) liquid-jet target, will generate a neutron rate of more than 1 × 1015 n/sec, with energies up to ∼45 MeV. We plan to use this high rate to measure integral neutron-induced reaction yields of all channels simultaneously, employing an original novel method that will identify the reaction-produced nuclei via accurate mass measurement. The neutron-energy dependence of the yields could be deduced by combining measurements at various deuteron energies. The measured cross-sections and yields at SARAF may predict the activation characteristics of construction materials of IFMIF-DONES and future fusion reactors. The deuteron beams will also be used directly to measure cross-sections via in-beam and offline methods. The high neutron and deuteron rates will extend SARAF’s reach to rare materials. The deuteron beam power density on the liquid GaIn target will be 100 kW/cm2(similar to IFMIF-DONES) on a 2 cm2spot. The resulting neutron flux on small secondary samples will be in the 1013 n/cm2/s level, only an order of magnitude less than IFMIF-DONES. Therefore, SARAF may serve as a pilot facility for fusion-related radiation damage studies, providing important information towards the design of IFMIF-DONES.

High-power inductive coupler with a high-voltage DC blocker

The authors of this study developed a high-power inductive coupler with a high-voltage DC blocker that is capable of delivering up to 100 kW of CW RF power at 176 MHz and 4 kV DC. The high-power indictive coupler was developed at the Soreq Nuclear Research Center for the radio-frequency quadrupole of the Soreq Applied Research Accelerator Facility. The purpose of the DC blocker is to enable the application of high-voltage DC bias to the inner conductors of the couplers in order to prevent the multipacting phenomena. A new, mechanically and thermally improved, version of the RF coupler is designed and implemented in conjunction with the DC blocker. The underlying design principles, indigenous development, and results of tests of the coupler are presented here.

Study of thermally and optically stimulated luminescence in LiF:Mg,Ti following neutron and beta irradiation

The results reported herein were motivated by the potential application of combined optically stimulated (OSL) and thermoluminescence (TL) measurements for separation between the beta and neutron dose components in mixed neutron-beta radiation fields. The advantages of OSL/TL are two-fold: (i) The OSL and TL readout can be carried out on the same sample and (ii) the greater efficiency of OSL to high ionization density (HID) radiation due to the excitation of F2 and F3 + centers. The beta calibration coefficients for LiF:Mg,Ti (TLD-700) were measured using a 90Sr/90Y source calibrated at the Soreq Nuclear Research Center nuclear facility. The estimation of the neutron calibration coefficients was carried out by irradiation with broad beam fast neutrons of median energy 5 MeV and a quasi-monoenergetic neutron field of mean energy 14.8 MeV at the Physikalisch Technische Bundenanstalt. The preferential excitation of OSL following HID irradiation has been demonstrated.

Particles Detection System with CR-39 Based on Deep Learning

We present a novel method that we call FAINE, fast artificial intelligence neutron detection system. FAINE automatically classifies tracks of fast neutrons on CR-39 detectors using a deep learning model. This method was demonstrated using a LANDAUER Neutrak® fast neutron dosimetry system, which is installed in the External Dosimetry Laboratory (EDL) at Soreq Nuclear Research Center (SNRC). In modern fast neutron dosimetry systems, after the preliminary stages of etching and imaging of the CR-39 detectors, the third stage uses various types of computer vision systems combined with a manual revision to count the CR-39 tracks and then convert them to a dose in mSv units. Our method enhances these modern systems by introducing an innovative algorithm, which uses deep learning to classify all CR-39 tracks as either real neutron tracks or any other sign such as dirt, scratches, or even cleaning remainders. This new algorithm makes the third stage of manual CR-39 tracks revision superfluous and provides a completely repeatable and accurate way of measuring either neutrons flux or dose. The experimental results show a total accuracy rate of 96.7% for the true positive tracks and true negative tracks detected by our new algorithm against the current method, which uses computer vision followed by manual revision. This algorithm is now in the process of calibration for both alpha-particles detection and fast neutron spectrometry classification and is expected to be very useful in analyzing results of proton-boron11 fusion experiments. Being fully automatic, the new algorithm will enhance the quality assurance and effectiveness of external dosimetry, will lower the uncertainty for the reported dose measurements, and might also enable lowering the system’s detection threshold.

High power high voltage bias-T for half wave resonators and radio frequency quadrupole couplers

High power high voltage bias-T units capable of delivering up to 100 kW CW RF power at 176MHz and up to 4kV DC were developed at the Soreq Nuclear Research Center for the Soreq Applied Research Accelerator Facility linac. Two separate bias-T units with different requirements were designed for the radio frequency quadrupole couplers and the half wave resonator couplers. The purpose of this bias-T is to prevent multipacting phenomena by application of a high voltage DC bias to inner conductors of RF couplers. Underlying design principles, indigenous development, and successful off-line and on-line tests results are presented.

A performance analysis of a new secondary standard dosimetry laboratory irradiation system

This study reviews the performance analysis of a commercial 137Cs irradiation system that has recently been installed in the Secondary Standard Dosimetry Laboratory (SSDL) at the Soreq Nuclear Research Center (SNRC).The SSDL is responsible for calibrating calibration facilities and for calibrating most protection level radiation monitoring instruments used in medical, nuclear, industrial and academic settings in Israel. Until recently, these monitoring instruments have been calibrated by a 40-year-old irradiation system.At the end of 2018, a new Hopewell Designs Inc. ® irradiation system was installed at our SSDL. This system employs the most advanced radiation safety standards, along with 21st century source automation methods. In this work, we review the system’s performance. Our analysis shows excellent results in terms of repeatability, robustness, linearity, and beam uniformity. Additionally, the system accuracy was tested by participating in an international IAEA postal audit, and here too, excellent results were obtained.

On spontaneous appearance of internal waves in an open-pool-type research reactor

Variation of temperature in time and space was recorded at multiple vertical locations in the course of initiation of a heated water layer in an open-pool research reactor of the Soreq Nuclear Research Center. The pool was initially filled with warm water, and heavier cooler water was then injected at the bottom of the facility. Different modes of coolant injection were employed in two different experiments. In both cases, a finite width thermocline that separated cool water at the lower part of the pool from the warm water in its upper part was observed. The thermocline gradually moved up eventually attaining a constant raise velocity. In both experiments, the thermocline characteristics were different, but wave-trains with notable temperature fluctuations were observed within the thermocline. The characteristic frequencies of oscillations were below the Brunt–Väisälä frequencies that characterize the density gradient within the thermoclines. The finite dimensions of the tank impose conditions in which standing internal waves with the length commensurate with tank size can be expected. The oscillations were thus associated with resonant internal waves excited by disturbances introduced by the coolant flow at the lower part of the pool. In both experiments, the measured wave spectra agree with the results of linear analysis of two-layer and three-layer stratification models.

Neutron Capture on the s -Process Branching Point Tm171 via Time-of-Flight and Activation

The neutron capture cross sections of several unstable nuclides acting as branching points in the s process are crucial for stellar nucleosynthesis studies. The unstable ^{171}Tm (t_{1/2}=1.92 yr) is part of the branching around mass A∼170 but its neutron capture cross section as a function of the neutron energy is not known to date. In this work, following the production for the first time of more than 5mg of ^{171}Tm at the high-flux reactor Institut Laue-Langevin in France, a sample was produced at the Paul Scherrer Institute in Switzerland. Two complementary experiments were carried out at the neutron time-of-flight facility (n_TOF) at CERN in Switzerland and at the SARAF liquid lithium target facility at Soreq Nuclear Research Center in Israel by time of flight and activation, respectively. The result of the time-of-flight experiment consists of the first ever set of resonance parameters and the corresponding average resonance parameters, allowing us to make an estimation of the Maxwellian-averaged cross sections (MACS) by extrapolation. The activation measurement provides a direct and more precise measurement of the MACS at 30keV: 384(40)mb, with which the estimation from the n_TOF data agree at the limit of 1 standard deviation. This value is 2.6 times lower than the JEFF-3.3 and ENDF/B-VIII evaluations, 25% lower than that of the Bao etal. compilation, and 1.6 times larger than the value recommended in the KADoNiS (v1) database, based on the only previous experiment. Our result affects the nucleosynthesis at the A∼170 branching, namely, the ^{171}Yb abundance increases in the material lost by asymptotic giant branch stars, providing a better match to the available pre-solar SiC grain measurements compared to the calculations based on the current JEFF-3.3 model-based evaluation.

A PERFORMANCE STUDY OF A NEW PERSONAL NEUTRON DOSIMETRY SYSTEM AT SNRC.

This research reviews the performance of a commercial personal fast neutron dosimetry system that has recently been installed in the External Dosimetry Laboratory at Soreq Nuclear Research Center (SNRC). Different kinds of neutron sources like ${}{}^{\mathbf{241}}\boldsymbol{Am}$-Be, ${}{}^{\mathbf{252}}\boldsymbol{Cf}$ and some neutron generators are used in the industry as well as in some research institutes and reactors. The workers in these places need to be regularly monitored for neutron radiation. These workers' individual neutron doses have been estimated at SNRC until recently by a 30-year-old in-house-made system based on the CR-39 ® solid state nuclear track (SSNT) detector technology. Last year, a new Landauer CR-39 ® neutron dosimetry system was adopted, the Neutrak system. This system employs a standard etching technique using NaOH solution, and measures the SSNT dosimeters automatically and in a repeatable manner. This paper reviews the performance of Landauer's commercial personal neutron dosimetry system. The system performance shows excellent results in terms of linearity, repeatability, robustness, etch parameters sensitivity and detection threshold.

Nondestructive measurements of residual 235U mass of Israeli Research Reactor-1 fuel using the Advanced Experimental Fuel Counter

In 2018, a measurement campaign took place with participants from Los Alamos National Laboratory (LANL), the Nuclear Research Centre-Negev (NRCN) and Soreq Nuclear Research Center (SNRC) at the Israeli Research Reactor-1 (IRR-1) in which 14 of the reactor’s used fuel assemblies (FAs) with varied amount of depletion were measured with the nondestructive assay instrument Advanced Experimental Fuel Counter (AEFC). Designed for safeguards purposes, the AEFC measures both neutrons emitted from the FA (passive neutrons) and fission neutrons induced by an external neutron source (in this experiment, 252Cf). Signals recorded with the AEFC include total neutron count rates (Singles), time-correlated neutron count rates (Doubles), and total gamma-ray count rates. The 235U content of the FAs was previously assessed by two independent methods: (1) measurement of the transparency of the FA to low-energy gamma rays from an activated rhenium source (rhenium gamma transmission, or the RGT method) and (2) calculation of the ∼30-year burnup history of the core using detailed three-dimensional Monte-Carlo core depletion calculations. The results from the FAs that had been measured via the RGT method were used to construct the calibration curves, which translate the AEFC count rates to 235U mass. Then, the calibration was evaluated using AEFC measurements of six additional FAs that were not measured via the RGT method. From the results, it was determined the Doubles calibration curve was more reliable than that of the Singles and follows a simple second-order polynomial fit for the whole range of residual 235U mass content, albeit with larger statistical uncertainty.Detailed uncertainties quantification was conducted for both the AEFC Singles and Doubles. This includes the analysis of statistical uncertainties, calibration uncertainty, and random uncertainties due to the sensitivity of the AEFC to several sources of uncertainty, namely the FA position, FA orientation, interrogation source position, and ambient pool temperature. An overall total uncertainty of 6 g of 235U is estimated for the Singles and Doubles, which is mainly due to calibration uncertainty (for the Singles) and statistical uncertainty (for the Doubles), and which constitutes 3%–6% of the 235U total mass in the FAs, depending on their level of depletion.

Depletion measurements using a new gamma transparency method of irradiated MTR-type fuel assemblies

An innovative method for the determination of the depletion in research reactor fuel assemblies (FAs) based on gamma transparency was developed, tested, and presented. The basic concept is the assessment of the uranium content from the attenuation of gamma rays passing through the FA, which is placed between a gamma-emitting source and a spectroscopic detector. With increasing depletion, more uranium transforms into fission products, which causes a significant decrease in the gamma-rays attenuation at energies higher than ∼116 keV, thus increasing the count rate at the detector. The attenuation from the fuel assembly was compared to that of a fresh (highest known uranium content, most opaque to gamma-rays) and dummy (Aluminum-only; most transparent) fuel assemblies, and subsequently, a logarithmic interpolation to determine the current uranium content was performed. In this study, a low-energy gamma ray of 155 keV (emitted from 188Re) was used, which exhibited significant contrast between the attenuation through the uranium versus its fission products, while obtaining sufficiently high count-rate at the detector at relatively short count durations. The 188Re source was generated prior to the experimental campaign through the 187Re(n,γ) reaction by irradiation of natural rhenium in IRR1 (Israel Research Reactor 1, 5MW open-pool HEU MTR-type fuel, Soreq Nuclear Research Center, Israel). Five FAs of different irradiation histories previously used in the IRR1 reactor were studied. The determined depletions were found to be in the range of 47%–59%, with absolute uncertainties of 2%–3%. The axial distribution of the depletion along the FAs was determined with high resolution (20 axial points per FA) using the low-energy photons and found to be in good agreement compared to the axial distributions obtained from fission products such as 137Cs. Experimental results of the present study were also compared to results of a different experiment that used the traditional gamma ray spectroscopy technique (i.e., using 137Cs activity) for one of the studied fuel assemblies (FS-5), giving good agreement in both the axial depletion distribution, as well as the whole assembly depletion. Thus, the method described in this paper demonstrates improved accuracy compared to other non-destructive spectroscopy-based methods. Furthermore, we managed to obtain spectroscopic signatures of relatively short-lived fission products such as 140Ba (T1∕2=12.7 days) from an FA that was irradiated 65 days before the measurement.

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).

Special Section: Selected Papers from the 28th Conference of the Nuclear Societies in Israel

The Nuclear Societies in Israel include the Israel Society for Radiation Protection (ISRP), the Israel Nuclear Society (INS), and the Israel Society for Medical Physics (ISMP). These three societies hold a biennial conference in which the scientific community in Israel dealing with nuclear science, technology, and engineering, as well as radiation uses and radiation protection, encounters. The 28th Conference of the Nuclear Societies in Israel (INS-28) was held between April 12 and 14, 2016, at the Dan Panorama Hotel in Tel Aviv, Israel. The INS-28 Conference included 25 plenaries, parallel and poster sessions, in which more than 160 studies in various scientific fields were presented.We are very grateful to Professor Igor Pioro, who serves as the Editor of the Journal of Nuclear Engineering and Radiation Science, for hosting this special section dedicated to INS-28.Our two largest societies, the INS and the ISRP, have advocated nuclear energy as one of the most environmentally friendly ways to produce baseline electricity for over a decade, especially with respect to global climate change. We are delighted to see that this message is beginning to infiltrate the “green” establishment, as presented at the conference by the Israeli Minister of National Infrastructures, Energy and Water Resources and by the Chief Scientist of the Israel Nature and Parks Authority.The INS was established in 1974 by several senior academics. The first President of the society was the late Professor Shimon Yiftach, from the Department of Nuclear Engineering at the Technion—Israel Institute of Technology and former director of the Soreq Nuclear Research Center (SNRC); and the first Secretary, and later President, was Professor Yigal Ronen, from the Unit of Nuclear Engineering, Ben-Gurion University of the Negev (BGU) and former Dean of the Faculty of Engineering Sciences at BGU. Under their leadership, the INS joined the European Nuclear Society (ENS) and signed cooperation agreements with the Canadian Nuclear Society (CNS) and the American Nuclear Society (ANS).The ISRP was founded in 1964, following the initiative of the late Professor Yehuda Feige, who is referred to as the father of radiation protection in Israel, set up the Radiation Protection Department at SNRC in 1957 and was the first President of the Society. Under his leadership, the ISRP became one of the first national societies to join the International Radiation Protection Association (IRPA). In 1980, the ISRP hosted the Fifth International Congress of IRPA in Jerusalem, thanks to the efforts of another President of the society, Professor Tuvia Schlesinger.Major subjects discussed at the INS-28 Conference included Nuclear Reactor Physics and Technology, Radiation Protection, Medical Physics, Radiation Dosimetry, Physical Properties of Nuclear Materials, Accelerators, Radiation Detectors and Measurements, Nuclear Security and Nuclear Forensics, Non-Ionizing Radiation, Simulations and Numerical Methods, Radiological Risk Assessment, Natural Radioactivity, Operational Radiation Protection, and Radioactive Waste and the Environment.Special attention was also given in the conference to some of the main issues facing the nuclear energy sector and the radiation protection community, among which one may mention the uncertainty regarding the health effects of exposure to low doses of radiation, dosimetry of the eye lens and its protection as well as the environmental and radiological consequences of nuclear accidents and radiation safety aspects of postaccident recovery.Finally, we hosted a symposium dealing with radiation protection in medicine, of both patients and medical staff. Since medical exposure is accounted for a significant contribution to exposure of the public and since the medical sector is the most exposed occupational sector, it was but natural for our societies to dedicate a special day to this issue. This day was recorded as the First Israeli Symposium on Radiation Protection in Medicine.As co-chairmen of the INS-28 Conference, we would like to thank all the authors for their contributions presented in this special section of the Journal of Nuclear Engineering and Radiation Science as well as the reviewers, the editorial staff, and the Editor, who made this special section possible.

Characterization of a Drift-Step-Recovery Diode Based on All Epi-Si Growth

Drift-step-recovery diodes (DSRDs) are fast-opening switches capable of delivering nanosecond-scale high-voltage (HV) pulses into a load. The HV capability is achieved by stacking DSRD dies in series. In this paper, we characterize a DSRD die based on silicon epitaxial layers, which was designed and manufactured at the Soreq Nuclear Research Center. In the static characterization, we have measured the diode's forward- and reverse-blocking voltages, and the junction capacitance. In the dynamic characterization, we have measured the peak voltage and its rise time for a single die, and up to a stack of 32 dies in series, where the stack was operated at current densities of up to ~1.3 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The shortest rise time was 0.65 ns from a stack of five dies. An HV increase of 250 V per die was obtained. The maximum measured peak voltage was 6.09 kV with a rise time of 2.2 ns, and these results being limited by the setup capability.

Thermoluminescence characteristics of Israeli household salts for retrospective dosimetry in radiological events

Following a nuclear accident or terror attack involving the dispersal of radioactive substances, radiation dose assessment to first responders and the members of the public is essential. The need for a retrospective assessment of the radiation dose to those possibly affected is, therefore, obligatory.The present study examines the potential use of Israeli household salt as a retrospective dosimeter (RD). The experiments were carried out on Israeli salt samples (NaCl) following a Nielsen market track survey based on scanning data representing the barcoded market, including organized and independent retail chains and a sample of private minimarkets and supermarkets. The technique used was thermoluminescence (TL) dosimetry. Salt samples were exposed to levels of dose from 0.5mGy to 300Gy at the Israeli Secondary Standard Dosimetry Laboratory of the Soreq Nuclear Research Center using a calibrated 137Cs source.Our emphasis has been on a detailed investigation of the basic dosimetric characteristics of the salts including: (i) glow curve analysis (ii) individual glow peak dose response (iii) reproducibility (iv) estimation of minimal measurable dose (v) effect of nitrogen readout, (vi) influence of humidity during pre-irradiation storage and (vii) light induced fading.The results are sufficiently favorable to lead to the conclusion that the Israeli household salts can serve as a pragmatic potential candidate for RD under certain restricted conditions. Occasional pre-calibration of the major salt brands in a dedicated laboratory may be essential depending on the required accuracy in the estimation of dose and consequent clinical evaluation.

SARAF Phase I linac operation in 2013–2014

Phase I of the SARAF superconducting RF linac is under operation at the Soreq Nuclear Research Center. The present status of Phase I main components is reported, as well as, the beam operation experience accumulated in 2013–2014. The latter include acceleration of a 2 mA and 1.6 mA CW proton beams at energies of 2 MeV and 3.9 MeV correspondingly and 1 mA pulsed, duty cycle of few %, deuteron beams up to 5.6 MeV. The recent experiments include operation of intense CW proton beams on the liquid lithium target.

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.

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.

Drift-Step-Recovery Diode Characterization by a Bipolar Pulsed Power Circuit

A stack of drift- step-recovery diodes (DSRDs) can produce high-voltage pulses with a rise rate of the order of 1 kV/ns. Their building blocks, i.e., the DSRD dies, are designed and manufactured at Soreq Nuclear Research Center based on silicon epitaxial layers. A characterization circuit for DSRDs is presented. The circuit features a power MOSFET, which serves to pump the DSRD in the forward direction and then to pulse it in the reverse direction, and a bias voltage source to balance the forward pumping current with respect to the reverse discharge. Placing the DSRD in series between the MOSFET and the load results in temporal and polarity separation of the MOSFET and DSRD pulses at the load, thus allowing viewing of the net DSRD signal. High-voltage probes are employed to measure the MOSFET and load voltages. Based on this measurement, we formulate the voltage and current extraction of the circuit signals. A 1-ns 190-V epi-Si DSRD die was characterized by this circuit. We show that the DSRD pulses when its reverse discharge is equal to the forward charge, as expected. The DSRD switching loss was measured. An accurate 98% energy balance, which includes the input and output energies with respect to the MOSFET and DSRD switching losses, was obtained. The relationship between the bias voltage, the pumping current, and the main supply voltage is provided. The experimental results are supported by a numerical simulation of the DSRD in the circuit using the Synopsys technological computer-aided design incorporated with a SPICE solver.

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.