Neutron Production Target Research Articles

Potential for improvement of a neutron producing target for time-of-flight measurements

The Geel electron linear accelerator (GELINA) white neutron source is designed for time-of-flight (TOF) measurements with a high neutron energy resolution. A project has been launched in order to improve the accuracy of the high-resolution neutron cross-section measurements. The experimental accuracy is highly dependent on the neutron intensity and the time spread of the neutrons leaving the target. The main project objective is not only to design a new neutron producing target that reduces the time spread of the neutrons of a given energy, but also to maintain or possibly enhance the neutron production. The present GELINA target has been simulated recently with coupled electron–photon–neutron MCNP4C3 calculations and, subsequently, compared with experimental results. Based on the very good benchmarking results, this code is now used to design and optimise the new target and to assess all relevant parameters influencing the neutron flux and resolution functions. In this paper, the reader will find an explanation of the methodology on which the research of a new target is based. We compared the neutron fluxes of the compact geometries of various materials for different angles with the fluxes of the existing target. Further, we also made a comparison of the resolution functions for the present design versus a compact design. Finally, a dedicated figure of merit was used to make a qualitative comparison of various designs. The results presented in this paper reveal that there is a possibility to design a new target with superior characteristics so that all project objectives will be fulfilled.

Optimization of a neutron production target and a beam shaping assembly based on the 7Li( p, n) 7Be reaction for BNCT

In this work a thick LiF target was studied through the 7Li( p, n) 7Be reaction as a neutron source for Accelerator-Based Boron Neutron Capture Therapy (AB-BNCT) to provide a testing ground for numerical simulations aimed at producing an optimized neutron production target and beam shaping assembly design. Proton beams in the 1.88–2.0 MeV energy range were produced with the tandem accelerator TANDAR ( TANDem ARgentino) at the Comisión Nacional de Energía Atómica (CNEA) in Buenos Aires, Argentina. A cylindrical water-filled head-phantom, containing a boric acid sample, was irradiated to study the resulting neutron flux. The dose deposited in the boric acid sample was inferred through the Compton-suppressed detection of the gamma radiation produced from the 10B( n, αγ) 7Li capture reaction. The thermal neutron flux was evaluated using bare and Cd-covered activation gold foils. In all cases, Monte Carlo simulations have been done showing good agreement with the experimental results. Extensive MCNP simulation trials have then been performed after the preliminary calculation tool validation in order to optimize a neutron beam shaping assembly. These simulations include a thick Li metal target (instead of LiF), a whole-body phantom, two different moderator–reflector assemblies (Al/AlF 3/LiF, Fluental ®, as moderator and lead as reflector and a combination of Al, PTFE (polytetrafluoroethylene) and LiF as moderator and lead as reflector) and the treatment room. The doses were evaluated for proton bombarding energies of 1.92 MeV (near to the threshold of the reaction), 2.0 MeV, 2.3 MeV (near the reaction resonance) and 2.5 MeV, and for three Fluental ® and Al/PTFE/LiF moderator thicknesses (18, 26 and 34 cm). In a later instance, the effect of the specific skin radiosensitivity (an RBE of 2.5 for the 10B( n, α) 7Li reaction) and a 10B uptake 50% greater than the healthy tissue one, was considered for the scalp. To evaluate the doses in the phantom, a comparison of the different assemblies and beam energies was done using a Tumor Control Probability (TCP) model. The simulations show the advantage of irradiating with near-resonance-energy protons (2.3 MeV) because of the relatively high neutron yield at this energy, which at the same time keeps the fast neutron healthy tissue dose limited and leads to the shortest treatment times. A moderator of 34 cm length (either Fluental or Al/PTFE/LiF) has shown the best performance among the studied cases.

Lithium neutron producing target for BINP accelerator-based neutron source

Pilot innovative accelerator-based neutron source for neutron capture therapy is under construction now at the Budker Institute of Nuclear Physics, Novosibirsk, Russia. One of the main elements of the facility is lithium target, that produces neutrons via threshold 7Li( p, n) 7Be reaction at 25 kW proton beam with energies 1.915 or 2.5 MeV. In the present report, the results of experiments on neutron producing target prototype are presented, the results of calculations of hydraulic resistance for heat carrier flow and lithium layer temperature are shown. Calculation showed that the lithium target could run up to 10 mA proton beam before melting. Choice of target variant is substantiated. Program of immediate necessary experiments is described. Target design for neutron source constructed at BINP is presented. Manufacturing the neutron producing target up to the end of 2004 and obtaining a neutron beam on BINP accelerator-based neutron source are planned during 2005.

An optimized neutron-beam shaping assembly for accelerator-based BNCT

Different materials and proton beam energies have been studied in order to search for an optimized neutron production target and beam shaping assembly for accelerator-based BNCT. The solution proposed in this work consists of successive stacks of Al, polytetrafluoroethylene, commercially known as Teflon®, and LiF as moderator and neutron absorber, and Pb as reflector. This assembly is easy to build and its cost is relatively low. An exhaustive Monte Carlo simulation study has been performed evaluating the doses delivered to a Snyder model head phantom by a neutron production Li-metal target based on the 7Li( p, n) 7Be reaction for proton bombarding energies of 1.92, 2.0, 2.3 and 2.5 MeV. Three moderator thicknesses have been studied and the figures of merit show the advantage of irradiating with near-resonance-energy protons (2.3 MeV) because of the relatively high neutron yield at this energy, which at the same time keeps the fast neutron healthy tissue dose limited and leads to the lowest treatment times. A moderator of 34 cm length has shown the best performance among the studied cases.

Results from the commissioning of the n_TOF spallation neutron source at CERN

The new neutron time-of-flight facility (n_TOF) has been built at CERN and is now operational. The facility is intended for the measurement of neutron induced cross-sections of relevance to Accelerator Driven Systems (ADS) and to fundamental physics. Neutrons are produced by spallation of the 20 GeV/c proton beam, delivered by the Proton Synchrotron (PS), on a massive target of pure lead. A measuring station is placed at ≈185 m from the neutron producing target, allowing high-resolution measurements. The facility was successfully commissioned with two campaigns of measurements, in November 2000 and April 2001. The main interest was concentrated in the physical parameters of the installation (neutron fluence and resolution function), along with the target behavior and various safety-related aspects. These measurements confirmed the expectations from Monte Carlo simulations of the facility, thus allowing to initiate the foreseen physics program.

Novel Applications of Particle Accelerators to Radiotherapy

Charged hadrons (protons and heavier ions) have very definite advantages over photons as far as radiotherapy applications are concerned. They allow for much better spatial dose localization due to their charge, relatively high mass and nature of the energy deposition process. In the frame of an attempt to promote the introduction of hadrontherapy in Argentina we have installed and started using an external beam facility at our tandem accelerator TANDAR. The advantages of heavy ions can only be fully exploited for tumors of well defined localization. In certain types of malignancies, however, the region infiltrated by tumor cells is diffuse, with no sharp boundaries and with microscopic ramifications. In such cases (particularly in certain brain cancers) a more sophisticated scheme has been suggested called boron neutron capture therapy (BNCT). In this work, the use of the Tandar accelerator to produce neutrons for feasibility studies for BNCT through low-energy proton beams on a thick LiF target is being briefly described. Studies on the 13C(d,n) reaction and a comparison with other neutron-producing reactions are also mentioned. Simulation work to optimize an accelerator-based neutron production target is discussed. A project is being prepared to develop a small proton accelerator in Argentina. Technical specifications of this machine will be briefly discussed.

A study on the utilization of tritide titanium targets for monoenergetic neutron production

Proton beam interaction with a tritide titanium target in an electrostatic accelerator was studied by measuring the produced neutron rate as a function of bombarding time. The results are analyzed in the frame of target temperature increase, extension of existing low energy interaction model and SRIM simulation. Special emphasis is put on the evaluation of the release of radiotoxic tritium from the solid target.

Accelerator‐based neutron brachytherapy

The development and evaluation of a new approach to neutron brachytherapy is described. This approach, accelerator-based fast neutron brachytherapy, involves the interstitial or intracavity insertion of a narrow, evacuated accelerator beam tube such that its tip, containing the neutron-producing target, is placed in or near the tumor. Tumor irradiation via brachytherapy should result in a reduction in the healthy tissue complication rate observed when poorly collimated and/or low energy external neutron beam are used for treatment. Use of a variable energy accelerator provides an advantage over isotope sources for neutron brachytherapy in that the neutron beam can be turned on and off and the neutron energy spectrum varied for different treatment applications. A prototype accelerator-based fast neutron brachytherapy device, 10 cm long and 6 mm outer diameter, has been constructed and evaluated in terms of its dosimetric output, treatment time, and practical feasibility. The prototype device is a tube-in-tube design with cooling water running between the inner and outer tubes to cool a beryllium target located at the tip of the inner tube. Cooling experiments were performed and coupled with Monte Carlo simulations to determine treatment times as a function of heat load for various neutron-producing reactions. Using the 9Be(d,n) 10B reaction at Ed= 1.5 MeV, 66 RBE-Gy (12 Gy physical dose) can be delivered to the boundary of a 4.5-cm-diam treatment volume in 8 min at a heat load of 130 W. Other reactions offer similar treatment times at somewhat higher bombarding energies and also show higher potential for dose enhancement with the boron-10 neutron capture reaction due to their softer neutron spectra. Dose distributions in a water phantom were measured with the prototype brachytherapy tube using the dual-ion chamber technique for the 9Be(d,n) 10B reaction at Ed = 1.5 MeV. The measurements and simulations agree within uncertainties and demonstrate that fast neutrons contribute more than 90% of the dose to the target volume.

Study of Properties of the Neutron Production Target Induced from 150-MeV Incident Proton Energy for the China ADS

A study of the properties of the neutron production target for the China accelerator-driven system induced from 150-MeV incident proton energy and a 3-mA beam current is performed. The selection of the material and geometry design for the target; the physics properties of the target involved in this work including the neutron yield, energy, and space distribution for the neutrons leaked from the target; and the spallation fragment of charge and mass distribution have been investigated using the SHIELD code. An analysis of the target radiation damage and energy deposit is also carried out.

A sensitive neutron spectrometer for the National Ignition Facility

We are developing a sensitive neutron spectrometer for the National Ignition Facility laser at Livermore. The spectrometer will consist of a 1020 channel single-neutron-interaction time-of-flight detector array fielded 23 m from the neutron-producing target. It will use an existing detector array together with upgraded electronics for improved time resolution. Measurements of neutron yield, ion and electron temperatures, and density-radius product are all possible under certain conditions using one-, two-, or three-step reaction processes. The locations of the most important potential sources of scattered neutron backgrounds are determined as the first step in designing collimation to reduce these backgrounds.

Modification of the University of Washington Neutron Radiotherapy Facility for optimization of neutron capture enhanced fast-neutron therapy.

A modified neutron production target assembly has been developed to provide improved performance of the proton-cyclotron-based neutron radiotherapy facility at the University of Washington for applications involving neutron capture enhanced fast-neutron therapy. The new target produces a neutron beam that yields essentially the same fast-neutron physical depth-dose distribution as is produced by the current UW clinical system, but that also has an increased fraction of BNCT enhancement relative to the total therapeutic dose. The modified target is composed of a 5-millimeter layer of beryllium, followed by a 2.5-millimeter layer of tungsten, with a water-cooled copper backing. Measurements of the free-field neutron spectrum of the beam produced by the new target were performed using activation foils with a direct spectral unfolding technique. Water phantom measurements were performed using a tissue-equivalent ion chamber to characterize the fast-neutron depth-dose curve and sodium activation in soda-lime glass beads to characterize the thermal-neutron flux (and thus the expected neutron capture dose enhancement) as a function of depth. The results of the various measurements were quite consistent with expectations based on the design calculations for the modified target. The spectrum of the neutron beam produced by the new target features an enhanced low-energy flux component relative to the spectrum of the beam produced by the standard UW target. However, it has essentially the same high-energy neutron flux, with a reduced flux component in the mid-range of the energy spectrum. As a result, the measured physical depth-dose curve in a large water phantom has the same shape compared to the case of the standard UW clinical beam, but approximately twice the level of BNCT enhancement per unit background neutron dose at depths of clinical interest. In-vivo clinical testing of BNCT-enhanced fast-neutron therapy for canine lung tumors using the new beam was recently initiated.

Development and construction of a neutron beam line for accelerator-based boron neutron capture synovectomy

A potential application of the 10B(n, alpha)7Li nuclear reaction for the treatment of rheumatoid arthritis, termed Boron Neutron Capture Synovectomy (BNCS), is under investigation. In an arthritic joint, the synovial lining becomes inflamed and is a source of great pain and discomfort for the afflicted patient. The goal of BNCS is to ablate the synovium, thereby eliminating the symptoms of the arthritis. A BNCS treatment would consist of an intra-articular injection of boron followed by neutron irradiation of the joint. Monte Carlo radiation transport calculations have been used to develop an accelerator-based epithermal neutron beam line for BNCS treatments. The model includes a moderator/reflector assembly, neutron producing target, target cooling system, and arthritic joint phantom. Single and parallel opposed beam irradiations have been modeled for the human knee, human finger, and rabbit knee joints. Additional reflectors, placed to the side and back of the joint, have been added to the model and have been shown to improve treatment times and skin doses by about a factor of 2. Several neutron-producing charged particle reactions have been examined for BNCS, including the 9Be(p,n) reaction at proton energies of 4 and 3.7 MeV, the 9Be(d,n) reaction at deuteron energies of 1.5 and 2.6 MeV, and the 7Li(p,n) reaction at a proton energy of 2.5 MeV. For an accelerator beam current of 1 mA and synovial boron uptake of 1000 ppm, the time to deliver a therapy dose of 10,000 RBEcGy ranges from 3 to 48 min, depending on the treated joint and the neutron producing charged particle reaction. The whole-body effective dose that a human would incur during a knee treatment has been estimated to be 3.6 rem or 0.75 rem, for 1000 ppm or 19,000 ppm synovial boron uptake, respectively, although the shielding configuration has not yet been optimized. The Monte Carlo design process culminated in the construction, installation, and testing of a dedicated BNCS beam line on the high-current tandem electrostatic accelerator at the Laboratory for Accelerator Beam Applications at the Massachusetts Institute of Technology.

Measurement of neutron inelastic scattering cross-section for the first level of 238U in hundreds of keV range

Neutron inelastic scattering cross-section for the first level of 238U ( Ex=44.9 keV) was measured for eight incident energy points from 350 to 855 keV at laboratory angle of 120° by using a time-of-flight spectrometer. Care was taken for the peak separation of the first level from the ground state, and for the absolute normalization of the cross-section. The peak separation was achieved by employing a thin neutron production target ( ΔE⩽10 keV), a long flight path (4–6 m), and by experimental simulation of the elastic scattering peak with a lead sample. Validity of the absolute value and internal consistency in the experiment were confirmed by measuring carbon with well-known cross-sections and 238U at another emission angle, 135°. The angle-integration was performed by employing the angular distribution of JENDL-3.2. The present experimental data for the first level and the ground state are close to the JENDL-3.2 data.

Photoneutron source based on a compact 10 MeV betatron

Accelerator-based photoneutron sources have enjoyed wide use and offer the advantages of long term stability, ease of control and absence of radioactive materials. We report here measurements of the yield of photoneutrons from a neutron generator using a compact betatron. Electrons were accelerated to energies up to 10 MeV and produced a bremsstrahlung beam with a dose rate of 0.16 Gy/min (at 10 MeV, 1 m from the bremsstrahlung target) to irradiate LiD, Be, depleted U, and Pb neutron-producing targets. The angular distributions of photoneutrons produced by bremsstrahlung beams were measured with a “long” counter and integrated to determine neutron yield. In addition, neutron time of flight spectra were recorded from all targets using a 15.5 m flight path perpendicular to the photon beam. The maximum observed yields were 4.6×10 7 n/s obtained with 1 kg of LiD, 5.7×10 7 n/s from a 3.3 kg Be block, 6.2×10 6 n/s from 1.5 kg of depleted U, and 7.0×10 6 n/s from 10.7 kg of Pb. Optimization of target dimensions, shape, and positioning is expected to increase the yield from the LiD target by a factor of 35, while optimization of the other targets is expected to yield at most a factor of 10. With the increased yield and a deuteride target, this compact betatron-based system could find application in the interrogation of waste containers for fissile material.

The Los Alamos Neutron Science Center (LANSCE)

In October 1995, with the closeout of the nuclear physics user program and the increased national need for neutrons, Los Alamos National Laboratory refocused the mission of its linear accelerator complex. The new mission called for the development and use of spallation neutron sources for research and applications. As a result, the accelerator complex formerly known as the Los Alamos Meson Physics Facility (LAMPF) is now called the Los Alamos Neutron Science Center (LANSCE). LANSCE, an acronym that formerly referred to the Manuel Lujan Jr. Neutron Scattering Center (now the Lujan Center), consists of a high-power 800–MeV proton linear accelerator (linac), a Proton Storage Ring (PSR), neutron production targets at the Lujan Center and the Weapons Neutron Research (WNR) facility, a Proton Radiography (PRad) Facility, a highpower materials irradiation area (LASREF), and an isotope production facility (IPF). With the ability to produce protons and neutrons, LANSCE is ideal for research in condensed matter science and engineering, accelerator science, nuclear science, and radiography. LANSCE uses these research capabilities to contribute to the Department of Energy Stockpile Stewardship Program and to support a national user program open to scientists from universities, industry, and federal laboratories. The layout of LANSCE can be seen at right.

A practical target system for accelerator-based BNCT which may effectively double the dose rate.

A dose-limiting component of a proton accelerator-based source of epithermal neutrons is the neutron production target. Possible targets are lithium, producing high yield but having low melting point and thermal conductivity, and beryllium, presenting less engineering problems but a much smaller neutron yield. We propose that a hybrid Be-Li target would provide the best of both worlds, with the upstream beryllium component producing neutrons and providing containment to the lithium, and the downstream liquid lithium in turn producing further neutrons as well as cooling the beryllium. The engineering considerations associated with such a target system are within the range of current technology. Calculations suggest a yield of such a practical target that is at least double that from pure beryllium.

A light localizer for use at large distances from a radiation source

In the superconducting cyclotron used for neutron therapy at Harper Hospital, space limitations prevented the use of a simple conventional light localizing system, employing a single mirror and a light source. The superconducting magnet coil is close to the neutron producing target and the vacuum window separating the acceleration chamber from atmospheric pressure is situated approximately 56 cm from this target. Therefore, the light localizing system must be designed to incorporate lenses, mirrors, an optical fiber light pipe and adjustment motors, in order to make the system compact enough to fit in the available space and to allow easy access for adjustment and alignment. The design of the light localizing system is explained in detail.

27Al(n,xγ)reactions for neutron energies from 3 to 400 MeV

The prompt \ensuremath{\gamma} radiation from the interaction of fast neutrons with aluminum was measured using the white neutron beam of the WNR facility at the Los Alamos National Laboratory. Aluminum samples were positioned at about 20 m or 41 m distance from the neutron production target. The spectra of the emitted \ensuremath{\gamma} rays were measured with a high-resolution HPGe detector. The incident neutron energy was determined by the time-of-flight method and the neutron fluence was measured with a ${}^{238}\mathrm{U}$ fission chamber. From the \ensuremath{\gamma}-ray spectra excitation functions for prominent \ensuremath{\gamma}-ray transitions in various residual nuclei (in the element range from F to Al) were derived for neutron energies from 3 to 400 MeV. Up to 200 MeV incident neutron energy the results are compared with the predictions of nuclear model calculations performed with the code GNASH. This code combines compound nucleus calculations using Hauser-Feshbach statistical theory with preequilibrium calculations from an exciton model including multiple preequilibrium processes. For the majority of the \ensuremath{\gamma}-ray transitions there is reasonable agreement between the measured and the calculated cross sections.

98/01702 Development of coprecipitated nickel-alumina catalyst for pyrocatalytic gasification of biomass: Garcia, L. et al. Biomass Energy Environ., Proc. Eur. Bioenergy Conf., 9th, 1996, 2, 1359–1364. Edited by Chartier, P., Elsevier, Oxford, UK

Associated particle imaging (API) is a powerful technique for special nuclear material (SNM) detection and characterization of fissile material configurations. A sealed-tube neutron generator has been under development by Lawrence Berkeley National Laboratory to reduce the beam spot size on the neutron production target to 1 mm in diameter for a several-fold increase in directional resolution and simultaneously increases the maximum attainable neutron flux. A permanent magnet 2.45 GHz microwave-driven ion source has been adopted in this time-tagged neutron source. This type of ion source provides a high plasma density that allows the use of a sub-millimeter aperture for the extraction of a sufficient ion beam current and lets us achieve a much reduced beam spot size on target without employing active focusing. The design of this API generator uses a custom-made radial high voltage insulator to minimize source to neutron production target distance and to provide for a simple ion source cooling arrangement. Preliminary experimental results showed that more than 100 μA of deuterium ions have been extracted, and the beam diameter on the neutron production target is around 1 mm.

A monochromatic neutron facility for (n,p) reactions

A neutron facility for the study of (n,p) reactions was set up at the Indiana University Cyclotron Facility. The neutrons were produced by the reaction 7Li(p,n) 7Be. The energy spread of the neutron beam is dominated by the thickness of the 7Li production target. At the (n,p) target the collimated neutron-beam intensity is about 10 5 n/s/cm 2 for a 1 μA proton beam at 120 MeV on a 7Li target 100 mg/cm 2 thick. The distance between the neutron production target and the (n,p) target, 3.14 m, makes it possible by time-of-flight to eliminate neutrons degraded below the maximum energy by scattering. Protons from the (n,p) reaction are detected simultaneously at three laboratory angles (7.5°, 12.5°, and 17.5°), in three particle telescopes each consisting typically of three hyperpure Ge elements. All three telescopes are mounted in a common cryostat. Spectra are obtained at other angles including zero by magnetically deflecting the (n,p) protons. Spectra recorded at E n = 118 MeV for (n,p) reactions on several light nuclei are presented.