J. Vujic

Designing accelerator-based epithermal neutron beams for boron neutron capture therapy

The 7Li(p,n)7Be reaction has been investigated as an accelerator-driven neutron source for proton energies between 2.1 and 2.6 MeV. Epithermal neutron beams shaped by three moderator materials, Al/AlF3, 7LiF, and D2O, have been analyzed and their usefulness for boron neutron capture therapy (BNCT) treatments evaluated. Radiation transport through the moderator assembly has been simulated with the Monte Carlo N-particle code (MCNP). Fluence and dose distributions in a head phantom were calculated using BNCT treatment planning software. Depth-dose distributions and treatment times were studied as a function of proton beam energy and moderator thickness. It was found that an accelerator-based neutron source with Al/AlF3 or 7LiF as moderator material can produce depth-dose distributions superior to those calculated for a previously published neutron beam design for the Brookhaven Medical Research Reactor, achieving up to approximately 50% higher doses near the midline of the brain. For a single beam treatment, a proton beam current of 20 mA, and a 7LiF moderator, the treatment time was estimated to be about 40 min. The tumor dose deposited at a depth of 8 cm was calculated to be about 21 Gy-Eq.

Development of a sealed-accelerator-tube neutron generator

Sealed-accelerator-tube neutron generators are being developed in Lawrence Berkeley National Laboratory (LBNL) for applications ranging from neutron radiography to boron neutron capture therapy and neutron activation analysis. The new generation of high-output neutron generators is based on the D-T fusion reaction, producing 14.1-MeV neutrons. The main components of the neutron tube--the ion source, the accelerator and the target--are all housed in a sealed metal container without external pumping. Thick-target neutron yield computations are performed in this paper to estimate the neutron yield of titanium and scandium targets. With an average deuteron beam current of 1 A and an energy of 120 keV, a time-averaged neutron production of approximately 10(14) n/s can be estimated for a tritiated target, for both pulsed and cw operations. In mixed deuteron/triton beam operation, a beam current of 2 A at 150 keV is required for the same neutron output. Recent experimental results on ion sources and accelerator columns are presented and discussed.

Neutron beam optimization for boron neutron capture therapy using the D-D and D-T high-energy neutron sources

Abstract A monoenergetic neutron beam simulation study is carried out to determine the most suitable neutron energy for treatment of shallow and deep-seated brain tumors in the context of boron neutron capture therapy. Two figures-of-merit—the absorbed skin dose and the absorbed tumor dose at a given depth in the brain - are used to measure the neutron beam quality. Based on the results of this study, moderators, reflectors, and delimiters are designed and optimized to moderate the high-energy neutrons from the fusion reactions 2H(d,n)3He and 3H(d,n)4He down to a suitable energy spectrum. Two different computational models (MCNP and BNCT_RTPE) have been used to study the dose distribution in the brain. With the optimal beam-shaping assembly, a 1-A mixed deuteron/triton beam of energy 150 keV accelerated onto a titanium target leads to a treatment time of 1 h. The dose near the center of the brain obtained with this configuration is >65% higher than the dose from a typical spectrum produced by the Brookhaven Medical Research Reactor and is comparable to the dose obtained by other accelerator-produced neutron beams.

Production of low energy spread ion beams with multicusp sources

Abstract The use of multicusp sources to generate ion beams with narrow energy spread has been investigated. It is found that the presence of a magnetic filter can reduce the longitudinal energy spread significantly. This is achieved by creating a uniform plasma potential distribution in the discharge chamber region, eliminating ion production in the extraction chamber and in the sheath of the exit aperture and by minimizing the probability of charge exchange processes in the extraction chamber. An energy spread as low as 1 eV has been measured.

ANEMONA-A neutron transport code for general geometry reactor assemblies based on the method of characteristics and R-function solid modeler

Abstract The method of characteristics (MOC) solves the transport equation along straight lines, called characteristics, of the system. Along these lines the differential operator of the Boltzmann equation reduces to a total derivative. The MOC methodology does not impose any limitation on geometry and allows for an accurate treatment of highly heterogeneous systems. However, the actual treatment of arbitrary domains in terms of their flexible description and efficient ray tracing does impose difficulties which limited broader application of MOC in reactor analysis. Most of the existing MOC codes describe the geometry by lines and arcs with extensive input data, have difficulty in sub-meshing, require a pre-defined condition of closed rays at starting points limiting the boundary shape, and need a large number of polar angles which prolong the execution time. A new MOC code, ANEMONA, has been developed to remove all these geometrical limitations. This unified and easy-to-use method is based on an R -function solid modeler approach that can account for all heterogeneities with full flexibility in domain description, ray map generation, ray tracing and boundary types. The ANEMONAs methodology has many advantages: an easy description of arbitrary domains and meshing; allowed mismatch of the reflected rays; calculation of the reflected angular flux as the boundary mesh average; use of only two adequately chosen polar angles; and the reduction in computational time by energy-dependent azimuthal ray map. ANEMONA has been tested on a number of benchmark problems with excellent results.

Dose uncertainty due to computed tomography (CT) slice thickness in CT‐based high dose rate brachytherapy of the prostate cancer

In computed tomography (CT)-based high dose rate (HDR) brachytherapy, the uncertainty in the localization of the longitudinal catheter-tip positions due to the discrete CT slice thickness, results in a delivered dose uncertainty. Catheter coordinates were extracted from five patients treated for prostate cancer, and three simulation scenarios were followed to mimic the longitudinal imprecision of the catheter tips, hence the dwell positions. All catheters were displaced (1) forward, (2) backward, or (3) randomly distributed within the space defined by one CT slice thickness, for thicknesses ranging from 2 to 5 mm. Average and standard deviation values of the relative dose variations are reported for the various catheter displacement scenarios. Also, the dose points were grouped according to their relative position in the prostate, inner, peripheral and outer area of prostate and base, median and apex zones, in order to estimate the spatial sensitivity of the dose errors. For scenarios (1) and (2), the dose uncertainties due to the finite slice thickness increase linearly with the slice spacing, from 3% to 8% for the slice thickness values ranging from 2 to 5 mm, respectively. The more realistic scenario (3) yields average errors ranging from 0.7% to 1.7%. The apex and the base show larger dose errors and variability of dose errors than the median of the prostate. No statistical difference was observed among different transversal sections of the prostate. A CT slice thickness of 3 mm appears to be a good compromise showing an acceptable average dose uncertainty of 1%, without unduly increasing the number of slices.

Ion energy spread and current measurements of the rf-driven multicusp ion source

Axial energy spread and useful beam current of positive ion beams have been carried out using a radio frequency (rf)-driven multicusp ion source. Operating the source with a 13.56 MHz induction discharge, the axial energy spread is found to be approximately 3.2 eV. The extractable beam current of the rf-driven source is found to be comparable to that of filament-discharge sources. With a 0.6 mm diameter extraction aperture, a positive hydrogen ion beam current density of 80  mA/cm2 can be obtained at a rf input power of 2.5 kW. The expected source lifetime is much longer than that of filament discharges.

Considerations of Autocatalytic Criticality of Fissile Materials in Geologic Respositories

Potential routes to autocatalytic criticality in geologic repositories are systematically assessed. If highly enriched uranium (HEU) or {sup 239}Pu are transported and deposited in concentrations similar to natural uranium ore, in principle, criticality can occur. For some hypothesized critical configurations, removal of a small fraction of pore water provides a positive feedback mechanism that can lead to supercriticality. Rock heating and homogenization for these configurations can also significantly increase reactivity. At Yucca Mountain, it is highly unlikely that these configurations can occur; plutonium transport would occur primarily as colloids and deposit over short distances. HEU solute can move large distances in the Yucca Mountain setting; its ability to precipitate into critical configurations is unlikely because of a lack of active reducing agents. Appropriate engineering of the waste form and the repository can reduce any remaining probability of criticality.

Axial energy spread measurements of an accelerated positive ion beam

Abstract A multicusp ion source has been designed for use in ion projection lithography. Longitudinal energy spreads of the extracted positive hydrogen ion beam have been studied using a retarding field energy analyzer. It has been found that the filament-discharge multicusp ion source can deliver a beam with an energy spread less than 3 eV which is required for the ALG-1000 machine. The multicusp ion source can also deliver the current required for the application.

A columnar cesium iodide (CsI) drift plane layer for gas avalanche microdetectors

A new method is proposed to improve the spatial and time resolutions, and the detection efficiency with respect to the angle of the incident particles, for gas avalanche micro detectors. The new technique uses a thin (/spl sim/200 /spl mu/m) columnar CsI layer at the drift plane that acts as an efficient source of electrons produced by secondary emission due to the incident particles, and as an electron multiplier. In this paper, we present the measurements of the electron multiplication factor and detection efficiency of this layer, using a /sup 90/Sr /spl beta/-source.