Frank Harmon

A Monte Carlo dosimetry-based evaluation of the 7Li(p,n)7Be reaction near threshold for accelerator boron neutron capture therapy.

Advanced methods of boron neutron capture therapy (BNCT) use an epithermal neutron beam in conjunction with tumor-targeting boron compounds for irradiation of glioblastomas and metastatic melanomas. A common neutron-producing reaction considered for accelerator-based BNCT is 7Li(p,n)7Be, whose cross section increases very rapidly within several tens of keV of the reaction threshold at 1.88 MeV. Operation in the proton energy region near threshold will have an appreciable thick target neutron yield, but the neutrons produced will have relatively low energies that require little moderation to reach the epithermal range desirable for BNCT. Because of its relatively low projected accelerator cost and the portability of the neutron source/target assembly, BNCT based on the near-threshold technique is considered an attractive candidate for widespread hospital use. A systematic Monte Carlo N-Particle (MCNP) investigation of the dosimetric properties of near-threshold neutron beams has been performed. Results of these studies indicate that accelerator proton energies between 1.93 and 1.99 MeV, using 5 cm of H2O moderator followed by thin 6Li and Pb shields, can provide therapeutically useful beams with treatment times less than one hour and accelerator currents less than 5 mA.

Sc-47 production from titanium targets using electron linacs.

In this work we have studied the feasibility of photonuclear production of (47)Sc from (48)Ti via (48)Ti(γ,p)(47)Sc reaction. Photon flux distribution for electron beams of different energies incident on tungsten converter was calculated using MCNPX radiation transport code. (47)Sc production rate dependence on electron beam energy was found and (47)Sc yields were estimated. It was shown that irradiating a natural Ti target results in numerous scandium isotopes which can reduce the specific activity of (47)Sc. Irradiating enriched (48)Ti targets with a 22MeV 1mA beam will result in hundreds of MBq/g activity of (47)Sc and no other isotopes of scandium. Decreasing the size of the target will result in much higher average photon flux through the target and tens of GBq/g levels of specific activity of (47)Sc. Increasing the beam energy will also result in higher yields, but as soon as the electron energy exceeds the (48)Ti(γ,np)(46)Sc reaction threshold, (46)Sc starts being produced and its fraction in total scandium atoms grows as beam energy increases. The results of the simulations were benchmarked by irradiating natural titanium foil with 22MeV electron beam incident on the tungsten converter. Measured (47)Sc activities were found to be in very good agreement with the predictions.

Optimization of commercial scale photonuclear production of radioisotopes

Photonuclear production of radioisotopes driven by bremsstrahlung photons using a linear electron accelerator in the suitable energy range is a promising method for producing radioisotopes. The photonuclear production method is capable of making radioisotopes more conveniently, cheaply and with much less radioactive waste compared to existing methods. Historically, photo-nuclear reactions have not been exploited for isotope production because of the low specific activity that is generally associated with this production process, although the technique is well-known to be capable of producing large quantities of certain radioisotopes. We describe an optimization technique for a set of parameters to maximize specific activity of the final product. This set includes the electron beam energy and current, the end station design (an integrated converter and target as well as cooling system), the purity of materials used, and the activation time. These parameters are mutually dependent and thus their optimizatio...

Temperature rise in lithium targets for accelerator based BNCT using multi-fin heat removal

Thick lithium targets are excellent sources of neutrons for accelerator boron neutron capture therapy (BNCT), but the low melting point of lithium (181 °C) and a need for high proton currents make target heating a concern. However, because neutrons are not produced for proton energies below the 7Li(p,n)7Be reaction threshold of 1.88 MeV, the lithium targets need only be thick enough to slow the proton beam past this energy. This allows the majority of the proton energy deposition, including the Bragg peak, to occur in the copper backing, whose superior thermal properties reduce the total temperature rise. We have developed a model for predicting temperature rises in a BNCT target design that utilizes multiple rectangular fins. A theoretical model of the multi-fin heat removal is presented. Experiments confirm the results of these calculations, which indicate that multi-fin lithium targets for BNCT can successfully cool milliamp level proton beams.

Portable Electron Radiography System

The technique of charged particle radiography has been developed and proven with 800 MeV protons at LANSCE and 24 GeV protons at the AGS. Recent work at Los Alamos National Laboratory in collaboration with the Idaho Accelerator Center has extended this diagnostic technique to electron radiography through the development of an inexpensive and portable electron radiography system. This system has been designed to use 30 MeV electrons to radiograph thin static and dynamic systems. The system consists of a 30 MeV electron linear accelerator coupled to a quadrupole lens magnifier constructed from permanent magnet quadrupoles. The design features and operational characteristics of this radiography system are presented as well as the expected radiographic performance parameters.

Actively-induced, Prompt Radiation Utilization in Nonproliferation Applications

The Pulsed Photonuclear Assessment (PPA) technique, which has demonstrated the ability to detect shielded nuclear material, is currently based on utilizing delayed neutrons and photons between accelerator pulses. While most active interrogation systems have focused on delayed neutron and gamma-ray signatures, the current requirements of various Homeland Security issues necessitate bringing faster detection and acquisition capabilities to field inspection applications. This push for decreased interrogation times, increased sensitivity and mitigation of false positives requires that detection systems take advantage of all available information. Collaborative research between Idaho National Lab (INL), Idaho State Universitys Idaho Accelerator Center (IAC), Los Alamos National Laboratory (LANL), and Oak Ridge National Laboratory (ORNL), has focused on exploiting actively-induced, prompt radiation signatures from nuclear material within a pulsed photonuclear environment. To date, these prompt emissions have not been effectively exploited due to difficulties in detection and signal processing inherent in the prompt regime as well as an overall poor understanding of the magnitude and yields of these emissions. Exploitation of prompt radiation (defined as during an accelerator pulse/(photo)fission event and/or immediately after (les 1 mus)) has the potential to dramatically reduce interrogation times since the prompt neutron yields are more than two orders of magnitude greater than delayed emissions. Successful exploitation of prompt emissions is critical for the development of an improved robust, high-throughput, low target dose inspection system for detection of shielded nuclear materials.

LINAC-BASED PHOTONUCLEAR APPLICATIONS AT THE IDAHO ACCELERATOR CENTER

In this paper, current Idaho Accelerator Center (IAC) activities based on the exploitation of high energy bremsstrahlung photons generated by linear electron accelerators will be reviewed. These beams are used to induce photonuclear interactions for a wide variety of applications in materials science, activation analysis, medical research, and nuclear technology. Most of the exploited phenomena are governed by the familiar giant dipole resonance cross section in nuclei. By proper target and converter design, optimization of photon and photoneutron production can be achieved, allowing radiation fields produced with both photons and neutrons to be used for medical isotope production and for fission product transmutation. The latter provides a specific application example that supports long-term fission product waste management. Using high-energy, highpower electron accelerators, we can demonstrate transmutation of radio-toxic, long-lived fission products (LLFP) such as 99Tc and 129I into short lived species. The latest experimental and simulation results will be presented.

Exotic X‐ray Sources from Intermediate Energy Electron Beams

High intensity x‐ray beams are used in a wide variety of applications in solid‐state physics, medicine, biology and material sciences. Synchrotron radiation (SR) is currently the primary, high‐quality x‐ray source that satisfies both brilliance and tunability. The high cost, large size and low x‐ray energies of SR facilities, however, are serious limitations. Alternatively, “novel” x‐ray sources are now possible due to new small linear accelerator (LINAC) technology, such as improved beam emittance, low background, sub‐Picosecond beam pulses, high beam stability and higher repetition rate. These sources all stem from processes that produce Radiation from relativistic Electron beams in (crystalline) Periodic Structures (REPS), or the periodic “structure” of laser light. REPS x‐ray sources are serious candidates for bright, compact, portable, monochromatic, and tunable x‐ray sources with varying degrees of polarization and coherence. Despite the discovery and early research into these sources over the past ...

Feasibility of a Near-Threshold BNCT Neutron Source Based on Phantom Dosimetry

Near-threshold boron neutron capture therapy (BNCT) is an accelerator-based concept that produces neutrons from thick lithium targets using proton beam energies only tens of keV above the 7Li(p,n)7Be reaction threshold. Proton energies in this range lead to lower neutron yields than a higher proton energy, such as 2.5MeV, but lower energy neutrons are produced and hence less moderation is required. This allows thinner moderators that place the patient closer to the neutron source. A summary is presented here of calculations and experiments that have been performed that demonstrate the feasibility of near-threshold neutron sources for BNCT. A model for predicting nearthreshold differential neutron yields from thick targets of lithium metal and lithium compounds was developed. Neutron yields from this model were used as neutron sources for Monte Carlo (MCNP) simulations of a head phantom. Calculated dose components were experimentally verified using an acrylic phantom. Initial dose calculations using treatment planning software, which indicate near-threshold neutron sources are competitive with existing reactor BNCT beams, are also presented.

Accelerator neutron sources for neutron capture therapy using near-threshold charged particle reactions

Compact neutron sources for neutron capture therapy hold the promise of permitting wide availability for this therapeutic modality for cancer treatment. Most accelerator based neutron source concepts for this purpose are centered on (p,n) reactions using bombarding energies several hundred keV to 1-2 MeV above the reaction threshold producing high neutron yield. The neutron energies in the range of hundreds of keV to 1-2 MeV require considerable moderation and/or filtration, which reduces the output epithermal neutron flux, and fast neutron contamination is always present. Operating with proton energies closer to the threshold decreases neutron yield but allows for smaller, more efficient filters and moderators, which results in less reduction of the epithermal flux by the moderator/filter assembly. Work by this collaboration is examining the balance between total neutron yield and filter/modulator efficiency in order to achieve intense epithermal beams with low fast neutron contamination. As the first stage of this project, neutron yield and spectrum measurements on 9Be(p,n) and 7Li(p,n) reactions have been made and the results will be presented along with neutronic calculations for these systems. A radio frequency quadrupole accelerator is being used in this work.