Spectrum shaping of accelerator-based neutron beams for BNCT

We describe Monte Carlo simulations of three facilities for the production of epithermal neutrons for Boron Neutron Capture Therapy (BNCT) and examine general aspects and problems of designing the spectrum-shaping assemblies to be used with these neutron sources. The first facility is based on an accelerator-driven low-power subcritical reactor, operating as a neutron amplifier. The other two facilities have no amplifier and rely entirely on their primary sources, a D-T fusion reaction device and a conventional 2.5 MeV proton accelerator with a Li target, respectively.     

A study of neutron radiation quality with a tissue-equivalent proportional counter for a low-energy accelerator-based in vivo neutron activation facility

The accelerator-based in vivo neutron activation facility at McMaster University has been used successfully for the measurement of several minor and trace elements in human hand bones due to their importance to health. Most of these in vivo measurements have been conducted at a proton beam energy (E(p)) of 2.00 MeV to optimise the activation of the selected element of interest with an effective dose of the same order as that received in chest X rays. However, measurement of other elements at the same facility requires beam energies other than 2.00 MeV. The range of energy of neutrons produced at these proton beam energies comes under the region where tissue-equivalent proportional counters (TEPCs) are known to experience difficulty in assessing the quality factor and dose equivalent. In this study, the response of TEPCs was investigated to determine the quality factor of neutron fields generated via the (7)Li(p, n)(7)Be reaction as a function of E(p) in the range 1.884-2.56 MeV at the position of hand irradiation in the facility. An interesting trend has been observed in the quality factor based on ICRP 60, Q(ICRP60), such that the maximum value was observed at E(p)=1.884 MeV (E(n)=33±16 keV) and then continued to decline with increasing E(p) until achieving a minimum value at E(p)=2.0 MeV despite a continuous increase in the mean neutron energy with E(p). This observation is contrary to what has been observed with direct fast neutrons where the quality factor was found to increase continuously with an increase in E(p) (i.e. increasing E(n)). The series of measurements conducted with thermal and fast neutron fields demonstrate that the (14)N(n, p)(14)C produced 580 keV protons in the detector play an important role in the response of the counter under 2.0 MeV proton energy (E(n) ≤ 250 keV). In contrast to the lower response of TEPCs to low-energy neutrons, the quality factor is overestimated in the range 1-2 depending on beam energy <2.0 MeV. This study provides an insight to understanding the response of TEPCs in low-energy neutron fields where the neutrons are moderated using a polyethylene moderator.     

Characterisation of an accelerator-based neutron source for BNCT versus beam energy

Neutron capture in ¹⁰B produces energetic alpha particles that have a high linear energy transfer in tissue. This results in higher cell killing and a higher relative biological effectiveness compared to photons. Using suitably designed boron compounds which preferentially localize in cancerous cells instead of healthy tissues, boron neutron capture therapy (BNCT) has the potential of providing a higher tumor cure rate within minimal toxicity to normal tissues. This clinical approach requires a thermal neutron source, generally a nuclear reactor, with a fluence rate sufficient to deliver tumorcidal doses within a reasonable treatment time (minutes). Thermal neutrons do not penetrate deeply in tissue, therefore BNCT is limited to lesions which are either superficial or otherwise accessible. In this work, we investigate the feasibility of an accelerator-based thermal neutron source for the BNCT of skin melanomas. The source was designed via MCNP Monte Carlo simulations of the thermalization of a fast neutron beam, generated by 7 MeV deuterons impinging on a thick target of beryllium. The neutron field was characterized at several deuteron energies (3.0–6.5 MeV) in an experimental structure installed at the Van De Graaff accelerator of the Laboratori Nazionali di Legnaro, in Italy. Thermal and epithermal neutron fluences were measured with activation techniques and fast neutron spectra were determined with superheated drop detectors (SDD). These neutron spectrometry and dosimetry studies indicated that the fast neutron dose is unacceptably high in the current design. Modifications to the current design to overcome this problem are presented.     

Secondary photon fields produced in accelerator-based sources for neutron generation

Neutrons can be produced with low-energy ion accelerators for many applications, such as the characterisation of neutron detectors, the irradiation of biological samples and the study of the radiation damage in electronic devices. Moreover, accelerator-based neutron sources are under development for boron neutron capture therapy (BNCT). Thin targets are used for generating monoenergetic neutrons, while thick targets are usually employed for producing more intense neutron fields. The associated photon field produced by the target nuclei may have a strong influence on the application under study. For instance, these photons can play a fundamental role in the design of an accelerator-based neutron source for BNCT. This work focuses on the measurement of the photon field associated with neutrons that are produced by 4.0-6.8 MeV protons striking both a thin 7LiF target (for generating monoenergetic neutrons) and a thick beryllium target. In both cases, very intense photon fields are generated with energy distribution extending up to several MeV.     

First experiments on neutron detection on the accelerator-based source for boron neutron capture therapy

A pilot accelerator-based source of epithermal neutrons, which is intended for wide application in clinics for boron neutron capture therapy, has been constructed at the Budker Institute of Nuclear Physics (Novosibirsk). A stationary proton beam has been obtained and near-threshold neutron generation regime has been realized. Results of the first experiments on neutron generation using the proposed source are described.     

An accelerator-based epithermal neutron beam design for BNCT and dosimetric evaluation using a voxel head phantom

The beam shaping assembly design has been investigated in order to improve the epithermal neutron beam for accelerator-based boron neutron capture therapy in intensity and quality, and dosimetric evaluation for the beams has been performed using both mathematical and voxel head phantoms with MCNP runs. The neutron source was assumed to be produced from a conventional 2.5 MeV proton accelerator with a thick (7)Li target. The results indicate that it is possible to enhance epithermal neutron flux remarkably as well as to embody a good spectrum shaping to epithermal neutrons only with the proper combination of moderator and reflector. It is also found that a larger number of thermal neutrons can reach deeply into the brain and, therefore, can reduce considerably the treatment time for brain tumours. Consequently, the epithermal neutron beams designed in this study can treat more effectively deep-seated brain tumours.     

The effects of neutron radiation on nickel-based alloys

The effects of neutron radiation on nickel-based alloys in thermal reactors are defying predictions that were made based upon accelerated testing in fast reactors. As nickel-based alloy components face significant doses in aging thermal reactors and their role in Gen-IV reactor designs becomes prominent, the literature on the effects of radiation on such alloys must be reviewed to enable better structural integrity assessments for relevant components and optimise alloys with respect to irradiation embrittlement resistance. This paper reviews the available data on the effects of radiation, notably neutron radiation, on nickel-based alloys and discusses the possible mitigation strategies and design opportunities for radiation embrittlement-resistant alloys based on recent developments in alloy computational design.

This review was submitted as part of the 2016 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.     

An electron microbeam cell-irradiation system at KIRAMS: performance and preliminary experiments

An electron microbeam cell-irradiation (EMCI) system is now ready for routine operation in Korea. The system components include an electron gun operating at 1-100 keV, a beam transport chamber delivering a micron-sized beam, a cell image acquisition and positioning part and an automatic system control section. The present choice of source beam energy is 30 keV so that the radiation impact is conveyed to the targeted cells with a minimum spatial dispersion. The beam is available at 5 microm in diameter now, but can be changed in the range of 1-200 microm. The cellular dose is delivered with a standard deviation of 30% at 0.1 Gy, 10% at 1 Gy and 3% at 10 Gy. The cells are recognised by over 98% in a 1 mm x 1 mm area and the system is capable of irradiating up to 30,000 cells h(-1).     

Beam characterisation of the KIRAMS electron microbeam system

An electron microbeam system has been installed at the Korea Institute of Radiological and Medical Sciences (KIRAMS) for use in radiation biology studies. The electron beam is produced from a commercial electron gun, and the beam size is defined by a 5 microm diameter pinhole. Beam energy can be varied in the range of 1-100 keV, covering a range of linear energy transfer from 0.4 to 12.1 keV microm-1. The micrometer-sized electron beam selectively irradiates cells cultured in a Mylar-bottomed dish. The positioning of target cells one by one onto the beam exit is automated, as is beam shooting. The electron beam entering the target cells has been calibrated using a Passivated Implanted Planar Silicon (PIPS) detector. This paper describes the KIRAMS microbeam cell irradiation system and its beam characteristics.     

X-ray microbeam and recrystallization studies of plastic deformation around fatigue cracks

X-ray microbeam technique and recrystallization method were used to measure plastic zones around 20 kHz fatigue cracks in mild steel. Both methods render approximately the same extension of a plastic zone. In order to obtain the amount of deformation within a plastic zone measurements of integral line breadth and recrystallization temperature were calibrated by specimens deformed with known per cent static tension. But since static and cyclic loading cause different dislocation structures (especially the dislocation density is much more inhomogeneous in fatigued specimens) quantitative comparison is difficult. Furthermore the two methods display different values of the actual amount of deformation. X-ray line broadening indicates an average dislocation density, whereas recrystallization displays a maximum amount of deformation, which corresponds to the local dislocation concentrations.     

An analytical model for reversible radiation effects on polymer specific heat

A study is made of the energy balance in gas release by radiation in polymers as one of the possible mechanisms for reversible effects on the specific heat.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 46, No. 5, pp, 781–789, May, 1984.     

Improvements in the system for support of the uniformity of measurements of dosimetric quantities in neutron radiation fields

The state primary standard for neutron radiation is described. It consists of measuring instruments for dose absorption rate with a set of ionization chambers and a cylindrical proportional counter and an instrument for measuring the dose equivalent rate with a set of spherical moderators, intended, in particular, for metrological support of measurements of ambient and personal dose equivalent rates used for monitoring radiation safety.     

Shielding design studies for a neutron irradiator system based on a 252Cf source.

This study aims to investigate a shielding design against neutrons and gamma rays from a source of 252Cf, using Monte Carlo simulation. The shielding materials studied were borated polyethylene, borated-lead polyethylene and stainless steel. The Monte Carlo code MCNP4B was used to design shielding for 252Cf based neutron irradiator systems. By normalising the dose equivalent rate values presented to the neutron production rate of the source, the resulting calculations are independent of the intensity of the actual 252Cf source. The results show that the total dose equivalent rates were reduced significantly by the shielding system optimisation.     

Application of conventional and microbeam synchrotron radiation x-ray fluorescence and absorption for the characterization of human nails

The synchrotron radiation based spectroscopies X-ray fluorescence and X-ray absorption fine structure are used to detect illness-related changes in the elemental distribution and bonding environment of metals in human nails. The effective atomic number of a collection of nails is determined using two methods, the X-ray transmittance and the scattering method, and is found equal to 7.5 +/- 0.5. X-ray fluorescence maps of the elemental distributions, recorded with a lateral resolution of 5 microm, reveal that S, Ca and Zn are distributed homogeneously while Fe tends to cluster. In the Fe-rich clusters, which have a diameter in the range 15-30 microm, the Fe concentration is 10 times higher than in the matrix. The Zn K edge X-ray Absorption Fine Structure spectra reveal that Zn, in the nails from healthy donors and patients suffering from lung diseases, is four-fold coordinated with N and S and the Zn-N and Zn-S distances are equal to 2.03 A and 2.25 A, respectively. Finally the signature of various bonds in the C-, O- and N- K near edge X-ray absorption fine structure spectra is discussed.     

New calculations of the atmospheric cosmic radiation field--results for neutron spectra

The propagation of primary cosmic rays through the Earth's atmosphere and the energy spectra of the resulting secondary particles have been calculated using the Monte Carlo transport code FLUKA with several novel auxiliary methods. Solar-modulated primary cosmic ray spectra were determined through an analysis of simultaneous proton and helium measurements made on spacecraft or high-altitude balloon flights. Primary protons and helium ions are generated within the rigidity range of 0.5 GV-20 TV, uniform in cos2theta. For a given location, primaries above the effective angle-dependent geomagnetic cut-off rigidity, and re-entrant albedo protons, are transported through the atmosphere. Helium ions are initially transported using a separate transport code called HEAVY to simulate fragmentation. HEAVY interfaces with FLUKA to provide interaction starting points for each nucleon originating from a helium nucleus. Calculated cosmic ray neutron spectra and consequent dosimetric quantities for locations with a wide range of altitude (atmospheric depth) and geomagnetic cut-off are presented and compared with measurements made on a high-altitude aeroplane. Helium ion propagation using HEAVY and inclusion of re-entrant albedo protons with the incident primary spectra significantly improved the agreement of the calculated cosmic ray neutron spectra with measured spectra. These cosmic ray propagation calculations provide the basis for a new atmospheric ionising radiation (AIR) model for air-crew dosimetry, calculation of effects on microelectronics, production of cosmogenic radionuclides and other uses.     

Ion chamber system for neutron flux measurements

A helium-filled ion chamber detector for intensity measurements of high-intensity epithermal neutron bursts with instantaneous rates as high as 10¹¹ Hz is presented. This system consists of an ion chamber to detect a portion of the neutron beam, a current-to-frequency converter and CAMAC scalers to readout the chamber. The chambers and readout electronics have a small temperature sensitivity and have high noise immunity. The statistical precision of the system is measured to be 10⁻³ for each neutron beam pulse.     

Elastic neutron scattering studies at 96 MeV for transmutation

Elastic neutron scattering from (12)C, (14)N, (16)O, (28)Si, (40)Ca, (56)Fe, (89)Y and (208)Pb has been studied at 96 MeV in the10-70 degrees interval, using the SCANDAL (SCAttered Nucleon Detection AssembLy) facility. The results for (12)C and (208)Pb have recently been published, while the data on the other nuclei are under analysis. The achieved energy resolution, 3.7 MeV, is about an order of magnitude better than for any previous experiment above 65 MeV incident energy. A novel method for normalisation of the absolute scale of the cross section has been used. The estimated normalisation uncertainty, 3%, is unprecedented for a neutron-induced differential cross section measurement on a nuclear target. Elastic neutron scattering is of utmost importance for a vast number of applications. Besides its fundamental importance as a laboratory for tests of isospin dependence in the nucleon-nucleon, and nucleon-nucleus, interaction, knowledge of the optical potentials derived from elastic scattering come into play in virtually every application where a detailed understanding of nuclear processes is important. Applications for these measurements are dose effects due to fast neutrons, including fast neutron therapy, as well as nuclear waste incineration and single event upsets in electronics. The results at light nuclei of medical relevance ((12)C, (14)N and (16)O) are presented separately. In the present contribution, results on the heavier nuclei are presented, among which several are of relevance to shielding of fast neutrons.     

Development of a dedicated ion injector for accelerator-based nanoprobe facilities

The paper discusses possible ways of increasing beam brightness in ion injectors. The argon/helium ion injector comprising a newly designed RF ion source and, a Wien filter has been designed for use in accelerator-based nanoprobe facilities. The phase set degradation due to aberrations in the injector ion-optic system was simulated with allowance for multipole and fringing fields. The RF ion sources with different permanent magnet systems were tested. Experiments were performed with argon and helium. A plasma density of up to 3×10¹¹ cm⁻³ and beam brightness of ∼100 A/(m² rad² eV) were obtained. The ion current density inside an extracting electrode in the source was 10 mA/cm² for an emission hole diameter of 0.6 mm. Measurements of the current value and emittance were performed with ion source testing equipment permitting measurements of the ion beam current, emittance, mass composition, and RF power input into the plasma.