Microdosimetric spectra of the THOR neutron beam for boron neutron capture therapy

A primary objective of the BNCT project in Taiwan, involving THOR (Tsing Hua Open Pool Reactor), was to examine the potential treatment of hepatoma. To characterise the epithermal neutron beam in THOR, the microdosimetry distributions in lineal energy were determined using paired tissue-equivalent proportional counters with and without boron microfoils. Microdosimetry results were obtained in free-air and at various depths in a PMMA phantom near the exit of the beam port. A biological weighting function, dependent on lineal energy, was used to estimate the relative biological effectiveness of the beam. An effective RBE of 2.7 was found at several depths in the phantom.     

Determination of the neutron fluence spectra in the neutron therapy room of KIRAMS

High energy proton induced neutron fluence spectra were determined at the Korea Institute of Radiological and Medical Sciences (KIRAMS) using an extended Bonner Sphere (BS) set from the Korea Atomic Energy Research Institute (KAERI) in a series of measurements to quantify the neutron field. At the facility of the MC50 cyclotron of KIRAMS, two Be targets of different thicknesses, 1.0 and 10.5 mm, were bombarded by 35 and 45-MeV protons to produce six kinds of neutron fields, which were classified according to the measurement position and the use or no use of a beam collimator such as the gantry of the neutron therapy unit. In order to obtain a priori information to unfold the measured BS data the MCNPX code was used to calculate the neutron spectrum, and the influence of the surrounding materials for cooling the target assembly were also reviewed through this calculation. Some dosimetric quantities were determined by using the spectra determined in this measurement. Dose equivalent rates of these neutron fields ranged from 0.21 to 5.66 mSv h(-1)nA(-1) and the neutron yields for a thick Be target were 3.05 and 4.77% in the case of using a 35 and a 45-MeV proton, respectively.     

Spectrometer equipment for neutron spectra measurements in mixed neutron/photon fields

The paper describes spectrometer equipment in the IPPE experimental laboratory for neutron spectra measurement in mixed (n,γ) fields. The laboratory was founded in 1957 and it occupies a leading position in the field of nuclear facilities radiation spectrometry and benchmark experiments in Russia. Spectrometer equipment includes spectrometers based on the organic stilbene scintillator, hydrogen counter and Bonner balls. Basic fields of spectrometer application are mixed radiation neutron spectra measurement of radionuclide sources, of nuclear reactors and accelerators; study of neutron transfer through the material, including benchmark experiments and measurement of neutron spectra in the rooms of nuclear facilities.     

The neutron spectra of nuclear facilities as the superposition of physically validated spectra

Neutron energy spectra in the energy range 10⁻¹⁰–18 MeV for 100 neutron fields of nuclear reactors and neutron generators are re-established. A method of forming the a priori spectrum in the form of the superposition of physically validated neutron spectra is used in the calculation. __________ Translated from Izmeritel’naya Tekhnika, No. 6, pp. 60–66, June, 2006.     

The measurement of neutron and neutron induced photon spectra in fusion reactor related assemblies

The spectral neutron and photon fluence (or flux) measured outside and inside of assemblies related to fusion reactor constructions are basic quantities of fusion neutronics. The comparison of measured spectra with the results of MCNP neutron and photon transport calculations allows a crucial test of evaluated nuclear data as generally used in fusion applications to be carried out.The experiments concern mixed neutron/photon fields with about the same intensity of the two components. An NE-213 scintillation spectrometer, well described by response matrices for both neutrons and photons, is used as proton-recoil and Compton spectrometer. The experiments described here in more detail address the background problematic of two applications, an iron benchmark experiment with an ns-pulsed neutron source and a deep penetration mock-up experiment for the investigation of the ITER in-board shield system. The measured spectral neutron and photon fluences are compared with spectra calculated with the MCNP code on the basis of FENDL-1 data. The agreement is better than 10% except for the photon fluence from the iron benchmark experiment, which is underestimated in the calculations by 25%.     

Fusion neutron energy spectra measured by time-of-flight spectrometers

Evaluation of measured neutron energy spectra provides information on the central ion velocity distribution; e.g. ion temperature and fast ions induced by NBI- or ICR-heating and on impurity content in fusion grade plasmas. Two self-contained high-energy resolution time-of-flight neutron spectrometers have been used at the JET- and JT-60U tokamaks during DD-operation. In the spectrometers, each neutron undergoes (n, p) elastic scattering in two sets of hydrogen based scintillators. Elapsed time between the two scatter events is measured and the energy of the neutron is evaluated. Interpretation of acquired data together with results obtained from other diagnostics yields information on central plasma parameters. Deduced ion temperatures, fast ion fractional contributions during auxiliary NBI- and ICR-heating and impurity content is presented. In general, good agreement is obtained between neutron data and data from other diagnostics.     

Measurement and unfolding of neutron spectra using Bonner spheres

Neutron spectra from Am-Be, 252Cf sources and 2H + 2H and 2H + 3H reactions have been measured using a Bonner sphere system in conjunction with a 4 x 4 mm2 LiI(Eu) detector, and unfolded using the two codes BUNKI and MAXED. The BON unfolding algorithm is used with BUNKI. It has been observed that end test percentage between 1 and 3 and smoothing factor between 0.05 and 0.1 are optimal choices for the BUNKI code. A temperature parameter 1.0 is used for MAXED. Comparison with standard spectra shows that the shape of the spectra is fairly well reproduced. A coupling between the two codes is made and the solution spectrum from BUNKI is used as the default spectrum in MAXED.     

A multiwire chamber system for measurements of neutron spectra

A new type of detector for measuring neutron flux and energy over a wide range of angles and energies is being developed. Measurements of neutron elastic and inelastic scattering as well as neutron energy continua are possible. Time-of-flight is not used for measuring outgoing neutron energy, and so for continuum measurements this system has some distinct advantages over conventional neutron detectors. Neutron energy measurement is carried out by measuring the energy and angle of the recoil proton produced by the neutron in a CH₂ converter. Spectra from ⁷Li(p, n)⁷Be at 62 MeV and ⁴⁰Ca(n, n′χ) at 65 MeV are presented.     

Self-shielding effects in neutron spectra measurements for neutron capture therapy by means of activation foils

The design and optimisation of a neutron beam for neutron capture therapy (NCT) is accompanied by the neutron spectra measurements at the target position. The method of activation detectors was applied for the neutron spectra measurements. Epithermal neutron energy region imposes the resonance structure of activation cross sections resulting in strong self-shielding effects. The neutron self-shielding correction factor was calculated using a simple analytical model of a single absorption event. Such a procedure has been applied to individual cross sections from pointwise ENDF/B-VI library and new corrected activation cross sections were introduced to a spectra unfolding algorithm. The method has been verified experimentally both for isotropic and for parallel neutron beams. Two sets of diluted and non-diluted activation foils covered with cadmium were irradiated in the neutron field. The comparison of activation rates of diluted and non-diluted foils has demonstrated the correctness of the applied self-shielding model.     

Measurement of neutron fluence spectra up to 150 MeV using a stacked scintillator neutron spectrometer

A stacked scintillator neutron spectrometer (S3N) consisting of three slabs of liquid organic scintillator is described. A pulsed beam providing a broad spectrum of neutron energies is used to determine the detection efficiency of the spectrometer as a function of incident neutron energy and to measure the pulse height response matrix of the system. Neutron spectra can then be determined for beams with any kind of time structure by unfolding pulse height spectra measured by the S3N. Examples of fluence spectrum measurements in the energy range 20-150 MeV are presented.     

Behavior of optical spectra of leucosapphire crystals under neutron irradiation

The paper addresses the dose and temperature dependences of the formation and decoloration of visible-spectrum absorption bands for leucosapphire monocrystals subjected to treatment in a reactor core; they reveal special behavior of the material’s optical spectra as a function of flux and temperature. When taking readings of the above-mentioned functions during the high-temperature isochronal annealing, the author has identified the main stages and substages of decoloration at 460, 570, 620 nm and detected the negative annealing of these bands at some temperatures. The defect formation process has been found to proceed in stages; the activation energies and concentration of the respective centers have been determined; an assumption is made as to the nature of these centers. It has been established that the radiation kinetics of generation of a color center at 460 nm and photoluminescence at 540 nm is represented by a power law; it is assumed that there might be a single mechanism responsible for inducing these bands. The author discusses the factors that may affect the behavior of the dose dependence of the luminescence intensity with increasing neutron fluence.     

Effect of neutron irradiation on optical spectra of sapphire crystals

We have studied the effects of neutron irradiation (fluences from 5 × 10¹⁵ to 5 × 10¹⁹ cm^?2) and thermal annealing on the optical spectra of sapphire crystals and the generation and annealing of visible luminescence and absorption centers in the crystals. The radiation-induced color centers with λ = 460, 570, 620, and 780 nm have been shown to be annealed in steps at temperatures from 70 to 1150°C. The process can be represented by two exponentials with activation energies of 0.05 and 0.30 eV. We have revealed antiannealing of the 460-, 570-, and 620-nm color centers at 200–300 and 300–450°C, and evaluated the thermal ionization potential U of the 400-and 460-nm color centers. The results suggest the possibility of charge transfer from the former center (U = 0.5 eV) to the latter (U = 0.2 eV). Increasing the neutron fluence from 2 × 10¹⁸ to 5 × 10¹⁹ cm^?2 is shown to increase the color center concentration by one order of magnitude. We have analyzed the nature of the radiation-induced centers and have shown that the concentrations of 460-nm absorption and 540-nm emission centers are power law functions of neutron fluence, with exponents of 0.79 and 0.81, respectively. These findings strongly suggest that the same mechanism underlies the generation of these centers.     

Determination of neutron spectra using the programs GNSR and SPECTRIX

We describe the capabilities and the application of two computer programs, which have been developed in order to facilitate common tasks in neutron spectrometry: GNSR (calculation of response matrices) and SPECTRIX (unfolding). Gas-filled Neutron Spectrometer Response calculates response functions and response matrices of various gas-filled neutron detectors. It can be configured to accommodate the appropriate gas-fillings and supports a number of different neutron beam configurations with a possibility to input calculated or measured neutron beam spectra. The program includes graphical capabilities as well as a context-sensitive help system. SPECTRIX implements several unfolding algorithms as well as support algorithms for unfolding and includes graphics capabilities and context-sensitive help. We apply both programs to a specific example: calculation of the response matrix of a ³He detector and unfolding of the neutron spectrum of a thick accelerator target using the calculated response matrix.     

Comparison between calculation and measured data on secondary neutron energy spectra by heavy ion reactions from different thick targets

Measured neutron energy fluences from high-energy heavy ion reactions through targets several centimeters to several hundred centimeters thick were compared with calculations made using the recently developed general-purpose particle and heavy ion transport code system (PHITS). It was confirmed that the PHITS represented neutron production by heavy ion reactions and neutron transport in thick shielding with good overall accuracy.     

Spallation neutron spectra measurements Part II: Proton recoil spectrometer

We present the experimental method conceived to measure high energy neutrons in the range (200 ≤ E ≤ 1600 MeV). The neutrons produce recoil protons in a liquid hydrogen converter. Momentum evaluation and identification of these protons are made by using a magnetic spectrometer equipped with plastic scintillators and three double-plane (X-Y) wire chambers. The response functions of the apparatus are determined using quasi-monoenergetic neutron beams produced by the break-up of deuterons or ³He on a Be target. The performance of the apparatus is illustrated in the form of a preliminary neutron spectrum.     

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.     

Determination of the ratios of polytypes in a silicon carbide grit using neutron diffraction

The distribution of polytypes in a sample of silicon carbide has been measured by the technique of neutron powder diffraction profile analysis. Three polytypes are shown to be present and their relative proportions have been calculated. The neutron technique was used because it was desired to examine the bulk properties of the sample and to use large (～ 1cm) samples.     

Investigation of combined unfolding of neutron spectra using the UMG unfolding codes

An investigation of the simultaneous unfolding of data from neutron spectrometers using the UMG codes MAXED and GRAVEL has been performed. This approach involves combining the data from the spectrometers before unfolding, thereby performing a single combined unfolding of all the data to yield a final combined spectrum. The study used measured data from three proton recoil counters and also Bonner sphere and proton recoil counter responses calculated from their response functions. In each case, the spectrum derived from combined unfolding is compared with either the spectrum obtained from merging the independently unfolded spectra or the spectrum used to calculate the responses. The advantages and disadvantages of this technique are discussed.