Thermal Neutron Flux Depression Research Articles

Determination of the 10B Dose in BNCT Using LiF-TLD

The 10B dose in boron neutron capture therapy (BNCT) was usually determined by multiplying the thermal neutron flux by the 10B concentration and the dose conversion factor. In this kind of application, the thermal neutron flux was commonly measured using gold foil activation techniques with and without the cadmium cover, assuming that the neutron spectrum has a Maxwellian distribution in the thermal range. This always generated uncertainties because the thermal neutron energy spectrum has no Maxwellian distribution in the body. The potential to determine the 10B dose by using a single LiF thermoluminescent dosimeter (TLD) is studied.The 10B dose in BNCT derives from the reaction of the thermal neutron with the 10B element. It always dominates the irradiation dose if the 10B concentration is higher than 20 ppm. Since the trends of the 10B absorption cross sections are similar to 6Li in the thermal neutron range, the LiF-TLD can be used for 10B dose determination in BNCT if the reaction of the thermal neutron with 6Li dominates the TLD response. The MCNP code is used to simulate the energy deposition in various LiF-TLDs and to show the suitability of LiF-TLD used for 10B dose determination in BNCT.The preliminary MCNP simulation shows that the TLD response strongly depends on the 6LiF content in the TLD. Comparing the TLD response, the 10B reaction, and the thermal neutron flux, they show the same distribution as a function of depth in a phantom irradiated with the BNCT neutron. On the other hand, not only is there a thermal neutron flux depression due to self-shielding within the TLD chip, but also there is significant perturbation around the TLD if the 6LiF content in the TLD is high enough. To balance these two factors, TLD-100 was recommended as a 10B dosimeter for BNCT.

The measurement of thermal neutron flux depression for determining theconcentration of boron in blood

Boron neutron capture therapy (BNCT) is a form of targeted radiotherapy thatrelies on the uptake of the capture element boron by the volume to be treated.The treatment procedure requires the measurement of boron in the patient'sblood. The investigation of a simple and inexpensive method for determiningthe concentration of the capture element 10B in blood is describedhere. This method, neutron flux depression measurement, involves thedetermination of the flux depression of thermal neutrons as they pass througha boron-containing sample.It is shown via Monte Carlo calculations and experimental verification that,for a maximum count rate of 1×104 counts/s measured by thedetector, a 10 ppm 10B sample of volume 20 ml can be measured with astatistical precision of 10% in 32±2 min. For a source activity ofless than 1.11×1011 Bq and a maximum count rate of less than1×104 counts/s, a 10 ppm 10B sample of volume 20 mlcan be measured with a statistical precision of 10% in 58±3 min. Ithas also been shown that this technique can be applied to the measurement ofthe concentration of any element with a high thermal neutron crosssection such as 157Gd.

Boron self-shielding effects on dose delivery of neutron capture therapy using epithermal beam and boronophenylalanine.

Previous dosimetry studies for boron neutron capture therapy have often neglected the thermal neutron self-shielding effects caused by the 10B accumulation in the brain and the tumor. The neglect of thermal neutron flux depression, therefore, results in an overestimation of the actual dose delivery. The relevant errors are expected to be more pronounced when boronophenylalanine is used in conjunction with an epithermal neutron beam. In this paper, the boron self-shielding effects are calculated in terms of the thermal neutron flux depression across the brain and the dose delivered to the tumors. The degree of boron self-shielding is indicated by the difference between the thermal neutron fluxes calculated with and without considering a 10B concentration as part of the head phantom composition. The boron self-shielding effect is found to increase with increasing 10B concentrations and penetration depths from the skin. The calculated differences for 10B concentrations of 7.5-30 ppm are 2.3%-8.3% at 2.3 cm depth (depth of the maximum brain dose) and 4.6%-17% at 7.3 cm depth (the center of the brain). The additional self-shielding effects by the 10B concentration in a bulky tumor are investigated for a 3-cm-diam spherical tumor located either near the surface (3.3 cm depth) or at the center of the brain (7.3 cm depth) along the beam centerline. For 45 ppm of 10B in the tumor and 15 ppm of 10B in the brain, the dose delivered to the tumors is approximately 10% lower at 3.3 cm depth and 20% lower at the center of the brain, compared to the dose neglecting the boron self-shielding in transport calculations.

Accounting for the Thermal Neutron Flux Depression in Voluminous Samples for Instrumental Neutron Activation Analysis

At the Delft University of Technology Interfaculty Reactor Institute, a facility has been installed to irradiate cylindrical samples with diameters up to 15 cm and weights up to 50 kg for instrumen...

Multiparameter neutron activation analysis

An advanced neutron activation technique has been developed for the accurate analysis of elemental and isotopic fissile material required in nuclear safeguards, nuclear material standardization and other applications. It is based on reactor neutron flux spectrum differentiation by cadmium screening and multistandard calibration, including the solution of a second order equation system or of computerized calibration curve fitting, taking into account the thermal neutron flux depression. Some discrepancies at high enrichments have still to be eliminated in order to achieve the required measurement accuracy.

Inelastic neutron scatter iron concentrations of the Moon from orbital gamma ray data

A set of regional Fe values were obtained from the inelastic scatter peak of the Apollo 15 and 16 orbital gamma ray spectra. These Fe (n, n′ γ) values were determined from a reaction that does not depend upon thermal neutrons and should therefore be a test of thermal neutron flux depression. No consistent effects of thermal neutron flux depression are apparent. The majority of the higher energy Fe (n, γ) values are substantiated by the Fe (n, n′ γ) results; differences of more than one sigma between iron (n, n′ γ) values and Fe (n, γ) values are attributed to the difficulty in stripping the variation in natural radioactivity from the inelastic scatter Fe band. The Fe (n, γ) data set is, therefore, shown to be reliable and to reflect the actual Fe variation on the lunar surface.

Comments on “Thermal-Neutron Flux Depression in Cylindrical UO2Fuel Rods”

Comments on “Thermal-Neutron Flux Depression in Cylindrical UO2Fuel Rods”

Reply to “Comments on Thermal-Neutron Flux Depression in Cylindrical UO2 Fuel Rods”

Reply to “Comments on Thermal-Neutron Flux Depression in Cylindrical UO2 Fuel Rods”

Further Comments on “Thermal-Neutron Flux Depression in Cylindrical UO2Fuel Rods”

"Further Comments on “Thermal-Neutron Flux Depression in Cylindrical UO2 Fuel Rods”." Nuclear Technology, 50(1), p. 110

Thermal-Neutron Flux Depression in Cylindrical UO2Fuel Rods

The thermal-neutron flux depression was measured in cylinders containing materials that mock up various enrichments of uranium dioxide. Six chemical compounds were mixed in various proportions to match the macroscopic absorption and scattering cross sections and the average cosine of the scattering angle for UO/sub 2/. Cylinder diameter was varied from 0.635 to 1.27 cm (0.25 to 0.50 in.) and the mockup enrichment was varied from 5 to 100%. For enrichments up to 20%, the measured flux depression was greater than that predicted by simple diffusion theory. At higher enrichments, diffusion theory significantly overestimates the flux depression.

Neutron Depth Dose Measurements in a Tissue-Equivalent Phantom for an Incident Pu-Be Spectrum

An elliptically shaped polythene phantom, filled with tissue-equivalent material, was exposed to collimated beams of fast neutrons from a Pu-Be source. Absorbed doses were measured at depth in the phantom for beams incident on both the major and minor axes. A Bragg-Gray type detector, made from plastic phosphor and polyvinyltoluene sheet, wasused to measure the proton recoil absorbed dose from the fast neutrons and a LiI(Eu) crystal to detect the thermal neutrons. Various correction factors to account for thermal neutron flux depression and self shielding from the crystal, neutron attenuation by the probe, etc., are discussed in detail. The absorbed dose at depth in tissue due to the 14N(n,p)14C, 1H(n,γ)2D reactions were derived from the corrected fluxes.The experimental doses are compared with the theoretical values of Snyder and Neufeld for broad beam irradiations, and with various other published results. Quality factors are determined and dose equivalents calculated.

Re: “Thermal Neutron Flux Depression by Absorbing Foils” and “Flux Perturbations by Thermal Neutron Detectors”

"Re: “Thermal Neutron Flux Depression by Absorbing Foils” and “Flux Perturbations by Thermal Neutron Detectors”." Nuclear Science and Engineering, 11(4), pp. 451–452

Thermal Neutron Flux Depression by Absorbing Foils

The perturbation of a thermal neutron flux field by an absorbing foil is considered for the case of a foil of thickness t and of lateral dimensions>>L, where L is the diffusion length of thermal neutrons in the medium. The inte-gral equation for one-velocity transport of neutrons in the medium containing the foil is solved by a variational method in which the eigenvalue is closely related to the foil activation. The results are compared with the predictions of the Bothe and Skyrme theories. The Bothe and Skyrme theories are compared for the case of the finite disk-shaped foil and are shown to differ primarily in the transport correction. This difference may be important in cases where L is not very large compared with the mean free path of neutrons in the medium. On the basis of these considerations, a new analytic approximation for the activation of a finite foil is proposed. (auth)