Gold Foil Activation Research Articles

An MCNP-based model for the evaluation of the photoneutron dose in high energy medical electron accelerators

The development of a computational model for the treatment head of a medical electron accelerator (Elekta/Philips SL-18) by the Monte Carlo code mcnp-4C2 is discussed. The model includes the major components of the accelerator head and a pmma phantom representing the patient body. Calculations were performed for a 14 MeV electron beam impinging on the accelerator target and a 10 cm×10 cm beam area at the isocentre. The model was used in order to predict the neutron ambient dose equivalent at the isocentre level and moreover the neutron absorbed dose distribution within the phantom. Calculations were validated against experimental measurements performed by gold foil activation detectors. The results of this study indicated that the equivalent dose at tissues or organs adjacent to the treatment field due to photoneutrons could be up to 10% of the total peripheral dose, for the specific accelerator characteristics examined. Therefore, photoneutrons should be taken into account when accurate dose calculations are required to sensitive tissues that are adjacent to the therapeutic X-ray beam. The method described can be extended to other accelerators and collimation configurations as well, upon specification of treatment head component dimensions, composition and nominal accelerating potential.

Performance of upgraded thermal neutron guides with supermirrors at JRR-3

Thermal neutron guides at the JRR-3 reactor have been upgraded from natural nickel mirror guides to Ni/Ti supermirror ones with two times critical angle as large as that of natural nickel mirror. We have performed the characteristics experiment of neutron beams transmitted through the supermirror neutron guides. The neutron spectra have been measured at the end of the thermal neutron guide by the time-of-flight method. It was observed that the characteristic wavelength for the neutron beam was shifted from 2 to 1.3 A ̊ . Neutron flux at the end of the thermal neutron guide was measured by the gold foil activation method. It was measured 6.11×10 11 n/m 2 s and increases 4.32 times after upgrade. We have conducted Monte Carlo simulation in consideration of the reflectivity of fabricated supermirrors and their alignment errors for the purpose to examine the performance of supermirror neutron guides.

First results with the upgraded IN5 disk chopper cold time-of-flight spectrometer

The primary spectrometer of the cold time-of-flight instrument IN5 at the ILL has been upgraded over the last 2 years. After commissioning in spring 2003 the absolute gain in flux when compared to the former spectrometer setting has been measured by gold-foil activation. Within standard configuration, namely, an incident wavelength of λ=5 A ̊ and chopper speeds chosen such that they correspond to identical energy resolution (δE≃100 μeV) and time frame, the flux has been found an order of magnitude higher on the upgraded instrument. Apart from the gain in flux the new spectrometer allows to work with appreciably improved resolution (up to a factor 2) at identical wavelength.

Performance of upgraded thermal neutron guides with supermirrors at JRR-3

Thermal neutron guides at the JRR-3 reactor have been upgraded from natural nickel mirror guides to Ni/Ti supermirror ones with two times critical angle as large as that of natural nickel mirror. We have performed the characteristics experiment of neutron beams transmitted through the supermirror neutron guides. The neutron spectra have been measured at the end of the thermal neutron guide by the time-of-flight method. It was observed that the characteristic wavelength for the neutron beam was shifted from 2 to 1.3Å. Neutron flux at the end of the thermal neutron guide was measured by the gold foil activation method. It was measured 6.11×1011n/m2s and increases 4.32 times after upgrade. We have conducted Monte Carlo simulation in consideration of the reflectivity of fabricated supermirrors and their alignment errors for the purpose to examine the performance of supermirror neutron guides.

Application of Imaging Plate to Measurement of Radiation Spatial Distribution

An imaging plate (IP) was applied to 1) detecting of low-level contamination that was difficult to detect by a conventionalway, 2) measurement of spatial distributions of thermal neutrons and 3) profile measurement of fast neutronsand proton beam of low intensity. For 1), a large-size IP (BAS-Ills) was disposed to the sample, and the position andintensity of the contamination was identified unambiguously by scanning IP. For 2), activation foils were combinedwith IP to avoid 7-rays from neutron sources and radioactive accelerator components. We placed small gold activationfoils at many points of interest in an accelerator room and disposed them on IP after irradiation. This method enables tomeasure neutron spatial distribution in a single scanning. We extended this method to profiling of fast neutron by usingan aluminum plate and proton beams with intensity too low to measure by beam current but too strong to measure bypulse counting by using a thin copper foil.

高エネルギー加速器研究機構における黒鉛パイル熱中性子校正場の開発

A thermal neutron reference field using a graphite pile of 190cm×190cm×250cm was developed at the High Energy Accelerator Research Organization. Inside the pile, the thermal neutron flux was measured by the gold foil activation technique using the Cadmium difference method, and outside the pile by the 3He proportional counter using the Cadmium difference method. Besides the gold foil, the optical fiber coupled to the ZnS(Ag) scintillator with the Li converter was used to measure the neutron flux inside the pile to verify its availability. The absolute values of thermal neutron fluxes measured inside the pile could be obtained with an accuracy of 7.5-7.8%. The measured thermal neutron fluxes inside the pile were compared with those calculated by use of the MCNP-4B Monte Carlo code. The calculated thermal neutron fluxes agreed very well with the measured ones. Outside the pile, the measured thermal neutron flux was 7.38×10-6 (n/[s·cm2·source neutron]), and the contribution of neutrons other than thermal neutrons was less than 1%. The thermal neutron fluxes measured by the optical fiber with the scintillator showed very good agreement with those measured with the gold foil activation method. The optical fiber detector was found to be quite useful to measure the thermal neutron flux distribution in the pile because of its much higher detection efficiency than that of gold foil activation.

Genetic effects induced by neutrons in Drosophila melanogaster I. Determination of absorbed dose

A method to obtain the absorbed dose in Drosophila melanogaster irradiated in the thermal column facility of the Triga Mark III Reactor has been developed. The method is based on the measurements of neutron activation of gold foils produced by neutron capture to obtain the neutron fluxes. These fluxes, combined with the calculations of kinetic energy released per unit mass, enables one to obtain the absorbed doses in Drosophila melanogaster.

Characteristics of the KUR Heavy Water Neutron Irradiation Facility as a neutron irradiation field with variable energy spectra

The Heavy Water Neutron Irradiation Facility (HWNIF) of the Kyoto University Research Reactor (KUR) was updated in March 1996, mainly for the improvement in neutron capture therapy (NCT). A striking feature of the updated facility is that the energy spectrum of the neutron beam can be controlled from almost pure thermal to epi-thermal, within 5 min by remote control under a continuous reactor operation. This feature is advantageous not only to medical science such as NCT, but also to the other research fields such as physics, engineering, biology, etc. The performance of the updated facility as a neutron irradiation field with variable energy spectra, was characterized. Thermal neutron flux, cadmium ratio, gamma-ray dose rate, etc., at the normal irradiation position for various irradiation modes were determined, mainly on the basis of the measurement using gold activation foils and thermo-luminescent dosimeters (TLDs). The emphasis was on the performance of the new neutron energy spectrum shifter and cadmium thermal neutron filter, that control the mixing ratio of thermal and epi-thermal neutrons, through the change in the heavy water thickness of the spectrum shifter and the aperture size of the cadmium filter. The evaluation of neutron energy spectra at the normal irradiation position was also performed for three representative irradiation modes, in which the neutron intensities are largest of all the irradiation modes. In addition, the irradiation characteristics of two irradiation devices, namely the Irradiation Rail Device and the Remote Patient Carrier, which were updated concurrently with the facility update, were evaluated.

Flux measurements at the new swiss spallation neutron source SINQ

Abstract During the initial beam tests of SINQ in December 1996 and March/April 1997, extensive flux measurements were made at the neutron guide system and at thermal beam ports, applying gold-foil activation, Gd-conversion and imaging plate radiography, time-of-flight spectroscopy, and flux measurements at the SANS-specimen position. The tests have proven that SINQ-the first continuous spallation neutron source with a 1 MW power potential-fulfills the expectations regarding machine performance, moderation efficiency, shielding quality and, last not least, neutron flux levels. The latter compare favorably with those of currently operating medium-flux reactors. In particular, the neutron guide system of SINQ has taken full advantage of supermirror coating with considerable gains in spectral range and integral flux at the instrument positions. The flux measurements were complemented by quantitative determination of beam contaminations by γ-radiation and epithermal and high-energy neutrons, yielding satisfac...

Dosimetry and fluence measurements with a new irradiation arrangement for neutron capture therapy of tumours in mice

Incorporation of 10B in human tumours treated with fast neutrons would increase the local dose in the tumour. In tissue the neutrons are thermalized mainly causing the neutron capture reaction 10B ( n, α) 7Li. The dose enhancement can be calculated using the thermal neutron fluence measured by activation of gold foils in a phantom. At a phantom depth of 5 cm a dose enhancement of (0.056 ± 0.0028)% for each μg of 10B per gram of tissue was determined in the tumour. A therapeutic gain by this dose enhancement in fast neutron therapy should be examined for a solid tumour in a mouse. The feasibility for neutron capture therapy of tumours in mice depends on the distribution of the tumour dose and on the effective shielding of the bodies of the mice. Therefore, a special holding device for simultaneous irradiation of four tumours was developed. The dose distribution in the tumours and in the surrounding bodies of mice was measured with TLD-300 in a special mouse phantom.

Dose Equivalent Measurement for Thermal Neutrons and Gamma Rays with a Small BGO Scintillator Connected to an Optical Fibre

A small dose equivalent counter for thermal neutrons and gamma rays for measuring the dose equivalent for a patient in boron neutron capture therapy is described. The system consisted of a small BGO scintillator partially covered with a Cd chip connected to an optical fibre. The thermal neutron response of the detector wasadjusted to correspond to the ICRP value (conversion factor from thermal neutron fluence to dose equivalent) by changing the size of the Cd chip. The dose equivalent of thermal neutrons and gamma rays was simultaneously measured in free air mixed field. The observed values were compared with the values deduced from the thermal neutron fluence determined by the gold foil activation method and the gamma ray dose by TLDs.

Boron neutron capture enhancement of 252Cf brachytherapy

Dosimetric and radiobiological studies were undertaken to investigate the potential enhancement in dose, dose distribution and cell killing effectiveness of 252Cf brachytherapy achievable when boron-10 enriched compounds are incorporated into simulated 252Cf brain implants. Thermal neutron distributions in a human head phantom containing a 252Cf source were measured by gold foil activation and calculated using a 1-dimensional transport code. This information was then used to modify measured event size distributions for 252Cf neutrons to determine the corresponding increase in dose and dose equivalent throughout the phantom. The addition of subtoxic levels of boron-10 to a typical 252Cf implant was found to significantly enhance both the absorbed dose and the high LET event frequency at distances of 3 to 5 cm from individual sources. Some unexpected geometric considerations are discussed. Reduced survival of cultured Chinese hamster cells correlated with the predicted increase in absorbed dose from the capture events with a concentration of about 60 μg 10B per ml in the culture medium. It was found that boron increased a (the “single-hit” parameter of the linear quadratic survival model) by 32% and decreased β (the “double-hit” parameter) by 8%. The αβ ratio increased to 4.34 Gy in the presence of boron, from 3.03 Gy in its absence. This translated to an 8% reduction in californium dose needed to effect 10% cell survival. It is concluded that there is a sufficiently high thermal neutron fluence present during californium brachytherapy for boron neutron capture dose augmentation to be feasible.

Neutron sensitivity of vanadium self powered neutron detectors

The neutron sensitivity of vanadium self powered neutron detectors (Type 5503-B), manufactured by Studsvik, Sweden, has been measured. Using the calculational model developed by Warren, the neutron sensitivity of these detectors has been computed taking into account additional factors viz. flux depression caused by the detector and the interaction of 52V gamma rays with the emitter, the correction factors due to which have been evaluated to be 0.957 and 1.03 respectively. The evaluation of different parameters that are necessary in this connection and the neutron flux density determination at the position of irradiation of these detectors by gold foil activation, are presented. The measured and calculated values of neutron sensitivity are observed to agree to within 4%.

Error analysis in reactor-core neutron beam density measurements by gold-foil activation

Error analysis in reactor-core neutron beam density measurements by gold-foil activation

Neutron leakage measurements from a medical linear accelerator.

The McCall method has been used to measure neutron leakage from the Mevatron 77, 18- and 15-MV photon beams. Gold foil activation has been used employing a beta counting technique for the 18-MV beam and a gamma counting technique for both the 18- and 15-MV beam. The two counting techniques were used to evaluate their relative merit. The measurements were made at various locations in the patient-treatment plane for different field sizes. The results show that the thermal-neutron dose equivalent contributes only about 1%-2% of the total neutron dose equivalent. At 100 cm, the neutron dose equivalent for the 18-MV beam is approximately six times that of the 15-MV beam, slightly exceeding the 0.1% of the useful beam criteria used by some of the regulatory agencies. In light of the uncertainty in fluence to dose equivalent conversion factors, the increased dose equivalent above 0.1% is insignificant.

測定器校正用熱中性子場の製作と特性試験

A graphite pile was constructed for routine calibration of radiation protection instruments. An isotropic thermal neutron irradiation field can be set up at the cavity in the pile. Also a collimated thermal neutron beam irradiation field can be set up at the outside facing the rectangular hole of pile. The sizes of irradiation fields are 200×276×170mm3 and 160×160mm2 respectively. Thermal neutron fluences are measured by the gold foil activation method. The over all error of fluence measurements is less than±2%. Intercomparison between ETL and JAERI on thermal neutron fluence rate measurements was done and the results have agreed within experimental errors. Distribution of fluence rate in the irradiation field are uniform with ±3%.

An absolute measurement of the efficiency of a BF 3 proportional counter

We report an absolute measurement of the efficiency of a BF 3 counter in narrow beam geometry in the energy range frame 0.024 eV to 0.111 eV. Agreement with calculated efficiency values is excellent. No significant end effects are noted. The basic feature of this experiment is the measurement of the incident neutron flux by gold foil activation and counting in an NaI well counter which was calibrated with gold foils irradiated in the National Bureau of Standards Standard Thermal Neutron Flux.

Corrections in the gold foil activation method for determination of neutron beam density

A finite foil thickness and deviation in the cross section from the 1 ν law imply corrections in the determination of neutron beam densities by means of foil activation. These corrections, which depend on the neutron velocity distribution, have been examined in general and are given in a specific example for a reactor beam transmitted through a 30 cm Bi filter. The effective cross section differs 0.5% from the capture cross section at 2200 m/s. For a 20 mg/cm 2 Au foil the correction for beam attenuation and hardening through the foil is 0.7% and the activity correction is 1.5%.

Activation of Gold Spheres and Foils in Nonisotropic Neutron Incidence

Gold spheres and foils were activated in a nonisotropic neutron flux in the center of boxes that were cadmium-lined on five sides and located in the graphite thermal column of the JUGGERNAUT reacto...

A Method for Measuring Absolute Reactor Power through Neutron Fluctuation

A new principle is presented for obtaining absolute reactor power by processing the random fluctuation of neutron flux based on the stochastic nature of nuclear reactions. The required combination of instruments to carry out experiments is described, and experimental results obtained in a swimming pool reactor are reported. The power spectral density of the output current of an ion chamber located near the reactor core is determined by reactor kinetic parameters such as delayed neutron yield, life time, ν (mean number of neutrons generated per fission) and counter efficiency as well as by the total number of neutrons in the core, which is a measure of absolute power. Using either logarithmic amplifier or reactivity meter, absolute reactor power can be measured without any information about detector efficiency. This method has such merits as easiness and simplicity in operation, ability to measure absolute power in the range 0.01∼100 W where other methods are inapplicable, and negligible effect of changes in core configuration or in detector position. The results of actual reactor experiments with this method proved to agree fairly well with those of absolute measurement by gold foil activation.