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.     

Neutron dose rate evaluation for medical linear accelerators

During X-ray therapeutic irradiation with energies above the threshold of (X,n) reactions in the structural materials of medical accelerators, a photoneutron fluence is generated. In Brazil, no measurements of neutron doses in radiotherapy rooms are being done yet, when licensing these equipment. Consequently, it is very important to obtain accurate analytical formulae and/or simulation of these dose rates, in order to estimate the increase in dose received by the patient and staff, as well as to correctly project the additional shielding for the treatment room. In this work, we present MCNP simulation of dosimetric quantities at the isocentre of some models of high-energy linear accelerators, and compare it with the values given by the manufacturers, finding good agreement between both.     

Monte Carlo simulation of a medical linear accelerator for radiotherapy use

A Monte Carlo code MCNPX (Monte Carlo N-particle) was used to model a 25 MV photon beam from a PRIMUS (KD2-Siemens) medical linear electron accelerator at the Centre Antoine Lacassagne in Nice. The entire geometry including the accelerator head and the water phantom was simulated to calculate the dose profile and the relative depth-dose distribution. The measurements were done using an ionisation chamber in water for different square field ranges. The first results show that the mean electron beam energy is not 19 MeV as mentioned by Siemens. The adjustment between the Monte Carlo calculated and measured data is obtained when the mean electron beam energy is approximately 15 MeV. These encouraging results will permit to check calculation data given by the treatment planning system, especially for small fields in high gradient heterogeneous zones, typical for intensity modulated radiation therapy technique.     

Evaluation of the photoneutron field produced in a medical linear accelerator

The doses and spectra of photoneutrons produced in a medical linear accelerator with photon energies of 10 and 15 MV were evaluated. The Monte Carlo code, MCNPX, was used to simulate the transport of these photoneutrons around the head for 10 and 15 MV photons. The fully-described geometry of the accelerator head was used in this calculation. The photoneutron energy spectra and doses for various photon field sizes were calculated at each of 20 positions. The results indicate that the maximum dose equivalents are observed in 20 x 20 cm(2) case among photon fields. It was found the neutron average energy at isocenter for a 0 x 0 cm(2) field is 0.38 MeV for 10 MV and is 0.45 MeV for 15 MV. The neutron doses at 10 positions around the head in the treatment room of the operation facility at 10 and 15 MV were measured using the bubble detectors. Measurements were compared with the calculations under the same geometry in the experiment. It was found that the majority of the calculated results agreed to within the standard deviations of the measurements. These above results can be applied in the verification of maximum allowed neutron leakage percentage of treatment dose defined in the IEC. We have been employing them to derive the empirical formula for neutron dose equivalent level at the maze entrance of medical accelerator treatment rooms in a study that is still underway.     

Absolute neutron emission rate measurement of a 252Cf source by the manganous sulfate bath method

A standard program for the absolute neutron emission rate measurement of a ²⁵²Cf source by the manganous sulfate bath method has been completed for the standardization of neutron source measurements. The MnSO₄ circulation system enables the ⁵⁶Mn activity to be measured during activation of the bath by the neutron source. The neutron capture cross section ratio of hydrogen to manganese has been determined to be 0.02506 by varying the manganese concentration in the MnSO₄ solution. From the graphical analysis of a straight line fitted to the six sets of data, the observed neutron emission rate was 2.544 × 10⁷ n/s for the ²⁵²Cf source at the reference date, January 1, 1987.     

The content of 250Cf and 248Cm in 252Cf neutron sources and the effect on the neutron emission rate

One of the most common radionuclide neutron sources used for the calibration of detectors is (252)Cf. However, these sources also contain (250)Cf, which is present in the material from which the sources are made, and (248)Cm, which is formed as the daughter of (252)Cf via alpha-decay. Both decay by spontaneous fission with longer half-lives than (252)Cf. Consequently, as the source becomes older, the emission rate does not follow the decay curve of (252)Cf. Fits have been made to emission rate measurements of (252)Cf sources at NPL spanning over 30 y to deduce their (250)Cf and (248)Cm content. The emission rate of a source can be significantly underestimated if the presence of (250)Cf and (248)Cm is not taken into account, and this has been investigated for a typical (252)Cf source. The importance of this problem to other calibration laboratories and users of (252)Cf sources is emphasised.     

Measurements of the neutron dose near a 15 Mv medical linear accelerator

In this work, simplified recombination methods for routine estimation of dose equivalent in mixed (gamma and neutrons) radiation field outside the irradiation field of linear medical accelerators is considered. The author's earlier reported method of H(10) measurements, involving determination of the recombination index of radiation quality, Q(4) by tissue-equivalent recombination chamber was combined with the new method for determination of the photon to neutron dose ratio D(X)/D(n) from the ratio of ion collection efficiencies measured in the investigated radiation field and in two reference fields of gamma and neutron radiations. The method is suitable when the neutron contribution to the total absorbed dose, D(n)/D, is >3%.     

Shielding for a cyclotron used for medical isotope production in China

Monte Carlo and discrete ordinate calculations have been performed to determine the doses at several locations in a positron emission tomography (PET) facility in China, where the radiation source is a cyclotron that is used for the production of the isotopes necessary for PET scans. The energy-dependent neutron source term is obtained by calculations using the ALICE code, and is interpolated for input to Monte Carlo and discrete ordinate calculations. The building that houses the cyclotron has a labyrinth of walls to minimise dose to operators and to other occupants of the building. Unbiased Monte Carlo calculations did not converge after more than one week of CPU time, whereas direction biasing alone resulted in convergence in several days. A study of several biasing techniques indicated that about a factor of 3 in computational efficiency is obtained using evaluated biasing methods. The use of adjoint fluxes for biasing Monte Carlo calculations can improve computational efficiencies by one or two orders of magnitude for some problems.     

Co-crystallization for enhanced dissolution rate of bicalutamide: preparation and evaluation of rapidly disintegrating tablets

Objectives: Enhance the dissolution rate of bicalutamide via co-crystallization with sucralose (sweetener), with the aim to develop rapidly disintegrating tablets with subsequent prompt dissolution.

Significance: Bicalutamide is antiandrogenic agent for the treatment of prostate cancer but has low and variable oral bioavailability, mainly attributed to poor dissolution. Co-crystallization with benign excipients is promising for dissolution enhancement with the additive serving dual functions. The benefit will become greater if dissolution enhancement is associated with the development of orodispersible tablets which is suitable for elderly patients who are the most vulnerable for prostate cancer.

Methods: Bicalutamide was dissolved in acetone in the presence of increasing molar ratios of sucralose. The solvent was evaporated while mixing to deposit crystals that were subjected to wet co-grinding until drying. The developed solids were characterized using Fourier transform infrared spectroscopy, differential thermal analysis and X-ray diffraction in addition to monitoring bicalutamide dissolution.

Results: Instrumental analysis provided evidences for co-crystallization which was initiated at 1:1 molar ratio of bicalutamide to sucralose with complete co-crystallization at 1:4 molar ratio. The co-crystals provided faster bicalutamide dissolution compared with the unprocessed drug and that recrystalized from acetone in the absence of sucralose. The formulation containing bicalutamide with sucralose at 1:4 molar ratio was selected for tablet formulation into which superdisintegrants were included. The developed tablets exhibited flash disintegration with subsequent fast dissolution of bicalutamide.

Conclusions: The study introduced co-crystallization of bicalutamide with sucralose as an efficient tool to enhance the dissolution rate and to develop rapidly dissolving tablets for intraoral administration.     

Variation of the synchronous phase in a multichannel accelerator for light ions

Measurements on acceleration of intense, low-energy ion beams with two multichannel acceleration structures are presented. While in the first structure the absolute value of the synchronous phase is kept constant like in standard linear accelerators, it is monotonously decreased in the second structure to improve the ion capture efficiency. In both structures a maximum He⁺-ion current of 2 mA per channel has been accelerated from 40 to 120 keV. The bunch forming process and the particle losses are studied by means of simulations.     

Corrosion rate of steel in concrete: evaluation of confinement techniques for on-site corrosion rate measurements

Earlier on-site investigations and laboratory studies have shown that varying corrosion rates are obtained when different commercially available instruments are used. The different confinement techniques, rather than the different electrochemical techniques used in the instruments, are considered to be the main reason for the discrepancies. This paper presents a method for the quantitative assessment of confinement techniques. The assessment is based on monitoring the operation of the corrosion rate instrument and following the current distribution between the electrode assembly on the concrete surface and a segmented reinforcement bar embedded in the concrete. The applicability of the method was demonstrated for two commercially available corrosion rate instruments based on different confinement techniques. The method provided an explanation of the differences in performance of the two instruments. Correlated measurements of linear polarisation resistance and macro-cell currents allowed the determination of calibration factors. Both instruments overestimated the corrosion rate of passive reinforcement, but underestimated the corrosion rate of reinforcement with intense localised corrosion.     

A two-step method for the evaluation of corrosion rate in metals

We analyze the negative influence of the polarization capacity of corroding electrodes on the accuracy of evaluation of the corrosion rate. Due to the influence of deposits, the impedance of the electrode has an abnormally high capacity [(10⁴ - 10⁵) μF/cm²]. The difficulties encountered in measuring the polarization resistance and, hence, the corrosion rate in the process of galvanostatic polarization are caused by the uncertainty of the time of complete charging of the capacity and, hence, the time of termination of measurements. We deduce the analytic dependence of the true value of corrosion rate on various polarization currents in the process of measurements and check it with the help of an experimental model of the source of direct current with galvanostatic polarization and recording the polarization voltage on the equivalent circuit of two-electrode corrosion transducer. The results obtained by using the proposed two-step method are independent of the polarization capacity and the duration of the transition process, whereas the period of measurements is much shorter than the period of complete (98%) charging of the polarization capacity and admissible for the corrosion measurements.     

Analysis and evaluation of a novel quality assurance device for ultrasonic medical imaging systems

A novel test device that has the potential to facilitate day-to-day maintenance and quality assurance of ultrasonic medical imaging systems was developed and tested. The device consists of small echogenic particles deposited on a thin film substrate in patterns similar to those found on conventional eye charts, which can be scanned to produce recognizable images that can be quantitatively evaluated in a quick and repeatable fashion. An evaluation system consisting of this device and a related assessment methodology was tested at several locations by both physicians and sonographers. The results of the testing and responses of the users were both encouraging and insightful.     

Amidinothiourea as a secondary accelerator in a binary system for the sulphur vulcanization of natural rubber

A study of a binary accelerator system using amidinothiourea as a secondary accelerator with mercaptobenzothiazyl disulfíde (MBTS), in sulphur vulcanization of natural rubber has been undertaken. This system was found to boost MBTS accelerator activity considerably. From the results, a nucleophilic reaction mechanism was established for this system. Different compositions of the amidinothiourea were tried in different mixes using standard recipes, and reference mixes were also vulcanized. Different cure characteristics of the mixes and important physical properties of the vulcanizates were evaluated. Optimum concentrations of amidinothiourea required were derived and the experimental mixes were found to have better cure characteristics, including an advantageous reduction in the optimum cure time. Many of the physical properties of the vulcanizates studied were also found to be better compared to those of the reference mixes. Evaluation of these properties both before and after ageing showed good retention of these properties as well. An attempt was also made to correlate the variation in physical properties to chemical crosslink formation in the various vulcanizates.     

Spray coating as a powerful technique in preparation of solid dispersions with enhanced desloratadine dissolution rate

Solid dispersion systems have been widely used to enhance dissolution rate and oral bioavailability of poorly water-soluble drugs. However, the formulation process development and scale-up present a number of difficulties which has greatly limited their commercial applications. In this study, solid dispersions (SDs) of desloratadine (DSL) with povidone (PVP) and crospovidone (cPVP) were prepared by spray coating technique. The process involved the spray application of 96% ethanol solution of DSL and PVP/cPVP, and subsequent deposition of the coprecipitates onto microcrystalline cellulose pellets during drying by air flow in a mini spray coater. The results from the present study demonstrated that the spray coating process is efficient in preparing SDs with enhanced drug dissolution rate and it is highly efficient in organic solvent removal. Both PVP and cPVP greatly improved drug dissolution rate by SDs, with PVP showing better solubilization capability. Very fast drug dissolution rate is achieved from SDs containing PVP regardless of differences in K grade. SD with smaller particles of cPVP have higher drug dissolution rate in comparison to the cPVP with larger particles. Results from physical state characterization indicate that DSL in SDs exist in the amorphous (high free-energy) state which is probably stabilized by PVP/cPVP. After 6-month accelerated stability study, DSL remains amorphous, while PVP and cPVP act as anti-plasticizing agents, offering efficient steric hindrance for nucleation and crystal growth.