ESTIMATION OF NEUTRON SCATTER CORRECTION FOR CALIBRATION OF PERSONNEL DOSIMETER AND DOSERATEMETER AGAINST 241Am-Be SOURCE-MONTE CARLO SIMULATION AND MEASUREMENTS.

Neutron scatter contributions as a function of distance along the transverse axis of 241Am-Be source were estimated by three different methods such as shadow cone, semi-empirical and Monte Carlo. The Monte Carlo-based FLUKA code was used to simulate the existing room used for the calibration of CR-39 detector as well as LB6411 doseratemeter for selected distances from 241Am-Be source. The modified 241Am-Be spectra at different irradiation geometries such as at different source detector distances, behind the shadow cone, at the surface of the water phantom were also evaluated using Monte Carlo calculations. Neutron scatter contributions, estimated using three different methods compare reasonably well. It is proposed to use the scattering correction factors estimated through Monte Carlo simulation and other methods for the calibration of CR-39 detector and doseratemeter at 0.75 and 1 m distance from the source.

Study on the measurement of photo-neutron for15 MV photon beam from medical linear accelerator under different irradiation geometries using passive detectors.

AIM OF STUDY: The photo-neutron dose equivalents of 15 MV Elekta precise accelerators were measured for different depths in phantom, for various field sizes, at different distances from the isocenter in the patient plane and for various wedged fields. MATERIALS AND METHODS: Fast and thermal neutrons are measured using passive detectors such as Columbia Resin-39 and pair of thermoluminescent dosimetry (TLD) 600 and TLD 700 detector from Elekta medical linear accelerator. RESULTS: It is found that fast photo-neutron dose rate decreases as the depth increases, with a maximum of 0.57 ± 0.08 mSv/Gy photon dose at surface and minimum of 0.09 ± 0.02 mSv/Gy photon dose at 15 cm depth of water equivalent phantom with 10 cm backscatter. Photo neutrons decreases from 1.28 ± 0.03 mSv/Gy to 0.063 ± 0.032 when measured at isocenter and at 100 cm far from the field edge along the longitudinal direction in the patient plane. Fast and thermal neutron doses increases from 0.65 ± 0.05 mSv/Gy to 1.08 ± 0.07 mSv/Gy as the field size increases; from 5 cm × 5 cm to 30 cm × 30 cm for fast neutrons. With increase in wedge field angle from 0° to 60°, it is observed that the fast neutron dose increases from 0.42 ± 0.03 mSv/Gy to 0.95 ± 0.05 mSv/Gy.s CONCLUSIONS: Measurements indicate the photo-neutrons at few field sizes are slightly higher than the International Electrotechnical Commission standard specifications. Photo-neutrons from Omni wedged fields are studied in details. These studies of the photo-neutron energy response will enlighten the neutron dose to radiation therapy patients and are expected to further improve radiation protection guidelines.

Applications of a bis-urea phenylethynylene self-assembled nanoreactor for [2 + 2] photodimerizations.

Confined environments can be used to alter the selectivity of a reaction by influencing the organization of the reactants, altering the mobility of trapped molecules, facilitating one reaction pathway or selectively stabilizing the products. This manuscript utilizes a series of potentially photoreactive guests to interrogate the utility of the one-dimensional nanochannels of a porous host to absorb and facilitate the reaction of encapsulated guests. The host is a columnar self-assembled phenylethynylene bis-urea macrocycle, which absorbs guests, including coumarin, 6-methyl coumarin, 7-methyl coumarin, 7-methoxy coumarin, acenaphthylene, cis-stilbene, trans-stilbene, and trans-ß-methylstyrene to afford crystalline inclusion complexes. We examine the structure of the host:guest complexes using powder X-ray diffraction, which suggests that they are well-ordered highly crystalline materials. Investigations using solid-state cross-polarized magic angle spinning (13)C{(1)H}CP-MAS NMR spectroscopy indicate that the guests are mobile relative to the host. Upon UV-irradiation, we observed selective photodimerization reactions for coumarin, 6-methyl coumarin, 7-methyl coumarin, and acenaphthylene, while the other substrates were unreactive even under prolonged UV-irradiation. Grand Canonical Monte Carlo simulations suggest that the reactive guests were close paired and preorganized in configurations that facilitate the photodimerization with high selectivity while the unreactive guests did not exhibit similar close pairing. A greater understanding of the factors that control diffusion and reaction in confinement could lead to the development of better catalysts.

Self-assembled phenylethynylene bis-urea macrocycles facilitate the selective photodimerization of coumarin.

There is much interest in designing molecular sized containers that influence and facilitate chemical reactions within their nanocavities. On top of the advantages of improved yield and selectivity, the studies of reactions in confinement also give important clues that extend our basic understanding of chemical processes. We report here, the synthesis and self-assembly of an expanded bis-urea macrocycle to give crystals with columnar channels. Constructed from two C-shaped phenylethynylene units and two urea groups, the macrocycle affords a large pore with a diameter of ∼9 Å. Despite its increased size, the macrocycles assemble into columns with high fidelity to afford porous crystals. The porosity and accessibility of these channels have been demonstrated by gas adsorption studies and by the uptake of coumarin to afford solid inclusion complexes. Upon UV-irradiation, these inclusion complexes facilitate the conversion of coumarin to its anti-head-to-head (HH) photodimer with high selectivity. This is contrary to what is observed upon the solid-state irradiation of coumarin, which affords photodimers with low selectivity and conversion.

Macrocycles with switchable exo/endo metal binding sites.

We report herein the synthesis and metal complexation properties of two macrocyclic hosts that contain two 2,2'-bipyridines and two urea groups. These hosts take advantage of the conformationally mobile 5,5'-positions of the bipyridines to give metal binding sites that are dynamic. By simple bond rotation, these hosts can exchange an interior (endo) situated metal binding site for an exterior (exo) binding site. We examine the solid-state structures of the two free hosts and two coordination complexes ([Cd(host 1)(H(2)O)(NO(3))(2)] and [Ag(2)(host 2)](SO(3)CF(3))(2)) using X-ray crystallography. Analysis of these crystal structures suggests that the bipyridine groups within the hosts are able to rotate to access multiple conformations including the desired exo and endo conformations. We also investigate the binding affinity of these new ligands in solution by UV-vis titrations with a series of metal nitrate salts (Ag, Cd, Zn, Ni, Mn, Fe, Co, Cr, and Cu) to afford discrete metal complexes. Some complexes showed a slow subsequent assembly to yield coordination polymers. Thus, these systems may afford unique insights into the process of metal organic framework formation.