[en] Purpose: To obtain an improved precise gamma efficiency calibration curve of HPGe (High Purity Germanium) detector with a new comprehensive approach. Methods: Both of radioactive sources and Monte Carlo simulation (CYLTRAN) are used to determine HPGe gamma efficiency for energy range of 0–8 MeV. The HPGe is a GMX coaxial 280 cm"3 N-type 70% gamma detector. Using Momentum Achromat Recoil Spectrometer (MARS) at the K500 superconducting cyclotron of Texas A&M University, the radioactive nucleus "2"4 Al was produced and separated. This nucleus has positron decays followed by gamma transitions up to 8 MeV from "2"4 Mg excited states which is used to do HPGe efficiency calibration. Results: With "2"4 Al gamma energy spectrum up to 8MeV, the efficiency for γ ray 7.07 MeV at 4.9 cm distance away from the radioactive source "2"4 Al was obtained at a value of 0.194(4)%, by carefully considering various factors such as positron annihilation, peak summing effect, beta detector efficiency and internal conversion effect. The Monte Carlo simulation (CYLTRAN) gave a value of 0.189%, which was in agreement with the experimental measurements. Applying to different energy points, then a precise efficiency calibration curve of HPGe detector up to 7.07 MeV at 4.9 cm distance away from the source "2"4 Al was obtained. Using the same data analysis procedure, the efficiency for the 7.07 MeV gamma ray at 15.1 cm from the source "2"4 Al was obtained at a value of 0.0387(6)%. MC simulation got a similar value of 0.0395%. This discrepancy led us to assign an uncertainty of 3% to the efficiency at 15.1 cm up to 7.07 MeV. The MC calculations also reproduced the intensity of observed single-and double-escape peaks, providing that the effects of positron annihilation-in-flight were incorporated. Conclusion: The precision improved gamma efficiency calibration curve provides more accurate radiation detection and dose calculation for cancer radiotherapy treatment.

[en] Drought stress (DS) usually causes a serious yield reduction in maize production. Nitrogen (N) has been reported to be able to alleviate DS damage in previous studies; however, which N form is beneficial for plant growth in drought-stress maize and its mechanism is still poorly understood. The objective of this study is to examine the effects of different nitrogen forms on growth, photosynthesis and stem juice characteristics of maize (Zea mays L.) under drought, and find out better and convenient measurers for drought resistance ability of the C/sub 4/ crop. Pot experiments were carried out using two maize cultivars (Zhengdan 958 and Jundan 20) under integrated root-zone drought stress (IR-DS) by irrigating 10% PEG-6000 solution and non-DS conditions grown for 33 days, and their tested indexes were determined at elongation stage. Dry matter (DM), net photosynthetic rate (Pn), stomatal conductance (gs) and oxidation-reduction potential (ORP) in stem juice were decreased while pH, electric conductivity (EC), soluble protein concentration (SPC) and ABA concentration (ABAC) in stem juice increased of both cultivars in all N forms treatments under IR-DS above non-DS. However, the responses of two examined cultivars to DS were different: significantly higher DM production, Pn and drought index (DI) were observed for ZD958 than JD20, therefore the former could be treated as a drought tolerance cultivar comparatively. Better correlations were obtained amongst the above parameters, especially for DM, Pn and pH, EC, ORP, ABAC in stem juice under IR-DS than non-DS. By comparison with sole ammonium (NH/sub 4/sup +/), sole nitrate (NO/sub 3//sup -/) and the mixed supply of NH/sub 4/sup +/ and NO/sub 3/ both obviously improved DM and Pn as well as pH, EC, SPC and ABAC whereas decreased Gs and ORP in both drought-stressed cultivars. The effects of NO/sup -3/ supplied were more evident than NH/sub 4/+ + NO/sub -3/ supplied in the above responses. These impacts were superior in ZD958 than JD20. Further analysis of variation indicated that the impact of N form treatment on almost all parameters measured were, in general, less than those of water regime. It is, therefore, concluded that pH, EC and ORP in stem juice could be regarded better and convenient indicators for evaluating drought resistance ability of the C/sub 4/ crop. The NO/sub 3/ supplied could be more beneficial for enhancing photosynthesis and plant growth by improving stem juice characteristics than NH/sub 4/sup +/ supplied, especially for a drought tolerant cultivar. (author)

[en] In this study, we investigated the effects of exogenously applied nitric acid (NO) source i.e., sodium nitroprusside (SNP), NO scavenger i.e., 2-(4-carboxypheny-4,4,5,5-tetramethylimidazoline-1-1oxyl-3-oxide, potassium salt (cPTIO), NO inhibitor i.e., NaN 3, and NOS inhibitor i.e., N-nitro-larginine methyl ester hydrochloride (L-NAME) on eglycinbetaine (GB) metabolism in root tips and leaves of maize seedlings under drought stress (DS). The accumulation of NO peaked earlier than that of GB in shoot tips and leaves. The maximum NO content in root tips was attained earlier than that in leaves, while the reverse was observed with respect to GB content. The concentrations of NO, GB and choline in root tips and leaves of maize seedlings under DS were greater in the plants which received exogenous SNP application as compared to those in plants without SNP application. Exogenous SNP application also increased betaine aldehyde dehydrogenase (BADH) activity in leaves. The GB metabolism was negatively influenced by exogenous applications of either PTIO, NaN/sub 3/ or L-NAME. The combined application of NaN/sub 3/ and L-NAME had the most serious negative effects. These results demonstrate that GB metabolism in drought stressed maize plan ts was significantly enhanced by exogenous NO application. The accumulation of greater level of NO in maize seedlings contributed to increase GB accumulation by regulating BADH activity and choline content. This study provides a direct evidence of regulation of GB metabolism in maize plants under drought stress by exogenous application of NO. (author)

[en] Foliar urea has been proved to act a better role in alleviation of the negative effects of drought stress (DS). However, the modulation mechanism of foliar urea are not conclusive in view of nitric oxide (NO) burst and glycine betaine metabolism and their relationship. Two maize ( Zea mays L.) cultivars (Zhengdan 958, JD958, Jundan 20, ZD20) were grown in hydroponic medium, which were treated with spraying of urea concentration of 15 g L/sup -1/ and two water regimes (non-stress and DS simulated by the addition of polyethylene glycol (PEG, 15% w/v, MW 6000). The ten-day DS treatment increased betaine aldehyde dehydrogenase (BADH) activity, choline content and nitric oxide (NO) content acted as the key enzyme, initial substrate and a nitrogenous signal substance respectively in GB synthesis metabolism, thus, induced to great GB accumulation. The accumulation of NO reached the summit earlier than that of GB. The more positive/less negative responses were recorded in JD958 as compared with ZD20 to DS. Addition of foliar ur ea could increase accumulation of choline and BADH activity as well as NO content, thereby, increase GB accumulation under DS. These positive effects of urea applying foliarly on all parameters measured were more pronounced in cultivar JD20 than those in ZD958 under drought. It is, therefore, concluded that increases of both BADH activity and choline content possibly resulted in enhancement of GB accumulation. Foliar urea application could provoke better GB accumulation by modulation of GB metabolism, possibly mediating by NO burst as a signal molecule during drought, especially in the drought sensitive maize cultivar. (author)

[en] Optical toroidal resonance in structural variations of the double-disk metamaterial, in viewpoint of an equivalent LC-circuit model, is numerically studied to optimize its geometrically-dependent optical characteristics, such as the enhancement capability of local-field confinement and the extinction coefficient. Responses of high-order magnetic vortex resonances (i.e., the double- and triple-fold vortex modes), in addition to the fundamental toroidal dipole, are excited in accordance with a standing-wave model of oscillating current. The induced strengths for these vortex responses are sensitive, in different manners, to the incident angle of the light, though their frequencies are angularly independent.

[en] A partial perpendicular anisotropy and a reproducible inverted minor hysteresis loop with a negative remanence were observed in Ta/NiFe/Ta. The minor inverted hysteresis loop appears only when the magnetic field is perpendicular to the film plane normal or deviates at several degrees from the normal of the film plane. This phenomenon is ascribed to the antiferromagnetic-like coupling between the co-existent minor magnetic moment with perpendicular anisotropy at the Ta/NiFe interface and major magnetic moment with in-plane anisotropy in the bulk of NiFe layer

[en] We report the ferromagnetic resonance studies on patterned NiFe films with arrays of submicron elements. It is shown that the shape anisotropy and nonuniform magnetization excitations vary with decreasing size of the patterned elements systematically

[en] Ferromagnetic resonance linewidth, ΔH, of polycrystalline Ta50 A/Ni₈₀Fe₂₀/Ta50 A films and patterned sandwich structures (NiFeCo60 A/Cu30 A/NiFeCo30 A) of submicron size was studied. ΔH_parallel is found to be in the range of 32-41 Oe for single-layer films with thickness larger than 100 A, which seems to be mainly due to intrinsic damping. For films thinner than 100 A, ΔH_parallel increases sharply, approaching a peak around 50 A and then drops. For patterned films, ΔH_parallel is found to increase with the decrease of element size. Magnetization measurements show a similar trend between coercivity and the element size

[en] Purpose: Elekta Infinity is the one of the latest generation LINAC with unique features. Two Infinity LINACs are recently commissioned at our institution. The dosimetric and mechanical characteristics of the machines are presented. Methods: Both Infinity LINACs with Agility MLC (160 leaves with 0.5 cm leaf width) are configured with five electron energies (6, 9, 12, 15, and 18 MeV) and two photon energies (6 and 15 MV). One machine has additional photon energy (10 MV). The commissioning was performed by following the manufacturer's specifications and AAPM TG recommendations. Beam data of both electron and photon beams are measured with scanning ion chambers and linear diode array. Machines are adjusted to have the dosimetrically equivalent characteristics. Results: The commissioning of mechanical and imaging system meets the tolerances by TG recommendations. The PDD₁₀ of various field sizes for 6 and 15 MV shows < 0.5% difference between two machines. For each electron beams, R₈₀ matches with < 0.4 mm difference. The symmetry and flatness agree within 0.8% and 0.9% differences for photon beams, respectively. For electron beams, the differences of the symmetry and flatness are within 1.2% and 0.8%, respectively. The mean inline penumbras for 6, 10, and 15 MV are respectively 5.1±0.24, 5.6±0.07, and 5.9±0.10 mm for 10x10 cm at 10 cm depth. The crossline penumbras are larger than inline penumbras by 2.2, 1.4, and 1.0 mm, respectively. The MLC transmission factor with interleaf leakage is 0.5 % for all photon energies. Conclusion: The dosimetric and mechanical characteristics of two Infinity LINACs show good agreements between them. Although the Elekta Infinity has been used in many institutions, the detailed characteristics of the machine have not been reported. This study provides invaluable information to understand the Infinity LINAC and to compare the quality of commissioning data for other LINACs

[en] The fundamental magnetism of the Fe film of 4 ML thick epitaxially grown on GaAs(100) was studied using ex situ ferromagnetic resonance (FMR) at room temperature. In parallel geometry the resonance fields were found to be strikingly different along [0 bar 11] (2170 Oe) and [011] (3360 Oe) direction, exhibiting a large uniaxial anisotropy with the easy and hard directions along [0bar 11] and [011], respectively. For perpendicular geometry the data of FMR show an abnormally low resonance field of 5050 Oe. It implies a pronounced reduction of effective magnetization. This can be explained by a combination of the shape anisotropy of the nanoclusters with their large perpendicular anisotropy. The g factor of these nanoclusters is even smaller than the bulk value. The linewidth is also obviously anisotropic, 220 Oe in easy direction, [0bar 11] and smaller by 50% in hard direction, [011]. [copyright] 2001 American Institute of Physics