Ravi Bhandari

Positron annihilation study of post-irradiation annealing of defects in α-irradiated molybdenum

Abstract Measurements of positron lifetime and Doppler broadening have been used to study the nature of radiation damage and defect annealing in α-irradiated molybdenum. Vacancies, dislocation/vacancy loops and microvoids were detected in as-irradiated samples. Positron trapping at these defects was predicted to be near saturation by the model proposed for the trapping process. The behaviour on annealing showed vacancy migration and formation of vacancy clusters in the region 150–500°C. Above 650°C microvoid coarsening to form voids was observed in two stages. Dislocation/vacancy loops started to anneal out at ∼650°C, but significant recovery of them was observed above 950°C. Various aspects regarding the effect of helium on defect annealing, positronium (Ps) formation in metal voids and its decay scheme are discussed along with a brief review of previous investigations. The results indicate that Ps decays primarily from a singlet state in clean voids, whereas a triplet state decay is possible in the pres...

Defect studies in alpha-irradiated and deformed niobium by positron annihilation

Abstract The positron annihilation technique has been used to study the recovery of defects in alpha-irradiated Nb. A comparative study on plastically deformed Nb has also been performed. An antirecovery behaviour of Doppler broadening lineshape parameter, S , is observed only in alpha-irradiated specimens between 350–600°C. This has been identified to arise from the presence of He and to the formation of He-vacancy complexes. A further sharp rise in S occurring in both irradiated and deformed specimens at about 600°C is presumed to be due to the release of vacancies from dissociating vacancy-impurity complexes. Above 650°C, S decreased towards the reference value. In irradiated specimens this recovery is seen till 950°C in two stages, i.e. rearrangement in the configuration and dissociation of He-vacancy complexes, respectively.

Assessment of Corticotomy Facilitated Tooth Movement and Changes in Alveolar Bone Thickness - A CT Scan Study

INTRODUCTION Corticotomy is an effective method of accelerating the orthodontic treatment. The aim of this study was to compare the treatment time for the extraction space closure, between corticotomy assisted and conventional orthodontic tooth movement and to check the alveolar bone thickness before and after corticotomy procedure in the corticotomy group. SETTINGS AND DESIGN Cross-sectional clinical study. MATERIALS AND METHODS Twenty patients (age>15 y) requiring orthodontic treatment with upper anterior retraction in the extraction space of 1(st) premolar were selected and were randomised into control and corticotomy group each group consisted of 10 subjects. Pre retraction, corticotomy was performed in the maxillary anterior segment. The pre and post retraction CT scans were recorded and the thickness of the alveolar plates were measured at crestal level (S1), mid root level (S2) and apical level (S3) PreTreatment (T1). The same measurements were repeated after incisor retraction was completed PostTreatment (T2). STATISTICAL ANALYSIS Students t-test, Pearson correlation coefficient. RESULTS There was a significant difference in retraction time (days) between control and corticotomy groups (p<0.001). Also, there were significant difference in total alveolar bone thickness at the crest region for all the four incisor teeth (p<0.05). A significant difference was observed in total alveolar bone thickness at the S2 and S3 level for 11, 21 and 11, 12 and 22 (p<0.05) respectively. CONCLUSION Alveolar corticotomies not only accelerates the orthodontic treatment but, also provides the advantage of increased alveolar width to support the teeth and overlying structures.

Vacuum System of the Large Cyclotrons at VECC

Variable Energy Cyclotron Centre (VECC) has two large cyclotrons, K-130 cyclotron and K-500 cyclotron. The first beam in the room temperature K-130 cyclotron (RTC) was accelerated in June 1977. The cyclotron accelerated and delivered alpha and proton beams consistently to the cyclotron users for several years. Heavy ion beams were available in this cyclotron from 1997 to 2007. Presently, the cyclotron is working as a primary source for RIB production. The cyclotron has an acceleration chamber volume of about 28 m3. The total length of beam line is about 65 m. Vacuum of the order of 1 x 10−6 mbar is presently maintained in the cyclotron and beam line using diffusion pumps. It is one of the largest vacuum systems operating in India. It is consistently being operated 24 x 7 round the year giving beam to the cyclotron users. A K-500 superconducting cyclotron (SCC) with K=520 has been constructed at Kolkata. SCC will be used to accelerate beams to 80 MeV/A for light heavy ions and about 10 MeV/A for medium mass heavy ions. Three turbo molecular pumps are connected to the acceleration chamber. Three cryopanels placed inside the lower dees in the valley gap of the superconducting magnet are available in the accelerating chamber for achieving high vacuum. The acceleration chamber having a volume of about 1.0 m3 was operated using turbomolecular pumps, liquid nitrogen cooled panels and liquid helium cooled cryopanels at different stages during beam commissioning. Differential pumping is provided across the RF liner to avoid distortion. The first beam line of about 21 m has been installed in the cyclotron. The outer vacuum chamber of the cyclotron magnet cryostat has active pumping. The vacuum system of the superconducting cyclotron is also operating reliably round the clock throughout the year. The paper describes the details of the vacuum systems of the large cyclotrons at VEC Centre Kolkata India, its commissioning and operating experience.

Shear Bond Strength of Ceramic Brackets with Different Base Designs: Comparative In-vitro Study

INTRODUCTION Knowledge about the Shear Bond Strength (SBS) of ceramic brackets with different base design is essential as it affects bond strength to enamel. AIM The aim of the present study was to evaluate and compare the effect of base designs of different ceramic brackets on SBS, and to determine the fracture site after debonding. MATERIALS AND METHODS Four groups of ceramic brackets and one group of metal brackets with different base designs were used. Adhesive precoated base of Clarity Advanced (APC Flash-free) (Unitek/3M, Monrovia, California), microcrystalline base of Clarity Advanced (Unitek/3M, Monrovia, California), polymer mesh base of InVu (TP Orthodontics, Inc., La Porte, IN, United States), patented bead ball base of Inspire Ice (Ormco, Glendora, California), and a mechanical mesh base of Gemini Metal bracket (Unitek/3M, Monrovia, California). Ten brackets of each type were bonded to 50 maxillary premolars with Transbond XT (Unitek/3M). Samples were stored in distilled water at room temperature for 24 hours and subsequently tested in shear mode on a universal testing machine (Model 3382; Instron Corp., Canton, Massachusetts, USA) at a cross head speed of 1mm/minute with the help of a chisel. The debonded interface was recorded and analyzed to determine the predominant bond failure site under an optical microscope (Stereomicroscope) at 10X magnification. One way analysis of variance (ANOVA) was used to compare SBS. Tukeys significant differences tests were used for post-hoc comparisons. The Adhesive Remnant Index (ARI) scores were compared by chi-square test. RESULTS Mean SBS of microcrystalline base (27.26±1.73), was the highest followed by bead ball base (23.45±5.09), adhesive precoated base (20.13±5.20), polymer mesh base (17.54±1.91), and mechanical mesh base (17.50±2.41) the least. Comparing the frequency (%) of ARI Score among the groups, chi-square test showed significantly different ARI scores among the groups (χ2 = 34.07, p<0.001). CONCLUSION Different base designs of metal and ceramic brackets influence SBS to enamel and all were clinically acceptable.

K-130 Cyclotron vacuum system

The vacuum system for K-130 cyclotron has been operational since 1977. It consists of two sub-systems, main vacuum system and beam line vacuum system. The main vacuum system is designed to achieve and maintain vacuum of about 1 × 10−6 mbar inside the 23 m3 volume of acceleration chamber comprising the Resonator tank and the Dee tank. The beam line vacuum system is required for transporting the extracted beam with minimum loss. These vacuum systems consist of diffusion pumps backed by mechanical pumps like roots and rotary pumps. The large vacuum pumps and valves of the cyclotron vacuum system were operational for more than twenty five years. In recent times, problems of frequent failures and maintenance were occurring due to aging and lack of appropriate spares. Hence, modernisation of the vacuum systems was taken up in order to ensure a stable high voltage for radio frequency system and the extraction system. This is required for efficient acceleration and transportation of high intensity ion beam. The vacuum systems have been upgraded by replacing several pumps, valves, gauges and freon units. The relay based control system for main vacuum system has also been replaced by PLC based state of the art control system. The upgraded control system enables inclusion of additional operational logics and safety interlocks into the system. The paper presents the details of the vacuum system and describes the modifications carried out for improving the performance and reliability of the vacuum system.

Doppler broadening studies on alpha-irradiated molybdenum

Abstract The positron annihilation technique (PAT) was used to study the annealing behaviour of defects prodeuced by 30.0 MeV alpha-irradiation in molybdenum. The small increase in the Doppler broadening lineshape parameter, S, in the temperature region 150–300°C is assigned to the migration of vacancies and the formation of vacancy clusters. In the temperature region 300–750°C S showed a prominent decrease. this later recovery can be seen in two steps, from 300°C to 550°C and from 650°C to 750°C, indicating two different types of defects were present. The first step can be assigned to the agglomeration of vacancy clusters whereas the second step may be due to the thermal coarsening of microvoids or due to He bubble growth.