Teflon Discs Research Articles

The energy dependence of CaF/sub 2/:Mn/Teflon thermoluminescent dosimeters

Thermoluminescent dosimeters (TLDs) which consist of Teflon disks, 0.635 cm in diameter by 0.038 cm thick, containing 5% CaF/sub 2/:Mn in the form of 20- mu m grains distributed throughout the volume, are convenient, disposable, single-reading dosimeters which are frequently used in radiation hardness testing. Their photon energy response differs markedly from that of solid CaF/sub 2/:Mn TLDs for photon energies below approximately 500 keV. An accurate model of the energy response is required for the application of such dosimeters to testing at moderate energy flash X-ray machines (E/sub e/ approximately=1.0-1.5 MeV). The energy response of these dosimeters was measured using heavily filtered X-rays from a 300-keV constant potential X-ray generator and calculated for photon energies from 10 to 500 keV using the transport code ACCEPT. The results indicate good agreement between measurements and calculations. >

Custom overdenture retainer

A simple technique for fabricating stud overdenture retainers in the form of custom attachment castings with teflon retention discs has been described. This system allows for selective placement of the retention sphere on the root surface and control of retention through the size of the retention sphere. Using transfer location projections, exact transfer of the castings to the working model is possible. When necessary, the teflon discs may easily be replaced at the chairside.

Response Characteristics of CaSO<sub>4</sub>:Dy Thermoluminescence Dosemeter Badge to Diagnostic X Rays and Dose Evaluation Procedures

The diagnostic X ray (50-120 kVp) response characteristics of a TLD (thermoluminescence dosemeter) badge currently in use for countrywide personnel monitoring of radiation workers in India is described. The badge consists of three CaSO4:Dy Teflon discs, below: (i) Cu+Al filters, (ii) Perspex, and (iii) an open window. Its response underneath the protective lead apron(s) as well as its angular dependence are reported. Two different dose evaluation procedures are described and their relative accuracies are discussed.

Airlift systems for in vivo NMR spectroscopy of plant tissues

Maintaining an adequate supply of oxygen and other nutrients is a problem that arises whenever NMR is applied to living systems (I). The problem can be acute in NMR studies of plant tissues (2, 3) because the highly vacuolated nature of many plant cells, with the cytoplasmic fraction often occupying only 5-20% of the tissue volume (4) means that it is often necessary to work with densely packed samples to detect the cytoplasmic metabolites. Clearly the desire to alleviate the sensitivity problem by increasing the sample fresh weight must be balanced against the increasing difficulty of maintaining the tissue in a satisfactory physiological state. The high cell density required for good quality spectra makes the oxygenation of plant cell suspensions particularly difficult and many of the oxygenation schemes that have been described for animal and bacterial cells (5) are unsuitable for plant cells. Oxygenation by bubbling air or oxygen into the suspension is only satisfactory at low cell densities and the 3’P spectra that have been recorded from plant cells maintained in this way have been characterized by poor signal-to-noise (6-8). An alternative approach, which has the advantage that it is easy to change the composition of the suspending medium, is to circulate an oxygenated solution through the NMR tube, but while this works well for excised plant tissues (9) its application to cell suspensions is less straightforward. Good quality spectra have sometimes been obtained from plant cell suspensions by this method (10) but the tendency for the cells to pack near the exit tube can make it difficult to establish a reliable circulation and to ensure uniform oxygenation throughout the sample. Similar problems have been encountered when using a circulating system to oxygenate plant cells in solenoidal, rather than Helmholtz, probeheads: the method worked well when the cells were immobilized in agarose or alginate, but it was much less satisfactory when the cells were suspended in free solution (1 I, 12). An ingenious solution to the problems of circulation at high cell densities has been reported (13) in which the cells were effectively immobilized by compression at a very high cell density between two perforated Teflon discs and then oxygenated by forcing a solution through the tube at 50 ml min-‘. This method gave very good spectra (I3), but it is arguable that the behavior of the cells in this system could be affected by the extreme conditions imposed on the tissue.

Torsional Buckling Analysis and Damage Tolerance of Graphite/Epoxy Shafts

The measured torsional buckling load of Graphite/Epoxy shafts is found in good agree ment with theoretical predictions based on a general shell theory that includes elastic cou pling effects and transverse shearing deformations. The direction of the applied torque and the lay-up stacking sequence drastically affects the buckling load (up to 80%). Transverse shearing deformations are found significant when the number of circumferential waves in the buckling pattern is larger than 3. The residual strength of shafts with holes is also measured, and the dominant failure mode remains torsional buckling. The buckling mode patterns are seemingly unaffected by damage but buckling loads are lower compared to un damaged specimens. Material failure concentrated around the hole does not occur until the hole size is approximately one-third of the shaft's diameter. Teflon disks inserted between plies to simulate delaminations decrease the buckling load, but not significantly. In con clusion, stiffness characteristics, rather than strength characteristics dominate the behavior of thin-walled, undamaged shafts under torsional load, and this appears to remain true for shafts with sizable damage (up to about one-third of the shaft's diameter).

On-target deposition of aerially applied deltamethrin

Abstract The mean amount of deltamethrin deposited on natural substrates (i.e., soil and vegetation) in three experiments was 63, 73, and 76% of applied. Composite samples taken from 16 sites/replicate gave representative and repeatable measures of deposition. The use of Teflon discs as artificial targets to measure aerial deposition was investigated. The discs had only 34–71% mean trapping efficiency for the spray droplets. Deposition was quite variable among different sampling sites within a field (CV=73–83%). Much of this variation was caused by irregular overlapping of spray swaths.

Studies on the effect of grain size on the response of a CaSO4:Dy Teflon disc dosemeter for monoenergetic electrons and beta rays

CaSO4:Dy Teflon discs are fabricated and used in the author's laboratory for routine personal monitoring and research. CaSO4:Dy powder and Teflon are mixed in the ratio 1:3 by weight. The discs are 13.5 mm in diameter and 0.8 mm thick. The response of this radiation detector is studied using cavity theory. CaSO4 is considered to be the cavity and Teflon the medium. The relationship between the absorbed dose in the cavity to that in the medium is calculated using Burlin's (1966) expression and Almond's (1970) modified expression.

Evaluation of repair of mechanical perforations of the pulp chamber floor using biodegradable tricalcium phosphate or calcium hydroxide

The effects of three materials on the biological repair of a defect created by perforating the pulp chamber floor of mandibular posterior teeth in dogs was studied clinically and histologically. The three materials used were tricalcium phosphate, calcium hydroxide, or Teflon discs. The calcium hydroxide caused a more destructive reaction than did the Teflon or tricalcium phosphate groups. The size of the tooth in relation to the size of the perforation was directly proportional to the prognosis; larger teeth had the best results.

Adsorption isotherms of insulin onto various materials.

Insulin adsorption isotherms on various materials have been measured to begin to clarify the role of material surface energy in insulin aggregation. Using 125I-insulin and direct gamma-counting of the exposed material, more insulin (per unit area) was adsorbed to hydrophobic materials (Teflon, Silastic) than to the hydrophilic ones (polyacrylamide, glass) from insulin solutions ranging from 0.1 to 100 U/ml. For example, after 30 min at room temperature, Teflon disks adsorbed 4.0 X 10(4) microU/cm2 and glass beads adsorbed 3.0 X 10(3) microU/cm2 from a phosphate-buffered, 100-U/ml solution (pH 7.4). The Teflon value exceeded, by a factor of six, an estimated plateau surface concentration based on the molecule area (750 A2), suggesting the occurrence of multilayer adsorption. Changing the buffer to acetate and lowering the pH to 3.5 resulted in an increased surface concentration, while the addition of glutamic or aspartic acid at pH 3.5 reduced the surface concentration to be comparable with that observed on glass and less than that observed on Teflon from phosphate buffer. Increasing the temperature to 37 degrees C resulted in a small decrease in adsorption, consistent with the exothermic adsorption of other proteins on similar materials. However, the significance of the adsorbed molecules in nucleating aggregate formation relative to other contributing factors remains to be assessed.

Some dosimetric parameters of MgB 4O 7:Dy sintered pellets and CaSO 4:Dy Teflon discs—A comparative study

The effect of u.v. and sunlight exposures on MgB 4O 7:Dy and CaSO 4:Dy have been studied. Both phosphors are found to have intrinsic sensitivity to u.v. radiation and their γ ray induced TL fades faster if exposed to sunlight. The intrinsic TL response for an u.v. (253.7 nm) exposure of 1500 Jm −2 was found equivalent to that caused by 1 and 5 mGy of 60Co γ rays for MgB 4O 7:Dy and CaSO 4:Dy, respectively. The fading caused by 48000 lx of sunlight exposure was 99.7% and 56% in MgB 4O 7:Dy pellets and CaSO 4:Dy Teflon TLD discs respectively. MgB 4O 7:Dy pellets did not exhibit any added advantage over CaSO 4:Dy Teflon discs except its near tissue equivalence.

The effect of ultra-violet and visible radiation on CaSO4:Dy in Teflon discs used for personnel gamma-radiation monitoring.

The influence of ultra-violet and visible radiation (UV/VIS) on the read-out obtained from the main dosimetry peak of CaSO4:Dy in Teflon discs exposed to the gamma-radiation levels measured by a personnel-monitoring service has been studied as a function of wavelength, irradiance and radiant exposure. The results have been explained by the net effect of three separate factors, namely (i) an inherent response, (ii) a phototransfer and (iii) an optical bleaching. The response obtained from unirradiated discs illuminated with fluorescent room lighting became saturated after 16 hrs at a value corresponding to 0.33 microC kg-1 whilst gamma-irradiated discs faded to 90% of their initial exposure for the same illumination period.

Photon energy dependence of self-fabricated TLD-CaSO4 : Dy/Teflon discs.

Thermoluminescent dosimeter-CaSO4/Teflon discs were prepared by the authors. Series of experiments were applied to investigate the photon energy dependence of these TLD discs. The results showed that the energy dependence of TLD discs is almost the same as that of TLD powder. The decrease of the proportion of TL phosphor embedded in teflon does reduce the sensitivity but not affect the energy dependence of TLD discs unless the grain size of TL phosphor is smaller than several microns. A personnel beta-gamma dosimeter in connection with proper filters was designed. The photon energy dependence can be corrected and the effects of beta and gamma can be distinguished in this dosimeter.

Low dose measurements with CaSO 4:Dy Teflon dosimeters

For low dose measurements with CaSO 4:Dy Teflon dosimeters, the flushing of N 2 during readout reduces the spurious TL from heater strip, as well as the storage time dependent spurious TL from CaSO 4:Dy Teflon discs. The N 2 flow is also essential to attain linearity in TL response vs γ-exposure. With N 2 flow, the performance of these dosimeters in environmental monitoring has been found highly satisfactory.

Light Induced Fading in CaSO4:Dy Teflon Thermoluminescence Discs

The light induced fading in CaSO4:Dy Teflon discs exposed to intense sunlight and fluorescent tube light is reported.

Ten-year pathological study of Beall mitral valve prostheses.

Detailed pathological studies are reported on 20 cloth-covered Beall mitral valve prostheses, which had to be removed for valve failure nine months to 10 years after implantation. Results in all patients except one indicated that there was a progressive and persistent granulomatous reaction that continued up to at least 10 years. Although the rate of tissue ingrowth into the valve decreased with time, the total volume of tissue formation increased. Cloth wear and disruption and deformity of the teflon discs was frequently seen. In these 20 valves the desired result--namely, the complete and smooth endothelialisation of the prosthesis--was certainly not achieved.

Response of Sensitised CaSO<sub>4</sub>:Dy Phosphor and Phosphor Embedded Teflon TLD Discs to X and Gamma Rays

The response of CaSO4:Dy phosphor embedded Teflon TLD discs to photons of effective energy in the range from 8 keV to 1.25 MeV has been studied. The dependence of TL sensitivity and glow curve structure of sensitised phosphor on LET is discussed. The sensitised CaSO4:Dy Teflon discs exhibit reduced photon energy dependence in the energy range from 8 to 30 keV mainly due to the high LET of low energy secondary electrons (in CaSO4, the mean stopping power secondary electrons of 8 keV (Eff) photons is about 9 keV µm-1 and of 60Co photons is about 0.55 keV µm-1). For photons above 30 keV the LET of secondary electrons and the lower ZEff of Teflon jointly contribute to the reduction of energy dependence.

The Effects of Photon Energy and Irradiation History on the Efficiency of Dose Re-Estimation in CaSO<sub>4</sub>:Dy Teflon Discs Using Repeated Read-outs

The Effects of Photon Energy and Irradiation History on the Efficiency of Dose Re-Estimation in CaSO<sub>4</sub>:Dy Teflon Discs Using Repeated Read-outs

Precision thermoluminescence dosimetry using CaSO 4 : Dy teflon dosimeters

Repeated use of CaSO 4 : Dy teflon dosimeters indicates a variation in TL output of up to 10% due to warpage of the teflon discs. This is corrected by use of a special planchet.

Effect of local radioactivity on trachael mucous velocity of sheep.

The effects of local isotope radiation and x-ray radiation on mucociliary transport of sheep were investigated. X-ray radiation from fluoroscopy equivalent to that used to determine tracheal mucous velocity in intact animals (approximately 3 min) did not alter transport of teflon discs in an isolated sheep trachael preparation. In conscious sheep, 2 ml of a solution containing 8 mCi of macroaggregates of albumin labeled with technetium-99m (99mTc) and 99mTc-pyrophosphate were instilled under direct vision into the trachea via a fiberoptic bronchoscope, providing an actual deposition on the tracheal epithelium of 1.8 to 2 mCi. Physiologic saline and unlabeled macroaggregates of albumin were instilled as control solutions. Tracheal mucous velocity measured by movement of teflon discs increased by 93.2 per cent with 99mTc-labeled macroaggregated albumin and 86.7 per cent with 99mTc-pyrophosphate (P less than 0.01), but no significant increase occurred after saline or unlabeled macroaggregates of albumin. Discrepancies in the values of trachael mucous velocity determined using inert particles and radioactive tracer solution or aerosols might be related in part to the local effects of the tracer radiation.

Thermal history and reusability of CaSO4:Dy Teflon TLD discs

The TL sensitivity of CaSO4:Dy Teflon discs was found not to change after isothermal annealing for 1 h in the temperature range 220-400 degrees C. The reusability of these TLD discs after two separate annealing treatments (240 degrees C and 400 degrees C, each for 1 h) were studied for 20 cycles of use. The former annealing treatment resulted in a 12% loss in TL sensitivity after 20 cycles, mainly due to the increase in optical density of these discs. A decrease in the maximum temperature from 300 to 250 degrees C while clamped for read-out in the TLD reader has prevented any change in the optical density of these discs, and their TL sensitivity did not show any change after 20 cycles. This study has demonstrated that under controlled conditions, the Teflon TL dosemeters can be reused a number of times without any change in TL sensitivity.