Re-entrant Well Research Articles

Progress on next generation gamma-ray Cherenkov detectors for the National Ignition Facility.

Fusion reaction history and ablator areal density measurements for Inertial Confinement Fusion experiments at the National Ignition Facility are currently conducted using the Gamma Reaction History diagnostic (GRH_6m). Future Gas Cherenkov Detectors (GCDs) will ultimately provide ∼100x more sensitivity, reduce the effective temporal response from ∼100 to ∼10 ps, and lower the energy threshold from 2.9 to 1.8 MeV, relative to GRH_6m. The first phase toward next generation GCDs consisted of inserting the existing coaxial GCD-3 detector into a reentrant well which puts it within 4 m of the implosion. Reaction history and ablator gamma measurement results from this Phase I are discussed here. These results demonstrate viability for the follow-on Phases of (II) the use of a revolutionary new pulse-dilation photomultiplier tube to improve the effective measurement bandwidth by >10x relative to current PMT technology; and (III) the design of a NIF-specific "Super" GCD which will be informed by the assessment of the radiation background environment within the well described here.

Next generation gamma-ray Cherenkov detectors for the National Ignition Facility.

The newest generation of Gas Cherenkov Detector (GCD-3) employed in Inertial Confinement Fusion experiments at the Omega Laser Facility has provided improved performance over previous generations. Comparison of reaction histories measured using two different deuterium-tritium fusion products, namely gamma rays using GCD and neutrons using Neutron Temporal Diagnostic (NTD), have provided added credibility to both techniques. GCD-3 is now being brought to the National Ignition Facility (NIF) to supplement the existing Gamma Reaction History (GRH-6m) located 6 m from target chamber center (TCC). Initially it will be located in a reentrant well located 3.9 m from TCC. Data from GCD-3 will inform the design of a heavily-shielded "Super" GCD to be located as close as 20 cm from TCC. It will also provide a test-bed for faster optical detectors, potentially lowering the temporal resolution from the current ∼100 ps state-of-the-art photomultiplier tubes (PMT) to ∼10 ps Pulse Dilation PMT technology currently under development.

Optimization of SPRT measurements of freezing in a zinc fixed-point cell

A numerical model of solute and heat transport in extremely pure materials is described. Its purpose is to characterize the effect of impurities on the freezing curves of metals containing impurities at the level of less than 1 part per million. It is used to simulate experiments performed using a commercially available zinc fixed-point cell for SPRT calibrations. The aim is to determine the effect of different vertical temperature gradients on the freezing curve and to find out whether a range of conditions could be determined where there was a good fit between theory and experiment. For this fixed-point cell, agreement between the model and experiment improves as the distribution coefficient k → 0. It is found that the model only agrees with the measured freezing curves over the entire freeze for a narrow range of furnace settings where the temperature profile is most uniform. We suggest that this is because if the furnace settings are not optimized, the solid does not grow uniformly, and freezing may continue in regions remote from the SPRT after the material in the vicinity of the SPRT has finished freezing, so distorting the freezing curve. This effect is not present in the model and so the method presented here enables optimization of the furnace to ensure the SPRT is surrounded by a liquid–solid interface over the entire freezing range. We find that the optimum thermal environment is extremely sensitive to the furnace settings; the optimum thermal environment is found when the temperature is slightly cooler at the top of the cell, as measured in the re-entrant well of the cell. We note that optimizing the freezing process is a necessary step towards using a thermal analysis to correct for the effects of impurities in the sample.

SU-FF-T-77: Measurement of the Average Seed Strength in a Mick Cartridge

Purpose: Develop an efficient method for the measurement of the average strength of brachytherapy seeds according to the 2008 recommendations of the AAPM Low Energy Brachytherapy Source Calibration Working Group. Method and Materials: The reentrant well ionization chamber is calibrated, at ADCL, for a single radioactive seed in a single-seed holder. When the seeds, loaded in Mick cartridges, are received from the manufacturer, it is necessary, only one time, to open only one cartridge. The strength of each seed in this cartridge is measured in the reentrant well chamber using the single-source holder and the ADCL calibration factor. As the seeds are removed from the Mick cartridge, one by one, the well chamber measurement is taken of the Mick cartridge, with the remaining seeds, in the Mick cartridge holder. Since the strength of each seed has been measured, the total strength of these remaining seeds in the Mick cartridge can be calculated. Results: Calibration coefficients have been determined for a Mick cartridge containing any number from 1 to 15 125I or 103Pd seeds. Using these coefficients the average strength of the seeds in the cartridge, with any number of seeds, can be determined with one single measurement in the reentrant well chamber. After the initial process, used to calculate the calibration coefficients, there is no more need to remove any seed from any cartridge. Typically, one-minute measurements of the electric charge generated in the reentrant well chamber are used for each Mick cartridge. Conclusion: A time-saving method for measurement of the average activity of 125I and 103Pd seeds has been developed. Only a one-minute measurement of the electric charge generated in a reentrant well chamber is needed to calculate the average strength of the radioactive seeds in the Mick cartridge loaded with any number from 1 to 15 seeds.

Realization of the Gallium Triple Point at NMIJ/AIST

The triple point of gallium has been realized by a calorimetric method using capsule-type standard platinum resistance thermometers (CSPRTs) and a small glass cell containing about 97 mmol (6.8 g) of gallium with a nominal purity of 99.99999%. The melting curve shows a very flat and relatively linear dependence on 1/F in the region from 1/F = 1 to 1/F = 20 with a narrow width of the melting curve within 0.1 mK. Also, a large gallium triple-point cell was fabricated for the calibration of client-owned CSPRTs. The gallium triple-point cell consists of a PTFE crucible and a PTFE cap with a re-entrant well and a small vent. The PTFE cell contains 780 g of gallium from the same source as used for the small glass cell. The PTFE cell is completely covered by a stainless-steel jacket with a valve to enable evacuation of the cell. The melting curve of the large cell shows a flat plateau that remains within 0.03 mK over 10 days and that is reproducible within 0.05 mK over 8 months. The calibrated value of a CSPRT obtained using the large cell agrees with that obtained using the small glass cell within the uncertainties of the calibrations.

Radioactivity standardization in South Africa

South Africa's national radioactivity measurement standard is maintained at a satellite laboratory in Cape Town by the National Metrology Laboratory (NML) of the Council for Scientific and Industrial Research. Standardizations are undertaken by a number of direct methods utilizing liquid scintillation counting (LSC). The successful application of LSC to the 4 πβ– γ coincidence method is reviewed. The activity unit is maintained through radionuclide specific calibration factors relating to a pressurized re-entrant well type ionization chamber. A comparison is made between normalized factors given by the manufacturer and deduced factors obtained by a method used to transfer calibration factors from the International Reference System of the International Bureau of Weights and Measures based on the NML's own absolute standardizations.

Analysis of 4 πγ ionization chamber response using EGS4 Monte Carlo code

The response of three types of 4 πγ re-entrant well pressure ionization chambers filled with argon or nitrogen were calculated by means of the EGS4 Monte Carlo code as a function of photon energy and compared with experimental results. Excellent agreement was found between calculations and experimental results. This code was also used for the evaluation of corrections required in the change of source conditions such as the displacement of source position. In the medium and high energy region, the chamber response increased when the source was moved to off-center position from center whereas the response decreased in low energy region. This phenomena can be explained by a change ofgeometry and the absorption of photons in an inner cylindrical absorber tube of the chamber.

A low-power hypsometer-type barometer for remote weather stations

The paper describes a low-power barometer intended for remote weather stations, but also meeting the requirements for manned stations and airports, in which the condensation temperature of carbon disulphide (CS <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> ) is used to determine the barometric pressure <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(p)</tex> . A heated cylindrical bulb with a re-entrant well for a thermistor through the bottom and an internal radiation shield is 1/3 filled with CS <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> . Helical springs assist CS <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> migration on wetted surfaces and allow liquid and vapor to pass one another in the small diameter exit-condenser tube. A miniature Dewar flask gives thermal insulation and a 0.01-mm beryllium-copper diaphragm transmits the external pressure. The condensation temperature is read with a simple Wheatstone bridge and dc amplifier giving an output <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V_{0} = 0.5(p- 100 Pa)</tex> V. Pulsed power with the pulse length controlled by a second thermistor on the outlet tube is used for efficiency. Long term tests of a number of barometers have given power levels around 40 mW at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20\deg</tex> C and indicated maximum drifts of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\pm50</tex> Pa/year, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\pml0</tex> Pa/day, and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\pm2</tex> Pa short term.

A re-entrant well gas flow proportional counter for radioactivity measurements of solutions containing 35S and other radionuclides

A re-entrant well gas flow proportional counter for radioactivity measurements of solutions containing 35S and other radionuclides

Equilibration of the Ice-Water Temperature Standard

A test has been made of the accuracy with which an ice-water system establishes the equilibrium temperature. A device consisting of a bulb containing ice and water with provision for volumetric measurement is used as a thermometer. This was placed in the re-entrant well of a chamber similar to those used to establish the triple point. The temperature established by the well chamber was varied by varying the pressure of the water in it. It was found that when the temperature was made to differ from the equilibrium temperature of the thermometer by as little as 3×10−6°C, growth or melting occurred in the thermometer.

Analysis of the τ-meson decay

Stacks of Ilfords G.5 emulsions were exposed to secondary particles from a target bombarded by 4.8 to 5.7 Bev protons. A re-entrant well permitted placement of the emulsions at 90 to the beam direction and within 10 inches of the hombarded target. With this geometry the proper flight time for 125-Mev K- particles to be brought to rest in the emulsion was about 1.5 x 10-2 sec. Some of the results obtained from two such exposures are described. (A.C.)