16 research outputs found
Coded aperture imaging of nuclear fusion in the plasma focus device
No full textThe Coded Aperture Imaging (CAI) technique has been used to image the deuterium-deuterium (D-D) fusion source in the NX2 plasma focus (PF) device, using protons emitted from the D(d,p)T reaction. CAI is a form of multiplexed pinhole imaging which uses many small pinholes arranged in specific patterns so as to obtain images with the good spatial resolution expected for a small pinhole, together with high brightness and signal-to-noise ratio (SNR) associated with a much larger open aperture. Our theoretical study of CAI led us to the conclusion that for extended sources, such as the PF pinch, coded mask patterns of relatively low open fraction (i.e. significantly less than 50%) are optimal. Therefore the mask patterns used in our experimental work are based on Singer cyclic difference sets. Monte Carlo simulations of CAI and pinhole imaging were performed in order to validate our theoretical SNR formulae and test our decoding algorithm. CR-39 polymer nuclear track detectors were used to register the ~3 MeV protons emitted from the PF pinch. As an additional test of the CR-39 detector scanning procedures, theoretical formulae, and CAI decoding algorithm, various source shapes were simulated experimentally using a 226Ra alpha particle source. As anticipated, the obtained alpha particle CAI images were found to be considerably better than the corresponding single-pinhole images. Significantly, this work represents the first application of the CAI technique to imaging the fusion source in a plasma focus device.The time-integrated neutron yield and anisotropy for each NX2 plasma focus shot were measured using fast- neutron detectors based on the production of radioactive 6He via the 9Be(n,α)6He reaction. The newly fabricated detectors comprise a beryllium metal sheet sandwiched between two large-area xenon-filled proportional counters. A methodology was developed for calculating the absolute neutron response function of these beryllium activation detectors, based on: the 9Be(n,α)6He reaction cross-section, energy calibration of the proportional counters, and numerical simulations (using MCNP5) of neutron and beta-particle interactions.In one series of experiments, five CAI cameras with identical masks were employed simultaneously: one placed on-axis (0°) and four at 45° to the plasma focus axis at the distances of 10.3 cm and 4.9 cm from the source respectively, to investigate the spatial distribution of D-D fusion in the NX2 PF device operated in pure D2 gas and with a PF bank energy of 1.6 kJ. These conditions represent the neutron-optimized regime, for which the neutron yield is typically 1-3×108 n/shot. In a second series of experiments, two larger coded masks: mask-1345 (91×15 array and 341 holes each 0.3 mm side) and mask-4680 (104×45 arrays and 585 holes each 0.27 mm side) were placed at 90° to the plasma focus axis to investigate the fusion source in pure deuterium and deuterium-krypton admixture working gases. The number of proton tracks registered on the CR-39 detectors per shot was typically (1-3)×105. The results clearly show the different size, density and shape of the fusion source in pure D2 and D2-Kr admixture operation. For this second series of experiments, an x-ray pinhole imaging system with suitable filtering was employed simultaneously to record the associated xray images of the hot dense pinched plasma column for comparison with the fusion source images. A plastic scintillation detector was used to measure the time-resolved neutron signal, while the time-resolved hard x-ray pulse was measured using both cesium-iodide and barium-fluoride scintillation detectors.The results show little apparent correlation between the shape of the D-D fusion source and the corresponding soft x-ray image of the plasma pinch. For pure D2 discharges the proton CAI images show a reasonably symmetrical “cigar-shaped” fusion source surrounding the pinch column, without any appreciable indication of m = 0 or m = 1 plasma instabilities. Soft x-ray emission from the pinch column is much stronger for D2-Kr admixture operation than for pure D2 operation, and micro-pinches are frequently present within the main pinch column. Despite this, there are again no discernable indications of plasma instabilities in the associated proton CAI images. Proton CAI images show that the D-D fusion source is much narrower (i.e. of smaller diameter) for D2-Kr admixture operation of the NX2 than it is for pure D2 operation. It is concluded that a collective mechanism of deuteron acceleration occurs throughout the length of the pinch column in the NX2 device, and therefore the fusion is distributed rather evenly around the pinch column. Although m = 0 instabilities in the pinch column may initiate deuteron acceleration, the fast deuterons and resulting fusion are not concentrated within, or around, m = 0 instabilities
Neutron anisotropy measurements in dense plasma focus device by means of deuteron beam obstacle
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Measurment of plasma displacements by a designed sine-coil and Mirnov coils in the IR-T1 tokamak
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Coded aperture imaging of alpha source spatial distribution
No full textThe Coded Aperture Imaging (CAI) technique has been applied with CR-39 nuclear track detectors to image alpha particle source spatial distributions. The experimental setup comprised: a 226Ra source of alpha particles, a laser-machined CAI mask, and CR-39 detectors, arranged inside a vacuum enclosure. Three different alpha particle source shapes were synthesized by using a linear translator to move the 226Ra source within the vacuum enclosure. The coded mask pattern used is based on a Singer Cyclic Difference Set, with 400 pixels and 57 open square holes (representing ρ = 1/7 = 14.3% open fraction). After etching of the CR-39 detectors, the area, circularity, mean optical density and positions of all candidate tracks were measured by an automated scanning system. Appropriate criteria were used to select alpha particle tracks, and a decoding algorithm applied to the (x, y) data produced the de-coded image of the source. Signal to Noise Ratio (SNR) values obtained for alpha particle CAI images were found to be substantially better than those for corresponding pinhole images, although the CAI-SNR values were below the predictions of theoretical formulae. Monte Carlo simulations of CAI and pinhole imaging were performed in order to validate the theoretical SNR formulae and also our CAI decoding algorithm. There was found to be good agreement between the theoretical formulae and SNR values obtained from simulations. Possible reasons for the lower SNR obtained for the experimental CAI study are discussed.Accepted versio
The Effect of Specific Heat Ratio on Neutron Yield
No full textIt is known that the radius ratio of a plasma pinch depends on the specific heat ratio γ of the pinch plasma, where radius pinch ratio is defined as: radius of plasma pinch column/radius of anode. The lower the specific heat ratio the lower would be the pinch radius ratio with corresponding increased compression and pinch density. The deuterium plasma focus pinch is invariably fully ionized and has a specific heat ratio of practically the highest possible value of 5/3. If the deuterium plasma focus could have its specific heat ratio reduced below 5/3 we might expect its radius ratio to be correspondingly reduced, increasing the pinch density, thus improving the D–D fusion neutron yield. To demonstrate this effect we run the Lee model code in deuterium but hypothetically fix the specific heat ratio, reducing it at each run. The results show that indeed the radius ratio is reduced, increasing the compression, and the neutron yield is substantially increased. The effect is used to explain the observed neutron yield enhancement when a deuterium plasma focus doped with a small amount of krypton
BioPET: dedicated plant and soil micro-organism positron emission tomography system
No full textThe Nuclear Imaging Detector Development group at the Department of Physics, University of Regina is developing Canada's first positron emission tomography (PET) system for plant physiological function studies In its current configuration the system consists
of four detectors heads, each with active area of 48x48 mm2 In a typical arrangement the four detectors are grouped in two pairs of opposing heads for detecting coincident events from positron annihilation, however the system geometry, data acquisition and image reconstruction are flexible and can be easily reconfigured on-demand to accommodate other scanner configurations The detector pairs can be scanned along the vertical axis allowing to image plants up to 50 cm tall, while the plant within the Field of View (FOV) of the scanner can be rotated on a precision controlled motorized platform to provide a complete 3D coverage of the plant Current development and characterization of the plant PET system will be presented
