179,313 research outputs found
Optimizing Compton camera performance
No full textAmore realistic simulation approach is used to study the behavior of the Compton camera in
this thesis than previous studies to date. The Compton camera differs from gamma cameras
in that the collimator is replaced by a detector known as the ‘scatterer’. Gamma rays may be
Compton scattered in the scatterer and subsequently detected by an ‘absorber’ which is the
equivalent of the detector in a gamma camera. By measuring the energies and the positions
of the points on the scatterer and the absorber where the incident and scattered gamma rays
interacted with the detectors, an image of the source can be reconstructed. Because there is
no collimator present, the potential sensitivity of the Compton camera is much higher than
the gamma camera, resulting in reduced acquisition times.
Most of the work described in this thesis was done with the GEANT4 Monte Carlo
simulation software. GEANT4 has been proven to be very robust and efficient in modelling
physics problems of radiation transport and interactions with matter in complex geometries.
Four major studies are carried out to estimate and optimize the performance of this
novel equipment. The first study takes a look at the scatterer’s imaging parameters with
the aim of prescribing an optimal scatterer material and geometry. In the second study,
the contribution of the absorber to the overall Compton camera performance is evaluated,
considering detector material, interaction type and geometry. The third study explores the
limitations imposed by the detector energy threshold and dead time on the Compton camera
performance, using a simplified model of the general electronic architecture. An evaluation
of Compton camera for scintimammography was performed in the fourth study. For
this study, three dual-head Compton camera models (Si/CZT, Si/LaBr₃:Ce and Si/NaI(Tl)
Compton cameras) were simulated, and the effect of scintillation photons’ interactions with
the photomultipliers was implemented.
The results show that silicon of about 1 cm thickness would be adequate as the Compton
camera scatterer. Analyses suggest however, that the choice of silicon is not completely
flawless. Doppler broadening for this detector material contributes as much as 7.3 mm
and 2.4 mm to full-width-at-half-maximum (FWHM) image resolution at 140.5 keV and
511 keV respectively. On the other hand, detector spatial resolution which accounts for
the least image degradation at 140.5 keV is found to be the dominant degrading factor
at 511 keV, suggesting that the absorber parameters play major roles in image resolution
at higher diagnostic energies. Findings further suggest that cadmium zinc telluride (CZT)
would be themost suitable detector as the absorber since thematerial demonstrated the highest efficiency and least positioning error due to multiple interactions as well as good spatial
resolution. The inclusion of the energy threshold and detector dead time at 140.5 keV, reduced
the Compton camera detection efficiency by 48% and 17% respectively, but improved
the image resolution from 10.7 mm to 9.5 mm at the source-to-scatterer distance of 5 cm.
At 511 keV, the inclusion of these parameters reduced the efficiency by 6% and 13% respectively,
but made no significant difference on the camera resolution. For a challenging
detection case in scintimammography, 5 mm breast tumours of tumour/background uptakes
of 10:1 and 6:1 at 511 keV were used. The best signal-to-noise ratio (SNR) was attained
for the Si/CZT Compton camera model, with the SNR values of 12.2 and 5.3.
It is therefore envisioned that with an optimal camera geometry, improved reconstruction
technique and adequate filter algorithm, the combination of Si and CZT as the scatterer
and the absorber of the Compton camera would make a very promising imaging system for
nuclear medicine studies at higher gamma ray energies where the collimated SPECT systems
perform very poorly due to increased septal penetration. It is equally evident from the
studies that with improved technology, new detectors such as LaBr₃:Ce could replace the
traditional NaI(Tl) detector as imaging detectors
Forts and Fort Life in New Caledonia under Hudson's Bay Company Regime
No full textCompton, Pyms Nevins. “Forts and fort life in New Caledonia under Hudson’s Bay Company regime”. 1878. Hubert Howe Bancroft Collection. Bancroft Library, University of California, Berkeley. BANC MSS P-C 5
Hand-coloured engraved portrait of Spencer Compton, second earl of Northampton (1601–1643)
No full textHand-coloured engraved portrait of Spencer Compton, second earl of Northampton (1601-1643) . Drawn by G.P. Harding, engraved by Woodman. Titled: 'Spencer Compton, 2nd Earl of Northampton, slain at Hoptonheath, March 19th A.D. 1642. From the original by C. Jansen, in the collection of the Marquess of Northampton.
Compton scattering from the deuteron above pion-production threshold
No full textThe electromagnetic polarizabilities of the nucleon are fundamental nucleon-structure observables that characterize its response to external electromagnetic fields. The neutron polarizabilities can be accessed from Compton-scattering data on light nuclear targets. Recent measurements of the differential cross section for Compton scattering on the deuteron below the pion-production threshold have decreased the uncertainties in the neutron polarizabilities, yet the proton polarizabilities remain known substantially more accurately. As the sensitivity of the cross section to the polarizabilities increases with incident photon energy, measurements above the pion threshold may offer a way for an improved determination of the neutron polarizabilities. In this Rapid Communiciation, the first measurement of the cross section for coherent Compton scattering on the deuteron above the pion-production threshold is presented
Norwegian Whaler, C. A. Larsen, Port Aransas, Texas.
No full textRecto: [inscribed on negative] Norwegian Whaler, C. A. Larson, Capacity 17250 Tons. Port Aransas, Texas. Compton
Recommended from our members
Correspondence
No full textA letter from Clyde C. Clack to Carl B. Compton regarding advertising
Recommended from our members
Correspondence
No full textA letter from Charlotte C. Burt to Carl B. Compton regarding information concerning university transfer credits
Correspondence
No full textA letter from Charlotte C. Burt to Carl B. Compton regarding information concerning university transfer credits
GLOBAL COMPTON HEATING AND COOLING IN HOT ACCRETION FLOWS
No full textThe hot accretion flow ( such as advection-dominated accretion flow) is usually optically thin in the radial direction, therefore the photons produced at one radius can travel for a long distance without being absorbed. These photons can thus heat or cool electrons at other radii via Compton scattering. This effect has been ignored in most previous works on hot accretion flows and is the focus of this paper. If the mass accretion rate is described by. (M) over dot = (M) over dot (0)(r/r(out))(0.3) and r(out) = 10(4)r(s), we find that the Compton scattering will play a cooling and heating role at r less than or similar to 5 x 10(3)r(s) and r greater than or similar to 5 x 10(3)r(s), respectively. Specifically, when (M) over dot (0) > 0.1L(Edd)/c(2), the Compton cooling rate is larger than the local viscous heating rate at certain radius; therefore the cooling effect is important. When (M) over dot (0) > 2L(Edd)/c(2), the heating effect at r(out) is important. We can obtain the self-consistent steady solution with the global Compton effect included only if (M) over dot (0) less than or similar to L(Edd)/c(2) for r(out) = 50r(s), which corresponds to L less than or similar to 0.02L(Edd). Above this rate the Compton cooling is so strong at the inner region that hot solutions cannot exist. On the other hand, for r(out) = 10(5)r(s), we can only get the self-consistent solution when (M) over dot (0) less than or similar to L(Edd)/c(2) and L < 0.01L(Edd). The value of this critical accretion rate is anticorrelated with the value of r(out). Above this accretion rate, the equilibrium temperature of electrons at r(out) is higher than the virial temperature as a result of strong Compton heating, so the accretion is suppressed. In this case the activity of the black hole will likely "oscillate" between an active and an inactive phase, with the oscillation timescale being the radiative timescale of the gas at r(out)
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