Search

We have developed a compact Compton gamma-ray imager with a large field of view and a low channel-count that is capable of quickly localizing gamma-ray sources in the few hundred keV - several MeV range. The two detector planes (scatter and absorber) employ bars of NaI(Tl) read out by photomultiplier tubes (PMTs) located at each end. The long-range imaging performance has been tested from 392 keV to 1274 keV. An angular resolution measure of 2.72 +/- 0.06 degrees and an efficiency of (1.79 +/- 0.04) x 10^(-3) at 662 keV is obtained. A Cs-137 (662 keV) source equivalent to a 10 mCi source 40 m away can be located in 60 seconds with an uncertainty of about a degree. No significant degradation in imaging performance is observed for source angles up to 40 degrees off axis.

We are developing a rugged and person-transportable Compton gamma imager for use in security investigations of radioactive materials, and for radiological incident remediation. The imager is composed of layers of scintillator with light collection for the forward layers provided by silicon photomultipliers and for the rear layer by photomultiplier tubes. As a first step, we have developed a 1/5th-scale demonstration unit of the final imager. We present the imaging performance of this demonstration unit for Cs-137 at angles of up to 30 degrees off-axis. Results are also presented for Sn-113 and Na-22. This represents the first demonstration of the use of silicon photomultipliers as an embedded component for light collection in a Compton gamma imager., Comment: 19 pages, 6 figures

In response to the Fukushima nuclear reactor accident, on March 20th, 2011, Natural Resources Canada conducted aerial radiation surveys over water just off of the west coast of Vancouver Island. Dose-rate levels were found to be consistent with background radiation, however a clear signal due to Xe-133 was observed. Methods to extract Xe-133 count rates from the measured spectra, and to determine the corresponding Xe-133 volumetric concentration, were developed. The measurements indicate that Xe-133 concentrations on average lie in the range of 30 to 70 Bq/m3., Comment: 14 pages, 5 figures, accepted for publication in Journal of Environmental Radioactivity

We are designing an all-scintillator Compton gamma imager for use in security investigations and remediation actions involving radioactive threat material. To satisfy requirements for a rugged and portable instrument, we have chosen solid scintillator for the active volumes of both the scatter and absorber detectors. Using the BEAMnrc/EGSnrc Monte Carlo simulation package, we have constructed models using four different materials for the scatter detector: LaBr_3, NaI, CaF_2 and PVT. We have compared the detector performances using angular resolution, efficiency, and image resolution. We find that while PVT provides worse performance than that of the detectors based entirely on inorganic scintillators, all of the materials investigated for the scatter detector have the potential to provide performance adequate for our purposes., Comment: Revised text and figures, Presented at SORMA West 2008, Published in IEEE Transactions on Nuclear Science

In the ZEUS experiment at HERA, photoproduction processes have been studied for photon-proton centre-of-mass energies in the range 100 < W_gamma p < 300 GeV and jet transverse energies extending to E_T^jet ~ 70 GeV. The data contribute to our understanding of QCD dynamics, and also provide new constraints on the photon's parton density., Comment: 7 pages, 12 figures, talk given at ICHEP'98, writeup also available at http://ppewww.ph.gla.ac.uk/preprints/98/05/

The cross section for producing three high transverse energy jets with a three-jet invariant mass of M_3J > 50 GeV has been measured in photoproduction at HERA. The angular distribution of the three jets is presented and found to be inconsistent with a uniform population of the available phase space. In contrast, parton shower models and O(alpha alpha_s^2) pQCD calculations are able to describe the three jet dynamics., Comment: 4 pages, 3 eps figures; to appear in the proceedings of the 6th International Workshop on Deep Inelastic Scattering and QCD

An excess of events with a rapidity gap between jets, over what would be expected from non-diffractive processes, has been observed at HERA. A process based on a perturbative QCD calculation of colour singlet exchange has been added to HERWIG. With this addition, HERWIG is able to describe the number of events with a gap between jets over the number without a gap. This gap fraction is predicted to rise at large rapidity intervals between jets which would only be visible if the detector coverage were increased., Comment: 5 pages including 3 figures. To appear in the conference proceedings of the Workshop "Future Physics at HERA"

Recent results obtained from studies of diffractive processes in hard photoproduction performed by the ZEUS collaboration using data delivered by HERA in 1993 and 1994 are presented. In particular, we have found that $(7 \pm 3)$\% of events with two jets at a pseudorapidity interval of 3.5 to 4 are inconsistent with a non-diffractive production mechanism. These events may be interpreted as arising due to the exchange of a colour singlet object of negative squared invariant mass ($-t$) around 40~GeV$^2$. We have also probed the structure of the exchanged colour singlet object in low--$t$ diffractive scattering. By comparing the results from photoproduction and electroproduction processes we find that between 30\% and 80\% of the momentum of the exchanged colour singlet object which is carried by partons is due to hard gluons., Comment: 13 pages. 13 postscript figures + 1 postscript preprint logo + 1 LaTeX file. Files tarred, gzipped and uuencoded into 1 file. Also available at http://ppewww.ph.gla.ac.uk/preprints/96/03/

A search for photoproduction events which contain a rapidity gap between the two highest transverse energy jets has been conducted at HERA using the ZEUS detector. The jets have transverse energies greater than 6~GeV, and are separated by pseudorapidity intervals of up to four units. The fraction of events containing a gap is measured as a function of the gap-width. It is expected that this gap-fraction will fall exponentially with the gap-width, until the dominant gap-production mechanism becomes colour singlet exchange, at which point it will plateau. An indication of a plateau in the measured gap-fraction has been found, at a higher level than that expected from electroweak exchange., Comment: 7 pages, 4 postscript figures, lines now wrapped to 80 characters