4,150 research outputs found
Experimental Measurements and Computer Simulation of Fission Product Gamma-Ray Spectra
No full textAirborne gamma ray spectrometry using high volume scintillation detectors, optionally in conjunction with Ge detectors, has potential for making rapid environmental measurements in response to nuclear accidents. An experimental investigation and computer simulation have been used to characterise the response of such detectors to short lived fission products. Small samples of 235U were irradiated in a research reactor for short periods, to generate fission product sources. Gamma ray spectra were recorded over a period of one year using both high volume scintillation detectors and semiconductor detectors. The main gamma emitting nuclides have been identified, and their associated signals in each detector defined. Simulation work has been used to calculate the corresponding spectra which would have been observed at airborne survey heights, or from fission product sources irradiated for a prolonged period. While Ge detectors can record a fuller range of isotopes, of interest in characterising release terms, the NaI spectra also have potential for providing data on a range of radiologically important isotopes at all stages
Fleming, R.L. Sr., Fleming, R.L., Jr. & Bangdel, L.S. — Birds of Nepal, with reference to Kashmir and Sikkim. Katmandu, Nepal, chez le senior author (Box 229), 1976
No full textBourlière François. Fleming, R.L. Sr., Fleming, R.L., Jr. & Bangdel, L.S. — Birds of Nepal, with reference to Kashmir and Sikkim. Katmandu, Nepal, chez le senior author (Box 229), 1976. In: La Terre et La Vie, Revue d'Histoire naturelle, tome 31, n°2, 1977. p. 348
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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Picture of Geo. Ray, Ratliff, Sefcic, Wilson R.L, Smith E.C, Carnes and Coker at a halt on way to N.C
No full textPicture of Geo. Ray, Ratliff, Sefcic, Wilson R.L, Smith E.C, Carnes and Coker at a halt on way to N.
An Aerial Gamma Ray Survey of the Surrounding Area of Sizewell Nuclear Power Station
No full text<p>An airborne gamma ray survey of the surroundings of the Sizewell nuclear power station was conducted to define the present levels of radiation background for reference purposes. A twin engine helicopter fitted with a high volume NaI detector and two semiconductor detectors was used. A 20x30km area around the site was surveyed with 500 m line spacing, with an inner zone of 6x6 km being investigated with 250 m line spacing.</p>
<p>More than 10,000 gamma ray spectra were recorded between 1st and 3rd October 1996 at a survey height of 200-300 feet above ground level and used to prepare maps showing the distribution of 137Cs, 40K,214Bi,208Tl and gamma ray dose rate. The data set has been retained digitally in an archive which can be used in the future should the need arise to measure change resulting from long term site operations, or for emergency response.</p>
<p>137Cs levels are typically around 2 kBq m-2, derived mainly from weapons' testing fallout, with slight Chernobyl input. Peak levels of some 6 kBq m-2 around the tidal inundation limits of estuarine and marsh areas may be associated with marine discharges from nuclear fuel reprocessing at Sellafield. Natural radionuclides show a distribution which reflects the geological and geomorphological variations within the landscape, and are the major contributors to dose rates within the survey zone. The gamma ray dose rate maps also show the position of the 41Ar plume emitted from the Magnox station at Sizewell during the survey. At the time in question the plume was projected over the sea in the SE direction. The gamma ray survey data crossing the plume show clear evidence of dispersion downwind with plume broadening as a result of gaseous diffusion with increasing distance. Ground level gamma ray dose rates were recorded at 8 routine district monitoring sites using a mini-series 680 survey meter operated by Sizewell staff and a portable scintillation spectrometer. The mean dose rates recorded with the 680 meter (24.7 nGy hr-1) are in good agreement with the results of ground based (27.4 nGy hr-1) and the nearest airborne gamma dose rate results (24.6 nGy hr-1).</p>
<p>Airborne survey methods are uniquely well suited to rapid environmental data capture from large areas. This has important emergency response potential which is increasingly recognised in the context of nuclear sites. The data recorded in this study provide a frame of reference against which future changes can be measured.</p>
Gamma ray astronomy in the low energy range
No full textA low energy gamma ray telescope, and the results of its observations are described. The telescope consisted of four sodium iodide crystals, each of 120 cm2 area, occulted by lead discs. Charged particles are rejected using scintillation, anti-coincidence shields.The telescope was flown from Palestine, Texas in 1974. Gamma rays were observed from the Crab Nebula, the pulsar NP0 532, and the atmospheric background. No evidence for a gamma ray burst was found, giving an upper limit of less than 5.8 x 103 bursts per year of intensity greater than 1.6 x 10-6 ergs cm-2.Various types of existing, and planned, gamma ray detectors, and successful gamma ray observations are reviewed. Detailed calculations of the detection efficiency of a Double Compton telescope are presented and these results led to the development of the MISO low energy gamma ray telescope.Preliminary calculations of the detection efficiency of an Anticollimated Double Compton telescope are presented, which suggest that further, more detailed investigation of this type of detector would prove fruitful
An Aerial Gamma Ray Survey of Springfields and the Ribble Estuary in September 1992
No full text<p>A short aerial gamma ray survey was conducted in the vicinity of the Springfields site and Ribble Estuary from 1st-5th September 1992, to define existing background radiation levels, against which any future changes can be assessed. A twin engine AS 355 "Squirrel" helicopter chartered from Dollar Helicopters was used for this work. It was loaded with a 16 litre NaI(Tl) gamma ray detector and spectroscopy system on the 31st August and during the following days over 2700 separate spectra were recorded within a survey area of 20 x 12 km. Gamma ray spectra were recorded every 5 seconds at survey speed and altitude of 120 kph and 75 m respectively. A flight line spacing of 0.3km was chosen for the main survey area. On the 3rd September a low altitude, high spatial resolution (flight line spacing 100m and altitude 30m) was made over Banks Marsh (an area frequented by local wild fowlers).</p>
<p>Survey results have been stored archivally and used to map the naturally occurring radionuclides 40K, 214Bi and 208Tl together with 137Cs and total gamma ray flux. In addition, for the first time, estimates of 234mPa in terms of deconvoluted count rate (normalised to 100m altitude) were made in the presence of 228Ac interference probably in disequilibrium with its parent thorium series.</p>
<p>The maps provide a clear indication of the distribution and sources of environmental radioactivity in the Ribble at the time of the survey. The Ribble estuary is subject to regular and ongoing ground based studies by BNF, MAFF, HMIP, and University based groups, as a result of the authorised discharges of low level radioactivity from the Springfields site. The results of this survey complement this ground based work, and add to confidence that the estuarine system, it's associated sediments, tide washed pastures, salt marshes and river banks, have been thoroughly examined. There is support for earlier conclusions that the Cs on the salt marshes is the dominant source of external gamma exposure, and that the Springfields contribution to these locations is minor in comparison with this, Sellafield derived, signal. Upstream the situation is more complex, particularly where the dynamic sources of beta radiation are considered. As far as critical group assessments are concerned the survey provides clear evidence that the areas affected by 137Cs, where external gamma dose and possible food chain effects are of greatest interest, are in the lower reaches of the Ribble, whereas, at the time of the survey the 234mPa distribution was in the upper reaches of the river. This not only confirms the findings of ground based work, but provides some assurance that the different exposure paths (external gamma dose, skin dose) are not entirely synergistic. The discovery of possible transient sources of natural 228Ac in the salt marsh environment as a consequence of Th series disequilibrium immediately following spring tides is extremely interesting. If substantiated by further studies using semiconductor detectors this provides a new insight into the dynamic radiation environment of tide washed contexts.</p>
<p>Aerial survey can potentially provide a rapid and cost effective means of studying environmentally dynamic sources such as 234mPa. In the case of the Ribble it would be necessary to reduce survey height to below 50m ground clearance to improve spatial resolution. Possible inconvenience to residents and property owners of such low altitude flights would have to be considered in addition to the potential value of environmental knowledge of the behaviour of short lived nuclides in a dynamic system such the Ribble estuary. There is nonetheless considerable potential for time series studies of this location.
Recent flight trials by SURRC incorporating high efficiency germanium semiconductor detectors have verified the feasibility and potential a hybrid scintillation⁄ semiconductor spectrometer. Such a device can resolve any ambiguities arising from overlapping gamma ray peaks. This is particularly relevant to the confirmation of 228Ac in salt marshes. Ground based sampling at the time of measurement would enable concentration calibrations to be made for these dynamic sources. Further ground based measurements would be desirable to establish the extent to which low energy photons contribute to external gamma ray dose rates from sources with pronounced subsurface activity maxima.</p>
An Aerial Gamma Ray Survey of Torness Nuclear Power Station on 27-30 March 1994
No full text<p>An aerial gamma-ray survey of the environment of Torness Nuclear Power Station was commissioned by Scottish Nuclear Limited, and conducted by the Scottish Universities Research and Reactor Centre. The area surveyed encloses a 31km square, with Torness Nuclear Power Station at the centre, flown with a line spacing of 500m. A secondary area, in closer proximity to the nuclear site, was flown with 250m spacing.</p>
<p>Over 6000 gamma ray spectra were recorded with a high volume spectrometer operated from a helicopter over a three day period in March 1994. Spectral data were recorded together with satellite navigation (GPS) and radar altimetry data. The results provide a comprehensive record of the radiation environment around Torness and have been used to map the distribution of natural and man-made radionuclides, forming a baseline to enable future environmental changes may be assessed.</p>
<p>The natural radionuclides 40K, 214Bi and 208Tl are highly correlated with each other and show a distribution which reflects both the underlying geological and geomorphological features of the area. The main structural boundaries of the Dunbar-Gifford and Lammermuir faults can be partly discerned in the maps, as can some igneous intrusions. Areas with peat or alluvium cover appear as negative features in the radiometric maps.</p>
<p>Radiocaesium 137Cs levels range from below 4 kBq m-2 to over 20 kBq m-2. Upland areas near Coldingham Common, Black Castle Hill and Dunbar Common show the highest values, similar in deposition pattern and level to Chernobyl activity observed in the West of Scotland and elsewhere. Published national maps derived from meteorological and ground sampling data predicted much lower levels for these locations. However core samples taken after the survey have confirmed the presence of the activity, and the attribution to Chernobyl. This finding demonstrates both the effectiveness of the method for rapid location of radioactive deposition, and the need for baseline studies to determine present levels. Count rates from a spectral window corresponding to 60Co were also mapped. The results are close to detection limits and show a slight correlation with natural sources. Therefore they are more probably due to residuals remaining after separation of spectral interferences than to low level 60Co contamination.</p>
<p>Gamma ray dose rates range from approximately 0.1 to 0.6 mGy a-1 with a mean value of 0.34 mGy a-1, and are derived mainly from natural sources. Ground level measurements were taken at nine district monitoring points within the area using a 3x3" NaI spectrometer and a survey meter (Series 6/80) used routinely by SNL. Both ground based data sets were in good agreement with each other and with the aerial survey after accounting for instrumental and cosmic ray background contributions.</p>
<p>There is no evidence that Torness Power Station has affected the surrounding radiation environment, within the operational and sensitivity limits of the aerial survey.</p>
<p>The longer term impact of the site can be assessed by future surveys. Moreover under emergency conditions it would be possible to utilise this method for rapid mapping of the area on a timescale which cannot be matched using alternative approaches. </p>
A search for TeV gamma ray emission from X-ray binary stars
No full textThis work is concerned with the detection of pulsed TeV gamma ray emission from a number of X-ray binary systems by the use of the atmospheric Cerenkov technique. Chapters 1 and 2 give an overview of the development of gamma ray astronomy, with emphasis placed on progress made in the detection of TeV gamma rays by their Cerenkov radiation in the atmosphere. Chapters 3 and 4 describe the University of Durham atmospheric Cerenkov telescopes, which were used to make the observations reported in this work, and the standard data processing and analysis procedures adopted. The main part of the thesis deals with the application of these techniques to observations of five X-ray binaries. After a review of the properties of such objects in Chapter 5, Chapters 6 and 7 deal specifically with the results for two of the systems considered to be among the most likely candidates to give a detectable TeV photon flux; Centaurus X-3 and Vela X-1. A study of all data recorded on Cen X-3 over the course of six years suggests the presence of a weak gamma ray flux pulsed at the X-ray period. Previous reports of stronger emission near the ascending node of the orbit are confirmed here. For Vela X-1, the analysis of a dataset recorded during a single dark moon interval reveals evidence for two short outbursts of pulsed TeV gamma ray emission. Chapter 8 reports the series of observations made of SMC X-1, 4U1626-67 and X0G21-72, and upper limits are placed on the TeV gamma ray emission from each. Finally, the results reported here are compared with the predictions of a number of theoretical models, some of which are found to give good agreement with the limits and detections derived in this work. A discussion of the status of this field and future observational prospects is also given
Very high energy cosmic gamma rays from radio and x-ray pulsars
No full textThis thesis is concerned with the detection of very high energy cosmic gamma-rays from isolated pulsars and X-ray binary sources using the atmospheric Cerenkov technique. A general introduction to gamma ray detection techniques is followed by adscription of the properties of atmospheric Cerenkov radiation and a discussion of the principles of the atmospheric Cerenkov technique. The Mark I and Mark II gamma-ray telescopes operated in Dugway, Utah by the University of Durham between 1981 and 1984 are briefly described. There follows a discussion of the results from observations at many different wavelengths of Cygnus X-3. This object was observed by the Durham group between 1981 and 1983 in Dugway Utah and also in Durham during autumn 1985. The detection in the Dugway data of the 4.8 hr X-ray period and the possible detection of a19.2 day intensity variation are considered. The discovery of a 12.59 ms pulsar in data taken on Cygnus X-3 in 1983 is described. Evidence is presented which suggests this periodicity is also present at a weaker level in earlier data and also in the data taken in Durham in 1985.Results from observations of PSR1937+21 , PSR1953+29and six radio pulsars , are presented. The design and construction of the Mark III telescope, now operating in Narrabri , N.S.W. , is described in detail. Preliminary results from observations with the Mark III telescope of three objects, LMC X-4, the Vela pulsar and CentaurusX-3, are presented, with particular reference to periodicities inherent in the sources. An observation of the supernova in the Large Magellanic Cloud is discussed. A brief discussion of the mechanisms by which V.H.E. gamma-rays may be produced in isolated pulsars and X-ray binary pulsars is given, followed by a description of the future prospects for the Mark III and Mark IV telescopes
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