共查询到20条相似文献，搜索用时 312 毫秒
1.
N. Yamashita T. H. Prettyman D. W. Mittlefehldt M. J. Toplis T. J. McCoy A. W. Beck R. C. Reedy W. C. Feldman D. J. Lawrence P. N. Peplowski O. Forni H. Mizzon C. A. Raymond C. T. Russell 《Meteoritics & planetary science》2013,48(11):2237-2251
We have completed a mapping study of 7.6 MeV gamma rays produced by neutron capture by Fe at the surface of the main belt asteroid 4 Vesta as measured by the bismuth germanate scintillator of the Gamma Ray and Neutron Detector (GRaND) on the Dawn spacecraft. The procedures used to determine Fe counting rates are presented, along with a global map, constituting the necessary initial step to quantify Fe abundances. While the final calibration of orbital data to absolute concentrations has not been determined, the range of fully corrected Fe counting rates is compared with that of Fe in howardites. We find that the global distribution of corrected Fe counting rates is generally consistent with mineralogy and composition determined independently by other instruments on the Dawn spacecraft, including measurements of pyroxene absorption bands by the Visible and Infrared Spectrometer and Framing Camera, and an index of diogenitic materials provided by neutron absorption measurements by GRaND. In addition, there is a distinctive low Fe region in the western hemisphere that was not reported by reflectance or optical observations, possibly indicating the presence of a cumulate eucrite component in Vesta's regolith. 相似文献
2.
3.
4.
A new use for a 2-dimensional position sensitive diode (PSD) is described. A duolateral PSD was used with a microchannel plate
image intensifier as a proof-of-concept photon counting (event driven) imager for astronomical imaging and photometry. This
produced an imager capable of counting 25–30 kcps over the astronomical bands B, V & R, with an overall efficiency of ∼19%. 相似文献
5.
The maximum entropy method has been used to reconstruct images in a wide range of astronomical fields, but in its traditional form it is restricted to the reconstruction of strictly positive distributions. We present an extension of the standard method to include distributions that can take both positive and negative values. The method may therefore be applied to a much wider range of astronomical reconstruction problems. In particular, we derive the form of the entropy for positive/negative distributions and use direct counting arguments to find the form of the entropic prior. We also derive the measure on the space of positive/negative distributions, which allows the definition of probability integrals and hence the proper quantification of errors. 相似文献
6.
The ARGO-YBJ experiment is almost completely installed at the YangBaJing Cosmic Ray Laboratory (4300 m a.s.l., Tibet, P.R.
China). The lower energy limit of the detector (E∼1 GeV) is reached with the scaler mode, i.e., recording the single particle
rate at fixed time intervals. In this technique, due to its high altitude location and large area (∼6700 m2), this experiment is the most sensitive among all present and past ground-based detectors. In the energy range under investigation,
signals due to local (e.g. solar GLEs) and cosmological (e.g. GRBs) phenomena are expected as significant enhancements of
the counting rate over the background. Results on the search for GRBs in coincidence with satellite detections are presented.
For the ARGO-YBJ Collaboration. 相似文献
7.
The issue of crater retention age estimates on planetary surfaces is discussed with an attempt to quantify the effect of overlapping primary and secondary impact crater populations in restricted crater diameter ranges. The approach to this problem is illustrated with a simple model production function where the secondary crater input is artificially enhanced. Extrapolation of such a secondary crater model distribution to a global record results in extraordinarily high crater frequencies that do not exist on Mars, and implies the need of detailed studies of the size-frequency distribution for remote secondary craters, to date poorly known. A key case, the martian crater Zunil and its secondary crater field, illustrate that reasonable predictions for the secondary crater size-frequency distribution at small (<100 m) crater diameters affected the standard model crater retention age for the Cerberus plains less than the statistical uncertainty. These observations show that age determination based on appropriate crater counting statistics is valid in a wide primary crater diameter range. 相似文献
8.
Low level counting of alpha and beta particles has been carried out on a 5 long sediment core from the lake La Bouchet in France, spanning the past around 24 000 years. A total of around 60 data points were spectrally analysed with the successive approximations method. Several whole or quasi-periodicities were located for the two radioactivity data sets, in different time intervals; for the alpha counting, periods of 1000–1200, 2000, 6000 and 24 000 years and for the beta counting periods of 1000, 3000, 4000, 8000 and 24 000 years were obtained. The non-stationarity nature of the obtained periods are discussed in relation to (a) palaeoclimatic fluctuations impact on mineral carrying natural radiosotopes partitioning of the sediment deposition of the last glacial and (b) other solar-terrestrial agents. 相似文献
9.
Radio meteor observations by Ham-band beacon or FM radio broadcasts using “Ham-band Radio meteor Observation Fast Fourier
Transform” (HROFFT) an automatic operating software have been performed widely in recent days. Previously, counting of meteor
echoes on the spectrograms of radio meteor observation was performed manually by observers. In the present paper, we introduce
an automatic meteor echo counting software application. Although output images of the HROFFT contain both the features of
meteor echoes and those of various types of noises, a newly developed image processing technique has been applied, resulting
in software that enables a useful auto-counting tool. There exists a slight error in the processing on spectrograms when the
observation site is affected by many disturbing noises. Nevertheless, comparison between software and manual counting revealed
an agreement of almost 90%. Therefore, we can easily obtain a dataset of detection time, duration time, signal strength, and
Doppler shift of each meteor echo from the HROFFT spectrograms. Using this software, statistical analyses of meteor activities
is based on the results obtained at many Ham-band Radio meteor Observation (HRO) sites throughout the world, resulting in
a very useful “standard” for monitoring meteor stream activities in real time. 相似文献
10.
S. Biswas N. Durgaprasad R. K. Singh M. N. Vahia J. S. Yadav A. Dutta J. N. Goswami 《Journal of Astrophysics and Astronomy》1994,15(1):85-94
The Anuradha cosmic ray experiment in Spacelab-3, flown in the orbit at 350 km with an inclination of 57° for about six days, was used to measure the low energy galactic cosmic ray (GCR) heavy ions using a specially designed CR-39 detector module incorporating the arrival time information of the particles. The abundances of sub-iron (Sc-Cr) and iron particles in the low energy interval of 30–300 MeV/N were determined from the measurements made in four different depths of the CR-39 detector module of 150 layers. From these studies we obtained sub-iron (Sc-Cr) to iron abundance ratios of 0.8 to 1.2 in 30–300 MeV/N energy range. It is found that these ratios are enhanced by a factor of two as compared to interplanetary ratios of about 0.5. It is shown that the enhancement of the ratio inside the earth’s magnetosphere is probably due to the degree of ionization of low energy Sc to Cr and Fe ions in the galactic cosmic rays and to the rigidity filtering effects of the geomagnetic field. Further studies are needed to understand fully the phenomena and their implications. 相似文献
11.
Background modelling for LEO satellites with high orbital inclination is not an easy task. The diffuse background component is dominated by the background coming from strong interactions with Earth magnetosphere trapped particles. Magnetic shielding is variable along the orbits and crosses through the SAA induce high radioactivity decay counting ratios. The aim of this paper is to present a model for the background total counting ratio of the 17 operative CdZnTe detectors on LEGRI in the short time scales and for observing periods outside crosses through SAA having enough time to cool LEGRI after the last SAA transit. Fluxes measured have been modelled in terms of the McIlwain parameter L using polynomial fits and exponential laws in terms of the geomagnetic cut-off rigidity used on COMPTEL background modelling. A discussion of the results is also included. 相似文献
12.
Eric Lantz Jean-Luc Blanchet Luca Furfaro Fabrice Devaux 《Monthly notices of the Royal Astronomical Society》2008,386(4):2262-2270
A multi-imaging strategy is proposed and experimentally tested to improve the accuracy of photon counting with an electron multiplying CCD (EMCCD), by taking into account the random nature of its on-chip gain and the possibility of multiple photodetection events on one pixel. This strategy is based on Bayesian estimation on each image, with a priori information given by the sum of the images. The method works even for images with large dynamic range, with more improvement in the low light level areas. In these areas, two thirds of the variance added by the EMCCD in a conventional imaging mode are removed, making the physical photon noise predominant in the detected image. 相似文献
13.
14.
Trapped protons are responsible for the main component of LEGRIbackground. Detailed theoretical model has demonstrated that theproton-induced counting rate is two orders of magnitude larger thanthe counting rate of the diffuse gamma-ray flux. The continuous passesof LEGRI through the SAA (7 times everyday) makes very difficult thebackground modelling. Long and short lived isotopes contribute in verydifferent time scales to the proton-induced background component.The goal of this paper is to present a comparison between the long-livedbackground noise theoretical predictions and the experimental data. Theresults show an unexpected good agreement between the predicted and theobserved counting rates. 相似文献
15.
16.
Michail I. PETAEV 《Meteoritics & planetary science》1996,31(6):807-815
Abstract— Olivine grains from selected meteorites (the Springwater pallasite, the Lowicz mesosiderite, the ALH 84025 brachinite, the Krymka LL3 chondrite, and the Calcalong Creek lunar meteorite) and terrestrial rocks (San Carlos forsterite and Rockport fayalite) were studied by optical microscopy and high-precision electron microprobe analysis. Detailed microprobe traverses revealed regular igneous zoning in the Krymka and Calcalong Creek olivines. Traverses across the San Carlos forsterite grain are flat and display no chemical variations larger than the 2σ range of counting error (±0.2 mol% Fa). Traverses across olivine grains in the ALH 84025, Lowicz, and Springwater meteorites show regular patterns of periodic or wavy chemical variations well exceeding the 2σ uncertainty range. However, no lamellar structure was seen in backscattered electron images. It is suggested that the periodic chemical variations may be due to spinodal decomposition of primary, more or less homogeneous grains. I conclude that the absence of earlier reports of such variations simply means that olivine grains in equilibrated meteorites have not been examined closely enough to detect them. 相似文献
17.
P. Senthil Kumar A. Senthil Kumar J.N. Goswami A.S. Kiran Kumar 《Planetary and Space Science》2011,59(9):870-879
Analysis of the Chandrayaan-1 Terrain Mapping Camera image of a 20 km×27 km area in the Mare Imbrium region revealed a cluster of thousands of fresh and buried impact craters in the size range of 20-1300 m. A majority of the large fresh craters with diameter ranging from 160 to 1270 m exhibit near-circular mounds (30-335 m diameter and 10-40 m height) in the crater floor, and their size depends on the host crater size. The origin of this cluster of secondary craters may be traced to Copernicus crater, based on global lunar image and the analysis of Chandrayaan-1 Hyper Spectral Imager data. Our findings provide further evidence for secondary crater formation by low-velocity impact of a cloud of clustered fragments. The presence of central mounds can also distinguish the secondary craters from the primary craters and refine the chronology of lunar surface based on counting of small craters. 相似文献
18.
Solar Physics - Normal mode spectra and neutrino counting rates are calculated for a set of chemically-inhomogeneous solar models. Each model has a core with a high concentration of heavy elements;... 相似文献
19.
Electron densities have been measured from [Sii] 6716/6731 Å line ratios for a grid of points over the surface of the planetary nebula NGC 7009 using a photon counting detector. The radial dependence of the electron density has been modelled, and the relationship provides possible evidence that the planetary nebula shell is driven by a strong stellar wind. 相似文献
20.
在光子计数资料的处理方法设计不完善时，有可能会大大影响光电等高仪的观测结果。本文提出了一套从平滑，搜索到定中心的光子计数资料处理方法，通过与原方法进行了９天（含满月夜）的同步处理比较，新的处理方法得到的星数是原方法结果的１．４８倍，单星测定精度提高０″０２，并且基本克服了天光的影响。 相似文献