Engineering Papers⌕ Search

Engineering topics

Spada, G.

Publications and source records attributed to Spada, G..

11 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Lower-mantle viscosity constrained by seismicity around deglaciated regions

It is shown here that seismicity around the margins of deglaciated areas provides a constraint on the viscosity of the lower mantle. Calculations using a spherical, viscoelastic earth model show that the present-day magnitude of the stress fields induced in the lithosphere beneath the Laurentide and Fennoscandian ice sheets is very sensitive to the value of the lower-mantle viscosity. Stress of about 100 bar, sufficient to cause seismicity, can still remain in the lithosphere for lower-mantle viscosities greater than about 10 to the 22nd Pa-s; for lower-mantle viscosities of about 10 to the 21st Pa-s, only a few tens of bars of stress persist in the lithosphere today. This influence of lower-mantle viscosity on the state of stress in the lithosphere also has implications for the migration of stress from earthquakes, and hence for earthquake recurrence times.

Spada, G.↗
NASA NTRS1985

An experimental assessment of the imaging quality of the low energy gamma-ray telescope ZEBRA

One gamma-ray detection plane of the ZEBRA telescope, consisting of nine position sensitive scintillation crystal bars designed to operate over the spectral range 0.2 to 10 MeV, has been constructed in the laboratory. A series of experimental images has been generated using a scaled down flight pattern mask in conjunction with a diverging gamma-ray beam. Point and extended sources have been imaged in order to assess quantitatively the performance of the system.

Butler, R. C.↗
NASA NTRS1974

Limits on rapid X-ray pulsing in X-ray binaries

A recent sounding rocket experiment in which the X-ray variations of Cir X-1 and Vela X-1 were investigated at time resolutions down to 1 msecond is described. Sporadic flaring activity in the X-ray sources was observed on time scales from 1 to several seconds, corresponding to intensity changes of about 20% in Cir X-1 and up to 100% in Vela X-1. Periodic, aperiodic, or quasi-periodic pulsations were not detected with time constants less than 1 second. The limits of rapid X-ray pulsations are shown to range from 3 to 15% for Cir X-1, Cyg X-1, Vela X-1, and 3U 1700-37.

Spada, G.↗
NASA NTRS1973

Description of small-scale fluctuations in the diffuse X-ray background.

An analytical study of the fluctuations on a small angular scale expected in the diffuse X-ray background in the presence of unresolved sources is presented. The source population is described by a function N(S), giving the number of sources per unit solid angle and unit apparent flux S. The distribution of observed flux, s, in each angular resolution element of a complete sky survey is represented by a function Q(s). The analytical relation between the successive, higher-order moments of N(S) and Q(s) is described. The goal of reconstructing the source population from the study of the moments of Q(s) of order higher than the second (i.e., the rms fluctuations) is discussed.

Cavaliere, A.↗
NASA NTRS1973

X-ray pulse profile and celestial position of Hercules X-1.

The celestial position of the binary X-ray source Her X-1 has been measured with a precision of 30 sec and, within the uncertainties, agrees with the position of the optical variable HZ Herculis. An average light curve for the 1.24-sec periodicity has been obtained with about 10-msec resolution. The average pulse is a double-peaked structure which shows significant intensity changes in time scales down to 30 msec.

Doxsey, R.↗