D.J. Morris

Publications (6)4.33 Total impact

• Article: Evidence for 56Co Line Emission from the Type Ia Supernova 1991T using COMPTELa

  D.J. MORRIS, K. BENNETT, H. BLOEMEN, W. HERMSEN, G.G. LICHTI, M. L. McCONNELL, J.M. RYAN, V. SCHÖNFELDER
  Annals of the New York Academy of Sciences 12/2006; 759(1):397 - 400. · 3.15 Impact Factor
• Article: COMPTEL measurements of the omnidirectional high-energy neutron flux in near-earth orbit.

  D J Morris, H Aarts, K Bennett, J A Lockwood, M L McConnell, J M Ryan, V Schonfelder, H Steinle, G Weidenspointner
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  ABSTRACT: On four occasions, twice in 1991 (near solar maximum) and twice in 1994 (near solar minimum), one COMPTEL D1 detector module was used as an omnidirectional detector to measure the high-energy (> 12.8 MeV) neutron flux near an altitude of 450 km. The D1 modules are cylindrical, with radius 13.8 cm and depth 8 cm, and are filled with liquid scintillator (NE213A). The combined flux measurements can be fit reasonably well by a product of the Mt. Washington neutron monitor rate, a linear function in the spacecraft geocenter zenith angle, and an exponential function of the vertical geomagnetic cutoff rigidity in which the coefficient of the rigidity is a linear function of the neutron monitor rate. When pointed at the nadir, the flux is consistent with that expected from the atmospheric neutron albedo alone. When pointed at the zenith the flux is reduced by a factor of about 0.54. Thus the production of secondary neutrons in the massive (16000 kg) Compton Gamma-Ray Observatory spacecraft is negligible. Rather, the mass of the spacecraft provides shielding from the earth albedo.
  Advances in Space Research 01/1998; 21(12):1789-92. · 1.18 Impact Factor
• Conference Proceeding: Activation in the COMPTEL double-scattering gamma-ray telescope

  D.J. Morris, M.L. McConnell, J.M. Ryan, R. Diehl, U.G. Oberlack, V. Schonfelder, M. Varendorff, G. Weidenspointner, H. De Boer, K. Bennett
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  ABSTRACT: The COMPTEL gamma-ray telescope has been operating in low Earth orbit for six years, since the launch of the Compton Gamma-Ray Observatory in April 1991. Comparisons of data for different orbits and epochs show evidence of activation on time scales from minutes (²⁷Mg, τ_1/2=9.5 min) to years (²²Na, τ _1/2=2.58 yr). The activation is correlated with both the orbital altitude and solar cosmic-ray modulation. Because it requires coincident measurements in two different detectors, COMPTEL is most susceptible to instrumental background events in which two or more photons are produced simultaneously
  High Energy Radiation Background in Space, 1997 Conference on the; 08/1997
• Article: Neutron measurements in near-Earth orbit with COMPTEL

  D.J. Morris, H. Aarts, K. Bennett, J. A. Lockwood, M.L. McConnell, J. M. Ryan, V. Schoenfelder, H. Steinle, X. Peng
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  ABSTRACT: The fast neutron flux in near-Earth orbit has been measured with the COMPTEL instrument on the Compton Gamma Ray Observatory (CGRO). For this measurement one of COMPTEL's seven liquid scintillator modules was used as an uncollimated neutron detector with threshold of 12.8 MeV. The measurements cover a range of 4.8 to 15.5 GV in vertical cutoff rigidity and 3 deg to 177 deg in spacecraft geocenter zenith angle. One of the measurements occurred near the minimum of the deepest Forbush decrease ever observed by ground-level neutron monitors. After correction for solar modulation, the total flux is well fitted by separable functions in rigidity and zenith angle. With the spacecraft pointed near the nadir the flux is consistent with balloon measurements of the atmospheric neutron albedo. The flux varies by about a factor of 4 between the extremes of rigidity and a factor of 2 between the extremes of zenith angle. The effect of the spacecraft mass in shielding the detector from the atmospheric neutron albedo is much more important than its role as a source of additional secondary neutrons. The neutron spectral hardness varies little with rigidity or zenith angle and lies in the range spanned by earlier atmospheric neutron albedo measurements.
  08/1995;
• Source

  Available from: articles.adsabs.harvard.edu

  Article: Neutron induced background in the COMPTEL detector on the Gamma Ray Observatory

  D. J. Morris, H. Aarts, K. Bennett, M. Busetta, R. Byrd, W. Collmar, A. Connors, R. Diehl, G. Eymann, C. Foster
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  ABSTRACT: Interactions of neutrons in a prototype of the Compton imaging telescope (COMPTEL) gamma ray detector for the Gamma Ray Observatory were studied to determine COMPTEL's sensitivity as a neutron telescope and to estimate the gamma ray background resulting from neutron interactions. The IUCF provided a pulsed neutron beam at five different energies between 18 and 120 MeV. These measurements showed that the gamma ray background from neutron interactions is greater than previously expected. It was thought that most such events would be due to interactions in the upper detector modules of COMPTEL and could be distinguished by pulse shape discrimination. Rather, the bulk of the gamma ray background appears to be due to interactions in passive material, primarily aluminum, surrounding the D1 modules. In a considerable fraction of these interactions, two or more gamma rays are produced simultaneously, with one interacting in the D1 module and the other interacting in the module of the lower (D2) detector. If the neutron interacts near the D1 module, the D1 D2 time of flight cannot distinguish such an event from a true gamma ray event. In order to assess the significance of this background, the flux of neutrons in orbit has been estimated based on observed events with neutron pulse shape signature in D1. The strength of this neutron induced background is estimated. This is compared with the rate expected from the isotropic cosmic gamma ray flux.
  01/1992; 3137:102-108.
• Article: Activation in the COMPTEL Double-Scattering Gamma-Ray Telescope

  D. J. Morris, M. L. McConnell, J. M. Ryan, R. Diehl, U. G. Oberlack, V. Schönfelder, M. Varendorff, G. Weidenspointer, H. De Boer, K. Bennett
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  ABSTRACT: COMPTEL is a double-scattering telescope designed for gamma-ray astronomy in the energy range 0.75-30 MeV. It has two sets of detector modules arrayed in two planes separated by about 1.5 m. The upper (D1) detector consists of seven cylindrical modules filled with NE213A liquid scintillator, each with radius 13.8 cm and depth 8 cm and viewed by eight photomultipliers around the side. The lower (D2) detector consists of 14 cylindrical NaI modules, each with radius 14 cm and depth 7.5 cm and viewed from below by seven photomultipliers. Each detector is surrounded by plastic charged-particle shields. A photon is detected when it scatters in the D1 detector followed by an interaction of the scattered photon in the D2 detector. Ideally, the scattered photon is absorbed in a D2 module. The total energy measured in the two detectors is then the energy of the incident photon, and the ratio of the energies in the two detectors, together with the interaction locations in the detectors, allow the direction of the incident photon to be localized to a circle on the sky. The time between the two interactions and the pulse shape in the D1 detector are used to discriminate against events which are not caused by single photons from within the telescope's normal field of view, extending about 40 deg. from the telescope axis.
  -1:26.