No abstract

The diffuse extragalactic γ-ray background in the MeV region is believed to be due to photons from radioactivity produced in supernovae throughout the history of galaxies in the universe. In particular, γ-ray line emission from the decay chain 56 Ni → 56 Co → 56 Fe provides the dominant photon source (Clayton & Silk 1969). Although iron synthesis occurs in all types of supernovae, the contribution to the background is dominated by Type Ia events due to their higher photon escape probabilities. Estimates of the star formation history in the universe suggest a rapid increase by a factor ∼ 10 from the present to a redshift z p ∼ 1.5, beyond which it either remains constant or decreases slowly. Little is known about the cosmological star formation history for redshift exceeding z ∼ 5. We integrate the observed star formation history to determine the Cosmic Gamma-Ray Background (CGB) from the corresponding supernova rate history. In addition to γ-rays from short-lived radioactivity in SNIa and SNII/Ib/Ic we also calculate the minor contributions from long-lived radioactivities ( 26 Al, 44 Ti, 60 Co, and electron-positron pair annihilation). The time-integrated γ-ray spectrum of model W10HMM (Pinto & Woosley 1988a, Pinto & Woosley 1988b was used as a template for Type II supernovae, and for SNIa we employ model W7 (Nomoto et al. 1984). Although progenitor evolution for Type Ia supernovae is not yet fully understood, various arguments suggest delays of order 1−2 Gy between star formation and the production of SNIa's. The effect of this delay on the CGB is discussed. We emphasize the value of γ-ray observations of the CGB in the MeV range as an independent tool for studies of the cosmic star formation history. If the delay between star formation and SNIa activity exceeds 1 Gy substantially, and/or the peak of the cosmic star formation rate occurs at a redshift much larger than unity, the γ-ray production of SNIa would be insufficient to explain the observed CGB and a so far undiscovered source population would be implied. Alternatively, the cosmic star formation rate would have to be higher (by a factor 2-3) than commonly assumed, which is in accord with several upward revisions reported in the recent literature.

No abstract

The cold neutron imaging and diffraction instrument IMAT at the second target station of the pulsed neutron source ISIS is currently being commissioned and prepared for user operation. IMAT will enable white-beam neutron radiography and tomography. One of the benefits of operating on a pulsed source is to determine the neutron energy via a time of flight measurement, thus enabling energy-selective and energy-dispersive neutron imaging, for maximizing image contrasts between given materials and for mapping structure and microstructure properties. We survey the hardware and software components for data collection and image analysis on IMAT, and provide a step-by-step procedure for operating the instrument for energy-dispersive imaging using a two-phase metal test object as an example.

A fast time response, wide dynamic range neutron flux monitor has been developed toward the LHD deuterium operation by using leading-edge signal processing technologies providing maximum counting rate up to ∼5 × 109 counts/s. Because a maximum total neutron emission rate over 1 × 1016 n/s is predicted in neutral beam-heated LHD plasmas, fast response and wide dynamic range capabilities of the system are essential. Preliminary tests have demonstrated successful performance as a wide dynamic range monitor along the design.