Milagrito, a detector sensitive to very high energy gamma rays, monitored the northern sky from 1997 February through 1998 May. With a large field of view and a high duty cycle, this instrument was well suited to perform a search for TeV gamma-ray bursts (GRBs). We report on a search made for TeV counterparts to GRBs observed by BATSE. BATSE detected 54 GRBs within the field of view of Milagrito during this period. An excess of events coincident in time and space with one of these bursts, GRB 970417a, was observed by Milagrito. The excess has a chance probability of 2.8x10-5 of being a fluctuation of the background. The probability for observing an excess at least this large from any of the 54 bursts is 1.5x10-3. No significant correlations were detected from the other bursts.
We evaluated a compact, high resolution PET dctector module using avalanche photodiode (APD) arrays to replacc bulky position sensitive PMTs. The newly developed APD array is a planar processed 4x4 array which has a 2x2 mm' pixel size with 0.4 mm gaps between pixcls, about 60% quantum efficiency at 420 nm wavelength, and uniform high gain (> 1000) across all channels. A 4x4 array of 2xZx10 mm3 LSO crystals was coupled to an APD array. Different readout elcctronics and signal multiplexing schemes were explored. All crystals in the dctcctor array wcre clearly identified in the flood source histogram, with average pcak-to-valley ratios of about 121 using a charge sharing resistor network. The encrgy resolution was measured to be -14% at 511 keV in the detector array. The measurcd timing resolution was 2.6 ns in coincidence with a LSOlPMT detector. By optimizing thc readout electronics currently being uscd, it is likely that dctcctor performencc can be further improvcd. We have also determined depth-or-interaction (DOI) by rcading out two APD arrays connected to thc ends of a 2 x 2~2 2 mm' LSO crystal.Preliminary measurcments show good DO1 measurement capability with DO1 posilioning unccrtainty betwccn 4 and 6.5 mm.
The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a new ground-based atmospheric Cherenkov telescope for gamma-ray astronomy. STACEE uses the large mirror area of a solar heliostat facility to achieve a low energy threshold. A prototype experiment which uses 32 heliostat mirrors with a total mirror area of ∼ 1200 m 2 has been constructed. This prototype, called STACEE-32, was used to search for high energy gamma-ray emission from the Crab Nebula and Pulsar. Observations taken between November 1998 and February 1999 yield a strong statistical excess of gamma-like events from the Crab, with a significance of +6.75σ in 43 hours of on-source observing time. No evidence for pulsed emission from the Crab Pulsar was found, and the upper limit on the pulsed fraction of the observed excess was < 5.5% at the 90% confidence level. A subset of the data was used to determine the integral flux of gamma rays from the Crab. We report an energy threshold of E th = 190 ± 60 GeV, and a measured integral flux of I(E > E th ) = (2.2 ± 0.6 ± 0.2) × 10 −10 photons cm −2 s −1 . The observed flux is in agreement with a continuation to lower energies of the power law spectrum seen at TeV energies.
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