R. Mirzoyan

The Crab Nebula and Pulsar between 500 GeV and 80 TeV: Observations with the HEGRA Stereoscopic Air Cerenkov Telescopes

The Crab supernova remnant has been observed regularly with the stereoscopic system of 5 imaging air Cherenkov telescopes that was part of the High Energy Gamma Ray Astronomy (HEGRA) experiment. In total, close to 400 hours of useful data have been collected from 1997 until 2002. The spectrum extends up to energies of 80 TeV and is well matched by model calculations in the framework of inverse Compton scattering of various seed photons in the nebula including for the first time a recently detected compact emission region at mm-wavelengths. The observed indications for a gradual steepening of the energy spectrum in data is expected in the inverse Compton emission model.The average magnetic field in the emitting volume is determined to be

Studies with a prototype high resolution PET scanner based on LSO-APD modules

(161.6\pm0.8mathrm{stat}\pm18_\mathrm{sys}) \mu

Design and performance of the angle integrating Čerenkov array AIROBICC

G. The presence of protons in the nebula is not required to explain the observed flux and upper limits on the injected power of protons are calculated being as low as 20 % of the total spin down luminosity for bulk Lorentz factors of the wind in the range of

Ultrathin entrance windows for silicon drift detectors

10^4-10^6

Detection of gamma rays above 1 TeV from the Crab Nebula by the second HEGRA imaging atmospheric Cherenkov telescope at La Palma

.The position and size of the emission region have been studied over a wide range of energies. The position is shifted by 13\arcsec to the west of the pulsar with a systematic uncertainty of 25\arcsec. No significant shift in the position with energy is observed. The size of the emission region is constrained to be less than 2\arcmin at energies between 1 and 10 TeV. Above 30 TeV the size is constrained to be less than 3\arcmin.No indications for pulsed emission has been found and upper limits in differential bins of energy have been calculated reaching typically 1-3 % of the unpulsed component.

The first telescope of the HEGRA air Cherenkov imaging telescope array

Lutetium oxyorthosilicate (LSO) scintillation crystals coupled to avalanche photodiodes (APDs) have been proven to be an interesting detector module for high resolution positron emission tomography (PET). Compact detector modules were built to test their performance in a prototype tomograph. Each detector module consisted of sixteen 3.7/spl times/3.7/spl times/12.0 mm/sup 3/ LSO crystals, which are one-to-one coupled to a two dimensional APD array (2/spl times/8 diodes, 3 mm O, 4 mm spacing). The signals were read out by a low noise, fast, charge sensitive preamplifier, a 50 ns shaping amplifier and CAMAC electronic modules. Two detector modules were mounted on a rotatable gantry, to simulate a detector ring. Long term runs over 110 hours have shown that the detector remains stable over a long period of time. The energy resolution (FWHM) was 13.4% for 511 keV gamma rays and the intrinsic spatial resolution was 2.2 mm (FWHM). The gain dependence of the APD on temperature had been determined as 3.5% K. The time resolution was 3.2/spl plusmn/0.1 ns (FWHM). Phantom measurements showed the suitability of the new LSO/APD detector for high resolution animal PET.

A search for gamma radiation above 24 TeV energy from cosmic point sources

Abstract An air-shower array consisting of 49 open huts with photomultiplier tubes viewing the night sky over a solid angle of about 1 steradian is described. The array is operating in conjunction with the HEGRA installation. The main objectives of the experiment are VHE to UHE γ-ray astronomy and a determination of the chemical composition of cosmic rays. The design principles, construction and performance in angular resolution, energy threshold and determination of shower parameters are discussed.

ON THE INFLUENCE OF AFTERPULSING IN PMTS ON THE TRIGGER THRESHOLD OF MULTICHANNEL LIGHT DETECTORS IN SELF-TRIGGER MODE

Abstract Detectors with ultrathin entrance windows have been fabricated, which show an overall improvement of the detector performance in the optical and X-ray region as well as for heavy ions. The quantum efficiency was higher than 60% within the entire wavelength range between 200 nm and 800 nm. In the soft X-ray region the spectroscopic resolution could be improved significantly. For the MnKα line a peak to valley ratio of 5700:1 was achieved. Measurements with241Am α-particles revealed an effective “dead” layer width of less than 150A. The compatibility of the technology to produce thin entrance windows with the planar process allows its application on various pn-junction detector designs. A new silicon drift detector with a total area of 21 mm2 was successfully tested and operated at count rates up to 3 × 107s−1cm−2. At room temperature, the devices have shown an energy resolution for the MnKα line of 227 eV (FWHM) with shaping times of 250–500 ns, decreasing to 152 eV at −20°C. The fast readout in combination with a large detector area, a homogeneous entrance window and an exceptionally low noise without the need of an extensive cooling system makes them especially suited for spectroscopic applications in non-laboratory environments.

A method to enhance the sensitivity of photomultipliers for air Cherenkov telescopes

Abstract The results of observations of the Crab Nebula by the second HEGRA imaging atmospheric Cherenkov telescope during the period 1994/1995 are presented. The signal, detected at a level of 10 σ , allows one to estimate the flux and the spectral index of the γ -ray energy spectrum above 1 TeV. The analysis, based on the comparison of measurements with comprehensive Monte Carlo simulations, reveals a γ -ray flux F γ (≥ 1 TeV) ⋍ 8 × 10 −12 photons cm −2 s −1 , and indicates a rather steep integral spectrum in the energy region 1–3 TeV with a power-law index α ⋍ 1.6–1.8.

Search for isotropic γ radiation in the cosmological window between 65 and 200 TeV

Abstract In search of VHE γ ray emission from cosmic point sources a system of imaging Cherenkov telescopes is constructed at present on the Canarian island of La Palma; the first telescope has been operational since 1992. The Cherenkov light from air shower particles is collected by a 5 m2 reflector. The camera at the focus contains 37 photomultipliers which sample the images of the Cherenkov flashes. The subsequent image analysis allows the discrimination of γ ray induced events from the much more abundant charged cosmic ray induced showers. The telescope has an effective energy threshold for γ showers of about 1.5 TeV. During the first year of operation a signal from the Crab nebula was detected.