H. Bergeret

Measurement of the Crab Flux above 60 GeV with the CELESTE Cerenkov Telescope

We have converted the former solar electrical plant THEMIS (French Pyrenees) into an atmospheric Cerenkov detector called CELESTE, which records gamma rays above 30 GeV (7 × 1024 Hz). Here we present the first sub-100 GeV detection by a ground-based telescope of a gamma-ray source, the Crab Nebula, in the energy region between satellite measurements and imaging atmospheric Cerenkov telescopes. At our analysis threshold energy of 60 ± 20 GeV we measure a gamma-ray rate of 6.1 ± 0.8 minute-1. Allowing for 30% systematic uncertainties and a 30% error on the energy scale yields an integral gamma-ray flux of I(E > 60GeV) = 6.2 × 10-6 photons m-2 s-1. The analysis methods used to obtain the gamma-ray signal from the raw data are detailed. In addition, we determine the upper limit for pulsed emission to be less than 12% of the Crab flux at the 99% confidence level, in the same energy range. Our result indicates that if the power law observed by EGRET is attenuated by a cutoff of form e, then E0 < 26 GeV. This is the lowest energy probed by a Cerenkov detector and leaves only a narrow range unexplored beyond the energy range studied by EGRET.

CELESTE: an atmospheric Cherenkov telescope for high energy gamma astrophysics

Abstract CELESTE is an atmospheric Cherenkov telescope based on the sampling method which makes use of the de-commissioned THEMIS solar electrical plant in the French Pyrenees. A large (2000 m 2 ) mirror surface area from 40 independent heliostats followed by a secondary optic, a trigger system using analog summing techniques and signal digitization with 1 GHz flash ADCs make possible the detection of cosmic γ-rays down to 30 GeV . This paper provides a detailed technical description of the CELESTE installation.

High quantum yield from photofield emitters

Abstract Nd-YAG lasers, operated in the ultraviolet, visible and infrared regions of the electromagnetic radiation spectrum at ps and ns pulse widths, have been used to produce high photoemission electron currents from single and array emitters. Photoemitted current densities of 10 12 and 10 10 A/cm 2 were obtained, respectively, in the ps and ns regimes. Quantum yields of 0.75 were obtained for IR light and exceeding unity for green and ultraviolet light in both regimes. Explanation of such high quantum yields is strongly tied to the field effect and to other physical properties of the microemitters.

A binary Al/Li alloy as a new material for the realization of high-intensity pulsed photocathodes

Abstract We propose a new material for the fabrication of high-current photocathodes: a binary Al/Li alloy acting as a lithium dispenser cathode. This material would have the great advantage to allow regeneration of the Li layer after poisoning or air exposure, by a simple heating process. In a first experiment, we have measured the photoemission energy threshold, W Φ , of a piece of Al/Li alloy and the quantum yield, Y , as a function of the photon energy. After a heating process (340°C for 12 h) we obtained W Φ = 2 eV and Y = 6 × 10 −4 for 4.6 eV photon energy. In a second experiment another sample was illuminated with a 40 ps frequency-tripled YAG laser. After two heating processes, we obtained electron bunches containing 1 nC with an incident laser energy of 100 μJ per pulse.

Pulsed photocurrents from lanthanum hexaboride cathodes in the ns regime

Abstract Photocathode emission investigation represents an essential part in developing new rf sources and new bunched electron guns. In order to determine the photoemissivity of different materials with work functions which can be easily lowered, we studied lanthanum hexaboride (LaB 6 )as a first step. The results of measurements with a nonosecond NdYAG laser, using wavelengths from 1064 to 355 nm, show that pulsed photocurrent densities can reach 7 kA/cm 2 . Quantum yields evolved from 10 −7 to 10 −3 with the lowering of the work function when cleaning the surface under laser exposition. The results also show that with the use of high density photon beams of ≈ 45 J/cm 2 , lanthanum desorption occurs at the surface. Suggestions are given for a cure of this desorption problem.

Short pulse photoemission from a dispenser cathode

Abstract Pulsed photoemission in the picosecond regime has been obtained from a standard thermionic dispenser cathode (WBaCa) at temperatures below the measurable thermoemission threshold. A picosecond Nd : YAG mode locked laser has been used at both green and UV light. Micropulse charges up to 0.5 nC have been measured on a wideband coaxial pickup located behind the anode. They correspond to an electron saturation limit from an approximately 20 mm 2 illuminated cathode area with a surface field of 3 MV/m. The effective cathode efficiency at small laser energies, defined as the number of electrons impinging on the coaxial pickup divided by the number of photons impinging on the cathode, is about 2 × 10 −5 .

Laser-induced electron emission from granular gold films

Abstract We report on measurements of the laser-driven electron emission from a granular gold film. The light source was a picosecond Nd:YAG laser. We illuminated the sample using the harmonics of this laser fundamental frequency (wavelengths 532, 355 and 266 nm). By comparison with a smooth gold thin film, the emitted current is three orders of magnitude higher for the granular gold film at 532 nm. This enhancement is due to plasmon resonances in the gold particles. We discuss the possibility of using such a granular gold film as an electron source in a photoinjector.

Two‐photon photoeffect from resonant silver structures

We report the two‐photon photoelectric sensitivities of a granular silver film and a silver covered diffraction grating at the photon energy ℏω=3.5 eV of a frequency‐tripled Nd:YAG laser when surface plasmons are excited. Values higher than 3×10−15 A cm2 W−2 have been obtained with both structures which correspond to a one‐photon quantum yield of 1×10−5 under a 1‐GW‐cm−2 incident laser power density. These photoemitters could be used as high current density electron sources by illuminating them with picosecond or subpicosecond lasers in the 2–4‐eV spectral range.

Photofield emission in the picosecond regime

Abstract Some results of pulsed photofield emission in the picosecond regime using UV and green lights with microphotocathodes are given below.