Yosef Eisen

Evidence for field enhanced electron capture by EL2 centers in semi-insulating GaAs and the effect on GaAs radiation detectors

The performance of Schottky contact semiconductor radiation detectors fabricated from semi‐insulating GaAs is highly sensitive to charged impurities and defects in the material. The observed behavior of semi‐insulating GaAs Schottky barrier alpha particle detectors does not match well with models that treat the semi‐insulating material as either perfectly intrinsic or as material with deep donors (EL2) of constant capture cross section compensated with shallow acceptors. We propose an explanation for the discrepancy based on enhanced capture of electrons by EL2 centers at high electric fields and the resulting formation of a quasineutral region in the GaAs. Presented is a simple model including field enhanced electron capture which shows good agreement with experimental alpha particle pulse height measurements.

Present status of undoped semi-insulating LEC bulk GaAs as a radiation spectrometer

Abstract Bulk GaAs has undergone extensive research by several groups in order to ascertain its usefulness as a room temperature radiation spectrometer. The results of an experimental program studying the properties of detectors fabricated from bulk GaAs are summarized in this paper. Electric field models of the active region are compared with measured results. Limitations of bulk LEC GaAs as a material for radiation spectrometers are discussed.

Development of bulk GaAs room temperature radiation detectors

Various configurations of Schottky diode detectors were fabricated with bulk crystals of liquid encapsulated Czochralski (LEC) semi-insulating undoped GaAs material. Basic detector construction utilized one Ti/Au Schottky contact and one Au/Ge/Ni alloyed ohmic contact. Pulsed X-ray analysis indicated pulse decay times dependent on bias voltage. Pulse height analysis disclosed nonuniform electric field distributions across the width of the detectors, best explained as a consequence of native deep level donors (EL2) in the crystal. Pulse height spectra measured from an /sup 241/Am alpha-particle source at room temperature resulted in a resolution ranging from 2.2% to 3.1% at full width at half maximum (FWHM) for different detectors with a typical resolution of 2.5%. Low-energy gamma rays measured under room-temperature operating conditions resulted in observed full energy peaks of 60 keV and 122 keV photons with measured FWHMs of 22 keV and 40 keV, respectively.<<ETX>>

Bulk GaAs room temperature radiation detectors

Abstract Bulk GaAs, a wide band gap semiconductor, shows potential as a room temperature radiation detector. Schottky diode detectors were fabricated from LEC bulk GaAs crystals. The basic construction of these diodes employed the use of a Ti/Au Schottky contact and a Au/Ge/Ni alloyed ohmic contact. Pulse height characteristics of these diodes indicate active regions of more than 100 μm. Pulse height spectra were recorded from alpha particle irradiation of the Schottky contact surface resulting in a best energy resolution of 2.5% at 5.5 MeV. Low energy gamma rays measured under room temperature operating conditions resulted in photopeaks with 37% FWHM at 60 keV.

A real-time pulsed photon dosimeter

Abstract Radiation sources producing short pulses of photon radiation are now widespread. Such sources include electron and proton linear accelerators, betatrons, synchrotrons, and field-emission impulse generators. It is often desirable to measure leakage and skyshine radiation from such sources in real time, on a single-pulse basis as low as 8.7 nGy (1 μR) per pulse. This paper describes the design and performance of a prototype, real-time, pulsed photon dosimeter (PPD) capable of single-pulse dose measurements over the range from 3.5 nGy to 3.5 μGy (0.4 to 400 μR). The PPD may also be operated in a multiple-pulse mode that integrates the dose from a train of radiation pulses over a 3-s period. A pulse repetition rate of up to 300 Hz is accommodated. The design is eminently suitable for packaging as a lightweight, portable, survey meter. The PPD uses a CdWO 4 scintillator optically coupled to a photodiode to generate a charge at the diode output. A pulse amplifier converts the charge to a voltage pulse. A digitizer circuit generates a burst of logic pulses whose number is proportional to the peak value of the voltage pulse. The digitizer output is recorded by a pulse counter and suitably displayed. A prototype PPD was built for testing and evaluation purposes. The performance of the PPD was evaluated with a variety of pulsed photon sources. The dynamic range, energy response, and response to multiple pulses were characterized. The experimental data confirm the viability of the PPD for pulsed photon dosimetry.

A liquid-lithium target project for production of high-intensity quasi-stellar neutrons

A windowless Liquid-Lithium Target (LiLiT) is under construction and development at Soreq NRC (Israel). The target is designed to be bombarded by a 2-4 mA proton beam (Ep= 1.8-2.5 MeV) from the high-intensity Soreq Applied Research Accelerator Facility (SARAF), a superconducting linear accelerator for light ions. The liquid-lithium forced flow at a velocity of ~20 m/s and a thickness of ~1.8 mm serves both as a power dump (10 kW) for the proton beam and as a neutron-producing target via the 7 Li(p,n) 7 Be reaction. As known from the work of the Forschungszentrum Karlsruhe group, the energy distribution of neutrons emitted for a proton energy Ep= 1.912 MeV, ~30 keV above the reaction threshold, and a thick Li target is very similar to that of a Maxwell-Boltzmann flux at a thermal energy of ~25 keV, well suited for activation measurements relevant to s-process nucleosynthesis. The neutron intensity expected under these conditions from the combination of the SARAF proton beam and the LiLiT thermal properties is of ~2×10 10 s -1 mA -1 , and is larger by more than one order of magnitude than currently available. The LiLiT setup is built as a loop circulating liquid lithium at a temperature of ~200 o C and producing a jet (acting as the target) onto a thin concave supporting wall, driven by a rotating magnet inductive electromagnetic pump. The liquid lithium is collected in a reservoir housing a heat exchanger with a mineral-oil closed loop. Circulation and thermal tests of the loop are presently in progress in an offline dedicated electron-gun laboratory and online installation at the SARAF accelerator is planned for end 2010. Characterization of the SARAF proton beam (beam energy, energy width and transverse profile) and of the neutron spectrum obtained under these conditions are studied in parallel using a solid-lithium (lithium fluoride) target at low beam intensities. The SARAF-LiLiT system will be used to measure stellar neutron capture cross sections for stable or radioactive targets demanding high neutron fluxes. Present status and plans are discussed.

FIRST NUCLEAR-ASTROPHYSICS EXPERIMENTS WITH HIGH-INTENSITY NEUTRONS FROM THE LIQUID-LITHIUM TARGET LiLiT

A high-intensity neutron source based on a Liquid-Lithium Target (LiLiT) and the Li(p,n) reaction was developed at SARAF (Soreq Applied Research Accelerator Facility, Israel). The setup is used for nuclear-astrophysics experiments owing to the quasi-Maxwellian shape of the neutron energy distribution at stellar thermal energies (kT ~ 30 keV). The LiLiT device consists of a forced-flown (> 2 m/s) film of liquid lithium (~200 C) whose free surface is bombarded by a proton beam. The lithium film acts both as the neutron-producing target and as a power beam dump. The setup was commissioned with a 1.2 mA proton beam at 1.91 MeV, producing a neutron yield (peaked at ~28 keV) of ~ 3 ×10 n/s, more than one order of magnitude larger than conventional Li(p,n)-based neutron sources. The target dissipates a peak power areal density of 2.5 kW/cm and a peak power volume density of 500 kW/cm with no significant temperature or vacuum pressure elevation in the target chamber. We present preliminary results of first activation measurements on Zr and Ce stable isotopes performed with the SARAF-LiLiT setup, using Au as neutron monitor and of the determination of their Maxwellian-averaged neutron capture cross section.

Energy-broadened proton beam for production of quasi-stellar neutrons from the 7Li(p,n)7Be reaction

Production of quasi-stellar neutrons by the 7Li(p,n)7Be reaction has been used for measuring s-process cross sections and efforts to upgrade the proton beam intensity with RF linear accelerators are presently ongoing. We investigated the effect of an energy-broadened proton beam, as is expected for a RF linear accelerator, on the produced 25-keV quasi-Maxwellian neutron spectrum and compared it to that of a Van de Graaff accelerator with well-defined proton energy. Neutron spectrum measurements from 0° to 80° were carried out with a pulsed proton beam at the IRMM Van de Graaff accelerator using time-of-flight techniques, both with a narrow energy spread (σ≈1.5 keV) proton beam and with an energy-broadened beam (σ≈20 keV) obtained by straggling through a Au-foil degrader. In the latter case, the neutron spectrum is closer to the Maxwellian flux distribution in the high neutron energy region.