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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71231/2/JAPIAU-75-12-7910-1.pd

Erratum: ‘‘Evidence for field enhanced electron capture by EL2 centers in semi‐insulating GaAs and the effect on GaAs radiation detectors’’ [J. Appl. Phys. 75, 7910 (1994)]

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69985/2/JAPIAU-77-3-1331-1.pd

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

Bulk GaAs room temperature radiation detectors

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 [mu]m. 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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29727/1/0000063.pd

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

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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31641/1/0000575.pd

Reactions along the astrophysical s-process path and prospects for neutron radiotherapy with the Liquid-Lithium Target (LiLiT) at the Soreq Applied Research Accelerator Facility (SARAF)

Neutrons play a dominant role in the stellar nucleosynthesis of heavy elements and the quest for accurate experimental determinations of neutron-induced reaction cross sections becomes more stringent with the refinement of nuclear and astrophysical models. We review here an experimental nuclear-astrophysics program using a high-intensity neutron source based on the 7Li(p, n)7Be reaction with a Liquid-Lithium Target (LiLiT) at the Soreq Applied Research Accelerator Facility (SARAF) Phase I. The quasi-Maxwellian neutron spectrum with effective thermal energy $kT \approx 30$ keV, characteristic of the thick-target 7Li(p, n) yield at proton energy $E_p \approx 1.92$ MeV close to its neutron threshold, is well suited for laboratory measurements of neutron capture reactions along the astrophysical s -process path. The high-intensity proton beam (in the mA range) of SARAF and the high power (few kW) dissipation of LiLiT result in the most intense source of neutrons available today at stellar-like energies. The principle, design and properties of the LiLiT device and recent measurements of Maxwellian Averaged Cross Sections (MACS) based on activation of targets of astrophysical interest are described. Decay counting or atom counting methods (accelerator mass spectrometry, atom-trap trace analysis) are used for the detection of short-lived or long-lived activation products, respectively. In a different realm of applications, the 7Li(p, n) reaction is a leading candidate as an accelerator-based neutron source for Boron Neutron Capture Therapy (BNCT). The high neutron yield achievable from a liquid-lithium target, its sustainability of operation under kW-power incident beams and the recent availability of small-size high-intensity accelerators are compatible with a hospital-based clinical facility. An effort towards the characterization and realization of a liquid-lithium target for BNCT is reviewed. Perspectives of pending and future developments towards SARAF Phase II, based on a 40MeV, 5mA CW proton/deuteron superconducting linear accelerator, are summarized

Telomere length kinetics assay (TELKA) sorts the telomere length maintenance (tlm) mutants into functional groups

Genome-wide systematic screens in yeast have uncovered a large gene network (the telomere length maintenance network or TLM), encompassing more than 400 genes, which acts coordinatively to maintain telomere length. Identifying the genes was an important first stage; the next challenge is to decipher their mechanism of action and to organize then into functional groups or pathways. Here we present a new telomere-length measuring program, TelQuant, and a novel assay, telomere length kinetics assay, and use them to organize tlm mutants into functional classes. Our results show that a mutant defective for the relatively unknown MET7 gene has the same telomeric kinetics as mutants defective for the ribonucleotide reductase subunit Rnr1, in charge of the limiting step in dNTP synthesis, or for the Ku heterodimer, a well-established telomere complex. We confirm the epistatic relationship between the mutants and show that physical interactions exist between Rnr1 and Met7. We also show that Met7 and the Ku heterodimer affect dNTP formation, and play a role in non-homologous end joining. Thus, our telomere kinetics assay uncovers new functional groups, as well as complex genetic interactions between tlm mutants