Doping and carrier transport in Ga1-3xIn3xNxAs1-x alloys

Si- and Be-doped Ga1-3xIn3xNxAs1-x (0 less than or equal tox less than or equal to 3%) layers are grown on GaAs substrates by gas-source molecular beam epitaxy with a nitrogen radical beam source. The carrier concentration and mobility are observed to decrease substantially with increasing nitrogen content in both p- and n-type GaInNAs films. After rapid thermal annealing at 750 degreesC, the Be dopants are almost fully activated in p-type material; yet only a small fraction of the Si dopants are activated in n-type GaInNAs films. At low temperature a broad photoluminescence band centered at 1.041 eV (about 120 meV below the band gap) is observed in n-type GaInNAs, which suggests the possible compensating centers present in Si-doped GaInNAs.Peer reviewe

Self-assembled GaIn(N)As quantum dots: Enhanced luminescence at 1.3 µm

Self-assembled GaIn(N)As quantum dots are fabricated on GaAs by atmospheric pressuremetalorganic vapor-phase epitaxy using dimethylhydrazine (DMHy) precursor as a nitrogen source. The incorporation of nitrogen into the islands is observed to be negligible. However, the areal density of the islands is increased by up to one order of magnitude compared to that of the respective GaInAs islands. The GaIn(N)As island size can also be controlled by varying the DMHy flow. An enhancement of the room-temperature luminescence at 1.3 μm is observed in the GaIn(N)As samples grown with DMHy.Peer reviewe

Standoff UV-C imaging of alpha particle emitters

Alpha particles are proven to be very useful in many areas of medicine, technology and science. Yet, they represent the biggest risk to soft biological tissues compared to all nuclear decay products when ingested or inhaled, implying thus stringent radiation protection measures in the management and monitoring of samples that emit them. In this paper we present a standoff optical imaging approach that enables sensing of a radiological threat items at facilities where alpha-emitting material is manufactured, handled, used and stored, at safe distances without putting personnel at risk or contaminating equipment. The optical imaging of alpha-emitting samples is based on the radiation induced air luminescence (radioluminescence) in the UV-C (solar-blind) spectral region which enables detection of alpha particles even under bright light conditions. We show that by adding trace amounts of nitric oxide into the nitrogen atmosphere surrounding the sample, alpha sources with specific activities as low as 1.5 Bq/cm(2) can be remotely imaged. This work provides a proof-of-concept implementation of a novel imaging approach that not only enhances capabilities of a state to take actions which effectively mitigate consequences of an radiological emergency for the society, health, environment and economy, but also benefits nuclear industry in decommissioning efforts.Peer reviewe

Matriisi-ilmaisin taktisen ydinaseen ja muiden radioaktiivisten aineiden havainnointiin

Kyky havaita ydinaineita ja muita radioaktiivisia aineita on tärkeä osa kansallista turvallisuutta. Tehokkaan havaitsemisen mahdollistavalla ilmaisimella tulee olla erittäin hyvä efektiivisyys lähteen tuottamalle neutroni- ja gammasäteilylle, jolloin säteilevä kohde voidaan havaita kaukaa. Samalla ilmaisinjärjestelmän tulee kyetä paikantamaan lähde automaattisesti. Nopea paikantaminen on ensiarvoisen tärkeää tilanteen hahmottamiseksi ja sen hallintaan saamiseksi. Tutkimuksessa osoitettiin, että tehokas, suuntaherkkä säteilyilmaisin voidaan rakentaa kustannustehokkaasti matriisiperiaatteella NaI(Tl)-tuikeilmaisimista. Matriisi-ilmaisimen toiminta demonstroitiin rakentamalla pienikokoinen neljään ilmaisinelementtiin perustuva säteilyilmaisinprototyyppi ja sille ohjelmisto, joka analysoi automaattisesti säteilylähteen tyypin ja suunnan. Prototyyppi kykeni määrittämään gammasäteilylähteiden suunnan alle 5° tarkkuudella ja fissioneutronilähteen suunnan noin 15° tarkkuudella. Simulaatioiden avulla selvitettiin, kuinka ilmaisimen herkkyyttä saadaan parannettua kasvattamalla ilmaisinelementtien kokoa. Simulaatiotulosten perusteella 1 000 000 neutronia sekunnissa lähettävä taktinen ydinase voidaan havaita 100 m päästä neljästä NaI(Tl)-tuikemateriaalista valmistetusta ilmaisinelementistä koostuvalla ilmaisimella, jonka ilmaisinelementtien kokonaismassa on 60 kg. Tällaisella ilmaisimella ydinase voidaan havaita noin viidessä minuutissa. Jos taktinen ydinase lähettää 200 000 neutronia sekunnissa, kasvaa havaitsemiseen tarvittava aika kahteen tuntiin.Tämä julkaisu on toteutettu osana Maanpuolustuksen tieteellisen neuvottelukunnan (MATINEn) tutkimusrahoituksen toimeenpanoa. (www.defmin.fi/matine) Julkaisun sisällöstä vastaavat tiedon tuottajat, eikä tekstisisältö välttämättä edusta puolustusministeriön näkemystä

Detection and Localization of Weapons Grade Plutonium with an Array of NaI Detectors

An array of scintillators is a multipurpose sensitive detection system of photons and neutrons while also providing directional sensitivity. Several simulations were performed with Geant4 software to optimize the design of such an array for neutrons. A crucial design criterion was an excellent detection and localization capability, and the system must be practical for operational field work. A large array, containing four 4 L NaI detectors, can fulfil these criteria. Such an array, clad with 3 - 5 cm of PVC, has a neutron counting efficiency which is of the same order of magnitude as the state-of-the art commercial neutron detectors based on helium tubes or plastic (LiF/ZnS). For a count time of 12 minutes, the array can detect 4 kg of weapons grade plutonium at a distance of 50 m. The detection time can be essentially shortened down to a few minutes by increasing the thickness of the PVC up to 10 cm.This publication is part of the implementation of research funding of the Scientific Advisory Board for Defence (MATINE). (www.defmin.fi/matine) The content is the responsibility of the producers of the information and does not necessarily represent the view of the Defence Ministry

Experimental investigation towards a periodically pumped single-photon source

Experiments towards a periodically pumped single-photon source are presented. The lateral piezoelectric field of a surface acoustic wave dissociates laser-generated two-dimensional excitons into electrons and holes. These carriers are separated by the wave potential and are transported over macroscopic length scales without recombining. When reaching a stress-induced quantum dot in the quantum well they periodically populate the zero-dimensional states and recombine, emitting single photons periodically in time according to the surface acoustic-wave frequency. We have successfully reduced the number of pumped quantum dots down to 100 and have detected a strong blinking photoluminescence signal. By further reducing the number of quantum dots down to 1 a periodically pumped single photon source could be realized.Peer reviewe

Observation of defect complexes containing Ga vacancies in GaAsN

Positron annihilation spectroscopy was used to study GaAsN/GaAs epilayers. GaAsN layers were found to contain Ga vacancies in defect complexes. The density of the vacancy complexes increases rapidly to the order of 1018 cm−3 with increasing N composition and decreases after annealing at 700 °C. The anticorrelation of the vacancy concentration and the integrated photoluminescence intensity suggests that the Ga vacancy complexes act as nonradiative recombination centers.Peer reviewe

Design of a novel instrument for active neutron interrogation of artillery shells

The most common explosives can be uniquely identified by measuring the elemental H/N ratio with a precision better than 10%. Monte Carlo simulations were used to design two variants of a new prompt gamma neutron activation instrument that can achieve this precision. The instrument features an intense pulsed neutron generator with precise timing. Measuring the hydrogen peak from the target explosive is especially challenging because the instrument itself contains hydrogen, which is needed for neutron moderation and shielding. By iterative design optimization, the fraction of the hydrogen peak counts coming from the explosive under interrogation increased from 53(-7)(-7)% to 74(-10)(+8)% (statistical only) for the benchmark design. In the optimized design variants, the hydrogen signal from a high-explosive shell can be measured to a statistics-only precision better than 1% in less than 30 minutes for an average neutron production yield of 10(9) n/s.Peer reviewe