1,740 research outputs found
Surface acoustic wave modulation of single photon emission from GaN/InGaN nanowire quantum dots
On-chip quantum information processing requires controllable quantum light
sources that can be operated on-demand at high-speeds and with the possibility
of in-situ control of the photon emission wavelength and its optical
polarization properties. Here, we report on the dynamic control of the optical
emission from core-shell GaN/InGaN nanowire (NW) heterostructures using radio
frequency surface acoustic waves (SAWs). The SAWs are excited on the surface of
a piezoelectric lithium niobate crystal equipped with a SAW delay line onto
which the NWs were mechanically transferred. Luminescent quantum dot (QD)-like
exciton localization centers induced by compositional fluctuations within the
InGaN nanoshell were identified using stroboscopic micro-photoluminescence
(micro-PL) spectroscopy. They exhibit narrow and almost fully linearly
polarized emission lines in the micro-PL spectra and a pronounced anti-bunching
signature of single photon emission in the photon correlation experiments. When
the nanowire is perturbed by the propagating SAW, the embedded QD is
periodically strained and its excitonic transitions are modulated by the
acousto-mechanical coupling, giving rise to a spectral fine-tuning within a
~1.5 meV bandwidth at the acoustic frequency of ~330 MHz. This outcome can be
further combined with spectral detection filtering for temporal control of the
emitted photons. The effect of the SAW piezoelectric field on the QD charge
population and on the optical polarization degree is also observed. The
advantage of the acousto-optoelectric over other control schemes is that it
allows in-situ manipulation of the optical emission properties over a wide
frequency range (up to GHz frequencies).Comment: arXiv admin note: text overlap with arXiv:1902.0791
Neutron activation as an independent indicator of expected total yield in the production of 82Sr and 68Ge with 66 MeV protons
Introduction
A method based on neutron activation is being developed to assist in resolving discrepancies between the expected yield and actual yield of radionuclides produced with the vertical-beam target station (VBTS) at iThemba LABS.
The VBTS is routinely employed for multi-Ci batch productions of the radionuclide pairs 22Na/68Ga and 82Sr/68Ga using standardized natMg/natGa and natRb/natGa tandem targets, respectively [1]. The metal-clad target discs are bombarded with a primary beam of 66 MeV protons at an intensity of nominally 250 ”A. The encapsulation materials are either Nb (for Mg and Ga) or stainless steel (for Rb) which serve to contain the molten target materials during bombardment and act as a barrier to the high-velocity cooling water which surrounds the targets in a 4Ï geometry. The natRb/natGa targets are typically bombarded according to a two-week cycle while natMg/natGa targets are bombarded on an ad-hoc basis, depending on a somewhat unpredictable 22Na demand.
A too-large deviation between expected yield and actual yield has at times plagued this programme. These deviations can manifest both as an apparent loss or an apparent gain (relative to the expected yield) by up to about 15% in either direction. The resulting uncertainty of up to 30% (in the worst case) from one production batch to another can be costly and is unacceptable in a large-scale production regimen. This phenomenon is believed to be brought about by two types of problems:
(1) Production losses, e.g. during the radio-chemical separation process or incomplete recovery of activated target material during the decapsulation step.
(2) Incorrect values obtained for the accumulated proton charge.
A problem of type (1) will always result in a loss of yield. A problem of type (2) can manifest as an apparent loss or gain. In an effort to get a handle on this second type of problem, neutron activation of suitable material samples, embedded in a target holder, is being investigated as an independent indicator of the total yield. For this purpose, samples of Co, Mn, Ni and Zn were activated during production runs and Co was found to be the most appropriate. Preliminary results will be presented after first discussing why the determination of the accumulated pro-ton charge is a problem with the VBTS.
Materials and Methods
The VBTS consists of a central region in which a target holder is located during bombardment as well as two half-cylindrical radiation shields which completely surround the target. The shields can be moved away from the central region on dedicated rails, e.g. when repairs or maintenance is required. FIGURE 1 shows the VBTS with the shields moved to the âopenâ position. As some components of the station are located below the vault floor, with the target position near floor level, it proved difficult to electrically isolate the VBTS as was done for the two horizontal-beam target stations at iThemba LABS [1]. The VBTS does not act as a Faraday cup like the other target stations. Instead, the beam current and accumulated charge is measured by means of a calibrated capacitive probe [1,2].
There appears to be a variation in the response of the capacitive probe, sensitive to the beam microstructure, in particular a dependence on the beam packet length. This problem is not yet fully resolved.
FIGURE 2 (a) shows the beamstop of a VBTS target holder with several Co samples mounted on the outside as well as one each of Ni, Mn and Zn. The samples are small âtabletsâ with a 10 mm diameter and 1 mm thickness. The reactions of interest are 59Co(n,Îł)60Co, 59Co(n,3n)57Co, nat-Ni(n,X)60Co, natNi(n,X)57Co, natZn(n,X)65Zn and 55Mn(n,2n)54Mn. The relevant half-lives are 60Co(5.271 a), 57Co(271.8 d), 65Zn(244.3 d) and 54Mn(312.2 d). The half-life should be long compared to the two-week cycle in order to reduce the dependence on the exact beam history, which is very fragmented over any production period. In this respect, 60Co is considered to be particularly attractive as its long half-life of more than 5 years leads to a negligible effect by the beam history.
Note that the tandem targets, shown in FIGURE 2 (b), are mounted just upstream of the beamstop â in fact, the targets and beamstop form a single unit before being fitted into the target holder.
At the end of bombardment, all samples were assayed for their characteristic Îł-emissions using standard off-line Îł-ray spectrometry with an HPGe detector connected to a Genie 2000 MCA. Calculations of the neutron fluence density in the central sample volume on the beamstop were also performed using the Monte Carlo radiation transport code MCNPX. For these calculations, the entire VBTS, a Rb/Ga target and the vault walls were included in the model.
Results and Conclusion
All samples activated significantly â copious amounts of 60Co were detected in the Co discs after a two-week run.
The neutron fluence density for the case of a 250 ”A, 66 MeV proton beam on a natRb/natGa tandem target is shown in FIGURE 3. The dominance of low-energy neutrons is evident, which is in part due to the large amount of paraffin-wax shielding material in close proximity to the target. While reactions such as the (n,2n) and (n,3n) would be sensitive to the more energetic part of the neutron spectrum, the (n,γ) capture reaction benefits from the large low-energy component. This explains the copious amounts of 60Co formed. It was therefore decided to only retain the central Co sample for subsequent bombardments, as shown in FIGURE 4.
The first results are shown in TABLE 1. The accumulated charge as obtained from the capacitive probe (Q), the specific 60Co activity (A) at the end of bombardment (EOB), and their ratio (A/Q) are presented in the table, together with the deviation of individual ratios relative to their average for the case of the Mg/Ga tandem tar-gets only. Note that all samples were counted until the statistical uncertainties were negligible. Any systematic uncertainties are ignored at this stage as they are considered to remain the same from one batch production to another.
For the sake of argument, the average value of the ratio is taken as the expected value. A positive deviation of the A/Q value is then indicative of a too-small value of the accumulated charge obtained from the capacitive probe, leading to a corresponding overproduction. Likewise, a negative value is indicative of a too-large value of the accumulated charge, leading to a corresponding underproduction.
It is certainly true that the data in TABLE 1 are currently very limited. It is envisaged, however, that with time the growing database of values will assist in reducing the uncertainty in determining the accumulated charge and reduce the discrepancies between predicted and actual yields significantly. TABLE 1 illuminates the underlying problem satisfactorily. The four Mg/Ga tandem target bombardments, on identical targetry, were performed successively. The neutron activation correlates well the with actual yields, pointing directly to the current integration as the main source of error.
The method already proves to be useful. An indication of an over or underprediction can be obtained prior to the target processing by recovering and measuring the Co disc. This in-formation can be used to make a decision concerning the present batch production and/or the subsequent one. One can either add beam to the present production target and/or in-crease/reduce the total beam on the subsequent production target to compensate for an expected overproduction or shortfall.
In conclusion, we would like to stress that the capacitive probes show great promise and that better understanding and/or possibly some development of their signal processing algorithm may improve their ability to measure the accumulated charge to the desired accuracy. Segmented capacitive probes used at iThemba LABS and elsewhere for beam position measurement [1,3] are not affected by beam microstructure as only the ratios of the signal strengths on the different sectors are important. In this case, changes in response affect all sec-tors equally and the ratios are unaffected
Publish and perish: a disturbing trend in the European Union's regulation of nutrition health claims made on foods
The cyclotron production of selected radionuclides using medium energy protons
Thesis (PhD)--Stellenbosch University, 2008.ENGLISH ABSTRACT: Radiochemical research involving ion exchange chromatography is of paramount
importance to the future of radionuclide production at the Radionuclide Production
Group (RPG) of iThemba LABS. It is required for the production of high-activity
yields of radionuclides to effectively remove impurities and for the safety of the
operators performing such productions. The radiochemical separations of some new
products from their target material, as well as experiments to determine whether
production is viable, are described.
67Ga is currently being produced at the RPG and makes use of zinc targets. With the
production of ultra-pure 67Ga, it was necessary to remove any Fe(III) impurities from
the final product, such that it may be possible to label peptides with this product. The
use of Amberchrom CG161M for this purpose was found to be satisfactory.
Interest was shown in 88Y by an overseas company for the manufacture of sources.
While a method involving extraction of the radionuclide and the ion exchange thereof
using Chelex 100 chelating resin had been published, problems with the production
persisted. Three methods, using ion exchange chromatography, were devised to
produce the radionuclide, with two of them being adopted for production purposes.
Thick-target nuclear data have also recently been accumulated in collaboration with
colleagues from ATOMKI, Debrecen, Hungary.
There is a large demand for 82Sr for the manufacture of 82Sr/82Rb generators for
medical use. A method was developed to manufacture this radionuclide with thicker
(32 g) target material, bombarded in the Vertical Beam Target Station (VBTS), and to
separate 82Sr from its target material with the use of Purolite S950 chelating resin.
68Ge/68Ga generators are becoming increasingly important in the world of
radiopharmaceuticals. A project to develop a local generator was funded by the
Innovation Fund and research was performed to produce 68Ge, such that the generator
could be manufactured. This involved bombarding thicker Ga targets in the VBTS
and performing the chemical separation using AG MP-1 anion exchange resin. The final product was loaded onto generators, although tests performed on different
materials to the ones being marketed are also reported in this work.
A project was initiated to study the cluster radioactive decay of 223Ac via 14C and 15N
emission. To produce 223Ac for these observations, a Th target was bombarded. The
227Pa was separated from the target material using AG MP-1 macroporous anion
exchange resin and used as a source, which decayed to 223Ac. The chemical
separation and the drying of the final product onto a source plate were completed
within approximately 70 minutes from the end of bombardment. The work was
performed in collaboration with JINR, Russia, and University of Milan and INFN,
Italy.
133Ba has a half-life of over 10 years and is an expensive radionuclide to produce. It
has been used in medical and biological studies and there still appears to be a demand
for it. A method was devised, utilizing AG50W-X4 cation exchange resin, to separate
133Ba from its CsCl target material.
Agricultural specialists in the past have shown an interest in 28Mg, to determine the
uptake of the element in fruit. It has long been regarded by some of the local
researchers as an interesting project to investigate. It has been determined that the
product can be produced in reasonable quantities using LiCl target material, with ten
targets being bombarded in series using a 200 MeV proton beam delivered by the
Separated Sector Cyclotron. A method, involving the use of Purolite S950 chelating
resin, was devised to separate 28Mg from its target material.AFRIKAANSE OPSOMMING: Radiochemiese navorsing, wat ioonuitruiling chromatografie behels, is van uiterste
belang vir die toekoms van die produksie van radionukliede by die
Radionukliedproduksiegroep (RPG) van iThemba LABS. Dit is nodig vir die hoë
aktiwiteit opbrengs van radionuklied produkte om onsuiwerhede te verwyder en vir
die veiligheid van die operateurs wat die produksies moet uitvoer. Die skeiding van
nuwe produkte van hulle skyfmateriaal, sowel as eksperimente om vas te stel of ân
produksie uitvoerbaar is, word in die werk beskryf.
67Ga word tans by RPG vervaardig en maak gebruik van sink as skyfmateriaal. Vir
die produksie van âultra-suiwerâ 67Ga was dit belangrik om enige Fe(III)
onsuiwerhede uit die finale produk te verwyder om sodoende peptiede merking te kan
uitvoer. Die gebruik van Amberchrom CG161M hars was voldoende vir dié
eksperiment.
ân Oorsese maatskappy het belangstelling getoon in 88Y vir die vervaardiging van
bronne. Alhoewel ân metode wat die ekstraksie van die radionuklied en die ioonuitruiling
daarvan met die gebruik van Chelex 100 chelerende hars reeds gepubliseer
was, het probleme met die produksie voortgeduur. Drie metodes is opgestel om 88Y te
produseer, waarvan twee van die metodes tans gebruik word vir produksie doeleindes.
Dik-skyf kerndata is ook versamel in samewerking met kollegas van ATOMKI,
Debrecen, Hongarye.
Daar is ân groot aanvraag vir 82Sr vir die vervaardiging van 82Sr/82Rb generators vir
mediese doeleindes. ân Metode is ontwikkel om die radionuklied te vervaardig van
dikker skyfmateriaal (32 g), in die Vertikale Bundelstasie gebombardeer, en om 82Sr
van sy skyfmateriaal te skei met die gebruik van Purolite S950 chelerende hars.
68Ge/68Ga generators is besig om toenemend belangrik te word in die wĂȘreld van
radiofarmasie. iThemba LABS kry baie navrae om die produk te vervaardig. Die
projek was ook deel van die voorlegging aan die âInnovation Fundâ en ân manier is
ondersoek om 68Ge te vervaardig, wat benodig word om so ân generator te laai. Dik Ga skyfmateriaal word in die Vertikale Bundelstasie gebombardeer en ân chemiese
skeiding is uitgevoer deur gebruik te maak van AG MP-1 anioonuitruiling hars. Die
finale produk is op die generators gelaai vir toetsdoeleindes. Toetse is ook op ân ander
tipe generator uitgevoer en word in die werk beskryf.
ân Projek is begin om âclusterâ radioaktiewe verval van 223Ac, via 14C en 15N emissie,
te bestudeer. ân Th-skyf is met protone gebombardeer om die 223Ac te produseer vir
die eksperiment. 227Pa is vervaardig en geskei van die skyfmateriaal. Dit is gedoen
met die gebruik van AG MP-1 makroporeuse anioonuitruiling hars en drooggemaak
op ân bronplaat, waar dit verval het na 223Ac. Die chemiese skeiding en die
droogmaak van die finale produk op ân bronplaat is uitgevoer binne 70 minute na
Einde van Bombardering (EVB). Die werk is deel van ân samewerking met kollegas
van JINR, Rusland, en die Universiteit van Milaan, sowel as INFN, Italië.
133Ba het ân halveertyd van oor die tien jaar en is ân duur produk om te vervaardig.
Dit is al gebruik in mediese en biologiese studies en daar is deesdae ân redelike
aanvraag daarvoor. ân Metode is uitgewerk om 133Ba te skei van die CsCl
skyfmateriaal met die gebruik van AG50W-X4 katioonuitruiling hars.
Spesialiste in landboustudies het in die verlede belangstelling getoon in 28Mg. Dit
word gebruik om die absorpsie van dié element in vrugte te ondersoek. Die produk
kan vervaardig word met die gebruik van LiCl skyfmateriaal: tot soveel as tien skywe
(agter mekaar) word gebombardeer met ân 200 MeV protonbundel te iThemba LABS.
ân Metode, wat Purolite S950 behels, is daargestel om 28Mg van die skyfmateriaal te
skei
An analysis of the Research Fellowship Scheme of the Royal College of Surgeons of England.
BACKGROUND: The Research Fellowship Scheme of the Royal College of Surgeons of England commenced in 1993 with the aim of exposing selected surgical trainees to research techniques and methodology, with the hope of having an impact on surgical research and increasing the cadre of young surgeons who might decide to pursue an academic career in surgery. Over 11 million pounds sterling (approximately US 20 million dollars) has been invested in 264 fellowships. The College wished to evaluate the impact of the Scheme on the careers of research fellows, surgical research, and patient care. As the 10th anniversary of the Scheme approached. STUDY DESIGN: Two-hundred and sixty research fellows whose current addresses were available were sent a questionnaire. Two-hundred and thirty-eight (91.5%) responded. RESULTS: Three-quarters of the research fellows conducted laboratory-based research, with most of the remainder conducting patient-based clinical research. One-third of the fellows who have reached consultant status have an academic component to their post. The total number of publications based on fellowship projects was 531, with a median impact factor of 3.5. Almost all fellows had been awarded a higher degree or were working toward this. Half of the fellows received subsequent funding for research, mostly awarded by national or international funding bodies. CONCLUSIONS: The Research Fellowship Scheme of the Royal College of Surgeons of England has successfully supported many trainee surgeons in the initial phase of their research career. It has helped surgical research by increasing the pool of surgeons willing to embark on an academic career. Indirectly, patient care has benefited by promoting an evidence-based culture among young surgeons. Such schemes are relevant to surgical training programs elsewhere if more young surgeons are to be attracted into academic surgery
Riboswitch RSthiT as a molecular tool in Lactococcus lactis
Previous RNA sequencing has allowed the identification of 129 long 5âČ untranslated regions (UTRs) in the Lactococcus lactis MG1363 transcriptome. These sequences potentially harbor cis-acting riboswitches. One of the identified extended 5âČ UTRs is a putative thiamine pyrophosphate (TPP) riboswitch. It is located immediately upstream of the thiamine transporter gene thiT (llmg_0334). To confirm this assumption, the 5âČ-UTR sequence was placed upstream of the gene encoding the superfolder green fluorescent protein (sfGFP), sfgfp, allowing the examination of the expression of sfGFP in the presence or absence of thiamine in the medium. The results show that this sequence indeed represents a thiamine-responsive TPP riboswitch. This RNA-based genetic control device was used to successfully restore the mutant phenotype of an L. lactis strain lacking the major autolysin gene, acmA. The L. lactis thiT TPP riboswitch (RS(thiT)) is a useful molecular genetic tool enabling the gradual downregulation of the expression of genes under its control by adjusting the thiamine concentration. IMPORTANCE The capacity of microbes with biotechnological importance to adapt to and survive under quickly changing industrial conditions depends on their ability to adequately control gene expression. Riboswitches are important RNA-based elements involved in rapid and precise gene regulation. Here, we present the identification of a natural thiamine-responsive riboswitch of Lactococcus lactis, a bacterium used worldwide in the production of dairy products. We used it to restore a genetic defect in an L. lactis mutant and show that it is a valuable addition to the ever-expanding L. lactis genetic toolbox
Identifying patients at risk of emergency admission for colorectal cancer.
BACKGROUND: Patients whose colorectal cancer is treated after an emergency admission tend to have late-stage cancer and a poor prognosis. We identified risk factors for an emergency admission by linking data from the National Bowel Cancer Audit (NBCA) and the English Hospital Episode Statistics (HES), an administrative database of all admissions to English National Health Service hospitals, which includes data on mode of admission. METHODS: We identified all adults included in the NBCA with a primary diagnosis of bowel cancer, excluding cancer of the appendix, between August 2007 and July 2011 whose record could be linked to HES. Multivariable logistic regression was used to estimate adjusted odds ratios (OR) for an emergency admission for colorectal cancer. All risk factors were adjusted for cancer site and calendar year. RESULTS: 97,909 adults were identified with a primary diagnosis of bowel cancer and 82,777 patients could be linked to HES. Patients who were older, female, of a non-white ethnic background, and more socioeconomically deprived, and those with dementia or cardiac, neurologic and liver disease had an increased risk of presenting as an emergency admission. The strongest risk factors were age (90 compared with 70 years: OR 2.99, 95% CI 2.84 to 3.15), dementia (OR 2.46, 2.18 to 2.79), and liver disease (OR 1.87, 1.69 to 2.08). CONCLUSIONS: Our study identifies risk factors that may impair health-seeking behaviour and access to healthcare. An earlier recognition of symptoms in patients with these risk factors may contribute to better outcomes
- âŠ