32 research outputs found
Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate
The H3+ molecular ion plays a fundamental role in interstellar chemistry, as
it initiates a network of chemical reactions that produce many interstellar
molecules. In dense clouds, the H3+ abundance is understood using a simple
chemical model, from which observations of H3+ yield valuable estimates of
cloud path length, density, and temperature. On the other hand, observations of
diffuse clouds have suggested that H3+ is considerably more abundant than
expected from the chemical models. However, diffuse cloud models have been
hampered by the uncertain values of three key parameters: the rate of H3+
destruction by electrons, the electron fraction, and the cosmic-ray ionisation
rate. Here we report a direct experimental measurement of the H3+ destruction
rate under nearly interstellar conditions. We also report the observation of
H3+ in a diffuse cloud (towards zeta Persei) where the electron fraction is
already known. Taken together, these results allow us to derive the value of
the third uncertain model parameter: we find that the cosmic-ray ionisation
rate in this sightline is forty times faster than previously assumed. If such a
high cosmic-ray flux is indeed ubiquitous in diffuse clouds, the discrepancy
between chemical models and the previous observations of H3+ can be resolved.Comment: 6 pages, Nature, in pres
Symptom prevalence, duration, and risk of hospital admission in individuals infected with SARS-CoV-2 during periods of omicron and delta variant dominance: a prospective observational study from the ZOE COVID Study
BACKGROUND: The SARS-CoV-2 variant of concern, omicron, appears to be less severe than delta. We aim to quantify the differences in symptom prevalence, risk of hospital admission, and symptom duration among the vaccinated population. METHODS: In this prospective longitudinal observational study, we collected data from participants who were self-reporting test results and symptoms in the ZOE COVID app (previously known as the COVID Symptoms Study App). Eligible participants were aged 16-99 years, based in the UK, with a body-mass index between 15 and 55 kg/m2, had received at least two doses of any SARS-CoV-2 vaccine, were symptomatic, and logged a positive symptomatic PCR or lateral flow result for SARS-CoV-2 during the study period. The primary outcome was the likelihood of developing a given symptom (of the 32 monitored in the app) or hospital admission within 7 days before or after the positive test in participants infected during omicron prevalence compared with those infected during delta prevalence. FINDINGS: Between June 1, 2021, and Jan 17, 2022, we identified 63 002 participants who tested positive for SARS-CoV-2 and reported symptoms in the ZOE app. These patients were matched 1:1 for age, sex, and vaccination dose, across two periods (June 1 to Nov 27, 2021, delta prevalent at >70%; n=4990, and Dec 20, 2021, to Jan 17, 2022, omicron prevalent at >70%; n=4990). Loss of smell was less common in participants infected during omicron prevalence than during delta prevalence (16·7% vs 52·7%, odds ratio [OR] 0·17; 95% CI 0·16-0·19, p<0·001). Sore throat was more common during omicron prevalence than during delta prevalence (70·5% vs 60·8%, 1·55; 1·43-1·69, p<0·001). There was a lower rate of hospital admission during omicron prevalence than during delta prevalence (1·9% vs 2·6%, OR 0·75; 95% CI 0·57-0·98, p=0·03). INTERPRETATION: The prevalence of symptoms that characterise an omicron infection differs from those of the delta SARS-CoV-2 variant, apparently with less involvement of the lower respiratory tract and reduced probability of hospital admission. Our data indicate a shorter period of illness and potentially of infectiousness which should impact work-health policies and public health advice. FUNDING: Wellcome Trust, ZOE, National Institute for Health Research, Chronic Disease Research Foundation, National Institutes of Health, and Medical Research Council
Experimental studies of the dissociative recombination processes for the dimethyl ether ions CD
Aims. Determination of branching fractions, cross sections and thermal rate coefficients for the dissociative recombination of CD3OCD2+ (0–0.3 eV) and (CD3)2OD+ (0–0.2 eV) at the low relative kinetic energies encountered in the interstellar medium.
Methods. The measurements were carried out using merged electron and ion beams at the CRYRING storage ring, Stockholm, Sweden.
Results. For (CD3)2OD+ we have experimentally determined the branching fraction for ejection of a single hydrogen atom in the DR process to be maximally 7% whereas 49% of the reactions involve the break up of the COC chain into two heavy fragments and 44% ruptures both C–O bonds. The DR of CD3OCD2+ is dominated by fragmentation of the COC chain into two heavy fragments. The measured thermal rate constants and cross sections are
k(T) = 1.7±0.5 × 10-6(T/300)
cm3 s-1, σ = 1.2±0.4 × 10-15(Ecm[eV]) cm2 and k(T) = 1.7±0.6 × 10-6(T/300) cm3 s-1, σ = 1.7±0.6 × 10-15(Ecm[eV]) cm2 for CD3OCD2+ and (CD3)2OD+, respectively
Study of low work function materials for hot cavity resonance ionization laser ion sources
The selectivity of a hot cavity resonance ionization laser ion source (RILIS) is most often limited by contributions from competing surface ionization of the hot walls of the ionization cavity. In this article we present investigations on the properties of designated high temperature, low work function materials regarding their performance and suitability as cavity material for RILIS. Tungsten test cavities, impregnated with a mixture of barium oxide and strontium oxide (BaOSrO on W), or alternatively gadolinium hexaboride (GdB6) were studied in comparison to a standard tungsten RILIS cavity as being routinely used for hot cavity laser ionization at ISOLDE. Measurement campaigns took place at the off-line mass separators at ISOLDE/CERN, Geneva and RISIKO/University of Mainz
Electron impact single detachment on the F
Electron Impact Single Detachment (EISD) of F- has been studied using the heavy ion storage
ring CRYRING at the Manne Siegbahn Laboratory, Stockholm, Sweden. F- ions stored in the
ring were merged with an electron beam in one of the ring sections. Neutral F atoms produced in
the EISD process were detected in the zero-degree direction using a surface barrier detector. The
threshold for the detachment process was found to be around 7.6 eV, thus more than twice the
binding energy of F-. The cross-sections increased smoothly up to 55 eV where it reached a
maximum of . At higher energies a slow decrease of the cross-section was observed,
which follows the energy dependence predicted by the Bethe-Born approximation. The
experiment showed that CRYRING can be used favourably for studies of anions, and several
experiments are forthcoming
Experimental studies of the dissociative recombination of CD
Aims. We determine branching fractions, cross sections and thermal rate
constants for the dissociative recombination of CD3CDOD+ and
CH3CH2OH at the low
relative kinetic energies encountered in the interstellar medium.
Methods. The experiments were carried out by merging an ion and electron
beam at the heavy ion storage ring CRYRING, Stockholm, Sweden.
Results. Break-up of the CCO structure into three heavy fragments is not
found for either of the ions. Instead the CCO structure is retained in 23 ± 3% of the DR
reactions of CD3CDOD+ and 7 ± 3% in the DR of
CH3CH2OH, whereas rupture
into two heavy fragments occurs in 77 ± 3% and 93 ± 3% of the DR events of the
respective ions. The measured cross sections were fitted between 1–200 meV yielding the
following thermal rate constants and cross-section dependencies on the relative kinetic
energy:
σ(Ecm [eV] ) = 1.7 ± 0.3 × 10-15(Ecm [eV] ) − 1.23 ± 0.02 cm2
and
k(T) = 1.9 ± 0.4 × 10-6(T / 300) − 0.73 ± 0.02 cm3 s-1
for CH3CH2OH as well as
k(T) = 1.1 ± 0.4 × 10-6(T / 300) − 0.74 ± 0.05 cm3 s-1
and
σ(Ecm [eV]) = 9.2 ± 4 × 10-16(Ecm [eV] ) − 1.24 ± 0.05 cm2
for CD3CDOD