Detector Dead Time Determination and Optimal Counting Rate for a Detector Near a Spallation Source or a Subcritical Multiplying System

The operation of accelerator-driven systems or spallation sources requires the monitoring of intense neutron fluxes, which may be billions-fold more intense than the fluxes obtained with usual radioactive sources. If a neutron detector is placed near a very intense source, it can become saturated because of detector dead time. On the contrary, if it is placed far away from the source, it will lose counting statistics. For this reason, there must exist an optimal position for placing the detector. The optimal position is defined as the one with the minimal relative uncertainty in the counting rate. In this work, we review the techniques to determine the detector dead time that can be applied with an accelerator-driven subcritical system or a spallation source. For the case of a spallation source, counting rates do not follow Poisson's statistics because of the multiplicity of the number of neutrons emitted by incident proton. It has been found a simple expression that relates the optimal counting rate with the source multiplicity and the uncertainty in the determination of the dead time

Minority status and mental distress: a comparison of group density effects

Background It has been observed that mental disorders, such as psychosis, are more common for people in some ethnic groups in areas where their ethnic group is less common. We set out to test whether this ethnic density effect reflects minority status in general, by looking at three situations where individual characteristics differ from what is usual in a locality. Method Using data from the South East London Community Health study (n = 1698) we investigated associations between minority status (defined by: ethnicity, household status and occupational social class) and risk of psychotic experiences, common mental disorders and parasuicide. We used a multilevel logistic model to examine cross-level interactions between minority status at individual and neighbourhood levels. Results Being Black in an area where this was less common (10%) was associated with higher odds of psychotic experiences [odds ratio (OR) 1.34 95% confidence interval (CI) 1.07–1.67], and attempted suicide (OR 1.84 95% CI 1.19–2.85). Living alone where this was less usual (10% less) was associated with increased odds of psychotic experiences (OR 2.18 95% CI 0.91–5.26), while being in a disadvantaged social class where this was less usual (10% less) was associated with increased odds of attempted suicide (OR 1.33 95% CI 1.03–1.71). We found no evidence for an association with common mental disorders. Conclusions The relationship between minority status and mental distress was most apparent when defined in terms of broad ethnic group but was also observed for individual household status and occupational social class

Review and comparison of effective delayed neutron fraction calculation methods with Monte Carlo codes

The calculation of the effective delayed neutron fraction, beff , with Monte Carlo codes is a complex task due to the requirement of properly considering the adjoint weighting of delayed neutrons. Nevertheless, several techniques have been proposed to circumvent this difficulty and obtain accurate Monte Carlo results for beff without the need of explicitly determining the adjoint flux. In this paper, we make a review of some of these techniques; namely we have analyzed two variants of what we call the k-eigenvalue technique and other techniques based on different interpretations of the physical meaning of the adjoint weighting. To test the validity of all these techniques we have implemented them with the MCNPX code and we have benchmarked them against a range of critical and subcritical systems for which either experimental or deterministic values of beff are available. Furthermore, several nuclear data libraries have been used in order to assess the impact of the uncertainty in nuclear data in the calculated value of beff

Analysis of C/E results of fission rate ratio measurements in several fast lead VENUS-F cores

During the GUINEVERE FP6 European project (2006-2011), the zero-power VENUS water-moderated reactor was modified into VENUS-F, a mockup of lead cooled fast spectrum system with solid components that can be operated in both critical and subcritical mode. The Fast Reactor Experiments for hybrid Applications (FREYA) FP7 project was launched in 2011 to support the designs of the MYRRHA Accelerator Driven System (ADS) and the ALFRED Lead Fast Reactor (LFR). Three VENUS-F critical core configurations, simulating the complex MYRRHA core design and one configuration devoted to the LFR ALFRED core conditions were investigated in 2015. The MYRRHA related cores simulated step by step design peculiarities like the BeO reflector and in pile sections. For all of these cores the fuel assemblies were of a simple design consisting of 30 % enriched metallic uranium, lead rodlets to simulate the coolant and Al2O3 rodlets to simulate the oxide fuel. Fission rate ratios of minor actinides such as Np-237, Am-241 as well as Pu-239, Pu-240, Pu-242 and U-238 to U-235 were measured in these VENUS-F critical assemblies with small fission chambers in specially designed locations, to determine the spectral indices in the different neutron spectrum conditions. The measurements have been analyzed using advanced computational tools including deterministic and stochastic codes and different nuclear data sets like JEFF-3.1, JEFF-3.2, ENDF/B7.1, ENDF/B6.8, JENDL-4.0 and TENDL-2014. The analysis of the C/E discrepancies will help to improve the nuclear data in the specific energy region of fast neutron reactor spectra

Thermal neutron background at Laboratorio Subterráneo de Canfranc (LSC)

The thermal neutron background at Laboratorio Subterráneo de Canfranc (LSC) has been determined using several He proportional counter detectors. Bare and Cd shielded counters were used in a series of long measurements. Pulse shape discrimination techniques were applied to discriminate between neutron and gamma signals as well as other intrinsic contributions. Montecarlo simulations allowed us to estimate the sensitivity of the detectors and calculate values for the background flux of thermal neutrons inside Hall-A of LSC. The obtained value is (3.5±0.8)×10 n/cms, and is within an order of magnitude compared to similar facilities.This work was supported partially by the Spanish Ministerio de Ciencia e Innovación and its Plan Nacional de I+D+i de Física de Partículas projects: FPA2016-76765-P and FPA2018-096717-B-C21. The authors want to acknowledge the help provided by the staff at LSC in the preparation and support for this work

Diatom assemblages and water quality assessment in the Duero Basin (NW Spain)

P. 39-50n order to determine water quality status in the Duero basin (NW Spain), epilithic diatom samples were collected and analyzed in 137 stations in August 2004 following normalized standard protocols. The floristic particularities of the diatom assemblages in the basin are discussed. A total of 429 diatom taxa were identified in the basin and 90 taxa (21%) were new for the Iberian Peninsula. Especially noticeable was the presence of Achnanthidium rivulare, only known until now from North America. Ordination revealed the existence of five species assemblages related t 3-o the following environmental factors: 5-Day Biochemical Oxygen Demand (BOD ), [PO ], and conductivity. Light and scanning electron microscopymicrographs are provide5d for s 4ome common, infrequent or exotic diatom species present in the Duero basin. Three diatom indices (Specific Pollution Index SPI, European Index CEC and Biological Diatom Index BDI) were applied. All of them correlated significantly with water physical and chemical variables, but SPI achieved the best correlations, and is therefore recommended as the reference diatom index for this Mediterranean basin.S

Measurement of the 12C(n,p)12B cross section at n-TOF at CERN by in-beam activation analysis

The integral cross section of the 12C(n,p)12B reaction has been determined for the first time in the neutron energy range from threshold to several GeV at the n-TOF facility at CERN. The measurement relies on the activation technique with the β decay of 12B measured over a period of four half-lives within the same neutron bunch in which the reaction occurs. The results indicate that model predictions, used in a variety of applications, are mostly inadequate. The value of the integral cross section reported here can be used as a benchmark for verifying or tuning model calculations.Peer reviewedFinal Accepted Versio

Ethnicity-specific factors influencing childhood immunisation decisions among Black and Asian Minority Ethnic groups in the UK: a systematic review of qualitative research

Background: Uptake of some childhood immunisations in the UK is lower among those from some Black and Asian Minority Ethnic (BAME) backgrounds. This systematic review of qualitative research sought to understand the factors that are associated with ethnicity that influence the immunisation decisions of parents from BAME backgrounds living in the UK. Methods: Databases were searched on 2 December 2014 for studies published at any time using the terms ‘UK’ and ‘vaccination’ and ‘qualitative methods’ (and variations of these). Included articles comprised participants who were parents from BAME backgrounds. Thematic synthesis methods were used to develop descriptive and higher order themes. Themes specific to ethnicity and associated factors are reported. Results: Eight papers were included in the review. Most participants were from Black (n=62) or Asian (n=38) backgrounds. Two ethnicity-related factors affected immunisation decisions. First, factors that are related to ethnicity itself (namely religion, upbringing and migration, and language) affected parents' perceived importance of immunisations, whether immunisations were permitted or culturally acceptable and their understanding of immunisation/the immunisation schedule. Second, perceived biological differences affected decision-making and demand for information. Conclusions: Factors related to ethnicity must be considered when seeking to understand immunisation decisions among parents from BAME backgrounds. Where appropriate and feasible, vaccination information should be targeted to address beliefs about ethnic differences held by some individuals from some BAME backgrounds

Measurement of 73 Ge(n,γ) cross sections and implications for stellar nucleosynthesis

© 2019 The Author(s). Published by Elsevier B.V.73 Ge(n,γ) cross sections were measured at the neutron time-of-flight facility n_TOF at CERN up to neutron energies of 300 keV, providing for the first time experimental data above 8 keV. Results indicate that the stellar cross section at kT=30 keV is 1.5 to 1.7 times higher than most theoretical predictions. The new cross sections result in a substantial decrease of 73 Ge produced in stars, which would explain the low isotopic abundance of 73 Ge in the solar system.Peer reviewe

Measurement of the 240Pu(n,f) cross-section at the CERN n-TOF facility : First results from experimental area II (EAR-2)

The accurate knowledge of the neutron-induced fission cross-sections of actinides and other isotopes involved in the nuclear fuel cycle is essential for the design of advanced nuclear systems, such as Generation-IV nuclear reactors. Such experimental data can also provide the necessary feedback for the adjustment of nuclear model parameters used in the evaluation process, resulting in the further development of nuclear fission models. In the present work, the 240Pu(n,f) cross-section was measured at CERN's n-TOF facility relative to the well-known 235U(n,f) cross section, over a wide range of neutron energies, from meV to almost MeV, using the time-of-flight technique and a set-up based on Micromegas detectors. This measurement was the first experiment to be performed at n-TOF's new experimental area (EAR-2), which offers a significantly higher neutron flux compared to the already existing experimental area (EAR-1). Preliminary results as well as the experimental procedure, including a description of the facility and the data handling and analysis, are presented