Hole misalignment and gain performance of Gaseous Electron Multipliers

It is well known and has been shown that the gain performance of Gaseous Electron Multipliers (GEM) depends on the size of the holes. With an optical scanner it is possible to measure the dimensions of the holes, and to predict the performance of GEMs. However, the gain prediction of GEMs that are manufactured with a double mask etching technique is not straightforward. With the hole size information alone, it is not possible to make precise prediction of the gain. We show that the alignment of the photo-masks between the two sides of the GEM foils plays a crucial role. A misalignment of a few microns can lower the gain substantially. The study is performed by using the Helsinki high definition optical scanner for quality control of GEM foils, and this will show its true potential.Peer reviewe

Passive neutron albedo reactivity measurements of spent nuclear fuel

The upcoming disposal of spent nuclear fuel in Finland creates new challenges for nuclear safeguards. Part of the national safeguards concept for geological repositories, developed by STUK — Radiation and Nuclear Safety Authority, is non-destructive assay (NDA) verification of all fuel items before disposal. The proposed verification system is a combination of PGET (Passive Gamma Emission Tomography), PNAR (Passive Neutron Albedo Reactivity) and weight measuring NDA-instruments. PGET takes a pin-level image of the fission products inside of a fuel assembly and PNAR verifies the multiplication of the assembly, a quantity that correlates with the fissile content. PGET is approved by IAEA (International Atomic Energy Agency) for safeguards measurements, but the feasibility of PNAR has not yet been established. A first of its kind PNAR prototype instrument was built in a collaboration coordinated by STUK. This paper concludes the results of the first measurements of spent BWR (Boiling Water Reactor) nuclear fuel with the prototype in July 2019. Based on the measurements, the ability of the PNAR instrument to detect the presence of fissile material in a repeatable manner in a reasonable amount of time was demonstrated. Furthermore, the instrument was able to detect differences in multiplication between partially and fully spent fuel assemblies, and axial differences in multiplication within a single assembly.Peer reviewe

Study of interpad-gap of HPK 3.1 production LGADs with Transient Current Technique

The Phase-2 upgrade of the Large Hadron Collider (LHC) to High-Luminosity LHC (HL-LHC) allows an increase in the operational luminosity value by a factor of 5-7 that will result in delivering 3000 fb(-1) or more integrated luminosity. Due to high luminosity, the number of interactions per bunch crossings (pileup) will increase up to a value of 140-200. To cope with high pileup rates, a precision minimum ionising particles (MIPs) timing detector (MTD) with a time resolution of similar to 30-40 ps and hermetic coverage up to a pseudo-rapidity of vertical bar eta vertical bar = 3 is proposed by the Compact Muon Solenoid (CMS) experiment. An endcap part (1.6 <vertical bar eta vertical bar <3) of the MTD, called the endcap timing layer, will be based on low-gain avalanche detector (LGAD) technology. LGADs provide a good timing resolution due to a combination of a fast signal rise time and high signal-to-noise ratio. The performance of the ETL depends on optimising the crucial features of the sensors, namely; gain, signal homogeneity, fill factor, leakage current, uniformity of multiple-pad sensors and long term stability. The paper mainly focuses on the study of the fill factor of LGADs with varying temperature and irradiation at varying proton fluences as these sensors will be operated at low temperatures and are subjected to a high radiation environment. The 3.1 production of LGADs from Hamamatsu Photonics K.K. (HPK) includes 2x2 sensors with different structures, in particular, different values of narrower inactive region widths between the pads, called the no-gain region. In this paper, the term interpad-gap is used instead of no-gain region in order to follow the conventional terminology. These sensors have been designed to study their fill factor, which is the ratio of the area within the active region (gain region) to the total sensor area. A comparative study on the dependence of breakdown voltage with the interpad-gap width for the sensors has been carried out. Using infrared light (as the electron-hole pair creation by IR laser mimics closely to the traversing of MIPs) from the Scanning-Transient Current Technique (Scanning-TCT) set-up shows that the fill factor does not vary significantly with a variation in temperature and irradiation at high proton fluences.Peer reviewe

Probing the Effects of Strong Electromagnetic Fields with Charge-Dependent Directed Flow in Pb-Pb Collisions at the LHC

The first measurement at the LHC of charge-dependent directed flow (v(1)) relative to the spectator plane is presented for Pb-Pb collisions at root s(NN) = 5.02 TeV. Results are reported for charged hadrons and D-0 mesons for the transverse momentum intervals p(T) > 0.2 GeV/c and 3 <p(T) <6 GeV/c in the 5%-40% and 10%-40% centrality classes, respectively. The difference between the positively and negatively charged hadron v(1) has a positive slope as a function of pseudorapidity eta, d Delta v(1)/d eta = [1.68 +/- 0.49(stat) +/- 0.41(syst)] x 10(-4). The same measurement for D-0 and (D) over bar (0) mesons yields a positive value d Delta v(1)/d eta = [4.9 +/- 1.7(stat)] +/- 0.6(syst)] x 10(-1), which is about 3 orders of magnitude larger than the one of the charged hadrons. These measurements can provide new insights into the effects of the strong electromagnetic field and the initial tilt of matter created in noncentral heavy ion collisions on the dynamics of light (u, d, and s) and heavy (c) quarks. The large difference between the observed Delta v(1) of charged hadrons and D-0 mesons may reflect different sensitivity of the charm and light quarks to the early time dynamics of a heavy ion collision. These observations challenge some recent theoretical calculations, which predicted a negative and an order of magnitude smaller value of d Delta v(1)/d eta for both light flavor and charmed hadrons.Peer reviewe

Multiplicity dependence of K*(892)(0) and phi(1020) production in pp collisions at root s=13 TeV

The striking similarities that have been observed between high-multiplicity proton-proton (pp) collisions and heavy-ion collisions can be explored through multiplicity-differential measurements of identified hadrons in pp collisions. With these measurements, it is possible to study mechanisms such as collective flow that determine the shapes of hadron transverse momentum (p(T)) spectra, to search for possible modifications of the yields of short-lived hadronic resonances due to scattering effects in an extended hadron-gas phase, and to investigate different explanations provided by phenomenological models for enhancement of strangeness production with increasing multiplicity. In this paper, these topics are addressed through measurements of the K* (892)(0) and phi(1020) mesons at midrapidity in pp collisions at root s = 13 TeV as a function of the charged-particle multiplicity. The results include the p(T) spectra, p(T)-integrated yields, mean transverse momenta, and the ratios of the yields of these resonances to those of longer-lived hadrons. Comparisons with results from other collision systems and energies, as well as predictions from phenomenological models, are also discussed. (C) 2020 European Organization for Nuclear Research. Published by Elsevier B.V.Peer reviewe

Evidence of Spin-Orbital Angular Momentum Interactions in Relativistic Heavy-Ion Collisions

The first evidence of spin alignment of vector mesons (K*(0) and phi) in heavy-ion collisions at the Large Hadron Collider (LHC) is reported. The spin density matrix element rho(00) is measured at midrapidity (vertical bar y vertical bar <0.5) in Pb-Pb collisions at a center-of-mass energy root(s)NNof 2.76 TeV with the ALICE detector. rho(00) values are found to be less than 1/3 (1/3 implies no spin alignment) at low transverse momentum (p(T) <2 GeV / c) for K*(0) and phi at a level of 3 sigma and 2 sigma, respectively. No significant spin alignment is observed for the K-S(0) meson (spin = 0) in Pb-Pb collisions and for the vector mesons in pp collisions. The measured spin alignment is unexpectedly large but qualitatively consistent with the expectation from models which attribute it to a polarization of quarks in the presence of angular momentum in heavy-ion collisions and a subsequent hadronization by the process of recombination.Peer reviewe

Measurement of the (anti-)He-3 elliptic flow in Pb-Pb collisions at root s(NN)=5.02TeV

The elliptic flow (v(2)) of (anti-)He-3 is measured in Pb-Pb collisions at root s(NN )= 5.02TeV in the transverse-momentum (p(T)) range of 2-6 GeV/c for the centrality classes 0-20%, 20-40%, and 40-60% using the event-plane method. This measurement is compared to that of pions, kaons, and protons at the same center-of-mass energy. A clear mass ordering is observed at low p(T), as expected from relativistic hydrodynamics. The violation of the scaling of v(2) with the number of constituent quarks at low p(T), already observed for identified hadrons and deuterons at LHC energies, is confirmed also for (anti-)He-3. The elliptic flow of (anti-)He-3 is underestimated by the Blast-Wave model and overestimated by a simple coalescence approach based on nucleon scaling. The elliptic flow of (anti-)He-3 measured in the centrality classes 0-20% and 20-40% is well described by a more sophisticated coalescence model where the phase-space distributions of protons and neutrons are generated using the iEBE-VISHNU hybrid model with AMPT initial conditions. (C) 2020 European Organization for Nuclear Research. Published by Elsevier B.V.Peer reviewe

Studies of J/psi production at forward rapidity in Pb-Pb collisions at root s(NN)=5.02 TeV

The inclusive J/psi production in Pb-Pb collisions at the center-of-mass energy per nucleon pair sNN= 5.02 TeV is presented and compared with previous measurements at sNN = 2.76 TeV as a function of the centrality of the collision, and of the J/psi transverse momentum and rapidity. The inclusive J/psi RAA shows a suppression increasing toward higher transverse momentum, with a steeper dependence for central collisions. The modification of the J/psi average transverse momentum and average squared transverse momentum is also studied. Comparisons with the results of models based on a transport equation and on statistical hadronization are carried out.Peer reviewe

Production of He-4 and (4) in Pb-Pb collisions at root(NN)-N-S=2.76 TeV at the LHC

Results on the production of He-4 and (4) nuclei in Pb-Pb collisions at root(NN)-N-S = 2.76 TeV in the rapidity range vertical bar y vertical bar <1, using the ALICE detector, are presented in this paper. The rapidity densities corresponding to 0-10% central events are found to be dN/dy4(He) = (0.8 +/- 0.4 (stat) +/- 0.3 (syst)) x 10(-6) and dN/dy4 = (1.1 +/- 0.4 (stat) +/- 0.2 (syst)) x 10(-6), respectively. This is in agreement with the statistical thermal model expectation assuming the same chemical freeze-out temperature (T-chem = 156 MeV) as for light hadrons. The measured ratio of (4)/He-4 is 1.4 +/- 0.8 (stat) +/- 0.5 (syst). (C) 2018 Published by Elsevier B.V.Peer reviewe

Effect of Water Gap and Fuel Assembly Positioning in Passive Neutron Albedo Reactivity Measurements for Spent Fuel Encapsulation Safeguards

The Passive Neutron Albedo Reactivity (PNAR) Ratio is proportional to the net neutron multiplication of a spent fuel assembly. In the planned integrated non-destructive assay instrument for Finnish encapsulation safeguards, a PNAR instrument is used to confirm the presence of fissile material. In this study, the sensitivity of fuel-type-specific PNAR Ratio measurements to the size of the water channel of the instrument is determined using MCNP5 Monte Carlo simulations. Based on the study results, use of the smallest possible water channel is recommended to maximize the dynamic range of the instrument. In the Finnish fuel encapsulation context, this means using water gap sizes of 5 mm and 3 mm for measurements of boiling water reactor (BWR) and water-water energetic reactor (VVER-440) fuel, respectively. Based on the neutron emission rates of the Finnish spent fuel inventory, we recommend maximizing count rates by having detectors all around the fuel assembly, i.e., 4 detectors for BWR fuel and 6 detectors for VVER-440 fuel. With these water gap sizes, and neutron detectors all around the fuel assembly, the variation of the PNAR Ratio measurement caused by the uncertainty on the position of the fuel in the instrument is estimated to be 0.06% for BWR fuel and 0.13% for VVER-440 fuel.Peer reviewe