Benchmarking the Particle Background in the LHC Experiments

The experiments for the Large Hadron Collider LHC at CERN have to work for 15 years in the presence of a very high particle background of photons in the energy range from 100\,keV to 10\,MeV and neutrons in the range from thermal energies ($\approx 0.025\,$eV) to 20\,MeV. \\ The background is so high that it becomes a major design criterion for the ATLAS ex\-peri\-ment, a general purpose experiment at LHC that will be operational in the year 2005. The exact level of this background is poorly known. At present an uncertainty factor of five has to be assumed to which the limited knowledge of the shower processes in the absorber material and the ensueing neutron and photon production is estimated to contribute with a factor 2.5. \\ So far, the background has been assessed only through extensive Monte Carlo evaluation with the particle transport code FLUKA. The lack of relevant measurements, which were not done up to now, are to a large extent responsible for this uncertainty. Hence it is essential to benchmark the background predictions with measurements in order to reduce the uncertainties resulting from the shower processes. This work describes in detail the benchmarking measurements and analysis of these backgrounds in an experimental arrangement that approaches rather closely the layout and shielding in the ATLAS detector. The absolute yield and energy of the particles ema\-nating from the final stages of the hadronic shower were measured using a Bi$_4$Ge$_3$O$_{12}$ detector. \\ In this study particular care was taken to guard against spurious effects, which could mask the measurements of the photon background. Typically we expect to measure a photon per $10^4$ incident hadrons which is equivalent to a reduction factor in energy of $\approx 10^8$. At first, calibration measurements with well known radioactive sources were carried out in order to evaluate the response to photons and neutrons of the used detector. The photon results show excellent agreement with the simulations, while the neutrons show some FLUKA specific discrepancies that are however well understood. The actual benchmarking task comprised measurements with different beam intensities and momenta, different positions and absorber thicknesses in order to reduce systematic effects and assess residual activities from other sources. \\ The careful analysis of the measurements including a detailed evaluation of the systematic uncertainties provides a good understanding of all effects due to residual activities, dead-time corrections and other rate effects of the set-up. Comparing the measurements with detailed FLUKA simulations shows that under all different measurement conditions the agreement is on the 20\,\% level. These studies also give answer to the nature of the particles emanating from the absorber. \\ Finally a method to obtain the measured photon rates and energies from the total measured and simulated numbers was developed. The comparison of the measurements with the FLUKA calculations can hence be used to reduce the uncertainties resulting from the shower processes, so that the background simulations can predict the ATLAS background with higher reliability

CERN Neutrinos to Gran Sasso (CNGS): First Beam

The CNGS, CERN Neutrinos to Gran Sasso project, aims at directly detecting muon-neutrino to tau-neutrino oscillations. An intense muon-neutrino beam (10 to the 17 muon neutrinos)is generated at CERN per day and directed towards the Gran Sasso National Laboratory, LNGS, in Italy, 732 km away from CERN. In LNGS large and complex detectors will allow to detect, in particular, the rare tau-neutrinos created by âoscillation' from muon-neutrinos on their way between CERN and LNGS. On average around three tau-neutrino events are predicted per year in each of the ~2000 ton detectors. The construction of the CNGS beam facility started in September 2000, and the first neutrino beam has been produced in July 2006. In the presently approved physics programme, it is foreseen to run the facility for five years

The Two-Screen Measurement Setup to Indirectly Measure Proton Beam Self-Modulation in AWAKE

The goal of the first phase of the AWAKE \cite{AWAKE1,AWAKE2} experiment at CERN is to measure the self-modulation \cite{SMI} of the $\sigma_z = 12\,\rm{cm}$ long SPS proton bunch into microbunches after traversing $10\,\rm{m}$ of plasma with a plasma density of $n_{pe}=7\times10^{14}\,\rm{electrons/cm}^3$. The two screen measurement setup \cite{Turner2016} is a proton beam diagnostic that can indirectly prove the successful development of the self-modulation of the proton beam by imaging protons that got defocused by the transverse plasma wakefields after passing through the plasma, at two locations downstream the end of the plasma. This article describes the design and realization of the two screen measurement setup integrated in the AWAKE experiment. We discuss the performance and background response of the system based on measurements performed with an unmodulated Gaussian SPS proton bunch during the AWAKE beam commissioning in September and October 2016. We show that the system is fully commissioned and adapted to eventually image the full profile of a self-modulated SPS proton bunch in a single shot measurement during the first phase of the AWAKE experiment.Comment: 5 pages 8 figure

Polycrystalline CdTe Detectors: A Luminosity Monitor for the LHC

The luminosity at the four interaction points of the Large Hadron Collider must be continuously monitored in order to provide an adequate tool for the control and optimization of the collision parameters and the beam optics. At both sides of the interaction points absorbers are installed to protect the super-conducting accelerator elements from quenches causes by the deposited energy of collision products. The luminosity detectors will be installed in the copper core of these absorbers to measure the electromagnetic and hadronic showers caused by neutral particles that are produced at the proton-proton collision in the interaction points. The detectors have to withstand extreme radiation levels (10^8 Gy/yr at the design luminosity) and their long-term operation has to be assured without requiring humain intervention. In addition the demand for bunch-by-bunch luminosity measurements, i.e. 40MHz detection speed, puts severe constraints on the detectors. Polycrystalline CdTe detectors have a high potential to fulfill the requirements and are considered as LHC luminosity monitors. In this paper the interaction region is shown and the characteristics of the CdTe detectors are presented

Novel parameter-based flexure bearing design method

A parameter study was carried out on the design variables of a flexure bearing to be used in a Stirling engine with a fixed axial displacement and a fixed outer diameter. A design method was developed in order to assist identification of the optimum bearing configuration. This was achieved through a parameter study of the bearing carried out with ANSYS®. The parameters varied were the number and the width of the arms, the thickness of the bearing, the eccentricity, the size of the starting and ending holes, and the turn angle of the spiral. Comparison was made between the different designs in terms of axial and radial stiffness, the natural frequency, and the maximum induced stresses. Moreover, the Finite Element Analysis (FEA) was compared to theoretical results for a given design. The results led to a graphical design method which assists the selection of flexure bearing geometrical parameters based on pre-determined geometric and material constraints

Indirect Self-Modulation Instability Measurement Concept for the AWAKE Proton Beam

AWAKE, the Advanced Proton-Driven Plasma Wakefield Acceleration Experiment, is a proof-of-principle R&D experiment at CERN using a 400 GeV/c proton beam from the CERN SPS (longitudinal beam size sigma_z = 12 cm) which will be sent into a 10 m long plasma section with a nominal density of approx. 7x10^14 atoms/cm3 (plasma wavelength lambda_p = 1.2mm). In this paper we show that by measuring the time integrated transverse profile of the proton bunch at two locations downstream of the AWAKE plasma, information about the occurrence of the self-modulation instability (SMI) can be inferred. In particular we show that measuring defocused protons with an angle of 1 mrad corresponds to having electric fields in the order of GV/m and fully developed self-modulation of the proton bunch. Additionally, by measuring the defocused beam edge of the self-modulated bunch, information about the growth rate of the instability can be extracted. If hosing instability occurs, it could be detected by measuring a non-uniform defocused beam shape with changing radius. Using a 1 mm thick Chromox scintillation screen for imaging of the self-modulated proton bunch, an edge resolution of 0.6 mm and hence a SMI saturation point resolution of 1.2 m can be achieved.Comment: 4 pages, 4 figures, EAAC conference proceeding

Expected signal for the TBID and the ionization chambers downstream of the CNGS target station

Downstream of the carbon graphite target of the CNGS (CERN Neutrinos to Gran Sasso) facility at CERN a secondary emission monitor called TBID (Target Beam Instrumentation Downstream) is installed to measure the multiplicities and the left/right as well as up/down asymmetries of secondary particles from the target. Calculations show that the titanium windows used to close off the TBID vacuum tank might not withstand the highest beam intensities with small spot sizes expected at CNGS, in case the proton beam accidentally misses the 4-5 mm diameter target rods. Therefore it has been suggested to place two ionisation chambers as a backup for the TBID, located left and right of the TBID monitor. Monte Carlo simulations with the particle transport code FLUKA were performed firstly to obtain the fluence of charged particles in the region of interest and secondly to estimate the induced radioactivity (background signal) in this area. This allows to assess the actual signal/noise situation and thus to determine the optimal position of the ionisation chambers. This paper presents the results of these calculations

LHC Beam Loss Monitors

At the Large Hadron Collider (LHC) a beam loss system will be installed for a continuous surveillance of particle losses. These beam particles deposit their energy in the super-conducting coils leading to temperature increase, possible magnet quenches and damages. Detailed simulations have shown that a set of six detectors outside the cryostats of the quadrupole magnets in the regular arc cells are needed to completely diagnose the expected beam losses and hence protect the magnets. To characterize the quench levels different loss rates are identified. In order to cover all possible quench scenarios the dynamic range of the beam loss monitors has to be matched to the simulated loss rates. For that purpose different detector systems (PIN-diodes and ionization chambers) are compared

Response of a BGO detector to photon and neutron sources: simulations and measurements

In this paper Monte Carlo simulations (FLUKA) and measurements of the response of a BGO detector are reported. %For the measurements different radioactive sources were used to irradiate the BGO crystal. For the measurements three low-energy photon emitters $\left({}^{60}\rm{Co},\right.$ ${}^{54}\rm{Mn},$ $\left. {}^{137}\rm{Cs}\right)$ were used to irradiate the BGO from various distances and angles. The neutron response was measured with an Am--Be neutron source. Simulations of the experimental irradiations were carried out. Our study can also be considered as a benchmark for FLUKA in terms of its reliability to predict the detector response of a BGO scintillator