CMS-DP-2014/016: FLUKA studies for a new sampling calorimeter in the forward ECAL

Contents:

Objective

• iCMS entry
• Corresponding conference note: conference note iCMS entry
• HL-LHC (High-Luminosity LHC): 5 x higher instantaneous Luminosity (up to 5*10^34 cm-2s-1 )
• Replacement of the ECAL Endcaps
• Future EE calorimeter challenged in terms of performance and radiation hardness:
  • Harsh radiation environment and energetic hadron fluences, pile-up
• Sampling calorimeter is one option for a new electromagnetic calorimeter

FLUKA studies for a sampling calorimeter for EE

Goal:

• FLUKA designed for calculations of flux maps, particle spectra, energy depositions, dose distributions, activation etc.
• Rather “minimum bias” scenario in CMS than “detector response” studies (Geant)
• Implementation of a configurable sampling calorimeter for the HL-LHC
• Determination of dose rates and particle fluxes within CMS for and in the presence of different calorimeters
• Examination and tracking of
  • Creation of isotopes that might produce a prohibitive background signal
  • Simulation of thermal neutrons as some elements have high neutron cross section
  • E.g. how much energy is deposited by the decaying isotopes?
• CMS description in FLUKA:
  

Electromagnetic sampling calorimeter

• Configurable materials, number and size of layers
• Divided in 5 η bins: 1.48-1.6-1.8-2.2-2.6-2.9
• FLUKA CMS geometry with sampling calorimeter:
  * Current study based on a 25 X0 sampling calorimeter
• Total length maximal 22 cm
• If smaller than 22 cm the rest of the current EE region (Lead Tungsten) is filled with a low density material, also used for the tracker region (rho=0.047g/cm3: 30% Fe, 23% Al, 30% C, some Air)
• Sampling calorimeter with alternating layers of * 20 X0 of passive absorber Tungsten (14 layers) * 5 X0 of an heavy inorganic scintillator: cerium fluoride (CeF3), LYSO, YSO (15 layers) * Corresponding sampling fraction, sampling resolution and Moliere Radius simulated with Geant4 standalone simulation * The Geant4 simulation shows, that the chosen parameters give reasonable results * The layer number and size is chosen such that any conflicts with the energy calculations of the Monte Carlo program FLUKA can be excluded

Studies with different sampling calorimeter geometries

• All plots are created with FLUKA2011.2b.5 and based on the CMS FLUKA nominal geometry version 1.0.0.0

FLUKA activation studies for HL-LHC up to LS5

• Parameters for FLUKA simulation:
  

Irradiation cycle until LS5 as used by BRIL radiation simulation group

* CMSirrProf_Sophie.xls: Irradiation cycle until LS5 as used by BRIL radiation simulation group

Additional material for plots for approval

• Lu_all_reactions.pdf: Lutetium high neutron capture cross section

• X4R19961_x4.pdf: Lutetium high neutron capture cross section

• Comparison of forward and backscattering neutrons at front surface of calorimeter:
  
• neutron_scatteringcomparison_140319.eps: neutron_scatteringcomparison_140319.eps
• electron_backwardscattering_140319.eps: electron_backwardscattering_140319.eps

• allparticlespectra_EEYSO_eta1.6_140319.eps: allparticlespectra_EEYSO_eta1.6_140319.eps

• CMSpp_EELYSO_Neutron2d_140319.eps: CMSpp_EELYSO_Neutron2d_140319.eps

• CMSpp_EECeF3_Dose2d_140319.eps: CMSpp_EECeF3_Dose2d_140319.eps

Approved plots

Technical specifications which all plots have in common:

Primary proton-proton collisions with an energy of 7TeV per beam. Inelastic collision cross section used for normalization is 80 mb. Used simulation cut offs in ECAL endcap region: Hadrons 100 keV, Neutrons 0.01 meV, Photons 50 keV, Electrons 100 keV. In the surrounding regions, photons and electrons have higher cutoffs, 100 keV and 300 keV respectively. The FLUKA geometry version 1.0.0.0 is the available baseline geometry version, corresponding to the situation prior to LS1, the EE is modeled with a single volume of Lead Tungstate. Version 1.0.1-1.0.3 for the respective calorimeter options are based on version 1.0.0.0. Only the region of the current Lead Tungstate crystals in the ECAL forward region is modified ( 320 cm - 342 cm in z). The sampling calorimeter consists of 15 layers of scintillator material and 14 layers of Tungsten. It has a total radiation length of 25 X_0, split into 5 X_0 for the scintillator and 20 X_0 for the absorber. If the total length is smaller than 22 cm, the rest of the volume will be filled with a region of very low mass density (ρ=0.047g/cm3), reflecting a region filled with cables and electronics.

For the simulations, the current FLUKA version 2011.2b.5 was used: http://www.fluka.org/fluka.php?id=release_notes&mm2=3

Link to previously approved BRIL plots with the baseline geometry version 1.0.0.0: https://twiki.cern.ch/twiki/bin/view/CMSPublic/BRILRadiationSimulation Link to the online plotting tool: https://cms-project-fluka-flux-map.web.cern.ch/

Link to evaluated nuclear data file (ENDF): http://www.nndc.bnl.gov/exfor/endf00.jsp

1. part: Validation of FLUKA geometry with a sampling calorimeter as replacement for ECAL forward calorimeter

Figure

Caption

pdf version

Validation of FLUKA geometry with a sampling calorimeter as replacement for ECAL forward calorimeter The implementation of the sampling calorimeter in the ECAL forward region is based on the FLUKA geometry version 1.0.0.0, the available baseline geometry version, corresponding to the situation prior to LS1. Only the region of the current Lead Tungstate crystals in the ECAL forward region is modified ( 320 cm - 342 cm in z). The sampling calorimeter versions are tagged with respect to the scintillator choice, version 1.0.1-1.0.3 for YSO, LYSO,CeF3, respectively. The sampling calorimeter consists of 15 layers of scintillator material and 14 layers of Tungsten. It has a total radiation length of 25 X_0, split into 5 X_0 for the scintillator and 20 X_0 for the absorber. If the total length is smaller than 22 cm, the rest of the volume will be filled with a region of very low mass density (ρ=0.047g/cm3, 30% Fe, 23% Al, 30% C, some Air), reflecting a region filled with cables, electronics and cooling devices. Identical to nominal geometry 1.0.0.0: the transition region between barrel and endcap region consists of an averaged material, accounting for the mechanics and cables in this region. The preshower in front of the sampling calorimeter acts as a placeholder for a possible timing device and the polyethylene moderator, shielding the tracker. Directly behind the sampling calorimeter, the Aluminium plate for the mounting of the current Lead Tungstate crystals is used as a placeholder for a similar support structure. The electronics, mechanics and cooling for this calorimeter are implemented as the same low density material (ρ=0.047g/cm3) together with a polyethylene volume for shielding.

2. part: benchmark of simulation: overall particle flux and dose rate

Figure

Caption

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EM fluence for 14 TeV pp collisions Monte Carlo estimation of the electron and positron fluence in CMS with a forward ECAL sampling calorimeter (EE) under HL-LHC conditions using FLUKA. The plot shows the electron and positron fluence in the EE for an integrated luminosity of 3000.0 fb-1 as a function of the radiation length X_0. The fluence is averaged over a region of 48 cm < R < 109 cm, corresponding to 1.8 < η < 2.6 for z = 320 cm. The z-coordinate is converted to radiation length by scaling with the effective radiation length for each calorimeter. The conversion factors are: 0.88 cm/X0 for PbWO4, 0.88 cm/X0 for YSO/W, 0.508 cm/X0 for LYSO/W and 0.616 cm/X0 for CeF3/W. The bin size is 10% of the effective radiation length of the respective calorimeter to visualise differences between the active and passive layers. The grid starts at the end of the first scintillator layer, each line indicates the end of a scintillator layer/start of an absorber layer. The fluence for Lead Tungstate is depicted as a reference. The average fluence is for all options similar, the development of the EM shower in a sampling calorimeter is visible. The difference in density of the scintillator materials is clearly visible up to 11 X_0. Most of the electromagnetic particles in a minimum bias environment have an energy of a few MeV up to 1 GeV and because of that the maximum electromagnetic particle flux develops in the first 4 X_0, particularly in the second scintillator layer.

eps version pdf version

Absorbed dose for 14 TeV pp collisions Monte Carlo estimation of the "absorbed dose" for CMS with a forward ECAL sampling calorimeter (EE) under HL-LHC conditions using FLUKA. The plot shows the dose absorbed in the EE for an integrated luminosity of 3000.0 fb-1 as a function of the radiation length X_0. The absorbed dose rate is averaged over 48 cm <R <109 cm, corresponding to 1.8 < η < 2.6 at z = 320 cm. The z-coordinate is converted to radiation length by scaling with the effective radiation length for each calorimeter. The conversion factors are: 0.88 cm/X0 for PbWO4, 0.88 cm/X0 for YSO/W, 0.508 cm/X0 for LYSO/W and 0.616 cm/X0 for CeF3/W. The bin size is 10% of the effective radiation length of the respective calorimeter to visualise differences between the active and passive layers. The grid starts at the end of the first scintillator layer, each line indicates the end of anscintillator layer/start of an absorber layer. The calculated dose corresponds to the dose absorbed in the implemented material. Any material not implemented in the geometry might have a different dose due material dependent interaction coefficients. The absorbed dose for Lead Tungstate is depicted as a reference. The average of the absorbed dose is for all options comparable. The absorbed dose is the deposited energy per unit mass of the medium. The higher dose rate in LYSO/W is explainable by the high thermal neutron cross section of Lutetium (23.1 barn). The absorbed dose is higher in the scintillator layers than in the Tungsten layers, opposite to the other options. The thermal neutron fluence from the low mass density regions at both ends of the sampling calorimeters is higher than the one in the scintillator materials. Due to the high thermal neutron cross section of Lutetium, the absorbed dose rate in LYSO increases significantly at both ends of the calorimeter. A thin absorber layer made out of Boron polyethylene is advisable to moderate neutrons and absorb the thermal ones.

eps version pdf version

Neutron energy spectrum at minimum bias shower maximum Monte Carlo estimation of the neutron fluence energy spectrum for CMS with a forward ECAL sampling calorimeter (EE) under HL-LHC conditions using FLUKA. The plot shows the fluence energy spectrum of neutrons for the maximal particle flux, which is in the second scintillator layer in 2.2 < η < 2.6. for 3000 1/fb. This track-length estimator is given as a differential distribution in energy, the value of each energy bin is thus multiplied by the width of the bin. In the region or low-energy neutrons, the number and width of the bins is given by the thermal neutron cross section library in FLUKA which consists of 260 data sets from 0.01 meV (simulation cutoff ) up to 20 MeV. Thermal neutrons carry an energy up to a few eV, followed by a resolved resonance region up to a few keV. This region shows different resonances for each option due to the different elements. LYSO/W shows a significantly lower fluence for thermal neutrons, as it has a high thermal neutron capture cross section, resulting in a higher absorbed dose. From a few MeV upwards all options show a similar shape for the neutron fluence. The spectrum contains also high energy neutrons with energies of a few GeV.

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Energy spectrum of backscattered neutrons Monte Carlo estimation of the neutron fluence energy spectrum for CMS with a forward ECAL sampling calorimeter (EE) under HL-LHC conditions using FLUKA. The plot shows the fluence energy spectrum of backscattered neutrons from the first scintillator layer to air in 2.2 < η < 2.6. for 3000 1/fb. This boundary crossing estimator is integrated over the full solid angle and given as a differential distribution in energy, the value of each energy bin is thus multiplied by the width of the bin. Only backscattered neutrons from the first scintillator layer are recorded. In the region of low-energy neutrons, the number and width of the bins is given by the thermal neutron cross section library in FLUKA which consists of 260 data sets from 0.01 meV (simulation cutoff ) up to 20 MeV. Thermal neutrons carry an energy up to a few eV, followed by a resolved resonance region up to a few keV. This region shows different resonances for each option due to the different elements. LYSO/W shows a significantly lower fluence for thermal neutrons, as it has a high thermal neutron cross section, resulting in a higher absorbed dose. From a few MeV upwards all options show a similar shape for the neutron fluence.

3. part: activation study for sampling calorimeter

Figure

Caption

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Absorbed dose rate in LYSO/W sampling calorimeter Monte Carlo estimation of the absorbed dose rate due to the prompt electromagnetic shower and of the decay products for an LYSO/ Tungsten sampling calorimeter for the forward ECAL sampling calorimeter (EE) under HL-LHC conditions using FLUKA. The absorbed dose rate is projected to the calorimeter length in z, from 320 cm - 332.7 cm. The volume behind the sampling calorimeter is filled with a region of low mass density (ρ=0.047g/cm^3), reflecting a region filled with cables and electronics. The plot compares the absorbed dose rate for a collision rate of 1.44*10^13 pp-int./h with the absorbed dose rate by decaying particles at the irradiation stop after 2475 fb^-1, starting from LS3 until LS5. The absorbed dose rate is visualised for two regions in R-Z: 48 cm - 71 cm and 109 cm -135 cm, corresponding to η = 1.6 - 1.8 and 2.2 - 2.6 at z = 320 cm. The absorbed dose rate in the higher η region is on average one order of magnitude higher than the one in the lower η region. The inelastic cross section of the prompt EM shower is normalised to running conditions whereas the part from the decay products at the irradiation stop of LS5 is normalised to a higher cross section as an upper limit for activation studies (recommendation from Radiation Protection group). The absorbed dose rate of the decay products is dominated by short lived radionuclides. Both rates differ in average by two orders or magnitude, leading to the conclusion that the constantly deposited energy by decaying isotopes is still in a manageable order of magnitude.