FLUKA studies for a new sampling calorimeter in the forward ECAL
Contents:
Objective
- HL-LHC (High-Luminosity LHC): 10 x higher instantaneous Luminosity (up to 10^35 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 (ca. 10 x higher), pile-up
- One option of a new electromagnetic calorimeter would be a sampling 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?
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 by Sasha Ledovsky. This specific simulation was done by Francesco Pandolfi:
* 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 simulation: HL-LHC irradiation
- FLUKA parameters for HL-LHC running conditions:
FLUKA activation studies for HL-LHC up to LS5
- FLUKA parameters for activation study:
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
PLOTS FOR APPROVAL
1. part: Validation of FLUKA geometry with a sampling calorimeter as replacement for ECAL forward calorimeter
- samp_calorimeter_forEE.png:
The implementation of the sampling calorimeter in the ECAL forward region is based on the FLUKA geometry version 1.0.0.0, corresponding to the situation prior to LS1, where 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 1.0.0.1-1.0.0.3 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, divided in 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 low density material, which is also used for the tracker region. 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 moderator to shield the tracker. Directly behind the sampling calorimeter, the Aluminium plate for the mounting of the current Lead Tungsten crystals is used as a placeholder for a similar support structure. The electronics, mechanics and cooling for this calorimeter are implemented as an averaged material, the same low density material as used for the tracker region, together with a polyethylene layer for shielding.
The text of the captions will be improved on Wednesday afternoon
2. part: benchmark of simulation: overall particle flux and dose rate
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, 100keV and 300keV respectively.
The FLUKA geometry version 1.0.0.0 corresponds to the situation prior to LS1, the EE is modeled with a single volume of Lead Tungstate. Version 1.0.0.1-1.0.0.3 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 changed ( 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, divided in 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 low density material, identical to the one used for the tracker region.
* ElectronPositronfluence.png:
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 bin size is 10% of the effective radiation length of the respective calorimeter to visualise differences between the active and passive layers. The fluence for Lead Tungstate is depicted as a reference. The sampling fraction and density of the scintillator materials influences the distribution up to 7 X_0.
Most of the electromagnetic particles in a minimum bias environment have an energy of a few MeV up to 1 GeV. Thus, the maximum electromagnetic particle flux is in the first 4 X_0, particularly in the second scintillator layer.
- Absorbed dose for sampling calorimeter:
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 3000.0fb-1 as a function of the radiation length X_0. The bin size is 10% of the effective radiation length of the respective calorimeter to visualise differences between the active and passive layers. 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 absorbed dose is directly correlated with the deposited energy and the difference of the sampling fraction between the different sampling calorimeters is clearly visible. Most of the electromagnetic particles in a minimum bias environment have an energy of a few
MeV up to 1
GeV. Thus, the maximum electromagnetic particle flux is in the first 4 X_0, particularly in the second scintillator layer.
The increase of the dose rate in LYSO/W at the end is explainable looking at the lower thermal neutron fluence compared to the other materials at the front surface of the sampling calorimeter. LYSO is the only scintillator congaing a heavy element, Lutetium with a thermal neutron capture cross section of 23 barn, the ratio of the absorbed dose rate between absorber and scintillator layer is reversed with respect to the other sampling calorimeters.
The LYSO layer show a slightly higher absorbed dose than the tungsten layer. Tungsten also absorbed some radiation as visible at the ends of the sampling calorimeters where the scintillator layer faces low density regions.
- Backscattering neutrons at front surface of sampling calorimeters per proton collision:
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 η 2.2-2.6. per proton-proton collision. This boundary crossing estimator is given as a double differential distribution in energy and solid angle, the value of each energy bin is thus multiplied by the width of the bin and 2 pi. In the region or low-energy neutrons, the number and width of the bins is limited by the thermal neutron cross section in FLUKA (260 data sets from 0.01 meV up to 20
MeV ).
Thermal neutrons carry en energy up to a few eV, followed by a resolved resonance region up to a few 1 keV, shaped by the levels in compound nucleus Z* with (A+1). The different sampling calorimeter option differ due to their different composition, LYSO+ Tungsten shows significantly less thermal neutrons which result in a higher absorbed dose.
3. part: activation study for sampling calorimeter
- neutronspectra.png:
Monte Carlo estimation of the neutron fluence energy spectrum with respect to its energy for CMS with a forward ECAL sampling calorimeter (EE) under HL-LHC conditions using FLUKA. The plot shows the neutron fluence energy spectrum in the EE for an integrated luminosity of 3000.0fb-1 in the scintillator layer with the highest particle flux. This track-length fluence estimator is given as a differential distribution of fluence in energy, the values of each energy bin are thus multiplied by the width of the bin. In the region or low-energy neutrons, the number and width of the bins is limited by the thermal neutron cross section in FLUKA (260 data sets from 0.01 meV up to 20
MeV).
- Absorbed_dose_rate_YSO_sampcalorimeter:
Monte Carlo estimation of the absorbed dose rate of the prompt electromagnetic shower and of the decay products for an YSO/ 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 322 cm - 342 cm. The bin size is 10% of the effective radiation length of the calorimeter to visualise differences between the active and passive layers.
The plot compares the absorbed dose rate for a collision rate of 1.44*10^11 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 η bins: 1.6-1.8 and 2.2-2.6. Both rates differ by two orders or magnitude. Taking into account the conservative radiation profile and the increased cross section by 25%, the difference is still in a reasonable region.