The occupancy in the HCal endcap coming from neutrons produced in the showers from inco-herent pairs is clearly too large. In addition to the increased granularity in the HCal it has also to be studied, whether it is possible to improve the shielding of the calorimeter from particles produced in the BeamCal. Currently the support tube inside the HCal endcap is represented by iron in the simulation. Iron, however, is not an optimal shield against neutrons. It could be pos-sible to reduce the number of neutrons reaching the HCal endcap by adding a neutron absorbing material where possible.
Another possibility is removing the neutron source, by placing the BeamCal further down-stream of the IP. This, however, clashes with the requirement of the intra-train-feedback system and QD0 to be positioned as close to the IP as possible.
It should also be studied, if the number of particles reaching the endcap yoke is decreased by adding the cavern and tunnel walls to the simulation, which could reduce the number of particles scattering into the yoke endcap from the outside of the detector.
8 Electron Tagging and Beam-Induced Background in the BeamCal
The beam calorimeter (BeamCal) is the most forward calorimeter in the detector. It covers a polar angle range of 10 mrad<θ<42 mrad. The details of the BeamCal are described in Sec-tion 4.5.2. The large energy deposits from the incoherent e+e− pairs and resulting ionising and non-ionising radiation doses require the use of radiation hard sensors. Because of the large en-ergy deposit in each bunch crossing the showers from physics events are not directly visible—i.e.
the BeamCal cannot be used like the other calorimeters in the detector. However, by estimat-ing the average energy deposit from the incoherent e+e−pairs some information about possible energy deposits from high energy electrons can be extracted.
8.1 Energy Deposit and Total Ionising Dose in the BeamCal
The BeamCal is the main absorber of incoherent e+e− pairs in the detector. There are several ten-thousand electrons and positrons with a total energy of 33 TeV per BX hitting each of the two BeamCal detectors. The total deposited energy in the sensors is 0.6 TeV/BX. This energy deposit is broadly spread across the volume of the BeamCal, so that almost all pads of the BeamCal will see an energy deposit in every bunch crossing. As for the other subdetectors, it will not be technically possible to read out the BeamCal after each bunch crossing. The deposited energy of several bunch crossings will have to be integrated, and the bunch train will be divided into 6 to 15 readout windows.
The total energy of particles fromγγ→hadron events inside the BeamCal is 150 GeV/BX and results in a deposited energy of only 1.5 GeV/BX, four-hundred times smaller than the energy deposited by the incoherent e+e−pairs. Therefore the energy deposit fromγγ→hadron events will be neglected for the following estimates of the background and for the electron tagging studies.
Figure 8.1 shows the energy deposit in the fourth and tenth layer of the two BeamCal detectors from ten bunch crossings of incoherent e+e−pairs. The different distributions in the forward and backward calorimeters are coming from the opposite charges of the repelled particles depending on the beam direction, and on the magnetic field and crossing angle.
Figure 8.2a shows the relative longitudinal distribution of the energy deposited by incoherent e+e− pairs. The energy deposit peaks in layer 4, and falls off quickly until layer 20. The last layer shows a peak in the deposited energy, because of particles scattering into the backside of the BeamCal.
The relative radial distribution in layer 4 with respect to the rings of the BeamCal is shown in Figure 8.2b. The energy deposits per ring fall quickly with increasing radius, as can be expected from the angular distribution of the incoherent e+e−pairs.
X’ [mm]
Deposited Energy per Pad [GeV]
10-3
10-2
10-1
1 10
(a) Forward BeamCal: Layer 4
X’ [mm]
Deposited Energy per Pad [GeV]
10-3
10-2
10-1
1 10
(b) Backward BeamCal: Layer 4
X’ [mm]
Deposited Energy per Pad [GeV]
10-3
10-2
10-1
1 10
(c) Forward BeamCal: Layer 10
X’ [mm]
Deposited Energy per Pad [GeV]
10-3
10-2
10-1
1 10
(d) Backward BeamCal: Layer 10
Figure 8.1: Energy deposit from 10 BX of incoherent e+e− pair background in the fourth and tenth layer of the forward and backward BeamCal.
8.1 Energy Deposit and Total Ionising Dose in the BeamCal
Layer
10 20 30 40
A.U.
10-3
10-2
10-1
Flux Energy/TID
(a) Per layer
Ring
0 5 10 15
A.U.
10-5
10-4
10-3
10-2
10-1
1
Flux Energy/TID
(b) Per ring
Figure 8.2: Fractional deposit of energy, total ionising dose and equivalent neutron flux (a) per layer of the BeamCal and (b) per ring in the fourth layer for the energy and TID and in the fifth layer for the equivalent neutron flux.
The energy deposit per pad from incoherent e+e− pairs of a single BX are shown in Fig-ure 8.3a. The maximum energy deposit in a single pad is about 2 GeV/BX, a few percent of the pads receive more than 1 MeV/BX, but the majority of pads, especially at larger radii, record only small amounts of deposited energy.
The energy deposited in a single pad from a minimum ionising particle is about 150 keV.
Figure 8.3b shows the energy deposits per pad from 150 GeV muons traversing the BeamCal;
the MIP-peak at 150 keV is clearly visible.
8.1.1 Total Ionising Dose
The deposited energy in the sensors is also used to estimate the total ionising dose per year∗. The dose is the deposited energy divided by the mass of the sensor volume in which the energy is deposited. The TID per year in the fourth layer of the BeamCal and the yearly dose for all pads are shown in Figure 8.4. The maximum expected dose, located at the inner edge of the BeamCal, is around 1 MGy/yr, further outside the dose falls to a few Gray per year.
8.1.2 Equivalent Neutron Flux
To estimate the neutron flux from incoherent e+e−pairs, the damage-factors for silicon from [83]
are used to scale the neutrons to the 1 MeV equivalent. The NIEL damage cross-section from neutrons in diamond is smaller than in silicon. However, the relative scaling between the damage-factors at different energies is similar in diamond and silicon [90]. Therefore the sili-con damage-factors can be used to approximate the 1 MeV neutron equivalent flux in diamond
∗Assuming 100 days of operation with nominal beams.
Energy [GeV/BX]
10-10 10-8 10-6 10-4 10-2 1 102
1/N dN
10-4
10-3
10-2
(a) Incoherent e+e−pairs
Energy [keV]
0 200 400 600 800 1000
1/N dN
0 0.01 0.02
(b) Minimum ionising particles
Figure 8.3: (a) distribution of energy deposits in all pads of the BeamCal from incoherent e+e− pairs and (b) deposited energy per pad from 150 GeV muons.
X’ [mm]
-100 0 100
Y [mm]
-100 0 100
Total Ionising Dose [kGy/yr]
10-3
10-2
10-1
1 10 102
103
104
(a) TID in layer 4
Total Ionising Dose [kGy/yr]
10-9 10-6 10-3 1 103
1/N dN
10-4
10-3
10-2
(b) TID per pad
Figure 8.4: TID per year (a) in the fourth layer of BeamCal and (b) in all pads of the BeamCal.