# Background

Information on thresholds and integration time can be found here Google sheet
Old version

An excellent explanation about timing for different detectors PDF

This section describes Synchrotron-Radiation (SR) studies carried out for the EPIC experiment. Two types of events will be described below. On one hand we have physical events, which correspond to what we expect to measure in the lab. On the other hand we have technical hepmc events, which correspond to all the information stored in a hepmc file in between lines that begin with the letter "E". We will refer to these as real and technical events, respectively.

The Synrad+ simulations provide a series of single-photon technical events in hepmc files. Each photon comes from a different vertex and has, besides the photon momentum vector and vertex coordinates, a weight that maps a given photon to a flux (photons/sec). The energy spectrum of these photons is shown below.

An event generator was constructed by creating a histogram with a photon per bin, and the bin content corresponding to the weight of that photon. To generate an event, the user begins by predefining a time integration window ${\displaystyle T}$ within which SR photons will be collected. Subsequently, photons are sampled from the aforementioned histogram until the sum of all inverse weights is greater than the predefined time integration window. That is, we continue sample photons as long as:

${\displaystyle \sum _{k=1}^{N}{\frac {1}{w_{i}}}

The figure below shows an example event for a 100-ns time integration window. The base of the arrow represents the vertex from which a given photon emerges and enters the detector, the length of the arrow corresponds to the relative size of the momentum component being represented, and the color scale indicates the total momentum of the photon, with darker colors corresponding to lower momenta.

Example 100-ns-wide SR event

The code that does this sampling can be found here. The output corresponds to a hepmc file with technical events matching real events for the given integration window. At the moment, the sampling is done based on single photons generated for an electron beam of energy ${\displaystyle E_{e}=10\ {\rm {GeV}}}$ and a current of 2.5 A. These hepmc files can subsequently be propagated through Geant (e.g. in DD4HEP) to determine the number of hits recorded in different subdetectors.

After generating 400k 100-ns-wide events (with this time integration window, events have on average 250 photons) and passing them through Geant in DD4HEP with the following command:

npsim --runType batch --numberOfEvents -1 --compactFile \${DETECTOR_PATH}/epic.xml --inputFiles input.hepmc --outputFile output_file.edm4hep.root


we get a root file with recorded hits, which can be used to estimate the expected number of hits per event in different subdetectors. See image below. Subdetectors not included in that plot did not register any SR hits from the 400k events propagated.

Hit frequency for different subdetectors.

This plot was generated with the EPIC "brycecanyon" detector configuration and with a 5 µm gold layer inside the beampipe. Below, we can see a comparison between this configuration and the case with no gold coating inside the beampipe, for the three innermost silicon layers:

Scatter plot of SR hits in the three innermost silicon layers of the EPIC detector. These hits are collected from 100-ns-wide SR events. Left: no gold coating in the beampipe. Right: 5 µm gold coating.

The gold coating reduces the expected hits in these layers by two orders of magnitude.

### Electron Beam - Gas

Study for ATHENA proposal is documented in https://wiki.bnl.gov/athena/index.php?title=Beam_backgrounds#Electron_Beam

Input hepmc for the simulation is:

Macros which were used to run the simulation are located in:

Output file from the ATHENA simulation:

The large hadronic cross section of the p/A + H2restgas interactions is a concern. Secondary interactions of the particles produced in hadron beam gas events with aperture limitations, i.e. magnets, beam pipe, masks are also one of the main sources of neutrons that thermalize within the detector hall. In the following we will discuss the results on the occupancy / rates from the different ATHENA subdetectors due to proton beam gas events.

We use Charles Hetzel’s vacuum simulation after 10000Ahr (All pumps on option), the range of the beam gas we consider is from -5.5m to 5m. We use the Pythia8 fixed target events including beam effects ( cross angle, crab cavity, beam energy spread, angular beam divergence, bunch length) for our simulation. The cross section for proton H2restgas interactions is 78.54mb (Pythia8). The background collision rate from proton beam gas is 31.45kHz (275GeV), 30.74kHz (100GeV), 30.96kHz (41GeV).

 Here we show the vertex Z distribution for the proton beam gas background collision.

We use the "ddsim" in the EPIC default configuration for the full simulations. In the following plot, we show the hit rate in each sub detectors.

In the following table we show the numbers of the hits per second for sub detector from dd4hep (no threshold appled):

 VertexBarrel 259000 DIRCBar 2.39E+03 MRICH 1.55E+05 EcalEndcapP 2.18E+07 DRICH 3400 TrackerEndcap 1.40+E6 MPGDTrackerBarrel 8.87E+04 EcalEndcapN 2.66E+06 EcalBareel 2.43e+07 EcalBareelScFi 1.32E+07 HcalBarrel 6.16E+06 HcalEndcapP 1.02E+08 HcalEndcapN 390000

In the following table we show the numbers of the hits per second for sub detector after the threshold applied:

 VertexBarrel 259000 DIRCBar 2.39E+03 MRICH 5.17E+04 EcalEndcapP 3.15E+05 DRICH 1130 TrackerEndcap 1.40E+06 MPGDTrackerBarrel 8.86E+04 EcalEndcapN 4.39E+05 EcalBareel 2.26e+07 EcalBareelScFi 5.77E+04 HcalBarrel 1.35E+06 HcalEndcapP 1.53E+07 HcalEndcapN 39800

For Ecal_Barrel, the readout unit is 0.5mmX0.5mm pixel. For Ecal_endcap, there is no segmentation, the readout for the hits is 1 fiber. For HCal, the readout unit is 100mmX100mm for barrel and endcap, the hits for Hcal are per layer.

Position of hits is shown in following series of plots. The position is given in transverse x and y coordinates and in radius in the transverse plane and z coordinate with energy threshold applied.

Tracking detectors, VertexBarrel
Electromagnetic calorimeters, EcalBarrelSciGlass+EcalEndcapP + EcalEndcapN +EcalEndcapPInsert
Hadron calorimeters, HcalBarrel + HcalEndcapN + HcalEndcapP