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ATHENA Far-Forward Detector Working Group


  • Alex Jentsch (ajentsch@bnl.gov)
  • John Arrington (jarrington@lbl.gov)

Meetings: INDICO on Mondays @ 1:30pm EDT, unless noted otherwise via email.

ATHENA FF layout Nov 2021 v1.pdf

Far-Forward Sub-systems

  • B0 Silicon Tracker & Photon Tagger

The B0 tracker and photon tagger currently consists of 4 silicon tracking layer, with 2-3 layers being comprised of MAPS (< 20um spatial resolution), and 1-2 layers being comprised of AC-LGADs spaced evenly by 30cm inside the B0pf magnet bore in IP-6. After the silicon tracking system, a simple photon tagger in the form of a preshower detector is included. This layer consists of 2 radiations lengths of Pb converter, followed by a layer of AC-LGADs. The magnet is still under design so details of the engineering aspects of the detectors are not currently able to be considered, except in a general sense.

The detector will need to be inserted/removed on the IP-side of the magnet bore. The detector space is warm and open to air, so routing the cabling and cooling components will be a little less challenging. The detector can slide in and out of the bore using a rail system like many other forward vertex trackers, e.g. STAR FST in the Forward Upgrade.

The below figure shows the basic angular acceptance for 100 GeV protons in the B0 tracking detector, with the current considerations for the available space. The space occupied by the electron components are still under design, so the acceptance loss in that region is still not finalized.

The below figure shows the baseline acceptance for the B0 detector for 100 GeV protons. The left pane is the generated proton sample, the middle pane is is the accepted protons, and the right pane is the occupancy in the first layer of the silicon.

B0 acceptances 100 gev.png

  • Roman Pots

Location: Station 1 @ z = 26m, station 2 at z = 28m

The Roman Pots detector subsystem consists of two stations, each with two layers, of AC-LGAD sensors bump bonded to ASICs, and surrounded by RF shielding foils. The whole package is then injected (vertically only) directly into the beam-line vacuum. The detectors will be injected a few millimeters from the beam (at 10Sigma), with the distance determined by the optics configuration in use for that run period. Currently the detector is ~12cm tall and ~26cm wide in total.

  • Off-Momentum Detectors


2 stations (separated by 2m) right after B1apf, injected as Roman Pots detectors into beam line (horizonatally).

The off-momentum detectors (OMD) are designed to capture charged particles (e.g. primarily protons) from nuclear breakup, where the protons have a different magnetic rigidity than the beam being used. These protons are then bent outside the beam pipe vacuum at some point along the drift. Implementing the OMD as horizontal Roman Pots detectors allows for a smoother transition for acceptance between the conventional Roman Pots detector, and the OMD.

  • Zero-Degree Calorimeter

The Zero-Degree Calorimeter (ZDC) is designed to capture neutrons and photons from various nuclear breakup and incoherent nuclear events. The ZDC will have two distinct calorimetry layers, as well as a silicon layer for charged particle vetoing. The ECAL will be comprised of WScFi towers with dimensions of 2.5cm x 2.5cm x 17cm, while the HCAL will be a 7 interaction length Pb/Sc sampling calorimeter with 10cm x 10cm towers. Additionally, a silicon or scintillating fiber imaging layer will be place near the hadron shower max to enable more precise determination of neutron transverse momentum via high angular resolution. The ECAL delivers an energy resolution of 10-12%/Sqrt(E) + 3%, while the HCAL (combined with the ECAL) delivers 30-40%/Sqrt(E) + 2%, which is better than what was assumed in the Yellow Report.

The image below shows a 100 GeV neutron showering in the combined ECAL + HCAL used for the full ZDC.

ZDC EICroot neutron shower.png

Acceptance Summary

Caption text
Detector Acceptance Notes
Zero-Degree Calorimeter (ZDC) 𝜽 < 5.5 mrad (πœ‚ > 6) About 4.0 mrad at Ο• ~ πœ‹
Roman Pots (2 stations) 0.0* < 𝜽 < 5.0 mrad (πœ‚ > 6) 0.65 < 𝑝_(𝑧, π‘›π‘’π‘π‘™π‘’π‘œπ‘›)/𝑝_(𝑧, π‘π‘’π‘Žπ‘š) < 1.0 -- *10Οƒ cut
Off-Momentum Detectors (2 stations) 0.0 < 𝜽 < 5.0 mrad (πœ‚ > 6) Roughly 0.3 < 𝑝_(𝑧, π‘›π‘’π‘π‘™π‘’π‘œπ‘›)/𝑝_(𝑧, π‘π‘’π‘Žπ‘š) < 0.6
B0 Detector 5.5 < 𝜽 < 20.0 mrad (4.6 < πœ‚ < 5.9) Silicon tracking + EM preshower