Plot Approval Yat Long Chan (CUHK) Igor Mandic (Ljubljana) Charlie Young (SLAC)

In the monitoring system in the ATLAS experiment various semiconductor sensors are used to measure integrated radiation doses. The basic unit of the system is a Radiation Monitor Sensor Board (RMSB) which hosts radiation detectors that can be read-out online. RMSBs are placed at 14 locations in the Inner Detector at 4 sets of locations with same r and |z| coordinates but at different azimuth angles. Sensors are read out once per hour and the data are stored in the DCS database. Total ionizing dose (TID) is measured with radiation sensitive p-MOS transistors (RADFETs). It is measured from the increase of gate voltage at a given drain current. The relation between absorbed dose D and gate voltage increase V varies between different types of RADFETs and must be determined experimentally. It can be approximated by D = a · (V)b, where a and b are calibration constants to be determined for each type and production batch of RADFETs. The precision of dose measurements with this system is about 20%. Displacement (bulk) damage in silicon is monitored with diodes using two different methods: measurement of leakage current in a reverse biased diode and measurement of forward voltage at given forward current. The leakage current increase in reverse biased diode (I) after irradiation is directly proportional to the non-ionizing energy loss (NIEL). This can be expressed in units of 1 MeV neutron equivalent fluence. Small thickness ensures that the diode can be fully depleted with less than 30 V up to fluences of n/cm 2.

Comparison of predicted and measured total ionizing dose (TID) in units of Gy in the Inner Detector as a function of time in 2011 – Measurements with RadFETs are given by the solid lines for four sets of locations, and uncertainties given by the solid bands include measurement spread and systematic contributions. Predictions shown as dotted lines are from a FLUGG- based simulation

Comparison of predicted and measured non-ionizing energy loss (NIEL) in expressed in 1-MeV neutron-equivalent fluence in the Inner Detector as a function of time in 2011 – Measurements with diodes are given by the solid lines for four sets of locations, and uncertainties given by the solid bands, some of which overlap, include measurement spread and systematic contributions. Predictions shown as dotted lines are from a FLUGG-based simulation

A new beam pipe will be installed in ATLAS during LHC Long Shutdown 1. Besides having a smaller radius near the interaction point to accommodate the Insertable B-Layer of the pixel detector, much of the beam pipe material is changed from stainless steel to aluminium. Background simulation using the FLUGG-based application predicts a reduction of ~30% in the background in the Muon Spectrometer. The magnitude of change in background may seem surprising since the beam pipe is a very small portion of the material inventory in the ATLAS cavern; however, this can be understood based on p-p event topology and the shielding layout. The majority of p-p collisions are low p T, and much of the outgoing energy from the primary interaction is in the forward direction, with the peak around  = for √s = 14 TeV. While the present stainless steel beam pipes is thin radially, it is O(1) hadronic interaction length at this glancing incidence angle. The new beam pipe is made of aluminium. Fewer hadronic showers are initiated on the beam pipe, and those outgoing particles that do not interact in the beam pipe shower significantly further away from the interaction point (IP), because the inner radius of shielding is much larger than the beam pipe radius. Since ATLAS shielding becomes progressively thicker away from the IP, these showers are on average better contained and therefore contribute less to background.

Energy deposition in units of Gy/cm3/s as a function of z and r in the ATLAS detector as predicted by the FLUGG-based background simulation program. The detector geometry is that for 2012 data-taking but with the new beam pipe to be used with the Pixel Insertable B-Layer. There is a reduction of 30% to 40% in energy deposition.

Energy deposition in units of Gy/cm3/s as a function of z and r in the ATLAS detector as predicted by the FLUGG-based background simulation program. The detector geometry is that for 2012 data-taking. The structure of the ATLAS detector, beam line shielding and the curved back wall of the cavern are clearly visible.