Beam Conditions Monitor in

ATLAS

I. Dolenc1, V. Cindro1, H. Frais-Kölbl6, A. Gorišek1, E. Griesmayer6, H. Kagan4, G. Kramberger1, I. Mandić1, M. Mikuž1,2, H. Pernegger3, W. Trischuk5, P. Weilhammer3, M. Zavrtanik1

1Jožef Stefan Institute, Ljubljana Slovenia; 2University of Ljubljana, Slovenia; 3CERN, Geneva, Switzerland; 4Ohio State University, Columbus, USA; 5University of Toronto, Toronto, Canada, 6Fotec, Wiener Neustadt, Austria

Mail to: irena.dolenc@ijs.si

ABSTRACT

In order to monitor beam conditions and detect signs of beam instabilities which could cause damage to their detectors, LHC experiments have decided to develop their own system in addition to those provided by accelerator. ATLAS Beam Conditions Monitor (BCM) is the first monitoring system that is based on single beam bunch crossing measurement rather than integrating the accumulated particle flux. It consists of eight detector modules with polycrystalline CVD diamond pad sensors, placed symmetrically around the interaction point along the beam axis. By use of fast electronics, time-of-flight measurement will provide unique distinction between normal beam events of colliding protons and anomalous events which induce background and could in worst case damage the ATLAS detector. Additionally BCM modules will be used to measure interaction rate providing an estimation of LHC luminosity.

In January 2007 eight final modules were installed onto ATLAS Beam Pipe Support Structure.

PURPOSE

To detect signs of beam anomalies in ATLAS such as

Beam scraping the TAS collimator

Beam gas interactions

Provide information about p-p interactions

Luminosity monitor

WORKING PRINCIPLE

Distinguish background events (TAS, beam-gas interactions) from

interaction events by the time difference between two successive signals

Two detector stations placed symmetrically around IP at ∓z:

Interactions - signals every 25ns

Background - signals at ∆t=2∆z/c on one side

Background optimally distinguished with ∆t=12.5ns ⇒ ∆z=1.9m

REQUIREMENTS

Multiple interactions every 25ns  fast signal

Rise time <1ns

Width <3ns

Baseline restoration in <10ns

One 7TeV proton hitting TAS collimator gives ~1MIP/cm2  single MIP sensitivity

Installation close to the beam pipe at large 

Radiation hardness of sensors up to 30MRad and 1015 particles/cm2

No maintenance

REALIZATION

Two stations, each with eight detector modules at =0o, 90o, 180o, 270o

Mounted on Beam Pipe Support Structure at z=183.3cm

Sensors at r~55mm

BCM modules

38cm

183cm

44m

ATLAS

Pixel

BCM

TAS events,

∆t=12.5, 37.5, .. ns

interactions,

∆t=0, 25, .. ns

2 BCM detector stations

Polycrystalline CVD diamond SENSORS

Fast and short signal

High charge carrier velocity – sensors will be operated at 2V/m, close to velocity saturation

Short trapping times even before irradiation

Low leakage current – no cooling required

Irradiation hard - shown to withstand fluences up to 2.21015p/cm2

Characteristics of BCM diamonds

Developed by CERN RD42/Element Six Ltd., metalized with

radiation hard process at Ohio State University

Size 1010cm2, thickness ~500m

Charge collection distance ~220m

Sensor assembly:

two diamonds back-to-back at 45o to increase the signal

MIP

SIGNAL PROCESSING

DETECTOR MODULE

BACK – END: NINO chip

Timing amplifier-discriminator

1ns peaking time, 25ps jitter

Designed at CERN for ALICE ToF

Pulse width depends on input charge (time-over-threshold LVDS output)

Radiation tolerant

Signal split in to 2 inputs (1:12) to increase dynamic range

2nd stage:

Mini Circuits, GALI-52,

GHz, 20dB

Sensor

2 back-to-back pCVD diamonds operated at ±1000V

Front end: 2 stage Fotec amplifier

1st stage:

Agilent MGA62563,

500MHz, 22dB

BCM MODULE TESTS

Beam test at MGH Boston in October 2004

Proton beam 200MeV and 125MeV (signal ≥2.3MIPs)

Single and two back-to-back sensors at 90o and 45o

to the beam

90o 45o: signal increase by ~√2

single diamond  double diamond: signal increase by

factor 2, noise by ~30%, SNR improved by ~50%

Bandwidth optimization

Bench test with 90Sr as MIP:

Full bandwidth (500MHz)  200MHz BW;

SNR improved by factor ~1.3

SIGNAL

NOISE

MIP

MIP

MIP

MIP

single diamond

single diamond

double diamond

double diamond

BCM MODULE TESTS

Beam test in CERN PS in summer 2006

CERN PS T11 and T9 pion beam (3.5 and 12GeV/c) – ~MIP signal

Setup:

Analogue readout (BCM  ORTEC ampl.  CAEN ADC)

Time-over Threshold readout (BCM  NINO  CAEN ADC)

Final high and low voltage power supplies, cables, connectors

Tracking: Bonn Si telescope (4 horizontal, 4 vertical planes;

resolution ~6m)

Analogue data:

sensor uniformity (fraction of tracks with

sig.>100 in 22.8mm2 box, box moved in

steps of 0.2mm over the sensor plane):

Time-over-Threshold data:

efficiency (tracks inside central diamond region 35mm2) vs. NINO threshold

occupancy (tracks missing diamond by >2mm) vs. NINO threshold

2.8 days @B=2T

time [s]

Bench tests

30MBq 90Sr source (~ MIP signal), signals recored with

LeCroy LC564A oscilloscope

Module stability tests (signal stable to few % during 24h)

Noise independent of HV

Good reproducibility of signals

Leakage currents in pCVD diamond

Leakage current increases by factor >100 on timescale of days; erratic behaviour

on time scale of minutes; also observed in BaBar at lower fields 1V/m

Module (inclined by 45o) placed in strong magnetic field Current stays <10nA

QUALIFICATION TEST of FINAL BCM MODULES

Procedure:

Module PCBs properly cleaned (Vigon EMF solution)

Accelerated aging for one module (14h @ 140oC  >10years @20oC),

infant mortality test for other modules (12h @ 80oC)

Thermal cycling (10 temperature cycles between -25oC and 40oC)

No change in SNR observed

Bench tests with 90Sr as source of MIP signal

Signals recoreded with LeCroy LC564A oscilloscope, 200MHz BWL applied

Signal: maximum value in 2ns interval

Noise: baseline fluctuations in 20ns interval before signal

No change in SNR observed after thermo-mechanical tests

Summary table for qualification tests of eight final modules:

noise

sampling

signal:

max in [34,36]ns

Offline FFT analysis of BCM MIP signal (applying

1st order filter):

Full BWL

200MHz BWL

MPV=3.7mV

MPV=2.5mV

=0.66mV

=0.33mV

NINO board

FPGA

Counting cavern

optical fiber

~100m

ATLAS PP2

BCM module

ATLAS Pixel

coaxial cable

~15m

~30MRad

~100kRad

occupancy vs.

NINO thr.

efficiency vs.

NINO thr.

region for efficiency estimation

region for occupancy estimation

NINO

hit-map

signal spectrum

inside

3.75.2 mm2 box

metalization projection

Slide 1

Afna Gunca

2007-02-07T12:36:50