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Machine-Detector Interface (MDI) report W. Kozanecki, CEA-Saclay Operational issues radiation aborts radiation-dose history injection & stored-beam background history Background characterization characterization experiments long-term projections & vulnerabilities outgassing storms

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machine detector interface mdi report
Machine-Detector Interface (MDI) report

W. Kozanecki, CEA-Saclay

  • Operational issues
    • radiation aborts
    • radiation-dose history
    • injection & stored-beam background history
  • Background characterization
    • characterization experiments
    • long-term projections & vulnerabilities
    • outgassing storms
  • Simulations
  • Accelerator performance enhancements

Stable-beam “genuine” radiation aborts:

    • 0.9 / day
    • ~ 60% of these may actually be sympathetic
  • “Genuine” injection aborts: 0.4 / day be compared to an average of 7 aborts/day from all sources

run 4 radiation dose history

HER trickle starts

Run-4 radiation-dose history

HER trickle starts

Outgassing storms

trickle injection background
Trickle-injection background

Veto windows

  • The background generated by the trickle injection is concentrated in a narrow time window corresponding to revolutions of the injected bunch.
  • BABAR vetoes this window during data taking to avoid high dead time
  • BABAR rejects a larger region at the analysis phase to guarantee good data quality.
  • The total loss is around 1.5%

Monitor using injection-gated triggers (1 ms x 20 ms)

Injection- & trickle- background history

DCH trigs LER trickle

EMC trigs (always on) LER trickle

EMC trigs (always on) HER trickle

DCH trigs HER trickle


IDCH, msrd/pred

Normalized DCH current

Stored-beam background history



SVT ocp’cy @ f = p (HEB-sensitive)

SVT ocp’cy @ f = 0 (LEB-sensitive)


background sources in p e p ii
Background sources in PEP-II
  • Synchrotron radiation (this bkg negligible in PEP-II, but not in KEKB)
  • Beam-gas (bremsstrahlung + Coulomb)
    • HEB only: BHbg ~ IH * (pH0 + PHDyn * IH) Note: p0 = f(T) !
    • LEB only: BLbg ~ IL * (pL0 + PLDyn * IL) Note: p0 = f(T) !
    • beam-gas x- term: BLHbg ~ cLH * IL * IH (LEB+HEB, out of collision) (?)
  • Luminosity (radiative-Bhabha debris) – major concern as L 
    • BP ~ dP * L (strictly linear with L)
  • Beam-beam tails
    • from LER tails: BL, bb ~ IL * fL(xL,H+/-)
    • from HER tails: BH, bb ~ IH * fH(xL,H+/-)
  • Trickle background: BLi ,BHi(injected-beam quality/orbit + beam-beam)
  • Touschek: BLT(signature somewhat similar to bremsstrahlung; so far small)
background characterization measurements

Data: Jan 04 (bef. therrmal outgassing crisis)

Background characterization measurements

Step 1: Beam-current scans

 single-beam terms


Total occupancy

  • HER single beam
  • LER single beam
  • Beam-beam term
  • present in all subdetectors
  • fluctuations, short - & long-term
  •  parametrization optimistic ?

Step 2: L & beam-beam terms

EMC cluster multiplicity

SVT occupancy (FL1 M01-f)




Step 3: Background Parametrizations

  • DCH example: total current & occupancies

Step 4: Background Extrapolations

60 L

Tracking efficiency drops by roughly 1% per 3% occupancy

PEP-II parameter projections

LER contribution very small


Luminosity background e+ e- e+e-g

  • elm shower debris
  • neutrons!
    • no contribution from coasting HEB or LEB
    • maydominate DCH, DIRC rate
neutron background
Neutron Background

Effort underway to measure neutron background in BaBar

BF3 counter installed on fwd Q4

Sees large rate (>10 kHz) during colliding beams, not single beam

Rate only seen with polyethylene moderator~1MeV neutrons

Neutrons thought to be from radiative Bhabhas hitting Q2 septum mask

and inside support tube

- Shielding of BaBar is being investigated

dch trg

When combined with higher trigger rates, long read-out time leads to unacceptable deadtime.

A major DCH elx upgrade is now in progress.







Background strongly - dependent

By 2007 predict 80% occupancy right in MID-plane

In layer 1, 10% will be above 20% occupancy











Integrated dose will be more than 1 Mrad/year by 2007


Background now is ~75% HEB [LEB negligible (!)]

In 2007, it will be 50% HER, 50% L

  • It has recently been realized that
    • in the SVT (but not in other subdetectors), a large fraction of the “Luminosity”background is most likely due to a HER-LER beam-gas X-term (but: similar extrap’ltn).
    • the HER single-beam background in run 4 is about 2x what it was in 2002  improve?

Outgassing storms

  • New (?) major background source: thermally-enhanced beam-gas
    • in incoming LER straight (exacerbated by NEG activation; OK for now)
      • sensitive to LER current; several time constants in a time-dependent mix
      • suspect: NEGs (MS’s talk), ion pumps, collimator jaws, misc. vac. pipe secs
      •  SVT dose + occupancy (E-MID); minor impact on dead time
    • in incoming HER straight (triggered the NEG activation; OK for now)
      • sensitive to HER current, very long time constants
      •  BaBar dead time + SVT occupancy (W-MID)
    • in (or very close to) the shared IR vacuum system
      • sensitive to both beam currents; at least 2 time constants
      • suspect: NEG + complicated IR ‘cavity’ (Q2L  Q2R) + HOM interference
      •  BaBar dead time + SVT occupancy (W-MID + E-MID)
  • HOM dominant heating mechanism
    • mostly long to very long time constants (30’-3 h): suggests low power
    • sensitive to: bunch pattern, VRF, collimator settings, Z(IP), hidden var’s
  • Many “??”(minor, inocuous changes  large effects, good or bad)
time evolution of the thermal outgassing background

Fill: March 28, 12-3 pm


Data points

End of injection










3 h

Days in March (April 1=32)

Time evolution of the thermal outgassing background
  • The different time dependences of the pressure readings allowed to fit the background sensor (Backward East diamond) as a linear combination of 4 LER gauges, on a fill by fill basis
  • The sensitivity coefficients for each gauge were then extracted. They represent the N2-equivalent pressure integral with the same time dependence as the gauge reading.

The background problem was not related to a Pressure increase (as indicated by the gauge readings) but to a huge increase in background sensitivy


HOM interference in IR

Data: 12 Apr 04

VGCC2187 (HER sensitive)

VGCC3027 (incoming LEB)

BW diamond [+ dead time] (HEB sensitive)

BE diamond (LEB sensitive)


HOM interference in IR

VGCC2187 (HER sensitive)

VGCC3027 (incoming LEB)

Collision phase

<ZIP> (BaBar)

BE diamond (LEB sensitive)

BW diamond (HEB sensitive)


Background simulations

Background is coming from:

HER & LER beam-gas, luminosity, and beam-beam tails

Important to understand/quantify these backgrounds

What will be the effect of the IR upgrade on beam-gas (& b-b) bgds?

Can the luminosity background be explained by radiative Bhabhas ?

Which way does it enter the detector ? What is the n spectrum?

Can we shield or reduce this background source ?

Can we mitigate beam-beam backgrounds with improved collimation ?

Substantial effort in reviving/updating simulation infrastructure! Status:

Turtle optics updated to 2003 HER beam line (LER in progress)

Description of masks & apertures under detailed review (bugs found)

Beam line up to Q5 (mostly) implemented in Geant4 simulation of BaBar detector –validation of geometry & magnetic tracking in progress

post-2005 configuration is awaiting a finalized IRdesign

Significantly more detailed simulation

compared to the old Geant3 simulation used until 2000



Coulomb scattering background in the HER (Turtle level, ’04 config.)

Where do scattered e-come from ?

Where do scattered e-hit?


G4 simulation: status

  • Simulation integrated inside the BaBar standard Geant-4 environment.
  • magnets added in the region from backward-Q5 to forward-Q5
  • background sensors (PIN diodes, diamonds, quartz & CsI) added & validated
  • beamline elements from -Q5 to +Q5 included
  • validation of magnetic field description (incl. Q1-Q5) in progress

Decision made to import beamline elements from CAD files 

tools written to allow automatic translation from Solid Edge files to G4 C++

Correction of some G4 geometry ‘features’ (!)

Main steps

Retrieval of CAD files: HER OK, LER downstream Q4-Q5 not yet available

Simplification & translation of CAD files to C++ source: done on all available parts

Debugging of geometry problems : done up to Q2s


Ongoing Background Simulation Studies


  • Single beam background comes from Coulomb scattering and bremsstrahlung
  • Study relative contributions at various locations at the IP and where in the ring the original scattering happens

So far only studied at Turtle level: HER (2003), LER (1998)

Radiative Bhabha background

  • Where does the electrons/positrons end up after scattering?
  • What kind of backgrounds are produced and can it be shielded?
  • How much neutron radiation is generated? What is the neutron spectrum?
  • Will the 2005 IR upgrade make it worse?

This simulation effort is just starting up.

Beam-beam collimation

  • Beam-beam background can be reduced with collimators
  • LER collimator usage limited by background in IFR end cap
  • Can the collimators be moved downstream of BaBar?

First results look encouraging!


BaBar involvement in Accelerator Performance Improvements (I)

  • Background analysis & mitigation [BP, MC/TG, NB, JM/JV, RM, LP, WK/GW]
  • Background simulations [RB, MB, GC, WL, SM, PR/AS, WK + SLAC (TF/GB)]
  • Fast monitoring of machine backgrounds  available online in PEP-II CS [MW, C’OG, AP, GDF,...]
    • injection & trickle quality variables: SVT, DCH, EMC
    • subdetector occupancies: SVT, DCH, EMC, DIRC
    • BaBar dead time
    • more operator-friendly displays (& controls) of radiation inhibits/aborts
  • BaBar-based machine diagnostics
    • time distribution of injection triggers [LP, BP, ...]
    • Online centroids & sizes of luminous region using Babar dimuons [C’OG, BV, AP, IN, MB,...]

<xIP >

Luminous-region history

<yIP >

<zIP >

<sLx >

<sLz >


BaBar involvement in Accelerator Performance Improvements (II)

  • Beam dynamics
    • beam-beam simulations [IN (Caltech), YC (Slac ARD), WK]
    • beam-beam experiments, monitoring of beam-beam performance [WK]
    • e & b* measurements using dimuons [just starting]
  • Instrumentation
    • Gated camera: now operational in both in LER & HER [DD, Slac Exptl Grp C]
    • LER interferometer software [AO, Orsay]
    • Development of an X-ray beam-size monitor for the LER [Caltech + LBL + SLAC]
    • SVTRAD sensor & electronics upgrade [BP et. al. (Stanford); MB/DK et. al. (Irvine) (initiated & funded by BaBar)]
    • CsI background sensors , n detectors & shielding [JV, Slac Exptl Grp B]
summary i
Summary (I)
  • Stable-beam (genuine) radiation aborts are down to < 1/day
  • Trickle injection
    • is a major success in terms of improving
      • machine stability + abort frequency  integrated L
      • overall injection quality
      • accumulated SVT dose
    • The associated detector backgrounds appear largely negligible (most – but not all – of the time)
  • Present stored-beam bgds (dose rate, data quality, dead time)
    • OK most of the time (& better w/ trickle)- for now (thermal outgassing!)
  • Background characterization experiments
    • were highly valuable in identifying the origin, magnitude & impact of single- & two-beam backgrounds.
    • On the long term, the dominant backgrounds are expected to be, in order of decreasing importance:
      • radiative-Bhabha debris (all subdetectors), incl. a significant neutron flux
      • HER beam-gas (SVT, TRG), especially if thermal outgassing resurfaces
      • beam-beam tails & their fluctuations (DCH, EMC, TRG, IFR  wall!)
summary ii
Summary (II)
  • In the medium term (2005-07), the main vulnerabilities are
    • beam-gas backgrounds from HOM-related thermal outgassing as I+,-
    • high dead time associated with DCH data volume & trigger rates (addressed by DCH elx upgrade)
    • high occupancy and radiation ageing in the mid-plane of the SVT,
      • possibly leading to a local loss of tracking coverage.
      •  reduce the HER single-beam background back to 2002 levels (/1.5-2) ?
    • a high flux of ~ 1 MeV neutrons in the DCH (wire aging from large pulses, possibly also contributions to occupancy)
  • Background simulations
    • large investment in reviving/updating tools + rebuilding the group
    • ‘almost’ ready to evaluate backgrounds in IR upgrade
    • manpower limited
  • BaBar-based accelerator performance enhancement
    • common BaBar-PEPII diagnostics greatly improved, starting to pay off
    • very significant involvement of BaBarians in beam instrumentation & simulation
run 4 radiation abort history
Run-4 radiation-abort history

(automated script)

  • Stable-beam aborts: 280
    • stable beams: 56% of the time
    • includes trickle
    • radiation-driven manual aborts (trapped events) not included
  • Injection aborts & inhibits: 301
    • inject: 24% of the time
    • note: dominated by pre-trickle
radiation signatures stable beam aborts sympathetic
Radiation signatures: stable-beam aborts, sympathetic

Compiled by B. Petersen

 1000 X stored beam

 150 X stored beam

 5000 X stored beam

15 / 22

6 / 22

1 / 22

radiation signatures stable beam aborts i radiation only
Radiation signatures: stable-beam aborts (I), radiation only

Compiled by B. Petersen

 50 X stored beam

 80 X stored beam



radiation signatures stable beam aborts ii radiation only
Radiation signatures: stable-beam aborts (II), radiation only ?

Compiled by B. Petersen

 10000 X stored beam

 10000 X stored beam

 1000 X stored beam




Could 17/28 radiation-only aborts be sympathetic?

typical radiation signatures injection aborts
Typical radiation signatures: injection aborts

Most of the recent injection aborts look like this

 5000 X stored beam

 100 X stored beam

 50 X stored beam

injection aborts a typical example eoic summary for 3 23 04
Injection aborts: a typical example (EOIC summary for 3/23/04)
  • tune management
  • non-reproducibility of thermally-induced IP motion
  • difficult for 1 operator to “keep all balls in the air”
stable beam aborts remediation avenues
Stable-beam aborts: remediation avenues
  • Need better understanding/characterization
    • 40-50% of the radiation aborts were found to be sympathetic... by manually scanning logs
    • 60% of the ‘radiation-only’ aborts may be sympathetic as well
    • < # ‘radiation-only’ aborts > ~ 1/day  2-3 % inefficiency (counting all, and adding manual aborts for trapped events)
    •  can we learn to use the radiation signature to diagnose the source?
    •  can we automate the categorization of aborts?
      • easier (automated?) identification/logging of T/L instabilities
      • data entry
  • Improved diagnostics: SVTRAD 1.5 elx upgrade coming soon

Implies replacement of mid-plane modules during 2005 shutdown

SVT: projected integrated dose

Dose projections assume negligible injection background

her single beam background possible improvements
HER single-beam background: possible improvements ?
  • now (Jan 2004) ~ 1.6 x Feb 2002 (@ 1 A)
    • mostly linear with IH dominated by base pressure (thermal outg’sg)
    • dynamic pressure term (~ SR  IH2) unchanged since Feb 2002 - no plausible improvement (short of $$)
  • radial ion pumps repair: regain ~ 20% ?
    • requires removing support tube ( > July 2005)
    • feasibility of repair tbc
  • high-vacuum (TSP) section (PR02 7039 to 7042)
    • TSP’s flashed on 5 + 27 Jan ’04 – no detectable improvement in pressure (p0 ~ 5E-10)
  • There may be some to gain from more frequent NEG activation
    • at best, will return to Feb 2002 levels, but not for long
    • # NEG cycles is finite  NEG activation expensive
drift chamber current as function of luminosity during a x scan all currents constant
Drift Chamber current as function of Luminosity during a X scan (all currents constant)

DCH current (microA)


dch trg background extrapolations
DCH/TRG background extrapolations
  • HER single-beam & lumi (bkg + physics) terms dominate
  • Trickle: only average shown. Must be able to accomodate large fluctuations.
  • Beam-beam: only best case shown. Data taken since then show beam-beam can easily be 2 x larger – not counting short-term fluctuations.
  • LER single beam: small (mostly beam-gas), no fluctuations expected

Looked at number of crystals with any/significant energy and clusters

Small quadratic term from single beam data

# of crystals used in cluster finding

Currently physics events have ~110 digis and 8 clusters

Long term impact on physics analysis not clear yet


12 hours

Thermal time constants

VGCC3027  (incoming LEB)

BE diamond  (LEB sensitive)

LER current

VGCC2187 (HER sensitive)

 BW diamond [+ BBR dead time] (HEB sensitive)

detailed study of the time evolution of the thermal outgassing related background
Detailed study of the time evolution of the thermal outgassing related background

Fill March 28, 12pm-3 pm

  • The different time dependences of the pressure readings allowed to fit the background sensor (Bacward East diamond) as a linear combination of 4 Pumps*LER, on a fill by fill basis
  • The 4 pumps are located on the incoming LER straight and all exhibit HOM related thermal outgassing (eg, change of pressure associated with change of bunch length)
  • A very satisfactory description of the background was thus obtained in all cases
  • The sensitivity coefficients for each pump were then extracted. They represent the N2-equivalent pressure integral with the same time dependence as the pump reading.


Data points

End of injection








3 hours

evolution of the sensitivity coefficients
Evolution of the sensitivity coefficients
  • The coefficients are normalised to their pre-NEG activation values , indicated by the red line (1 point per long fill)
  • The background problem was not related to a Pressure increase (as indicated by the pump readings) but to a huge increase in background sensitivy
  • The problem was solved by:
    • -continued processing
    • Collimator jaw opening
    • Change in bunch pattern

These changes has different actions on the various background drivers





Days in March (April 1=32)

Days in March (April 1=32)


BE diamond (LEB sensitive)

VGCC3027 (incoming LEB)

NEG actvtd

NEG actvtd

NEG actvtd

NEG actvtd

BW diamond (HEB sensitive)

VGCC2187 (HER sensitive)

Mismatch (x 10-100) betw. time evolution of msrd p and of bkgd

demonstrated by detailed analysis of local pressure contributions to background signals


NEG actvtd

NEG actvtd

NEG actvtd

NEG actvtd

Large variety of processing times, mechanisms, & bkg sensitivities


Backgrounds: long-term projections II

SR simulations

(an intrinsic part of the new-IR design)

  • Beam-gas simulations
  • ring: Turtle
  • IR  Geant4


Lattice mods? (dynamic aperture)

  • 2 themes...
    • validate IR upgrade design
      • make sure that what we install in ’05 does not suffer from built-in flaws...
      • least for those processes we can calculate (SR, beam-gas)
    • understand / improve backgrounds in present machine
  • ...that are intimately intertwined
    • validation requires credibility
      • update “old” simulations to incorporate what we learnt
      • simulations of present machine/detector configuration better get the ‘right’ answer (when confronted with measurements)...
      • ...if we want to believe predictions for the upgraded IR
    • improve those backgrounds we canNOT calculate
      • both for today’s and for tomorrow’s sake!
architecture of background simulations 1
Architecture of background simulations (1)
  • Synchrotron Radiation
    • MAGBENDS / QSRAD: stand-alone programs
    • SR background calculations: an intrinsic component of IR re-design
    • shouldn’t these be interfaced to GEANT?
  • Beam-gas
    • step 1: LP-TURTLE transports particles around 1 ring turn
      • full model of ring optics (treated as transport line)
      • start with ‘nominal’ beam at IP
      • beam-gas scattering randomly around ring (bremsstrahlung or Coulomb scattering)  transport ‘secondaries’ (e’, g)
      • simplified model of IR apertures (simple geometry, no showering!)
      • those particles lost ‘near’ the IP are
        • saved @ scoring plane
        • input to step 2
    • step 2: full GEANT simulation of detector + near-IR (+- 8.5 m)
      • see Mario Bondioli’s talk
architecture of background simulations 2
Architecture of background simulations (2)
  • Beam-beam
    • full simulation of beam-beam tails impractical
    • focus on collimation studies
      • optimize collimator placement/relocation (SM)
      • understand main characteristics of collimator secondaries (HB)
      • provide guidance for machine experiments
    • use Turtle machinery
  • Strategy considerations
    • improve/update description of magnetic fields & apertures (TF, GC)
    • many fundamental features easier to understand at Turtle level
      •  first round of IR-upgrade design validation will be done this way (RB)
    • GEANT-level simulation essential (MB, GB, GC)
      • to benchmark computations against data
      • to make sure there are no “alligators” hiding in new design
    • absolute background predictions always suspect
      • even when benchmarked against experiments. However...
      • ...ratios (new design /present machine) much more reliable.

G4 simulation: general strategy

1) Input from Turtle

2) Track evolution with Geant4 in a complete material

/ field description.

3) Simulation of the particle interaction with sub-detectors

and specific background sensors.

4) Integration in the BaBar framework

5) Validation



A) Trajectory validation

Verification of nominal orbit

Validation of orbit envelope using beam size at the

IP as estimator

Check of position of the extrapolated tracks using cross sections (no-hit)

B) Geometry/apertures validation

Check the position of hits in Geant vs Turtle

C) Volumes validation

Select a given volume: (ex: a SVT front-end chip)

Identify the ingoing-outgoing positions of crossing tracks to trace boundaries and verify the expected positions

D) Comparison with real data

lost particle backgrounds

Coulomb scattering in Arcs (y-plane)



in last 26 m


Vacuum pipe / mask apertures

Lost-particle backgrounds

Normalized to:

- uniform pressure profile of 1 nT

- 1 A beam current


the background zones reflect the combined effect of

Zone 3

X (mm)

Zone 2

X (mm)

Zone 1

X (mm)

Zone 4


The “Background Zones” reflect the combined effect of....
  • beam-line geometry (e.g. bends)
  • optics at the source and at the detector
  • aperture restrictions, both distant(good!) & close-by (bad!)


Bremmsstrahlung in field-free region

Coulomb scattering in Arcs


benchmarking of simulations comparing predicted and measured background levels
Benchmarking of simulations: comparing “predicted” and measured background levels
  • Radiation patterns
    • for a given sensor type: independent of absolute calibration
    • among different sensors: compare fractional derivatives
  • Absolute background levels
    • sensor calibration!
    • absolute pressure profile !
  • Global consistency/sanity checks
    • operational experience in MCC
pressure bump experiment neg heating in babar straight

Vacuum gauge reading (nT)

Abort diode signal (mR/s)

Pressure-bump experiment: NEG heating in BaBar straight
  • Create localized P-bumps
    • NEG heating
    • DIPS on/off
  • Measure response of  background monitors
  • Compare relative measured & simulated monitor response to validate Monte Carlo
  • Different
  • regions
  • ==>
  • diff. patterns
  • diff. abs. levels
understanding the absolute level of her backgrounds sep 99
Understandingthe absolute level of HER backgrounds (Sep 99)
  • Compare measured & predicted dose rates in HER:
  • Monte Carlo lost-particle simulation (Turtle + BBSIM) validated by p-bump experiments
  • Computed pressure profile in detector straight section (N2-equivalent, not vac.-gauge units!)
  • Average ring pressure (from lifetime) for arcs & distant straights
belle backgrounds
Belle Backgrounds

During the Super-B workshop (Hawaii, Jan 04) background experiences were compared between Belle and BaBar

Belle Backgrounds:

Stable beam backgrounds generally lower than at BaBar

SVT and DCH backgrounds well described by single beam background or even lower (Touschek effect)

TOF (+50%) and Muon (x5) see a “luminosity” background

Injection backgrounds did not seem to be a concern

Comparing Backgrounds:

Quantitative background comparison is not simple due to difference between the two detectors

Currently a common set of “background measures” are being defined and should be available in the Summer

belle backgrounds cont d

SVT radiation doses are almost directly comparable

(Belle's new SVD2.0 is a little closer to the beam pipe)

BaBar (Stable) 400 kRad/yr 500 kRad/yr

BaBar (Injection) 150 kRad/yr 400 kRad/yr

BaBar (Total) 550 kRad/yr 900 kRad/yr

Belle Backgrounds, Cont'd

Corresponds to 2 times Run 4 dose