Background sources

1
Machine Background Status issues in BaBar/PEP-II
W. Kozanecki, CEA-Saclay

• Background sources
• Characterization experiments
• Long-term projections vulnerabilities

2
Background sources in PEP-II

• Synchrotron radiation (this bkg negligible in
  PEP-II)
• 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
  (LEBHEB, 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)

3
Background characterization measurements
Data Jan 04 (bef.
therrmal outgassing crisis)
Step 1 Beam-current scans ?
single-beam terms
4
Step 2 L beam-beam terms
EMC cluster multiplicity
SVT occupancy (FL1 M01-f)
5
DCH

• 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
6
EMC
Looked at number of crystals with any/significant
energy 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
7
Luminosity background e e- ? e e- g

• elm shower debris
• neutrons!
• no contribution from coasting HEB or LEB
• may dominate DCH, DIRC rate

8
Neutron Background
Effort underway to measure neutron background in
BaBar
BF3 counter installed on fwd Q4 Sees large rate
(gt10 kHz) during colliding beams, not single beam
Rate only seen with polyethylene moderator ?1
MeV neutrons
Neutrons thought to be from radiative Bhabhas
hitting Q2 septum mask and inside support
tube - Shielding of BaBar is being investigated
9
EMC default digi map
luminosity background (N. Barlow)
f index
W
10
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.
11
SVT
Yearly 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 extrapltn).
• the HER single-beam background in Jan 04 is
  about 2x what it was in 2002 ? improve?

12
HEB current scan
Data 27 Jan 04
13
HEB scan evidence for Touschek beam-beam
background
14
Outgassing storms (April 04)

• New (?) major background source
  thermally-enhanced beam-gas
• in incoming LER straight (exacerbated by NEG
  activation now limits Ib)
• sensitive to LER current several time constants
  in a time-dependent mix
• suspect NEGs, 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 now limits Ib)
• 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
  (now limits Ib )
• 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 vars
• Many ?? (minor, inocuous changes ? large
  effects, good or bad)
• detailed analysis by GW

15
Thermal time constants
VGCC3027 ? (incoming LEB)
BE diamond ? (LEB sensitive)
LER current
VGCC2187 (HER sensitive)
? A potential limitation for run 5!
? BW diamond BBR dead time (HEB
sensitive)
16
HOM interference in IR
Data 13 Apr 04
VGCC2187 (HER sensitive)
VGCC3027 (incoming LEB)
Collision phase t(e-) - t(e)IP
ltZIPgt (BaBar)
BE diamond (LEB sensitive)
BW diamond (HEB sensitive)
17
Thermal outgasssing now limits the beam current
Babar dead time ()
VGCC2187 (nT, HEB side)
HER current
BE diamond (mR/s)
VGCC3027 (nT, incoming LEB)
LER current
18
Summary (I)

• 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)
• Improved understanding of background abort
  sources
• genuine radiation aborts down to lt 1 /day
• clear reproducible correlations between diamond
  dose rates, on-line SVT occupancies, dead time,
  and pressure measurements in incoming HER LER
  straights
• lumi background is really due to lumi!
  (except in the SVT maybe)
• Stored-beam backgrounds (dose rate, data quality,
  dead time)
• OK most of the time ( better w/ trickle) until
  recently
• thermal outgassing now limits beam currents
  (primarily in the HER)

19
Summary (II)

• 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 that due to
  thermal outgassing
• beam-beam tails their fluctuations (DCH, EMC,
  TRG, IFR ? wall!)
• 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 growing data
  volume trigger rates
• Mainly HER beam-gas (TRG, SVT)
  radiative-Bhabha debris (DCH)
• high occupancy and radiation ageing in the
  mid-plane of the SVT
• ? local loss of tracking coverage (?)
• closely monitor the HER single-beam background
  keep it similar to 2002 levels
• high n flux ( 1 MeV) correlates with L, some
  spikes? is it an issue?

20
Spare slides
21
Run-4 radiation-abort history
B. Petersen L.Piemontese

• 60 of stable-beam radiation aborts
  sympathetic
• 2/19 4/29 lt 0.9 (genuine) rad. aborts/day
  (out of 7 total avrg)

22
Stored-beam background history
IDCH, msrd/pred
DCH current normalized to Jan 04 background
data
04
20
20
SVT ocpcy _at_ f p (HEB-sensitive)
SVT ocpcy _at_ f 0 (LEB-sensitive)
10
23
B Petersen N. Barlow
M. Cristinziani/T.
Glanzman J. Malcles
Evolution of HER single-beam background, 2002-04
Jan 2004
EMC clusters
Apr 2004
DCH current
24
SVT projected integrated dose
Dose projections assume negligible injection
background
25
DCH current vs. Luminosity during a X scan (all
currents constant)
DCH current (microA)
Luminosity (1E33)
26
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

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Time evolution of the thermal outgassing
background

• The different time dependencies of the pressure
  readings allow to fit the measured background
  level (BE diamond) as a linear combination of 4
  LER Pumps, on a fill by fill basis
• The 4 pumps are located in the incoming LER
  straight and all exhibit HOM-induced thermal
  outgassing (e.g. change of pressure associated
  with change of bunch length)
• A very satisfactory description of the background
  was 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.

29
Evolution of the sensitivity coefficients (Apr 04
outgassing storms)

• The coefficients are normalized to their pre-NEG
  activation values , indicated by the red line (1
  point per long fill)
• The background problem was not related to an
  increase in local pressure reading (at the pump)
  but to a huge increase in background sensitivity
• The problem was solved (serendipitously) by
• continued processing
• opening collimator jaw(s)
• changing in bunch pattern
• These changes had different actions on the
  various background drivers

VP3044
VGCC3027
200
10
Days in March (April 1 day 32)
30
Mismatch (x 10-100) betw. time evolution of msrd
p and of bkgd
demonstrated by detailed analysis of local
pressure contributions to background signals
31
Large variety of processing times, mechanisms,
bkg sensitivities
32
Lost-particle backgrounds
Normalized to - uniform pressure profile of 1
nT - 1 A beam current
IP
33
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
Bremmsstrahlung in field-free region
Coulomb scattering in Arcs
Bremmsstrahlung