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 (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)

3. Background characterization measurements
Data: Jan (bef. therrmal outgassing crisis)
Background characterization measurements
Step 1: Beam-current scans
 single-beam terms

4. Step 2: L & beam-beam terms EMC cluster multiplicity
SVT occupancy (FL1 M01-f)
Beam-beam term
present in all subdetectors
fluctuations, short - & long-term
 parametrization optimistic ?
Total occupancy
HER single beam
LER single beam

5. LER contribution very small
Step 3: Background Parametrizations
DCH example: total current & occupancies
Step 4: Background Extrapolations
IDCH =
60 L
Tracking efficiency drops by roughly 1% per 3% occupancy
DCH
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. 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 ~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)
Fwd
Bkwd
q index E
EMC default digi map: luminosity background (N. Barlow)
f index W

10. A major DCH elx upgrade is now in progress.
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. Yearly dose will be more than 1 Mrad/year by 2007
SVT
Yearly dose will be more than 1 Mrad/year by 2007
Backward:
Forward:
Top
East
West
Bottom
NOW
2004
2005
2006
2007
Background now is ~75% HEB [LEB negligible (!)]
In 2007, it will be 50% HER, 50% L
Background strongly  - dependent
By 2007 predict 80% occupancy right in MID-plane
In layer 1, % will be above 20% occupancy
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 Jan 04 is about 2x what it was in 2002  improve?

12. Data: 27 Jan 04
HEB current scan

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 var’s
Many “??” (minor, inocuous changes  large effects, good or bad)
detailed analysis by GW

15. Thermal time constants  A potential limitation for run 5!
VGCC3027  (incoming LEB)
BE diamond  (LEB sensitive)
LER current
12 hours
VGCC (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)
VGCC (incoming LEB)
Collision phase = [t(e-) - t(e+)]IP
<ZIP> (BaBar)
BE diamond (LEB sensitive)
BW diamond (HEB sensitive)

17. Thermal outgasssing now limits the beam current
Babar dead time (%)
VGCC (nT, HEB side)
HER current
BE diamond (mR/s)
VGCC (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 < 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 ( ), 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
Run-4 radiation-abort history
~ 60% of stable-beam radiation aborts “sympathetic”
2/19 – 4/29: < 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 f = p (HEB-sensitive)
SVT f = 0 (LEB-sensitive)
10%

23. Evolution of HER single-beam background, 2002-04
B Petersen N. Barlow M. Cristinziani/T. Glanzman J. Malcles
Evolution of HER single-beam background,
Jan 2004
Apr 2004
Feb 2002
SVT occupancy
Jan 2004
EMC clusters
Apr 2004
Jan 2004
Feb 2002
Apr 2004
DCH current (mA)
HER current (A)
DCH current

24. SVT: projected integrated dose
Dose projections assume negligible injection background
Implies replacement of mid-plane modules during 2005 shutdown

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

28. 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.
Fill March 28, 12pm-3 pm
3 h 1 2
Data points
VP3044
VGCC3027
Fit
VP3147
End of injection
mRad/s

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
BE diamond (LEB sensitive)
NEG actvtd
VGCC (incoming LEB)
BW diamond (HEB sensitive)
VGCC (HER sensitive)

31. Large variety of processing times, mechanisms, & bkg sensitivities
NEG actvtd
NEG actvtd
NEG actvtd
NEG actvtd

32. Lost-particle backgroundsIP
Coulomb scattering in Arcs (y-plane)
e- Brems-strahlung
in last 26 m
(x-plane)
Vacuum pipe / mask apertures
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!)
X (mm)
Zone 1
X (mm)
Zone 2
Bremmsstrahlung
Bremmsstrahlung in field-free region
Zone 3
X (mm) IP
Zone 4
Coulomb scattering in Arcs
Bremmsstrahlung