Short summary of background/MDI report (1)• Steady background rates in 2021ab was
acceptable for Belle II (p.5,6)• TOP rate limit relaxed to 3MHz and cantolerate ILER=1.2A now• or 1.5A if limit is further relaxed to 4.5MHz• Extrapolated background rates seems
acceptable for most subdetectors (p.7)• large uncertainty due to collimation etc.
though…

• Simulation for storage background
getting more accurate (p.8)• data/MC for important backgroundsources are now O(~1)
1Short summary of background/MDI report (2)• Good progress on neutrons from thetunnel: measurements and simulation• additional shield design started (p.10)• tighter upstream collimator can reduce loss
on the collimator near IP and neutron rates
on Belle II EKLM (p.12)• improved tunnel GEANT4 geometry (p.13-14)

2

Short summary of background/MDI report (3)IP• Catastrophic beam loss caused severe damage
on collimator and BelIe II

Loss monitors
in the ring(diamonds, CLAWS)

RF arc sensor
is sensitive tobeam loss• post-mortem abort analysis to find the initial beamloss location, using loss monitor data (need moresensors?) → p.23-26• faster abort request by CLAWS at IP (~4us fasterthan IP diamonds) →p.22• PIN diodes
at each collimators

HER
RF D4
LER
RF D5

FUJI
KEKB control

D06H3IN
abort kicker was issued
0~10us faster thanks to CLAWS(4.4us faster on average)

Bunch current monitors (BCM)“Libera” turn-by-turn BPMs

Abort kicker/beam dump

D06H3OUT

3

Beam background and MDI
H. Nakayama (KEK)4

Beam background status in 2021ab• Summary: Recent background rates in
Belle II acceptable – all well below
administrative limits and therefore
not limiting beam currents• Main background concerns in shortterm are catastrophic losses andstable operation
Note: “3 MHz/PMT” is the limit for the sum of
single beam and injection backgrounds. It does
not include the luminosity background.
5

Storage backgrounds: near term outlook• TOP group already analyzed BG composition
using June 16th 2021 campaign• On average, the total single beam BG increased
by a factor of 1.23 ± 0.06 over 2020
• LER Touschek increased by factor 2.35 ± 0.11
• Relaxed collimator settings are used in this run
to allow higher injection efficiency and longer
lifetime (TOP limit was relaxed)• The data/MC ratios of luminosity background are
0.76 ± 0.08 on average (was 0.98 ± 0.04 in 2020).
This is an unexpected discrepancy withsimulation, but consistent with other detectors.
Analysis artifact? Good news if correct.
Kazuki Kojima
Anja Novosel

Needs to understand - affects design luminosity
outlook6

Storage backgrounds: long term outlook
Slavomira
Stefkova(*) will be relaxed by a factor of about 2 (TBC) with hit time selection.

~3% (*)The extrapolation assume: 1) collimator settings
re-optimized for an optimal tradeoff between
beam lifetime and backgrounds at design optics,
2) Vacuum pressure of 1nTorr in both rings."

Preliminary summary by Sven May 2021. To be updated based on June 2021 studies.
Still assuming L= 8 x 1035 cm-2s-1 and original design optics, collimation

7

Storage background Simulation Accuracy Natochii
2020
BG study• Diamonds, PXD, SVD, CDC, TOP: Four of five storage background components have data/MC of order 1 !
• KLM and ARICH included into common analysis for first time. Still need refinement at detector level.
• Plan: Include 2021 measurements. Add ECL. Improved error analysis. Then publish the results.
• Puzzle: luminosity background in 2021 data seems 20-25% lower than predicted in some subdetectors• We use these data/MC factors to correct extrapolations of backgrounds to design luminosity.
• Systematic errors are small for Lumi and LER (largest backgrounds at design parameter), reducing theextrapolation uncertainty.
• Effort for injection BG simulation has already started
8

Neutron background9

Neutron shielding

Natochii• Neutrons generated both inside andoutside Belle II• New background MC samples w/ truth
can be used by detector groups to study
their neutron sensitivity• Neutron background reduction can
reduce SEU rates of CDC/TOP FPGAs• Shielding design for 2022 installation
urgent → new inter-detector groupformed• Neutron dosimeters installed to assess
the situation• Strategy is to install as muchpolyethylene as possible• Background group will simulate
preliminary /conceptual designs

10Cavern Neutrons• Greatly improved understanding• Spectral and directional
measurements with TPCs (fastneutron detectors)• EKLM hit distributions• New far beamline “cavern
backgrounds” included in simulation• Corrected simulation of beam particle
interaction withcollimator jaws

Schueler• Most neutrons from two
mechanisms• RBB photon hotspots at
downstream of IP• Touschek losses at collimators
just upstream of IP
11

Cavern Neutrons• Armed with this improved
knowledge, performedexperiment to verify• Achieved 40% EKLM Touschek
neutron reduction via
modified collimation• Also directly observed the
reduced fast-neutron flux inthe tunnel with the TPCs
• This confirms the Touschekcollimator neutron hypothesis• Data/MC comparison indicatessome materials are missing in
our simulation → need to fix
the geometry in GEANT4 (seefollowing pages)

TPCs (Schueler)

KLM, April 27 (Meier)
Close D2H2
collimator to
reduce loss on
D2H4 (closer to IP)
40%
reduction

Natochii
No polyethylene
wall here

Shaded by
QCS structure

12

GEANT4 geometry (OLD)

Dmitri, Andrii

13

GEANT4 geometry (NEW)

Dmitri, Andrii

New cavern geometry looks much more realistic.

14

Are Belle II detectors seeing delayed neutrons
from injection background losses at collimators?
• Utilizing new 2Hz Poisson trigger which
extends into injection veto period.
• CDC occupancy evolves during one
revolution (10 μs) and on ms time scales• Only in the first bin (<0.5ms after injection)
there’s a significant delayed component• MeV neutrons originating from thehorizontal collimator D01H5 ?
• Check if MC sample can reproduce such
events• Another hypothesis: instability due to somecoupling between the injected bunch andlater bunches?

Maiko Takahashi

Artifact due to event-of-doom buster
Delayed bkg during first 50 turnInjected bunch passes IR

15

Catastrophic beam loss events

16

Abort summary pageH. Nakayama

http://kekb-co-web.kek.jp/doc/Image/BELLE/abort_summary/web/abortlist.html• Aggregates relevant information in one page
• abort time, earliest abort source, link to abort
timestamp page• beam currents, number of bunches, Belle HV
status• bellows diamond doses, IP diamond plot andintegrated dose
• BOR/BCM plots, abort BPM plots• time difference since the last injection, BT orbit
plots• LINAC trouble information (Pulmag misfire, Kly
down, RF gun pressure burst)• earthquakes, LER/HER pressure bursts

Very useful for beam abort diagnosis
17

Beam aborts in 2021ab
1) HER beam aborts not in coincidence with injection
are most frequent. Especially, HER was quite unstable inlate April.

2) LER beam aborts not in coincidence with injection
are also frequent. Some are catastrophic and caused
QCS quenches and hardware damage (see next page)1)★

3) HER aborts due to injection increased by installing
CLAWS abort on May 28th, but became less after June
18th (CLAWS threshold adjusted).

★

D2V1
replaced(June 9th)

3)

★
★★
★

4)

1)

4) LER aborts due to injection increased in mid June,
probably due to LINAC energy feedback trouble.

2)
CLAWS abort
since May 28th

★ : QCS quench due to HER beam loss
★ : QCS quench due to LER beam loss

Understanding the cause of LER/HER aborts (not incoincidence with injections) is very important forsafe & stable operation

Catastrophic beam loss abort events in 2021b(which caused QCS quenches)

BOR/BCM

1 turn=10us

Most of them are caused byhuge beam loss in LER,
several turns before the abort.

Dangerous for Belle II innersensors. In some cases,
diamonds on IP beam pipes
saw >1500mrad (saturated)
and PXD was damaged

This event caused a severe damage on LER D2V1 collimator 19

Severe damage on LER D02V1 collimatorthafter the huge beam loss on June 6
Beam Beam

Beam- After the huge beam loss event on June 6th, LER BG increased significantly- D02V1 collimator jaws were severely damaged (deep scar on the bottom jaw)
- We lost 3~4 days for the collimator replacement work and the baking runs
Understanding the cause of huge beam loss events is essential for the stable
operation at high beam currents. Where in the ring the beam abnormality initiallyoccurs? Adding more sensors to the key collimators will help to understand theinitial beam loss position.

20

Severe damage on D6H3 collimator,
probably due to kicker misfire event on May 14th
• Kicker misfire happened on May 14th
• QCS quench occured• Pressure burst at D6H3 was also observed• However, we realized the severe damage only after the
run end (in July), not just after the incident

D06H3IN

D06H3OUT• Loss monitor PINs got easy saturated and can’t tell if thebeam loss is huge or moderate• Dust particles falling off from D6H3 could be a cause ofcatastrophic events in LER?
• Ikeda-san’s post-mortem abort analysis using loss monitor
data is ongoing
Terui-san’s report on July 15th

• If yes, why huge loss disappeared after D2V1 replacement?

21

Even faster beam abort: CLAWS• 4+4 CLAWS sensors (on QCSL and QCSR) are used to issue beam aborts (since May 26th, 2021)• Thanks to CLAWS, abort kicker can be fired faster by ~4.4us on average• For some LER aborts, CLAWS are outperformed by LER RF D5-F, which is located at better ring position• Adding new beam loss sensors (upstream of IP and downstream of initial beam loss) might be able to makeabort even faster

abort kicker was issued
0~10us faster thanks to CLAWS(4.4us faster on average)
3x3 cm2 plastic scintillator+ Hamamatsu SiPM

CLAWS sensors for beam abort

22

Where’s the location of initial beam loss?
IP

Loss monitors
in the ring

Diamonds(diamonds, CLAWS)

CLAWSInternal delay~6us

Timing between sensors aresynchronized by 1) abort kicker
trigger signal, 2) abort request
signal, or 3) “all beam kicked out”
timingsaturated,
Internal
delay~0.2usRF arc sensor
is sensitive tobeam loss• PIN diodes
at each collimators

2)

thresholdcrossing

RF D5-E

2)
thresholdcrossing

1 turn=10us

10us

PIN diodes on collimators

HER
RF D4
LER
RF D5

2) & 3)

Long integration time

1us

1)

10us

FUJI

Bunch Current Monitor (BCM)“Libera” BPM

LER BCMall beam
kicked out

KEKB control
Bunch current monitors (BCM)“Libera” turn-by-turn BPMs

3)

Loss from theprevious turn

10us

10us

all beam
kicked out?

3)

Abort kicker/beam dump

- By comparing beam loss timing among several sensors along the ring, we can findthe possible region where initial beam loss occurred.
- If we can add new beam loss sensors at some important collimators, it might help us
pin-down the initial beam loss position of dangerous aborts.

23

IP sensors(diamond, CLAWS)

Beam abort timing chart (LER)
In case the earliest abort request issued by IP sensors (CLAWS/diamond)a) Disturbed bunch arrives at the sensor position

HER
RF D4
LER
RF D5

• PIN diodes
at each collimatorsb) Abort request signal arrives at the Central Control Room (CCR)e+c) All bunces are kicked out and no beam loss seen by the sensor

FUJId) Abort kicker signal arrives at the sensor position
KEKB control(CCR)
Bunch current monitors (BCM)“Libera” turn-by-turn BPMs

BOR/BCMdt=0 in Abort
Timestamp Log

IP sensors

4.9us

a)

(CLAWS,diamond)d) 3.2us later than c)

4.7us

~10us
10.6usAbort kicker

BOR/BCMd) arrivesc)~7us (dia)~1us (claws)

LER RF-D5

Injection point

Abort kicker/beam dump

2.6us
0.4~5.4us

CCR
Where is the initial
beam loss location?

b)

Waiting time for the next abort gap(recorded by WhiteRabbit since June 2021)

If the earliest abort was issued
by CLAWS,a)→c) takes 1+10+(0.4~5.4)+2.6+4.7 = 18.7~23.7 us
If issued by diamonds,a)→c) takes 7+10+(0.4~5.4)+2.6+4.7 = 24.7~29.7 us

IP(diamond, CLAWS)

Beam abort timing chart (LER)
In case the earliest abort request issued by RF D5 arc sensora) Disturbed bunch arrives at the sensor position

HER
RF D4
LER
RF D5

• PIN diodes
at each collimatorsb) Abort request signal arrives at the Central Control Room (CCR)c) All bunces are kicked out and no beam loss seen by the sensor

e+
FUJI

d) Abort kicker signal arrives at the sensor position
KEKB control (CCR)
Bunch current monitors (BCM)“Libera” turn-by-turn BPMs

BOR/BCM
7.7us

IP sensors
LER RF-D5

dt=0 in Abort
Timestamp Loga)

BOR/BCM

c)

~1us
1.9us

d)

c) and d) arrive almost
at the same time

4.9us

~6us

Abort kicker
Injection point

Abort kicker/beam dumpa)→c) takes
1+6+(0.4~5.4)+2.6+1.9= 11.9~16.9 us.

2.6us

CCR
Where is the initial
beam loss location?

0.4~5.4usb)

Waiting time for the next abort gap(recorded by WhiteRabbit since June 2021)

LER abort kicker:
RF D5-F: s~2180m
RF D4-H: s~2320m

20us

2021/06/02 20:13:51(QCS quench):beam dump

- beam current+9.7us

RF D5-F~10us• Earliest abort request arrived at CCR was issued by CLAWS• Abort gap waiting time at CCR was 4.4us for this abort• All bunches kicked out after 1us+10us+4.4us+2.6us+4.7us = 22.7us (a→c)• Diamond abort request arrived at D2 4.9us later than CLAWS. It means CLAWS
and diamond issued their aborts by seeing almost same bunch. In diamondplot, all bunches should be kicked out at t=24us (abort was issued at t=0 in theplot) (a→c). The observed signal waveform has a peak around t=23us and thetail follows for ~6us.
• BCM start to see the beam loss in the same turn with initial loss in diamondand CLAWS. The observed beam loss in RF D5-F starts 1 turn later than
CLAWS/diamond. (Why?)• Initial beam loss seems to occur at: downstream of BCM and upstream of IP(i.e. right half of the ring ).

CLAWS

~6us

Diamond~24us

BOR(hor)BOR(ver)
BCM• D6,D2 loss monitor waveforms are being analyzed by Ikeda-san.
~20usSummary• Steady state beam backgrounds in 2021ab mostly acceptable for Belle II• TOP PMT replacement still expected in 2022, of course• First full single-beam + luminosity BG study since 1 year ago• At higher luminosity, BG extrapolation update• Systematic overestimate of luminosity bkg in all detectors (~20-25 % effect) not yet understood• Long term extrapolations: most detectors safe
• Large uncertainties related to collimation etc..
• CDC and TOP uncertain  more work needed, ECL not really studied but “probably fine”• Good progress on neutron studies, shielding design is starting• Promising new efforts on injection backgrounds (see backup)• Catastrophic losses / abort events dangerous for inner detectors• Abort summary page is prepared• CLAWS can issue faster beam abort request than diamonds• Post-mortem analysis will indicate the initial beam loss positions
27

backup28

LER collimator aperture setting history
beta x k

Relaxed on
July 1st
beta*k~30

width

How muchnarrowerthan QC1
aperture?

Twocollimatorswith 80%
aperture29

1.44

Sum(beta*k) = 47

3.39
6.78

30

Background Publications Status
Submitted• A. Natochii et al., Improved simulation of beam backgrounds and collimation at SuperKEKB, PRAB
Review by authors / authorship process ongoing• Z. Liptak et al., Measurements of Beam Backgrounds in SuperKEKB Phase 2, NIMA
• M. Hedges et al., First deployment of novel vector tracking nuclear recoil detectors for directional
neutron measurements at SuperKEKB, NIMA
• J. Schueler et al., Directional cavern neutron background measurements at SuperKEKB, NIMA
Just started / planned• Phase 3 paper: simulation, data/MC up to now, BG19c forecast, NIMA.
• Snowmass paper: background forecast + mitigation plan to reach 2026. Now due March 15, 2022

31

Cavern Background Simulation Improvements
NatochiiMore realistic
handling of
beam loss at
the LER D2H4
collimator• Initial motivation: significantly affects neutron production in tunnel close to Belle II, and thereby EKLM
• But also found to have subtle effects on other Belle II detectors• First feedback: TOP ~ no effect, PXD: ~20% reduction in occupancy, CDC: ~20% hit rate increase32

Injection Backgrounds• Two new efforts

Natochii• Natochi: Simulation of injection
background now can produce IR
backgrounds• Schwenker: Machine learning canidentify injection backgroundswith rather good accuracy• What we still need
• Validation of simulated injection
backgrounds vs inj. parameters• Identify most predictive
parameters• Method for extrapolating injection
backgrounds

Schwenker

33

20us

2021/06/06 16:06:10(QCS quench):beam dump

- beam current+19us

RF D5-F~10us• CCRに届いたのはCLAWSが最速。
• このLERアボートの CCRでのAbortGap 待ち時間は 5.000 us• ビームがなくなるのは、1us+10us+5.0us+2.6us+4.7us = 23.3us後(a→c)• CLAWSとdiamondのD2到達時間差は21.1usなので、 diamondはCLAWSより
15us遅れのバンチを見てアボートを出している。diamondでビームがな
くなるのはdiamondがアボートを出したt=0から約8us後になるはず。観
測された信号波形では、8us後はflat top(サチってる最中)で、さらに約15usくらいtailを引いている。
• RF D5-Fでは、約1ターン遅れてロスが見え始めている。
• まとめ：CLAWSが最初にロスを見ている。1ターン遅れでdiamond, RF D-5が反応している。
• D6,D2のLoss Monitorはどうか？(→池田さん)

CLAWS
~23us
~6us

Diamond

BOR(hor)BOR(ver)
BCM

~18us

D2V1 BGO 追加
2021/06/28 19:43 (LER storage 起因):beam dump

- beam current

RF D5-F+5.6usD2V1 BGO• CCRに届いたのはDiamondが最速。DiamondがCLAWSより早いのは珍しい。
• このLERアボートの CCRでのAbortGap 待ち時間は 1.864 us• Diamondでビームがなくなるのはdiamondがアボートを出したt=0から
7us+10us+1.9us+2.6us +4.7us = 26.2us後(a→c)。観測された信号波形では、28us付
近が最大のピークで、さらにtailが8us程度残っているように見える。
• D2V1 BGOでは、abort kicker signal はビームがなくなってから3us遅れてくる。
diamondアボートを出したバンチ通過の26.2+3.2 = 29.4us後(a→d)。観測では8us後だから、diamondより2ターン遅れてロスが見え始めていることになる。
• CLAWSとdiamondのD2到達時間差は-4.6usなので、 CLAWSはdiamondより約1ター
ンあとのバンチを見てアボートを出している。ただし、CLAWS波形を見ると、ダイアモンドと同じバンチでも信号は見えている（閾値以上の持続時間が短かったため、アボートを出し損ねている）。
• RF D5-Fでは、2ターン遅れてロスが見え始めている。
• BORは、diamondより1ターン遅れてhorizontalに揺れ、そのあたりからBCMでビームロスが見え始めている。

• まとめ：diamond・CLAWSが最初にロスを見ている。1ターン遅れてBORが水平方向の軌道変動を感じ、2ターン遅れでRF D-5, D2V1のBGOが反応している(垂直方向のロス？)。
• D6,D2のLoss Monitorはどうか？(→池田さん)

3us遅れ

8us

CLAWS

Diamond(207mrad)

dia.
abort

~6us

+1turn+2turnBOR(hor)BOR(ver)
BCM loss
15us

Abort kicker

Data/MC ratio (2020 measurements)

Data/MC ratio (2020+2021 measurements)

37