# Performance of the diamond-based beam-loss monitor system of Belle II

S. Bacher  G. Bassi  L. Bosisio  G. Cautero  P. Cristaudo  M. Dorigo  A.
Gabrielli  D. Giuressi  K. Hara  Y. Jin  C. La Licata  L. Lanceri  R. Manfredi
H. Nakayama  K. R. Nakamura  A. Natochii  A. Paladino  G. Rizzo  L. Vitale  H.
Yin  Dipartimento di Fisica, Università di Trieste, I-34127 Trieste, Italy
INFN, Sezione di Trieste, I-34127 Trieste, Italy  Elettra Sincrotrone Trieste
SCpA, AREA Science Park, I-34149 Trieste, Italy  now at: Scuola Normale
Superiore, I-56126 Pisa, Italy  now at: Kavli Institute for the Physics and
Mathematics of the Universe (WPI), University of Tokyo, Kashiwa 277-8583,
Japan  INFN Sezione di Pisa and Dipartimento di Fisica, Università di Pisa,
I-56127 Pisa, Italy  High Energy Accelerator Research Organization (KEK),
Tsukuba 305-0801, Japan  H. Niewodniczanski Institute of Nuclear Physics,
Krakow 31-342, Poland  Institute of High Energy Physics, Vienna 1050, Austria
University of Hawaii, Honolulu, Hawaii 96822, USA

###### Abstract

We designed, constructed and have been operating a system based on single-
crystal synthetic diamond sensors, to monitor the beam losses at the
interaction region of the SuperKEKB asymmetric-energy electron-positron
collider. The system records the radiation dose-rates in positions close to
the inner detectors of the Belle II experiment, and protects both the detector
and accelerator components against destructive beam losses, by participating
in the beam-abort system. It also provides complementary information for the
dedicated studies of beam-related backgrounds. We describe the performance of
the system during the commissioning of the accelerator and during the first
physics data taking.

###### keywords:

sCVD diamond sensor , beam-loss monitoring , accelerator interlocks

††journal: Nuclear Instruments and Methods A

##  1 Introduction

Beam-loss monitoring is an important component of every particle-accelerator
system. For the SuperKEKB [1] electron-positron collider and the Belle II
experiment [2, 3] this function is crucial, specially in the interaction
region, since the accelerator is aiming at an unprecedented design luminosity
of $8\times 10^{35}$ cm-2s-1 [1] with highly-focused intense beams. The
detector has a projected lifetime of at least a decade, to collect a very
large sample of electron-positron annihilation events, and explore extensions
of the Standard Model of fundamental interactions.

The inner Belle II detectors are silicon-based pixel and microstrip sensors
with integrated front-end electronics. The Belle II silicon technology is
designed to sustain a dose of $10$ to $20$ Mrad, that is $0.1-0.2$ MGy, before
significant performance degradation. It is therefore necessary to continuously
monitor the dose rates from beam losses, and keep the radiation budget under
control while the experiment integrates the planned luminosity.

High dose rates, absorbed in short time intervals, may cause irreversible
local damage to essential parts of the inner detector, and disrupt the
accelerator operation by inducing quenches in the neighboring superconducting
magnets of the final-focus system. A beam-loss monitor in this region must
therefore be able to deliver beam-abort triggers to the accelerator control
system, with a latency comparable with the revolution period of the
accelerator rings.

In addition, knowledge of the intensity and timing patterns of beam losses in
the interaction region offers essential information to optimize the collider
operations and increase luminosity.

To meet these challenges we designed, built, and have been operating a
radiation monitoring system based on single-crystal diamond sensors, grown by
chemical vapour deposition (sCVD sensors), and read-out by custom-designed
electronics. This paper describes the diamond system requirements,
construction, and its early operations and performance.

##  2 Belle II interaction region and vertex detector

The SuperKEKB layout is sketched in Figure 1 [4] with some features of the
beam abort system. Information from hardware components such as the RF,
vacuum, magnets systems and the beam loss monitor signals are collected at
local control rooms (LCR); their abort requests, signalling abnormal
accelerator and beam conditions, are sent to the central control room (CCR) to
trigger the abort kicker magnets.

Figure 1: The SuperKEKB beam loss monitor and abort system, from reference
[4].

The Belle II detector [2, 3] is a spectrometer surrounding the intersection of
the $7$ GeV “high-energy” electron ring (HER) and $4$ GeV “low-energy”
positron ring (LER). The inner silicon-based vertex detector (VXD) has two
layers of pixel detectors (PXD) directly mounted on the beam pipe, and four
layers of microstrip silicon vertex detector (SVD) ladders supported by the
cones and rings sketched in Figure 2. The superconducting final-focus magnets
(QCS) of SuperKEKB extend partly inside the spectrometer, close to the beam-
pipe bellows of the intersecting rings.

##  3 The diamond-based beam-loss monitor system

The commissioning and data taking phases of SuperKEKB and Belle II are
summarized in Table 1.

Table 1:  The three running phases of SuperKEKB and Belle II.  Phase/Year | Collisions / Detector
---|---
1 / 2016 | No collisions / BEAST II [5]
2 / 2018 | Collisions / BEAST II $\&$ Belle II, no VXD
3 / 2019 $-$ | Collisions / Full Belle II with VXD
Figure 2: Sketch of the location of the $28$ diamond detectors on the beam
pipe, the SVD support cones, and the beam-pipe bellows, as shown by the on-
line display of dose rates, available to Belle II and SuperKEKB operators.

Figure 3: Photographs of diamond detectors (red dashed boxes) mounted on the
beam pipe (top), on the backward SVD support cone (bottom left), and on the
bellows close to the backward QCS (bottom right).

Four prototype diamond detectors and their readout were installed and operated
during the initial accelerator commissioning Phase 1 in 2016, as part of the
BEAST [5] project for initial beam-background studies; eight detectors were
installed on the final beam pipe for the Phase 2 commissioning run, with first
collisions and a provisional inner detector installed, in 2018. The final
system consists of $28$ diamond detectors, mounted on the beam pipe (BP) near
the interaction point (IP), on the SVD support cones, and on the beam-pipe
bellows, close to the QCS magnets (Figures 2 and 3). We installed it in 2018
and have been operating it since then, for the physics data taking (Phase 3)
with the complete Belle II apparatus.

###  3.1 System requirements

The KEKB peak luminosity was exceeded and a new world record of $2.4\times
10^{34}$ cm-2s-1 was established by SuperKEKB during Phase 3 in June 2020. It
was achieved with a product of beam currents that was less than 25% that of
KEKB, and by the initial implementation of the “nano-beam scheme” and the
“crab waist scheme” [6, 7]: the vertical height of the beams at the
interaction point was squeezed to about 220 nanometers. To approach the design
luminosity, almost a factor $40$ higher, the beam height at the IP will have
to decrease to approximately 50 nanometers, and the product of beam currents
increase to four times that of KEKB [1].

This luminosity enhancement is associated with harsher background conditions.
The main beam-background sources are Touschek scattering, radiative Bhabha
scattering, electron-positron pair production in photon-photon scattering, and
off-momentum particles from beam-gas interactions [5]. The intensities of
these backgrounds are strongly dependent on the beam optics. At the design
luminosity, QED backgrounds will become relevant.

We based the initial requirements of the monitoring system on background
simulations and on the extrapolation of the experience of KEKB [8], the BaBar
experiment at PEP-II (SLAC) [9, 10], CDF at the Tevatron (FermiLab) [11],
ATLAS [12] and CMS [13] at LHC. To design the read-out electronics, we assumed
a sensor current to dose-rate conversion factor in the range $0.1-1$
(mrad/s)/nA; after calibrations, our sCVD sensors turned out to have
sensitivities larger by an order of magnitude, of about $35$ (mrad/s)/nA on
average.

We expected to amplify and digitize diamond currents from pA up to several mA,
to be sensitive to both very low beam losses with radiation levels below $1$
mrad/s, and large spiky losses due to noisy beam injections or machine
operation accidents, of the order of $10$ krad/s or more.

The time response, range, and precision of the associated read-out electronics
were initially constrained as follows.

  * 1.

The time scale relevant for beam abort is the $10$ $\mu$s revolution time of
electrons and positrons in the SuperKEKB rings: the sampling and comparison of
signals with abort thresholds must match the corresponding frequency of $100$
kHz, or better.

  * 2.

Localised radiation damage in SVD microstrip detectors may occur when a
radiation dose in excess of O($1$ rad) is delivered in a time interval of
O($1$ ms) [10]. To produce “fast” abort requests, the digitised diamond
current is integrated (averaged) in moving sums, updated and compared with an
abort threshold every $10$ $\mu$s. With an integration time of about $1$ ms, a
precision at the level of about $10$ nA is required on a $5$ mA measurement
range for such aborts. The integration time and thresholds must be
programmable, to satisfy evolving requirements.

  * 3.

Finally a recording rate of $10$ Hz is chosen for dose rate monitoring,
archiving and integration. The sensitivity obtained by averaging the dose
rates on these time scales is also available for “slow” abort requests, that
could be delivered to the accelerator by the slow control software rather than
by the diamond electronics directly.

###  3.2 Diamond detectors

Diamond sensors, with a bias voltage applied on two opposite electrodes,
operate functionally as solid-state ionisation chambers, delivering a current
induced by the drift of electrons and holes produced by ionisation in the
diamond bulk. Their compact size and radiation hardness are well-suited for
radiation monitoring in the interaction region; the temperature-independent
response allows operation without additional temperature monitoring and
related corrections.

The assembly, test, and calibration of the $28$ sCVD diamond detectors are
detailed in ref. [14].

The electronic grade sCVD sensors were grown by Element Six [15] in the
standard size $4.5\times 4.5\times 0.5$ mm3; two (Ti + Pt + Au),
$(100+120+250)$ nm electrodes were deposited on both faces by CIVIDEC [16]. We
mounted each sensor on a small Rogers [17] printed circuit board, providing
both the mechanical support and the electrical screening, completed by a thin
aluminum cover. The inner conductors of two thin coaxial cables were soldered
to two pads, connected to the sensor electrodes by conductive glue (back side)
and gold wire ball bonding respectively. The outer conductors of the cables
were connected together and with the external detector shield, as shown in
Figure 4.

Figure 4: Diamond sensor packaged into a detector unit. The sensor is glued on
a Rogers printed-circuit board; electrical contact is established between the
two electrodes (front/back) and the inner conductors of two miniature coaxial
cables; outer shielding is also provided.

Each detector was characterized by measuring [14]

  * 1.

the current $I$ as a function of the bias voltage $V$, in the dark and in the
presence of ionization by a $\beta$ radioactive source;

  * 2.

the stability of the current signal in time;

  * 3.

the transport properties of charge carriers, electrons and holes, using a $5$
kBq 241Am source of monochromatic $\alpha$-particles.

  * 4.

a current to dose-rate calibration factor $k$, using an almost point like $3$
MBq 90Sr radioactive $\beta$ source located at varying distance.

The dose rate, expressed in mrad/s, is proportional to the measured current
$I_{m}$ (nA),

$\displaystyle\frac{dD}{dt}$ $\displaystyle=$
$\displaystyle\frac{1}{m}\frac{dE}{dt}=\frac{1}{m}\frac{I_{m}}{G}\frac{E_{eh}}{q_{e}}=\frac{F}{G}I_{m}=kI_{m}$
(1) $\displaystyle F$ $\displaystyle=$ $\displaystyle\frac{E_{eh}}{mq_{e}},$
(2)

where $m$ is the sensor mass, $E_{eh}$ the average ionization energy per
electron-hole pair, $q_{e}$ the elementary charge. The unit conversion factor
$F$ is computed as $F=34.9$ (mrad/s)/nA. A bias voltage of $100$ V is chosen
for full charge-collection efficiency. For a typical measured current of $1$
nA, the dose rate is $34.9$ mrad/s if $G=1$.

The dimensionless factors $G$ describe possible deviations of each diamond
sensor from the ideal behaviour in stationary conditions: 100% efficiency in
the collection of charge carriers from the assumed active volume, zero net
trapping-detrapping rate, and absence of charge injection from the electrodes.
They are obtained from the ratios of the measured current to the corresponding
energy deposit per unit time and sensor mass, estimated by a detailed
simulation of the source and of the set-up. Similar measurements and
simulations on a reference silicon diode are used to account for the source
activity and to reduce the uncertainties of the set-up model. The measured $G$
values cluster around $G\simeq 1$, with maximum variations of about $50\%$.

The uncertainty in the determination of the calibration factors $k$ is
dominated by systematic effects; we estimate an accuracy of $8\%$ [14]. The
linear behaviour (equation 1) is confirmed by measurements performed on a sub-
sample of diamond detectors with a 60Co $\gamma$ source at dose rates and
currents larger by two orders of magnitude.

###  3.3 Electronics

The $28$ diamond detectors are controlled by seven diamond control units
(DCU). Each DCU pilots four diamond detectors, as sketched in the block
diagram of Figure 5 .

Figure 5: Diamond control unit (DCU) block diagram. Arrows on the left side
indicate currents from diamond sensors and arrows on the down side indicate
high voltage applied on the diamond sensors.

The digital core is a board hosting a Cyclone V FPGA by Intel [18], which
receives commands via an Ethernet interface, drives four HV modules
independently through a DAC, and accepts input data from an analog module with
amplifiers and ADC conversion, as sketched in Figure 5.

The DCU is also able to deliver VXD abort requests separately for the electron
higher-energy ring (HER) and the positron lower-energy ring (LER), and
receives the SuperKEKB abort signals.

The diamond currents are amplified by trans-impedance amplifiers, digitised by
a 16-bit ADC [19] at $50$ Msamples/s, and processed by FPGAs in the DCUs, as
shown in the simplified block diagram of Figure 6.

Figure 6: Diamond control unit (DCU): simplified block diagram of the FPGA
firmware.

Three amplifier gain values can be selected by resistors in the feedback loop
of the front-end operational amplifier [20], to provide three different
current measurement ranges, indexed as 0, 1, and 2 in the following. The
analogue bandwidth at the lowest gain (range 2) is about $10$ MHz, matched to
large and fast signals; it is reduced to the order of $10$ kHz at the highest
gain (range 0) used for monitoring smaller signals at $10$ Hz.

The oversampling 16-bit ADC followed by digital integration performs similarly
to slower ADCs with a larger number of bits. At the design stage, this
solution was preferred for its flexibility in choosing the level of digital
integration.

The sum of 125 samples is obtained every 2.5 $\mu$s, at 400 kHz. These sums,
called in the following “400 kHz data”, are written in a 4 Gbit DDR circular
buffer memory. These numbers refer to an upgraded version of the firmware, in
use since January 2020; in the previous version, sums of $500$ samples were
obtained at $100$ kHz, every $10$ $\mu$s.

Two “moving sums” of “$400$ kHz data” are updated for each diamond sensor at
each memory cycle, by subtracting the oldest added value of “400 kHz data” and
adding the newest one. Corresponding to integrated doses over programmable
time intervals, they are compared with programmable thresholds, initially
intended to provide abort requests on two different time scales: (1) “fast” or
“acute” aborts for sudden large spikes in beam losses, typically in the
millisecond time range, and (2) “slow” or “chronic” aborts for moderate, but
increasing beam losses. Operations experience showed that abort requests are
delivered mainly on quickly rising beam losses. Integration-time windows were
therefore shortened, producing “very fast” and “fast” aborts, as described in
Section 5.

If a moving sum exceeds the corresponding programmed dose threshold, a logical
signal is generated; in total, eight signals are available per DCU. Individual
masks can be applied to exclude noisy channels, if needed. “Abort request”
signals are generated separately for LER and HER then sent to SuperKEKB, when
a programmed minimum number of unmasked signals above threshold is reached.
After activating the beam abort kicker magnets, the accelerator control system
broadcasts “SuperKEKB Abort” signals, which are input to DCUs.

Incoming “SuperKEKB LER Abort” and “SuperKEKB HER Abort” signals from
SuperKEKB stop the memory writing. The $400$ kHz data can then be read out and
written to a file for “post-abort” analysis of the beam losses preceding the
abort.

Data at $400$ kHz are further added up in groups of 40000, to provide sums of
5000000 ADC values that can be read out at $10$ Hz (“$10$ Hz data”).

Prototypes were extensively characterized during commissioning: Table 2 shows
the measured characteristics of the three options for current-measurement
ranges, selected by changing the amplifier gain, as implemented in the final
production version of the DCUs. The range 0 ($36$ nA) allows precise
monitoring of relatively small beam losses, while the range 2 ($4.5$ mA)
avoids saturation in the detection of large radiation spikes: the latter is
used if the DCU is specifically dedicated to provide beam abort requests. The
quoted noise is measured in the complete experimental set up during normal
accelerator operations. The intrinsic electronics noise, measured in a test
bench in a laboratory, is about 10 (4) times better at 100 kHz (10 Hz).

Table 2:  Options for current-measurement ranges of diamond control units, with the corresponding rms noise values, measured in $100$ kHz data (third column) and in $10$ Hz data (fourth column).  Range | Current | Rms noise | Rms noise
---|---|---|---
index | range | @100 kHz | @10 Hz
0 |  $36$ nA |  $0.23$ nA |  $0.8$ pA
1 |  $9$ $\mu$A |  $3$ nA |  $70$ pA
2 |  $4.5$ mA |  $0.22$ $\mu$A |  $40$ nA

###  3.4 Slow control

The slow control handles the DCU configuration and the monitoring data in
EPICS 3.14 [21] code. Process variables (PV) are employed to regularly query
$10$ Hz data. After subtraction of pedestals, which are ADC values averaged
over a prior no-beam time, and multiplication with calibration factors, values
of dose-rates are obtained. All dose rate PVs are archived.

A finite-state machine controls the various phases, with transitions between
configurations, $10$ Hz data readout, and buffer memory readout for post-abort
analysis. This last state transition is triggered by the arrival of “SuperKEKB
Abort” signals, which stop the advancement of the buffer memory pointers.
After completion of buffer memory readout, the system resumes the $10$ Hz data
readout.

Time-dependent dose rates are displayed in both the Belle II and the SuperKEKB
main control rooms (Figure 2). Also the pre-abort history of dose rates with
2.5 $\mu$s time resolution is available to operators for post-abort analysis
of the beam conditions, as discussed in Section 5. Individual diamond
detectors are labelled as a_b_c according to their position; a = BP, SVD, QCS
describes the location on beam pipe (BP), SVD support cones, or QCS bellows; b
= FW, BW denotes the forward or backward position with respect to the
interaction point, in the electron beam direction; c represents the azimuthal
angle in degrees, in the standard Belle II laboratory reference frame.

##  4 Dose rate measurements

The dose rates from the 28 diamond detectors, measured and archived at 10 Hz,
provide on-line monitoring of beam losses and accelerator-related backgrounds,
and are correlated with the data of Belle II inner detectors. We describe here
the main features and results of these measurements.

###  4.1 Pedestals and noise

Offsets (pedestals) and noise of each readout channel are obtained by repeated
measurements without circulating beams. A few minutes are sufficient to
precisely determine the central value and noise value for the chosen current-
measurement range, from histograms similar to the examples in Figure 7. Table
3 gives a summary of measured noise for a typical diamond sensor calibration
factor $k=30$ (mrad/s)/nA.

Figure 7: Examples of noise measurements performed without circulating beams
for diamond detectors installed on beam pipe, expressed in mrad/s: (top left)
range 0, $100$ kHz; (top right) range 2, $100$ kHz; (bottom left) range 0, 10
Hz; (bottom right) range 2, 10 Hz. Entries in the top plots are read from the
$100$ kHz buffer memory, spanning $1$ s (100000 entries), pedestal-subtracted
and converted from ADC to dose-rate units; the slight asymmetry in these
distributions is due to an intrinsic cutoff of the largest negative
fluctuations in the almost-unipolar front-end range. Entries in the bottom
plots are 10 Hz data collected in 17 minutes ($\sim$10000 entries); the shift
of the average from zero corresponds to a pedestal drift since the previous
measurement.

Figure 8: Example of time evolution of pedestals and noise (mrad/s) over three days without beams. The central values represent pedestals (averages of 10 Hz data) evaluated in short time intervals of 10 minutes every three hours; error bars show the standard deviations of the corresponding approximately Gaussian distributions, representative of noise, for measurements performed in range 2 (top) and in range 0 (bottom). Table 3:  Typical measured rms-noise values expressed in dose rate units for the three measurement ranges; the approximate noise for $100$ kHz data in range 2 is relevant for the “very fast abort” threshold with $10$ $\mu$s integration time (Section 5.2).  Range | Dose rate | Rms noise | Rms noise
---|---|---|---
index | range | @100 kHz | @ 10 Hz
0 |  $1.1$ rad/s |  $6.8$ mrad/s |  $0.024$ mrad/s
1 |  $270$ rad/s |  $90$ mrad/s |  $2.2$ mrad/s
2 |  $140$ krad/s |  $7$ rad/s |  $1.2$ rad/s

The measured noise includes a random component that scales with the data
averages from 100 kHz to 10 Hz, and common-mode components, picked up from the
environment, which are only partially averaged out. Common-mode noise depends
on the selected range and shows a pattern correlated with the detector
cabling: substantial attenuation is obtained by placing ferrite bead filters
on the DCU input signal cables.

The measured average values (pedestals) are mostly stable (Figure 8), drifting
typically less than one or two times the standard deviations of beam-off
pedestals during the time interval between programmed accelerator beam-off
periods: a weekly or bi-weekly update of pedestals is sufficient for most
purposes, including the pedestal subtraction for monitoring of instantaneous
dose rates. For off-line computations requiring a more accurate subtraction,
we use the data archived during short accidental beam-off periods to determine
smaller pedestal shifts more frequently.

###  4.2 Instantaneous dose rates

Diamond current signals, converted to dose rate units after pedestal
subtraction, are continuously monitored in all accelerator conditions. They
are particularly relevant as indicators of beam quality during injection and
collimator tuning, and for beam optics studies.

###  4.3 Integrated dose

The integrated radiation doses are approximately computed on-line, using the
$10$ Hz monitoring data, and made available on a daily basis. Small pedestal
shifts are then determined (Section 4.1) and used off-line for a more precise
estimate of the integrated dose, as reported in Figure 9 for the most exposed
diamond detectors, located on the beam pipe (BP) and the QCS bellows. The
diamond detectors mounted on SVD cones received radiation doses lower by
almost an order of magnitude, concentrated mostly in the horizontal plane of
the circulating beams.

The most exposed diamond detector QCS_FW_225 received an estimated total dose
of about $960$ krad in Phase 3 until July 2020.

The pre-amplifier of diamond detector BP_FW_325 was damaged during a beam
incident with very large losses in May 2020, as shown by the dose for that
channel in Figure 9(a), no longer increasing after the incident. The pre-
amplifier was repaired and the diamond read-out channel recovered for the next
run period.

(a) Beam pipe

(b) QCS backward

(c) QCS forward

Figure 9: Integrated dose during Phase 3 (March 13, 2019 to July 1, 2020), in
diamond detectors located on the beam pipe (a) and close to the final-focusing
superconducting magnets in the backward (b) and forward (c) regions.

###  4.4 Accelerator tuning and beam backgrounds studies

During Phase 2 several studies were dedicated to investigate the origin of
beam-related backgrounds, by correlating detector outputs with the accelerator
conditions. Diamond detectors contributed to these studies, as shown in the
following examples [22].

Figure 10: (left) Dose rate of the diamond detector with highest signal as a
function of beam currents, separately for LER and HER, during single-beam
measurements at different dates in Phase 2; (right) residual gas pressure
increase with the beam current, during the same measurements.

The left panels in Figure 10 show the dose rates, measured by the diamond
detector on the beam pipe showing the highest signal, as functions of the
circulating beam current, separately for LER and HER, at different dates.

Single beam backgrounds, shown here, are dominated by two types of
contributions. Beam losses due to beam interactions with the residual gas, the
so called beam-gas backgrounds, scale with the product of the beam current and
the residual gas pressure. Touschek background, due to intra-bunch scattering,
scales quadratically with the beam current.

Two features are evident in these plots: a progressive lowering of dose rates
with increasing date and a quadratic dependence of dose rate on beam current.

The former feature is explained by several factors. In particular, the
accelerator vacuum was improved by “beam scrubbing” the vacuum pipes, with
consequent decrease of the beam losses due to beam interactions with the
residual gas, and collimator tuning contributed to limit the beam losses.

The quadratic dependence from beam current, valid for Touschek losses, is
qualitatively explained also for the beam-gas background, since the residual
gas pressure is increasing with the beam current, as shown in the right panels
of Figure 10.

A quantitative analysis separates the contributions of beam-gas and Touschek
backgrounds, using data from dedicated studies with single beams, as functions
of beam size and number of bunches at fixed beam current $I$ [5]. Beam-gas
backgrounds remain constant in these conditions, while Touschek intra-bunch
scattering is expected to increase when the density of particles in each bunch
increases, by decreasing the beam size or lowering the number of bunches at
the same total current. This is qualitatively shown in Figure 11 and Figure 12
and quantitatively expressed in the parameterization [5]

$\frac{dD}{dt}\frac{1}{P_{e}I}=S_{bg}Z_{e}+S_{T}\frac{I}{P_{e}\sigma_{y}},$
(3)

where $P_{e}$ is the residual gas average effective pressure, $Z_{e}$ the
effective atomic number of the gas mixture, $\sigma_{y}$ the vertical beam
size, $S_{bg}$ and $S_{T}$ coefficients related to the relative contributions
of beam-gas and Touschek backgrounds respectively.

Figure 11: Dose rate of one diamond detector as a function of the reciprocal
of the LER beam size. Both variables are normalized using the constant LER
beam current and the effective vacuum pressure. The linear fit extracts the
beam-gas (constant) and Touschek (linearly increasing) contributions, as shown
in equation 3. Sharing the same beam current in a larger number of bunches
(1576 instead of 789) decreases the Touschek contribution, as expected. Figure
12: Dose rate in one diamond detector as a function of the reciprocal of the
LER beam size, similar to Figure 11, for two different LER beam currents. The
two sets of measurements overlap, as expected for the dose rate normalized on
the beam current.

Similar studies have been repeated in Phase 3 at each major step of the
evolution in accelerator conditions, such as increasing beam currents,
reducing beam size at the interaction point, optimization of collimator
settings and of the parameters of continuous injection.

Dedicated studies, based on the combination of the pattern of signals from
diamond detectors and the extrapolation of particle tracks from Belle II
tracking detectors, were used to identify specific background sources such as
those due to beam tails “scraping” the vacuum-chamber walls.

“Beam-dust events”, characterized by localized pressure bursts accompanied by
radiation bursts, often triggered beam aborts and losses of operation time,
more frequently in the initial operation of the accelerator. They were studied
by looking for large sudden signals from diamonds, in coincidence with other
detectors, and interpreted as collisions between beams and small particles
such as dust, coming off the vacuum-chamber material.

###  4.5 Comparison with simulations

Simulations of beam-related backgrounds are needed for two main purposes. The
space and time distribution of the radiation field in the interaction region
correlates the radiation doses measured by diamond sensors with the radiation
dose received by the neighboring Belle II inner detectors, PXD and SVD.
Moreover, comparisons between simulations and measurements validate the
simulations, which can be then used to identify and estimate the background
sources and to extrapolate their effects in the future accelerator conditions.

Single-beam background simulation studies start with the simulation of a
single circulating beam and its interactions (Touschek, Bremsstrahlung, and
Coulomb scattering) based on the SAD code [23], followed by tracking of lost
particles and secondaries in the beam pipe and the surrounding material with
the GEANT4 simulation of Belle II [24]. The geometry and materials of the
diamond detectors are included in the simulation. The energy deposited per
unit time by particles crossing each diamond detector is converted into a dose
rate, using the measured calibration constants.

Figure 13: Comparison between dose rate data and simulations: an example from
a study of backgrounds from single LER beam. (Top) LER beam current $I$ as a
function of time; red line: beam-current decay period; $n_{b}$: number of
bunches. (Middle) Corresponding dose rate from beam losses recorded by
QCS_FW_135; black squares: non-fitted data, red circles: fitted data; blue
line: fit results for the study. (Bottom) Ratio of recorded and simulated data
for eight QCS and three BP diamond detectors, as a function of their
positions; red circles: LER; blue squares: HER.

Simulations are repeated for the specific beam optics and collimator settings
of successive data taking periods and accelerator tuning studies. Figure 13
shows an example from LER single-beam background studies in June 2020. After
initial injection, the beam is kept for some minutes at constant current with
continuous injection, and then the current decays without injection (red line
in Figure 13(top)). This pattern is repeated three times, with different
number of bunches $n_{b}$ filled into the machine (Figure 13(top)). The beam
losses recorded by the diamond detectors follow this pattern closely; the
injection-related background is well separated and can be excluded from a fit
(blue line in Figure 13(middle)) taking into account beam current $I$, number
of bunches, vertical beam size $\sigma_{Y}$, beam-gas ($B_{0}$ and $B_{1}$)
and Touschek $T$ background components, as shown in the insert of the figure.
The data agree with simulations within an order of magnitude (Figure
13(bottom)); the comparison helps understanding the effect of collimators,
alignments and other accelerator parameters in the simulation.

###  4.6 Saturation effects

In the continuous injection mode of SuperKEKB, the intensities of the
circulating beam bunches are topped up repeatedly over short time intervals,
at a frequency of up to $25$ Hz. Immediately after each injection, beam losses
in the interaction region grow until the beam oscillations due to the
injection perturbation are dumped away. As a result, beam losses and diamond
detector signals vary in time; a significant part of the radiation dose is
delivered during short time intervals, adding up to a few milliseconds per
second, depending on the specific injection pattern and frequency.

The dose-rate monitor at 10 Hz and the total dose computation are based on
digital sums of the 50 MHz ADC data over $100$ ms time intervals (Section 3.3,
Section 3.4). If saturation of the current signals occurs in range 0 over much
shorter time intervals, it becomes partially hidden by averaging in $10$ Hz
data, leading to an underestimate of radiation dose rates and integrated
doses.

Figure 14: (Top) An example of saturation in range 0, observed in a one-second
time interval. The $100$ kHz data are read during continuous injection at $25$
Hz for diamond detector BP_BW_325. Ten out of $25$ injection-related peaks
have a saturated value of about $1.3$ rad/s, at least for one $10$ $\mu$s
sampling each; (bottom) one of the above peaks saturates the measurement range
during $40$ $\mu$s.

These effects were studied in November 2019 by repeatedly dumping the DCU
buffer memories during continuous injection, to observe the beam loss patterns
in 100 kHz data with $10$ $\mu$s resolution, for both range 0 ($36$ nA) and
range 1 ($9$ $\mu$A). The example in Figure 14 shows that saturation of range
0 could be observed in some BP and QCS diamond detectors, exposed to higher
beam losses. Saturation typically lasted for one or a few 100 kHz samples
(10-100 $\mu$s). This observation suggests to switch to range 1 for some DCUs
in future operations, to avoid saturation at higher beam currents; an upgrade
of the electronics toward expanding the dynamic range is also considered in
the long term.

###  4.7 SVD occupancy and injection inhibit

The occupancy of the inner layer of the SVD double-sided microstrip detector
of Belle II is a critical parameter for track reconstruction. The performance
of the pattern recognition algorithms deteriorate significantly beyond an
occupancy of about $3$ %.

An approximately linear correlation was observed between SVD occupancy and
$10$ Hz data from BP diamond detectors: an example is shown in Figure 15. The
expected occupancy could therefore be predicted in real time by a combination
of BP diamond signals. An “injection inhibit” flag was prepared and
implemented in EPICS, to suspend the continuous injection in large background
conditions corresponding to unacceptable occupancy.

Figure 15: (Top) Correlation between the dose rate in diamond detector
BP_BW_215 and the occupancy of the inner SVD microstrip detector layer,
measured during single-beam background studies with changing beam currents.
(Bottom) the beam-pipe diamond detectors’ dose rates measured at 10 Hz give an
estimated occupancy (blue), compared with the SVD occupancy (red), measured at
1 Hz. Their ratio (pink) is close to unity and stable in time. On the 3rd of
May 2020, the estimated occupancy exceeded the limit and triggered a diamond
“injection inhibit” (green).

##  5 Beam aborts

$24$ diamond detectors were dedicated to monitoring, and the corresponding
DCUs preset on the most sensitive current measurement range 0 (section 3.3).
Four detectors mounted on the beam pipe were dedicated for the abort function,
with range 2 selected in the corresponding DCU.

###  5.1 Abort timing

Minimizing the time delays in the abort system is essential for the protection
of Belle II and of accelerator components in the interaction region: during
the interim between the initial detection of abnormal beam losses and the
complete dumping of the beams, radiation levels may increase up to damaging
values [25], as shown in the example of Figure 16.

Figure 16: An example of large beam loss, which occurred before the SuperKEKB
reduction of abort delays and the DCU firmware improvement described in the
text. The integrated dose in a moving window of $1$ ms, updated every $10$
$\mu$s, is shown as a function of time: a dose of $6$ rad is integrated in the
almost $50$ $\mu$s interim between abort threshold crossing and beam dump
completion.

On the accelerator end, optical fibres are used for long-distance propagation
of abort signals. Their propagation delay was minimized by optimizing the
signal paths in the accelerator central control system; in 2020, a second
“abort gap” [26] was also introduced in the circulating bunch trains, to
effectively reduce the waiting time for the activation of abort kicker
magnets.

The generation of abort-request signals in the diamond-based system was
initially cycling every $10$ $\mu$s, to compare the moving sums of data stored
at $100$ kHz with the abort thresholds. At the beginning of 2020, the modified
DCU firmware (section 3.3) contributed to the delay-minimization effort, by
storing data in the buffer memory at $400$ kHz and performing the abort-
threshold comparison every $2.5$ $\mu$s. As a result, the internal delay of
the DCU between a large step-like fast-rising input signal and the delivery of
the output abort request is halved to about $6$ $\mu$s. This time includes
both the effect of the analog front-end bandwidth and the overhead of an
additional confirmation cycle after threshold-crossing.

###  5.2 Abort thresholds settings

The diamond abort thresholds were adjusted according to the evolving
accelerator conditions. In Phase 2, several quenches of QCS superconducting
magnets were accompanied by SuperKEKB aborts, coming too late to prevent large
energy depositions in the superconducting coils by the rapidly increasing beam
losses. A study of the recorded diamond signals in these events showed that in
most cases a lower diamond abort-threshold could have triggered an earlier
abort of the beams, reducing in this way the energy deposition by beam losses
and preventing the corresponding QCS quenches. As a result of this study, the
diamond abort thresholds were lowered, still remaining well above the noise
levels (Table 4, first line).

Table 4:  Abort thresholds in Phase 2 (2018) and Phase 3 (starting in 2019), adopted after initial tests and the adjustments preventing QCS quenches. Range 2 and multiplicity “at least one sum above threshold” have been adopted, except for the first period (range 1, multiplicity “at least two”). The abort cycle initially had a frequency of $100$ kHz, modified to $400$ kHz since 18 Feb 2020 (section 3.3).  date | label | integr. | threshold
---|---|---|---
|  | time |
28 May 2018 | fast |  $1$ ms |  $20$ mrad
| slow |  $1$ s |  $400$ mrad
5 Jun 2019 | fast |  $1$ ms |  $140$ mrad
| slow |  $1$ s |  $4.4$ rad
12 Jun 2019 | fast |  $1$ ms |  $20$ mrad
| slow |  $1$ s |  $10$ rad
15 Oct 2019 | fast |  $1$ ms |  $60$ mrad
| very fast |  $40$ $\mu$s |  $8$ mrad
28 Oct 2019 | fast |  $1$ ms |  $80$ mrad
| very fast |  $40$ $\mu$s |  $80$ mrad
18 Feb 2020 | fast |  $1$ ms |  $80$ mrad
| very fast |  $10$ $\mu$s |  $8$ mrad

We also observed that shorter integration time-intervals for the moving sums
(section 3.3) allowed to advance the abort request in case of rapidly rising
beam losses, with the advantage of minimizing the received radiation dose.
Moreover, the reduction of the abort-cycle time in the DCU firmware from $10$
to $2.5$ $\mu$s contributed to the effort of shortening the delays in all the
components of the SuperKEKB abort system. Table 4 shows the evolution of
diamond beam abort parameter settings during Phases 2 and 3.

###  5.3 Abort features and statistics

(a)  (b) “Very fast” abort (c) “Fast” abort

Figure 17: (a) Histogram of the number of aborts per day (scale on the left)
in February-June 2020, and cumulative number of diamond abort requests
(continuous line, scale on the right). (b, c) Two examples of post-abort
buffer memory dump: dose rate as a function of time, whose zero corresponds to
the reconstructed threshold-crossing time.

A total of 305 abort requests were delivered by the diamond-based system in
2019, 655 in 2020 spring runs. Most aborts were determined by the “fast”
threshold initially, and by the “very fast” threshold after its
implementation. The latest “very fast” threshold setting, with a time window
of $10$ $\mu$s for the moving sum (four times $2.5$ $\mu$s), corresponds to an
average dose rate of $0.8$ krad/s in the time interval, or to a single $3.2$
krad/s spike concentrated in one $2.5$ $\mu$s sample.

Figure 17(a) shows the number of aborts per day in 2020. The higher peaks
correspond to accelerator tuning. The decreasing slope of the cumulative
distribution indicates a decrease in the average number of aborts per day
after the initial period, correlated with improvements in accelerator
conditions, namely tighter collimators and stable injections.

Figure 17(b,c) shows two examples of post-abort memory dump for four aborting
diamond detectors, after pedestal subtraction and conversion to dose units.
The first event corresponds to a very sharp rise of the dose rate, which
reaches a large value of about 5 krad/s immediately after crossing the abort
threshold and before the beam abort process is completed. The second event
shows a much less frequent occurrence of more slowly rising dose rate, whose
integral over $1$ ms crosses the “fast” threshold of $80$ mrad.

##  6 Conclusions and outlook

The Belle II diamond-based beam-loss monitoring and beam-abort system,
designed and assembled at the INFN Trieste laboratories, was installed and
operated successfully in the interaction region of the SuperKEKB electron-
positron collider. It has been reliably providing dose-rate monitoring data to
keep the radiation budget of Belle II inner detectors under control, and has
given valuable feedback for accelerator studies and tuning. The participation
in the beam-abort system, with the delivery of a substantial fraction of the
abort requests, has ensured an effective protection of the Belle II inner
detectors and of the SuperKEKB superconducting final-focus magnets.

To meet the challenge of SuperKEKB long-term operation, with the projected
future large increase in luminosity, possible improvements of the diamond
system are considered. In particular, the performance of the beam-loss
monitoring would be improved by electronics with a wider dynamic range and by
a more flexible access to dose-rate data with high time resolution in the ring
buffer memories.

## Acknowledgements

The construction of the diamond-based beam loss monitor and beam abort system
was funded by Istituto Nazionale di Fisica Nucleare (INFN) in the framework of
the Belle II experiment. The electronics was designed by the Instrumentation
and Detectors Laboratory of Elettra Sincrotrone Trieste ScpA. We thank our
colleagues of the SVD, PXD, and BEAST groups within the Belle II collaboration
for fruitful discussions and comments, and the Machine-Detector Interface
group (MDI) of Belle II and SuperKEKB, an essential discussion forum for the
operation of the system.

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