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

At a Glance

Metadata	Details
Publication Date	2021-02-17
Journal	Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
Authors	S. Bacher, G. Bassi, L. Bosisio, G. Cautero, P. Cristaudo
Institutions	University of Trieste, Istituto Nazionale di Fisica Nucleare, Sezione di Trieste
Citations	28
Analysis	Full AI Review Included

Executive Summary

The following points summarize the performance and technical achievements of the single-crystal synthetic diamond (sCVD) beam-loss monitor (BLM) system implemented at the SuperKEKB/Belle II interaction region:

Critical Protection Function: The BLM system successfully monitors radiation dose rates near the inner detectors (VXD/SVD) and superconducting final-focus magnets (QCS), participating in the beam-abort system to prevent irreversible damage.
High-Speed Interlocks: The system delivers “very fast” abort requests with a cycle time of 2.5 µs (400 kHz frequency), utilizing a 10 µs integration window, achieving an internal delay of approximately 6 µs. This speed is essential for mitigating rapid, destructive beam loss spikes.
Wide Dynamic Range: Custom electronics (DCUs) provide three current measurement ranges, from 36 nA (high sensitivity) up to 4.5 mA (saturation avoidance), allowing monitoring of radiation levels from less than 1 mrad/s up to 10 krad/s.
Dose Tracking and Lifetime Management: Continuous 10 Hz monitoring and archiving provide integrated dose data, allowing operators to manage the radiation budget. The most exposed detector recorded an integrated dose of 960 krad during Phase 3 operations.
Accelerator Diagnostics: The high-resolution dose rate data (10 µs pre-abort history) provides crucial feedback for accelerator tuning, enabling the separation of background sources (e.g., Touschek scattering vs. beam-gas interactions) and optimization of collimator settings.
Validation of sCVD Technology: The system confirms sCVD diamond sensors are highly suitable for harsh accelerator environments due to their radiation hardness, compact size, and stable, temperature-independent response.

Technical Specifications

Parameter	Value	Unit	Context
Sensor Type	Single-Crystal Diamond (sCVD)	N/A	Electronic grade, Element Six.
Sensor Dimensions	4.5 x 4.5 x 0.5	mm³	Standard size.
Bias Voltage	100	V	Chosen for full charge-collection efficiency.
Dose Rate Conversion Factor (k)	~35	(mrad/s)/nA	Average sensitivity of sCVD sensors.
ADC Resolution	16	bit	Used in Diamond Control Unit (DCU).
ADC Sampling Rate	50	Msamples/s	Used for digital integration.
Abort Cycle Frequency	400	kHz	Rate of threshold comparison (2.5 µs cycle).
”Very Fast” Abort Integration Time	10	µs	Moving sum window for abort trigger.
DCU Internal Delay (Abort)	~6	µs	Delay between fast input signal and output abort request.
Dose Rate Monitoring Rate	10	Hz	Archiving and slow control update rate.
Lowest Current Range (R0)	36	nA	Range for precise monitoring of small losses.
Highest Current Range (R2)	4.5	mA	Range used for dedicated abort function (saturation avoidance).
Rms Noise (R0, 10 Hz)	0.8	pA	Noise floor for the most sensitive range.
Highest Integrated Dose (Phase 3)	960	krad	Recorded by QCS_FW_225 detector (Mar 2019 - Jul 2020).
Target Detector Lifetime Dose	10 to 20	Mrad	Belle II silicon detector design limit.

Key Methodologies

The BLM system relies on sCVD sensors and custom FPGA-based electronics (DCUs) to achieve high-speed monitoring and interlock functionality:

Sensor Construction and Packaging: Electronic grade sCVD sensors were used, featuring Ti/Pt/Au electrodes deposited on both faces. Each sensor was mounted on a Rogers printed circuit board (PCB) for mechanical support and electrical screening.
Calibration and Characterization: Sensors were calibrated using radioactive beta (⁹⁰Sr) and alpha (²⁴¹Am) sources to determine the current-to-dose-rate conversion factor (k) and assess charge carrier transport properties and collection efficiency (G).
Signal Acquisition and Amplification: Diamond currents are fed into trans-impedance amplifiers offering three selectable gain ranges (36 nA, 9 µA, 4.5 mA) to accommodate the wide dynamic range of expected beam losses.
High-Speed Digital Integration: The amplified analog signals are digitized by a 16-bit ADC at 50 Msamples/s. The DCU FPGA performs digital integration by calculating moving sums of 125 samples, generating “400 kHz data” every 2.5 µs.
Fast Abort Logic: The moving sums are continuously compared against programmable dose thresholds. If a threshold is exceeded, an abort request signal is generated for the LER or HER ring, with a minimum integration time of 10 µs for the “very fast” abort setting.
Post-Abort Diagnostics: Upon receiving a SuperKEKB Abort signal, the DCU stops writing to its circular buffer memory (4 Gbit DDR), allowing operators to read out the 400 kHz data history (2.5 µs resolution) immediately preceding the event for root-cause analysis.
Background Separation Studies: Dose rate measurements were correlated with accelerator parameters (beam current, bunch number, beam size, vacuum pressure) to quantitatively separate beam-gas backgrounds (linear dependence on current) from Touschek scattering backgrounds (quadratic dependence on current).

Commercial Applications

The technology and methodologies developed for the Belle II diamond-based BLM system are applicable to several high-tech and industrial sectors:

High-Energy Physics and Accelerator Science: Direct application in beam diagnostics, machine protection systems, and radiation monitoring for future colliders (e.g., FCC, CEPC) or high-intensity light sources.
Nuclear and Space Radiation Monitoring: Utilizing sCVD diamond’s intrinsic radiation hardness and stability for long-term, high-precision dosimetry in harsh environments, such as nuclear reactors or satellite systems.
Medical Dosimetry and Radiotherapy: High-speed, high-resolution diamond detectors are ideal for monitoring pulsed radiation beams (e.g., proton therapy, linear accelerators) to ensure precise dose delivery to patients.
Industrial Non-Destructive Testing (NDT): Diamond detectors can be used for high-flux X-ray or gamma-ray detection in industrial imaging and quality control systems requiring fast response times.
High-Power Electronics: While the paper focuses on sensing, the underlying sCVD growth technology (Element Six) is critical for producing diamond substrates used in high-power RF devices and thermal management solutions due to diamond’s superior thermal conductivity.

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.nima.2021.165157

References

2013 - Accelerator design at SuperKEKB
2018 - Detectors for extreme luminosity: Belle II [Crossref]
2017 - Beam loss and abort diagnostics during SuperKEKB phase-I operation
2019 - First measurements of beam backgrounds at SuperKEKB [Crossref]
2020 - Highlights from superkekb commissioning for early stage of nano-beam scheme and crab waist scheme
2013 - Progress in KEKB beam instrumentation systems
2004 - Radiation hardness and monitoring of the BABAR vertex tracker [Crossref]
2005 - Radiation monitoring with CVD diamonds in babar [Crossref]