Calibration of diamond detectors for dosimetry in beam-loss monitoring
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-04-27 |
| Journal | Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment |
| Authors | G. Bassi, L. Bosisio, P. Cristaudo, M. Dorigo, A. Gabrielli |
| Institutions | University of Trieste, Istituto Nazionale di Fisica Nucleare, Sezione di Trieste |
| Citations | 15 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study details the assembly, characterization, and calibration of 28 single-crystal Chemical Vapor Deposition (sCVD) diamond detectors intended for high-radiation dosimetry and Beam-Loss Monitoring (BLM) at the SuperKEKB collider.
- Material and Design: The sensors are sCVD diamond (4.5 x 4.5 x 0.50 mm3) with Ti+Pt+Au electrodes, packaged for extreme radiation hardness (expected lifetime dose > 10 Mrad).
- Performance Validation: Transient-Current Technique (TCT) confirmed homogeneous charge transport properties across the sample set, yielding average electron and hole saturation velocities of 0.9 x 107 cm/s and 1.3 x 107 cm/s, respectively.
- Optimal Operation: An optimal bias voltage of ±100 V was determined, achieving a steady-state charge-collection efficiency (CCE) close to 100%. Dark current at this bias is reliably less than 1 pA.
- Calibration Method: A robust current-to-dose-rate calibration factor (k) was established using a 90Sr beta source and a silicon diode reference, significantly reducing systematic uncertainties related to source activity and simulation geometry.
- Accuracy and Stability: The average calibration factor corresponds to 35 (mrad/s)/nA, determined with a relative systematic uncertainty of 8%. The detectors demonstrated stable current response (within 1-5% fluctuation) over several days of continuous irradiation.
- Asymmetry Handling: Approximately half the sensors exhibited asymmetric Current-Voltage (I-V) profiles or hysteresis; for these, the polarity leading to current saturation was chosen for reliable operation.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Detector Material | sCVD Diamond | N/A | Grown by Chemical Vapor Deposition (CVD) |
| Sensor Dimensions | 4.5 x 4.5 x 0.50 | mm3 | Face dimensions and thickness |
| Electrode Composition | Ti+Pt+Au | N/A | Layered structure (100+120+250 nm thickness) |
| Optimal Bias Voltage | ±100 | V | Selected for near 100% Charge Collection Efficiency (CCE) |
| Maximum Dark Current | 1 | pA | Measured at ±100 V bias |
| Electron Saturation Velocity (vsat) | 0.9 x 107 | cm/s | Average from TCT measurements |
| Hole Saturation Velocity (vsat) | 1.3 x 107 | cm/s | Average from TCT measurements |
| Average Ionization Energy (Eeh) | 13.1 | eV | Energy required to create one electron-hole pair |
| Average Calibration Factor (k) | 34.9 | (mrad/s)/nA | Dose rate per measured current |
| Total Systematic Uncertainty | 8 | % | Relative uncertainty on calibration factor k |
| Largest Uncertainty Source | Current Transients/Fluctuations | 5 % | Contribution to systematic uncertainty on k |
| Operational Dose Rate Range | Few urad/s to hundred krad/s | N/A | Required range for SuperKEKB monitoring |
| Detector Mass (m) | 37 | mg | Used in calculation of calibration factor F |
Key Methodologies
Section titled âKey Methodologiesâ-
Detector Assembly and Initial QC:
- sCVD sensors were mounted on Rogers PCBs. Inner coaxial cable conductors were soldered, and outer shields were fixed using conductive glue for mechanical stability.
- Initial dark current measurements were performed up to ±800 V to ensure the current remained below 1 pA at the target operating voltage (±100 V).
-
Charge Carrier Transport (TCT):
- The Transient-Current Technique (TCT) was used to study electron and hole transport properties.
- A collimated 241Am alpha source (5.485 MeV) was used to generate localized electron-hole pairs (penetration depth ~12 ”m).
- Pulse shape analysis determined drift velocity and mobility as a function of the electric field, confirming homogeneous transport properties.
- Pulse integral analysis determined the average ionization energy (Eeh), found to be 13.1 eV.
-
I-V Profile and Stability Testing:
- Current-Voltage (I-V) profiles were measured under continuous irradiation from a 90Sr beta source across a ±500 V range.
- The optimal operating voltage (±100 V) was selected where the current reached a stable plateau, indicating maximum CCE.
- For detectors showing asymmetric I-V profiles or hysteresis, the polarity leading to current saturation was chosen.
- Current stability was monitored over several hours to days, confirming fluctuations were typically < 1% for symmetric sensors and < 5% for asymmetric sensors (at optimal polarity).
-
Current-to-Dose Calibration (Reference Method):
- Detectors were irradiated with a 90Sr beta source, and the signal current I(d) was measured as a function of source-detector distance (d).
- A calibrated silicon diode was used as a reference detector, measuring Ir(d) under identical conditions.
- The measured signal-to-reference ratio R = I(d)/Ir(d) was compared to the expected ratio Rexp, calculated via detailed FLUKA simulation, to determine the characteristic constant G.
- The final calibration factor k was calculated using G and fundamental constants (k = F/G), resulting in an average value of 35 (mrad/s)/nA with 8% systematic uncertainty.
Commercial Applications
Section titled âCommercial ApplicationsâThe characterized sCVD diamond detectors are suitable for applications requiring extreme radiation hardness, high stability, and precise dose rate measurement:
- High-Energy Physics (HEP) Accelerators:
- Beam-Loss Monitoring (BLM) and machine protection systems in colliders (e.g., SuperKEKB, LHC).
- Dosimetry for sensitive detector components operating in high-background environments.
- Medical Dosimetry:
- High-precision measurement of absorbed dose in complex radiation fields (e.g., proton, heavy ion, or photon therapy).
- Micro-dosimetry due to diamondâs tissue-equivalent properties and small active volume.
- Nuclear and Reactor Monitoring:
- In-situ radiation monitoring in nuclear facilities where high neutron and gamma fluxes necessitate radiation-hard sensors.
- Space Radiation Monitoring:
- Dosimetry for satellites and spacecraft, leveraging diamondâs stability and resistance to displacement damage in long-duration missions.
Tech Support
Section titled âTech SupportâOriginal Source
Section titled âOriginal SourceâReferences
Section titled âReferencesâ- 1975 - Preparation and characteristics of natural diamond nuclear radiation detectors [Crossref]
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- 2005 - Radiation monitoring with CVD diamonds in babar [Crossref]
- 2006 - A diamond-based beam condition monitor for the CDF experiment
- 2008 - The ATLAS beam conditions monitor [Crossref]