Precision Electron-Beam Polarimetry at 1 GeV Using Diamond Microstrip Detectors

At a Glance

Metadata	Details
Publication Date	2016-02-01
Journal	DSpace@MIT (Massachusetts Institute of Technology)
Authors	A. Narayan, D. Jones, J. C. Cornejo, M. M. Dalton, W. Deconinck
Citations	3
Analysis	Full AI Review Included

Precision MPCVD Diamond Microstrip Detectors for High-Precision Electron-Beam Polarimetry

Document based on: Narayan et al., “Precision Electron-Beam Polarimetry at 1 GeV Using Diamond Microstrip Detectors,” Physical Review X 6, no. 1 (2016).

Executive Summary

The research details the successful deployment of custom CVD diamond microstrip detectors for high-precision electron-beam polarimetry, a critical component for next-generation parity-violating physics experiments. Key findings that validate the use of MPCVD diamond in extreme conditions include:

Record Precision: Achieved a systematic uncertainty of 0.59% in electron beam polarization measurement at 1.16 GeV, surpassing the stringent requirements of the JLab Q_weak experiment.
Radiation Hardness Verified: The diamond detectors operated stably for two years while exposed to a cumulative electron radiation dose of 100 kGy with no measurable degradation in performance or charge collection.
Pioneering Application: This represents the first successful use of CVD diamond microstrip detectors specifically for particle tracking in a live, high-radiation experimental setting.
Detector Granularity: The use of high-granularity detectors (200 µm pitch Ti-Pt-Au strips) enabled measurement of the full Compton electron spectrum, significantly improving fit robustness and overall precision.
Future Feasibility: The demonstrated stability and precision confirm that diamond tracking detectors are the superior choice for future high-luminosity experiments (e.g., SOLID and MOLLER), which require polarization uncertainties below 0.4%.
Methodology: Precision was enabled by combining the radiation-hard diamond detectors with a high-intensity Fabry-Pérot laser cavity and a noise-suppressing, FPGA-based track-finding DAQ system.

Technical Specifications

The following hard data points were extracted relating to the diamond detector application and achieved performance:

Parameter	Value	Unit	Context
Beam Energy	1.16	GeV	CW Electron Beam (JLab Hall C)
Statistical Precision	< 1	% / hour	Routinely achieved measurement rate
Total Systematic Uncertainty	0.59	%	Total precision achieved
Future Uncertainty Goal	0.4	%	Required for SOLID and MOLLER experiments
Detector Material	Synthetic CVD Diamond	N/A	High-purity tracking substrate
Diamond Plate Dimensions	$21 \times 21 \times 0.5$	mm³	Size of individual detector planes
Diamond Thickness	500	µm	Thickness of the SCD wafer
Metalization Structure	Ti-Pt-Au	N/A	Electrode composition
Strip Pitch	200	µm	Center-to-center electrode spacing
Electrode Gap	20	µm	Insulating space between metal strips
Operating Bias Voltage	-300	V	High Voltage (HV) applied to detector back side
Radiation Exposure	100	kGy	Total dose received over 2-year running period
Signal-to-Background Ratio	O(10)	N/A	Achieved due to diamond insensitivity to synchrotron radiation
Intracavity Laser Power	~1.7	kW	Effective optical power at the interaction region

Key Methodologies

The precision measurement relies on robust material science and careful engineering integration, including:

Beam Steering and Interaction: A magnetic chicane was employed to vertically displace the 1.16 GeV electron beam, allowing it to interact at 1.3° with a high-intensity photon target.
Photon Source: Circularly polarized 532 nm laser light was injected into a Fabry-Pérot optical cavity, achieving a high effective gain (~200) and 1.7 kW of optical power at the interaction point.
Diamond Detector Fabrication: $21 \times 21 \times 0.5 \text{ mm}^3$ plates of synthetic CVD diamond were prepared.
- Metalization: A novel Ti-Pt-Au metalization scheme was used to deposit 96 horizontal microstrip electrodes on the front face, with a 200 µm pitch (180 µm metal, 20 µm gap).
- Mounting: Plates were attached to alumina carrier boards using silver epoxy, with Au traces and Al wire bonds connecting the strips to readout electronics.
Signal Detection and Readout: Scattered electrons were detected by four planes of diamond microstrip detectors operated under a -300 V bias.
- DAQ System: An FPGA-based Data Acquisition (DAQ) system implemented a real-time track-finding algorithm to achieve single-electron mode and suppress electronic noise by a factor of 100-200.
Polarization Control: Laser polarization uncertainty was minimized (0.18%) using a technique based on the optical reversibility theorem, analyzing reflected light at the cavity entrance to maximize the degree-of-circular-polarization (DOCP) in situ.

6CCVD Solutions & Capabilities

This research validates the critical role of high-quality CVD diamond in achieving extreme precision in high-energy physics. 6CCVD offers the specialized materials and customization services required to replicate this success and advance towards the 0.4% uncertainty goal for future experiments (SOLID/MOLLER).

Applicable Materials

To replicate or extend the performance detailed in this paper, researchers require electronic-grade, highly controlled SCD.

Single Crystal Diamond (SCD): Required for superior charge collection efficiency and maximum radiation hardness. The stability demonstrated over 2 years (100 kGy) confirms that high-purity SCD is the only viable material for long-term tracking in such environments.
Polycrystalline Diamond (PCD) / Boron-Doped Diamond (BDD): While SCD is ideal for tracking detectors, PCD or BDD may be suitable for high-current beam condition monitors or other ancillary radiation detectors requiring specialized thermal or electrochemical properties.

Customization Potential

6CCVD’s internal capabilities directly address the specific engineering requirements of high-precision microstrip detectors:

Requirement from Paper	6CCVD Solution & Capability	Engineering Benefit
Custom Dimensions	Plates up to 125 mm (PCD) / Precision Laser Cutting	We readily accommodate the $21 \times 21 \text{ mm}$ size and offer custom laser shaping/cutting of SCD/PCD to match complex detector geometries and mounting constraints.
Material Thickness	SCD Thickness: 0.1 µm to 500 µm	We provide the required 500 µm (0.5 mm) thick electronic-grade SCD, ensuring consistency in charge collection distance and material budget.
Specific Metalization	In-House Ti-Pt-Au Deposition	The Ti-Pt-Au (Titanium/Platinum/Gold) electrode stack is a standard capability. We ensure optimal adhesion (Ti), diffusion barrier (Pt), and low-resistance contact (Au). Customization available for Ti, Pt, Pd, W, Cu.
Surface Quality	Ultra-Polished Surfaces (Ra < 1 nm for SCD)	Low surface roughness is critical for reproducible micro-lithography of 200 µm pitch strips and ensuring low noise characteristics in high-granularity detectors.
Global Support	Global Shipping (DDU/DDP)	Reliable, secure global delivery ensures sensitive detector materials arrive safely, supporting international collaborations like those detailed in the research.

Engineering Support

The challenges of achieving sub-percent precision rely heavily on material consistency and controlled fabrication.

Consultation for Precision: 6CCVD’s in-house PhD engineering team specializes in the material science of CVD diamond for high-energy physics and radiation detection applications. We can assist researchers in material selection and specification tailoring necessary to meet the demanding polarization uncertainty goals of 0.4% or better for projects like SOLID and MOLLER.
Process Control: We guarantee high crystal quality and precise thickness control required to minimize strip-to-strip efficiency variations, a factor noted in the paper as a potential concern for future, more demanding applications.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

We report on the highest precision yet achieved in the measurement of the polarization of a low-energy, O(1 GeV), continuous-wave (CW) electron beam, accomplished using a new polarimeter based on electron-photon scattering, in Hall C at Jefferson Lab. A number of technical innovations were necessary, including a novel method for precise control of the laser polarization in a cavity and a novel diamond microstrip detector that was able to capture most of the spectrum of scattered electrons. The data analysis technique exploited track finding, the high granularity of the detector, and its large acceptance. The polarization of the 180-μA, 1.16-GeV electron beam was measured with a statistical precision of <1% per hour and a systematic uncertainty of 0.59%. This exceeds the level of precision required by the Q[subscript weak] experiment, a measurement of the weak vector charge of the proton. Proposed future low-energy experiments require polarization uncertainty <0.4%, and this result represents an important demonstration of that possibility. This measurement is the first use of diamond detectors for particle tracking in an experiment. It demonstrates the stable operation of a diamond-based tracking detector in a high radiation environment, for two years.

Tech Support

Original Source

DOI: None