On the Single-Photon-Counting (SPC) modes of imaging using an XFEL source

At a Glance

Metadata	Details
Publication Date	2015-12-14
Journal	Journal of Instrumentation
Authors	‪Zhehui Wang
Institutions	Los Alamos National Laboratory
Citations	13
Analysis	Full AI Review Included

Technical Documentation & Analysis: Single-Photon-Counting XFEL Detectors

This document analyzes the requirements for high-speed, high-efficiency X-ray Free-Electron Laser (XFEL) detectors, focusing on the critical role of Chemical Vapor Deposition (CVD) diamond in achieving Single-Photon-Counting (SPC) modes.

Executive Summary

The research outlines the necessity of using low-Z semiconductors, particularly diamond, in a Multilayer 3D detector architecture to meet the demanding requirements of the proposed 42-keV MaRIE XFEL source (GHz frame rate, >50% efficiency).

Application Focus: High-speed, hard X-ray imaging detectors operating in Single-Photon-Counting (SPC) or Weak SPC modes.
Material Superiority: Diamond (C) is identified as a superior material due to its extremely high saturated electron drift speed (270 µm/ns), enabling the necessary GHz frame rates.
Architecture Requirement: To achieve high efficiency (>50%), the total sensor thickness must be large (42 mm), necessitating a Multilayer 3D architecture composed of many thin-film diamond layers.
Thin Film Specification: Strong SPC mode requires ultra-thin diamond layers, specifically 14 µm thick, while Weak SPC mode requires 17 µm thick layers (for N^max = 10³ photons/pixel).
Fabrication Challenge: The proposed architecture requires hundreds of high-purity, ultra-thin diamond sensors (< 100 µm) integrated with ASICs, a capability 6CCVD is uniquely positioned to fulfill.
Compton Mitigation: Diamond’s low atomic number (Z) and required layer separation (~2 mm) help mitigate efficiency loss and duplicated counts caused by Compton scattering, which dominates in Si at 42 keV.

Technical Specifications

The following parameters define the operational environment and material requirements for diamond (C) sensors in the proposed 42 keV XFEL detector system.

Parameter	Value	Unit	Context
Target XFEL Photon Energy	42	keV	MaRIE XFEL source
Required Detection Efficiency	> 50	%	Minimum requirement
Required Frame Rate	1	GHz	High-speed imaging
SCD Saturated Drift Speed (v_d)	270	µm/ns	Charge collection velocity in C(Diamond)
SCD CSDA Electron Range (42 keV)	10.2	µm	Lower limit for pixel size
SCD Thickness (3 Attenuation Lengths)	42	mm	Required total thickness for >50% efficiency
Maximum Strong SPC Thickness (T_SPC)	14	µm	Max layer thickness for N^max = 10³ photons/pixel
Maximum Weak SPC Thickness (T_WSPC)	17	µm	Max layer thickness for N^max = 10³ photons/pixel
Required Layer Separation (C)	~2	mm	To reduce double-scattering fraction to < 5%
Pixel Size (Δx=Δy)	25 - 100	µm	Based on sample size and resolution requirements

Key Methodologies

The research focuses on defining the physical and architectural requirements for achieving Single-Photon-Counting (SPC) modes in hard X-ray detection.

Direct Detection Analysis: Analysis of semiconductor sensors (C, Si, Ge, GaAs, CdTe) for direct X-ray conversion, focusing on the increasing dominance of Compton scattering over photoelectric absorption at 42 keV.
SPC Mode Definition:
- Strong SPC Mode: Defined by allowing no more than one photon interaction (absorption + inelastic scattering) per pixel (N_p ≤ 1).
- Weak SPC Mode: A relaxed condition allowing multiple absorptions but no more than one Compton scattering per pixel.
Thickness Determination: Calculation of the maximum allowed sensor thickness (T_SPC and T_WSPC) based on the X-ray flux density (up to 10¹⁰ µm^-2) and the SPC interaction limits.
Multilayer 3D Architecture Proposal: Introduction of a Multilayer 3D detector architecture to overcome the conflict between the need for high total thickness (for efficiency) and low individual layer thickness (for fast charge collection and high frame rate).
Thin Film Camera Requirement: Each layer must function as a self-sufficient 2D camera, requiring sensor thickness < 100 µm and integrated thin ASICs.
Double-Scattering Mitigation: Calculation of the necessary layer-to-layer separation (2 mm for diamond) to reduce duplicated counts caused by Compton scattered photons.

6CCVD Solutions & Capabilities

6CCVD is the ideal partner for developing and scaling the high-purity diamond sensors required for next-generation XFEL Single-Photon-Counting detectors. Our capabilities directly address the material specifications and architectural challenges outlined in this research.

Applicable Materials

To achieve the high charge collection speed (270 µm/ns) and radiation hardness necessary for GHz XFEL operation, High Purity Single Crystal Diamond (SCD) is the required material.

Optical Grade SCD: Ideal for the sensor layer due to its extremely low defect density, ensuring maximum carrier mobility and minimizing charge trapping, which is critical for achieving the required sub-nanosecond charge collection times.
SCD Substrates: Can be provided up to 10 mm thick for specialized support structures or as starting material for subsequent thinning processes.

Customization Potential

The Multilayer 3D architecture demands hundreds of ultra-thin, precisely dimensioned diamond films integrated with ASICs. 6CCVD specializes in meeting these exact specifications:

Research Requirement	6CCVD Capability	Technical Advantage
Ultra-Thin Films (14 - 17 µm)	SCD and PCD thickness control from 0.1 µm to 500 µm.	Enables precise fabrication of T_SPC and T_WSPC layers necessary for SPC modes.
Large Area/High Volume	Custom dimensions for plates/wafers up to 125 mm (PCD).	Supports scaling up the detector area and high-volume production of multiple layers required for the ‘billion-pixel’ detector concept.
Integrated Contacts	Internal metalization services: Au, Pt, Pd, Ti, W, Cu.	Essential for creating the planar or 3D deeply entrenched electrodes required for charge collection and integration with the onboard ASICs.
Surface Quality	Polishing capability: Ra < 1 nm (SCD).	Ensures optimal interface quality for metalization and minimizes surface defects that could interfere with charge collection efficiency or noise.

Engineering Support

The transition from a 2D hybrid structure to a Multilayer 3D detector architecture presents significant engineering challenges, particularly concerning material purity, electrode design, and thinning processes.

Material Selection: 6CCVD’s in-house PhD team provides expert consultation on selecting the optimal SCD grade and orientation to maximize saturated drift velocity and minimize noise for Single-Photon-Counting XFEL Detector projects.
Custom Fabrication: We offer advanced processing, including precision laser cutting and thinning, to achieve the required layer dimensions (< 100 µm) and aspect ratios necessary for 3D electrode configurations.
Global Supply Chain: We ensure reliable, global shipping (DDU default, DDP available) to support international research collaborations and XFEL facility development.

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

View Original Abstract

In this study, the requirements to achieve high detection efficiency (above 50%) and gigahertz (GHz) frame rate for the proposed 42-keV X-ray free-electron laser (XFEL) at Los Alamos are summarized. Direct detection scenarios using C (diamond), Si, Ge and GaAs semiconductor sensors are analyzed. Single-photon counting (SPC) mode and weak SPC mode using Si can potentially meet the efficiency and frame rate requirements and be useful to both photoelectric absorption and Compton physics as the photon energy increases. Multilayer three-dimensional (3D) detector architecture, as a possible means to realize SPC modes, is compared with the widely used two-dimensional (2D) hybrid planar electrode structure and 3D deeply entrenched electrode architecture. Demonstration of thin film cameras less than 100-μm thick with onboard thin ASICs could be an initial step to realize multilayer 3D detectors and SPC modes for XFELs.

Tech Support

Original Source

DOI: https://doi.org/10.1088/1748-0221/10/12/c12013