Determination of X-ray flux using silicon pin diodes

Robin L Owen,Elspeth F Garman,James M Holton,Clemens Schulze-Briese

doi:10.1107/s0909049508040429

Abstract

Accurate measurement of photon flux from an X-ray source, a parameter required to calculate the dose absorbed by the sample, is not yet routinely available at macromolecular crystallography beamlines. The development of a model for determining the photon flux incident on pin diodes is described here, and has been tested on the macromolecular crystallography beamlines at both the Swiss Light Source, Villigen, Switzerland, and the Advanced Light Source, Berkeley, USA, at energies between 4 and 18 keV. These experiments have shown that a simple model based on energy deposition in silicon is sufficient for determining the flux incident on high-quality silicon pin diodes. The derivation and validation of this model is presented, and a web-based tool for the use of the macromolecular crystallography and wider synchrotron community is introduced.

Highlights

With the recent resurgence of interest in radiation damage progression and avoidance in crystals used for macromolecular crystallography (MX), knowledge of the dose absorbed during an experiment is becoming increasingly important
The theoretical model used for converting measured current into flux is described, followed by the details of experiments to calibrate a pin diode against a scintillator, measure diode thickness, assess the possible error in the simple model introduced by charge carrier recombination, and to check device linearity
A prerequisite for estimation of the dose absorbed by a crystal is the incident photon flux

Summary

Introduction

With the recent resurgence of interest in radiation damage progression and avoidance in crystals used for macromolecular crystallography (MX), knowledge of the dose absorbed during an experiment is becoming increasingly important. Dose is the amount of energy per unit mass (J kgÀ1 or Gy) deposited in the sample and is proving to be the fundamental coordinate of radiation damage progression (Ravelli & Garman, 2006). This is because at cryotemperatures of around 100 K the damage appears to depend on the accumulated energy absorbed by the sample, regardless of the time taken to deposit it [i.e. is largely independent of dose rate (Owen et al, 2006; Leiros et al, 2006; Sliz et al, 2003)].

Methods

Results

Conclusion