Active beam attenuators for synchrotron radiation

Attenuating materials are employed on synchrotron beam lines to achieve photon flux modulation as well as beam hardening of the x-ray spectrum. In some experiments it is desirable to maintain the intensity of the beam on the sample at a constant value as the current in the synchrotron decays during a fill cycle. This is often done by attenuating the incident beam with a set of discrete, thin foils. To change the amount of material in the beam path typically involves inserting or redrawing a number of foils. The use of discrete foils imposes practical limits on the number of thickness values available as well as the smallest thickness increment. Micro-machined attenuators can avoid these restrictions by offering either continuous variation of the material thickness or through the implementation of a large number of small thickness steps. Through a combination of photolithography and appropriate device geometry sub-micron thickness increments can be fabricated. Device geometries such as staircase, low-angle triangle, and overlapping triangles will be described. Fabrication of these devices by direct micro-machining of materials such as silicon as well as micro-molding of various polymers can be done relatively easily. In addition, by manufacturing a silicon diode into the attenuator the absorbed fraction of the beam can be continuously monitored. By using a feedback loop where the attenuator thickness is varied based on a downstream beam monitor, it should be possible to maintain the photon flux on a sample to vary by less than 0.5%. The performance of a variety of these devices at the Advanced Light Source is presented.