Hi Richard,
Interesting topic raise you did... On a philosophical level, I would define a microbeam as a beam which matches in size with a microcrystal. Then question becomes what is a microcrystal? If we wanted to be purely scientific, though, we should not be measuring anything in mm or microns and everything should be in cm, shouldn't it? And don't get me even started about inches... But all the jokes aside, when "standard" beams used to be 200 microns or even larger, it was probably natural to call 50 micron beam a microbeam. However, many beamlines now have a standard beam which is on the order of 20-60 microns and therefore the meaning of "microbeam" is also evolving. APS should not be broadly blamed for introducing "minibeam" - it was just our beamlines (GM/CA-CAT) that did that. Reasons for discriminating 5-10 micron beams (minibeam) from ca 1 micron (microbeam) might have been not so much their size but what it involved to achieve these sizes. 5 micron and 1 micron beams, at least in our facility, required drastically different beamline optics, tolerances on the goniometer performance, hutch temperature stability, number of sleepless nights etc etc etc. Therefore, we wanted to internally discriminate between these two, very different efforts. Then the term "leaked" out in the community. Who wants to branch out into discussing the habit of using "microbeam" and "microfocus" interchangeably? Cheers, N. Ruslan Sanishvili (Nukri), Ph.D. GM/CA-CAT Biosciences Division, ANL 9700 S. Cass Ave. Argonne, IL 60439 Tel: (630)252-0665 Fax: (630)252-0667 rsanishv...@anl.gov -----Original Message----- From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf Of Richard Gillilan Sent: Tuesday, April 21, 2009 11:00 AM To: CCP4BB@JISCMAIL.AC.UK Subject: [ccp4bb] How small is a microbeam? Just an interesting question of semantics that annoyingly comes up from time to time when people are comparing x-ray beam diameters. What counts as "microbeam?" Of course "micro" has the precise meaning in SI as being a factor of 10^-6. The problem is that the prefix "micro" just means "extremely small" in common usage. The term is used very confusingly everywhere. Take microwaves. Microwaves have wavelengths from 1 millimeter to 1 meter. Go figure. They're just "extremely small" radio waves. Now I believe that it is more widely accepted that "nanofabrication" is making objects that are measured in nanometers. So shouldn't microbeams rightly be x-ray beams with diameters measured in microns (i.e. < 1 mm and >= 1 micron). Of course this makes all crystallography beams microbeams and everything smaller than 1 micron a nanobeam. That won't be popular. I've always called anything smaller than 50 microns microbeam because that's about as small of an aperture-based collimator as we could make. So a user should ask for "microbeam" if regular collimator is too large. I was always puzzled at the APS habit of calling this "minibeam", but it's starting to sound better all the time. But in practice, I think "microbeam" sometimes means "smaller beam than yours." So microbeam used to be 30 microns, 10 or 5, now maybe 1 micron. Pretty soon no microbeam at all. I think maybe I'll stick with "small", "smaller than usual", and someday "extremely small." I'd love to hear people's opinion on the topic. Richard Gillilan MacCHESS
