A question about some old news (found it on sci.astro.research). I know that a lot of what will follow is half baked. So, please be patient with me. I'm just asking questions.
(From: AIP listserver (email@example.com).)
Until now impractical because lenses absorb too much or refract too little at X-ray wavelengths, one has been developed by scientists at the European Synchrotron Radiation Facility (ESRF) in Grenoble. The compound lens consists simply of a series of closely-spaced 0.6- mm holes drilled in a piece of aluminum. With this lens, a 14- keV beam of x rays was focused to a an 8-micron spot size. (A. Snigirev et al., Nature, 7 November 1996.)
Kwiat, Weinfurter and Zeilinger ("Interaction-Free measurement" in Physical Review Letters, Vol.74, no.24, pp. 4763-4766, June 12, 1995, co-authored with Herzog and Kasevich; "Quantum Seeing in the Dark", Scientific American, November 1996) made reference to the theoretical possibility of reducing the intensity of x-rays used in medical photography without sacrificing the quality of the image produced.
(In case someone reading hasn't seen the article yet, the question it poses is how one could tell if an object was present in a chamber without any interaction occurring (probably).)
The approach used in "interaction free measurement", is to perform a modified double slit experiment give a photon a choice of pathways, with the object to be detected placed along one of the pathways (if it is present at all). If it is present, and opaque, an individual photon passing through can't interfere with itself.
I'm wondering: If the object is translucent, will we see a contribution to the wave function at the other end of the apparatus, from the obstructed pathway, whose amplitude will be proportional to the probability that absorption won't occur ?
Yielding an intermediate interference pattern consisting of a dot (as one would see if no self-interference occurred), at which a photon has a probability of (1/2) (prob. of absorption) of being detected at, superimposed with a normal interference pattern whose amplitude is multiplied by (1 - prob(absorption)), (1/2) (prob. of absorption) being the probability that the photon will go down the obstructed pathway (1/2 chance) and then get absorbed by the target ? Allowing one to determine the absorption probability by looking at the intensity of the dot on the screen ? The practical difficulty they foresaw lay in the nonexistence of x-ray optics. I was wondering if this this difficulty seemed less insurmountable, now ? For what they wanted, it would be necessary to produce an optical component capable of rotating polarized x-ray radiation. In particular, I was wondering if one could produce a high resolution CAT scanner that would do less damage to the patient. Producing holes drilled in a thin sheet, to sub micrometer accuracy, would actually be well within the current capabilities of microfabrication technology. (An individual microdevice on a chip, the last time I checked, could be as small as .55 microns, a few years ago. By now, it's almost certainly smaller. Needless to say, photolithography was NOT the technique used - that gets you down to about 1 micron). Meaning that one could conceivably produce an extremely regular hole, though the process would be expensive, and slow. Microfabrication techniques, naturally, have generally been used on thin films in the past. Meaning, that you might be in for a LONG wait while those holes form. But it COULD be done. Given the obvious problem of bubble formation if wet etching is attempted on a metal substrate (acid on metal), plasma etching using a reactive gas such as Fluorine would seem a natural choice. As for finding a type of resist that will adhere to aluminum, the electrical connections on a chip tend to be made out of deposited aluminum, so whichever brand of photoresist is used to pattern those connections might serve nicely. Now, if it turns out that the brand you obtain doesn't stand up to plasma etching conditions very well, you might still be able to make due if you can find a second variety which does, and adheres to the first variety, which ends up being reduced to playing the role of glue. Probably, though, there will be a simpler, cheaper, easier to work with material available to promote adhesion in this fashion. I was also wondering if reflecting optics might not be possible. In my extremely naive introduction to Computational Electromagnetics course (Maxwell's laws only, no quantum theory of radiation involved) I remember that one could show that an idealized perfect conductor would reflect incident electromagnetic radiation. Would a superconductor be close enough to being one of those to reflect x-rays ? If so, how intense a flux of x-rays could such a reflector handle before superconductivity broke down. (Excessively strong EM fields destroy superconductivity, right ?) Would it be high enough, for practical x-ray photography to be done with a beam of such low intensity ? While even high temperature superconductors require liquid nitrogen temperatures (right ?) this problem might not be insurmountable. Perhaps one might set up sort of a thermos arrangement, with one monocrystalline quartz container inside the other, the x-ray mirror inside the inner container, the connection between the two containers and the liquid nitrogen hookup to the inner container passing behind the mirror, the hookup consisting of a many thin, flexible sheets of material rather than a few bolts (to accommodate thermal expansion - the outside container is at room temperature), the containers being properly silverized (this is a thermos, after all), a good hard vacuum being present between the containers. Like I said, though, very half baked. I'm not even sure which phase transitions SiO2 undergoes as it gets that cold, one of them might be a destructive one. While SiO2 is generally polycrystalline in industrial use, a modification of the Czochralski method has been used to produce monocrystalline Gallium Arsenide (a layer of slag is left over the melt in order to prevent evaporation), so maybe it would be possible to do so with another nonatomic substance, without getting little flecks of elemental silicon embedded because some of the oxygen has diffused out of the melt. A monocrystalline container's contribution to the image produced by a collimated x-ray beam passing through it would be a regular, predictable diffraction pattern which one might be able to remove from the final image. The technology already exists to produce a very regular parabolic reflector (the same used to produce telescope mirrors). So the idea, again, half baked and untested, I have in mind is to form a parabolic surface, and then chemical vapor deposit a superconducting film on it. This, incidentally, has already been done, though on a flat surface, with a relatively high temperature superconductor. In this case, one would have to do the deposition quite slowly, of course, because one will have to change the orientation of the parabolic surface a good many times to get anything resembling an even coating, as the thickness should be dependent on the inclination of the surface during deposition. Slow, and expensive. If it works, at all. Put a point source of x-rays inside the inner chamber, and project onto the mirror, with a cup around one side of the source, blocking the departure of x-rays which aren't reflected off the mirror. The source is placed at the focus of the paraboloid defining the shape of the reflector. High school geometry - after reflection, the x-rays will follow parallel paths out of the mirror. Again, old technology reapplied. What you've done is build a searchlight, only with x-rays, instead of visible light. Same design principle, though. The high accuracy attainable by surface grinding translates into extremely good performance in producing parallel x-rays. You can see, now, why we wanted as even a layer of superconducting material deposited as possible. Irregularities in the deposition will scatter x-rays in undesired directions. The shadows wouldn't be as crisp, if you will, because the lighting is more diffuse. Result : collimated x-rays, and a crisper image ? Allowing for increased resolution if applied to computer aided tomography ? So, the question.....am I out of my mind on this ? Is it feasible ? What should be changed ? I hope the description I gave isn't too vague. My own background : thesis stage PhD Math, MS Electrical Engineering, but a terminal Bachelor's in Physics, alas. So, I might be of assistance in answering fabrication questions, maybe; would be of more help on computational ones, and definitely need your help on theoretical physics questions. As I'm sure you could tell. E-mail is appreciated, but followups even more. ____________________________________________________________________________ Newsgroups: sci.electronics.design Organization: The University of Iowa Joseph Dunphy (firstname.lastname@example.org) wrote: > The practical difficulty they foresaw lay in the nonexistence of x-ray > optics. I believe my father has a patent on X-ray and gamma ray optics made using lead (that's Pb) phase-plates. He was working fairly hard with various radiologists on using these for radioisotope imaging, but new technologies like CAT scans and PET scans came along and people lost interest in his approach. What's a phase plate? It's a set of precisely dimensioned concentric rings; an appropriate phase plate will diffract light to a focus in exactly the same way a lens refracts it. A phase plate that actually brings gamma rays to a focus would require infinitesimal dimensions, but my father's trick was to use a practically sized phase plate to record an X-ray or gamma ray hologram that could be viewed with visible light. The phase plate dimensions depended on the difference in wavelength between the light used to record the hologram and the light used to view it. An interesting trick, and the results were subject to most of the problems we associate with monochromatic holograms (grainy image, difficulty in viewing, etc). Doug Jones email@example.com _____________________________________________________________________ Don't worry about the mental oven being set too cool (i.e. half-baked ideas)--what you ask has some MAJOR physical limitations, but at least you aren't talking about why we aren't extracting gold from Mars.....You do have interesting ideas, and deserve better than the 'does anybody read this' snottiness of someone who posted on the thread. The most serious limitation to the proposal you make is in the 'punch' of an Xray photon and the availability of electronic states in solids. In reflection of visible light, there is little opportunity for the light to cause serious electronic phenomena to occur in the metal (i.e. you are usually WAAAAY under the photoelectric threshold). With an Xray (say, a C xray at ~286eV), you have plenty of energy to cause core-electron excitations. Even if the excitation doesn't cause photoemission, you will have essentially lost the wavelength 'identity' of your incident Xray. Much of your light (Xrays) gets lost in causing electronic excitation in the mirror, leaving little for your output beam. This is not to say that xray focussing is not possible. At the synchrotron sources (I'm most familiar with the IBM line at Brookhaven), Xray focussing down to ~300Å is performed with Fresnel Zone Plates (similar to those used in camera eyepieces), with the proviso that this is a focus point, not a collimated beam. Other focussing techniques include use of diffraction mirrors, such as are used in the PHI Q2000 ESCA spectrometer. In this case, the Xrays diffract (not reflect) from the mirror, and there are losses in the optics due to photoelectron formation (can't make an omelette without breaking a FEW eggs). Regards: David Neiman Pump it down, Bake it out, Drop the beam, Or do without! __________________________________________________________________________ From: Johannes Ullrich To: Joseph Dunphy
Subject: Re: Some questions about x-ray optics, and CAT Joseph Dunphy wrote: > So, if you are reading this, and are posting from a faculty account, > or > some other account that would indicate professional credentials, could > you > give an indication that you saw this, even if you don't feel like > commenting on it ? It would be nice to know that I'm not just tilting > at > windmills, here. Hi. I am not usually reading this newsgroup. However, I am scanning newsgroups for certain keywords (e.g. x-ray optics) using DejaNews. I found this technique very effective in cutting down on the noise level usually found in newsgroups. I subscribe to a few newsgroups which are more specific to my interests and have less trafic. I am currently working as a researcher for 'X-Ray Optical Systems (www.xos.com)' and have a Ph.D. in physics (thesis work related to x-ray optics). A few comments on your post (the article from the AIP newsserver and the CAT scanner idea): First of all: There are many different kinds of x-ray optics. The special difficulty is the nonexistence of a material that is transparent to x-rays and has a index of refraction that is significantly different from 1 (vacuum). Glasses as they are used for visible light, have transperancies of over 99 % for many meters of thickness. Typically, the index of refraction is larger than 1.5 maybe even as high as 3 (just to give some numbers, I am not that aware of actual specifications for different numbers). Moreover: Visible light interacts with outer electrons. This allows the variation of the index of refraction and absorbtion by slightly changing the composition of a glass. X-rays interact with inner core electrons which are only slightly effected by the chemical environment of the material. A refractive lens for x-rays would have to be very thick (meters) and all the x-rays would be absorbed within the lens. The article below shows an experiment that uses a series of lenses which overcome this problem (partially). It works because the material (Al) does not absorb x-rays too badly and the energy used (17 keV) is high enough to penetrate Al with reasonable losses. Now something very special about x-rays. The index of refraction is actually smaller than one. A typical index of refraction is 1-10^(-6) (e.g. almost 1 but a little bit less). The second article: Sounds kind of strange to me and I have to think more about it. But one problem with x-rays is the short coherence length of typically a few micron (depends on the source). > : The practical difficulty they foresaw lay in the nonexistence of > x-ray > : optics. I was wondering if this this difficulty seemed less > : insurmountable, now ? For what they wanted, it would be necessary > to > : produce an optical component capable of rotating polarized x-ray > : radiation. Could their approach be modified to produce such an > optical > : component ? There are x-ray polarizers (crystals) I am not sure if magnetic multilayers can be used to rotate the polarization. > : reflect incident electromagnetic radiation. Would a superconductor > be > : close enough to being one of those to reflect x-rays ? If so, how > : intense a flux of x-rays could such a reflector handle before X-rays would not 'see' the superconductor. However, x-rays can be reflected at grazing incidence and this is done a lot. The one problem is the small critical angle for total reflection (typically about 0.2 deg.). X-ray mirrors are know to be subject to radiation damage. This occurs mostly at high intensity synchrotron sources. X-rays can be reflected at larger angles from crystalline material (x-ray diffractions). But only one particular energy will be reflected. > : While even high temperature superconductors require liquid > nitrogen > : temperatures (right ?) this problem might not be insurmountable. Many x-ray detectors use liquid nitrogen. Not a technical problem. There are some cooled x-ray mirrors at synchrotron. It is tricky but can be done. > : > : Result : collimated x-rays, and a crisper image ? Allowing for > : increased resolution if applied to computer aided tomography ? Overall: The problem of x-ray imaging is not a parallel bea, but the contribution of scatter. X-rays are scattered inside the patient. Scatter grids (lead strips) are used after the patient to remove some of that scattered radiation. The fan beam as obtained from a point source is very adequate for most imaging. One problem is the size of the source. the smaller the source the crisper the image. Hope this help. Let me know if you want to know more. Johannes. Subject: Re: producing x-rays with old vacuum tube RE: Homemade X-ray machine. The potential hazards here cannot be overemphasized. Any device that produces ionizing radiation has the potential for long term harm. Whenever a vacuum tube type device and voltages over 10kV or so are put together there is the potential to produce X-rays. Old TVs with tubes in the HV section had shielding to protect against X-rays from the HV rectifier and regulator. The CRT including moderm CRTs are constructed with leaded glass for a similar reason. I am not saying that such experimentation should be avoided - just that you understand all of the ramifications and take appropriate precautions. Radiation exposure is something you cannot use an Undo command on. Having said all that. if you are really want more info on these sorts of projects, I suggest you contact: From: Steve Hansen Newsgroups: sci.electronics Subject: Vacuum Newsletter #2 - Generating X-Rays with Receiving Tubes ************************************************************************** * ----------------------------------- ---------------- * * the Bell Jar (electronic version) #2 (October 1994) * * ----------------------------------- ---------------- * * * * This newsletter contains material which has been extracted from the * * hard-copy newsletter of the same name. Devoted to the vacuum * * experimenter, the intent of "the Bell Jar" is to broaden interest * * in vacuum technology through useful discussions of theory and * * technique, and to present ways in which a variety of apparatus may * * be assembled using common and inexpensive materials. Information * * on "the Bell Jar" may be obtained by sending email to the editor, * * Steve Hansen, at firstname.lastname@example.org or by writing to 35 Windsor Dr., * Amherst, NH 03031. Please feel free to circulate this electronic * * version, intact, to others who might be interested in the subject * * matter. New numbers will be mailed at approx. quarterly intervals. * * Email subscriptions are free and may be obtained by contacting the * * editor. Comments, contributions and criticisms are always welcome. * * Copyright 1994, Stephen P. Hansen. * **************************************************************************