Making Tracks

Science Nugget: August 04, 2000


Well, from the point of view of academic family, we X-ray astronomers spring from research groups specializing in cosmic rays. Not only did these remarkable particles (the discovery of which led to an early Nobel prize) serve as a laboratory for basic physics, they're also astronomical. No doubt the linkage of particle physics and cosmology has deep roots! In fact X-rays are really photons, the bread and butter of regular astronomers, and not material particles, but some of the experimental techniques remind one more of high-energy physics.

A phenomenon

The CCD detector of SXT recorded the track of a high-energy charged particle. It does this frequently, even though particles are few and the camera is in fact shielded from them in any case. Yohkoh flies well below most of the Van Allen belts, and is shielded by the Earth's magnetic field from all but the most energetic primary cosmic rays. But here is what we saw recently:

The above figure shows a curious cosmic-ray track observed by Yohkoh SXT at 15:17:34 UT, July 25, 2000. The copy at the bottom has a straight line fitted roughly to the end pixels. The curved features at the top are coronal magnetic loops. There are a couple of peculiarities. First, the track is slightly curved - at an eyeball estimate, about 1.4 pixels at maximum. The SXT pixels are 18.28 microns, so this is about 25 microns of curvature. Second, the track is brightest at the two endpoints. The length of the track is about 1.4 mm.

Following the discovery of this track, the SXT group and friends have been exchanging e-mail about what it represents. All no doubt agree that it is not important science, but just interestingly unusual. Here are some of the comments that have resulted:


  • The CCD may have a non-flat surface, which could give the appearance of out-of-plane curvature.
  • We don't seem to know exactly how thick the CCD pixels are (B. Handy points out that typically, pixels are about as thick as they are wide - so perhaps 18 microns).
  • Other bright points in the image are probably particle hits with velocities not so tangential to the CCD surface.
  • The Yohkoh orbit database is not up-to-date, so the exact location isn't known yet, but of course we get these hits in the SAA. (But see the nice map of the Yohkoh orbit, showing the orbit in grey color, and the Yohkoh satellite as a small icon over Argentina, for a confirmation of this).
  • We haven't done any kind of systematic search, and don't know how unusual this is - but see below please.
  • From the curvature, a back-of-the-envelope calculation gives a speed of 300 km/s, if a proton - about 1 MeV?.
  • Low-energy protons should not be found within the CCD, since it's shielded from them (as pointed out by Y. Ogawara).
  • Perhaps the morphology of the track gives the answer. The track is bright/faint/bright, whereas a particle entering from the outside might be expected to be faint/bright/faint as it grazes its way through. So, this might be a knock-on secondary particle actually created in the CCD, and also (amazing if true!) stopping there after its heroic 1.4-mm run.
  • Curvature appears to increase from bottom to top, suggestion that the the particle was moving in that direction and slowing down via collision losses (Loren Acton).
  • In principle the density of ionization in the CCD, coupled with knowledge of the particle's "rigidity" (like the momentum), tells one the atomic number of the particle (T. Takahashi).
  • The presence of an almost-parallel track suggests a shower of particles originating somewhere outside the CCD, perhaps in the camera housing or shielding (T. Takahashi).

    How rare?

    First of all (theoreticall!) how much exposure does the CCD have? It turns out that the CCD contains a volume of about .006 cm^3 of Si, and that it has been exposed for tracks perhaps for a total of 5 million seconds since launch. Spreading the CCD's 6 microliters out into a layer roughly as thick as the range of a cosmic ray (say 50 gm/cm^2), one can estimate a paltry exposure of some 600 cm^2-sr-sec for the whole nine years. No wonder people don't use CCDs as primary detectors for high-energy cosmic rays! At this exposure we might expect to see only about one "star" interaction event (ie, a true nuclear reaction - would be spectacular, no doubt) in the whole data set.

    Instead, what we almost certainly see here is a low-energy secondary particle produced by scattering within the Si of a proton (say, 100 MeV) from the Van Allen belt - from the nice map of the Yohkoh orbit you'll see that Yohkoh was flying above Argentina at the time of the event, and this is as close as the orbit comes to the magnetosphere (it turns out).

    A quick search for similar events turned up these beauties:

    On the left, a nice straight track, with the faint-bright-faint character expected of a particle arcing through the CCD tangentially; on the right, a champion long track with a straight line fitted. This beauty is more than 6 mm long - it vanishes at the top of the CCD. The left image is "half resolution", 2x2 pixel sums; the right image is "quarter resolution", 4x4 sums. Both tracks above are a lot fainter than the curved track. This means that it had a higher density of ionization - hmmm. If anybody would like to try to estimate the particle Z (is it perhaps one of the CCD's own Si atoms, or a spallation product from one of them, or just a humdrum proton?) ask the undersigned and I'll provide an estimate of charge per unit length.

    "Snowstorm" at the Lagrangian point

    As a postscript to this nugget, but it really deserves a full writeup of its own, we show you the "snowstorm" detected by SOHO following the X-class flare of July 14, 2000. The sequence of events is as follows: a huge flare erupts on the solar disk, spotted by various observers; the CME resulting from this then appears in the SOHO coronagraphs; en route to the Earth or wherever, the shock wave caused by the CME accelerates high-energy particles; these particles zip right over to SOHO and cause a cloud of streaks in their next set of images. Yohkoh is protected from these particles because of its low orbit, but the Lagrangian point is out there fully exposed, some 1% of the way from the Earth to the Sun. So, one image is worth a thousand words, but be careful - the link is 1 MB:

    Sam Freeland provided this image, and noted at the same time that one or two long curved tracks are present (bottom; upper left). In fact there are many things to see, including what look like actual vertices (eg, at pixel coordinates [805, 444]).

    August 2, 2000

    Hugh Hudson <>;