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The Question

(Submitted November 14, 1996)

  1. Why is the shape of shower different for a gamma-ray compared to a cosmic ray? Is it to do with the initial interaction of the gamma-ray produces a positron/electron pair that go off at some angle.

  2. At the CGRO learning center I was expecting to see plots of the pool of Cerenkov light to be elliptical for a gamma-ray and roughly circular for a cosmic ray. I don't see this in the plots shown, especially the cosmic ray plot, which shows a scattered distribution that I don't understand.

  3. On the actual imaging telescope why are there multiple mirrors instead of just one dish with the PMTs behind focal plane. Also why are the individual mirrors hexagonal and not say squares or pentagons?

  4. Since the pool of light is much bigger than the area of telescope how can the shape of the pool be determined? Does the intensity of light fall off or increase in concentric contours from the edge of the pool to the center?

The Answer

> 1. why is the shape of shower different for a gamma-ray
> compared to a cosmic ray? Is it to do with the initial
> interaction of the gamma-ray produces a positron/electron
> pair that go off at some angle.
>

Yes, that is the key. The initial interaction of the gamma-ray is a pair production interaction which has a relatively small opening angle for the created pair. On the other hand, the first cosmic ray interaction is some exotic nuclear interaction where many different particles can result, each splintering off in different directions, some at large angles. Further interactions tend to accentuate this. Even though the gamma-ray showers broaden lower down in the atmosphere as the particles lose energy, the cosmic-ray showers are still much more extended.

> 2. At the CGRO learning center I was expecting to see
> plots of the pool of Cerenkov light to be elliptical
> for a gamma-ray and roughly circular for a cosmic ray.
> I don't see this in the plots shown, especially the
> cosmic ray plot, which shows a scattered distribution
> that I don't understand.
>

The plots at the learning center are actually hit positions of photons from simulated air showers. If many more events were plotted, and some smoothing were done, the shapes you describe would start to be apparent.

> 3. On the actual imaging telescope why are there multiple
> mirrors instead of just one dish with the PMTs behind
> focal plane. Also why are the individual
> mirrors hexagonal and not say squares or pentagons?
>

Some observatories use mirrors originally intended for other work - such as solar power studies. For this type of work, and air Cerenkov work, it is the size of the mirrored surface, not the quality of the mirror which is important. Using the Whipple Observatory example, it would be very expensive to manufacture a smooth mirror with a 10 meter diameter. On the other hand, a 10 meter diameter mirror which is a mosaic of smaller mirrors is cheap, has a large collecting area, and provides adequate optics for the relatively broad structures being measured (the size of the image is typically a few tenths of a degree). The shape of the individual facets is not really important.

> 4. Since the pool of light is much bigger than the area of
> telescope how can the shape of the pool be determined?
> Does the intensity of light fall off or increase in concentric
> contours from the edge of the pool to the center?
>

Detectors are not actually measuring the shape of the entire pool (which is roughly a large pancake), but the "shape" arising from the angular distribution of light in the local part of the pool that the mirror reflects. This shape changes depending upon where you are in the "pool". Even gamma-ray showers look circular at the center of the pool. The light intensity in the pool does have an interesting radial dependence. For gamma-ray initiated showers, if you measured the intensity from the center of the shower, you would find that it was approximately constant out to a distance of 100 meters or so, falling off rapidly after that.

Daryl Macomb
Compton Gamma Ray Observatory team

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