"west" writes:


"Craig Markwardt" wrote in message
news

"west" writes:
I have access to several types of used Nuclear Medicine gamma cameras.
I've
often entertained the idea to convert one for astro-gamma imaging. I'm
not
much of an astronomer but am somewhat knowledgeable with gamma cameras.
The
main problem that has always stopped me from pursuing this idea, is the
fact
that the large sodium-iodine crystal can fracture if there's an ambient
temperature change of 6 degrees F in 1 hour. Outdoor temperature
fluctuations ... (well I'm sure you understand).
I apologize if this is not the correct NG for my question. Perhaps I can
be
directed to a more appropriate one? Thank you.

Your main problem will be that extraterrestrial gamma rays do not
penetrate the atmosphere to sea level. That's why most gamma-ray (and
X-ray) instruments are launched into space, or at least lofted by high
altitude balloon.
....
I think I need a refresher in spectrum-frequency. Ham radio signals (HF) are
reflected by the ionosphere. That's how Hams communicated around the world.
However, VHF, UHF, etc., are shorter wavelength signals that penetrate
through all atmospheric layers and are lost in space. Communications is line
of sight only. The highest frequency or shortest wavelength of the spectrum
that I am familiar with is gamma waves. Why can they not penetrate the
earth's atmosphere? Also, what are cosmic rays and how do they compare to
gamma rays? Thank you kindly for any input.

The atmosphere is more complicated than you describe. In certain
wavelength bands the atmosphere is opaque; this is due to the
composition of the atmosphere and the nature of electromagnetic
interactions.

For low energy gamma-rays (E 512 keV) the primary interaction would
be photoelectric absorption. A gamma-ray interacts with an atom and
ejects an electron, thereby losing energy. These reactions proceed
until the gamma ray has lost enough energy that it can no longer eject
any more electrons. The ejected electrons and the recombining ions
can produce IR/optical/UV light. Extremely high energy photons can
also make showers which reach the ground, at least the Cerenkov light
from the electrons does.

The point is that the original gamma ray doesn't make it to the
ground.

Best of luck,
CM