Some colleagues and I were planning to launch a stratospheric balloon in December. A thought occurred to me: what if we were to put some sort of xray camera in the payload? We are expecting a maximum altitude of about 80,000 feet. Would it work any better at that altitude?
Long ago I was part of a group that flew high altitude balloons to study x-rays from pulsars etc. How much you detect and at what energy depends upon how much atmosphere is above your detector. Lower energy x-rays are absorbed more than higher energy x-rays. Realistically you won't detect anything except maybe the Sun at energies below 20 keV and at altitude below 25km. We aimed for altitudes near 40km. To detect high energy x-rays the cheapest method is to use a proportional counter. Not something you can buy off-the-shelf.
Also the Sun only emits a lot of high energy x-rays during solar flares.
An interesting experiment would be to fly a geiger counter see http://earthtosky.net/