"Flying with Scintillators"
I was on a flight from the Caribbean to Boston yesterday, and used my new Radiacode 103G gamma ray spectrometer to monitor background radiation before and during the flight. It uses a sensitive GAGG:Ce scintillator (Gadolinium Aluminum Gallium Garnet, with Cerium doping). Readings varied from a low of 25 CPM while on a ferry, to 1.38 kCPM up at 37,855 feet (~11.5 km). With only 215 gm/cm^2 of air left above me at our cruising altitude, I was very near the production peak of the "hard" cosmic ray component (muons, pions, kaons, and protons), of which only a small fraction of muons make it down to sea level. At my cruising altitude, the "soft" cosmic ray component (gamma ray photons, and pair production electrons and positrons) is actually stronger than the hard one, although the jet fuselage partially attenuates this. Examining the spectrum showed a low energy X dash ray peak, but also a single line out at the maximum energy bin (bin 1024, nominally corresponding to 3.0 MeV). The hard component of cosmic rays has energies peaking in the several GeV range, far beyond the scale of the Radiacode 103G. They will only lose a fraction of their energy while passing thru the small (1 cubic centimeter) volume of the 103G's scintillator, but even this amounts to over 5 MeV. As it happens, the 103G lumps all off-scale detections into the single highest bin of its MCA (Multi Channel Analyzer), so these detections show as a single very narrow (1 bin) line, which you can see circled in the attached photo.
I was flying JetBlue, in one of their Mint seats. These are very nice; on each side of the aisle, they alternate with only one or two seats per row. I was in a single seat row, with plenty of space to spread out. It occurred to me that the 103G wasn't my only nuclear physics gear, so I dug into the carry on bag in the overhead bin, and brought out a GammaSpectacular GS-8000-Max MCA (8192 channels), a Scionix 38B57 scintillator (NaI, thallium doped, with coupled PMT), and a cable. At first, I thought I was stuck, since the cable had a SHV connector on one end (for the GS-8000), and a standard BNC on the detector end. But my Scionix was fitted with a Ludlum C connector, as I have a Ludlum 3000 survey meter coming. Not to worry; I recalled that I had also packed a Ludlum C to BNC adapter; problem solved! Within a few minutes, I was able to examine the Scionix NaI(Tl) detector's output on my laptop, using both BecqMoni and Impulse software. It appears that the GS-8000 doesn't lump out of scale detections into its top bin, so all that I could see were the low energy X dash rays, presumably XRF from the jet's aluminum fuselage. The main peak was at 56 keV, with a small bump at 447 keV. I had recently programmed the bias voltage to 606 volts, which put the CS-137 line spot-on at 662 keV. Since this detector is larger (3.8 diameter x 5.7 cm long), and the energy loss as per the Bethe-Bloch equation in NaI(Tl) is about 7 MeV/cm, cosmic ray detections would have been way off scale, at ~ 27 MeV with the detector horizontal.
I then recalled that I had a yet larger scintillator in the carry on above me. That is a "Gamma Dog Max", built by Charles David Young for hot rock prospecting, with a 6.3 cm diameter x 6.3 cm long NaI(Tl) scintillator. Just to add to the fun, I brought that down as well, but decided not to fire it up and draw more attention when the GDM would start growling at all the muons. But it rounded out the scintillator party, and made for a nice photo-op.
Separately, I am building a triple shielded (Pb/Sn/Cu), dual detector muon telescope with coincidence circuitry. In this case, I am deliberately screening for only high energy events, and hope to replicate the zenith angle and east-west effects first discovered back in the 1930's.
