Strange energy peak at high altitude (cosmic rays?)

[Magnus7]

I have been travelling by plane with my small RadiaCode 103 gamma spectrometer a couple of times.
Since I have been trying this before with a Geiger Counter I was happy to see the drop in radiation when plane took of until an increase starts at a couple of 100 meters again. So far so good but at higher altitude I get a high peak at the lowest channels.
The low peak is very unexpected so I brought some lead on a flight lately to shield the scintillator to filter out the low energies.
However, the low energy peak was only reduced insignificantly by the shield, but higher energies were reduced so obviously the peak is false. The lead seemed to work fine because the airport security personnel found the “black spots” on x-ray as they said and I was interviewed on why 😊
My guess is that something at 11000 meter altitude with high energy hits crystal or SiPM and makes it register erroneously at the low channels.
The peak starts to show up at around 6000 – 7000 meters altitude, then increasing to cruising altitude of 11000 meter.
Cosmic rays are known to exist frequently at those altitudes so my guess is that it might be the source, if so I would be happy to understand what kind of energetic radiation/particles gives my peak and also the mechanism on how they light up the CsI(Tl) crystal in that case.
I have a larger gamma spectrometer from Gammaspectacular also but I can unfortunately not bring that one on a commercial plane.
I have seen exact same behavior on several flights of different make and models of planes.
Has anyone else used a gamma spectrometer in a plane? (I have heard others using RadiaCode of other models in planes without seeing this)
Do you have any theories on what is going on?

I have attached a screen shot of the peak.

/Magnus Linnér
Amateur user in Sweden

[ColoRad-o]

Hi Magnus! Take a look at https://www.faa.gov/sites/faa.gov/files ... 202108.pdf and https://rosap.ntl.bts.gov/view/dot/20607

[Magnus7]

Thank you ColoRad-o for the links, I just browsed them now but will read them carefully later!

I think it seems most likely that something related to Cosmic Rays is the source of the strange behavior.
Since I do not have any contacts with my old university in Lund I hoped for luck and wrote to the Max IV Lab in Lund and asked for a possible explanation on what is going on in the crystal but they did not (not unexpectedly) have time to give it a thought.
I also contacted the RadiaCode maker and they had no idea.
What I hope for is to understand what's happening inside the crystal giving the result of a false peak.

Magnus Linnér
M.Sc.E.E Malmoe, Sweden

[ColoRad-o]

Google AI comes through!

Search Labs | AI Overview

A "low energy peak" observed in gamma ray detection at high altitudes is , where the atmospheric interaction at higher altitudes leads to a greater probability of lower energy gamma photons reaching the detector, resulting in a noticeable peak at the lower energy range of the spectrum. Key points about this phenomenon:

Atmospheric scattering:
As cosmic gamma rays enter the atmosphere, they can interact with air molecules, causing them to scatter and lose energy, leading to a higher proportion of lower-energy gamma rays at high altitudes. 

Altitude dependence:
The intensity of this low-energy peak will generally increase with altitude due to the thinner atmosphere allowing more scattered gamma rays to reach the detector. 
Background radiation:

This "low energy peak" can be considered part of the background radiation measured by gamma ray detectors at high altitudes, which needs to be accounted for in data analysis. 

Footprint and height corrections for UAV-borne gamma-ray spectrometry studies
https://www.sciencedirect.com/science/a ... 1X21000175

A Rapid Gamma-Ray Glow Flux Reduction Observed From 20 km Altitude
https://agupubs.onlinelibrary.wiley.com ... 0mechanism.

Gamma-ray glow preceding downward terrestrial gamma-ray flash
https://www.nature.com/articles/s42005-019-0168-y

Let us know what you find out!--DMW

[Magnus7]

Thanks again ColoRad-o!
My "low energy peak" is false. I brought lead on a following flight and wrapped around the scintillator crystal to filter out low energy gamma and x-rays. Other energies were significantly attenuated by the lead as expected but my "low peak" was almost unchanged. So I assume the peak comes from noise generated by high energetic rays/particles that can pass through the airplane fuselage and also the lead.
I attached a screenshot of the spectrum with lead. Green is without filter, orange is with lead wrap.
It is no problem at all but my curiousity has been triggered.
I will gladeley read your links!
/Magnus