[ RadSafe ] low energy gamma-spikes

[marco bähler]

dear group

I live close to an old bwr and went for a walk with my BNC 935 / 3inch NaI. it was towards the end of the annual refueling period.
500m downwind, calm wind, on the open field  suddenly I recorded several short alarms, lasting between one and 12 seconds. countrates up to 15000cps. energy almost exclusively around 25 keV. I am wondering what this could have been. 
answers from the authorities range from "noise due to high tension lines" (actually 200m away) to "probably x-ray exams somewhere" and  "no hear no see no say" or "cosmic particle showers". 
one answer referred to the gm-tube-equipped doserate-monitoring network which "recorded nothing". I think it is quite common for compensated gm tubes not to see 25 keV.
Is someone of YOU familiar with such spikes (me thinks I can call them clouds) and knows possible sources/causes? (Cd109??)

kind regards and thankyou for pondering 

marco bähler
(euratom level 5b, 2004 utrecht)
uetikon
switzerland