Week ago shutdown core give unpleasant amounts of counts to Cu modified GM tube (and it have also "unreactive" fuel inside)..
Without knowing what You realy did we can only speculate about the nature of Your radiation.
Any Li7+H --> Be --> 2 Alphas can transmute Ni into Cu which radiates B+/-. Ni58 goes to Zn62 which has a half time of 9h, which is long enough for Your measurements.
Also H* can freely travel through any confinement.
If you use steel reactors, you also might encounter monople like particles. (To test this just leave an old Kodak film somewhere around in the room (one week) and develop it -and look for traces made of circles.)
Myons are heavy electrons that react with a strong B-field. You might try to concentrate them...If a strong field doesn't change the particle count then they are of neutral nature of very heavy.