This is the smalles geiger tube I could find. It’s so cute but… so insensitive to the background. It’s a good tube to build around it a personal dosimeter. It’s not suitable to monitor background radiation.

SBM21 Micro geiger tube

Tech. spec.:
 
·Radiation determine: beta soft/hard; gamma
·Voltage of start of counting: 260V - 320V;
·Operating voltage (recommended): 380 - 420V;
·Length of flat part of counting curve (plateau): 100 V
·Slope of the counting data: < 0.15%/V;
·Own background: < 0.2 imp/s;
·Max. counting range: 650 imp/s;
·Sensetivity: 6.5 - 9.5 imp/uR standard 137Cs, p = 10 uR/s;
·Working temperature range: -50…+60 C
·Weight: 0,8g.
·Dimensions: counter length 21mm, diameter 6mm