This is a γ-selective Geiger counter from the “M” professional serie used also in demonstration devices for measuring the age of minerals by the activity of the isotope potassium-40. It’s a classic all-glass construction with platinum cathode.

Tech. spec.:

·Radiation determine: (γ) gamma soft/hard
·Voltage of start of counting: 720 – 780V ;
·Operating voltage (recommended): 820 – 880V;
·Length of flat part of counting curve (plateau): 200 V
·Slope of the counting curve: < 0.1 %/V;
·Sens. background: < 65 counts/min;
·Maximum counting range: 5*10E4 counts/min
·Load resistance: 8 – 15 MOhm
·Counter resistance: 4 – 8 MOhm
·Allowable stray input capacitance: < 25 pF
·Working temperature range: -40…+50 C
·Anode material: Platinum (Pt)
·Cathode material: Copper (Cu)
·Weight: ~30g.
·Dimensions: counter length 175mm, diameter 21.5mm