This is another glass russian Geiger tube made for professional use, not for military like all the M-series tubes. The MS-6 is gamma-selective and have a copper cathode. The internal wire is the anode and is made of platinum.
I’ve found two different but similar specs for this tube from two different eBay sellers
First eBay seller 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: < 120 counts/min; · Maximum radiation: 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) · Dimensions: counter length 260mm, diameter 21.5mm
Second eBay seller spec:
Beginning of the counting voltage - 720V-780V. The working voltage interval - 820V-880V. The length of the plateau of the counting characteristics - not less than 200V. The slope of the plateau of the counting characteristic - 0.1%/V. The maximum count rate - 25000imp/min. The biggest overload irradiation - 50000imp/min. Sensitivity (when irradiated with 0.1 μR/s) - 4500imp/min-5500imp/min. The largest background - 120imp/min. Service life - 3∙108imp. Temperature - from -40ºС to +50ºС. Dimensions: diameter - 23mm length - 266mm Weight - 65g